Investigation of dissociative photoionization pathways for orientation-selected molecules by phase-controlled laser fields

Kenichi Okutsu,Hideki Ohmura,Naoaki Saito

doi:10.1063/5.0029997

Abstract

Intense (5.0 × 1012 W/cm2) nanosecond phase-controlled laser fields consisting of fundamental and second-harmonic light induce orientation-selective molecular tunneling ionization in a randomly orientated molecular ensemble in a gas phase. The selection of oriented molecules enables one to elucidate dissociative photoionization pathways while eliminating loss of information due to orientational averaging. Here, we have investigated the dissociative ionization of hydrofluorocarbon molecules induced by phase-controlled two-color laser fields. From the phase-dependent behavior of photofragment emission from orientation-selected molecules, dissociation pathways were elucidated experimentally. Bond dissociation energies obtained by quantum chemical calculations support the identified dissociation pathways.

Highlights

Interaction between intense light and matter, such as nonlinear optical response, can be described by the conventional perturbation theory of quantum mechanics as multiphoton processes at a considerably high order
The backward peak of CH3+ and the forward peak of its counterpart CHF2+ predominated at φ = 0. This forward–backward asymmetry was reversed at φ = π. These results show that dissociative ionization was induced by the phase-controlled ω+2ω laser fields while discriminating between the head and tail orientations of the molecule
The OSM-tunneling ionization (TI) of two HFC molecules has been investigated by using intense nanosecond phase-controlled ω+2ω laser fields

Summary

INTRODUCTION

Interaction between intense light and matter, such as nonlinear optical response, can be described by the conventional perturbation theory of quantum mechanics as multiphoton processes at a considerably high order. When TI of molecules with the asymmetric HOMO structure is induced by an asymmetric ω+2ω field, electrons are much more likely to be removed from the large-amplitude part of the HOMO in the direction opposite to that of the electric field vector at field maxima so that OSM-TI occurs among randomly oriented molecular ensembles. As an application of OSM-TI, we demonstrate the elucidation of dissociative ionization pathways of polyatomic molecules induced by phase-controlled laser fields.. An HFC molecule does not contain chorine, so HFC does not damage atmospheric ozone, but it remains a serious greenhouse gas with a global warming potential hundreds to thousands of times that of CO2 These particular HFCs are studied from the viewpoint of the release of radicals in the same manner as for CFCs and many reactions with NOx molecules and O atoms in previous studies.. HFC molecules are suitable for investigating dissociative ionization pathways clearly

EXPERIMENTAL AND THEORETICAL METHODS

HFC-152a

HFC-134a

CONCLUSIONS