Measurements of Internal State Distributions

A very interesting application of LIF is the measurement of population distributions N(v., J. ) in rovibronic molecular levels under situations which differ from thermal equilibrium. Examples are chemical reactions of the type AB + C - AC + B, where a reaction product AC with internal energy is formed in a reactive collision between the partners AB and C. The measurement of the internal state distribution N q(v,J) can often provide useful information on the reaction paths and on the potential surfaces of the collision complex (AB)C. The fact that for some of these reactions -population invevsion has been observed allowing the operation of chemical lasers [8.71] may elucidate the importance of such studies. A better knowledge of the reaction mechanisms can help to optimize the conditions for maximum inversion. 

When the laser wavelength x is tuned to a molecular transition E. - Ej, the LIF collected from a solid angle 6/4tt on the photomultiplier cathode yields, according to (8.10), a photon counting rate 

When the laser is tuned successively to two different absorbing transitions 1 k and 2 - m, the ratio of the LIF signals 

This technique has been first applied by ZARE and his group [8.75] to determine the internal state distribution of molecular products formed in chem- 

Fiq 8 27. (a) Vibrational population distribution N(v ) of BaCl for two different collision energies of the reactants Ba + HCl. (b) Mean vibrational and rotational energy of reactively scattered BaCl as function of the collision energy [8.76b] 

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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... 

The use of laser-induced fluorescence as a molecular beam detector for the measurement of internal state distributions of reaction products is presented and applied to the reactions of barium with the hydrogen halides. It is found that most of the reaction exoergicity appears as translational energy of the products and that the total reactive cross section is positively correlated with the average fraction of the exoergicity appearing as vibrational excitation. 

First measurements of internal-state distributions of reaction products using LIF have been performed by Zare et al. [189-191], One example is the formation of... 

Cruse HW, Dagdigian PJ, Zare RN. 1973. Crossed-beam reactions ofbariumwithhydrogen halides. Measurement of internal state distributions by laser-induced fluorescence . Faraday Discuss. Chem. Soc. 55 277-292. 

A survey on several experiments where LIF has been applied to measurements of internal state distributions in various chemical reactions or in photodissociation products can be found in the review by KINSEY [8,70]. 

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... 

The development of techniques to measure the internal state distribution of cold trapped molecules,... 

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. 

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

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 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. 

Hofacker and Levine have proposed a non-adiabatic model in which the product vibrational distributions are described by a single parameter g, taken as a measure of the attractive character of the potential energy surface. In this model, g /2 is equal to Eyfhcw. We find that the BaX vibrational distributions are fit moderately well to the functional form given by Hofacker and Levine and that the parameter g increases monotonically along the series. Such a model shows promise as a means of relating the details of the product internal state distribution to the potential energy surface of the reaction. 

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