# Strong-field photoelectron momentum imaging of OCS at finely resolved incident intensities

## TL;DR

This study uses high-resolution photoelectron momentum imaging to analyze strong-field ionization of OCS, revealing detailed resonance structures and confirming an empirical rule relating orbital momentum and ionization photon count.

## Contribution

It introduces a method to suppress focal volume effects, enabling clear identification of Freeman resonances and reassignment of photoelectron lines in molecular ionization.

## Key findings

- Resonant Rydberg states identified at various intensities.
- Empirical rule confirmed for molecular ionization.
- Focal volume averaging effects minimized.

## Abstract

Photoelectron momentum distributions from strong-field ionization of carbonyl sulfide with 800 nm central-wavelength laser pulses at various peak intensities from $4.6$ to $13\times10^{13}$ W/cm$^2$ were recorded and analyzed regarding resonant Rydberg states and photoelectron orbital angular momentum. The evaluation of the differentials of the momentum distributions with respect to the peak intensity highly suppressed the impact of focal volume averaging and allowed for the unambiguous recognition of Freeman resonances. As a result, previously made assignments of photoelectron lines could be reassigned. An earlier reported empirical rule, which relates the initial state's orbital momentum and the minimum photon expense to ionize an ac Stark shifted atomic system to the observable dominant photoelectron orbital momentum, was confirmed for the molecular target.