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Physical Review Letters

Controlling the Branching Ratio of Photoionization Products under Two-Color Excitation: Competition between ac Stark Splitting and Two-Path Interference

Takashi Nakajima, Jian Zhang, P. Lambropoulos

Published Nov 3, 1997• 11 citations
Publisher

Abstract

We investigate the variation of the photoionization yields into different ionic channels by means of the one-photon-near-resonant two-photon ionization scheme under two-color excitation. We present a general formulation and the results of specific calculations pertaining to the Ca atom with realistic parameters. A significant change of the ionization into ${\mathrm{Ca}}^{+}$ $4s$, $3d$, and $4p$ channels has been observed as a detuning is varied, which agrees qualitatively well with the observation by Wang, Chen, and Elliott [Phys. Rev. Lett. 77, 2416 (1996)] for the Ba atom. The importance of the laser intensity effects is also addressed.

DOI
10.1103/physrevlett.79.3367

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Saturation effects in multiphoton ionization

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We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.

Annual Meeting Optical Society of America•Jan 1, 1985•0 citations
We investigate the variation of the photoionization yields into different ionic channels by means of the one-photon-near-resonant two-photon ionization scheme under two-color excitation. We present a general formulation and the results of specific calculations pertaining to the Ca atom with realistic parameters. A significant change of the ionization into Ca+{\mathrm{Ca}}^{+}Ca+ 4s4s4s, 3d3d3d, and 4p4p4p channels has been observed as a detuning is varied, which agrees qualitatively well with the observation by Wang, Chen, and Elliott [Phys. Rev. Lett. 77, 2416 (1996)] for the Ba atom. The importance of the laser intensity effects is also addressed.
We report photoionization measurements that help us interpret mechanisms leading to the control of the branching ratio for resonantly enhanced, two-color, two-photon ionization experiments. We measure intermediate state populations and photoelectron angular distributions to explore the ionization dynamics. Our measurements support the interpretation that ac Stark shifts are the primary mechanism of the product state control previously observed in a similar experimental setup. We discuss the role of interference in the ionization continuum and the effect of the ac Stark shift on the product state control.
A quantitative analysis of part of the experimental data reported by Wang, Chen and Elliott [1,3] who studied in barium coherent control through two-color resonant interfering paths is reported. Dynamics of the two-color photoionization process, described as an adiabatic process in the rotating wave approximation, is governed by the coherent excitation of the 6s6p and 6s7p 1P1 intermediate states. Interference effects are found to play a minor role. The required atomic parameters are obtained from a theoretical approach based on a combination of jj-coupled eigenchannel R-matrix and Multichannel Quantum Defect Theory.
We present the results of our study on coherent control of photoionization of atomic barium. Our study focused on the understanding of the controllability, especially due to the effect of the coherent interaction between the atomic system and the laser field. The first half of the study investigates the mechanisms of the control behind the previously observed laser phase-insensitive product state control. The controllability of this excitation scheme, two-color two-photon resonantly enhanced excitation, was analyzed from two aspects, the role of ac Stark shift introduced by the strong laser field and the multi-pathway quantum mechanical interferences. We have analyzed the excitation scheme from the analysis of the photoelectron angular distribution measured using the excitation scheme and the monitoring of the intermediate state population. Analysis of the data as well as the numerical simulation showed clear understanding of the role of two mechanisms in the product state control reported. We also investigated the control of the phase lag during the product state control. We conducted the control of the phase lag in the study of asymmetric photoelectron angular distribution, which arises from the concurrent even-odd parity outgoing electron wave excitation. The phase lag was controlled in full range, 2π, and the results were analyzed in terms of the role of autoionizing resonance structures as well as the nature of outgoing electron waves at different locations of the autoionizing resonances.
Three-photon ionization spectra of Ba were studied which result from two-photon resonances between the 6s2{s}^{2}s2 1{\mathrm{}}^{1}1S0{S}_{0}S0​ ground state and the 6snd J=2,6s(n+1)s 1{\mathrm{}}^{1}1S0{S}_{0}S0​ (16\ensuremath{\le}n40) Rydberg states and 5d7d 1{\mathrm{}}^{1}1D2{D}_{2}D2​,3{\mathrm{}}^{3}3P0{P}_{0}P0​,2 perturber states. A tunable laser with field strength ${10}^{\mathrm{\ensuremath{-}}(4\mathrm{\ensuremath{-}}5)}$ a.u. was used to drive the excitation in a low-density atomic beam (n\ensuremath{\lesssim}3\ifmmode\times\else\texttimes\fi{}108{10}^{8}108 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). Resonant line shapes were systematically studied as a function of light polarization and intensity. Studies with linearly polarized light find all resonant line shapes manifest the same peak shift and asymmetry which is traced to the ac Stark shift of the ground state. The resonant peak height, shift, and linewidth scale linearly with light intensity which is accounted for with a rate-equation formulation.For the case of circular polarization, a distinct minimum of ionization occurs between each J=2 doublet of the 6snd series. One level (generally 3D2^{3}\mathrm{D}_{2}3D2​) is significantly broadened in comparison to the laser linewidth (0.1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) with increasing intensity while its sister level remains narrow (\ensuremath{\sim}0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Further, light shifts of the 3D2^{3}\mathrm{D}_{2}3D2​ states are generally \ensuremath{\gtrsim}1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to the red while shifts of 1D2^{1}\mathrm{D}_{2}1D2​ states are negligible. This behavior switches for the n=27 doublet. Lastly, the dynamic shifts are not quadratic with the field strength. These effects are attributed in part to cancellation due to channel interference and a two-photon Raman process which mixes each J=2 doublet. These processes act in conjunction with the ac Stark effect to produce the observed profile behavior. The shifts and coupling of the excited states are attributed to the influence of the 6pnd ${J}^{\ensuremath{\pi}}$${=3}^{\mathrm{\ensuremath{-}}}$ autoionization series and are shown to be sensitive to the singlet-triplet mixing coefficient \ensuremath{\beta} of the J=2 doublets. A perturbation analysis is introduced which accounts for the salient features of the data, although it does not include bandwidth and saturation effects. Calculations of light shifts of the 6snd J=2 levels based on this analysis require \ensuremath{\beta} be modified from previous assignments based on hfs measurements for n in the vicinity of the 5d7d 1{\mathrm{}}^{1}1D2{D}_{2}D2​ perturber.
We have demonstrated control over the branching ratio for photoionization into two different continuum channels of atomic barium using an interference between two distinct photoexcitation routes. Each pathway for photoionization is a two-photon process, and is resonantly enhanced by an intermediate state. We have found the unexpected situation of destructive interference when both lasers are resonant with their respective transitions. The magnitude of the interference, as well as the exceptionally broad wavelength range over which the interference persists, is also quite surprising.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.