Stimulated Raman transitions from the metastable 2s state of hydrogen

Abstract

We consider the hydrogen atom H() exposed to a short laser pulse with a central frequency ranging from 136 eV to 1.5 keV ( au) at the intensity of W cm−2. We study stimulated Raman scattering transitions to the ground state (anti-Stokes) and to the upper ns, np and nd states (with ) (Stokes). Nondipole (retardation) effects are included up to (c is the speed of light in a vacuum). The calculation of the transition probabilities, based on the integration of the time-dependent Schrödinger equation, is confronted with results obtained by applying perturbation theory. The two methods are in very good agreement. We show that retardation effects play an important role for frequencies larger than a few hundred eV and pulse durations of the order of the femtosecond. In this regime, as the frequency increases, the contribution of the term dominates over (where and denote the vector potential of the field and the momentum operator, respectively). Results are presented for various values of the pulse frequency and duration.