Dynamics of the defect-mediated desorption of alkali halide surfaces
Dynamic processes leading to desorption of Rb and I atoms from the RbI (100) surface co-irradiated with 1 keV electrons and visible light (with a wavelength corresponding to the F-center absorption band) have been studied by means of mass-selected time-of-flight (TOF) spectroscopy. Depending on th...
Збережено в:
| Опубліковано в: : | Физика низких температур |
|---|---|
| Дата: | 2012 |
| Автори: | , , , |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2012
|
| Теми: | |
| Онлайн доступ: | https://nasplib.isofts.kiev.ua/handle/123456789/117433 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Dynamics of the defect-mediated desorption of alkali halide surfaces / M. Szymonski, A. Droba, P. Struski, F. Krok // Физика низких температур. — 2012. — Т. 38, № 8. — С. 970-975. — Бібліогр.: 23 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
nasplib_isofts_kiev_ua-123456789-117433 |
|---|---|
| record_format |
dspace |
| spelling |
Szymonski, M. Droba, A. Struski, P. Krok, F. 2017-05-23T15:01:42Z 2017-05-23T15:01:42Z 2012 Dynamics of the defect-mediated desorption of alkali halide surfaces / M. Szymonski, A. Droba, P. Struski, F. Krok // Физика низких температур. — 2012. — Т. 38, № 8. — С. 970-975. — Бібліогр.: 23 назв. — англ. 0132-6414 PACS: 62.72.J–, 68.35.Dv, 68.43.–h https://nasplib.isofts.kiev.ua/handle/123456789/117433 Dynamic processes leading to desorption of Rb and I atoms from the RbI (100) surface co-irradiated with 1 keV electrons and visible light (with a wavelength corresponding to the F-center absorption band) have been studied by means of mass-selected time-of-flight (TOF) spectroscopy. Depending on the sample temperature, substantial enhancement of the desorption yield, as well as pronounced changes in the TOF spectra of the emitted atoms have been found. The TOF spectra of halogen atoms consist of two components: thermal (which can be fitted with Maxwellian distribution) and non-thermal one. The non-thermal peak is temperature-independent. There is no non-thermal component for alkali atoms. The comparison of TOF spectra for I atoms emitted from electron bombarded sample with and without simultaneous light irradiation indicates that the yield increase is caused by thermally desorbed atoms, while the non-thermal peak remains unchanged. Presented results confirm well the predictions of the theoretical model of desorption proposed earlier, known as the defect-mediated (F and H center) desorption of alkali halides. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Low Temperature Spectroscopy and Radiation Effects Dynamics of the defect-mediated desorption of alkali halide surfaces Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Dynamics of the defect-mediated desorption of alkali halide surfaces |
| spellingShingle |
Dynamics of the defect-mediated desorption of alkali halide surfaces Szymonski, M. Droba, A. Struski, P. Krok, F. Low Temperature Spectroscopy and Radiation Effects |
| title_short |
Dynamics of the defect-mediated desorption of alkali halide surfaces |
| title_full |
Dynamics of the defect-mediated desorption of alkali halide surfaces |
| title_fullStr |
Dynamics of the defect-mediated desorption of alkali halide surfaces |
| title_full_unstemmed |
Dynamics of the defect-mediated desorption of alkali halide surfaces |
| title_sort |
dynamics of the defect-mediated desorption of alkali halide surfaces |
| author |
Szymonski, M. Droba, A. Struski, P. Krok, F. |
| author_facet |
Szymonski, M. Droba, A. Struski, P. Krok, F. |
| topic |
Low Temperature Spectroscopy and Radiation Effects |
| topic_facet |
Low Temperature Spectroscopy and Radiation Effects |
| publishDate |
2012 |
| language |
English |
| container_title |
Физика низких температур |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
Dynamic processes leading to desorption of Rb and I atoms from the RbI (100) surface co-irradiated with
1 keV electrons and visible light (with a wavelength corresponding to the F-center absorption band) have been
studied by means of mass-selected time-of-flight (TOF) spectroscopy. Depending on the sample temperature,
substantial enhancement of the desorption yield, as well as pronounced changes in the TOF spectra of the emitted
atoms have been found. The TOF spectra of halogen atoms consist of two components: thermal (which can
be fitted with Maxwellian distribution) and non-thermal one. The non-thermal peak is temperature-independent.
There is no non-thermal component for alkali atoms. The comparison of TOF spectra for I atoms emitted from
electron bombarded sample with and without simultaneous light irradiation indicates that the yield increase is
caused by thermally desorbed atoms, while the non-thermal peak remains unchanged. Presented results confirm
well the predictions of the theoretical model of desorption proposed earlier, known as the defect-mediated (F and
H center) desorption of alkali halides.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/117433 |
| citation_txt |
Dynamics of the defect-mediated desorption of alkali halide surfaces / M. Szymonski, A. Droba, P. Struski, F. Krok // Физика низких температур. — 2012. — Т. 38, № 8. — С. 970-975. — Бібліогр.: 23 назв. — англ. |
| work_keys_str_mv |
AT szymonskim dynamicsofthedefectmediateddesorptionofalkalihalidesurfaces AT drobaa dynamicsofthedefectmediateddesorptionofalkalihalidesurfaces AT struskip dynamicsofthedefectmediateddesorptionofalkalihalidesurfaces AT krokf dynamicsofthedefectmediateddesorptionofalkalihalidesurfaces |
| first_indexed |
2025-11-26T21:25:26Z |
| last_indexed |
2025-11-26T21:25:26Z |
| _version_ |
1850776692730101760 |
| fulltext |
© M. Szymonski, A. Droba, P. Struski, and F. Krok, 2012
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8, pp. 970–975
Dynamics of the defect-mediated desorption of alkali
halide surfaces
M. Szymonski, A. Droba, P. Struski, and F. Krok
Research Centre for Nanometer-Scale Science and Advanced Materials (NANOSAM)
Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University
4 Reymonta Str., Krakow 30-059, Poland
E-mail: ufszymon@cyf-kr.edu.pl
Received April 23, 2012
Dynamic processes leading to desorption of Rb and I atoms from the RbI (100) surface co-irradiated with
1 keV electrons and visible light (with a wavelength corresponding to the F-center absorption band) have been
studied by means of mass-selected time-of-flight (TOF) spectroscopy. Depending on the sample temperature,
substantial enhancement of the desorption yield, as well as pronounced changes in the TOF spectra of the emit-
ted atoms have been found. The TOF spectra of halogen atoms consist of two components: thermal (which can
be fitted with Maxwellian distribution) and non-thermal one. The non-thermal peak is temperature-independent.
There is no non-thermal component for alkali atoms. The comparison of TOF spectra for I atoms emitted from
electron bombarded sample with and without simultaneous light irradiation indicates that the yield increase is
caused by thermally desorbed atoms, while the non-thermal peak remains unchanged. Presented results confirm
well the predictions of the theoretical model of desorption proposed earlier, known as the defect-mediated (F and
H center) desorption of alkali halides.
PACS: 62.72.J– Point defects and defect clusters;
68.35.Dv Composition, segregation; defects and impurities;
68.43.–h Chemisorption/physisorption: adsorbates on surfaces.
Keywords: electron, stimulated desorption, alkali halide surtaces, mass-selected spectroscopy.
1. Introduction
Electron stimulated desorption (ESD) of alkali halides
has been studied intensively for several decades as a model
example of inelastic process converting local electronic ex-
citations into a nuclear motion of the sample constituents,
and leading to emission of atomic and molecular particles
from the eroded surface [1–3]. Similar type of desorption
processes caused by electronic transitions has been found in
another class of insulating materials, i.e., frozen rare gas
solids [4,5]. More recently, such desorption processes have
been applied for controlled nanostructuring of wide band
gap insulators with the aim of fabricating templates for de-
positing metal and/or molecular structures fully decoupled
electronically from the supporting substrate [6]. In fact, alka-
li halides nanostructured by electrons have already been
used as templates for metal clusters, as well as complex mo-
lecule deposition and immobilization [7–9]. Such structures
could be applied in molecular electronics.
Kinetic energy of the incident beam of electrons is lost
in the alkali halide crystal for electronic excitations, such
as electron-hole pairs, and/or surface excitons. Such prima-
ry excitations lead to production of so called “Frenkel de-
fects”, i.e., F-center and H-center pairs (F-center is an elec-
tron trapped in halogen vacancy, while the H-center is an
interstitial halogen atom), in the crystal bulk and at the
surface. H-center is a very mobile defect, which, activated
by the crystal thermal energy, diffuses to the surface lead-
ing to emission of halogen atoms. The F-center, when pro-
duced in its ground state, cannot efficiently diffuse until
activated to the excited state by interaction with an imping-
ing electron, or absorption of light. Arriving at the surface,
F-center could neutralize surface alkali ion, and cause its
desorption. Whereas the first process, recombination of the
H-center with the surface, can cause desorption from any
surface site, the recombination of the F-center is more site
selective. Puchin et al. [10] have performed APES calcula-
tions, that show that only an excited F-center recombining
at low-coordinated surface site (such as terrace edge, kink
or corner) can cause desorption of alkali atom.
The study of time dependence of the yield of emitted
particles, and the atomic force microscopy (AFM) mea-
surements have demonstrated that electron-stimulated de-
Dynamics of the defect-mediated desorption of alkali halide surfaces
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8 971
sorption from alkali halide surfaces proceeds in a layer-by-
layer mode by growth and linking of pits (of monoatomic
depth) on the (100) crystal surface [11,12]. It was also
demonstrated, that the flux of emitted atoms is anticorre-
lated with the concentration of F-centers at the surface, or
in the subsurface region. Similar process has been ob-
served for the desorption stimulated by UV photons
[13,14] and by low energy ions [15]. It has been found,
that simultaneous irradiation of the surface with visible
light leads to increase of the desorption coefficient, al-
though photons of such energies are unable to cause de-
sorption. They are, however, able to excite the immobile F-
centers trapped in bulk or accumulated on the surface [16],
and in this way increase the emission of both alkali atoms
(desorbed due to F-centers neutralization) and halogen
atoms (since the F-centers in ground state act as traps for
diffusing H-centers).
Due to recent advancements of the atomic force micro-
scopy, in particular its dynamic version of frequency mod-
ulated AFM performed in an ultrahigh vacuum environ-
ment, investigations of the interactions of electrons and
ions with crystal surfaces have attracted a renewed interest.
Using the AFM technique, it was for the first time possible
to study not only what already left the surface but to ex-
amine the atomic structure of the crystal right after the
desorption process without exposing it to the reactive am-
bient environment [11,17]. This atomistic approach to
processes leading to surface modification allowed for es-
tablishing a consistent theoretical model describing both
the particle emission from bombarded alkali halides, and
the surface topography changes due to beam-induced elec-
tronic excitation/ionisation [9,18]. Finally, it was found
that a visible light illumination within the absorption band
of the F-centers in desorbed alkali halides could considera-
bly increase the yield of the UV photon stimulated desorp-
tion [19]. Since most of the experimental observations,
reported so far, yielded qualitatively similar results for
both photon- (PSD) and electron- (ESD) stimulated de-
sorption, one would expect that the yield enhancement due
to visible light co-excitation should occur for ESD as well
as it does for PSD. In fact, such an effect has been found
by our group [20] but the results of more systematic mea-
surements have not been reported yet. Therefore, we use
this unique occasion of publishing the special Festschrift
issue of the Journal “Fizika Nizkikh Temperatur”, dedicat-
ed to honor Professor Elena V. Savchenko, to contribute
with a concluding report on the topic being so close to Ele-
na’s scientific interest over the years. In this paper, we
report on the first velocity distribution measurements of
halogen and alkali atoms desorbed from RbI (100) surface
due to ESD with and without co-excitation by visible pho-
ton flux. We demonstrate that the thermal component of
ESD emitted alkali and halogen atoms is mediated by the
excitation and diffusion of ground-state F-centers created
during surface irradiation with electron beam leading to an
increase of the desorption coefficient for both elements.
Experimental
The RbI crystals used in our experiment are cleaved in
air, and subsequently placed in a vacuum chamber. The
pressure in the system is lower than 1·10–8 mbar. The sam-
ple is then heated to a 650 K and annealed in vacuum for
several hours to ensure that the surface is free from any
contaminations. It has been shown by many previous expe-
riments that such treatment is sufficient for obtaining a
clean, well defined (100) surface, with almost no defects
(evaporation of low-coordination number atoms is more
probable, so the surface defects are removed) [12]. The
crystal is irradiated with a 1 keV electron beam, typically at
about 5 to 6 μA per cm2, i.e., of 4·1013 electrons/s per cm2.
The emitted Rb and I atoms are detected and analyzed by
the quadrupole mass spectrometer equipped with an electron
impact ioniser. The sample temperature is varied in the
range 330–390 K as measured by a thermocouple mounted
to the sample holder. As a source of visible light, a conven-
tional halogen bulb of 150 W is used. Luminescence of the
irradiated crystal is detected by a monochromator with the
photon – counting photomultiplier system. Desorbed atom
flux signal and the luminescence intensity could be record-
ed simultaneously as a function of time.
Results
The dependence of the Rb and I desorption yield on the
time of irradiation (the electron dose) is presented in
Fig. 1. The oscillations of the desorption yields are con-
nected with the varying topography of the sample surface:
the desorption coefficient increases with increasing num-
Fig. 1. (Color online) The electron stimulated desorption signal
of Rb and I atoms as a function of the irradiation time (the elec-
tron dose), measured at sample temperature of 370 K. The elec-
tron beam of spot size of about 1 mm2 was scanned over the area
of 1.5 cm2 (electron density ∼3.3 μC/cm2 per second).
120000
100000
80000
60000
20000
0
0 30 60 90
D
es
o
rp
ti
o
n
y
ie
ld
,
a.
u
.
Time of irradiation, s
40000
I atoms
Rb atoms
1
2
( )1
( )2
M. Szymonski, A. Droba, P. Struski, and F. Krok
972 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8
ber of surface steps and edges, since they act as traps for
diffusing defects and enable the F-centers to cause ejection
of an alkali atoms. The desorption yield reaches its maxi-
mum when half of the monolayer is removed. Then, the
desorption signal decreases and the minimum of the de-
sorption process is observed when the whole upper mono-
layer is being removed. After a few periods of oscillations,
the process does not follow the layer-by-layer mode any-
more, and the oscillations are damped. The connection
between the desorption yield changes and the surface to-
pography has been confirmed previously by the AFM mea-
surements [11,12].
In Fig. 2 the changes in partial desorption yields caused
by co-excitation, with the electron beam and visible light
(photons of energies about 2–2.5 eV), are presented for
both, Rb and I atoms. Since the measurement is performed
with the electron beam irradiating a single spot (contrary to
the case shown in Fig. 1, where the beam is scanned over
the sample surface), there is no the oscillatory behavior of
the desorption signals due to fast erosion of the surface
topography averaging the number of the desorption sites
(for more explanation see Discussion section). It is clear
that additional illumination of the sample with light causes
significant increase of the desorption yield for both halo-
gen and alkali atoms.
In further steps of our work mass selected time-of-flight
(TOF) spectroscopy is used to study velocity distributions
of the desorbing atoms. It has been demonstrated in many
experiments [20–22], that the TOF spectra of halogen
atoms consist of two components: the thermal (which can
be fitted with a Maxwellian distribution function) and the
non-thermal one. The non-thermal peak, at energy of about
0.25 eV, corresponds to the halogen atoms emitted as a
result of the decay of self-trapped excitons at the surface.
This component is temperature-independent, and does not
exhibit the oscillatory behavior on the irradiation time.
Atoms desorbed in that process have anisotropic angular
distribution, so that they are ejected preferentially along
the <100> axis. It has been already shown that such hyper-
thermal halogen emission occurs for most of the alkali ha-
lide crystals, except the sodium compounds [20]. The
comparison of time-of-flight spectra for I atoms emitted
from electron bombarded RbI sample with and without
simultaneous white light co-excitation at 4 different tem-
peratures of the sample is shown in Fig. 3. The pronounced
increase of the yield of thermally desorbed atoms is clearly
visible, whereas the non-thermal peak remains unchanged
at all investigated temperatures.
The corresponding TOF spectra of rubidium atoms de-
sorbed by electron beam with and without light illumina-
tion are shown in Fig. 4. As already established in our pre-
vious reports there is no the non-thermal component for
alkali atoms. Although it is already shown that the simul-
taneous illumination with the white light photons causes
the increase in the total emission of Rb atoms (the QMS
signal is enhanced as shown in Fig. 2), the Maxwellian
peak in the TOF spectrum taken for both temperatures has
lower intensity for the experiment with light co-excitation
than for the electron bombardment alone. As it is described
in the section Discussion, the excited F-centers require
some time to diffuse to the surface and cause the desorp-
tion. The timescale of their diffusion, which depends also
on the sample temperature, is much longer than 1 ms (time
of the single TOF measurement), that is why most atoms,
emitted as a result of a single electron pulse, are detected
after some considerable delay time, and they are not shown
in the TOF spectrum displayed in Fig. 4. There is also
some excitation and emission occurring between the elec-
tron pulses, since the sample is light irradiated constantly,
not only during the electron bombardment. The delayed
thermal peak is also not exactly the Maxwellian distribu-
tion, what is clearly seen for lower sample temperatures.
Fig. 2. Changes of I (a) and Rb (b) quadrupole mass spectrometer signals for electron irradiated RbI(001) surface with
and without simultaneous co-excitation of the F-centers with visible light.
0
Time of irradiation, s
250
200
150
50D
es
o
rp
ti
o
n
y
ie
ld
,
1
0
a.
u
.
3
100
0
Time of irradiation, s
140
100
20
60
Electrons
ON Electrons
OFF
Light Light
Light Light
ON
ON
ONOFF
OFF
OFF
Electrons
Electrons
12 36 4824 26 52 78
a b
D
es
o
rp
ti
o
n
y
ie
ld
,
1
0
a.
u
.
3
Dynamics of the defect-mediated desorption of alkali halide surfaces
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8 973
Discussion
It has been established that the electron-induced desorp-
tion yield of the alkali halide crystal depends, firstly, on
the topography of the surface and, secondly, on the con-
centration of Frenkel defects in the subsurface region of
the electron irradiated crystals [9,18]. As for the first ef-
fect, the influence of the surface topography on the effi-
ciency of ESD process, the following scenario has been
proposed to explain the oscillatory behavior of the desorp-
tion signal on the irradiation time seen in Fig. 1. Electron
bombardment of the crystal leads to production of the pairs
of defects, i.e., mobile excited F*-centers (electrons
trapped in the halogen vacancies) and H-centers (halogen
interstitial atoms). These defects diffuse independently
inside the crystal. When the H-center reaches the surface,
the halogen atom is promptly emitted. On the other hand,
excited F*-centers recombining with the surface, induce
Fig. 3. (Color online) Time-of-flight spectra of iodine atoms emitted from RbI surface irradiated with 1 keV electrons, measured for various
sample temperatures. The black curves (1) present the TOF spectra obtained without the simultaneous illumination with visible light whereas
the red curves (2) are the corresponding TOF spectra obtained with simultaneous white light illumination of the electron irradiated sample. All
spectra are recorded for observation angle 0 degrees (along normal to the surface plane). The electron beam incidence angle is ∼ 45°.
Io
d
in
e
y
ie
ld
,
1
0
a.
u
.
2
Io
d
in
e
y
ie
ld
,
1
0
a.
u
.
3
Io
d
in
e
y
ie
ld
,
1
0
a.
u
.
3
Io
d
in
e
y
ie
ld
,
1
0
a.
u
.
3
45
40
35
30
25
20
15
10
5
0
12
10
8
6
4
2
0
6
4
5
3
2
1
0
16
14
12
10
8
6
4
2
0
Time of flight, ms
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Time of flight, ms
Time of flight, ms
Time of flight, ms
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
50 C� 70 C�
100 C�
120 C�
1
2 1
2
1
2
1
2
Fig. 4. (Color online) TOF spectra of rubidium atoms emitted from RbI(001) surface irradiated with 1 keV electrons, measured for two
different sample temperatures. The black curves (1) present the TOF spectra obtained without the simultaneous illumination with visible
light whereas the red curves (2) are the corresponding TOF spectra obtained with the simultaneous light illumination of the electron
irradiated samples. The delay in atom emission decreases with increasing temperature.
0 0.2 0.4 0.6 1.0 1.2 1.4 1.6 1.80 0.2 0.4 0.6 08 1.0 1.2 1.4 1.6 1.8
Time of flight, ms
R
u
b
id
iu
m
y
ie
ld
,
1
0
a.
u
.
2
11
10
9
8
7
6
5
4
3
2
1
0
75 C�
120 C�
30
25
20
15
10
5
0
R
u
b
id
iu
m
y
ie
ld
,
1
0
a.
u
.
2
Time of flight, ms
0.8
1
2
1
2
M. Szymonski, A. Droba, P. Struski, and F. Krok
974 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8
emission of the alkali atoms, and consequently lead to
formation of molecular vacancies on the surface. Further-
more, it has been shown by theoretical calculations [10]
that the excited F*- center cannot initiate desorption unless
it is trapped at low coordinated sites (like terrace edges,
kinks). Thus, at the surface, depending on its roughness,
different fractions of excited F*-centers are reflected back
to the bulk where they de-excite and stabilize. The steady-
state concentration of immobile F-centers in the subsurface
region is built up after a certain time of irradiation. This
concentration is controlled by the surface recombina-
tion/reflection rate for the diffusing F*-centers, which in
turn depends on the surface step density. Since the desorp-
tion occurs in the layer-by-layer mode, oscillating surface
step density leads to the observed oscillatory behavior of
the desorbed flux of alkali atoms (see Fig. 1). Varying
concentration of immobile F-centers at the surface proxy-
mity modulates probability of the H–F annihilation what
eventually results in yield modulation of the complementa-
ry desorption process of the halogen component. Conse-
quently, the emission of both constitutes of the alkali ha-
lide crystal is influenced by the surface topography.
As for the second effect, the role of concentration of
Frenkel defects, it turns out that the desorption efficiency
depends on the mobility and lifetime of those defects in the
bulk. Therefore, changing F-center properties in the bulk
should lead to changes of the desorption yield. Such an
experiment with simultaneous irradiation of RbI(001)
sample with electrons and white light is reported in this
work in Figs. 2–4. For the co-excitation with light, a noti-
ceable increase of the desorption yield for both sample
constituents is observed. This can be explained by assum-
ing that the ground-state F-centers, accumulated in the bulk
due to electron irradiation, are very efficiently excited by a
white light to their 2p-electronic state and become very
mobile, so they escape very fast via desorption process
from the surface (contributing to the increase of alkali
component of desorption yield) instead of staying in the
bulk and constituting traps for migrating H-centres. Thus,
by F-center excitation with light, the rate of F–H recombi-
nation process in the bulk is decreased. Since the F–H re-
combination in the bulk is a competing process to the H-
centre diffusion to the surface, the decrease of its efficien-
cy results in enhancement of the desorption yield of the
halogen thermal component.
This interesting effect of F–H center bulk recombination
on the desorption yield of surface constituents is further
supported by investigations of the π-luminescence intensity
emitted by the desorbed sample. Since the desorption
process should be competitive to the F–H recombination in
the bulk of the crystal, one would expect that phases of the
oscillatory signal of desorption and luminescence are oppo-
site. Results of the simultaneous measurements of the de-
sorption yield and the luminescence intensity are shown in
Fig. 5. Although the excitation of the accumulated F-centers
leads to an increase in the desorption and luminescence sig-
nals but both signals are in phase. Feasible explanation of
such a relationship requires that F-center participating in the
radiative annihilation of the F–H pair is in the electronically
excited state. Nonradiative decay, in turn, occurs only if the
F-center at the moment of recombination is in its ground
state. Consequently, when the desorption signal passes
through its maximum, the concentration of excited, mobile
F-centers in the vicinity of the surface is high. At the mini-
mum of the desorption signal majority of beam generated
F-centers is reflected from the relatively undefected sur-
face [11] accumulating in the bulk as immobile, ground
state defects. Consequently, the recombination of H-centers
with the accumulated ground state F-centers is nonradiative.
Conclusions
Mass selected time-of-flight spectroscopy has been used
for studying desorption processes occurring on RbI (100)
surfaces due to simultaneous electron and white light co-
excitation. Our results demonstrate that F-centers play a
crucial role in the ESD process. Although the ground-state
F-centers are immobile, theirs excitation either by the
primary beam itself, or by illumination with light within
the absorption F-band, results in efficient transport of the
excited defects from the bulk to the surface and detailed
balancing of the thermal desorption component of halogen
versus alkali atoms. Therefore, simultaneous irradiation of
the crystal by electrons and visible light within the F-center
absorption band increases average F-center mobility, and
therefore, has a profound effect on both the morphology of
the surface and the desorption process efficiency. Such
processes could be applied to increase efficiency and pre-
cision of the controlled surface nanostructuring of ionic
0
250
200
150
50D
es
o
rp
ti
o
n
y
ie
ld
,
a.
u
.
100
Electrons
Electrons
Light
Light
ON
OFF
ON
OFF
350
300
0 25020015050 100 300
Time of irradiation, s
Rubidium
Luminescence
Fig. 5. Dependence of Rb desorption signal and the correspond-
ing π-luminescence on time of electron irradiation for RbI(001)
surface with simultaneous co-excitation with the white light. The
moments of turning on and off of the light are indicated by ar-
rows. Electron beam energy is 1 keV and the sample temperature
is of 100 °C.
Dynamics of the defect-mediated desorption of alkali halide surfaces
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 8 975
insulators. In contrast, prompt emission of non-thermal
halogen atoms due to electronic excitations with energy
higher than the bandgap energy, and surface recombination
of the excited electron-hole pairs, is not affected by addi-
tional illumination with the white light.
1. For recent reviews see: N. Itoh and A.M. Stoneham,
Materials Modification by Electronic Excitation, Cambridge
University Press (2001); E. Gnecco and M. Szymonski,
Nanoscale Processes on Insulating Surfaces, World Scien-
tific Publishing Co., Singapore (2009).
2. M. Szymonski, Radiat. Eff. 52, 9 (1980).
3. W.P. Hess, A.G. Joly, K.M. Beck, V.P. Sushko, and A.L.
Shluger, Surf. Sci. 564, 62 (2004).
4. E.M. Williams and R.E. Palmer, Desorption Induced by
Electronic Transitions, Elsevier, New York (1997).
5. E.V. Savchenko, G.B. Gumenchuk, I.V. Khyzhniy, A.N.
Ponomaryov, and V.E. Bondybey, Surf. Sci. 602 3204 (2008).
6. M. Goryl, FB de Mongeot, F. Krok, A. Vevecka-Priftaj, and
M. Szymonski, Phys. Rev. B76, 075423 (2007).
7. L. Nony, E. Gnecco, A. Baratoff, A. Alkauskas, R. Ben-
newitz, O. Pfeiffer, S. Maier, A. Wetzel, E. Meyer, and C.
Gerber, Nano Lett. 4, 2185 (2004).
8. L. Nony, R. Bennewitz, O. Pfeiffer, E. Gnecco, A. Baratoff,
E. Meyer, T. Eguchi, A. Gourdon, and C. Joachim, Nano-
technology 15, S91 (2004).
9. M. Goryl, B. Such, F. Krok, K. Meisel, J.J. Kolodziej, and
M. Szymonski, Surf. Sci. 593, 147 (2005).
10. V. Puchin, A. Shluger, Y. Nakai, and N. Itoh, Phys. Rev.
B49, 11364 (1994).
11. B. Such, J. Kolodziej, P. Czuba, P. Piatkowski, P. Struski, F.
Krok, and M. Szymonski, Phys. Rev. Lett. 85, 2621 (2000).
12. B. Such, P. Czuba, P. Piatkowski, and M. Szymonski, Surf.
Sci. 451, 203 (2000).
13. H. Hoeche, J.P. Toennies, and R. Vollmer, Phys. Rev. B50,
679 (1994).
14. M. Szymonski, A. Droba, M. Goryl, J.J. Kolodziej, and F.
Krok, J. Phys.: Condens. Matter 18, S1547 (2006).
15. F. Krok, J.J. Kolodziej, B. Such, P. Piatkowski, and M.
Szymonski, Nucl. Instr. Meth. B212, 264 (2003).
16. T. Kubo, A. Okano, J. Kanasaki, K. Ishikawa, Y. Nakai, and
N. Itoh, Phys. Rev. B49, 4931 (1994).
17. J. Kolodziej, B. Such, P. Czuba, F. Krok P. Piatkowski, P.
Struski, M. Szymonski, R. Bennewitz, S. Schar, and E. Meyer,
Surf. Sci. 482–485, 903 (2001).
18. M. Szymonski, J. Kolodziej, B. Such, P. Piatkowski, P.
Struski, P. Czuba, and F. Krok, Prog. Surf. Sci. 67, 203
(2001).
19. M. Szymonski, P. Struski, A. Siegel, J.J. Kolodziej, B. Such,
P. Piatkowski, P. Czuba, and F. Krok, Acta Phys. Polonica
B33, 2237 (2002).
20. A. Droba, Ph. D. Thesis, Jagiellonian University, Krakow
(2006).
21. J. Kolodziej, Z. Postawa, P. Czuba, P. Piatkowski, and M.
Szymonski, Radiat. Eff. 128, 47 (1994).
22. M. Szymonski, J. Kolodziej, P. Czuba, P. Piatkowski, and A.
Poradzisz, Phys. Rev. Let. 67, 1906 (1991).
23. A. Alexandrov, M. Piacentini, R.G. Tonkyn, M.T. Sieger, N.
Zema, and T.M. Orlando, Surf. Sci. 451, 208 (2000).
|