Noise reduction in laser photolysis studies of photolabile samples using an optical multichannel analyzer

Abstract

An optical multichannel analyzer system for obtaining high-quality spectra of intermediates formed by nanosecond laser photolysis is described. The system is useful for samples which undergo irreversible photochemistry and which are available in limited amounts. Sample volumes as small as 25 μl are possible and a syringe pump driven by a linear stepper motor is used to deliver fresh sample after each photolysis pulse. The microchannel-plate intensified detector used here in gated mode can collect an entire spectrum in 5 ns, but proper synchronization of the detector scans is required to avoid severe distortion which can appear as random noise in signal averaging. Noise from various sources such as phosphor lag in the detector, time jitter, source intensity fluctuations, and the photon character of light are discussed. Experimental design for noise reduction is described including diagnostic procedures and light source filtering to produce more uniform signal-to-noise ratio throughout the spectrum. Finally, a comparison is made between the noise observed to occur between detector channels and the noise predicted to occur based on the photoelectron gain of the intensifier. The disagreement between these is discussed in terms of the spatial resolution of the detector.