Photo-Induced Structural Changes at a Surface of Organic Single Crystals Observed by Vibrational Sum Frequency Generation Spectroscopy

K. Fukumotoa, b, N. Fukazawaa, T. Ishikawaa, S. Koshiharaa, b, H.M. Yamamotoc, d, e, R. Katoc and K. Ondaf aDepartment of Materials Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan bJST-CREST, Kawaguchi, Saitama 332-0012, Japan cRIKEN, Wako, Saitama 351-0198, Japan dJST-PRESTO, Kawaguchi, Saitama 332-0012, Japan eDepartment of Elec. Chem., Tokyo Institute of Technology, Nagatsuta, Yokohama, 226-8502, Japan fDepartment of Environ. Chem. and Engineering, Tokyo Institute of Technology, Nagatsuta, Yokohama 226-8502, Japan

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Phase transitions from insulator to metal induced by thermal excitation and by photo-irradiation at the surface of β'-(BEDT-TTF)(TCNQ) are investigated using vibrational sum frequency generation spectroscopy. The results are compared with linear reflectivity measurements which detect the information mainly from bulk. IR absorption spectra of ag vibrational modes of TCNQ induced by the dimerization are observed using vibrational sum frequency generation spectroscopy, and its intensity is found to become smaller by thermal excitation indicating the transition to the metal phase. Time-dependent degree of dimerization is also observed by pump-probe experiments with almost the same statistics as the static measurements. On the other hand, in the reflectivity experiments, the reduction of dimerization is also observed by elevating the temperature, however, the spectral shapes are deformed for the time-resolved measurements. These results imply that the different dynamics occur at the surface and in the bulk.

DOI: 10.12693/APhysPolA.121.313 PACS numbers: 78.40.Me, 68.35.Rh, 68.35.Ja