Dissociation channel

Continuum states require a completely different treatment than bound states. Their definition and general behavior is the topic of this section. 

The possible dissociation channels for the fragmentation of a triatomic molecule were discussed in Section 1.4. The linear ABC molecule can fragment into three chemical channels, A+B+C, A+BC(n), and AB(n )+C with the diatoms being produced in particular vibrational states denoted by quantum numbers n and n, respectively. Furthermore, each of the fragment atoms and molecules can be created in different electronic states. The total energy Ef = Ei + hu is the same in all cases and therefore the different channels are simultaneously excited by the monochromatic light pulse. The dissociation channels differ merely in the products and in the way the total energy partitions between translation and vibration. 

For simplicity we consider at this point only the case of a single product channel, 

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The acronym SEC refers to the case where the reference wave function is of the MCSCF type and tire correlation energy is calculated by an MR-CISD procedure. When the reference is a single determinant (HE) the SAC nomenclature is used. In the latter case the correlation energy may be calculated for example by MP2, MP4 or CCSD, producing acronyms like MP2-SAC, MP4-SAC and CCSD-SAC. In the SEC/SAC procedure the scale factor F is assumed constant over the whole surface. If more than one dissociation channel is important, a suitable average F may be used. 

By contrast, the dissociation channel leading to H + HCO (the radical channel) has no barrier and a dissociation threshold [49] of 30,328.5 cm. In 1993, van Zee et al. [50] found that excitation above this threshold led to CO rotational distributions that were bimodal. In addihon to the high-/ CO... 

The total dissociation cross sections of silane and disilane have been taken from Perrin et al. [201]. An uncertainty in the present knowledge of the silane chemistry is the branching ratio of the silane dissociation channels [192]. Here, the branching ratio is taken from Doyle et al. [197], who suggest using the branching ratio determined by Perkins et al. [202] for photolysis, viz., a branching of... 

Figure 8. Phase lag spectrum of HI in the vicinity of the 5d(n, 8) resonance. In panel (a), the circles show the phase lag between the ionization and dissociation channels. The diamonds and triangles separate the phase lag into contributions from each channel, using H2S ionization as a reference. Panels (b) and (c) show the conventional one-photon (m3) and three-photon (3a>i) photoionization spectra. (Reproduced with permission from Ref. 45, Copyright 2002 American Institute of Physics.)...

Polyatomic molecules provide a still richer environment for studying phase control, where coupling between different dissociation channels can occur. Indeed, one of the original motivations for studying coherent control was to develop a means for bond-selective chemistry [25]. The first example of bond-selective two-pathway interference is the dissociation of dimethyl-sulfide to yield either H or CH3 fragments [74]. The peak in Fig. 11 is indicative of a resonance embedded in an elastic continuum (case 4). 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

The elimination of two D atoms and two ring-opening dissociation channels were observed from the two-photon dissociation. 

Fig. 18. The momentum matches of two fragments in each dissociation channel (a) m/e = 77 and 18, (b) ra/e = 78 and 17, (c) ro/e = 79 and 16, (d) ro/e = 80 and 15. The thick lines represent light fragments, and the thin lines represent heavy fragments.

The 0(3P2) formed in the minor dissociation channel was probed through a (2+1) REMPI scheme at photolysis wavelengths of 226, 230, 240 and 266 nm. Abel-transformed images similar to those used for this analysis... 

Fig. 10. Energy level diagram for methyl radical. The energetically accessible dissociation channels in the UV photolysis at 193 nm are shown. (From North et al.112)...

Electronic states and photodissociation dynamics of chloromethyl radical have been studied recently.114-117 Because of the chlorine substitution, there are several low-lying valence excited states (such as l2Ai and 22Bi, which mainly involve the orbitals on the CC1 bond) in addition to the 3s Rydberg state (22Ai), and more dissociation channels are available. 

Fig. 18. Energy levels and correlation diagram of vinyl radical C2H3 and its dissociation channels. (From Refs. 37, 38, 132 and 135)...

The spectroscopy, structure, photochemistry, and unimolecular reactions of allyl radical have been studied extensively and reviewed recently.145 Possible dissociation channels of allyl radical, their energetics, and the potential energy barriers of the C3H5 system are shown in Figs. 20 and 21.145,146... 

Fig. 20. Possible dissociation channels of allyl radical and their standard heats of formation relative to allyl. The loss of H2 generally proceeds via a high activation barrier and is thus considered unlikely. (From Fischer et ai.14B)...

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