Optical Interference Effect in Strong-field Electronic Coherence Spectroscopy

TL;DR

This study explores how strong infrared laser pulses induce and detect electronic coherences in argon and nitrogen ions, highlighting the importance of optical interference effects and pulse characteristics in interpreting coherence signals.

Contribution

It demonstrates the experimental detection of strong-field-induced electronic coherences in ions and clarifies how optical interference and pulse shape influence the measurements.

Findings

Optical interference affects coherence detection differently in argon and nitrogen ions.

Double ionization can diminish the visibility of generated electronic coherences.

Pulse shape and spectral properties are crucial for isolating coherence signals.

Abstract

We have investigated strong-field-induced electronic coherences in argon and molecular nitrogen ions created by high-intensity, few-cycle infrared laser pulses. This is a step toward the long-sought goal of strong-field coherent control in molecular chemistry. We employed high-intensity, few-cycle infrared laser pulses in a pump-probe setup to investigate a recent prediction that electronic coherences in nitrogen molecules change the ion yields vs. pump-probe delay. [Yuen and Lin, Phys. Rev. A 109, L011101 (2024)]. The predicted coherence signals in molecular nitrogen could not be resolved above the optical interference of the pump and probe pulses; a simultaneous measurement clearly resolved the induced cation fine-structure coherence in strong-field-ionized argon. The results of our comparison with simulations suggest that optical interference effects manifest differently in each ionic species and must be carefully accounted for when interpreting experimental data. We found that nonsequential double ionization in the low-intensity region of the focal volume can reduce the visibility of coherence generated by two-pulse sequential ionization, and we quantify the importance of pulse shape and spectral characteristics for isolating the desired coherence signals.