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Physical Review Letters

Spectroscopy of negative ions utilizing multiphoton detachment in a Raman coupling regime

Philip Trøst Kristensen, Henrik Stapelfeldt, Péter Balling, T. Andersen, Harold K. Haugen

Published Nov 22, 1993• 16 citations
Publisher

Abstract

Multiphoton detachment of ${\mathrm{Te}}^{\mathrm{\ensuremath{-}}}$ is studied utilizing two optical fields of wavelengths \ensuremath{\sim}694 and \ensuremath{\sim}1064 nm, respectively. Three-photon detachment is resonantly enhanced when the Raman condition is satisfied for coupling of the ${\mathrm{Te}}^{\mathrm{\ensuremath{-}}}$ 5${\mathit{p}}^{5}$ $^{2}$${\mathit{P}}_{3/2}$ and $^{2}$${\mathit{P}}_{1/2}$ fine-structure states. The fine-structure splitting is determined to be 5004.7\ifmmode\pm\else\textpm\fi{}0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which represents a 25-fold improvement over earlier measurements. The possibilities for extended spectroscopic measurements, as well as general perspectives for multiphoton experiments at high intensities, are briefly discussed.

DOI
10.1103/physrevlett.71.3435

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Two-laser multiphoton ionisation under the autoionising-like resonance condition is studied theoretically within an extended atomic model dictated by the experiment of Feldmann et al (1986). Ordinary coupling between a selected pair of bound states via the continuum only is supplemented in the author's model by Raman-type coupling involving the atomic spectrum below the ionisation threshold. In the case considered this additional Raman coupling enhances the ionisation rate to a much greater extent than the coupling via the continuum can do. The Raman coupling modifies the autoionising-like shape parameter and is decisive as far as the resonance peak profile is concerned. Exploiting this Raman coupling within the atomic model with four effective bound states, the author gives explanations for all the details of the ionisation spectrum detected by Feldmann et al, i.e. the number of resonance peaks, their strength, fine structure, shape, relative heights and sensitivity to the laser field.

Journal of Physics B Atomic and Molecular Physics•Oct 14, 1987•6 citations
Multiphoton detachment of Te\ensuremath−{\mathrm{Te}}^{\mathrm{\ensuremath{-}}}Te\ensuremath− is studied utilizing two optical fields of wavelengths \ensuremath{\sim}694 and \ensuremath{\sim}1064 nm, respectively. Three-photon detachment is resonantly enhanced when the Raman condition is satisfied for coupling of the Te\ensuremath−{\mathrm{Te}}^{\mathrm{\ensuremath{-}}}Te\ensuremath− 5p5{\mathit{p}}^{5}p5 ^{2}$${\mathit{P}}_{3/2} and ^{2}$${\mathit{P}}_{1/2} fine-structure states. The fine-structure splitting is determined to be 5004.7\ifmmode\pm\else\textpm\fi{}0.2 cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\ensuremath−1, which represents a 25-fold improvement over earlier measurements. The possibilities for extended spectroscopic measurements, as well as general perspectives for multiphoton experiments at high intensities, are briefly discussed.
Efficient excess-photon detachment to lowest nonvanishing order is observed with long (\ensuremath{\sim}0.7 ns) optical pulses and intensities of \ensuremath{\le}6\ifmmode\times\else\texttimes\fi{}109{10}^{9}109 W/cm2{\mathrm{cm}}^{2}cm2. A Feshbach (``window'') resonance in ${\mathrm{Cs}}^{\mathrm{\ensuremath{-}}}$ suppresses single-photon detachment, while simultaneously enhancing the probability of absorption of one or more excess photons. The perspectives for future spectroscopic excess-photon detachment and studies of direct two-electron multiphoton detachment are briefly discussed.
The aim of this thesis is to extend the study of multiphoton processes to the heavier negative ions of the alkali-metal atoms and thus, Na"- and K"- were natural candidates for this investigation. The main idea is to achieve an homogeneous description of negative alkaline ions and their response to an intense laser field. This idea has been suggested by recent experimental findings for negative alkaline ions which have revealed a rich structure of their scattering and photodetachment spectra, showing the existence of high-lying temporary negative ion states. Moreover, dramatic effects due to excess photon detachment have been observed yielding additional information on the dynamics of these highly correlated systems
We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5s2{\mathit{s}}^{2}s2 is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}1010{10}^{10}1010 W/cm2{\mathrm{cm}}^{2}cm2) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.
Multiphoton excitation with elliptically polarized light brings out more information in the photodetachment or photoionization processes than linearly polarized light. This information can be reached by recording photoelectron angular distributions for several values of the ellipticity angle, and fitting the whole set of data with an analytic formula. We give here the appropriate formula, valid for any number N of exciting photons, with the assumptions of (1) an initially even and momentumless (J equals 0) atom or ion and (2) that after excitation of an electron, the residual core be left in an odd state. These are the conditions met by multiphoton detachment from halogen negative ions. With elliptically polarized light of (lambda) equals 1064.2 nm, we have measured the angular distributions of three-photon detached electrons from the ion I-, and four-photon detached electrons from Cl- and F-. The experimental results are quantitatively analyzed and commented on.
Two-laser multiphoton ionisation under the autoionising-like resonance condition is studied theoretically within an extended atomic model dictated by the experiment of Feldmann et al (1986). Ordinary coupling between a selected pair of bound states via the continuum only is supplemented in the author's model by Raman-type coupling involving the atomic spectrum below the ionisation threshold. In the case considered this additional Raman coupling enhances the ionisation rate to a much greater extent than the coupling via the continuum can do. The Raman coupling modifies the autoionising-like shape parameter and is decisive as far as the resonance peak profile is concerned. Exploiting this Raman coupling within the atomic model with four effective bound states, the author gives explanations for all the details of the ionisation spectrum detected by Feldmann et al, i.e. the number of resonance peaks, their strength, fine structure, shape, relative heights and sensitivity to the laser field.