UV resonance Raman spectroscopy. Multi-wavelength UVRR spectra have been collected at the BL10.2-IUVS beamline of Elettra Sincrotrone Trieste by exploiting the experimental setup described in detail in Ref. Different excitation wavelengths in the deep UV range (220-270 nm) were employed. These excitation conditions were chosen to obtain suitable resonance Raman signals for the examined samples and, at the same time, the best features in terms of the spectral resolution and signal-to-noise ratio in the whole concentration range considered. In particular, the reported quantitative information specifically refers to the UVRR spectra acquired using 266 nm as excitation wavelength. The Raman signal was collected in back-scattered geometry using a single pass of a Czerny-Turner spectrometer (Trivista 557, Princeton Instruments, 750 mm focal length) and detected with a UV-optimized CCD camera. The spectral resolution was set at 1.2 cm -1 /pixel. The calibration of the spectrometer was standardized using cyclohexane (spectroscopic grade, Sigma-Aldrich). The power of the beam on the sample was measured to be 0.3-0.4 mW with a spot area of about 1 mm 2 . Solid samples (D, U, and D-U hyd) were dissolved in pure water. Such freshly prepared solutions were placed in suitable suprasil-quartz cuvettes with an optical path of 10 mm. Atmosphere in cuvette was made inert by nitrogen flow, and then measured under a magnetic stirrer. During the Raman measurements, the temperature of the solutions was controlled using a sample holder equipped with a thermal bath coupled to a resistive heating system to keep the temperature of the sample at a fixed value with the stability of ±0.1 • C. The collected UVRR spectra were subtracted from a 2-order polynomial baseline and the cosmic rays were removed. We have assessed the intensity of the bands of interest through an integration algorithm applied to the wavenumber regions of interest.