Experimental and theoretical study of three-photon ionization of He(1s2p3Po)

Abstract

A joint experimental and theoretical study of three-photon ionization of the $1s2p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}^{o}$(${M}_{L}=0,\ifmmode\pm\else\textpm\fi{}1$) states of helium is presented. The ion yield is recorded in the 690--730 nm wavelength range for different laser pulse energies, using an excited helium beam produced by photodetachment of helium negative ions. Two series of asymmetric peaks due to two-photon resonances with $1snp$ and $1snf$ Rydberg states are observed. In one series, the peaks have tails towards higher frequencies, while in the other series the tails change direction for higher Rydberg states. An effective Hamiltonian is built in the dressed state picture, and a numerical model simulating the traversal of the helium atom across the laser pulse is developed. The simulated and observed ion yields are in good qualitative agreement. The observed behavior is shown to result from the contributions of two different resonantly enhanced multiphoton ionization processes, depending on the magnetic quantum number ${M}_{L}$ of the initial state. The asymmetry reversal is explained by the strong $1s2p$--$1s3s$ dynamic Stark mixing for ${M}_{L}=0$.