High-precision spectroscopy of molecular iodine: From optical frequency standards to global descriptions of hyperfine interactions and associated electronic structure

A widely tunable and high-resolution spectrometer based on a frequency-doubled Ti:sapphire laser is used to explore sub-Doppler transitions of molecular iodine in the wavelength range 523--498 nm. We investigate the natural width of the hyperfine components at various transitions and its wavelength dependence is mapped out in this region. The narrowest natural width observed is ∼52 kHz near 508 nm. The observed excellent signal-to-noise ratio should lead to high-quality optical frequency standards that are better than those of the popular 532-nm system. In addition, we employ a self-referenced femtosecond optical comb to measure the absolute frequency of the length standard at 514.67 nm, which is based on the a3 hyperfine component of transition P(13) 43-0. This technique improves the precision of the frequency measurement by two orders of magnitude as compared with previous wavelength-based results.; The hyperfine spectra of B ← X transitions in the wavelength range 500--517 nm are investigated systematically. Four effective hyperfine parameters, eqQB, CB, dB, and delta B, are determined for an extensive number of rovibrational levels spanning the intermediate region 42 < upsilon' < 70 in the B0+u (3piu) state. Near vibrational levels upsilon ' = 57--60, the 1g( 1pig) electronic state strongly perturbs the B0+u (3piu) state through rotational coincidence, leading to effects such as abnormal variations in the hyperfine parameters and strong u-g mixing recorded at the transition P(84) 60-0. Various perturbation effects in the B0+u (3piu) state identified so far are summarized.; We have also performed a high-resolution analysis of the six electronic states that share the same dissociation limit with the excited electronic state B0+u (3piu) in molecular iodine. These six states are coupled to the B0+u (3piu) state via hyperfine interactions. The four hyperfine parameters are calculated using available potential energy curves and wave functions constructed from the separated-atom basis set. We obtain a maximum separation of the respective contributions from all six electronic states and compare each individual contribution with high-precision spectroscopic data, allowing an independent verification of the relevant electronic structure.