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Optics and Spectroscopy

Polarization characteristics of radiation of an ensemble of excited particles in a strong magnetic field

A. G. Petrashen

Published May 1, 2007• 0 citations
Publisher

Abstract

The induction of oriented states of angular momenta is shown to be possible under an axially symmetric excitation in a strong magnetic field.
DOI
10.1134/s0030400x07050025

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We know now how alignment and orientation of angular momenta arise in the excited state as a result of optical excitation from the primarily isotropic distribution of the ensemble of angular momenta. This phenomenon reflects, in essence, the anisotropy of absorption probability G (θ,ϕ). Since spatial structures of the type shown in Fig. 2.2 appear in the excited state the same structure must be ‘eaten out’ from the angular momentum distribution in the ground (initial) state. If the gap created is not closed quickly enough, the ensemble of the remaining angular momenta of the absorbing molecules in the ground state must necessarily also be polarized. If, from another point of view, fluorescent decay leads to a considerable population of the ground state level c, (see, e.g., Fig. 1.2), then molecules on this level may exhibit angular momenta polarization, being particularly noticeable for thermally unpopulated, high lying levels. In the present chapter we intend to discuss the conditions necessary for the creation of ground state angular momenta polarization, the possibilities of its experimental observation, and to develop the theoretical description of broad line radiation interaction with molecules further. Angular momenta polarization via depopulation For many years there existed the widespread opinion that in molecules we obtain an anisotropic distribution of angular momenta only in the excited state in the absorption process, whilst in the ground state isotropy is conserved [124]. This conjecture is based essentially on the assumption of the existence of a so-called ‘weak’ excitation. At first sight it may appear, for instance, from Eq. (2.20) that, as is usual in non-linear spectroscopy, such an admission is correct if condition Γ p /Γ ≫ 1 is valid.
The mixing of state multipoles for an atomic state with unresolved fine structure in a magnetic field is investigated in its simplest case: the conversion of alignment to orientation. The combined action of the external field and the spin-orbit coupling results in a build-up of orientation and hence in the emission of circularly polarised light although the excitation process is axially symmetric and is invariant with respect to the reversal of the symmetry axis and thus the system could only show alignment initially. The case investigated is the collisional excitation of the He 3 3 D state by He + ions. After correction of some errors in the theoretical literature the agreement between the calculated and the observed circular polarisation of the fluorescence radiation from the collisionally excited He atoms is very good. Owing to possible objections in two previous experiments this is the first unambiguous demonstration of the alignment to orientation conversion effect.