Submillimeter-wave measurements of N2 and O2 pressure broadening for HO2 radical generated by Hg-photosensitized reaction

The N2 and O2 pressure broadening coefficients of the pure rotational transitions at 625.66 GHz (NKaKc=101▒9-100▒10, J=10.5-10.5) and 649.70 GHz (NKaKc=102▒9-92▒8, J=9.5-8.5) in the vibronic ground state X2A′ of the perhydroxyl (HO2) radical have been determined by precise laboratory measurements. For the production of HO2, the mercury-photosensitized reaction of the H2 and O2 precursors was used to provide an optimum condition for measurement of the pressure broadening coefficient. The Superconducting Submillimeter-wave Limb Emission Sounder (SMILES) was designed to monitor the volume mixing ratio of trace gases including HO2 in the Earth's upper atmosphere using these transitions. The precise measurement of pressure broadening coefficient γ in terms of the half width at half maximum is required in order to retrieve the atmospheric volume mixing ratio. In this work, γ coefficients of the 625.66 GHz transition were determined for N2 and O2 at room temperature as γ(N2)=4.085±0.049 MHz/Torr and γ(O2)=2.578±0.047 MHz/Torr with 3σ uncertainty. Similarly, the coefficients of the 649.70 GHz transition were determined as γ(N2)=3.489±0.094 MHz/Torr and γ(O2)=2.615±0.099 MHz/Torr. The air broadening coefficients for the 625.66 GHz and 649.70 GHz lines were estimated at γ(air)=3.769±0.067 MHz and 3.298±0.099 MHz respectively, where the uncertainty includes possible systematic errors. The newly determined coefficients are compared with previous results and we discuss the advantage of the mercury-photosensitized reaction for HO2 generation. In comparison with those of other singlet molecules, the pressure broadening coefficients of the HO2 radical are not much affected by the existence of an unpaired electron.

► N2 and O2 pressure broadening were determined for 625.66 and 649.70 GHz lines of HO2. ► These data will allow retrievals of HO2 from SMILES data. ► Advantage of the Hg-photosensitized reaction for HO2 generation is discussed.