Product branching ratio, photodissociation

Control over the product branching ratio in the photodissociation of Na2 into Na(3s) + Na(3p), and Na(3s) + Na(3d) is demonstrated using a two-photon incoherent interference control scenario. Ordinary pulsed nanosecond lasers are used and the Na2 is at thermal equilibrium in a heat pipe. Results show a depletion in the Na(3d) product of at least 25% and a concomitant increase in the Na(3p) yield as the relative frequency of the two lasers is scanned. 

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... 

Fig. 1.7. Branching ratio for the production of OD and OH fragments in the photodissociation of HOD in the first absorption band. The energy is measured with respect to H -I- OH(re), where re is the equilibrium bond distance of OH. The dashed curve indicates a simple kinematical limit (see the text) and the data point represents the measured value of Shafer, Satyapal, and Bersohn (1989) for the photolysis at 157 nm.

The photodissociation of symmetric triatomic molecules of the type ABA is particularly interesting because they can break apart into two identical ways ABA — AB + A and ABA — A + BA. Figure 7.18(a) shows a typical PES as a function of the two equivalent bond distances. It represents qualitatively the system IHI which we will discuss in some detail below. We consider only the case of a collinear molecule as illustrated in Figure 2.1. The potential is symmetric with respect to the C -symmetry line 7 IH = i HI and has a comparatively low barrier at short distances. The minimum energy path smoothly connects the two product channels via the saddle point. A trajectory that starts somewhere in the inner region can exit in either of the two product channels. However, the branching ratio ctih+i/cti+hi obtained by averaging over many trajectories or from the quantum mechanical wavepacket must be exactly unity. 

As in Example 4.2, the branching ratio between the two product channels can be controlled by appropriate vibrational pre-excitation of HOD [16]. For example, when the initial state is a vibrationally excited state of HOD corresponding to four quanta in the HO-D stretch, the channel D + OH is exclusively populated in a subsequent unimolecular photodissociation reaction induced by a UV-photon. The energy of the UV-photon must, however, lie within a rather narrow energy range. 

Assuming that a large fraction of these radicals remains at the surface, they can further react to form saturated molecules like CH4, NH3 or H20. Reactions of these molecules with radicals, which have some excitation energy, either as a consequence of their formation or from the photodissociation of saturated molecules, can then lead to more complex organic molecules. The branching ratio, which determines the chemical composition of the products thus formed, depends on the surface abundance of atoms and radicals and any possible ejection mechanisms which may interrupt the reaction sequence. 

The properties measured in photofragmentation experiments (photodissociation or photoionization) may be divided into scalar quantities, such as photofragmentation cross sections and branching ratios, and vectorial quantities, such as angular distributions of the photofragments and their alignment or orientation. Moreover, these properties may be measured independently or in coincidence where two or more properties associated with either or both photofragmentation products are measured simultaneously, event-by-event. 

We assume that the fine-structure distribution measured at F = 8 x 10 photons/cm remains the same throughout the entire range of photon flux. Because the population of S( P2) is dominant in the production of S( F), the fine-structure distribution used should have a minor effect on the values reported for the branching ratios S( P)/S( D) and for the photodissociation cross sections. 

Short-lived reaction intermediates and products resulting fi-om light flash photolysis have been detected using TOF-MS [76]. A flash lamp was used to induce photochemical reactions of the reactant gases in a reaction vessel. The generated species, such as radicals, could be immediately (in approximately milliseconds) detected by TOF-MS [76]. In other work, a laser beam was combined with an ion cyclotron resonance mass analyzer to follow the process of photodissociation [77]. The dissociation rates and branching ratios for naphthalene ion were measured by means of the time-resolved photodissociation approach. The above-mentioned approaches [76,77] are limited to detection of species generated from gas-phase substrates. 

Basic questions are analyzed, as is the case for the photochemistry of formaldehyde. Contrary to previous results, direct quantum dynamics simulations showed that the H2 + CO H + HCO branching ratio in the Si/Sq nonadiabatic photodissociation of formaldehyde is controlled by the direction and size of the mean momentum of the wavepacket when it crosses the seam of conical intersection. In practice, if the wavepacket falls down from the barrier to the conical intersection with no initial momentum the system leads to H2 + CO, while an extra momentum toward products favors... 

---