Zero kinetic energy pulsed field ionization (ZEKE-PFI) spectroscopy of electronically and vibrationally excited states of I-2(+): The A(2)Pi(3/2,u) state and a new electronic state, the a(4)Sigma(u)(-) state

Both one- and two-color zero kinetic energy-pulsed field ionization (ZEKE-PFI) spectra of the first electronically excited state of I-2(+) (A (II3/2,u)-I-2) as well as a new electronic state, the a (4) Sigma(u)(-), state, have been recorded for the first time. In the one-color (two photon) experiment, transitions to the quartet state are formally spin-forbidden and this is reflected in the weak transition intensity observed compared with the A (II3/2,u)-I-2 state. However, in the two-color (1+2') experiment, which used the valence B (II0u)-I-3+ state as an intermediate, transitions into both A (II3/2,u)-I-2 and a (4) Sigma(u)(-) states are fully allowed and appear in the spectrum with comparable intensity. The a (4) Sigma(u)(-) State appears in the one-color experiment by virtue of spin-orbit coupling with excited electronic states for which direct ionization from the neutral ground state is fully allowed. Values for <(omega) over bar (e)x(e)> of 0.46+/-0.01 cm(-1) for the A (II3/2,u)-I-2 State and 0.38+/-0.02 cm(-1) for the a (4) Sigma(u)(-) State were derived, together with lower limits for <(omega) over bar (e)> of 138+/-2 and 128+/-2 cm(-1), respectively. In addition, highly vibrationally excited levels of the X (IIOmega,g)-I-2 state have been probed in the one-color experiments, by exploiting a propensity for extensive electronic autoionization involving Rydberg states based on the excited X (II1/2,g)-I-2 and A (IIOmega,u)-I-2 states as well as the dissociative B-2 Sigma(g)(+) state. In the case of the X (II3/2,g)-I-2 ground state, the vibrational progression extends as far as v(+) = 90 while that associated with the X (II1/2,g)-I-2 state extends as far as v(+) = 40. (C) 1996 American Institute of Physics.