Atom production

Richeboeuf, L., Pasquiers, S., Legentil, M. et al. (1998) The influence of H2 and C2H6 molecules on discharge equilibrium and F-atom production in a phototriggered HF laser using SF6, J. Phys. D Appl. Phys. 31, 373-89. 

Fig. 21. The product D-atom velocity-flux contour map, d

Since H-atom products from chemical reactions normally do not carry any internal energy excitation with its first excited state at 10.2 eV, which is out of reach for most chemical activations, the high-resolution translational energy distribution of the H-atom products directly reflects the quantum state distribution of its partner product. For example, in the photodissociation of H2O in a molecular beam condition,... 

The excitation of the ground state H-atom product (n = 1) is made by the following two-step excitation scheme ... 

H2 molecular beam. The H-atom products were detected by the Rydberg tagging TOF technique using the same scheme described in the last paragraph with a rotatable MCP detector. Figure 4 shows the experimental scheme of the crossed beam setup for the 0(1D) + H2 reactive scattering studies. The scheme used for the H + D2(HD) studies is very similar to that used in the 0(1D) + H2 except that the H-atom beam source is generated from HI photodissociation rather than the 0(1D)-atom beam source from 02 photodissociation. 

The time-of-flight spectrum of the H-atom product from the H20 photodissociation at 157 nm was measured using the HRTOF technique described above. The experimental TOF spectrum is then converted into the total product translational distribution of the photodissociation products. Figure 5 shows the total product translational energy spectrum of H20 photodissociation at 157.6 nm in the molecular beam condition (with rotational temperature 10 K or less). Five vibrational features have been observed in each of this spectrum, which can be easily assigned to the vibrationally excited OH (v = 0 to 4) products from the photodissociation of H20 at 157.6 nm. In the experiment under the molecular beam condition, rotational structures with larger N quantum numbers are partially resolved. By integrating the whole area of each vibrational manifold, the OH vibrational state distribution from the H2O sample at 10 K can be obtained. In... 

Photodissociation of D2O on the A surface at 157 nm has also been investigated. The time-of-flight spectrum of the D-atom product from the D2O... 

Fig. 7. The total translational energy distribution of the H-atom product from (a) the mixed sample using 1 18 mass ratio, (b) pure H2O sample using 1 18 mass ratio.

Similarly, the TOF spectrum of the D-atom product from the mixed sample has also been measured. Figure 8(a) shows the translational energy distribution for the D-atom product from the mixed sample. In order to show the contribution from the D20 photodissociation, Fig. 8(b) also shows the translational energy distribution for the photodissociation of the pure D20 sample converted from the D-atom TOF spectrum using a mass ratio... 

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... 

Recently, the photodissociation process, HOD + hv — OD + H, has also been studied at the 121.6 nm using the experimental technique described above. Contributions from H2O were then subtracted from the results of the mixed sample. The experimental TOF spectra of the H atom from HOD were then converted into translational energy spectra in the center-of-mass frame. Figure 17 shows the translational energy spectra of the H-atom products at 121.6 nm excitation using two different polarization schemes... 

TOF spectra of the H atom products have been measured at 18 laboratory angles (from 117.5° to —50° at about 10° intervals). Figure 19 shows a typical TOF spectrum at the laboratory (LAB) angle of —50° (forward direction). By definition, the forwardness and backwardness of the OH product is defined here relative to the 0(7D) beam direction. The TOF spectrum in Fig. 19 consists of a lot of sharp structures. All these sharp structures clearly correspond to individual rotational states of the OH product, indicating that these TOF spectra have indeed achieved rotational state resolution for the 0(1D)+H2 — OH+H reaction. By converting these TOF spectra from the laboratory (LAB) frame to the center-of-mass (CM) frame... 

Fig. 19. Time of flight spectra of the H atom product from the ()(1 I)) + H2 —> OH + H reaction at -50° laboratory scattering angle at the collision energy of 1.3kcal/mol.

Fig. 21. The H atom product angular distribution in the laboratory frame for the...

Time-of-fhght spectra of the D atom products have been measured at many laboratory angles at both collision energies. Translational energy distributions can be derived by direct conversion of these TOF spectra. For the experiment carried out at 2.0 kcal/mol, Fig. 28(a) shows the total product angular distribution from 0 = —60° to 117.5°, which correspond to the forward (—60°), the sideward (30°) and the backward (117.5°) scattering directions. The direction of the D2 beam is at 0 = 0°, while the direction of the 0(XD) beam is at 0/. 90°. By definition, the forwardness and back-... 

Fig. 28. The D atom product angular distribution in the laboratory frame for the 0(1D) + D2 — OD + D reaction at two collision energies (a) 2.0 kcal/mol, and (b) 3.2 kcal/mol.

Fig. 35. The three-dimensional plot of the differential cross-sections for the H + HD H2 + D reaction at the collision energy of 1.200 eV (detecting the D-atom product).

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

Fig. 17. H-atom product channel translational energy distributions of the ethyl photodissociation, with the 245-nm photolysis radiation polarization (a) parallel to the TOF axis (b) at magic angle and (c) perpendicular to the TOF axis, and (d) anisotropy parameter /3(Et). In (b), the de-convoluted fast component, P[(i T), and slow-component, Pii(E ), are plotted in dashed and dotted lines, respectively. (From Amaral et al,39)...

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