Nonadiabatic atomic ionization by intense subcycle laser pulses

The atomic photoionization by a subcycle linearly polarized laser pulse is studied with numerical solutions of the time-dependent Schrödinger equation for a hydrogenlike atom. In this regime, the presumption for the adiabatic ionization theories that atoms are ionized directly from an initial bound state to the continuum state will fail. The nonadiabatic ionization channels turn out to play important roles: the bound electrons can climb up the energy ladder and get ionized from a certain bound state other than the original one. This process leaves significant fingerprints in carrier-envelope-phase-sensitive phenomena like total ionization yield and momentum asymmetry of the photoelectrons. For the modeling of subcycle pulses, the inconsistency of the popular vector potential definition is noticed and a modified version with an analytic envelope is presented that is capable of describing pulses with arbitrary pulse width.