Angular Distributions in Molecular Photoionization

Abstract

Photoelectron angular distributions (PADs) in molecular photoionization can provide detailed information about the geometry of a molecule, the initial electronic state and the electronic state of the product molecular ion. In general, the molecular frame photoelectron angular distribution (MFPAD) can be a function of four angles for linearly and circularly polarized light and five angles for elliptically polarized light. In this chapter, we show how, for dipole transitions, these multidimensional distributions can be reduced to a limited number of one-dimensional functions. PADs must be determined relative to the molecular frame of reference and we consider experimental distributions that can be obtained with post-ionization analysis of the molecular frame orientation, using coincidence detection of the photoelectron and at least one ionic fragment. We also consider concurrent orientation, where a multiphoton process modifies the alignment of the molecule, helping the analysis of the PAD. And finally, we consider the form of the PAD when the molecule is preoriented, as can be achieved with impulsive alignment by intense infrared lasers. IntroductionThe ionization of molecules by absorption of light is significantly different from absorption where the final state is a bound state. In the case of ionization, the continuous nature of the final state leads to a lack of quantization for the total energy of the system. The infinite degeneracy of the continuum allows ejection of the photoelectron in any direction relative to the molecular frame (MF), that is, in any direction in the coordinate system of the molecule. The differential cross-section for this photoemission process is the molecular frame photoelectron angular distribution (MFPAD). This observable results from the coherent superposition of all partial waves contributing to the continuum electronic wavefunction: it therefore provides a very sensitive probe of the potentials and interactions governing the quantum dynamics of the emitted photoelectrons. In molecules, the MFPAD is a function of Attosecond and XUV Physics, First Edition. Edited by Thomas Schultz and Marc Vrakking.

Theoretical description of the ionization processes with a discrete basis set representation of the electronic continuum

“…With the development of more advanced experimental techniques, the interest in the molecule-fixed photoelectron angular distribution grows [33,34]. There are three main experimental approaches to measure MFPAD [35,36]. The post-orientation method assumes the axial recoil approximation where molecule is excited to the dissociative state.…”