Photoionization of helium nanodroplets doped with rare gas atoms

Photoionization of He droplets doped with rare gas atoms (Rg=Ne, Ar, Kr, and Xe) was studied by time-of-flight mass spectrometry, utilizing synchrotron radiation from the Advanced Light Source from 10 to 30 eV. High resolution mass spectra were obtained at selected photon energies, and photoion yield curves were measured for several ion masses (or ranges of ion masses) over a wide range of photon energies. Only indirect ionization of the dopant rare gas atoms was observed, either by excitation or charge transfer from the surrounding He atoms. Significant dopant ionization from excitation transfer was seen at 21.6 eV, the maximum of He 2p 1P absorption band for He droplets, and from charge transfer above 23 eV, the threshold for ionization of pure He droplets. No Ne+ or Ar+ signal from droplet photoionization was observed, but peaks from HenNe+ and HenAr+ were seen that clearly originated from droplets. For droplets doped with Rg=Kr or Xe, both Rg+ and HenRg+ ions were observed. For all rare gases, Rg2+ and HenRgm+ (n,m> or =1) were produced by droplet photoionization. Mechanisms of dopant ionization and subsequent dynamics are discussed.     

Diatomic potential well depths from analyses of high-resolution electron energy spectra for autoionizing collision complexes

High-resolution energy spectra of electrons released in Penning ionization collisions of metastable rare gas atoms Rg*(ns) (Rg = He, Ne, Ar, Kr, Xe) with several open-shell and closed-shell atoms are analyzed to determine the well depth of the potential energy curve which describes the respective autoionizing collision complex. We thereby elucidate trends in the chemical interaction of Rg* with closed-shell target atoms A(ns ²) and establish a basis for detailed comparison with the respective interactions involving open-shell, ground state alkali atoms A(ns). From electron energy spectra due to␣associative ionization (RgH⁺ formation) in Rg* + H(1s) collisions, we determine binding energies for the RgH⁺(¹Σ) ground state potential (Rg = Ne, Ar, Kr, Xe) with uncertainties around 0.03 eV. Received: 30 June 1998 / Accepted: 5 August 1998 / Published online: 28 October 1998     

Atmospheric-pressure Penning ionization mass spectrometry

A preliminary study on the atmospheric-pressure Penning ionization (APP(e)I) of gaseous organic compounds with Ar* has been made. The metastable argon atoms (Ar*: 11.55 eV for (3)P(2) and 11.72 eV for (3)P(0)) were generated by the negative-mode corona discharge of atmospheric-pressure argon gas. By applying a high positive voltage (+500 to +1000 V) to the stainless steel capillary for the sample introduction (0.1 mm i.d., 0.3 mm o.d.), strong ion signals could be obtained. The ions formed were sampled through an orifice into the vacuum and mass-analyzed by an orthogonal time-of-flight mass spectrometer. The major ions formed by APP(e)I are found to be molecular-related ions for alkanes, aromatics, and oxygen-containing compounds. Because only the molecules with ionization energies less than the internal energy of Ar* are ionized, the present method will be a selective and highly sensitive interface for gas chromatography/mass spectrometry.     

Absorption spectra of e-beam-excited Ne, Ar, and Kr, pure and in binary mixtures

A technique using the broadband emission of a laser plume as probe radiation is applied to record UV-visible (190-510 nm) absorption spectra of Ne, Ar, and Kr, pure and in binary mixtures under moderate e-beam excitation up to 1?MW/cm(3). In all the rare gases and mixtures, the absorption spectra show continuum related to Rg(2) (+) homonuclear ions [peaking at λ～285, 295, and 320 nm in Ne, Ar, and Kr(Ar/Kr), respectively] and a number of atomic lines related mainly to Rg(?)(ms) levels, where m is the lowest principal quantum number of the valence electron. In argon, a continuum related to Ar(2) (?) (λ～325?nm) is also recorded. There are also trains of narrow bands corresponding to Rg(2) (?)(npπ?(3)Π(g))←Rg(2) (?)(msσ?(3)Σ(u) (+)) transitions. All the spectral features mentioned above were reported in literature but have never been observed simultaneously. Although charge transfer to a homonuclear ion of the heavier additive is commonly believed to dominate in binary rare-gas mixtures, it is found in this study that in Ne/Kr mixture, the charge is finally transferred from the buffer gas Ne(2) (+) ion not to Kr(2) (+) but to heteronuclear NeKr(+) ion.     

Energy transfer kinetics of the np5(n + 1)p excited states of Ne and Kr

Energy transfer rate constants for Ne(2p(5)3p) and Kr(4p(5)5p) atoms colliding with ground state rare gas atoms (Rg) have been measured. In part, this study is motivated by the possibility of using excited rare gas atoms as the active species in optically pumped laser systems. Rg(np(5)(n + 1)s) metastable states may be produced using low-power electrical discharges. The potential then exits for optical pumping and laser action on the np(5)(n + 1)p ? np(5)(n + 1)s transitions. Knowledge of the rate constants for collisional energy transfer and deactivation of the np(5)(n + 1)p states is required to evaluate the laser potential for various Rg + buffer gas combinations. In the present study we have characterized energy transfer processes for Ne (2p(5)3p) + He for the six lowest energy states of the multiplet. Rate constants for state-to-state transfer have been determined. Deactivation of the lowest energy level of Kr (4p(5)5p) by He, Ne, and Kr has also been characterized. Initial results suggest that Kr (4p(5)5p) + Ne mixtures may be the best suited for optically pumped laser applications.     

Rare gas effects on hyperfine coupling constants of BO, AlO, and GaO

Using density functional theory methods and large basis sets, we calculated hyperfine coupling constants (HFCCs) for the (11)B, (17)O, (27)Al, and (69)Ga nuclei of the radicals BO, AlO, and GaO (XO), embedded in 2-14 rare gas (Rg) Ne and Ar atoms. Kr atoms were included for AlO. The distance of the Rg atoms from XO was varied from 4 to 12 bohr. Matrix effects cause A(iso)(X) to increase, accompanied by decreases in A(dip)(X) and A(dip)(O), while A(iso)(O) remains close to zero. Changes are largest for AlO, slightly smaller for GaO, and very small for BO, in line with the molecular polarizabilities. Observed changes of A(iso)(X) and A(dip)(X) for BO in Ne matrixes and for AlO in Ne, Ar, and Kr matrixes are reproduced in complexes with 12 Rg atoms at distances of 5-6 bohr or 14 Rg atoms at distances of 6-7 bohr. For GaO, experimental data are available only in Ne matrixes. Theoretical results obtained for HFCCs of (17)O could not be verified due to insufficient experimental information. Estimates of HFCCs in matrixes not yet experimentally studied and for GaO in the gas phase have been made. Due to the interaction with rare gas atoms, p-spin density on the X and O atoms of XO is converted into s-spin density on X, thereby causing an increase (in magnitude) of A(iso)(X), accompanied by decreases in A(dip) of X and O. The higher polarizability of XO along the bond axis is reflected in complexes that have axial Rg atoms showing larger changes in HFCCs than comparable complexes without axial Rg atoms.     

H atom—rare gas interaction potentials from an electron gas model

A recently proposed electron gas model is applied to calculate the interaction potentials for HRg (Rg  He, Ne, Ar, Kr). In each case the results are in satisfactory with available experimental values.     

The state dependence of the interaction of metastable rare gas atoms Rg*(ms3 P 2,3 P 0) (Rg=Ne,Ar, Kr,Xe) with ground state sodium atoms

Using crossed beams of metastable rare gas atoms Rg*(ms³ P ₂,³ P ₀) (Rg=Ne, Ar, Kr, Xe) and ground state sodium atoms Na(3s ² S _1/2), we have measured the energy spectra of electrons released in the respective Penning ionization processes at thermal collision energies. For Rg*(³ P ₂)+Na(3s), the spectra are quite similar for the different rare gases, both in width and shape; they reflect attractive interactions in the entrance channel with well depthsD* _e [meV] decreasing slowly from Rg=Ne to Xe as follows: 676(18); 602(23); 565(26); 555(30). For Rg*(³ P ₀)+Na(3s), the spectra vary strongly with the rare gas, indicating a change in the character of the interaction from van der Waals type attraction (Ne) to chemical binding for Kr and Xe with well depthsD* _e [meV] of: 51(19); 107(25); 432(30); 530(50). These findings are explained through model calculations of the respective potential curves, in which the exchange and the spin orbit interaction in the excited rare gas and the molecular interaction between the two valences-electrons in terms of suitably chosen singlet and triplet potentials are taken into account. These calculations also explain qualitatively the experimental finding that the ratiosq ₂/q ₀ of the ionization cross sections for Rg*(³ P ₂)+Na and Rg*(³ P ₀)+Na vary strongly with the rare gas from Ne to Xe as follows: 15.8(3.2); 2.6(4); 1.4(2); 1.6(4).     

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI _S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI _P (γ = ΔI _S/ΔI _P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s ³ P ₂), Ar* (4s ³ P ₂), and Kr* (5s ³ P ₂) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2³ S, 2¹ S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.     

Theoretical Study of Rg·NO (Rg=He,Ne, Ar and Kr) Complexes

Rg·NO (Rg=He, Ne, Ar and Kr) complexes were studied using ab initio calculations. The neutral Rg·NO complex geometry and vibrational frequencies were calculated with the cc-pVDZ basis set at the CCSD(T) level of theory. The calculations show that the geometry of the Rg·NO complexes is a skewed T-shape with the Rg atom on the oxygen side of the NO molecule, and that the RgNO bond angle increases with mass. The dissociation energies (DE) and ionization energies (IE) of the neutral Rg·NO complexes, and the dissociation energies of Rg·NO+ ionic complexes were calculated using Gaussian-2 (G2) methods and a high accuracy energy model. The ionization energies of the neutral Rg·NO complexes range from 9.265 eV for He·NO to 9.132 eV for Kr·NO and the dissociation energies of Rg·NO+ range from 0.017 eV for He·NO+ to 0.156 eV for Kr·NO+, in line with the expectation based on the increasing polarizability of the Rg atom.     

Intramolecular penning ionization in organic molecule-rare gas clusters

A supersonic beam of organic molecules (benzonitrile (Bn), toluene (Tol), benzene (Bz) and pyridine (Pyr)) seeded into Ne or Ar was ionized by VUV radiation from the Berlin electron storage ring (BESSY). Photoionization efficiency curves were measured for Bn, Bn·Ne, Bn·Ar, Tol, Tol·Ar, Tol·Ne, Bz, Pyr, Pyr-Ar, and Pyr-Ne. Resonance peaks in these spectra at the positions of the atomic rare gas resonance lines confirm our previous measurements on Bn seeded into Ar and Kr, showing that the excitation energy of the rare gas atom is transferred to the attached organic molecule within a heterogeneous cluster, leading to ionization of the molecule and fragmentation of the cluster.     

Elimination of the concentration dependence in mass isotopomer abundance mass spectrometry of methyl palmitate using metastable atom bombardment

An important problem in mass isotopomer abundance mass spectrometry (MIAMS) is the dependence of measured mass isotopomer abundances on sample concentration. We have evaluated the role of ionization energy on mass isotopomer abundance ratios of methyl palmitate as a function of sample concentration. Ionization energy was varied using electron impact ionization (EI) and metastable atom bombardment (MAB). The latter generates a beam of metastable species capable of ionizing analyte molecules by Penning ionization. We observed that ionization of methyl palmitate by EI (70 eV) showed the greatest molecular ion fragmentation and also showed the greatest dependence of relative isotopomer abundance ratios on sample concentration. Ionization using the 3P2 and 3P0 states of metastable krypton (9.92 and 10.56 eV, respectively) resulted in almost no molecular ion fragmentation, and the isotopomer abundances quantified were essentially independent of sample concentration. Ionization using the 3P2 and 3P0 states of metastable argon (11.55 and 11.72 eV, respectively) showed molecular ion fragmentation intermediate between that of EI and MAB(Kr) and showed an isotopomer concentration dependence which was less severe than that observed with EI but more severe than that observed with MAB(Kr). The observed decrease in the dependence of isotopomer abundance on sample concentration with a decrease in molecular ion fragmentation is consistent with the hypothesis that proton transfer from a fragment cation to a neutral molecule is the gas phase reaction mechanism responsible for the concentration dependence. Alternative explanations, e.g., hydrogen abstraction from a neutral molecule to a molecular cation, is not supported by these results. Moreover, the MAB ionization technique shows potential for eliminating one source of error in MIAMS measurements of methyl palmitate, in particular, and of fatty acids methyl esters, in general.     

Electron impact ionization of CCl4 and SF6 embedded in superfluid helium droplets

Electron impact ionization of helium nano-droplets containing several 10⁴ He atoms and doped with CCl₄ or SF₆ molecules is studied with high-mass resolution. The mass spectra show significant clustering of CCl₄ molecules, less so for SF₆ under our experimental conditions. Positive ion efficiency curves as a function of electron energy indicate complete immersion of the molecules inside the helium droplets in both cases. For CCl₄ we observe the molecular parent cation CCl₄⁺ that preferentially is formed via Penning ionization upon collisions with He*. In contrast, no parent cation SF₆⁺ is seen for He droplets doped with SF₆. The fragmentation patterns for both molecules embedded in He are compared with gas phase studies. Ionization via electron transfer to He⁺ forms highly excited ions that cannot be stabilized by the surrounding He droplet. Besides the atomic fragments F⁺ and Cl⁺ several molecular fragment cations are observed with He atoms attached.     

Emission and sputtering characteristics of Ne-Ar mixed gas glow discharge plasmas.

The emission characteristics of several Cu lines emitted from a Ne-Ar mixed gas glow discharge plasma were investigated. The addition of small amounts of Ar to a Ne plasma increases the sputtering rate of a Cu sample because Ar ions, which work as the impinging ions for cathode sputtering, are predominantly produced through Penning ionization collisions between Ne metastables and Ar atoms. Ar addition also elevates the number density of electrons in the plasma. These changes occurring in the Ne-Ar mixed gas plasma result in enhanced emission intensities of the Cu lines. The Cu II 270.10-nm and the Cu II 224.70-nm lines yield different intensity dependence on the Ar partial pressure added. This phenomenon is because these Cu II lines are excited principally through different charge transfer processes: collisions with Ne ions for the Cu II 270.10-nm line and collisions with Ar ions for the Cu II 224.70-nm line. The shape of sputtered craters in the Ne-Ar glow discharge plasma was measured. The depth resolution was improved when Ar was added to a Ne plasma because the crater bottoms were flatter with larger Ar partial pressures.     

A structural investigation of [C6H6]2+ and [C6H6]+˙ ions involved in charge exchange reactions

Mass-analysed ion kinetic energy spectra for collisional activation (CA) of [C₆H₆]⁺˙ formed via electron capture by [C₆H₆]²⁺ ions in collision with neutral benzene molecules have been compared for the C₆H₆ isomers benzene, 1,5-hexadiyne and 2,4-hexadiyne. Comparisons of fragment abundance and total CA fragment yields were also made for [C₆H₆]⁺˙ ions generated by electron ionization (EI). CA conditions of ion velocity and collision gas pressure were identical in these comparisons. In general the fragment abundance patterns for the ions formed by charge exchange were very similar to those for singly charged benzene ions generated by EI. However, significant variations in CA fragment yield (the ratio of the total CA fragment ion abundance to the abundance of the incident unfragmented ions) were observed. It is not clear from the results whether these variations reflect structurally different ions or ions of different internal energies. The CA spectra of [C₆H₆]⁺˙ ions derived from charge exchange reactions between the benzene dication and the target gases He, Ne, Ar, Kr and Xe have also been recorded and, once again, very similar fragment abundance patterns were observed along with large variations in total CA fragment yields. Charge exchange efficiency measurements are reported for reactions between the benzene dication and the targets He, Ne, Ar, Kr, Xe and C₆H₆ (benzene) and also for the doubly charged ions derived from the linear C₆H₆ isomers. In the latter case Xe and benzene targets were used. The energetics and efficiency measurements for the former reactions suggest that for targets such as He and Ne the processes probably involve excited states of the doubly charged ions. The efficiencies measured for the latter reactions were distinctly different for the three C₆H₆ isomers and may indicate a strong dependence of charge exchange cross-section on doubly charged ion structure.     

Effect of Ar/H2 and Kr/H2 mixtures as discharge gas on the ion intensity in dc glow discharge mass spectrometry

 In dc glow discharge mass spectrometry, the addition of small amounts of H₂ to pure Ar as discharge gas has greatly increased the ion intensities of elements compared with the conventional method using pure Ar. This phenomenon was also observed for the addition of H₂ to pure Kr. The reason for the increase of the ion intensities of elements was studied by using a Kr gas mixture containing 0.2% (v/v) H₂. The ion intensities of the elements P, Se and As (whose first ionization potentials are higher than the energy levels of the excited state of Kr) did not increase even if the Kr/H₂ gas mixture was used. The results show that the addition of H₂ significantly contributed to increasing the number of metastable argon or krypton atoms (Penning ionization). Received: 4 November 1995/Revised: 5 January 1996/Accepted: 10 January 1996     

Effect of Ar/H2 and Kr/H2 mixtures as discharge gas on the ion intensity in dc glow discharge mass spectrometry

 In dc glow discharge mass spectrometry, the addition of small amounts of H₂ to pure Ar as discharge gas has greatly increased the ion intensities of elements compared with the conventional method using pure Ar. This phenomenon was also observed for the addition of H₂ to pure Kr. The reason for the increase of the ion intensities of elements was studied by using a Kr gas mixture containing 0.2% (v/v) H₂. The ion intensities of the elements P, Se and As (whose first ionization potentials are higher than the energy levels of the excited state of Kr) did not increase even if the Kr/H₂ gas mixture was used. The results show that the addition of H₂ significantly contributed to increasing the number of metastable argon or krypton atoms (Penning ionization). Received: 4 November 1995/Revised: 5 January 1996/Accepted: 10 January 1996     

Gas-phase ion-molecule bimolecular and clustering reactions in dilute solutions of acetonitrile in xenon,krypton, argon,neon and helium

Gas-phase bimolecular and clustering reactions of acetonitrile in Xe, Kr, Ar, Ne and He were studied at high chemical ionization pressures in the new coaxial ion source at Auburn. With electron energies near the ionization threshold, the mass spectra are exceedingly simple and are comprised of [CH₄CH]⁺ and clusters of [CH₄CN]⁺ with various ligands such as H₂O and CH₃CN. At higher electron energies many other peaks appear. The intensities of the new peaks depend upon the ionization potential of the charge transfer gas, the ionizing electron energy and the ion source conditions, and are due to reactions of fragment ions. Residence time distributions at electron energies above the ionization threshold (∼ 30 eV) demonstrate that two molecular structures are present in the ion beam at m/z 42, one presumably is protonated acetonitrile ([CH₃CNH]⁺) while the evidence indicates that the second species does not contain acidic hydrogens. With ionizing electron energies near threshold (∼ 10. 5 eV) only one structure is observed. Studies with electron energies near the ionization threshold under high-pressure chemical ionization conditions result in greatly simplified mass spectra and are possible only because of the coaxial geometry of the ion source.     

Cooperativity between S···π and Rg···π in the OCS···C6H6···Rg (Rg = He, Ne, Ar, and Kr) van der Waals complexes

The complexes OCS···C(6)H(6), C(6)H(6)···Rg, and OCS···C(6)H(6)···Rg (Rg = He, Ne, Ar, and Kr) have been studied by means of MP2 calculations and QTAIM analyses. The optimized geometries of the title complexes have C(6v) symmetry. The intermolecular interactions in the OCS···C(6)H(6)···Rg complexes are comparatively stronger than that in the OCS···C(6)H(6) complex, which prove that the He, Ne, Ar, and Kr atoms have the ability to form weak bonds with the benzene molecule. In QTAIM studies, the π-electron density of benzene was separated from the total electron density. The molecular graphs and topological parameters of the OCS···πC(6)H(6), πC(6)H(6)···Rg, and OCS···πC(6)H(6)···Rg complexes indicate that the interactions are mainly attributed to the electron density provided by the π-bonding electrons of benzene and the top regions of the S and Rg atoms. Charge transfer is observed from the benzene molecule to SCO/Rg in the formation of the OCS···C(6)H(6), C(6)H(6)···Rg, and OCS···C(6)H(6)···Rg complexes. Molecular electrostatic potential (MEP) analyses suggest that the electrostatic energy plays a pivotal role in these intermolecular interactions.