Michael Allan

Study of triplet states and short-lived negative ions by means of electron impact spectroscopy

Abstract An elementary introduction of general aspects of electron impact Spectroscopy is given, including instrumentation and application of the technique to the study of short-lived negative ions (resonances) and spin and dipole forbidden transitions of molecules. The major part of this paper is devoted to the application of a recently developed type of instrument, the trochoidal electron spectrometer. This instrument is capable of measuring electron transmission spectra and also inelastic cross sections (a superposition of 0° and 180° scattering). The present work demonstrates that the properties of this instrument render it a broadly applicable extension of the standard instrumentation, despite the limitation of a fixed scattering angle. Its exceptional sensitivity and low energy capability make it superior to other types of instruments for many applications. Selected results are described from electron energy-loss, electron transmission, and energy-de-pendence of vibrational and electronic excitation measurements, recorded in most instances with the new instrument on N2, CO, CO2, and nine representative polyatomic organic molecules. Dissociative attachment spectra are also presented in several cases. One section focusses on phenomena encountered in electron impact excitation of high vibrational levels. Existing data are reviewed and original research is presented. Characterization of the properties of the new instrument, and comparison with other instrument types, is accomplished by showing spectra recorded using the trochoidal electron spectrometer, which can be compared with older results obtained with other types of instruments given in the literature. Technical details of the trochoidal electron spectrometer are given in the Appendix.

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

Publisher Summary This chapter discusses resonance and threshold phenomena in low-energy electron collisions with molecules and clusters. Low-energy collisions of electrons with atoms and molecules are among the most important elementary processes in gaseous environments such as discharges, arcs, gas lasers, gaseous dielectrics and the earths atmosphere. The dynamics behavior of low-energy electron-molecule collisions is discussed. The dynamical behavior of slow electrons traversing gases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances). Some aspects of resonance and threshold phenomena are discussed. The theoretical description of electron-molecule collisions generally requires an adequate description of electronic, vibrational and rotational degrees of freedom. However, if the typical collision time is short compared to the rotational period, the molecule can be treated as having a fixed orientation during the collision process, and the result for the cross-section can be averaged over orientations. Treatment of vibrational dynamics is usually more important and more challenging to the theory. In the electron energy region important for applications, many inelastic processes such as vibrational excitation and dissociative electron attachment are driven by negative-ion resonances. The theoretical description of vibrational dynamics in these cases is usually based on the nonlocal complex potential describing the nuclear motion in the intermediate negative-ion state.

Radative relaxation of the B̃(π-1)EXCITED ELECTRONIC STATES OF THE RADICAL CATIONS OF REXAFLUOROBENZENE, PENTAFLUOROBENZENE, 1,2,3,4-, 1,2,3,5-, 1,2,4,5-TETRAFLUOROBENZENE, 1,3,5-, 1,2,4-TRIFLUOROBENZENE AND 1,3-DIFLUOROBENZENE

The lifetimes of the zeroth and some vibiationally excited levels of the B(π-1) states of the radical cations of hexafluorobenzene (1), pentafluorobenzene (2), 1,2,3,4- (3), 1,2,3,5- (4) and 1,2,4,5-(5)-tetrafluorobenzene, 1,3,5- (6) and 1,2,4- (7)-trifluorobenzene in the aaseous phase have been measured. The cations were produced by 20—30 eV electron beam excitation of the samples at pressures < 10-3 torr The 00 level lifetimes (± 2 ns) found were 1 -48 us, 2 -47 ns, 3 - 50 ns, 4 - 50 ns, 5 - 30 ns, 6 - 58 os and 7 - 10 ns. The emission of 1,3-difluoralienzene radical cation (8) has also now been detected and the emission spectra of the fluorobenzenes 1-8,B → A, X band systems, are discussed. The emission intensities of the B →A, X transitions have been determined for 2-8 relative to 1. The lack of detectable emission from the corresponding excited (ir1) states of the radIcal cations of 1,4-and 1,2-difluorobeozene, fluorobeuzene, besizene and benzene-d6 indicates that the quantum yields of emission are < 10-5. The implications of the positive and negative results are considered with respect to the electronic structures of these species and the possible non-radiative pathways accessible to such excited cations.

Emission spectra of the radical cations of diacetylene (Ã2Πu→X̃2Πg), triacetylene (Ã2Πg→X̃2Πu), and tetraacetylene (Ã2Πu→X̃2Πg,O00), and the lifetimes of some vibronic levels of the à states

Abstract The emission spectra of the radical cations of tricetylene, A 2 Π g →X 2 Π u band system, and of tetraacetylene, A 2 Π u →X 2 Π g O o o band, excited in the gaseous phase by controlled electron impact, are reported. The emission spectrum of diacetylene cation, A 2 Π u →X 2 Π g band system, with corrected band intensities is also presented. The lifetimes of the lowest vibrational level of the A 2 Π excited states have been measured as 71±3 ns, 17±2 ns and ⩽6 ns for the cations of diacetylene, triacetylene and tetraacetylene respectively. The lifetimes of the measured excited vibronic levels are shorter. Emission bands from the radical cation of cyanoacetylene and cyanogen were however not detected and these results are discussed.

Absolute angle-differential elastic and vibrational excitation cross sections for electron collisions with tetrahydrofuran

Absolute angle-differential elastic and vibrational excitation cross sections for electron collisions with tetrahydrofuran were measured in the energy range 0.1–20 eV, extending existing measurements to lower energies. The elastic cross sections were measured as a function of scattering angle from 10° to 180° at energies of 2 eV, 6 eV, 10 eV and 20 eV, and as a function of electron energy at 45°, 90°, 135° and 180°. The agreement with previous measurements and with the published theoretical work was generally satisfactory. The Ramsauer–Townsend minimum was observed at low energies, down to 0.24 eV at 180°. Three additional minima were observed at 1.13, 4.74 and 15.3 eV in the 180° elastic cross section. Vibrational excitation cross sections are reported as a function of electron energy from the threshold to 16 eV. They reveal threshold peaks and broad bands at 6.2 and 10.8 eV, attributed to shape resonances, in agreement with theoretical predictions, although the calculated energies are generally somewhat higher. A broad enhancement of the CH2 scissoring vibration is observed around 2.6 eV, implying a low-lying (shape) resonance similar to that observed earlier in cyclopropane.

Dissociative electron attachment in cyclopentanone, γ-butyrolactone, ethylene carbonate, and ethylene carbonate-d4: Role of dipole-bound resonances

The title compounds allow the study of the effect of the dipole moment and the energy of the lowest shape resonance on dissociative electron attachment, since both the dipole moments (2.9, 4.5, and 5.3 Debye) and the π* attachment energies (1.15, 1.98, and 2.94 eV) increase progressively along the series. An unexpected observation was made in ethylene carbonate, the molecule with the largest dipole moment, where two fragments (CO3− and C2H3O−) are formed at low energies (1–1.5 eV), well below the first π* attachment energy. We assign these bands to dissociation of a vibrationally excited dipole bound anion formed upon electron attachment. Furthermore, the number of fragments at low energies (below 5 eV) was generally found to increase with the number of oxygen atoms in the molecules, presumably because of the larger number of possible fragments with large electron affinity. Finally, “scrambling” of atoms was found in the fragmentation of ethylene carbonate even at low energies, indicating that the initial...

Measurement of the elastic and v = 0 → 1 differential electron–N2 cross sections over a wide angular range

Absolute differential elastic and vibrational excitation cross sections have been measured for N2 in the scattering angle ranges starting between 0° and 20° and extending to 180°, at energies between 0.8 and 5 eV. The results agree with many previous measurements, in particular for angles around 90°, but discrepancies were found in some cases for angles close to 0° and 180°. Integral and momentum-transfer cross sections have been derived and compared with previous beam and swarm measurements. The procedures for determining the instrumental response function and for assuring optimal beam overlap over wide ranges of angles and energies are discussed.

On the photoelectron spectrum of p-benzoquinone

A high-resolution photoelectron spectrum of p-benzoquinone in the low energy (9.5–11.5 eV) region is reported and analyzed with the aid of simulations based on high-level ab initio calculations. The results generally support the notion that the two prominent spectral features in this region are each due to a pair of final ion states. The lower energy feature beginning near 10 eV is due to oxygen lone-pair ionizations, while that beginning near 11 eV comes from π electron removal. Contrary to previous interpretations of the spectrum, however, the results of this study indicate that the two π states are nearly degenerate, with the strongest peak in the photoelectron spectrum representing a convolution of the corresponding pair of 0–0 ionizations.

σ* resonances in electron impact‐induced vibrational excitation of n‐propane, cyclopropane, ethylene oxide, cyclopentane, and cyclohexane

Electron‐energy‐loss spectra in the range of vibrational excitation, and excitation functions for selected vibrational peaks, were measured for the title compounds. Angular distributions of the vibrationally inelastic peaks were measured for n‐propane and cyclopropane. The results in n‐propane are similar to the published results in ethane, only one very broad band is observed in all channels, with gradual onset at about 3 eV and a maximum around 8 eV. In contrast, narrower resonances emerge in all cyclic compounds. The effect is most pronounced in cyclopropane, where two resonances appear, at 2.6 and 5.5 eV. The latter is exceptional in several respects. It is narrow and thus relatively long lived for a shape resonance of this energy. It causes ring stretch excitation with very high selectivity and pronounced angular distribution, which is reproduced very well by the theory of Read and Andrick, revealing dominance of a partial wave with an unusually high angular momentum, l=3, m=3, and unambiguously iden...

Energy partitioning in the O−/CO2 dissociative attachment

Abstract The 4.4 and 8.2 eV dissociative attachment bands in CO2 were reinvestigated with particular attention to O− ion kinetic energy release. On the 4.4 eV band a structure associated with the vibrational excitation of the CO fragment was observed when only zero kinetic energy O− ions were collected. A narrower structure with a spacing of 0.10±0.01 eV, assigned to the intermediate CO2− was observed without ion kinetic energy discrimination. On the zero kinetic energy spectrum in the 8.2 eV region a structure associated with the vibrational excitation of the CO fragment up to ν = 21 is reported for the first time. The O− kinetic energy spectrum at the 8.2 eV band is reported, corrected for the ion optics transmission function. The 8.2 eV resonance was also observed in threshold electron energy loss spectra. It decays by ejection of a near zero energy electron into the low-lying π - π ? excited states of CO2.