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Journal of Physics B Atomic Molecular and Optical Physics

Polarized laser spectroscopy of two-step electric-dipole excitation of atoms

Koichi Nittoh, Akira Kuwako, Tadashi Ikehara, Toshio Watanabe, Itaru Yoguchi, T. Yoshida, Hiroto Adachi

Published Nov 28, 1994• 3 citations
Publisher

Abstract

A simple and accurate method of measuring the total angular momentum J of atomic energy levels is described. The method is essentially based on the J and photon-polarization dependence of the laser-induced two-step excitation in a three-level system. Four different combinations of polarizations in two lasers, which are tuned to each of the two Bohr frequencies, are utilized. They are co-circulating, counter-circulating polarizations and mutually parallel and perpendicular linear polarizations. It has been confirmed that the J-value of a level in the three-level system can be determined uniquely from relative strengths of the fluorescence originating in the top level. The determination of J is carried out by recognition of patterns in the excitation behaviour, which is peculiar to each J-value. The merit of the present method lies in the high reliability inherent in the pattern recognition procedure and also in the relative ease of the experiment.

DOI
10.1088/0953-4075/27/22/011

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A simple and accurate method of measuring the total angular momentum J of atomic energy levels is described. The method is essentially based on the J and photon-polarization dependence of the laser-induced two-step excitation in a three-level system. Four different combinations of polarizations in two lasers, which are tuned to each of the two Bohr frequencies, are utilized. They are co-circulating, counter-circulating polarizations and mutually parallel and perpendicular linear polarizations. It has been confirmed that the J-value of a level in the three-level system can be determined uniquely from relative strengths of the fluorescence originating in the top level. The determination of J is carried out by recognition of patterns in the excitation behaviour, which is peculiar to each J-value. The merit of the present method lies in the high reliability inherent in the pattern recognition procedure and also in the relative ease of the experiment.
An improved method for the angular momentum measurement of autoionizing levels of complex atoms based on photoionization polarization spectroscopy is reported. In this method, a variation of the ion yield is measured as a function of the combination of the polarizations of the lasers used to excite the autoionizing level through a ladder of intermediate resonant states. Here, eight combinations of different polarizations in three lasers are used, which include not only circular but also linear polarizations. It has been confirmed that the angular momentum of the autoionizing level can be determined uniquely by relative ion-yield measurements for any combination of angular momenta of the levels of the excitation ladders by choosing appropriate intensities of the lasers.
Using a pair of lasers near 6000 A, the J = 6 ground state is excited to a J = 6 intermediate level and then to a J = 5 upper state. Non-resonant fluorescences from both excited states are separately detected. Results of a set of model calculations are tabulated. Level 3 average populations are always very small, since the system decays rapidly into inaccessible metastable states. This means that chemistry experiments are difficult. The differences between parallel and perpendicular polarization are modest. The fluorescence dips converge to a value near 50%. The level 3 populations do not approach a common value. This means that the most reliable experimental measurement does not determine the level populations. The most striking changes appear in the last two, most exact, models. The difference between the models is that one includes, for the first time, the linear frequency chirp produced by the fanlike spread of the focussed laser beams. This seemingly innocuous change halves the population left in the ground state. It appears that analogues of adiabatic passage and adiabatic rapid passage respectively are being observed.
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.
Obtaining an accurate value of the electric dipole moment μ is essential in the fields of laser application technologies. A direct way of measuring the electric dipole moment μ is to observe the Rabi-oscillation which manifests itself in the coherent photo-excitation behavior of atoms. In the case of the elements which have large angular momenta, identifying the Rabi-oscillation in their excitation behavior becomes rather difficult. We proposed an accurate and straightforward method of determining the electric-dipole moment μ between multifold degenerate levels. The point is to remove the degeneracy by applying an external magnetic field with the aid of the Zeeman effect and, then, to realize a degeneration free coherent excitation. As a result, we can observe the Rabi-oscillations explicitly in the excitation vs. laser-fluence curves. The present method provides a reliable basis of experimental determination of μ. As an example, we applied the present method to a transition to 0–17,362 cm−1 level in uranium and obtained the value μ=0.86±0.06 (Debye).
Most conventional methods of determining the dipole moment μ are unsatisfactory when applied to transitions between degenerate levels; this is because the overlapping of transitions among magnetic sub‐levels modifies the photo‐excitation behavior in a complex way. To overcome this difficulty, two novel methods of determination are presented. One makes use of differently polarized laser light, and yields a set of excitation curves specific to each polarization. By treating μ as a parameter and fitting the curves through calculations, one can obtain the value of μ. In the other method, a magnetic field is applied to the atom under study in order to raise its degeneracy. As a result, simple Rabi oscillations manifest themselves and one can derive μ directly from their frequencies. These two methods of determining μ are tested on absorption lines in uranium, demonstrating that they work quite well and improve the accuracy of measured dipole moments.