HSCO + and DSCO + : a multi-technique approach in the laboratory for the spectroscopy of interstellar ions
Valerio Lattanzi, Silvia Spezzano, Jacob C. Laas, Johanna Chantzos, Luca Bizzocchi, Kin Long Kelvin Lee, Michael C. McCarthy, Paola Caselli
AAstronomy & Astrophysics manuscript no. HSCO + ˙final˙astroph c (cid:13) ESO 2019April 30, 2019
HSCO + and DSCO + : a multi-technique approachin the laboratory for the spectroscopy of interstellar ions Valerio Lattanzi , Silvia Spezzano , Jacob C. Laas , Johanna Chantzos , Luca Bizzocchi , Kin Long Kelvin Lee , ,Michael C. McCarthy , , and Paola Caselli Max-Planck-Institut f¨ur extraterrestrische Physik, Giessenbachstraße 1, D-85748 Garching, Germany Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, Massachusetts 02138, USAe-mail: [email protected]
Preprint online version: April 30, 2019
ABSTRACT
Context.
Protonated molecular species have been proven to be abundant in the interstellar gas. This class of molecules is also pivotalfor the determination of important physical parameters for the ISM evolution (e.g. gas ionisation fraction) or as tracers of non-polar,hence not directly observable, species. The identification of these molecular species through radioastronomical observations is directlylinked to a precise laboratory spectral characterisation.
Aims.
The goal of the present work is to extend the laboratory measurements of the pure rotational spectrum of the ground electronicstate of protonated carbonyl sulfide (HSCO + ) and its deuterium substituted isotopomer (DSCO + ). At the same time, we show howimplementing di ff erent laboratory techniques allows the determination of di ff erent spectroscopical properties of asymmetric-top pro-tonated species. Methods.
Three di ff erent high-resolution experiments were involved to detected for the first time the b − type rotational spectrum ofHSCO + , and to extend, well into the sub-millimeter region, the a − type spectrum of the same molecular species and DSCO + . Theelectronic ground-state of both ions have been investigated in the 273–405 GHz frequency range, allowing the detection of 60 and 50new rotational transitions for HSCO + and DSCO + , respectively. Results.
The combination of our new measurements with the three rotational transitions previously observed in the microwave regionpermits the rest frequencies of the astronomically most relevant transitions to be predicted to better than 100 kHz for both HSCO + and DSCO + up to 500 GHz, equivalent to better than 60 m / s in terms of equivalent radial velocity. Conclusions.
The present work illustrates the importance of using di ff erent laboratory techniques to spectroscopically characterise aprotonated species at high frequency. Each instruments addressed complementary part of the same spectroscopic challenge, showingthat a similar approach can be adopted in the future when dealing with similar reactive species. Key words.
Molecular data – Methods: laboratory: molecular – Techniques: spectroscopic – Radio lines: ISM
1. Introduction
Carbonyl sulfide (OCS) has been observed in the ISM in sev-eral types of objects (Goldsmith & Linke 1981; Li et al. 2015),and it is the only S-bearing molecule so far unambiguously de-tected in interstellar ices (Boogert et al. 2015). Furthermore, ithas been observed in the coma of the comet 67P (Bockel´ee-Morvan et al. 2016) and, recently, OCS has been mapped to-wards the pre-stellar core L1544 showing a spatial distributioncomparable to methanol (Spezzano et al. 2017), possibly hintingat a common formation path onto the icy mantels of dust grains,as suggested by Loison et al. (2012). In molecular clouds, OCSmolecules could undergo protonation or deuteronation via reac-tions with H + and its deuterated isotopomers.The presence of protonated OCS in the ISM has been sug-gested by several groups (Fock & McAllister 1982; Turneret al. 1990). OCS possesses a large proton a ffi nity (632 kJ / mol),higher than either CO or CO , which are 576 kJ / mol and551 kJ / mol, respectively (Hayhurst & Taylor 2001), whose pro-tonated variants are known to exist in space (Bizzocchi et al.2017). Radioastronomical observations of protonated carbondioxide have been used in the past also to constrain the abun-dance of CO in di ff erent interstellar environments. Vastel et al. (2016) derived an indirect estimate of the [CO ] / [CO] from[HOCO + ] / HCO + in the L1544 pre-stellar core; similar argu-ments were used by Sakai et al. (2008) to study the Class 0 pro-tostar IRAS 04368 + / THzemission of HOCO + towards SgrB2(N), which, in conjunctionwith previously published observations of HCO + isotopologues,allowed the authors to infer the gas-phase CO abundance in theregion. Protonated species and their deuterated variants can alsogive important information about the deuteration in interstellarclouds. DOCO + was searched in many environments where itsparent species was also detected; new accurate rest frequenciesfrom laboratory spectroscopic studies on DOCO + (Bizzocchiet al. 2017) ruled out a tentative detection reported by Vastelet al. (2016) towards the L1544 pre-stellar core.According to the estimates by Fock & McAllister (1982),the molecular abundance in dense clouds of protonated carbonylsulfide may be comparable to that of HOCO + , which is also iso-valent with HSCO + . Carbonyl sulfide can be protonated both onthe sulfur (HSCO + ) and the oxygen side (HOCS + ), with the for-mer being more stable by ∼
20 kJ / mol (Wheeler et al. 2006). a r X i v : . [ a s t r o - ph . GA ] O c t attanzi et al.: Protonated OCS Highly reactive molecules, such as ions and radicals, are dif-ficult to produce and detect at high spectral resolution in thelaboratory. Surprisingly, the higher–energy isomer, HOCS + , wasdetected in the laboratory first: Nakanaga & Amano (1987) ob-served this cation with infrared spectroscopy with a hollow cath-ode discharge, followed by Ohshima & Endo (1996), who fur-ther characterised its spectrum in the cm-wave range by meansof a Fourier-transform microwave spectrometer coupled with apulsed-discharge nozzle. Twenty years later, the rotational spec-trum of the lower energy isomer, HSCO + , along with its deu-terium and S isotopic substituted species, was finally measuredby McCarthy & Thaddeus (2007), using a similar experiment tothat used by Ohshima & Endo (1996).Rest frequencies derived from the current laboratory datasetsare not accurate enough to confidently search for either isomer ofprotonated OCS in the ISM above 50 GHz. A search for HOCS + towards star-forming regions at 3 mm ( ∼
90 GHz) was carriedout some time ago despite the large uncertainties of the rest fre-quencies ( ∼
800 kHz, corresponding to ∼ / s at 90 GHz),and was ultimately unsuccessful (Turner et al. 1990). This illus-trates the necessity for highresolution laboratory spectroscopy,which provides frequencies to su ffi cient accuracy to allow iden-tification of molecular carriers of emission features, particularlyin cold, quiescent astronomical sources with very narrow line-widths. Motivated by the lack of precise data in the mm- andsubmm-wave range, a high–resolution laboratory study of theground state isomer was carried out. This work has resulted inprecise measurements of HSCO + and DSCO + up to ∼
400 GHz.
2. Experiment
The spectroscopic study of HSCO + and DSCO + performedhere has involved three di ff erent spectrometers: two at theCenter for Astrochemical Studies (CAS) at the Max PlanckInstitute for Extraterrestrial Physics in Garching (DE), and oneat the Harvard–Smithsonian Center for Astrophysics (CfA) inCambridge (USA). Chronologically, the first part of the experiment was begunwith the new frequency-modulated free-space absorption cellspectrometer, CASAC (the Center for Astrochemical StudiesAbsorption Cell), recently developed at our institute (Bizzocchiet al. 2017). The main radiation source is a frequency synthe-siser (Keysight E8257D), synchronised to a 10 MHz rubidiumfrequency standard (Stanford Research Systems) for accuratefrequency and phase stabilisation. The radiation from the syn-thesiser is then coupled to a Virginia Diodes (VDI) solid stateactive multiplier chain, which allows a great frequency agilityand a full coverage of the 75–1100 GHz frequency range. Theradiation is fed through a Pyrex tube, 3 m long and 5 cm sec-tion, equipped with two hollow stainless steel electrodes, 10 cmlong, connected to a DC power supply (5 kW). The 2 m activeregion of the discharge, defined by the distance of the two elec-trodes, can be cooled by liquid nitrogen and magnetically con-fined using a 3-layer solenoidal coil wrapped around the glasstube and which can generate a magnetic field coaxial with the ra-diation beam up to ∼
300 G. This technique is particularly suitedfor producing positive molecular ions since the magnetic fieldincreases the length of the ion rich negative glow by restrict-ing inside a small diameter tube the ionising electrons acceler-ated by the large cathode drop of an anomalous glow discharge (De Lucia et al. 1983). The frequency modulation of the radi-ation is obtained by encoding its signal with a sine-wave at arate of 15 kHz ; the signal, after interacting with the molecu-lar plasma, is hence detected by a liquid-He cooled InSb hotelectron Bolometer (QMC Instruments Ltd.). A lock-in amplifier(SR830, Stanford Research Systems) is used for demodulatingthe detector output at twice the modulation frequency (2 f de-tection) resulting in a second derivative profile of the absorptionsignal recorded by the computer controlled acquisition system.The chemical conditions for the production of the protonatedcarbonyl sulfide were first taken from those producing the largestamount of HOCO + , a similar and isoelectronic species recentlyobserved in our laboratory (Bizzocchi et al. 2017); once the pro-duction of the latter was optimised, the CO sample was replacedwith OCS. The experimental conditions that produced the largestsignal of HSCO + were further optimised and found to be a 1:1mixture of OCS and H , diluted in a bu ff er gas of Ar, producinga total pressure, as measured at the output of the absorption cell,of 20 mTorr; for the DSCO + molecule experiment, the H wasreplaced by the D sample. Further parameters that were crucialfor the signal quality were a DC discharge with 5 mA at ∼ ∼
200 G and a temperature of the glass wallof ∼
130 K. The latter has been the most critical factor: the opti-mal conditions were found to be varying during the experiment,more so than in other similar studies.
The CfA Fourier Transform Microwave (FTM) spectrometerwas used to observe the b -type spectrum of HSCO + . The sameinstrument was used in the original detection of the protonatedspecies (McCarthy & Thaddeus 2007) and modified accordinglyto perform the double-resonance (DR) experiment; the full de-scription of the instrument and the DR implementation can befound elsewhere (e.g. McCarthy et al. 2000; Lattanzi et al. 2010).Briefly, protonated ions are created in the throat of a small su-personic nozzle by applying a low-current dc discharge (750 V,0.3 mA) to a short (300 µ s) gas pulse created by a Series 9 noz-zle. The molecular beam was formed by 1.2% of OCS in H diluted further in a 1 to 5 ratio with pure He, reaching a to-tal flow rate of 20 cm min − at standard temperature and pres-sure. The stagnation pressure behind the valve was 2.5 kTorrand the valve operated with a repetition rate of 6 Hz. The fre-quency coverage of the FTM spectrometer (5–43 GHz) can be“extended” by means of DR measurements: the FTM cavity iskept fixed to a known resonant frequency corresponding to aspecific transition (the “probe”) of the target molecule, and, dur-ing the free induction decay of the FTM signal, a second radi-ation source (the “pump”) is scanned in frequency until a reso-nance is reached with another transition sharing a common ro-tational level. Depending on the frequency range of the pumpsignal, microwave-microwave (MW–MW) or millimiterwave-microwave (MMW–MW) DR techniques are defined. In thesearch for the b- type spectrum of HSCO + the MMW–MW ex-periment was performed coupling the output of a frequency syn-thesiser (Keysight N5173B-540) with a frequency tripler. Thesignal was then amplified (Millitech AMP-10-10040) and sentto a second frequency tripler unit (VDI Inc. WR2.8X3) to reachthe frequency range of 250–375 GHz. The nominal input poweris 15 dBm, and the final high frequency output is on the order ofseveral dBm. Finally, the last part of the measurements for the HSCO + cation was conducted with the newly developed Free Unit JetExperiment in Garching. A deeper elucidation of this spectrom-eter will be the subject of a follow-up work (Lattanzi et al. inpreparation), while here we will briefly report the basics of theinstrumentation. A molecular beam is created by mixing the out-put of several mass flow controllers (MKS Instruments) that areindividually connected to the sample bottles, and then inject-ing this mixture downstream into a vacuum chamber throughthe 1–mm pinhole of a pulsed valve (Series 9, Parker Hannifin).The expansion of the molecular beam into the chamber is su-personic thanks to the high pressure gradient (up to 10 ) be-tween behind the valve (few kTorr / few bar) and the cham-ber ( ∼ − Torr / ∼ − bar); this supersonic expansion allows toadiabatically cool the molecular beam, reaching temperatures inthe range of 7 to 20 K ca., depending on the bu ff er gas used. Thevacuum inside the chamber is obtained by means of a large di ff u-sion pump (DIP 8000, Oerlikon Leybold), supported by a com-bination of mechanical pumps (roots blower and rotary–vanepump, Oerlikon Leybold). The coupling of the molecular beamto the mm– and submm–wave radiation is obtained through aroof–top mirror placed inside the chamber, which also containsthe aperture through which the molecular sample is injected. Theprobing radiation enters the vacuum section through a teflonwindow on the opposite side of the roof–top mirror, interactstwice with the gas and finally reflects back outside the chamber.The radiation source (harmonic multiplication of the signal gen-erated by a frequency synthesiser) and the detector (hot electronbolometer) are the same as those used for the CASAC exper-iment, allowing the same frequency coverage and agility. Theproduction of unstable species is achieved by attaching a high-voltage low-current DC nozzle to the front of the valve, throughwhich the molecules pass right after the pulsed valve and prior tofree expansion, where the molecular sample is quickly stabilisedin the region dubbed the “zone of silence”. All the measurementsare performed in absorption, with frequency modulation of thesignal, as described above for the CASAC.The experimental conditions were first optimised on the well–known protonated ion HCO + . Finally, a highly diluted mixture ofOCS in H (0.3%) was injected through the pulsed valve, operat-ing at a repetition rate of 15 Hz and open for 1 ms. The dischargenozzle was operated for 1.5 ms at 1.5 kV ( (cid:46) µ Torr. The continuous–wave (CW) signal ofthe synthesiser, synced to a 10 MHz rubidium frequency stan-dard (Stanford Research Systems), is frequency–modulated at30 kHz and, after the interaction with the molecular beam, ac-quired by the hot electron bolometer before passing through alock-in amplifier (MFLI, Zurich Instruments). Here the signalis demodulated at twice the modulation frequency with a 30 µ stime constant, and digitised so that integration of the molecularsignal and baseline subtraction can be performed to yield the fi-nal spectrum. All the timings (discharge, lock-in demodulation,and frequency stepping) were triggered to the repetition rate ofthe valve.
3. Analysis
The protonated carbonyl sulfide system includes two stableisomeric forms, HSCO + and HOCS + , separated by 4.9 kcal / mol.The two isomers are connected by a transition state with the pro-ton localised above the central carbon atom, lying 68.9 kcal / mol above the ground state HSCO + (Wheeler et al. 2006). The latteris a closed-shell near-prolate ( κ = − . a- and b- type rotational spectra ( µ a = .
57 D and µ b = .
18 D). The OCS angle is nearly linear at approximately175 ◦ (Fig. 1). HS C O+
Fig. 1.
Theoretical geometry of protonated carbonyl sulfide de-termined by Fortenberry et al. (2012).The search for HSCO + in the millimeter band was guidedby the rotational constants previously reported by McCarthy &Thaddeus (2007) in the cm-wave band along with the centrifu-gal distortion constants derived by Fortenberry et al. (2012). Thelow–frequency measurements detected the three lowest K a = B e f f ( = [ B + C ] /
2) constant. B e f f and the predicted quartic distortion con-stant D J were subsequently used in the initial search to observethe a − type rotational transition 27 , − , near 304 GHz .In the first 50 MHz –wide search we found just one candidatefeature, deviating only by ∼ orOCS was removed from the sample mixture. The decisive testfor confirming the assignment was then to search the harmonicprogression of the rotational transitions, and finally we were ableto detect a total of 11 K a = + inthe millimeter band was then supported by the detection of ad-ditional 33 rotational transitions in the K a ladders, up to K a = A rotational constant remainedlarge, as expected considering that the new set of millime-ter / submillimeter lines all belonged to the a -type spectrum ofthe ion, and were hence less sensitive to the contribution of the A constant. The uncertainty on the aforementioned parameterwas around 20 MHz, compared to those for the B and C rota-tional constants, which were on the order of a few kHz. With theparameters derived from the a -type spectrum analysis, a searchwith the CASAC experiment for the b -type spectrum of HSCO + was then carried out. This search, however, led to a non detec-tion, owing to both a large uncertainty on the predicted frequen-cies and a weaker dipole moment, which results roughly in linesexpected with half intensities compared to the a -type spectrum.The b -type spectrum was then searched for using the MMW–MW double resonance techniques. The strongest depletion of the probe transition, and hence the largest DR signal, is achievedwhen the pump rotational transition is connected to the lowestenergy state of the probe rotational transition; also for this reasonthe best option was to look for the fundamental b − type transition1 , –0 , , which was predicted around 285 GHz and whose cor-responding a − type transition 1 , –0 , were previously detectedin the same FTMW spectrometer. After a few searches aroundthe predicted value, a clear DR signal was finally detected at afrequency ∼
140 MHz away from the prediction (0.05%). Oncethe fundamental rotational b − type was detected, and hence the A rotational constant was locked to its value with a reasonableuncertainty, the search for the other transitions of the same spec-trum, detectable within our spectral coverage, was straightfor-ward and we were able to detect a total of 5 b − type transitions(e.g. Fig.2). Frequency [MHz] S i g n a l [ a . u . ] HSCO + (33
4, *
4, * ) ExperimentBest Fit probe pump1 Fig. 2.
Laboratory spectra of HSCO + : ( top ) the J K a , K c = , ∗ − , ∗ (overlapping) doublet around 371 GHz, acquired in 725 sintegration time with 3 ms time constant; the strong spikes vis-ible in plot are due to instabilities of the dc discharge. ( bot-tom ) The b -type J K a , K c = , − , rotational transition around274 GHz, acquired in double resonance with 45 kHz step andin a total integration time of 20 minutes (30 shots per integra-tion step at 5 Hz). The red dashed lines represent the best fitto a speed-dependent Voigt and Gaussian profile (respectively).A schematic diagram indicating the two transitions that share acommon energy level is shown in the inset.With the DR measurements as a guide, we decided to testour newly developed free–unit jet experiment with the search of other low–energy b − type HSCO + rotational transitions. Theexperimental conditions described above allowed the detec-tion of a total of 16 b − type features, including the line previ-ously observed with the DR experiment, belonging to Q − and P − branches between 274 and 373 GHz (e.g. Fig.3). Frequency [MHz] S i g n a l [ a . u . ] HSCO + (4 ) ExperimentBest Fit
Fig. 3.
Laboratory spectrum of the HSCO + J K a , K c = , − , ro-tational transition around 318 GHz, acquired with the free–unitjet experiment. The integration time is ∼
15 minutes with 30 µ stime constant. The red dashed line represents the best fit to aspeed-dependent Voigt profile, as discussed in the text. Each ro-tational transition has a double–peaked line shape, the result ofthe Doppler shift of the supersonic molecular beam relative tothe two traveling waves that compose the radiation beam.The rotational spectrum of HSCO + was analysed with theWatson S-reduced Hamiltonian, including all the quartic cen-trifugal distortion terms and two sextic terms, H JK and H KJ . Thequartic parameter D K was not constrained in our dataset due toa strong correlation with the A rotational constant; the overallrms improved slightly ( (cid:46) ab initio value. All 63 rotational transitions, including thethree observed in the previous microwave experiment, were re-produced by our model with a final rms uncertainty of 39 kHz,which corresponds approximately to the average experimentaluncertainty (Table 1).Using the absorption cell, we employed a similar experi-mental approach for studies of DSCO + , replacing hydrogen withdeuterium. The experimental search started with the hunt for the29 , − , line, as the prediction from the microwave mea-surements estimated this line around 318 GHz. In a matter ofa few days we collected 50 a − type rotational transitions, up tothe K a = + rotational spectrum was fitted toan rms uncertainty of 23 kHz (Table 2).All the experimental lines, except for the DR spectra, were fittedusing the line profile model summarised in Dore (2003) and im-plemented in our in-house analysis software. More precisely, aspeed-dependent Voigt profile was applied to retrieve the centralfrequency of the 2 f absorption line; both the complex compo-nent of the Fourier-transform of the dipole correlation function(i.e. the dispersion term) and a third–order polynomial were alsotaken into account to model the line asymmetry and baseline pro-duced by the background standing-waves between non-perfectlytransmitting windows of the absorption cell in the CASAC mea-surements. The experimental uncertainty is estimated to be in therange of 30-50 kHz, depending on the line width, the achievedsignal-to-noise ratio (S / N), and the baseline.
In the supplementary material, available at the CDS, the fulllist of assigned transitions is available for both HSCO + andDSCO + , along with the respective frequencies and estimated un-certainties.
4. Discussion and Conclusions
A combination of spectroscopic techniques has been used toextend the frequency coverage of the protonated OCS system,a cation of potential radioastronomical interest. This new setof experiments allow the detection of 60 and 50 new rotationaltransitions for HSCO + and DSCO + , respectively. First detectionof b − type transitions for HSCO + permitted the A rotationalconstant to be precisely determined, free from correlations withother spectroscopic parameters. The experimental rotationalconstants to ab initio values are in striking agreement: both B and C agree to 0.01%, while A only di ff ers by 0.07% (Table 1).The agreement of the experimental and theoretical quarticcentrifugal distortion terms is in the range of a few percent,while the agreement is better than 25% for the capital D (cid:48) s andlower d ’s, respectively. This level of agreement is similar to thatobtained with the same level of theory (CcCR QFF) recentlyreported for HOCO + (Bizzocchi et al. 2017). For example, theexperimental D J fourth-order distortion constant di ff ers by 2.5%from the ab-initio values; recent studies on rigid molecules withtwo to three heavy atoms showed that this parameter, whencalculated at a high level of theory, is usually accurate to about3% or better (Lattanzi et al. 2011; Spezzano et al. 2012). InTable 1 the HSCO + spectroscopic parameters are also comparedwith those derived by Br¨unken et al. (2009) for HSCN. Thisparallel confirms, once again, (1) how close are the rotationalconstant values and their centrifugal distortion corrections fortwo isoelectronic species; (2) how the non-detection of the b − type transitions (as in the work by Br¨unken et al. 2009)a ff ects the accuracy of the A rotational constant. A similar levelof agreement between theory and experiment is found for thespectroscopic parameters of the deuterated species. Howeverthe A rotational constant needs to be treated more carefully,since the b − type spectrum is presently lacking for this species(Table 2).The present work illustrates the importance of using di ff erentbut complementary techniques to spectroscopically characterisea protonated species at high frequency. The CASAC experimentaddressed the “warmer” part of the spectrum, extending themicrowave measurements to considerably higher J (up to J =
36) and K a (up to K a =
6) levels. However, attemptsto address other aspects of the rotational spectrum with thisexperiment were unsuccessful, for several reasons. Althoughmoving to a lower frequency range provided more powerfrom the radiation source, the targeted transitions sought arepoorly populated in the relatively warm molecular plasma ofthe CASAC ( ∼
130 K). Attempts to enhance the populationof these lower states by decreasing the temperature of theabsorption cell resulted in the freeze–out of the OCS sample onthe glass wall and an overall decrease of the absorption signal.Extension to higher frequency also proved impractical becausethe power of the millimeter–wave source slowly decreases withincreasing frequency, and the temperature of the plasma is nolonger optimal to probe this higher–lying transitions. Theseconsiderations only apply to the a − type spectrum of the ion,which has a linear-like progression with increasing frequency,similar to that of strictly linear molecules. The b − type spectrumhas a more complex progression but provides access to a wide energetic regimes (Figure 4). Owing to a combination of thelarge uncertainty of the predicted b − type transitions and thetechnical challenge of stabilising the discharge system for alonger integration time required for this search, no b − typeabsorption features were observed in the CASAC experiment.Despite the intrinsic limitations of finding connected transitionssharing the same lower energy rotational level, DR experimentsare generally unambiguous, since perturbations to the probetransition can only arise when the pump frequency is coincidentwith a connected transition. In addition, this experiment greatlybenefits from the use of the same spectrometer where HSCO + was already observed, meaning that the experimental conditionswere already optimised. Frequency [GHz] S i m u l a t e d Sp e c t r a [ a . u ] HSCO + @ 15K a type b type Frequency [GHz] S i m u l a t e d Sp e c t r a [ a . u ] HSCO + @ 130K a type b type Fig. 4.
Simulated spectra of HSCO + . Thermal conditions werechosen representing the average temperatures of the Free–UnitJet ( top ) and CASAC ( bottom ) experiment, respectively. The y axis scale is the same for both plots.With the rotational constants reasonably well constrainedfrom a combination of CASAC and DR measurements, the spec-troscopic analysis was extended even further by detection of16 additional b − type rotational transitions using a new jet ex-periment in Garching. This new jet experiment uses a similarnozzle source to that routinely employed in the FTM experi-ments (i.e. a free expansion of a pulsed supersonic molecularjet) and, although less sensitive than the DR implemented in theFTM spectrometer, the former has notable advantages in termsof frequency agility, and flexibility with respect to rotationaltemperature of the expansion. Its wide frequency coverage per-mits the detection of rotational lines through the millimeter andsub-millimeter molecular spectra, with the only limitation be-ing the choice of the bu ff er gas used for the jet expansion, and hence the resulting e ff ective rotational temperature. For this ex-periment, the choice of bu ff er gas is fixed, namely H , for proto-nated species. Owing to the large A rotational constant, theseconditions limit the measured lines to K a = → J / K a failed presumably because of the low population of higherenergy rotational levels in the jet expansion. Nevertheless, thespectroscopic catalogues for HSCO + and DSCO + are largely im-proved, now allowing radioastronomical search to be performedwith much greater confidence. With the new spectroscopic data,the rest frequencies of the astronomically most interesting lineshave either been measured or can be predicted to better than100 kHz for both HSCO + and DSCO + up to 500 GHz , equiva-lent to better than 60 m / s in terms of equivalent radial velocity. Acknowledgements.
The authors wish to thank Mr. Christian Deysenroth andMr. Martin Gillhuber for the thorough assistance in the engineering of the molec-ular spectroscopy laboratories at the MPE / Garching. We would also like to thankEdward Tong of the Submillimeter Array Receiver laboratories at the Harvard-Smithsonian Center for Astrophysics for loaning the Millitech amplifier usedfor the 300 GHz DR measurements. M.C. McCarthy and K.L.K. Lee thank NSFgrant AST-1615847 for financial support.
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Table 1.
Spectroscopic parameters derived for HSCO + Parameter unit This work Previous a ab initio b HSCN c A MHz 279431.995(10) – 279236.6 289737(64) B MHz 5696.74645(86) 5636.8660(20) d C MHz 5576.97793(66) 5636.8660(20) d D J kHz 1.51961(25) 3.10(10) d D JK kHz 136.393(93) – 137 151.53(11) D K MHz 15.654 e – 15.654 – d Hz -34.03(17) – -30.14 -35.91(21) d Hz -4.720(85) – -3.829 -5.17(31) H JK Hz 0.320(49) – 0.361 0.518(26) H KJ Hz -176.9(10) – -180.366 -170.3(86) H K kHz 0.64769 e – 0.64769 – σ rms kHz 41 σ w f Notes.
Values in parentheses represent 1 σ uncertainties, expressed inunits of the last quoted digit. ( a ) McCarthy & Thaddeus (2007). ( b ) Fortenberry et al. (2012). ( c ) Br¨unken et al. (2009). ( d ) The actual parameters fitted in McCarthy & Thaddeus (2007) are B ef f = ( B + C ) / D ef f = D J + ( B − C ) / { A − ( B + C ) / } ,assuming D J = ( e ) Fixed to the ab initio value. ( f ) Dimensionless rms , defined as σ w = (cid:114) (cid:80) i (cid:18) δ ierri (cid:19) N , where the δ ’s arethe residuals weighted by the experimental uncertainty ( err ) and N thetotal number of transitions analysed. Table 2.
Spectroscopic parameters derived for DSCO + Parameter unit This work Previous a ab initio b A MHz 145158.6(22) – 145002.7 B MHz 5618.1838(19) 5509.6970(20) c C MHz 5401.1678(19) 5509.6970(20) c D J kHz 1.42281(27) 12.00(10) c D JK kHz 132.643(86) – 132 D K MHz 3.951 d – 3.951 d kHz -0.05913(48) – -0.05318 d kHz -0.01525(11) – -0.01302 H JK Hz 0.439(52) – – H KJ Hz -42.68(81) – – σ rms kHz 23 σ we Notes.