Simulation of Nuclear Dynamics of C₆₀: From Vibrational Excitation by Near-IR Femtosecond Laser Pulses to Subsequent Nanosecond Rearrangement and Fragmentation

Abstract

Impulsive Raman excitation of C₆₀ by single or double near-IR femtosecond pulses of λ = 1,800 nm was investigated by using a time-dependent adiabatic state approach combined with the density functional theory method. We confirmed that the vibrational energy stored in a Raman active mode of C₆₀ is maximized when T _p ∼ T _vib/2 in the case of a single pulse, where T _p is the pulse length and T _vib is the vibrational period of the mode. In the case of a double pulse, mode selective excitation can be achieved by adjusting the pulse interval τ. The energy of a Raman active mode is maximized if τ is chosen to equal an integer multiple of T _vib, and it is minimized if τ is equal to a half-integer multiple of T _vib. The energy stored can be larger than the barrier heights for rearrangement or fragmentation processes. The picosecond or nanosecond dynamics of resulting Stone-Wales rearrangement (SWR) and fragmentation are also investigated by using the density functional-based tight-binding semiempirical method. We present how SWRs are caused by the flow of vibrational kinetic energy on the carbon network of C₆₀. In the case where the h_g(1) prolate-oblate mode is initially excited, the number of SWRs prior to fragmentation is larger than in the case of a_g(1) mode excitation for the same excess vibrational energy. Fragmentation by C₂-ejection is found to occur from strained, fused pentagon/pentagon defects produced by a preceding SWR, which confirms the earliest mechanistic speculations of Smalley et al. (J. Chem. Phys. 88, 220, 1988). The fragmentation rate of C₆₀ → C₅₈ + C₂ in the case of h_g(1) prolate-oblate mode excitation does not follow a statistical description as employed for instance in the Rice-Ramsperger-Kassel (RRK) theory, whereas the rate for a_g(1) mode excitation does follow predictions made by RRK. We also found for the h_g(1) mode excitation that the nonstatistical nature still remains in the distribution of barycentric velocities of fragments C₅₈ and C₂. This result suggests that it is possible to control rearrangement and subsequent bond breaking in a “nonstatistical” way by initial selective mode excitation.