Transient absorption spectroscopy using tunable diode lasers

Abstract

This thesis describes experimental techniques which have been developed to monitor transient infrared absorptions using lead-salt tunable diode lasers. The techniques are easily implemented, yield sensitivities which are limited by detector noise at 10 level of absorbance, and have a response time on the order of one microsecond. The transient absorption detection techniques are essentially high frequency versions of the sweep integration technique. TDL modulation rates of 100 kHz and 500 kHz allow for absorption sampling rates of 200 kHz and 500 kHz respectively. In order to reproducibly achieve near-detector-noise—limited sensitivities for 100 kHz TDL modulation rates, an automated analog subtraction circuit has been developed which removes the effects of minor TDL power variations. At the 500 kHz modulation rate digital filtering techniques are used to remove the effects of this power variation. The transient absorption detection technique could become an important tool for the detection of unstable species such as free radicals or molecular ions. The spectroscopy of these transient species could reveal new insights in important research areas such as atmospheric and interstellar chemistry, biology and semiconductor materials processing. In addition, the theoretical aspects of noise reduction techniques commonly used in the area of laser spectroscopy are examined in this thesis. Using a Gaussian statistical analysis, the theoretical relationships for both bandwidth limiting and digital averaging noise reduction methods are experimentally confirmed.