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Studies of the dynamical behaviour of adsorbed molecules by slow neutron spectrometry
Volume 25A, number 6
PHYSICS
was financially supported by the Foundation for Fundamental Research (F.O.M.), subsidized by the Netherlands Organization for Pure Scientific Research (Z .W .O.).
investigation
References 1. D.Onderdelinden et al., Proc. 7th Int. Conf. on Phenomena in ionized gases, Beograd, 1965, Vol. I, p. 157. 2. E. S. Masbkova et al., Soviet Phys. -Solid State (English transl.) 5 (1964) 2516.
LETTERS
25 September 1967
3. D. D. Cdintsov, Soviet Phys. -Solid Dtate (English transl.) 5 (1963) 813. 4. S. A.Drentje, Nucl. Instr. and Methods, to be published. 5. C. H. Weijsenfeld, see ref. 1, Vol. I, p. 151; and Philips Res. Repts. suppl. no. 2 (1967). 6. M. W. Thompson and R. S. Nelson, Proc. Roy. Sot. A259 (1961) 458. 7. S. A. Drentje, Nucl. Instr and Methods 38 (1965) 271. 8. S. A. Drentje, Phys. Letters 24A (1967) 12.
*****
STUDIES
OF
THE
DYNAMICAL BEHAVIOUR OF ADSORBED BY SLOW NEUTRONSPECTROMETRY*
MOLECULES
G. VERDAN Division
of Experimental Neutron Physics, Reactor Physics EURATOIUI Ispra. Varese. Italy
Deparfment.
Received 15 August 1967
The scattering of cold neutrons from H2, CH4, C2H4 and C2H2 adsorbed on activated charcoal was measured at low temperatures. The time-of-flight spectra of the scattered neutrons reveal the dynamical behaviour of the adsorbed molecules.
Unfortunately, the classical methods of investigating adsorption phenomena [l] give no direct informations about the dynamical behaviour of the adsorbed atoms or molecules. In the last few years IR and NMR-spectroscopy have been applied to get some direct information about the molecular dynamics. However these methods are usually restricted to frequencies larger than 200 cm-l (corresponding to 25 meV). For this reason W. Kley proposed to use the cold neutron scattering method. It is well known that this technique provides information concerning the motion of molecules in solids, liquids and gases [2-31. Beryllium filtered neutrons were scattered by the sample with an angle of 96’ and analysed * The contents of this paper have been reported at the “Diskussionstagung Uber Neutronenphysik an Forschungs-realctoren”, held at Jtilich, Germany, April 25-28, 1967.
after a flight path of 3 m. All the data have been corrected for background, sample thickness, air attenuation in the flight path and detector efficiency [4]. The samples were prepared from Merck-charcoal with a surface area of 1100 m2/g Another series not reported here was carried out with SARAN-charcoal with a surface area of 680 m2/g. Though the surface homogeneity of this charcoal is better, no significant differences in the measured spectra have been observed. Fig. 1 show the time-of-flight spectra of the neutrons scattered by the adsorbed H2, CH , C2H4 and C2H2 at 85’K, 89OK, 85’K and 85 %K respectively. The spectra were obtained by subtracting the pure adsorbent intensity from the adsorbate-adsorbent intensity. The adsorbed quantities were 12-15s of the monolayer capacity for all the samples. The solid curves are the computed differential cross section of the free molecule, according to the Krieger-Nelkin theory [5]. The accuracy of the Krieger Nelkin 435
Volume
25A. number
PHYSICS
6
C2H2
TIME b302015
OF
h-%--54
FLIGHT 3
ENERGY
(16j.1sec 2 1 TRANSFER
AT85’K
chonnelsl O--(r-n&)
Fig. 1. Time-of-flight spectra of neutrons scattered by adsorbed molecules. The solid curves are calculated on the basis of the Krieger and Nelkin model.
curves have been verified by free gas measurements at room temperature. The spectra of all four adsorbates reveal the three following facts : First, one cannot observe any distinct peaks in the inelastic part, due to the vibration of the adsorbed molecule against the surface. From thermodynamical estimations, these “adsorption” 436
-
LETTERS
25 September
1967
frequencies should be found at about 10 meV for CH4, C2H4 and C2H2 and at 40 meV for H2 [l]. In the case of H2 the low occupation of this level (hu/kT = 6) could be the reason of the nonobservation. On the other hand in the case of CH4, C2H4 and C2H2, a possible explanation may be the large broadening of the expected peaks because of anharmonicity of the potential and because of surface heterogeneity. Second, the maximum intensity observed in all four cases is shifted to higher energies compared with the Krieger Nelkin curve. This may be explained by the fact that the external molecular motions a& hindered, that means the adsorbed molecules behave like a quasi-crystalline body, which gives a certain Debye like inelastic contribution. Third, in the elastic part of the spectra, two different behaviours can be observed. In the case of H2 and CH4 the quasi-elastic peak is broadened. This can be attributed toa certain surface diffusion of the adsorbed molecules, like in a two-dimensional gas or liquid. This is in good agreement with the model of a mobile adsorbed film. On the other hand, in the case of C2H4 and C2H2 no broadening of the elastic peak is visible within the spectrometer resolution. The adsorption seems to be localized. That means that for sample temperature higher or even equal to the melting point of the free adsorbate (eq. HZ, CH4), there exists diffusion along the surface of the adsorbent, whilst on the contrary for sample temperatures much belwo the melting point of the free adsorbate (eq. C2H4, C2H2) the diffusion of molecules along the surface is “frozen in”. In analysing the broadening of the elastic peak at different sample temperatures, it should be possible to measure the temperature dependence of the mobility of the adsorbed molecules. An article with complete discussions of the results is in preparation. The author would like to thank Dr. W. Kley for helpful discussions and for the idea of this work.
References
1. S. Ross and J. P. Oliver, On physical adsorption (Interscience Publishers, New York, 1964). 2. Proceedings of the Symposium on inelastic scattering of neutrons, Bombay (I.A.E.A., Vienna, 1965). 3. Symposium on inelastic scattering of neutrons by condensed systems, Brookhaven National Laboratory, September 20-22, 1965, BNL 940 (C-45). 4. G. Verdan and Neutof, A program to correct neutron time-of-flight spectra, EUR 2779 e. 5. T. J.Krieger and M.S.Nelkin, Phys. Rev. 106 (1957) 290.
PHYSICS
was financially supported by the Foundation for Fundamental Research (F.O.M.), subsidized by the Netherlands Organization for Pure Scientific Research (Z .W .O.).
investigation
References 1. D.Onderdelinden et al., Proc. 7th Int. Conf. on Phenomena in ionized gases, Beograd, 1965, Vol. I, p. 157. 2. E. S. Masbkova et al., Soviet Phys. -Solid State (English transl.) 5 (1964) 2516.
LETTERS
25 September 1967
3. D. D. Cdintsov, Soviet Phys. -Solid Dtate (English transl.) 5 (1963) 813. 4. S. A.Drentje, Nucl. Instr. and Methods, to be published. 5. C. H. Weijsenfeld, see ref. 1, Vol. I, p. 151; and Philips Res. Repts. suppl. no. 2 (1967). 6. M. W. Thompson and R. S. Nelson, Proc. Roy. Sot. A259 (1961) 458. 7. S. A. Drentje, Nucl. Instr and Methods 38 (1965) 271. 8. S. A. Drentje, Phys. Letters 24A (1967) 12.
*****
STUDIES
OF
THE
DYNAMICAL BEHAVIOUR OF ADSORBED BY SLOW NEUTRONSPECTROMETRY*
MOLECULES
G. VERDAN Division
of Experimental Neutron Physics, Reactor Physics EURATOIUI Ispra. Varese. Italy
Deparfment.
Received 15 August 1967
The scattering of cold neutrons from H2, CH4, C2H4 and C2H2 adsorbed on activated charcoal was measured at low temperatures. The time-of-flight spectra of the scattered neutrons reveal the dynamical behaviour of the adsorbed molecules.
Unfortunately, the classical methods of investigating adsorption phenomena [l] give no direct informations about the dynamical behaviour of the adsorbed atoms or molecules. In the last few years IR and NMR-spectroscopy have been applied to get some direct information about the molecular dynamics. However these methods are usually restricted to frequencies larger than 200 cm-l (corresponding to 25 meV). For this reason W. Kley proposed to use the cold neutron scattering method. It is well known that this technique provides information concerning the motion of molecules in solids, liquids and gases [2-31. Beryllium filtered neutrons were scattered by the sample with an angle of 96’ and analysed * The contents of this paper have been reported at the “Diskussionstagung Uber Neutronenphysik an Forschungs-realctoren”, held at Jtilich, Germany, April 25-28, 1967.
after a flight path of 3 m. All the data have been corrected for background, sample thickness, air attenuation in the flight path and detector efficiency [4]. The samples were prepared from Merck-charcoal with a surface area of 1100 m2/g Another series not reported here was carried out with SARAN-charcoal with a surface area of 680 m2/g. Though the surface homogeneity of this charcoal is better, no significant differences in the measured spectra have been observed. Fig. 1 show the time-of-flight spectra of the neutrons scattered by the adsorbed H2, CH , C2H4 and C2H2 at 85’K, 89OK, 85’K and 85 %K respectively. The spectra were obtained by subtracting the pure adsorbent intensity from the adsorbate-adsorbent intensity. The adsorbed quantities were 12-15s of the monolayer capacity for all the samples. The solid curves are the computed differential cross section of the free molecule, according to the Krieger-Nelkin theory [5]. The accuracy of the Krieger Nelkin 435
Volume
25A. number
PHYSICS
6
C2H2
TIME b302015
OF
h-%--54
FLIGHT 3
ENERGY
(16j.1sec 2 1 TRANSFER
AT85’K
chonnelsl O--(r-n&)
Fig. 1. Time-of-flight spectra of neutrons scattered by adsorbed molecules. The solid curves are calculated on the basis of the Krieger and Nelkin model.
curves have been verified by free gas measurements at room temperature. The spectra of all four adsorbates reveal the three following facts : First, one cannot observe any distinct peaks in the inelastic part, due to the vibration of the adsorbed molecule against the surface. From thermodynamical estimations, these “adsorption” 436
-
LETTERS
25 September
1967
frequencies should be found at about 10 meV for CH4, C2H4 and C2H2 and at 40 meV for H2 [l]. In the case of H2 the low occupation of this level (hu/kT = 6) could be the reason of the nonobservation. On the other hand in the case of CH4, C2H4 and C2H2, a possible explanation may be the large broadening of the expected peaks because of anharmonicity of the potential and because of surface heterogeneity. Second, the maximum intensity observed in all four cases is shifted to higher energies compared with the Krieger Nelkin curve. This may be explained by the fact that the external molecular motions a& hindered, that means the adsorbed molecules behave like a quasi-crystalline body, which gives a certain Debye like inelastic contribution. Third, in the elastic part of the spectra, two different behaviours can be observed. In the case of H2 and CH4 the quasi-elastic peak is broadened. This can be attributed toa certain surface diffusion of the adsorbed molecules, like in a two-dimensional gas or liquid. This is in good agreement with the model of a mobile adsorbed film. On the other hand, in the case of C2H4 and C2H2 no broadening of the elastic peak is visible within the spectrometer resolution. The adsorption seems to be localized. That means that for sample temperature higher or even equal to the melting point of the free adsorbate (eq. HZ, CH4), there exists diffusion along the surface of the adsorbent, whilst on the contrary for sample temperatures much belwo the melting point of the free adsorbate (eq. C2H4, C2H2) the diffusion of molecules along the surface is “frozen in”. In analysing the broadening of the elastic peak at different sample temperatures, it should be possible to measure the temperature dependence of the mobility of the adsorbed molecules. An article with complete discussions of the results is in preparation. The author would like to thank Dr. W. Kley for helpful discussions and for the idea of this work.
References
1. S. Ross and J. P. Oliver, On physical adsorption (Interscience Publishers, New York, 1964). 2. Proceedings of the Symposium on inelastic scattering of neutrons, Bombay (I.A.E.A., Vienna, 1965). 3. Symposium on inelastic scattering of neutrons by condensed systems, Brookhaven National Laboratory, September 20-22, 1965, BNL 940 (C-45). 4. G. Verdan and Neutof, A program to correct neutron time-of-flight spectra, EUR 2779 e. 5. T. J.Krieger and M.S.Nelkin, Phys. Rev. 106 (1957) 290.