Spin-orbit relaxation of highly excited species

5 SPIN-ORBIT RELAXATION OF HIGHLY EXCITED SPECIES 

The rate of spin-orbit relaxation of NO (AT2/ ) has been measured using ultrasonic absorption by Bauer et al. 6, and is included in Table 11. 

Krause et a/.123-125 have recently reported a series of measurements of the spin-orbit relaxation of the alkali metals in their first excited states (2P). The technique, for example for atomic caesium with AE = 554 cm-1, consists of irradiating the metal vapour with light from a monochromator to excite only one of the 2P states. The vapour pressure of the metal is controlled at 10-6 torr to avoid imprisonment of the resonance radiation. The components of the fluorescence light are measured with a photomultiplier by isolating the 2P - 2S lines with interference filters. In the presence of added gases which cause the transitions 

Spin-orbit relaxation of Rb(52P) appears to be abnormally slow in all the inert gases except helium (cross-section 10 17 cm2). In fact, according to Pitre et a/.123, the relaxation rate in Kr and Xe is slower than for the equivalent transitions of atomic Cs, which correspond to a three-fold larger change in internal energy. Pitre et al. discuss complications in the rubidium experiments, including the formation of van der Waals complexes with the inert gases, in order to account for the apparently abnormal relaxation rates. Efficient removal of Rb(5 2P) by radiationless processes could upset the derived rate coefficients. The results were discussed in relation to Zener s semi-classical equivalent of equation (14). 

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