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The effects of the nature of the coupling gas and its pressure on infrared photothermal beam deflection spectra of solids
InfrnredPh,v.s. Vol. Printed
in Great
23. No. 4. pp. 199-206. Britam
0020.3891;83 $3.00+0.00 Pergamon Press Ltd
1983
THE EFFECTS OF THE NATURE OF THE COUPLING GAS AND ITS PRESSURE ON INFRARED PHOTOTHERMAL BEAM DEFLECTION SPECTRA OF SOLIDS M. J. D. Low, T. H. ARNOLDand A. G. Department
of Chemistry,
New York
University.
New York,
SEVERDIA NY
10003, U.S.A.
(Received 16 March 1983) Abstract-An entire aspirin tablet was confined in a pressure cell and infrared spectra were recorded with a Fourier transform beam deflection spectrometer using different coupling gases. The intensity S of spectra at 1atm increased linearly in the order He < Ne < Hz < N? _ Ar < CO: i CH,, with increasing refractive index, n, except CO:. As the pressure P was increased to 7 atm, S and S/noise increased _ l.Sfold, but with H, the increases were -4-fold. The data indicate that the thermal conductivity k and dn/dP rather than dnjdP of the gases are the main causes of increases in S. The values for CO? were anomalously low, suggested to be caused by physical adsorption of CO? by the sample. Relatively large samples can be examined, and wo~h~hi~e S/noise enhancement can be obtained with pressure cells.
INTRODUCTION Recently Boccara et al. (I) described a new spectroscopic technique based on what they called the “mirage” detector. Briefly, if a solid is illuminated with light it can absorb, the temperature of the solid will increase and so will the temperature of the gas near the solid’s surface, so that a refractive index gradient will form near the surface. A light beam grazing the surface will thus be deflected; hence the term mirage. If the illumination is periodically interrupted, the surface will periodically heat and cool, increasing and decreasing the extent of the refractive index gradient, so that the light beam is periodically deflected, and the extent of the then easily measured deflection can be related to the extent of the absorption of Iight by the solid, i.e. to the photothermal effect. If narrow-band radiation from a monochromator or tuned laser, or broad-band radiation from an interferometer, is used to illuminate the sample, a spectroscopic technique of remarkable sensitivity and versatility results. This new technique of photothermal beam deflection spectroscopy (PBDS) has been used in the visible region of the spectrum to examine crystalline and amorphous silicon,(2’ absorption in optically thin media,‘3) in situ trace gas detection, (4)for absorption and dichroic measurements,(s.6) corrosion,“’ and Murphy and Aamodt@) and Jackson et al. (9) have developed PBDS theory. In the infrared range, PBDS has been usefully applied to chemical systems and has shown itself to be a valuable tool for surface studies.“0-‘6’ As with other spectroscopies, it is desirable to increase the sensitivity of PBDS, and Fournier et al.“’ have shown that one way this can be done is to use the “immersed mirage effect”, i.e. to replace the gas by a liquid so that the beam deflection is enhanced, the refractive index and index gradient being larger with a liquid than with a gas; the beam deflection can be increased by three orders of magnitude in this fashion. However, although this more sensitive technique involving the use of a solid sample submerged in a liquid will be useful for studies of corrosion,“’ adsorption in situ at the liquid/solid interface (as well as the obvious extension to the liquid/liquid interface), weakly absorbing materials and microsamples, it is obvious that the “coupling fluid” must be (a) transparent to the radiation used to produce the photothermal effect, as well as to the light beam used to detect the refractive index gradient, and (b) inert and pure so that the sample is not altered. The criterion involving transparency is readily met for work in the visible and near-infrared regions; there are numerous suitable liquids. However, all liquids which might be considered for use as coupling fluids absorb more or less strongly somewhere in the infrared (ir.) range (molten alkali halides or liquified inert gases seem too esoteric at present) so that, as is common practice in i.r. transmission spectroscopy, the liquid layer has to be very thin indeed so that some transparency remains at regions of strong absorption. With very thin liquid layers it would seem 199
difficult to be able to pass a light beam over the surface of the sample. If the layer were of some “practical” thickness. say 0.5 mm. certain regions of the i.r. range would be opaque, but then the same sample might be examined using two or three different liquids. much as is done with mull spectra. The second criterion of inertness and purity must be adhered to for work in all spectral regions. It is of crucial importance for surface studies. partly because the USCof liquids can lead to much higher amounts of impurities than with gases. and partly because surfaces of interest such as those “activated” so that the chemisorption of of adsorbents or catalysts have usually been deliberately what might be an otherwise innocuous liquid may be possible. Some difficulties can thus bc foreseen for the USCon the immersed mirage system. especially for surface studies in the i.r. region. However, an analogous immersed mirage system suggests itself, the fluid being a compressed gas. Although the beam deflections produced in the latter would be far smaller than those in a liquid. such a system consisting of a solid in contact with compressed gas is attractive and merits attention because precisely such systems are used to study heterogeneously catalyzed reactions. We have consequently explored the effects of gas pressure and the nature of the gas on the response of a mirage system. EXPERIMENTAL
The i.r. Fourier transform PBDS spectrometer used and its operation have been described elsewhere.““’ The pressure cell A. shown in the largely self-explanatory Fig. I, was used for part of the measurements. Dimensions are not critical for the relatively low pressures employed. except that the distance between the KBr window and the sample surface should be kept short: the i.r. beam converges rapidly. so that a large window would be needed if that distance were great. thus increasing the chance of window failure. Not shown in Fig. I is a third Swagelok fitting brazed to the cell body in between and at right angles to the other fittings. The copper tube coming from the gas regulators and tanks was fastened to that fitting. Although the cell was tested several times to 24 atm. measurements were limited to 8 atm (100 psig = 7.8 atm. nominally taken at 8 atm) to avoid rupturing the KBr window. All the measurements were also carried out with another cell B which differed from cell A in that the glass windows for the probe beam were not held by screw-on fittings. Instead, the windows were epoxied to flanges inside the cell body in the same way as the KBr window. The sample was an entire aspirin tablet which was held to the cell closure by epoxy cement. The tablet was chosen because prior experience had shown that relatively high photothermal signals could be obtained fairly easily with such a sample. and a portion of its spectrum was highly structured while another portion relatively featureless so that noise could be assessed easily. The choice was also influenced by a practical consideration, in that the size of the tablet is extremely large in comparison to the sizes of samples used for conventional i.r. transmission or photoacoustic
Infrared beam from Interferometer ,
l/B”
glass
+
wlndow \
Epoxy
100 turns/ screw
# ‘\
3/16”KBr
wlndow
+‘I 1
Translator
Laser
It-
~~.~
Brass cell -4x3x3cm ~ pm-;
-lOcm
I.lF
I
bodv ,.
~rpcrlmental
Pos ItIon
detector
3rd flttlng at 90” to other flttlngs In center of body is not shown \ertlp
Inch
Effects of the coupling gas on i.r. PBDS of solids
201
spectroscopies: although the use of the pressure cell imposes some limitations on sample size,“@ it would be demonstrated that samples of relatively large size could still be examined. Research Grade gases were used (Linde), and pressures were monitored with conventional pressure regulators and gauges. All spectra shown except three were recorded with 100 scans at a resolution of 8 cm-‘, and have not been corrected for instrument or source functions. In practice, the sample was installed and the cell was bolted to a simple positioning device (e.g. Fig. 3, Ref. 9) permitting horizontal, vertical and tilt adjustments. The cell was then moved so that the photothermal interferogram was maximized,(‘O’ at ambient pressure of air. As the signal magnitude depends critically on the position of the sample with respect to the i.r. and laser beams, no further mechanical adjustments were made while the entire sequence of measurements with all gases at the various pressures was carried out. In order to test cell A and the various procedures, some experiments were carried out with the sample at ambient temperature, i.e. with the i.r. beam blocked (Fig. I). It must be remembered that with the set-up shown schematically in Fig. 1, the sample is not at ambient temperature: if the interferometer is not scanning, an i.r. beam of constant intensity falls onto the sample, so that the sample’s temperature rises slightly. Then, when the interferometer is scanned, the then modulated i.r. beam causes the temperature of the sample to fluctuate about that slightly raised temperature. These effects are easily observable, as follows. The i.r. beam is blocked, the sample installed, and the signal maximized by moving the position sensing detector. With the sample at ambient temperature there is no temperature or refractive index gradient over the sample’s surface and the laser probe beam is “straight”. If the i.r. beam is then allowed to fall on the sample’s surface, with the interferometer scanning or not scanning, the laser beam is immediately deflected upward (the detector has to be moved up in order to null the system) as the sample warms. There is an upward drift of the beam (which may last from h 10 set to several minutes, depending upon the nature of the sample) as the sample tends toward a steady state slightly above ambient temperature. The photothermal interferogram is observed when the interferometer is scanned. If the i.r. beam is blocked at this stage, the laser beam moves down as the sample returns to room temperature. After the cell had been installed and the signal maximized, as described, the i.r. beam was blocked and the air in the cell replaced with one of the gases at 1 atm, and the signal was nulled. The gas pressure was then increased. With all gases it was found that there was a small deflection of the probe beam down toward the surface of the sample. The extent of the deflection increased with increasing pressure, and as the refractive index of the gas increased. As it was suspected that the effect was in part caused by a movement of one or both glass windows due to distortion of the gaskets, the cell B having fixed windows was constructed and tested. With cell B there were slight upward deflections of the beam as the gas pressure was increased. The effect is attributed to a very slight wedging of the windows, so that the cells acted as prisms, e.g. with cell A (Fig. l), the top of the windows were slightly inclined toward one another, forming a prism of gas which caused the beam to deflect down. The deflections were N 10m4to 10-j degree at the highest pressures and will not be considered further, but point to a potential source of error for measurements of this type. The other measurements were carried out with the sample exposed to the i.r. beam of the scanning interferometer, as follows. A gas was introduced at ambient pressure and after a few minutes the signal was nulled by moving the detector to point N,, measured by a scale affixed to the translator screw (Fig. l), and a spectrum was recorded. The pressure was then increased to P,, causing the laser beam to rise. The detector was moved upward, and the new null position N, established and another spectrum recorded. The beam deflection at P, is then (N,,-N,). The measurements were repeated at various pressures for each of the gasses. RESULTS
AND
DISCUSSION
The results of the beam deflection measurements are shown in Fig. 2, and are not unexpected or unusual, in view of the refractive index, ~1,values summarized in Table 1. The data roughly fall into two groups for gases differing in n by an order of magnitude. The plots are measures of the beam deflections under the various conditions and are indications of dn/dP, but what is measured
is the summation of the various heat transfer efYects heat transfer erects at and near the sample’s surface under what are essentially steady state or static conditions. The dynamic situation. (he effects produced by the modulated i.r. beam. is quite direrent, as shown by Fig. 3. The data of Fig. 3 were derived from the spectra which were recorded: some sample spectra are shown in Fig. 4. The spectra were normalized and the computer was instructed to produce the values of the strongest and weakest signals observed with each spectrum, and the differences of the maximum and minimum values were plotted. Essentially what is shown is the variation of the intensity. S. of the strongest band in the spectrum near 1300 cm ’ ax functions of gas and pressure (with CH,, it is the band near 1200 cm ‘). It is apparent from Fig. 3 that the increases in the intensities. S. of the spectra which might be expected on the basis of values oft? (see Table 1) and tht: trends of Fig. 2 are not fully realiad. At atmospheric pressure there is a linear increase of S with increasing II except for the CO, datum, but at higher pressures the data scatter extensively on S--n plots. With each gas there is an increase in S with increasing pressure, resulting in an increase in the signal-to-noise ratio. However. there are variations in that (A) with H, there is 21dramatic increase in S with increasing pressure. much larger than that found with other gases: (B) the rate of increase of S for He is greater than that found with most other gases; and (C) the data for CO2 seem anomalously low at all pressures. In view of the relations of Fig. 2. it would seem that changes in t7 with pressure play only ;t minor role in causing the intensity--pressure changes of Fig. 3; for these. it is likely that the thermal conductivity. k, plays the major role through its effects on the thermal ~ii~usivit~. I3 = k A;>. where (1 is the density and t?,, is the heat capacity. II determines the rate of heat propagation in transient state processes.“7 “” The changes of k with pressure at constant temperature can be expressed by an equation used by Vargaftig,““’ I, = x,, + 1%;’ where k,, is the thermal conductivity of the gas at atmospheric pressure and the same temperature, R a constant specific to the gas. G the specific weight of the gas. and I‘ a constant related to the
Tzthlc
<;>I,
R&lC?iVC mden. tl””
Hs (‘II 4
1.OO(i I ix I .000036 1.000443
NC N1
1.000067 I .00037
A; C‘O,
I .0002xI I .00044s
II,
I
Thcmmal c(~llducti~it~ j$ .p:z
,()d. [j”“’
1
,.1,
0
/).Y
-M 360
7 9i _. 1.5x
I .I h 1.17
7. i
x1 I IO 62
Il.49 I J c,.i* 0. I Oh.!
I .‘O 1..‘.1*
I I ‘7
I .23 I .2h I.26
I 0.0 05
3.7 40
*Estimated from dtrta ol’ ReIi (IO) ~rnd (20) i-C’31 WC cm (‘, 10 1’. :it 26.7 C‘.
O.Oh4h 0. I MI
s. I
Effects of the
couplinggas on i.r. PBDS
of solids
203
P, lie v Ns 0 Nz A Ar
IO
i.r -PBDS
of i.r.-PBDS
Fig. 3. Intensity
20
slgnal
spectra
0
CH4
l
Hz
30
(arbitrary
of aspirin
units)
as functions
of gas and pressure.
molecular weight of the gas. Values of B and 4’ are given in Table 1, which also shows values of D/D,, i.e. the thermal diffusivities of the gases normalized to that for N,. at 1 atm. It is apparent that compared to the other gases, for H, and to a lesser extent for He. the thermal conductivity is very high, its increase with increasing pressure is also very high in view of the large value of B, leading to high values of the derived quantity, the thermal diffusivity, thus providing an explanation for the trends A and B. Effectively, it is thought that the relatively high thermal diffusivity with H, and resultant high thermal diffusion length, L = (20/w)‘!‘, where MIis the chopping frequency. causes the thermal and refractive index disturbances over the surface to be more extensive, so that a larger portion of the laser probe beam is affected, thus leading to a larger signal. It is interesting and pertinent to note that the trend of signal strength observed with PBDS at 1 atm, He
spectroscopy (PAS). Adams et LZ~.,(‘~) using a
He>Ar>N,>COz
I
1
I
iOO0
2000
1000
cm-’ Fig. 4. i.r.-PBDS spectra of aspirin. The 3 spectra shown over the entire i.r. range are notarized to the 8 atm Hz spectrum. The arrows point to segments which have been scale-expanded so that the maxima of the spectra are the same, so that the segment show the relative amounts of noise present. The insert shows segments of normalized spectra obtained with Ne. The ordinates are displaced.
while Eaton
and Stuart””
similarly
observed
the trend
He > H, > Ar > 0, > N, > CH, with a carbon black sample. Aamodt c’t rrl.“” predicted the trend He > N, for many experimental conditions. Low and Parodi observed (unpublished) the signals to vary in the order He > H, .b air. with charcoal and many other samples. and Wang”“’ reported the trend Hc > H, > Ar - N, > CO-. with samples of K,Cr,O,. CuO. BaS. and a plastic coated with black paint. Essentially the same trends were observed in all these cases. PAS measurements with solids using various gases at other than attnospheric pressure do not seem to have been reported. The various observations on the effects of filler gases on the strength of the photoacoustic signals have been summarized by Ganguly and Rae:“” there appear to be some discrepancies between PAS experiment and theories mainly concerned with the quantitative aspects of the relation between the strength of the PAS signal and the properties of the gases, but the generality that the signal increases as the heat capacity of the gas decreases is valid. In contrast. with PBDS the gas heat capacity has only a minor eff‘ect. With PAS. Wong observed (Table I of Ref. 26) the trend. CO, > Ar - N, - air > H, > He with activated charcoal. a reversal of the trend found with his other samples. Wong. who did not note the contrast between his data with the carbon and those of others.“‘-“’ attributed the reversal of trends to the effects of gas physically adsorbed by the activated charcoal. He thought that “the adsorbed gas forms a region of high gas density very close to the solid surface and hence significantly enhances the pressure variation. The amplified pressure variation then propagates subsequently to the cell space where the gas density is much lower”.““’ It is not known or presently pertinent why the observations with carbon conflict: what is of interest is the suggested effects of adsorption. which can also explain some of the present PBDS results. The generality that any solid surface will weakly bind any gas is ;I valid one: the extent of the adsorption depends on the nature and properties of the adsorbent and adsorbate. and decreases with increasing temperature but increases with increasing gas pressure.“’ “I Sotne data quoted by Brunauer (Table I of Ref. 17) show that I g of charcoal of unspecitied surface area would adsorb 2 cm’ of Hz, 8 cm’ of N,. I6 cm’ of CH, and 48 cm’ of CO, at I5 C, and similar data were given by Gregg (Table 3. I of Ref. 28) for the physical adsorption of the same gases on charcoal at 25 (I and I atm pressure. The adsorption values become correspondingly smaller when account is taken of the fact that the specific surface areas of most solids are one to two orders of magnitude smaller than those of charcoals (e.g. Tables 6.20, 6.22 of Ref. 30) but the values. although quite small. are not negligible. There is also a general relation bctwcen adsorption and condensibility, i.e. boiling point and critical adsorption occurs to a greater extent the higher the adsorbate’s temperature. Note that the adsorption of CO, occurred above the boiling point ( ~ 7X C) bum neat the critical temperature (3 I C). but much above the boiling points and critical temperatures of the other gases (for CH,. the respective values are - 164 and --X7 C). and that the adsorption of CO, is much greater than that of the other gases. In the present experiments. at I atm pressure and ambient temperature the extent of adsorption of all gases except CO, would be quite umall, as in the examples cited above. When the prcssurc was then increased, all gases would be caused to adsorb more extensively. with CO1 adsorbing the most. The desorption of a part of this bound CO, may be the cause of the anomalously low S values which were observed. The following mechanism suggests itself. If a radiation pulse is adsorbed bq a solid bearing a layer or multilayers of physically adsorbed gas. the temperature of the surface rises slightly. causing some gas to desorb. a portion of the photothermal energy release being taken up as heat of desorption (the heat of adsorption of CO, is of the order of 8 kcal:‘mol and that of CH, of the order of 4 kcal/mol. for example), so that the subsequent heating of the gas o\‘er the surface is diminished with respect to that occurring in the absence of the adsorbed layer. The refractive index gradient and consequent beam deflection would thus be stnaller than would otherwise be obtained. The mechanism may be considerably more complex. e.g. the pas dcsorbcd
Effects of the coupling
gas on i.r. PBDS of solids
205
Hydrogen
3000
IO00
2000 cm-’
Fig. 5. Spectra
of aspirin
with H? coupling
gas. The ordinates
are displaced
initially may cause a local increase in pressure and n, causing the probe beam to deflect down, and will be explored with instrumentation giving data on phase angle and modulation frequency. From the practical point of view, recording PBDS spectra at super-atmospheric pressures is generally beneficial in that larger signals are obtained than at atmospheric pressure, but may not be worthwhile as in the case of He and Ne because of the initially low signals. There is little to choose between the effects of Ar and N,, except that Ar is inert. However, Nz is relatively unreactive and suitable for most samples, is readily available in highly purified form, and cheap. An improvement by a factor of - 1.5 is obtainable with N, and Ar by an 8-fold pressure increase but at lower pressures of -3-4 atm which would be tolerated by simple glass cells, the S/N improvement is marginal and of doubtful utility in view of the increased experimental complexity. A quite worthwhile improvement can be obtained by using CH,, a relatively unreactive gas; at 1 atm there is a gain of - 50% with respect to Nz or air, and a - 100% gain is achievable by raising the pressure to 2-3 atm, readily achievable with simple cells. However, the absorption spectrum of CH, becomes superimposed on the PBDS spectrum of the solid, as shown by comparison of the CH, and Hz spectra of Fig. 4. The superimposition. for which no compensation is possible if absorption by the gas is complete, may or may not be acceptable, depending on the adsorptions of the solid sample. Significant improvements can be obtained with Hz, with unreactive samples. With reference to the signal obtained with Nz or air at I atm, a - I.5fold improvement can be obtained at 3-4 atm and a 2.5fold improvement by raising the pressure to 8 atm. In terms of measurement time, t, as S/N is proportional to f I”, the 2.5-fold improvement means that t can be reduced by a factor of 6. The dramatic improvement brought about by increasing the H, pressure is illustrated by the spectra of Fig. 5: the spectra have comparable S and S/N values, yet the measurement times differ by a factor of IO. Judging by the results obtained with COz, which are also of heuristic interest, easily condensible and hence easily adsorbable gases should not be used as coupling gases. Indeed, the presence of such gases in streams of otherwise innocuous coupling gases, or the presence of weakly bound adsorbates on high surface area materials such as chars or atmospheric pollutant particles, may complicate PBDS measurements. A~kno~led~rmmts~Support is gratefully acknowledged.
by AROD
Contract
DAAG
29-79-C-0135
and NSF Grants
CPE-792100
and CHE-RI 1778
REFERENCES I.
Boccard
A. C.. D. Fournier
Topicd
Mwfiq
and J. Badoz, A@.
on Plrotoucoustic
(unpublished). 2. Jackson W. B.. N. M. Amer.
Spectroscopy,
D. Fournier
Phys. Let/. 36, 130 (1980);
Iowa
State
and A. C. Boccara
University,
D. Fournier. l-3 August,
. 2nd Internotional
A. C. Boccara and J. Badoz, 1979. Dige.vt, Paper ThA I
Topical
Meeting
on Photoacoustic
ZOh
M. J. D. Low (‘I rri.
in Great
23. No. 4. pp. 199-206. Britam
0020.3891;83 $3.00+0.00 Pergamon Press Ltd
1983
THE EFFECTS OF THE NATURE OF THE COUPLING GAS AND ITS PRESSURE ON INFRARED PHOTOTHERMAL BEAM DEFLECTION SPECTRA OF SOLIDS M. J. D. Low, T. H. ARNOLDand A. G. Department
of Chemistry,
New York
University.
New York,
SEVERDIA NY
10003, U.S.A.
(Received 16 March 1983) Abstract-An entire aspirin tablet was confined in a pressure cell and infrared spectra were recorded with a Fourier transform beam deflection spectrometer using different coupling gases. The intensity S of spectra at 1atm increased linearly in the order He < Ne < Hz < N? _ Ar < CO: i CH,, with increasing refractive index, n, except CO:. As the pressure P was increased to 7 atm, S and S/noise increased _ l.Sfold, but with H, the increases were -4-fold. The data indicate that the thermal conductivity k and dn/dP rather than dnjdP of the gases are the main causes of increases in S. The values for CO? were anomalously low, suggested to be caused by physical adsorption of CO? by the sample. Relatively large samples can be examined, and wo~h~hi~e S/noise enhancement can be obtained with pressure cells.
INTRODUCTION Recently Boccara et al. (I) described a new spectroscopic technique based on what they called the “mirage” detector. Briefly, if a solid is illuminated with light it can absorb, the temperature of the solid will increase and so will the temperature of the gas near the solid’s surface, so that a refractive index gradient will form near the surface. A light beam grazing the surface will thus be deflected; hence the term mirage. If the illumination is periodically interrupted, the surface will periodically heat and cool, increasing and decreasing the extent of the refractive index gradient, so that the light beam is periodically deflected, and the extent of the then easily measured deflection can be related to the extent of the absorption of Iight by the solid, i.e. to the photothermal effect. If narrow-band radiation from a monochromator or tuned laser, or broad-band radiation from an interferometer, is used to illuminate the sample, a spectroscopic technique of remarkable sensitivity and versatility results. This new technique of photothermal beam deflection spectroscopy (PBDS) has been used in the visible region of the spectrum to examine crystalline and amorphous silicon,(2’ absorption in optically thin media,‘3) in situ trace gas detection, (4)for absorption and dichroic measurements,(s.6) corrosion,“’ and Murphy and Aamodt@) and Jackson et al. (9) have developed PBDS theory. In the infrared range, PBDS has been usefully applied to chemical systems and has shown itself to be a valuable tool for surface studies.“0-‘6’ As with other spectroscopies, it is desirable to increase the sensitivity of PBDS, and Fournier et al.“’ have shown that one way this can be done is to use the “immersed mirage effect”, i.e. to replace the gas by a liquid so that the beam deflection is enhanced, the refractive index and index gradient being larger with a liquid than with a gas; the beam deflection can be increased by three orders of magnitude in this fashion. However, although this more sensitive technique involving the use of a solid sample submerged in a liquid will be useful for studies of corrosion,“’ adsorption in situ at the liquid/solid interface (as well as the obvious extension to the liquid/liquid interface), weakly absorbing materials and microsamples, it is obvious that the “coupling fluid” must be (a) transparent to the radiation used to produce the photothermal effect, as well as to the light beam used to detect the refractive index gradient, and (b) inert and pure so that the sample is not altered. The criterion involving transparency is readily met for work in the visible and near-infrared regions; there are numerous suitable liquids. However, all liquids which might be considered for use as coupling fluids absorb more or less strongly somewhere in the infrared (ir.) range (molten alkali halides or liquified inert gases seem too esoteric at present) so that, as is common practice in i.r. transmission spectroscopy, the liquid layer has to be very thin indeed so that some transparency remains at regions of strong absorption. With very thin liquid layers it would seem 199
difficult to be able to pass a light beam over the surface of the sample. If the layer were of some “practical” thickness. say 0.5 mm. certain regions of the i.r. range would be opaque, but then the same sample might be examined using two or three different liquids. much as is done with mull spectra. The second criterion of inertness and purity must be adhered to for work in all spectral regions. It is of crucial importance for surface studies. partly because the USCof liquids can lead to much higher amounts of impurities than with gases. and partly because surfaces of interest such as those “activated” so that the chemisorption of of adsorbents or catalysts have usually been deliberately what might be an otherwise innocuous liquid may be possible. Some difficulties can thus bc foreseen for the USCon the immersed mirage system. especially for surface studies in the i.r. region. However, an analogous immersed mirage system suggests itself, the fluid being a compressed gas. Although the beam deflections produced in the latter would be far smaller than those in a liquid. such a system consisting of a solid in contact with compressed gas is attractive and merits attention because precisely such systems are used to study heterogeneously catalyzed reactions. We have consequently explored the effects of gas pressure and the nature of the gas on the response of a mirage system. EXPERIMENTAL
The i.r. Fourier transform PBDS spectrometer used and its operation have been described elsewhere.““’ The pressure cell A. shown in the largely self-explanatory Fig. I, was used for part of the measurements. Dimensions are not critical for the relatively low pressures employed. except that the distance between the KBr window and the sample surface should be kept short: the i.r. beam converges rapidly. so that a large window would be needed if that distance were great. thus increasing the chance of window failure. Not shown in Fig. I is a third Swagelok fitting brazed to the cell body in between and at right angles to the other fittings. The copper tube coming from the gas regulators and tanks was fastened to that fitting. Although the cell was tested several times to 24 atm. measurements were limited to 8 atm (100 psig = 7.8 atm. nominally taken at 8 atm) to avoid rupturing the KBr window. All the measurements were also carried out with another cell B which differed from cell A in that the glass windows for the probe beam were not held by screw-on fittings. Instead, the windows were epoxied to flanges inside the cell body in the same way as the KBr window. The sample was an entire aspirin tablet which was held to the cell closure by epoxy cement. The tablet was chosen because prior experience had shown that relatively high photothermal signals could be obtained fairly easily with such a sample. and a portion of its spectrum was highly structured while another portion relatively featureless so that noise could be assessed easily. The choice was also influenced by a practical consideration, in that the size of the tablet is extremely large in comparison to the sizes of samples used for conventional i.r. transmission or photoacoustic
Infrared beam from Interferometer ,
l/B”
glass
+
wlndow \
Epoxy
100 turns/ screw
# ‘\
3/16”KBr
wlndow
+‘I 1
Translator
Laser
It-
~~.~
Brass cell -4x3x3cm ~ pm-;
-lOcm
I.lF
I
bodv ,.
~rpcrlmental
Pos ItIon
detector
3rd flttlng at 90” to other flttlngs In center of body is not shown \ertlp
Inch
Effects of the coupling gas on i.r. PBDS of solids
201
spectroscopies: although the use of the pressure cell imposes some limitations on sample size,“@ it would be demonstrated that samples of relatively large size could still be examined. Research Grade gases were used (Linde), and pressures were monitored with conventional pressure regulators and gauges. All spectra shown except three were recorded with 100 scans at a resolution of 8 cm-‘, and have not been corrected for instrument or source functions. In practice, the sample was installed and the cell was bolted to a simple positioning device (e.g. Fig. 3, Ref. 9) permitting horizontal, vertical and tilt adjustments. The cell was then moved so that the photothermal interferogram was maximized,(‘O’ at ambient pressure of air. As the signal magnitude depends critically on the position of the sample with respect to the i.r. and laser beams, no further mechanical adjustments were made while the entire sequence of measurements with all gases at the various pressures was carried out. In order to test cell A and the various procedures, some experiments were carried out with the sample at ambient temperature, i.e. with the i.r. beam blocked (Fig. I). It must be remembered that with the set-up shown schematically in Fig. 1, the sample is not at ambient temperature: if the interferometer is not scanning, an i.r. beam of constant intensity falls onto the sample, so that the sample’s temperature rises slightly. Then, when the interferometer is scanned, the then modulated i.r. beam causes the temperature of the sample to fluctuate about that slightly raised temperature. These effects are easily observable, as follows. The i.r. beam is blocked, the sample installed, and the signal maximized by moving the position sensing detector. With the sample at ambient temperature there is no temperature or refractive index gradient over the sample’s surface and the laser probe beam is “straight”. If the i.r. beam is then allowed to fall on the sample’s surface, with the interferometer scanning or not scanning, the laser beam is immediately deflected upward (the detector has to be moved up in order to null the system) as the sample warms. There is an upward drift of the beam (which may last from h 10 set to several minutes, depending upon the nature of the sample) as the sample tends toward a steady state slightly above ambient temperature. The photothermal interferogram is observed when the interferometer is scanned. If the i.r. beam is blocked at this stage, the laser beam moves down as the sample returns to room temperature. After the cell had been installed and the signal maximized, as described, the i.r. beam was blocked and the air in the cell replaced with one of the gases at 1 atm, and the signal was nulled. The gas pressure was then increased. With all gases it was found that there was a small deflection of the probe beam down toward the surface of the sample. The extent of the deflection increased with increasing pressure, and as the refractive index of the gas increased. As it was suspected that the effect was in part caused by a movement of one or both glass windows due to distortion of the gaskets, the cell B having fixed windows was constructed and tested. With cell B there were slight upward deflections of the beam as the gas pressure was increased. The effect is attributed to a very slight wedging of the windows, so that the cells acted as prisms, e.g. with cell A (Fig. l), the top of the windows were slightly inclined toward one another, forming a prism of gas which caused the beam to deflect down. The deflections were N 10m4to 10-j degree at the highest pressures and will not be considered further, but point to a potential source of error for measurements of this type. The other measurements were carried out with the sample exposed to the i.r. beam of the scanning interferometer, as follows. A gas was introduced at ambient pressure and after a few minutes the signal was nulled by moving the detector to point N,, measured by a scale affixed to the translator screw (Fig. l), and a spectrum was recorded. The pressure was then increased to P,, causing the laser beam to rise. The detector was moved upward, and the new null position N, established and another spectrum recorded. The beam deflection at P, is then (N,,-N,). The measurements were repeated at various pressures for each of the gasses. RESULTS
AND
DISCUSSION
The results of the beam deflection measurements are shown in Fig. 2, and are not unexpected or unusual, in view of the refractive index, ~1,values summarized in Table 1. The data roughly fall into two groups for gases differing in n by an order of magnitude. The plots are measures of the beam deflections under the various conditions and are indications of dn/dP, but what is measured
is the summation of the various heat transfer efYects heat transfer erects at and near the sample’s surface under what are essentially steady state or static conditions. The dynamic situation. (he effects produced by the modulated i.r. beam. is quite direrent, as shown by Fig. 3. The data of Fig. 3 were derived from the spectra which were recorded: some sample spectra are shown in Fig. 4. The spectra were normalized and the computer was instructed to produce the values of the strongest and weakest signals observed with each spectrum, and the differences of the maximum and minimum values were plotted. Essentially what is shown is the variation of the intensity. S. of the strongest band in the spectrum near 1300 cm ’ ax functions of gas and pressure (with CH,, it is the band near 1200 cm ‘). It is apparent from Fig. 3 that the increases in the intensities. S. of the spectra which might be expected on the basis of values oft? (see Table 1) and tht: trends of Fig. 2 are not fully realiad. At atmospheric pressure there is a linear increase of S with increasing II except for the CO, datum, but at higher pressures the data scatter extensively on S--n plots. With each gas there is an increase in S with increasing pressure, resulting in an increase in the signal-to-noise ratio. However. there are variations in that (A) with H, there is 21dramatic increase in S with increasing pressure. much larger than that found with other gases: (B) the rate of increase of S for He is greater than that found with most other gases; and (C) the data for CO2 seem anomalously low at all pressures. In view of the relations of Fig. 2. it would seem that changes in t7 with pressure play only ;t minor role in causing the intensity--pressure changes of Fig. 3; for these. it is likely that the thermal conductivity. k, plays the major role through its effects on the thermal ~ii~usivit~. I3 = k A;>. where (1 is the density and t?,, is the heat capacity. II determines the rate of heat propagation in transient state processes.“7 “” The changes of k with pressure at constant temperature can be expressed by an equation used by Vargaftig,““’ I, = x,, + 1%;’ where k,, is the thermal conductivity of the gas at atmospheric pressure and the same temperature, R a constant specific to the gas. G the specific weight of the gas. and I‘ a constant related to the
Tzthlc
<;>I,
R&lC?iVC mden. tl””
Hs (‘II 4
1.OO(i I ix I .000036 1.000443
NC N1
1.000067 I .00037
A; C‘O,
I .0002xI I .00044s
II,
I
Thcmmal c(~llducti~it~ j$ .p:z
,()d. [j”“’
1
,.1,
0
/).Y
-M 360
7 9i _. 1.5x
I .I h 1.17
7. i
x1 I IO 62
Il.49 I J c,.i* 0. I Oh.!
I .‘O 1..‘.1*
I I ‘7
I .23 I .2h I.26
I 0.0 05
3.7 40
*Estimated from dtrta ol’ ReIi (IO) ~rnd (20) i-C’31 WC cm (‘, 10 1’. :it 26.7 C‘.
O.Oh4h 0. I MI
s. I
Effects of the
couplinggas on i.r. PBDS
of solids
203
P, lie v Ns 0 Nz A Ar
IO
i.r -PBDS
of i.r.-PBDS
Fig. 3. Intensity
20
slgnal
spectra
0
CH4
l
Hz
30
(arbitrary
of aspirin
units)
as functions
of gas and pressure.
molecular weight of the gas. Values of B and 4’ are given in Table 1, which also shows values of D/D,, i.e. the thermal diffusivities of the gases normalized to that for N,. at 1 atm. It is apparent that compared to the other gases, for H, and to a lesser extent for He. the thermal conductivity is very high, its increase with increasing pressure is also very high in view of the large value of B, leading to high values of the derived quantity, the thermal diffusivity, thus providing an explanation for the trends A and B. Effectively, it is thought that the relatively high thermal diffusivity with H, and resultant high thermal diffusion length, L = (20/w)‘!‘, where MIis the chopping frequency. causes the thermal and refractive index disturbances over the surface to be more extensive, so that a larger portion of the laser probe beam is affected, thus leading to a larger signal. It is interesting and pertinent to note that the trend of signal strength observed with PBDS at 1 atm, He
spectroscopy (PAS). Adams et LZ~.,(‘~) using a
He>Ar>N,>COz
I
1
I
iOO0
2000
1000
cm-’ Fig. 4. i.r.-PBDS spectra of aspirin. The 3 spectra shown over the entire i.r. range are notarized to the 8 atm Hz spectrum. The arrows point to segments which have been scale-expanded so that the maxima of the spectra are the same, so that the segment show the relative amounts of noise present. The insert shows segments of normalized spectra obtained with Ne. The ordinates are displaced.
while Eaton
and Stuart””
similarly
observed
the trend
He > H, > Ar > 0, > N, > CH, with a carbon black sample. Aamodt c’t rrl.“” predicted the trend He > N, for many experimental conditions. Low and Parodi observed (unpublished) the signals to vary in the order He > H, .b air. with charcoal and many other samples. and Wang”“’ reported the trend Hc > H, > Ar - N, > CO-. with samples of K,Cr,O,. CuO. BaS. and a plastic coated with black paint. Essentially the same trends were observed in all these cases. PAS measurements with solids using various gases at other than attnospheric pressure do not seem to have been reported. The various observations on the effects of filler gases on the strength of the photoacoustic signals have been summarized by Ganguly and Rae:“” there appear to be some discrepancies between PAS experiment and theories mainly concerned with the quantitative aspects of the relation between the strength of the PAS signal and the properties of the gases, but the generality that the signal increases as the heat capacity of the gas decreases is valid. In contrast. with PBDS the gas heat capacity has only a minor eff‘ect. With PAS. Wong observed (Table I of Ref. 26) the trend. CO, > Ar - N, - air > H, > He with activated charcoal. a reversal of the trend found with his other samples. Wong. who did not note the contrast between his data with the carbon and those of others.“‘-“’ attributed the reversal of trends to the effects of gas physically adsorbed by the activated charcoal. He thought that “the adsorbed gas forms a region of high gas density very close to the solid surface and hence significantly enhances the pressure variation. The amplified pressure variation then propagates subsequently to the cell space where the gas density is much lower”.““’ It is not known or presently pertinent why the observations with carbon conflict: what is of interest is the suggested effects of adsorption. which can also explain some of the present PBDS results. The generality that any solid surface will weakly bind any gas is ;I valid one: the extent of the adsorption depends on the nature and properties of the adsorbent and adsorbate. and decreases with increasing temperature but increases with increasing gas pressure.“’ “I Sotne data quoted by Brunauer (Table I of Ref. 17) show that I g of charcoal of unspecitied surface area would adsorb 2 cm’ of Hz, 8 cm’ of N,. I6 cm’ of CH, and 48 cm’ of CO, at I5 C, and similar data were given by Gregg (Table 3. I of Ref. 28) for the physical adsorption of the same gases on charcoal at 25 (I and I atm pressure. The adsorption values become correspondingly smaller when account is taken of the fact that the specific surface areas of most solids are one to two orders of magnitude smaller than those of charcoals (e.g. Tables 6.20, 6.22 of Ref. 30) but the values. although quite small. are not negligible. There is also a general relation bctwcen adsorption and condensibility, i.e. boiling point and critical adsorption occurs to a greater extent the higher the adsorbate’s temperature. Note that the adsorption of CO, occurred above the boiling point ( ~ 7X C) bum neat the critical temperature (3 I C). but much above the boiling points and critical temperatures of the other gases (for CH,. the respective values are - 164 and --X7 C). and that the adsorption of CO, is much greater than that of the other gases. In the present experiments. at I atm pressure and ambient temperature the extent of adsorption of all gases except CO, would be quite umall, as in the examples cited above. When the prcssurc was then increased, all gases would be caused to adsorb more extensively. with CO1 adsorbing the most. The desorption of a part of this bound CO, may be the cause of the anomalously low S values which were observed. The following mechanism suggests itself. If a radiation pulse is adsorbed bq a solid bearing a layer or multilayers of physically adsorbed gas. the temperature of the surface rises slightly. causing some gas to desorb. a portion of the photothermal energy release being taken up as heat of desorption (the heat of adsorption of CO, is of the order of 8 kcal:‘mol and that of CH, of the order of 4 kcal/mol. for example), so that the subsequent heating of the gas o\‘er the surface is diminished with respect to that occurring in the absence of the adsorbed layer. The refractive index gradient and consequent beam deflection would thus be stnaller than would otherwise be obtained. The mechanism may be considerably more complex. e.g. the pas dcsorbcd
Effects of the coupling
gas on i.r. PBDS of solids
205
Hydrogen
3000
IO00
2000 cm-’
Fig. 5. Spectra
of aspirin
with H? coupling
gas. The ordinates
are displaced
initially may cause a local increase in pressure and n, causing the probe beam to deflect down, and will be explored with instrumentation giving data on phase angle and modulation frequency. From the practical point of view, recording PBDS spectra at super-atmospheric pressures is generally beneficial in that larger signals are obtained than at atmospheric pressure, but may not be worthwhile as in the case of He and Ne because of the initially low signals. There is little to choose between the effects of Ar and N,, except that Ar is inert. However, Nz is relatively unreactive and suitable for most samples, is readily available in highly purified form, and cheap. An improvement by a factor of - 1.5 is obtainable with N, and Ar by an 8-fold pressure increase but at lower pressures of -3-4 atm which would be tolerated by simple glass cells, the S/N improvement is marginal and of doubtful utility in view of the increased experimental complexity. A quite worthwhile improvement can be obtained by using CH,, a relatively unreactive gas; at 1 atm there is a gain of - 50% with respect to Nz or air, and a - 100% gain is achievable by raising the pressure to 2-3 atm, readily achievable with simple cells. However, the absorption spectrum of CH, becomes superimposed on the PBDS spectrum of the solid, as shown by comparison of the CH, and Hz spectra of Fig. 4. The superimposition. for which no compensation is possible if absorption by the gas is complete, may or may not be acceptable, depending on the adsorptions of the solid sample. Significant improvements can be obtained with Hz, with unreactive samples. With reference to the signal obtained with Nz or air at I atm, a - I.5fold improvement can be obtained at 3-4 atm and a 2.5fold improvement by raising the pressure to 8 atm. In terms of measurement time, t, as S/N is proportional to f I”, the 2.5-fold improvement means that t can be reduced by a factor of 6. The dramatic improvement brought about by increasing the H, pressure is illustrated by the spectra of Fig. 5: the spectra have comparable S and S/N values, yet the measurement times differ by a factor of IO. Judging by the results obtained with COz, which are also of heuristic interest, easily condensible and hence easily adsorbable gases should not be used as coupling gases. Indeed, the presence of such gases in streams of otherwise innocuous coupling gases, or the presence of weakly bound adsorbates on high surface area materials such as chars or atmospheric pollutant particles, may complicate PBDS measurements. A~kno~led~rmmts~Support is gratefully acknowledged.
by AROD
Contract
DAAG
29-79-C-0135
and NSF Grants
CPE-792100
and CHE-RI 1778
REFERENCES I.
Boccard
A. C.. D. Fournier
Topicd
Mwfiq
and J. Badoz, A@.
on Plrotoucoustic
(unpublished). 2. Jackson W. B.. N. M. Amer.
Spectroscopy,
D. Fournier
Phys. Let/. 36, 130 (1980);
Iowa
State
and A. C. Boccara
University,
D. Fournier. l-3 August,
. 2nd Internotional
A. C. Boccara and J. Badoz, 1979. Dige.vt, Paper ThA I
Topical
Meeting
on Photoacoustic
ZOh
M. J. D. Low (‘I rri.