Lifetime measurements of interference-narrowed sodium Stark resonances

We report measurements of the temporal decay of sodium Stark resonances over regions of interference narrowing and compare with calculations using Wentzel-Kramers-Brillouin quantum-defect (WKB-QD) Stark theory. The measurements are performed with a beam of sodium atoms in a uniform electric field of ∼3.2 kV/cm excited via the 3 ${}_{}{}^2{}_{}{}^{}P$ state to ∼285 ${\mathrm{cm}}^{\mathrm{-}1}$ ($n^{\mathrm{*}}$∼20) below the zero-field ionization threshold, and hence substantially above the classical saddle-point energy. For mixed states at level crossings, the discrete-continuum couplings interfere, yielding resonances that are stabilized against ionization over a small interval of the field. Measurements of the maximum lifetimes of ‖m‖=1 levels here are not limited by laser bandwidth, as were our previous spectral observations of m=0 resonances. For three narrowing regions studied here, the observed maximum lifetime values of 127–191 nsec are 10–20 % less than values obtained from WKB-QD calculations, while experimental error limits are ∼5%. After considering estimates for competing processes, namely radiative decay and transfer by blackbody radiation, a slight discrepancy remains.