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Abstract:
The problem of the choice of gauge in the relativistic strong-field approximation (SFA) is analyzed. The main motivation is to obtain a relativistic ionization amplitude in the SFA which in the nonrelativistic limit coincides with the conventional well-accepted results of the SFA in the length gauge. A gauge-invariant formulation of the SFA is derived which is applicable both in the nonrelativistic as well as in the relativistic regime of laser-induced strong-field ionization phenomena. The gauge invariance is achieved by means of employment of an eigenstate of the physical energy operator for the initial atomic state. As an example, a comparison of predictions of the gauge-invariant theory with conventional SFA results in the radiation gauge is given for above-threshold ionization in the relativistic regime.
