Coadsorption of sodium and water on MgO„100…/Mo„100…
studied by ultraviolet photoelectron and metastable impact
electron spectroscopies
J. Günster, G. Liu, V. Kempter,a) and D. W. Goodmanb)
Department of Chemistry, Texas A&M University, P. O. Box 300012, College Station, Texas 77842-3012
~Received 4 October 1997; accepted 2 March 1998!
The coadsorption of D2 O and Na on the MgO~100!/Mo~100! surface has been studied by metastable
impact electron spectroscopy and ultraviolet photoelectron spectroscopy ~He I!. The initial layer of
D2 O adsorbed on the MgO~100! surface at 100 K ‘‘wets’’ the surface. For multilayer adsorption, the
outermost water molecules exhibit an electronic structure which is very similar to gas phase water.
Na dosed onto a D2 O- precovered MgO surface leads to the formation of a hydroxide species, most
likely NaOH. This hydroxide species is stable to 530 K. © 1998 American Vacuum Society.
@S0734-2101~98!06103-X#
I. INTRODUCTION
II. EXPERIMENT
The adsorption of water has been the subject of numerous
experimental and theoretical investigations. In particular, the
adsorption of water on the model basic oxide—magnesium
oxide—has attracted considerable attention recently ~see
Refs. 1–3 for a review!. This system has been studied utilizing techniques such as reflection adsorption infrared spectroscopy ~RAIRS!, temperature programmed desorption
~TPD!, helium atom scattering ~HAS!, high-resolution electron energy-loss spectroscopy ~HREELS! and low energy
electron diffraction ~LEED!. From these investigations
emerges a rather complete picture of the adsorption dynamics; however, very little information exists regarding the
electronic structure of the valence band region of water adsorbed onto solid surfaces. Recently, surface sensitive spectroscopic techniques such as metastable impact electron
spectroscopy ~MIES! and ultraviolet photoelectron spectroscopy ~UPS! have proved to be very useful in monitoring the
electronic structure of molecular species adsorbed on insulating films.4–8 For an introduction to MIES and its various
applications in molecular and surface spectroscopies, see the
recent review by Harada et al.9
In the present article MIES and UPS have been used to
characterize the adsorption of water on the MgO~100! surface at 100 K. In addition the effect of Na coadsorption has
been investigated. In order to avoid charging of the surface
during the measurements, ultrathin MgO films were grown
on a Mo~100! substrate. As shown in previous
investigations,10–13 thin MgO films grow epitaxially on a
Mo~100! surface. In addition, these surfaces reproduce the
adsorption behavior of a MgO~100! single crystal surface
with respect to the adsorption of water.3
The experiments were carried out in an ultrahigh vacuum
~UHV! system ~base pressure ,2310210 Torr! consisting of
two interconnected chambers: one for sample treatment and
TPD and the other for electron spectroscopy. In the latter
chamber, there exist facilities for x-ray photoelectron spectroscopy ~XPS!, Auger electron spectroscopy ~AES!, UPS
and MIES measurements. MIES and UPS spectra were measured simultaneously using a cold-cathode discharge source
which has been described previously.13,14 Briefly, a helium
cold-cathode gas discharge provides both ultraviolet photons
~He I! and metastable He* 2 3 S/21 S (E * 519.8/20.6 eV! atoms with thermal kinetic energies. The triplet-to-singlet ratio
has been measured by He* –Ar impact as 7:1, but very efficient conversion into He* 23 S has been observed on metallic
and semiconducting surfaces.15–17 Metastable and photon
contributions within the beam were separated by means of a
time-of-flight method using a mechanical chopper. The collection of a MIES/UPS spectrum requires approximately
180 s. The AES, MIES and UPS spectra were acquired with
incident photon/metastable beams 45° with respect to the
surface normal in a constant pass energy mode using a
double pass cylindrical mirror analyzer ~CMA!, PHI Model
15-255G. It is worth noting that the MIES spectra in the
present study were acquired with a CMA which integrates
over a relatively wide angular distribution. These results are
very similar to those measured previously on MgO surfaces7
using a hemispherical analyzer with a relatively small acceptance angle. The energies denoted by E F in the spectra correspond to electrons emitted from the Fermi level of the
Mo~100! substrate. In the following spectra all binding energies are referenced to E F . Since the metallic Mo substrate
and the analyzer are in electrical contact, the Fermi energy
appears at a constant position. Biasing of the sample ~245
V! permits the work function change of the surface to be
measured directly from the high binding-energy cutoff of the
spectra.
MgO films were grown by depositing Mg in 1310 26
Torr O2 ambient on the Mo~100! surface at 550 K. The Mg
a!
Also with: Physikalisches Institut, Technischen Universität Clausthal,
Leibnizstrasse 4, D-38678 Clausthal-Zellerfeld, Germany.
b!
Author to whom correspondence should be addressed; electronic mail:
[email protected]
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©1998 American Vacuum Society
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Günster et al. : Coadsorption of sodium and water on MgO„100…/Mo„100…
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source was made from a high-purity Mg ribbon wrapped
around a tantalum filament. In order to ensure a stoichiometric MgO layer, the as-prepared surface was further annealed to 700 K in a 1310 27 Torr O2 background for 20
min. As shown in previous investigations, MgO films prepared under these conditions grow epitaxially on the
Mo~100! substrate.10,12,18
Water was dosed following the previously described
procedure.3 D2O ~CIL, 99.9%! was used after further purification in the vacuum manifold via several freeze–pump–
thaw cycles ~vacuum distillation!. In order to minimize H–D
exchange, the manifold was baked repeatedly for several
hours in an D2O background. Na was deposited onto the
surface using a SAES Getters source.
III. RESULTS
The MIES and UPS spectra of a MgO-covered Mo~100!
surface acquired during D2O exposure are presented in Fig.
1. As determined by AES, the thickness of the MgO layer is
approximately 5 monolayers ~ML!. D2O was dosed ~0.6 L
per minute! onto the substrate which was held at 100 K.
MIES and UPS spectra were recorded continuously during
the exposure. Due to the insulating character of the clean
MgO~100! surface no intensity between E F and 3.8 eV binding energy is apparent in the bottom spectra of Fig. 1. These
data show that there are no occupied states in resonance with
the impinging He 2s electrons which could lead to MIES
spectra dominated by an Auger capture process. Therefore,
contributions from the Auger capture process need not be
considered, and a direct comparison between the MIES and
UPS data is possible.19
The MIES and UPS spectra of the clean MgO~100! surface are in good agreement with those reported previously.7
The structure in the bottom spectrum of Fig. 1, denoted by
O(2p), corresponds to emission from the O 2p valence band
of the MgO~100! substrate.7,20 During D2O dosing a continuous change from a spectrum typical for a MgO surface ~bottom spectrum! to a spectrum typical for water in the gas
phase21 ~uppermost spectrum! is observed in the sequence of
MIES data of Fig. 1. Binding energies and molecular orbital
assignments for the three water induced structures 1b 1 , 3a 1
and 1b 2 , based on gas-phase photoelectron spectra22,23 and
Penning ionization electron spectra ~PIES!24, are also presented in Fig. 1. After an exposure of 12 L D2O ~dotted
MIES spectrum!, the MIES spectra are dominated by the
water-induced features. We attribute this 12 L exposure to
the completion of the first water monolayer ~ML! and the
onset of second layer adsorption. After a 15 L exposure the
water background pressure was increased by a factor of 2 as
indicated in Fig. 1, from 5310 29 Torr to 1310 28 Torr.
After a 33 L exposure the water pressure was increased yet
again by a factor of 5 ~5310 28 Torr!. At an exposure between 15 and 33 L, the intensities of the water-induced features level off. At 33 L, a shift of the MIES and UPS spectra
indicates charging of the surface.
On the other hand, the sequence of UPS spectra in Fig. 1
shows a transition from a spectrum typical for a MgO surface
JVST A - Vacuum, Surfaces, and Films
FIG. 1. MIES and UPS spectra from a 100 K MgO~100!/Mo~100! substrate
as a function of D2O exposure. The bottom spectrum shows the clean
MgO~100! surface, and the uppermost spectrum shows the water-covered
surface. The dashed spectrum corresponds to 1 ML of adsorbed D2O.
~bottom spectrum! to a spectrum typical for condensed water. In the exposure regime between 15 and 33 L, the spectral
features corresponding to 1b 1 and 1b 2 are well developed.
However, 3a 1 appears to be broadened considerably, which
is typical for a condensed water layer.25
The MIES results obtained during exposure of Na to the
water-covered MgO surface held at a substrate temperature
of 100 K are presented in Fig. 2. The Na exposure and the
direction of data collection are noted in the figure. The spectra are displayed with the Na-free, water-covered MgO/Mo
spectrum at the top, followed by various Na coverages. In
the binding energy range between 6 and 18 eV a spectral
change occurs, from that dominated by the three waterinduced features, 1b 1 , 3a 1 and 1b 2 , to that with two additional peaks denoted by A 1 and A 2 . During the Na exposure
an attenuation of the water-induced features and their shift to
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Günster et al. : Coadsorption of sodium and water on MgO„100…/Mo„100…
998
comparison with UPS ~He I! data acquired for sodium
hydroxide28 and water adsorbed onto a sodium-doped
Cu~111!27 suggests that features A 1 and A 2 at 7 and 11.1 eV
binding energy, respectively, correspond to the ionization of
the first and third orbitals, respectively, of the Na-bound OH.
As shown in Fig. 3, the hydroxide-induced features are stable
on the surface up to 530 K. After flashing the substrate to
710 K, the MIES and UPS spectra ~bottom spectra in Fig. 3!
are similar to those acquired for a clean MgO~100! surface.
IV. DISCUSSION
FIG. 2. MIES spectra from a D2O-covered surface as a function of Na
exposure where the uppermost spectrum shows the water-covered
MgO~100! surface without Na. The direction of the data collection and the
relative Na exposure are indicated by the arrow. It is noted that the dotted
spectrum shows the same structure between 6 and 18 eV as the substrate
shown in Fig. 3 which has been flashed to 150 K.
higher binding energies is apparent. The respective UPS
measurements, which are not shown here, exhibit the same
spectral features but are less well pronounced. The appearance of the feature near E F at the highest Na exposures,
denoted by AU, and its satellite at 2.5 eV higher binding
energy, likely corresponds to the Na coverage at which the
formation of islands with metallic character begins to
occur.8,26 Flashing the substrate to 190 K ~as shown in Fig.
3! results in a MIES spectrum that is dominated by A 1 and
A 2 and is very similar to the UPS spectrum acquired from
water adsorbed on Cu~111! covered by a multilayer Na.27 A
FIG. 3. MIES spectra from a D2O-covered MgO surface following the addition of Na ~uppermost spectrum! as a function of the anneal temperature.
J. Vac. Sci. Technol. A, Vol. 16, No. 3, May/Jun 1998
The adsorption of water on the clean MgO~100! surface
~Fig. 1!, in the coverage regime up to 1 ML ~dotted spectrum!, yields only weakly developed features with MIES and
UPS. This is consistent with the formation of a strong interaction ~chemisorption! between the adsorbed molecules and
the MgO surface.3 For coverages higher than 1 ML three
distinct features, corresponding to the ionization of the outermost water molecules, are apparent in MIES. According to
previous MIES and UPS investigations of condensed water
on Cu21 and gas phase measurements,22,23 the features at
binding energies of 7.2, 9.2 and 13.6 eV are assigned to 1b 1 ,
3a 1 and 1b 2 , respectively. These binding energies correspond to ionization potentials @IP5~excitation energy!
2~electron kinetic energy!# of 11.2, 13.2 and 17.6 eV, respectively. These values are in agreement with those
measured for water adsorbed on MoS2 by Yu et al.25 but are
approximately 1 eV lower compared to the results of Campbell et al.29 Since no MgO substrate intensity is apparent in
MIES after dosing 12 L water, it is assumed that the initial
layer of water adsorbed at 100 K ‘‘wets’’ the surface. Due to
the very high surface sensitivity of MIES it is not possible to
distinguish one layer of condensed water from the uppermost
water layer at higher coverages. Therefore, it cannot be excluded that three-dimensional island growth of water, subsequent to the formation of the first uniform layer, has occurred. The most significant difference between the UPS and
MIES measurements on condensed water is the appearance
of a narrow 3a 1 peak in MIES. This feature is only seen in
the UPS measurements for gas phase water.
A broadening of the 3a 1 feature in the UPS measurements for condensed water is most likely due to the overlap
of the 3a 1 water molecular orbitals forming hydrogen
bonds.30 The close agreement between the UPS gas phase
3a 1 spectral feature and the MIES spectra suggests a binding
geometry of the outermost water molecules, which minimizes the overlap of the 3a 1 molecule orbitals. However, a
determination of the precise geometry for this structure is
beyond the scope of this investigation.
Na adsorption on the water-precovered MgO~100! surface
~Fig. 2! leads to the formation of a sodium hydroxide species
which is stable on the surface to 530 K ~see Fig. 3!. After
dosing Na, two groups of features in the binding energy
ranging between 6 and 18 eV are apparent in MIES ~bottom
spectrum in Fig. 2!. One group (1b 1 , 3a 1 and 1b 2 ) corresponds to intact water molecules, and one (A 1 and A 2 ) corresponds to a hydroxide species . The appearance of both
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Günster et al. : Coadsorption of sodium and water on MgO„100…/Mo„100…
species on the surface subsequent to a saturation exposure of
Na indicates that even at high Na coverages intact water
molecules survive on the surface. After flashing the substrate
to 190 K, which is the typical desorption temperature for
multilayer water,3 the MIES spectrum ~see Fig. 3! is dominated by the hydroxide features. The similarity of these spectra with those acquired for solid NaOH suggests, in accordance with Ref. 27, that a three-dimensional D2O derived
species is formed on the substrate. A detailed discussion and
a comparison to measurements on a Na-precovered MgO
surface will be published elsewhere.
V. CONCLUSIONS
Because of the enhanced surface sensitivity of MIES, it is
superior to UPS for monitoring chemisorption phenomena at
low adsorbate coverages. It is found that the initial layer of
D2O adsorbed on the MgO~100! surface at 100 K wets the
surface. For multilayers of adsorbed D2O, the outermost water molecules in the condensed water phase are arranged
such that the 3a 1 molecular orbitals are significantly less
perturbed than those of bulk water. Dosing Na onto the
D2O-precovered MgO surface leads to the formation of a
hydroxide species, most likely NaOH. This hydroxide species is stable to 530 K.
ACKNOWLEDGMENT
The authors acknowledge with pleasure the support of this
work by the National Science Foundation ~Contract No.
DMR 9423707!.
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