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Anal. Chem. 1985, 57,95-99
Effect of Heat from Rear Surface of a Film Sample on Spectral Features in Fourier Transform Infrared Photoacoustic Spectroscopy Norio Teramae*' and Shigeyuki Tanaka Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
The effect of heat generated at the rear surface of a film sample on spectral features in Fourier transform infrared photoacoustic spectroscopy was Investigated theoretlcaily and experhnentally. The effect was found to be a maln c a w , glvlng undesirable photoacoustlc (PA) spectral features for a bliayered fllm, and was found to make the PA spectrum of a monolayeredfilm structureless. Careful positioning of a fllm sample in a PA cell was required to obtain a reasonable PA spectrum.
In recent years, there has been a growing interest in photoacoustic spectroscopy (PAS)as a new analytical tool for the study of solid, liquid, and gaseous samples (1). Most of the studies of PAS, however, have been cancerned with the ultraviolet and visible spectral ranges, for which powerful excitation sources are readily available. On the other hand, it is well-known that the spectral measurements in the infrared region give valuable information about molecular structures and that infrared spectroscopy is particularly useful for the characterizatiop of organic materials. More recently, the spectral range of PAS has been successfully extended into the mid-infrared region by using both dispersive (2,3)and Fourier transform infrared (FTIR) (4-10) spectrometers. Compared with the conventional measuring technique for PAS (11, the Fourier transform method has complicated characteristics inherent to the rapid scanning interferometry. That is, the modulation frequency varies with the frequency of the radiation being emitted from the source (11). Accordingly, the Helmholtz resonancg effect (12, 13) is apt to affect the FTIR-PA spectra (14, 15), and the effect of the variation of sampling depth with the spectral frequency appears in the FTIR-PA spectra (16,17). Although FTIR-PAS has several complicated aspects as mentioned above, it is convenient to use the Fourier trqnsform method for measuring PA spectra in the mid-infrared region. This is true because the PA signal iptensity is proportional to the power of the source incident upon a sample, 'and FTIR spectrometers have the advaptage of higher energy throughput when compared to conventional dispersive spectrometers. By using this advantage, FTIR-PAS has been applied to the qualitative (18-20) and quantitative (21,22) analysis of powders, to the study of conducing polymers (23-25), and to the analysis of surface species (26-31). It is of interest to characterize the surface of film samples with FTIR-PAS, since many of the techniques including electron spectroscopies (32) are not well suited for the analysis of thin polymer films because of the decomposition of the sample under the ultrahigh vacuum or the irradiation of an electron beam. In this regard, information obtained from the 'Temporary address until August, 1985: Leigh Hall, Box 551, Chemistry Department, University of Florida, Gainesville, Florida 32611.
FTIR-PA spectra of films has been compared with that obtained from attenuated total reflectance spectra (33). In this paper, we make some experimental and theoretical discussions about the characteristic features of the FTIR-PA spectra of filmlike samples.
EXPERIMENTAL SECTION The measurement system for FTIR-PAS was similar to the one we used in earlier experiments (27,28) and was composed of a Digilab FTS-15 FTIR spectrometer and the PAS attachment described below. The light source used was a nichrome wire. Infrared radiation from the source was collimated by mirrors and modulated with a Michelson interferometer in which a movable mirror was scanned at a constant velocity of 0.16 cm/s. The wavenumber range 4000-400 cm-l corresponds to the acoustic frequency range 1280-128 Hz, since the modulation frequency f (Hertz) corresponds to the spectral frequency v (inverse centimeters) through the relationship (11) f = 2Vv, where Vis the velocity of a movable mirror in centimeters/second. The modulated infrared beam was reflected by a plane mirror placed in the sample compartment of the spectrometer to a right-angle off-axis toroidal mirror which afforded a 6 1 reduction of the beam and focused the beam onto the sample in a PA cell. The toroidal mirror and the PA cell were placed inside an acoustically insulated chamber which was set up outside the spectrometer. The PA signals from the microphone attached to the PA cell were amplified with a Brookdeal 9454 ac amplifier and were fed to the HgCgTe detector amplifier section of the FTIR spectrometer. The samples studied were polymer films. In this case, a PA cell with a large cell cavity is not necessary. According to the results on the optimum dimensions of a P A cell, the maximum PA signal occurs at l g / p g = 1.4 (34) or 1.8 (35), where 1, and pg are the length of the gas column in a PA cell and the gas thermal diffusion length, respectively. Taking these results into consideration, a PA cell was newly designed so as to minimize the dead volume and to enhance the sensitivity. A schematic drawing of the PA cell used in this study is shown in Figure 1. The cell was made of brass, and its internal volume was approximately 0.04 cm3 (4-mm i.d. and 3-mm depth). A KBr plate (10-mm diameter and 2 mm thick) was used as a cell window. The condenser microphone used was a 1/2-in.Bruel & Kjaer (B & K) Model 4165, with a B & K Model 2619 preamplifier powered by a B & K Model 2807 power supply. The sample can be placed in a PA cell from either the top or the bottom. Most PA cells have their own acoustic resonant frequencies. The response of a PA cell will be affected by the volume of a sample placed in the PA cell, since the sample may alter the volume of the coupling gas in the PA cell and may alter the resonant frequency. In the case of our PA cell, the effect of the volume of a film sample on the response of the P A cell can be neglected, since the volume of the cell cavity can be kept almost constant when placing a sample on the sample holder from the bottom. The cell can be used for powder samples and rubbers as well. The response of the PA cell was examined with a He-Ne laser, a light chopper, and a lock-in amplifier using carbon black as a sample. The PA signal from the microphone was inversely proportional to the modulation frequency in the measured range of 8-840 Hz. Accordingly, our PA cell can be regarded as a nonresonant PA cell. This result means that the cell has a flat response in the spectral region of interest and that the serious
0003-2700/85/0357-0Q95$0 1.50/0 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 I n f rored
Beam
Figure 1.
Schematlc drawing of a PA cell.
0 8 0
HAVENUMBERS
spectra of layered films. PE, PET, and Adh represent polyethylene, poly(ethy1ene terephthalate),and adhesive tape, respectively. Top layers of the films for A, B,and C are PE, PET, and PE, respectively. Flgure 3. FTIR-PA
HAVENUMBERS
Effectof the number of repeated scans (NSS) on FTIR-PA spectrum of 36 pm thick poly(ethy1ene terephthalate) film: NSS = 9 (A), 25 (B), 400 (C), and 1800 (D). Figure 2.
change of phase at the resonant frequency (12, 13) can be disregarded. In the measurements of FTIR-PAS, the output signal level (center burst in the interferogram) from the microphone was ca. 10 mV (peak-to-peak)at 0.16 cm/s mirror velocity with a nichrome wire source and a carbon black sample. The noise level was ca. 20 pV (peak-to-peak). Thus, a maximum SIN of 500 could be obtained from a single scan. The S I N ratio would be enhanced if helium was the coupling gas.
RESULTS AND DISCUSSION It has already been reported that the PA signal intensity of f i i l i k e samples is weaker than that of powder samples (36, 37), since the filmlike samples have a lower surface area. As an example of the performance of our instruments, Figure 2 shows unsmoothed PA spectra of 36 pm thick poly(ethy1ene terephthalate) (PET) film. In this figure, the number of repeated scans used for measuring the spectra of A, B, C and D is 9,25,400, and 1600, respectively. It takes about 10 s to measure the spectrum of A at 8-cm-' resolution. As shown in Figure 2, it took only a few minutes to obtain high-quality PA spectra. One of the advantages of PAS which is often mentioned is that spectra corresponding to the absorption spectra can be obtained with minimal sample preparations and, in fact,
by simply putting a sample into a PA cell. However, the PA spectra of multilayered film were found to show distorted spectral features around the strong absorption bands when the sample was simply placed on the sample holder as shown in Figure 1A. An example of this phenomenon is shown in Figure 3A which represents the PA spectrum of a bilayered film of polyethylene(PE)/PET. The top layer of the film is P E about 40 pm thick, and the second layer is PET about 10 pm thick. In the Figure 3A, it is noticeable that the PA signals around the strong absorption bands at 1725,1270, and 1120 cm-l do not correspond to the absorption spectral features and show a pattern similar to self-absorption. Taking into account the velocity of the moving mirror and the thermal properties of P E T film, whose thermal diffusivity is assumed to be about cm2/s, the thermal diffusion length can be estimated as about 5-16 pm for a spectral range of 4000-400 cm-l. In this case, since the top layer is about 40 pm thick, one might have expected that a PA spectrum corresponding to the absorption spectrum of the top layer, P E would be observed, judging from both the one-dimensional PA (R-G) theory (38) for homogeneous materials and/or several extended PA theories (39-42) for layered materials. However, the observed PA spectrum shown in Figure 3A does not fulfill this expectation, and the contribution from the subsurface layer, PET, to the PA spectrum can be easily recognized. In order to explain the above phenomenon, it might be relevant to take the following characteristics of FTIR into account: (i) there is insufficient Rhase correction in the FTIR spectrometer used here, though the PA spectrum shown in Figure 3A is presented as a power spectrum, and (ii) the effect of a thin air layer under the sample is not negligible, since the modulation frequency of the FTIR spectrometer is higher than that of the conventional PA spectrometer. When an additional film was attached to the rear surface of the sample with an adhesive tape, its PA spectrum was f o p d to correspond to the absorption spectrum of the top layer as shown in Figure 3B,C. This obseraation suggests that the heat generated at the rear surface of a sample makes a significant contribution to the PA signal of films and that the air layer and the higher modulation frequency play an important role in FTIR-PA spectra of film samples.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
t
1
Infrared Beam
97
\&E KEir window
Schematic drawing of a PA cell and the positions for a film sample: A, at the entrance window; B, in the cell cavity, and C at the exit window. Figure 4.
In order to confirm the above suggestion, dependence of the PA spectra on the position of the sample in a PA cell was examined. Figure 4 shows a schematic drawing of the PA cell and the placing position of the film sample. At position A in Figure 4,only the heat generated from the rear surface of the sample can contribute to the PA signal. When the sample is placed at position B, the heat generated from both the top and the rear surfaces of the sample can contribute to ;the PA signal. At position C, only the heat from the top surface of the sample can contribute to the PA signal. As shown in Figure 4,a KBr window was used as backing material so as to minimize the effect of the reflecting light from the backing material on the PA signal. Figure 5 shows the dependence of the PA spectra on the position in which the sample is placed. The spectra at the top, middle, and bottom were obtained by placing the sample at positions A, B, and C as in Figure 4. In the top spectrum (A) a spectral pattern similar to self-absorption can be recognized in the vicinity of strong absorption bqnds. The middle spectrum (B) is quite similar to the one shown in Figure 3A. From this observation, the spectral feature similar to selfabsorption in Figure 3A could well have been caused by the heat generated at the rear surface of the sample. The bottom spectrum (C) in Figure 5, in which the contribution to the PA signal from the heat at the rear surface can be discounted, corresponds to the absorption spectrum of the top layer, PE. From these results, it is clear that when measuring the PA spectrum of a filmlike sample, simply putting the sample into a PA cell cavity is not a very good method for obtaining a FTIR-PA spectrum corresponding to the absorption spectrum. It can be concluded from these experimental results that obtaining reasonable FTIR-PA spectra of films requires careful positioning of samples to eliminate the contribution from the heat generated at the rear surface of a sample as shown in Figures 1B or 4C or requires adhesion of a suitable material to the rear surface of a sample as shown in Figure 3B,C. In order to evaluate the heat generated from the rear surface of a sample as mentioned in the preceding section, the PA signal of a film was examined theoretically, using a monolayered film as a model sample. Considering a one-dimensional gas piston model for the PA signal, the temperature generated from the front surface of the sample, OF, was given by Rosencwaig and Gersho (38)and was expressed as
eF=
+
( r - I)(b + 1)e"l - (r + l ) ( b - l)e-"' 2(b - r)e+ E (1) (g + l ) ( b + 1)e"l - (g - l ) ( b - l)e-"l
where E = pIo/2k,(P2 -,):a b = kbab/ksa,, g = kp,/k,a,, r = (1- j ) P / 2 u s , and u = (1+ j)us. Parameters Io, p, 1, ai, and k denote the intensity of light, optical absorption coefficient, sample thickness, thermal diffusion coefficient of material i
i
f
HAVENUMBERS
Dependence of FTIR-PA spectrum of a bilayered film, PE/PET, on the sample positions in a PA cell. The top layer is PE. Spectrum of A, B, and C are obtained by placing the sample at A, B, and C in Figure 4. Figure 5.
(g, gas; s, sample; b, backing), and thermal conductivity, respectively. An expression for the heat generated at the rear surface, OB,was obtained by solving the thermal diffusion equation using the same approach as in the R-G theory (38). The solution is given by eB= 2(g r ) - ( r l)(g l)e"'e-@l- (r - l)(g - l)e-uLe-@t
+
+
(g
+
+ l ) ( b + I)e'l
- (g - l ) ( b - l)e-"l
E
(2) Since the expressions given by eq 1 and 2 are somewhat difficult to interpret intuitively, special cases are examined by using the same approximation as in the R-G theory. In this study, we take notice of the PA signal of a thermally thick sample ( I > l/us), so that the following two cases are examined, assuming the existence of an air layer between the rear surface of a sample and a backing material: (i) opically transparent and thermally thick (case 1-C) and (ii) optically opaque and thermally thick (case 2 4 ) . Resultant complex PA magnitude signals, Q, generated from the front and the rear surfaces of a sample are expressed as follows: case 1-C (1/p
> 1 > l/as)
front surface: Q r (-j/2a,)(p5/k,)~ps
(3)
= (-j/2us)(p5/k,)@p,
(4)
= (-j/2us)(ps/k,)@p,
(5)
rear surface: Q case 2-C (1 >
1/p > l/u,)
front surface: Q
rear surface: Q N= 0 (6) As can be seen from eq 3 and 4, the same signal including phase arises from both surfaces in the case of 1-C and the signal is proportional to the optical absorption coefficient. On the other hand, in the case of 2-C, no PA signal arises from the rear surface, though the same signal as in the case of 1-C is generated from the front surface. If we consider a Gaussian absorption band, it is expected from these expressions that a Gaussian spectral feature is always obtained from the front surface and that a PA spectral feature similar to self-ab-
98
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
51
ABSORPTION COEFFICIENT
A
Q
a
0
DATA POINT
tG; H
51
C
B A
3 4 ai
E
Q ”
+ 3 t;
co
PHASE
2
-1 RUI
-0
z 2 2 z H -I
ABSORPTION COEFFICIENT Figure 8. Computergenerated plots for the PA power (solld Ilne) and phase (broken Ilne) slgnals from the front (upper) and the rear (lower) surfaces of a film sample. Sample thickness is 100 pm, and the thermal diffusion length is 8 pm.
sorption will be obtained from the rear surface for an intense absorption band whose center part includes the 2-C case. In Figure 6 , we show the computer-generated plots derived directly from eq 1and 2 for the magnitude and phase of the PA signal as a function of the optical absorption coefficient. In this figure, we consider the case that the sample thickneqp is 100 pm and the thermal diffusion length is 8 pm. The magnitude (solid h e ) is shown as log-log plots and the phase (broken line) ’is shown as semi-log plots. AE the absorption coefficient increases up to the value where the optical absorption length (1, = 1/p) is coincident with the sample thickness, PA amplitude signals from both surfaces increase lineply, an! the phwe signals keep a constant value of -90° for 60th surfaces. If the absorption coefficient increases further above the value of lo2 cm-l, the phase signal varies gradually from -90° to -45’ for both the front and the rear surfaces but the amplitude of the signals from the rear surface decreases dfastically in contrrast with those from the front surface. These calculated results agree well with the expectation deduced from the special cases. Considering a Gaussian absorption profile, the corresponding PA signql amplitudes were calculated as shown in Figure 7. The PA amplitude signals arising from the front surface appear as’elmost keeping the Gaussian absorption profile. On the other hand, the PA amplitude signals arising from the rear surface appear to have a self-absorption-like profile. Taking into account the fact that the phase varies gradually while the absorption coefficient keeps almost the same value for both surfaces, observed P A signals of films can
g 1 H
Cr)
Q
a.
m - 0
5U0
1om0
DATA POINT Figure 7. Computer-generated plots for PA power signals from the front (upper) and the rear (lower) surfaces of a film sample, assuming Gaussian absorption bands. The intensity scale of curves C, D, and E in the upper is expanded by a factor of 2.5. The maximum absorption coefficients are 1000 (A), 500 (B),100 (C), 50 (D), and 20 (E) in inverse centimeters. Sample thickness and the thermal diffusion length are the same as in Figure 6.
be regarded as a simple sum of the signals from the front and rear surfaces. It is, therefore, surmised that diffuse PA spectral features would be observed for the closely neighboring strong absorption bands, if the existence of an air layer under the sample is not negligible and the heat arising from the rear surface contributes to the PA sign$ greatly. In this case, the observed FTIR-PA spectrum would be structureless, if many sharp and strong absorption bands are closely present. In order to c o n f i i the above expectation,FTIR-PA spectra of a PET film was measured for several positions in a PA cell, and the results are shown in Figure 8. The top spectrum (A) was measured without considering the effect of the heat from the rear surface and by simply putting the sample into a PA cell as usual, that is, as shown in Figure 1A. Spectra B-D were obtained by placing the sample at the position of (A-C) in Figure 4, respectively. As shown in Figure 8B, PA spectral features in the vicinity of the strong absorption bands appeared as having self-absorptioQ-like features, although PA spectral features at around the weak absorption bands are similar to the absorption profiles. It is also recognized that the spectrum of C resembles to that of A and that the spectrum D shows the most distinct spectral features. When the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
e
99
(IO) Gardeiia, J. A,, Jr.; Jiang, D.-2.; McKenna, W. P.; Eyring, E. M. Appl.
0
2000 1500 WA VENUPlBER S
e900
1000
L--
Flgure 8. Dependence of FTIR-PA spectrum of a PET film on the sample positions In a PA cell. Spectrum of A is obtained by simply putting the sample on a sample holder. Spectra B, C, and D are obtained by placing the sample at A, B, and C in Flgure 4, respecthrely.
spectrum of D is compared with A or C, it is concluded that the heat from the rear surface is the main reason that the PA spectrum of a film sample is structureless. Registry No. Poly(ethy1eneterephthalate) (SRU), 2503859-9; polyethylene (homopolymer), 9002-88-4.
LITERATURE CITED (1) Rosencwaig, A. “Photoacoustics and Photoacoustic Spectroscopy”; Wiiey-Interscience: New York, 1980. (2) Low, M. J. D.; Parodi, 0. A. Spectrosc. Lett. 1078, 7 7 , 581-588. (3) Low, M. J. D.; Parodi, 0. A. J . Photoacoust. 1082, 1 , 131-144. (4) Rockley, M. G. Chem. Phys. Lett. 1070, 66, 455-456. (5) Rockley, M. 0.;Davis, D. M.; Richardson, H. H. Science (Washington, D.C.)1980, 270, 918-920. (6) Vidrine, D. W. Appl. Spectrosc. 1980, 34, 314-319. (7) Krishnan, K. Appl. Spectrosc. 1981, 35, 549-557. (8) Vidrine, D. W. I n “Fourier Transform Infrared Spectroscopy”; Ferraro, J. R., Baslie, L. J., Eds.; Academic Press: New York. 1982; Voi. 3, Chapter 4. (9) McCleliand, J. F. Anal. Chem. 1983, 55, 89A-105A.
Surf. SCi. 1983, 75, 36-49. (11) Griffiths. P. R. “Chemical Infrared Fourier Transform Spectroscopy”; Wiiey-Interscience: New York, 1975. (12) Fernellus, N. C. Appl. Opt. 1079, 78, 1784-1787. (13) McCienny, W. A,; Bennett, C. A., Jr.; Russwurm, G. M.; Richmond, R. Appl. Opt. 1981, 20, 650-653. (14) Teramae, N.; Hiroguchi, M.; Tanaka, S. Chem. Lett. 1981, 1091. (15) Klnney, J. B.; Staiey, R. H. Anal. Chem. 1083, 55, 343-348. (16) Chalmers, J. M.; Stay, B. J.; Kirkbright, G. F.; Spillane, D. E. M.; Beadie, B. C. Analyst (London) 1081, 706, 1179-1186. (17) Teng, Y. C.; Royce, B. S. H. Appl. Opt. 1982, 27, 77-80. (18) Rockiey, M. G.; Richardson, H. H.; Davis, D. M. J . Photoacoust. 1982, 7 , 145-149. (19) Gardeiia, J. A., Jr.; Eyring, E. M.; Klein, J. C.; Carvalho, M. B. Appl. Spectrosc. 1082, 36, 570-573. (20) Nelson, J. H.; Macdougaii, J. J.; Bagiin, F. G.; Freeman, D. W.; Nadler, M.; Hendrix, J. L. Appl. Spectrosc. 1982, 36, 574-576. (21) Lloyd, L. B.; Yeates, R. C.; Eyring, E. M. Anal. Chem. 1082, 5 4 , 549-552. (22) Rockiey, M. 0.;Woodard, M.; Richardson, H. H.; Davis, D. M.; Purdie, N.; Bowen. J. M. Anal. Chem. 1983, 55, 32-34. (23) Yaniger, S. I.; Riseman, S.M.; Frigo, T.; Eyring, E. M. J . Chem. Phys. 1982, 76, 4298-4299. (24) Will, F. G.; McDonald, R. S.; Gleim, R. D.; Winkle, M. R. J . Chem. P h y ~ 1983, . 78, 5847-5852. (25) Yaniger, S. I.; Rose, D. J.; McKenna, W. P.; Eyring, E. M. Appl. Spectrosc. 1084, 38, 7-11. (26) Riseman, S. M.; Massoth, F. E.; Dhar, G. M.; Eyring, E. M. J . Phys. Chem. 1082, 86, 1760-1763. (27) Teramae, N.; Yamamoto, T.; Hiroguchi. M.; Mastui, T.; Tanaka, S. Chem. Lett. 1982, 37-40. (28) Teramae, N.; Hiroguchl, M.; Tanaka, S. Bull. Chem. Soc. Jpn. 1982, 55, 2097-2100. (29) Kinney, J. B.; Staley, R. H. J . Phys. Chem. 1983, 87, 3735-3740. (30) Gardeila, J. A., Jr.; Jiang, D.Z.; Eyring, E. M. Appl. Spectrosc. 1983, 37, 131-133. (31) Porter, M. D.; Karweik, D. H.; Kuwana, T.; Theis, W. B.; Norris, G. B.; Tiernan, T. 0. Appl. Spectrosc. 1984, 38, 11-16. (32) Somorjai, 0. A.; Zeaera, F. J . Phys. Chem. 1082, 86, 3070-3078. (33) Krishnan, K.; Hill, S.; Hobbs, J. P.; Sung, C. S. P., Appl. Spectrosc. 1982, 36, 257-259. (34) Aamodt, L. C.; Murphy, J. C.; Parker, J. G. J . Appl. Phys. 1977, 48, 927-933. (35) Tam, A. C.; Wong, Y. H. Appl. Phys. Lett. 1980, 36, 471-473. (36) Adams, M. J.; King, A. A.; Kirkbright, G. F. Analyst (London) 1078, 70 7 . 73-85. (37) Freeman, J. J.; Friedman, R. M.; Reichard, H. S. J. Phys. Chem. 1980, 84, 315-319. (38) Rosencwaig, A.; Gersho, A. J . Appl. Phys. 1078, 4 7 , 64-69. (39) Fernellus, N. C. J . Appl. Phys. 1980, 57, 650-654. (40) Helander, P.; Lundstrom, I.; McQueen, G. J. Appl. Phys. 1081, 52, I 146-1 151. (41) Fujii, Y.; Moritani, A.; Nakai, J. Jpn. J . Appl. Phys. 1081, 2 0 , 361-367 and Eratta 20, 1005. (42) Morita, M. Jpn. J . Appl. Phys. 1981, 20, 835-842.
RECEIVED for review August 22, 1984. Accepted October 1, 1984.
