AB-INITIO IONIZATION AND EXCITATION-SPECTRA INVOLVING IO(X-2-PI) AND IO-3-SIGMA(-), A(1)DELTA, B(1)SIGMA(+))((X)

An equilibrium bond length of r(e) = 1.894 Angstrom and harmonic vibra tional frequency of omega(e) = 664 cm(-1) are computed for IO(X-2 Pi) at the nonrelativistic all-electron CCSD(T)/6-311+G(3df) level of theo ry. These are within 1% of the experimental Omega-averaged values of 1 .877 +/- 0.005 Angstrom (3 sigma) and 670 +/- 13 cm(-1) (3 sigma), res pectively. Similar calculations predict r(e)=1.824 Angstrom and omega( e)=764 cm(-1) for IO+ (X-3 Sigma(-)). For the singlet excited electron ic states of IO+, which require a multireference treatment, CASPT2(8,6 )/6-311+G(3df) predicts r(e)=1.841 Angstrom and omega(e)=738 cm(-1) fo r IO+(a(1) Delta) and r(e)=1.870 Angstrom and omega(e)=679 cm(-1) for IO+(b(1) Sigma(+)). The equilibrium bond length increases and the harm onic vibrational frequency decreases from the ground to the excited st ates of IO+ as observed for the isovalent O-2 molecule. A spin-spin sp litting of 1060 +/- 160 cm(-1) (3 sigma) is assigned to the two sublev els of the ground state of iodine monoxide cation, IO+ (X-1(3) Sigma(0 )(-), X-2(3) Sigma(+/-1)(-)), on the basis of a reinterpretation of an experimental photoionization efficiency spectrum of IO. This reassign ment is required by the present vibrational constants for IO+, Franck- Condon factors, and four-component, relativistic multireference config uration interaction (CI) calculations of the spin-spin splitting. An a diabatic ionization energy of 9.60 eV is computed at the CCSD(T) level of theory for the IO+(X-3 Sigma(0)(-)) <-- IO(X-2 Pi(3/2)) transition and is in reasonable agreement with the experimental photo-ionization threshold of 9.735 +/- 0.017 eV (3 sigma). Adiabatic excitation energ ies of 0.72 and 1.18 eV are calculated at the CASPT2 level of theory f or IO+(a(1) Delta) <-- IO+(X-3 Sigma(0)(-)) and IO+(b(1) Sigma(+)) <-- IO+(X-3 Sigma(0)(-)) transitions, respectively. The photoionization s pectrum of IO and excitation spectrum of IO+ have also been simulated.