The surface reflectivity measurements have been performed with a home-built system, operating between 10 and 300 K and in a spectrometric range from 365 nm to 1050 nm. A halogentungsten light source (Leica CLS 150 XD tungsten halogen source with a fixed power of 0.5 mW/cm 2 ) was used as the spectroscopic light. The measurements were calibrated by barium sulphate as a reference sample. With this reflectivity technique, the absolute reflectivity (AR) at a specific wavelength  is plotted as AR ( = (Rsample() -Rdark()) / (Rref() -Rdark()). The corresponding spectra AR () can be viewed as a mirror image of the absorbance spectra: i.e., when the sample absorbs efficiently (or weakly) the light, a low (or high, respectively) value of AR is measured. Irradiations were done with the spectroscopic white light (0.5 mW/cm 2 ) and a set-up of 14 different Light Emitting Diodes (high power LEDs from Thorlabs) operating between 365 nm and 1050 nm with an adjustable power (up to 160 mW/cm²). As the samples are potentially very photo-sensitive, the light exposure time was minimized during the experiments keeping the samples in the dark except during the spectra measurements when white light is shined on the sample surface (Power = 0.5 mW/cm 2 ). For all the excitation/deexcitation experiments performed at 10 K, the sample was initially placed at 300 K and then at the low temperature keeping the sample in the dark to avoid any excitation. Heating and cooling measurements were carried out at 5 K/min, whereas the relaxation experiment was performed with a sweep rate of 0.5 K/min. For white light irradiation, the source described above was used, but continuously with a power of 0.5 mW/cm 2 and the light is only switched on for 1 second to record the reflectivity spectrum at a given temperature, and then the light is switched off during the next temperature step.