Probing ultrafast nuclear dynamics in dissociative double ionization of H2 and D2 using a multicycle elliptically polarized femtosecond laser pulse

Ultrafast nuclear dynamics in laser-induced dissociative double ionization of H2 is challenging to investigate with a multicycle elliptically polarized femtosecond laser pulse due to the difficulty of distinguishing electrons released in neighboring optical cycles. In this study we experimentally reveal these dynamics by employing the same laser technique and overcoming this limitation through comparison with the isotopic counterpart of D2. By analyzing the internuclear-distance-dependent sum momentum of the ionic fragments, we reconstruct the time-dependent nuclear motions with subcycle temporal resolution. Furthermore, we observe a decrease in the momentum magnitude of the second released electron with increasing internuclear distance, attributed to the combined effects of the instantaneous laser field strength and the Coulomb attraction of the molecular ion at the moment of ionization. Our findings highlight the critical role of isotope effects in elucidating ultrafast nuclear dynamics in laser-induced molecular fragmentation.