Photodissociation branching ratio

Z. Chen,M. Shapiro, P. Brumer, Interference control of photodissociation branching ratios. Two-color frequency tuning of intense laser fields, Chem. Phys. Lett. 228 (1994) 289. 

The mechanism of the FOIST based selective control of IBr photodissociation has been further probed by the use of and V iii the TDWP calculation of IBr absorption spectrum (Fig. 5) and branching ratio (Fig. 6). 

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... 

However, a shortcoming with the VUV photoionization approach is that absolute PI cross-sections are very often not known, and therefore branching ratios cannot be estimated. As matter of fact, studies of photodissociation processes by soft PI using synchrotron light are usually accompanied by measurements carried out using classic (hard) El ionization, where much data have to be taken at all possible fragment masses in order to estimate branching ratios.14-16,20... 

The early work on the photolysis of water was in the gas phase employing one photon. The branching ratio of the photodissociation into H + OH and H2 + O was reported by McNesby et al. [28] as 3 1 at a photon energy of 10.03 eV. Ever since, that ratio has been consistently revised in favor of the H + OH reaction with the final result of Stief et al. [29] giving 0.99 0.01 for 6.70-8.54 eV photon energy and 0.89 0.11 for the interval 8.54-11.80 eV. In the absence of direct determination these ratios often are assumed valid in the liquid phase. In the early work of Sokolev and Stein [30], mainly the photodissociation quantum yield in liquid water was measured, but a small photoionization yield of -0.05 was attributed to the process... 

Schmidt, S., R. N. Schindler, and T. Benter, Photodissociation Dynamics of CIO and CIOOCI Branching Ratios, Kinetic Energy and Quantum Yield of Primary Photoproducts, Presented at the XXIII Informal Conference on Photochemistry, May 10-15,... 

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. 

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. 

Fig. 13.6. Calculated branching ratios cth+oh/ d+OH following the photodissociation of the 0m) (m = 0,2 and 4) vibrational states of HOD through the first continuum. The first quantum number gives the excitation of the O-D bond (n = 0 in the present case) and the second one, m, indicates excitation of the O-H bond. The open circle is the experimental result of Shafer, Satyapal, and Bersohn (1989) for the photolysis of 00) at 157 nm. The filled circle is the result for initial state 04) and photolysis wavelength A2 = 218.5 nm and the (upward) arrow indicates the lower limit for state 04) and A2 = 239.5 nm. See Figure 11.7 for an illustration of the experimental set-up. The arrow on the energy axis marks the energy of the barrier of the 4-state PES. E = 0 corresponds to three atoms in their ground state. Reproduced from Vander Wal et al. (1991).

The potential energy surfaces of the ground as well as the first electronically excited state of HOD are shown in Fig. 7.1.1. When the photodissociation is induced by ultraviolet (UV) light corresponding to an excitation to the first electronically excited state of HOD, the branching ratio between H + OD and D + OH depends on the frequency of the radiation but in such a way that one will always get, at least, about twice as much H + OD as D + OH. 

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. 

As the excitation process in an external field can be regarded as being a nonadiabatic transition between dressed adiabatic states [32], effective laser control can be achieved by manipulating the parameters of these nonadiabatic transitions directly. Based on this idea, two control schemes have been proposed. The first one is a control scheme for the branching ratio during the molecular photodissociation, achieved by utilizing the phenomenon of complete reflection [24,43,44], The second is to control the population transfer by using a laser pulse with periodically swept parameters [24-29], In both cases the best parameters of the laser pulse can be easily estimated from the ZN theory of nonadiabatic transitions. 

Rq q, expression, so that the relative phases of the two pulses do not affect the restf This approach was applied to realistic systems such as the control of the Br to,If branching ratio in die photodissociation of IBr [94], and the control of Li2 phpl dissociation [95], discussed later. To gain insight into the control afforded byfp... 

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 probability of photodissociation depends on the spectral and intensity distribution of the light flux, the absorption cross section and the photolysis branching ratio(s). The key molecular property is the transition dipole that governs the intensity or oscillator strength of the transition. Due to violation of space and spin... 

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