M. Braunstein

Vibrational branching ratios and shape resonant photoionization dynamics in N2O

Vibrational branching ratios and photoelectron asymmetry parameters for alternative vibrational modes in the photoionization of N_2O(7σ^(−1)) have been studied using accurate photoelectron continuum orbitals. Earlier dispersed ionic fluorescence measurements [E. D. Poliakoff, M. H. Ho, M. G. White, and G. E. Leroi, Chem. Phys. Lett. 130, 91 (1986)] revealed strong non‐Franck–Condon vibrational ion distributions for both the symmetric and antisymmetric stretching modes at low photoelectron energies. Our results establish that these features arise from a σ shape resonance which, based on its dependence on internuclear geometry, must be associated with the molecular framework as a whole and not with either of its fragments, N–N or N–O. This behavior accounts for the more pronounced deviations of the vibrational branching ratios from Franck–Condon values observed in the symmetric than in the antisymmetric mode. The σ continuum also supports a second shape resonance at higher energy which does not influence the vibrational branching ratios but is quite evident in the photoelectron asymmetry parameters around a photon energy of 40 eV. These vibrationally resolved studies of the photoelectron spectra of this polyatomic system provide an interesting example of the rich shape resonant behavior that can be expected to arise in polyatomic molecules with their alternative vibrational modes.

Shape resonance effects in the rotationally resolved photoelectron spectra of O2

We report the results of theoretical and experimental studies of the rotationally resolved photoelectron spectra of O_2 at low temperature leading to the v^+=0, 1, and 2 levels of the X  ^2Π_g state of O^+_2. A delayed, pulsed field ionization technique is used in conjunction with a coherent VUV radiationsource to obtain high resolution spectra near threshold. The data are compared with theoretical results obtained using static‐exchange photoelectron orbitals and a full description of the mixed Hund’s case (a)–(b) ionic ground state. Agreement with experiment is good, especially for the v^+=1 and v^+=2 levels. Analysis of the rotational branch intensities yields detailed information on the angular momentum composition of the shape resonance near threshold. We also show that the dependence of the electronic transition moment on internuclear distance caused by the shape resonance leads to a significant dependence of the rotational branch intensity on ion vibrational level.

Shape-resonance-induced non-Franck–Condon effects in (2+1) resonance enhanced multiphoton ionization of the C 3Πg state of O2

We show that strong non-Franck–Condon effects observed in (2+1) resonance enhanced multiphoton ionization of the C 3Pig state of O2 are due to the ksigmau shape resonance previously observed in single-photon studies of diatomic molecules. Calculated vibrational branching ratios for the v=2,3 levels of the C 3Πg state are in reasonable agreement with experiment. Certain discrepancies remain in comparing theoretical results with the measured spectra, and possible electron-correlation effects which underly this are discussed.

Cross sections and photoelectron asymmetry parameters for photoionization of H2O

The iterative Schwinger variational method is used to obtain cross sections and photoelectron asymmetry parameters for photoionization of the three outermost valence orbitals (1b1, 3a1, and 1b2) of H2O for photon energies from near threshold to 50 eV. A comparison of these calculated results with available experimental data is encouraging.

Studies of the photoionization cross sections of CH4

We present cross sections and asymmetry parameters for photoionization of the 1t_2 orbital of CH_4 using static‐exchange continuum orbitals of CH^+_4 to represent the photoelectron wave function. The calculations are done in the fixed‐nuclei approximation at a single internuclear geometry. To approximate the near‐threshold behavior of these cross sections, we assumed that the photoelectron spectrum is a composite of three electronic bands associated with the Jahn–Teller components of the distorted ion. The resulting cross sections reproduce the sharp rise seen at threshold in the experimental data and are in good agreement with experiment at higher energy. The agreement between the calculated and measured photoelectron asymmetry parameters is, however, less satisfactory.

Shape resonance and non‐Franck–Condon effects in (2+1) resonant enhanced multiphoton ionization of O2 via the C 3Πg state

We report vibrationally resolved photoelectron angular distributions for photoionization of the C3Πg Rydberg state of O2. Comparison is made with recent experimental measurements of angular distributions which employ (2+1) resonant enhanced multiphoton ionization of the C3Πg state. The present theory treats the process as single-photon ionization from an unaligned Rydberg state, and qualitatively accounts for much of the observed trends. Non-Franck–Condon effects induced by the kσu shape resonance lead to a substantial dependence of the angular distributions on the vibrational state of the X2Πg ion. Discrepancies between our theoretical results and experiment are qualitatively discussed and tentatively attributed to residual electron correlations.

Shape resonance behavior in 1πg photoionization of O2

We report calculations of vibrationally resolved cross sections and photoelectron angular distributions for photoionization of O_2 leading to the X^ 2 Π_g (ν^+ =0–4) states of O^+_2 using Hartree–Fock continuum photoelectron orbitals. These studies were motivated by recent results which show that a σ_u shape resonance plays a dominant role in producing non‐Franck–Condon vibrational distributions in resonant multiphoton ionization of O_2 via the C ^3Π_g (1π_g3sσ_g) Rydberg state. In the present study, we investigate how this shape resonance influences photoionization dynamics in single‐photon ionization. Below 21 eV photon energy, we find significant non‐Franck–Condon effects in the vibrational branching ratios as well as in the vibrationally resolved photoelectron angular distributions. Substantial autoionization hinders a direct comparison between theory and experiment.

Shape resonances in the photoionization of N2O

We report the results of studies of the cross sections and photoelectron asymmetry parameters for photoionization of the 7σ level of N2O using Hartree–Fock photoelectron continuum orbitals. These studies were motivated by recent measurements which showed significant non‐Franck–Condon vibrational distributions at low photoelectron energies where previously only autoionizing resonances, but no shape resonance, had been identified. Our results establish that there are two σ shape resonances in the 7σ ionization continuum, a pronounced resonance at low photoelectron energies, and another at higher energy which is essentially obscured in the vibrationally unresolved cross sections. The shape resonant structure that emerges from these studies differs significantly from the predictions of previous model studies. Studies in progress reveal a rich and unusual dependence of these resonances on changes in internuclear distances.

Multiplet‐specific shape resonant features in photoionization of NO

We report photoionization cross sections and asymmetry parameters for the 5σ and 4σ levels of NO obtained using numerical photoelectron continuum functions. For the 5σ orbital multiplet‐specific potentials lead to shape‐resonant cross sections which show significant nonstatistical multiplet behavior. This behavior, which reflects the sensitivity of shape resonances to the exchange component of the molecular ion potential, is most prominently characterized by resonance peaks at substantially different photoelectron kinetic energies in the two multiplet channels. Comparison of the calculated cross sections with available experimental data for NO shows somewhat poorer agreement than has been seen in the photoionization of closed‐shell systems such as N2 and CO. This difference reflects the more complex nature of photoionization processes in open‐shell molecules.

Rotationally resolved photoionization of molecular oxygen

We report the results of theoretical studies of the rotationally resolved photoelectron spectra of ground state O_2 leading to the X ^2Π_g state of O^+ 2 via the absorption of a single vacuum ultraviolet photon. These studies elaborate on a recent report [M. Braunstein e t a l., J. Chem. Phys. 9 3, 5345 (1990)] where we showed that a shape resonance near threshold creates a significant dependence of the rotational branching ratios on the ion vibrational level. We also showed that analysis of the rotational branches yields detailed information on the angular momentum composition of the shape resonance. We continue this analysis giving a comprehensive derivation of the rotationally resolved cross sections and photoelectron angular distributions. We discuss the selection rules implied by these expressions and present very high resolution cross sections (J→J^+) obtained using static‐exchange photoelectron orbitals and explicitly taking into account the internuclear distance dependence of the electronic transition moment. These cross sections illustrate the selection rules and show more explicitly the angular momentum composition of the shape resonance. We also present rotationally resolved photoelectron angular distributions which would be expected at low energy.