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Applied Spectroscopy

Magneto-Optic Spectroscopy on Neon Using Laser-Induced Forward Scattering

Åsa Marie Lindberg

Published Dec 1, 1994• 1 citation
Publisher

Abstract

The forward-scattered light observed when a single-mode dye laser interacts with the three neon transitions 1 s 5 ( J = 2)-2 p s ( J = 2), 1s 3 ( J = 0)-2 p 5 ( J = 1), and 1 s 4 ( J = 1)-2 p 7 ( J = 1) has been investigated as an applied longitudinal magnetic field is varied. The 1 s 5 and the 1 s 3 states are metastable. This condition allows a comparison of the spectra for the cases 1 s 5 ( J = 2)-2 p 8 ( J = 2) and 1 s 3 ( J = 0)-2 p 5 ( J = 1) in order to discuss whether the complex line shapes obtained for the J = 2 to J = 2 atomic system are due to higher-order coherences or whether they originate from coherences between m = ±1 states. Experimental and calculated line shapes are presented. The calculations have been performed with the use of a semiclassical model for the light/matter interaction. A J = 0 to J = 1 (or a J = 1 to J = 1) model reproduces the overall shapes of the recorded spectra for all three transitions. The results show that, even when laser beam spatial effects and the sample isotopic effects are taken into account, interpretations of forward-scattering spectra in order to determine atomic parameters can reliably be done for simple J = 0 to J = 1 systems only.

DOI
10.1366/0003702944027912

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The forward-scattered light observed when a single-mode dye laser interacts with the three neon transitions 1 s 5 ( J = 2)-2 p s ( J = 2), 1s 3 ( J = 0)-2 p 5 ( J = 1), and 1 s 4 ( J = 1)-2 p 7 ( J = 1) has been investigated as an applied longitudinal magnetic field is varied. The 1 s 5 and the 1 s 3 states are metastable. This condition allows a comparison of the spectra for the cases 1 s 5 ( J = 2)-2 p 8 ( J = 2) and 1 s 3 ( J = 0)-2 p 5 ( J = 1) in order to discuss whether the complex line shapes obtained for the J = 2 to J = 2 atomic system are due to higher-order coherences or whether they originate from coherences between m = ±1 states. Experimental and calculated line shapes are presented. The calculations have been performed with the use of a semiclassical model for the light/matter interaction. A J = 0 to J = 1 (or a J = 1 to J = 1) model reproduces the overall shapes of the recorded spectra for all three transitions. The results show that, even when laser beam spatial effects and the sample isotopic effects are taken into account, interpretations of forward-scattering spectra in order to determine atomic parameters can reliably be done for simple J = 0 to J = 1 systems only.
We present experimental and theoretical investigations of a Gaussian laser beam interacting with Zeeman-tuned J=1\ensuremath{\rightarrow}J=0 atomic systems. In our experiment a single-mode cw dye-laser beam traverses a neon discharge in a longitudinal magnetic field. The excited neon transitions used for these studies are 1s4{\mathit{s}}_{4}s4​(J=1,g=1.464)\ensuremath{\rightarrow}2p3{\mathit{p}}_{3}p3​(J=0) and 1s2{\mathit{s}}_{2}s2​(J=1,g=1.034)\ensuremath{\rightarrow}2p1{\mathit{p}}_{1}p1​(J=0). In the laser-induced forward-scattered light, dark and bright ring-shaped structures are observed. The ring structures are investigated as a function of the magnetic detuning of the m=\ifmmode\pm\else\textpm\fi{}1 sublevels and as a function of the laser-beam power. We find that the line shapes of the spectra, i.e., the forward-scattered light intensities as a function of the magnetic detuning, are sensitive to the transverse intensity distribution of the laser beam. Analytical expressions based on a semiclassical theory are derived and the calculated results on the ring structures and the spectra are compared with the experimental results. Good agreement between the calculated and the experimental results are obtained only when the transverse effects are taken into account.
We present analytical predictions based on a semiclassical theory concerning a single-mode cw laser interaction with longitudinally Zeeman tuned J=1--J=0 atomic systems. In the theoretical model, the laser frequency detuning from the inhomogeneously broadened absorber line center is considered. Equation for the interaction signal are derived for all rotations of a polarizating analyzer transmisson axis with respect to an original laser beam linear polarization. Results are presented for the forward-scattering case when the analyzer transmission axis is orthogonal to the beam polarization. Calculated results on bright and dark ring-shaped light structures are compared with experimental ones originating from a Gaussian beam interaction with a neon 1s4{\mathit{s}}_{4}s4​(J=1, g=1.464)--2p3{\mathit{p}}_{3}p3​(J=0) transition. Calculated results on forward-scattering spectra are presented, i.e., the forward-scattering light intensity as a function of the longitudinal magnetic tuning when the laser frequency is fixed to selected detunings from the absorber line center. Spectra are presented both for the case when the inhomogeneously broadened line originates from a single absorbing isotope and for the case when it originates from overlapping contributions of two isotopes. The results show that laser frequency detunings of tens of megahertz from the absorber line center(s) show up in the forward-scattering spectra. The conclusion is that reliable data on atomic parameters of a nonlinearly absorbing sample can be extracted from experimental spectra only when the sample is monoisotopic and when the laser frequency detuning from the absorber line center is known.
An important effect to be considered in forward-scattering experiments is demonstrated. Ring-shaped structures are observed in the transverse intensity distribution of the forward-scattered light induced by an intense CW single-mode dye laser in an excited neon system. These structures depend upon the laser detuning from the absorber line centre, the magnetic field strength and the neon gas pressure. A preliminary interpretation of the phenomenon is given and its consequences for the interpretation of forward-scattering spectra are pointed out.
Summary form only given, as follows. We investigated the hidden alignment of Ne plasma in a magnetic field observing the optical characteristics of the medium-absorption and refraction. A tunable single frequency laser beam was used as a probe beam. Depending on the direction of the applied magnetic field and the geometry of observation, the dependence of the plasma birefringence or dichroism on the magnetic field was registered. The dye laser was tuned to /spl lambda/=607.4 nm (2p/sub 3/-1s/sub 4/ Ne transition). The dependence on the external magnetic field of the laser beam intensity passed through the Ne discharge cell was registered, for various laser frequencies in the limits of the absorption line. At the edge of the absorption line, the signal of the dichroism consists of two Lorentzian components while at the center of the absorption line the wide component is absent. The wide component width depends on neon pressure as: H(P)=(1.64/spl plusmn/0.03) gauss + P(torr) (0.33/spl plusmn/0.08) gauss/torr. This value is in a good agreement with other results concerning the lifetime of the 1s/sub 4/ state. The mechanism of alignment formation in this state is not clear. This state could not be aligned by light absorption following a quantitative calculation. The irradiation of atoms is isotropic within the limits of all the absorption line because of the high magnitude of absorption for the 1s/sub 4/-1s/sub 0/ transition. We consider the optical pumping as the most probable mechanism for alignment of this state. The magnitude of absorption on the s-p neon transitions are not high. The intensity of the spontaneous emission beam along the tube is greater than in the perpendicular direction. If the quantization axis coincides with the tube axis, this beam empties the magnetic sublevels with M=/spl plusmn/1, thus creating longitudinal alignment. The absence of a signal at the center of the absorption line can be explained by the high selfabsorption on the s-p transitions. The width of the narrow component of the signal is approximately 0.7 gauss. Its pressure dependence is small. We have established experimentally the absence of signals of such width in the spontaneous emission from 2p levels. Hence, all optical mechanisms of alignment transmission from higher or from lower states to the 1s/sub 4/ state are excluded. Apparently this signal arises from alignment of metastable state 1s/sub 5/. In our opinion the collisions are the most probable mechanism of alignment transmission.
A general equation is constructed for the magneto-optical spectra of the forward-scattered light observed through an analyzer when an absorption cell in a homogeneous magnetic field is illuminated by a linearly polarized laser beam at an arbitrary angle to the field direction. The Faraday effect and the Voigt effect are deduced as special solutions of this general equation. For the case of Voigt geometry, \ensuremath{\pi}-polarized light whose plane of polarization is parallel to the applied magnetic field gives a simple absorption profile. \ensuremath{\sigma}-polarized light oscillating perpendicularly to the static magnetic field contains an interference term between right and left circularly polarized light as well as absorption terms. For these eigenwaves (\ensuremath{\pi},\ensuremath{\sigma}) for the Voigt effect, the spectral shapes are independent of the angle of the analyzer, only their intensity is modified by the angle. For the general case, experimental results for cesium vapor and theoretical spectra show good agreement.
Laser-induced Zeeman coherences and non-linear processes in the neon 2p 5 (J = 1) to 1s 5 (J = 2) transition have been studied experimentally and theoretically. By varying the observation geometry and choice of polarisation, the fluorescence observed in the 2p 5 to 1s 3 (J = 0) transition becomes sensitive to different processes within the atom. A detailed theoretical treatment enables profiles to be matched to the experiment using an appropriate choice of parameters. Moreover, analysis of the equations of motion for the system allows the origins of the various features of the fluorescence profiles to be precisely identified in terms of the generation of higher-order Zeeman coherences in the 1s 5 level and coherent trapping of part of the atomic population there. The population trapping can be interpreted and analysed, for a particular choice of geometry, in terms of simpler subsystems of the full transition.