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The threshold photoelectron spectra of SO2 were recorded over the photon energy range (12–50 eV) under high-resolution conditions using synchrotron radiation. Extensive vibrational ‘filling-in’ is found in the Franck-Condon gap regions between the three lowest ionic-state band systems that is attributed to autoionization of Rydberg states lying in these regions. The outer-valence ionization region (12–18 eV) consists mainly of the vibrationally well-defined electronic states of ( X ˜ 2A1), ( A ˜ 2B2), ( B ˜ 2A2), ( C ˜ 2B1), ( D ˜ 2A1) and ( E ˜ 2B2) being derived from the molecular orbitals: (8a1), (5b2), (1a2), (2b1), (7a1) and (4b2), respectively, but there are also some features that can be associated with the two weak satellite structures (1 2B1) and ( 3 B 2 2 ). On the other hand, in the inner-valence ionization region (18–50 eV) the opposite is true, i.e., a well-defined electronic state structure is limited to the ( F ˜ 2A1) state of SO2+ with a (6a1) molecular orbital origin, with the preponderance of the remaining structure due to broad satellite features involving ionization from the inner-valence (3b2) and (5a1) molecular orbitals.

We have performed an extensive study of the threshold photoelectron spectrum of N 2 over the valence ionization region 15.0–28.0 eV under high-resolution conditions. In addition, we measured the high-resolution photoion spectrum of N 2 over the same photon energy range. These complimentary views of ionization in molecular systems provide different perspectives of the mechanism for the production of both ions and threshold electrons. The studies are presented in the context of our current understanding of the relationship between: (i) the autoionization of neutral Rydberg states and direct photoionization to (ii) the production of electrons and the ionic states of nitrogen at threshold. In both cases (ions and threshold electrons) we have found a considerable amount of new information on the vibrational states of a number of neutral and ionic electronic states. The inner valence ionization region of nitrogen was also investigated using threshold photoelectron spectroscopy over the photon energy range (27.7–52.7 eV) and is presented from the perspective of the formation of mixed-configuration ionic satellite states and the possibility of the involvement of ionic Rydberg states converging on the double ionization of nitrogen.

High-resolution (6 meV and 12 meV) threshold photoelectron (TPE) spectra of acetaldehyde and acrolein (2-propenal) have been recorded over the valence binding energy region 10–20 eV, employing synchrotron radiation and a penetrating-field electron spectrometer. These TPE spectra are presented here for the first time. All of the band structures observed in the TPE spectra replicate those found in their conventional HeI photoelectron (PE) spectra. However, the relative band intensities are found to be dramatically different in the two types of spectra that are attributed to the different dominant operative formation mechanisms. In addition, some band shapes and their vertical ionization potentials are found to differ in the two types of spectra that are associated with the autoionization of Rydberg states in the two molecules.

The carboxylic acids (formic and acetic) have been studied using threshold photoelectron (TPE) and total photoion yield (TPIY) spectroscopies; simultaneously obtained spectra of formic acid (HCOOH) were recorded over the entire valence ionisation region from 11–21 eV at a resolution of ∼12 meV. Higher resolution spectra (∼6 meV) were also obtained in the energy region of the lowest two cationic states. Analysis of the TPE spectrum in this energy range agreed very favorably with the best available conventional photoelectron (PE) spectrum of formic acid. Autoionising Rydberg structure was observed in the TPIY spectrum of formic acid and is attributed primarily to the presence of the n p a′ ← 8a′ Rydberg series converging on to the 3 2 A′ ionic state of formic acid. Preliminary results, at a resolution of ∼8 meV, were obtained for acetic acid (CH 3 COOH) over the onset of the ionisation energy region. The TPE spectrum was found to be very similar to the best published photoelectron spectrum, but no Rydberg structure was observed in the TPIY spectrum.

The photoabsorption ion-yield spectra of the titled molecules have been recorded in the threshold ionization region between the two spin-orbit components of the ground states of the molecular ions. All ion-yield spectra display rather simple autoionization structure superimposed on a smoothly rising continuum in the ionization energy region. The structure was analyzed in terms of autoionizing Rydberg states and their vibrational profiles simulated using Rydberg–Klein–Rees (RKR) derived potential energy curves and calculated Franck–Condon factors using Gaussian line-shape functions. All of the structure observed is attributed to spin-orbit autoionization. There seems to be a propensity for autoionization of sσ Rydberg states in all of the interhalogen molecules studied.

The threshold photoelectron spectra of H2O and D2O have been recorded over the ionization region (12–40 eV) under high-resolution conditions using synchrotron radiation. Extensive vibrational ‘filling-in’ is found in the Franck–Condon gap regions between the three lowest ionic-state band systems that is attributed to autoionization of Rydberg states lying in these binding-energy regions. Vibronic coupling effects are found in the A 2A1 band system of H2O+ and D2O+ in general agreement with a previous experiment. An unusual intensity enhancement is found in the B 2B2 band system in the threshold photoelectron spectra of H2O and D2O that is attributed to an enhanced production of H2O+ and D2O+ ions due to the autoionization of Rydberg states lying in the binding energy region of this electronic state. In the inner valence ionization region two broad features have been found that are attributed to the (2a1)−1 ‘main-line’ state and the ( 1 b 1 ) 1 4 a 1 ∗ resonant state.

Using a crossed-beams set-up, we have studied the electron energy spectra due to Penning and associative ionization in thermal energy collisions of state-selected metastable Ar*(4s 3P2, 3P0) and Kr*(5s 3P0) atoms with ground-state Hg atoms at an electron energy resolution of 30 meV. In all three cases the energy spectra exhibit a prominent peak located close to the energy difference between the metastable excitation energies and the Hg ionization energy; they show a sharp drop to lower and an extended tail towards higher electron energy with shapes which differ for the three systems. The energy-integrated ionization cross section for Ar*(3P0) exceeds that for Ar*(3P2) by about 30%. Excitation transfer to autoionizing Hg** states, resulting in a sharp additional structure, is only observed—as a weak channel—in Ar*(3P0) + Hg collisions. For the three investigated systems calculations of the electron energy spectra have been carried out within the local optical potential approximation. As input we use ab initio potential curves for the ionic exit channels as well as for the complex entrance channel potentials. Using these potentials (involving a scaled width for Kr* + Hg), impressive agreement with the experimental electron spectra is observed. In contrast, earlier empirical model potentials for the entrance channel are found to be inadequate. The calculated ionization rate coefficients at T = 300 K amount to (3.72, 5.26) × 10−10 cm3 s−1 for Ar*(3P2, 3P0) + Hg, respectively.

Photo-double ionization of hydrogen iodide has been investigated over the photon energy range 29.0–32.5 eV by threshold photoelectrons coincidence (TPEsCO) spectroscopy and by ab initio calculations that included spin–orbit interaction. Good agreement is found between experiment and theory for the adiabatic ionization potentials for the formation of the X 3 Σ − , a 1 Δ and b 1 Σ + states of HI2+ and for the vibrational separations and relative vibrational intensities within the states. The spin–orbit splitting of the X 3 Σ − state of HI2+ is calculated to be 0.17924 eV for ν2+=0. Because this splitting is close to the calculated vibrational separations within the spin–orbit components of this state [e.g., for ν2+=0–1, 0.22382 eV in 3 Σ 0 − and 0.22163 eV in 3 Σ 1 − ], there is an overlapping of vibrational structure within the TPEsCO spectrum at the resolution used that is confirmed by the simulation spectrum.

A high-resolution (2–15 meV) threshold photoelectron (TPE) spectroscopic study of BrCN was performed using synchrotron radiation and a penetrating-field electron spectrometer over the outer valence ionization region of the molecule. Significant differences were found between the TPE spectrum and the published high-resolution (∼6 meV) HeI photoelectron spectrum of BrCN [Chem. Phys. 212 (1996) 457]. Autoionization of Rydberg states and the 6σ valence state of BrCN were found to be primarily responsible for the formation of the observed band systems of BrCN + in this study.

The threshold photoelectron spectrum of benzene has been recorded up to 26.5 eV photon energy under high-resolution conditions using synchrotron radiation and employing the penetrating-field threshold electron collection method. By means of a direct comparative study with a recent HeI photoelectron spectrum of benzene of equally high resolution, numerous autoionization effects are observed in the formation of the ionic band systems of benzene in the outer valence ionization region in the threshold photoelectron spectrum. The Rydberg states responsible for these effects are identified. Autoionization does not appear to play a role in the formation of the two lowest-energy, inner-valence bands of C6H6+.Key words: threshold photoelectron spectroscopy, photoelectron spectroscopy, benzene, ionization, autoionization, Rydberg states.

An in-depth photoionization study of the inner-valence electrons in HCl and DCl has been performed using synchrotron radiation. A series of photoelectron spectra of HCl were obtained at a resolution of 23 meV over the binding energy range 25-30.5 eV at various excitation energies and at two different electron collection angles relative to the plane of polarization of the undulator radiation. In addition, photoelectron spectra of DCl were recorded at two different excitation energies. These spectra were compared directly with the threshold photoelectron spectra of HCl and DCl that were recorded previously under similar resolution conditions (similar to30 meV). This comparative study reveals new information on the nature of the numerous band systems observed in this binding energy region. In addition, we present the experimental confirmation of the theoretical prediction given by Andersson et al (2001 Phys. Rev. A 65 012705) that a vibrational progression showing interference structure would appear in the main inner-valence ionization band in the photoelectron spectrum of DCl at a resolution of 10 meV.

Photo-double ionization of HBr and DBr has been investigated in the 32–37 eV energy range using synchrotron radiation and the threshold photoelectrons coincidence (TPEsCO) method. The TPEsCO spectra of HBr and DBr, encompassing the formation of the X 3Σ−, a 1Δ and b 1Σ+ states of HBr2+ and DBr2+, were recorded using a pair of penetrating-field electron spectrometers. All electronic band systems exhibited vibrational structure. In addition, the threshold photoelectron spectra of HBr and DBr for the formation of singly charged HBr and DBr were recorded simultaneously.

A combined high-resolution (6 meV) HeI photoelectron (PE) and ab initio theoretical study of the A 2Σ+ ion system of HI and DI has been conducted to elucidate the origin of a peculiar “holelike” feature in the vibrational distribution found in the high-resolution (6 meV) threshold photoelectron (TPE) spectrum of HI. The PE and TPE spectra were found to yield essentially identical results. Ab initio potential energy curves for the low-lying cationic states of HI have been calculated for the first time with and without spin–orbit contributions included. It has been found that the diabatic A 2Σ+ state of HI+ is strongly predissociated due to spin–orbit coupling with the 4Σ−, 2Σ−, and 4Π repulsive states leading to a complex set of adiabatic curves. It is shown that the adiabatic A 2Σ1/2+ state is only slightly bound (by 260 cm−1 after suitable adjustments of the positions of the various repulsive potentials relative to that of the A state are made based on observed atomic spectral data) and should support at...

The threshold photoelectron spectrum of boron trifluoride has been recorded over the entire outer valence ionization region (15–37 eV) under high-resolution conditions using synchrotron radiation and employing the penetrating-field threshold electron collection method. By means of a direct comparative study with the photoelectron spectrum of BF 3 over the same binding energy region numerous autoionization effects are identified in the ionic band systems in the threshold photoelectron spectrum. Extensive vibrational “filling-in” is found in the Franck–Condon gap regions between the four lowest ionic-state band systems that are attributed to resonance autoionization of Rydberg states lying in these binding-energy regions. Extensive vibronic coupling effects are found in all band systems of BF 3 + in general agreement with theory. Analysis of the vibrational structure in most band systems of BF 3 + suggests that interstate vibronic coupling via pseudo-Jahn–Teller interactions may be more complicated than previously assumed. There is some evidence that a shape resonance involving the 5e ′ molecular orbital of BF 3 is important in populating the (D 2 E ′ ) and (E 2 A 1 ′ ) states of BF 3 + as observed in the attendant band systems of the threshold photoelectron spectrum.

Vibrationally resolved threshold photoelectrons in coincidence (TPEsCO) spectra of the X 3 Σ − and a 1 Δ states of HCl2+ and DCl2+ are compared with ab initio simulations. The four resonances observed in the TPEsCO spectra of both the X 3 Σ − and a 1 Δ states of HCl2+ are assigned as three quasi-bound vibrational levels and one continuum resonance with an energy greater than the potential barrier maximum. The four clearly identified resonances observed in the TPEsCO spectra of both the X 3 Σ − and a 1 Δ states of DCl2+ are assigned as quasi-bound vibrational levels.

A high-resolution (1–11 meV) threshold photoelectron spectroscopic study of ICl has been performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Accurate vibrational constants have been obtained for both spin–orbit components of the ICl+ (X 2 Π i ) state through the observation of extended vibrational bands in the Franck–Condon gap regions of the molecular ion. The appearance of these vibrational bands is attributed to resonant autoionization of Rydberg states residing in this energy region of the ionic state. The adiabatic (v+=0) ionization potentials for formation of the ground state of the ICl+ ion are found to be 10.076±0.002 eV for (X 2 Π 3/2 ) and 10.655±0.002 eV for (X 2 Π 1/2 ) yielding a spin–orbit splitting in this state of 0.579±0.002 eV. The adiabatic (v+=0) ionization potential for formation of the (A 2 Π i ) state of ICl+ is estimated to be at ∼12.5 eV. Additional autoionization effects are noted in the formation of the (A 2 Π i ) and (B 2 Σ + ) band systems in ICl+.

The threshold photoelectron (TPE) spectrum of SF6 has been recorded over the photon energy range 25-140 eV using synchrotron radiation and a penetrating-field electron spectrometer. In addition, the photo-double ionization spectrum of SF6 has been obtained over the energy range 30.7-49.3 eV using the threshold photoelectrons coincidence technique. The TPE spectrum in the inner-valence ionization region is found to be significantly different to the conventional photoelectron spectrum in the same binding energy region. Indirect autoionization of neutral Rydberg and shape-resonance states of SF6 dominate the formation of the TPE spectrum. Inner-valence SF6 + ion states appear to play a major role in the formation of the SF6 2+ ion states. The onset of double ionization in SF6 was found to occur at 31.98±0.02 eV.

The pulsed-field ionization (PFI) photoelectron (PE) spectrum of HF has been recorded at the chemical dynamics beamline of the advanced light source over the photon energy range 15.9-16.5 eV using a time-of-flight selection scheme at a resolution of 0.6 meV. Rotationally-resolved structure in the HF+(X 2 pi 3/2, 1/2, v+ = 0, 1) band systems are assigned. The spectral appearance of these systems agrees with a previous VUV laser PFI-PE study. Importantly, extensive rotationally-resolved structure between these two vibrational band systems is also observed. This is attributed to ion-pair formation via Rydberg states converging on the v+ = 1 vibrational levels of the HF+(X 2 pi 3/2, 1/2) spin-orbit states. These Rydberg states are assigned to the 1 sigma+ part of the nd-complexes (sigma, pi, and delta). Ion-pair formation is observed in this study by the detection of F- ions. Some partially rotationally-resolved structure in a previously published threshold photoelectron spectrum is similarly attributed to ion-pair formation (F- detection) through a combination of the v+ = 17 level of the (A 2 sigma+) 3s sigma Rydberg state and the (X 2 pi 3/2, 1/2, v+ = 1) 7d Rydberg states. On the basis of the present study, an accurate experimental value for the dissociation energy of the ground state of HF has been obtained, D0(HF) = 5.8650(5) eV.

Photo-double ionization of DCl has been investigated in the 35.5–38 eV energy range using synchrotron radiation on beamline 10.0.1.2 of the Advanced Light Source by the threshold photoelectrons coincidence (TPEsCO) method. The TPEsCO spectrum of DCl, encompassing the formation of the ground (X 3 Σ − ) and first-excited (a 1 Δ ) states of DCl2+, was recorded at good resolution (∼13 meV) using a pair of penetrating-field electron spectrometers. Four vibrational bands were identified in both electronic-state systems. In addition, the threshold photoelectron (TPE) spectrum of DCl for the formation of singly-charged DCl was recorded simultaneously in the same energy range. Indirect resonance autoionization of Rydberg ion states appears to play a substantial role in the formation of the (X 3 Σ − ) state of DCl2+ while the (a 1 Δ ) state appears to be formed primarily by direct photo-double ionization.

The threshold photoelectron (TPE) spectrum of hydrogen fluoride has been recorded using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 30–52 eV encompassing the inner valence ionization region. The TPE spectrum is characterized by a dominant, broad band system and a weaker broad band system both of which are assigned to the `main line' for (2sσ)−1 ionization, with some vibrational structure evident on the more intense one of these. The results of this study contrast with the findings for the TPE spectra of the inner valence ionization region of the heavier halogen acids and this is explained on the basis of differences between potential energy curves for the dicationic states. Observation of a broad, structureless band system on the low energy side of the `main-line' band systems is attributed to indirect autoionization of neutral Rydberg states.

We have recorded threshold photoelectron spectra of HF and DF over the outer valence ionization region under good resolution conditions (3-6 meV) using synchrotron radiation and employing the penetrating-field electron detection technique. The spectra show extensive vibrational structure in the X(2i) system in the first Franck-Condon gap region that is attributed to resonance autoionization of Rydberg states lying in this energy range. The Rydberg states responsible for these effects are identified as [A(2+)]ns 1+, with n = 4-6. Analyses of all the vibrational data contained in these spectra for the X(2i) state using a modified isotopic vibrational Dunham equation have led to greatly improved spectroscopic constants for this state in HF+ and DF+. The same method was also used to determine improved spectroscopic constants for the A(2+) state. Autoionization is found not to be important in the formation of the A(2+) state due to the absence of Rydberg states in the energy region of the A state. Based on the observations and interpretations of the present work, combined with literature data, we have constructed a potential-energy diagram of the relevant states of HF and HF+. Partially rotationally resolved threshold photoelectron spectra of HF+ and DF+ over the v+ = 0 vibrational band of the X 2i system show pronounced effects due to autoionization processes.

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(ns2) 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.

We have measured the threshold photoelectron spectra of HCl and DCl with varying resolution (3–40 meV) using tunable synchrotron radiation and employing the penetrating-field electron-spectroscopic technique over a wide photon energy range (up to 34 eV) that includes the inner-valence ionization region in the molecules. The observed spectra display features produced by both direct ionization and autoionization processes. Autoionization is found to play a particularly important role in the outer-valence ionization region of these molecules. For example, in the formation of the v+=0 band of the X ( 2 Π 3/2 ) subsystem, in the populating of high vibrational levels of the X ( 2 Π i ) system in the Franck–Condon gap region between the X ( 2 Π i ) and A ( 2 Σ + ) states, and in the predissociated levels of the A ( 2 Σ + ) state. The production of extended vibrational structure in the X ( 2 Π i ) system is attributed to resonance autoionization of 4sσ, 5sσ and 6sσ 1 Σ + Rydberg states. These and other threshold photoelectron results suggest a propensity for threshold electron production from sσ Rydberg states. Photoionization of HCl and DCl in the inner-valence ionization region between 25 and 30 eV yields nine vibrationally resolved band systems that include the `main line' 3sσ−1 band system and four sets of pairs of vibrationally very similar satellite ion systems. Absolute vibrational numbering in these systems was greatly facilitated by having the isotopomer threshold photoelectron spectra.

The threshold photoelectron spectra of HBr and DBr have been recorded with varying resolution (3–20 meV) over a wide photon energy range (11.5–31.5 eV) using synchrotron radiation and a penetrating-field electron spectrometer, encompassing both the outer- and inner-valence ionization regions. Extensive vibrational structure has been observed in the X( 2 Π i ) band systems for both isotopomers in the Franck–Condon gap region, between the X( 2 Π i ) and A( 2 Σ + ) states, that is attributed to the resonance autoionization of 5sσ, 6sσ and 7sσ 1 Σ + Rydberg states. Resonance autoionization is also found to play a role in the vibrational bands within the A( 2 Σ + ) system that is associated with both non-predissociated and predissociated vibrational levels of the A( 2 Σ + ) state. This is evident by the observed increased broadening of these vibrational bands in comparison to those formed in conventional photoelectron spectroscopy obtained at a similar resolution. Photoionization of HBr and DBr in the inner-valence ionization region between 23 and 29 eV shows four sets of pairs of very similar, vibrationally resolved band systems that are identified as being due to satellite ion states and a single, vibrationally resolved, band system assigned to the `main-line' 4sσ−1 ionization. Absolute vibrational numbering in these systems was greatly facilitated by having the isotopomer threshold photoelectron spectra. Evidence for the direct excitation of repulsive ion states in the 17.5–23.0 eV range has been found.

The threshold photoelectron spectrum of hydrogen iodide has been recorded at high resolution (4–30 meV) using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 10.2–31.0 eV. Extended vibrational structure in the first Franck-Condon gap region between the X (2Πi) and A (2Σ+) states of HI+ has been observed and analyzed to yield improved spectroscopic constants for the X (2Πi) state. The Rydberg states responsible for the production of this structure, through resonance autoionization, are identified as [A (2Σ+]nsσ 1Σ+ for n = 6–8. A vibrational progression in the A (2Σ+) band system in HI+ leading up to the dissociation continuum of this state has been identified for the first time, yielding spectroscopic constants for the state. The A (2Σ+) state of HI+ appears to be formed by an avoided crossing between the diabatic A (2Σ12+) and 4Π12 potentials. Spectroscopic constants are derived for the ‘main’ band system for 5sσ−1 inner-valence ionization of HI.

The threshold photoelectron spectrum of molecular fluorine has been recorded in the 5.6–21.9 eV photon energy range, at resolutions ranging from 3 to 12 meV, using synchrotron radiation and a penetrating-field electron spectrometer. In addition to observing the three known band systems of F 2 + at higher resolution than previously achieved with conventional photoelectron spectroscopy, extensive vibrational structure is found in the Franck-Condon gaps between the main electronic systems of F 2 + . This extended vibrational structure is attributed to resonance autoionization of neutral Rydberg states.

We have carried out experimental and theoretical studies of Penning ionization processes occuring in thermal energy collisions of state-selected metastable He(2 3S) and He(2 1S) atoms with ground state alkaline earth atoms X (X= Mg, Ca, Sr, Ba). Penning ionization electron energy spectra for these eight systems, measured with a crossed-beam set-up perpendicular to the collision velocity at energy resolutions 40-70 meV, are reported ; relative populations of the different ionic X(+) (ml) states are presented and well depths De for the He +X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He(2 3S) + Mg/Ca/Sr/Ba : 130/250/240/260 meV ; : He(2 1S) + Mg/Ca/Sr/Ba : 300/570/550/670 meV. The spectra show substantial differences for the free ionic states X(+)(2S), X(+)(2P) and X(+)(2D) and reveal that transitions to a repulsive potential - attributed to He + X(+)(2P) 2Sigma formation - are mainly involved for the X(+)(2P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He(2 3S) + Ca and He(2 1S) + Ca. The respective well depths De are calculated to be 243(15) meV and 544(15) meV ; the ionization cross sections at the experimental mean energy of 72 meV amount to 101 x10(-16) cm(2) and 201x10(-16) cm(2), respectively. Very good overall agreement with the experimental electron spectra is observed.

The excitation functions for ion‐pair formation in supersonically cooled HF and DF have been measured using synchrotron radiation with photon resolutions of 0.12 and 0.06 nm. The excitation functions for both molecules are characterized by an intense, sharp peak, essentially coincident with the thermodynamic onset for ion‐pair formation, followed by weaker, broader structure to higher energy. All of the structure is interpreted as arising from the photoexcitation of Rydberg states of the neutral molecules that are predissociated by the V 1Σ+ ion‐pair state potential. Ab initio calculations using multichannel quantum defect theory to include both predissociation and autoionization enabled assignments of the observed structure to be made and the construction of simulated ion‐pair excitation function spectra in an energy region encompassing ∼0.25 eV of the lowest energy part of the experimental excitation functions. The intense first peak in the ion‐pair excitation function of both molecules is assigned to a...

The threshold photoelectron spectrum of HCl has been recorded using synchrotron radiation over the energy range encompassing both outer and inner valence ionization. A large enhancement in the intensity of the v+ = 0–1 bands of the HCl+ 2∏32 subsystem was observed in comparison with the same vibrational bands of the 2∏12 subsystem which are attributed to autoionization processes. Resonance autoionization was also found to be responsible for the populating of upper vibrational levels (v+ = 2–14) of the X 2∏i system in the Franck-Condon gap region preceding the onset of the A 2∑+ band system. The observation of vibrational-band broadening for v+ = 7–10 of the A 2∑+ system is attributed to combined autoionization processes between repulsive neutral Rydberg states and bound vibrational levels of the A 2∑+ state and the continuum of repulsive ion states, the latter of which are responsible for predissociation in the A 2∑+ state of HCl+. In the inner valence ionization region (20.0–40.0 eV), pronounced resonance structure was observed which correlates well with the calculated energy positions of 4σ−1 satellite ions states of HCl. In addition, the direct populating of repulsive ion states in the 20.0–25.2 eV range is suggested by the observation of several broad, structureless band features, and the detection of three vibrational bands in this same energy range suggests the presence of a bound neutral Rydberg state that undergoes autoionization into the continuum of a repulsive ion state.

The threshold photoelectron spectrum of CF4 up to 60.5 eV has been recorded using synchrotron radiation and a penetrating-field electron spectrometer of high resolution. Our results confirm the broad spectral features of the X 2T1, A2T2 and B 2E state bands of CF4+ reported previously with He I PES. The influence of autoionizing super-excited states of CF4 has been observed as an extension of the high-energy tail of the B 2E state, as a structured band in the energy region between the B 2E and C 2T2 states, and as additional vibrational structure extending the D 2A1 state band. Improved spectra of the C 2T2 and D 2A1 states have been obtained as a consequence of the high resolution of this study. The overall electronic state band profiles observed here up to about 26eV agree with a recently published low-resolution threshold photoelectron spectrum of CF4 [Creasey et al., Chem. Phys., 174 (1993) 441]. Between 26 and 60.5 eV a continuum-like background was observed with some broad-band features superimposed on it. One broad peak, centered on 40.3 eV, is tentatively assigned to the formation of the Ẽ 2T2 and F 2A1 states of CF4+, but the formation of the double-charged ion, CF42+, may also be involved.

The threshold photoelectron spectrum of molecular iodine has been measured at a resolution of 6–8 meV using synchrotron radiation in the region 9–14 eV and a penetrating-field electron spectrometer. The spectrum reveals resolved vibrational structure in each spin-orbit component of the X (2Πg,i) state of I2+, which arises as a result of both direct photoionization and autoionization processes. In addition, extensive resolved vibrational structure has been observed in the A (2Πu,i) state of I2+ for the first time in any electron spectroscopic study. The average vibrational spacings were as follows: 220 (8) cm −1 in X (2Πg,32), 214(8) cm−1 in X (2Πg12), and 120(8) cm−1 averaged over both spin-orbit components in A (2Πu,i).

An electron spectrometric study has been performed on HCl using metastable helium and neon atoms as well as neon resonance photons. High resolution electron spectra were obtained with two different beam apparatuses for a mixed He(21 S, 23 S) beam, a pure He(23 S) beam, and, for the first time, state-selected pure Ne(3s 3 P 2) and pure Ne(3s 3 P 0) beams, and for NeI resonance photons. For the system He(23 S)+HCl the vibrational populationsP(υ′) of the formed HCl+ (X 2∏ i , υ′) and HCl+ (A 2Ω+, υ′) ions are found to differ from the Franck-Condon factors for unperturbed potentials, indicating slight bond stretching in HCl upon He(23 S) approach. For He(21 S)+HCl the vibrational peak shapes and vibrational populations are substantially different from the He(23 S) case, pointing to an additional, charge exchanged interaction (He++HCl−) in the entrance channel of the former system. For the first time, we have detected the electrons in both the He(21 S)+HCl and He(23 S)+HCl spectra associated with the major mechanism for the formation of Cl+ ions: energy transfer to repulsive HCl** Rydberg states, dissociating toH(1s) and autoionizing Cl**(1 D 2 nl) atoms. For both Ne(3 P 2)+HCl and Ne(3 P 0)+HCl, the populationsP(υ′) of both final molecular states HCl+ (X, A) agree closely with the Franck-Condon factors at the average relative collision energyĒ coll=55 meV and, for HCl+ (A 2Ω+), also atĒ coll=130 meV.

Rotational contours of the (2+1) REMPI spectra of iodine due to resonance excitation to the [ 3 2 ] c 5d; 1g Rydberg state have been recorded at room temperature. Rotational parameters, B itv , were estimated from the band-head structure and used to derive the band origin by spectral simulations. T e =59683.9±1.5 cm −1 and ω e =248.5±0.6 cm −1 were obtained from vibrational analyses. Several anomalies observed in the vibrational and band-head structures are discussed and interpreted as being due to the homogeneous interaction between the Rydberg state and the β(1g) ion pair state.

The 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum of ICl has been recorded in the two-photon energy region 57 500-64 200 cm -1 . A wide range of vibrational levels (υ'=203-387) in the E0 + - ( 3 P 2 ) ion-pair state has been observed using jet-cooling, and data for these levels are reported and analyzed. Pronounced dip-resonance structure was observed which results from coupling between the E0 + ( 3 P 2 ) ion-pair state and the b 6 0 + Rydberg state. Parts of the b 6 , b' 6 ←X Rydberg systems have also been observed directly

Combined theoretical and experimental studies of He I photoelectron spectra for photoionization of the X^1Σ^+ (v″ = 0) ground state HF leading to the X^2Π (v^+ = 0–2) and A ^2Σ^+ (v^+ = 0–3) ionic states, including detailed analyses of the rotational bandshapes, are reported. Agreement between the measured bandshapes and the calculated spectra, convoluted with the experimental resolution function (22 meV FWHM), is excellent. Ionization potentials of 16.046(1) eV and 19.116(2) eV are obtained for formation of the X and A ionic states, respectively. The vibrational spacings, deduced from the measured data by careful bandshape analysis, are in very good agreement with spectroscopic values.

Vacuum-ultraviolet fluorescence excitation and resonance enhanced multiphoton ionization (REMPI) spectra of IBr, cooled in a free-jet expansion, are reported. Resonance structure, observed in the region λ>179 nm, is greatly reduced on cooling and is ascribed to heterogeneous coupling betweenthe E(0 + ) ion-pair state and the a 6 Rydberg state. Observation of a new Rydberg series b 6 '[ 2 Π 1/2 ] c 6s(0 + )←X(0 + ) confirms the origin of the dip-resonance structure in the fluorescence excitation spectrum at wavelengths

Excitation functions for both positive and negative ions (X + + Y − ), formed by photodissociation in the vicinity of the onset of molecular ionization, for the jet-cooled interhalogen systems BrCl, ICl and IBr are reported. The overall shape of the excitation functions indicates that a number of overlapping Rydberg systems are accessed leading to dissociation. From analyses of the ion-pair excitation functions of the three molecules, by means of simulation calculations, the dominant Rydberg systems involved are found to be X( 2 Π 3/2 ) c nsσ and X( 2 Π 1/2 ) c nsσ: other weaker Rydberg systems, with configurations X( 2 Π 3/2 ) c npπ, A( 2 Π 3/2 ) c npπ, A( 2 Π 1/2 ) npπ, A( 2 Π 3/2 ) c nsσ and A( 2 Π 1/2 ) c nsσ are also found to be present. It is concluded that the formation of ion pairs occurs by the same process in all three interhalogens, i.e. via predissociation of bound Rydberg states by ion-pair states

The rotationally resolved (2+1) REMPI spectrum of the vibrational band (v′=0, v″=1) in the Dalby system of I 2 has been recorded and analysed by simulation calculation. The rotational structure is interpreted and spectroscopic parameters for v′=0 of the Rydberg state [ 2 Π 1 2 ] c 6s;1g derived.

Dispersed fluorescence spectra from BrCl E(O+) have been recorded between 290 nm and 400 nm following excitation with synchrotron radiation in the wavelength region 144–147 nm. The spectrum due to excitation with 146 nm radiation has been analysed by simulation calculations to interpret spectral structure and to estimate the shape of the potential energy curves involved.

Photo-double ionization of gaseous water has been investigated in the 30–53 eV energy range using synchrotron radiation and the threshold photoelectron s coincidence (TPE s CO) method. The TPE s CO spectrum of H 2 O, encompassing the formation of the X ˜ 3 B 1 , 1 1 A 1 , b ˜ 1 B 1 and 2 1 A 1 states of H 2 O 2+ , was recorded. The TPE s CO spectrum is very broad with no discernable vibrational structure. In addition, the TPE spectrum of H 2 O for the formation of H 2 O + was recorded simultaneously. Indirect double ionization of H 2 O by dissociative autoionization of H 2 O + is found to play a major role in the TPE s CO spectrum well below the lowest vertical double IP.

An electron spectrometric study has been performed on HBr using metastable helium and neon atoms as well as helium resonance photons. High resolution electron spectra were obtained for a mixed He(21S, 23S) beam, a pure He(23S) beam, a mixed Ne(3s3P2,3P0) beam, and for HeI VUV light. From the comparison of vibrational populations of HBr+ (X, v′) and HBr+ (A, v′), formed by either He* and Ne* Penning ionization (PI) or HeI photoionization, we conclude that HBr+ (X) formation by PI exhibits only little perturbation of HBr potentials, whereas HBr+ (A) formation by PI exhibits substantial bond stretching of HBr due to metastable atom attack preferably from the H end. For He(21S) + HBr theX- andA-state vibrational peak shapes are substantially broader than for the He(23S) + HBr case pointing to an additional, charge exchanged interaction (He+ + HBr−) in the entrance channel of the former system which is also responsible for a broad feature found at lower electron energies in the He(21S, 23S) induced PI electron spectra. For the first time, we have detected the low energy electrons in both the He(21S) + HBr and He(23S) + HBr spectra, associated with the major mechanism for the formation of Br+ ions: energy transfer to repulsive HBr** Rydberg states, dissociating to H(1s) and autoionizing Br** atoms. The HBr+ (X)2II3/2:2II1/2 fine structure branching ratios vary significantly with the ionizing agent in a similar way as for the isoelectronic, atomic target case krypton.

Excitation functions for ion-pair (I− and I+) formation from jet cooled I2 in the region 120–140 nm (i.e. from threshold to just above the first ionisation limit of I2, are reported. The mechanism for ion-pair formation is discussed in terms of coupling between Rydberg states and ion-pair states correlating with I− (1S0 and I+ (3PJ).

The resonance enhanced (2+1) multiphoton ionization spectrum of Br 2 has been recorded in the region 61000–85000 cm −1 and gerade Rydberg states up to n =11 have been identified. Two series are seen, based on the Ω= 3 2 g and 1 2 g core states. All of the higher states are members of the n s series but four components of the [Ω] 4d multiplet close in energy to [Ω] 6s have been identified for the first time. The quantum defects are very similar to those of the Br atom and to those of the lower members of the d series in CH 3 Br. The s series is at least an order of magnitude more intense than the d series, but intensities within each [Ω c ] n s←-X and [Ω c ]4d←X multiplet present some unexpected features.

Absorption and fluorescence excitation spectra for ICl in the vacuum-ultraviolet (125-195 nm) have been recorded using tunable synchrotron radiation. Fluorescence occurs mainly from the E(0{sup +}) ion-pair state, and pronounced resonance structure is observed. These resonances result from an unusual three-state interaction. Both homogeneous and heterogeneous coupling between the E(0{sup +}) ion-pair state and degenerate Rydberg states takes place. Pronounced broadening of the Rydberg levels occurs through heavy predissociation by a repulsive valence state or states.

Excitation functions for Br − and Br + formation from jet-cooled Br 2 , in the region 120-114 nm (i.e. from threshold to above the ionisation limit of Br 2 ), are reported. The mechanism for ion-pair formation is discussed in terms of homogeneous coupling between the 8pπ Rydberg state and the D(0 u + ) ion-pair state, correlating with Br − ( 1 S 0 ) + Br + ( 3 P 2 ).

Detailed analysis of the HOMO bandshape in the photoelectron spectrum of gaseous C60 reveals a dynamic Jahn-Teller effect in the ground state of C60+. The direct observation of three tunneling states asserts a D3d geometry for the isolated cation, originating from a strong vibronic coupling. These results show that the ionic motion of the ions plays an important role in the electron-phonon interaction.