The v = 1 -> 0 radiative lifetime of NH (X triplet-Sigma-, v=1,N=0) is determined to be tau_rad,exp. = 37.0 +/- 0.5 stat +2.0 / -0.8 sys miliseconds, corresponding to a transition dipole moment of |mu_10| = 0.0540 + 0.0009 / -0.0018 Debye. To achieve the long observation times necessary for direct time-domain measurement, vibrationally excited NH (X triplet-Sigma-, v=1,N=0) radicals are magnetically trapped using helium buffer-gas loading. Simultaneous trapping and lifetime measurement of both the NH(v=1, N=0) and NH(v=0,N=0) populations allows for accurate extraction of tau_rad,exp. Background helium atoms are present during our measurement of tau_rad,exp., and the rate constant for helium atom induced collisional quenching of NH(v=1,N=0) was determined to be k_q < 3.9 * 10^-15 cm^3/s. This bound on k_q yields the quoted systematic uncertainty on tau_rad,exp. Using an ab initio dipole moment function and an RKR potential, we also determine a theoretical value of 36.99 ms for this lifetime, in agreement with our experimental value. Our results provide an independent determination of tau_rad,10, test molecular theory, and furthermore demonstrate the efficacy of buffer-gas loading and trapping in determining metastable radiative and collisional lifetimes.
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