Time-resolved spectroscopy of a pulsed H2–F2 laser with well-defined initial conditions

Abstract

Timeresolved infrared diode laser spectroscopy of the ν5 band of the cyanomethyl radical (H2CCN)The time-resolved spectral output of a pulsed H2"Fz laser has been investigated as a function of initiating atomic fluorine concentration. Fast initiation, as well as variation of initial F-atom concentration. was accomplished by photodissociation of F z in a flowing Hz-Fz-N z mixture utilizing a variable-amplitude 30-nsee second harmonic pulse from a Q -switched ruby laser. Initial HF concentration due to prereaction was reduced to ;....0.01% of the final HF content by cryogenic cooling of the reagent premixer. The 30-cm-long reactor cell was contained within a 113-cm stable resonator cavity having a, 70% output coupling coefficient. Lasing was observed on a number of P -branch transitions in the four lowest vibrational bands of HF for a gas mixture Hz: F z : N z =7.5 : 7.5 : 75 Torr. For each of the nine dominant lines, variation of initiation time, pulse duration. peak intensity, and total energy was observed as a function of initiating F-atom concentration. Strong initial intensity transients observed for highest gain transitions are indicative of relaxation oscillation effeets not included in conventional HF laser simulation codes based on the threshold gain constraint. Trends in temporal J progression predicted by such codes were observed, but simultaneous lasing on multiple lines within a given vibrational band indicates that usual assumptions of Boltzmann rotational distribution and gain saturation to threshold are overrestrictive. The observations show strong correlation between peak intensity and relative gain of the individual transition. Energy content of individual lines does not exhibit such correlation. 5360 FROM / a-SWITCHED RUBY LASER O.75-m GRAtiNG SPECTROGRAPH FIG. 1. Schematic diagram of the experimental arrangementfor the laser-initiated chemical laser. A 60-nsec ruby pulse is frequency doubled, filtered, and inserted into a stable optical cavity. The resulting fluorine dissociation initiates the F2 + B2 reaction chain and BF lasing, which is recorded by an array of 10 fast ir detectors at the grating spectrograph.