Bright continuously-tunable VUV source for ultrafast spectroscopy

TL;DR

This paper introduces a continuously tunable femtosecond VUV light source around 21 eV, enabling precise ultrafast spectroscopy of electron dynamics with high spectral flexibility and sufficient intensity for nonlinear experiments.

Contribution

The work presents a novel monochromatic VUV source tunable across the He absorption bands using high-harmonic generation driven by an OPCPA laser, filling a gap in spectral tunability for ultrafast spectroscopy.

Findings

Verified tunability through interatomic Coulombic decay experiments

Achieved high intensities suitable for nonlinear processes

Demonstrated collective autoionization in He nanodroplets

Abstract

Ultrafast electron dynamics drive phenomena such as photochemical reactions, catalysis, and light harvesting. To capture such dynamics in real-time, femtosecond to attosecond light sources are extensively used. However, an exact match between the excitation photon energy and a characteristic resonance is crucial. High-harmonic generation sources are exceptional in terms of pulse duration but limited in spectral tunability in the VUV range. Here, we present a monochromatic femtosecond source continuously tunable around 21 eV photon energy utilizing the second harmonic of an OPCPA laser system to drive high-harmonic generation. The unique tunability of the source is verified in an experiment probing the interatomic Coulombic decay in doped He nanodroplets across the He absorption bands. Moreover, we achieved intensities sufficient for driving non-linear processes using a tight focusing of the VUV beam. We demonstrated it on the observation of collective autoionization of multiply excited pure He nanodroplets.