The v = 3 <-- 0 S(0)-S(3) Electric Quadrupole Transitions of H2 near 0.8 μm

International audience; The very weak S(0)-S(3) electric quadrupole transitions of the second overtone band of molecular hydrogen have been recorded in the laboratory by continuous-wave cavity ring-down spectroscopy near 0.8 μm. The ultrahigh sensitivity of the spectrometer (αmin ~ 1 &#215; 10-10 cm-1) allows us to detect the considered transitions at a relatively low sample pressure (50-750 torr). The line positions, intensity, and pressure-shift coefficients are derived from a fit of the line shape using a Galatry profile. Compared with literature values, the relative differences between the experimental and theoretical transition intensities are reduced by one order of magnitude, reaching a value of about 2% mainly dependent of the line-shape function adopted for the profile fitting. The thermal equilibrium relative intensity of the S(1) to S(0) line is determined with an accuracy of 0.4%, which can be used to probe the ortho- to para-H2 concentration ratio. Our measurements confirm the quality of the high-level ab initio calculations, including the relativistic and quantum electrodynamics corrections.