The adsorption and self-assembling properties of terephthalic
acid
(TPA) molecules deposited on Cu(001) at room temperature have been
systematically studied with both experimental and theoretical tools.
The system forms two phases at room temperature: the metastable β-phase
and the stable (3 × 3) one. In the case of the β phase,
low-energy electron diffraction and scanning tunneling microscopy
(STM) results indicate that it has a (9√2 × 2√2) R45° unit cell with exactly the same molecular coverage
as the (3 × 3) phase. In addition, the high-resolution X-ray
photoelectron-spectroscopy spectra of the O 1s core level indicate
that the irreversible β → (3 × 3) transition involves
the following two processes: (i) deprotonation of the complete carboxyl
groups remaining in the metastable phase and (ii) eventual rearrangement
of the molecules into the 3 × 3 configuration. We explored possible
molecular configurations for the β phase with different degree
of deprotonation (including structures with Cu adatoms) by means of
density functional theory calculations. Our theoretical results indicate
the formation of strong bonds between the O atoms in carboxylates
and the Cu atoms of the surface, which causes a bending of the molecules
and a buckling of the first Cu layer. In the (3 × 3) phases,
we show that the bending produces observable effects in the molecular
STM images. Moreover, the strong interaction between the carboxylates
and the Cu atoms at the step edges drives the reorientation of the
surface steps along the ⟨100⟩ crystallographic directions.
Phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. In the present work, the excited states of these zwitterionic clusters have been studied experimentally with two-color pump probe methods on the nanosecond time scale and by ab initio electronic-structure calculations. The experiments reveal the existence of a long-lived excited electronic state with a lifetime in the 50-100 ns range, much longer than the excited state lifetime of bare phenol and small clusters of phenol with ammonia. The ab initio calculations indicate that this long-lived excited state corresponds to a biradicalic system, consisting of a phenoxy radical that is hydrogen bonded to a hydrogenated ammonia cluster. The biradical is formed from the locally excited state of the phenolate anion via an electron transfer process, which neutralizes the charge separation of the ground state zwitterion.
We nd that for adsorbed terephthalic acid (TPA) molecules, surface Sn alloying deactivates the Cu(001) surface by decoupling the adsorbed molecules from the substrate. This eect is investigated for the case of the 0.5 ML phase of the Sn/Cu (001) surface alloy by applying fast X-ray Photoemission Spectroscopy, Scanning Tunnel-
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