ESR investigation of hydrogen and deuterium atoms in impurity-helium solids
Impurity-helium solids (Im-He solids) are porous solids created by injecting a beam of mixed helium and impurity gases into superfluid ⁴He. In this work we use electron spin resonance (ESR) techniques to investigate Im-He solids containing atoms and molecules of hydrogen and/or deuterium. We have pe...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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irk-123456789-1288592018-01-15T03:02:54Z ESR investigation of hydrogen and deuterium atoms in impurity-helium solids Kiselev, S.I. Khmelenko, V.V. Bernard, E.P. Lee, D.M. 3-й Международный семинар по физике низких температур в условиях микрогравитации Impurity-helium solids (Im-He solids) are porous solids created by injecting a beam of mixed helium and impurity gases into superfluid ⁴He. In this work we use electron spin resonance (ESR) techniques to investigate Im-He solids containing atoms and molecules of hydrogen and/or deuterium. We have performed studies of low temperature (T ~ 1.35 K) tunnelling chemical reactions in which deuterium atoms replace the hydrogen atoms bound in H₂ or HD molecules to produce large (up to 7.5×10¹⁷cm⁻³) and relatively stable concentrations of free hydrogen atoms. The time dependence of H and D atom concentrations has been investigated for Im-He samples with different initial ratios of hydrogen and deuterium ranging from 1:20 to 1:1.The satellite ESR lines associated with the dipolar coupling of electron spins of H and D atoms to the nuclear moments of the hydrogen nuclei found in neighboring molecules have been observed in Im-He solids. The forbidden hyperfine transition of atomic hydrogen involving the mutual spin flips of electrons and protons has also been observed. 2003 Article ESR investigation of hydrogen and deuterium atoms in impurity-helium solids / S.I. Kiselev, V.V. Khmelenko, E.P. Bernard, D.M. Lee // Физика низких температур. — 2003. — Т. 29, № 6. — С. 678-683. — Бібліогр.: 22 назв. — англ. 0132-6414 PACS: 67.40.Yv, 67.40.Mj, 61.10.Eq, 61.46.+w http://dspace.nbuv.gov.ua/handle/123456789/128859 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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3-й Международный семинар по физике низких температур в условиях микрогравитации 3-й Международный семинар по физике низких температур в условиях микрогравитации |
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3-й Международный семинар по физике низких температур в условиях микрогравитации 3-й Международный семинар по физике низких температур в условиях микрогравитации Kiselev, S.I. Khmelenko, V.V. Bernard, E.P. Lee, D.M. ESR investigation of hydrogen and deuterium atoms in impurity-helium solids Физика низких температур |
| description |
Impurity-helium solids (Im-He solids) are porous solids created by injecting a beam of mixed helium and impurity gases into superfluid ⁴He. In this work we use electron spin resonance (ESR) techniques to investigate Im-He solids containing atoms and molecules of hydrogen and/or deuterium. We have performed studies of low temperature (T ~ 1.35 K) tunnelling chemical reactions in which deuterium atoms replace the hydrogen atoms bound in H₂ or HD molecules to produce large (up to 7.5×10¹⁷cm⁻³) and relatively stable concentrations of free hydrogen atoms. The time dependence of H and D atom concentrations has been investigated for Im-He samples with different initial ratios of hydrogen and deuterium ranging from 1:20 to 1:1.The satellite ESR lines associated with the dipolar coupling of electron spins of H and D atoms to the nuclear moments of the hydrogen nuclei found in neighboring molecules have been observed in Im-He solids. The forbidden hyperfine transition of atomic hydrogen involving the mutual spin flips of electrons and protons has also been observed. |
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Article |
| author |
Kiselev, S.I. Khmelenko, V.V. Bernard, E.P. Lee, D.M. |
| author_facet |
Kiselev, S.I. Khmelenko, V.V. Bernard, E.P. Lee, D.M. |
| author_sort |
Kiselev, S.I. |
| title |
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids |
| title_short |
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids |
| title_full |
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids |
| title_fullStr |
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids |
| title_full_unstemmed |
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids |
| title_sort |
esr investigation of hydrogen and deuterium atoms in impurity-helium solids |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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2003 |
| topic_facet |
3-й Международный семинар по физике низких температур в условиях микрогравитации |
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http://dspace.nbuv.gov.ua/handle/123456789/128859 |
| citation_txt |
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids / S.I. Kiselev, V.V. Khmelenko, E.P. Bernard, D.M. Lee // Физика низких температур. — 2003. — Т. 29, № 6. — С. 678-683. — Бібліогр.: 22 назв. — англ. |
| series |
Физика низких температур |
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2025-07-09T10:02:58Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6, p. 678–683
ESR investigation of hydrogen and deuterium atoms in
impurity-helium solids
S.I. Kiselev, V.V. Khmelenko, E.P. Bernard, and D.M. Lee
Laboratory of Atomic and Solid State Physics
Cornell University, Ithaca, NY 14853-2501, USA
E-mail: epb22@cornell.edu
Received December 19, 2002
Impurity–helium solids (Im–He solids) are porous solids created by injecting a beam of mixed
helium and impurity gases into superfluid 4He. In this work we use electron spin resonance (ESR)
techniques to investigate Im–He solids containing atoms and molecules of hydrogen and/or deute-
rium. We have performed studies of low temperature (T � 1.35 K) tunnelling chemical reactions in
which deuterium atoms replace the hydrogen atoms bound in H2 or HD molecules to produce large
(up to 7.5�1017cm–3) and relatively stable concentrations of free hydrogen atoms. The time de-
pendence of H and D atom concentrations has been investigated for Im–He samples with different
initial ratios of hydrogen and deuterium ranging from 1:20 to 1:1.The satellite ESR lines associ-
ated with the dipolar coupling of electron spins of H and D atoms to the nuclear moments of the
hydrogen nuclei found in neighboring molecules have been observed in Im–He solids. The forbid-
den hyperfine transition of atomic hydrogen involving the mutual spin flips of electrons and pro-
tons has also been observed.
PACS: 67.40.Yv, 67.40.Mj, 61.10.Eq, 61.46.+w
1. Introduction
Investigations of hydrogen and deuterium atoms,
stabilized in solid matrices at low temperatures, have
attracted the attention of scientists for many years.
Possible quantum effects associated with these sys-
tems are of special interest. Quantum behavior is ex-
pected when the thermal de Broglie wavelength of the
atoms becomes comparable with their interparticle
spacing. For the case of H atoms in the gas phase, this
condition is satisfied for a concentration of
2.6�1018cm–3 at a temperature of 30 mK [1]. For the
case of a solid phase, the temperature for the onset of
any quantum effect may be lower because of the possi-
bility of a larger effective mass. It is always desirable
for investigations of quantum behavior to generate the
highest possible concentrations of H or D atoms in
solid matrixes. Unfortunately, from the earliest inves-
tigations of H atoms in solid H2, it became clear that
some tunnelling processes and molecular recombina-
tion could lead to a decrease in the concentration of
stabilized H atoms [2]. The detailed investigations of
the processes of quantum diffusion and tunnelling re-
actions of H atoms in solid H2 at T = 1.35–4.2 K were
performed by Ivliev et al. [3] and later by Miyazaki et
al. [4–6]. They established that the decay of H atoms
could take place in solid H2 by tunnelling migration,
in which H atoms tunnel through a chain of H2 mole-
cules according to the reaction
H + H2 � H2 + H (1)
thereby travelling through the solid H2 to recombine
with another H atom. The reaction rate constant k1, for
reaction (1) was found to be k1
3 1 118� � �cm mol s– –
at T = 4.2 K [4]. This value remains the same even
when the temperature is lowered to 1.9 K, confirming
that tunnelling reactions are involved. The behavior of
D atoms in solid D2 is different. The rate constant for
the reaction
D + D2 � D2 + D (2)
is four orders of magnitude lower; k2 �
� � � �
� � �18 10 3 3 1 1. cm mol s [7] leading to a much
lower recombination rate. Therefore it is possible to
create a larger concentration of D atoms in solid D2
as compared with concentrations of H atoms in solid
H2 [7–9]. The exchange tunnelling reactions
© S.I. Kiselev, V.V. Khmelenko, E.P. Bernard, and D.M. Lee, 2003
D + H2 � HD + H (3)
D + HD � D2 + H (4)
lead to the possibility of creating high concentrations
of H atoms in solid mixtures of H2 and D2 at low
temperatures [8,10,11]. When an H atom is sur-
rounded by shell of D2 molecules, it becomes very
stable because it can neither migrate through the
solid D2 nor react further with D2 by the reaction
H + D2 � HD + D (5)
at low temperatures, since it is an endothermic reaction.
Gordon et al. [8,12] suggested that high concentrations
of H atoms could be stabilized at low temperatures by
means of reactions (3) and (4). In their approach, a gas
mixture of hydrogen, deuterium and helium gas was
transported through a radio frequency discharge onto
the surface of superfluid He contained in a small
beaker, at a temperature 1.5 K. The jet of impurity and
helium gases penetrates the surface of the superfluid
He, and then forms a snow-like solid which settles to
the bottom of the collection beaker. This solid became
known as an Im–He solid [13,14]. In the case of heavy
impurities, Im–He solids are built from a loosely con-
nected aggregation of nanoclusters of impurities each
surrounded by one or two layers of solid helium. These
aggregates form extremely porous solids into which li-
quid helium easily and completely penetrates. This sys-
tem, having the high thermal conductance and the high
thermal capacitance of superfluid helium, allows prepa-
ration and storage of very high concentrations of stabi-
lized atoms. To this date the structures of Im–He solids
for light impurities such as hydrogen and deuterium
atoms and molecules are not fully determined. Our
preliminary x-ray investigations of D2–He samples
showed the presence of nanoclusters of D2 in these so-
lids [15]. Therefore it is reasonable to assume that the
structure of Im–He solids formed from light impurities
is similar to that of Im–He solids formed from heavy
impurities (Ne, N2, Kr) [16,17]. In this work we have
studied impurity–helium solids formed by light impuri-
ties, namely hydrogen and deuterium atoms and mole-
cules. The method of electron spin resonance (ESR)
was used for detailed studies of H and D atoms stabi-
lized in Im–He solids. We have performed studies of
the exchange tunnelling chemical reactions (3) and (4)
to produce large (up to 7.5�1017 cm–3) and relatively
stable concentrations of H atoms. The kinetics of these
reactions have been investigated for Im–He samples
formed by introducing gas mixtures with different ini-
tial ratios of H2 to D2, ranging from 1:20 to 1:1, into
He II. We determined the exact positions of the H and
D lines by using precise measurements of the magnetic
field. Satellite ESR lines associated with the dipolar
coupling of the electron spins of H and D atoms to the
nuclear moments of hydrogen nuclei in neighboring
molecules have been observed. This observation allows
us to determine the distances between stabilized H or D
atoms and neighboring HD or H2 molecules in Im–He
solids [18]. From the analysis of line widths and the
saturation behavior of H and D signals, we estimate the
spin-spin relaxation time T2 and the spin-lattice relax-
ation time T1 of H and D atoms in Im–He solids.
2. Experimental method
The experiments were performed in a Janis cryostat
with a variable temperature insert (VTI). The lower
part of the cryostat was installed between the pole
pieces of a Varian electromagnet for these ESR investi-
gations. The home-made insert for the creation and in-
vestigation of Im–He solids with stabilized atoms
shown in Fig. 1 was placed into the VTI. The details of
the experimental procedure were described in our pre-
vious work [17,19]. For sample preparation, a gas mix-
ture of H2, D2, and He was transported from a room
temperature gas handling system to the cryogenic re-
gion. To provide H and D atoms, high power radio fre-
quency was applied to the electrodes around the quartz
capillary carrying the mixed gases. The jet
( )~d dN t 5 1019 1
�
�s of impurity atoms and molecules
as well as helium gas emerged from a small (0.75 mm)
orifice and then penetrated the surface of superfluid he-
lium in the collection beaker. The temperature during
sample preparation was 1.5 K. The liquid helium level
in the beaker was maintained by a fountain pump con-
nected to the main helium bath of the VTI. At the top
of the beaker was a funnel that caught the sample as it
emerged from the quartz capillary. A set of teflon
blades was employed to scrape the sample from the fun-
nel while the beaker was rotated so that the sample
could fall to the bottom of the cylindrical part of the
beaker. During a period of 10 minutes an impurity–he-
lium solid sample with volume 0 35 3. cm was formed in
the beaker. Following this process, the beaker with the
sample was lowered into the ESR cavity, which was
situated near the bottom of the VTI in the homoge-
neous field region of the electromagnet. We used a cy-
lindrical cavity operating in a TE011 mode. A ruby
crystal was attached to the bottom of the cavity. The
ruby was used as a secondary standard for the calibra-
tion of the measurements of the number of stabilized H
and D atoms in Im–He solids. The calibration of the
absolute value of number of atoms was made by using a
DPPH sample with a known number (2.4�1017) of spins
with measurements being carried out at T = 1.35 K.
Special measurements were made to determine the depend-
ence of the signal of the small calibration sample of DPPH
on the position along the axis of the cavity. This
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6 679
dependence is consistent with the calculated distribu-
tion of the microwave magnetic field in the cavity. ESR
signals were measured using a CW reflection homodyne
spectrometer ( . ,fr � 912 GHz fmod � 100 kHz). All
measurements have been done for Im–He samples at
temperatures of 1.35 K and 1.8 K. A calibrated Ge ther-
mometer was used for temperature measurements. Con-
tinuously pumping the VTI while supplying liquid he-
lium from the main bath allows us to conduct long term
investigations of the Im–He samples. In this paper we
present reaction kinetics studies of H and D atoms
which continued for � 8 hours at T = 1.35 K.
3. Results
Figure 2 shows the ESR derivative spectra of H and
D atoms in Im–He solids prepared from an initial gas
mixture in the ratio of H2:D2:He = 1:4:100. The
positions of observed lines are shown in Table.
Table
Observed positions of ESR lines for H and D atoms in
Im–He solids at T = 1.8 K and a frequency 9.12 GHz
Line Transition (F, m
F
) Field, G
H
1
( , ) ( , )11 00� 2976.3
H
2
( , ) ( , )10 1 1� � 3484.7
H
f
( , ) ( , )10 00� 3205.7
D
1
( , ) ( , )3 2 3 2 1 2 1 2� 3171.7
D
2
( , ) ( , )3 2 1 2 1 2 1 2� � 3247.4
D
3
( , ) ( , )3 2 1 2 3 2 3 2� � � 3326.9
The allowed H and D lines are each accompanied
by two satellite lines. The intensity of the small for-
bidden hydrogen line Hf is about 200 times smaller
than that of the allowed hydrogen lines. Figure 3
shows the microwave power saturation behavior of H
and D atoms in Im–He solids. Unlike the behavior of
H atoms produced by radiolysis in solid H2 [11], H
atoms in Im–He solids saturate at a larger microwave
power � 16 �W. ESR signals were normally measured
at a microwave power � � �W, significantly below the
saturation limit.
680 Fizika Nizkikh Temperatur, 2003, v. 29, No. 6
S.I. Kiselev, V.V. Khmelenko, E.P. Bernard, and D.M. Lee
Fig. 2. ESR spectra of H and D atoms for an Im–He solid
prepared from the gaseous mixture H2:D2:He = 1:4:100.
Spectra observed at T = 1.8 K, 182 min after sample col-
lection. The width of each of the main hydrogen and deu-
terium lines is 3 G. Seven fold magnification of the for-
bidden line is shown in the inset.
Quartz
insert
Electrodes for
RF discharge
Beaker holder
Impurity-Helium
sample
Coaxes
Fountain pump
ESR cell
Coulling
loop
Ruby
Fountain pump
Modulation
coils
ESR cavity
Capacitance
level meter
Level of liquid
helium
Quartz beaker
Teflon blades
Source of atom
and molecules
Electrical
connectors
Fig. 1. Low temperature insert for Im–He sample prepara-
tion and ESR investigations. The quartz beaker is lowered
into the ESR 9.12 GHz TE011 resonant cavity for measure-
ments after sample preparation.
The spin-spin relaxation time T2 is calculated from
the line-width below saturation by means of the ex-
pression [20]:
T
Hpp
2 1 2
2
3
�
(6)
where is the gyromagnetic ratio, and
Hpp is the
line width obtained from the peak to peak separation
for the derivative of the ESR signal. The line width
of ESR lines for H and D atoms was found to be 3 G
(see Fig. 2), leading to a value of T2 = 2.2�10–8 s for
H and D atoms in our Im–He solids. The observed
line width is far larger than the inhomogeneous
broadening expected from our magnet (� 0.1 G). We
used a saturation method for determination of the
spin-lattice relaxation time T1 for H and D atoms in
Im–He solids. For estimating T1, we determined the
dependence of the signal amplitude on the square root
of microwave power as plotted in Fig. 3 for hydrogen
and deuterium atoms. Values of T1 were determined
by means of an expression for the maximum values of
the peak amplitude of the derivative signal [20]:
T
H
H
pp
g
1
7
1
2
198 10
�
�
�.
(7)
where g is the spectroscopic splitting factor. For H and
D atoms g � 2. We performed calculations of the mi-
crowave magnetic field according to the equation [20]:
H PQ
V
Vs
c
1
2 32 10� �
� � (8)
where P is the microwave power; Q is the quality fac-
tor; � is the filling factor; Vc is the cavity volume, and
Vs is the sample volume. When we substitute the
geometric parameters for our cavity, expression (8) be-
comes H P1
2 8� . From the plot in Fig. 3 we obtained
the value of the power Pmax � 16 �W for which the
amplitude of the derivative of the ESR signal of the H
atoms has a maximum value. From the expression (7)
we then find a value of T1
32 3 0 5 10� � �
�( . . ) s for H
atoms in Im–He solids. For D atoms (Pmax = 25 �W, see
also Fig. 3) we find a value T1
315 0 5 10� � �
�( . . ) s .
The relatively small values of T1 for D and H atoms in
Im–He solids show that the atoms are stabilized in solid
clusters of mixtures HD and D2 molecules, rather than
being isolated in liquid and solid helium. In latter case,
the T1 values should be much larger [21].
We also investigated the evolution of Im–He solids
containing H and D atoms as well as H2, HD, and D2
molecules in an attempt to maximize the H atom con-
centration. The investigations were performed for a
variety of initial gas mixtures of H2, D2, and He. The
initial H2:D2 ratio was varied from 1:20 to 1:1. The
ratio of the concentration of the impurity gases to the
He gas in the mixtures was always equal to 1:20. The
yields of H and D atoms leaving the radio frequency
discharge were proportional to the concentrations of
H2 and D2 in the initial gas mixture [12]. Figure 4
shows the time evolution of the H and D concentra-
tions in Im–He solids formed by two different initial
mixtures. Immediately after preparation of all the
Im–He samples, a large enhancement of the concen-
tration of H atoms relative to D atoms was observed
compared with the ratio of H2 to D2 in the initial gas
mixture. This fact indicates that at the earliest stages
of sample preparation at T = 1.5 K, a fast exchange
tunnelling reaction leads to a large reduction in the
number of D atoms and a corresponding increase in
the number of H atoms. According to the calculation
by Takayanagi et al. [22] the rate constant for reac-
tion (3) is found to be 5 4 10 2 3 1 1. – – –
� � �cm mol s , so
that the enhancements of the H atom concentrations
are attributed to this reaction with a time constant on
the order of a few minutes for our samples. Later on, a
steady increase of the ratio H:D takes place over a pe-
riod of hours, during the storage of our Im–He sample
at T = 1.35 K. The kinetics of the changing concentra-
tions of H and D atoms for the sample prepared from
the initial gas mixture H2:D2:He = 1:20:420 is similar
to that observed by Lukashevich et al. [10] and
Miyazaki et al. [11] for atoms trapped in solid H2–D2
mixtures. The concentration of H atoms grows, but
the concentration of D atoms is decaying, just as in
our Im–He studies. At this stage we believe that the
behavior of atoms in our solid is governed by reaction
(4) with rate constant 19 10 3 3 1 1. – – –
� � �cm mol s [22]
which further reduces the number of D atoms and in-
ESR investigation of hydrogen and deuterium atoms in impurity-helium solids
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6 681
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0 2 4 6 8 10 12
P 1/2 , ( W)� 1/2
A
m
p
lit
u
d
e
, a
rb
.u
n
its
Fig. 3. Microwave power saturation behavior for different
ESR lines of H and D atoms in Im–He solid at T = 1.35 K
(see Fig. 2): H1 (�), D1 (�), Hf (�). Im–He solid was
formed by initial gas mixture H2:D2:He = 1:4:100.
creases the number of H atoms. Figure 4,a shows that
the increase in the number of H atoms is smaller than
the decrease in the number of D atoms. This observa-
tion could be explained by recombination of H atoms.
In this process, the hydrogen atoms migrate through
the solid via reaction (1) or the reaction
H + HD � HD + H. (9)
A succession of these reactions allows the transport of
H atoms to neighboring positions in the Im–He solid
where they recombine to form H2 or HD molecules
via the reactions:
H + D � HD (10)
H + H � H2. (11)
This mechanism is supported by the results of investi-
gations of Im–He samples prepared from the gas mix-
ture H2:D2:He = 1:2:60 (see Fig. 4,b). In this sample
the concentration of H2 and HD molecules is about
one order of magnitude larger compared with the for-
mer sample, so therefore reactions (1) and (9) should
accelerate the recombination processes (10) and (11).
It can be seen that the decay of D atoms is more rapid
and the H atom population also decays steadily
throughout the experiment. The exchange tunnelling
reactions can be used to produce very large concentra-
tions of atomic hydrogen in our samples. As discussed
earlier, these large concentrations could allow us to
enter the regime where the thermal de Broglie wave-
length becomes comparable to the spacing between H
atoms, provided that the temperature is low enough.
In our experiments we have investigated samples ob-
tained from a variety of initial gas mixtures to deter-
mine the one which yields the highest hydrogen atom
concentration. The experimental plots shown in
Fig. 5 correspond to the H and D atom average con-
centrations after a storage time � 500 minutes follow-
ing initial sample preparation. For the optimal gas mix-
ture H2:D2:He = 1:4:100, the largest H atom
concentration (7.5 � 3.0)�1017 cm–3 was obtained. We
are planning investigations with even longer storage
times of this optimal sample to obtain the largest possi-
ble concentration of H atoms due to exchange tunnel-
ling reactions.
4. Conclusion
The observation of satellite lines and the relatively
short longitudinal relaxation times T1 for H and D
atoms in impurity–helium solids show that the H and
D atoms are stabilized in clusters of solid mixtures of
H2, D2, and HD. We did not observe any signatures of
682 Fizika Nizkikh Temperatur, 2003, v. 29, No. 6
S.I. Kiselev, V.V. Khmelenko, E.P. Bernard, and D.M. Lee
7.5
6.0
4.5
3.0
1.5
0
0.1 0.2 0.3 0.4 0.5
1
7
C
o
n
ce
n
tr
a
tio
n
(
x
1
0
/
c
m
)
3
[H ]/([H ]+[D ])2 2 2
Fig. 5. The dependence of the average concentrations of H
atoms (�) and D atoms (�) in Im–He solids on the frac-
tion of hydrogen gas in the make up gas mixture,
[H2]/([H2]+[D2]). For each point the concentrations
were determined after a waiting period of 500 minutes
following sample collection.
b
5
4
3
2
1
0
100 200 300 400 500
1
7
C
o
n
ce
n
tr
a
tio
n
(
x
1
0
/
c
m
)
3
Time in minutes
Fig. 4. Time dependence of the concentration of H
atoms (�) and D atoms (�) in Im–He solids at tempe-
rature T = 1.35 K prepared from different initial mixtu-
res: H2:D2:He = 1:20:420 (a); H2:D2:He =1:2:60 (b).
0 100 200 300 400 500
3
6
9
12
15
18
1
7
C
o
n
ce
n
tr
a
tio
n
(
x
1
0
/
c
m
)
3 à
single atoms perfectly isolated by helium. Observa-
tion of these isolated atoms requires the application of
much lower microwave power levels to avoid satura-
tion. Our spectrometer was not capable of providing
such low powers.
The time dependence of the populations of H and D
atoms in the Im–He solids at T = 1.35 K proves the oc-
currence of the exchange tunnelling reaction D + HD
� H + D2. The exchange tunnelling reactions are ca-
pable of producing very high concentrations of hydro-
gen atoms. Our estimations from ESR line intensity
measurements indicate that the largest concentration
we have obtained is (7.5 � 3.0)�1017 cm–3. For this
concentration the onset of quantum overlap pheno-
mena may be found at T � 13 mK which can be reached
by means of a dilution refrigerator.
Acknowledgements
The authors are grateful to NASA for supporting
this research via grant NAG 8-1445. We also thank
P. Borbat, G. Codner, H. Padamsee, V. Shemelin,
and R. Silsbee for extremely useful conversations and
help with the experiments.
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ESR investigation of hydrogen and deuterium atoms in impurity-helium solids
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6 683
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