Low-energy protons from strong-field breaking of hydrogen

For molecules exposed to a strong laser field, upon the releasing of the electrons the bond will break, which is overwhelmingly crucial for grasping the causality of chemical reactions. Previous studies on energy spectra of electrons and nuclear fragments have refreshed the understanding of molecular ultrafast dynamics continuously. Particularly, low-energy protons generated from dissociating H2, one of the well-observed phenomena in light-molecule interaction, have stimulated the investigation of the molecular bond softening. However, the lack of time information makes it hard to unambiguously understand the molecular bond breakage behavior. 

A joint research team led by Prof. Jian Wu from East China Normal University and Prof. Feng He from Shanghai Jiao Tong University explored the generation of low-energy protons in dissociative ionization of H2 in a time-energy-resolved manner. The low-energy protons are identified to be produced via the dissociation from high vibrational states. The researchers constructed a multicycle polarization-skewed laser pulse to trigger and trace the ionization and dissociation of the molecule with extremely high time resolution. 

Analyzing the relationship between the physics information extracted from the molecular frame photoelectron angular distributions and the energy spectrum of the nuclear fragments, the researchers discovered that the low-energy protons are produced via the dipole-transition at large bond lengths, contrary to the well-known bond-softening scenario.

Both classical and quantum simulations on molecular dissociation dynamics verified the experimental observations and the intriguing dissociation pathway. The field-dressed transient electronic states are periodically modulated by a strong near-infrared laser pulse, opening up a one-photon or multiphoton transition channel at large bond lengths.

This work reveals the physical origin of the low-energy proton generation in strong-field dissociation of molecules and highlights the influence of laser-dressed states in strong-field physics. The outstanding time-energy–resolved approach proposed here has great potential to become a standard solution for other complex molecules.