HD spectroscopy in the Lyman-Werner bands (91 - 112 nm) measured with VUV-FTS at Synchrotron SOLEIL

H2 and HD can be destroyed both by collisions and by UV photons (photodissociation). In reality, however, direct photodissociation of H2 from the ground state (X 1Σ+g) by UV is very limited because transitions to the first excited state (b 3Σ+u) are spin-forbidden and also because higher energy photons (< 91.2 nm) are more likely to be consumed to photoionize the abundant H atoms and H2 in gas clouds. The remaining process for H2 and HD is indirect photodissociation by UV fluorescence through the Lyman (B-X) or Werner (C-X) band systems (longward of the Lyman limit, primarily in the 91 – 112 nm region). So spectroscopic characterization is critical to understanding the probabilities of H2 and HD photodissociation as compared to photoionization, but we note that no reliable linelist is available from direct laboratory measurements for HD in the region.To fill this gap, we have obtained high-resolution spectra of HD and H2 by using the VUV-FTS end station at the DESIRS beamline of Synchrotron SOLEIL, which adopts wavefront-division interferometry (as an alternative to the Michelson type) configured with an undulator synchrotron radiation source. The entire L-W band (11.2 – 13.6 eV; 91 – 112 nm) has been collectively covered using a windowless gas cell by sequentially scanning six sub-windows at a resolving power R ~ 250,000. We also obtained a few HD spectra using a LiF windowed cell at a lower spectral resolution (R~ 125,000) in the lowest energy region (~90,300 cm-1). The windowless spectrum set provides high-fidelity relative intensity measurements, while the windowed cell spectra produce absolute intensity measurements for the first time. In this work, we have determined spectroscopic line parameters of HD (e.g., line position, line intensity every transition observed) using a non-linear least squares multispectral fitting algorithm for all observed transitions simultaneously.In this preliminary analysis, the HD line positions and intensities retrieved from the windowless spectra will be presented and compared with model calculations available to us. The preliminary comparison shows an offset up to ~85% for 15 transitions in the 90,157 – 94850 cm-1 alone, but in a very small variation (< 5 %) from transition to transition, confirming the high-fidelity relative intensity. Moreover, we have determined line intensities for previous unobserved transitions. A continued effort to obtain more and higher quality spectra for HD and H2 using the windowed cell is in progress.Acknowledgements: United States Government sponsorship is acknowledged (NASA-SMD-ROSES: NNH23ZDA001N-APRA). Synchrotron SOLEIL Proposal#20240046 is also acknowledged.