Fast mode excitation, vibrational modes

We have also learned that VMP is an effective tool in molecular spectroscopy and molecular dynamics studies. It is effective, in particular, for determination of IVR lifetimes and for studying the vibrational spectroscopy of states that are difficult to study applying other methods. The above-mentioned limit of the size of the molecule is irrelevant here. For observing the mode selectivity in VMP, the vibrational excitation has to survive IVR in order to retain the selectivity since the subsequent electronic excitation has to be from the excited vibrational state. In contrast, monitoring vibrational molecular dynamics relies only on the efficacy of the excitation of the specific rovibrational state. When IVR is fast and rovibrational distribution reaches equilibrium, the subsequent electronic excitation will still reflect the efficacy of the initial rovibrational excitation. In other words, whereas fast IVR precludes mode selectivity, it facilitates the unraveling of the vibrational molecular dynamics. 

Since the pulse time is so short (see Sec. 3.6.2.2.3) one can coherently excite many vibrational modes at a time and monitor relaxation processes in real time. The first reported femtosecond time-resolved CARS experiments (Leonhardt et al., 1987 Zinth et al., 1988) showed beautiful beating patterns and fast decays of the coherent signal for several molecular liquids. The existence of an intermolecular coherence transfer effect was suggested from the analysis of the beating patterns (Rosker et al., 1986). Subsequent studies by Okamoto and Yoshihara (1990) include the vibrational dephasing of the 992 cm benzene mode. A fast dephasing process was found that is possibly related to... 

The competition between intramolecular vibrational relaxation and chemical reaction has been discussed in terms of the applicability of transition state theory to the kinetic analysis [6], If the environment functions mainly as a heat bath to ensure thermalization among the vibrational modes in the excited complex, then transition state theory is a good approximation. On the other hand, when the reaction is too fast for thermalization to occur the rate can depend upon the initial vibronic state. Prompt reaction and prompt intersystem crossing are, by definition, examples of the latter limit. 

In addition to measurements of lifetime of these vibrational excited states, time-resolved nonlinear IR could also give precise information on the mechanisms of deexcitation of these states. It could thus be shown that relaxation of the first excited state of modes of water molecules in liquid water was mainly due to resonance interactions of these modes with excited bending modes (65). As a result of the analysis of ID IR spectra shown above, Fermi resonance with bending modes allows the energy of the first excited state of to be transferred to the overtone of the bending band. It offers a fast relaxation path toward vibrational levels of a lower energy. Time-resolved nonlinear IR spectroscopy shows that this process is the main relaxation mechanism of and is at the origin of an unexpected increase of the relaxation time when temperature increases (66, 67). 

Other experimental techniques have been used to study the very fast relaxation of dye molecules in solution. Ricard and Ducuing studied rhodamine molecules in various solvents and observed vibrational rates ranging from 1 to 4 ps for the first excited singlet state. Their experiment consisted of two pulses with a variable delay time between them the first excites molecules into the excited state manifold and the second measures the time evolution of stimulated emission for different wavelengths. Ricard found a correlation between fast internal conversion and vibrational relaxation rates. Laubereau et al. found a relaxation time of 1.3 0.3 ps for coumarin 6 in CCI4. They used an infrared pulse to prepare a well-defined vibrational mode in the ground electronic state, and monitored the population evolution with a second pulse that excited the system to the lowest singlet excited state, followed by fluorescence detection. 

This kinetic approximation assumes a single vibrational temperature 77 for CO2 molecules and, therefore, is sometimes referred to as quasi equilibrium of vibrational modes. As one can see from (5-20), most of the vibrationally excited molecules can be considered as being in quasi continuum in this case. Vibrational kinetics of polyatomic molecules in quasi continuum was discussed in Chapter 3. The CO2 dissociation rate is limited not by elementary dissociation itself, but via energy transfer from a low to high vibrational excitation level of the molecule in the W-relaxation processes. Such a kinetic situation was referred to in Chapter 3 as the fast reaction limit. The population of highly excited states with vibrational energy E depends in this case on the number of vibrational degrees of freedom 5 and is proportional to the density of the vibrational states p E) a. The... 

For the bound Naa B system, 1.25 ps and 120 fs one-color real-time TPI spectroscopy, at moderate intensities, yielded preferential excitations of the relatively slow pseudorotation (3 ps) and the fast symmetric stretch mode (310 fs) respectively. Three-dimensional quantum chemical and quantum dynamical ab initio investigations fully corresponded to these experimental results. The time-dependent wave packet dynamics elucidate the effect of ultrafast state preparation on the molecular dynamics. Hence, these experiments manifest efficient control of molecular dynamics using the pulse duration as a control parameter. Since known cw spectra show the pseudorotation features only, this result demonstrates also that cw and femtosecond spectroscopy have complementary sensitivities for the excitation of different vibrational modes, thus neatly confirming the original conjecture of Zewail (see e.g. [425]). 

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