The wave packet motion and intramolecular vibrational redistribution in CHX3 molecules under infrared multiphoton excitation

Abstract

We report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared-multiphoton excitation. The vibrational Hamiltonian is based on results from high resolution spectroscopy and ab initio calculations of the potential hypersurfaces for these molecules. The quantum dynamical calculations involve accurate solutions of the time dependent quantum equations of motion by means of both Floquet and quasiresonant approximations. We find mode selective redistribution between the CH stretching and bending modes on a time scale of 50 to 100 fs. Other modes participate only on much longer time scales (>1 ps), as was shown previously by analysis of the spectra. For the real, strongly anharmonic systems (k′sbb≂30 to 100 cm−1 ), the redistribution is nonclassical with fast spreading to a quasimicrocanonical distribution, which is particularly pronounced if a narrow range of energies (for example, the N=6 polyad) is initially excited. The effect can be interpreted as an intrinsic quantum statistical behavior induced by anharmonicity. In comparison, a weakly anharmonic hypothetical model system (ksbb≤2 cm−1) leads to quasiclassical motion of the wave packet with quasiperiodic exchange between stretching and bending motions. We present an approximate analytical investigation of the Fermi modes underlying the dynamics which provides a semiquantitative understanding of the Fermi-resonance spectra. On the basis of these results, we discuss possibilities of mode selective reaction control in unimolecular processes with laser excitation and some aspects of intramolecular ‘‘chaos.’’

Three-dimensional infrared spectroscopy of isotope-diluted ice Ih

“…In accordance with earlier work [10,11], we find however that the adiabatic picture is still quite correct for the first excited state, where the effect of anharmonicity is much smaller. A similar crossover behavior has been observed in models of vibrational energy flow in polyatomic molecules, where a transition between a localized energy flow regime at low excitations to a highly delocalized and facile regime at higher energies has been observed [79,80].…”

Stereomutation dynamics in hydrogen peroxide

“…Mode selective chemistry has been discussed for some very simple systems such as O 3 [27] and H 2 O isotopomers as well as other examples [28][29][30][31][32][33]. Among the important early developments we may mention also the demonstration, on the basis of fs wavepackets derived from high resolution spectroscopic experiment, of mode selective intramolecular vibrational redistribution as a primary process in chemical kinetics [34,35] one-dimensional quantum wavepackets [36,37] as well as multidimensional kinetics of polyatomic molecules in isolation and under condensed phase condition, a development which was well honored as time proceeded [38][39][40][41].…”

Radiative excitation of the harmonic oscillator with applications to stereomutation in chiral molecules

“…This becomes possible, because on the very short times considered here dissipative processes such as spontaneous emission, radiationless electronic transitions or intramolecular vibrational redistribution (°'IVR" [46,59]) may often be negligible. Thus the one dimensional stereomutation dynamics considered here might be at least partly realistic.…”

“…In that sense, our present study is a continuation of our previous investigations of harmonic oscillator dynamics under coherent electromagnetic ("laser") field driving [ [42]. Although our study uses clearly an oversimplified 1-dimensional representation of the very complex anharmonic multidimensional vibrational dynamics in polyatomic molecules [43][44][45][46], it provides some relevant insight into chiral molecule model wavepacket dynamics on very short time scales relating to the current fashions in "Femtosecond Chemistry" [11,47].…”

“…We have checked the validity of the approximation by solving (10) also using the Floquet-Liapounoff approach [46,48]. Parity violating potentials have been neglected in the present model, as they are expected to be insignificant on the femtosecond time scales of evolution considered here.…”