Multiphoton absorption in crystalline biphenylene

The TP absorption spectrum of crystalline biphenylene has been measured at 77 K in the region of S2. Vibronically induced transitions are observed due to b3u vibrations and to combinations with a(g) modes built on them. Calculated TP cross-sections are in good agreement with experiment. In the crystal absorption of more than two photons may be also observed with sufficiently high incident intensities using wavelengths around 650 nm. It is proposed that the process occurs stepwise, that is with the mechanism S0-->(TP) S2,vib --> S1 -->(OP) S5.