Photodissociation indirect dissociation

In contrast to indirect dissociation, which is the topic of Chapter 7, direct photodissociation is relatively simple to understand. The reflection principle describes qualitatively the fully state-resolved photofragmentation cross sections a E, n, j) as a multi-dimensional mapping of the initial coordinate distribution in the electronic ground state ... 

The main characteristics of indirect dissociation are resonances in the time-independent picture and recurrences in the time-dependent approach. Resonances and recurrences are the two sides of one coin they reveal the same dynamical information but provide different explanations and points of view. To begin this chapter we discuss in Section 7.1, on a qualitative level, indirect photodissociation of a one-dimensional system. A more quantitative analysis follows in Section 7.2. The time-dependent and the time-independent views of indirect photodissociation are outlined and illustrated in Sections 7.3 and 7.4, respectively, with emphasis on vibrational excitation of the NO moiety in the photodissociation of CH30N0(S i). Section 7.5 accentuates the relation between... 

As estimated from the widths of the partial absorption cross sections, the lifetime varies from 15 fe to 70 fs which corresponds, on the average, to merely one to four vibrational periods of NO within the T complex. The photodissociation of ClNO(Ti) is thus a hybrid of direct and indirect dissociation. 

Since direct dissociation on Si is precluded by a large barrier, indirect dissociation following a radiationless transition to o, facilitated by a seam of conical intersections, has been suggested. For that reason the 5o(l A) — 5i(2 A) seam of conical intersection in HNCO has been the subject of much recent work. ° The seam exists for both cis and trans arrangements of HNCO. The trans structures are relevant to the indirect photodissociation, noted above, while the cis structures may be relevant to the stability of a cis moiety on the 2 A potential energy surface, which has yet to be... 

From this starting point, the authors develop equations leading to the evaluation of the real symmetric K matrix to specify the scattering process on the repulsive surface and propose its determination by a variational method. Furthermore, they show explicitly the conditions under which their rigorous equations reduce to the half-collision approximation. A noteworthy result of their approach which results because of the exact treatment of interchannel coupling is that only on-the-energy-shell contributions appear in the partial linewidth. Half-collision partial linewidths are found not to be exact unless off-the-shell contributions are accidentally zero or (equivalently) unless the interchannel coupling is zero. The extension of the approach to indirect photodissociation has also been presented. The method has been applied to direct dissociation of HCN, DCN, and TCN and to predissociation of HCN and DCN (21b). 

According to the dressed oscillator model, the normal modes describing the dissociative state are assumed to be part of the set of normal modes for the initial bound state. However, the initial and final states (G and D for direct photodissociation, or Q and D for indirect photodissociation) are each characterized by their own set of normal modes, that are related to each other by a linear transformation (2,40). 

This expression has been analyzed for the dissociation of ICN. The initial thermal distribution corresponds to large j. A distribution peaked around j 25 was obtained, in good agreement with experimental data. The analysis has been generalized to describe the case of a bent triatomic molecule (53,5A). Moreover, these authors consider the scalar coupling which corresponds to indirect photodissociation. 

The Si PES, calculated by Nonella and Huber (1986), has a shallow minimum above the ground-state equilibrium, or expressed differently, a small potential barrier hinders the immediate dissociation of the excited S complex. Although the height of the barrier is less than a tenth of an eV, it drastically affects the dissociation dynamics, even at energies which significantly exceed the barrier. The excited complex lives for about 5-10 internal NO vibrational periods before it breaks apart. The photodissociation of CH3ONO through the Si state exemplifies indirect photodissociation or vibrational predissociation (Chapter 7). 

In indirect photofragmentation, on the other hand, a potential barrier or some other dynamical force hinders direct fragmentation of the excited complex and the lifetime amounts to at least several internal vibrational periods. The photodissociation of CH3ONO via the 51 state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH30N0(5i) breaks apart it first performs several vibrations within the shallow well before a sufficient amount of energy is transferred from the N-0 vibrational bond to the O-N dissociation mode, which is necessary to surpass the small barrier. 

The photodissociation of methyl nitrite in the first absorption band, CH30N0(Si) — CH3O + NO(n, j), exemplifies indirect photodissociation (Hennig et al. 1987). Figure 1.11 shows the two-dimensional potential energy surface (PES) of the S electronic state as a function of the two O-N bonds. All other coordinates are frozen at the equilibrium values in the electronic ground state. Although these two modes suffice to illustrate the overall dissociation dynamics, a more realistic picture... 

The transition from direct to indirect photodissociation proceeds continuously (see Figure 7.21) and therefore there are examples which simultaneously show characteristics of direct as well as indirect processes the main part of the wavepacket (or the majority of trajectories, if we think in terms of classical mechanics) dissociates rapidly while only a minor portion returns to its origin. The autocorrelation function exhibits the main peak at t = 0 and, in addition, one or two recurrences with comparatively small amplitudes. The corresponding absorption spectrum consists of a broad background with superimposed undulations, so-called diffuse structures. The broad background indicates direct dissociation whereas the structures reflect some kind of short-time trapping. 

The general theory for the absorption of light and its extension to photodissociation is outlined in Chapter 2. Chapters 3-5 summarize the basic theoretical tools, namely the time-independent and the time-dependent quantum mechanical theories as well as the classical trajectory picture of photodissociation. The two fundamental types of photofragmentation — direct and indirect photodissociation — will be elucidated in Chapters 6 and 7, and in Chapter 8 I will focus attention on some intermediate cases, which are neither truly direct nor indirect. Chapters 9-11 consider in detail the internal quantum state distributions of the fragment molecules which contain a wealth of information on the dissociation dynamics. Some related and more advanced topics such as the dissociation of van der Waals molecules, dissociation of vibrationally excited molecules, emission during dissociation, and nonadiabatic effects are discussed in Chapters 12-15. Finally, we consider briefly in Chapter 16 the most recent class of experiments, i.e., the photodissociation with laser pulses in the femtosecond range, which allows the study of the evolution of the molecular system in real time. 

Study the dissociation dynamics of such a system, the development of simple models can best be accomplished using semiclassical or classical techniques. In Section IV C a curve-hopping model is developed, based on a collisional reorientation of the electronic angular momentum. It assumes that a bath atom collides with just one of the diatoms and reorients its electronic angular momentum on a time scale that is short compared to the relative motion of the diatoms. The model is applied to iodine photodissociation dynamics in Section IV D. The dissociation dynamics of polyatomic systems with their internal degrees of freedom is more complex than for diatomics. If these degrees of freedom are not thermally equilibrated and are coupled to the dissociation coordinate, then their dynamics cannot simply be projected out, but rather they can act as an indirect source of excitation of the dissociation coordinate. 

Elsewhere in this volume Gentry discusses arguments in favor of an indirect photodissociation mechanism for these and other clusters. In such a mechanism the excitation energy would decay first to the van der Waals vibrational modes followed by a slower dissociation step. The direct and indirect mechanisms are very similar in that the energy acceptor modes for the two mechanisms are the same for both pictures. The point of disagreement (if there is one) is whether the intermediate state is more naturally thought of as those of the cluster or those of the products. Our intuition is that such states are so short lived that their identification as states is of limited use in describing the dynamics of the molecule. 

Full equilibration of ions at a known temperature in IMS allows measuring temperature-dependent rate constants for structural transitions, from which accurate activation energies and preexponential factors could be determined in an assumption-free manner using Arrhenius plots. In contrast, structural characterization techniques implemented in vacuum, such as various laser spectroscopies (threshold photoionization,photodissociation,or photoelectron spectroscopy ), MS/MS by collisional or other dissociation, or chemical reactivity studies lack a direct ion thermometer. In those methods, ion temperature is estimated as the source temperature (possibly with semiempirical adjustments) or gauged using various indirect thermometers, and vibrationally or electronically hot ions are the ever-... 

The photodissociation of an adsorbed molecule may occur directly or indirectly. Direct absorption of a photon of sufficient energy produces a Franck-Con-don transition from the ground to an electronically excited repulsive or predissociative state. When the excited state is repulsive, bond breaking is very fast ( 10 —10 s) and dissociation competes... 

In indirect photodissociation processes, the initial absorption occurs into a bound discrete excited state which subsequently interacts with the continuum of a final dissociating state. The process of predissociation, in which the bound potential curve is crossed by a repulsive state of a different symmetry, is illustrated in Figure 16. The cross section consists in this case of a series of discrete peaks, broadened by the predissociation process. 

Indirect photodissociation can be the result of more than one type of delay. First, the energy may not be made directly available for the motion along the reaction coordinate. If there is time for energy scrambling tiien the dissociation will be a la RRKM, as will be discussed further in Sections 111-1.1 A. There is however another type of delay. It is when electronic energy is made available as vibrational energy of a lower-lying electronic state. 

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