Ultrafast time-resolved coincidence momentum imaging of molecular photoionization processes using tabletop extreme ultraviolet pulses

The coupling of electronic and nuclear motion in molecules photoexcited to the ionization continuum steers both photoionization [1] and nonradiative relaxation pathways such as dissociation [2–4]. Detailed measurements of such processes require techniques that are sensitive to specific electronic transitions, the symmetries of electronic states, and the ultrafast molecular dynamics that occur in excited states. Momentum imaging of electrons and mass-resolved ions in coincidence is a powerful technique that enables multiple reaction channels to be investigated through the measurement of photoelectron angular distributions in the body-fixed frame of the molecule. Such experiments require sufficiently high-repetition rate light sources exceeding the coincidence detection rate, otherwise the assignment of electrons and ions to one or another molecule will be ambiguous. Recent developments of high pulse energy, high repetition rate lasers, tabletop XUV instrumentation, and electron-ion momentum spectroscopy, deliver new capabilities to measure the dynamics of electronic resonances in molecules in unprecedented detail, with electronic-state specificity.

We report our recent progress in measuring ultrafast time and energy-resolved photoelectron angular distributions with femtosecond tabletop XUV pulses generated by high harmonic generation driven by 515 nm femtosecond laser pulses at 55 kHz in krypton and argon gas. We will present the performance parameters and first results of electron-ion coincidence momentum imaging [5] from this novel experimental configuration that enables future measurements of ultrafast time-resolved molecular frame photoelectron angular distributions.

[1] Larsen KA et al. 2020 Phys. Rev A 102 063118

[2] Larsen KA et al. 2020 J. Chem. Phys. 153 021103

[3] Slaughter DS et al. 2021 Phys. Rev. Research 3 033191

[4] Larsen KA et al. 2023 J. Chem. Phys. 158 024303

[5] Shaikh M et al. 2026 in preparation