The theoretical and experimental values of the 5f, 6f, 7f, and 8f radiative lifetimes of neutral potassium are reported. The reduced matrix elements for all allowed electric-dipole nf 5/2 -nЈd 5/2 , nf 5/2 -nЈd 3/2 , and nf 7/2 -nЈd 5/2 transitions with n =5-8 in K are calculated 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, 6f, 7f, and 8f states in neutral K and their uncertainties. The contributions from the nf 5/2 -nЈg 7/2 , nf 7/2 -nЈg 7/2 , and nf 7/2 -nЈg 9/2 transitions to the lifetimes of the 6f, 7f, and 8f states are evaluated using the third-order manybody perturbation theory and are found to be very small. The theoretical results are 5f = 117͑4͒ ns, 6f = 195͑4͒ ns, 7f = 301͑6͒ ns, and 8f = 441͑9͒ 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 = 117͑3͒ ns, 6f = 190͑6͒ ns, 7f = 309͑8͒ ns, and 8f = 428͑10͒ ns are in excellent agreement with the present theoretical calculations.
All-experimental self-quenching cross-sections σ q nf are reported for K(nf) states (n = 5, 6, 7, 8). The experiment was performed at varied temperatures of K vapour in a spectral cell. Time-resolved fluorescence was observed following pulsed step-wise excitation with dipole and quadrupole transitions K(4s) → K(4p) → K(nf). The values: σ q 5f = 2.4 ± 1.7, σ q 6f = 4.5±1.4, σ q 7f = 8.1±1.8, and σ q 8f = 18.2±3.0 (in units of 10 −13 cm −1) were obtained from Stern-Volmer-type plots.
We report measurements of the cross sections σt-mix for angular momentum mixing of the K(n2F) (n = 6, 7, 8) states with the K(n, d > 3) states, induced in thermal collisions with parent ground state K(4 2S) atoms. The experiment was performed in a cell heated to ca. 450 K. The n2F states were excited in two steps: 4 2S-> 42P-4. n2F by using a diode laser for the first step (a dipole-transition) and a pulsed dye laser for the second step (a quadrupole-transition). The measured values σl-mix (62F) = 3. 6 ± 2. 2 , σ l-m i x (7 2 F) = 4. 2 ± 2. 6 a n d o l-m i x (82 F) = 8. 0 ± 4. 6 (i n u n i t s o f 10-13 cm-1) are close to the corresponding values of the geometrical cross sections.
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