Fine structure splitting, alkali atoms

In light alkali atoms, Li and Na, the fine structure splitting of a low state is typically much larger than the radiative decay rate but smaller than the interval between adjacent states. In zero field the eigenstates are the spin orbit coupled tsjnij states in which and s are coupled. However, in very small fields and s are decoupled, and the spin may be ignored. From this point on all our previous analysis of spinless atoms applies. How the passage from the coupled to the uncoupled states occurs depends on how rapidly the field is applied. It is typically a simple variant of the question of how the m states evolve into Stark states. When... 

As early as in 1925, Goudsmit and Uhlenbeck revealed that certain lines of the optical spectra of hydrogen and the alkali atoms are composed of a closely spaced pars of lines. They supposed that this fine structural splitting is related to an intrinsic angular momentum of an electron. 

Soh Sohlberg, K., Yarkany, D.R. On the origin of the heavy atom effect in the fine structure splitting of the 1 state of alkali metal 2p-rare gas Van der Waals molecules, J. Chem Phys. 107 (1997) 7690— 7694. 

Table 2.1. Fine-structure splittings in states of the alkali atoms... 

The observation of spectral lines under high resolution revealed that many possessed a fine structure, and this led to the concept of electronic sublevels. These were named s, p, d and f levels, the letters having their origin in the atomic spectra of the alkali metals in which four series of lines were observed, which were known as sharpy principal, diffuse and fundamental. In 1896 some lines had been found to be split in a magnetic field by Pieter Zeeman (1865-1943), and this phenomenon was now explained in terms of electron spin. Each electron was now described in terms of four quantum numbers principal (n), orbital (/), magnetic m) and spin (5). In 1925 Wolfgang Pauli (1900-1958) put forward his exclusion principle, which stated that no two electrons in a given atom could have all four quantum numbers the same. 

In Chap. 7 we have discussed how hyperfine structure can be determined by level-crossing spectroscopy. Clearly, alkali atom states can readily be studied using this technique after stepwise excitation. We will here instead choose an example illustrating fine-structure measurements. In Fig. 9.17 the example of the inverted sodium 4d 5/2,3/2 state is given. From the measured level-crossing positions the fine-structiue splitting can be calculated using the Breit-Rabi formula for the fine structure, (2.31). 

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