Products vibrational state distribution

In highly exothermic reactions such as this, that proceed over deep wells on the potential energy surface, sorting pathways by product state distributions is unlikely to be successful because there are too many opportunities for intramolecular vibrational redistribution to reshuffle energy among the fragments. A similar conclusion is likely as the total number of atoms increases. Therefore, isotopic substitution is a well-suited method for exploration of different pathways in such systems. 

Figure 12. Potential energy contour plots for He + I Cl(B,v = 3) and the corresponding probability densities for the n = 0, 2, and 4 intermolecular vibrational levels, (a), (c), and (e) plotted as a function of intermolecular angle, 0 and distance, R. Modified with permission from Ref. 40. The I Cl(B,v = 2/) rotational product state distributions measured following excitation to n = 0, 2, and 4 within the He + I Cl(B,v = 3) potential are plotted as black squares in (b), (d), and (f), respectively. The populations are normalized so that their sum is unity. The l Cl(B,v = 2/) rotational product state distributions calculated by Gray and Wozny [101] for the vibrational predissociation of He I Cl(B,v = 3,n = 0,/ = 0) complexes are shown as open circles in panel (b). Modified with permission from Ref. [51].

Gray and Wozny [101, 102] later disclosed the role of quantum interference in the vibrational predissociation of He Cl2(B, v, n = 0) and Ne Cl2(B, v, = 0) using three-dimensional wave packet calculations. Their results revealed that the high / tail for the VP product distribution of Ne Cl2(B, v ) was consistent with the final-state interactions during predissociation of the complex, while the node at in the He Cl2(B, v )Av = — 1 rotational distribution could only be accounted for through interference effects. They also implemented this model in calculations of the VP from the T-shaped He I C1(B, v = 3, n = 0) intermolecular level forming He+ I C1(B, v = 2) products [101]. The calculated I C1(B, v = 2,/) product state distribution remarkably resembles the distribution obtained by our group, open circles in Fig. 12(b). 

Figure 18. Vibrational product state distribution of O2 following the dissociation of HO2, both quantum and classical, together with the predictions of PST. (Reprinted, with permission of the American Institute of Physics, from Ref. 37.)...

Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-93]. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pump-probe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon... 

The knowledge of the internal-energy distribution is of equal interest for the practical applications indicated in the preceding paragraphs. First spectroscopic obervations of the IR emission from the molecule BC, which is related to the vibrational-state population, were reported by Karl and Polanyi13 on the system Hg + CO. These measurements were subsequently improved and extended.14-16 Recent time-resolved experiments with IR-laser absorption17- 18 and emission techniques19-21 yield more reliable results on the product-state distribution. 

The most important conclusions of these dynamical studies is that van der Waals clusters behave in a statistical manner and that IVR/VP kinetics are given by standard vibrational relaxation theories (Beswick and Jortner 1981 Jortner et al. 1988 Lin 1980 Mukamel and Jortner 1977) and unimolecular dissociation theories (Forst 1973 Gilbert and Smith 1990 Kelley and Bernstein 1986 Levine and Bernstein 1987 Pritchard 1984 Robinson and Holbrook 1972 Steinfeld et al. 1989). One can even arrive at a prediction for final chromophore product state distributions based on low energy chromophore modes. If rIVR tvp [4EA(Ar)i], a statistical distribution of cluster states is not achieved and vibrational population of the cluster does not reflect an internal equilibrium distribution of vibrational energy between vdW and chromophore states. If tvp rIVR, and internal vibrational equilibrium between the vibrational modes is established, and the relative intensities of the Ar = 0 torsional sequence bands of the bare chromophore following IVR/VP can be accurately calculated. A statisticsl sequential IVR/VP model readily explains the data set (i.e., rates, intensities, final product state distributions) for these clusters. 

The fragment excited-state NO(A2S+) is a molecular 3r Rydberg state, and we shall refer to this as NO(A, 3s). The observed NO(A, 3.v) product state distributions supported the notion of a planar dissociation involving restricted intramolecular vibrational energy redistribution (IVR) [176]. A scheme for studying NO dimer photodissociation dynamics via TRPES is depicted in Fig. 25. The NO(A, 3.v) + NO(X) product elimination channel, its scalar and vector properties, and its evolution on the femtosecond time scale have been discussed in a number of recent publications (see Ref. [175] and references cited therein). 

Vibrational product state distributions have been obtained for reactions studied in crossed molecular beams using the technique of beam electric resonance spectroscopy [109]. This method uses the focusing action of electric quadrupole and dipole fields to measure the radio frequency Stark spectrum of the reaction products, which must possess a dipole moment. This has restricted this technique to reactions producing alkali halides. 

Vissers GWM, Groenenboom GC, van der Avoird A (2003) Spectrum and vibrational predissociation of the HF dimer. II. Photodissociation cross sections and product state distributions. J Chem Phys 119 286-292... 

In this chapter we elucidate the state-specific perspective of unimolec-ular decomposition of real polyatomic molecules. We will emphasize the quantum mechanical approach and the interpretation of the results of state-of-the-art experiments and calculations in terms of the quantum dynamics of the dissociating molecule. The basis of our discussion is the resonance formulation of unimolecular decay (Sect. 2). Summaries of experimental and numerical methods appropriate for investigating resonances and their decay are the subjects of Sects. 3 and 4, respectively. Sections 5 and 6 are the main parts of the chapter here, the dissociation rates for several prototype systems are contrasted. In Sect. 5 we shall discuss the mode-specific dissociation of HCO and HOCl, while Sect. 6 concentrates on statistical state-specific dissociation represented by D2CO and NO2. Vibrational and rotational product state distributions and the information they carry about the fragmentation step will be discussed in Sect. 7. Our description would be incomplete without alluding to the dissociation dynamics of larger molecules. For them, the only available dynamical method is the use of classical trajectories (Sect. 8). The conclusions and outlook are summarized in Sect. 9. 

In this section, we shall focus exclusively on the scalar properties of the fragments and consider the vibrational and rotational product state distributions (PSD s) following the dissociations of HCO, NO2, and H2CO discussed in Sects. 5 and 6. An in-depth introduction to the vast and fascinating field of product state analysis can be found in Ref. 20 (Chapters 9, 10, and 11). Recently, the PSD s of several representative groups of molecules were reviewed in Ref. 306. 

Internal energy partitioning between vibration and rotation is very different for Ai and A2 symmetries 18% of the internal energy goes into rotation for the Ai symmetry, in contrast with 50% for the A2 symmetry. This reflects itself in the product state distributions of Fig. 11, which have a maximum for low rotational quantum number j in the Ai symmetry, but for j near 15 for the A2 case. 

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