The D¹Pi\ state of HD and the mass scaling relation of its predissociation widths

Absorption spectra of HD have been recorded in the wavelength range of 75 to 90 nm at 100 K using the vacuum ultraviolet Fourier transform spectrometer at the Synchrotron SOLEIL. The present wavelength resolution represents an order of magnitude improvement over that of previous studies. We present a detailed study of the D^1\Pi - X^1\Sigma^+_g system observed up to v'=18. The Q-branch transitions probing levels of \Pi^- symmetry are observed as narrow resonances limited by the Doppler width at 100 K. Line positions for these transitions are determined to an estimated absolute accuracy of 0.06 cm^{-1} . Predissociation line widths of \Pi^+ levels are extracted from the absorption spectra. A comparison with the recent results on a study of the D^1\Pi state in H_2 and D_2 reveals that the predissociation widths scale as \mu^-2J(J+1), with \mu the reduced mass of the molecule and J the rotational angular momentum quantum number, as expected from an interaction with the B'^1\Sigma_u^+ continuum causing the predissociation.