Dynamics photofragmentation

These photofragmentation T matrix elements contain all the information about the photofragmentation dynamics. We will now discuss how they may be extracted from the time-dependent wavepacket calculations. 

Beswick, J.A. (1991). Photofragmentation dynamics, in Structure, Interactions, and Reactivity, ed. S. Fraga (Elsevier, Amsterdam). 

Drobits, J.C. and Lester, M.I. (1988a). Near threshold photofragmentation dynamics of ICL-Ne A state van der Waals complexes, J. Chem. Phys. 88, 120-128. 

Grunewald, A.U., Gericke, K.-H., and Comes, F.J. (1987). Photofragmentation dynamics of hydrogen peroxide Analysis of two simultaneously excited states, J. Chem. Phys. 87, 5709-5721. 

The first quantitative measurement of the distribution of excited atomic states produced in the multiphoton dissociation of a metal carbonyl has been made for Cr(CO)6. Photodissociation does not yield spin- or parity-differentiated states, rather the state distribution appears to be statistical. Photofragmentation dynamics of Cr(CO)6 in the gas phase have been measured and two channels of dissociation revealed. One of these is a rapid predissociation (efficiency 36%) and the other a slow process (efficiency... 

As will be seen in Chapter 8, equations identical to Eq. (7.2.13) and Eq. (7.2.17) describe the photoelectron angular distributions observed in photoionization processes. However, owing to the difference in photofragment masses (atom vs. electron), the photoelectron angular distributions sample the photofragmentation dynamics very differently from photodissociation angular distributions. 

Experimental determinations of photofragmentation dynamics have been matched by increasing interest in models for the photodissociation process. Several papers have been concerned with the calculation of angular distribution... 

The experimental techniques used and the results obtained for all three systems will be discussed in greater detail in the following sections. The final section concludes the review with a brief discussion of future prospects for the use of radioisotope detection in the study of collision and photofragmentation dynamics. 

As seen above, laser assisted and controlled photofragmentation dynamics can conceptually be viewed in two different ways. The time-dependent viewpoint offers a realistic time-resolved dynamical picture of the basic processes that are driven by an intense, short laser pulse. For pulses characterized by a long duration (as compared to the timescales of the dynamics), the laser field can be considered periodic, allowing the (quasi-) complete elimination of the time variable through the Floquet formalism, giving rise to a time-independent viewpoint. This formalism not only offers a useful and important interpretative tool in terms of the stationary field... 

The accurate TD approach to tetraatomic systems was first applied to the photofragmentation dynamics of H2HF H2 + HF [42, 43] and HOOH OH + OH [44], in which the two diatomic vibrations were frozen but all the other four internal degrees of freedom were treated exactly. These two theoretical studies established the numerical feasibilities for the accurate TD wave-packet treatment for tetraatomic dynamics problems. Within quick succession, accurate quantum dynamics calculations for the tetraatomic reaction AB + CD A + BCD were reported for the benchmark reaction H2 + OH H + H2O [45-47] and its isotopic reactions DH F OH D + H2O, H F DOH [48] and D2 F OH D F DOD [49]. For the H2 F OH reaction, cumulative reaction probabilities have also been computed by calculating the flux directly without summing over individual reaction probabilities [50-52]. Additionally, accurate dynamics calculations for reactions of HO F CO H F CO2 [53], H2 F CN H F HCN [54, 56], and D2 F CN D F DCN [55] have been reported. With the exception of Refs. [50, 56] which use the time-independent iterative approach to calculate cumulative reaction probabilities, the other works all applied the TD wavepacket approach. In addition, the TD calculation for the reverse reaction of H F H2O H2 F OH has been reported [57]. More recently, state-to-state TD calculations have been reported for the H2 F OH reaction [58, 59] and its reverse reaction H F H2O [60]. 

Probing product state distributions by multiphoton ionization is one of the most sensitive methods for the analysis of both bimolecular and photofragmentation dynamics. For example, by using REMPI one can measure the rotational state distribution in the N2 fragment produced in the photofragmentation of N2O it was found that the maximum in the rotational state population is near J 70. This reveals that although the ground electronic state is linear, the excited state is bent and thus the recoil from the O atom results in rotational excitation of the N2 molecule... 

For the reaction A + BC AB + C, the partition of the total angular momentum J between the initial and flnal momentum of the colliding particles L, V and the rotational momenta of the reactant and product molecules j,f has been shown to be very useful in the diagnosis of the reaction dynamics. The main problem is that even if one lets two molecular beams collide with well-defined speeds and directions, one cannot select the impact parameter and its azimuthal orientation about the initial relative velocity vector. A currently popular way to circumvent this lack of resolution is to use vector correlations, particularly in laser studies, photofragmentation dynamics and, more generally, the so-called field of dynamical stereochemistry . One of the most commonly used correlations is that between the product rotation angular momentum and the initial and final relative velocity vectors. 

In photofragmentation dynamics experiments, the rotational alignment is obtained by analysis of laser-polarized broadband spectra. Thus, the LIF of a fragment I due to a photodissociation process is given by (Greene and Zare, 1982)... 

The binding forms of these clusters are often weak interactions of the van der Waals type. These forces are responsible for important phenomena, such as deviations of real gases from ideal behaviour and condensation of atoms and molecules into the liquid and crystalline states. These weakly bound molecules, often called van der Waals molecules, have become a model system not only to study intra-molecular energy transfer processes, but also photofragmentation dynamics. A whole chapter of this part, therefore, is dedicated to laser studies of the photodissociation of van der Waals molecules. 

The pump and probe technique using picosecond lasers also allows us to investigate the I2 (C02) cluster ion photofragmentation dynamics in real time. An example of such an investigation is shown in Figure 25.5 for cluster size n= 12-17, where the absorption recovery of I2 is displayed as a function of the pump-probe delay. 

Photofragmentation dynamics of adsorbed molecules surface-aligned photoreactions... 

In Chapter 27 we studied the photochemistry of the adsorbate state, and we discussed the occurrence of localized atomic scattering as a key feature characterizing the photofragmentation dynamics of adsorbed molecules. Furthermore, it was shown how the concept of localized atomic scattering extended to that of localized atomic reaction, in which the new bond created at the surface takes place in an adjacent location to the old (broken) bond. 

Selectivity with respect to reagents leads to specificity with respect to products, exemplified by non-statistical branching ratios, anisotropy of angular distribution and "surprising" product state distributions. The importance of vector correlations (from crossed beam experiments with polarized laser-induced fluorescence detection) is touched upon, as well as the subject of polarized laser photofragmentation dynamics (half-collisions) leading to (non-statistical) specificity in product polarizations. 

Nesbitt, D.J.H., H. Petek, M.F. Foltz, S.V. FUseth, D.J. Bamford, and C.B. Moore (1985), Photofragmentation dynamics of ketene at 306 nm Initial vibrational and rotational state distributions of CO product by vacuum UV laser-induced fluorescence, J. Chem. Phys., 83, 223-229. 

Shinohara, H., and N. Nishi (1982), Laser photofragmentation dynamics of an acrolein supersonic molecular beam at 193 run, J. Chem. Phys., 77, 234—245. 

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