THRESHOLD AND COULOMB EFFECTS IN MULTIPHOTON IONIZATION

The effect of the long-range Coulomb potential on a very-low-energy final state of ionization is known to be important in a variety of atomic reactions. It is here extended to intense field multiphoton ionization. A trial wave function which is a generalization of the Coulomb-Volkov state is chosen with a parameter which is determined variationally. It is then used in a form of the S matrix and yields ionization in the polarization direction (for linear polarization) which is of the order 10(8) larger than that obtained for the Volkov final state. For larger angles of ionization the Coulomb contribution falls off rapidly but still dominates the total rate. This enhancement is not the result of the usual amplification of the wave function at the origin by the long-range Coulomb potential.