Electronic-vibrational excitations of a hydrogen-bonded molecule in supersonic expansions: 1,4-dihydroxyanthraquinone and its deuterated derivatives

In this paper we report the results of an experimental study of the S₀ → S₁ and S₁ → S₀ transitions of 1,4-dihydroxyanthraquinone [H,H] and its deuterated derivatives [H,D] and [D,D], explored by laser spectroscopy in seeded, pulsed, supersonic expansions of He and of Ar. We were able to provide quite detailed information on the vibrational level structure. Out of the twenty-five A₁ totally symmetric vibrations, we were able to identify sixteen A₁ modes in S₀ and seventeen A₁ modes in S₁. In addition, we observed nine B₂ out-of-plane modes involved in vibronic coupling. Evidence for the strengthening of the hydrogen bond upon S₀ → S₁ electronic excitation was inferred from the increase of the frequencies of the vibrational modes of the {A figure is presented} rings upon electronic excitation, as well as from the large blue deuterium isotopic shifts, which originate from the weakening of the ν(OH)] stretching upon S₀ → S₁ excitation. On the basis of the analysis of energetics and the relative intensities of overtones and of combination bands, we concluded that electronic excitations to S₁ result in small configurational changes. No spectroscopic evidence for nuclear motion in a double-well potential of the hydrogen bond in the S₁ state could be inferred. Furthermore, our spectroscopic data do not provide any evidence for intramolecular H-atom transfer in the electronically-vibrationally excited states of this isolated molecule.