Bloch-Siegert shifts and widths for multiphoton resonances: In-plane field configuration

Bloch-Siegert shifts and widths of resonances in two-level systems are evaluated within an adiabatic approximation applicable in the multiphoton regime. The level splitting is controlled by a static field. Most attention is paid to the case of a circular driving field with the static field in the plane of rotation. This nonconventional setup was used in a recent experiment on intrashell resonances in Rydberg atoms where shifts and widths for N-photon resonance with N as large as 23 were observed [D. Fregenal et al., Phys. Rev. A 69, 031401(R) (2004)]. For this in-plane configuration the resonance widths are drastically enhanced compared to their values in the conventional configuration (linearly polarized driving field perpendicular to static field), whereas the shifts are only weakly affected.