Nonadiabatic tunnel ionization: Looking inside a laser cycle

We obtain a simple closed-form analytical expression for ionization rate as a function of instantaneous laser phase phi (t), for arbitrary values of the Keldysh parameter gamma, within the usual strong-field approximation. Our analysis allows us to explicitly distinguish multiphoton and tunneling contributions to the total ionization probability. The range of intermediate gamma similar to1, which is typical for most current intense field experiments, is the regime of nonadiabatic tunneling. In this regime, the instantaneous laser phase dependence differs dramatically from both quasistatic tunneling and multiphoton limits. For cycle-averaged rates, our results reproduce standard Keldysh-Iike expressions.