Time-resolved Coulomb-explosion imaging of nuclear wave-packet dynamics induced in diatomic molecules by intense few-cycle laser pulses

Abstract

We studied the nuclear dynamics in diatomic molecules (N${}_{2}$, ${\mathrm{O}}_{2}$, and CO) following their interaction with intense near-IR few-cycle laser pulses. Using Coulomb-explosion imaging in combination with the pump-probe approach, we mapped dissociation pathways of those molecules and their molecular ions. We identified all symmetric and asymmetric breakup channels for molecular ions up to ${\mathrm{N}}_{2}^{5+}$, ${\mathrm{O}}_{2}^{4+}$, and CO${}^{4+}$. For each of those channels we measured the kinetic energy release (KER) spectra as a function of delay between the pump and probe pulses. For both ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$ the asymmetric (3,1) channel is only observed for short (20 fs) delays and completely disappears after that. We interpret this observation as a signature of electron localization taking place in dissociating molecular tri-cations when their internuclear separation reaches about 2.5 times the equilibrium bond length. This is a direct confirmation that electron localization plays an essential role in the universal mechanism of enhanced ionization in homonuclear diatomic molecules. Using classical and quantum mechanical simulations of the time-dependent KER spectra, we identify the pathways and intermediate states involved in the laser-induced dissociation of those molecules.