Quasistatic limit of the strong-field approximation describing atoms in intense laser fields: Circular polarization

In the recent work of Vanne and Saenz [Phys. Rev. A 75, 063403 (2007)] the quasistatic limit of the velocity gauge strong-field approximation describing the ionization rate of atomic or molecular systems exposed to linearly polarized laser fields was derived. It was shown that in the low-frequency limit the ionization rate is proportional to the laser frequency omega (for a constant intensity of the laser field). In the present work I show that for circularly polarized laser fields the ionization rate is proportional to omega(4) for H(1s) and H(2s) atoms, to omega(6) for H(2p(x)) and H(2p(y)) atoms, and to omega(8) for H(2p(z)) atoms. The analytical expressions for asymptotic ionization rates (which become nearly accurate in the limit omega -> 0) contain no summations over multiphoton contributions. For very low laser frequencies (optical or infrared) these expressions usually remain with an order-of-magnitude agreement with the velocity gauge strong-field approximation.