Imaging multiphoton ionization dynamics of CH3I at a high repetition rate XUV free-electron laser

Yu-Chen Cheng ,Jan Lahl ,Cédric Bomme ,Dimitrios Rompotis ,R Moshammer ,Bastian Manschwetus ,Michael Meyer ,Kirsten Schnorr ,Georg Schmid ,Severin Meister ,Benjamin Erk ,Artem Rudenko ,Per Johnsson ,Rebecca Boll ,Sven Augustin ,Pavel K Olshin ,Sylvain Maclot ,Bart Oostenrijk ,Daniel Rolles ,H Redlin ,Mathieu Gisselbrecht

doi:10.1088/1361-6455/abc6bc

Abstract

XUV multiphoton ionization of molecules is commonly used in free-electron laser experiments to study charge transfer dynamics. However, molecular dissociation and electron dynamics, such as multiple photon absorption, Auger decay, and charge transfer, often happen on competing time scales, and the contributions of individual processes can be difficult to unravel. We experimentally investigate the Coulomb explosion dynamics of methyl iodide upon core–hole ionization of the shallow inner-shell of iodine (4d) and classically simulate the fragmentation by phenomenologically introducing ionization dynamics and charge transfer. Under our experimental conditions with medium fluence and relatively long XUV pulses (∼75 fs), we find that fast Auger decay prior to charge transfer significantly contributes to the charging mechanism, leading to a yield enhancement of higher carbon charge states upon molecular dissociation. Furthermore, we argue for the existence of another charging mechanism for the weak fragmentation channels leading to triply charged carbon atoms. This study shows that classical simulations can be a useful tool to guide the quantum mechanical description of the femtosecond dynamics upon multiphoton absorption in molecular systems.

Highlights

Y-C Cheng et alFree-electron-lasers (FELs) [1,2,3,4] have opened new avenues to investigate ultrafast dynamics with short wavelength photons using non-linear light–matter interaction processes [5, 6]
Under our experimental conditions with medium fluence and relatively long XUV pulses (∼75 fs), we find that fast Auger decay prior to charge transfer significantly contributes to the charging mechanism, leading to a yield enhancement of higher carbon charge states upon molecular dissociation
The thickness of the shaded areas represents the upper and lower limits of the kinetic energy release (KER) obtained from the different pathways that contribute to the charge transfer

Summary

Introduction

Y-C Cheng et alFree-electron-lasers (FELs) [1,2,3,4] have opened new avenues to investigate ultrafast dynamics with short wavelength photons using non-linear light–matter interaction processes [5, 6]. Detailed descriptions of charge dynamics are needed to understand the fragmentation process, but they represent a great theoretical challenge due to a large number of parameters, some of which might be poorly known [6, 15]. Simplified models, such as the generalised two-parameter model introduced by Motomura et al [16] to describe the charge buildup and the charge transfer processes, may provide satisfactory agreement with experiments, but can only provide a qualitative understanding.

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Discussion

Conclusion