Optical pumping of metastable NH radicals into the paramagnetic ground state

We here report on the optical pumping of both ${}^{14}\mathrm{NH}$ and ${}^{15}\mathrm{NH}$ radicals from the metastable $a{}^{1}\ensuremath{\Delta}$ state into the $X{}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$ ground state in a molecular beam experiment. By inducing the hitherto unobserved spin-forbidden $A{}^{3}\ensuremath{\Pi}\ensuremath{\leftarrow}a{}^{1}\ensuremath{\Delta}$ transition, followed by spontaneous emission to the $X{}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$ state, a unidirectional pathway for population transfer from the metastable state into the electronic ground state is obtained. The optical pumping scheme demonstrated here opens up the possibility to accumulate NH radicals in a magnetic or optical trap.