4. W. F. Hillebrand and G. E. F. Lundell, Appfied Inorganic Analysis (J. Wiley and Sons, New York, 1929), p. 380. 5. S. A. Matthes, Introduction to Microwave Sample Preparation (American Chemical Society, Washington, D.C., 1988), Chap. 3, p. 34. 6. H. M. Kingston and P. J. Walter, Spectroscopy 7 (9), 20 (1992). 7. R. K. Winge, V. J. Peterson, and V. A. Fassel, Appl. Spectrosc. 33, 206 (1979). Observation of FT-IR/Photoacoustic and FT-IR/Photoacoustic Reflection-Absorption Spectra of Langmuir-Blodgett Films on a Metal Substrate S. OCHIAI, J. F. McCLELLAND,* K. KOBAYASHI, and K. TAKAOKA Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011-3020, U.S.A. (S.O., J.F.M.); and Department of Chemistry, Musashi Institute of Tech- nology, Setagaya-ku, Tokyo 158, Japan (K.K., K.T.) Index Headings: FT-IR/Photoacoustic; Photoacoustic-RAS; LB film. INTRODUCTION Recently, much attention has been paid to studies of the applications of thin films deposited on solid substrates for microelectronics. In order to clarify the properties and functions of thin films, researchers have developed many techniques. Vibrational spectroscopy, using Fourier transform infrared (FT-IR) instrumentation in particular, has been applied successfully to such films. For the de- termination of molecular orientation of films deposited on a flat metal substrate, reflection-absorption spectros- copy (RAS) is frequently used. 1-6 Sometimes, the trans- mission spectrum is needed if detailed information about molecular orientation of the films is to be obtained. To obtain the transmission spectra of the thin films, one should deposit materials on a mid-IR transparent sub- strate with a flat surface, such as a ZnSe or a BaF2 window, under the same conditions used with the deposition on the metal substrate. Dote and Mowry 7 have reported, however, that the adsorbate-surface bonding orientation of a Langmuir-Blodgett (LB) monolayer differs with dif- ferent substrates. Chatzi et al. s have applied FT-IR/photoacoustic (FF- IR/PA) spectroscopy to quantitative analysis and orien- tation of LB films deposited on PTFE (polytetrafluoro- ethylene). Furthermore the FT-IR/PA spectra of LB films of cadmium arachidate on a glass plate have been ob- served by Nakanaga et al. 9 In this study, FT-IR/PA/RA (FT-IR/PA detection in which the reflection-absorption technique is used) is dem- onstrated for LB films deposited on a metal substrate. Received 28 February 1994; accepted 25 July 1994. * Author to whom correspondence should be sent. FT-IR/PA and FT-IR/PA/RA spectra can be taken on the same LB film. By the use of FT-IR/PA and FT-IR/ PA/RA methods, spectra corresponding to transmission and RA spectra can be identified for the same LB film on a metal substrate. The molecular orientation of the LB film can be analyzed with the use of these spectra. EXPERIMENTAL Arachidic acid was obtained from Sigma Chemical Co. and used without further purification. Purified chloro- form was used as a spreading solvent. LB films of cad- mium arachidate were prepared on gold-coated brass from a subphase surface (pH 7.0) containing cadmium ions at surface pressure of 30.0 mN m -1. All FT-IR measurements were performed on a Bio- Rad FTS-60A spectrometer equipped with an MTEC Model 200 photoacoustic detector. During the PA/RA spectra measurement, a polarizer was set in the optical path in front ofphotoacoustic detector and a sample plate was inclined so that the angle between the IR beam axis and sample surface normal was 75 °. A sample plate size of 5 × 10 mm was used. A schematic drawing of the PA/ RA cell appears in Fig. 1. Two types of PA/RA cells were used. In one cell, the interior surfaces of the PA cup and the sample plate holder surfaces are bare metal [PA/RA(A) cell]. The other has graphite-coated inside surfaces [PA/ RA(B) cell]. The former geometry results in the direct detection of a PA/RA signal from the sample, whereas the latter results in detection of the PA signal primarily generated by the graphite coating when it absorbs the reflected beam with RA information. All infrared spectra were measured at 8-cm -1 resolution and with helium coupling gas. RESULTS AND DISCUSSION When an FT-IR/PA spectrum of LB films is obtained by means oflR light at normal incidence to the substrate, only absorption bands of the LB film with a dipole tran- sition parallel to the substrate surface appear in the spec- modulated and polarized IR light I I sample plate sample plate holder PA cup • graphite coating for version B is absent in version A FIG. 1. A schematic drawing of the PA/RA cell. Volume 48, Number 10, 1994 0003-702S/94/4S10-1287$2.00/0 APPLIED SPECTROSCOPY 1287 © 1994 Societyfor Applied Spectroscopy