Physical Review A

Product-state control in barium

Rekishu Yamazaki, Daniel S. Elliott

Published Feb 15, 2006• 2 citations

Abstract

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.

DOI: 10.1103/physreva.73.023415

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Physical Review A

Product-state control in barium

Rekishu Yamazaki, Daniel S. Elliott

Published Feb 15, 2006• 2 citations

Publisher

Abstract

DOI: 10.1103/physreva.73.023415

Related papers

Product-state control through interfering two-photon ionization routes

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We have extended our studies of control of the branching ratio for photoionization of atomic barium. This control is based on an interference between two distinct photoexcitation routes, each of which is a resonantly enhanced two-photon process. We report our experimental studies of the dependence of the interference on atomic beam density, laser power, selection of intermediate states, and relative polarization of the two field components. We observe that the potential for product control increases dramatically with increasing atomic beam density and depends strongly on the laser pulse energies as well.We have extended our studies of control of the branching ratio for photoionization of atomic barium. This control is based on an interference between two distinct photoexcitation routes, each of which is a resonantly enhanced two-photon process. We report our experimental studies of the dependence of the interference on atomic beam density, laser power, selection of intermediate states, and relative polarization of the two field components. We observe that the potential for product control increases dramatically with increasing atomic beam density and depends strongly on the laser pulse energies as well.

Physical Review AOct 1, 199710 citations

Coherent control of photoionization of atomic barium

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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.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.

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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.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.

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We present results from a time-dependent two-level-plus-continuum model of multiphoton ionization in order to better understand the short-pulse photoelectron spectrum resulting from multiphoton ionization and the role of intermediate states. Both resonant and nonresonant population transfer to the upper state are found to play an important role in the multiphonon ionization process. The dynamic modeling reveals the potential for broadening and shifting peaks in the photoelectron spectrum. However, we conclude that despite the complex dynamics of the excitation, short-pulse photoelectron spectra can be used to measure ac Stark shifts of both ponderomotively and nonponderomotively shifted states in strong laser fields.We present results from a time-dependent two-level-plus-continuum model of multiphoton ionization in order to better understand the short-pulse photoelectron spectrum resulting from multiphoton ionization and the role of intermediate states. Both resonant and nonresonant population transfer to the upper state are found to play an important role in the multiphonon ionization process. The dynamic modeling reveals the potential for broadening and shifting peaks in the photoelectron spectrum. However, we conclude that despite the complex dynamics of the excitation, short-pulse photoelectron spectra can be used to measure ac Stark shifts of both ponderomotively and nonponderomotively shifted states in strong laser fields.

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Path dependence in resonant multiphoton excitation to autoionizing states of barium

James S. Keller, John E. Hunter, R. Stephen Berry

We measure photoion spectra and angular distributions of photoelectrons from the two-color, two-photon resonant ionization of barium atoms in order to probe the effect of the photoionization pathway on autoionization processes. The (6s6p ${)}^{1}$ P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ and the (5d6p ${)}^{1}$ P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ states are used as intermediates in the photoexcitation of barium to energies in the range of a series of 5dnd autoionizing states. Dramatic differences in the profiles of autoionizing lines are observed for excitation pathways involving different intermediate states. Angular distributions of photoelectrons collected across the range of autoionizing resonances demonstrate strong variations that follow constraints imposed by the angular momentum of the autoionizing peaks and also suggest significant spin-orbit interactions in the photoionization step.We measure photoion spectra and angular distributions of photoelectrons from the two-color, two-photon resonant ionization of barium atoms in order to probe the effect of the photoionization pathway on autoionization processes. The (6s6p${)}^{1}$P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ and the (5d6p${)}^{1}$P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ states are used as intermediates in the photoexcitation of barium to energies in the range of a series of 5dnd autoionizing states. Dramatic differences in the profiles of autoionizing lines are observed for excitation pathways involving different intermediate states. Angular distributions of photoelectrons collected across the range of autoionizing resonances demonstrate strong variations that follow constraints imposed by the angular momentum of the autoionizing peaks and also suggest significant spin-orbit interactions in the photoionization step.

Physical Review AMar 1, 199119 citations

{\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.

{)}^{1}

P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ and the (5d6p

{)}^{1}

P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ states are used as intermediates in the photoexcitation of barium to energies in the range of a series of 5dnd autoionizing states. Dramatic differences in the profiles of autoionizing lines are observed for excitation pathways involving different intermediate states. Angular distributions of photoelectrons collected across the range of autoionizing resonances demonstrate strong variations that follow constraints imposed by the angular momentum of the autoionizing peaks and also suggest significant spin-orbit interactions in the photoionization step.