? doubling in 2? states of diatomic molecules

The Lambda doubling constants of the 2 Pi states of OH, BeH, CH and NO are calculated from Hartree-Fock wavefunctions to test the validity of Van Vleck's hypothesis of 'pure precession'. Excellent agreement with experiment is found for OH and BeH while for CH the discrepancies are relatively small. In the case of NO, however, no agreement is found and it is concluded that the hypothesis only holds when the molecular situation resembles that in atoms.

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