Photoionization Mass Spectrometric Study of HOCl: Photoionization Efficiency Spectrum and Ionization Energy

The photoionization efficiency (PIE) spectrum of HOCl was measured over the wavelength range λ = 102−115 nm, using a discharge flow-photoionization mass spectrometer (DF-PIMS) apparatus coupled to a synchrotron radiation source. The PIE spectra displayed steplike behavior near threshold. This study represents the first determination of the HOCl photoionization efficiency spectrum and the photoionization threshold. A value of 11.12 3 ± 0.01 8 eV was obtained for the adiabatic ionization energy (IE) of HOCl from analysis of photoion thresholds, corresponding to the HOCl + (X 2 A‘ ‘) ← HOCl(X 1 A‘ ‘) transition. Our PIMS result is identical to the only previous experimental measurement and in good agreement with a recent ab initio calculation. From our result for IE(HOCl), a value of 999.4 ± 3.6 kJ mol -1 was calculated for Δ f H o 0 (HOCl + ), and from the latter, the proton affinity of ClO at T = 0 K, PA 0 (ClO), was determined to be 629.6 ± 3.6 kJ mol -1 . At 298 K, the computed values for Δ f H o 298 (HOCl + ) and PA 298 (ClO) are 996.5 ± 3.6 and 635.1 ± 3.6 kJ mol -1, respectively.