The (v = 1 \to 0) radiative lifetime of (NH (X^3 \Sigma^-, v=1, N=0)) is determined to be (\tau_{rad,\text{exp.}} = 37.0 \pm 0.5_{\text{stat}}{}^{+2.0} {-0.8\text{syst}}) miliseconds, corresponding to a transition dipole moment of (|\mu{10}| = 0.0540_{{-0.0018}^{+0.0009}}) D. To achieve the long observation times necessary for direct time-domain measurement, vibrationally excited (NH (X^3 \Sigma^-, v=1)) radicals are magnetically trapped using helium buffer-gas loading. The rate constant for background helium-induced collisional quenching was determined to be (k_{v=1}<3.9 \times 10^{-15}cm^3s^{-1}), which yields the quoted systematic uncertainty on Ď„rad,exp.. With a new ab initio dipole moment function and a Rydberg-Klein-Rees potential, we calculate a lifetime of 36.99 ms, in agreement with our experimental value.
