Observing Femtosecond Fragmentation Using Ultrafast X-ray-Induced Auger Spectra

Thomas Wolf,Ingo Fischer,John Bozek,Francesco Tarantelli,Todd Martinez,Raimund Feifel,Brian Mcfarland,Isabella Wagner,Saikat Nandi,Nora Berrah,Christoph Bostedt,Joe Farrell,Fabian Holzmeier,James Cryan,Melanie Mucke,Phil Bucksbaum,Markus Gühr,Ryan Coffee

doi:10.3390/app7070681

Abstract

Molecules often fragment after photoionization in the gas phase. Usually, this process can only be investigated spectroscopically as long as there exists electron correlation between the photofragments. Important parameters, like their kinetic energy after separation, cannot be investigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine. We observe the appearance of clearly distinguishable signatures from thymine′s neutral photofragment isocyanic acid. Furthermore, we observe a time-dependent shift of its spectrum, which we can attribute to the influence of the charged fragment on the Auger electron. This allows us to map our time-dependent dataset onto the fragmentation coordinate. The time dependence of the shift supports efficient transformation of the excess energy gained from photoionization into kinetic energy of the fragments. Our method is broadly applicable to the investigation of photofragmentation processes.

Highlights

The speed of a photoexcited chemical reaction is determined by gradients of potential energy surfaces and the mass of reaction products or precursors
We conclude by pointing out the unique long-range sensitivity of Auger electron spectroscopy to the presence of charged particles, which allows one to map time-dependent signatures of fragments onto a dissociation coordinate
Future experiments are planned to exploit this powerful X-ray induced Auger electron technique

Summary

Introduction

The speed of a photoexcited chemical reaction is determined by gradients of potential energy surfaces and the mass of reaction products or precursors. We focus on investigating a specific type of reaction, photoion fragmentation as it occurs in the presence of ionizing radiation in the upper atmosphere or in space [7]. Organic molecules can be photoionized by absorbing either one or multiple photons, which combined can overcome the ionization potential (typically on the order of 9 eV [8]). When sufficient energy is deposited in the molecule, it is placed on a cationic potential energy surface with repulsive character in at least one internal degree of freedom. The positive charge localizes on one of the fragments.

Methods

Results

Conclusion