Probing the vibronic coherence of charge migration in N₂ with few-cycle pulses

Charge migration following ionization of a molecule is a primary initiator of molecular dynamics but has been difficult to measure experimentally. One proposed method for observing charge migration is through sequential double ionization (SDI) in a strong laser field, which we employ here. In this method, a pulse which forms the cation is followed by a second pulse to drive the cation through one of several possible pathways to the same final dication. For particular molecular electronic states, this process can produce interference signals as a result of coherence between the ionic states formed by the pump laser pulse. Recent theoretical work using a density matrix approach for SDI predicts that the coherence among multiple electronic states of N₂⁺ following ionization of the N₂ molecular ground state can be observed as oscillations in the dication yield. These oscillations should appear at several distinct frequencies, characteristic of the splitting between the X, A, and B states of the cation. Here, we study the proposed coherence experimentally by initiating SDI with 6 fs pulse pairs with variable pump-probe delays sampled finely enough to resolve the calculated oscillation frequency. This study attempts a direct comparison to theory of using SDI to probe charge migration.