Multiply enhanced site selective CARS spectroscopy

Abstract

Three tunable lasers are used to perform site selective coherent anti-Stokes Raman (CARS) spectroscopy in pentacene:p-terphenyl crystals at 2 K. The vibrational and vibronic levels of pentacene in the individual substitutional sites of p-terphenyl can be selectively examined by matching the resonances of a particular site. The signals from a site and the selectivity between sites are much higher than CARS performed with two tunable lasers. The spectra are dominated by lines from processes where the vibrational and vibronic states involved in the four-wave mixing are associated with the same molecular mode. There are also important lines from processes where different vibrational and vibronic modes are involved in the nonlinear mixing. The intensities of lines from the resonant site are strongly dependent on the laser energy because of saturation of the electronic transition.