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Journal of Physics B Atomic and Molecular Physics

Stark spectroscopy of forbidden two-photon transitions: a sensitive probe for the quantitative measurement of small electric fields

P P Herrmann, John A. Hoffnagle, Nick Schlumpf, V. L. Telegdi, Antoine Weis

Published May 14, 1986• 20 citations
Publisher

Abstract

The authors have investigated both theoretically and experimentally the intensities of the forbidden two-photon lines 6S 1/2 to 41P 3/2 ,61P 3/2 in an atomic beam of caesium exposed to an electric field. At low field strengths the line intensities depend quadratically on the electric field and are quantitatively well described by Stark mixing parameters obtained from perturbation theory. At higher field strengths the line intensities exhibit an unexpected linear field dependence which is in good agreement with the results from a more general calculation. The authors applied the results obtained to measure the components of a small spurious electric field in a thermionic ionisation detector.

DOI
10.1088/0022-3700/19/9/009

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Journal of Physics B Atomic and Molecular Physics•Sep 28, 1987•16 citations
The authors have investigated both theoretically and experimentally the intensities of the forbidden two-photon lines 6S 1/2 to 41P 3/2 ,61P 3/2 in an atomic beam of caesium exposed to an electric field. At low field strengths the line intensities depend quadratically on the electric field and are quantitatively well described by Stark mixing parameters obtained from perturbation theory. At higher field strengths the line intensities exhibit an unexpected linear field dependence which is in good agreement with the results from a more general calculation. The authors applied the results obtained to measure the components of a small spurious electric field in a thermionic ionisation detector.
The Stark splitting of the second excited level of atomic hydrogen (with principal quantum number n=3n=3n=3) has been measured by pulsed laser spectroscopy in order to determine small electric fields by optical means. Doppler-free excitation with two counterpropagating laser beams at 205nm205\phantom{\rule{0.3em}{0ex}}\mathrm{nm}205nm allows us to measure the n=3n=3n=3 Stark spectra either by an optogalvanic signal after ionization by a third laser photon or by detection of the Balmer-$\ensuremath{\alpha}$ fluorescence photons emitted by the excited atoms. Three different cases of laser polarization have been investigated in order to evaluate the possibilities and limitations of this one-step laser excitation method for electric field determination, especially at field strengths below 200V∕cm200\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}200V∕cm. The method provides the best field sensitivity achieved with lower excited levels that can be reached directly by two-photon absorption from the ground state (in contrast to the more sensitive Rydberg levels). For this purpose, an advanced tunable pulsed uv laser with narrow bandwidth almost at the Fourier limit has been used. The high spectral resolution allowed us to deduce electric field strengths as small as 50V∕cm50\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}50V∕cm from the frequency shift between two selected isolated Stark components. As a main advantage, this simple analysis of the measured spectra does not rely on knowledge of the line intensities which may be strongly affected by the measuring conditions.
For pt.I see ibid., vol.17, p.4135 (1984). Using a previously described new method to evaluate light-induced level shifts, it is experimentally demonstrated that the two possible intermediate 6D 2 states in four-photon resonant five-photon ionisation of mercury are mixed at high laser intensities. Due to this mixing, the AC Stark shifts of these levels are no longer quadratic in field amplitude. This is in good agreement with a simple theoretical model. This makes a direct determination of the oscillator strengths for two mercury lines possible.
We present experimental and theoretical investigations of the He spectral series 2S1−nQ1 (n=3÷9, Q=S,P,D,…,n−1) and 2P1−nQ1 (n=3÷9, Q=S,P,D,…,n−1) in electric fields up to 1635 kV/cm. Apart from the allowed transitions with |ΔL|=1, the transitions with |ΔL|=0,2,3,…—without field strictly forbidden—were observed. Several He patterns become similar to hydrogen patterns, which means they are nearly symmetric and show in higher fields nearly linear Stark shifts. The applied fields are high enough that patterns belonging to neighboring principal quantum numbers (for n>6) begin to overlap, which leads to interesting level-anticrossing effects. The experimental results are compared with numerical calculations, taking into account mixing between states of different principal quantum numbers and between singlet and triplet states. The agreement between experimental and theoretical results is quite good.
We report on an experimental and theoretical study of the dynamic (ac) Stark effect on a forbidden transition. A general framework for parametrizing and describing off-resonant ac-Stark shifts is presented. A model is developed to calculate spectral line shapes resulting from resonant excitation of atoms in an intense standing light wave in the presence of off-resonant ac-Stark shifts. The model is used in the analysis and interpretation of a measurement of the ac-Stark shifts of the static-electric-field-induced $6{s}^{2}\phantom{\rule{0.2em}{0ex}}^{1}S_{0}\ensuremath{\rightarrow}5d6s^{3}D_{1}$ transition at 408nm408\phantom{\rule{0.3em}{0ex}}\mathrm{nm}408nm in atomic Yb. The results are in agreement with estimates of the ac-Stark shift of the transition under the assumption that the shift is dominated by that of the 6s21S06{s}^{2}\phantom{\rule{0.2em}{0ex}}^{1}S_{0}6s21S0​ ground state. A detailed description of the experiment and analysis is presented. A biproduct of this work is an independent determination (from the saturation behavior of the 408-nm transition) of the Stark transition polarizability, which is found to be in agreement with our earlier measurement. This work is part of the ongoing effort aimed at a precision measurement of atomic parity-violation effects in Yb.
Two synchronised pulsed single-mode tunable laser systems are used to excite atomic hydrogen in an atomic beam. A vacuum ultraviolet (VUV) laser populates first the 2p 2 P 3/2 sublevel from the ground 1s 2 S 1/2 state. A second-step UV laser selects substates of the n=33 manifold in the presence of a weak electric field (23.6 V cm -1 ). Stark patterns of this manifold are recorded for different polarisation configurations of the exciting light beams. The n=44 substates are detected by the standard field ionisation technique. The measured relative intensities of the Stark components agree with the theoretical excitation probabilities in the two-step process, calculated to the first order of perturbation theory.