Trace Gas Analysis by Pulsed Laser Absorption Spectroscopy

Optical absorption measurements of trace level (ppb) pollutants in ambient air samples and chemical reaction products in laboratory flow reactors have been made using a new technique employing pulsed laser sources. This new technique allows optical absorption measurements to be made using either broad band or narrow band pulsed lasers and offers a temporal resolution and absorption detection sensitivity significantly greater than can be attained by using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. We present measurements that demonstrate the great sensitivity of this technique and the utility of this approach as a sensitive probe of complex chemical environments. Optical techniques based upon atomic and molecular absorption and fluorescence are routinely used for the in-situ analysis of environmental samples both in the field and in the laboratory. For many applications involving large polyatomic molecules, absorption measurements are potentially superior to those based on optical emission of which the quantum yield could be reduced by the rapid quenching of the excited upper state through energy redistribution processes. To study these systems, sensitive new optical absorption techniques are needed. In the limit of weak absorption the transmitted optical intensity (I) decreases exponentially with absorption pathlength (l) in accordance with Beer's law, I = Io×e , where the exponential decay constant, k, is the absorption coefficient at the frequency of the incident beam and Io is the incident intensity. The ability to accurately measure the ratio of I to Io typically limits the measurement to minimum losses of 0.01% to 0.001%. As a rule, such precision absorption measurements require sophisticated FIGURE 1. Schematic representation of the experimental layout used to evaluate the Cavity Lossmeter absorption technique. A 10 ns optical pulse from a dye laser pumped by a nitrogen laser is mode matched to the sample absorption cavity and coupled into it by weak transmission through the cavity mirrors(T≅1010). The light coupled in is trapped in a closed cavity with the only loss mechanisms being intracavity absorption and transmission loss at the mirror surfaces. A signal proportional to the intracavity optical intensity is obtained by measuring the transmitted signal through the rear mirror. This signal is digitized and fit to an exponential waveform to obtain the ring-down time constant, which can be expressed as a loss per pass. optical systems and sources that have stabilized output intensity. In the most sophisticated laboratory systems, the required intensity stability has been achieved by using several types of continuous lasers (e.g. infrared lasers, diode lasers, and tunable cw dye lasers. Typical experimental configurations also employ some form of frequency modulation to discriminate against low frequency noise. The same level of sensitivity has not yet been possible for experimental systems based upon pulsed laser sources for several reasons. First, the pulse to pulse amplitude variation of most pulsed laser sources is typically larger than 10%. Therefore, the detectors used need a greater dynamic range and their effective signal resolution suffers. In addition, frequency modulation techniques are not feasible with the short pulsewidths (typically 10 to 30 nsec) of these lasers. Because of these limitations, sensitive absorption measurements have not been made over the full spectral range accessible only with the combination of pulsed lasers and non-linear frequency conversion techniques. The Cavity Lossmeter system (Deacon Research, Palo Alto, CA) employs a new technique that allows an increase of several orders of magnitude in sensitivity of absorption measurement over existing techniques while taking the advantages of pulsed laser sources. Its sensitivity is better than 0.0001% (1 ppm) and its operation range is between 0.0001% and about 5%. Its spectral range is limited only by the availability of short pulse (several tens of nanosecond or shorter) lasers and high reflectors (better than 99% reflectivity) at a given wavelength. Principles of operation Details of the development and design of the basic Cavity Lossmeter system described here have been discussed in a recent paper. A schematic representation of the system approach and configuration is shown in Figure 1. The technique is based on the measurement of the rate of absorption of a tailored light pulse by a sample located within a closed optical cavity consisted of two parallel, aligned mirrors with very high reflectivity. A pulse of light is coupled into the cavity from which a small portion (~50 parts-permillion) is transmitted through one of the mirrors on each transit cycle and directed into a photodetector. The measured intensity of the small, out-coupled light pulse is proportional to that of the circulating pulse within the cavity. In the absence of any other loss mechanism, the rate of intra-cavity intensity loss is dI/dt = Ct × c/L × I (1) where Ct is the coefficient of transmission for the mirrors at the laser wavelength, c is the speed of light, and L is the cavity length. The solution to this differential equation is a simple exponential I = Io × exp[-Ct×c/L×(t-to)] (2) where Io is the initial intensity injected into the cavity at time to. It can be seen that additional losses in the cavity simply add to the magnitude of the decay constant, Ct. In order to make the determination of the loss rate, the signal is amplified, digitized, and transferred to a microcomputer. The computer extracts the exponential decay time (τ) of the signal and calculates the total loss (Γ) of the