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Physical Review A

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

Matthieu Génévriez, Xavier Urbain, Mohand Brouri, A. P. O’Connor, Kevin M. Dunseath, Mariko Terao-Dunseath

Published May 30, 2014• 2 citations
Publisher

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 ${M}_{L}=0$.

DOI
10.1103/physreva.89.053430

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A joint experimental and theoretical study of four-photon ionization of Li is performed using a pump-probe setup with the laser frequency tuned in between the excitation energies of the 2s2s2s--4p4p4p and 2s2s2s--4f4f4f three-photon transitions. The three-dimensional photoelectron momentum pattern is recorded as a function of the delay time between two otherwise identical laser pulses. The process is modeled by numerically solving the time-dependent Schr\"odinger equation. While complicated interference patterns arise in the electron spectrum when the two pulses partially overlap in time, the expected quantum-beat phenomenon between Li atoms in either the 4p4p4p or the 4f4f4f state are seen when the fast oscillations involving the 2s2s2s initial state are averaged over. The qualitative agreement between the predicted and observed beating patterns is very good.
We have made several different types of measurements of the three-photon ionization of Li produced by 3-ps laser pulses and describe the results using a Floquet picture. Over the photon frequency range 15 000 to 15 800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, Li represents a strongly coupled three-state system with the 2s ground state coupled to the 2p and 3d states by one and two photons, respectively. Energy analysis of the photoelectrons allows the measurement of the intensity dependent shift of the 2s Floquet state during the laser pulse. The shift shows a strong frequency dependence that is not predicted by first-order perturbation theory. We have also measured the total ionization spectrum over several ranges of frequency, as well as the angular distribution of the ionization and the first above-threshold ionization peak for frequencies where the ground state is near resonance with the 4s and 4d excited states. Calculations based on the Floquet Hamiltonian indicate that all of these processes may be understood in terms of a Floquet description.
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