Complex organic molecules in protoplanetary disks : X-ray photodesorption from methanol-containing ices. Part I -- Pure methanol ices
R. Basalgète, R. Dupuy, G. Féraud, C. Romanzin, L. Philippe, X. Michaut, J. Michoud, L. Amiaud, A. Lafosse, J-H. Fillion, M. Bertin
aa r X i v : . [ a s t r o - ph . I M ] F e b Astronomy & Astrophysicsmanuscript no. 39676corr © ESO 2021February 22, 2021
Complex organic molecules in protoplanetary disks: X-rayphotodesorption from methanol-containing ices
Part I - Pure methanol ices
R. Basalgète, R. Dupuy, G. Féraud, C. Romanzin, L. Philippe, X. Michaut, J. Michoud, L. Amiaud, A.Lafosse, J.-H. Fillion, M. Bertin Sorbonne Université, Observatoire de Paris, PSL University, CNRS, LERMA, F-75014, Paris, France Univ Paris Saclay, CNRS UMR 8000, ICP, F-91405, Orsay, France Univ Paris Saclay, CNRS, ISMO, F-91405, Orsay, FranceReceived 14 October 2020 ; Accepted 5 January 2021
ABSTRACT
Context.
Astrophysical observations show complex organic molecules (COMs) in the gas phase of protoplanetary disks. X-rays emit-ted from the central young stellar object (YSO) that irradiate interstellar ices in the disk, followed by the ejection of molecules in thegas phase, are a possible route to explain the abundances observed in the cold regions. This process, known as X-ray photodesorption,needs to be quantified for methanol-containing ices. This paper I focuses on the case of X-ray photodesorption from pure methanolices.
Aims.
We aim at experimentally measuring X-ray photodesorption yields (in molecule desorbed per incident photon, displayed asmolecule / photon for more simplicity) of methanol and its photo-products from pure CH OH ices, and to shed light on the mechanismsresponsible for the desorption process.
Methods.
We irradiated methanol ices at 15 K with X-rays in the 525 - 570 eV range from the SEXTANTS beam line of the SOLEILsynchrotron facility. The release of species in the gas phase was monitored by quadrupole mass spectrometry, and photodesorptionyields were derived.
Results.
Under our experimental conditions, the CH OH X-ray photodesorption yield from pure methanol ice is ∼ − molecule / photon at 564 eV. Photo-products such as CH , H CO, H O, CO , and CO also desorb at increasing e ffi ciency. X-rayphotodesorption of larger COMs, which can be attributed to either ethanol, dimethyl ether, and / or formic acid, is also detected. Thephysical mechanisms at play are discussed and must likely involve the thermalization of Auger electrons in the ice, thus indicatingthat its composition plays an important role. Finally, we provide desorption yields applicable to protoplanetary disk environments forastrochemical models. Conclusions.
The X-rays are shown to be a potential candidate to explain gas-phase abundances of methanol in disks. However, morerelevant desorption yields derived from experiments on mixed ices are mandatory to properly support the role played by X-rays innonthermal desorption of methanol (see paper II).
Key words.
Astrochemistry, Protoplanetary disks, X-ray photodesorption, X-ray induced-chemistry
1. Introduction
Methanol (CH OH) is a main complex organic molecule (COM)observed in the interstellar medium (ISM) and is considered aprecursor for the formation of larger COMs such as, potentially,amino-acids (Garrod et al. 2008; Elsila et al. 2007). Its detectionin the gas phase of the ISM is often used as a reference toprobe other gaseous COMs (Bergner et al. 2019), but its weakemission lines make the observations di ffi cult (e.g., Carney et al.2019). In protoplanetary disks, methanol, formaldehyde (H CO)and formic acid (HCOOH) have been detected in the gas phasearound di ff erent young stellar objects (YSOs). In the ClassII TW Hydrae protoplanetary disk, a peak column density of ∼ × cm − , suggested to be located at the CO snowline,was derived for methanol in the gas phase (Walsh et al. 2016),and formic acid was also detected with a disk-averaged columndensity of ∼ × cm − (Favre et al. 2018). In the disk Send o ff print requests to : [email protected] around the young Herbig Ae star HD 163296 (Carney et al.2019), an upper limit on the disk-averaged column density ofmethanol was found to be ∼ × cm − and a CH OH / H COabundance ratio < CO and CH OH were estimated at ∼ . − × cm − and < . − . × cm − , respectively, with a CH OH / H COabundance ratio < CO ring located beyond the COsnow line (Podio et al. 2019). In the circumstellar envelope ofthe post-asymptotic giant branch (AGB) object HD 101584(Olofsson et al. 2017), H CO, H
CO, and CH OH were alsoobserved.Methanol is the only COM that has been detected as asignificant constituent of the icy dust grains that can be foundat several stages of star formation, with an estimated abundancebetween ∼
1% and ∼
25% relative to H O (Taban et al. 2003;
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Gibb et al. 2004; Boogert et al. 2015). Beyond the CO snowline, at T ∼
17 K (Öberg et al. 2005), in the coldest outerregions of protoplanetary disks, most COMs are believed to beformed directly onto dust grains, but they can also originatefrom competitive gas-phase reaction channels depending on thelocal physical conditions. This is in contrast with methanol,which is generally thought to be entirely formed on the icygrain mantles by successive hydrogenation of CO in the upperCO-rich phase of interstellar ices (Watanabe & Kouchi 2002),although this is still debated (Dartois et al. 2019). In these coldregions, nonthermal processes should then be invoked to explainthe presence of gaseous methanol.It is expected that photons and / or cosmic rays coming fromvarious sources could trigger the ejection of methanol from theicy dust grains into the gas phase and participate in the overallgas-to-ice balance of these cold regions. For the specific case ofphotons, this mechanism is known as photodesorption. Severalexperimental studies have been conducted to quantitativelyconstrain these processes in order to explain astrophysicalobservations, especially for methanol-containing ices. In oneof these experiments, heavy ion Xe + irradiations wereperformed on methanol in pure ice and embedded in a water-icematrix (Dartois et al. 2019). A sputtering yield of methanolclose to that of the main water-ice matrix (Dartois et al. 2018),which is ∼ sputtered molecule per incident ion, wasestimated for each studied ice. When it was embedded in a CO ice, Dartois et al. (2020) found that this sputtering yield is aboutsix times higher. Experimental studies of UV photodesorptionin the 7 - 10.5 eV range were first conducted for pure methanolice at 20 K by Öberg et al. (2009). More recent experiments byBertin et al. (2016) in the 7-14 eV range suggested an e ffi ciencyfor methanol UV photodesorption from pure methanol ice of ∼ − molecule / photon. This desorption was found to bebelow the detection threshold ( < − molecule / photon) whenmethanol was mixed with CO ice for a wide range of dilutionfactors (from 1 in 4 to 1 in 50; Bertin et al. 2016). Accordingly,Cruz-Diaz et al. (2016) derived an upper limit of 3 × − molecule / photon for the UV photodesorption yield of methanolfrom pure methanol ice from 8 K to 130 K (in the 6.88-10.9eV range). In addition to these previous mechanisms, chemicaldesorption, which is the desorption induced by exothermicreactions, is a possible route for explaining the gas-phaseenrichment in the ISM (Cazaux et al. 2016; Minissale et al.2016b; Ligterink et al. 2018). However, chemical desorptionof methanol by H addition onto CO, H CO, and CH OH iceswas not detected (upper limit < ∼ ∼ erg.s − . Dependingon the YSO emission spectrum and on the geometry andcomposition (dust and gas densities) of the protoplanetary disk,X-rays can penetrate the disk more deeply than UV photonsand therefore irradiate more molecular ices (Agundez et al.2018; Walsh et al. 2015). As protoplanetary disks are generallyformed within embedded YSO clusters, Adams et al. (2012)suggested that the X-ray background field originating fromthese clusters could also increase the X-ray flux that irradiatesthe molecular ices in the outer region of the disk (e.g., for r & Oand its photo-fragments in protoplanetary disks (Dupuy et al.2018). In this experimental study, an average X-ray photodes-orption yield for H O from pure water ice was estimatedfrom ∼ − to ∼ − molecule / photon for di ff erent regions inprotoplanetary disks. X-ray induced electron-stimulated des-orption (XESD) was proposed as a possible mechanism for thephotodesorption of neutral molecules from water ice: followingX-ray photoabsorption, the excited molecular state decaysby releasing an Auger electron of ∼
500 eV that thermalizesthrough the ice and creates secondary valence ionizations orexcitations of the neighboring molecules, ultimately leading totheir desorption at the ice surface. X-ray photodesorption inthe 250-1250 eV range has also been studied for H O:CO:NH ices (Jiménez-Escobar et al. 2018), but no methanol desorptionwas detected. Irradiation of a mixture of H O:CO:NH (2:1:1)covered by a layer of CO:CH OH (3:1) with 250-1250 eVX-rays at high flux (120 minutes at 7 . × photon / s) did notshow significant methanol desorption compared to CO, CO ,HCO, and H CO desorption (Ciaravella et al. 2020). X-rayphotodesorption (at 537 eV) of ions from pure methanol ice at55 K was also studied by Andrade et al. (2010) : a photodesorp-tion yield of ∼ × − ions / photon was estimated for H CO + and CH OH + , for instance. Finally, X-ray photodesorption ofneutral molecules from methanol-containing ices has not beenconstrained so far.This is paper I of an experimental work dedicated to thestudy and quantification of the photodesorption from methanol-containing ices in the X-ray energy range. It focuses on theX-ray photodesorption from pure methanol ices with the aim toprovide absolute yields for the main desorbing neutral species,including methanol itself, but also smaller fragments, or evenmore complex molecules, and to shed light on the involvedmicroscopic mechanisms responsible for their ejection. Thework is based on the use of the monochromatized output of theSEXTANTS beam line (SOLEIL synchrotron facility, St Aubin,France) in the O K-edge region of the CH OH molecule (525 -570 eV). Although pure CH OH ices are not likely to be foundin the ISM, studying them is the first necessary step towardstudying those of more complex and astrochemically relevantbinary ices containing methanol. This study is presented inpaper II.In paper I, section 2 introduces the experimental setup andprocedures. Section 3 presents the main experimental results weobtained on the pure methanol ices, including X-ray absorptionprofiles and energy-resolved photodesorption yields derivedfrom our experiments. Section 4 focuses on the molecularmechanisms at the origin of the photodesorption and on com-parisons with the vacuum UV (VUV) photodesorption frompure methanol ices. Finally, section 5 presents and discusses theextrapolation of our results to the astrophysical conditions.
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2. Experimental procedures
Experiments were conducted using the SPICES 2 setup (SurfaceProcesses and ICES 2). It consists of an ultra-high vacuum(UHV) chamber (base pressure ∼ − mbar) within whicha rotatable copper substrate (polycrystalline oxygen-free high-conductivity copper) can be cooled down to T ∼
15 K by aclosed-cycle helium cryostat. Its temperature is controlled with0.1 K precision, and it is electrically insulated from its sampleholder by a Kapton foil, allowing the measurement of the draincurrent generated by the electrons escaping its surface afterX-ray absorption, referred to as the total electron yield (TEY)in the following. The molecular ices are formed using a dosingsystem: a tube positioned a few millimeters away from thesubstrate allows injecting a partial pressure of methanol gas onthe cold substrate surface without notably modifying the basepressure in the chamber, resulting in frozen methanol ice. Theice thickness is calibrated with the temperature-programmeddesorption (TPD) technique (Doronin et al. 2015), with arelative precision of about 10 %. The ice thickness is expressedin monolayer (ML), equivalent to a molecule surface densityof 10 molecules.cm − . In our experiments, we studied ices of ∼
100 ML, deposited at 15 K.The X-ray photon source of the SEXTANTS beam line ofthe SOLEIL synchrotron facility was connected to the SPICES2 setup to run our experiments. We used photons in the range of525-570 eV, corresponding to the ionization edge of the O(1s)electron, with a resolution of ∆ E =
150 meV, where E is thephoton energy. The flux, measured with a calibrated siliconphotodiode, was approximately 1 . × photon.s − , with littlevariation except for a significant dip around 534 eV due to theO 1s absorption of oxygen pollution on the optics of the beamline. The beam was sent at a 47 ◦ incidence on the ice surface ina spot of ∼ .While the ices were irradiated, the photodesorption of neutralspecies was monitored by recording the desorbed molecules inthe gas phase using a quadripolar mass spectrometer (QMS).Each gas-phase species was probed by monitoring the masssignal of its corresponding intact cation: for example, CH OHand HCOOH were recorded by selecting mass 32 and 46,respectively. Di ff erent irradiation procedures were made :- short irradiations at 534, 541, and 564 eV were conductedto measure the photodesorption yields at these fixed energies.The irradiation time per fixed energy was approximately 10seconds (higher than the acquisition time of the QMS, which is100 ms). This mode allowed us to probe the photodesorptionfrom the ices with a relatively low irradiation fluence, mainly toprevent the photo-aging of the condensed systems from having asignificant e ff ect on the detected signals: the fluence received bythe ice during these short irradiations is ∼ × photon / cm .- continuous irradiations from 525 to 570 eV in steps of0.5 eV (scans) allowed us to record the photodesorption spectraas a function of the photon energy. The irradiation time per scanwas approximately 10 minutes and the fluence received by theice was ∼ × photon / cm . During these irradiations, theTEYs were also measured as a function of the photon energywith a scan step of 0.5 eV.These irradiations were performed successively on the same sample, allowing us to explore the aging e ff ect on the extractedphotodesorption yields. Finally, at the end of these irradiationexperiments, TPD experiments (from 15 K to 200 K) were usedto evaporate all the molecules from the substrate surface beforea new ice was formed. In order to compute the photodesorption yields (in moleculedesorbed per incident photon, displayed as molecule / photon formore simplicity in the following) from the QMS signals, thefollowing procedure was conducted:- the signal given by the QMS was corrected for the pho-ton flux profile and for the apparatus function of the QMS for agiven mass to take the transmission and detection e ffi ciency ofthe QMS into account.- using TPD experiments (Doronin et al. 2015), we deter-mined a proportionality factor k X between the moleculardesorption flux and the QMS signal. This calibration wasmade on CH OH where the di ff erence between the monolayerand multilayer thermal desorption regime is quite clear andthus allows depositing a single monolayer of methanol anddetermining the corresponding integral mass signal. Usingthis technique, we assumed that the proportionality factor forphotodesorbed methanol is similar to the factor for thermaldesorption, assuming a similar angular distribution of thedesorbates. This method has previously been employed in thecase of X-ray photodesorption from water ices (Dupuy et al.2018) and was confirmed in the UV range, where an infraredcalibration procedure can also be used for systems with littlephotodissociation, such as pure CO ice.- the photodesorption yields for the other neutral species,such as fragments for which TPD calibration is not possible, canbe deduced from the CH OH calibration by taking into accountthe di ff erences in electron-impact ionization cross sections andapparatus functions of the mass filter. This results in using thefollowing formula (Dupuy et al. 2017): k X = σ ( X + / X ) σ ( CH OH + / CH OH ) × AF ( X ) AF ( CH OH ) × k CH OH (1)where X is the neutral species considered, AF ( X ) is the ap-paratus function of our QMS for the given species, k X is itsproportionality factor, and σ ( X + / X ) is the electron-impactionization cross section for the X neutral species, taken at 70eV. The values for these di ff erent cross sections were foundin the literature (Freund et al. 1990; Srivastava et al. 1996;Straub et al. 1998; Joshipura et al. 2001; Liu & Shemansky2006; Vacher et al. 2009; Zawadzki 2018).- for molecules that can originate from the cracking oftheir parent molecules in the QMS, we used the crackingpatterns available on the National Institute of Standards andTechnology (NIST) chemistry Webbook to correct the signals.More importantly, the cracking of CH OH into CH OH + and / orCH O + was used to compute the photodesorption yield ofmethanol (see section 3).- uncertainties on the photodesorption yields were com-puted by taking the signal-to-noise ratio for the correspondingmass channel of our QMS into account. Article number, page 3 of 11 & Aproofs: manuscript no. 39676corr
In section 4 we compute the X-ray photodesorption yields perabsorbed photon for pure methanol ice, which is an interestingfigure with which to discuss the photodesorption mechanismsusing the following formula: Γ abs = Γ inc − e − σ N (2)where σ is the photoabsorption cross section, N is the columndensity of molecules that we consider to be involved in the pho-todesorption process, and Γ inc is the photodesorption yield perincident photon derived from our experiments at a given en-ergy. We assumed that the X-ray absorption cross section ofcondensed phase methanol is equal to that in gas phase at 564eV, which is ∼ molecule.cm − . Our experimental photodesorption spectra are limited to the 525-570 eV range, while X-ray emission spectra of YSOs are typi-cally in the 0.1-10 keV range. In order to provide photodesorp-tion yields for astrochemical models, we extrapolate our resultsin section 4 to higher photon energies. We observed that the X-ray photodesorption from our ices is following the absorption ofthe O 1s core electrons in the 525-570 eV range (see section 3.1).Above 570 eV, the ionization threshold of O 1s core electrons isexceeded and the X-ray absorption is dominated by O 1s ioniza-tion, also resulting in the release of an Auger electron followedby a cascade of low-energy secondary electrons within the ice.We can therefore extrapolate our experimental photodesorptionspectra above 570 eV by assuming that they follow the samevariations as the X-ray O 1s ionization cross section, which isassumed to be similar to that of gas-phase methanol (Berkowitz2002). For the 525-570 eV range, the photodesorption spectrafollow the TEY, starting from the estimated yields at 564 eV, asobserved in section 4. Then, for a given X-ray emission spectrum φ ( E ), where E is the photon energy, we can estimate an averagephotodesorption yield Y avg using the following formula: Y avg = R Γ inc ( E ) φ ( E ) dE R φ ( E ) dE (3)where Γ inc ( E ) is our measured photodesorption yield. In proto-planetary disks, as X-rays are attenuated by dust and gas de-pending on the region considered, we also provide an averagephotodesorption yield for attenuated X-ray spectra φ att ( E ) corre-sponding to di ff erent H column densities using the Beer-Lambertlaw, φ att ( E ) = φ ( E ) e − σ pe ( E ) n H (4) where n H is the H column density and σ pe ( E ) is the photoelectriccross section of gas and dust in a typical T Tauri protoplanetarydisk estimated by Bethell & Bergin (2011). Fig. 1. (a) TEY as a function of photon energy for methanol in gasphase, in liquid microjets at 298 K (Wilson et al. 2005), and in con-densed phase at 15 K (measured in our experiments). (b) TEY of puremethanol ice, as deposited (fresh ice), and after having been processedby an irradiation fluence of 1.10 ph / cm (processed ice).
3. Results
The TEY measured from our experiments are shown in Figure1.(b). The di ff erent features observed are labeled and can beassimilated to the X-ray absorption of the ice: after a photonis absorbed by a core O 1s electron, the decay of the resultingmolecular excited state leads to the release of an Auger electronof ∼
500 eV. The thermalization of this Auger electron byinelastic scattering within the ice creates secondary valenceexcitations and ionizations of neighboring molecules, leading toa cascade of secondary electrons. The current generated by theescape of these electrons from the ice surface can be quantifiedper incident photon and provides information about the coreelectronic structure of O-bearing molecules in condensed phasenear the O K edge.In Figure 1.(a) we compare our TEY measurement forpure methanol ice (100 ML, at 15 K) with X-ray absorptionspectroscopy of gas-phase methanol and liquid methanol micro-jets (Wilson et al. 2005). The first two peaks of the gas-phasemethanol (at 534 eV and 537 eV) are attributed to 3s and 3p
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Fig. 2.
Photodesorption spectra for masses 30, 32, 44, and 46 in molecule / photon from pure methanol ice at 15 K, with the associated molecules.The measurements at fixed energy are represented by the squares with error bars. The scan experiments are represented by solid lines. The TEYsmeasured during the scan experiments are also shown as dashed lines in arbitrary units. Information about the fluence received by the ice beforeeach measurement is displayed in the bottom panel. Rydberg orbitals, with some σ ∗ character of the O-H and C-Obonds, respectively. From the gas phase to the liquid phase,these peaks are broadened as a result of the greater orbitaloverlap with neighboring molecules in the liquid state, and thegas-phase first peak is also blueshifted by ∼ ff erences are observed between the TEY of liquid methanolat 298 K and methanol in condensed phase at 15 K, exceptfor the small peak at 531.5 eV that is labeled peak 1 in Figure1.(b). We lack published data on the X-ray absorption of solidmethanol in the considered energy range, therefore it is di ffi cultto propose an attribution for this feature. Its low dependenceon the irradiation fluence indicates a structure that is associatedeither with intact methanol ice, that is, not to a photoproduct, orwith the substrate itself. However, the mean depth reachable bythe photoelectrons is estimated to be about 30 ML (see section2.3). This is three times lower than the methanol-ice thickness,which makes a possible contribution of the substrate to theTEY negligible. Finally, the broad peak near 537 eV (labeled 3in Figure 1.(b)) decreases when the ice is irradiated, showingthat the destruction of CH OH occurs in the ice. From the photodesorption yields that are presented in the next sections,we can estimate that about 1 ML of methanol is photodesorbedfor a fluence of 10 photon / cm . The ice thickness is 100 MLand the probed thickness is ∼
30 ML, therefore the TEY is notexpected to be strongly a ff ected by the photodesorption. Instead,we attribute the decrease in peak 3 to the photodissociation ofthe methanol and to photo-chemistry. This is further confirmedby the appearance of peak 2 with an ongoing irradiation fluence( > × photon / cm ). This peak can be attributed to the1s − π ∗ resonance (1 σ -2 π transition) of CO molecules in con-densed phase, which are formed by X-ray induced chemistry.This feature is also observed for CO in gas phase (Püttner et al.1999) and for pure CO ice (Dupuy 2019). Similar photoaginge ff ects have been observed in X-ray irradiation experiments ofpure methanol ice (La ff on et al. 2010; Chen et al. 2013). In Figure 2 we report the photodesorption yields from puremethanol ice in molecule desorbed per incident photon (dis-
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Table 1.
X-ray photodesorption yields (in 10 − molecule / photon) at 564 eV from pure methanol ice at 15 K and for a dose between 5.10 and2.10 photon / cm . Mass - photodesorbed species Yield Mass - photodesorbed species Yield15 - CH . ± .
30 - H CO 1 . ± .
16 - CH , O 1 . ± .
32 - CH OH 7 . ± .
17 - OH ND *
44 - CO ±
18 - H O 7 . ± .
46 - HCOOH, C H O isomers 0 . ± .
28 - CO (1 . ± . × ND = Not detected: the desorption signal measured for the considered species is lower than our signal-to-noise ratio, meaningthat we did not detect its desorption from the ice. Considering the noise profile in the mass channel 17 of our QMS, if OHphotodesorption occurs, the photodesorption yield is < × − molecule / photon. (1) The yield is given without being able to correct for the cracking of CH into CH in the QMS. These values may thereforebe overestimated.played molecule / photon for more simplicity) we derived fromour measurements. We do not display all available data for moreclarity, as we did not observe behaviors di ff erent from the datawe present. The TEY measurements are also shown in arbitraryunits (only the energy dependence is of interest when comparingwith the photodesorption). The photodesorption yields derivedfrom the irradiations at fixed energy are consistent with thosemeasured during the scan experiments, except when the fluencereceived by the ice di ff ers. The reason is that the induced chem-istry occurs in the ice. We discuss this in the next sections. Theremaining relevant data we obtained are summarized in Table 1,where the yields are derived from our fixed energy experiments.As a lower fluence is used compared to the scan experiments,the aging e ff ect is limited for these yields: the fluence receivedby the ice before measurement ranges from 5 × to 2 × photon / cm . These yields are derived at a fixed energy of 564 eV.CO and CO are the most strongly desorbing species witha photodesorption yield at 564 eV of ∼ / photon and ∼ / photon, respectively. OH X-ray photodesorp-tion is not detected during irradiations. For the desorption signalon the mass 16, we were not able to distinguish between CH or atomic O. For the mass 15, we give the raw data that are notcorrected for any cracking pattern (especially from the mass 16,which could be attributed to CH photodesorption) and it maybe overestimated.As the QMS signal registered on the mass 32 could corre-spond to O or CH OH desorption, we used the signal on themass 31 to estimate the weight of CH OH photodesorptionon the mass channel 32. We assumed that the desorption ofCH OH or CH O radical (that would contribute to the masschannel 31) is negligible in our experiments with pure methanolice, so that the signal on the mass 31 only originates fromthe cracking of desorbing CH OH into CH OH or CH O(which are the main fragments) in the ionization chamber ofthe QMS. This hypothesis agrees well with our conclusionsabout the photodesorption mechanisms at play in section 4.1and appears reasonable considering the data available fromsimilar experiments: the non-negligible formation of CH OHis clearly visible by infrared spectroscopy (Chen et al. 2013)when pure methanol ice is irradiated at 550 eV at similar fluence, but UV photodesorption of CH OH or CH O wasnot observed ( < × − molecule / photon; Bertin et al. 2016)from pure methanol ice. We then found that a maximumof ∼
75% of the signal on the mass 32 could correspond toCH OH photodesorption (after correction for the cracking ofCH OH into CH OH or CH O, this brings the signal on themass 31 to below our detection threshold, which is 5 × − molecule / photon), at 541 and 564 eV and for fresh ice (fluence < × photon / cm ). The remaining signal on the mass 32( ∼ photodesorption, whose yieldat 564 eV is then ∼ . × − molecule / photon. When thefluence increased to 3.10 photon / cm (see Figure 2), we foundthat only ∼
30% of the signal on the mass 32 could correspondto CH OH photodesorption at 541 eV.We also observed a photodesorption signal on the masschannel 46, which can be attributed to either HCOOH (formicacid) and / or C H O isomers (ethanol and dimethyl ether). Thesemolecules have been detected by infrared spectroscopy whenpure methanol ice at 14 K was irradiated with X-rays of 550 eV(Chen et al. 2013).
4. Discussion
Figure 2 shows a correlation between the photodesorptionspectra from pure methanol ice and the TEY as a function ofthe photon energy (except for the broad peak observed on thephotodesorption spectrum of the mass channel 32 between545 and 565 eV, which is due to an unstable background noiseand does not reflect any particular physical mechanism). Thephotodesorption process is thus linked to the X-ray absorption ofthe ice and could be caused by two main mechanisms. The firstmechanism is direct desorption, which means that the desorptionoccurs after the decay of an excited molecular state due to thephotoabsorption on the ice surface. The second mechanism iscalled X-ray induced electron-stimulated desorption (XESD).In this case, the desorption originates from the multiple eventscaused by the cascade of secondary electrons generated by theX-ray absorption of O 1s core electrons within the ice followed
Article number, page 6 of 11asalgète et al.: X-ray photodesorption from methanol-containing ices by the thermalization of the Auger electron. It is not clearwhether direct desorption or XESD dominates the desorptionprocess because the fact that the photodesorption spectra followthe TEY is compatible with either process. However, in thefollowing discussion, we develop arguments in favor of XESD.For simpler molecular ices such as water ice or CO ice, X-rayphotodesorption experiments (Dupuy et al. 2018; Dupuy 2019)have demonstrated that XESD was the dominant process.When the fluence received by our pure methanol ice is in-creased, the TEY and photodesorption spectrum shapes aremodified in a similar way. The main modification is that a peaknear 535 eV arises (see Figure 1 or 2) that is as high as the broadpeak near 540 eV that we attributed to CH OH molecules. It canbe explained by the formation of CO inside the ice as a result ofX-ray induced chemistry. This means that the photodesorptionprocess can originate from either CO (534 eV) or CH OH(541 eV) X-ray photoabsorption and lead to quite similarphotodesorption e ffi ciency at these energies for a processedice. In Figure 2, for instance, the photodesorption yields onthe mass channel 32 at 534 eV and at 541 eV are 0 . × − molecule / photon and 0 . × − molecule / photon, respectively,for a fluence of 2 × photon / cm , whereas they di ff er by afactor of ∼ ffi cient COmolecules are present in the ice, CO X-ray absorption at 534 eVleads to the same photodesorption e ffi ciency of the mass 32 asCH OH X-ray absorption at 541 eV. This is also observed forCO photodesorption yields at 534 eV and 541 eV, which are7 . × − molecule / photon and 6 . × − molecule / photon,respectively, for a fluence of 2 × photon / cm , whereasthey di ff er by a factor of ∼ < × photon / cm and at 3 × photon / cm in Figure 2, the CO photodesorption first increasesfrom 2 . × − molecule / photon to 7 . × − molecule / photon.We also observed this phenomenon for CO photodesorptionyield (the data are not shown for more clarity), which increasedfrom 1 . × − molecule / photon to 3 . × − molecule / photon.Second, the estimated yield for the X-ray photodesorption ofCH OH from pure methanol ice decreased by almost one orderof magnitude from 9 . × − molecule / photon to 1 . × − molecule / photon. This indicates that the photodesorption ofCH OH is higher for a lower fluence received by the ice whenmore intact methanol molecules are present in the ice. Thisaging process favors the photodesorption of simpler moleculessuch as CO or CO. La ff on et al. (2010) estimated with NEX-AFS spectroscopy (at the C K-edge) that X-ray irradiation at150 eV of pure methanol ice at 20 K leads to a survival rate of50% for methanol after an absorbed dose of 1.1 MGy. In ourfixed-energy experiments, we irradiated pure methanol ice withfluences between 5 × photon / cm and 2 × photon / cm .Because we irradiated a volume of 0.1 cm ×
100 ML, with amean energy of ∼
550 eV, and when we consider a volumic massof condensed methanol of ∼ .
64 g.cm − (at 20 K; Luna et al.2018) and an X-ray absorption cross section of ∼ ∼ ∼
15 MGy, whichis quite similar to the absorbed doses in La ff on et al. (2010). This indicates that we could expect a methanol destructionrate of about 50% for our low-fluence experiments. In similarexperiments, when irradiating a H O:CH :NH (2:1:1) icemixture covered by a layer of CO:CH OH (3:1) with 250-1250eV X-rays during 120 minutes with a flux of 7 . × photon / s,higher by almost two order of magnitudes than our experiments,Ciaravella et al. (2020) did not detect a desorption signal onthe mass channel 31 (attributed to methanol desorption) andestimated that only ∼
20% of methanol molecules remainedintact in the first minutes of the irradiation. The irradiation fluxtherefore appears to be critical for detecting methanol desorptionin X-ray irradiation experiments of methanol-containing ices. Alower X-ray flux appears to favor methanol desorption becausethe methanol destruction rate is lower. This destruction ofmethanol molecules could also have a significant e ff ect on theformation and desorption of more complex molecules.In Table 2 we compare the photodesorption yields per ab-sorbed photons in the X-ray (at 564 eV, from Table 1) and theUV domains (at 10.5 eV; from Bertin et al. (2016)) for CO,H CO, and CH OH from pure methanol ice using the methoddescribed in section 2.3. To compute the UV photodesorptionyield per absorbed photon, we used the same formula as for theX-ray photodesorption, and we considered the UV photodesorp-tion yields measured at 10.5 eV in Bertin et al. (2016), whichare 1 . × − , 1 . × − and 1 . × − molecule per incidentphoton for CO, H CO, and CH OH, respectively. The columndensity involved in the UV photodesorption is well constrainedfor pure CO ices (Bertin et al. 2012) and equal to ∼ Table 2.
Photodesorption yields in molecule per absorbed photon in UV(at 10.5 eV) and X-ray domains (at 564 eV) from pure methanol ice.
Ice at 15 K Molecule X-rays UV (1)
Pure CH OH CO 8.0 5 . × − H CO 0.08 4 . × − CH OH 0.49 5 . × − from Bertin et al. (2016)The photodesorption yields per absorbed photons in Table2 show that the X-ray yields are approximately three ordersof magnitude higher than the UV yields. This di ff erence isstill significant in the photodesorption yields per absorbed eV:as X-rays carry ∼
500 eV and UV photons carry ∼
10 eV, theCH OH X-ray and UV photodesorption yield is 1 . × − molecule / absorbed eV and 5 . × − molecule / absorbed eV,respectively, which gives a X-ray yield higher than a UV yieldby more than one order of magnitude. This is also the samedi ff erence as for CO X-ray and UV photodesorption yields,which are 1 . × − molecule / absorbed eV and 5 . × − molecule / absorbed eV, respectively. For H CO, this di ff erenceis smaller than one order of magnitude: H CO X-ray and UVphotodesorption yields are 1 . × − molecule / absorbed eVand 4 . × − molecule / absorbed eV, respectively. Article number, page 7 of 11 & Aproofs: manuscript no. 39676corr
For pure methanol ice, X-ray photodesorption per absorbed eVis therefore more e ffi cient by one order of magnitude than UVphotodesorption. This could be explained by a di ff erence inthe induced chemistry. The processing of pure methanol ice byhigh-energy electrons (of ∼ ff erence observed in thephotodesorption e ffi ciency and the mechanisms at play. Theprocessing of pure methanol ice by UV photons (ice at 20 Kirradiated by 7–10.5 eV photons; Öberg 2016), X-rays (ice at 14K irradiated by monochromatic X-rays at 300 eV and 550 eVand by broadband 250–1200 eV X-rays; Chen et al. 2013), andhigh-energy electrons (ice at 11 K irradiated by 5 keV electrons;Bennett et al. 2007) leads to similar photo-products within theice. However, there are some slight di ff erences that may beimportant. For example, kinetic modeling of induced chemistryshows some di ff erences in the dissociation branching ratios ofCH OH between UV (Öberg 2016) and electron irradiations(Bennett et al. 2007). Moreover, Chen et al. (2013) comparedthe number of molecules produced per absorbed eV in puremethanol ice between UV (from Öberg (2016)), X-rays, and5 keV electron (from Bennett et al. (2007)) experiments andsuggested a lower product e ffi ciency by UV irradiation thanby X-rays and 5 keV electron irradiations. These di ff erencesbetween UV and secondary low-energy electron inducedchemistry may translate into a di ff erence between X-ray andUV photodesorption e ffi ciency for pure methanol ice that favorsthe X-ray e ffi ciency over the UV e ffi ciency (Table 2).The way in which the energy is deposited within the iceand its consequence on the induced chemistry could provide aninteresting route for explaining X-ray and UV photodesorptione ffi ciency from pure methanol ice. When an X-ray is absorbedin the ice, most of the energy ( ∼
500 eV) goes into the Augerelectron, and the deposited energy is spatially localized aroundthe thermalization path of this Auger electron. Depositing anequivalent amount of energy ( ∼
500 eV) with UV photons inthe ice may yield a di ff erent spatial distribution of the total de-posited energy as it is expected to be homogeneously distributedin the ice. At the low temperatures considered here (T ∼
15 K),where the di ff usion of molecules is limited, this localization ofthe deposited energy would therefore favor reactions betweenneighboring excited molecules, radicals or ions in the case ofX-ray absorption compared to UV absorption. Moreover, X-rayinduced chemistry could involve a richer reaction network as aresult of ion chemistry because molecular ionizations are easilyproduced in the thermalization path of the Auger electron,whereas UV photons in the range of 7-14 eV are expectedto produce fewer ions because their energy is close to theionization energy of CH OH.Finally, for the specific case of CH OH photodesorption,Bertin et al. (2016) suggested that the UV photodesorptionof methanol from pure methanol ice originates from theexothermic recombination of CH O / CH OH into CH OHfollowed by its desorption (also suggested by Öberg (2016)).An exothermic recombination was also proposed in similar experiments as a possible route for the UV photodesorptionof H CO from formaldehyde-containing ices (Féraud et al.2019). Bennett et al. (2007) suggested that the main dissociationchannels of CH OH in condensed phase by 5 keV electronslead to CH OH, CH O, and CH formation. CH OH / CH Oreactions with H atoms were also suggested as a recombinationchannel to re-form methanol. This recombination process couldbe responsible for methanol X-ray photodesorption from puremethanol ice. Moreover, the reaction CH OH e − −−→ CO +
4H incondensed methanol is very e ffi cient for low-energy electrons(Lepage et al. 1997), as suggested in La ff on et al. (2010), whichis certainly why a significant CO formation is observed in theTEY evolution with increasing fluence in our experiments.Thus, as X-ray induced chemistry may produce a large amountof H atoms in pure methanol ice, the recombination of CH OHand / or CH O with H atoms to re-form methanol, followed byits desorption, should be more e ffi cient than the same processfor UV-irradiated pure methanol ice, and this recombinationprocess should be favored by a spatially localized X-ray inducedchemistry, as explained before. This supports our finding thatthe CH OH photodesorption yield / absorbed eV from puremethanol ice is larger for X-rays than for UV photons and isconsistent with our previous results that stated a lower X-rayphotodesorption e ffi ciency of methanol when fewer intactmethanol molecules are present in pure methanol ice as a resultof an increasing X-ray fluence. In the previous section, we have derived the photodesorptionyields at fixed energy under our experimental conditions.In this section, we extrapolate these yields to astrophysicalenvironments by applying the method described in section 2,considering three di ff erent X-ray emission spectra: one of aClassical T Tauri star (CTTS), TW Hya, from Nomura et al.(2007), one of a Herbig Ae star, HD 104237, from Skinner et al.(2004), and one that represents the X-ray emission spectrumof a cluster of YSOs, computed by Rab et al. (2018). As theseX-rays are attenuated by the disk material (gas and dust)when they reach the cold regions of the disk, we multipliedthem by an attenuation factor (from Bethell & Bergin (2011))depending on the H column density. The resulting spectrashown in Figure 3.(a), (b), and (c) thus represent an estimate ofthe local X-ray field for di ff erent regions of the disk. In Figure3.(d) we reproduce the photodesorption spectrum of CH OHfrom 0.525 keV to 0.570 keV by starting from the estimatedyield from pure methanol ice in Table 1 at 564 eV (7 . × − molecule / photon) and by taking the same variations as the TEYin the 525-570 eV range, according to what we discussed inprevious sections. From 0.570 keV to 10 keV, we assumed thatthe photodesorption spectrum followed the X-ray absorptioncross section of condensed phase methanol, the latter assumedto be similar to the X-ray absorption cross section of gas-phasemethanol linked to the O 1s ionization (taken from Berkowitz(2002)) as it is related to a core absorption and should not besignificantly modified between gas phase and condensed phase.The final computations are presented in Table 3, where wedisplay the estimated astrophysical average photodesorptionyields of methanol in di ff erent regions of protoplanetary disks.As these yields are computed in photodesorbed molecule perincident photon, the di ff erences in X-ray luminosity betweenYSOs could play an important role for the actual photodesorp-tion in protoplanetary disks. For example, Imanishi et al. (2003) Article number, page 8 of 11asalgète et al.: X-ray photodesorption from methanol-containing ices
Table 3.
Average astrophysical photodesorption yield of methanol from pure methanol ice at 15 K in molecule / photon for di ff erent YSO X-rayspectra and for di ff erent attenuation n H column density, computed as detailed in section 2. The X-ray spectra used are presented in Figure 3. TW Hya (1)
Herbig Ae (2)
XBGF (3)
Source spectrum 3.9 ± × − ± × − ± × − n H = cm ± × − ± × − ± × − n H = cm ± × − ± × − ± × − n H = cm ± × − ± × − ± × − n H = cm ± × − ± × − ± × − X-ray spectrum from Nomura et al. (2007) (2)
X-ray spectrum of Herbig Ae star HD 104237 from Skinner et al. (2004) (3)
X-ray spectrum from Rab et al. (2018)observed that Class I YSOs might have higher X-ray luminositydistributions than Class II and Class III YSOs. Hamaguchi et al.(2005) suggested that Herbig Ae / Be stars could emit X-rayswith higher luminosity than low-mass pre-main sequence starssuch as T Tauri stars.This method outputs values of the same order of magni-tude for the di ff erent X-ray emission spectra. However, weshould note that this computation has some limitations:- the attenuation factor of gas and dust applied to the X-ray emission spectra could vary from disk to disk becauseof di ff erences in gas and dust densities and disk geometry,as has been shown in protoplanetary disk modeling (e.g.,Agundez et al. 2018; Walsh et al. 2015).- the photodesorption yield at 564 eV was derived underour experimental conditions using an X-ray flux of ∼ photon / s. According to what we discussed in the previoussections, using a higher flux (e.g., higher by approximately twoorders of magnitude) in similar experiments (Ciaravella et al.2020) outputs di ff erent and important results: the X-ray pho-todesorption of methanol, formic acid, dimethyl ether, and / orethanol, for example, appears to be more easily detected inour experiments, with a lower X-ray flux. In protoplanetarydisk environments, the local X-ray flux that could be found inthe cold regions (T <
30 K), which is between 10 − and 10 − erg / cm / s (i.e., between 10 and 10 photon / cm / s) in the caseof a T Tauri-like disk modeling (Walsh et al. 2012), for instance,is lower by many orders of magnitude than the flux used underexperimental conditions. As irradiating molecular ices with ahigh X-ray flux during a short time (experimental conditions)may result in significantly di ff erent outputs than irradiatingthese ices with a lower flux but during a much longer time andwith other physical processes at play (astrophysical conditions),the experimental results should be extrapolated to astrophysicalenvironments with care.In the previous sections, we have demonstrated that X-rays are more e ffi cient than UV photons in photodesorbingCOMs (methanol, formic acid, dimethyl ether, and / or ethanol)from pure methanol ice per absorbed photon (this is also the caseper incident photon) and under experimental conditions. Whenthese experimental results are extrapolated to protoplanetarydisk environments, regarding the estimated astrophysical UV photodesorption yield of methanol from pure methanol ice inBertin et al. (2016), which is 1 . × − molecule / photon, ourresults in Table 3 also suggest that given an incident photon,X-rays are at least as e ffi cient as UV photons in photodesorbingmethanol from pure methanol ice. However, as these yieldsare computed per incident photon, we should also considerthe di ff erences between the local UV flux and the local X-rayflux in protoplanetary disks. In the T Tauri protoplanetary diskmodel implemented by Walsh et al. (2012), the cold regions (T <
50 K) near the disk midplane lie at a radius greater than 10AU. For Z / R < − and 10 − erg.cm − .s − (10 and 10 photon.cm − .s − ), and the UV flux is negligible ( < photon.cm − .s − ). According to our results, X-ray photodes-orption from the typical interstellar ices that could be foundin this region should participate in the richness and diversityof the gas phase in addition to possible cosmic ray-inducedsputtering (Dartois et al. 2018, 2019). In the outer cold regions,away from the midplane (for Z / R > to 10 photon.cm − .s − ). In these regions, our estimated astrophysicalphotodesorption yields also indicate that X-ray photodesorptionper incident photon is as e ffi cient as UV photodesorption. Inaddition to the stellar X-ray emission, Rab et al. (2018) showedusing a T Tauri-like protoplanetary disk model and results fromAdams et al. (2012) that the X-ray background field (XBGF)possibly originating from clusters of YSOs surrounding theprotoplanetary disk could dominate in terms of flux the centralyoung star X-ray emission at the very outer cold regions of thedisk (at a radius >
10 AU, for any Z / R ). This background fieldcould also act as an e ffi cient source for X-ray photodesorption.In Table 4 we display the astrophysical photodesorptionyields corresponding to the main molecules observed in our ex-periments. These yields are derived with the same method asfor Table 3. As we do not see any significant e ff ect of the X-ray emission spectrum used on the estimated yields in Table 3,we decided to use the X-ray emission spectrum of the TW Hyastar (Nomura et al. 2007) alone, with an attenuation factor corre-sponding to n H = cm . The yields corresponding to other at-tenuation factors can be deduced by applying the same variationsas in Table 3. These yields, however, should be considered withcaution because they are extracted from pure methanol ices thatare unlikely to be found in the interstellar medium. More realisticsystems, in which methanol would be embedded in a frozen ma- Article number, page 9 of 11 & Aproofs: manuscript no. 39676corr
Fig. 3.
Normalized (with respect to the area) X-ray spectra of (a) TWHya (Nomura et al. 2007), (b) Herbig Ae HD 104237 (Skinner et al.2004), and (c) X-ray background field (XBGF) of YSO clusters(Rab et al. 2018). The source spectrum is represented along with its at-tenuation for di ff erent H column densities. (d) Extrapolated photodes-orption yield spectra from our experimental results and using the ab-sorption cross section of gas-phase methanol from Berkowitz (2002) trix composed of the main species of the interstellar ices (suchas H O, CO, or CO ), may exhibit photodesorption e ffi ciencydi ff erent from pure methanol ice. We established in section 4that the photodesorption of neutrals in the X-ray range is muchlikely carried out by the thermalization of high-energy electronsinto the molecular solids, and involves subsequent chemistry, asis demonstrated by the desorption of more complex moleculesthan CH OH. It is therefore expected that the composition ofthe ice a ff ects the photodesorption e ffi ciency. In order to accessmore relevant photodesorption yields, it is then necessary to con-strain this composition e ff ect by studying model binary ices con- Table 4.
Average astrophysical photodesorption yield inmolecule / photon extrapolated from our experimental results us-ing the method described in section 2 for di ff erent molecules at15 K. The X-ray emission spectrum used is the TW Hya one fromNomura et al. (2007), to which we applied an attenuation factorcorresponding to n H = cm . Ice DesorbedMolecule X-ray astrophysicaldesorption yieldPure methanol H O 2.3 ± × − CO 3.9 ± × − CO ± × − H CO 3.6 ± × − CH OH 2.4 ± × − HCOOH, C H O 2.4 ± × − taining methanol. This has been achieved, and the outcomes arepresented in a second article (paper II).
5. Conclusion
Pure methanol ice was irradiated by monochromatic X-rays inthe range of 525-570 eV. Intact methanol, other COMs, andsimpler molecules were found to photodesorb due to X-ray ab-sorption of core O(1s) electrons, quantified by TEY measure-ment, which leads to a cascade of low-energy secondary elec-trons within the ice. X-ray photodesorption yields were derivedand found to be intimately linked to X-ray induced chemistry,which indicates that X-ray induced electron-stimulated desorp-tion (XESD) may be the dominant mechanism in X-ray pho-todesorption from these ices. However, electron-stimulated des-orption experiments are mandatory to conclude on the dominantmechanism. The main conclusions of this paper are listed below.1. CH OH X-ray photodesorption from pure methanol ice isfound to be e ffi cient with a yield of ∼ − molecule des-orbed by incident photon and is assumed to be due to therecombination of CH OH and / or CH O into CH OH.2. X-ray photodesorption of formic acid, ethanol, and / ordimethyl ether is detected with a yield of ∼ − moleculedesorbed per incident photon.3. Destruction of methanol molecules by photolysis and / or ra-diolysis seems to defavor the detection of methanol des-orption in X-ray irradiation experiments, and the fluencereceived by the ice or the X-ray flux used under experi-mental conditions may be a critical parameter for detectingmethanol desorption.4. X-ray photodesorption is found to be more e ffi cient bymore than one order of magnitude than UV photodesorption(Bertin et al. 2016) for pure methanol ice per incident or ab-sorbed photon, which is assumed to be due to a di ff erencein the induced chemistry. When these experimental resultswere extrapolated to a protoplanetary disk environment, wefound that X-rays are at least as e ffi cient as UV photons indesorbing molecules from interstellar ices depending on theregion considered.The yields we presented, extracted from pure methanol ices,should be considered with caution because the photodesorption Article number, page 10 of 11asalgète et al.: X-ray photodesorption from methanol-containing ices e ffi ciency may depend on the composition of the more complexinterstellar ices containing methanol. This is discussed in paperII. Acknowledgements.
This work was done with financial support from the RegionIle-de-France DIM-ACAV + program and by the European Organization for Nu-clear Research (CERN) under the collaboration Agreement No. KE3324 / TE. Wewould like to acknowledge SOLEIL for provision of synchrotron radiation fa-cilities under Project Nos. 20181140, and we thank N. Jaouen, H. Popescu andR. Gaudemer for their help on the SEXTANTS beam line. This work was sup-ported by the Programme National “Physique et Chimie du Milieu Interstellaire”(PCMI) of CNRS / INSU with INC / INP co-funded by CEA and CNES. Financialsupport from the LabEx MiChem, part of the French state funds managed by theANR within the investissements d avenir program under Reference No. ANR-11-10EX-0004-02, is gratefully acknowledged.
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