The radiative lifetime of metastable CO ($a^3\mathrm{\Pi },v=0$)

We present a combined experimental and theoretical study on the radiative lifetime of CO in the $a^3{\mathrm{\Pi }}_{1,2},v=0$ 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 $2.63\pm 0.03$ ms for the $a^3{\mathrm{\Pi }}_1,v=0,J=1$ level. From spin-orbit coupling between the $a^3\mathrm{\Pi }$ and the $A^1\mathrm{\Pi }$ state a 20% longer radiative lifetime of 3.16 ms is calculated for this level. It is concluded that coupling to other ${}^1\mathrm{\Pi }$ states contributes to the observed phosphorescence rate of metastable CO.