Overtone induced dissociation

The central O—O bond in hydrogen peroxide has a dissociation energy of 17,350 cm-1, an amount of energy which can be more than achieved by excitation of the v = 6 overtone of an O—H bond. As a consequence, there have been a number of experimental and theoretical studies of overtone induced dissociation of hydrogen peroxide. 

Because the reaction has been the subject of considerable experimental activity, the overtone-induced dissociation of H202 has been frequency studied (124,125). In a model study based on HOO dissociation, Uzer and co-workers (126) compared classical and quantum dynamics. 

The velocity side of the initial conditions can be handled by choosing the velocities in all the coordinates from the Boltzmann distribution at the appropriate temperature. For some nonthermally activated processes, for example, overtone-induced dissociations or vibrational excitation, the energy can be placed into the system at this time by adjusting the velocity, and hence the kinetic energy, of the appropriate coordinate. 

An overtone induced dissociation consists of two parts Energy is introduced into a local vibrational mode by using (usually) visible light to excite a high overtone of that bond. The amount of energy introduced in this fashion is sufficient to dissociate another, weaker bond in the molecule. (For example, in HOOH, energy is introduced into the OH bond in order to dissociate the OO bond.) The second part of the process involves the energy flow from the local mode into the dissociative bond. 

ABSTRACT. The mechanisms for energy flow from overtone excited HC and HO local modes have been elucidated in two mode model Hamiltonians of benzene and trihalomethanes and in a six mode model of HOOH molecule. Intramolecular vibrational relaxation (IVR) from the excited 2 1 Fermi resonance is shown to be very sensitive to the stretch-bend potential energy coupling in connection with the stability of the HC stretch periodic orbit. The overtone induced dissociation of HOOH, which is a slow process in comparison with the initial HO overtone relaxation, is explained in terms of the details of the potential energy surface. 

We have also studied the overtone induced dissociation of HOOH by classical trajectory calculations using different Hamiltonians (12). Our results illustrate the importance of bending modes and provide evidence for incomplete relaxation of energy. The experimental data suggest a strong coupling of the torsional vibration to the HO stretch mode due to the dependence of the torsional barrier heights on the level of HO excitation, but the trajectory studies are not yet conclusive. The importance of rotation has been stressed by Sumpter and Thompson(12). 

In section II, we describe the results of a classical trajectory calculation on two mode model for benzene relaxation in conection with the sensitivity of IVR to potential coupling in terms of the classical phase space structure. The same kind of approach is used in section III for a Hamiltonian model of fluoroform. Preliminary results of classical trajectory calculations of overtone induced dissociation in HOOH using two types of potential surface are presented in section IV. We end with a brief discussion of some possible extensions of our work and outstanding problems. 

We have carried out classical trajectory calculations of overtone induced dissociation in HOOH using two types of potential surface(18) to explore the sensitivity of unimolecular decay lifetimes to surface details. 

Fig. 1.3. Schematic illustration of unimolecular decay induced by electronic excitation. In (a) the photon creates a bound level in the upper electronic state which subsequently decays as a result of a radiationless transition (rt) to the electronic ground state. In (b) overtone pumping directly creates a quantum state above the threshold of the electronic ground state. In both cases the dissociation occurs in the electronic ground state.

Another example of a dramatic difference in the population of the two spin-orbit states is the dissociation of HN3 in the electronic ground state induced by overtone pumping (Foy, Casassa, Stephenson, and King 1988 ... 

HOD. A 722.5 nm laser pulse (A,i) excites the third overtone stretch of OH. After a short delay, a pulse of ultraviolet radiation of frequency V2 (wavelength X2) dissociates the molecule, and a third pulse with a wavelength near 308 nm (X3) probes the OH or OD fragments by laser-induced fluorescence. It is observed that with a dissociation wavelength of 266 or 239.5 nm, the products are almost exclusively H + OD,... 

Figure 12 The classical power spectrum of the OH stretch of OHC1 initially excited to the v = 6 overtone of the OH mode. This provides enough energy to dissociate the weak O—Cl bond but the two modes, which very much differ in their frequency, are not effectively coupled. Two power spectra are shown corresponding to propagating classical trajectories for the first ps after excitation and for the time interval between 1 and 2 ps after excitation, (a) Computation for the isolated molecule, (b) Computations for the molecule in liquid Ar at the (quite high) reduced density of 0.83. Note the solvent induced changes in the spectrum showing that the environment can affect the course of IVR. (Adapted from Y. S. Li, R. W. Whitnell, K. R. Wilson, and R. D. Levine, J. Phys. Chem. 97 3647 (1993).)...

Figure 4.3 Energy level diagram for infrared-optical double resonance excitation of HOOH to the 6pqh vibrational overtone level and subsequent laser-induced fluorescence detection of the OH dissociation products. The 6vqh level is at 18,943 cm above the ground state. Since the O—O bond dissociation energy of HOOH is 17,035 cm molecules dissociating from 6vqh have 1913 cm of excess vibrational energy to be partitioned between the two OH fragments Luo and Rizzo, 1990).

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,... 

Cold, trapped HD+-ions are ideal objects for direct spectroscopic tests of quantum-electrodynamics, relativistic corrections in molecules, or for determining fundamental constants such as the electron-proton mass ratio. It is also of interest for many applications since it has a dipole moment. The potential of localizing trapped ions in Coulomb crystals has been demonstrated recently with spectroscopic studies on HD+ ions with sub-MHz accuracy. The experiment has been performed with 150 HD+ ions which have been stored in a linear rf quadrupole trap and sympathetically cooled by 2000 laser-cooled Be+ ions. IR excitation of several rovibrational infrared transitions has been detected via selective photodissociation of the vibra-tionally excited ions. The resonant absorption of a 1.4/itm photon induces an overtone transition into the vibrational state v = A. The population of the V = A state so formed is probed via dissociation of the ion with a 266 nm photon leading to a loss of the ions from the trap. Due to different Franck-Condon factors, the absorption of the UV photon from the v = A level is orders of magnitude larger than that from v = 0. 

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