Polyatomic molecules photodissociation dynamics

Flowever, in order to deliver on its promise and maximize its impact on the broader field of chemistry, the methodology of reaction dynamics must be extended toward more complex reactions involving polyatomic molecules and radicals for which even the primary products may not be known. There certainly have been examples of this notably the crossed molecular beams work by Lee [59] on the reactions of O atoms with a series of hydrocarbons. In such cases the spectroscopy of the products is often too complicated to investigate using laser-based techniques, but the recent marriage of intense syncluotron radiation light sources with state-of-the-art scattering instruments holds considerable promise for the elucidation of the bimolecular and photodissociation dynamics of these more complex species. 

Schinke R and Huber J R 1993 Photodissociation dynamics of polyatomic molecules. The relationship between potential energy surfaces and the breaking of molecular bonds J. Rhys. Chem. 97 3463... 

R. Schinke. Photodissociation Dynamics Spectroscopy and Fragmentation of Small Polyatomic Molecules. Cambridge University Press, Cambridge (1995). 

The final rotational state distributions of the products in the fragmentation of a polyatomic molecule contain additional clues about the intra- and intermolecular dynamics, especially about the coupling in the exit channel. In realistic as well as model studies it has been observed that the rotational state distributions of the photodissociation products reflect the angular dependence of the wave function at the transition state and the anisotropy of the PES in the exit channel [4, 9, 10]. HO2 is no exception. 

Besides its practical importance, photodissociation — especially of small polyatomic molecules — provides an ideal opportunity for the study of molecular dynamics on a detailed state-to-state level. We associate with molecular dynamics processes such as energy transfer between the various molecular modes, the breaking of chemical bonds and the creation of new ones, transitions between different electronic states etc. One goal of modern physical chemistry is the microscopical understanding of molecular reactivity beyond purely kinetic descriptions (Levine and Bernstein 1987). Because the initial conditions can be well defined (absorption of a single monochromatic photon, preparation of the parent molecule in selected quantum states), photodissociation is ideally suited to address questions which are unprecedented in chemistry. The last decade has witnessed an explosion of new experimental techniques which nowadays makes it possible to tackle questions which before were beyond any practical realization (Ashfold and Baggott 1987). 

Analysis of the correlations of fi, v, and j with Eo and among each other is necessary for a full understanding of photodissociation dynamics, especially for polyatomic molecules with more than three atoms. Vector correlations play an increasingly important role in experimental investigations. This section covers only the most elementary aspects and for deeper insight the reader is referred to the numerous reviews on this fascinating topic (Zare 1972 Simons 1977 Houston 1987, 1989 Simons 1987 Hall and Houston 1989). 

Transitions between electronic states are formally equivalent to transitions between different vibrational or rotational states which were amply discussed in Chapters 9 11. Computationally, however, they are much more difficult to handle because they arise from the coupling between electronic and nuclear motions. The rigorous description of electronic transitions in polyatomic molecules is probably the most difficult task in the whole field of molecular dynamics (Siebrand 1976 Tully 1976 Child 1979 Rebentrost 1981 Baer 1983 Koppel, Domcke, and Cederbaum 1984 Whetten, Ezra, and Grant 1985 Desouter-Lecomte et al. 1985 Baer 1985b Lefebvre-Brion and Field 1986 Sidis 1989a,b Coalson 1989). The reasons will become apparent below. The two basic approaches, the adiabatic and the diabatic representations, will be outlined in Sections 15.1 and 15.2, respectively. Two examples, the photodissociation of CH3I and of H2S, will be discussed in Section 15.3. 

Gelbart, W.M. (1977). Photodissociation dynamics of polyatomic molecules, Ann. Rev. Phys. Chem. 28, 323-348. 

Photodissociation of small polyatomic molecules is an ideal field for investigating molecular dynamics at a high level of precision. The last decade has seen an explosion of many new experimental methods which permit the study of bond fission on the basis of single quantum states. Experiments with three lasers — one to prepare the parent molecule in a particular vibrational-rotational state in the electronic ground state, one to excite the molecule into the continuum, and finally a third laser to probe the products — are quite usual today. State-specific chemistry finally has become reality. The understanding of such highly resolved measurements demands theoretical descriptions which go far beyond simple models. 

From the point of view of chemical reaction dynamics, the most interesting case is that of unbound excited states or excited states coupled to a dissociative continuum that is, photodissociation dynamics. The dissociative electronically excited states of polyatomic molecules can exhibit very complex dynamics, usually involving nonadiabatic processes. The TRPES and TRCIS may be used to study the complex dissociation dynamics of neutral polyatomic molecules, and below we will give two examples of dissociative molecular systems that have been studied by these approaches, NO2 and (NO)2. 

Photochemical Reactions.—The use of correlation diagrams in chemical dynamics has been discussed in a review article,510 and the problem of potential-surface crossing in diatomic511 and polyatomic molecules has been widely considered in several papers.512 A paper has appeared concerned with the quantum-mechanical expression to describe the relaxation processes in a chemically reacting gas under monochromatic (laser) excitation.513 Other quantum theories of molecular photodissociation have also been published.514 These are too extensive for detailed consideration here, but in general provide solvable models for small-molecule reactions. 

As already mentioned, water and hydrogen sulfide are often regarded as archetypal, small polyatomic systems for the study of molecular photodissociation dynamics. It is not surprising, therefore, that several groups have extended such studies to include their homologues, methanol (CH3OH) and methanethiol (CH3SH). Such studies have been stimulated further by the industrial and atmospheric relevance of these molecules. 

Small polyatomic organosulfur molecules are excellent molecular systems for detailed experimental studies of photodissociation dynamics because of their high photodissociation cross sections in a wide UV range, matching the output of commercial lasers. Because of this and other practical interests, the UV photochemistry of simple stable organosulfur molecules has received much experimental [40-61] and theoretical [48-50,56-59,62-69] attention in recent years. As summarized below, the UV photodissociation of simple polyatomic organosulfur species displays bond-breaking selectivity similar to that observed in the photodissociation of CHjIBr. 

Finally, we note that Figure 17.2 and the above brief discussion of acetylene photodissociation (Section 17.1) serve to illustrate the increasing complexity of PESs, which are already quite challenging even for molecules containing only four atoms. We shall discuss other possible ways of deciphering the complex dynamics involved in the photodissociation of large polyatomic molecules at the end of Chapter 19, and some further examples of polyatomic photodissociation dynamics will be discussed in Chapter 18, which is concerned with multiphoton excitation. 

Time-resolved ionization offers several advantages as a probe of these wavepackets [41, 42, 343, 360]. For example, the ground state of an ion is often better characterized than higher excited states of the neutral molecule, particularly for polyatomics. Ionization is also universal and hence there are no dark states. Furthermore, ionization provides both ions and photoelectrons and, while ion detection provides mass and kinetic-energy resolution in time-resolved studies [508], photoelectron spectra can provide complementary information on the evolution of the wavepacket [22, 63, 78, 132, 201, 270, 271, 362, 363, 377]. Its utihty for real-time probing of molecular dynamics in the femtosecond regime has been nicely demonstrated in studies of wavepackets on excited states of Na2 [22], on the B state of I2 [132], and on the A state of Nal [201]. Femtosecond photoelectron-photoion coincidence imaging studies of photodissociation dynamics have been reported [107]. 

Schinke R (1995) Photodissociation dynamics spectroscopy and fragmentation of small polyatomic molecules. Cambridge University Press, Cambridge... 

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