Competition between predissociation and autoionization in molecular hydrogen above the ionization threshold: A high-resolution spectroscopy and velocity map imaging study

Abstract

The dynamics of molecular hydrogen above its ionization threshold represent a fundamental prototype for understanding the competition between predissociation and autoionization in superexcited states. We investigate this competition in the 125 600–126 000 cm−1 region using a combined high-resolution extreme ultraviolet pump–ultraviolet (UV) probe spectroscopy and velocity map imaging technique. The core of our methodology lies in the unambiguous distinction between decay channels by comparing product signals with and without the UV probe laser. This enables the first direct, quantitative measurement of state-specific relative branching ratios among predissociation, autoionization, and (1 + 1′) resonance-enhanced multiphoton ionization. Our key findings reveal that predissociation is the dominant decay channel for most of the studied resonances, implying a faster rate compared to autoionization. Furthermore, the high anisotropy parameter (β ≈ +2) extracted from the fragment angular distributions unambiguously confirms that the effective dissociation proceeds via a parallel transition, with the dissociating state having predominant 1Σu+ symmetry. These results provide a definitive, multidimensional experimental dataset that quantifies the competition between quantum decay pathways in a prototype molecular system.