The ngr;1(Ondash;H stretching) band of the HO2radical was observed in absorption by using a difference frequency output obtained from an Ar ion laser and a cw dye laser as a source, combined with a multiplehyphen;reflection discharge cell. Zeeman modulation was employed to improve the sensitivity. The HO2radical was generated directly in the cell by a dc discharge in a mixture of allyl alcohol and oxygen. About 280 lines were observed and analyzed to precisely determine the rotational constants, centrifugal distortion constants, and spinhyphen;rotation interaction constants in the ngr;1=1 state. The band origin was determined to be 3436.1951(4) cmminus;1, which is 22 cmminus;1higher than the value previously obtained by a matrix isolation study.
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