UV–visible absorption spectra were obtained in the range 190–1100 nm, at 1 nm resolution, using an Agilent 8453 spectro-photometer. The band 190–290 nm was saturated and it was re-moved from all the spectra, thus only the range from 290 to 1100 was used in the data analysis. The radiation source was a combi-nation of a deuterium-discharge lamp for the ultraviolet wave-length range and a tungsten lamp for the visible wavelength range. The cells are rectangular quartz cuvettes with a path length of 5 mm.