Chopper technique for measuring a delayed fluorescence spectrum superimposed on a excitation spectrum

Abstract

For situations where the pump and fluorescence wavelengths are comparable with each other a simple chopper technique is described which eliminates tech. problems in cases where the fluorescence decay time is ¡Ã20 ¥ìs. The basic element is a high speed mech. chopper having a max. rotational range of 0-100 rps and running in air. It uses a specially designed Al wheel which has an odd no. of radial apertures and its inner parts have a smaller angular aperture size (¡­20¡Æ ¡¾ 3')than the outer part (36¡Æ ¡¾ 3'). The exciting beam is focused on the left side of the wheel and chopped by the inner slots, while the fluorescence is focused on the right side of the wheel and is chopped during its decay by the outer slots at the same frequency but with a mech. phase shift of 180¡Æ. In this way all the scattered or stray light coming from the exciting beam and dying with it is eliminated; the detector will only see the true luminescence. This technique was successfully used to measure excitation and emission spectra and decay time behavior of isolated and pairs of Cr3+ in GdAlO3 when excited in their lowest transition energies.