Internal state distribution

Coherent anti-Stokes Raman spectroscopy (CARS) [59] has also found utility in the detemiination of the internal state distributions of products of chemical reactions. This is one of several coherent Raman spectroscopies based on the... 

Sauder D G and Dagdigian P J 1990 Determination of the internal state distribution of NC produced from the H + NG2 reaction J. Chem. Phys. 92 2389-96... 

Irvine AML, Smith I W M, Tuckett R P and Yang X-F 1990 A laser-induced fluorescence determination of the complete internal state distribution of CH produced in the reaction H + NG2 CH + NC J. Chem. Phys. 93 3177-86... 

Dagdigian P J, Varley D F, Liyanage R, Gordon R J and Field R W 1996 Detection of DCI by multiphoton ionization and determination of DCI and HCI internal state distributions J. Chem. Phys. 106 10 251-62... 

Sinha M P, Schulz A and Zare R N 1973 Internal state distribution of alkali dimers in supersonic nozzle beams J. Chem. Phys. 58 549-56... 

The occurrence of predissociation opens up a new family of observable quantities. It is possible to measure not only linewidths or lifetimes, but also the internal state distributions of the fragments. All these quantities are sensitive to the intennolecular potential and can be used to test or refine proposed potential surfaces. 

The spectroscopy methods such as LIF and REMPI are utilized not only to detect the free radicals as discussed above, but also to directly measure the internal state distributions of the photoproducts in the photodissociation of free radicals. In this approach, the photochemistry is carried out in the free radical beam under single-collision conditions with well-defined... 

With the surface ionization source it is generally assumed that the reactant ion internal state distribution is characterized by the source temperature and that the majority of the reactant ions are in their ground electronic state. This contrasts with the uncertainty in reactant state distributions when transition metal ions are generated by electron impact fragmentation of volatile organometallic precursors (10) or by laser evaporation and ionization of solid metal targets (11). Many examples... 

Ultraviolet Photodissociation Studies of Organosulfur Molecules and Radicals Energetics, Structure Identification, and Internal State Distribution (Cheuk-Yiu Ng). 

For molecular desorption, laser spectroscopic studies of the desorbing molecule can give full internal state distributions, Df Ef, 6f, v, J, f M ), Ts), where f M ) is some distribution function describing the rotational orientation/alignment relative to the surface normal. For thermal desorption in non-activated systems, most atoms/molecules have only modest (but important) deviations from a thermal distribution at Ts. However, in associative desorption of systems with a barrier, the internal state distributions reveal intimate details of the dynamics. Associative desorption results from the slow thermal creation of a transition state, with a final thermal fluctuation causing desorption. Partitioning of the energy stored in V into... 

Using fs laser excitation at 620 nm, a 2PC in Y of 0.5ps [399] implicates hot electrons, probably thermalized at Te, as the mechanism for desorption induced by the fs laser (Section 2.6.2). Rotational state distributions are nearly Boltzmann characterized by Tf. The 2PC of internal state distributions was also obtained. Rather surprisingly, significant differences in these 2PC were obtained for T and the state-resolved yield for the two spin-orbit states and this was qualitatively rationalized by a DIMET picture [399]. Where overlap in experiments exist, the qualitative results are similar to those for fs laser induced desorption of NO/Pd(lll) [400,401]. For this latter system, the absolute yield Y is large at typical fluencies used in the experiments and a very hot vibrational distribution was observed (Tv = 2900 K). 

A. Internal State Distributions of the Fragments of Molecular Photodissociation... 

There are two alternative methods for determining internal-state distributions in single-source photoionization experiments, depending on the prevailing type of ionization process. 

The internal state distribution of the product 0H(X II) was probed by Jacobs et al. (38) using laser induced fluorescence. 

Other than the earlier work reviewed by Ashfold et al. (3), only three studies on the photodissociation dynamics have been reported for this molecule (153,154,158). The first study reported the quantum state distribution of the CN radical obtained in an effusive molecular beam and in a static gas cell, while the second study reported the observations in a pulsed molecular beam. The dynamics remains the same despite the fact that the initial internal state distribution of the C1CN molecule changes. This of course shows that hot bands are not important in the photodissociation of this molecule at this wavelength. 

Spiglanin, T.A., Perry, R.A., and Chandler, D.W. (1987). Internal state distributions of CO from HNCO photodissociation, J. Chem. Phys. 87, 1568-1576. 

Vasudev, R., Zare, R.N., and Dixon, R.N. (1983). Dynamics of photodissociation of HONO at 369 nm Motional anisotropy and internal state distribution of the OH fragment, Chem. Phys. Lett. 96, 399-402. 

Standard methods are used to propagate each Om in time. For the z and Z coordinates we make use of the fast fourier transform [99], and for the p coordinate we use the discrete Bessel transform [100]. The molecular component of asymptotic region at each time step, and projected onto the ro-vibrational eigenstates of the product molecule, for a wide range of incident energies included in the incident wave packet [82]. The results for all ra-components are summed to produce the total ER reaction cross section, a, and the internal state distributions. 

The reactions of alkaline earth atoms with alkali halide molecules are especially noteworthy because laser-induced fluorescence has been employed in these crossed-molecular beam experiments to measure the product internal state distributions as a function of scattering angle. For Ba + KC1 and Ca + NaCl, both the atomic and diatomic products were detected. 

The formation of the alkaline earth cyanide is the major pathway in the reaction M + BrCN. The other channel (giving MBr + CN) is observed for the reactions of Ba and Sr. The ratio of the cross section is o(BaCN)/ a(BaBr) 25-100 and a(SrCN)/o(SrBr) 250-1000 [363]. It was not possible to measure internal state distribution for the alkaline earth salts, but for the CN product of the minor channel, the vibrational distribution was found to be N(d = 1)/N(p = 0) <. 0.2 and Txot = 1250K for Ba + BrCN and TIot = 750 K for Sr + BrCN. The reaction dynamics appear to be consistent with an electron jump mechanism which would favour the breakup of the M+(BrCN) ion pair to give MCN + Br. 

The observation of statistical internal state distributions for the ground state MO product from the reactions M + NO and S02, prompts the suggestion that a long-lived complex is involved in these reactions [393]. For the chemiluminescent reactions La + 02, N02 and N20, it is found that as the exoergicity increases, more highly excited states of LaO are populated [390] however, the reaction La + 03 deviates from this trend. 

In a particularly novel extension of the studies of CsF, Freund, Fisk, Herschbach and Klemperer [55] have used electric resonance to probe the internal state distribution of CsF formed by the crossed beam reaction of Cs and SF6. Transitions involving./ = 1 to 4 and v = 0 to 4 were observed, and details of the kinematics thereby unravelled. A block diagram of the electric resonance spectrometer is shown in figure 8.33. 

The dynamics of the reactions of 0( P) with cyclohexane, cyclohexene, and cyclohexa-1,4-diene have been studied by measurement of the product OH(X II) internal state distributions in a molecular beam/LIF apparatus. The rotational state distributions were found to be similar for all three reactions and consistent with small (1—3%) partitioning of the available energy, indicating that H-abstraction occurs only when the O atom is collinear with the C-H bond under attack. Comparisons with model predictions suggested that some of the extra energy available in the more exoergic reactions between 0( P) and the unsaturated hydrocarbons is released into internal excitation of the hydrocarbon radical product, resulting in only a modest increase in OH vibrational excitation. 

Commentary on Crossed-Beam Reactions of Barium with Hydrogen Halides Measurement of Internal State Distributions by Laser-induced Fluorescence, H. W. 

Measurement of Internal State Distributions by Laser-Induced Fluorescence... 

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