Control of H_{2} Dissociative Ionization in the Nonlinear Regime Using Vacuum Ultraviolet Free-Electron Laser Pulses

Area of Science:

• Physical Chemistry
• Atomic and Molecular Physics
• Quantum Dynamics

Background:

• Nonlinear two-photon single ionization is a fundamental process in atomic and molecular physics.
• Understanding the role of nuclear degrees of freedom is crucial for controlling ionization dynamics.
• Short and intense vacuum ultraviolet (VUV) pulses offer unique opportunities to probe ultrafast molecular processes.

Purpose of the Study:

• To investigate the influence of nuclear motion on nonlinear two-photon single ionization of H_{2}.
• To explore the selective excitation of vibronic states using tunable VUV free-electron laser pulses.
• To identify and control new photoionization pathways through ab initio calculations and experimental measurements.

Main Methods:

• Experimental investigation using tunable, narrow-bandwidth VUV pulses from the FERMI free-electron laser.
• Selective excitation of single resonant vibronic intermediate neutral states in H_{2}.
• Theoretical ab initio calculations to analyze photoelectron and ion energy spectra obtained via velocity map imaging.

Main Results:

• Observed a sustained enhancement of dissociative ionization, exceeding nondissociative ionization.
• Demonstrated control over ionization pathways by selecting progressively higher vibronic states.
• Identified new photoionization pathways by analyzing energy spectra for varying pulse durations (around 100 fs).

Conclusions:

• Nuclear degrees of freedom play a significant role in nonlinear two-photon ionization of H_{2}.
• Selective excitation of vibronic states allows for controlled enhancement of dissociative ionization.
• The experimental timescale bridges ultrafast dynamics and steady-state excitations, offering insights into molecular response.