Experimental and theoretical study of the nf-level lifetimes of potassium

The theoretical and experimental values of the 5f ,6 f ,7 f, and 8f radiative lifetimes of neutral potassium are reported. The reduced matrix elements for all allowed electric-dipole nf5/2-nd5/2, nf5/2-nd3/2, and nf7/2-nd5/2 transitions with n =5–8 in K arecalculated using the relativistic linearized coupled-cluster method with single and double excitations of Dirac-Fock wave functions included to all orders in many-body perturbation theory. The resulting electric-dipole matrix elements are used to evaluate the lifetimes of the 5f ,6 f ,7 f, and 8f states in neutral K and their uncertainties. The contributions from the nf5/2-ng7/2, nf7/2-ng7/2, and nf7/2-ng9/2 transitions to the lifetimes of the 6f ,7 f, and 8f states are evaluated using the third-order manybody perturbation theory and are found to be very small. The theoretical results are 5f = 1174 ns, 6f = 1954 ns, 7f = 3016 ns, and 8f = 4419 ns. The experiment is performed in a cell using time-resolved fluorescence spectroscopy. The nf states are excited stepwise, 4s → 4p →nf, using two pulsed synchronous dye lasers for the dipole and quadrupole transitions, respectively. The measured values 5f = 1173 ns, 6f = 1906 ns, 7f = 3098 ns, and 8f = 42810 ns are in excellent agreement with the present theoretical calculations.