Ultraviolet photodissociation of the van der Waals dimer (CH{sub 3}I){sub 2} revisited. II. Pathways giving rise to neutral molecular iodine
- Institute of Chemical Kinetics and Combustion, Institutskaja Street 3, Novosibirsk 630090, Russia and Novosibirsk State University, Novosibirsk 630090 (Russian Federation)
The formation of neutral I{sub 2} by the photodissociation of the methyl iodide dimer, (CH{sub 3}I){sub 2}, excited within the A band at 249.5 nm is evaluated using velocity map imaging. In previous work [J. Chem. Phys. 122, 204301 (2005)], we showed that the formation of I{sub 2}{sup +} from photodissociation of the methyl iodide dimer takes place via ionic channels (through the formation of (CH{sub 3}I){sub 2}{sup +}). It is thus not possible to detect neutral I{sub 2} by monitoring I{sub 2}{sup +}. Neutral I{sub 2} is detected in this study by monitoring I atoms arising from the photodissociation of I{sub 2}. Iodine atoms from I{sub 2} photodissociation have a characteristic kinetic energy and angular anisotropy, which is registered using velocity map imaging. We use a two-color probe scheme involving the photodissociation of nascent I{sub 2} at 499 nm, which gives rise to I atoms that are ionized by (2+1) resonance enhanced multiphoton ionization at 304.67 nm. Our estimate of the yield of nascent I{sub 2} is based on the comparison with the signal from I{sub 2} at a known concentration. Using molecular beams with a small fraction of CH{sub 3}I (1% in the expanded mixture) where smaller clusters should prevail, the production of I{sub 2} was found to be negligible. An upper estimate for the quantum yield of I{sub 2} from (CH{sub 3}I){sub 2} dimers was found to be less than 0.4%. Experiments with a higher fraction of CH{sub 3}I (4% in the expanded mixture), which favor the formation of larger clusters, revealed an observable formation of I{sub 2}, with an estimated translational temperature of about 820 K. We suggest that this observed I{sub 2} signal arises from the photodissociation of several CH{sub 3}I molecules in the larger cluster by the same UV pulse, followed by recombination of two nascent iodine atoms is responsible for neutral I{sub 2} production.
- OSTI ID:
- 20864300
- Journal Information:
- Journal of Chemical Physics, Vol. 125, Issue 13; Other Information: DOI: 10.1063/1.2345365; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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