Transition probabilities for the 1815 and 3344 angstrom forbidden lines ofNeIII

We have measured the radiative lifetime of the 2s(2)2p(4) S-1(0) metastable level of Ne2+ (Ne III) to be 223 +/- 11 ms at the 90% confidence level by observing the photons emitted at 1815 Angstrom by a decaying population of S-1(0) Ne2+ ions produced and stored in a radio-frequency ion trap. This is the first lifetime measurement for an excited term of a ground configurati on ion in the second row of the periodic table. The transition probabilitie s (A-values) for the forbidden transitions in the ground configurations of these ions are required for astrophysical line-ratio diagnostics. Using cal culated branching ratios, we estimate that A(lambda 1815)= 1.94 +/- 0.17 an d A(lambda 3344)= 2.55 +/- 0.19 s(-1). Because these numbers have a sum wit h an experimentally determined uncertainty of 5%, they will provide more ac curate results than the calculated A-values for determining electron temper ature and density from astrophysical Ne III line ratios.