26 June 1998 Ž . Chemical Physics Letters 290 1998 125–130 On the nature of the laser irradiation induced reversible softening of phonon modes in C single crystals 60 K.P. Meletov a , E. Liarokapis b , J. Arvanitidis c , K. Papagelis c , D. Palles b , G.A. Kourouklis d, ) , S. Ves c a Institute of the Solid State Physics RAS, ChernogoloÕka, Moscow region, 142432, Russian Federation b Department of Physics, National Technical UniÕersity, Athens 157 80, Greece c Physics Department, Aristotle UniÕersity of Thessaloniki, GR-540 06, Thessaloniki, Greece d Physics DiÕision, School of Technology, Aristotle UniÕersity of Thessaloniki, GR-540 06, Thessaloniki, Greece Received 19 January 1998; in final form 6 April 1998 Abstract The average temperature rise in the laser excitation spot of the C single crystal has been determined using the Stokes to 60 Ž. Ž. anti-Stokes integrated peak intensity ratio for the H 1 phonon mode. The reversible softening of the A 2 pentagon pinch g g mode was found to be due to the heating of the sample caused by the laser irradiation, in agreement with experimental results obtained for uniformly heated samples. These findings are in quantitative agreement with results obtained by numerical calculations of the local temperature rise, which indicates the highly non-uniform temperature distribution in the laser excitation spot associated with the small thermal conductivity of solid C . q 1998 Elsevier Science B.V. All rights 60 reserved. 1. Introduction Ž. Ž . The response of the A 2 pentagon pinch PP - g mode, which is the most intense band in the Raman spectrum of solid C , to a variety of perturbations 60 has been used for the study of many diverse proper- ties of solid fullerene and fullerene-based materials. Among them are the study of the temperature and pressure induced phase transitions, the effects caused by the intercalation of solid C with alkali metals 60 w x 1–5 , the dimerization processes in solid C under 60 laser illumination and the effects caused by high- w x pressure and -temperature treatment 6,7 . These last ) Corresponding author. E-mail: gak@vergina.eng.auth.gr effects are clearly manifested by the considerable Ž. softening of the A 2 PP-mode. g Ž. The position of the A 2 mode, reported initially g wx by Bethune et al. 8 , in the room-temperature Ra- man spectrum of air-exposed C films is equal to 60 1469 cm y1 . It was also reported that the room-tem- perature Raman spectrum of the oxygen-free C 60 contains a broad peak at 1459 cm y1 , which is more intense than the 1469 cm y1 peak. The recovery of the 1469 cm y1 peak is possible upon exposure of the wx sample to oxygen 9 . The appearance of the 1459 y1 wx cm peak was explained, by Rao et al. 6 , as the manifestation of the photoassisted dimerization of the oxygen-free C films under intense laser irradia- 60 tion. It has been also shown that oxygen-exposed C 60 films are more resistant to the laser irradiation and 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 98 00495-3