Radiative Lifetime Measurement of theBa+MetastableD3/2State

The extremely long lifetimes of the metastable $D$ states of the ${\mathrm{Ba}}^{+}$ ion make it difficult to measure them accurately due to their high sensitivity to collisional effects. Thus a discrepancy exists between theories and experiments for the ${\mathrm{Ba}}^{+}$ $D_{3/2}$ state. We have measured the $D_{3/2}$ lifetime under a nearly ideal condition, i.e., single trapped ${\mathrm{Ba}}^{+}$ ions in ultrahigh vacuum. The obtained value of $79.8\pm 4.6\mathrm{s}$ resolves the existing discrepancy. The collisional quenching is found insignificant in our experiment, but there are indications of a non-negligible fine-structure mixing effect.