The energy dependence of HO(2) radical formation from the irradiation of benzene (C(6)H(6)) in the presence of oxygen (O(2)) at 248 nm is studied. We investigate the origin of the HO(2) radicals, that is, whether they originate from the reaction of O(2) with products obtained by one- or two-photon excitation of C(6)H(6). The concentration-time profiles of HO(2) radicals are monitored by continuous-wave cavity ring-down spectroscopy (cw-CRDS) coupled to a laser photolysis reactor. HO(2) radicals are detected in the first vibrational overtone of the OH stretch at 6638.20 cm(-1), using a distributed feedback (DFB) diode laser. Two well-distinguished HO(2) radical-formation phases are observed: a fast initial formation of HO(2) radicals followed by a slower secondary formation. While the concentration of the initially formed HO(2) species increases linearly with the excitation energy, the concentration of the secondary slow HO(2) radicals appears to vary in accordance with a two-photon process.
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