Experimental evidence on the source of negative peaks in coherent anti-Stokes Raman spectra

Under certain conditions, when the laser pump frequency approaches an electronic transition frequency, negative peaks have been observed in coherent anti-Stokes Raman spectra. Two conflicting explanations of this phenomenon have been offered. We have noted that these two explanations lead to a different dependence of the intensity of the negative peaks on the power in the pump beam. We have experimentally determined this power dependence in the case of a 5×10−3M solution of N, N-diethyl-p-nitrosoaniline in benzene and found that the intensity of the negative peaks varies with the square of the power in the pump beam. This result is consistent with the explanation that the negative peaks arise from a cross term between the background susceptibility and the imaginary part of the Raman susceptibility and not from the inverse Raman effect.