Nonperturbative generalization of the Fermi golden rule

The problem of the ejection of an s electron bound in a short-range potential well by a time-independent external electromagnetic field is considered. A simple formula for the probability of the decay of such a state in a tunneling-like process is derived. This formula is applied to the case of simultaneous action of uniform electric and magnetic fields. It is demonstrated that, in the case of a strong magnetic field, the previously reported results need to be refined.