Interatomic Coulombic decay in helium nanodroplets
M. Shcherbinin, A. C. LaForge, V. Sharma, M. Devetta, R. Richter, R. Moshammer, T. Pfeifer, M. Mudrich
IInteratomic Coulombic decay in helium nanodroplets
M. Shcherbinin , A. C. LaForge , V. Sharma , M. Devetta , R. Richter , R. Moshammer , T. Pfeifer , M. Mudrich Physikalisches Institut, Universit¨at Freiburg, Germany Indian Institute of Technology Hyderabad, India Istituto di Fotonica e Nanotecnologie – Milano, Italy Elettra Sincrotrone, Trieste, Italy Max-Planck-Institut f¨ur Kernphysik, Heidelberg, Germany and Department of Physics and Astronomy, Aarhus University, Denmark (Dated: October 12, 2018)Interatomic Coulombic decay (ICD) is induced in helium (He) nanodroplets by photoexciting the n = 2 excited state of He + using XUV synchrotron radiation. By recording multiple coincidenceelectron and ion images we find that ICD occurs in various locations at the droplet surface, insidethe surface region, or in the droplet interior. ICD at the surface gives rise to energetic He + ions aspreviously observed for free He dimers. ICD deeper inside leads to the ejection of slow He + ions dueto Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formationof He + k complexes. Isolated atoms or molecules excited by energetic ra-diation typically decay through intramolecular processessuch as the emission of an electron or photon. In con-trast, in weakly bound complexes, locally generated elec-trons can additionally interact with neighboring atomsor molecules, leading to new interatomic or intermolecu-lar interactions. Interatomic Coulombic decay (ICD) is aparticularly interesting decay process which occurs whenlocal electronic decay is energetically forbidden [1]. Thus,ICD offers a new, ultrafast decay path where energy isexchanged with a neighboring atom leading to its ioniza-tion. Since its discovery, ICD has been observed in a widevariety of weakly-bound systems from He dimers [2, 3]and rare-gas clusters to biologically relevant systems suchas water clusters; for reviews see [4, 5]. Today, the focusis on condensed-phase systems where ICD is involved incomplex relaxation mechanisms [6–8], which can gener-ate genotoxic low-energy electrons and radical cations [9].Recently, it was suggested to utilize this property of ICDfor cancer treatment [10, 11].Here we present the first study of ICD in helium (He)nanodroplets. He nanodroplets are generally consideredas an ultracold, inert spectroscopic matrix for embed-ded, isolated molecules and clusters [12, 13]. Upon ion-ization by intense or energetic radiation, however, Hedroplets turn into a highly reactive medium, inducingreactions and secondary ionization processes of the em-bedded species [14]. Their homogeneous quantum liq-uid density profile, and the simple structure of atomicconstituents, make He droplets particularly beneficial asbenchmark systems for elucidating correlated decay pro-cesses. Recent examples include the collective autoion-ization of multiply excited pure He droplets [15, 16] andthe creation of doubly charged species by one-photon ion-ization of doped He droplets [17]. In this work we fullycharacterize the product states generated by ICD andsecondary processes in He nanodroplets using coincidenceimaging techniques. The experiments were performed using a Hedroplet machine attached to a velocity map imagingphotoelectron-photoion coincidence (VMI-PEPICO) de-tector at the GasPhase beamline of Elettra-SincrotroneTrieste, Italy. The apparatus has been described in detailelsewhere [18, 19]. Briefly, a beam of He nanodropletsis produced by continuously expanding pressurized He(50 bar) of high purity He out of a cold nozzle (10-28 K)with a diameter of 5 µ m into vacuum. At these expan-sion conditions, the mean droplet sizes range between (cid:104) N (cid:105) = 700 and ∼ × He atoms per droplet. Inthe main detector chamber, the He droplet beam crossesthe synchrotron beam perpendicularly in the center ofa combined VMI and time-of-flight (TOF) detector. Bydetecting either electrons or ions with the VMI detec-tor in coincidence with the corresponding particles of op-posite charge with the TOF detector, we obtain eitherion mass-correlated electron spectra or mass-selected ionkinetic energy (KE) distributions by Abel inversion ofthe VMIs [20]. The XUV photon energy is tuned nearthe first excited level of He + , hν (cid:38) E (He + ∗ , n = 2) =65 . + hν → HeHe + ∗ + e − sat → He + + He + + e − sat + e − ICD generates two electrons and two He + ions flying apartdue to Coulomb repulsion. Here, He + ∗ denotes a Heion in an excited state with principal quantum num-ber n >
1. The satellite photoelectron e − sat is emit-ted directly with kinetic energy E sat = hν − E (He + ∗ )upon simultaneous ionization and excitation of a Heatom. The ICD electron e − ICD is created by energy trans-fer from He + ∗ to the neighboring He atom resulting in E ICD = E (He + ∗ ) − × E i − KER ∼ . E i = 24 . + -He + fragments.When ICD takes place in He droplets, the primary pro-cess is likely to occur between the ionized atom He + ∗ and a r X i v : . [ phy s i c s . a t m - c l u s ] J u l Electron intensity (arb. units)
E l e c t r o n k i n e t i c e n e r g y ( e V ) h n = 6 7 . 5 e V < N > = 2 7 0 0 0 5 0 0 0 1 0 0 0 2 Ion mass m2 (amu)
I o n m a s s m ( a m u ) H e + - H e H e + - H e H e - H e
Figure 1. Ion-ion-electron coincidence time-of-flight massspectrum recorded at hν = 67 . (cid:104) N (cid:105) = 4000 He atoms. The inset shows photoelectronspectra measured in coincidence with He + for various (cid:104) N (cid:105) andfor free He [3]. its nearest neighbor due to the steep dependence of theICD rate on interatomic distance [3]. Three-body effectsand more complex interactions give only small contribu-tions [21]. However, in He droplets the outgoing ions caninteract with the surrounding He atoms and eventuallyform stable ionic complexes He + k [22, 23].The simultaneous formation of two He + k ions is indeedclearly observed. Fig. 1 displays coincidences of one elec-tron and two ions with masses m (horizontal axis) and m (vertical axis) as bright spots. The visible lines be-tween integer values are due to false coincidences. Whilewe see He + k progressions up to k = 36 for (cid:104) N (cid:105) ∼ , +1 − , highlighted by circles. Unfortu-nately, coincidences involving two identical ion massescannot be resolved with our setup.Photoelectron spectra recorded in coincidence withions He + and He +2 − (not shown) at hν = 67 . (cid:104) N (cid:105) strongly resemble one another and closely matchthat of free He , see inset in Fig. 1. The shown spectrumfor free He is obtained from the measured KER distri- bution using the unique relation between KER and pho-toelectron energies given by the Coulomb potential [3].The sharp line at 2.2 eV represents e − sat and the asym-metrically broadened feature extending from 6 to 16 eVreflects e − ICD created by ICD at various inter-atomic dis-tances [3]. The close resemblance of the ICD featuremeasured in droplets and that of free He confirms thatICD proceeds as a binary process with little effect of thedroplet on the outgoing electron.The crucial influence of the He droplet on the ICDprocess is revealed by the KE distributions of ions in-ferred from ion VMIs. Fig. 2 shows the mass-selected ionKE of He + k complexes recorded for different experimen-tal paramteters. For comparison, the ion KE spectrummeasured for free He at hν = 68 .
86 eV is shown inFig. 2 d) [2]. The distribution peaked around 4.2 eV isattributed to KER from Coulomb explosion of the pair ofHe + ions generated by ICD [2, 3]. The KE distributionsof He + ions measured with droplets feature a slightlybroader structure in the same energy range. Thus, partof the He + ions created by ICD of pairs of He atomsin He droplets are emitted nearly unperturbed. This ismost likely to occur at the droplet surface where the Hedensity is low.Aside from this clearly ICD-related feature, the He + KE spectra contain an additional broad peak at about1 eV and a very narrow peak near 0 eV. The peak near0 eV is present for all photon energies exceeding E i , seethe spectrum recorded at hν = 26 eV shown in Fig. 2a) as a red (lowest) line. Moreover, it is most domi-nant in the regime of small He droplets where a substan-tial fraction of free He atoms accompanies the dropletbeam. Thus, we attribute this peak to direct photoion-ization of atomic He. The broad peak around 1 eV ispredominantly due to ICD in He droplets. This can beconcluded from comparing with the spectrum recordedslightly below the ICD threshold at hν = 64 . hν = 64 . hν = 26 eV where only direct single droplet ionizationcan occur.The prominent feature around 1 eV in the He + ionspectra evidences efficient energy loss for He + ions indroplets, as the coincidence electron spectra show no in-dications for a corresponding upshift in energy. Obvi-ously, friction-like multiple elastic scattering of He + withHe atoms inside the droplets may lead to He + energyloss. However, the ratio of peak integrals of the featurearound 1 eV in proportion to that at 4.7 eV only slightly d ) H e + h n = 6 7 . 5 e V < N > = Ion intensity (arb. units) e ) H e + f ) H e + h n , < N > = a ) H e + b ) H e + c ) H e + I o n k i n e t i c e n e r g y ( e V ) < N > = n = 7 7 e V H e + H e + + H e +2 H e + + H e +3 h n = 2 6 e V H e + g ) H e + h ) H e + < N > = n = 7 7 e V H e H e +2 + H e + H e +2 + H e +3 H e +2 + l a r g e rh n = 2 6 e V H e +2 i ) H e + < N > = n = 7 7 e V H e +3 H e +3 + H e + H e +3 + H e +2 H e +3 + l a r g e rh n = 2 6 e V H e +3 Figure 2. Kinetic energy distributions of He +1 − ions for various photon energies (left column), He droplet sizes (center), anddouble-ion coincidences (right column). See text for details. rises from 2.2 to 2.7 when varying the He droplet size (cid:104) N (cid:105) from 700 to 5 × . In contrast, the ratio of thenumber of He atoms in the bulk of the droplets to thosein the surface region ( <
90 % of bulk density) increasesfrom 2 to 54 [24]. Thus, the 1 eV feature must be relatedto ICD occuring in the surface region of the droplets.What is the origin of the massive loss of KE of He + when ICD occurrs in He droplets? We propose the follow-ing mechanism, illustrated in Fig. 3 a): Initially one He + ∗ ion (labeled 2 ) is excited in step I, approaches a neigh-boring He atom 1 in step II, and decays by ICD (III).In the Coulomb explosion of He + ions 1 and 2 (IV),each He + ion flies away from the other until it reachesits neighboring neutral He atom 3 located in the lineof flight. There, an energetic billiard-like collision takesplace in which the He + ion transfers its KE to the Heatom and thus stops moving if the collision is central(V). Subsequently, Coulomb explosion of the two He + ICD ions restarts from a larger distance as if ICD oc-cured between non-nearest neighbors [25], giving rise toa lower final KE.This model is supported by a classical trajectory sim-ulation for a linear configuration of atoms He-He + -He + -He. Fig. 3 b) shows the trajectory of He + ion 2 as a red (lowest) solid line, and of the neighboring He atom3 as a black (upper) solid line for initial distances be-tween neutral atoms of 3.6 ˚A and between the ICD ionsof 1.7 ˚A, respectively [3, 26]. In contrast to freely movingHe + ions (dashed line), in the linear four-atom system, acentral collision takes place at t = 37 fs. The correspond-ing ion KE, shown in Fig. 3 c), is massively reduced bythe collision and converges towards 0.8 eV, in good agree-ment with the experimental finding. When we run thissimulations for a distribution of initial distances betweenHe + ions given by the measured KER spectrum of thefree He [3], and for a distribution of initial He-He dis-tances corresponding to the He density distribution for (cid:104) N (cid:105) = 1000 [24], we obtain the red (lower) smooth spec-tral feature shown on the right hand side of Fig. 3 c). Itnicely matches the low-energy edge of the 1 eV-featurein the experimental droplet spectrum. To simulate thehigh-energy part, non-central as well as many-body col-lisions would have to be included, which falls beyond thescope of this work. When determining the initial He-Hedistance distribution we assume the active surface layerfor the described collision process to be constrained to-wards the bulk of the droplet by the mean free path ofHe + in He droplets of 3 ˚A, inferred from the gas-phase P o s i t i on ( Å ) He + ion ❷ He atom ❸ He + in free He b ) c ) K i ne t i c ene r g y ( e V ) Time (fs)
Experimental Simulation Dimer KER/2 P o t en t i a l ene r g y ( e V ) He + *(n=2) -- He He + -- He + He -- He a) He + -- He III
1 Distance ❶-❷ (Å)
Distance ❷-❸(Å)
II I IV V
Figure 3. a) Schematic potential energy level diagram (seetext). b) Classical trajectories of a He + ion colliding with aneighboring He atoms in the course of Coulomb explosion forthe linear configuration He-He + -He + -He. c) Correspondingkinetic energies and the simulated and experimental energydistributions (bottom right). elastic collision cross section [27]. Since the He densitydistribution inside this layer only weakly varies with (cid:104) N (cid:105) ,the simulated energy distribution is robust against vari-ations of (cid:104) N (cid:105) .In case ICD occurs deeper inside the droplets, ICD isfollowed by He + k ion complex formation. This we con-clude from the sharply rising ratio of detected He + k toHe + ions for k > (cid:104) N (cid:105) = 700to 5 × . Most likely ion complex formation is assistedby elastic stopping collisions to generate slow He + ionssurrounded by He atoms as a precursor.The He +2 and He +3 KE distributions strikingly differfrom those of He + in that only low energy ions ( (cid:46) + k complexes are formed from the stoppedHe + by subsequent aggregation of He atoms inside thedroplet. Similar to the low-energy part of the He + KE spectra [Fig. 2 a)], the He +2 and He +3 spectra fea-ture two partially overlapping peaks. However, for He +2 and He +3 the low-energy component ( ∼ . +2 , ∼ . +3 ) is already present when singly ion- izing the droplets at hν = 26 eV [red (lowest) linesin Fig. 2 b) and c)]. In contrast to atomic He + , He + k ionic complexes ( k >
1) can be ejected out of neutral Hedroplets with substantial KE (cid:46) . ∼ . +2 , ∼ .
35 eV for He +3 ) are already present at hν = 64 . +2 , ions in the same He droplet,either by ICD or by electron impact ionization. Accord-ingly, for small droplets with (cid:104) N (cid:105) = 1200 [turquoise (low-est) lines in Fig. 2 e) and f)], these components are sig-nificantly reduced because electron-impact ionization isimprobable and ICD is likely to occur near the dropletsurface where at least one ion promptly escapes.A further confirmation for the 1 eV feature in the He + KE spectra stemming from ICD is obtained from ana-lyzing the data with regard to multiple ion coincidences.Fig. 2 g) shows the KE distributions of He + detectedin coincidence with He +2 or He +3 molecular ions [pink andlight blue (intermediate) lines], along with He + single co-incidence spectra at hν = 26 and 77 eV [blue (upper) andred (lowest) lines]. Aside from differing signal-to-noiseratios, the ion-ion coincidence spectra, which are char-acteristic for ICD, closely match the single coincidenceKE spectrum. Thus, even pairs of free ions, He + +He + ,generated by ICD at the droplet surface, are subjectedto elastic stopping collisions.In stark contrast to the KE spectra of He + , the He + k double ion coincidence spectra for k = 2 , (cid:96) of the second ion detected in the He + k +He + (cid:96) events.While the He + k spectra recorded in coincidence with He + closely match the single droplet ionization spectra at hν = 26 eV, those recorded in coincidence with largercomplexes ( (cid:96) > k −
1) are shifted to higher energies by0.25-0.35 eV. Consequently, the single-coincidence He +2 , spectra are superpositions of low and high-energy com-ponents, where the high-energy peaks clearly dominate.The different energetics of He + k ion ejection may arisefrom the dynamics following ICD. In the case that oneHe + k complex forms inside the droplet and one He + (cid:96) , (cid:96)