Anal. Chem. 1982, 5 4 , 239-242
239
Laser-Induced Thermal Diffraction for Calorimetric Absorption Measurements M. J. Pelletier, H. R. Thorshelm, and J. M. Harris" Department of Chemlstty, University of Utah, Salt Lake Clty, Utah 84 112
The laser-Induced gratlng or real-tlme hologram Is proposed as a potentlally useful thermooptlcal element for calorlmetrlc trace absorptlon measurements. The method has an attrlbute In common wlth fluorescence detection, that the optlcal slgnal Is observed on a nearly zero lntenslty background, even though the effect arlses from the nonradlatlve fractlon of the excited state decay. I n addltlon, the method has excellent spatlal selectlvlty wlth no loss of sensltlvlty. A prellmlnary study of the technlque uslng a CW Ar+ laser excltatlon source Is reported. The mlnlmum detectable sample, llmlted by the shot nolse on the PM dark current, has an absorbance per unlt length of a = 7 X I O 4 cm-' for a sampled volume of 0.17 FL. The more sensltlve analytlcal performance expected of the method, when used wlth pulsed lasers Is also dlscussed.
The incorporation of laser sources into instrumentation for molecular fluorescence spectroscopy has produced outstanding advances in sensitivity and detection limit for trace level samples (1, 2). The small limits of detection are a result of the linear sensitivity relationship between fluorescence signal detected and the large intensity available from the laser source. For the large host of sample chromophores having vanishingly small fluorescenceyields, this sensitivity advantage does not lead to similar limits of detection due to blank limitations imposed by the fluorescence of the solvent or sample matrix (3). When the sample absorbance in this situation remains larger than that of the blank, the limit of detection may be lowered by a measurement of the heat given off by the sample, arising from the predominantly nonradiative relaxation of the excited states produced. This calorimetric approach to determining small absorbances forms a class of techniques (4, 5) which differ by the method of measuring the temperature increase in the sample. Among the most sensitive of these methods are those which rely on the temperature difference within the sample to produce a perturbation in optical pathlength. This refractive index perturbation can be detected interferometrically; similarly, it can be probed using a laser beam resulting in a change in divergence caused by a thermal lens (6) or by photothermal deflection of the beam (7) by a prismlike thermo-optical element. In this work, another thermo-optical element, a laser-induced thermal grating, is considered for calorimetric trace absorption measurements. The literature of laser-induced diffraction phenomena or real-time holography is quite mature (8-12) since the technique allows the measurement of thermal diffusion and exciton transport, the excitation of high-frequency sound waves, and the determination of picosecond excited state lifetimes. Despite the plethora of exotic applications of this technique, the relationship between sample absorbance and diffracted light intensity produced has not been demonstrated. Furthermore, the unique capabilities of laser-induced thermal diffraction as a method for measuring weak optical absorption have not been considered.
THEORY The principle of detection in laser-induced diffraction is to force absorption of radiation to create a spatially periodic 0003-2700/82/0354-0239$01.25/0
disturbance in the refractive index of the sample. This disturbance then behaves as a transmission grating, causing diffraction of a second, probe laser beam into a detector placed in such a way as to view only the diffracted radiation. The spatially periodic excitation is achieved by splitting and recombining an excitation laser beam, while introducing a path difference not larger than the coherence length. An interference pattern is thus formed at the intersection where a sample would be placed, as shown in Figure 1. Since absorption of radiation can only take place where the resulting electromagnetic field is nonzero, excited states are produced at the planes of constructive interference, separated by a spacing A = X,/2 sin ( 8 / 2 ) (1) where A, is the wavelength of the excitation beam and 0 is the angle of intersection. The subsequent spatial modulation of the refractive index occurs as the excited-state species thermally relax and increase the temperature of the surrounding solvent. A sensitive method to detect the resulting transmission grating is by diffraction of a beam from a second laser intersecting the particular volume of sample in which the grating is formed, as shown in Figure 2. The advantage of this configuration is that the detector can be placed in a position where it can only view the diffracted radiation from the probe laser beam. The resulting optical signal is, therefore, observed on a nearly zero-intensity background, which could allow a shot-noise limited measurement of a nonradiative excited state decay process. If the probe laser has a wavelength, A,, the angle of the probe beam with respect to the thermal grating, 4 / 2 , must be optimized according to the Bragg equation. 4 = 2 si& ( X P / 2 h ) = 2 sin-l [(A,/&) sin ( 0 / 2 ) ] (2) Under these conditions, diffraction of the probe beam into first order will depend on the square of the phase difference across the fringes (13) I+/Io= (rAnb/AJ2
