TIME-RESOLVED FREQUENCY-MODULATION SPECTROSCOPY OF PHOTOCHEMICAL TRANSIENTS

We report the use of time-resolved frequency modulation (FM) spectrosc opy for the measurement of photochemically generated radicals. CN radi cals from the 193 nm photodissociation of cyanogen (NCCN) have been de tected using a phase-modulated cw Ti:sapphire ring laser, probing sing le rotational lines of the A (2) Pi <-- X (2) Sigma system. The combin ation of sensitivity with time and frequency resolution is more than a dequate to record Doppler-broadened line shapes of collisionless photo fragments. Significant signal-to-noise enhancement is demonstrated com pared to a dual-beam transient absorption technique.