High resolution photoelectron (ZEKE-PFI) spectrum of IBr: the role of repulsive intermediate states

We present the high resolution two-photon one-colour photoelectron (ZEKE-PF I) spectrum of IBr excited via a repulsive intermediate state. Both spin-or bit components ((X) over tilde(2) Pi(3/2) and (X) over tilde(2) Pi(1/2)) of the ground electronic state have been observed. The adiabatic ionisation e nergies have been determined to be 79 018 +/- 1 cm(-1) for the (X) over til de(2)pi(3/2) state and 83 680 +/- 1 cm(-1) for the (X) over tilde(2) Pi(1/2 ) state, giving a spin-orbit splitting of 4662 +/- 2 cm(-1). An extended pr ogression allowed resolution of the vibrational isotope shift and vibration al constants have been determined for both isotopomers ((IBr)-Br-79 and (IB r)-Br-81) in the (X) over tilde(2) Pi(3/2) state: <(omega)over tilde>(e) = 303 +/- 1 cm(-1) and <(omega)over tilde>(e)x(e) = 1.0 +/- 0.1 cm(-1) for (I Br+)-Br-79 and omega(e) = 300 +/- 1 cm(-1) and <(omega)over tilde>(e)x(e) = 0.9 +/- 1 cm(-1) for (IBR+)-B-81. The vibrational progression in the ((X) over tilde(2)Pi(1/2)) state extended only as far as nu(+) = 3 and, as a res ult, the isotope shift was not resolved. The vibrational constants for the upper spin-orbit component were determined as <(omega)over tilde>(e) = 300 +/- 1 cm(-1) and <(omega)over tilde>(e)x(e) = 2.0 +/- 0.2 cm(-1). The role of repulsive intermediate states, at the one photon level, in the formation of extended vibrational progressions, is discussed. (C) 1998 Elsevier Scie nce B.V. All rights reserved.