R.I. Hall

Double photoionization of CO2, OCS, C2H2, CF4, and C6H6 studied by threshold photoelectrons coincidence (TPEsCO) spectroscopy

Abstract Double photoionization of CO 2 , OCS, C 2 H 2 , CF 4 and C 6 H 6 molecules has been studied in an electron-electron coincidence experiments, where nearly zero kinetic energy (⩽20 meV) electrons are detected. This first application of TPEsCO spectroscopy to polyatomic molecules confirms the ability of this experimental technique to determine accurate double ionization potentials and locate excited electronic states. The spectrum of benzene reveals what appears to be the first observation of vibrational structure in a polyatomic dication.

Threshold photoelectrons coincidence spectroscopy of N22+ and CO2+ ions

Abstract Threshold photoelectrons coincidence (TPEsCO) spectra of the nitrogen and carbon monoxide dications have been recorded with improved resolution (≈ 40 meV) allowing us to clearly resolve the lowert three vibrational progressions in each species. For the nitrogen dication we have identified, for the first time, the 3 Σ g − state predicted by theory and determined its constants. In the case of the CO dication, we have determined that the ν = 0 level of the 1 1 Σ + state is almost degenerate with X 3 Π, ν = 1 and is thus one vibrational quantum lower than previously believed.

High resolution threshold photoelectron spectroscopy of rare gas dimers

Information on the structure of the ions of neon, argon, krypton and xenon dimers has been obtained using threshold photoelectron spectroscopy and synchrotron radiation under conditions of high resolution. Vibrational structure has been well resolved for the ground state of all these species thus allowing accurate values for the spectroscopic constants to be derived. Structure corresponding to excited states of the dimer ions dissociating to the 2P3/2,1/2 levels of the atomic ion have also been identified and their dissociation energies determined.

Vibrational structure of the N+2 ground state observed by threshold photoelectron spectroscopy

The long vibrational progression of the ground state of N+2 was observed in a high resolution threshold photoelectron spectrum obtained using the penetrating field technique and synchrotron radiation. Vibrational states were observed up to v′=67 for the first time and the final vibrational level was deduced to be v′=77 from extrapolation. The complete molecular constants of this state were obtained and the entire potential curve was then drawn.

Double photoionization of below the double ionization potential

Three separate experiments demonstrate that an indirect process of double ionization can populate part of the potential energy surface of the ion at energies below the lowest bound vibrational level. The process seems to involve autoionization at a curve crossing of a singly ionized (or possibly neutral) state with a repulsive part of the adiabatic ground state doubly charged ion surface.

Photoionization of acetylene near the threshold

Abstract The photoionization of C 2 H 2 has been studied by threshold photoelectron (TPE) spectroscopy and constant photoelectron energy (CPE) spectroscopy in the energy region between 11.3 and 20eV. TPE spectroscopy has enabled a detailed observation of the first three electronic states of the ion as well as the non-Franck-Condon region. Further investigation by CPE spectroscopy at photoelectron energies between 0.2 and 3.7eV has established that the main process occurring in this region is the autoionization of superexcited states of C 2 H 2 . A dynamic effect in the autoionization process has been proposed to explain the excitation of a quasicontinuum of vibrational levels of the ionic ground state in the region of 13eV.

Structure and fragmentation dynamics of N22+ ions in double photoionization experiments

A detailed analysis of the spectroscopy of the N 2+ dication has been performed at the vibrational level. Two coincidence experiments have been used for that purpose: a pulsed VUV lamp associated with a magnetic bottle timeof-flight spectrometer (time-of-flight photoelectron–photoelectron coincidence method) and synchrotron radiation associated with a threshold electron selector (threshold photoelectrons coincidence method). Combined with ion detection, this last technique reveals, from one side, new properties of N 2+ dissociation and, from the other side, what appears to be the first observation of fluorescence by this method allowing us to study the competition between fluorescence and dissociation in the D 1 � + u vibrational levels. (Some figures in this article are in colour only in the electronic version)

Investigation of the CS22+ dication using threshold photoelectrons coincidence spectroscopy

Abstract A threshold photoelectrons coincidence (TPEsCO) spectrum of CS2 has been recorded in the 26.8–29.8 eV range with a resolution of 40 meV using a synchrotron radiation source. The X 3 Σ − g , a 1 Δ g , b 1 Σ + g and c 1 Σ − u states of CS22+ have been observed in the spectrum and their double ionisation potentials determined. Vibrational structure of these states was also observed for the first time and comparison with recent theoretical results allowed its tentative assignment to vibrational progressions associated with the symmetric stretching mode. The experimental spectrum also revealed important contributions from indirect photoionisation processes in the formation of CS22+ from the neutral CS2 molecule.

High-resolution threshold photoelectron spectra of molecular oxygen in the 18–24 eV region

A high-resolution threshold photoelectron spectrum of molecular oxygen has been studied between 18 and 24 eV using the penetrating field technique and a synchrotron radiation source. Nine ionic states are observed, and three of them for the first time. Higher vibrational levels of the b 4Σg− state and the 2Φu state are also studied. Furthermore, the doublet structure of the 2Φu state is resolved for the first time. Two new states with quartet multiplicity are assigned to the 4Πg and the 4Πu states which dissociate to the limit, [O(3P)+O+(2D)], at 22.059 eV. Also we tentatively assign another newly observed band to the 2Πu state. The long vibrational progression of the 3 2Πu state is clearly observed in the region 22.3–23.8 eV with doublet splitting which increases with increasing of the vibrational quantum number.

THRESHOLD PHOTOELECTRON SPECTROSCOPY OF HCL UP TO 40 EV

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.