Threshold Photoelectron Photoion Coincidence Spectra Research Articles

DELICIOUS III: A multipurpose double imaging particle coincidence spectrometer for gas phase vacuum ultraviolet photodynamics studies

We present a versatile double imaging particle coincidence spectrometer operating in fully continuous mode, named DELICIOUS III, which combines a velocity map imaging device and a modified Wiley-McLaren time of flight momentum imaging analyzer for photoelectrons and photoions, respectively. The spectrometer is installed in a permanent endstation on the DESIRS vacuum ultraviolet (VUV) beamline at the French National Synchrotron Radiation Facility SOLEIL, and is dedicated to gas phase VUV spectroscopy, photoionization, and molecular dynamics studies. DELICIOUS III is capable of recording mass-selected threshold photoelectron photoion coincidence spectra with a sub-meV resolution, and the addition of a magnifying lens inside the electron drift tube provides a sizeable improvement of the electron threshold/ion mass resolution compromise. In fast electron mode the ultimate kinetic energy resolution has been measured at ΔE/E = 4%. The ion spectrometer offers a mass resolution--full separation of adjacent masses--of 250 amu for moderate extraction fields and the addition of an electrostatic lens in the second acceleration region allows measuring the full 3D velocity vector for a given mass with an ultimate energy resolution of ΔE/E = 15%, without sacrificing the mass resolution. Hence, photoelectron images are correlated both to the mass and to the ion kinetic energy and recoil direction, to access the electron spectroscopy of size-selected species, to study the photodissociation processes of state-selected cations in detail, or to measure in certain cases photoelectron angular distributions in the ion recoil frame. The performances of DELICIOUS III are explored through several examples including the photoionization of N2, NO, and CF3.

Threshold ionization and dissociation oft-butylamine

The threshold photoelectron spectrum of t-butylamine, recorded between 8 and 28 eV, is reported for the first time. The spectrum was compared to orbital ionization energies calculated at the OVGF/cc-pVTZ level of theory. The adiabatic and vertical ionization energies of the outermost orbital (made up primarily of the nitrogen p orbital) are 8.48 ± 0.02 and 9.40 ± 0.02 eV, respectively. Threshold photoelectron photoion coincidence spectra were recorded between 8.5 and 35 eV, and appearance energies for 15 fragment ions were obtained. The lowest energy dissociation is the loss of a methyl group to form the (CH3)2CNH2+ion. RRKM fitting of this dissociation leads to a ΔfH298for (CH3)2CNH2+of 603 ± 3 kJ mol–1. This value is ~10 kJ mol–1higher than the previously derived value of Lossing et al. (based on an electron ionization appearance energy value) and the G3B3 estimate. Together with the proton affinity value of 932.3 kJ mol–1, the present ΔfH298leads to a ΔfH298for 2-propanimine (CH3)2C=NH of 5.3 kJ mol–1, which can be compared to a value of –5.7 kJ mol–1(based on the Lossing et al. (CH3)2CNH2+ΔfH298) and a G3B3 value of –0.3 kJ mol–1. At photon energies above 26 eV, there is evidence for the dissociative double ionization of t-butylamine forming either the singlet or triplet state of the dication.

Dissociation of energy-selected CH 3CN + in a region 15.1–16.5 eV: Vibrationally enhanced dissociation and mechanisms

Threshold photoelectron–photoion coincidence spectra of CH 3CN were measured in a region 15.1–16.5 eV. We observed a small vibrational enhancement on dissociation of CH 3CN + and obtained kinetic energy releases for channels C 2HN + + H 2 and CH 2 + + HCN . The kinetic energies released in channel C 2HN + + H 2 are substantial; a linearly extrapolated dissociation threshold of 14.68 ± 0.01 eV agrees with a Gaussian-3 B3LYP prediction 14.70 eV for formation of HCCN + + H 2. In contrast, the small releases for channel CH 2 + + HCN agree with a statistical calculation. We describe a plausible dissociation mechanism with the Gaussian-3 B3LYP method to rationalize experimental results.

The photoionization dynamics of the three structural isomers of dichloroethene

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron spectrum, threshold photoelectron photoion coincidence spectrum and ion breakdown diagram of the 1,1, cis-1,2 and trans-1,2 isomers of C2H2Cl2 have been recorded in the range 9–23 eV. The energies of the peaks in the threshold photoelectron spectrum are in good agreement with outer-valence Greens function calculations. The major difference between the isomers, both predicted and observed experimentally is that the and states of are approximately degenerate for 1,1 and trans-1,2, but well separated for the cis-1,2 isomer. The ground and low-lying valence states of are bound, with higher-lying states dissociating to C2H2Cl+ or . The translational kinetic energy release into C2H2Cl+ + Cl is determined as a function of energy. Isolated-state behaviour for the low-lying electronic states of becomes more statistical as the energy increases.

Selected ion flow tube cation–molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic-C5F8

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C5F8 with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H2O+ for which the reaction efficiency is only 55%. Apart from H2O+ and N+, the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion-molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of c-C5F8 measured by TPEPICO spectroscopy occurs at 12.25 +/- 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 +/- 0.10 eV), He (I) photoionisation (11.30 +/- 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 +/- 0.002 eV). The ground electronic state of c-C5F8+ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C5F8 recorded from 12-23 eV shows detection of the parent ion and the daughter ions C4F6+ and C5F7+, with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of c-C5F8 at 298 K has been determined to be -1495 kJ mol(-1).

Isomeric Effects in the Gas-Phase Reactions of Dichloroethene, C2H2Cl2, with a Series of Cations

A study of the reactions of a series of gas-phase cations (NH(4)(+), H(3)O(+), SF(3)(+), CF(3)(+), CF(+), SF(5)(+), SF(2)(+), SF(+), CF(2)(+), SF(4)(+), O(2)(+), Xe(+), N(2)O(+), CO(2)(+), Kr(+), CO(+), N(+), N(2)(+), Ar(+), F(+), and Ne(+)) with the three structural isomers of dichloroethene, i.e., 1,1-C(2)H(2)Cl(2), cis-1,2-C(2)H(2)Cl(2), and trans-1,2-C(2)H(2)Cl(2) is reported. The recombination energy (RE) of these ions spans the range of 4.7-21.6 eV. Reaction rate coefficients and product branching ratios have been measured at 298 K in a selected ion flow tube (SIFT). Collisional rate coefficients are calculated by modified average dipole orientation (MADO) theory and compared with experimental data. Thermochemistry and mass balance have been used to predict the most feasible neutral products. Threshold photoelectron-photoion coincidence spectra have also been obtained for the three isomers of C(2)H(2)Cl(2) with photon energies in the range of 10-23 eV. The fragment ion branching ratios have been compared with those of the flow tube study to determine the importance of long-range charge transfer. A strong influence of the isomeric structure of dichloroethene on the products of ion-molecule reactions has been observed for H(3)O(+), CF(3)(+), and CF(+). For 1,1-C(2)H(2)Cl(2) the reaction with H(3)O(+) proceeds at the collisional rate with the only ionic product being 1,1-C(2)H(2)Cl(2)H(+). However, the same reaction yields two more ionic products in the case of cis-1,2- and trans-1,2-C(2)H(2)Cl(2), but only proceeds with 14% and 18% efficiency, respectively. The CF(3)(+) reaction proceeds with 56-80% efficiency, the only ionic product for 1,1-C(2)H(2)Cl(2) being C(2)H(2)Cl(+) formed via Cl(-) abstraction, whereas the only ionic product for both 1,2-isomers is CHCl(2)(+) corresponding to a breaking of the C=C double bond. Less profound isomeric effects, but still resulting in different products for 1,1- and 1,2-C(2)H(2)Cl(2) isomers, have been found in the reactions of SF(+), CO(2)(+), CO(+), N(2)(+), and Ar(+). Although these five ions have REs above the ionization energy (IE) of any of the C(2)H(2)Cl(2) isomers, and hence the threshold for long-range charge transfer, the results suggest that the formation of a collision complex at short range between these ions and C(2)H(2)Cl(2) is responsible for the observed effects.

Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube reactions of CHF3: comparison of product branching ratios§

The threshold photoelectron and threshold photoelectron photoion coincidence spectra of CHF3 in the range 13.5–24.5 eV have been recorded. Ion yields and branching ratios have been determined for the three fragments , and CF+. The mean kinetic energy releases into fragment ions involving either C–H or C–F bond cleavage have been measured, and compared with statistical and impulsive models. behaves in a non-statistical manner characteristic of the small-molecule limit, with the ground electronic state and low-lying excited states of being largely repulsive along the C–H and C–F coordinates, respectively. The rate coefficients and product ion branching ratios have been measured at 298 K in a selected ion flow tube for the reactions of CHF3 with a large number of gas-phase cations whose recombination energies span the range 6.3 through 21.6 eV. A comparison between the branching ratios from the two experiments, together with an analysis of the threshold photoelectron spectrum of CHF3, shows that long-range charge transfer probably occurs for the Ar+ and F+ atomic ions whose recombination energies lie above ca. 15 eV. Below this energy, the mechanism involves a combination of short-range charge transfer and chemical reactions involving a transition state intermediate. §In honour of Professor John Simons’ 70th birthday Festschrift.

Threshold photoelectron–photoion coincidence spectroscopy study of CHCl2F+, CHClF+2 and CH2ClF+: Steric influence of the chlorine, fluorine and hydrogen atoms

The threshold photoelectron spectrum and threshold photoelectron-photoion coincidence spectra of CHCl2F, CHClF2 and CH2ClF are reported in the range 11.3-24.8 eV. Tunable photoionizing radiation with a resolution of 0.3 nm is provided from a synchrotron source with a vacuum-UV monochromator. The coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Energetic comparisons suggest that the major products of the titled molecules dissociate in a similar manner at low photon energy, with the parent and first fragment ion, corresponding to cleavage of the weakest bond, appearing at their thermochemical thresholds. The second major ion, corresponding to cleavage of the second weakest bond, is formed ca. 1 eV higher than its predicted threshold, this disparity implying state-selected dissociation. CHCl2F and CHClF2 fragment in a similar manner at higher photon energies, with minor ions formed by the cleavage of three bonds possessing lower appearance energies than fragment ions formed by the cleavage of two bonds. CH2ClF displays the more expected behaviour, namely sequential bond cleavage as the photon energy increases. These observations can be rationalised in terms of the height of the barrier on the exit channel, as determined by the steric bulk of the leaving group. For the three titled molecules, mean translational kinetic energy releases have also been measured into the channels involving C-F or C-Cl bond fission. These data infer that impulsive dissociations occur at lower energy, with a trend towards statistical behaviour with increasing photon energy. Competition between statistical and impulsive processes is observed, for example C-Cl vs. C-F bond cleavage in CHCl2F+ and CHClF2+.

Fragmentation of the valence electronic states of PSCl3+ studied by threshold photoelectron–photoion coincidence spectroscopy

The threshold photoelectron spectrum and threshold photoelectron–photoion coincidence spectrum of PSCl3 are reported in the range 9.5–22.5 eV. Tunable ionising radiation with a resolution of 0.3 nm is provided from a synchrotron source coupled to a vacuum–UV monochromator. The coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. The ground state of PSCl3+ is bound with respect to dissociation, whereas the five lowest excited electronic states dissociate by P–Cl bond cleavage to PSCl2+. An upper limit of 629 ± 30 kJ mol−1 is determined for the enthalpy of formation of PSCl2+ at 298 K. For energies above ca. 15 eV, fragment ions corresponding to multiple bond cleavages are observed; PSCl+ and PCl2+, PS+ and PCl+. Because of uncertainties in thermochemistry, it is possible only for PS+ to determine the neutrals that form with this fragment cation. Translational kinetic energy releases have been measured into PSCl2+ + Cl at the energies of the Franck–Condon maxima of the valence states of PSCl3+, and the results compared with models assuming statistical and impulsive dissociation. The kinetic energy data suggest that the six lowest excited states of PSCl3+ decay by a mechanism intermediate between statistical dissociation from the ground state of PSCl3+ following rapid internal conversion and impulsive dissociation from the initially-excited electronic state.

Heats of Formation of the Acetyl Radical and Ion Obtained by Threshold Photoelectron Photoion Coincidence

The dissociative photoionization onsets for the production of CH 3 CO + + CH 3 _from acetone and CH 3 CO + + CH 3 CO_from butanedione have been measured by threshold photoelectron photoion coincidence (TPEPICO) in which time-of-flight (TOF) mass spectra are obtained as a function of the ion internal energy. The use of velocity focusing for threshold electrons and the subtraction of hot electron coincidences from the TPEPICO spectra allow the 0 K dissociation onset to be measured with a precision of I kJ/mol. The experimental onset for CH 3 _loss from CH 3 COCH 3 was measured to be 10.563 ′ 0.010 eV and the onset for CH 3 CO_ loss from CH 3 COCOCH 3 was found to be 10.090 ′ 0.006 eV. A 298 K heat of formation of the CH 3 CO + of 659.4 ′ 1.1 kJ/mol is obtained by combining the measured dissociation onset with the well-established heats of formation of acetone and the methyl radical. A 298 K heat of formation of the CH 3 CO_ radical of -9.8 ′ 1.8 kJ/mol is obtained by combining the measured dissociation onset with the well-known heat of formation of butanedione and the measured heat of formation of CH 3 CO + . The acetone and butanedione ionization energies were measured to be 9.708 ′ 0.004 and 9.21 ′ 0.05 eV, respectively.

Suppression of hot electrons in threshold photoelectron photoion coincidence spectroscopy using velocity focusing optics

Velocity focusing of electrons is combined with photoelectron photoion coincidence (PEPICO) spectroscopy to achieve a true threshold PEPICO signal without contributions from energetic electrons. Ions are generated by a continuous vacuum ultraviolet light source. Electrons, extracted by a field of 20 V/cm, pass through a 13 cm drift region and are dispersed in space on a multichannel plate detector by velocity focusing optics. The ions are extracted in the opposite direction by the same electric field, further accelerated by a second field, and collected after passing through a 30 cm drift region. Ions are measured in coincidence with electrons collected from the central 3.2 mm electrode as well as a ring electrode (inner and outer diameters of 5.6 and 8.1 mm). The central ring electrode contains mostly true threshold electrons along with a background of “hot” electrons, whereas the outer ring electrode collects only hot electrons. By subtracting the latter from the former, true threshold photoelectron photoion coincidence spectra are obtained. The major advantages of this approach are the high electron energy resolution with the use of high direct current extraction fields, and the complete suppression of energetic electrons.

Decay processes of the C2 2Π1/2u states of Ar2+, Kr2+, and Xe2+

The threshold photoelectron photoion coincidence spectra of the rare gas dimers Ar2, Kr2, and Xe2 have been measured in the photon energy region between the first and second dissociation limits of the dimer ions. It has been found that the C2 2Π1/2u states of Kr2+ and Xe2+ completely dissociate via an optical transition to the repulsive B2 2Π1/2g state, while that of Ar2+ does not dissociate.

Vibrationally resolved threshold photoelectron–photoion coincidence spectra of ArXe

High-resolution threshold photoelectron–photoion coincidence spectra of ArXe were measured in the photon energy region 11.9–15.9 eV using the penetrating field technique and synchrotron radiation. Six vibrational progressions were observed and ascribed to the production of the six electronic states of ArXe+ ions (X 1/2, A1 3/2, A2 1/2, C1 3/2, B 1/2, and C2 1/2) via photoionization of the ground state ArXe dimer. The lowest vibrational level ν=0 of the ground X 1/2 state and some of the other vibrational levels were observed for the first time in the present measurements. The vibrational constants were obtained from observed vibrational energy levels. The dissociation energy of the ground state (X 1/2) was obtained as D00=1432±8 cm−1.

The design and performance of a threshold-photoelectron–photoion coincidence spectrometer for the study of unimolecular decomposition in polyatomic ions

The design, construction and performance of a threshold-photoelectron–photoion coincidence (TPEPICO) spectrometer for the study of unimolecular decomposition in polyatomic ions is described. The spectrometer incorporates a hemispherical electrostatic energy analyser and a time-of-flight (TOF) mass spectrometer. The entrance lens to the hemispherical analyser has been designed to have a high collection efficiency for low energy electrons but to discriminate strongly against energetic electrons. This arrangement has resulted in a resolution of about 3.5 meV being achieved for the threshold electron peak recorded at the krypton 2P 3/2 ionisation limit. A pulsed electric field is used to extract the ions from the interaction region and propel them towards the TOF analyser. Computer modelling has been used to trace the electron and ion trajectories through their respective analysers. These simulations have enabled the effective interaction volume to be defined, and this has allowed the transmission efficiency of energetic fragment ions, formed through a process which also yielded a threshold electron, to be quantified. The ion TOF peak shape has been examined as a function of initial kinetic energy and as a function of ion residence time. The contribution of energetic fragments, having specific initial spatial and directional properties, to the TOF peak shape has been determined by tracing the paths of individual ions. The actual performance of the spectrometer is illustrated by a TPEPICO spectrum of the krypton isotopes. Experimental breakdown curves for furan are presented as an example of the use of the apparatus to study unimolecular decomposition in polyatomic ions. By introducing a delay between the detection of the threshold electron and the application of the ion extraction field, breakdown curves can be recorded as a function of ion residence time in the source region. The procedure for analysing the data is described, and the experimental factors that need to be taken into account to obtain a meaningful comparison with theoretical predictions are discussed.

Charge transfer from neutral perfluorocarbons to various cations: long-range versus short-range reaction mechanisms

The bimolecular reactions of the high recombination energy cations Ar +, F +, and Ne + with four fully saturated (CF 4, C 2F 6, C 3F 8, and n-C 4F 10) and three unsaturated (C 2F 4, C 3F 6, and 2-C 4F 8) perfluorocarbons (PFCs) are reported. The cation branching ratios obtained from these reactions, and from the reactions with O 2 +, H 2O +, N 2O +, O +, CO 2 +, CO +, N +, and N 2 + [reported by us, Jarvis et al., J. Phys. Chem. 100 (1996) 17166], are compared with those determined from the threshold photoelectron–photoion coincidence spectra of the PFCs at the recombination energies of the reagent cations. This comparison provides information that helps to interpret the dynamics of charge transfer, and whether it occurs via a long-range or a short-range mechanism. Energy resonance and good Franck-Condon factors connecting the ground electronic state of a reactant neutral molecule to one of its ionic states, at the recombination energy of the reagent cation, are generally considered to be sufficient criteria for long-range charge transfer to occur. However, the results from this study imply that good Franck-Condon factors are not critical in determining the efficiency of a long-range charge transfer. Instead, the results suggest that, in addition to the requirement for energy resonance, the electron taking part in the charge-transfer process must be removed from a molecular orbital which is unshielded from the approaching reagent cation. This enables the cation to exert an influence on the electron at large impact parameters.

The use of threshold photoelectron–photoion coincidence spectroscopy to probe the spectroscopic and dynamic properties of the valence states of CCl3F+, CCl3H+ and CCl3Br+

Using tunable vacuum-ultraviolet radiation from a synchrotron source in the range 10–25 eV, threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to determine the decay pathways of the valence electronic states of CCl3X+ (X=F, H, Br). TPEPICO spectra are recorded continuously as a function of photon energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are also recorded with improved time resolution, allowing total mean translational kinetic energy releases, 〈KE〉t , into some dissociation channels to be determined. By comparing 〈KE〉t values for single bond-fission processes (i.e. cleavage of a C–Cl or C–X bond) with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. Excited states of all three parent cations show evidence for isolated-state behaviour, and the 〈KE〉t values suggest a relationship between the part of the molecule where ionisation occurs and the bond that breaks to form daughter ion plus neutral atom products; impulsive values of 〈KE〉t are more likely to be obtained when the breaking bond lies close to the part of the molecule where ionisation occurs, statistical values when ionisation occurs further away from the breaking bond. At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond. With CCl3F and CCl3Br, the appearance energies of the daughter ions are close to the thermochemical energy for production of that ion with isolated neutral atoms, suggesting strongly that these ions form by bond-fission processes only. With CCl3H, at certain energies some fragment ions can only form with molecular neutral fragments (e.g. CCl2++HCl), involving bond-breaking and bond-making processes. It is suggested that this phenomenon is related to the small size of the hydrogen atom, and hence less steric hindrance in a tightly constrained transition state along the reaction coordinate.

Vibrationally resolved threshold photoelectron–photoion coincidence spectra of KrXe

High resolution threshold photoelectron–photoion coincidence spectra of KrXe were measured in the photon energy region 11.7–14.7 eV using the penetrating field technique and synchrotron radiation. Six vibrational progressions were observed and ascribed to the production of the six electronic states of KrXe+ ions (X 1/2, A1 3/2, A2 1/2, C1 3/2, B 1/2, and C2 1/2) via photoionization of the ground state KrXe dimer. The vibrational structures for the X 1/2, A2 1/2, B 1/2, and C2 1/2 states were observed for the first time in the present measurements. The molecular constants for vibration were obtained from the analysis of the deduced vibrational energy levels. The potential curves of each state were drawn by using the Morse function.

Fragmentation of the valence electronic states of NF 3+ studied by threshold photoelectron–photoion coincidence spectroscopy

The valence threshold photoelectron spectrum of NF 3 is reported for the first time in the literature, and threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has measured, state-selectively, the decay dynamics of the valence states of NF 3 + in the range 13–23 eV. Vacuum–UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions and the breakdown diagram to be obtained. A comparison of the integrated threshold photoelectron and the total ion signals as a function of energy suggests that, in the range 16–19 eV, autoionisation via Rydberg states of NF 3 makes a significant contribution to the production of threshold electrons. The 50% crossover energy for production of NF 2 + from NF 3 + is determined to be 14.10±0.05 eV. The first onsets for NF 2 + and NF + production are 13.95±0.05 and 17.6±0.1 eV, respectively. The majority of the Franck–Condon region of the X ̃ 2 A 1 ground state of NF 3 + is stable with respect to dissociation to NF 2 +, whereas the unresolved A ̃ 2 E / B ̃ 2 A 2 states and most of the C ̃ 2 E state dissociate exclusively to NF 2 +. The D ̃ 2 A 1 and E ̃ 2 E states dissociate to NF +. Translational kinetic energy releases have been measured in NF 2 + and NF + at the energies of the Franck–Condon maxima of the valence states of NF 3 +. The results are compared with models assuming statistical and impulsive dissociation. The Ã/B̃ states of NF 3 + dissociate directly from the excited-state potential energy surface to NF 2 +, whereas the higher-lying C̃ state probably dissociates off the ground-state surface following rapid internal conversion. It is not possible to correlate unambiguously the formation of NF + with either F 2 or 2F, although on energetic grounds the latter products are more likely. Assuming that the neutral products are 2F, no information is obtained whether the two N–F bonds break simultaneously or sequentially.

Pulsed-Field Ionization Zero-Kinetic-Energy Photoelectron Spectra of Ar2

The pulsed-field ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectra of Ar2 were measured between 117 000 cm−1 (14.5 eV) and 128 300 cm−1 (15.9 eV) using the penetrating field technique and synchrotron radiation at resolutions of up to 1 meV. The A2Σ+1/2u ground state and B2Π1/2g and C2Π3/2u excited states were assigned to those spectra. The first adiabatic ionization potential of Ar2 was determined to be 14.4572 ± 0.0007 eV. A Le Roy–Bernstein plot and a Birge–Sponer plot were used to determine the dissociation limit of the A2Σ+1/2u ground state. The vibrational level of the C2Π3/2u excited state was observed for the first time. The effective lifetime for high-n Rydberg states converging on the vibrational levels of the C2Π1/2u excited state is estimated to be shorter than those of the A2Σ+1/2u, B2Π1/2g, and C2Π3/2u states, because the spectrum could not be observed on the PFI-ZEKE experiment. The threshold photoelectron–photoion coincidence spectra of Ar2 were also measured and compared with the spectra obtained from the PFI-ZEKE experiment.

Vibrationally resolved threshold photoelectron–photoion coincidence spectra of ArKr

Threshold photoelectron–photoion coincidence (TPEPICO) spectra of ArKr were measured between the ionization threshold and 16.0 eV by using the penetrating field technique and synchrotron radiation. The ground state (X 1/2 state) vibrational progression of ArKr+ was observed from v′=2 to 27 and the first ionization energy was estimated to be 108 703±12 cm−1 by extrapolation to v′=0. The other excited states constructed from the ground state atom and ion were also observed. The ionization energies to the A1 3/2, A2 1/2, C1 3/2, B 1/2, and C2 1/2 states from the neutral ground state were 112 274, 117 388, 126 211, 126 614, and 127 952 cm−1, respectively.