Temporal Effects in Multiphoton Ionization of Lithium

The temporal effects of multiphoton ionization are investigated using a two-color method which determines both the time and intensity at which the process occurs. We show that the total ionization efficiency depends strongly on the time at which an atom makes a transition to an excited state during an intense laser pulse. This result clearly shows that the ac-Stark-shifted bound-state resonances not only enhance the cross section for photoionization, but that the excited bound states provide temporary storage states for the atomic population during the photoionization process

Channel Closing in Multiphoton Ionization of Mg

Experimental data are presented showing the channel closing of four-photon ionization of Mg. It is shown that, for circularly polarized light, the ionization versus intensity spectra exhibit sharp breaks from the normal I4 intensity dependence at the critical intensity where the channel closing occurs. Above the critical intensity, the population of Rydberg states which survives the laser pulse is observed. The residual Rydberg population is found to be greatly reduced for linearly polarized light due to the relatively large probability of ionization of the low-angular-momentum Rydberg states. The data are in good agreement with a model which includes averaging over the spatial profile of the laser

Floquet Description of Multiphoton Processes in Li

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 cm-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