TIME-RESOLVED PULSE-COUNTING LOCK-IN DETECTION OF LASER-INDUCED FLUORESCENCE IN THE PRESENCE OF A STRONG BACKGROUND EMISSION

We describe a time-resolved pulse-counting system well adapted for the detection of continuous laser induced fluorescence (LIF) signals in r epetitive phenomena, when a strong background emission is present. It consists of 256 channels coupled to a first in first out memory and in terfaced to a 486 DX 33 PC, for data storage. It accepts time-averaged count rates up to 450 kcount/s. Time between channels can be set from 12.5 ns to several mu s and the dead time between two consecutive cyc les of the physical phenomena is less than 20 ns. In phase with a chop per, which modulates the laser beam, it adds the observed photon signa l to the channel memories when the beam is on and substracts it when t he beam is stopped, acting like a lock-in amplifier which detect only the modulated part of the signal. The minimum detectivity on the LIF s ignal is only limited by the shot noise of the plasma induced emission signal. As an application, we studied the time variation of the Ar-((2)G(9/2)) metastable ions, detected by LIF, in two types of plasmas. Their radiative lifetime and collisional quenching frequencies were d educed from their decay rate in the afterglow of a pulsed Helicon reac tor. We also observed the evolution of their density in a 455 kHz capa citively coupled argon discharge. (C) 1996 American Institute of Physi cs.