Vibrational overtone excitation

A number of studies of unimolecular reactions induced by single-photon vibrational overtone excitation have appeared this year. The basic motivation behind such experiments has been the possibility of observing mode-selective photochemistry resulting from the apparently highly localized nature of overtones of X-H stretching vibrations (X = C, O, etc.) in polyatomic molecules. A series of studies by Jasinski, Frisoli, and Moore examined the isomeriza-... 

A class of reactions that has been studied frequently by classical mechanics in the gas phase is unimolecular dissociations and isomerizations induced by vibrational overtone excitation. However, until very recently, there have been no simulations of such processes in condensed phases. This situation has recently changed with the simulations by Li et al. i of the dissociation of HOCl in Ar and the work of Finney and Martens i on the dissociation of HOOH in Ar clusters. 

See, for example, D. L. Bunker, /. Chem. Phys., 40,1946 (1963). Monte Carlo Calculations. IV. Further Studies of Unimolecular Dissociation. D. L. Bunker and M. Pattengill,/. Chem. Phys., 48, 772 (1968). Monte Carlo Calculations. VI. A Re-evaluation erf Ae RRKM Theory of Unimolecular Reaction Rates. W. J. Hase and R. J. Wolf, /. Chem. Phys., 75,3809 (1981). Trajectory Studies of Model HCCH H -P HCC Dissociation. 11. Angular Momenta and Energy Partitioning and the Relation to Non-RRKM Dynamics. D. W. Chandler, W. E. Farneth, and R. N. Zare, J. Chem. Phys., 77, 4447 (1982). A Search for Mode-Selective Chemistry The Unimolecular Dissociation of t-Butyl Hydroperoxide Induced by Vibrational Overtone Excitation. J. A. Syage, P. M. Felker, and A. H. Zewail, /. Chem. Phys., 81, 2233 (1984). Picosecond Dynamics and Photoisomerization of Stilbene in Supersonic Beams. II. Reaction Rates and Potential Energy Surface. D. B. Borchardt and S. H. Bauer, /. Chem. Phys., 85, 4980 (1986). Intramolecular Conversions Over Low Barriers. VII. The Aziridine Inversion—Intrinsically Non-RRKM. A. H. Zewail and R. B. Bernstein,... 

The examples discussed above illustrate the utility of vibrational overtone excitation by red sunlight in atmospheric photochemistry. The low absorption cross-section of vibrational overtones limits the importance of such light-initiated chemistry. However, when reactive electronic states are high in energy (as is the case with most alcohols and acids) or when UV radiation is suppressed at high solar zenith angles, vibrational overtone initiated photochemistry has been used to explain discrepancies between measurements and model results. 

In the vibrational overtone excitation in HN3(Vi =5, 6), NH in the electronic ground state can be produced in a spin-forbidden dissociation [89 to 91]. 

Callegari A, Rebstein J, Muenter J S, Jost R and Rizzo T R 1999 The spectroscopy and intramolecular vibrational energy redistribution dynamics of HOCI in the u(OH) = 6 region, probed by infrared-visible double resonance overtone excitation J. Chem. Phys. 111 123-33... 

In multiplying by we use, again, examples of the vibrations of NH3. The result depends on whether we require when (a) one quantum of each of two different e vibrations is excited (i.e. a combination level) or (b) two quanta of the same e vibration are excited (i.e. an overtone level). In case (a), such as for the combination V3 - - V4, the product is written E x E and the result is obtained by first squaring the characters under each operation, giving... 

Tables for all degenerate point groups, giving the symmetry species of vibrational combination states resulting from the excitation of one quantum of each of two different degenerate vibrations and of vibrational overtone states resulting from the excitation of two quanta of the same degenerate vibration, are given in the books by Herzberg and by Hollas, referred to in the bibliography.

In addition there is the possibility of combination tones involving transitions to vibrationally excited states in which more than one normal vibration is excited. Fundamental, overtone and combination tone transitions involving two vibrations and Vj are illustrated in Figure 6.11. 

Chemiluminescence is observed from several different emitting species, depending on the analyte and reaction conditions. Vibrational overtone bands of HF in the wavelength region of =500-900 nm are observed under nearly all conditions and are often the dominant spectral feature, the (3,0), (4,0) (5,1), and (6,2) bands being the most intense, while for some reaction conditions emissions from levels up to v = 8 are observed [63], It is likely that hydrogen atoms are produced in the reaction and form vibrationally excited HF in the reaction reported by Mann et al. [62] ... 

Donaldson, D. J., G. J. Frost, K. H. Rosenlof, A. F. Tuck, and V. Vaida, Atmospheric Radical Production by Excitation of Vibrational Overtones via Absorption of Visible Light, Geophys. Res. Lett., 24, 2651-2654 (1997). 

A rule of thumb for hydride stretches [56, 57] is that the intensities of the vibrational overtone and combination transitions decrease, approximately, as IQ-Ay jjjg drop-off in intensity for the first few quanta of excitation may be even steeper, by another factor of 10. This implies that, in a specific spectral interval, the strongest vibrational transitions from the vibrationless ground state level correspond to the transition with the smallest Av and the greatest anharmonicity. However, as shown later, even these small absorption cross sections of vibrational overtone transitions can be sufficient for overtone preexcitation. 

The quenching of the metal excited state by coupling to the O-H vibrational overtones has already been mentioned. Horrocks and Sudnick (22) showed that this effect (shown in Pig. 3) can be utilized in the elucidation of the structure of the complex in solution. 

It is also possible that the unimolecular reaction takes place with the molecule in the electronic ground state, but it requires very intense fields to generate so-called multiphoton or direct overtone transitions that is, transitions from the vibrational ground state of the type 0 —> n, where n > 1. The opening of the cyclo-butene ring to form butadiene is an example of a unimolecular reaction induced by direct overtone excitation ... 

Similarly, in the case of bimolecular reactions, Zare s group [466] confirmed theoretical predictions and demonstrated experimentally [467-469] that by exciting either the OH or the OD bond in HOD one can selectively enhance product forma- tion in a subsequent H + HOD reaction. Specifically, when the OH bond is excited , the reaction yields H2 + OD, whereas when the OD bond is excited, H reacts with HOD to form the HD + OH product. In these experiments, the OH was prepared either by overtone excitation [57, 58] to the fourth vibrational level v = 4 or by excitation to the u = 1 state by Raman pumping [102]. As yet to be verified experimentally is the computational prediction of Manz et al. [124, 125] that strong optimized pulses can also achieve selective excitation of higher lying vibrational) ... 

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