Characterization of some suitable deflecting liquids in photothermal deflection spectroscopy Marco Montecchi and Enrico Masetti

ENEA-Casaccia, Thin Film Laboratory, C. P. 2400, I-00100 Rome A. D., Italy. Received 20 November 1989. 0003-6935/90/283989-02$02.00/0. © 1990 Optical Society of America. Generally CCl4 is used in photothermal deflection spec­

troscopy as the deflecting medium. We looked for alterna­tive deflecting liquids. This Letter reports the main fea­tures of some suitable liquids. Keywords: Photothermal deflection spectroscopy, deflecting liquids, thermal diffusi-υity of liquids.

Photothermal deflection spectroscopy (PDS) allows the measurement of absorptance low values. The coating ab-sorptance value depends on the refractive index of the medi­um surrounding the device. If a suitable deflecting liquid is used to enhance the mirage effect, the value of the measured PDS absorptance is not equal to that of the device interfaced with air. Thus a correction is needed. This is a easy task for single layer coating, but it is not so for multilayer coatings. Recently we showed that the absorptance value of a multi­layer packed between air and an infinitely thick substrate may be obtained by performing two PDS measurements using deflecting liquids having different refractive indices.1

Moreover, the air-substrate interface contribution can be taken into account using the PDS measurements of direct and inverse absorptances. For such a purpose, we looked for PDS suitable deflecting liquids in the 0.25-1.6-μm wave­length range, an alternative to the commonly used CC14.

The transmittances of CS2, trimethilpentane (Iso-octane), and Aceton have had quite interesting results.2 In Table I their useful optical ranges are reported. These optical ranges give only general information. In fact, the real ex­tremes of each range depend on the absorptance of the coat­ing too. When the probe beam deflection generated by the coating absorptance becomes similar to the one caused by the liquid absorptance, the output will be affected by a spurious signal. Consequently, a higher coating absorbtance leads to a wider useful optical range of the liquid. On the other hand, the contribution of liquid absorptance in the PDS signal is easily detected because its phase, measured by a synchro­nous detection technique, is in advance of 3/4π with respect to the coating phase contribution. Summing up, the select­ed deflecting liquid can be employed until the signal phase is

TABLE I. Useful Optical Ranges of the Examined Deflecting Liquids

insignificant in advance. It is possible to characterize the liquids of interest on the ground of the mathematical rela­tions describing the photothermal deflection phenomenon. For a transversal PDS apparatus the fundamental Fourier component of the signal as a function of time and distance between probe beam and coating is3

with

where Tr is the detector transduction factor, L is the interac­tion length between the optical heated region and probe beam, λ is thermal conductivity, ρ is mass density, c is specif­ic heat, An is the absorptance of the sample interfaced with the deflecting liquid having the refractive index n, I0 is pump intensity, v is chopper frequency, and k is thermal diffusivity. The subscripts s and l stand for substrate and deflecting liquid, respectively. According to Eq. (1), the module of the PDS signal [R(z) = R0 exp(—hz)] increases exponentially as the probe beam approaches the sample surface as a function of the parameter h = √πv/kl. Then, by measuring R(z) for some z-values and different chopper frequencies, the ther­mal diffusivity kl and the value of R0 can be deduced. R(z = 0) = R0 is independent of chopper frequency. The quantity R0/An, for a given substrate, only depends on the liquid deflecting capability. We will define the ratio (R0/An)/ (R0CCl4/ACCl4) as the relative deflecting power (RDP). It gives us information on the PDS signal strength for a certain liquid. A sample consisting of an a-C:H single film, with a thickness of 1500 Å, deposited on a quartz substrate 1 mm thick was examined for comparing different liquids. The wavelength of the pump beam was 600 nm. The correspond­ing refractive indices of substrate and film were, respective­ly, 1.462 and 2.13; the film extinction coefficient was 0.179. Data have been worked out using different chopper frequen-

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TABLE II. Main Physical Constants of the Considered Materials

TABLE III. Thermal Diffuslvlty and Relative Deflecting Power of the Examined Liquids

cies in the 1-5-Hz range. In Table II some of the main features of the substrate and of liquids of our interest report­ed in the literature4,5 are summarized. In Table III we report the experimental values of k and RDP along with the values calculated from Eqs. (2) and (4) using data in Table II. By comparison it is possible to point out that the experimental values are in good agreement with the calculated ones. It can be noticed that Iso-octane and Aceton are quite similar in both refractive index and signal strength when the stan­

dard distance of 125 μm between the sample surface and probe beam is considered.

Summarizing: We analyzed three deflecting liquids as alternatives to the commonly used CCl4, and we determined their main features. Considering the useful range in wave­length and the deflecting power, CC14 and CS2 seem to be the most suitable liquids in PDS measurements.

References 1. M. Montecchi, E. Masetti, and G. Emiliani, "Measurement of

Optical Absorptance in Multilayer Coatings by Photothermal Deflection Spectroscopy," (submitted to Appl. Opt. 29 1990).

2. Transmittance as measured by a Perkin-Elmer Lambda 9 spec­trophotometer filling a quartz cell (15 X 15 mm2) with the liquid.

3. N. M. Amer and W. B. Jackson, Semiconductor and Semimetals, Vol. 21B, R. K. Williardson and A. C. Beer, Eds. (Academic, London, 1984).

4. J. Stone, "Absorption of Light in Low-Loss Liquids," J. Opt. Soc. Am. 62, 327-333 (1972).

5. American Institute of Physics Handbook, 3rd ed. (McGraw-Hill, New York, 1982).

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