Above-threshold ionization of cesium by 1.9-µm light

Multiphoton ionization of cesium atoms has been studied with both linearly and circularly polarized light pulses of 1.9-µm wavelength. Photoelectron energy spectra show that the use of circular polarization leads to a strong reduction of above-threshold ionization (ATI), in comparison to linear polarization, and not to suppression of low-order ATI peaks. This behavior is different from that expected for hydrogen under similar circumstances. Calculations reproduce the behavior of cesium and indicate that it can be attributed to the large size of the ground state of cesium.