MULTIPHOTON AND TUNNEL IONIZATION BY AN OPTICAL-FIELD WITH POLAR ASYMMETRY

A simplified form of multiphoton and tunneling ionization is applied t o the problem of ionization of atoms by a bichromatic field consisting of the coherent superposition of a fundamental field and its second h armonic: E(t)=0.5[E1 exp(-iomegat) + E2 exp(-2iomegat)] + c.c. This su perposition possesses a polar asymmetric average of the cube of the fi eld, [E3]=3/8(E1(2) E2 + E1(2)E2*). Two limits of the adiabaticity pa rameter gamma are considered, where gamma=omega(2mI)1/2/(Absolute of v alue of e E) and I is the ionization potential. The case gamma << 1 co rresponds to tunneling ionization by a quasistatic field. Here, the el ectron is released from the barrier with almost. zero velocity at the moment when the magnitude of the field strength Absolute value of E is maximum. Subsequent oscillations of the free electron in the strong b ut adiabatically decreasing field yield some residual velocity. Polar asymmetry of the distribution of that velocity and [v] are calculated as a function of the phase shift, arg(E1(2)E2), between the squared f undamental field E1(2) and its second harmonic E2. The other case, gam ma >> 1, corresponds to multiphoton ionization by the combined fields where n1HBARomega + n(2)2HBARomega almost-equal-to I. Interference of the amplitudes corresponding to opposite parities of the total number of quanta n1 + n2 with the same energy (n1 + 2n2)=const (almost-equal- to I/HBARomega) gives rise to the arg(E1(2)E2)-dependent polar asymme try of emitted electrons. Recent experiments on arg(E1(2)E2)-sensitiv e effects in multiphoton ionization are discussed.