D G Wilden

Electron energy-loss spectroscopy of forbidden transitions to valence and Rydberg states of formaldehyde

Abstract The electronic spectrum of formaldehyde has been studied by the technique of electron scattering covering the range of energy loss from 0 to 12 eV with

Rydberg states of C2H4 and C2D4: assignments using the technique of low-energy electron energy-loss spectroscopy

Energy-loss spectra of C2H4 and C2D4 are presented covering the energy-loss range 6-11 eV, using incident electron energies in the range 5-100 eV above threshold, and scattering angles up to 60 degrees . Members of several optically assigned Rydberg series were identified, and their intensities measured at a number of scattering angles. Each transition was then classified as electric dipole or quadrupole allowed, and this information was used to assign the Rydberg orbitals involved. Four new triplet Rydberg states have been observed, and their assignments are also discussed. For two of the triplet states, more than one vibrational level was observed, and where possible the frequencies of the symmetric C=C stretch and CH2 torsional modes were obtained. For the lowest triplet Rydberg state, the barrier to internal rotation of the CH2 groups was estimated to be 0.08 eV.

High resolution electron impact studies of electric dipole-forbidden states of benzene

Electron energy-loss spectra have been obtained, with a resolution of 0.030 eV for the electronic states of benzene below 11.5 eV. At high scattering angles prominent vibrational structure has been observed in the first three triplet bands (3B1u, 3E1u, and 3B2u). A progression of three levels starting at 6.10 eV has been observed for the first time, and assigned to the 3E2g valence state. Rydberg states converging to the first and second ionisation potentials have been studied and in both cases quadrupole transitions have been detected for the first time. A series of four such levels converge to the first ionisation potential with a quantum defect of 0.266, and the Rydberg orbital is identified as nd(a1g). Two quadrupole transitions converging to the second ionisation limit have a similar quantum defect of 0.226 and are believed to involve transitions from the 3e2g orbital to the same Rydberg orbital.

High resolution studies of dipole-forbidden states of N2 using low-energy electron energy-loss spectroscopy

Energy-loss spectra for molecular nitrogen have been measured with impact energies between 0.5 and 100 eV above the threshold for excitation and at scattering angles between 2 and 60 degrees . The energy-loss range between 5 and 25 eV was studied, with a typical resolution of approximately 0.025 eV. At energy losses below 11 eV the observed spectra can be fully assigned, with the exception of one progression of very weak peaks, using dipole-forbidden states previously observed in emission studies. The angular distribution of one of these states, w1 Delta u was measured. Between 14.30 and 15.90 eV, six energy-loss peaks were found to have angular distributions typical of dipole-forbidden transitions which cannot be related to previously observed states of N2. Above the ionisation limit, three new Rydberg series of states converging to the nu =0 level of the B2 Sigma u+ state of N2+ were detected at high scattering angles and low impact energies.

High-resolution electron energy-loss spectroscopy of ethylene: analysis of the vibrational structure of the triplet ( π to π*) state

Low-energy electron impact energy-loss spectra of the triplet (π to π*) state of ethylene have been obtained using scattering angles in the range 2 to 60 degrees . The high resolution obtained ( approximately 0.025 eV FWHM) has allowed many vibrational levels to be observed which have not been resolved in previous energy-loss or photoabsorption studies. Analysis of the vibrational structure indicates that a long progression of the C-C stretching mode is involved as well as at least three quanta of the CH2 torsional mode. The results obtained are compared with theoretical predictions about the geometry of the triplet (π to π*) state of ethylene.

Autoionization of N2 studied using an electron time-of-flight coincidence spectrometer

Autoionizing states of N2 have been studied using a new electron-electron coincidence technique, by simultaneous measurement of their excitation energies and ejection energies in decay to states of N2+. This method gives a much clearer understanding of molecular autoionization than methods which measure either energy separately. Low energy electron impact is employed using hemispherical analysers to select the incident beam and to detect scattered electrons which have excited a selected autoionizing level. In coincidence with these scattered electrons, ejected electrons are analysed by measurement of their flight times along a field-free region. Transitions between the selected autoionizing level and levels of N2+ appear as peaks in the time spectra. The design, construction and operation of the spectrometer are discussed and factors limiting the time and energy resolutions are considered. Results are presented showing time spectra for electrons ejected from five autoionizing states of N2 and good agreement is observed between the predicted and measured resolutions of the spectrometer.

Energy-loss spectroscopy of the higher excited states of acetylene

Electron energy-loss spectra have been obtained for electronic states of acetylene in the range 7-11 eV with resolution as high as 0.015 eV. Measurements taken at high scattered electron energies and low scattering angles enable quadrupole allowed singlet states to be observed, and the authors have assigned three such transitions as involving 1 Sigma g+ and 1 Delta g or 1 Pi g states. At low scattered electron energies and high scattering angles a substantial number of transitions involving triplet states are observed, all but one of these for the first time. By energy comparison with nearby singlet states the authors have tentatively assigned most of these to electronic states theoretically predicted to lie in this region.

An electron impact energy-loss study of triplet states of acetylene

Electron impact energy-loss spectra have been obtained for acetylene in the range 4 to 8 eV at various impact energies and scattering angles. At low impact energies structure attributed to two triplet states is observed as long vibrational progressions. Analysis of the structure indicated that in the ground vibrational level of each triplet state, the molecule has a trans-bent geometry, and the configurations 13Au and 13Bu are suggested. The lowest vibrational level observed for the 13Au state has an excitation energy of 4.36+or-70.007 eV.

A variable angle photoelectron spectrometer using a position-sensitive detector

Abstract A variable angle photoelectron spectrometer utilizing a position-sensitive multidetector is described. Photoelectron spectra of N 2 obtained using a synchrotron radiation source are presented. The data are acquired using a new technique in which the photoelectron yield is obtained as a function of both wavelength and photoelectron energy, and this gives detailed information on autoionization processes.

Electron impact studies of resonances and autoionizing states of neon

Measurements have been made of the energy spectrum of electrons ejected from autoionizing states of neon with excitation energies in the region 41-46 eV; in particular, the lowest doubly excited autoionizing state of neon, the 2s22p43s2(3P) state, has been observed positively. These studies have given detailed information on post-collision interaction effects. In addition, it has been shown that resonant excitation of autoionizing states via negative-ion states can be an important process and measurements have been made on a number of excited negative-ion states in the region 41-48 eV.