A kinetic investigation of Sr[5s6s( 1S O)] in energy pooling from Sr[5s5p( 3P J)] studied by time-resolved atomic emission following pulsed dye-laser excitation

Abstract

We present a kinetic study of Sr[5s6s( 1S o)] generated by energy pooling arising from self-annihilation of Sr[5s5p( 3P J )]. Sr[5s5p( 3P 1)] was produced by pulsed dye-laser excitation at λ = 689.3 nm (Sr[5s5p( 3P 1)] ← Sr[5s 2( 1S o)]) of strontium vapour at elevated temperatures (750–890 K) in the presence of excess helium buffer gas. Following rapid Holtzmann equilibration within the 5s5p( 3P J ) spin-orbit manifold, time-resolved emission was monitored at λ = 1124.1 nm (Sr[5s6s( 1S o)] → Sr[5s5p( 1P 1)]) using signal averaging methods. To our knowledge, this constitutes the first direct observation of this time-resolved emission, which has become feasible through the commercial development Hamamatsu) of a new long-wavelength photomultiplier tube (350–1200 nm) based on an AgOCs photocathode, whose operation and application in the present system are described in some detail. Subsequent time-resolved emission at λ = 460.7 nm Sr[5s5p( 1P 1)] → Sr[5s 2( 1S o)]) is thus seen to result from “cascading fluorescence” as originally proposed by Gallagher and coworkers. First-order decay coefficients, characterized across the above temperature range for Sr[5s6s( 1S o)], Sr[5s5p( 1P 1)] and Sr[5s5p( 3P J )], are shown to be in the ratio of 2:2:1 in accord with self-annihilation, energy pooling emission and cascading fluorescence. Integrated atomic emission intensity profiles for the transitions at λ = 1124.1 nm and 460.7 nm, coupled with optical and electronic sensitivity calibrations, yield an estimate of the Einstein coefficient for the λ = 1124.1 nm transition based on the above mechanism. Finally, the present optical system, using the new photomultiplier tube with the associated optics and gating circuitry, is seen to provide a basis for studying low-lying electronically excited states above the ground state (more than approximately 1.03 eV), particularly Ba[6s5d( 3D 1,2,3)] and Ba[6s5d( 3D 2)] by time-resol emission.