Inelastic neutron scattering of hydrogen trapped in solid argon

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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 Appliquée, Société française de physique / EDP, 1984, 19 (9), pp.755-757.

�10.1051/rphysap:01984001909075500�. �jpa-00245253�

755

Inelastic

neutron

scattering ^of hydrogen trapped ⁱⁿ solid argon

W. Langel

Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, ^France

Résumé. ²⁰¹⁴ La diffusion inélastique ^{des neutrons} par des molécules d’hydrogène isolées dans une matrice d’argon

et par 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.

Abstract ²⁰¹⁴ Molecular hydrogen ^was isolated in an argon matrix. Its inelastic ^neutron scattering ^{functions at}

momentum transfers of 1 to 7 Å-1 were 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,.

Revue Phys. Appl. 19 (1984) ^{755-757 SEPTEMBRE} 1984,

1. Introduction.

Trapping

molecules in an inert environment is a common

technique

^of

preparing

samples for diffe- rent 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 :

- In many ^cases, the interaction between the matrix and the

trapped

molecules is _very weak com-

pared

^{with the} energies of intramolecular transitions

(e.g.

^internal vibrations), and the molecule can be studied under conditions similar to those in the _gas

phase. ^{For neutron}

scattering

^{this means, that sam-} ples ^{of a}

high particle

^{densities can be}

prepared,

ⁱⁿ

which the intermolecular interactions are still

quite

small.

- The existing small intermolecular interactions

cause shifts of rotational lines and local modes of the

trapped

molecules. By

measuring

^these effects, ^inter-

molecular potentials ^{can be found.}

The neutron scattering of molecular

hydrogen

^was

intensively

studied until now, both

experimentally

and

theoretically

[2]. ^{In solid}

hydrogen,

^{a maximum}

in the phonon

density

^{of states was found at 5.4 meV} [3]. ^{The ortho} para- transition in the solid is found at energies ^{of 13.5 to 14.6} meV, whereas the _gas

phase value is 14.6 meV. A

major

part of the work on

H2 deals with the

liquid

^state. Whittemore and Dan-

ner [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.

2.

ExperimentaL

The main

difficulty

^{of a matrix}

experiment

^{is the} pre-

paration

^{of the} sample. ^In

optical

spectroscopy, where matrix

techniques

^{are most}

commonly applied, layers

^{of a} thickness of about 0.1 mm are studied,

which

obviously

^{must have a}

perfect optical quality.

They

^contain about 5 mmol of the host and 0.01 mmol of the

impurity.

^{For neutron}

scattering,

large samples

of some cm’ of volume containing about 500 mmol matrix material and 5 to 10 mmol of the

impurity (i.e.

^{5 x 1021} molec.) ^{have to be}

prepared

^The

impu- rity

should have a

high

scattering ^{cross section}

(e.g.

contain

hydrogen

atoms), since its concentration is

limited, ^{while the one} of the matrix should be small.

This holds for. the most important ^{hosts as} argon and

krypton,

^{as well as for neon and}

SF6,

^{but not}

for

hydrocarbon

glasses.

There are two methods

of preparing

^these samples :

Either matrix and

impurity

^are condensed into the

sample ^{container as a}

liquid

solution and then frozen to form a solid, ^{or a} gas mixture of both components is frozen

directly

^{at the} cold walls of the sample

container (vapour

deposition).

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 depo-

sition 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 appear- ed _necessary to check, ^{whether a}

separation

^{of host}

and

impurity during

sample

deposition

^{had taken}

place.

^{This was done}

by

^{two methods :}

- After

deposition,

^the sample ^was

pumped

^and

at the same time heated to a temperature ^{above the}

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 beco-

mes important

(about ^{40 %}

^{of the melting}

^point [1]).

- The results of the

scattering

^of

trapped hydro-

gen ^was

compared

with that of

scattering

^{from bulk}

solid

hydrogen

recorded under the same conditions

(s. below).

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-of-

flight

spectra ^were recorded with a channel width of 8 _ps.

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 scatter-} ing ^{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}

equili-

brium with the matrix.

3. Results and discussion.

Figure la shows the spectrum ^of

H2

^{in argon at low}

scattering angles.

^{It has to be}

compared

^{with the}

Fig. ^{1. -} Spectrum ^of2 % H2 ^{in argon at 6 K. a) Momentum}

transfer 1

A-1,

b) Momentum transfer 7 A-1.

spectrum ^{of solid}

hydrogen

^{at the same conditions}

(Fig. 2a).

^Both spectra ^{show lines at 14.5 meV as well in} energy

gain

^as in energy loss, ^which

clearly

^{have to}

be

assigned

^{to the ortho}

para-transition

^{of mole-}

cular

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 mole- cules

(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 _para- ortho-lines is far above the value

expected

^{in thermal}

757

Fig. ^{2. -} Spectrum ^{of solid} hydrogen ^{at 6 K.} a) ^Momentum

transfer 1

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 rota-}

tional 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

8 meV energy loss, ^{which is} very similar to the pho-

non spectrum ^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}

hydro-

gen

trapped

in the matrix and

vibrating

ⁱⁿ

phase

^with

the lattice.

Fig. ^{3. -} Spectrum of solid argon ^at 6 K, ^Momentum transfer 7 Â -1.

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

^mole-

cule 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

experi-

ments,

H2

shall be isolated in matrices with smaller lattice parameters

(neon)

^or stronger chemical inter- action with the

trapped

molecule in order to see

how sensitive the

H2

rotation is a test for the inter- action with its environment.

References

[1] MEYER, ^{B., Low} Temperature Spectroscopy (Elsevier)

1971.

[2] EGELSTAFF, ^P. A., Thermal Neutron Scattering (Aca-

demic Press) ^1965.

[3] BICKERMANN, A., SPITZER, H., STILLER, H., ^Z. Physik B

31 (1978) ^345.

[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.