Inelastic neutron scattering of hydrogen trapped in solid
argon
W. Langel
To cite this version:
W. Langel. Inelastic neutron scattering of hydrogen trapped in solid argon. Revue de
Physique Appliquee, 1984, 19 (9), pp.755-757. <10.1051/rphysap:01984001909075500>. <jpa00245253>
HAL Id: jpa-00245253
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Submitted on 1 Jan 1984
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Revue
Phys. Appl. 19 (1984) 755-757
Inelastic neutron
W.
SEPTEMBRE
scattering of hydrogen trapped
1984,
755
in solid argon
Langel
Institut
Laue-Langevin, 156X,
38042 Grenoble Cedex, France
La diffusion inélastique des neutrons par des molécules d’hydrogène isolées dans une matrice d’argon
l’hydrogène pur dans la phase solide a été étudiée. Les pics de la rotation libre de l’hydrogène sont mesurés
dans les deux échantillons, et aucun décalage de position n’est observé entre les deux spectres. Les spectres de
l’hydrogène pur montrent des effets du recul importants pour un transfert de moment élevé (7 Å-1), contrairement
aux spectres de l’hydrogène dans la matrice d’argon.
Résumé.
2014
et par
Molecular hydrogen was isolated in an argon matrix. Its inelastic neutron scattering functions at
transfers of 1 to 7 Å-1were compared with those of solid hydrogen. Both samples show free rotation
without line shift. At high momentum transfer (7 Å-1), the spectrum of pure hydrogen is affected by recoil, whereas
no recoil effects could be seen in matrix isolated H,.
Abstract
2014
momentum
1. Introduction.
molecules in an inert environment is a
technique of preparing samples for different kinds of spectroscopy [1]. Especially optical
absorption and fluorescence spectroscopy have been
applied to a large number of matrix-isolated molecules.
Molecular spectroscopy in matrices is interesting
for two reasons mainly :
Trapping
common
[4] worked at a high incident energy (65 meV)
and found broad lines, which they interpreted as due
to recoil effects.
In this work the inelastic neutron scattering of
hydrogen in a matrix will be described and compared
with the well understood spectra of pure hydrogen.
ner
2.
ExperimentaL
The main difficulty of a matrix experiment is the preIn many cases, the interaction between the
paration of the sample. In optical spectroscopy,
matrix and the trapped molecules is very weak com- where matrix
techniques are most commonly applied,
pared with the energies of intramolecular transitions layers of a thickness of about 0.1 mm are studied,
(e.g. internal vibrations), and the molecule can be which obviously must have a perfect optical quality.
studied under conditions similar to those in the gas
They contain about 5 mmol of the host and 0.01 mmol
phase. For neutron scattering this means, that sam- of the impurity. For neutron scattering, large samples
ples of a high particle densities can be prepared, in of some cm’ of volume containing about 500 mmol
which the intermolecular interactions are still quite matrix material and 5 to 10 mmol of the
impurity
small.
(i.e. 5 x 1021 molec.) have to be prepared The impuThe existing small intermolecular interactions
rity should have a high scattering cross section (e.g.
cause shifts of rotational lines and local modes of the
contain hydrogen atoms), since its concentration is
trapped molecules. By measuring these effects, inter- limited, while the one of the matrix should be small.
molecular potentials can be found.
This holds for. the most important hosts as argon
The neutron scattering of molecular hydrogen was and krypton, as well as for neon and SF6, but not
intensively studied until now, both experimentally for hydrocarbon glasses.
There are two methods of preparing these samples :
and theoretically [2]. In solid hydrogen, a maximum
in the phonon density of states was found at 5.4 meV Either matrix and impurity are condensed into the
[3]. The ortho para- transition in the solid is found sample container as a liquid solution and then frozen
to form a solid, or a gas mixture of both components
at energies of 13.5 to 14.6 meV, whereas the gas
A
is
frozen directly at the cold walls of the sample
value
is
meV.
14.6
of
the
work
on
major part
phase
the
state.
with
container
deals
Whittemore
and
Danliquid
(vapour deposition).
H2
-
-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01984001909075500
756
The first possibility is limited to impurities, which
dissolve readily in the matrix in the liquid phase and
do not separate and cluster during the freezing itself.
Whereas examples of such systems are well known [5],
many impurities have an extremely low solubility
in the matrix near the freezing point and therefore
cannot be isolated in a solid matrix by this method.
As the behaviour of hydrogen in freezing argon was
not known, the second method was chosen for the
present experiment. The samples were prepared in a
liquid helium cryostat An aluminium cylinder of a
length of 100 mm and an inner diameter of 15 mm was
connected to a stainless steel inlet tube. The cylinder
is cooled by 1 mbar of helium exchange gas. The
inlet tube was heated by a thermocoax wire. The
temperature at the sample can was measured by a
30 Ohms Fe-Rh-resistor.
The gas to be deposited, initially was mixed in a
stainless steel cylinder of a volume of 1 1. The gases
used were argon (L’Air Liquide, 99.995 % purity)
and hydrogen (L’Air Liquide, 99.995 % purity). The
mixing ratio was controlled by the partial pressures
of the gases in the cylinder. For that purpose, two
piezoresistive pressure sensors (Keller, 0 to 5 bar and
0 to 50 bar) were employed At least one day time was
given to the gas after ’filling to the cylinder in order
to obtain complete mixing. The gas flow during deposition was controlled by a TYLAN-gas flow controller
to be about 40 cm3/min. All essential parameters for
the condensation the sample were listed on a chart
recorder.
Since the evaporization temperatures of argon and
hydrogen are very different (21 K and 87 K), it appeared necessary to check, whether a separation of host
and impurity during sample deposition had taken
place. This was done by two methods :
-
After
a
3. Results and discussion.
Figure la shows the spectrum of H2 in argon at low
scattering angles. It has to be compared with the
and
above
the
temperature
deposition, the sample
at the same time heated to
The results were evaluated with the help of a newly
developed program on the DEC-10 computer of the
ILL. The spectra were first corrected for scattering
of the sample can and in a separate step for the scattering of the pure matrix, which had also been recorded.
In order to reduce the scatter, the raw data were
smoothed. They were, however, not symmetrized,
since the ratio of ortho- and para-modification of the
hydrogen in the samples was not in thermal equilibrium with the matrix.
was
pumped
boiling point of hydrogen. Only hydrogen trapped
in the argon thus could stay in the matrix. On the
other hand, clustering of the trapped hydrogen in the
argon matrix was avoided by keeping the temperature
below the range, where diffusion in the matrix becomes important (about 40 % of the melting point [1]).
The results of the scattering of trapped hydrogen was compared with that of scattering from bulk
solid hydrogen recorded under the same conditions
Fig.
1.
-
transfer 1
Spectrum of2 % H2 in argon at 6 K. a) Momentum
A-1, b) Momentum transfer 7 A-1.
-
(s. below).
spectrum of solid hydrogen at the same conditions
(Fig. 2a). Both spectra show lines at 14.5 meV as well in
The neutron scattering experiment was performed
at the time-of-flight spectrometer IN 4 at the ILL.
The primary spectrometer consisted of two rotating
graphite crystals. The incident energy was 31 meV.
The rotational speed of the two crystals was 7 194 rpm
resulting in one neutron pulse every 4.2 ms. The
secondary spectrometer has a flight path of 4 m’length.
The detectors covered scattering angles of - 9.6
to 28, 48.5 to 84, and 104 to 140 degrees. The time-offlight spectra were recorded with a channel width
of 8 ps.
energy gain as in energy loss, which clearly have to
be assigned to the ortho
para-transition of molecular hydrogen. The lines in the matrix and in pure
hydrogen have the same position within instrumental
resolution. The rotations of H2 are less *affected by the
environment of the molecule than those of other molecules (e.g. CH4 [5]), since H2 is geometrically very
small and since its rotational energy spacings are
large compared with the interaction to the host. The
ratio of intensities of the ortho- para- to the paraortho-lines is far above the value expected in thermal
757
8 meV energy loss, which is very similar to the phospectrum of pure argon (Fig. 3). As the spectrum
in figure 2 was corrected for the scattering of the argon
matrix, this feature is, however, entirely due to hydrogen trapped in the matrix and vibrating in phase with
the lattice.
non
Fig. 3.
transfer 7
-
2.
transfer 1
Fig.
-
Spectrum of solid hydrogen at 6 K. a) Momentum
Â-1, b) Momentum transfer 7 A-1.
equilibrium at 6 K. This is due to the well known fact,
that ortho- para-conversion is slow [6], and with
condensation a ratio of both modifications close to
that at ambient temperature is conserved.
Apart from the rotational lines, the two spectra
(Figs. 1 a, 2a) differ considerably. The spectrum of
solid hydrogen (Fig. 2a) shows some maxima, which
can be assigned to peaks in the phonon density of
states and combination lines of phonons and rotational transitions, in perfect agreement with [3].
For H2 in argon (Fig. la), a broad feature around
5 meV can be seen, which is probably due to the lattice
phonons of argon. At high angles, the spectra ôf the
two samples become completely different (Figs. 1 b
and 2b). The spectrum of solid hydrogen shows large
recoil broadening and shift, the sharp line at zero
energy transfer has disappeared. This is similar to
earlier results for liquid hydrogen [4]. In the spectrum
of H2 in the matrix, these recoil effects cannot be seen.
Instead, there is a feature with maxima at 3 and
Spectrum of solid
 -1.
argon at 6
K, Momentum
At present, we only can draw some qualitative
conclusions from the spectra recorded here. In solid
hydrogen, the translational movement of the molecules
is hindered by a barrier, which is small compared with
the incident neutron energy of 31 meV. Thus, recoil
effects can be seen. In the argon matrix, the hydrogen
molecules are more rigidly embedded into the lattice.
They are thus vibrating in phase with the lattice.
Even at high momentum transfers, no recoil effects
can be seen, since the effective mass is much higher
than that of a single molecule.
A strong interaction potential of the trapped molecule and the host seems to be in contradiction with
the observation of free rotation. The rotations of the
hydrogen molecule are, however, only hindered by the
angular dependent part of its interaction potential
with the host. A potential, which depends strongly
on the distance of the centre of the molecule from the
surrounding atoms, but hardly on its orientation,
would account for all observations. In future experiments, H2 shall be isolated in matrices with smaller
lattice parameters (neon) or stronger chemical interaction with the trapped molecule in order to see
how sensitive the H2 rotation is a test for the interaction with its environment.
References
[1] MEYER, B.,
Low
Temperature Spectroscopy (Elsevier)
1971.
P. A., Thermal Neutron
demic Press) 1965.
[2] EGELSTAFF,
Scattering (Aca-
[3] BICKERMANN, A., SPITZER, H., STILLER, H.,
31 (1978) 345.
Z.
Physik B
[4] WHITTEMORE, W. L., DANNER, H. R., IAEA Proceedings,
Vienna, 1 (1963) 273.
[5] KATAOKA, Y., PRESS, W., BUCHENAU, U., SPITZER, H.,
IAEA Proceedings, Vienna, 2 (1978) 311.
[6] SILVERA, I. F., Rev. Mod. Phys. 52 (1980) 393.