The radiative lifetime of metastable CO (a 3Pi, v=0).

We present a combined experimental and theoretical study on the radiative lifetime of CO in the a (3)Pi(1,2), v=0 state. CO molecules in a beam are prepared in selected rotational levels of this metastable state, Stark-decelerated, and electrostatically trapped. From the phosphorescence decay in the trap, the radiative lifetime is measured to be 2.63+/-0.03 ms for the a (3)Pi(1), v=0, J=1 level. From the spin-orbit coupling between the a (3)Pi and the A (1)Pi states a 20% longer radiative lifetime of 3.16 ms is calculated for this level. It is concluded that coupling to other (1)Pi states contributes to the observed phosphorescence rate of metastable CO.