Photoelectron emission from solid Ne tested by impurity adsorption
Electron emission was obtained from a solid Ne sample growing from the gas phase on a low temperature substrate. The surface of the sample was irradiated by the light of an open-source microwave discharge running in the gaseous Ne. A second gas flow of CH₄ was, simultaneously, passed onto the substr...
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irk-123456789-1171062017-05-20T03:03:37Z Photoelectron emission from solid Ne tested by impurity adsorption Dmitriev, Yu.A. 7th International Conference on Cryocrystals and Quantum Crystals Electron emission was obtained from a solid Ne sample growing from the gas phase on a low temperature substrate. The surface of the sample was irradiated by the light of an open-source microwave discharge running in the gaseous Ne. A second gas flow of CH₄ was, simultaneously, passed onto the substrate avoiding the discharge zone. Free electrons ejected into a vacuum chamber during the sample growth were detected by means of the electron cyclotron resonance (ECR) technique. The electron yield was found to be decrease at increasing CH₄ flow. Fitting curves to the experimental data showed that the surface CH₄ impurities played the major role in emission quenching. Atemperature effect was observed in which a 4.2 K sample was much more sensitive to CH₄ doping than a 1.6 K one. Based on the experimental results, a model was proposed of the surface sites where electrons escape the solid. 2009 Article Photoelectron emission from solid Ne tested by impurity adsorption / Yu.A. Dmitriev // Физика низких температур. — 2009. — Т. 35, № 4. — С. 350-354. — Бібліогр.: 12 назв. — англ. 0132-6414 PACS: 52.50.Sw, 79.60.–i, 79.75.+g http://dspace.nbuv.gov.ua/handle/123456789/117106 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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7th International Conference on Cryocrystals and Quantum Crystals 7th International Conference on Cryocrystals and Quantum Crystals Dmitriev, Yu.A. Photoelectron emission from solid Ne tested by impurity adsorption Физика низких температур |
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Electron emission was obtained from a solid Ne sample growing from the gas phase on a low temperature substrate. The surface of the sample was irradiated by the light of an open-source microwave discharge running in the gaseous Ne. A second gas flow of CH₄ was, simultaneously, passed onto the substrate avoiding the discharge zone. Free electrons ejected into a vacuum chamber during the sample growth were detected by means of the electron cyclotron resonance (ECR) technique. The electron yield was found to be decrease at increasing CH₄ flow. Fitting curves to the experimental data showed that the surface CH₄ impurities played the major role in emission quenching. Atemperature effect was observed in which a 4.2 K sample was much more sensitive to CH₄ doping than a 1.6 K one. Based on the experimental results, a model was proposed of the surface sites where electrons escape the solid. |
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Dmitriev, Yu.A. |
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Dmitriev, Yu.A. |
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Dmitriev, Yu.A. |
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Photoelectron emission from solid Ne tested by impurity adsorption |
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Photoelectron emission from solid Ne tested by impurity adsorption |
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Photoelectron emission from solid Ne tested by impurity adsorption |
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Photoelectron emission from solid Ne tested by impurity adsorption |
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Photoelectron emission from solid Ne tested by impurity adsorption |
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photoelectron emission from solid ne tested by impurity adsorption |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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7th International Conference on Cryocrystals and Quantum Crystals |
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Photoelectron emission from solid Ne tested by impurity adsorption / Yu.A. Dmitriev // Физика низких температур. — 2009. — Т. 35, № 4. — С. 350-354. — Бібліогр.: 12 назв. — англ. |
| series |
Физика низких температур |
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AT dmitrievyua photoelectronemissionfromsolidnetestedbyimpurityadsorption |
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2025-07-08T11:39:34Z |
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2025-07-08T11:39:34Z |
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1837078702422228992 |
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Fizika Nizkikh Temperatur, 2009, v. 35, No. 4, p. 350–354
Photoelectron emission from solid Ne tested by impurity
adsorption
Yu.A. Dmitriev
A.F. Ioffe Physico-Technical Institute, 26 Politekhnicheskaya str., St. Petersburg 194021, Russia
E-mail: dmitrievyurij@gmail.com
Received January 4, 2009
Electron emission was obtained from a solid Ne sample growing from the gas phase on a low temperature
substrate. The surface of the sample was irradiated by the light of an open-source microwave discharge run-
ning in the gaseous Ne. A second gas flow of CH4 was, simultaneously, passed onto the substrate avoiding
the discharge zone. Free electrons ejected into a vacuum chamber during the sample growth were detected
by means of the electron cyclotron resonance (ECR) technique. The electron yield was found to be decrease
at increasing CH4 flow. Fitting curves to the experimental data showed that the surface CH4 impurities
played the major role in emission quenching. A temperature effect was observed in which a 4.2 K sample was
much more sensitive to CH4 doping than a 1.6 K one. Based on the experimental results, a model was pro-
posed of the surface sites where electrons escape the solid.
PACS: 52.50.Sw Plasma heating by microwaves; ECR, LH, collisional heating;
79.60.–i Photoemission and photoelectron spectra;
79.75.+g Exoelectron emission.
Keywords: Ne solid, ECR, electron emission, surface and bulk impurities.
1. Introduction
In a very recent study [1], an effect has been found that
a small gas flow of He provided onto the cold substrate
where the gaseous Ne was condensed suppressed the elec-
tron photoemission from the solid Ne. It was not clear
immediately whether bulk or surface He impurities are re-
sponsible for the suppression, though a certain consider-
ation favored the surface effect. Indeed, He atoms trapped
in the bulk seem not be able to decrease considerably the
yield by quenching Ne excitons through the energy trans-
fer process because of the poor match between atomic He
levels and Ne exciton bands. The crucial role of the sur-
face He impurities in quenching photoelectron yield has
been verified by fitting experimental curves A(p) [2],
where A is the signal amplitude which is proportional to
the free electron yield and p is the gas pressure measured
at the warm end of the tube supplying the gaseous He to
the substrate; the pressure is proportional to the quantity
of He flow. To get further in understanding the roles of
bulk and surface impurities in the photoemission of free
electrons, it would be helpful to test this process using an
impurity with lower ionization potential and readily ad-
sorbed by a sample at liquid He temperatures. In the pres-
ent study, we utilize the molecular CH4 as such a probe.
The CH4 impurity may serve not only as a trap for free
electrons but, based on its comparatively small ioniza-
tion potential, may contribute to the free electron pro-
duction. Indeed, the impurity photoemission threshold is
E E Ei
g
i
ath � � , where Eg
i = 12.98 eV is the methane mole-
cule ionization potential and Ea = –1.3 eV is the negative
electron affinity of the Ne solid. Then, E i
th = 11.68 eV
which is smaller than the energy levels of the exciton
states of the Ne matrix. In turn, the direct excitation of the
impurity state above E i
th by the light of the Ne gas dis-
charge open source used for the sample irradiation in the
experiments can contribute to the photoelectric yield. At
the first glance, the net effect of the CH4 doping on the
photoelectron yield from solid Ne might be as negative,
i.e., decreasing the yield, as positive, i.e. increasing it.
However, taking into account the fact that the quantity of
electron photoemission from noble gas solids is far above
that of molecular solids [3], one would expect a minor
contribution of the CH4 to the yield when compared to the
effect of quenching the emission.
2. Experimental details
The electrons escaping into the vacuum from the sam-
ple were observed through the electron cyclotron absorp-
© Yu.A. Dmitriev, 2009
tion using a conventional EPR device [1,3]. The setup and
experimental procedure have been presented elsewhere
[3,4]. Briefly, they were as follows. The bottom of a
quartz finger filled with liquid helium served as a low
temperature substrate for the gases being condensed. The
bottom was located at the center of the microwave cavity
of an X-band EPR spectrometer. The cavity was eva-
cuated and cooled externally with liquid nitrogen vapor
providing a cavity temperature from 77 to 300 K. An
electrodeless high-frequency (15 MHz) discharge operat-
ing in pulsed regime was excited in the gaseous Ne which
was passed through a glass tube with an outlet of approxi-
mately 0.6 mm diameter. The products of the discharge
entered the cavity and condensed on the finger bottom,
forming Ne solid. The solid was subjected to the action of
the irradiation from the outlet, which, thus, operated as an
open-discharge source. The methane gas flow was sup-
plied to the substrate by a quartz tube inserted into the
cavity. The tube was placed outside the discharge zone.
The end of the quartz tube was located close (3 mm) to
the substrate. Both gases were cooled with liquid nitro-
gen vapor prior to deposition. The substrate temperature
was lowered down by pumping-out the liquid He bath.
The base pressure in the experimental chamber was
2·10–6 torr. Pure gases were used with the following im-
purity contents: 0.004% Ne and 0.1% CH4.
3. Results
Figure 1,a, solid circles, shows experimental data ob-
tained for the sample temperature of 1.6 K. Also there are
shown the fitting curves based on various mechanisms of
impurity CH4 effect on the electron photoemission from
solid Ne. The figure suggests decreasing the ECR signal
amplitude, A, with increasing impurity methane concen-
tration in solid Ne which is proportional to pressure, p,
measured at the warm end of the tube supplying the gas-
eous CH4 to the substrate. Therefore, the major effect of
doping Ne with CH4 is quenching electron emission from
the sample. Let us consider first the bulk effect. If q is the
rate of free electron production in the bulk, k is a rate con-
stant which relates to trapping these electrons by CH4 im-
purities, and a rate constant k1 is for the electron loss
through emission from the surface, then, in the steady-
state condition:
q – knN – k1n = 0 . (1)
Here n is the free electron concentration in the bulk, and N
is the CH4 concentration.
Hence, the A(p) dependence can be written in the form:
A p
a
b p
( ) �
�
1
11
(2)
with constants a1 and b1 to be obtained in the fitting pro-
cedure.
The dependence is plotted in the Fig. 1,a, providing
rather poor match to the experimental results, especially
at moderate and large CH4 flows.
Another model deals with surface CH4 impurities, sug-
gesting that the electron emission into the vacuum pro-
ceeds from the Ne surface free of adsorbed CH4 mole-
Photoelectron emission from solid Ne tested by impurity adsorption
Fizika Nizkikh Temperatur, 2009, v. 35, No. 4 351
10–3 10–2
p, torr
10–1
160
140
120
100
80
60
40
20
0
A
,
ar
b
.
u
n
it
s
1
2
a
A
,
ar
b
.
u
n
it
s
3
4
5
b
10–3
p, torr
10–2 10–1
160
140
120
100
80
60
40
20
0
Fig. 1. (a) The intensity of the ECR signal, A, for the 1.6 K
sample versus pressure, p, measured at the warm end of the tube
supplying the gaseous CH4 to the substrate. Filled circles —
experimental data; fitting curves: 1 — calculated in assump-
tion that bulk CH4 impurities play major role in photoemission
quenching, 2 — suggests a major role of the surface CH4 im-
purities. (b) The theoretical curves for A(p) dependence based
on the assumption that the surface CH4 impurities play a key
role in emission quenching; the experimental points are the
same as presented in Fig. 1,a; these experimental data are fit-
ted with curves: 3 and 4 account for CH4 microcrystal forma-
tion with and without a component independent of the pressure
p, respectively, 5 — accounts for both the CH4 microcrystal
formation and the direct ionization of the impurity CH4 mole-
cules.
cules. Let Sfree and Soccup be the surface areas which are
free of adsorbed CH4 atoms or occupied by these atoms,
respectively. It is reasonable to assume that Sfree/Soccup is
proportional to pNe/ pCH4
, where pNe is the gaseous Ne
pressure in the cavity and pCH4
is the methane pressure:
Sfree/Soccup = �pNe/pCH4
. (3)
Here � is the proportionality coefficient. Taking into ac-
count that Sfree + Soccup = const, one concludes:
S
p
p
free
CH
Ne
� 1 4
1
�
�
�
�
�
�
�
�
�
. (4)
Based on the suggestion that A and pCH4
are propor-
tional to Sfree and p, respectively, we again come to the
above A(p) dependence, Eq. (2). Therefore neither bulk
CH4 impurities nor those on the flat surface are responsi-
ble for quenching electron photoemission.
Next we test the model which applies well to photo-
emission from solid Ne quenched by impurity He [2]. We
used expression for A(p) modified as follows [2]:
A p
b
c p
a
d
( ) �
�
�2
2
2
1 2
. (5)
Equation (5) was obtained under suggestion that the
surface He atoms play the major role in quenching pho-
toemission. The component a2 accounted for the fact that,
due to new layers of solid Ne appearing again and again
during condensation, the part of the sample surface had
high enough temperature not to adsorb He atoms. There-
fore, no He «screening» occurred at these areas and the
electrons were allowed to live the sample. Fitting pro-
cedure gave the best value for d2 close to 2. An analysis
of the value of d2 led to the conclusion that the electrons
escape the sample from special regions on the surface
nearby the lines where two Ne planes cross and these re-
gions are, possibly, atomic step sites (step edges) at the
Ne surface which are responsible for the sample growth.
Now, we apply Eq. (5) with d2 = 2 to fit the experimental
data for photoemission from solid Ne quenched by CH4
impurities. Curve 2 (Fig. 1,a) presents a result of the fit-
ting procedure which suggests that we arrive at a reason-
able agreement between theory and experiment for small
and moderate CH4 flows. The physical meaning, how-
ever, of the component a2 is not as clear as in the case of
the He impurity. Another difference between the present
experimental results and those of Ref. 2 is a trend to
slightly higher A values at large flows observed for the
CH4 impurity and not observed for the He impurity. One
may suppose that at large CH4 flows solid methane
microcrystals start to be formed, which (being subjected
to UV radiation) contribute to photoelectron emission. It
is reasonable to assume that the CH4 crystal concentration
is proportional to p k p
d d3 31 2/ ( )� with d3 � 2. We have
no suggestion at the moment about the exact value of d3.
However, the fitting shows that d3 may vary across a large
range and, for example, quadratic pressure dependence,
p2, leads to result which differs insignificantly from that
of the cubic one, p3. Let us assume d3 = 3. Curves 3 and 4
(Fig. 1,b) are plotted using Eqs. (6) and (7), respectively,
which account for CH4 microcrystal formation:
A p
a
b p
c p
e p
( ) �
�
�
�
3
3
2
3
3
3
31 1
, (6)
A p a
b
c p
e p
f p
( ) � �
�
�
�
4
4
4
2
4
3
4
31 1
. (7)
It is readily seen from the Fig. 1 that the component a4
(independent of pressure p) has almost no effect at low
and moderate CH4 flows. On the other hand, curve 4 fits
better to the experimental data which means that some
third component should be presented in the A(p) expres-
sion and this component is linked to the methane flow. We
suppose that this component accounts for the direct ion-
ization of the impurity CH4 molecules and is proportional
to p k p/ ( )1 3� . In this way we come to the following ex-
pression for A(p):
A p
a
b p
c p
e p
f p
g p
( ) �
�
�
�
�
�
4
4
2
4
3
4
3
4
41 1 1
. (8)
Curve 5 (Fig. 1,b) plotted using Eq. (8) fits well to the ex-
perimental data. At moderate and large CH4 flows it over-
laps curve 4 thus making these two curves undistinguish-
able.
For 4.2 K samples, Fig. 2, the overall trend resembles
that of 1.6 K. The major difference is in that the observed
signal amplitude decreases with increasing CH4 flow
much more rapidly as compared with the process at 1.6 K.
352 Fizika Nizkikh Temperatur, 2009, v. 35, No. 4
Yu.A. Dmitriev
10–3 10–2
p, torr
A
,
ar
b
.
u
n
it
s
160
140
120
100
80
60
40
20
Fig. 2. The intensity of the ECR signal, A, versus pressure, p,
measured at the warm end of the tube supplying the gaseous
CH4 to the substrate: triangles — the 4.2 K sample, circles —
the 1.6 K samples.
The CH4 microcrystal formation is not seen in 4.2 K ex-
periments. This obviously is due to relatively small meth-
ane flows in the experiments. Henceforth, we modify the
fitting Eq. (8) by removing the second component:
A p
a
b p
c p
e p
( ) �
�
�
�
5
5
2
5
51 1
. (9)
An agreement between the theoretical model and experi-
ment, Fig. 2, may be considered as rather good. For com-
parison, the 1.6 K data are also presented in the figure.
4. Conclusion
The present study revealed that the net effect of doping
by an impurity with relatively small ionization potential
on the photoelectron yield from the solid Ne is to decrease
the free electron emission. Though the bulk CH4 impuri-
ties are believed to take some part in quenching emission,
their influence was not elucidated. The major role is play-
ed by the surface CH4 molecules. Interestingly, the same
inference was made for the solid Ne–He impurity system.
The similarity in emission behavior under doping with
such a different species like He and CH4 deserves special
attention. It is known that electrons are not self-trapped in
matrices like Ar [5] and Ne due to negative electron affin-
ities of these matrices. In nominally pure matrices the
electrons can be trapped only by such lattice defects as
vacancies, vacancy clusters or pores [5]. It is thought that
the defects are relatively shallow traps, while much deep-
er traps could be a guest atom or molecule with positive
electron affinity [5]. The atomic He, however, is a particle
with very small positive affinity, 0.0054 Ry [6] which is
equal 0.073 eV, and CH4 molecule has negative electron
affinity, Ea = –5 eV [7,8]. The mechanism for the decrease
in electron emission by impurity particles in our experi-
ments is not clear at present. One of the propositions for
this mechanism is that, despite the affinities of different
signs, both He and CH4 impurities turns out to be effec-
tive traps in solid Ne, which scavenge electrons, thus not
allowing them to escape into the vacuum. The observed
effect is related to the processes of electron trapping in in-
sulator materials [8,9]. The authors established a relation-
ship between the electron trap and the molecular proper-
ties of the material. They studied both physical (e.g.,
conformational disorder) and chemical defects (e.g., bro-
ken bonds and impurities) and showed that while typical
physical trap energies were of the order of 0.15 eV and all
are less than 0.3 eV (for polymeric insulators), the chemi-
cal defect trap energies reached about 1 eV even for impu-
rities with negative electron affinity for free molecules
[9]. The trap energy, Etrap, was defined as the energy
difference between the electron affinity of the system
with and without the defect, thus
Etrap = E Ea adefect reference– . (10)
Another proposition for the quenching mechanism of
electron emission by impurities is that the electron affin-
ity of the impurity does not account for the effect, i.e.,
a surface impurity of any kind may prevent bulk electrons
from appearing at the surface. To check this assumption,
additional experiments using impurities with positive af-
finity, like O2, Ea = 0.44 eV [10], NO2, Ea = 2.43 eV [10],
as well as those with negative affinity, H2 and CO, Ea =
= –1.8 eV [11], are in progress. When compared to each
other, the results will give us an answer to the problem
whether the electron affinity of an impurity links to the
quenching electron photoemission from solid Ne.
The present experimental results for the CH4–Ne sys-
tem and those reported earlier for He–Ne one [1] differs
significantly in the temperature behavior of the photo-
emission. Indeed, the Ne–He pair showed a faster drop in
the photoelectron yield for the 1.6 K samples as compared
to the 4.2 K ones, while an inverse situation was observed
for the Ne–CH4 pair. At 4.2 K, He atoms have too small
adsorption time on step sites where, possibly, the electron
emission occurs. The adsorption time grows exponen-
tially with lowering temperature. That is why He impurity
has much more prominent effect on the photoelectron
yield for the 1.6 K sample as compared the 4.2 K one. The
situation for CH4 is quite different, with long adsorption
time at both temperatures. The observed temperature
effect can, therefore, be explained by changing sample
quality at different deposition temperatures. In a recent
paper [12], the temperature stimulated luminescence and
temperature stimulated exoelectron emission from solid
Xe pre-irradiated by low-energy electrons were found to
be sensitive to the sample deposition temperature. The au-
thors pointed out that the effect is explained with growing
number of lattice defects on lowering deposition tempera-
ture. These defects serve as shallow traps for electrons.
Therefore, it is stressed [12] that the more defects in the
crystal, the lower concentration of electrons escaping
from traps at low temperatures and vice versa. In our ex-
periment with impurity CH4, low deposition temperature
favors electron emission which is evident from the fact
that quenching emission by the impurities at 1.6 K is less
effective than at 4.2 K. We suggest that low deposition
temperature leads to larger number of steps on the Ne sur-
face and that more CH4 flow is necessary to block emis-
sion occurring, as we suppose, from these steps. The sug-
gestion may be verified in experiments with other
impurities which are also readily adsorbed at liquid He
temperatures. Similar temperature behavior for all these
kinds of impurities would verify the suggestion.
Doping of Ne may lead not only to a decrease of elec-
tron emission but also to its increase. It follows from ex-
amining of Eqs. (8) and (9) that at moderate impurity
Photoelectron emission from solid Ne tested by impurity adsorption
Fizika Nizkikh Temperatur, 2009, v. 35, No. 4 353
flows A(p) may increase reaching a maximum. Whether
the gains in emission can be significant remains to be seen
in experiments which are planned now.
Partial financial support from Russian Foundation for
Basic Research under grant 08-02-90409-Ukr_à is ac-
knowledged.
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354 Fizika Nizkikh Temperatur, 2009, v. 35, No. 4
Yu.A. Dmitriev
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