Ultraviolet photochemistry of 2-bromothiophene explored using universal ionization detection and multi-mass velocity-map imaging with a PImMS2 sensor.

R A Ingle,R Turchetta,C Vallance,J W L Lee,S J King,M Brouard,M Bain,C S Hansen,M N R Ashfold,E Elsdon

doi:10.1063/1.4979559

Abstract

The ultraviolet photochemistry of 2-bromothiophene (C4H3SBr) has been studied across the wavelength range 265-245 nm using a velocity-map imaging (VMI) apparatus recently modified for multi-mass imaging and vacuum ultraviolet (VUV, 118.2 nm) universal ionization. At all wavelengths, molecular products arising from the loss of atomic bromine were found to exhibit recoil velocities and anisotropies consistent with those reported elsewhere for the Br fragment [J. Chem. Phys. 142, 224303 (2015)]. Comparison between the momentum distributions of the Br and C4H3S fragments suggests that bromine is formed primarily in its ground (2P3/2) spin-orbit state. These distributions match well at high momentum, but relatively fewer slow moving molecular fragments were detected. This is explained by the observation of a second substantial ionic product, C3H3+. Analysis of ion images recorded simultaneously for several ion masses and the results of high-level ab initio calculations suggest that this fragment ion arises from dissociative ionization (by the VUV probe laser) of the most internally excited C4H3S fragments. This study provides an excellent benchmark for the recently modified VMI instrumentation and offers a powerful demonstration of the emerging field of multi-mass VMI using event-triggered, high frame-rate sensors, and universal ionization.

Highlights

The history of the velocity-map imaging (VMI) technique spans two decades,1–3 yet new chapters are still being written as technological developments continue to extend the power and versatility of the technique
These findings are in agreement with the findings of a previous study that imaged the atomic Br fragments and, identify that most of these atoms are formed in their ground (2P3/2) spinorbit state
The present study detected a number of lighter product ions, including a dominant C3H+3 ion that was prevalent at short photolysis wavelengths

Summary

Introduction

The history of the velocity-map imaging (VMI) technique spans two decades, yet new chapters are still being written as technological developments continue to extend the power and versatility of the technique. One area in which this is perhaps most evident is the advent of event-triggered, high frame rate sensors These devices are able to acquire bursts of images with time resolutions on the order of tens of nanoseconds and address some longstanding challenges of velocitymap imaging. A prominent theme in the development of the VMI technique is how to record the two-dimensional (2-D) projection from the complete three-dimensional (3-D) velocity distribution of a mass-selected (by time-of-flight (TOF)) ensemble of ions. These ions are the products of photodissociation or scattering reactions, and evolve as expanding concentric spheres (Newton spheres) with radii a)R. By measuring the time it takes for electrons formed as a result of an

Methods

Results

Conclusion