J. Delwiche

Single photon double ionization studies of CS2 with synchrotron radiation

Single photon double ionization of CS2 has been investigated, using mass spectrometric coincidence techniques: both metastable and dissociative CS++2 states have been observed; three different dissociation pathways of CS++2 have been demonstrated, including one bond (S++CS+) and two bonds (S++C+S+ and S++C++S) breakings; simulation of the observed dissociation provided some insight into the dissociation mechanisms. Monochromatized synchrotron radiation enabled us to measure the excitation spectra of these relatively intense processes, in the 25–75 eV photon energy range. Our results provide an approach to the spectroscopy of the doubly charged CS++2 ion; comparison with a SCF‐LCAO‐MO calculation leads to a tentative assignment of the observed states.

Dissociations of the ethyne dication C2H2+2

Dissociations of the ethyne dication following its production by photoionization in the photon energy range of 35–65 eV have been investigated by the photoelectron–ion–ion coincidence technique using both synchrotron radiation and laboratory light sources. New quantum mechanical calculations identify and locate the electronic states of the molecular dication in this energy range and show that the dissociation products are formed in their ground states by heterogeneous processes. Five reaction channels leading to three molecular fragments have been identified and are interpreted as sequential processes, several faster than fragment rotation and one possibly involving dissociation of CH+ to H+ with a lifetime of the order of 25 fs.

On the photoelectron spectra of HBr and DBr

Abstract The photoelectron spectra of HBr and DBr with He I (58.4 nm) are discussed. The photoelectron spectrum of DBr confirms the predissociation of the A 2Σ+ electronic state. A rough estimate of the predissociation rate is calculated.

Intramolecular dynamics by photoelectron spectroscopy. I. Application to N+2, HBr+, and HCN+

The Fourier transform of an optical electronic spectrum leads to an autocorrelation function C(t) which describes the evolution in time of the wave packet created by the Franck–Condon transition, as it propagates on the potential energy surface of the electronic upper state. This correlation function is equal to the modulus of the overlap integral between the initial position of the wave packet and its instantaneous position at time. The original data resulting from an experimentally determined spectral profile must be corrected for finite energy resolution, rotational, and spin‐orbit effects. The behavior of the system can then be followed up to a time of the order of 10−13 s, i.e., during the first few vibrations which follow immediately the electronic transition. The method is applied to photoelectron spectra and the results are compared to the available information on potential energy surfaces of ionized molecules, in order to study their unimolecular dissociation dynamics. In the case of the X 2Σ+g, ...

Autoionization of OCS by threshold photoelectron spectroscopy

Autoionization of carbonyl sulfide between 12 and 16 eV has been investigated by photoionization using the pulsed synchrotron radiation from ACO Orsay’s storage ring. The threshold photoelectron spectrum and the total photoionization spectrum of carbonyl sulfide have been recorded at high resolution in the wavelength range between 112.0 and 65.0 nm (11–19 eV). Threshold energy electrons are observed in specific wavelength regions: (i) at excitation energies where the X, A, B, and C ionic states are formed by a direct process; (ii) in the A state region where resonant autoionization to A can be understood classically within the Franck–Condon approximation; (iii) in the A–X Franck–Condon gap between 90 and 110 nm, where resonant autoionization leads to very sharp electron energy distribution strongly peaked at zero energy. Here the mechanism must be more complex.

On the He(I) and Ne(I) photoelectron spectra of OCS

Abstract The He(I) (58.43 nm) and Ne(I) (73.59–74.37 nm) photoelectron spectra of carbonyl sulphide have been reinvestigated at an improved resolution and a high signal-to-noise ratio; deconvolution of the data was carried out to aid interpretation of the spectra. Improved values are deduced for the vibrational wavenumbers and the ionization energies. The spin—orbit components of the A 2 Π II level are well resolved and a value of 135 cm −1 (17 ± 5 meV) is proposed for the splitting. Perturbations in the relative intensities are observed in the Ne(I) spectrum; they suggest that the autoionizing Rydberg state located around 73.7 nm is excited by the 73.59 nm Ne(I) line and contributes to populate, with different probabilities, the ionic levels of lower energy, and particularly the X 2 Π state.

Autoionization observed in the O+2(A 2Πu→X 2Πg) and O+2(b 4Σ−g→a 4Πu) fluorescence excitation spectra

High resolution O+2 b 4Σ−g and A 2Πu partial photoionization cross sections are obtained in the 450 to 750 A range using the fluorescence excitation technique. The total photoionization cross section also is measured with the same photon resolution. Most of the fine structure observed in each individual channel is interpreted in terms of electronic and vibrational autoionization. The underlying continuum in the b 4Σ−g cross section shows a strong enhancement at 650 A (19 eV) photon energy (i.e., 0.8 eV above threshold) and a strong vibrationally resolved structure centered at 620 A (20 eV). They are interpreted as 3σgeσu resonances associated with 3σg electron ejection, the 19 eV being a shape resonance in the b 4Σ−g continuum, and the 20 eV a bound electronic state associated to B 2Σ−g and strongly autoionized into the b 4Σ−g continuum.

Photoelectron spectrometry of hydrogen sulfide

Abstract The high-resolution photoelectron spectrum of H 2 S shows mainly three vibrationally resolved bands corresponding to 2 B 1 , 2 A 1 and 2 B 2 states of H 2 S + with adiabatic ionization engergies of 10.43, 12.76 and 14.91 eV and frequencies of 2476, 1113 and 2000 cm −1 respectively. A sudden loss of vibrational structure in the second band is interpreted as due to predissociation process leading to S + ion formation.

Electronic excitation and optical cross sections of methylamine and ethylamine in the UV-VUV spectral region

High resolution UV–VUV photon absorption spectra of methylamine and ethylamine have been recorded between 5.0–9.0 eV (250–140 nm) using synchrotron radiation. In methylamine, the energies of the absorption bands are confirmed as centered at 5.7, 7.2, and 8.7 eV, respectively. In ethylamine the band centers are 5.8, 7.0, and 7.9 eV, respectively; the last band is seen here for the first time. Most of the transitions exhibit rich fine structure dominated by vibrational progressions involving excitation of an amino wagging vibration. The absolute photoabsorption oscillator strengths have been measured by photon absorption over the 5–9 eV range and by dipolar electron energy loss spectroscopy from 5–14 eV (250–90 nm).

On the high-resolution HeI photoelectron spectrum of Cl2O

Abstract The high-resolution HeI (58.4 nm) photoelectron spectrum of dichlorine monoxide, Cl 2 O, has been recorded in the region of the four lowest-energy ionic electronic states. Formation of the ion in its ground and excited electronic states is accompanied in each case by vibrational excitation. In particular, the vibrational structure of the first and second excited states of Cl 2 O + is resolved. Analysis of the vibrational progressions associated with formation of the various ionic states has been completed, allowing confirmation of the symmetry and bonding characteristics of the four highest-energy occupied molecular orbitals of Cl 2 O.