Bending energy level structure and quasilinearity of the (X)over-tilde+ 3B1 ground electronic state of NH+2

The bending level structure of the quasilinear (X) over tilde (+) B-3(1) ground electronic state of the amidogen cation NH2+ was studied by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy using a near-infrared vacuum-ultraviolet two-photon ionization sequence via selected rovibronic levels of the (A) over tilde (2)A(1) state of NH2. The careful selection of the intermediate levels permitted to optimize the transition intensities to the lowest vibrational levels of the cation in the photoionization step and to overcome the low sensitivity of previously employed single-photon ionization schemes. For the first time, all bending levels of the cationic ground state with quantum numbers upsilon(+)(2,lin)<= 4, N+<= 4, and vertical bar K+vertical bar <= 2 could be observed, enabling a detailed characterization of the large-amplitude bending vibration. The rotational structure corresponds to that of an effectively linear molecule in all observed vibrational levels. The bending vibrational structure which shows marked deviations from a harmonic behavior was analyzed in terms of a semirigid bender model. The bending potential function was obtained from a fit to the experimental data. The height of the barrier at the linear geometry and the bond angle at the potential minimum were determined to be 231.8(22) cm(-1) and 152.54(4)degrees, respectively, and all bending levels are located above the maximum of the barrier. (c) 2006 American Institute of Physics.