Journal of Physical Chemistry A, Vol.122, No.25, 5602-5609, 2018
Hydroxyl Radical Fluorescence and Quantum Yield Following Lyman-alpha Photoexcitation of Water Vapor in a Room Temperature Cell and Cooled in a Supersonic Expansion
Photoexcitation of water by Lyman-alpha (121.6 nm) induces a dissociation reaction that produces OH(A(2)Sigma(+)) + H. Despite this reaction being part of numerous studies, a combined understanding of the product and fluorescence yields is still lacking. Here, the rotational and vibrational distributions of OH(A) are determined from dispersed fluorescence following photoexcitation of both room-temperature and jet-cooled water vapor, for the first time in the same experiment. This work compares new data of state-resolved fluorescence with literature molecular branching ratios and brings previous studies into agreement through careful consideration of OH(A) fluorescent and predissociation lifetimes and confirms a fluorescent quantum yield of 8%. Comparison of the room-temperature and jet-cooled OH(A) populations indicate the temperature of H2O prior to excitation has subtle effects on the OH(A) population distribution, such as altering the rotational distribution in the nu' = 0 population and affecting the population in the nu' = 1 state. These results indicate jet-cooled water vapor may have a 1% higher fluorescence quantum yield compared to room-temperature water vapor.