Spectroscopy, overtone

Boyarkin O V and Rizzo T R 1996 Secondary time scales of intramolecular vibrational energy redistribution in CFgH studied by vibrational overtone spectroscopy J. Chem. Phys. 105 6285-92... 

The approach is ideally suited to the study of IVR on fast timescales, which is the most important primary process in imimolecular reactions. The application of high-resolution rovibrational overtone spectroscopy to this problem has been extensively demonstrated. Effective Hamiltonian analyses alone are insufficient, as has been demonstrated by explicit quantum dynamical models based on ab initio theory [95]. The fast IVR characteristic of the CH cliromophore in various molecular environments is probably the most comprehensively studied example of the kind [96] (see chapter A3.13). The importance of this question to chemical kinetics can perhaps best be illustrated with the following examples. The atom recombination reaction... 

Fehrensen B, Hippier M and Quack M 1998 Isotopomer selective overtone spectroscopy by ionization detected IR + UV double resonance jet-cooled aniline Chem. Phys. Lett. 298 320-8... 

S. A. Nizkorodov, M. Ziemkiewicz, D. J. Nesbitt, and A. E. W. Knight, Overtone spectroscopy of H20 clusters in the voh 2 manifold Infrared ultraviolet vibrationally mediated dissocia... 

Reddy, K. V., Heller, D. F., and Berry, M. J. (1982), Highly Vibrationally Excited Benzene Overtone Spectroscopy and Intramolecular Dynamics of CsH6, C6D6, and Partially Deuterated or Substituted Benzenes, J. Chem. Phys. 76, 2814. 

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.)...

Luo, X. and Rizzo, T.R. (1990). Rotationally resolved vibrational overtone spectroscopy of hydrogen peroxide at chemically significant energies, J. Chem. Phys. 93, 8620-8633. 

EXAMPLES OF INTERACTION OF THEORY AND EXPERIMENT 1.2.1 High-resolution overtone spectroscopy of many-atom systems... 

Brennan CJ. Femtosecond wavevector overtone spectroscopy of anharmonic lattice dynamics in ferroelectric crystals. Cambridge, MA Massachusetts Institute of Technology, 1997. 

Romero-Rochin V, Koehl RM, Brennani CJ, Nelson KA. Theory of anharmonic phonon-polariton excitation in LiTaOj by ISRS and detection by wavevector overtone spectroscopy. J Chem Phys 1999 111(8) 3559—3571. 

Howard, D. L. Jorgensen, P. Kjaergaard, H. G. Weak intramolecular interactions in ethylene glycol identified by vapor phase OH-stretching overtone spectroscopy, J. Am. Chem. Soc. 2005,127, 17096-17103. 

It has been pointed out that the central ( 2, transition does not experience any first-order quadrupole interaction. The absence of first-order broadening effects is a general property of symmetric (m, - m) transitions. There are cases where this can be a distinct advantage, the most direct instance being for integer spin nuclei (e.g. D and both 1=1) where there is no ( /2, — /2) transition. The main problem is to excite and detect such higher-order transitions, for which there are two separate approaches. The sample may either be irradiated and detected at the multiple quantum frequency (called overtone spectroscopy) or the MQ transition can be excited and a 2D sequence used to detect the effect on the observable magnetisation. 

Overtone spectroscopy developed for irradiates the sample at approximately twice the Larmor frequency (Tycko and Opella 1987). If the quadrupole interaction is sufficiently large that second-order quadrupole effects are significant, the (— 1 1) transition becomes weakly allowed. In powders the spectmm is still structured, allowing the interactions to be deduced, but is narrowed by a factor of 8vq/xq. 

Figure 1 The normal modes of motion for the three stretch modes of DCCH. (Adapted from T. A. Holme and R. D. Levine, Chem. Phys. Lett. 150 393 (1988).) The displacement of the atoms in each mode is shown by an arrow. Note that while all atoms contribute to all modes, the respective contributions do vary and the v, mode is almost a localized CH stretch. For recent studies of the overtone spectroscopy of HCCH and its isotopomers see J. Lievin, M. Abbouti Temsamani, P. Gaspard, and M. Herman, Chem. Phys. Lett. 190 419 (1995) M. J. Bramley, S. Carter, N. C. Handy, and 1. M. Mills, J. Mol. Spectrosc. 160 181 (1993) B. C. Smith and J. S. Winn, J. Chem. Phys. 89 4638 (1988) K. Yamanouchi, N. Ikeda, S. Tuschiya, D. M. Jonas, J. K. Lundberg, G. W. Abramson, and R. W. Field, J. Chem. Phys. 95 6330 (1991).)...

Leforestier (73) in studies of the overtone spectroscopy of CD H. Some of these results will be described later in Section III.D.2. 

In this section, we will review previous studies of the CH overtone spectroscopy of CD3H. There exist high-resolution, rotationally resolved, experimental data for the first three CH overtones ( uv,) with n = 2, 3, 4) in the infrared and near infrared region (91,94). In the visible region, there also exists data from photoacoustic laser spectroscopy (96,97) and from the intracavity laser absorption spectroscopy (ICLAS) technique, which provides absolute intensities (85,86). Compared to methane, analysis of the spectra is much easier for CD3H, because of the relatively isolated CH chromophore. 

A very demanding application of the computational methods discussed in Section II concerns the overtone spectroscopy and dynamics of energy flow in benzene. This section... 

Overtone spectroscopy and dynamics in fluoroform, CF3H. This molecule has been the focus of many experimental studies (3). Computational studies on the influence of the background modes on the CH chromophore dynamics are now underway on a new ab initio force field (167). 

Overtone spectroscopy and dynamics in CH4. The availability of new ab initio force field information (168) and the possibility of new experiments suggest that quantum dynamical studies of this molecule would be both timely and interesting. 

Because overtone excitation is a low-probability process, it is necessary to carry out the experiment with considerable sample density. Thus, most of the studies on overtone spectroscopy have been with sample pressures in excess of 0.1 Torr (gas density = n > 3 x 10 molec./cm ). An estimate of the number of molecules excited can be made with use of Beer s law which is N/N = exp(-o- n) = 1 — a , the latter approximation being valid for low-probability processes. In this expression, N and N are the intensities of the transmitted and incident light, respectively. Suppose that the cross section (o) for excitation of the fifth overtone in HOOH is lO cm (Scherer and Zewail, 1987), that the path length = 1 cm, and that the density, n is as given above. The probability for photon absorption will be 3 x 10 . If the laser delivers 10 6 photons/second, as many as 10 molecules can be excited per second. 

Vibrational overtone spectroscopy has been applied to methyl-group conformational analysis in aromatic molecules (Henry, 1987). In addition to conformational data, very accurate information on the C—H bond lengths are obtained, showing that the methyl C—H bond eclipsing the ring plane is slightly shorter than the other methyl C—H bonds, in excellent agreement with ah initio calculations. 

C. Hornberger, W. Demtroder, Photoacoustic overtone spectroscopy of acetylene in the visible and near infrared. Chem. Phys. Lett. 190, 171 (1994)... 

---