A A Wills

The production of autoionizing states of atomic oxygen by the photodissociation of O2

A study has been made of photodissociation processes in O2 which produce autoionizing fragments. Tunable VUV radiation was used to obtain photoelectron spectra. These contain structure due to electron emission from states of atomic oxygen decaying to the ground and first excitated states of the ion. The investigation has concentrated on the region of the thresholds for the production of the atomic autoionization, and spectra have also been recorded up to 60 eV photon energy.

Resonance structure in the neon photoelectron satellites

Doubly excited neutral states of neon, 2s22p4nlnl, have been investigated using photoelectron spectroscopy with synchrotron radiation. The selective decay of these levels into all the energetically accessible ionic states in the range 50-60 eV has been measured. In addition, decay of Ne+ states, with binding energies above the first double ionisation potential, by autoionisation to the Ne2+(3P) state has been observed.

Photoelectron study of the satellite ion states of HCl and the production of autoionizing chlorine atoms by photodissociation of HCl

The HCl+ satellite states, in the energy range 20-31 eV, have been studied by photoelectron spectroscopy. The energy resolution achieved has allowed vibrational levels to be observed for the first time which has shown the two lowest lying states to be dissociative. The use of tunable VUV radiation has provided detailed information on resonance processes occurring in the region of the ionization thresholds for the satellite ion states. These processes include neutral photodissociation producing autoionising chlorine atoms. Some of the observed atomic states are optically forbidden in transitions from the ground state of the atom.

A photoelectron study of resonance structure in the argon correlation satellites

Doubly excited neutral states of argon have been investigated by observing their decay into the 3p and 3s main ionisation lines and into the satellite ionic states. These were measured using photoelectron spectroscopy and monochromatic synchrotron radiation in the range 31.0 to 42.3 eV. The 3p and 3s main line constant ionic state spectra show structure due to the decay of doubly excited states but in general these are much less prominent than in the spectra for the satellites which are dominated by resonances, particularly close to threshold. There is also a strong selectivity with marked differences in the resonance structure observed in the decay to different satellite states. Four Rydberg progressions have been observed each of which is split into two series.

Resonances in the vibrationally resolved excitation functions of b 4Σg-, B 2Σ g- and X 2Πg states of O2+ between 18.5 and 25.0 eV

Branching ratios and vibrationally resolved partial cross sections of O2+ have been measured for the X 2 Pi g, b 4 Sigma g- and B 2 Sigma g- ionic states for photon energies between 18.5 and 25.0 eV,. Vibrational branching ratios for the b 4 Sigma g- and B 2 Sigma g- electronic states show non-Franck-Condon behaviour arising from autoionising Rydberg states and the sigma u shape resonance at around 21.5 eV. A comparison between experiment and existing calculations is presented.

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.

Selective population of spin-orbit levels in the autoionization of O2

Autoionization processes in O2 have been studied by measuring photoelectrons in the photon energy region between the O2+ X 2 Pi g and a 2 Pi u ionic states (12.4-15 eV). The present experimental results provide comprehensive information about autoionization dynamics since the decay routes from the spin-orbit components of the neutral autoionizing states to the two spin-orbit components of the ground ionic state are resolved. The intensity ratio of the O2+ X 2 Pi 1/2 and 2 Pi 3/2 components has been determined and strong Omega to Omega + selectivity has been observed. The observed selectivity is explained in terms of a model in which the orbital angular momentum along the internuclear axis of the positive ion core remains unchanged during the autoionization process. Using this model, Rydberg orbitals of the I, I and I autoionizing states are reassigned as 4s sigma g, 3d delta g and 3d sigma g, respectively, and the spin-orbit constant of the J state is determined.

Resonance structure in the 5p, 5s and satellite photoelectron lines of xenon

The decay of doubly excited resonances of neutral xenon into all the accessible main line and satellite ionic states has been studied using photoelectron spectroscopy and synchrotron radiation between 20.5 and 26.2 eV. The results show that resonances dominate the satellite cross sections in this region and that they decay selectively into individual J levels of the ion.

Auger emission from Xe above and below the 4d ionization threshold

The Auger electron yield from xenon has been measured in the vicinity of the 4d inner-shell ionization threshold. Just above this ionization threshold the Auger line profile is modified by the post-collision interaction effect involving a slow photoelectron. The process is followed through threshold into a region where the photoelectron is recaptured and either shaken up/down or remains in its Rydberg orbital during the Auger decay. By measuring the electron yield as a function of both incident photon energy and electron kinetic energy a comprehensive study of these processes is made.

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.