R I Hall

Highly excited hydrogen molecules desorbed from a surface: Experimental results

The rovibrational excitation of hydrogen molecules resulting from recombination of atoms on a metallic surface has been observed by means of an electronic collision method. We present the obtained H− experimental spectra reflecting the vibrational populations of the desorbed hydrogen molecules. This study brings to light the fundamental role of the nature of the metallic surface in the recombination process: excitation of higher vibrational levels (from v=5 to v=8) is observed only on those metal surfaces which strongly chemisorb atomic hydrogen. To explain these observations we propose the formation, on these surfaces, of a physisorbed H layer on the chemisorbed one.

Spectroscopy and metastability of CO22+ molecular ions

The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.

Electron impact excitation of the Rydberg states in O2 in the 7–10 eV energy‐loss region

Electron impact energy‐loss spectra of O2 in the 7–10 eV region has been investigated at low impact energies and high scattering angles. Under these conditions a number of new transitions have been found which do not appear in optical spectra. Bands at 8.595, 8.826, 9.045, and 9.27 eV have been assigned to the 3sσg 1Π (v′=0, 1, 2, and 3) excitations, respectively, and the idenfification of the corresponding 3Πg bands have been reconfirmed. New transitions appearing at 9.13, 9.32, 9.51, 9.58, and 9.78 eV cannot be unambiguously assigned on the basis of the presently available information.

Electron excitation of N2. II. Resonant phenomena associated with Rydberg states

For pt. I see ibid., vol.6, 862 of 1973. The study of the excitation function of the E state vibrational levels shows a strong threshold excitation, several bumps and extra structure. An angular analysis of these phenomena leads to the determination of the configuration and symmetry of four transient N2- states.

Energy and width of N-(3P) from observation of its formation by dissociative attachment to N2 and NO

A dissociative attachment process has been observed in both N2 and NO which yields N- ions in their unstable ground state (3P). An electron impact spectrometer has been used to detect the N- ions by collecting the electrons emitted when this ion decays to the 4S ground state of atomic nitrogen. The 3P state of N- is found to be located at an energy of 0.07+or-0.02 eV above the ground state of N and its natural width is determined to be 16+or-5 meV.

Electron impact excitation and assignment of the low-lying electronic states of N2O

Electron scattering spectra of CO2 are reported in the 7-10 eV energy-loss range, at energies of 0.2, 0.35, 0.6, 0.7 and 7.0 eV above threshold, and at a scattering angle of 90 degrees . Several new distinct overlapping continua with weak, diffuse bands superimposed are observed to lie in this energy-loss range. The experimental spectra are discussed in the light of ab initio configuration-interaction calculations by Winter et al (1973) of the vertical transition energies of CO2. The experimental spectra are shown to be consistent with excitations to the 3 Sigma u+, 1.3 Sigma u-, 1.3 Pi g, and 1.3 Delta u states of CO2.

Electron impact excitation of N2. I. Resonant phenomena associated with the A 3 Σ u+ and B 3 Π g valence states

The electron impact cross sections for vibrational excitation of the A 3 Sigma u+ and B 3 Pi g valence states of the N2 molecule show similar broad overlapping resonant peaks for impact energies from about 8.2 to 11 eV for the A 3 Sigma u+ state and from about 9 to 11.5 eV for the B 3 Pi g state. In each case, a sequence of minor oscillations is superimposed on these peaks. In the cross section of the vibrational levels of the B 3 Pi g state, the minor peaks appear at different energies and with slightly different spacings.

An Auger electron-threshold photoelectron coincidence spectrometer for studies of atomic and molecular dications

An Auger electron-threshold photoelectron coincidence spectrometer has been developed to obtain spectroscopic information on atomic and molecular dicationic states. The spectrometer consists of two electrostatic hemispherical analysers. Auger electrons are detected by one of the analysers with a position sensitive detector. The other analyser captures threshold photoelectrons over a 4π sr solid angle with a penetrating electric field. The signal-accumulation circuit is based on a fast-slow coincidence technique. In order to demonstrate both the performance and advantages of this type of Auger electron spectrometer, the Kr MNN Auger transitions are investigated. Auger electron spectra associated with each 3d subshell are obtained by coincidence detection with the corresponding threshold photoelectrons, thus providing unambiguously assignments for the initial core-hole states of the Auger lines. The total energy resolution in a coincident Auger electron spectrum reaches 80 meV (FWHM), a value which corresponds to a sub-natural linewidth as the natural width of the Kr 3d core-hole states is 101 meV.

Detection and measurement of ro-vibrational populations in molecular hydrogen

A technique has been developed for measuring ro-vibrational populations in H2 and its isotopes based on the observation of near-zero energy H- (D-) yields produced by the dissociative attachment process when a low energy electron beam interacts with the gas molecules. The method is sensitive in H2 to a number density of molecules of 1011 cm-3 for the v=0 level and rises to 5*105 cm-3 for high levels (v>5). In principle the maximum level that can be detected using this technique is v=9 for H2, v=10 for HD and v=13 for D2.

Initial vibrational state dependence of resonant excitation and ‘‘dissociative attachment’’ in electron–N2 scattering

A beam of vibrationally excited ground state nitrogen molecules has been generated using a microwave discharge. The effect of this vibrational energy on the different decay processes of the A 2Πu state of N−2 formed by electron impact in the 7–13 eV region has been studied using an electron‐impact spectrometer. Firstly, the ’’boomerang’’ oscillations occuring in the excitation cross section of the A 3Σ+u levels increase in amplitude when the initial level is v=1. This effect, as well as the measured cross sections are well described by calculations using the local complex potential model. Secondly, the cross section ratio for the ’’dissociative attachment’’, process yielding unstable N−(3P) from the v=0 and 1 levels has been evaluated indicating that the vibrational energy has almost no effect. The calculations further indicate that this ratio only rises to 4 when going from v=0 to 4 which is in sharp contrast to H2 where, in the 4 eV region the same amount of vibrational energy increases the dissociative...