Threshold photodissociation dynamics of NO2 studied by time-resolved cold target recoil ion momentum spectroscopy.

Xiaoyan Ding,M. Spanner,A. Stolow,P. B. Corkum,Kevin F. Lee,A. Staudte,D. M. Villeneuve,A. Yu. Naumov,R. Forbes,M. Kübel

doi:10.1063/1.5095430

Abstract

We study the near-threshold photodissociation dynamics of NO2 by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO2 to the optically bright Ã2B2 state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(X2Π) + O(3P) dissociation channel threshold, and therefore, following internal conversion, excited NO2 is energetically prepared both "above threshold" (dissociating) and "below threshold" (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO+ fragments with nonzero momentum at 200 fs delay and an increasing yield of NO+ fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO+ and O+ fragments impulsively created from the evolving "hot" neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO-O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled "inner region" to a loosely coupled "outer region" where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO+ angular distributions support our assignments.

Highlights

The near-threshold photodissociation of NO2 + hv → NO(X2Π) + O(3P) is the classic example of ground state unimolecular reaction dynamics
Single ionization probe We present data associated with the single ionization channel: NO2 pum→p (NO2)∗ prob→e NO++ O + e−, where a photoelectron is detected in coincidence with a NO+ fragment
We study NO2 photodissociation at 400 nm excitation and probe the ground state unimolecular dynamics as a function of pump-probe time delay using single and double strong field ionization in a COLTRIMS apparatus

Summary

Introduction

The near-threshold photodissociation of NO2 + hv → NO(X2Π) + O(3P) is the classic example of ground state unimolecular reaction dynamics. A recent ab initio study using new potential energy surfaces showed that internal conversion to the ground state occurs on a 50 fs time scale and that, by 200 fs, the initially coherent large amplitude bending motion in the electronic ground state broadly spreads into other coordinates through an intramolecular vibrational energy redistribution (IVR). The dissociation process has often been described by statistical or phase space theories of unimolecular decay, where the intramolecular vibrational redistribution is assumed to be “instantaneous” on the time scale of dissociation, leading to expectations of exponential decay of the energized molecule.. Classical trajectory simulations reported the existence of multiple time scales during the ground state dissociation, suggesting that the available phase space is divided into a strongly coupled “inner” and a loosely coupled “outer” region.. This study aims to provide a new observable that will probe this “loose” large amplitude region of the ground state dynamics

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