Raman spectra of single-walled carbon nanotubes ͑SWNTs͒ with diameters of 0.6-1.3 nm have been studied under high pressure. A "plateau" in the pressure dependence of the G-band frequencies was observed in all experiments, both with and without pressure transmission medium. Near the onset of the G-band plateau, the corresponding radial breathing mode ͑RBM͒ lines become very weak. A strong broadening of the full width at half maximum of the RBMs just before the onset of the G-band plateau suggests that a structural transition starts in the SWNTs. Raman spectra from SWNTs released from different pressures also indicate that a significant structural transition occurs during the G-band plateau process. Simulations of the structural changes and the corresponding Raman modes of a nanotube under compression show a behavior similar to the experimental observations. Based on the experimental results and the theoretical simulation, a detailed model is suggested for the structural transition of SWNTs, corresponding to the experimentally obtained Raman results in the high-pressure domain.

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The diffuse bands B３Σ-u→X３Σ-g(ν′≥18,0) of diatomic molecule S2 observed in the experiment are investigated. The electronic potential curves,including the spin-orbit coupling (SOC) effect of B３Σ-u and repulsive 1５Πu， 23Σ+u states are calculated. For the diffuse bands beginning at (18,0), a point of view different from others' results is presented in this work. Our results indicate that the SOC induced predissociation between B３Σ-u and 1５Πu， 23Σ+u plays the key role in the diffusion of spectra. Comparison with experimental results shows good agreement.