State- and Bond-Selected Photodissociation and Bimolecular Reaction of Water

It is possible to exploit the isolation of the O-H stretching vibration in H₂O and HOD to control the photodissociation and reaction dynamics in water molecules excited in the region of the third overtone (4ν _OH) of the O-H stretch. In vibrationally mediated photodissociation of H₂O, the selection of different initial stretching states having roughly the same energy leads to drastically different populations of the vibrational states of the OH photolysis product. By exciting the O-H stretching overtone in HOD, we can selectively photolyze that bond. In bimolecular reaction experiments, we react H₂O (4ν _OH) with H atoms to produce H₂ and OH. The reaction, which is endothermic, proceeds at an undetectable rate in our room temperature measurements. Vibrationally excited water, however, reacts at roughly the gas kinetic collision rate. Applying this technique to HOD (4ν _OH) allows us to demonstrate bond selected bimolecular chemistry in which the reaction produces only OD. This observation suggests a general approach to assessing bond controlled reactions in a variety of systems.