Ultraviolet photodissociation of the van der Waals dimer (CH3I)(2) revisited. II. Pathways giving rise to neutral molecular iodine

摘要

The formation of neutral I-2 by the photodissociation of the methyl iodide dimer, (CH3I)(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-2(+) from photodissociation of the methyl iodide dimer takes place via ionic channels (through the formation of (CH3I)(2)(+)). It is thus not possible to detect neutral I-2 by monitoring I-2(+). Neutral I-2 is detected in this study by monitoring I atoms arising from the photodissociation of I-2. Iodine atoms from I-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-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-2 is based on the comparison with the signal from I-2 at a known concentration. Using molecular beams with a small fraction of CH3I (1% in the expanded mixture) where smaller clusters should prevail, the production of I-2 was found to be negligible. An upper estimate for the quantum yield of I-2 from (CH3I)(2) dimers was found to be less than 0.4%. Experiments with a higher fraction of CH3I (4% in the expanded mixture), which favor the formation of larger clusters, revealed an observable formation of I-2, with an estimated translational temperature of about 820 K. We suggest that this observed I-2 signal arises from the photodissociation of several CH3I molecules in the larger cluster by the same UV pulse, followed by recombination of two nascent iodine atoms is responsible for neutral I-2 production. (c) 2006 American Institute of Physics.