Carbon vibrational relaxation time

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

We have presented experimental and theoretical results for vibrational relaxation of a solute, W(CO)6, in several different polyatomic supercritical solvents (ethane, carbon dioxide, and fluoroform), in argon, and in the collisionless gas phase. The gas phase dynamics reveal an intramolecular vibrational relaxation/redistribution lifetime of 1.28 0.1 ns, as well as the presence of faster (140 ps) and slower (>100 ns) components. The slower component is attributed to a heating-induced spectral shift of the CO stretch. The fast component results from the time evolution of the superposition state created by thermally populated low-frequency vibrational modes. The slow and fast components are strictly gas phase phenomena, and both disappear upon addition of sufficiently high pressures of argon. The vibrational... 

Smalley and co-workers have probed intramolecular vibrational relaxation by viewing the yields and the time-dependence of the fluorescence from Sj in alkylated benzenes. They focus attention on those ring modes whose vibrational frequencies are unshifted by alkylation these are vibrations with nodes at the alkylated ring carbon atom. The absorption lines are sharp, but as the alkyl chain is lengthened, the emission spectrum develops a broad relaxed component, while the intensity of the sharp unrelaxed resonance fluorescence diminishes in intensity as the intensity of the relaxed spectrum increases. The time-dependence of the relaxed and unrelaxed emission is found to be a single exponential decay, so unfortunately, the rapid intramolecular dephasing decay has not yet been followed. 

Sagnella, D. E. Straub, J. E. Jackson, T. A. Lim, M. Anfinrud, P. A., Vibrational population relaxation of carbon monoxide in the heme pocket of photolyzed carhonmonoxy myoglobin Comparison of time-resolved mid-lR absorbance experiments and molecular dynamics simulations. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 14324—14329. 

Addition of singlet CDa (. 1), from the photolysis of CDa C 0, to the fluoro-olefins, Rp CF CFa (Rp = CF3, CFs CFa CFa, or CFs CFa CFa CFa CFa-) in the presence of carbon monoxide yields chemically activated cyclopropane, which may relax or expel difluorocarbene (see Scheme 4). At low pressures, ranging from <650 Torr for Rp = CFa down to <0.25 Torr for the more complex Rp = CsFii, the rate of cyclopropane decay is independent of pressure, but then it increases with increasing pressure. Relaxation of internal vibration energy is not instantaneous, the time required being longest when the vibrational modes available are fewest. ... 

Structural changes associated with relaxation of the j3-carbon to planar geometry which would tend to induce vibrational excitation of the olefin (a time scale of 2-4 x 10" s is estimated from the vibrational frequency for a CHa rock). This step may occur in conjunction with the third step. 

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