Multiphoton sequential photodissociative excitation: a new method of remote atmospheric sensing.
Howard University, Chemistry Department, Washington, D.C.20059. Received 27 November 1978. 0003-6935/79/050590-03$00.50/0. © 1979 Optical Society of America. Recent work has shown that many molecular systems, when exposed to the 193-nm emission of an ArF laser, will produce excited photofragments, whose fluorescence may be easily detected. This is in spite of the fact that a single laser photon does not have enough energy to produce the observed photochemical reaction. The phenomenon has been ex­ plained in terms of competition in the excited state between predissociation and absorption of a second photon from the laser beam. While this point is discussed thoroughly in Refs. 1-3, one can get a feeling for its magnitude by comparing predissociative state lifetimes, which range from 1 psec to 10 psec, with the mean time for an allowed radiative transition of 10 psec when a cross section of 10 1 6 cm and an ArF laser intensity of 10 photons/cm sec is assumed. If the beam is focused, the competition will shift further toward the doubly excited states, from which the higher energy dissociation channel may be accessed. It should be stressed that all these transitions are allowed in contrast to virtual multiphoton processes. We report here on the use of this effect to detect NH3 in the atmosphere. Donelly et αl. have seen NH(AII i) → (X Σ–) emission at 336 nm when ammonia was exposed to the ArF laser. If spin is conserved, this cannot be the result of the following two-step process since reaction (lb) is spin forbid­ den: