The flash photolysis kinetic spectroscopy experiment has been transferred to the mid-infrared region of the spectrum using an excimer laser as the photolysis source and a color center laser as the spectroscopic probe. Both frequency and time information is available in this type of experiment, and this has permitted both spectroscopic and kinetics investigations to be undertaken.
The sensitivity of this technique was examined by monitoring absorption features due to Br, NH(,2), and OH radicals, and calculations of expected absorption strengths were verified in the Br atom system. Several sensitivity enhancement schemes were explored in this connection. Using balanced detectors for the OH radical study, a signal-to-noise ratio of 100:1 for a 1% absorption of the IR probe beam was achieved. Of the various problems encountered during the course of these investigations, the most significant was the presence of IR light in the excimer laser pulse. For this reason a single-pass, counterpropagating arrangement of the photolysis and probe beams was chosen and was used in all subsequent studies.
With the above method the vibrational fundamental of the a('1)(DELTA)NH radical was recorded for the first time. The radical was produced by 193 nm photolysis of HN(,3). The high degree of rotational excitation of the ('1)(DELTA)NH produced in this manner (corresponding to a rotational temperature of (TURN)10,000 K) allowed high-order centrifugal distortion parameters to be determined.
The reaction between NH(,2) and NO was examined in efforts to determine the branching ratio of the various product channels using NH(,2) produced by ArF photolysis of NH(,3). The contribution of the OH + N(,2)H channel was found to account for 13 (+OR-) 2% of the reaction. The N(,2) + H(,2) channel was measured using two different pairs of NH(,3) and H(,2)O lines, giving values of 0.85 (+OR-) 0.09 and 0.66 (+OR-) 0.03 for the ratio of H(,2)O formed to NH(,3) photolysed. The H(,2)O signals arising from both ground and vibrationally excited states showed distinct induction periods. These are interpreted as suggesting that the H(,2)O molecule is formed vibrationally very hot.