  The D^1\Pi_u - X^1\Sigma_g^+ absorption system of molecular deuterium has
been re-investigated using the VUV Fourier -Transform (FT) spectrometer at the
DESIRS beamline of the synchrotron SOLEIL and photon-induced fluorescence
spectrometry (PIFS) using the 10 m normal incidence monochromator at the
synchrotron BESSY II. Using the FT spectrometer absorption spectra in the range
72 - 82 nm were recorded in quasi static gas at 100 K and in a free flowing jet
at a spectroscopic resolution of 0.50 and 0.20 cm^{-1} respectively . The
narrow Q-branch transitions, probing states of \Pi^- symmetry, were observed up
to vibrational level v = 22. The states of \Pi^+ symmetry, known to be
broadened due to predissociation and giving rise to asymmetric Beutler-Fano
resonances, were studied up to v = 18. The 10 m normal incidence beamline setup
at BESSY II was used to simultaneously record absorption, dissociation,
ionization and fluorescence decay channels from which information on the line
intensities, predissociated widths, and Fano q-parameters were extracted.
R-branch transitions were observed up to v = 23 for J = 1-3 as well as several
transitions for J = 4 and 5 up to v = 22 and 18 respectively. The Q-branch
transitions are found to weakly predissociate and were observed from v = 8 to
the final vibrational level of the state v = 23. The spectroscopic study is
supported by two theoretical frameworks. Results on the \Pi^- symmetry states
are compared to ab initio multi-channel-quantum defect theory (MQDT)
calculations, demonstrating that these calculations are accurate to within 0.5
cm^-1.
