Dissociative Rydberg-state excitation of H2 and CO molecules induced by a femtosecond laser pulse

Rydberg state excitation of atoms and molecules induced by femtosecond laser fields has attracted considerable attention due to its critical role in processes such as neutral atom acceleration and near-threshold harmonic generation. In this study, we experimentally investigate the dissociative Rydberg state excitation of H2 and CO molecules induced by a femtosecond laser pulse centered at 400 nm. Coincidence measurements are conducted for the resulting photoelectrons, ionic, and Rydberg fragments. Electron-nuclear correlations are observed by analyzing the joint electron-nuclear energy spectrum, which shows multiple diagonal lines spaced by photon energy, indicating that the observed dissociative Rydberg state excitations are primarily accessed via multiphoton resonance excitation. The excitation pathways of H2 and CO molecules are explored in detail. Interestingly, the dissociative Rydberg state excitations of H2 and CO molecules exhibit distinct behavior compared to their respective dissociative double ionization channels. This notable difference is attributed to electron-nuclear correlations in photon energy sharing and is further explained through an analysis of the relevant potential energy curves. Our findings contribute to a deeper understanding of the dissociative Rydberg state excitation of molecules in femtosecond laser fields.