Selective Stimulation of Chemical Reactions by Laser Excitation

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Abstract
Among the many different applications to which lasers have been put, one of the more intriguing is their potential use in photochemistry - particularly, in inducing specific reaction pathways that are not accessible from conventional broad-band excitation. One of the first of such experiments was an attempt to use a ruby laser to carry out laser-flash photolysis of Br2, analogous to Porter's work on I2. Since the optical energy of the laser radiation (14,400 cm−l) was less than the Br2 dissociation energy, direct photodissociation was not possible; instead, excited Br2∗ (3πlu) molecules were produced, which were dissociated in subsequent collisions. The Br atoms formed in this way subsequently added to halogenated olefins such as C4F8 and C4F7Cl.1 The first actual laser flash-photolysis experiment, carried out in Porter's laboratory, used a Q-switched ruby laser to photodissociate dihydrophthalocyanine vapor at ca. 400°C; a multi-photon process is likely to be involved in this case.2 As high-power infrared lasers became available, these were used to effect the decomposition of monia, ethyl chloride, and other molecules absorbing at 10.6 μm.3,4 It seems clear, in retrospect, that all these reactions were essentially a laser-induced pyrolysis.

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