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Rotational tunnelling and ammonia sites in Ar and N2 matrices
Physica B 180 & 181 (1992) North-Holland
Rotational
674-676
tunnelling
M. Havighorst”,
and ammonia sites in Ar and N, matrices
M. Pragera and W. Langelb
“Institut fiir Festkorperforschung der KFA Jiilich, Postfach 1913, W-51 70 Jiilich, Germany bInstitut fiir Physikalische Chemie der Universitbt Siegen, Postfach 101240, W-5900 Siegen, Germany
Ammonia isolated in Ar and N, matrices with concentrations of 0.5, 1.1 and 4% was investigated by high resolution inelastic neutron scattering in a temperature range from 2 to 25 K. In Ar, substitutional monomeric and dimeric sites are found which are related to a three- and a one-dimensional rotor, respectively. The one-dimensional rotor transition is split due to coupling.
The three-dimensional
rotor
is missing
in the N, matrix.
1. Introduction
0
-1
2
1
3 20
Ammonia molecules isolated in inert gas matrices have been investigated for more than 20 years because of the supposed combination of quantum rotation and inversion. Most work was done by IR spectroscopic methods [l-5]. All authors conclude that the inversion mode of ammonia in argon is slightly hindered compared to the free molecule while a hindered rotation is partly interpreted as one-dimensional, partly as threedimensional. First inelastic neutron scattering experiments (INS) showed a single rotational transition which tentatively was assigned to a one-dimensional rotor [6]. To test these models, further INS measurements were done in an extended energy range. 2. Experimental
10
5
0
and results
-1
Gaseous ammonia and argon were premixed in a volume outside the cryostat. Ammonia concentrations of 0.5, 1.1 and 4 mol% were determined from the partial pressures of the two constituents. Ammonia is insoluble in argon. Thus solid solutions have to be produced by condensing the gas directly into the solid phase (matrix isolation technique). Eight liters of gas at standard conditions were condensed at a rate of 60cm3/min as in earlier INS experiments [6]. Inelastic neutron scattering spectra in the energy range ]A,??]< 3 meV were obtained using the time of flight (TOF) spectrometer IN5 of the Institut LaueLangevin, Grenoble. Incident wavelengths of A = 3.8 and 6.5 8, were used. Figure 1 shows the spectra of 0.5, 1.1 and 4% ammonia in argon at a sample temperature of T = 5 K with A = 3.8 A. In this comparison the intensities in the various spectra were scaled to that of the 0.5% sample. The inelastic lines do not shift with concentration. With increasing ammonia concentration, the 1.9 meV transition becomes dominant. The spectrum of 1.1% ammonia in argon taken at T = 5 K with A = 6.5 8, is given in fig. 2. The solid lines in figs. 1 and 2 represent fits with a Gaussian for each inelastic line and a resolution function 0921-4526/92/$05.00
15
0
1992 - Elsevier
Science
Publishers
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1
2
3
energy transfer ImeVl Fig. 1. Spectra of NH, in Ar with ammonia concentrations of 0.5, 1.1 and 4%. All data are scaled to the 0.5% spectrum. Sample temperatures T = 5 K. Spectrometer: IN5 of the ILL. Incident wavelength A = 3.8 A. The solid lines are fits with Gaussians. The solid line at the bottom describes the broad
background. for the elastic line. Further, a broad background is found below the sharp inelastic lines. It is described with a Boltzmann weighted Gaussian centered at zero energy transfer. In the temperature range from 2 to 25 K no shift of the inelastic lines was observed in the case of 1.1% ammonia in argon. The transition at 1.9 meV is damped faster than the others. The temperature and Table 1 Positions of the fitted Gaussians in figs. 1 and 2 and in N, matrices. Matrix
Energy
Ar
0.648
N*
B.V. All rights
reserved
transfer
(meV) 0.740 0.638
1.91 1.54
2.69 2.16
M. Havighorst et al. I Rotational tunnelling and ammonia sites in Ar and N2 matrices
12
10
8 6
6
-.5
energy
0
transfer
.5
ImeVl
Fig. 2. Spectrum of 1.1% NH, in Ar. Sample temperatures T = 5 K. Spectrometer: IN5 of the ILL. Incident wavelength A = 6.5 A. The solid lines are fits with Gaussians.
concentration table 1.
independent
line positions are given in
3. Discussion It is generally assumed that the ammonia molecules occupy substitutional sites, since the van der Waals radii of the argon atom and the NH, molecule are close to each other: rAr = 1.91 A, rNH3 = 1.96 A. In the case of statistical occupation of sites, monomers are dominant at very low concentration while dimers and higher multimers become increasingly important at higher concentration. We observe two different sets of tunnel lines in fig. 1: the intensity of the line at 1.91 meV increases with decreasing concentration, while that of the lines at 0.74 and 2.69meV becomes smaller. Also their temperature dependence (not shown here) is different. According to this systematic behaviour, we assign the transition at 1.91 meV to an isolated ammonia molecule (monomer). This molecule induces a dipole moment to the nearest argon neighbors. A simple calculation based on dipole-dipole interactions shows that there is no energetically preferred orientation of an ammonia monomer. For this reason and because of the large transition energy, the rotation of an isolated NH, molecule on a substitutional site is three-dimensional and almost free. The ground state of a totally free symmetric rotor with rotational constants of the ammonia molecule (B, = 0.782 meV around symmetry axis, B, = 1.23 meV) is a doublet at 2.01 and 2.46 meV with an intensity ratio of 3.2 : 1 [7]. If the 1.91 meV line is related to the 2.01 meV transition of the totally free rotor, one should also observe a weak line between 2.3 and
675
2.4 meV. Such a line seems to be hidden in the broad background. According to the concentration dependence of its intensity, the line at 0.74meV is assigned to dimers. Because of the similar temperature and concentration behavior the transition at 2.69meV should be part of the same rotor system. These two energies can be described by a sixfold potential of 12 meV but not by a three- or twelvefold potential. The 0.74 meV refer to the transition O-+ 1, the 2.69 meV to the transition 0+2. The dominating line of the three-dimensional rotor at 1.91 meV hinders the observation of the corresponding l-+2 transition at 1.97 meV The splitting of the ground state (see fig. 2) can be explained quantitatively by a coupling of the two neighboring ammonia molecules. The formalism to handle two coupled onedimensional rotors is described in detail in [9]. This, again, is a confirmation of assigning this transition to dimers. A first principles calculation of the structure of two interacting ammonia molecules (dimer) yielded a head to head configuration with the molecular symmetry axes parallel [8]. The nitrogen atoms interact with the protons of the second molecule similar to hydrogen bonds. This is the microscopic reason for the observed coupling. If the symmetry axes are oriented along the diagonal face in the FCC argon lattice, the twofold symmetry of the environment together with the threefold symmetry of the molecule leads to the observed effective sixfold potential for the one-dimensional rotor. The broad background is interpreted as the fingerprint of less well defined sites related to a broad distribution of rotational potentials. From the ratio of the inelastic to the elastic scattering, the occurrence probabilities of the tunnelling ammonia molecules can be estimated on the basis of the tunnelling structure factors. Only 19% of the ammonia molecules take part in the observed tunneling processes. Furthermore the dimers appear with a higher than statistical probability. A possible explanation could be a preformation of the ammonia molecules in the gas phase. There is a serious contradiction to earlier IR results. No ammonia inversion is needed to describe our spectra. Instead two different sites, not extracted from the IR spectra, are found which follows naturally from the model of substitutional defects. The spectra of ammonia in nitrogen look quite similar to those of ammonia in argon. The transitions are shifted to lower energies and are shown in table 1. The line around 1.5 meV is considerably weaker, however. For this reason we can explain all the lines by the presence of just a one-dimensional rotor with a sixfold potential. Due to the very different matrix, the microscopic picture for the one-dimensional rotor is different and likely related to a substitutional mono-
676
M. Havighorst et al. I Rotational tunnelling and ammonia sites in Ar and N2 matrices
merit defect. The interpretation of IR spectroscopic data also led to a one-dimensional rotor [4], but the predicted inversion is not observed in our INS data.
4. Outlook To confirm the model outlined, it is intended to study matrices with lower ammonia concentration and in a further extended energy range. The Q-dependence of the inelastic transitions will allow an even more definite discrimination between tunnelling and inversion. The ammonia dynamics in other rare gas matrices might yield helpful systematic information.
References ill D.E.
Milligan and R.M. Hexter, J. Chem. Phys. 34 (1961) 1009. PI J.A. Cugley and A.D.E. Pullin, Spectrochim. Acta A 29 (1973) 1665. M.E. Jacox and D.E. Milhgan, J. [31 L. Abouaf-Marguin, Mol. Spectrosc. 67 (1977) 34. [41 B. Nelander, Chem. Phys. 87 (1984) 283. and A. Lakhlifi, J. Chem. Phys. 91 (1989) [51 C.Girardet 1423 and 2171. bl M. Prager and C.J. Carlile, Chem. Phys. Lett. 173 (1990) 524. Doktorarbeit, Universitit Kiel (1991). [71 B. Asmussen, PI S. Siizer and L. Andrews, J. Chem. Phys. 87 (1987) 5131. 191 W. Hausler and A. Huller, Z. Phys. B 59 (1985) 177.
Rotational
674-676
tunnelling
M. Havighorst”,
and ammonia sites in Ar and N, matrices
M. Pragera and W. Langelb
“Institut fiir Festkorperforschung der KFA Jiilich, Postfach 1913, W-51 70 Jiilich, Germany bInstitut fiir Physikalische Chemie der Universitbt Siegen, Postfach 101240, W-5900 Siegen, Germany
Ammonia isolated in Ar and N, matrices with concentrations of 0.5, 1.1 and 4% was investigated by high resolution inelastic neutron scattering in a temperature range from 2 to 25 K. In Ar, substitutional monomeric and dimeric sites are found which are related to a three- and a one-dimensional rotor, respectively. The one-dimensional rotor transition is split due to coupling.
The three-dimensional
rotor
is missing
in the N, matrix.
1. Introduction
0
-1
2
1
3 20
Ammonia molecules isolated in inert gas matrices have been investigated for more than 20 years because of the supposed combination of quantum rotation and inversion. Most work was done by IR spectroscopic methods [l-5]. All authors conclude that the inversion mode of ammonia in argon is slightly hindered compared to the free molecule while a hindered rotation is partly interpreted as one-dimensional, partly as threedimensional. First inelastic neutron scattering experiments (INS) showed a single rotational transition which tentatively was assigned to a one-dimensional rotor [6]. To test these models, further INS measurements were done in an extended energy range. 2. Experimental
10
5
0
and results
-1
Gaseous ammonia and argon were premixed in a volume outside the cryostat. Ammonia concentrations of 0.5, 1.1 and 4 mol% were determined from the partial pressures of the two constituents. Ammonia is insoluble in argon. Thus solid solutions have to be produced by condensing the gas directly into the solid phase (matrix isolation technique). Eight liters of gas at standard conditions were condensed at a rate of 60cm3/min as in earlier INS experiments [6]. Inelastic neutron scattering spectra in the energy range ]A,??]< 3 meV were obtained using the time of flight (TOF) spectrometer IN5 of the Institut LaueLangevin, Grenoble. Incident wavelengths of A = 3.8 and 6.5 8, were used. Figure 1 shows the spectra of 0.5, 1.1 and 4% ammonia in argon at a sample temperature of T = 5 K with A = 3.8 A. In this comparison the intensities in the various spectra were scaled to that of the 0.5% sample. The inelastic lines do not shift with concentration. With increasing ammonia concentration, the 1.9 meV transition becomes dominant. The spectrum of 1.1% ammonia in argon taken at T = 5 K with A = 6.5 8, is given in fig. 2. The solid lines in figs. 1 and 2 represent fits with a Gaussian for each inelastic line and a resolution function 0921-4526/92/$05.00
15
0
1992 - Elsevier
Science
Publishers
0
1
2
3
energy transfer ImeVl Fig. 1. Spectra of NH, in Ar with ammonia concentrations of 0.5, 1.1 and 4%. All data are scaled to the 0.5% spectrum. Sample temperatures T = 5 K. Spectrometer: IN5 of the ILL. Incident wavelength A = 3.8 A. The solid lines are fits with Gaussians. The solid line at the bottom describes the broad
background. for the elastic line. Further, a broad background is found below the sharp inelastic lines. It is described with a Boltzmann weighted Gaussian centered at zero energy transfer. In the temperature range from 2 to 25 K no shift of the inelastic lines was observed in the case of 1.1% ammonia in argon. The transition at 1.9 meV is damped faster than the others. The temperature and Table 1 Positions of the fitted Gaussians in figs. 1 and 2 and in N, matrices. Matrix
Energy
Ar
0.648
N*
B.V. All rights
reserved
transfer
(meV) 0.740 0.638
1.91 1.54
2.69 2.16
M. Havighorst et al. I Rotational tunnelling and ammonia sites in Ar and N2 matrices
12
10
8 6
6
-.5
energy
0
transfer
.5
ImeVl
Fig. 2. Spectrum of 1.1% NH, in Ar. Sample temperatures T = 5 K. Spectrometer: IN5 of the ILL. Incident wavelength A = 6.5 A. The solid lines are fits with Gaussians.
concentration table 1.
independent
line positions are given in
3. Discussion It is generally assumed that the ammonia molecules occupy substitutional sites, since the van der Waals radii of the argon atom and the NH, molecule are close to each other: rAr = 1.91 A, rNH3 = 1.96 A. In the case of statistical occupation of sites, monomers are dominant at very low concentration while dimers and higher multimers become increasingly important at higher concentration. We observe two different sets of tunnel lines in fig. 1: the intensity of the line at 1.91 meV increases with decreasing concentration, while that of the lines at 0.74 and 2.69meV becomes smaller. Also their temperature dependence (not shown here) is different. According to this systematic behaviour, we assign the transition at 1.91 meV to an isolated ammonia molecule (monomer). This molecule induces a dipole moment to the nearest argon neighbors. A simple calculation based on dipole-dipole interactions shows that there is no energetically preferred orientation of an ammonia monomer. For this reason and because of the large transition energy, the rotation of an isolated NH, molecule on a substitutional site is three-dimensional and almost free. The ground state of a totally free symmetric rotor with rotational constants of the ammonia molecule (B, = 0.782 meV around symmetry axis, B, = 1.23 meV) is a doublet at 2.01 and 2.46 meV with an intensity ratio of 3.2 : 1 [7]. If the 1.91 meV line is related to the 2.01 meV transition of the totally free rotor, one should also observe a weak line between 2.3 and
675
2.4 meV. Such a line seems to be hidden in the broad background. According to the concentration dependence of its intensity, the line at 0.74meV is assigned to dimers. Because of the similar temperature and concentration behavior the transition at 2.69meV should be part of the same rotor system. These two energies can be described by a sixfold potential of 12 meV but not by a three- or twelvefold potential. The 0.74 meV refer to the transition O-+ 1, the 2.69 meV to the transition 0+2. The dominating line of the three-dimensional rotor at 1.91 meV hinders the observation of the corresponding l-+2 transition at 1.97 meV The splitting of the ground state (see fig. 2) can be explained quantitatively by a coupling of the two neighboring ammonia molecules. The formalism to handle two coupled onedimensional rotors is described in detail in [9]. This, again, is a confirmation of assigning this transition to dimers. A first principles calculation of the structure of two interacting ammonia molecules (dimer) yielded a head to head configuration with the molecular symmetry axes parallel [8]. The nitrogen atoms interact with the protons of the second molecule similar to hydrogen bonds. This is the microscopic reason for the observed coupling. If the symmetry axes are oriented along the diagonal face in the FCC argon lattice, the twofold symmetry of the environment together with the threefold symmetry of the molecule leads to the observed effective sixfold potential for the one-dimensional rotor. The broad background is interpreted as the fingerprint of less well defined sites related to a broad distribution of rotational potentials. From the ratio of the inelastic to the elastic scattering, the occurrence probabilities of the tunnelling ammonia molecules can be estimated on the basis of the tunnelling structure factors. Only 19% of the ammonia molecules take part in the observed tunneling processes. Furthermore the dimers appear with a higher than statistical probability. A possible explanation could be a preformation of the ammonia molecules in the gas phase. There is a serious contradiction to earlier IR results. No ammonia inversion is needed to describe our spectra. Instead two different sites, not extracted from the IR spectra, are found which follows naturally from the model of substitutional defects. The spectra of ammonia in nitrogen look quite similar to those of ammonia in argon. The transitions are shifted to lower energies and are shown in table 1. The line around 1.5 meV is considerably weaker, however. For this reason we can explain all the lines by the presence of just a one-dimensional rotor with a sixfold potential. Due to the very different matrix, the microscopic picture for the one-dimensional rotor is different and likely related to a substitutional mono-
676
M. Havighorst et al. I Rotational tunnelling and ammonia sites in Ar and N2 matrices
merit defect. The interpretation of IR spectroscopic data also led to a one-dimensional rotor [4], but the predicted inversion is not observed in our INS data.
4. Outlook To confirm the model outlined, it is intended to study matrices with lower ammonia concentration and in a further extended energy range. The Q-dependence of the inelastic transitions will allow an even more definite discrimination between tunnelling and inversion. The ammonia dynamics in other rare gas matrices might yield helpful systematic information.
References ill D.E.
Milligan and R.M. Hexter, J. Chem. Phys. 34 (1961) 1009. PI J.A. Cugley and A.D.E. Pullin, Spectrochim. Acta A 29 (1973) 1665. M.E. Jacox and D.E. Milhgan, J. [31 L. Abouaf-Marguin, Mol. Spectrosc. 67 (1977) 34. [41 B. Nelander, Chem. Phys. 87 (1984) 283. and A. Lakhlifi, J. Chem. Phys. 91 (1989) [51 C.Girardet 1423 and 2171. bl M. Prager and C.J. Carlile, Chem. Phys. Lett. 173 (1990) 524. Doktorarbeit, Universitit Kiel (1991). [71 B. Asmussen, PI S. Siizer and L. Andrews, J. Chem. Phys. 87 (1987) 5131. 191 W. Hausler and A. Huller, Z. Phys. B 59 (1985) 177.