Knowledge Bank

A new technique, time resolved frequency modulation spectroscopy, has been used to measure absorption spectra of photochemically generated radicals. As a first example, CN radicals from the 193 nm photodissociation of cyanogen (NCCN) have been detected using a phase modulated continuous wave Ti:sapphire ring laser, probig single rotational lines in the $A2\Pi -X2\Sigma$ band system. The sensitivity is significantly higher than could be obtained using a dual beam noise subtraction technique, and in our implementation, we retain sufficient band width to record Doppler broadened lineshapes of collisionless photofragments and accurately follow the time evolution of the lineshape as the fragment radicals collisionally relax to room temperature. We plan to use this spectrometer to record new spectra of $CH2$ in the $b-1B1-a-1A1$ band system and also to modify the diode laser absorption spectrometer at Brookhaven to use a variant of the FM Technique. Acknowledgement: Work at Brookhaven National Laboratory was carried out under Contract No. DE=ACO2. 76CH00016 with the U. S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences.