Peter Hammond

Highly-excited double Rydberg states of He-

By measuring the yield of metastable atoms resulting from electron-impact excitation of helium, in the range of incident electron energy from 19.8 to 24.6 eV, the authors have obtained evidence for He- resonances in which both the excited electrons have high values of the principal quantum number. The resonances occur in distinct groups, the lower members of which form Rydberg series characterised by effective core charges that are nonintegral. Classification schemes for these resonances are discussed.

The threshold electron spectrum of molecular nitrogen

The threshold electron spectrum of molecular nitrogen has been studied using a high-energy-resolution electron impact spectrometer incorporating a penetrating field analyser for low-energy scattered electrons. Spectra are presented for the energy region from 6 to 19.2 eV. The assignments of the observed features are discussed.

The threshold electron spectrum of carbon monoxide

A high energy resolution electron impact spectrometer incorporating a penetrating field analyser for low-energy scattered electrons has been used to study the threshold electron spectrum of carbon monoxide. Spectra are presented for the energy region from 5.8 to 19.5 eV. The assignments of the observed features are discussed. Evidence is found for effects due to long-range electron-electron correlations.

Energy partitioning in near-threshold excitation and ionisation of helium by electron impact

The processes of near-threshold ionisation and excitation of helium by electron impact have been studied experimentally by two different techniques to establish how the available excess energy is partitioned between the resulting free or bound electrons. The partitioning is found to be non-uniform and to be in agreement with that obtained from classical trajectory calculations.

High-lying long-lived doubly excited states near ionization thresholds of helium

Long-lived, excited neutral particles, arising from the interaction of monochromatic synchrotron radiation with ground state He atoms, have been observed at photon energies close to the (N = 1,2,3 and 4) ionization thresholds. The measurements have been made using an unconventional experimental arrangement in which charged particles, responsible for the dominant signal in conventional photon impact studies, are prevented from reaching the detector. For , it appears that the formation, via photoexcitation, of relatively long-lived, doubly excited Rydberg states is a necessary step in the production of the observed signal. Four processes that might account for the production of the excited neutral particles are considered. The simplest, that atoms in the initial doubly excited Rydberg states are themselves directly observable, is considered unlikely. However, the lifetimes of the photoexcited double Rydberg states may be increased through the effects of electric fields present in the apparatus. Transitions from these double Rydberg states, occurring either as a result of collisions or by fluorescence, could result in metastable, singly excited neutral particles. The presence of signal related to the existence of long-lived doubly excited states could be significant in the interpretation of other photon impact measurements.

Resonance structure in elastic scattering of electrons from atomic hydrogen

High-energy-resolution spectra of electrons scattered elastically from atomic hydrogen have been obtained for incident electron energies of 9.2-10.4 eV and at angles of 33 degrees , 54 degrees , 70 degrees and 90 degrees . The energy spread of the incident electron beam is estimated to be typically 25 meV FWHM. In addition to the 1S and 3P resonances which have been reported in earlier studies, a 1D resonance can be clearly seen and there is some evidence of a resonance state at a higher energy, probably corresponding to a 1S resonance state. The energies and resonance widths of all these features have been determined.

Angular distribution of the fluorescence of helium doubly photo-excited states converging on the He+(N = 2) ionization threshold

The asymmetry parameter β for fluorescence photons emitted during the radiative decay of the sp2,n+(1Po), sp2,n−(1Po), 2pnd(1Po) and 2pnd(3Do) series of helium doubly excited states, formed by the absorption of linearly polarized synchrotron radiation, has been measured. The results are compared with recent theoretical calculations (Žitnik et al 2002 Phys. Rev. A 65 032520). At high n the β parameter is shown to increase and oscillate due to the excitation of triplet states, optically accessible from the helium ground state due to spin–orbit effects.

A two-dimensional study of the autoionizing decay routes of Rydberg states converging on the ionization threshold in

The autoionizing decay pathways of the five-dipole allowed Rydberg series converging on the threshold in have been investigated using two-dimensional photoelectron spectroscopy. Measurements of electron yield as a function of both electron and photon energy have been carried out using tuneable synchrotron radiation. The vast majority of vibrational levels of the three bound electronic states of the ion that are accessible to the decaying Rydberg states have been studied with an experimental resolution of approximately 30 meV. This was sufficient to study transitions to individual vibrational levels. The comprehensive nature of the data presented has enabled various observations pertaining to both electronic and vibrational selectivity in the autoionizing decay processes to be made. The observed preference for the decay of and Rydberg states to the and states of the ion respectively may be explained if the emitted electron conserves its angular momentum. Members of the series result in the most intense features in the spectrum of the only available ion state that has a nuclear arrangement which is significantly different from that of the autoionizing states. It appears that dissociative, neutral states are more likely to be accessed from states and it seems plausible that these dissociative states play a role in the resonant population of the state. There is also evidence to suggest that this state has a bent equilibrium geometry.

A photoelectron study of the rotational selectivity of vibrational autoionization of H2

Rotationally selective vibrational autoionization in molecular hydrogen has been investigated using two-dimensional photoelectron spectroscopy. Measurements of electron yield as a function of both electron and photon energy have been carried out using tuneable synchrotron VUV radiation between the upsilon =0 and upsilon =1 vibrational thresholds of H2+X2 Sigma g+, 15.45-15.70 eV. Constant rotational transition energy spectra have been extracted revealing the decay routes of autoionizing states. No large changes in the rotational quantum number of the ion core during autoionization were observed.

Laser probing of the electron-impact excited c(2p) 3Πu manifold of states in H2

A stepwise laser excitation method has been used to probe individual ro-vibrational levels in the molecular hydrogen metastable c(2p) 3Πu manifold of states. Metastable states produced by electron-impact excitation are subsequently excited by the absorption of a single UV laser photon, producing a complex triplet nd Rydberg spectrum observed via field ionization and autoionization. The spectrum has been analysed to determine quantum numbers of the states associated with the transitions. Excitation functions for individual ro-vibrational states in the c(2p) 3Πu manifold are then determined as a function of electron-impact energy.