3P135
Adsorption of Benzene Derivatives on the Ag(110) at 90K Studied by Collision-energy-resolved
Electron Spectroscopy with He*(23S) Metastable Atoms(Department of Chemistry, Graduate School of Science, Tohoku University)○Andriy Borodin, Tadafumi Akazawa, Naoki Kishimoto, Takuya Horio and Koichi Ohno
Penning ionization electron spectroscopy (PIES) is a widely known method for investigating
the electron structure of the molecules in the valence band. This method has been even moredeveloped to a powerful tool for investigation of dynamic and anisotropy in interactions betweena target molecule and the metastable atoms of the probe gas (mainly He*(23S)). In PIES thespectra can be collected in dependence on two parameters: the common electron kinetic energy
and the energy of collision between a target and He*(23S). This technique, known as 2D-PIES,
has been widely used for the study of various molecules in gas phase experiments.
The cuts of two-dimensional data at selected collision energies provide the usual
PIES-spectra while cuts at selected ionization bands give the collision energy dependence ofpartial ionization cross sections (CEDPICS). In the case of a molecular orbital (MO) mainly
localizing around repulsive region, the CEDPICS-slope for this MO is positive, because themetastable atom with larger kinetic energy reaches the inner region (where the MO-overlapping
and the reaction probability is high) of the target against the repulsive interaction. On the otherhand, a negative CEDPICS can be assigned to an attractive interaction with He* around theextending region of corresponding MO, because the metastable atom is deflected to this area ofthe molecule by the attractive interaction. In this case, the number of deflected trajectories
decreases with the increase of collision energy. Since the electron distributions of individual MOs
are more or less localized on the different spatial parts of the molecule, different CEDPICSs forvarious ionic states can be connected to the anisotropy in interaction between He*(23S) and thetarget molecule. Knowing the value of the CEDPICS-slope, the chemical activity for various
parts or its functional parts of the target can also be estimated.
The spectroscopy with metastable atoms is also intensively used for investigation on surfaces.
This method is also known as MIES (Metastable Impact Electron Spectroscopy) or MAES
(Metastable Atom Electron Spectroscopy). On condition that no resonant transfer between He*
2s state and an occupied state of the target takes place, which is typical for dielectric adsorbates,
the most important electron process for the surfaces is Auger de-excitation (AD), which is similar
to Penning ionization process for gas phase targets. These processes take place when theinner-shell unoccupied 1s orbital of He*(23S) overlaps the field of MOs localizing outside thecollision boundary surface of the target, then 1s MO is filled by a target electron and the 2selectron is ejected and can be analyzed. Therefore, on the basis of similar kind for interaction ofmetastable atoms with surfaces and gas targets, application of the 2D-PIES method forinvestigations on a surface can also be useful.
Adsorption of benzene on Ag(110), which is investigated in the framework of the project for
2D-PIES on the surfaces, is shown in the Figure 1. Along with some irregularities in the electron

energy range 0 – 2.5 eV, which are very likely caused by peculiarity of the experimental setup
with a electron analyzer of retarding type, five bands inthe energy range 3 – 12 eV, which can be easily
assigned to benzene on the basis of the gas-phase
spectrum, can be distinctly observed. Due to highsurface sensitivity, the intensity of the bands at theadsorption reaches full intensity already at monolayer(ML) covering. Thus after exposition of ca. 35 L, aclosed layer is formed on the silver surface at thetemperature of the liquid nitrogen. For the 2D-PIESmeasurements, the films corresponded to the spectra
exposed with 15 L and 95 L, with covering ca. 0.5ML
and 2 ML respectively, were selected.
The results for the collision energy dependence ofpartial ionization cross sections are sown in the Figure
2 for three most intensive bands 1 – 3. The curves forcovering of 0.5 ML are drawn taking into account thebackground correction. As in the case of benzene in theFigure 1. C6H6 adsorption on
gas-phase, the π MOs 1 and 2 have negative CEDPICSthe Ag(110) surface at 90 K
slopes indicating an attractive interaction with He* forrelated target regions. The band 3, which can be
assigned to σCH MO, has a negative CEDPICS for thecase of 0.5 ML while in the gas-phase the σ MOs show
usually a repulsive character; taking into account avery strong attractive character of the Ag(110) surface,it may be an evidence of planar adsorption of thebenzene molecules, which has been already observed forsome metal surfaces. When a benzene molecule islocalized apart of the silver surface, like in the case of 3
ML covering, the repulsive character of the σ regions isalso indicated by CEDPICS. We can also conclude, that
there is an additional factor causing the more intensive
repulsive interaction of the σ bands: A comparison of thespectra for ~ 1 ML and ~ 3 ML (~ 35 L and 95 L
respectively), shows larger intensity in the region 3 – 7eV, which is assigned to the σ bands. It may indicate,
2 Collision energy dependence
that σ regions (i.e. edges) of the molecules in next layers Figure 2.
ofionization
intensity for bands 1 – 3
can be easier reached by the metastable He* molecules,
which means a different (not planar) kind of adsorption.
Therefore spectroscopy with metastable atoms, especially with resolving collision energy,
allows us to obtain electron structure of the adsorbed films as well as information on orientation
adsorbed molecules and on the interaction of different regions of the film with metastable atoms.