Stretching mode excitation

It is also possible to measure microwave spectra of some more strongly bound Van der Waals complexes in a gas cell ratlier tlian a molecular beam. Indeed, tire first microwave studies on molecular clusters were of this type, on carboxylic acid dimers [jd]. The resolution tliat can be achieved is not as high as in a molecular beam, but bulk gas studies have tire advantage tliat vibrational satellites, due to pure rotational transitions in complexes witli intennolecular bending and stretching modes excited, can often be identified. The frequencies of tire vibrational satellites contain infonnation on how the vibrationally averaged stmcture changes in tire excited states, while their intensities allow tire vibrational frequencies to be estimated. 

M. Shmilovits-Ofir, R. B. Gerber. Proton transfer and dissociation of GlyLysH(- -) following O-H and N-H stretching mode excitations dynamics simulations, J. Am. Chem, Soc., 133 ... 

Inspection of the spectra in fig.l shows that stretching mode excitation is not very efficient in inducing the - Pq transition but bending mode with 1=1 about the Hg-N-N axis have an efficiency close to unity as shown in the shaded areas of the figure la. 

A covalent bond (or particular nomial mode) in the van der Waals molecule (e.g. the I2 bond in l2-He) can be selectively excited, and what is usually observed experimentally is that the unimolecular dissociation rate constant is orders of magnitude smaller than the RRKM prediction. This is thought to result from weak coupling between the excited high-frequency intramolecular mode and the low-frequency van der Waals intemiolecular modes [83]. This coupling may be highly mode specific. Exciting the two different HE stretch modes in the (HF)2 dimer with one quantum results in lifetimes which differ by a factor of 24 [84]. Other van der Waals molecules studied include (NO)2 [85], NO-HF [ ], and (C2i J )2 [87]. 

The comparison of the vibrational modes energies, determined by i) Raman scattering, ii) fluorescence, iii) ab initio calculation, and iv) absorption, is reported in Table 6-5. We note that, as in the case of T4 [64], the C=C stretching mode of 1460 cm-1 in the ground slate decreases its frequency significantly (1275 cm-1) in the first excited electronic state. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

Vibrational excitation by electron impact of the background neutrals is an important process, because it is a major cause of energy loss for the electrons [reactions SVl (SiH4 stretching mode), SV2 (SIHt bending mode), and HV in Table II]. Moreover, the density of the vibrationally excited molecules has been reported to be important [211]. However, information about reaction coefficients of vibrationally excited molecules is scarce [192]. Here, only the vibrational excitation of SiHa and Ht is included [212, 213]. 

To uniquely associate the unusual behavior of the collision observables with the existence of a reactive resonance, it is necessary to theoretically characterize the quantum state that gives rise to the Lorentzian profile in the partial cross-sections. Using the method of spectral quantization (SQ), it is possible to extract a Seigert state wavefunction from time-dependent quantum wavepackets using the Fourier relation Eq. (21). The state obtained in this way for J = 0 is shown in Fig. 7 this state is localized in the collinear F — H — D arrangement with 3-quanta of excitation in the asymmetric stretch mode, and 0-quanta of excitation in the bend and symmetric stretch modes. If the state pictured in Fig. 7 is used as an initial (prepared) state in a wavepacket calculation, one observes pure... 

The spectrum of this compound displays a structured band at 437 nm. The vibrational structure was extensively studied and assigned to vibronic levels of a 6 -> n and a 6) 8 state with activity in the Mo-Mo and Mo-0 stretching modes (49-56). The MPI photofragment spectrum following excitation at 450 nm is dominated by Mo+ and Mo0+. In contrast, excitation at 337 nm accesses a structureless band assigned to the 5 > n (carbon) transition. Photofragmentation is dominated by with a weaker Mo0 ... 

Calculations have also given a better understanding of the anomalous frequency increase of the C=C stretch mode upon excitation to the 2 kg state in polyenes1,67,78. By comparing the calculated adiabatic and diabatic frequencies, this increase is explained in terms of... 

Figure 6. The complex OH stretching spectrum of methanol trimer (bottom) can be explained by sum (v5), difference (vD), and hot bands (vH) involving the OH fundamental (vF) and two umbrella modes of the methyl groups, which are nearly degenerate in the ground state but soften and split after OH stretching excitation. vR is the predominantly Raman active concerted stretching mode [16].

Figure 0.2 Direct overtone spectroscopy of C2H2 using Fourier transform spectroscopy. Here, at high resolution, the entire band of rotational transitions, which accompany a given vibrational transition, can be resolved. Here the band, in the visible range, corresponding to the direct excitation of v = 5 of the v3 stretch mode is shown. (Adapted from Herman et al., 1991. See also Scherer, Lehmann, and Klemperer, 1983, and Figure 8.4.)...

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