Photoionization Dynamics

Powis Group

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Photoelectron Spectroscopy

The emitted photoelectrons carry information in their energy and angular distribution. Traditionally this has been interpreted to yield information about the static electronic structure of the molecule and photoion. Studying the photoionization dynamics extends this. Establishing full control over the ionizing photon's characteristics – such as wavelength (energy) and momentum (direction and polarization), pulse intensity and duration – and simultaneously selecting the initial molecular quantum state and possibly the molecular orientation allows far more detail to be exposed — concerning the interaction of the photoelectron with other electrons and with motions of the atomic nuclei in the molecular system.

Photoionization Resonances

To a first approximation, once the photon energy exceeds an ionization threshold the photoionization cross-section will reach some characteristic value, which then slowly, smoothly, and monotonically decreases as the photon energy is raised. In practice the cross-section behaviour is often more structured than this idealised picture as a result of resonance effects. Corresponding structures are found in the angular distribution of electrons, often with an even more marked deviation from idealised behaviour.

Resonances provide us with insight into detailed quantum structure of a molecular system:

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Electronic autoionization occurs at certain discrete photon energies when the system is excited into an excited neutral quantum state that lies above an ionization limit. The excited state electronic configuration then spontaneously evolves into that of the lower lying state, which ionizes with ejection of an electron. Autoionization structure appears as characteristic narrow "spikes" in a spectrum. Rotational and vibrational excited states can similarly autoionize.
Shape Resonance
The outgoing photoelectron may sometimes become temporarily "trapped" close to the ion core. Shape resonant trapping can be ascribed to a barrier (i.e. shape) in the potential experienced by the electron that thereby impedes its departure and consequently increases the ionization cross-section. In atomic systems shape resonances are often attributed to a centrifugal barrier, and are associated with specific angular momentum states; in molecules other cage-like trapping effects may also be suggested. Shape resonances are typically broad – several eV is common.

The shape resonant trapping of an electron near a molecular ion core provides an opportunity for enhanced vibronic interaction and can lead to non-Frank–Condon ion vibrational distributions. Another significant consequence of the resonance is a rapid phase change in the outgoing electron wave, and this can strongly influence the observed photoelectron angular distributions.


Studies of Molecular Photoionization/Reaction Dynamics focus on the forces and torques experienced by the atomic nuclei and electrons during ionization (or reaction) in an attempt to understand the complete process or mechanism of ionization and reaction.

The recoil of various fragments and the various sources of angular momenta are vector properties, possessing both magnitude and direction. Examination of the vector correlations, linking the mutual interdependence of the various forces provide fresh insights into the fundamental electron-molecule interactions.

Cooper Minima

Another type of dynamical phenomenon that can be encountered in photoionization is the Cooper minimum. As the name implies this produces dips in the ionization cross-section and the photoelectron angular anisotropy parameter, β, provided the quantum numbers of the ionizing orbital satisfy certain conditions.

H2S Cooper Minimum

Although essentially atomic in origin, Cooper minima can sometimes persist when an appropriate atom is incorporated in a molecular system. We have studied this behaviour of Cooper minima in molecular photoionization to understand better the retention of atomic character after molecular bonding is effected.

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Interactive Tutorials

  • Shape Resonant Wave functions: trapping, amplitude and phase. Go...
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  • Various calculated properties of shape resonances. Go...