Electron-ion vector correlations for the study of photoionization of molecules in the UVX range: From synchrotron radiation to short light pulses

Abstract
Molecular frame photoemission is a very sensitive probe of the photoionization (PI) dynamics of molecules. The electron-ion vector correlation (VC) method takes advantage of dissociative photoionization (DPI) reactions of small molecules to measure such observables. It relies on the coincident detection of the photoelectron and the ion fragment emitted from the same DPI event and the determination of their velocity vectors. Most of the VC studies so far have been performed using synchrotron radiation (SR) light sources which benefit from a high repetition rate (1–10 MHz) favorable for coincidence experiments. In this paper we discuss the extension of this method to the study of PI processes induced by ultra-short VUV light sources, which provide the capability for investigating processes characterized by femtosecond or subfemtosecond dynamics. We first illustrate the VC method by the report of recent results of a comparative study of resonant photoionization of the H₂ and D₂ molecules induced by VUV circularly polarized synchrotron radiation at SOLEIL in the region involving resonant excitation of Q₁ and Q₂ doubly excited state series. This problem is of particular interest since autoionization and dissociation of selected Q₁ or Q₂ states occur on a comparable time scale of few femtoseconds, which implies a coupling between the electronic and nuclear motion. The extension of the VC method using femtosecond laser sources is then demonstrated by results for multiphoton PI of the Xe atom induced by 70 fs pulses centered at 400 nm delivered by the SOFOCKLE and PLFA sources (SLIC, CEA-Saclay) after frequency doubling of a 1 kHz Ti:sapphire laser.