Knowledge Bank

\maketitle The laser magnetic resonance (LMR) spectroscopictechnique has been used to investigate the rotational spectrum of the NH radical in its $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma -$ ground electronic state. An optically pumped molecular laser was used as a source of radiation in the 26.7683 to 123.3066 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ region of the far-infrared. The molecular transitions were brought into resonance with the laser frequency using magnetic fields up to 2 T. Using twenty-seven laser lines, spectra were recorded in both parallel ($\Delta MJ$ = 0) and perpendicular ($\Delta MJ$ = $\pm$1) polarizations. Observation of the $N\prime$ $\leftarrow$ $N\prime \prime$ = 1 $\leftarrow$ 0, 2 $\leftarrow$ 1, 3 $\leftarrow$ 2 and 4 $\leftarrow$ 3 transitions in the first and second excited vibrational levels of NH allows its molecular parameters (particularly the hyperfine constants) to be refined.