Electric-quadrupole transition of H2 determined to 10-9 precision

The S(3) electric-quadrupole transition of the (3-0) overtone of molecular hydrogen has been recorded with a continuous-wave cavity ring-down spectrometer and calibrated with sub-megahertz accuracy using a temperature-stabilized Fabry-Perot interferometer. The collision-induced frequency shift and the frequency at zero pressure limit are derived from a fit of the spectral profiles. The absolute transition frequency of the S(3) line is determined to be 376 531 818.1 MHz with a 1.6-MHz accuracy (delta nu/nu = 4 x 10(-9)) by measuring the shift to a Rb line at 795 nm. As the best determined transition frequency of neutral H-2 in the ground electronic state, this value agrees well with recent high-level quantum chemistry calculations including relativistic and quantum electrodynamics corrections.