Knowledge Bank

We have observed the vibration-rotation spectrum of the $\nu 3$ band of $NH3+$. The ion was produced in an air-cooled discharge cell with a gas mixture He: $H2:NH3(600:15:2)$ and with a total pressure of $\approx 6Torr$. The spectrum was observed using a difference frequency laser system combined with velocity modulation. About 100 lines have been measured and assigned. Spin-rotation splittings for most Q-branch lines were well resolved. A symmetric rotor Hamiltonian with $\ell$-resonance and $A1-A2$ splitting was used to analyze the spectrum. We find that $NH3+$, like the isoelectronic radical $CH3\mathrm{<mrow\; data-mjx-texclass="ORD">1,2</mrow>}$ is a planar molecule with $D3b$ symmetry and with an $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A2\prime \prime$ electronic ground state. We obtain the following preliminary vibration-rotation constants (in $cm-1$):[FIGURE]