Photoelectron angular distributions from strong-field ionization of oriented molecules

TL;DR

This study demonstrates how photoelectron angular distributions from oriented molecules, measured with ultrafast circularly polarized laser pulses, reveal detailed molecular properties and dynamics, advancing molecular imaging techniques.

Contribution

It introduces experimental and theoretical methods to measure and analyze molecular frame PADs from oriented molecules, highlighting their sensitivity to molecular orbitals and dipole moments.

Findings

PADs show anisotropies in oriented molecules absent in random orientations.

Nodal planes and dipole moments influence the PAD structures.

Results enable time-resolved probing of valence electron dynamics.

Abstract

The combination of photoelectron spectroscopy and ultrafast light sources is on track to set new standards for detailed interrogation of dynamics and reactivity of molecules. A crucial prerequisite for further progress is the ability to not only detect the electron kinetic energy, as done in traditional photoelectron spectroscopy, but also the photoelectron angular distributions (PADs) in the molecular frame. Here carbonylsulfide (OCS) and benzonitrile molecules, fixed in space by combined laser and electrostatic fields, are ionized with intense, circularly polarized, 30 femtosecond laser pulses. For 1-dimensionally oriented OCS the molecular frame PADs exhibit pronounced anisotropies, perpendicular to the fixed permanent dipole moment, that are absent in PADs from randomly oriented molecules. For 3-dimensionally oriented benzonitrile additional striking structures appear due to suppression of electron emission in nodal planes of the fixed electronic orbitals. Our theoretical analysis, relying on tunneling ionization theory, shows that the PADs reflect nodal planes, permanent dipole moments and polarizabilities of both the neutral molecule and its cation. The calculated results are exponentially sensitive to changes in these molecular properties thereby pointing to exciting opportunities for time-resolved probing of valence electrons dynamics by intense circularly polarized pulses. Molecular frame PADs from oriented molecules will prove important in other contexts notably in emerging free-electron-laser studies where localized inner shell electrons are knocked off by x-ray pulses.