Vibrational energy redistribution in polyatomic liquids: Ultrafast IR-Raman spectroscopy of acetonitrile

Ultrafast anti-Stokes Raman scattering after intense mid-IR excitation is used to study vibrational energy relaxation (VER) and vibrational cooling (VC) in neat liquid acetonitrile. The mid-IR pulse (3000 cm^-1) excites a combination of C-H stretching fundamentals and C-H bending overtones, which are coupled by Fermi resonance. Vibrational excitation decays from the pumped C-H stretch in 5 ps. Almost no energy is transferred from C-H stretch to C≡N stretch (2253 cm^-1). The C≡N stretch behaves as a VER "blocking group" which keeps vibrational energy in the CH₃-C moiety. A 5 ps buildup, which mirrors the C-H stretch decay, is seen in the C-H bending modes (∼1500 cm^-1) at about one-half the C-H stretch energy, and in the lowest energy vibration at 379 cm^-1, a C-C≡N bend. By diluting the acetonitrile with CCl₄, it is shown that the buildup of population in the C-C≡N bend mirrors the buildup of excitation of the bath. Monitoring the C-C≡N bend allows us to track the instantaneous fraction of energy which has been transferred from acetonitrile vibrations to the bath. The 5 ps buildup of the C-C≡N bend to about one-half its final value shows C-H stretch decay populates the v = 1 C-H bending vibrations, rather than the v = 2 C-H bending overtones. The decay of daughter C-H bend excitations to C-C stretching vibrations (918 cm^-1) and the decay of the C-C stretching excitation are also observed. Combined with the C-C≡N bending data, a rather complete picture of VER and VC in acetonitrile is obtained. VC in acetonitrile takes about 300 ps. An interesting energy recurrence phenomenon is observed. A C-H bending excitation (v = 2) is initially excited by the laser. That excitation is transferred to C-H stretching vibrations, whose decay repopulates the same bending vibration (v = 1).