Effect of pressure-dependent quantum interference on the ac Stark shifting of two-photon resonances.

Abstract

We describe ac Stark shift production and measurement in a two-laser mode where a first laser is tuned through a two-photon resonance between the ground state and an excited state in order to record a line shape for the corresponding resonantly enhanced multiphoton ionization. A second laser is tuned to a fixed wavelength which is close to a resonance between the upper state in the two-photon transition and a third state which has dipole-allowed transitions back to the ground state. At low concentrations the second laser leads to very large ac Stark shifts in the two-photon excitation and in the related resonantly enhanced multiphoton ionization. However, at high concentrations a destructive interference occurs involving the three-photon coupling between the third state and the ground state and the one-photon coupling due to the four-wave-mixing field at frequency 2[omega][sub [ital L]1][plus minus][omega][sub [ital L]2]. This interference leads to a pressure-dependent suppression of the amplitude for the third state. When this destructive interference occurs, the ac Stark shift due to the second laser undergoes a corresponding pressure-dependent suppression. The criteria for the pressure to be high enough to completely suppress the ac Stark shift is that the real part of phase mismatch [vertmore » bar][Delta][ital k][vert bar] at the angular frequency 2[omega][sub [ital L]1][plus minus][omega][sub [ital L]2] and (or) the absorption coefficient [beta] at the same frequency should be very large, so that with unfocused focused beams [vert bar][Delta][ital k][vert bar][ital L][gt]5[[Delta][sub [ital s]][sup (max)][tau]][sup 1/2], where [ital L] is the thickness of gas penetrated before the shift is measured, [Delta][sub [ital s]][sup (max)] is the maximum ac Stark shift at very low pressure, and [tau] is the pulse length of the lasers.« less