Delayed Ultrafast X-ray Auger Probing (DUXAP) of Nucleobase Ultraviolet Photoprotection

TL;DR

This paper introduces DUXAP, a novel ultrafast x-ray Auger spectroscopy technique that probes molecular excited state dynamics with high sensitivity, demonstrated on nucleobases to reveal ultrafast relaxation processes.

Contribution

The paper presents a new delayed ultrafast x-ray Auger probing method that enhances sensitivity to local electronic changes during molecular relaxation, specifically applied to nucleobase photoprotection.

Findings

Measured a 300 fs decay constant for thymine's excited state.

Revealed new propensity rules for Auger decay sensitive to local valence electrons.

Demonstrated potential of DUXAP for studying ultrafast molecular dynamics.

Abstract

We present a new method for ultrafast spectroscopy of molecular photoexcited dynamics. The technique uses a pair of femtosecond pulses: a photoexcitation pulse initiating excited state dynamics followed by a soft x-ray (SXR) probe pulse that core ionizes certain atoms inside the molecule. We observe the Auger decay of the core hole as a function of delay between the photoexcitation and SXR pulses. The core hole decay is particularly sensitive to the local valence electrons near the core and shows new types of propensity rules, compared to dipole selection rules in SXR absorption or emission spectroscopy. We apply the delayed ultrafast x-ray Auger probing (DUXAP) method to the specific problem of nucleobase photoprotection to demonstrate its potential. The ultraviolet photoexcited \pi\pi* states of nucleobases are prone to chemical reactions with neighboring bases. To avoid this, the single molecules funnel the \pi\pi* population to lower lying electronic states on an ultrafast timescale under violation of the Born-Oppenheimer approximation. The new type of propensity rule, which is confirmed by Auger decay simulations, allows us to have increased sensitivity on the direct relaxation from the \pi\pi* state to the vibrationally hot electronic ground state. For the nucleobase thymine, we measure a decay constant of 300 fs in agreement with previous quantum chemical simulations.