Strong field ionization to multiple electronic states in water

TL;DR

This study investigates how strong field ionization in water involves multiple electronic states, using isotope effects to distinguish nuclear motions, and employs simulations to interpret the experimental results.

Contribution

It introduces an isotope-based method to probe excited ionic states in water during strong field ionization, supported by quantum simulations.

Findings

Harmonic yield ratios reveal nuclear motion differences between isotopes.

Ionization of HOMO-1 causes significant bending motion and isotope effects.

Simulation results support the experimental interpretation.

Abstract

High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H2O and D2O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by simulating strong field ionization and high harmonic generation from the water isotopes using the time-dependent Schr\"odinger equation. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.