Direct photodissociation

Photodissociation, direct molecular dynamics, nuclear motion Schrodinger equation, 365-373... 

Y. Kurosaki. Hydrogen-atom production channels of acetaldehyde photodissociation direct DFT molecular dynamics study, J. Mol. Struct. - THEOCHEM 850 9—16 (2008). 

The experiments were perfonued in a static reaction cell in a large excess of N2 (2-200 bar). An UV laser pulse (193 mu, 20 ns) started the reaction by the photodissociation of N2O to fonu O atoms in the presence of NO. The reaction was monitored via the NO2 absorption at 405 mu using a Hg-Xe high-pressure arc lamp, together with direct time-dependent detection. With a 20-200-fold excess of NO, the fonuation of NO2 followed a pseudo-first-order rate law ... 

Dyer R B, Einarsdottir 6, Killough P M, Lopez-Garriga J J and Woodruff W H 1989 Transient binding of photodissociated CO to of eukaryotic cytochrome oxidase at ambient temperature. Direct evidence from time-resolved infrared spectroscopy J. Am. Chem. Soc. Ill 7657-9... 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules ia a gas at low pressure can be taken to be isolated for the short time between coUisions. Unimolecular reactions such as photodissociation or isomerization iaduced by photon absorption can sometimes be studied between coUisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity ia one direction and almost zero velocity ia perpendicular directions. Not only does this reduce coUisions, it also aUows bimolecular iateractions to be studied ia intersecting beams and iacreases the detail with which unimolecular processes that can be studied, because beams facUitate dozens of refined measurement techniques. (J) Means have been found to trap molecules, isolate them, and keep them motionless at a predetermined position ia space (11). Thus far, effort has been directed toward just manipulating the molecules, but the future is bright for exploiting the isolated molecules for kinetic and dynamic studies. 

S.3.4.3. Photodissociation It is also possible to use light as the activation method. Laser sources are especially useful because of their high intensity and narrow wavelength bandwidth. Photodissociation can be used to determine bond dissociation energies in ions directly, similar to what is done with threshold CID, or, alternatively, can be used in conjunction with direct ionization. 

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,... 

Fig. 9. The translational energy distributions of H2O photodissociation at 121 nm obtained with photolysis laser polarization parallel to the detection direction, (a) The upper trace was acquired experimentally, (b) The lower trace is the simulated distribution.

Fig. 15. The product translational energy distributions for the OH + D channel from the HOD photodissociation at 121.6 nm with the photolysis laser polarization parallel as well as perpendicular to the detection direction.

The spectroscopy methods such as LIF and REMPI are utilized not only to detect the free radicals as discussed above, but also to directly measure the internal state distributions of the photoproducts in the photodissociation of free radicals. In this approach, the photochemistry is carried out in the free radical beam under single-collision conditions with well-defined... 

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

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