On charged impurity structures in liquid helium
The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid helium is explained by neutralization reactions occurring in impurity nanoclusters. Op...
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irk-123456789-1284942018-01-11T03:02:44Z On charged impurity structures in liquid helium Pelmenev, A.A. Krushinskaya, I.N. Bykhalo, I.B. Boltnev, R.E. Низкотемпературная физика пластичности и прочности The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid helium is explained by neutralization reactions occurring in impurity nanoclusters. Optical spectra of excited products of neutralization reactions between nitrogen cations and thermoactivated electrons were rather different from the spectra observed at higher temperatures, when the luminescence due to nitrogen atom recombination dominates. New results on current detection during the IHC destruction are presented. Two different mechanisms of nanocluster charging are proposed to describe the phenomena observed during preparation and warmup of IHC samples in bulk superfluid helium, and destruction of IHC samples out of liquid helium. 2016 Article On charged impurity structures in liquid helium / A.A. Pelmenev, I.N. Krushinskaya, I.B. Bykhalo, R.E. Boltnev // Физика низких температур. — 2016. — Т. 42, № 3. — С. 289–296 . — Бібліогр.: 57 назв. — англ. 0132-6414 PACS: 78.60.Kn, 78.67.Bf, 36.40.Wa, 72.20.Jv http://dspace.nbuv.gov.ua/handle/123456789/128494 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Низкотемпературная физика пластичности и прочности Низкотемпературная физика пластичности и прочности |
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Низкотемпературная физика пластичности и прочности Низкотемпературная физика пластичности и прочности Pelmenev, A.A. Krushinskaya, I.N. Bykhalo, I.B. Boltnev, R.E. On charged impurity structures in liquid helium Физика низких температур |
| description |
The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have
been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid
helium is explained by neutralization reactions occurring in impurity nanoclusters. Optical spectra of excited
products of neutralization reactions between nitrogen cations and thermoactivated electrons were rather different
from the spectra observed at higher temperatures, when the luminescence due to nitrogen atom recombination
dominates. New results on current detection during the IHC destruction are presented. Two different mechanisms
of nanocluster charging are proposed to describe the phenomena observed during preparation and warmup
of IHC samples in bulk superfluid helium, and destruction of IHC samples out of liquid helium. |
| format |
Article |
| author |
Pelmenev, A.A. Krushinskaya, I.N. Bykhalo, I.B. Boltnev, R.E. |
| author_facet |
Pelmenev, A.A. Krushinskaya, I.N. Bykhalo, I.B. Boltnev, R.E. |
| author_sort |
Pelmenev, A.A. |
| title |
On charged impurity structures in liquid helium |
| title_short |
On charged impurity structures in liquid helium |
| title_full |
On charged impurity structures in liquid helium |
| title_fullStr |
On charged impurity structures in liquid helium |
| title_full_unstemmed |
On charged impurity structures in liquid helium |
| title_sort |
on charged impurity structures in liquid helium |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2016 |
| topic_facet |
Низкотемпературная физика пластичности и прочности |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/128494 |
| citation_txt |
On charged impurity structures in liquid helium / A.A. Pelmenev, I.N. Krushinskaya, I.B. Bykhalo, R.E. Boltnev // Физика низких температур. — 2016. — Т. 42, № 3. — С. 289–296 . — Бібліогр.: 57 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
AT pelmenevaa onchargedimpuritystructuresinliquidhelium AT krushinskayain onchargedimpuritystructuresinliquidhelium AT bykhaloib onchargedimpuritystructuresinliquidhelium AT boltnevre onchargedimpuritystructuresinliquidhelium |
| first_indexed |
2025-07-09T09:11:28Z |
| last_indexed |
2025-07-09T09:11:28Z |
| _version_ |
1837159984201203712 |
| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 3, pp. 289–296
On charged impurity structures in liquid helium
A.A. Pelmenev1, I.N. Krushinskaya2, I.B. Bykhalo1, and R.E. Boltnev2
1Branch of Talroze Institute for Energy Problems of Chemical Physics, Russian Academy of Sciences,
Chernogolovka 142432, Russia
2Joint Institute for High Temperatures, Russian Academy of Sciences,
Izhorskaya St. 13, Bd. 2, Moscow 125412, Russia
E-mail: boltnev@gmail.com
Received October 12, 2015, revised October 29, 2015, published online January 26, 2016
The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have
been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid
helium is explained by neutralization reactions occurring in impurity nanoclusters. Optical spectra of excited
products of neutralization reactions between nitrogen cations and thermoactivated electrons were rather different
from the spectra observed at higher temperatures, when the luminescence due to nitrogen atom recombination
dominates. New results on current detection during the IHC destruction are presented. Two different mecha-
nisms of nanocluster charging are proposed to describe the phenomena observed during preparation and warm-
up of IHC samples in bulk superfluid helium, and destruction of IHC samples out of liquid helium.
PACS: 78.60.Kn Thermoluminescence;
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters;
36.40.Wa Charged clusters;
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping.
Keywords: nanoclusters, impurity-helium condensates, thermoluminescence, superfluid helium, stabilization of
ions and radicals.
1. Introduction
Macroscopic impurity particles and systems formed di-
rectly in liquid helium-4 are known almost 70 years [1,2].
Such particles and systems have found many different ap-
plications: visualization of flows and quantum vortices in
superfluid helium [2–4]; study of energy transfer phenom-
ena in liquid helium [5]; development of new high-energy
density materials [6–8]; investigation of tunneling reac-
tions of hydrogen isotopes in impurity-helium condensates
[9]; structural studies of rare gas, molecular deuterium and
nitrogen nanoclusters [10–12]; study of cold neutron inter-
action with watergels [13,14]; synthesis of metallic na-
nowires in superfluid helium [15,16]. Laser ablation of
metallic target in solid helium-4 allows to create so-called
icebergs (helium crystals doped with metallic particles)
remaining metastable below the melting curve [17,18]. It
was shown that such icebergs contain rather high densities,
~ 1015 cm–3, of positive ions and electrons [19].
In this paper we present new results on spectroscopic
studies of thermoluminescence spectra of impurity-helium
condensates (IHC) submerged in superfluid helium (He II)
as well as the charge detection during the destruction of
IHC samples. We explain the thermoluminescence of im-
purity-helium condensates submerged in superfluid helium
as a result of neutralization reactions of thermoactivated
electrons with nitrogen cations. To describe the phenome-
na observed during preparation and warm-up of IHC sam-
ples in bulk superfluid helium, and destruction of IHC
samples out of liquid helium two different mechanisms of
nanocluster charging are proposed.
2. Experimental setup
Impurity-helium condensates are highly porous materi-
als formed, as shown in Fig. 1, by injection of a helium gas
jet containing admixtures into superfluid helium at the T =
= 1.5 K [20]. Atoms of heavier noble gases as well as mo-
lecules of N2, O2, H2, and NO can be used as admixtures
(~ 1%) to helium gas. A passage of a gas mixture through a
radio frequency (rf) discharge area permits to obtain radi-
cals, ions, metastable atoms and molecules by dissociation
of impurity molecules and excitation of atoms.
© A.A. Pelmenev, I.N. Krushinskaya, I.B. Bykhalo, and R.E. Boltnev, 2016
A.A. Pelmenev, I.N. Krushinskaya, I.B. Bykhalo, and R.E. Boltnev
A scheme of the low-temperature part of experimental
setup is shown in Fig. 1(a). While the jet transits the cold
gas above the He II surface, impurity nanoclusters (with
the characteristic size ~ 3–6 nm) form [12]. The nano-
clusters penetrate through the superfluid surface and ag-
gregate into an aerogel-like substance (Figs. 1(b) and 1(c))
with the impurity density of ~ 1020 atom/cm3 [10,12]. A spec-
trometer AvaSpec-ULS2048XL-USB2 allowed us to detect
emission within the spectral range from 200 to 1160 nm
with the resolution ≈2.5 nm. Emission from IHC sample
under study was fed to the spectrometer by means of opti-
cal fibers and vacuum feedthrough. The relative spectral
response of the detection system has been measured with the
calibration light source AvaLight-DH-BAL-CAL. The pres-
sure of helium vapor in the dewar within the range 0−10 kPa
was measured with a Rosemount gauge 3051TA2. A Lake
Shore thermometer calibrated within the range 1.4–100 K
was used for the temperature measurements. The ion cur-
rents accompanying the destruction of IHC samples were
collected by a ring-shaped electrode (pos. 6 in Fig. 1(a))
connected directly to a picoammeter Keithley 6485.
The green afterglow of the fresh samples (Fig. 1(b) and
1(c)) is due to so-called α-group corresponding to the pro-
hibited transition 2D–4S of nitrogen atom. The lifetime of
N(2D) atoms in matrices is greatly shortened, for example,
from ~ 4.4⋅104 s in the gas phase to ~ 300 s and ~ 40 s for
neon and nitrogen matrices, correspondingly [21,22]. As it
was shown earlier, luminescence of the IHC sample sub-
merged in bulk He II can be stimulated by the temperature
increase of ~ 0.1 K of He II [23]. The fast decay stage
(obeying to an exponential law with τ = 15–40 s) of the
thermostimulated luminescence was followed by long-
lived decay obeying to a hyperbolic law with much longer τ,
up to 8⋅103 s [23]. In the present work we studied
thermoluminescence spectra of IHC samples in He II during
warm-up from 1.5 to 2.2 K (we will call such spectra as
“cold” ones). A photon counting system was used to detect
isothermal decay of very weak long-lived emission (the
green afterglow) observed after sample preparation cessation
and step-like temperature increase. The thermoluminescence
spectra during destruction of IHC samples (“hot” spectra)
were detected when the samples under study were out of
liquid helium (it was evaporated from the beaker) at the
temperature exceeding 2.2 K.
3. Experimental results
3.1. Macrostructure of IHC samples prepared with rf
discharge applied
First of all, we will describe the effect of rf discharge
application on the macroscopic structure of the IHC sam-
ples. Small grains (~ 0.1 mm) associate into bigger frag-
Fig. 1. (Color online) (a) Scheme of experimental setup for preparation and study of IHC samples: atom source (1); electrodes for rf
discharge (2); liquid nitrogen (3); optical fiber (4); gas jet (5); ion collector (6); glass beaker filled with superfluid helium (7); thermom-
eter (8); fountain pump (9); nitrogen glass dewar (10); helium glass dewar (11); (b) and (c) IHC sample preparation by condensation of
the gas jet passed through an rf discharge (a gas mixture [N2]/[He] = 1/100 and [N2]/[Xe]/[He] = 1/1/400, correspondingly). White ar-
rows point to filament-like structures.
290 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 3
On charged impurity structures in liquid helium
ments on the beaker’s edge and bottom when the gas jet
was not undergone to the discharge. When the gas jet pass-
es through a discharge area one can see also that long fila-
ments (with the length ~ 1 cm) and flakes (with the diame-
ter ~ 1 mm) form in the bulk He II (Figs. 1(b) and 1(c)).
The flake and filament formation was not due to quantum
vortices in superfluid helium because it was observed
only when rf discharge was applied. Another effect of rf
discharge application was observation how some frag-
ments were dropping from the beaker edge. They rolled
down along the beaker wall sticking to the wall like there
was an electrostatic attraction between the IHC fragments
and the beaker wall. The attraction was strong enough to
deform the fragments. The IHC samples and their frag-
ments are rather elastic and very compressible due to
high porosity of the condensates [10].
3.2. The green afterglow decay
The temporal dependence of the α-group intensity
stimulated by a step-like temperature increase of the
sample prepared from gas mixture [N2]/[Ne]/[He] =
= 1/100/5000 is shown in Fig. 2. The α-group intensity
decay is best fitted by hyperbolic law.
3.3. Thermoluminescence spectra of IHC samples
“Cold” and “hot” spectra observed during warm-up and
destruction, respectively, of the IHC samples are shown in
Fig. 3. The samples under study were prepared from gas
mixtures [N2]/[He] = 1/100 and [N2]/[Ne]/[He] = 1/20/500.
The dominating features of the “cold” spectra were α-
group of N atom and the Vegard–Kaplan bands of N2
molecule ( 3 ,uA +Σ 1 0 ,gv X +′ = → Σ 2 12v′′≤ ≤ transi-
tions). New additional features were observed in the
“hot” spectra: the β-group of O atom (1S–1D transition),
the δ-line of N atom (2P–2D transition), as well as β- and
M-bands of NO molecule (B2Π–X2Π and a4Π–X2Π
transitions), correspondingly, in nitrogen-helium and
nitrogen-neon-helium samples. The “hot” spectra shown
in Fig. 3 were detected at T = 13–15 K.
3.4. Currents accompanying destruction of IHC samples
The currents detected during destruction of IHC sam-
ples containing stabilized radicals have revealed existence
of charges in the condensates [24,25]. To find out a sign of
charge carriers we have modified the geometry of elec-
trodes (Fig. 4(a)). The stainless steel collector has been set
above the beaker with the sample under study (Figs. 1(a)
and 4(a)) and directly connected to the “high” input of a
picoammeter. The second, “low”, input was grounded.
Such a scheme excludes the same sign of signals due to neg-
ative and positive charges detected, respectively, by anode
and cathode. Typical current signals detected during destruc-
tion of nitrogen-helium sample are shown in Fig. 4(c). The
unipolar current peaks reflect the signs and values of the
charges accumulated by sample fragments touching the ion
collector (anode). Bipolar signals correspond to currents
induced on the collector by passing charged particles. A
photo in Fig. 4(b) shows the destruction process of the ni-
trogen-neon-helium samples. Scatters of burning sample
fragments from the beaker accompany the explosions dur-
ing the sample destruction. Due to rather long exposure
time ~ 0.1 s, one can see trajectories of some sample frag-
ments leaving the beaker during the sample explosions. In
addition to the burning sample and fragments flying away,
one can see in the beaker a bright green flash — a cloud of
glowing dust.
Fig. 2. (Color online) (a) Temporal dependences of the temperature (red line) and the thermostimulated α-group emission (green dots)
of the sample prepared from a gas mixture [N2]/[Ne]/[He] = 1/100/5000; (b) the fit of the emission decay as I–1 ~ t (line).
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 3 291
A.A. Pelmenev, I.N. Krushinskaya, I.B. Bykhalo, and R.E. Boltnev
4. Discussion
4.1. Formation of charged nanoclusters and macroscopic
structure of impurity-helium condensates
The intense formation of filaments during preparation
of impurity-helium condensates due to application of ra-
diofrequency discharge (Fig. 1(b) and (c)) agrees very well
with the idea of ion presence in impurity nanoclusters. The
appearance of charged nanoclusters makes their interaction
stronger and anisotropic. The filament formation is not the
result of the coalescence of impurity grains and flakes due
to quantum vortices [15,26] because such filaments have
never been observed during preparation of impurity-helium
condensates without of radiofrequency discharge. We sug-
gest that electrons and ions are trapped in impurity
nanoclusters during cluster growth in a cold gas jet. We
observed spectra of ions 2N+ in the jet just above the He II
surface during IHC sample preparation [27]. A possibility
of creation of nitrogen cations N+, 2N ,+ 3N ,+ and 4N ,+ in
helium-nitrogen plasma is rather high mainly due to the
efficient Penning ionization by helium metastable species
(He+, *He , and *
2He ) [28–31], particularly in cold (at T <
< 100 K) afterglow, where ions Nn
+ with 1 ≤ n ≤ 9 had
been observed [32]. We should also keep in mind that the
nitrogen atoms, molecules, and nanoclusters moving along
Fig. 3. (Color online) A comparison between the thermoluminescence spectra during warm-up and destruction (blue and red lines, re-
spectively) of the IHC samples prepared from: (a) gas mixture [N2]/[He] = 1/100; (b) gas mixture [N2]/[Ne]/[He] = 1/20/500. The spec-
tral response of the detection system is shown by green dotted line.
Fig. 4. (Color online) Detection of current pulses during destruction of IHC samples: (a) disposition of the ion collector and the beaker
containing an IHC sample; (b) explosive destruction of an IHC sample; (c) the current pulses detected during destruction of the sample
prepared from [N2]/[He] = 1/100 gas mixture.
292 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 3
On charged impurity structures in liquid helium
the jet are still irradiated with VUV photons from helium
plasma within the discharge area (Fig. 1). Such photons are
also able to ionize nitrogen atoms and molecules [33].
Therefore, the presence of charged nanoclusters in IHC
samples prepared with rf discharge application should be
quite obviously.
We have estimated the charge values of sample frag-
ments from the current amplitudes (Fig. 4(c)): the current
of ~ nA produces the voltage ~ 10 µV (the input resistance
of the picoammeter is equal to 10 kOhm for the work range
below 20 nA). To charge the measurement circuit capaci-
tance ~ 100 pF up to the potential of 10 µV we need ~ 1 fC
or about of 104 elementary charges. Detection of IHC
fragments carrying ~ 104 elementary charges was very sur-
prising for us. Definitely, the growth of impurity clusters
around ionic centers and capturing electrons by the clusters
in the cold jet can’t be a reason of formation of macroscop-
ic aggregations consisting of nanoclusters bearing unipolar
electric charges. We suggest a capture of electrons from a
glass beaker surface by excited metastable N2(A3Σ) mole-
cules might be responsible for the charging of IHC sam-
ples. Formation of N2
–(X2Πg) through a capture of elec-
trons from dielectric and metallic surfaces by metastable
N2(A3Σ) molecules has been studied rather well [34–36].
The destruction of IHC sample starts with a sublimation of
the helium shells surrounding impurity nanoclusters and
preventing the recombination of nitrogen atoms stabilized
on the nanocluster surfaces. Nitrogen atom recombination
produces mainly metastable molecules N2(A3Σ) observable
due to intense luminescence [24,25,27]. Then N2(A3Σ)
molecules contacting with the beaker wall capture the elec-
trons. That is why the sample surface next to the beaker is
charging negatively. When the sample loses its integrity
because of explosions, the charged fragments fly away due
to Coulomb repulsion (Fig. 4(b)) and some of them are
detected by the ion collector (Fig. 4(c)).
Therefore, we can explain the IHC structure modifica-
tion and the results on current detections during IHC sam-
ple destructions by two different mechanisms of
nanocluster charging. The first one consists in trapping
positive ions and electrons by nanoclusters growing in the
cold jet. It forms quasineutral IHC samples. The second
mechanism provides a charge separation: nanoclusters cap-
ture electrons from the contacting surfaces due to the pecu-
liarity of N2(A3Σ) molecule interaction with dielectric and
metallic surfaces.
During IHC sample destruction we can see, beside of
the burning sample, some fragments flying away and a
cloud of glowing ultrafine particles. Keeping in mind that
some big and ultrafine particles are charged we can define
the whole system as cryogenic dusty plasma. From this
point of view, a behavior of such systems in external elec-
tric field is of great interest.
4.2. “Cold” and “hot” thermoluminescence spectra from
IHCs
The initial point of this work was the principal distinc-
tion between the “cold” and “hot” spectra observed from
nitrogen-helium and nitrogen-neon-helium condensates.
This distinction along with last results on currents detected
during destruction of IHCs [24,25] had allowed us to sug-
gest existence of two different mechanisms responsible for
“cold” and “hot” luminescence from IHCs.
The earlier explanation of the thermostimulated lumi-
nescence of IHC samples submerged in He II was related
to the model of van der Waals clusters consisting of single
impurity atom or molecule surrounded by a shell of local-
ized helium atoms [23]. The key point was the extremely
weak perturbation of the helium shell on the central meta-
stable N(2D) atom. The lifetime of such N(2D) atoms was
expected to be close to its gas phase value ~ 104 s. The
green emission on 2D–4S transition was stimulated by
“heavy” particle (N2 molecule, atoms of neon, argon, or
Kr) approaching N(2D) atom. The high sensitivity of the
IHC samples to the temperature changes has been ex-
plained by the low value (about of 40 K) of the energy
needed for helium shell rearrangement and meeting of
N(2D) atom and “heavy” particle [23]. Nevertheless, the
model did not allow us to explain the photostimulated lu-
minescence of IHC samples submerged in bulk He II.
Moreover, the model was not confirmed by structural stud-
ies of IHCs revealed nanoclusters as building units of the
condensates [10–12].
It is well known that each temperature increase of
cryocrystals of rare gases and molecular nitrogen is fol-
lowed by thermal mobilization of the electrons trapped in
shallow traps [24,37,38]. We suggest that the electrons are
localized in traps of varying depth (with the different acti-
vation energy). Some impurity centers as well as different
structural defects of cryocrystals and clusters can be such
electron traps. There are very broad distributions of elec-
tron traps in cryocrystals and clusters on the activation
energy: the activation energies of 3.5 and 20 meV were
determined for structure defects in neon nanoclusters [23]
and cryocrystals [39], respectively, while the electron af-
finity energy of O(3P) atom is of 1.46 eV [40]. Therefore,
thermo- or photostimulated electrons leave the traps and
initiate neutralization reactions. It was shown that such
electrons are responsible for luminescence observed in
cryocrystals [39,41]. We suggest similar processes were
observed in cryocondensates [42–44] and impurity
nanoclusters [23] at temperatures below 10 K when the
atoms are still immobile in solids.
Our analysis of the literature shows that the products of
neutralization reactions of N+ and 2N+ with electrons, and
dissociative recombination reactions of 3N ,+ and 4N+ with
electrons are mainly N(2D) atoms and N2 molecules in the
metastable lowest singlet 1( )ua −′ Σ and triplet 3( )uA +Σ
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 3 293
A.A. Pelmenev, I.N. Krushinskaya, I.B. Bykhalo, and R.E. Boltnev
states [45–50]. The bands corresponding to the transition
1 1
u ga X− +′ Σ → Σ were out of the spectrometer work range.
The absence of the β-group emission in the “cold” spec-
tra reflects the fact that the numbers of ions O+ stabilized
in neon and nitrogen nanoclusters are much less comparing
to nitrogen cations. Now, we should mention some addi-
tional pathways of formation of excited N and O atoms in
solid nitrogen matrix [22]:
3 4 2 1
2 2N N N( ) ( ) ( ) N ( ),u gA S D X+ +Σ + → + Σ (1)
3 4 2 1
2 2N N N( ) ( ) ( ) N ( ),u gA S P X+ +Σ + → + Σ (2)
3 3 1 1
2 2( ) ( ) (N O O N) ( ).u gA P S X+ +Σ + → + Σ (3)
There were no the features of N(2P) and O(1S) atoms
(the δ-line and the β-group, correspondingly) detected in
the “cold” spectra (Fig. 3). We suggest this fact is due
to inefficiency of the energy migration 3
2 ( )N uA +Σ +
1 1 3
2 2 2( ) ( ) ( )N N Ng g uX X A+ + ++ Σ Σ Σ→ + within nitrogen
nanoclusters because of high density of structure defects
[10]. This assumption is supported by identical “cold”
spectra detected from nitrogen-neon-helium sample. There
is no energy transfer possible from excited nitrogen mole-
cule to neither N nor O atom isolated in a neon matrix. So,
“cold” spectra contain the features only of neutralization
reaction products.
As one can see, the features of “cold” spectra can be
well explained by recombination of nitrogen cations and
electrons trapped in nitrogen and neon nanoclusters. The
high sensitivity of IHCs to temperature changes is a conse-
quence of the shallow traps (with the depth of ~ meV) formed
by structural defects. The suggested mechanism of thermo-
luminescence allows us also to explain an origin of photo-
stimulated emission of IHC samples submerged in He II [23].
Electrons can be releazed from the traps by photons of appro-
priate energy and promoted into the conduction band [51].
Such photomobilized electrons participate in exoelectron
emission [51–53], react with holes producing luminescence
[42,54,55] and anomalous low-temperature post-desorption
from the cryocrystal surfaces [45,56].
The thermoluminescence mechanism proposed agrees
well with the experimental results of the ESR studies of
IHCs containing nitrogen atoms: the number of stabilized
N(4S) atoms during the warm-up of the samples in bulk
He II remains almost constant [8].
Therefore, we suggest that neutralization reactions are re-
sponsible for luminescence of IHC samples submerged in
He II (“cold” spectra), while recombination of N(4S) atoms
dominates as energy source for thermoluminescence (“hot”
spectra) of “dry” IHC samples at higher temperatures.
The hyperbolic decay of the long-lived afterglow
(Fig. 2(b)) corresponds to electron release from the traps
uniformly distributed on the depths [57]. Thus, the thermo-
luminescence observed in IHC samples submerged in He II
can be explained by electrons releazed from the traps with
very broad distribution of the activation energy (~ meV in the
structure defects and ~ eV at atoms with large positive elec-
tron affinity energy) in impurity nanoclusters. The tempera-
ture decrease causes fast α-group decay with the characteristic
times much shorter than the lifetimes of metastable N(2D)
atoms stabilized in corresponding matrices [23].
5. Conclusions
In this paper, we have presented new experimental re-
sults of optical spectroscopy and current detection studies
of impurity-helium condensates. The phenomena observed
during formation (growth of impurity filaments and stick-
ing of IHC fragments to a glass surface) and destruction
(scattering of fragments and current pulse detection) of
impurity-helium condensates, as well as the distinctions
between the “cold” and “hot” spectra had allowed us to
propose existence of two different mechanisms of charging
impurity nanoclusters.
1. Thermoluminescence spectra of nitrogen-helium and
nitrogen-neon-helium condensates submerged in superfluid
helium had been observed for the first time.
2. The phenomena observed during impurity-helium
condensate formation can be explained by presence of ions
trapped in impurity nanoclusters. Electrons and positive
ions are trapped by the nanoclusters growing in cold heli-
um gas jet passed through a radiofrequency discharge.
3. The luminescence of IHC samples submerged in bulk
He II is caused by neutralization reactions with participa-
tion of thermo- and photoactivated electrons and nitrogen
cations stabilized in impurity nanoclusters.
4. Another mechanism of the charged nanoclusters for-
mation is proposed to describe the phenomena observed
during the destruction of IHC samples: intense recombina-
tion of nitrogen atoms during the IHC sample destruction
produces excited molecules 3
2 ( )N A Σ which capture elec-
trons from a substrate and charge the sample fragments up
to 104 elementary charges.
5. Charged big fragments and ultrafine particles form
systems which can be defined as cryogenic dusty plasma.
Such systems in gas and liquid phases of helium-4 are of
great interest for studying at temperatures of 1–10 K.
Acknowledgments
The work has been done under support of the Russian
Science Foundation (grant #14-50-00124) in Joint Institute
for High Temperatures.
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1. Introduction
2. Experimental setup
3. Experimental results
3.1. Macrostructure of IHC samples prepared with rf discharge applied
3.2. The green afterglow decay
3.3. Thermoluminescence spectra of IHC samples
3.4. Currents accompanying destruction of IHC samples
4. Discussion
4.1. Formation of charged nanoclusters and macroscopic structure of impurity-helium condensates
4.2. “Cold” and “hot” thermoluminescence spectra from IHCs
5. Conclusions
Acknowledgments
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