Optical orientation of atoms

In the first excited state, the orbital angular momentum is / = ft and can be oriented either antiparallel to the spin angular momentum or parallel to it, forming two states, 

The degree of polarization can be defined as the extent of the accumulation of atoms, with density Nm, in the states with the maximum m value (Cohen-Tannoudji and Kastler 1966)  

Experimentally, the optical orientation of atoms in the ground state is usually observed by means of the change of the polarization of their spontaneous radiation consequent upon optical excitation. 

2 Nonzero nuclear spin. Optical orientation of nuclei 

For atoms with a nonzero nuclear spin, optical pumping with a circularly polarized light makes it possible to orient the nucleus also. There are two cases of orientation of atoms in the ground state (1) the angular (spin) momentum of the electron and the nuclear spin are both other than zero ( Na, Rb, and Cs), so that the 

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Okunevich, A.I. (1981). Excited-state collisional relaxation by optical orientation of atoms with arbitrary electronic angular momentum,... 

The first ideas about the control of atoms by light (for the case of the orientation of atoms) were born even before the advent of the laser. I have in mind the fundamental work by Kastler on the optical control of the orientation of atoms produced by optical pumping (Kastler 1950). This work had an effect not only on the development of the laser, but also on the use of the laser light for the cooling of atoms. I have therefore considered it necessary to treat the optical orientation of atoms. 

Here, for simplicity s sake, the degeneracy of the quantum levels of a two-level system in a real atom is not taken into accoimt. Consideration of the degeneracy of levels due to the angular momentum of an atom in a given quantum state (s,p,d...) and the polarization of radiation (linear, circular, or natural) is essential (see, for example. Foot 2004). This effect is fundamental for the optical orientation of atoms and nuclei (Chapter 4) and the laser cooling of atoms by the polarization gradient method (Chapter 5). 

There are several aspects of the manifestation of coherent effects, both in the optical orientation of atoms and in the use of optically oriented atoms for studies into new... 

Alexandrov, E. B., Bonch-Bruevich, A. M., and Khodovoi, B. A. (1967). Possibihty of weak magnetic field measurements by optical orientation of atoms. Optics and Spectroscopy (Russia), 23, 282-286 [Optics and Spectroscopy, 23, 151—154]. 

Within this historical setting, the actual birth of stereochemistry can be dated to independent publications by J. H. van t Hoff and J. A. Le Bel within a few months of each other in 1874. Both scientists suggested a three-dimensional orientation of atoms based on two central assumptions. They assumed that the four bonds attached to a carbon atom were oriented tetrahedrally and that there was a correlation between the spatial arrangement of the four bonds and the properties of molecules, van t Hoff and Le Bell proposed that the tetrahedral model for carbon was the cause of molecular dissymmetry and optical rotation. By arguing that optical activity in a substance was an indication of molecular chirality, they laid the foundation for the study of intramolecular and intermolecular chirality. 

The use of d- and L-prefixes is a nomenclature for orientation of atomic structure of sugar and amino acid molecules. It is a structural definition and is not related to the optical properties. 

The simplest and most straightforward idea for producing collisionally polarized molecules in a thermal cell consists of using collisions with the participation of particles which are polarized in the laboratory frame. It seems that the earliest one was the method based on collisions of atoms which have been optically pumped (optically oriented in their ground state) by the Kastler method see Section 1.1, Fig. 1.1. If the gas constitutes a mixture of a molecular and an atomic component, the conditions being specially created in such a way as to produce such optical orientation of the atoms, we must expect, from considerations of spin conservation in molecular reactions, that polarization of the molecular component must also emerge. 

Let us first discuss a system which is traditional for optical pumping in the Kastler sense [106, 224, 226], namely an optically oriented alkali atom A (see Fig. 1.1) in a noble gas X buffer surrounding. It is important to take into account the fact that in alkali atoms, owing to hyperfine interaction, nuclear spins are also oriented. However, in a mixture of alkali vapor with a noble gas alkali dimers A2 which are in the 1SJ electronic ground state are always present. There exist two basic collisional mechanisms which lead to orientation transfer from the optically oriented (spin-polarized) atom A to the dimer A2 (a) creation and destruction of molecules in triple collisions A + A + X <—> A2 + X (6) exchange atom-dimer reaction... 

A commonly used technique for the investigation of depolarizing collisions in excited states is based on the orientation of atoms or molecules by optical pumping with a polarized laser and the measurement of the degree of polarization P = (7 - /j )/(/ -I- /j ) of the fluorescence emitted from the optically pumped or the collisionally populated levels (Sect. 8.2). 

The orientation of atoms by way of their optical pumping is a noncoherent process involving spontaneous emission. However, it is possible to transfer angular momentum from light to atoms in a coherent fashion. This possibility is based on off-resonant interaction between circularly polarized light and atoms, giving rise to light-induced... 

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