Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam
Spatially separated stable charge centers, self-trapped holes and trapped electrons, were generated in Ar cryocrystals by a low-energy electron beam. A combination of the cathodoluminescence (CL) and photon- stimulated exoelectron emission (PSEE) methods was used to monitor center formation and se...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2007
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| Цитувати: | Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam / G.B. Gumenchuk, A.N. Ponomaryov, A.G. Belov, E.V. Savchenko, V.E. Bondybey // Физика низких температур. — 2007. — Т. 33, № 6-7. — С. 694-700. — Бібліогр.: 26 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
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irk-123456789-1217912017-06-17T03:03:02Z Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam Gumenchuk, G.B. Ponomaryov, A.N. Belov, A.G. Savchenko, E.V. Bondybey, V.E. Electronic Processes in Cryocrystals Spatially separated stable charge centers, self-trapped holes and trapped electrons, were generated in Ar cryocrystals by a low-energy electron beam. A combination of the cathodoluminescence (CL) and photon- stimulated exoelectron emission (PSEE) methods was used to monitor center formation and selected relaxation channel – exoelectron emission. It was found that photon-promoted electron current decreased exponentially under irradiation with the laser operating in the visible range. Influence of the laser parameters (power and wavelength) on the characteristic lifetime of exoelectron emission is discussed. Effective bleaching of the low-temperature peaks of thermally stimulated exoelectron emission by the laser light in a visible range was observed. 2007 Article Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam / G.B. Gumenchuk, A.N. Ponomaryov, A.G. Belov, E.V. Savchenko, V.E. Bondybey // Физика низких температур. — 2007. — Т. 33, № 6-7. — С. 694-700. — Бібліогр.: 26 назв. — англ. 0132-6414 PACS: 72.20.Jv; 78.60.Kn; 79.75.+g http://dspace.nbuv.gov.ua/handle/123456789/121791 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
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English |
| topic |
Electronic Processes in Cryocrystals Electronic Processes in Cryocrystals |
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Electronic Processes in Cryocrystals Electronic Processes in Cryocrystals Gumenchuk, G.B. Ponomaryov, A.N. Belov, A.G. Savchenko, E.V. Bondybey, V.E. Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam Физика низких температур |
| description |
Spatially separated stable charge centers, self-trapped holes and trapped electrons, were generated in Ar
cryocrystals by a low-energy electron beam. A combination of the cathodoluminescence (CL) and photon-
stimulated exoelectron emission (PSEE) methods was used to monitor center formation and selected relaxation
channel – exoelectron emission. It was found that photon-promoted electron current decreased exponentially
under irradiation with the laser operating in the visible range. Influence of the laser parameters
(power and wavelength) on the characteristic lifetime of exoelectron emission is discussed. Effective
bleaching of the low-temperature peaks of thermally stimulated exoelectron emission by the laser light in a
visible range was observed. |
| format |
Article |
| author |
Gumenchuk, G.B. Ponomaryov, A.N. Belov, A.G. Savchenko, E.V. Bondybey, V.E. |
| author_facet |
Gumenchuk, G.B. Ponomaryov, A.N. Belov, A.G. Savchenko, E.V. Bondybey, V.E. |
| author_sort |
Gumenchuk, G.B. |
| title |
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam |
| title_short |
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam |
| title_full |
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam |
| title_fullStr |
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam |
| title_full_unstemmed |
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam |
| title_sort |
stimulated by laser light exoelectron emission from solid ar pre-irradiated by an electron beam |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2007 |
| topic_facet |
Electronic Processes in Cryocrystals |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121791 |
| citation_txt |
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam / G.B. Gumenchuk, A.N. Ponomaryov, A.G. Belov,
E.V. Savchenko, V.E. Bondybey // Физика низких температур. — 2007. — Т. 33, № 6-7. — С. 694-700. — Бібліогр.: 26 назв. — англ. |
| series |
Физика низких температур |
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| first_indexed |
2025-07-08T20:31:43Z |
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2025-07-08T20:31:43Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2007, v. 33, Nos. 6/7, p. 694–700
Stimulated by laser light exoelectron emission from solid
Ar pre-irradiated by an electron beam
G.B. Gumenchuk1, A.N. Ponomaryov2, A.G. Belov1,
E.V. Savchenko1, and V.E. Bondybey2,3
1
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine,
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: galina.gumenchuk@mytum.de
2
Institut für Physikalische und Theoretische Chemie, TU München, 4 Lichtenberg Str., Garching 85747, Germany
3
University of California, Irvine 92697, USA
Received November 20, 2006
Spatially separated stable charge centers, self-trapped holes and trapped electrons, were generated in Ar
cryocrystals by a low-energy electron beam. A combination of the cathodoluminescence (CL) and pho-
ton-stimulated exoelectron emission (PSEE) methods was used to monitor center formation and selected re-
laxation channel – exoelectron emission. It was found that photon-promoted electron current decreased ex-
ponentially under irradiation with the laser operating in the visible range. Influence of the laser parameters
(power and wavelength) on the characteristic lifetime of exoelectron emission is discussed. Effective
bleaching of the low-temperature peaks of thermally stimulated exoelectron emission by the laser light in a
visible range was observed.
PACS: 72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping;
78.60.Kn Thermoluminescence;
79.75.+g Exoelectron emission.
Keywords: exoelectron emission, cryocrystals, optical spectroscopy.
Introduction
Irradiation of solid insulators with vacuum ultraviolet
(VUV) light above the band gap energy Eg or with fast
particles results in changing properties of solids via exci-
tation of the electronic and atomic subsystems, defects
creation and following relaxation. These changes and es-
pecially last stage — relaxation cascade — are of high in-
terest for radiation physics and chemistry as well as for
materials science. A variety of relaxation processes have
been under extensive investigation in various classes of
materials [1–4]. Atomic cryocrystals — model insulating
materials — offer the best opportunity to study radiation
effects and various relaxation channels because of their
simple structure, weak interatomic forces, and strong
electron-phonon interaction. The final relaxation stage,
i.e., the processes occurring on completion of the irradia-
tion, is of special interest for understanding the radiation
effects, the dynamics of charge carriers, and stability of
radiation-induced defects. The primary states of the re-
laxation cascades in this case are states of self-trapped or
trapped holes and trapped electrons, as well as metastable
levels of the guests. A stimulating factor for these relax-
ation processes could be the heating of the sample or irra-
diation by visible light.
The methods of activation spectroscopy are especially
powerful tools for the investigation of relaxation in sol-
ids, and the method of thermally stimulated luminescence
(TSL) is the most common in use [5]. Indeed, TSL of
atomic cryocrystals has been studied in several publi-
cations — irradiation by x-rays [6,7], electron beams
[8–11], and synchrotron radiation [12]. The total and
spectrally resolved yields [6–13] of TSL were measured
and the activation energies of various electron traps were
estimated. Analysis of the thermally stimulated intrinsic
recombination luminescence in the VUV range, the
well-known M-band [1], was performed for solid Ar in
the range 15–30 K [7] and in a wider range 5–30 K
[9,13,14]. Relaxation processes involve not only charged
species but also neutral ones and that is why we combined
the methods of optical and current activation spectros-
copy. Thermally stimulated exoelectron emission (TSEE)
© G.B. Gumenchuk, A.N. Ponomaryov, A.G. Belov, E.V. Savchenko, and V.E. Bondybey, 2007
was recently detected from solid Ne [15,16] and Ar [13,
14]. The later studies demonstrated a correlation in the
yields of exoelectrons and VUV photons in the intrinsic
recombination emission from pre-irradiated solid Ar. It
was suggested [17] and then proved [10,11,14] that the
thermally stimulated recombination of neutral guest oxy-
gen atoms in the Ar matrix followed by O2
* formation and
radiative decay of the oxygen molecule resulted in the
emission of exoelectrons from solid Ar. These findings
posed the question concerning the influence of visible
light on the relaxation paths in the atomic cryocrystals.
Up to very recently there was an only study (to our best
knowledge) related with photon-stimulated processes in
atomic cryocrystals [18]. In those experiments the samples
were irradiated by pulsed synchrotron radiation. Between
synchrotron pulses the samples were exposed to laser
pulses. Photoelectrons and photon-stimulated luminescence
in the M-band of Xe and Kr were registered in those experi-
ments. It was found that the characteristic times of pho-
ton-stimulated luminescence from solid Xe and Kr (1700 ns
and 190 ns, respectively) are very close to those in the spon-
taneous luminescence from Rg
2
* dimers in the well-known
M-bands [1]. The excitation spectra after laser irradiation
were found to be identical with those for the spontaneous lu-
minescence from atomic cryocrystals [1].
To investigate the role of stable lattice defects and the
effect of visible light on the relaxation paths, other exper-
iments on photon-stimulated spectrally resolved recombi-
nation luminescence were performed. Solid Ar pre-irradi-
ated with a low-energy electron beam was exposed to the
irradiation of a He–Ne laser operated in the continuous
mode. Stimulated by photons in the visible range, spec-
trally resolved luminescence in the M-band was regis-
tered [19]. In this case we used laser light as the external
source to trigger the relaxation channels in pre-irradiated
solid Ar. The experiments were performed at low temper-
atures to exclude a possible contribution from thermally
stimulated processes. The characteristic decay time of
photon-induced intrinsic recombination luminescence
observed under continuous laser emission was found to
be around 200 s in contrast to the very short lifetime of the
radiative transition 3 1� �g
� �� u (1200 ns) for the sponta-
neous luminescence of solid Ar [1].
Additional information on photon-stimulated pro-
cesses was obtained in the experiments [20,24] with some
kind of an «internal light source». For that purpose, solid
Ar was doped with nitrogen and xenon to form not only
intrinsic but also extrinsic charged centers and metastable
ones. The sample was deposited under electron beam on a
cooled substrate. Under electron bombardment, N2 mole-
cules were fragmented and metastable nitrogen atoms
were efficiently formed. The green afterglow from atomic
nitrogen due to the well-known forbidden transition
2 4D S� , indicating the formation of N atoms, was used
as the «internal light source» to stimulate the relaxation.
Photon-stimulated exoelectron emission as well as spec-
trally resolved VUV recombination luminescence of in-
trinsic Ar2
� and extrinsic Xe 2
� centers with electrons (in-
dicated by the radiative transition of neutralized centers
Ar2
� and Xe 2
� to the ground state) were registered.
Simultaneously with exoelectron emission an afterglow
in the visible range from the doped solid Ar was observed.
It was found that the characteristic lifetimes of the
exoelectron emission and recombination luminescence
from intrinsic and extrinsic charged centers were around
20 s, which characterized the duration of afterglow from
atomic nitrogen in Ar matrix [20].
Analysis of the data obtained shows that the photon-
stimulated relaxation processes are branched into the se-
veral paths: (i) radiative recombination of intrinsic ionic
centers (self-trapped holes) of the dimer Ar2
� with elec-
trons released from the traps by visible light; (ii) radiative
recombination of ionic guests with electrons, and (iii) exo-
electron emission.
Here we present new results on the effect of the param-
eters of external source (laser light) on the characteristics
of the relaxation channel — exoelectron emission. Influ-
ence of the laser power and wavelength on the lifetime of
photon-stimulated exoelectron emission (PSEE) was
investigated.
Experimental
In view of the fact that the relaxation can be triggered
by photon irradiation or heating we combined the thermal
and optical activation spectroscopy methods in order to
investigate the whole set of relaxation paths and their in-
terrelations. The general sequence of experimental proce-
dures was as follows: (1) sample preparation under elec-
tron bombardment, (2) irradiation with a laser light and
measurement of the photon-stimulated exoelectron yield,
(3) upon completion of the laser irradiation the samples
were heated at a constant rate and thermally stimulated
exoelecron emission was recorded. Details of the experi-
ments are given below.
The samples of nominally pure solid argon (99.999 %)
were grown from the gas phase by depositing on a metal
substrate coated by a thin MgF2 layer, which was cooled
to 6 K by a two stage, close-cycle Leybold RGD 580
cryostat. The base pressure in the vacuum chamber (about
10
–8
mbar) was monitored with a Compact BA Pressure
Gauge PBR 260. The content of impurities such as O2,
N2, CO2 and H2O did not exceed 100 ppm. The sample
thickness (of about 100 �m) was determined by measur-
ing the pressure decrease in a known volume of the bulb
in the gas inlet system. The typical deposition rate was
kept at about 10
–2
�m/s.
For the creation of neutral and charged defects in the
films via excitation of the electronic and atomic subsys-
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam
Fizika Nizkikh Temperatur, 2007, v. 33, Nos. 6/7 695
tems the samples were deposited with a concurrent irradi-
ation by a low-energy (500 eV) electron beam. A hot
tungsten filament served as the source of electrons. The
electrons were accelerated to 500 eV and focused by a cy-
lindrical electrostatic lens placed in front of the substrate
and held at –200 V potential deflecting the electrons to-
ward the sample. The current density of the electron beam
was kept at 30 �A/cm
2
. It enabled us to generate charged
centers over the whole a sample.
To initiate the relaxation of a pre-irradiated matrix, we
used laser light as the source of photons. To cover the
whole visible range, a coherent Ar Ion Laser (Innova 70)
and a Coherent CR–599 Dye Laser operating with Rho-
damine 6G and pumped with Ar Ion Laser were used. The
laser light was introduced into the sample chamber via an
optical fiber. The laser power was varied in the range
10–75 mW, as measured in front of the sample, and laser
beam was defocused to the diameter of about 3 cm to cover
the whole the sample. The overheat did not exceed 0.2 K
under excitation by the most powerful (75 mW) laser light.
The temperature was measured with a calibrated si-
licon diode sensor, mounted at the substrate. The pro-
grammable temperature controller LTC 60 permitted us to
keep the temperature fixed during sample preparation, as
well as to control the desirable heating rate. During depo-
sition the temperature was kept at 6 K to minimize possi-
ble thermally stimulated processes. Note that the thre-
shold temperature for thermally stimulated processes in
solid Ar is about 10 K. During measurements of the pho-
ton-stimulated exoelectron emission, the temperature was
kept at 6 K. For the experiments on the influence of laser
light on the ensuing TSEE we used continuous heating
with a constant rate of 3 K/min. The TSEE yield was
measured in a range 6–45 K.
The emission of electrons from pre-irradiated samples
was detected with an Au-coated Faraday plate kept at a
small positive potential +9 V positioned at 5 mm in front
of the sample. The Faraday plate current was amplified by
a low-noise current amplifier FEMTO DLPCA-200, con-
verted into voltage, and the inverted voltage digitized in a
PC. Currents of about 100 fA can be easily measured in
our experiments.
To investigate the influence of sample structure on the
lifetime of the relaxation processes we performed experi-
ments with annealed samples. To this end, the Ar samples
were prepared as was described above. After laser light ir-
radiation each sample was annealed at 25 K during 5 min
and then cooled down to 6 K. After cooling the sample
was irradiated once again with the electron beam. The la-
ser was switched on again and exoelectron emission from
the annealed sample was measured. Then we repeated the
cycle of annealing and laser irradiation and measure-
ments of the exoelectron emission.
Results and discussion
Electron-hole pairs are created quite efficiently in
atomic cryocrystals under electron bombardment. Holes
are self-trapped in the lattice within 10
–12
s [1] due to the
electron-phonon coupling, become immobile at low tem-
peratures. As suggested theoretically [1] and proved exper-
imentally [21,22], self-trapped holes (STH) have the con-
figuration of a dimer ion and can be considered as Rg 2
�
centers in their own matrix. Frenkel pairs, interstitials and
vacancies, can be created in atomic cryocrystals via excita-
tion of the electronic subsystem by a low-energy electron
beam [9]. Electrons are not self-trapped in Ar matrix and
characterized by free-like behavior [1]. Because of a nega-
tive electron affinity Ea � �0 4. eV [1], with repulsive for-
ces prevailing the electron can be trapped in solid Ar only
by such lattice defects as vacancies, vacancy clusters, or
pores. Although these traps are thought to be relatively
shallow, the trapped electrons remain nevertheless stable at
low temperatures at least up to 10 K. The activation energy
needed to release electrons from traps can be transferred by
heating or photon irradiation. STH are less mobile than
electrons (by a factor of 10
–4
–10
–5
) and bound more
tightly therefore, they cannot be released in the same way
[1]. Solid Ar is a wideband material with the conduction
bandwidth of about 2.3–3.7 eV [23]. The depth of the most
shallow trap was estimated to be about 12 meV [8]. The
strongest peak at 15 K in the thermally stimulated lumines-
cence (TSL) of nominally pure Ar related to the exciton-in-
duced defects is characterized by an activation energy of
15 meV, according to [8], or 36 meV, according to [12].
The deep thermally disconnected trap could be a guest
atom or molecule with positive electron affinity. The typi-
cal example of the so-called «electron scavenger» is oxy-
gen. The oxygen atom has a binding energy of 2.61 eV and
for molecular oxygen this value is 1.59 eV, as one can
estimate by taking into account the polarization energy of
the Ar matrix [10]. In fact, photons of energies above
3�10
–2
eV can be used to release electrons from shallow
traps and promote their passage to the conduction band.
When an electron starts to move through the lattice,
there are at least two possibilities for further relaxation:
(i) to reach the surface and escape the sample directly
as was observed in [10,11,14,17] for nominally pure and
doped solid Ar;
(ii) to recombine radiativelly with positively charged
intrinsic Ar2
� and extrinsic (for example Xe 2
� ) centers via
the following reactions [19,20]:
Ar e Ar Ar Ar eV2 2 1 9 8� � � � � �* ( . )hv , (1)
Xe e Xe Xe Xe eV2 2 1 7 2� � � � � �* ( . )hv . (2)
To check the influence of visible-range photons on the
direct escape of electrons from the sample (channel i), we
696 Fizika Nizkikh Temperatur, 2007, v. 33, Nos. 6/7
G.B. Gumenchuk, A.N. Ponomaryov, A.G. Belov, E.V. Savchenko, and V.E. Bondybey
performed experiments on photon-stimulated exoelectron
emission. The yield of exoelectrons stimulated by a laser
of wavelength � � 514 nm and power W � 25 mW is
shown in Fig. 1. Switching on the laser light resulted in a
sharp signal rise with a subsequent slow decay. As was
shown in [20,25], the decay can be described in a no-
retrapping case by the expression:
N N g t gtc c� �0 ( ) exp ( ) , (3)
where g t( ) is the product of the density of photons irradi-
ating the sample and the effective interaction cross sec-
tion of the photons and the electrons in the traps; c is the
effective lifetime of electrons in the conduction band; N 0
is the initial concentration of electrons in the traps. Note
that the current is proportional to the concentration of
electrons in the conduction band. Since the rate of exo-
electron emission and the recombination luminescence in
the M-band depend on the concentration of electrons in
the conduction band, the same expression (3) can be ap-
plied to describe the decay of photon-stimulated recombi-
nation luminescence (1), (2) [20]. The characteristic life-
time for this recombination luminescence (several tens of
seconds) exceeds the «intrinsic» M-band lifetime (1.8 ns
for singlet and 1200 ns for triplet in Ar matrix [1]) by
many orders of value [19,20]. When we use the laser light
as a source of photons, g t( ) can be described as a product
of the power P and the effective interaction cross section
of photons and electrons in the traps. In that case, g t( ) is
a constant: g t g P( ) � �
. The decay shown in Fig. 1 can
be described by (3) with � �g 1, which characterizes the
time scale needed for electrons to pass the conduction
band and to escape the sample without any barrier be-
cause of the negative electron affinity of Ar (– 0.4 eV).
Using a laser as the source of photons we can easily
vary the laser power P at fixed wavelength in order to un-
derstand how the laser light power influences the lifetime
. For this purpose we performed a series of experiments
on PSEE with varyins laser power. The results are de-
picted in Fig. 2. At a laser power of 15 mW the decay
curve lifetime is about 42 s. With increasing laser power
the lifetime clearly decreases. At 75 mW the lifetime is
about 23 s. Note that the overheating during irradiation at
the highest laser power does not exceed 0.2 K, so we can
be sure that no additional thermal stimulation processes
occured. We can suppose that substantial error can be
caused by differences in sample structure. We found that
the lifetime determination experiment is quite sensitive to
sample quality. In Fig. 3 one can see the results of the
PSEE experiment performed with 2 cycles of annealing.
After completing the electron-beam irradiation the laser
was switched on and the exoelectron emission from the
sample was measured. In that case we used the laser at
20 mW power and a wavelength of 514 nm. After a cycle
of laser irradiation the sample was annealed at 25 K du-
ring 5 min and then was cooled down to 6 K and irradiated
once again by electron beam. The laser was switched on
again. The lifetime extracted from the exoelectron emis-
sion curve is about 13 s after the first annealing cycle and
8 s after the second cycle. The annealing procedure im-
proves sample structure and effects of electron scattering
on structure defects become less pronounced, which re-
sults in a shorter time the electrons needs to escape the
sample. And, indeed, shortens with each next annealing
cycle.
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam
Fizika Nizkikh Temperatur, 2007, v. 33, Nos. 6/7 697
C
u
rr
en
t,
p
A
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
–0.2
–100 0 100 200 300 400 500 600 700 800
laser on
a
b
C
u
rr
en
t,
p
A
Time, s
Time, s
0 20 40 60 80 100 120
0.6
0.5
0.4
0.3
0.2
0.1
0
–0.1
Fig. 1. (a) Exoelectron emission from electron-beam pre-irra-
diated solid Ar, stimulated by laser light (514 nm, 20 mW).
(b) a blowup of the initial portion of the decay curve.
50
45
40
35
30
25
20
15
10
5
0
10 20 30 40 50 60 70 80
Laser power, mW
L
if
et
im
e,
s
Fig. 2. The lifetime of the exoelectron emission versus the la-
ser power at fixed wavelength (514 nm).
The lifetime depends on two factors: (i) is inversely
proportional to the density irradiation of photons (laser
power), (ii) the effective interaction cross section of pho-
tons and electrons in the traps. This is why we can expect
the lifetime the photon-stimulated exoelectron emission
to depend on laser wavelength. To check this assumption,
we performed PSEE experiments with a tunable laser. In
this case, a Coherent Ar Ion Laser (Innova 70) and a Co-
herent CR–599 Dye Laser operating with Rhodamine 6G
and pumped with Ar ion laser were used to cover the
range from 450 to 640 nm. In Fig. 4,a we show decay
curves for the current from pre-irradiated samples stimu-
lated by laser light of wavelengths 622, 514 and 476 nm.
The laser power in all these experiments was fixed and in
front of the sample it was 25 mW. The lifetimes extracted
from these curves are: 36 s for the exoelectron emission
stimulated with the red laser, 24 s for the green laser, and
10 s, for the blue laser. Figure 4,b shows the PSEE life-
time versus the laser wavelength at fixed laser power. The
lifetime increases with laser wavelength. Thus we can say
that laser light with a shorter wavelength shortens the life-
time of the relaxation process. Changing the laser wave-
length can result in some changing of the effective inter-
action cross sections and, hence, the lifetime of the
relaxation processes. Moreover, the photons of higher en-
ergy create «hotter» electrons in the conduction band and
ensure preference for the PSEE with respect to the charge
recombination processes.
An interesting question is the interrelation between ther-
mally and optically stimulated relaxation processes. In this
aspect we performed experiments on the laser stimulated
exoelectron emission with subsequnt heating. The samples
were exposed to the laser light of variable power. After
600–700 s, when the PSEE current goes down to a noise
level, laser irradiation was completed. After that the samples
were heated at a constant rate of 3.2 K/min from 7 up to 45 K.
A thermally stimulated exoelectron emission was observed
and the behavior of the peaks was similar to that in our previ-
ous study [10,13,14]. Since the origin of the peaks on this
curve was discussed in those papers, we outlined below it
briefly. The first peak at 12 K is caused by the release of elec-
trons from traps in the subsurface layer or from grain bound-
aries [13], the peak at 15 K belongs to radiation-stimulated
defects. The shoulder at 23 K has a rather nontrivial origin
[11,26]. It was shown that at this temperature, residual neutral
oxygen atoms embedded in the matrix become mobile. Their
diffusion results in a recombination of neutral O atoms, creat-
ing O2
* molecules. Their formation is followed by a lumines-
cence in the range of the well-known Herzberg progression.
This light was treated as an «internal» source of photons to
help release electrons from traps. In our experiments it was
found (see Fig. 5) that the intensities of these low-tempera-
ture thermally stimulated peaks (LT TSEE) depend on the la-
ser power at fixed wavelength, which was used before heat-
ing. The heating after laser irradiation using a more powerful
laser leads to an effective bleaching of the LT TSEE peaks. It
means that laser light of 75 mW power at 514 nm, for exam-
ple, releases electrons from all kinds of shallow traps more ef-
fectively than laser light with a power of 15 mW at the same
wavelength, which results in a decreasing of the LT TSEE
698 Fizika Nizkikh Temperatur, 2007, v. 33, Nos. 6/7
G.B. Gumenchuk, A.N. Ponomaryov, A.G. Belov, E.V. Savchenko, and V.E. Bondybey
a
b
laser on
laser on
C
u
rr
en
t,
p
A
–100 0 100 200 300 400 500 600 700 800
Time, s
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
–0.2
Fig. 3. Effect of anneal cycling of pre-irradiated solid Ar. The
solid lines are exponential fitting curves for the first (a) and
second (b) annealing cycles.
� = 514 nm
� = 622 nm
� = 476 nm
C
u
rr
en
t,
p
A
5
4
2
1
0
3
Time, s
0 20 40 60 80 100 120
Laser wavelength, nm
450 500 550 600 650
C
h
ar
ac
te
ri
st
ic
li
fe
ti
m
e,
s
35
30
25
20
15
10
5
0
b
a
Fig. 4. a — The decay curves from pre-irradiated solid Ar stim-
ulated by laser light with different wavelengths at a fixed laser
power of 25 mW. b — Dependence of the lifetime for the exo-
electron emission on the laser wavelength at a fixed (25 mW) la-
ser power.
peaks. From Fig. 5,b one can see that around 40–60 mW the
ratio of the number of LT TSEE electrons to the number of
PSEE electrons tends to 1 and the more powerful laser re-
leases electrons, leaving fewer for the thermally stimulated
channel.
It was supposed [17,24] that laser light in the visible
range can release electrons not only from deep, thermally
disconnected traps, such as a guest atoms of oxygen, but
also from shallow traps. The present results on the bleach-
ing of thermally stimulated exoelectron current peaks by
laser light (514 nm) demonstrate that increasing in the la-
ser power results in a more effective bleaching of LT
TSEE. These findings give us a direct proof that shallow
traps in the pre-irradiated solid Ar are effectively depopu-
lated by photons of the visible range.
Summary
Using the current activation spectroscopy method in
combination with cathodoluminescence spectroscopy, we
investigated photon-induced relaxation processes in solid
Ar which was pre-irradiated by a low-energy electron
beam. Influence of the laser power and wavelength on the
characteristic lifetime of PSEE was investigated. To
avoid the contribution of thermally stimulated processes,
the experiments were performed at low (6 K) tempera-
tures. The characteristic lifetime proved to be quite sen-
sitive to the sample structure: is shorter for more perfect
samples.
The study of the bleaching of the TSEE peaks has
shown that the laser light ensures an effective release of
electrons not only from deep but also from shallow traps
in pre-irradiated solid Ar. From PSEE experiments it is
concluded that increasing the photon flux shortens the
characteristic relaxation time and bleaches more effec-
tively the LT TSEE peaks. With increasing laser wave-
length the time increases. These results are in good
agreement with the model of photon-stimulated processes
[25] which assumes the lifetime to be inversely propor-
tional to the product of the density of irradiating photons
and the effective interaction cross section of photons and
electrons in the traps.
Acknowledgments
The authors are pleased to thank K.S. Song, G.
Zimmerer and P. Feulner for very fruitful discussions and
helpful comments. Financial support from Deutsche
Forschungsgemeinschaft is greatfully acknowledged.
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75 mW
T
S
E
E
cu
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en
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p
A
15 mW1000
900
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100
0
–100
Temperature, K
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a
A
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aL
T
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/A
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