kinds of systems. The principles of four popular approaches are outlined in Figure 1. The refractive-index dis tribution generated by each experi ment approximates a conventional op tical element. That element is shown next to each configuration. By far the most common effects used are transverse photothermal de flection, also called the mirage effect, and the thermal-lens effect. Trans verse photothermal deflection probes the refractive-index change of the me dium in thermal contact with the sam ple, whereas the thermal-lens tech nique probes the change of refractive index of the sample directly. The ther mal lens can be observed as the defocusing of the laser that heats the solu tion, or as the defocusing of a second weak probe beam. Variants in which the lens is observed with a nonparaxial probe laser are increasingly com mon. Depending on the details of the experiments, and the laboratories re porting them, these techniques are called collinear photothermal deflec tion or crossed-beam thermal-lens measurements. Photothermal effects are closely re lated to the photoacoustic effect. In photoacoustic measurements, the heat generated by light absorption is de tected as a pressure change, using a microphone or other pressure trans
ducer in contact with the sample or a coupling fluid. Both photoacoustic and photothermal spectroscopy are in direct absorption measurements. To the extent that a sample is em bedded in a nonabsorbing matrix, photothermal and photoacoustic spec troscopies are zero background meas urement techniques. They should pro vide for nonfluorescent systems the same advantage that fluorimetry of fers, which is the absence of shot noise associated with measurement of a large background signal. This advan tage is more easily realized in gases than in liquids or solids. In principle, photoacoustic and photothermal meas urements are capable of measuring absorbances down to 10~9 units, al though this level is rarely reached. By that standard, nominally transparent materials, which may have absorbances of 10 - 6 -10 - 3 /cm, are strongly absorbing. In condensed-phase meas urements photothermal techniques are often limited by the noise associat ed with matrix absorption. The thermal-lens effect was first ob served in the early years of laser Ra man experiments by Gordon and co workers (2). They observed that the small amount of heat deposited in a liquid benzene sample caused local heating. The heat, in turn, formed a refractive-index distribution that be
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haved as a diverging lens. Because the thermal lens defocused the laser beam that generated it, even small absorbances were easily detected as changes in the spot size of the laser beam. Thermal-lens spectroscopy was soon applied in the physics community for measurement of visible light absorp tion by pure organic liquids. By the early 1970s, the technique was used by Flynn and his co-workers to study vi brational energy transfer in the gas phase (3). In the mid-1970s Albrecht and Swofford (4) introduced a con venient two-laser variant for liquid phase measurements, and Kliger and Twarowski (5,6) began a study of twophoton absorption using pulsed dye lasers and thermal-lens detection. Harris's INSTRUMENTATION article on thermal-lens theory in this J O U R NAL (7) introduced the technique to analytical chemists and sparked the current interest in applications of this technique. Transverse photothermal deflection was first reported by Boccara et al. (8) in 1980. The effect is an application of the mirages described by Marco Polo. In fact, Boccara et al. used the term "mirage effect" to describe their ex periments. The heating beam is pulsed or modulated and aimed ap proximately perpendicular to the sam ple surface. It is frequently, but not al-
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