J. M. Gingell

VUV optical absorption and electron energy-loss spectroscopy of ozone

Absolute VUV optical absorption cross sections for ozone have been measured between 325 and 110 nm (3.0 - 11.3 eV) using a synchrotron radiation source. Vibrational fine structure is resolved in Rydberg bands and comparison of this with the limiting bands in the photoelectron spectrum confirms that the order (increasing ionization energy) of the three lowest ionization bands is . Near-threshold electron energy-loss spectra have also been recorded. In these, in addition to the known triplet states between 1 and 2 eV, a low-lying triplet state has been located around 3.4 eV and several others between 6 and 9 eV. Characterization of the valence states (both optically allowed and forbidden) are discussed in relation to the results of early theoretical computations which seem to give a good account of the ozone spectrum.

VUV optical absorption and electron-energy-loss spectroscopy of formamide

Abstract Absolute absorption cross-sections for formamide have been measured using a synchrotron radiation source (5–11.2 eV; 250–110 nm), along with electron-energy-loss (EEL) spectra at (i) high incident electron energies and low scattering angles and (ii) near-threshold incident energies and large scattering angles. In the optical- and high-energy EEL data, the excitation energies of the historical amide absorption bands (W, R 1 , V 1 , R 2 and Q) are in agreement with expectation. Vibrational structure is assigned to the V 1 ( 1 ππ ∗ ) band. It is proposed that the Q band (∼ 9.2 eV) arises from superposition of transitions to several Rydberg states, with second 1 ππ ∗ state (V 2 , once related to the Q band) lying at a higher energy; the V 2 state may be visible in the EEL data. A number of Rydberg series converging to the lowest ionisation potential are suggested and the possibility of using these Rydberg data to assign the two first (closely spaced) ionisation potentials is discussed. The near-threshold EEL data fail to resolve the low-lying 3 ππ ∗ state (the V 1 state triplet) because of spectral congestion. Dissociative electron attachment occurs in formamide upon impact with electrons of energy around 6.3 eV.

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.

Absolute cross sections for the VUV optical absorption of Cl2O in the 6.5-9.7 eV energy range

Abstract The VUV photoabsorption spectrum of dichlorine monoxide has been recorded between 6.5 and 9.7 eV photon energies using the UK Daresbury Synchrotron Radiation facility. The spectrum consists of a broad structureless band centred at 7.25 eV followed by several vibrationally resolved high-intensity bands. The 7.25 eV band has been interpreted in terms of the excitation of a dissociative valence excited state while the higher-energy electronic bands (above 7.5 eV) have been classified into several ( n = 3, 4) Rydberg states linked with the ionic ground state and its lowest-energy excited states. An analysis of the vibrational progressions associated with the excitation of the various Rydberg states has been completed.

Photoabsorption and near-threshold electron energy-loss spectroscopy of OClO

The spectroscopy of chlorine dioxide, OClO, has been investigated using the techniques of dissociative electron attachment (DEA), near-threshold electron energy loss (EEL, 0-10 eV) and optical absorption (2.5-10.8 eV). Mass and energy analyses of the ions formed in the DEA processes show that at energies above about 3 eV, the product ions are predominantly near-thermal , suggesting that the accompanying oxygen atoms are excited electronically. All features observed in the EEL experiments are in regions of optical absorption, indicating that the optically dark doublet and quartet states have energies which overlap optically allowed transitions. The optical spectrum itself is in good qualitative agreement with previous measurements, but there are differences in relative absorption cross sections. Alternative interpretations of some spectral features are proposed.

Electronic excitation and oscillator strength of ethyl bromide by vacuum ultraviolet photoabsorption and electron energy loss spectroscopy

The high resolution vacuum ultraviolet photoabsorption spectrum of ethyl bromide has been recorded between 5 and 10.15 eV (248-122 nm) using synchrotron radiation. It exhibits a broad structureless valence band centred at 6.1 eV of low cross section followed by a region dominated by excitation of Rydberg states. A high resolution photoelectron spectrum (PES) of the lowest energy ionization band has been obtained and provides ionization energies necessary for identification of related Rydberg-excited states. Also, analysis of the vibrational fine structure in the PES has allowed identification of the normal vibrational modes excited and their wave numbers in the ion. These data, in turn, have been used in the assignment of the lowest energy photoabsorption bands arising from electron excitation into the 5s Rydberg orbital. The electron energy loss spectrum, recorded from 6.5 to 14.1 eV, under electric-dipole conditions, confirms the magnitude of the photoabsorption cross-section values obtained using the synchrotron radiation and extends the differential and optical oscillator strength values up to 14.004 eV

Vibrational excitation of methane by 15 and 30 eV intermediate-energy electron impact

Absolute measurements of differential electron-collision cross sections have been performed for the vibrational excitation of methane in its electronic ground state at impact energies of 15 eV and, for the first time, at 30 eV, in the 8°–95° angular range. The normalization to the absolute scale has been carried out using the inelastic/elastic ratios and the absolute elastic differential cross section obtained from a relative flow technique recently developed in our laboratory, in which the elastic differential cross section of methane was compared to that of nitrogen at each impact energy and at each scattering angle. Special attention has been given to the small scattering angular range (⩽40°) where minima have been observed in the vibrational cross sections and where simultaneous independent measurements have been made on two different electron spectrometers.

Vacuum ultraviolet spectrum of dinitrogen pentoxide

Absolute photoabsorption cross-sections at 195 +/- 3 K are reported for dinitrogen pentoxide, N2O5, between 150 and 240 nm. The spectrum consists of a broad intense band with a maximum at 160 nm. The lack of sharp features indicates that excitation of N2O5 results in dissociation over the whole of the spectral range studied. At wavelengths above 200 nm, our results are up to 30% smaller than the current NASA recommendation, but are in generally good agreement with a more recent measurement. Below 200 nm, there are no reported data with which to compare our work

Electronic excitation and oscillator strength of ethyl iodide by VUV photoabsorption and electron energy loss spectroscopy

A high resolution VUV photoabsorption spectrum of ethyl iodide has been recorded between 4 and 10.2 eV (310–120 nm) using synchrotron radiation. The spectrum consists of a broad structureless absorption band centered at 4.78 eV, followed by a region dominated by excitation of Rydberg states. A high resolution photoelectron spectrum (PES) of the lowest energy ionization band has been obtained and provides ionization energies necessary for identification of related Rydberg-excited states. Also, analysis of the vibrational fine structure in the PES has allowed identification of the normal vibrational modes excited and their wave numbers in the ion. These, in turn, have been used in the assignment of the lowest energy photoabsorption bands arising from electron excitation into the 6s Rydberg orbital. An electron energy loss spectrum has also been recorded from 5.8 to 14.2 eV, under electric-dipole conditions. It confirms the magnitude of the photoabsorption cross section values obtained using the synchrotron ...