Irene Nenner

Non‐Franck–Condon transitions in resonant autoionization of N2O

Autoionization of N2O between 12.89 and 16.4 eV was investigated by photoionization using the pulsed synchrotron radiation from ACO, Orsay’s storage ring. Measurements were performed of threshold photoelectron spectra, photoionization spectra, and of photoelectron energy spectra. The latter were obtained from photoelectron time of flight distributions at selected wavelengths. The results suggest that autoionization in the Franck–Condon gap between the ? 2Π and the ? 2Σ+ states of N2O+ proceeds via two distinct mechanisms. The major autoionization process (?90%) produces the ? 2Π state in its low vibrational levels, while a resonant autoionization path (?10%) produces vibrationally excited ? 2Π ions. The latter process is associated with the production of low energy electrons with a distribution peaking sharply at zero energy. This resonant autoionization process appears to be a general phenomena for polyatomic molecules.

Interactions between neutral dissociation and ionization continua in N2O

Autoionized Rydberg states converging to the A 2Σ+ and B 2Π states of N2O+ are shown to be predissociated into neutral fragments. The decay to excited neutral fragments is observed by their fluorescence in the visible and ultraviolet regions of the spectrum. These decay channels are in competition with autoionization, which was shown in a previous study to yield unexpectedly large numbers of low energy electrons. On the basis of the striking similarity between the fluorescence excitation and the threshold photoelectron spectra, models are presented which connect dissociative channels to the production of threshold electrons. Ionization of dissociating neutral states and Rydberg states converging to the ground electronic state of N2O+(X 2Π) are suggested as being responsible for the production of low energy electrons. The distribution of photoelectron energies resulting from such autoionization channels are calculated.

A threshold photoelectron–photoion coincidence study of the N2O+ dissociation between 15 and 20.5 eV

Branching ratios and the kinetic energy released in the various fragmentation channels of energy selected N2O+ (15–20.5 eV) were investigated by the technique of threshold photoelectron–photoion coincidence. Pulsed synchrotron radiation from ACO, Orsay’s storage ring, dispersed by a monochromator, was used as a photon source. Threshold electrons were energy selected on the basis of angular and temporal discrimination against energetic electrons. The energy region below 16.388 eV and the ? state were investigated in detail. Below the ? state the most abundant fragment ion is O+, while above the ? state NO+ dominates. Results for the ? and ? states are also reported.

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.

Physics of molecular ion formation.

A review of the basic processes in molecular photoionization is made in the light of the classical as well as recent new theoretical models and illustrated on O(2) and N(2)O.

Relaxation processes following excitation and ionization of SiF4 in the vicinity of the silicon 2p threshold. I. Electronic relaxation processes

The photoelectron and Auger spectra of SiF4 have been measured, using synchrotron radiation in the 100 to 125 eV photon energy range in the vicinity of the Si2pionization threshold. Partial photoionization cross sections have been obtained for outer, inner valence states and satellite states in the same energy range, together with the threshold photoelectron spectrum. At the energies of the discrete resonances observed below 112 eV, the core excited molecule is found to decay mostly by resonant Auger and to a small extent by autoionization. Among the resonant Auger pathways, those in which two electrons (rather than one) are ejected are found to play a dominant role. Above threshold, especially around the energy of the first continuum resonance, normal Auger processes are observed. Also, we offer a new interpretation of the Auger spectrum based on large configuration interaction in the final state and on the screening of the Si2p hole by the lone‐pair electrons of the fluorine atoms. We also find some evidence of cascade Auger processes which explain the formation of triply ionized molecules.

Electron spectroscopy study of the 27.5 eV satellite line in acetylene excited by synchrotron radiation

Abstract Valence electron spectra from acetylene using synchrotron radiation with 50 and 70 eV excitation energy are presented. The 27.5 eV satellite line is resolved into two components. The intensities of these satellite lines are compared to the previously measured intensities at 1487 eV excitation energy. β values for the satellite lines and for all the valence levels are determined at 70 eV excitation enery. A previously unknown satellite line is found at 31.0 eV binding energy. This line corresponds to an onset of the double ionization continuum.

Laser induced photodissociation of s‐tetrazine probed by photoelectron spectroscopy with synchrotron radiation

We have combined a cw visible laser with synchrotron radiation, respectively, to photodissociate the s‐tetrazine molecule and to probe, with an electron analyzer, the vibrational energy of the nascent fragments. No fragments other than N2 and HCN were detected on the time scale of our experiment. We find that 5.4%±0.5% of the nitrogen fragment departs with one quantum of vibration and at least 26% of each HCN fragment is vibrationally excited, mainly in the bending mode (n=1 to 6) and probably to a very small extent in the C–N stretching mode (n=1). These data are fully consistent with a pure three body fragmentation mechanism governed by the geometry of the transition state as calculated by Scheiner et al., J. Am. Chem. Soc. 108, 8160 (1986).

Synchrotron radiation study of vibrationally resolved partial photoionization cross sections of CO2 between 64 and 80 nm

Vibrationally resolved partial photoionization cross sections of CO2 have been measured for the X 2Πg ionic state for wavelengths between 64 and 80 nm and for A 2Πu and B 2Σ+u from 64 to about 2 nm below the respective thresholds. The autoionization of the Rydberg series converging to A and those converging to B is discussed in terms of population of two vibrational levels of the ground electronic state of the ion. The Rydberg series with limit at C 2Σ+g (000) are shown to decay mainly to B 2Σ+u in its (000) and (100) vibrational levels and to a lesser manner to X 2Πg. They do not populate the A 2Πu state.

Excitation spectra of neutral fragments emissions by photodissociation of OCS using VUV photons

The Rydberg series converging to the B 2Σ+ state of OCS+ are shown to be predissociated by dissociative states leading to CS (A 1Π) and to CO triplet levels. The decay of these excited fragments is observed by their fluorescence in the visible and ultraviolet ranges. The corresponding fluorescence cross section in the 280–630 nm range is roughly estimated not to exceed 3% of the absorption. These dissociative states are very likely involved in the mechanism of resonant autoionization.