J. Quanr. Specrrosc. Radiat. Transfer Vol. 39, No. 4, pp. 339-340, 1988 Printed in Great Britain. All rights reserved 0022-4073/88 $3.00 + 0.00 Copyright 0 1988 Pergamon Press plc zyxwvutsrqpo NOTE zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON OPTOGALVANIC OH (A +X) SPECTRUM SUBHASH DESHMUKH,’ SEONG-POONG LEE,’ ERHARD W. ROTHE,’ and G. P. RECK* ‘Department of Chemical Engineering and Research Institute for Engineering Sciences and %hemistry Department, Wayne State University, Detroit, MI 48202, U.S.A. zyxwvutsrqponmlkjihgfedc (Received 5 October 1987) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO Abstract-A pulsed, tunable dye-laser initiates an optogalvanic spectrum of OH between 3074 and 3098A. The OH is in a hollow cathode discharge that originates from a mixture of Ne and ~0.6% H,O. Data analysis indicates that the ground (X) electronic state of OH has a rotational temperature of ~250-300 K. This method appears to be a good diagnostic for the X state in discharges. EXPERIMENTAL STUDIES The OH radical is an important constituent in combustion processes, in discharges, in interstellar space, and in our atmosphere. Its spectrum is the basis of several diagnostic techniques. Most spectral analyses, including ours, use the A (‘X) - X(‘n) system described in the classic paper of Dieke and Crosswhite.’ We apply the optogalvanic *‘*t technique to analyze the OH generated in a hollow-cathode discharge from neon-water mixtures. Pulses of tuned laser light enter the discharge and cause A - X transitions. The ionization potential is x4 eV less for the A than for the X state and so the light serves to decrease the voltage across the discharge. In contrast to previous4*’ discharge studies that used emission from the A state, our method involves absorption by X state OH and therefore a different OH population is sampled. The light pulses are linearly polarized, have xO.2A resolution, x 10 ns duration, and x 1 mJ energy. They are the doubled output (3074-3098& of an excimer-pumped dye-laser operating with Sulforhodamine-B. The light goes through a discharge tube containing two uniaxial hollow- cylinder electrodes. A computer synchronizes the laser pulses and wavelength scan (i.e., moves its grating and its KDP doubler crystal) and the operation of a boxcar averager. The gas mixture is prepared by flowing Ne (x 760 torr) above ice water (vapor pressure x 4.6 torr). The equilibrium H,O fraction is x0.006, is too large to maintain a stable discharge, and is therefore reduced: its final value is unknown. The mixture flows through the cell and its pressure is read with a capacitance manometer. The discharge currents are adjusted within the range 2.5-5 mA and they flow through the cell and a series resistor R. The discharge is unstable outside this range. Any voltage variations at the point between the cell and R (i.e., the signal) go to the boxcar via a coupling capacitor. The data from the boxcar are transmitted to the computer for analysis. Figure 1 shows an OH spectrum: these positive peaks, which are sometimes as large as 50 mV, indicate a reduced discharge voltage. The three negative peaks near 3077 and 307981 are neon transitions: here the discharge voltage rises above its steady state value at certain times.6 The other negative peaks are noise. The OH line notation in Fig. 1 is that of Ref. 1, i.e., R2(K”), where the 2 indicates that J” = K” - f; K” is now usually designated as N”. Also shown is a simulation for 300K, using line intensities calculated from Eq. (3) of Ref. 7 and tabulated’ Einstein A constants (our transitions are not saturated). The agreement of the data with tabulated wavelengths is excellent. A comparison of simulations for 200, 300, 400, 500 and 600K shows that 300K is the best match to the overall peak heights. Most of these peaks involve two or more transitions. ?A11 of Ref. 3 deals with optogalvanic work. 339