Excitonic and vibrational nonlinear processes in a polydiacetylene studied by a few-cycle pulse laser

Abstract

High-density excitation of exciton induces the faster decay of absorbance change due to exciton-exciton interaction originated from the Auger recombination process. Experimental results clarified the dynamics of the exciton and the bimolecular quenching rate and the jump rate of exciton were determined to be 5.2 x 10(-9) cm(3) s(-1) and 5.2 x 10(-9) s(-1), respectively. The intensity modulation of electronic transition under high-density excitation with sub-5-fs pulse was analyzed to observe the Fourier power spectrum of the molecular vibration modes and revealed the existence of a combination tone (2726 cm(-1)) of the C-C (1241 cm(-1)) and the C=C (1485 cm(-1)) stretching modes. This indicates that the highly excited state induced by the Auger process results in the fusion of vibrational quantum states. Difference frequency generation of the vibrational modes also takes place in the same process due to the fission of the vibrational quanta stimulated by the interaction of the two vibrational modes. It was also found that the intensities of several prominent peaks corresponding to the modes of C=C and C-C stretching were proportional to the squared intensity of the probe pulse. Furthermore, the frequencies of the modes were shifted to low frequencies at high probe intensity. The process can be considered to be fifth-order cascaded Raman scattering via real excitation of vibronic excitons.