Carrier-envelope-phase-dependent dissociation of hydrogen

Abstract

We studied the dependence of dissociative ionization in H _2 on carrier-envelope phase (CEP) of few-cycle (6 fs) near-infrared laser pulses. For low-energy channels, we present the first experimental observation of the CEP dependence of combined dissociation yield (with protons emitted in both directions), as well as the highest degree of asymmetry reported to date (40%). The observed modulations in both asymmetry and combined yield could be understood in terms of interference between different n -photon dissociation pathways— n and ( n + 1) photon channels for asymmetry, n and ( n + 2) photon channels for yield—as suggested by the general theory of CEP effects (Roudnev and Esry 2007 Phys. Rev. Lett . 99 220406). The yield modulation is found to be π -periodic in CEP, with its phase strongly dependent on fragment kinetic energy (and reversing its sign within the studied energy range), indicating that the dissociation yield does not simply follow the CEP dependence of maximum electric field, as a naïve intuition might suggest. We also find that a positively chirped pulse can lead to a higher dissociation probability than a transform-limited pulse.