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Density dependence of the 5 μm infrared spectrum of NH3
J. Quant. Spectrosc. Radiat. TransJerVol.43, No. 4, pp. 319-326, 1990
0022-4073/90 $3.00+ 0.00 PergamonPressplc
Printed in Great Britain
DENSITY DEPENDENCE
OF THE 5 p m I N F R A R E D
SPECTRUM
OF
NH 3
CAMILLE CHAPADOS~, GORDON L. BJORAKER~, and GEORGE BIRNBAUM§ tD6partement de Chimie-Biologie, Universit6 du Qu6bec fi Trois-Rivi6res, Trois-Rivi6res, Qu6bec, Canada G9A 5H7, ~National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. and §National Institute of Standards and Technology, B344 Materials Building, Gaithersburg, MD 20899, U.S.A. (Received 19 April 1989; received for publication 27 October 1989)
Abstract--Measurements of dilute mixtures of NH 3 in H 2 were made in the window region 1900-2100cm -~ of the NH 3 spectrum to determine its behavior with increasing pressure of H2. The spectra of pure H2, pure NH3, and mixtures of the two, in the total pressure range from 2.38 to 8.17 atm at 309 K, were obtained with a 975 cm White cell. Synthetic spectra were calculated using precise line strengths, line positions, and a Lorentz profile. The experimental and calculated spectra are in reasonably good agreement, except that the former is superimposed on a rather flat background not given by the calculation. A possible mechanism for this background is suggested.
INTRODUCTION The Jovian atmosphere contains H2 as the predominant species and CH4, CHEH2, CH3D, NH3, PH3, H20, and GeH4 in small quantitites. ~'2 Although H2 has no permanent dipole moment, it becomes active in the far infrared 3,4 and infrared 5 regions due to transient dipoles induced during collisions that produce weak absorption at moderate pressures. On the contrary, NH3, which is a minor constituent of the Jovian atmosphere, has a permanent dipole moment and is a strong absorber in the far 6 and middle i.r. 7"s regions. The absorption by NH3 in the 5/~m region is due primarily to the high frequency wings of the Vgband centered at 1630 cm -~ and the 2v2 (s ~ a) band centered at 1882 c m - I . 9,1° The only study of the absorption in this region is due to Varanasi and Pugh, ~ who made measurements at a number of discrete frequencies of N H 3 broadened by H2, He, and N_,. In this work, we determine the spectrum of NH3 in the region 1900-2100cm -~ and study its behavior with increasing pressure of H2. Some of these results are compared with synthetic spectra that were calculated with precise line strengths and positions and a Lorentz profile of N H 3 lines in the region from 1800 to 2100 cm-~. Apart from the usefulness of this window region for probing the Jovian atmosphere, this work is relevant to the study of absorption in the far wings of i.r. bands, a subject which is actively under investigation) 2--~5 CALCULATED SPECTRA The absorption coefficient per unit path length of an i.r. band of uncoupled lines is according to the Lorentz formula
~(v) = y~ ~j(v) = ~ sj avj j g (v - ~j)~ + A~"
(1)
Here, aj(v) is in cm -~, j signifies a given transition, Sj is the intensity or integrated absorption coefficient, which is proportional to the density of absorbing molecules, vj (cm -~) is the resonance frequency, and Avj ( c m - J) is the linewidth, which depends on the number density of both absorber and pertuber [see Eq. (2)]. Precise line strengths and line positions are given in the work of Lellouch et aP ° who determined the broadening coefficient for each line, which varies between approx. 0.2 319
320
CAMILLECHAPADOSet al
and 0.6cm-~/atm in the region 1 8 0 0 - 2 1 0 0 c m ~. An average value of the NH3-NH3 Lorentz halfwidth of 0.42 cm J/atm was used for all lines. This value is large because NH3 is a polar molecule. For broadening by H 2, we used the value 0.072 cm ~, which is close to the average value for the lines of the v2 band of NH3 broadened by H2.16 The value of Av for binary collisions in a mixture of N H 3 - H 2 is Av,/P~o~ ~ = 0.072 + 0.3485 q(NH~),
(2)
where q (NH3) = P(NH3)/P~otal. Absorption by N H 3 in the 5/xm region is primarily due to the blue wing of the strong v4 band centered at 1630 cm l and to a much smaller extent to the lines of the very weak 2v 2 (s --*a) band centered at 1882 cm- ~.9.10The computed spectra were obtained from Eq. (1) for all of the lines from 1800 to 2100 cm ~ in the v4 and 2v2 band, with a resolution of 2.1 cm -~. However, for frequencies such that I v - x)l > 50cm-~, ~j(v) was set equal to zero, since the Lorentzian shape cannot be correct in the far wings. In fact, the absorption must decrease in the wings much more rapidly with wavenumber than that given by Eq. (1) if the spectral moments beyond the zeroth spectral moment are to be finite. ~5 There is however little justification for selecting a cut-off at 50 cm ~, but the computed results would not differ too much if the cut-off were changed by +__20 cm ~, for example. There is much experimental evidence that the far wings of pressure broadened bands vary nearly exponentially with frequency, ~7 a behavior that may be ascribed to a finite duration of collision. ~ Furthermore, overlapping lines cause interference effects which change the intensity of the trough between lines and even the shapes of the lines themselves, ~'~and moreover greatly decrease the far wing absorption. ~2 14 Calculations which include all these effects are not yet possible; however, E q ( 1) which gives an upper limit to the magnitude of far wing absorption is sufficient for our purpose here. EXPERIMENTAL
CONSIDERATIONS
The spectra were obtained with a Perkin Elmer model 180 infrared spectrophotometer, operated in the double beam mode with a programmed slits function. Two gratings were used to cover the region from 2300 to 1900 cm ~. The analog data were converted to numerical values, transferred to a 3600 Perkin-Elmer Data Station and stored on floppy disks. A frequency scan was taken first to o b t a i n / d r ) , the spectrum of the evacuated cell; then a scan with the sample was taken to obtain I(v). The intensity of the sample beam was adjusted to about 75% the intensity of the reference beam. The resolution was around 2 c m ~. Each region was scanned twice for a given sample. The calibration of the spectrometer was accomplished using the rotational lines of water vapor. ~ A 20 m variable-path White cell (Wilks Scientific Co.) was used. The Teflon-coated aluminum cells contains gold-coated glass mirrors and KBr lens windows. A pathlength of 9.75 m requiring 25 reflections on the mirrors was used: the loss of energy was around 70%. N H 3 is adsorbed strongly on the cell walls, mirrors, and the gas transfer line was made mostly of stainless steel and some copper. Before making measurements, the cell and transfer line were heated and flushed several times with H~ to reduce the amount of adsorbed N H , . The presence of NH3 was detected by monitoring its absorption lines near 10/~m where there are strong rotational vibrational transitions. Ultra high purity H2 (99.999 + %, Matheson Canada) and electronic grade N H 3 (99.998 + %, Matheson Canada) were used directly without purification. Four pressures of H2 were used: 2.38, 4.08, 6.13, and 8.17 atm. Two pressures of NH~ were used: 0.791 and 1.19atm. To obtain the mixtures, NH3 was first introduced into the cell and then H2 was added to obtain total pressures of 2.38, 4.08, 6.13, and 8.17 atm. After each addition of H2, the mixture was allowed to equilibrate. Two frequency scans of the empty cell and the gas-filled cell were taken. After the cell was emptied, and when previously containing NH3, it was washed several times with H2 and then pumped to 1 mtorr. The following day, the background absorption was checked by taking two scans. To minimize the variation of cell temperature due to variations in room temperature, the temperature of the cell was maintained at (308.7 4- 0.5) K with a heating coil. The transmissivities were averaged and smoothed to reduce the high frequency noise in the frequency scans.
The 5 #m spectrum of NH 3
321
I .0 I
i,1 (.3 Z I-I.--
~IU0 . O3 Z n"
. 900
2000
21 O0
P/cm
-
2200
2300
I
Fig. 1. Effect of pressure on the base line 1 (continuous line) in a White cell with a pressure of 1 mtorr; 2, with 2.38 arm of H2; 3, with 4.08 atm; 4, with 6.13 atm; 5, with 8.17 atm; dashed line, pumped overnight to a pressure of 1 mtorr. Temperature of gas cell here and in subsequent figures is (308.7 __+0.5 K).
The absorption coefficient was computed from
~x(v)
=
1/L ln[l'(v)/I(v)],
(3)
where L is the pathlength in cm. Here I(v) is the intensity with a sample in the gas cell, and I'(v) is the intensity I0 (v) obtained with the evacuated cell, which was used for measurements with pure H 2, or I'(v), is IH2(v), the intensity obtained with the cell filled with pure H 2 at the same pressure as that of the N H 3 or the N H 3 - H 2 samples. The molar absorption coefficient is ot(v)/p, where the density p (mol/cm 3) is determined from the pressure P (atm) by the following formula: p
=
P/RT
-
Bv(P/RT) 2.
(4)
Here, T is the temperature in Kelvin, R is the gas constant (82.055 cm 3 a t m / K mol) and Bv (in cm3/mol) is the second virial coefficient obtained from tablesfl ° For N H 3 at 298.16K, Bv = - 2 6 1 cm3/mol, and for H2 Bv = (14.8 + 0.05)cm3/mol at 300 K. The total density is the sum of the partial densities of N H 3 and H 2. RESULTS Figure 1 shows the spectra of H 2 obtained at (308.7 + 0.5) K using the White cell with a pathlength of 975 cm under the following conditions: (1) gas cell evacuated to 1 mtorr; and with H 2 a t (2) 2.38atm, (3) 4.08atm, (4) 6.13 atm, and (5) 8.17atm. The small absorption bands 40. I o
O. 1900 . . . .
20'00 . . . .
21'00 . . . .
22'00 . . . .
2300
P/cm-' Fig. 2. Absorption per unit density of NH 3 in a mixture of 0.789 atm of NH~ with H 2. 1, Pure NH3: H 2 added to obtain total pressures of 2, 2.38atm; 3, 4.08arm; 4, 6.13atm; 5, 8.17atm; here, or(v) = L -Iln[lo(v)/l(v)].
322
CAMILLECHAPADOSel al
40. ¸
i
o E 61
E 20 o
3 ,
1900
~
2000
21 00 P/cm
-
2200
2300
1
Fig. 3. The same as in Fig. 2, but the background due to an equivalent pressure of H, was subtracted. Here and in the following figures, air) L tln[Im{v),'l(v)]. observed between 1900 and 2000 cm ~are due to water vapor. The dashed line is the baseline taken the following day after pumping the cell overnight; it took about a day to reduce the pressure to 1 retort and to recover the position of the baseline. Its value was very similar to the initial value obtained with the evacuated cell the day before, thereby demonstrating the very good stability of the equipment. Taking the results in Fig. 1 to be independent of wavenumber, we obtain
log(Io/IH,) = 6.96 × 10 ~ PH, (atm).
(5)
The only source of absorption in h2 in the 5 ltm region is due to the high frequency wing of the collision-induced translational -rotational band which peaks in the region near 600 cm ~. From the measurements of Bouanich et al 2~ we deduce that at the highest H2 pressure used (8.2 atm), the collision-induced absorption in H2 is less by two orders of magnitude than the apparent absorption indicated in Fig. 1. Moreover, the relation between the pressure and absorbance given by Eq. (5) is linear whereas the relation would be quadratic if collision-induced absorption were involved. The observed pressure behavior could be the result of mechanical distortion due to the increasing pressure inside the gas cell, resulting in a vignetting of the beam and hence attenuation of the signal. Another explanation (and possibly a less likely one) could be the adsorption of H, on the surface of the mirrors, which would modify their reflective properties so as to defocus the beam. Either effect would be amplified by the 25 reflections of the incident beam on the mirrors. In any case. the results shown in Fig. 1 were confirmed by additional measurements taken after the experiment reported here was completed. Furthermore, to test for possible malfunction in the dual beam system of the spectrometer, the measurements were made using the spectrometer as a single beam instrument and the same results as in Fig. 1 were obtained. The absorption of 0.790 aim of NH~ is shown in Fig. 2. The spectra obtained with the addition of H, to give a total pressure of 2.38.4.08, 6.13, and 8.17 atm are given in traces 2-5, respectively. These spectra were obtained by using I~(v) for the evacuated cell. However, when the apparent absorption of an equivalent pressure of H 2 (cell distortion effect) was subtracted from the spectra by taking the ratio lm(v)/I(v) instead of lo(v)/I(v), we obtained the spectra presented in Fig. 3. Except for spectrum 5 obtained at a total pressure of 8.17 atm, all the other spectra tire on the average superimposed. The reason for the small departure of this spectrum from the others is unknown. We followed the same procedure with 1.188 atm of NH 3 and the addition of H~ to give the same total pressures as in the previous case. The spectra are given in Fig. 4, and within experimental error they are all on the average superimposed and very similar to the ones obtained with 0.790 atm of NH3 (Fig. 3). In Fig. 5, we compare the experimental spectra of pure N H 3 (trace 3, 0.790 atm: trace 4, 1.186 atm) with the calculated ones (trace I, 0.790 atm; trace 2, 1.188 atm) in the region from 1900 to 2100 cm ~. The experimental spectra presented in this figure are the ones given in Figs. 3 and 4 from which the spectrum of water vapor was subtracted, since minute quantities of water vapor
The 5 ~ m spectrum of N H 3
323
40. I m o ¢E
E20 o
~3
O,
,
.
.
.
1900
.
.
.
.
.
.
.
2000
.
.
.
.
.
.
21 O0 -
~'/cm
.
.
2200
2300
I
Fig. 4. Absorption per unit of density o f N H 3 of a mixture of 1.19 atm o f N H 3 with background subtracted for an equivalent pressure of H 2 as in Fig. 4. 1, 1.19 arm pure NH3; H 2 added to obtain a total pressure of 2, 2.38 atm; 3, 4.08 atm; 4, 6.13 atm; 5, 8.17atm.
was identified in the spectra of pure N H 3. The traces 5 and 6 (Fig. 5) give the difference between the experimental and calculated spectra for the 0.790 and 1.188 atm of pure N H 3. Within an estimated experimental error of roughly 0.5cm2/mol (NH3) , curves 3 and 4 are essentially indistinguishable from about 2000 to 2100cm ~. However, there is a distinguishable difference between the computed and experimental spectra in this region. We note that whereas Eq. (1) reproduces fairly well the line structure, it fails to give the experimentaly observed background absorption. From 2100 cm ~to lower wavenumbers, we note that the difference in the background absorption increases. The structure in the difference spectra could be due in part to the use in the calculation of a constant rather than the actual line widths. The theoretical spectra at the two pressures in Fig. 5 are very nearly identical except in the range 1900-1950 cm-~ where the strongest lines are located. Here there are barely discernible differences between peaks and between troughs at the two pressures. There differences are qualitatively in
O.
7 20.
lo.
43 4 2
0"900 . 1
.
3
.
.
.
.
.
.
P/cm
. -
.
.
.
O0
1
Fig. 5. Comparison between calculated and experimental spectra of pure NH3, the latter obtained from Figs. 3 and 4. Calculated: 1, 0.790 arm; 2, 1.19 arm. Experimental: 3, 0.789 atm; 4, 1.19 atm. Difference between experimental and calculated spectra: 5, 0.790 atm; 6, 1.19 atm. QSRT 43,4--E
324
CAMILLE CHAPADOSet al
accord with Eq. (1), which shows that with increasing pressure and hence increasing Av/, ~j(v)/p decreases at the peak of a line whereas it increases in the trough. Unlike the computed spectra, the experimental spectra are displaced, particularly below 2050 cm ~where curve 4 (1.88 atm NH~) lies above curve 3 (0.790 atm NH~). A greater background absorption for curve 4 is qualitatively understandable due to the higher pressure. As noted previously, the observed background absorption is much greater than that computed from the lines in the region 1900-2100cm ~. However, to this we must add the continuum absorption due to the v: (950cm ~) and v4 (1630cm J) bands. An estimate of an upper limit of this absorption may be obtained from Eq. (1), which for the case I v - v,I >> Av, becomes
~,(v)=~&6v/~(vt
v,)L
!6)
~
Here, v designates the vibrational band, S, is the integrated intensity of the band, which is proportional to the density of NH3, and v,. is the band center. At 1980 cm t where there is relatively little local structure we obtain from Eq. (6) 6.0 cm2/mol (P = 1.19 atm NH3), which is greater than the experimental value of about 4.0 cm-2/mol. However, for the reasons mentioned previously, the absorption from the v2 and v4 bands is expected to be much smaller than that given by Eq. (6). In Fig. 6, the experimental spectra of 0.789 atm of NH3 with H 2 to give total pressures of 2.38, 4.08, 6.13, and 8.17 atm, traces 5 8, respectively, are compared with the respective calculated spectra, traces 1 4. Unlike the case of pure N H 3 where the experimental ~(v)/p is greater at the higher pressure, it is difficult to discern any general trend of ~(v)/p with increasing pressure of H~, although surprisingly, it appears that the experimental absorption is smaller at 8.17 atm than at the lower pressures in the region 1960 2100cm ~. Within an estimated experimental error of 0.5cm2/mol (NH3), curves 5, 6 and 7 are essentially indistinguishable from about 2000 to 2100 cm ~. Curve 8 obtained at the highest pressure of H2 is distinguishably lower than curves 5, 6 and 7 in this region and aproaches zero absorption. The traces 9 11, which give the difference between the experimental and calculated spectra, are essentially indistinguishable from ~ 2030 to 2100 cm t. We note that the value of Av used in calculating the NH3---H2 spectra may be about
<3
0.
- 20. 5 I
O,
.~I0.
u
7
O.
1~ 6
900 . . . .
19"50 . . . .
~ 20"00
~, / c rn
-
2050
2 O0
I
Fig. 6. Comparison between calculated and experimental spectra, the latter obtained from Fig. 3, of a mixture of 0.790 atm of N H 3 with H 2 to obtain the following total pressures. Calculated: 1, 2.38 atm; 2, 4.08 atm; 3, 6.13atm; 4, 8.17atm. Experimental: 5, 2.38 atm; 6, 4.08 atm; 7, 6.13atm; 8, 8.17atm. Difference between experimental and calculated spectra: 9, 2.38atm; 10, 4.08atm; 11, 6.13atm: 12, 8.17 atm.
The 5 #m spectrum of NH 3
325
20% too small. :2 However, increasing Av by this amount is not expected to alter the above observations. DISCUSSION Varanasi and Pugh ~ measured the absorption coefficient of mixtures of N H 3 in He, N2, and Hz at nine wavenumbers between 2000 and 2200 cm -j and used an experimental arrangement and conditions that were similar to those employed here. The total pressures they used were between 4 and 8 arm, and the partial pressures of NH~ ranged from 0.I to 0.4 arm. The measurements were made at room temperature, and path lengths up to 10 m were employed. They found that the absorption coefficient of all the mixtures between 2000 and 2200 cm-~ were the same within an experimental error of 15%, independent of the broadening gas and its pressure, and also independent of the pressure of NH3. The general trend of our results with wavenumber are not too different from those of Varanasi and Pugh, ~ except for the magnitude of the absorption coefficient itself. At 2100 cm-1, for example, they obtained 6.4 cm2/mol, whereas we obtained about 2.0 cm2/mol. This difference may be understood if they experienced the same problem with the multiple traversal White cell that we report here. In fact, if we do not make the corrections for the changes in the transmission of the cell observed with the addition of just H2, we arrive at a value for the absorption coefficient at 2100 c m - I that is not too different from their value. They reached the conclusion that the absorption of NH3 broadened by H2, He, and N2 is (1) neither due to the far wings of the lines in the bands surrounding the 5 # m region, nor (2) due to any dimer absorption but entirely due to local lines. We certainly agree with (1). However, our computation of the absorption due to these local lines, while reproducing the observed structure, gives a baseline much less than that obtained experimentally. An interesting observation is that ~(v)/p (p = density of NH3) in the continuum region is the least at the highest pressure of H2. Such behavior might be attributed to the interference of overlapping lines, i.e., those of the 2v2 band. Nevertheless, the measured absorption at wavenumbers less than roughly 2050 cm 1 is greater than that given by the 2v 2 band which appears to be superimposed on a featureless background. This discrepancy may be attributed to an unsatisfactory theory or perhaps to the contribution of NH3 dimers, in which NH3 undergoes transitions in the 2v2 vibrational mode. The dimer spectrum of the v2 mode of NH3 was found to be dense and the line widths large. 23 It then may be supposed that the 2v2 mode should also have similarly dense spectrum leading to continuum absorption at the pressures used here. Acknowledgements--The authors thank G. T. Fraser for suggesting the possibilityof NH 3dimers as the origin of the 5 pm continuum. The authors also thank Glenn S. Orton for many valuable discussions and M. Trudel for assistance with the experiment. This work was supported, in part, by a contract from the Jet Propulsion Laboratory, sponsored by the Planetary Atmospheres Program, NASA, and the Natural Science and Engineering Research Council of Canada. REFERENCES 1. V. Kunde, R. Hanel, W. Maguire, D. Gautier, J. P. Baluteau, A. Marten, A. Chedin, N. Husson, and N. Scott, Astrophys. J. 263, 443 (1982). 2. G. L. Bjoraker, H. P. Larson, and V. G. Kunde, Icarus 66, 579 (1986). 3. P. Dore, L. Nencini, and G. Birnbaum, JQSRT 30, 245 (1983). 4. G. Bachet, E. R. Cohen, P. Dore, and G. Birnbaum, Can. J. Phys. 61, 591 (1983). 5. S. P. Reddy, in Phenomena Induced by Intermolecular Interactions, pp. 129-167, NATO ASI Series B: Physics Vol. 127, G. Birnbaum ed., Plenum Press, New York, NY (1985). 6. A. Leupolt, Infrared Phys. 14, 99 (1974). 7. D. C. McKean and P. N, Schatz, J. Chem. Phys. 24, 316 (1956). 8. W. S. Benedict, E. K. Plyler, and E. D. Tidwell, J. Chem. Phys. 29, 829 (1958). 9. S. Urban, V. Spirko, D. Papousek, R. S. McDowell, N. G. Nereson, S. P. Relov, L. I. Gershstein, A. V. Maslovskij, A. F. Krupnow, J. Curtis, and K. N. Rao, J. Molec. Spectrosc. 79, 445 (1980). 10. E. Lellouch, N. Lacome, G. Guelachvili, G. Tarrago, and T. Encrenaz, J. Molec. Spectrosc. 124, 33 (1987); a tape of the data kindly supplied by Dr E. Lellouch was used. 11. P. Varanasi and L. Pugh, JQSRT 13, 1225 (1973). 12. M, O. Bulanin, A. B. Dokuchaev, M. V. Tonkov, and N, N. Filippov, JQSRT 31, 521 (1984). 13. C. Boulet, J. Boissoles, and D. Robert, J. Chem. Phys. 89, 625 (1988). 14. P. W. Rosenkranz, J. Chem. Phys. 83, 6139 (1985). 15. G. Birnbaum, JQSRT 21, 597 (1979).
326 16. 17. 18. 19. 20. 21. 22. 23.
CAMILLECHAPADOSet al P. Varanasi, JQSRT 12, 1283 (1972). D. E. Burch, D. A. Gryvnak, R. R. Patty, and C. E. Bartky, JOSA 59, 267 (1969). N. Lacome, A. Levy, and C. Boulet, J. Chem. Phys. 80, 2429 (1984). International Union of Pure and Applied Chemistry, Tables ~?["Wal~enumbers.for Calibration qf Infrared Spectrometers, Butterworths, London (1961). J. H. Dymond and E. B. Smith, The Virial Co¢:~cients of Gases, Clarendon Press, Oxford (1969). J. P. Bouanich, C. Brodbeck P. Drossart, and E. Leltouch, JQSRT 42, 141 (1989). J. S. Margolis and S. Sarangi, JQSRT 16, 405 (1976). G. T. Fraser, D. D. Nelson, Jr., A. Charo, and W. Klemperer, J. Chem. Phys. 82, 2535 (1985).
0022-4073/90 $3.00+ 0.00 PergamonPressplc
Printed in Great Britain
DENSITY DEPENDENCE
OF THE 5 p m I N F R A R E D
SPECTRUM
OF
NH 3
CAMILLE CHAPADOS~, GORDON L. BJORAKER~, and GEORGE BIRNBAUM§ tD6partement de Chimie-Biologie, Universit6 du Qu6bec fi Trois-Rivi6res, Trois-Rivi6res, Qu6bec, Canada G9A 5H7, ~National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. and §National Institute of Standards and Technology, B344 Materials Building, Gaithersburg, MD 20899, U.S.A. (Received 19 April 1989; received for publication 27 October 1989)
Abstract--Measurements of dilute mixtures of NH 3 in H 2 were made in the window region 1900-2100cm -~ of the NH 3 spectrum to determine its behavior with increasing pressure of H2. The spectra of pure H2, pure NH3, and mixtures of the two, in the total pressure range from 2.38 to 8.17 atm at 309 K, were obtained with a 975 cm White cell. Synthetic spectra were calculated using precise line strengths, line positions, and a Lorentz profile. The experimental and calculated spectra are in reasonably good agreement, except that the former is superimposed on a rather flat background not given by the calculation. A possible mechanism for this background is suggested.
INTRODUCTION The Jovian atmosphere contains H2 as the predominant species and CH4, CHEH2, CH3D, NH3, PH3, H20, and GeH4 in small quantitites. ~'2 Although H2 has no permanent dipole moment, it becomes active in the far infrared 3,4 and infrared 5 regions due to transient dipoles induced during collisions that produce weak absorption at moderate pressures. On the contrary, NH3, which is a minor constituent of the Jovian atmosphere, has a permanent dipole moment and is a strong absorber in the far 6 and middle i.r. 7"s regions. The absorption by NH3 in the 5/~m region is due primarily to the high frequency wings of the Vgband centered at 1630 cm -~ and the 2v2 (s ~ a) band centered at 1882 c m - I . 9,1° The only study of the absorption in this region is due to Varanasi and Pugh, ~ who made measurements at a number of discrete frequencies of N H 3 broadened by H2, He, and N_,. In this work, we determine the spectrum of NH3 in the region 1900-2100cm -~ and study its behavior with increasing pressure of H2. Some of these results are compared with synthetic spectra that were calculated with precise line strengths and positions and a Lorentz profile of N H 3 lines in the region from 1800 to 2100 cm-~. Apart from the usefulness of this window region for probing the Jovian atmosphere, this work is relevant to the study of absorption in the far wings of i.r. bands, a subject which is actively under investigation) 2--~5 CALCULATED SPECTRA The absorption coefficient per unit path length of an i.r. band of uncoupled lines is according to the Lorentz formula
~(v) = y~ ~j(v) = ~ sj avj j g (v - ~j)~ + A~"
(1)
Here, aj(v) is in cm -~, j signifies a given transition, Sj is the intensity or integrated absorption coefficient, which is proportional to the density of absorbing molecules, vj (cm -~) is the resonance frequency, and Avj ( c m - J) is the linewidth, which depends on the number density of both absorber and pertuber [see Eq. (2)]. Precise line strengths and line positions are given in the work of Lellouch et aP ° who determined the broadening coefficient for each line, which varies between approx. 0.2 319
320
CAMILLECHAPADOSet al
and 0.6cm-~/atm in the region 1 8 0 0 - 2 1 0 0 c m ~. An average value of the NH3-NH3 Lorentz halfwidth of 0.42 cm J/atm was used for all lines. This value is large because NH3 is a polar molecule. For broadening by H 2, we used the value 0.072 cm ~, which is close to the average value for the lines of the v2 band of NH3 broadened by H2.16 The value of Av for binary collisions in a mixture of N H 3 - H 2 is Av,/P~o~ ~ = 0.072 + 0.3485 q(NH~),
(2)
where q (NH3) = P(NH3)/P~otal. Absorption by N H 3 in the 5/xm region is primarily due to the blue wing of the strong v4 band centered at 1630 cm l and to a much smaller extent to the lines of the very weak 2v 2 (s --*a) band centered at 1882 cm- ~.9.10The computed spectra were obtained from Eq. (1) for all of the lines from 1800 to 2100 cm ~ in the v4 and 2v2 band, with a resolution of 2.1 cm -~. However, for frequencies such that I v - x)l > 50cm-~, ~j(v) was set equal to zero, since the Lorentzian shape cannot be correct in the far wings. In fact, the absorption must decrease in the wings much more rapidly with wavenumber than that given by Eq. (1) if the spectral moments beyond the zeroth spectral moment are to be finite. ~5 There is however little justification for selecting a cut-off at 50 cm ~, but the computed results would not differ too much if the cut-off were changed by +__20 cm ~, for example. There is much experimental evidence that the far wings of pressure broadened bands vary nearly exponentially with frequency, ~7 a behavior that may be ascribed to a finite duration of collision. ~ Furthermore, overlapping lines cause interference effects which change the intensity of the trough between lines and even the shapes of the lines themselves, ~'~and moreover greatly decrease the far wing absorption. ~2 14 Calculations which include all these effects are not yet possible; however, E q ( 1) which gives an upper limit to the magnitude of far wing absorption is sufficient for our purpose here. EXPERIMENTAL
CONSIDERATIONS
The spectra were obtained with a Perkin Elmer model 180 infrared spectrophotometer, operated in the double beam mode with a programmed slits function. Two gratings were used to cover the region from 2300 to 1900 cm ~. The analog data were converted to numerical values, transferred to a 3600 Perkin-Elmer Data Station and stored on floppy disks. A frequency scan was taken first to o b t a i n / d r ) , the spectrum of the evacuated cell; then a scan with the sample was taken to obtain I(v). The intensity of the sample beam was adjusted to about 75% the intensity of the reference beam. The resolution was around 2 c m ~. Each region was scanned twice for a given sample. The calibration of the spectrometer was accomplished using the rotational lines of water vapor. ~ A 20 m variable-path White cell (Wilks Scientific Co.) was used. The Teflon-coated aluminum cells contains gold-coated glass mirrors and KBr lens windows. A pathlength of 9.75 m requiring 25 reflections on the mirrors was used: the loss of energy was around 70%. N H 3 is adsorbed strongly on the cell walls, mirrors, and the gas transfer line was made mostly of stainless steel and some copper. Before making measurements, the cell and transfer line were heated and flushed several times with H~ to reduce the amount of adsorbed N H , . The presence of NH3 was detected by monitoring its absorption lines near 10/~m where there are strong rotational vibrational transitions. Ultra high purity H2 (99.999 + %, Matheson Canada) and electronic grade N H 3 (99.998 + %, Matheson Canada) were used directly without purification. Four pressures of H2 were used: 2.38, 4.08, 6.13, and 8.17 atm. Two pressures of NH~ were used: 0.791 and 1.19atm. To obtain the mixtures, NH3 was first introduced into the cell and then H2 was added to obtain total pressures of 2.38, 4.08, 6.13, and 8.17 atm. After each addition of H2, the mixture was allowed to equilibrate. Two frequency scans of the empty cell and the gas-filled cell were taken. After the cell was emptied, and when previously containing NH3, it was washed several times with H2 and then pumped to 1 mtorr. The following day, the background absorption was checked by taking two scans. To minimize the variation of cell temperature due to variations in room temperature, the temperature of the cell was maintained at (308.7 4- 0.5) K with a heating coil. The transmissivities were averaged and smoothed to reduce the high frequency noise in the frequency scans.
The 5 #m spectrum of NH 3
321
I .0 I
i,1 (.3 Z I-I.--
~IU0 . O3 Z n"
. 900
2000
21 O0
P/cm
-
2200
2300
I
Fig. 1. Effect of pressure on the base line 1 (continuous line) in a White cell with a pressure of 1 mtorr; 2, with 2.38 arm of H2; 3, with 4.08 atm; 4, with 6.13 atm; 5, with 8.17 atm; dashed line, pumped overnight to a pressure of 1 mtorr. Temperature of gas cell here and in subsequent figures is (308.7 __+0.5 K).
The absorption coefficient was computed from
~x(v)
=
1/L ln[l'(v)/I(v)],
(3)
where L is the pathlength in cm. Here I(v) is the intensity with a sample in the gas cell, and I'(v) is the intensity I0 (v) obtained with the evacuated cell, which was used for measurements with pure H 2, or I'(v), is IH2(v), the intensity obtained with the cell filled with pure H 2 at the same pressure as that of the N H 3 or the N H 3 - H 2 samples. The molar absorption coefficient is ot(v)/p, where the density p (mol/cm 3) is determined from the pressure P (atm) by the following formula: p
=
P/RT
-
Bv(P/RT) 2.
(4)
Here, T is the temperature in Kelvin, R is the gas constant (82.055 cm 3 a t m / K mol) and Bv (in cm3/mol) is the second virial coefficient obtained from tablesfl ° For N H 3 at 298.16K, Bv = - 2 6 1 cm3/mol, and for H2 Bv = (14.8 + 0.05)cm3/mol at 300 K. The total density is the sum of the partial densities of N H 3 and H 2. RESULTS Figure 1 shows the spectra of H 2 obtained at (308.7 + 0.5) K using the White cell with a pathlength of 975 cm under the following conditions: (1) gas cell evacuated to 1 mtorr; and with H 2 a t (2) 2.38atm, (3) 4.08atm, (4) 6.13 atm, and (5) 8.17atm. The small absorption bands 40. I o
O. 1900 . . . .
20'00 . . . .
21'00 . . . .
22'00 . . . .
2300
P/cm-' Fig. 2. Absorption per unit density of NH 3 in a mixture of 0.789 atm of NH~ with H 2. 1, Pure NH3: H 2 added to obtain total pressures of 2, 2.38atm; 3, 4.08arm; 4, 6.13atm; 5, 8.17atm; here, or(v) = L -Iln[lo(v)/l(v)].
322
CAMILLECHAPADOSel al
40. ¸
i
o E 61
E 20 o
3 ,
1900
~
2000
21 00 P/cm
-
2200
2300
1
Fig. 3. The same as in Fig. 2, but the background due to an equivalent pressure of H, was subtracted. Here and in the following figures, air) L tln[Im{v),'l(v)]. observed between 1900 and 2000 cm ~are due to water vapor. The dashed line is the baseline taken the following day after pumping the cell overnight; it took about a day to reduce the pressure to 1 retort and to recover the position of the baseline. Its value was very similar to the initial value obtained with the evacuated cell the day before, thereby demonstrating the very good stability of the equipment. Taking the results in Fig. 1 to be independent of wavenumber, we obtain
log(Io/IH,) = 6.96 × 10 ~ PH, (atm).
(5)
The only source of absorption in h2 in the 5 ltm region is due to the high frequency wing of the collision-induced translational -rotational band which peaks in the region near 600 cm ~. From the measurements of Bouanich et al 2~ we deduce that at the highest H2 pressure used (8.2 atm), the collision-induced absorption in H2 is less by two orders of magnitude than the apparent absorption indicated in Fig. 1. Moreover, the relation between the pressure and absorbance given by Eq. (5) is linear whereas the relation would be quadratic if collision-induced absorption were involved. The observed pressure behavior could be the result of mechanical distortion due to the increasing pressure inside the gas cell, resulting in a vignetting of the beam and hence attenuation of the signal. Another explanation (and possibly a less likely one) could be the adsorption of H, on the surface of the mirrors, which would modify their reflective properties so as to defocus the beam. Either effect would be amplified by the 25 reflections of the incident beam on the mirrors. In any case. the results shown in Fig. 1 were confirmed by additional measurements taken after the experiment reported here was completed. Furthermore, to test for possible malfunction in the dual beam system of the spectrometer, the measurements were made using the spectrometer as a single beam instrument and the same results as in Fig. 1 were obtained. The absorption of 0.790 aim of NH~ is shown in Fig. 2. The spectra obtained with the addition of H, to give a total pressure of 2.38.4.08, 6.13, and 8.17 atm are given in traces 2-5, respectively. These spectra were obtained by using I~(v) for the evacuated cell. However, when the apparent absorption of an equivalent pressure of H 2 (cell distortion effect) was subtracted from the spectra by taking the ratio lm(v)/I(v) instead of lo(v)/I(v), we obtained the spectra presented in Fig. 3. Except for spectrum 5 obtained at a total pressure of 8.17 atm, all the other spectra tire on the average superimposed. The reason for the small departure of this spectrum from the others is unknown. We followed the same procedure with 1.188 atm of NH 3 and the addition of H~ to give the same total pressures as in the previous case. The spectra are given in Fig. 4, and within experimental error they are all on the average superimposed and very similar to the ones obtained with 0.790 atm of NH3 (Fig. 3). In Fig. 5, we compare the experimental spectra of pure N H 3 (trace 3, 0.790 atm: trace 4, 1.186 atm) with the calculated ones (trace I, 0.790 atm; trace 2, 1.188 atm) in the region from 1900 to 2100 cm ~. The experimental spectra presented in this figure are the ones given in Figs. 3 and 4 from which the spectrum of water vapor was subtracted, since minute quantities of water vapor
The 5 ~ m spectrum of N H 3
323
40. I m o ¢E
E20 o
~3
O,
,
.
.
.
1900
.
.
.
.
.
.
.
2000
.
.
.
.
.
.
21 O0 -
~'/cm
.
.
2200
2300
I
Fig. 4. Absorption per unit of density o f N H 3 of a mixture of 1.19 atm o f N H 3 with background subtracted for an equivalent pressure of H 2 as in Fig. 4. 1, 1.19 arm pure NH3; H 2 added to obtain a total pressure of 2, 2.38 atm; 3, 4.08 atm; 4, 6.13 atm; 5, 8.17atm.
was identified in the spectra of pure N H 3. The traces 5 and 6 (Fig. 5) give the difference between the experimental and calculated spectra for the 0.790 and 1.188 atm of pure N H 3. Within an estimated experimental error of roughly 0.5cm2/mol (NH3) , curves 3 and 4 are essentially indistinguishable from about 2000 to 2100cm ~. However, there is a distinguishable difference between the computed and experimental spectra in this region. We note that whereas Eq. (1) reproduces fairly well the line structure, it fails to give the experimentaly observed background absorption. From 2100 cm ~to lower wavenumbers, we note that the difference in the background absorption increases. The structure in the difference spectra could be due in part to the use in the calculation of a constant rather than the actual line widths. The theoretical spectra at the two pressures in Fig. 5 are very nearly identical except in the range 1900-1950 cm-~ where the strongest lines are located. Here there are barely discernible differences between peaks and between troughs at the two pressures. There differences are qualitatively in
O.
7 20.
lo.
43 4 2
0"900 . 1
.
3
.
.
.
.
.
.
P/cm
. -
.
.
.
O0
1
Fig. 5. Comparison between calculated and experimental spectra of pure NH3, the latter obtained from Figs. 3 and 4. Calculated: 1, 0.790 arm; 2, 1.19 arm. Experimental: 3, 0.789 atm; 4, 1.19 atm. Difference between experimental and calculated spectra: 5, 0.790 atm; 6, 1.19 atm. QSRT 43,4--E
324
CAMILLE CHAPADOSet al
accord with Eq. (1), which shows that with increasing pressure and hence increasing Av/, ~j(v)/p decreases at the peak of a line whereas it increases in the trough. Unlike the computed spectra, the experimental spectra are displaced, particularly below 2050 cm ~where curve 4 (1.88 atm NH~) lies above curve 3 (0.790 atm NH~). A greater background absorption for curve 4 is qualitatively understandable due to the higher pressure. As noted previously, the observed background absorption is much greater than that computed from the lines in the region 1900-2100cm ~. However, to this we must add the continuum absorption due to the v: (950cm ~) and v4 (1630cm J) bands. An estimate of an upper limit of this absorption may be obtained from Eq. (1), which for the case I v - v,I >> Av, becomes
~,(v)=~&6v/~(vt
v,)L
!6)
~
Here, v designates the vibrational band, S, is the integrated intensity of the band, which is proportional to the density of NH3, and v,. is the band center. At 1980 cm t where there is relatively little local structure we obtain from Eq. (6) 6.0 cm2/mol (P = 1.19 atm NH3), which is greater than the experimental value of about 4.0 cm-2/mol. However, for the reasons mentioned previously, the absorption from the v2 and v4 bands is expected to be much smaller than that given by Eq. (6). In Fig. 6, the experimental spectra of 0.789 atm of NH3 with H 2 to give total pressures of 2.38, 4.08, 6.13, and 8.17 atm, traces 5 8, respectively, are compared with the respective calculated spectra, traces 1 4. Unlike the case of pure N H 3 where the experimental ~(v)/p is greater at the higher pressure, it is difficult to discern any general trend of ~(v)/p with increasing pressure of H~, although surprisingly, it appears that the experimental absorption is smaller at 8.17 atm than at the lower pressures in the region 1960 2100cm ~. Within an estimated experimental error of 0.5cm2/mol (NH3), curves 5, 6 and 7 are essentially indistinguishable from about 2000 to 2100 cm ~. Curve 8 obtained at the highest pressure of H2 is distinguishably lower than curves 5, 6 and 7 in this region and aproaches zero absorption. The traces 9 11, which give the difference between the experimental and calculated spectra, are essentially indistinguishable from ~ 2030 to 2100 cm t. We note that the value of Av used in calculating the NH3---H2 spectra may be about
<3
0.
- 20. 5 I
O,
.~I0.
u
7
O.
1~ 6
900 . . . .
19"50 . . . .
~ 20"00
~, / c rn
-
2050
2 O0
I
Fig. 6. Comparison between calculated and experimental spectra, the latter obtained from Fig. 3, of a mixture of 0.790 atm of N H 3 with H 2 to obtain the following total pressures. Calculated: 1, 2.38 atm; 2, 4.08 atm; 3, 6.13atm; 4, 8.17atm. Experimental: 5, 2.38 atm; 6, 4.08 atm; 7, 6.13atm; 8, 8.17atm. Difference between experimental and calculated spectra: 9, 2.38atm; 10, 4.08atm; 11, 6.13atm: 12, 8.17 atm.
The 5 #m spectrum of NH 3
325
20% too small. :2 However, increasing Av by this amount is not expected to alter the above observations. DISCUSSION Varanasi and Pugh ~ measured the absorption coefficient of mixtures of N H 3 in He, N2, and Hz at nine wavenumbers between 2000 and 2200 cm -j and used an experimental arrangement and conditions that were similar to those employed here. The total pressures they used were between 4 and 8 arm, and the partial pressures of NH~ ranged from 0.I to 0.4 arm. The measurements were made at room temperature, and path lengths up to 10 m were employed. They found that the absorption coefficient of all the mixtures between 2000 and 2200 cm-~ were the same within an experimental error of 15%, independent of the broadening gas and its pressure, and also independent of the pressure of NH3. The general trend of our results with wavenumber are not too different from those of Varanasi and Pugh, ~ except for the magnitude of the absorption coefficient itself. At 2100 cm-1, for example, they obtained 6.4 cm2/mol, whereas we obtained about 2.0 cm2/mol. This difference may be understood if they experienced the same problem with the multiple traversal White cell that we report here. In fact, if we do not make the corrections for the changes in the transmission of the cell observed with the addition of just H2, we arrive at a value for the absorption coefficient at 2100 c m - I that is not too different from their value. They reached the conclusion that the absorption of NH3 broadened by H2, He, and N2 is (1) neither due to the far wings of the lines in the bands surrounding the 5 # m region, nor (2) due to any dimer absorption but entirely due to local lines. We certainly agree with (1). However, our computation of the absorption due to these local lines, while reproducing the observed structure, gives a baseline much less than that obtained experimentally. An interesting observation is that ~(v)/p (p = density of NH3) in the continuum region is the least at the highest pressure of H2. Such behavior might be attributed to the interference of overlapping lines, i.e., those of the 2v2 band. Nevertheless, the measured absorption at wavenumbers less than roughly 2050 cm 1 is greater than that given by the 2v 2 band which appears to be superimposed on a featureless background. This discrepancy may be attributed to an unsatisfactory theory or perhaps to the contribution of NH3 dimers, in which NH3 undergoes transitions in the 2v2 vibrational mode. The dimer spectrum of the v2 mode of NH3 was found to be dense and the line widths large. 23 It then may be supposed that the 2v2 mode should also have similarly dense spectrum leading to continuum absorption at the pressures used here. Acknowledgements--The authors thank G. T. Fraser for suggesting the possibilityof NH 3dimers as the origin of the 5 pm continuum. The authors also thank Glenn S. Orton for many valuable discussions and M. Trudel for assistance with the experiment. This work was supported, in part, by a contract from the Jet Propulsion Laboratory, sponsored by the Planetary Atmospheres Program, NASA, and the Natural Science and Engineering Research Council of Canada. REFERENCES 1. V. Kunde, R. Hanel, W. Maguire, D. Gautier, J. P. Baluteau, A. Marten, A. Chedin, N. Husson, and N. Scott, Astrophys. J. 263, 443 (1982). 2. G. L. Bjoraker, H. P. Larson, and V. G. Kunde, Icarus 66, 579 (1986). 3. P. Dore, L. Nencini, and G. Birnbaum, JQSRT 30, 245 (1983). 4. G. Bachet, E. R. Cohen, P. Dore, and G. Birnbaum, Can. J. Phys. 61, 591 (1983). 5. S. P. Reddy, in Phenomena Induced by Intermolecular Interactions, pp. 129-167, NATO ASI Series B: Physics Vol. 127, G. Birnbaum ed., Plenum Press, New York, NY (1985). 6. A. Leupolt, Infrared Phys. 14, 99 (1974). 7. D. C. McKean and P. N, Schatz, J. Chem. Phys. 24, 316 (1956). 8. W. S. Benedict, E. K. Plyler, and E. D. Tidwell, J. Chem. Phys. 29, 829 (1958). 9. S. Urban, V. Spirko, D. Papousek, R. S. McDowell, N. G. Nereson, S. P. Relov, L. I. Gershstein, A. V. Maslovskij, A. F. Krupnow, J. Curtis, and K. N. Rao, J. Molec. Spectrosc. 79, 445 (1980). 10. E. Lellouch, N. Lacome, G. Guelachvili, G. Tarrago, and T. Encrenaz, J. Molec. Spectrosc. 124, 33 (1987); a tape of the data kindly supplied by Dr E. Lellouch was used. 11. P. Varanasi and L. Pugh, JQSRT 13, 1225 (1973). 12. M, O. Bulanin, A. B. Dokuchaev, M. V. Tonkov, and N, N. Filippov, JQSRT 31, 521 (1984). 13. C. Boulet, J. Boissoles, and D. Robert, J. Chem. Phys. 89, 625 (1988). 14. P. W. Rosenkranz, J. Chem. Phys. 83, 6139 (1985). 15. G. Birnbaum, JQSRT 21, 597 (1979).
326 16. 17. 18. 19. 20. 21. 22. 23.
CAMILLECHAPADOSet al P. Varanasi, JQSRT 12, 1283 (1972). D. E. Burch, D. A. Gryvnak, R. R. Patty, and C. E. Bartky, JOSA 59, 267 (1969). N. Lacome, A. Levy, and C. Boulet, J. Chem. Phys. 80, 2429 (1984). International Union of Pure and Applied Chemistry, Tables ~?["Wal~enumbers.for Calibration qf Infrared Spectrometers, Butterworths, London (1961). J. H. Dymond and E. B. Smith, The Virial Co¢:~cients of Gases, Clarendon Press, Oxford (1969). J. P. Bouanich, C. Brodbeck P. Drossart, and E. Leltouch, JQSRT 42, 141 (1989). J. S. Margolis and S. Sarangi, JQSRT 16, 405 (1976). G. T. Fraser, D. D. Nelson, Jr., A. Charo, and W. Klemperer, J. Chem. Phys. 82, 2535 (1985).