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Journal de Physique Lettres

Polarization of calcium atomic fluorescence due to a coherence effect in the photodissociation of Ca2 molecules

J. Vigué, Philippe Grangier, G. Roger, Alain Aspect

Published Jan 1, 1981• 46 citations
Publisher

Abstract

A strong degree of polarization (64 %) has been observed in the fluorescence from calcium atoms excited by the photodissociation of Ca2 molecules in their ground state. This large value, at variance with the predictions by Van Brunt and Zare, is explained by taking into account a coherence effect between the sublevels of the excited state.

DOI
10.1051/jphyslet:019810042024053100

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Journal de Chimie Physique•Jan 1, 1997•0 citations
A strong degree of polarization (64 %) has been observed in the fluorescence from calcium atoms excited by the photodissociation of Ca2 molecules in their ground state. This large value, at variance with the predictions by Van Brunt and Zare, is explained by taking into account a coherence effect between the sublevels of the excited state.
We present a theoretical analysis of a previous experiment, in which a modulation in the time-resolved atomic fluorescence light following the photodissociation of Ca2 molecules by a pulsed laser was observed. First a calculation is performed on a simple model, involving two atoms coupled with the quantized radiation field. Then the validity of this model is discussed on molecular physics grounds. Finally, a quantitative analysis of the experiment is presented ; a good agreement is obtained if quasi-bound levels of the molecular ground state are taken into account.
In this paper a general quantum mechanical treatment of photodissociation and fluorescence from the fragments in the case of direct dissociation as well as in the case of photo-predissociation is presented. This formulation takes into consideration all the possible coherence effects coming from quantum interference between photodissociation amplitudes to different magnetic sublevels of the fragments. This results in large differences between classical and quantum results for the degree of polarization of the light emitted from photofragments. The theory is applied to the case of Ca2 which constitutes a striking experimental example of coherence effects in direct photodissociation. For this case the classical calculation predicts a polarization degree of 14 % while the experiment shows 64 %. The maximum theoretical value when coherence is taken into account is 78 %. The difference between the experimental result and the theoretical value is attributed to depolarization by saturation, although some other minor contribution may also be present.
Circular dichroism has been detected in the polarization of the fluorescence resulting from the decay of excited calcium ions. Synchrotron radiation was used to both ionize and excite calcium atoms from their ground state, and the photoejected electron was detected in coincidence with the polarized fluorescent photon. By introducing a degree of left, then right circularly polarized light into the incident photon beam the circular dichroism could be measured. The analysis of the results was carried out using recent developments of the statistical tensor method, and it was possible to determine the sign of the phase difference between the outgoing waves.
In photodissociation of a homonuclear diatomic molecule, conservation of linear and angular momenta requires that each atomic fragment follow identical but oppositely directed trajectories with respect to the molecular center of mass. For certain values of the incident photon energy, one of the atoms will be electronically excited while the other will be in its ground state. When a detector records the undispersed fluorescence from the decay of the excited atom it cannot distinguish which trajectory the excited atom followed and hence whether the photon possesses a blue or red Doppler shift corresponding to recoil towards or away from the detector. According to the principle of superposition in quantum mechanics, the wave function of the photon must be a sum over two paths corresponding to emission from an excited atom in either of the two recoil directions. Detection of such photons gives rise to an interference between the probability amplitudes for the two paths that is manifested in the output from the photodetector as a beat frequency. Since photodissociation of an ensemble of molecules gives rise to a distribution of recoil directions relative to the electric vector of the incident radiation, a range of beat frequencies must be present in the detected signal. This paper gives a calculation of expected output voltage waveforms from a photodetector viewing atomic fluorescence excited by molecular photodissociation. Expressions are given for partial Fourier transforms of the photodetector signals. The results show that the shape of the voltage waveforms depends strongly on the asymmetry parameter for recoil of the photofragments and on whether or not the magnetic sublevels of the excited atomic state are created in a coherent superposition.
Atomic fluorescence that accompanies the photodissociation of Ca2 molecules is modulated in time. This modulation was attributed to an interference between the fields emitted by the two atoms [1,2]. The effect of radiative resonant coupling between the atoms was considered to be negligible. It was suggested that the modulation of emission is analogous to the interaction of a single photon with two slits. However, the present article demonstrates that radiative resonant coupling is the main cause of the modulation, and the effect is not analogous to the interaction of single, space-time separated photons with two slits.