Measurement of the lifetime of theP13metastable state of Mg by dye-laser excitation

We have measured the lifetime of the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ metastable state of Mg and obtained a value of 2.2 ± 0.2 msec. The level was excited with a dye laser tuned to the intercombination line ($3s3p{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\leftrightarrow 3s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$). In addition to the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ state, the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ levels were populated by mixing collisions owing to the presence of an inert buffer gas. The populations of all three metastable levels were monitored as a function of time by observing the absorption of Mg resonance radiation (${}_{}{}^3{}_{}{}^{}P_{0,1,2}^{}{}_{}{}^{}\to {}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$) out of each level. The mixing time was short compared to the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ lifetime, so that the populations of the three metastable levels were in thermal equilibrium with each other. This produced an apparent lengthening of the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ lifetime, which would introduce a large systematic error if the degree of mixing were not known. Cenventionally, one observes the fluorescence from the excited levels to obtain their lifetimes. Because the lifetimes of the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ states are too long, fluorescence from these states cannot be observed, and the observation of fluorescence from the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ state alone provides insufficient information to determine its lifetime.