High resolution molecular beam study of the origin bands of the (A)over-tilde2Π-(X)over-tilde2Σ+ and (A)over-tilde"2Π1/2-(X)over-tilde2Σ+ systems of yttrium imide (Y14NH, Y15NH, and Y14ND)

The (0,0,0)-(0,0,0) bands of the (A) over tilde (2)Pi-(X) over tilde (2)Sigma(+) and (A) over tilde " (2)Pi(1/2)-(X) over tilde (2)Sigma(+) systems of three isotopomers of yttrium imide ((YNH)-N-14, (YNH)-N-15, and (YND)-N-14) have been studied by laser-induced fluorescence in a molecular beam apparatus. Rotational, fine, and nuclear magnetic hyperfine structures have been resolved and analyzed. The previously studied (B) over tilde (2)Sigma(+)-(X) over tilde (2)Sigma(+) (0,0,0)-(0,0,0) bands of the three isotopomers have been reanalyzed. Global fits of all observed bands, in which the ground state has been fitted to a Hamiltonian model, while the excited states have been represented by term values, have been performed for the three isotopomers. Subsequently, the individual bands have been fitted. The ground state parameters have been fixed at the values obtained in the global fits, while the upper states have been fitted to the Hamiltonian models. The (0,0,0) (A) over tilde (2)Pi state of (YNH)-N-14, (YNH)-N-15, and (YND)-N-14 is severely perturbed. Even though the nature of these perturbing states can only be speculated upon, the introduction of effective perturbers made it possible to deperturb the state successfully. The (A) over tilde " (2)Pi(1/2) state is unperturbed. The spectra can be reproduced to better than 120 MHz (0.004 cm(-1)). The analyses confirm that the molecule is linear in all four states with the nuclear arrangement Y-N-H. The bond lengths (r(0) structure) in the (X) over tilde (2)Sigma(+) ground state and the (A) over tilde " (2)Pi(1/2), (A) over tilde (2)Pi, and (B) over tilde (2)Sigma(+) excited states have been derived to be r(YN)=0.187785(17) nm, r(NH)=0.10039(14) nm; r(YN)=0.1927(1) nm, r(NH)=0.081(1) nm; r(YN)=0.19013(56) nm, r(NH)=0.1032(54) nm; and r(YN)=0.18848(52) nm, r(NH)=0.1236(46) nm, respectively. The electronic configurations for the (X) over tilde (2)Sigma(+) ground state and the (A) over tilde (2)Pi, (A) over tilde " (2)Pi(1/2), and (B) over tilde (2)Sigma(+) excited states are discussed and compared with ab initio calculations whenever possible. (C) 2000 American Institute of Physics. [S0021-9606(00)00633-4].