Rate constants for specific product channels from metastable Ar(3P2,0) reactions and spectrometer calibration in the vacuum ultraviolet

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Abstract
The product channels from interaction of metastable argon atoms (3P2,0) with a series of chlorine containing molecules (Cl2, NOCl, ClO, HCl, CCl4, PCl3, and SnCl4), as well as some other molecules (Br2, N2O, NO2, H2S, and COS), have been investigated by emission spectroscopy. The rate constants for product channels were established by comparison of the emission intensities from the excited state products to the emission intensities from excited state Kr atoms, which have known rate constants for excitation by metastable argon atoms. Comparison of the individual product rate constants with previously determined total quenching rate constants of Ar(3P2,0) gave the branching ratios for emitting product channels. Although our measurements include the wavelength range from 120–800 nm, emphasis is placed upon the vacuum ultraviolet region and upon the ArCl*, ArBr*, and ArO* product channels. The highest ArCl* yield, ∼50%, was obtained for Cl2 and ClO. Quenching mechanisms for the halogen containing reagents are discussed with an emphasis on analogy to reactions of alkali metal atoms with halogen containing molecules. The nature of the bound-free ArCl* emission also is discussed. Since the spectral response of the monochromator in the vacuum ultraviolet is of critical importance for assignment of rate constant values, the molecular branching ratio method employed for the calibration of the optical system is carefully described.