Structure of the ammonium radical from a rotationally resolved photoelectron spectrum Available to Purchase

High-resolution zero-kinetic-energy photoelectron spectroscopy has been used to record the transition between the lowest bound state $(3s$ ²$A1)$ of the perdeuterated ammonium radical $(ND4)$ and the $X̃ 1A1$ ground vibronic state of the perdeuterated ammonium ion $(ND4+)$⁠. The spectra obtained are the first rotationally resolved photoelectron spectra ever measured for a tetrahedral molecule. The analysis of the rotational structure is accompanied by a description of the observed symmetry selection rules and propensity rules for core rotational angular momentum changes that characterize the photoionization process. Rotational constants $(B0=2.8560±0.0037$ cm^-1 and $B0+=2.9855±0.0037$ cm⁻¹) and centrifugal distortion constants $(D0=(4.78±1.4)×10−5$ cm^-1 and $D0+=(4.77±1.5)×10−5$ cm⁻¹) have been determined for the $3s$ ²$A1$ state of $ND4$ and the $X̃ 1A1$ state of $ND4+,$ respectively. The ionic rotational constant is in good agreement with the value $B0+=2.9787±0.0029$ cm^-1 determined indirectly by Crofton and Oka (J. Chem. Phys. 86, 5983 (1987)) from the measurement of allowed transitions of the $ν3$ vibrational band of $ND4+.$ The neutral rotational constant differs markedly from the ab initio value $B0=3.0407$ cm^-1 of Havriliak and King (J. Am. Chem. Soc. 105, 4 (1983)) used by Alberti, Huber and Watson (J. Mol. Spectrosc. 107, 133 (1984)) as input data to fit the rotational structure of the Schüler band of $ND4$⁠. The adiabatic ionization potential of $ND4$ is determined to be $37490.7±1.5$ cm^-1 $(4.64826±0.00019$ eV). The large changes in core rotational angular momentum that accompany the removal of the photoelectron may be caused by the Cooper minimum in the $s→p$ photoexcitation/photoionization channel recently predicted by Smith and Chupka [Chem. Phys. Lett. 250, 589 (1996)] to lie in the vicinity of the ionization threshold.