THE $\tilde{B}{1/2} (^{2}\Sigma^{+}) - \tilde{X}^{2}\Sigma^{+}$ ELECTRONIC TRANSITIONS OF LaNH AND LaND

New high resolution spectra of the $\\tilde{B}1/2 (^{2}\\Sigma^{+}) - \\tilde{X}^{2}\\Sigma^{+}$ transitions of LaNH and LaND, near $16000 cm^{-1}$, have been obtained under jet-cooled conditions, following the reaction of laser-ablated lanthanum metal with $NH_{3}$ and $ND_{3}$. Rotational analyses have given the ground state structure as $r_{0}(La-N) = 1.932072(11) \\AA; r_{0}(N-H) = 1.01357(31) \\AA$. Strong vibronic and spin-orbit coupling is found to occur between the $\\tilde{B}$ state and the $\\tilde{A}^{2}\\Pi$ state near $12600 cm^{-1}$. This causes various interesting effects: the hyperfine line strengths in the $\\tilde{B} - \\tilde{X}$ transition are unusual, showing that, though it is nominally $^{2}\\Sigma - ^{2}\\Sigma$, both parallel and perpendicular transition moments are present; also the bending frequency of the $\\tilde{B}$ state is raised by $150 cm^{-1}$ compared to the ground state, while its vibrational structure shows large l-dependent ``spin-vibration'' splittings which can be represented by $A_{sv} l$, where $A_{sv} \\simeq -14 cm^{-1}$.