EXPERIMENTAL-DETERMINATION OF THE IONIZATION-ENERGY OF IO(X(2)PI(3 2)) AND ESTIMATIONS OF DELTA-H-F-DEGREES(0)(IO+) AND PA(IO)/

Photoionization efficiency (PIE) spectra of IO(X(2) Pi(i)) were measur ed over the wavelength range lambda = 115.0-130.0 nm and in the ioniza tion threshold region lambda = 126.0-130.0 nm, using a discharge flow- photoionization mass spectrometer apparatus coupled to a synchrotron r adiation source. Iodine oxide was generated by the reactions of O(P-3) atoms with I-2 and CF3I. The PIE spectra displayed step-function beha vior. From the half-rise point of the initial step, a value of 9.73(5) +/- 0.01(7) eV was obtained for the adiabatic ionization energy IE) o f IO, corresponding to the IO+(X(3) Sigma(-)) <-- IO(X(2) Pi(3/2)) tra nsition. As this appears to be the first experimental determination of IE(IO), a trend analysis has been employed to provide further insight into the experimentally derived value. The separation between the fir st two steps in the PIE spectrum (which might be perturbed slightly du e to autoionization) was used to derive the vibrational spacing of the cation, 1060 +/- 160 cm(-1). A Rydberg progression, presumably conver ging to the excited (1) Delta state of the cation, has a convergence l imit that lies 0.596 eV above the ground state of the cation. A select ed value for the bond dissociation energy of IO, D-0(o)(IO) = 226 kJ m ol(-1), leads to Delta(f)H(0)(o)(IO) = 128 i 4 kJ mol(-1). From this v alue for the heat of formation of IO, estimates are made for Delta(f)H (0)(o)(IO+), Delta(f)H(0)(o)(HOI) and the proton affinity of IO.