Bond Softening and Breathing Mode Excitation in C 60 Followi

Motivated by the experiments of Dexheimer et al. and Fleischer et al., we have performed simulations of the response of a C 60 molecule subjected to 12 fs laser pulses with fluences ranging from 0.07 to 4.40 kJ=m 2. We observe a symmetric excitation of ele

Bond Softening and Breathing Mode Excitation in C60 Following an Ultrashort Laser PulseDepartment of Physics, Texas A&M University, College Station, Texas 77843Motivated by the experiments of Dexheimer et al. and Fleischer et al., we have performed simulations of the response of a C60 molecule subjected to 12 fs laser pulses with uences ranging from 0.07 to 4.40 kJ=m2. We observe a symmetric excitation of electrons which softens the intramolecular bonding. As a result, there is a vibrational excitation which is almost purely in the breathing mode at low temperature. At the highest uence, the vibrational amplitude is very large, with the diameter of the molecule oscillating between about 7.2 and 8.8 A.

Ben Torralva and Roland E. Allen

During the past few years, vibrational excitation 1 2 and photofragmentation 3 4 5 have been observed for C60 molecules subjected to laser pulses with durations as short as 12 femtoseconds. Motivated by these experiments, and by potential applications to the photochemistry of fullerenes, we have performed simulations at both moderate and high uences. We have also studied collisions of atoms with C60 molecules following such ultrashort and ultraintense pulses, to consider the possibility of laser-promoted encapsulation. In the present paper, however, we focus on vibrational excitation. Details of the calculation will be given in a longer article. Here we mention only that the technique is tight-binding electron-ion dynamics 6 7 and that the tight-binding model is that of Xu et al. 8 The results for several di erent uences at two di erent temperatures are shown in Figs. 1-4. The rst simulation, depicted in Fig. 1, is for a uence of 0.07 kJ=m2. The temperature T was chosen to be 0 K, so that the e ect of this lower uence laser pulse would not be intermingled with the e ects of thermal vibrations. One can see that there is a sharp peak near 500 cm?1, a frequency which is in very good agreement with the experimental value for the breathing mode of C60. Furthermore, a computer animation of the motion in real space and time shows that the response of the molecule to the laser pulse is indeed a symmetric breathing motion. Analysis of the results shows that the electrons undergo a symmetric promotion from bonding to antibonding molecular states. It is the consequent bond softening which then leads to vibrational excitation. Figure 2 shows the response at 300 K to a more intense pulse. Although the spectrum is complicated by thermal vibrations, the most prominent peak is again associated with the breathing mode. A comparison with the power 1;;;;

Motivated by the experiments of Dexheimer et al. and Fleischer et al., we have performed simulations of the response of a C 60 molecule subjected to 12 fs laser pulses with fluences ranging from 0.07 to 4.40 kJ=m 2. We observe a symmetric excitation of ele

spectrum before the pulse (not shown here) demonstrates that the dominant new strength comes from this mode. Figure 3 shows the response to a still more intense pulse. Despite the thermal vibrations, it is clear that it is again the breathing mode which is primarily excited. Notice however, that the frequency is shifted due to anharmonic e ects, which result from the large am

plitude of the vibrations. It is also clear that the response of the electrons is quite nonlinear. I.e., the electronic excitations involve multiphoton processes. Finally, Fig. 4 shows the remarkably large amplitude vibrations whose Fourier spectrum is given by Fig. 3. The unusual behavior beyond 400 fs is a precursor of fragmentation. It should be mentioned that we have observed more rapid photofragmentation at still higher uences, with emission of C2 dimers, and these results will be reported elsewhere. This work was supported by the Robert A. Welch Foundation.

Acknowledgements References

1. S. L. Dexheimer, D. M. Mittleman, R. W. Schoenlein, W. Vareka, X. -D. Xiang, A. Zettl, and C. V. Shank, In Ultrafast Phenomena VIII, Editors: J. L. Martin, A. Migus, G. A. Mourou, A. H. Zewail (Springer-Verlag, Berlin, 1993). 2. S.B. Fleischer, B. Pevzner, D.J. Dougherty, H.J. Zeiger, G. Dresselhaus, M.S. Dresselhaus, E.P. Ippen, and A.F. Hebard, Appl. Phys. Lett. 71, 2734 (1997). 3. S. Hunsche, T. Starczewski, A.l. Huillier, A. Persson, C. -G. Wahlstrom, B. van Linden van den Heuvell, and S. Svanberg, Phys. Rev. Lett. 77, 1966 (1996). 4. K. Hansen and O. Echt, Phys. Rev. Lett. 78, 2337 (1996). 5. H. Hohmann, C. Callegari, S. Furrer, D. Grosenick, E. E. B. Cambell, and I.V. Hertel, Phys. Rev. Lett. 73, 1919 (1994). 6. R. E. Allen, Phys. Rev. B 50, 18629 (1994). 7. J. S. Graves and R. E. Allen, Phys. Rev. B (in press). 8. C. H. Xu, C. Z. Wang, C. T. Chan, and K. M. Ho, J. Phys. Condens. Matter 4, 6047 (1992).

Motivated by the experiments of Dexheimer et al. and Fleischer et al., we have performed simulations of the response of a C 60 molecule subjected to 12 fs laser pulses with fluences ranging from 0.07 to 4.40 kJ=m 2. We observe a symmetric excitation of ele

Fourier Power Spectrum

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Frequency [cm]-115002000

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Motivated by the experiments of Dexheimer et al. and Fleischer et al., we have performed simulations of the response of a C 60 molecule subjected to 12 fs laser pulses with fluences ranging from 0.07 to 4.40 kJ=m 2. We observe a symmetric excitation of ele

Fourier Power Spectrum

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