STRUCTURE OF THE AMMONIUM RADICAL FROM A ROTATIONALLY RESOLVED PHOTOELECTRON-SPECTRUM

High-resolution zero-kinetic-energy photoelectron spectroscopy has bee n used to record the transition between the lowest bound state (3s (2) A(1)) of the perdeuterated ammonium radical (ND4) and the (X) over til de (1)A(1) ground vibronic state of the perdeuterated ammonium ion (ND 4+). The spectra obtained are the first rotationally resolved photoele ctron spectra ever measured for a tetrahedral molecule. The analysis o f the rotational structure is accompanied by a description of the obse rved symmetry selection rules and propensity rules for core rotational angular momentum changes that characterize the photoionization proces s. Rotational constants (B-0 = 2.8560 +/- 0.0037 cm(-1) and B-0(+) = 2 .9855 +/- 0.0037 cm(-1)) and centrifugal distoration constants (D-0 = (4.78 +/- 1.4) x 10(-5) cm(-1) and D-0(+) = (4.77 +/- 1.5) x 10(-5) cm (-1)) have been determined for the 3s (2)A(1) state of ND4 and the (X) over tilde(1)A(1) state ND4+, respectively. The ionic rotational cons tant is in good agreement with the value B-0(+) = 2.9787 +/- 0.0029 cm (-1) determined indirectly by Crofton and Oka (J. Chem. Phys. 86, 5983 (1987)) from the measurement of allowd transitions of the v(3) vibrat ional band of ND4+. The neutralrotational constant differs markedly fr om the ab initio value B-0= 3.0407 cm(-1) of Havriliak and King (J. Am . Chem. Sec. 105, 4 (1983)) used by Alberti, Huber and Watson (J. Mel. Spectrosc. 107, 133 (1984)) as input data to fit the rotational struc ture of the Schuler band of ND4. The adiabatic ionization potential of ND4 is determined to be 37490.7 +/- 1.5 cm(-1) (4.64826 +/- 0.00019 e V). The large changes in core rotational angular momentum that accompa ny the removal of the photoelectron may be caused by the (looper minim um in the s --> p photoexcitation/photoionization channel recently pre dicted by Smith and Chupka [Chem. Phys. Lett. 250, 589 (1996)] to lie in the vicinity of the ionization threshold. (C) 1997 American Institu te of Physics.