- Email: [email protected]
Infrared spectroscopy of CO trapped in an argon matrix revisited
24 September 1999
Chemical Physics Letters 311 Ž1999. 153–158 www.elsevier.nlrlocatercplett
Infrared spectroscopy of CO trapped in an argon matrix revisited Hisashi Abe a
a,)
, Harutoshi Takeo
a,b
, Koichi M.T. Yamada
a
National Institute for AdÕanced Interdisciplinary Research (NAIR), 1-1-4 Higashi, Tsukuba, Ibaraki, 305-8562, Japan b National Institute of Materials and Chemical Research (NIMC), 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 15 April 1999; in final form 15 June 1999
Abstract The infrared spectra of carbon monoxide in an argon matrix at cryogenic temperatures have been reinvestigated by FTIR spectroscopy over a wide range of experimental conditions, and new assignments are suggested for the peaks at 2136.7, 2138.5, and 2140.1 cmy1. The former two peaks are assigned to the CO monomer, while the last peak is attributed to the CO dimer. A temperature dependence, observed for the absorption at 2138.5 and 2140.1 cmy1, is explained by the coupling between librational modes and the lattice phonons. The increase in the CO dimer concentration upon annealing suggests the rearrangement of Ar toward the closely packed structure. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon monoxide ŽCO. is one of the simplest molecules, with only one vibrational feature expected in the mid-infrared region. However, the interpretation of the infrared ŽIR. spectra of CO in an Ar matrix has been the subject of controversy. Although a number papers have been published on the subject w1–12x, the assignment of the observed spectra has not been fully settled. The difficulties in the assignment are caused mainly by the following two anomalous phenomena observed in its spectrum w6x: Ž1. the intensity of the CO monomer spectrum in an Ar matrix depends on the sample temperature, and Ž2. aggregate species, such as the CO dimer or CO–H 2 O ŽH 2 O is a typical impurity., which exhibit additional absorption features in the same wavenumber region.
)
Corresponding author. E-mail: [email protected]
The two phenomena given above are responsible for the complicated behavior of the spectral features: center positions, intensities, and widths of absorption lines depend strongly on the experimental parameters such as concentration, deposition rate, deposition temperature, temperature cyclings, and impurities. To overcome the difficulties, we have measured the spectra for much wider choices of experimental parameters in the present study than those reported previously. Three peaks at 2136.7, 2138.5, and 2140.1 cmy1 Žnamed hereafter peaks A, B, and C, respectively. are observed. We suggest that peaks A and B be assigned to the CO monomer, while peak C be assigned to the dimer. Comparison between the present assignments and those made previously will be discussed in Section 4. A significant temperature dependence has been observed for the peak intensity of peaks B and C. This phenomenon will be explained in terms of the coupling between the librational motion of CO and the lattice vibration of the
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 4 7 - 7
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
154
solid Ar. The growth of the CO dimer upon annealing will be also discussed from a viewpoint of the rearrangement of the Ar crystal.
2. Experimental details A schematic diagram of the experimental apparatus is shown in Fig. 1. The matrix gas Ar Ž99.9995%. and the seeded gas CO Ž99.95%., both obtained commercially, were mixed with a total pressure less than 1 bar in a pyrex bulb connected to a vacuum line made of stainless steel tubing. The gaseous mixtures were prepared at least 4 h before the experiment by using standard manometric techniques. The storage time of 4 h is enough for the Ar–CO system to establish homogeneous mixing, since no differences were observed between the spectra measured after 4 h and after 24 h. The pressure ratios of the matrix gas to the seeded gas Ž MrS . used in this work were 100, 500, 1000, and 10 000. The premixed Ar–CO gases were deposited on a CsI substrate at a cryogenic temperature maintained
by a closed-cycle helium-refrigerator ŽJanis model CCS-150.. The substrate was fixed to a holder made of copper. Indium gaskets, which improve thermal conductance, were sandwiched between the substrate and the holder. An electric heater was wrapped around the cryotip under the substrate holder. The temperature of the sample was adjusted in the range from 8 to 324 K by a temperature controller ŽLakeShore model 330.. The temperature was monitored by two Si diode sensors mounted at the bottom and the top of the holder. A turbo-drag pump ŽBalzers model TSH 064D. was employed for evacuation. The pressure inside the cryostat is typically 10y7 torr at 8 K as monitored with a cold cathode vacuum gauge ŽBalzers model PKR250.. As illustrated in Fig. 1, the sampling and deposition vacuum line is equipped with a liquid nitrogen and a cold ethanol Ž- 173 K. trap to avoid water contamination in the sample as much as possible. Deposition was carried out slowly through a 1r8 in. diameter stainless steel tube with a spray-on angle normal to the substrate surface. The deposition rate was controlled to 0.3 mmol hy1 with a micrometer
Fig. 1. The experimental apparatus used in the present study is schematically illustrated.
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
needle valve by monitoring the pressure just downstream of the valve by a Baratron diaphragm gauge. The slow deposition rate was adopted even for very dilute samples of MrS s 10 000 in order to make the spectra as sharp as possible. Deposition temperatures in the present experiments were varied from 8 to 30 K. The IR spectra were recorded in the range from 1800 to 4000 cmy1 using a vacuum Fourier-transform IR spectrometer ŽBOMEM DA 3.36. at a resolution of 0.1 cmy1 . The IR radiation was taken from the spectrometer and focused onto the cold substrate. The transmitted radiation was again focused on an liquid nitrogen-cooled InSb detector. The optical path from the FTIR spectrometer to the detector was purged by N2 to remove atmospheric water. The incident angle of the infrared radiation was about 458 to the surface of the substrate.
3. Assignments The typical MrS ratio dependence of the spectra observed in the present study is shown in Fig. 2 for the CO stretching region. These spectra were obtained for the samples deposited at 8 K and they were recorded at the same temperature without annealing. The absorbance is scaled for the sake of
Fig. 2. The typical MrS ratio dependence of the spectra Žat 8 K. observed in the present study is demonstrated. The absorbance is scaled for the sake of comparison.
155
comparison. The two peaks at 2136.7 and 2138.5 cmy1 Žpeaks A and B. can be seen in all of those spectra, while a peak at 2140.1 cmy1 Žpeak C. appears only in the spectrum of MrS s 100 sample. It is, therefore, reasonable to assign peak C to the first smallest aggregation of CO in the Ar matrix, that is, the CO dimer. Peaks A and B, on the other hand, can be attributed to the CO monomer since these peaks are observed even at highly diluted samples and the relative intensity of the features is independent of MrS ratio.
4. Comparison with previous assignments 4.1. Peak A (2136.7 cm y 1) The interpretation of peak A is the most controversial issue of the three peaks. Dubost and AbouafMarguin w6x assigned this peak to the CO–N2 complex on the basis of their N2-doping experiments. Diem et al. w10x, and recently Han and Kim w12x, attributed the peak to the CO dimer. Givan et al. w11x claimed that peak A stemmed from the CO polymer. These interpretations, however, are inconsistent with our two observations: first, peak A is observed even for highly diluted samples, and the relative intensities between A and B are independent of the MrS ratio. Therefore peak A should not be assigned to aggregate species, such as ŽCO. 2 or ŽCO. n . Secondly, the amount of N2 as an impurity in our system originating form the degassing is estimated to be one order of magnitude less than that of H 2 O which was identified as the most dominant impurity in our experiments. In Fig. 2, however, no absorption signal of CO–H 2 O can be seen in the expected region around 2149 cmy1 w6,11x. Furthermore, no significant change in the spectrum has been observed for the samples doped with N2 with the concentration almost three orders of magnitude higher than that expected as an impurity. These facts indicate that peak A observed in the present experiment does not originate from the CO–N2 complex. Another assignment was given by Givan et al. w11x who claimed that peak A represents the CO monomer in unstable sites on the basis of their temperature cycling experiment. These authors re-
156
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
ported that the peak eventually diminished in intensity after annealing for 25 min at 28 K. However, in our experiment, the line did not show such behavior. Fig. 3 displays the observed time dependence of the spectral feature of CO in an Ar matrix maintained at 30 K. This figure shows that the CO molecules corresponding to peak A are stable and its absorption does not disappear in 60 min at 30 K. 4.2. Peak B (2138.5 cm y 1) Our assignment of peak B agrees with those made by Maki w1x and Dubost and Abouaf-Marguin w6x. Leroi et al. w3x and Davies and Hallam w5x claimed, however, that peak B is due to the CO polymer. This assignment is now discarded for reasons given above. The peak which they had assigned to the CO monomer, i.e. the peak near 2149 cmy1 , should be assigned to the CO–H 2 O complex, as already pointed out in Ref. w6x. The assignment made in Refs. w3,5x is based on the following observations: Ž1. the 2149 cmy1 line appears as the most intense peak for highly diluted samples deposited at 20 K, and Ž2. its relative intensity to peak B decreases with the increase in the concentration of CO. This has also been observed in the present study. We explain the phenomena in a different way: the formation of the CO–H 2 O complex is very efficient for the 20 K
deposition and most of the CO molecules form dimers with the impurity H 2 O if the number of the CO molecules is small Žhighly diluted sample.. Thus, by increasing the concentration of CO up to a certain value, the number of CO–H 2 O complexes increases. Since the concentration of H 2 O is limited in the present experiment Žestimated ratio of ArrH 2 O is ; 3000., the number of CO monomers exceeds that of the CO–H 2 O complex for high concentrations of CO. As expected, we have observed for the sample of MrS s 1000 deposited at 20 K that the line of the CO monomer is stronger than that of the CO–H 2 O complex. 4.3. Peak C (2140.1 cm y 1) Diem et al. w10x did not observe any distinct peaks near 2140 cmy1 . The highest concentration of the sample which they used was MrS s 1000. The ratio of the number of dimers to monomers Ž DrM . for this concentration can be estimated to be 0.012 Žsee Eq. Ž1. in Section 5.2... It is therefore difficult to observe the peak clearly for the sample of MrS s 1000. This is consistent with our experiment with the sample of MrS s 1000: peak C is not clear even after annealing to 30 K.
5. Discussion 5.1. The reÕersible change of peak intensity on the temperature
Fig. 3. The observed time development of spectral feature of CO in an Ar matrix maintained at 30 K is shown. The peak at 2136.7 cmy1 does not change in least 60 min at this temperature.
The effects of temperature cycling on the MrS s 100 sample is shown in Fig. 4a–c. The reversible change of peak intensity with temperature is clearly demonstrated in the spectra. The peak intensity of B decreases significantly at T s 30 K. This spectral behavior can be explained in terms of the librational motion of trapped molecules. The libration frequency seems to be so low that the libration motion is coupled strongly with the lattice vibrations of the solid Ar. By increasing the temperature, the molecules are distributed thermally to those coupled levels, and the line becomes broader and loses its peak intensity. Since this behavior can be seen not only in peak B but also in peak C, the CO dimer
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
157
annealing from Fig. 3c. Thus the number of dimers clearly increases upon annealing. This growth in the dimer concentration is considered from a statistical point of view. If the matrix has a fcc or hcp structure, and if seeded substances of interest are randomly distributed in substitutional matrix sites with a concentration designated by P, the ratio of DrM Žthe dimer defined here is a pair of two molecules which occupy two adjacent sites, and whose other adjacent sites are vacant. would be given by w20x 6
DrM s 12 P Ž 1 y P . .
Fig. 4. The effects of temperature cycling are shown for CO in an Ar matrix with MrSs100, sample deposited at 8 K: Ža. recorded at 8 K before annealing, Žb. recorded at 30 K, and Žc. recorded at 8 K after annealing to 30 K.
seems to be able to librate in an Ar matrix with a low frequency, probably because of its floppiness w13x. Peak A, on the other hand, does not show such behavior in spite of the fact that the peak is assigned to the CO monomer. We therefore presume that the librational motion of CO corresponding to peak A is of much higher frequency than the lattice vibrations of the surrounding Ar atoms. The temperature dependence of the spectral line broadening in the matrix has been discussed in the literature Žsee, e.g., Refs. w14–19x..
Ž 1.
In the case of MrS s 100 Žnamely P ; 0.01., this ratio is ; 0.113, which is almost comparable to, but somewhat larger than, the experimental value for after annealing of 0.085. The growth of the dimer upon annealing thus can be explained not only by introducing diffusion of CO in an Ar matrix but also by considering the annealing effect on the phase of the crystal as follows: just after deposition at 8 K, deposited particles on the substrate should be frozen almost immediately and should not be close packed. Upon annealing, crystal of the Ar–CO mixture would be rearranged toward the most stable structure. It is presumed that the Ar–CO mixture of MrS ) 100 forms crystals of fcc at low temperature w21x. The experimental value of DrM for after annealing does not exceed the one obtained from the statistical calculation. This fact suggests that diffusion of CO molecules in an Ar matrix does not occur easily upon annealing up to 30 K, in the conditions of the present study.
5.2. Growth of the CO dimer upon annealing Integrated intensities corresponding to peaks A, B and, C were obtained by fitting the spectral data shown in Fig. 4a,c with a Lorentzian shape function. The sum of the first two ŽA and B. integrated intensities and the last ŽC. one are denoted hereafter as Im and Id , respectively; the subscript ‘m’ represents the CO monomer and ‘d’ the CO dimer. Assuming that the transition moment of the CO stretching vibration does not depend on the size of the CO aggregation, the ratio of DrM in an Ar matrix can be estimated from the ratios of IdrIm . The ratios obtained from the observations are 0.048 before annealing measured from Fig. 3a, and 0.085 after
Acknowledgements We wish to express our thanks to Dr. K. Sugawara and Prof. M. Nakata for valuable comments and fruitful discussions. The help of Dr. F. Ito in the experimental work is acknowledged with thanks.
References w1x A.G. Maki, J. Chem. Phys. 35 Ž1961. 931. w2x H. Vu, M.R. Atwood, B. Vodar, J. Chem. Phys. 38 Ž1963. 2671.
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H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
w3x G.E. Leroi, G.E. Ewing, G.C. Pimentel, J. Chem. Phys. 40 Ž1964. 2298. w4x A.J. Barnes, H.E. Hallam, G.F. Scrimshaw, Trans. Faraday Soc. 65 Ž1969. 3172. w5x J.B. Davies, H.E. Hallam, J. Chem. Soc., Faraday Trans. II 68 Ž1972. 509. w6x H. Dubost, L. Abouaf-Marguin, Chem. Phys. Lett. 17 Ž1972. 269. w7x R.N. Perutz, J.J. Turner, J. Chem. Soc., Faraday Trans. II 69 Ž1973. 452. w8x G.J. Jiang, W.B. Person, K.G. Brown, J. Chem. Phys. 62 Ž1975. 1201. w9x H. Dubost, Chem. Phys. 12 Ž1976. 139. w10x M. Diem, T. Tso, E.K.C. Lee, Chem. Phys. 73 Ž1982. 283. w11x A. Givan, A. Loewenschuss, C.J. Nielsen, J. Chem. Soc., Faraday Trans. 92 Ž1996. 4927.
w12x H.S. Han, K. Kim, J. Mol. Struct. ŽTHEOCHEM. 418 Ž1997. 1. w13x M.D. Brookes, A.R.W. McKellar, Chem. Phys. Lett. 287 Ž1998. 365. w14x G.E. Ewing, J. Chem. Phys. 37 Ž1962. 2250. w15x K.O. Lee, Can. J. Phys. 49 Ž1971. 2018. w16x H. Metiu, G.E. Korzeniewski, D. Hone, Chem. Phys. Lett. 58 Ž1978. 473. w17x H. Dubost, A. Lecuyer, R. Charneau, Chem. Phys. Lett. 66 Ž1979. 191. w18x H. Dubost, R. Charneau, M. Harig, Chem. Phys. 69 Ž1982. 389. w19x I.L. Garzon, ´ E. Blaisten-Barojas, J. Chem. Phys. 83 Ž1985. 4311. w20x R.E. Behringer, J. Chem. Phys. 29 Ž1958. 537. w21x C.S. Barrett, L. Meyer, J. Chem. Phys. 43 Ž1965. 3502.
Chemical Physics Letters 311 Ž1999. 153–158 www.elsevier.nlrlocatercplett
Infrared spectroscopy of CO trapped in an argon matrix revisited Hisashi Abe a
a,)
, Harutoshi Takeo
a,b
, Koichi M.T. Yamada
a
National Institute for AdÕanced Interdisciplinary Research (NAIR), 1-1-4 Higashi, Tsukuba, Ibaraki, 305-8562, Japan b National Institute of Materials and Chemical Research (NIMC), 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 15 April 1999; in final form 15 June 1999
Abstract The infrared spectra of carbon monoxide in an argon matrix at cryogenic temperatures have been reinvestigated by FTIR spectroscopy over a wide range of experimental conditions, and new assignments are suggested for the peaks at 2136.7, 2138.5, and 2140.1 cmy1. The former two peaks are assigned to the CO monomer, while the last peak is attributed to the CO dimer. A temperature dependence, observed for the absorption at 2138.5 and 2140.1 cmy1, is explained by the coupling between librational modes and the lattice phonons. The increase in the CO dimer concentration upon annealing suggests the rearrangement of Ar toward the closely packed structure. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon monoxide ŽCO. is one of the simplest molecules, with only one vibrational feature expected in the mid-infrared region. However, the interpretation of the infrared ŽIR. spectra of CO in an Ar matrix has been the subject of controversy. Although a number papers have been published on the subject w1–12x, the assignment of the observed spectra has not been fully settled. The difficulties in the assignment are caused mainly by the following two anomalous phenomena observed in its spectrum w6x: Ž1. the intensity of the CO monomer spectrum in an Ar matrix depends on the sample temperature, and Ž2. aggregate species, such as the CO dimer or CO–H 2 O ŽH 2 O is a typical impurity., which exhibit additional absorption features in the same wavenumber region.
)
Corresponding author. E-mail: [email protected]
The two phenomena given above are responsible for the complicated behavior of the spectral features: center positions, intensities, and widths of absorption lines depend strongly on the experimental parameters such as concentration, deposition rate, deposition temperature, temperature cyclings, and impurities. To overcome the difficulties, we have measured the spectra for much wider choices of experimental parameters in the present study than those reported previously. Three peaks at 2136.7, 2138.5, and 2140.1 cmy1 Žnamed hereafter peaks A, B, and C, respectively. are observed. We suggest that peaks A and B be assigned to the CO monomer, while peak C be assigned to the dimer. Comparison between the present assignments and those made previously will be discussed in Section 4. A significant temperature dependence has been observed for the peak intensity of peaks B and C. This phenomenon will be explained in terms of the coupling between the librational motion of CO and the lattice vibration of the
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 4 7 - 7
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
154
solid Ar. The growth of the CO dimer upon annealing will be also discussed from a viewpoint of the rearrangement of the Ar crystal.
2. Experimental details A schematic diagram of the experimental apparatus is shown in Fig. 1. The matrix gas Ar Ž99.9995%. and the seeded gas CO Ž99.95%., both obtained commercially, were mixed with a total pressure less than 1 bar in a pyrex bulb connected to a vacuum line made of stainless steel tubing. The gaseous mixtures were prepared at least 4 h before the experiment by using standard manometric techniques. The storage time of 4 h is enough for the Ar–CO system to establish homogeneous mixing, since no differences were observed between the spectra measured after 4 h and after 24 h. The pressure ratios of the matrix gas to the seeded gas Ž MrS . used in this work were 100, 500, 1000, and 10 000. The premixed Ar–CO gases were deposited on a CsI substrate at a cryogenic temperature maintained
by a closed-cycle helium-refrigerator ŽJanis model CCS-150.. The substrate was fixed to a holder made of copper. Indium gaskets, which improve thermal conductance, were sandwiched between the substrate and the holder. An electric heater was wrapped around the cryotip under the substrate holder. The temperature of the sample was adjusted in the range from 8 to 324 K by a temperature controller ŽLakeShore model 330.. The temperature was monitored by two Si diode sensors mounted at the bottom and the top of the holder. A turbo-drag pump ŽBalzers model TSH 064D. was employed for evacuation. The pressure inside the cryostat is typically 10y7 torr at 8 K as monitored with a cold cathode vacuum gauge ŽBalzers model PKR250.. As illustrated in Fig. 1, the sampling and deposition vacuum line is equipped with a liquid nitrogen and a cold ethanol Ž- 173 K. trap to avoid water contamination in the sample as much as possible. Deposition was carried out slowly through a 1r8 in. diameter stainless steel tube with a spray-on angle normal to the substrate surface. The deposition rate was controlled to 0.3 mmol hy1 with a micrometer
Fig. 1. The experimental apparatus used in the present study is schematically illustrated.
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
needle valve by monitoring the pressure just downstream of the valve by a Baratron diaphragm gauge. The slow deposition rate was adopted even for very dilute samples of MrS s 10 000 in order to make the spectra as sharp as possible. Deposition temperatures in the present experiments were varied from 8 to 30 K. The IR spectra were recorded in the range from 1800 to 4000 cmy1 using a vacuum Fourier-transform IR spectrometer ŽBOMEM DA 3.36. at a resolution of 0.1 cmy1 . The IR radiation was taken from the spectrometer and focused onto the cold substrate. The transmitted radiation was again focused on an liquid nitrogen-cooled InSb detector. The optical path from the FTIR spectrometer to the detector was purged by N2 to remove atmospheric water. The incident angle of the infrared radiation was about 458 to the surface of the substrate.
3. Assignments The typical MrS ratio dependence of the spectra observed in the present study is shown in Fig. 2 for the CO stretching region. These spectra were obtained for the samples deposited at 8 K and they were recorded at the same temperature without annealing. The absorbance is scaled for the sake of
Fig. 2. The typical MrS ratio dependence of the spectra Žat 8 K. observed in the present study is demonstrated. The absorbance is scaled for the sake of comparison.
155
comparison. The two peaks at 2136.7 and 2138.5 cmy1 Žpeaks A and B. can be seen in all of those spectra, while a peak at 2140.1 cmy1 Žpeak C. appears only in the spectrum of MrS s 100 sample. It is, therefore, reasonable to assign peak C to the first smallest aggregation of CO in the Ar matrix, that is, the CO dimer. Peaks A and B, on the other hand, can be attributed to the CO monomer since these peaks are observed even at highly diluted samples and the relative intensity of the features is independent of MrS ratio.
4. Comparison with previous assignments 4.1. Peak A (2136.7 cm y 1) The interpretation of peak A is the most controversial issue of the three peaks. Dubost and AbouafMarguin w6x assigned this peak to the CO–N2 complex on the basis of their N2-doping experiments. Diem et al. w10x, and recently Han and Kim w12x, attributed the peak to the CO dimer. Givan et al. w11x claimed that peak A stemmed from the CO polymer. These interpretations, however, are inconsistent with our two observations: first, peak A is observed even for highly diluted samples, and the relative intensities between A and B are independent of the MrS ratio. Therefore peak A should not be assigned to aggregate species, such as ŽCO. 2 or ŽCO. n . Secondly, the amount of N2 as an impurity in our system originating form the degassing is estimated to be one order of magnitude less than that of H 2 O which was identified as the most dominant impurity in our experiments. In Fig. 2, however, no absorption signal of CO–H 2 O can be seen in the expected region around 2149 cmy1 w6,11x. Furthermore, no significant change in the spectrum has been observed for the samples doped with N2 with the concentration almost three orders of magnitude higher than that expected as an impurity. These facts indicate that peak A observed in the present experiment does not originate from the CO–N2 complex. Another assignment was given by Givan et al. w11x who claimed that peak A represents the CO monomer in unstable sites on the basis of their temperature cycling experiment. These authors re-
156
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
ported that the peak eventually diminished in intensity after annealing for 25 min at 28 K. However, in our experiment, the line did not show such behavior. Fig. 3 displays the observed time dependence of the spectral feature of CO in an Ar matrix maintained at 30 K. This figure shows that the CO molecules corresponding to peak A are stable and its absorption does not disappear in 60 min at 30 K. 4.2. Peak B (2138.5 cm y 1) Our assignment of peak B agrees with those made by Maki w1x and Dubost and Abouaf-Marguin w6x. Leroi et al. w3x and Davies and Hallam w5x claimed, however, that peak B is due to the CO polymer. This assignment is now discarded for reasons given above. The peak which they had assigned to the CO monomer, i.e. the peak near 2149 cmy1 , should be assigned to the CO–H 2 O complex, as already pointed out in Ref. w6x. The assignment made in Refs. w3,5x is based on the following observations: Ž1. the 2149 cmy1 line appears as the most intense peak for highly diluted samples deposited at 20 K, and Ž2. its relative intensity to peak B decreases with the increase in the concentration of CO. This has also been observed in the present study. We explain the phenomena in a different way: the formation of the CO–H 2 O complex is very efficient for the 20 K
deposition and most of the CO molecules form dimers with the impurity H 2 O if the number of the CO molecules is small Žhighly diluted sample.. Thus, by increasing the concentration of CO up to a certain value, the number of CO–H 2 O complexes increases. Since the concentration of H 2 O is limited in the present experiment Žestimated ratio of ArrH 2 O is ; 3000., the number of CO monomers exceeds that of the CO–H 2 O complex for high concentrations of CO. As expected, we have observed for the sample of MrS s 1000 deposited at 20 K that the line of the CO monomer is stronger than that of the CO–H 2 O complex. 4.3. Peak C (2140.1 cm y 1) Diem et al. w10x did not observe any distinct peaks near 2140 cmy1 . The highest concentration of the sample which they used was MrS s 1000. The ratio of the number of dimers to monomers Ž DrM . for this concentration can be estimated to be 0.012 Žsee Eq. Ž1. in Section 5.2... It is therefore difficult to observe the peak clearly for the sample of MrS s 1000. This is consistent with our experiment with the sample of MrS s 1000: peak C is not clear even after annealing to 30 K.
5. Discussion 5.1. The reÕersible change of peak intensity on the temperature
Fig. 3. The observed time development of spectral feature of CO in an Ar matrix maintained at 30 K is shown. The peak at 2136.7 cmy1 does not change in least 60 min at this temperature.
The effects of temperature cycling on the MrS s 100 sample is shown in Fig. 4a–c. The reversible change of peak intensity with temperature is clearly demonstrated in the spectra. The peak intensity of B decreases significantly at T s 30 K. This spectral behavior can be explained in terms of the librational motion of trapped molecules. The libration frequency seems to be so low that the libration motion is coupled strongly with the lattice vibrations of the solid Ar. By increasing the temperature, the molecules are distributed thermally to those coupled levels, and the line becomes broader and loses its peak intensity. Since this behavior can be seen not only in peak B but also in peak C, the CO dimer
H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
157
annealing from Fig. 3c. Thus the number of dimers clearly increases upon annealing. This growth in the dimer concentration is considered from a statistical point of view. If the matrix has a fcc or hcp structure, and if seeded substances of interest are randomly distributed in substitutional matrix sites with a concentration designated by P, the ratio of DrM Žthe dimer defined here is a pair of two molecules which occupy two adjacent sites, and whose other adjacent sites are vacant. would be given by w20x 6
DrM s 12 P Ž 1 y P . .
Fig. 4. The effects of temperature cycling are shown for CO in an Ar matrix with MrSs100, sample deposited at 8 K: Ža. recorded at 8 K before annealing, Žb. recorded at 30 K, and Žc. recorded at 8 K after annealing to 30 K.
seems to be able to librate in an Ar matrix with a low frequency, probably because of its floppiness w13x. Peak A, on the other hand, does not show such behavior in spite of the fact that the peak is assigned to the CO monomer. We therefore presume that the librational motion of CO corresponding to peak A is of much higher frequency than the lattice vibrations of the surrounding Ar atoms. The temperature dependence of the spectral line broadening in the matrix has been discussed in the literature Žsee, e.g., Refs. w14–19x..
Ž 1.
In the case of MrS s 100 Žnamely P ; 0.01., this ratio is ; 0.113, which is almost comparable to, but somewhat larger than, the experimental value for after annealing of 0.085. The growth of the dimer upon annealing thus can be explained not only by introducing diffusion of CO in an Ar matrix but also by considering the annealing effect on the phase of the crystal as follows: just after deposition at 8 K, deposited particles on the substrate should be frozen almost immediately and should not be close packed. Upon annealing, crystal of the Ar–CO mixture would be rearranged toward the most stable structure. It is presumed that the Ar–CO mixture of MrS ) 100 forms crystals of fcc at low temperature w21x. The experimental value of DrM for after annealing does not exceed the one obtained from the statistical calculation. This fact suggests that diffusion of CO molecules in an Ar matrix does not occur easily upon annealing up to 30 K, in the conditions of the present study.
5.2. Growth of the CO dimer upon annealing Integrated intensities corresponding to peaks A, B and, C were obtained by fitting the spectral data shown in Fig. 4a,c with a Lorentzian shape function. The sum of the first two ŽA and B. integrated intensities and the last ŽC. one are denoted hereafter as Im and Id , respectively; the subscript ‘m’ represents the CO monomer and ‘d’ the CO dimer. Assuming that the transition moment of the CO stretching vibration does not depend on the size of the CO aggregation, the ratio of DrM in an Ar matrix can be estimated from the ratios of IdrIm . The ratios obtained from the observations are 0.048 before annealing measured from Fig. 3a, and 0.085 after
Acknowledgements We wish to express our thanks to Dr. K. Sugawara and Prof. M. Nakata for valuable comments and fruitful discussions. The help of Dr. F. Ito in the experimental work is acknowledged with thanks.
References w1x A.G. Maki, J. Chem. Phys. 35 Ž1961. 931. w2x H. Vu, M.R. Atwood, B. Vodar, J. Chem. Phys. 38 Ž1963. 2671.
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H. Abe et al.r Chemical Physics Letters 311 (1999) 153–158
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