Frequency measurements of hyperfine splittings in ground rovibronic statesof I-2 by stimulated resonant Raman spectroscopy

Measurements of hyperfine splittings in the ground electronic state of I-2 have been performed by stimulated Raman spectroscopy. An argon laser emitti ng at 514.5 nm, drives the coherence between hyperfine levels of the J " = 13 or J " = 15 rotational levels of the ground vibronic state, via resonant excitation of the hyperfine transitions of the optical resonances (43-0) P (13) or R(15). We study the influence of the various experimental parameter s on the line shape: the beam geometry, the laser modulation spectrum, the laser power, the molecular frequency shifts. We show that only beam aberrat ions can give rise to a significant asymmetry of the line shape, which cont ributes to an error in the determination of the resonance frequency. From a theoretical expression of the line shape taking into account the beam geom etry, a detailed study of this error is performed. The theoretical predicti ons and the experimental results are in very good agreement. From the measu rements, improved sets of hyperfine interaction constants for the I-2 molec ule have been calculated for J " = 13 and J " = 15. These constants are ide ntical for both levels, except for quadrupole coupling constant eqQ which e xhibits a J-dependence. which we attribute to the centrifugal distortion of the molecule.