Strong overtones and combination bands in ultraviolet resonance Raman spectroscopy

Ultraviolet resonance Raman spectroscopy is carried out using a continuous wave frequency-doubled argon ion laser operated at 229, 244, and 257 nm in order to characterize the overtones and combination bands for several classes of organic compounds in liquid solutions. Contrary to what is generally anticipated, for molecules such as pyrene and anthracene, strong overtones and combination bands can show up; it is demonstrated that their intensity depends critically on the applied laser wavelength. If the excitation wavelength corresponds with a purely electronic transition-this applies to a good approximation for 244-nm excitation in the case of pyrene and for 257-nm excitation in the case of anthracene-mostly fundamental vibrations (up to 1700 cm