Observation of theS0(3)pure rotational quadrupole transition ofH2with a tunable diode laser

The $S_0\mathrm{}\left(3\right)\mathrm{}$ pure rotational quadrupole transition of ${\mathrm{H}}_2$ has been observed using a tunable PbSnSe diode laser and a long-path absorption cell containing ${\mathrm{H}}_2$ at pressures in the range 0.1-0.9 atm. The observed linewidths (80-170 MHz full width at half maximum) were considerably narrower than the Doppler width (268 MHz for 297 K) owing to the effects of collisional (Dicke) narrowing. The line position was measured to be 1034.6702 ± 0.0007 ${\mathrm{cm}}^{-1\mathrm{}}$, and its intensity was found to agree well with that expected from ab initio calculations of the quadrupole moment of ${\mathrm{H}}_2$.