Full Characterization of a Molecular Cooper Minimum Using High-Harmonic Spectroscopy

Timothy Scarborough,Kenneth Schafer,Louis Dimauro,Timothy Gorman,Pierre Agostini,Mette Gaarde,Péter Sándor,Sanjay Khatri,François Mauger

doi:10.3390/app8071129

Timothy Scarborough, Kenneth Schafer + Show 7 more

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https://doi.org/10.3390/app8071129

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Abstract

High-harmonic generation was used to probe the spectral intensity and phase of the recombination-dipole matrix element of methyl chloride (CH3Cl), revealing a Cooper minimum (CM) analogous to the 3p CM previously reported in argon. The CM structure altered the spectral response and group delay (GD) of the emitted harmonics, and was revealed only through careful removal of all additional contributors to the GD. In characterizing the GD dispersion, also known as the “attochirp” we additionally present the most complete validation to date of the commonly used strong-field approximation for calculating the GD, demonstrating the correct intensity scaling and extending its usefulness to simple molecules.

Highlights

Much of the focus of attosecond science involves the development and calibration of precise measurement techniques in atoms [1]
With high-harmonic generation (HHG) spectra composed of odd harmonic orders of the driving field, the spacing in the XUV frequency comb scales as the inverse of the laser wavelength, and MIR sources allow for a finer energetic sampling
Before applying the strong-field approximation (SFA) attochirp removal to the Cooper minimum (CM) of methyl chloride, we first confirmed its validity in the case it was initially meant to describe: the ionization of an atomic target

Summary

Introduction

Much of the focus of attosecond science involves the development and calibration of precise measurement techniques in atoms [1]. As the field moves toward the investigation of dynamics in more complex targets, it becomes important to determine the applicability of techniques developed for atoms to the molecular case, and when found lacking, to update these methods to properly resolve dynamics. One example of such dynamics which garnered considerable attention in recent years is molecular charge migration [5], in which, following ionization, the electrons in a molecular ion undergo a coherent spatial oscillation, driven entirely by electron correlations. One reasonable approach is to look for changes to known, calibrated structural features in high-harmonic spectra to infer additional information about dynamics

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