Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals
Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy electrons are studied with the focus on the role of diffusion controlled atom-atom recombination reaction of oxygen in the relaxation cascades. The results of correlated in real time measurements of thermally s...
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irk-123456789-1208912017-06-14T03:05:05Z Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals Savchenko, E.V. Belov, A.G. Gumenchuk, G.B. Ponomaryov, A.N. Bondybey, V.E. Cryocrystals Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy electrons are studied with the focus on the role of diffusion controlled atom-atom recombination reaction of oxygen in the relaxation cascades. The results of correlated in real time measurements of thermally stimulated phenomena are presented. The experiments have been performed using activation spectroscopy methods — thermally stimulated exoelectron emission and spectrally resolved thermally stimulated luminescence. Solid evidence of the radiative mechanism of electron detrapping triggering the relaxation cascades is obtained. 2006 Article Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals / E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, V.E. Bondybey // Физика низких температур. — 2006. — Т. 32, № 11. — С. 1417–1421. — Бібліогр.: 15 назв. — англ. 0132-6414 PACS: 72.20.Jv, 79.75+g, 78.60.Kn http://dspace.nbuv.gov.ua/handle/123456789/120891 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Cryocrystals Cryocrystals |
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Cryocrystals Cryocrystals Savchenko, E.V. Belov, A.G. Gumenchuk, G.B. Ponomaryov, A.N. Bondybey, V.E. Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals Физика низких температур |
| description |
Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy electrons
are studied with the focus on the role of diffusion controlled atom-atom recombination reaction
of oxygen in the relaxation cascades. The results of correlated in real time measurements of
thermally stimulated phenomena are presented. The experiments have been performed using activation
spectroscopy methods — thermally stimulated exoelectron emission and spectrally resolved
thermally stimulated luminescence. Solid evidence of the radiative mechanism of electron detrapping
triggering the relaxation cascades is obtained. |
| format |
Article |
| author |
Savchenko, E.V. Belov, A.G. Gumenchuk, G.B. Ponomaryov, A.N. Bondybey, V.E. |
| author_facet |
Savchenko, E.V. Belov, A.G. Gumenchuk, G.B. Ponomaryov, A.N. Bondybey, V.E. |
| author_sort |
Savchenko, E.V. |
| title |
Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals |
| title_short |
Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals |
| title_full |
Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals |
| title_fullStr |
Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals |
| title_full_unstemmed |
Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals |
| title_sort |
oxygen-driven relaxation processes in pre-irradiated ar cryocrystals |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2006 |
| topic_facet |
Cryocrystals |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120891 |
| citation_txt |
Oxygen-driven relaxation processes in pre-irradiated
Ar cryocrystals / E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, V.E. Bondybey // Физика низких температур. — 2006. — Т. 32, № 11. — С. 1417–1421. — Бібліогр.: 15 назв. — англ. |
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Физика низких температур |
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| first_indexed |
2025-07-08T18:49:31Z |
| last_indexed |
2025-07-08T18:49:31Z |
| _version_ |
1837105752656838656 |
| fulltext |
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11, p. 1417–1421
Oxygen-driven relaxation processes in pre-irradiated
Ar cryocrystals
E.V. Savchenko1, A.G. Belov1, G.B. Gumenchuk1, A.N. Ponomaryov2,
and V.E. Bondybey2,3
1B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy
of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: savchenko@ilt.kharkov.ua
2Institut f�r Physikalische und Theoretische Chemie, TU M�nchen, Lichtenbergstr, 4,
Garching 85747, Germany
3University of California, Irvine 92697, USA
Received September 13, 2006
Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy elec-
trons are studied with the focus on the role of diffusion controlled atom-atom recombination reac-
tion of oxygen in the relaxation cascades. The results of correlated in real time measurements of
thermally stimulated phenomena are presented. The experiments have been performed using acti-
vation spectroscopy methods — thermally stimulated exoelectron emission and spectrally resolved
thermally stimulated luminescence. Solid evidence of the radiative mechanism of electron detrap-
ping triggering the relaxation cascades is obtained.
PACS: 72.20.Jv, 79.75+g, 78.60.Kn
Keywords: thermally stimulated luminescence, thermally stimulated exoelectron emission, relaxation
processes, radiation effects, radical recombination, cryocrystals.
Introduction
Oxygen is one of the most abundant elements in the
universe that exhibits in many respects unique proper-
ties attracting considerable interest of scientists over
many years. A.F. Prikhot’ko studied extensively ele-
mentary excitation of solid oxygen [1] and a lot of the
credit must go to her research group which discovered
a number of novel phenomena in solid oxygen. The
brilliant findings of A.F. Prikhot’ko inspired us when
we went to the heart of the matter on the role of oxy-
gen in the relaxation pattern of pre-excited cryo-
crystals. Exposure of insulating materials to ionizing
radiation results in formation of electron-hole pairs,
defects of structure, molecule fragments. Relaxation
channels following the primary electronic excitation
involve such processes as electron-hole recombination,
trapping or self-trapping of charge carriers, emission
of electrons and photons etc. Some fraction of energy
absorbed by solids under irradiation is then stored by
charged and neutral centers formed in the lattice.
After the irradiation is completed a further relaxation
can be stimulated by heating the samples or optically
— by photons. Both factors can induce relaxation pro-
cesses in electronic and atomic subsystems. Clear un-
derstanding of the radiation effects changing the solid
properties is of great importance for material and sur-
face sciences as well as for radiation chemistry. Very
often electronic and atomic relaxation processes occur-
ring in pre-irradiated solids are considered separately
ignoring the interconnection between these channels
of relaxation. Our recent studies demonstrated the
fundamental importance of the interconnection be-
tween atomic and electronic processes in the formation
and branching of relaxation paths [2].
Ar cryocrystals were chosen as matrices for several
reasons: (i) because of a small atomic radius of oxygen
(0.12 nm) it can be easily stabilized at low tempera-
tures in Ar solids without any significant distortion of
the lattice; (ii) solid Ar and O2 have close sublimation
temperatures (about 30 K); (iii) a high mobility and
© E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, and V.E. Bondybey, 2006
free-like behavior of electrons in solid Ar [3] together
with a large electron escape depth of about 500 nm [4]
enables one to gain information about relaxation pro-
cesses in the bulk and at the surface; (iv) a negative
electron affinity of solid Ar (–0.3 eV [3]) facilitates es-
caping the electrons from the surface. Moreover, the
first successful experiments on thermally stimulated
exoelectron emission (TSEE) [5] offered the prospect
for developing of low-temperature comprehensive acti-
vation spectroscopy technique as applied to cryocrystals.
We combined TSEE studies with measurements of
the total and spectrally resolved yields of thermally
stimulated luminescence (TSL). Partial yields in the vis-
ible and in the vacuum ultraviolet (VUV) ranges were
detected. In addition, the experiments on photon-stimu-
lated exoelectron emission (PSEE) were performed.
Since both TSL and TSEE are quite sensitive to sample
structure and concentration of guest centers there is a
clear need to make synchronous measurements of TSL,
TSEE on the same sample. We present the results of
«correlated activation spectroscopy» studies of relax-
ation processes in pre-irradiated Ar solids doped with
oxygen. The studies are aimed to elucidating in detail
the role of oxygen in the relaxation cascades.
Experimental
In this study we used the low-temperature modifi-
cation of activation spectroscopy technique developed
recently by our group, namely the real time-correlated
measurements of relaxation processes in cryogenic so-
lids with simultaneous measurements of TSEE, spec-
trally resolved TSL or partial TSL yields in a chosen
wavelength range. The samples were grown from the
gas phase by deposition on a metal substrate coated
with a thin layer of MgF2. The substrate was cooled
by a closed-cycle 2-stage Leybold RGD 580 cryostat.
The high-purity (99.999%) Ar gas was used. A base
pressure in the vacuum chamber was 10–8 mbar. The
samples were doped with minute amounts of O2
(10–4). The deposition was performed with a concur-
rent irradiation by 500 eV electrons to generate charge
centers throughout the layer. The current density was
kept at 30 �A/cm2. A typical deposition rate was
10–1
�m/s. We deposited the Ar films of 50–100 �m
thickness. The sample thickness and the deposition
rate were determined by measuring the pressure de-
crease in a known volume of the gas-handling system.
The temperature was measured with a calibrated sili-
con diode sensor, mounted on the substrate. The rela-
xation processes in the Ar samples were studied in the
temperature range from 7 to 42 K. The programmable
temperature controller LTC 60 allowed us to maintain
desired temperatures of deposition, irradiation and
heating regimes. Two heating modes were used: linear
heating at a constant rate of 3.2 K/min and step-wise
heating of the samples with a step of 2 K and a 5 min
interval between successive steps. After the sample
preparation the substrate was turned to the position
for measurements. We measured simultaneously the
yields of thermally TSEE from the samples and spec-
trally-resolved TSL in the visible range or the yield of
TSL in the VUV range. The exoelectron yield was
measured with an Au-coated Faraday plate kept at a
small positive potential +9 V and connected to a cur-
rent amplifier FEMTO DLPCA 100. A centrally lo-
cated hole in the Faraday plate permitted us to detect
spectra from the sample through an optical window.
The spectra were recorded by a spectrometer (Multi-
channel S2000 Spectrometer Ocean Optics based on
CCD detectors) operating in the range 170–1100 nm
with a resolution not worse than 1.3 nm in the
millisecond time window. This spectrometer allows
monitoring the temporal evolution of the spectra in
the operating range. The program developed permit-
ted us to detect synchronously spectra over the entire
operating range, TSEE current, temperature and pres-
sure in the chamber.
Results and discussion
Figure 1 shows typical curves of the yields of ther-
mally stimulated exoelectron emission and recombina-
tion thermally stimulated luminescence in the VUV
range stemmed from the reaction
Ar Ar Ar Ar2 2
� �
� � � � � �e E h� � (9.7 eV). (1)
The measurements were performed simultaneously on
the same sample in the temperature range 7–42 K
using heating at a constant rate of 3.2 K/min. The
clearly seen structure of the curves reasonably agrees
1418 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11
E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, and V.E. Bondybey
5 10 15 20 25 30 35 40 45
0
20
40
60
80
100
120
140
0
2
4
6
8
10
12
14
TSEE
Temperature, K
VUV TSL
V
U
V
T
S
L
in
te
n
si
ty
, a
rb
. u
n
its
T
S
E
E
cu
rr
e
n
t,
p
A
Fig. 1. Yields of TSL in the VUV range and TSEE taken
from the sample of Ar which was annealed at 25 K and
then irradiated at 7 K.
with the previously measured yield of TSL detected
in the M-band [6,7] and the yield of TSEE [5] with
the only difference that the curves in Fig. 1 show
more distinct features. The low-temperature peak at
11.5 K is related to the electron traps in a subsurface
layer or at inner interfaces of the sample. The peak at
about 15 K exhibiting the pronounce dose dependence
was assigned to radiation-induced defects [5,6,8,9]. A
rather strong TSEE maximum observed between 37
and 41 K is presumably due to electrons trapped too
deep in the sample to be promoted to the conduction
band, which are however released on the sample eva-
poration. The corresponding maximum in the VUV
TSL turned out to be quite weak.
The origin of the broad 23 K peak in thermally
stimulated phenomena was under discussion over
10 years. This peak was detected in the total yield of
TSL [5,8] and in intrinsic recombination emission —
spectrally resolved TSL in the M-band [6] taken from
Ar pre-irradiated by a 1 keV electron beam as well as
from Ar pre-irradiated by x-rays [7]. Note, that the
peak at 23 K was also registered in the thermally sti-
mulated conductivity [9] and in thermally stimulated
exoelectron emission [5]. On the other hand, the in-
tensity of this peak showed a clear connection with
the presence of oxygen in the samples enhancing TSL
and TSEE peaks at 23 K with increasing oxygen con-
tent. Monitoring of spectrally resolved TSL in the vi-
sible range taken from pre-irradiated Ar solids re-
vealed the emergence of O2
� emission near 23 K in
oxygen-containing samples. Different mechanisms of
the 23 K peak formation were suggested to explain
this puzzling behavior of this peak in the yields of
thermally stimulated phenomena in Ar solids. For in-
stance it has been suggested in [9] that this peak
is caused by recombination of O– ions with neutral
O atoms. Another scenario was discussed in [10,11]. It
was supposed that neutral O atoms on heating started
to diffuse through the Ar lattice forming oxygen mole-
cules O2
� . The radiative transition of O2
� into the
ground state was considered as a stimulating factor to
electron detrapping which is the primary step of
branching relaxation channels.
To verify the discussed above [9–11] mechanisms of
triggering relaxation cascades by recombination reac-
tions we performed the experiments with synchronous
measurements of thermally stimulated chemilumi-
nescence of O2
� stemmed from the diffusion-controlled
atom-atom recombination reaction and TSEE yield as
well as yields of TSEE and TSL in the VUV range
originated from the intrinsic charge recombination re-
action (1). The samples were doped with O2 (10–4)
and the generation of O atoms under irradiation was
monitored spectroscopically. Figure 2 shows a part of
the luminescence spectrum in the visible range re-
corded under electron beam at low temperature (7 K).
The molecular bands of oxygen (the C Xu g
3 3
� ��
�
transition) became relatively weak just after 10 min
irradiation. The strongest feature in the spectrum be-
longs to the O atoms effectively formed in the sample
under electron beam. An increase of the O line inten-
sity with exposure time is illustrated in the insert.
In order to study the origin of the 23 K peak we
«removed» all the peaks at temperatures lower than
the temperature of emerging the peak under study.
For the purpose we used the so-called «cleaning tech-
nique» [12] and irradiated oxygen containing samples
at 18 K. On completing the irradiation we recorded
the yield of TSEE and the TSL yield in the VUV
range shown in Fig. 3. Both curves exhibit a maxi-
mum near 23 K thus demonstrating the correlation of
the exoelectron emission and the emission of the VUV
photons of the intrinsic charge recombination reaction
(1). The VUV TSL curve turn out to be much broader
than the TSEE curve which is to say that some addi-
tional electron traps are involved in the bulk recombi-
nation channel. The corresponding VUV TSL curve
taken from the sample annealed at 25 K (Fig. 1) ex-
hibits a more narrow peak which correlates with the
23 K peak in TSEE. At the same time the correlated in
real time measurements of the TSEE yield and spec-
trally resolved TSL in the operating range of the spec-
trometer demonstrated in a conclusive way the corre-
lation in the TSEE yield of the sample irradiated at
18 K and the observed simultaneously photon emission
Oxygen driven relaxation processes in pre-irradiated Ar cryocrystals
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1419
300 400 500 600 700
0
5
10
15
20
25
30
35
550 560 570
0
10
20
30
40
50
60
Wavelength, nm
In
te
n
si
ty
,a
rb
.u
n
its
1
2
Fig. 2. Luminescence spectrum of the Ar sample doped
with O2 (10–4) recorded in the range of O2 and O emis-
sions. The spectrum was detected during deposition under
electron beam. The O line detected after 10 (1) and 20 (2)
min exposure is shown in the insert.
of O2
� (the C Xu g
3 3
� ��
� transition). Two cycles of
the experiments were performed to check carefully the
correlation (with liner and step-wise heating). Figure 4,a
displays the oxygen molecule progression in TSL mea-
sured in step at 23 K. The simultaneously detected
TSEE current is depicted in Fig. 4,b together with the
behavior of the chosen band of the progression (�� = 0,
��� = 10). The experiment with the linear heating
(Fig. 5) confirmed the correlation between the mole-
cular oxygen emission and the yield of the TSEE cur-
rent near 23 K.
Our experimental data on relaxation processes in
pre-irradiated Ar solids doped with oxygen enable us
to restore the following scenario to explain the pattern
puzzling at first sight.
The charge recombination reactions in pre-irradi-
ated rare-gas atomic cryocrystals are controlled by the
mobility of electrons. The weakest bound in the traps
electrons are mobilized at the lowest temperatures and
escape the crystal or neutralize positive intrinsic
(self-trapped holes) ions. These charge recombination
reactions result in the appearance of the VUV emis-
sion. At a definite, higher temperature, the O atoms
present also start to diffuse, and a complex sequence
of events may take place. Recombination of neutral O
atoms results in the formation of molecular O2
� in
bound excited electronic states. This is followed by a
rapid relaxation, and eventually by radiative transi-
tion into the ground state. The emitted visible range
photons, in turn, can be absorbed and provide the en-
ergy needed to detrap electrons from deeper traps pro-
moting them to the conduction band. This is some
kind of «internal photoeffect» followed by (i) ejec-
tion of exoelectrons from the surface, (ii) conversion
of visible light into VUV photons via recombination
of electrons with self-trapped holes by the reaction
(1), or (iii) conversion of visible light into photons of
other range via recombination of electrons with some
extrinsic positively charged centers. All these elec-
tron-driven processes, which are outlined in the
scheme below, were detected in our experiments.
1420 Fizika Nizkikh Temperatur, 2006, v. 32, No. 11
E.V. Savchenko, A.G. Belov, G.B. Gumenchuk, A.N. Ponomaryov, and V.E. Bondybey
5 10 15 20 25 30 35
0
20
40
60
80
100
120
140
160
180
200
0
10
20
30
40
50
TSEE
Temperature, K
VUV TSL
V
U
V
T
S
L
in
te
n
si
ty
, a
rb
. u
n
its
T
S
E
E
cu
rr
e
n
t,
p
A
Fig. 3. Yields of TSL in the VUV range and TSEE taken
from the oxygen-containing sample pre-irradiated at 18 K.
16000 18000 20000 22000 24000 26000
5
0
10
15
20
25
30
35
40
45
50
Wavenumber , cm–1
0 1000 2000 3000 4000 5000
0
10
20
30
40
50
60
70
0
100
200
300
400
500
TSEE
39K
37K35K
33K
31K
29K
27K
25K
23K
21K
19K
Time, s
Tim
e, s
17K
O line2TSL
T
S
E
E
cu
rr
e
n
t,
p
A
T
S
L
i
, a
rb
.u
n
its
n
te
n
si
ty
In
te
n
si
ty
, a
rb
.u
n
its
a
b
2800
2500
Fig. 4. TSL yield in the visible range and TSEE yield
measured at step-wise heating of the Ar sample doped
with O2 (10–4). The sample was pre-irradiated at 18 K.
The TSL spectrum measured at the 23 K step is shown in
(a); (b) displays the temperature behavior of the 0–10 mo-
lecular band of the C Xu g
3 3
� ��
� transition.
5 10 15 20 25 30
0
50
100
150
200
250
300
350
400
511 nm
TSL O
TSEE
Temperature, K
T
S
L
in
te
n
si
ty
,a
rb
. u
n
its
T
S
E
E
cu
rr
e
n
t,
p
A
0
40
80
120
160
200
240
2
Fig. 5. Yields of O2 emission in TSL and TSEE taken
from the oxygen-containing sample pre-irradiated at 18 K.
where Rg is a rare-gas atom.
The results obtained provide solid evidence for the
electronic relaxation stimulation via chemiluminescent
reactions. It is worthy of note that in contrast to the
local energy release under nonradiative radical recom-
bination the revealed radiative mechanism provides a
long-range energy transfer for hundreds of lattice con-
stants. Based on these results other radiative mecha-
nisms of relaxation cascades triggering in pre-irra-
diated solids can be predicted, e.g., stimulation of
relaxation by radiative transitions of metastable atoms
or molecules. The first results demonstrating reliabi-
lity of this mechanism are presented in [14]. The ex-
periments on stimulation of relaxation processes by la-
ser light [2,15] provide an additional proof of the
mechanisms discussed.
Summary
The combination of correlated in real time measure-
ments of spectrally-resolved thermally stimulated lu-
minescence and exoelectron emission from pre-irradi-
ated Ar solids doped with O2 has given conclusive
evidence in favour of relaxation cascades stimulation
via chemiluminescent reactions. The verified mecha-
nism provides a long-range energy transfer and dem-
onstrates the existence of a new relaxation channel
(triggering of electronic relaxation processes by diffu-
sion controlled atom-atom recombination reaction fol-
lowed by light emission).
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Oxygen driven relaxation processes in pre-irradiated Ar cryocrystals
Fizika Nizkikh Temperatur, 2006, v. 32, No. 11 1421
Photon-stimulated exoelectron emission (i)
e�
O + O � O2
�
� O2 + h� � e � Rg2
� + e � Rg2
�
� Rg + Rg + h�1(VUV) + �E (ii)
e
�
Guest+ + e � Guest* � Guest + h�2, (iii)
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