  We report the observation of transient velocity-selective coherent population
trapping (VSCPT) in a beam of metastable neon atoms. The atomic momentum
distribution resulting from the interaction with counterpropagating sigma_+ and
sigma_- radiation which couples a J_g=2 <-> J_e=1 transition is measured via
the transversal beam profile. This transition exhibits a stable VSCPT dark
state formed by the two |J=2,m=+/-1> states, and a metastable dark state
containing the |J=2,m=+/-2> and |J=2,m=0> states. The dynamics of the formation
and decay of stable and metastable dark states is studied experimentally and
numerically and the finite lifetime of the metastable dark state is
experimentally observed. We compare the measured distribution with a numerical
solution of the master equation.
