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UHV-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane (TMP 1 -TCNQ 1 ) on gold have been studied using ultraviolet photoelectron spectroscopy (UPS), X-ray-diffraction (XRD), infrared (IR) spectroscopy and

We used Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) to investigate the decay of Ne2 after K-shell photoionization. The breakup into Ne 1+ / Ne 2+ shows interatomic Coulombic decay (ICD) occurring after a preceding atomic Auger decay. The molecular frame angular distributions of the photoelectron and the ICD electron show distinct, asymmetric features, which imply localization of the K-vacancy created at one of the two atomic sites of the Ne2 and an emission of the ICD electron from a localized site. The experimental results are supported by calculations in frozen core Hartree-Fock approach. PACS numbers:Are inner-shell holes in homonuclear diatomic molecules localized at one of the atoms or delocalized over the two equivalent sites? This question has been discussed highly controversial in literature for more than 35 years now (see e.g. [1,2,3,4,5,6,7,8,9,10,11] and our discussion below). Here we report on an experiment answering that question for the Ne 2 Van-der-Waalsmolecule. Intuitively strong arguments for both opinions may be found: the K-shell wave function is very tightly confined to the nuclei and the overlap between inner-shell orbitals at different atoms of a molecule is usually negligibly small [12]. Hence, the geometry of the problem suggests to think of individual localized atomic wave functions. The symmetry of the problem however suggests the opposite: both sites of the diatomic molecule are indistinguishable and therefore the total molecular wave function has to have well defined gerade (g) or ungerade (u) symmetry. In order to construct the molecular manybody wave function it seems natural to employ only symmetry adapted single electron orbitals. A core level hole would then have well defined g or u symmetry and hence be delocalized over the two sites. This approach is used in today's state of the art theory concerning the photoionization and decay of Ne 2 [13].To address this question of a possible localization in a quantum mechanically meaningful and experimentally accessible way one has to relate it to a measurable observable. Prime candidate is the energy of the state measured by photoelectron spectroscopy. Theoretical claims for core hole localization based on the energy date back to classic works of Snyder [1] and Bagus [2]. They showed that allowing symmetry broken -i.e. localized -basis states for inner-shell vacancies in a Hartree-Fock calculation lowers the energy and yields better agreement with the experiment. It was later argued that this is a peculiarity of the Hartree-Fock approach [3]. More sophisticated calculations on N 2 today very well reproduce the experimentally observed energy splitting of about 100 meV between the 1σ g and 1σ u core-ionized states [4,5]. This is generally taken as evidence for the delocalized character of the inner-shell hole.As an alternative observable being sensitive to core hole localization the angular distribution of the photoelectrons in the laboratory system was suggested [6]. Corresponding experiments on nitrogen molecules show good ag...

At photon energies near the Ne K edge it is shown that for 1s ionization the Auger electron, and for 2s ionization the fast photoelectron, launch vibrational wave packets in a Ne dimer. These wave packets then decay by emission of a slow electron via interatomic Coulombic decay (ICD). The measured and computed ICD electron spectra are shown to be significantly modified by the recoil induced nuclear motion.

In the search for novel organic charge transfer salts with variable degrees of charge transfer we have studied the effects of two modifications of the recently synthesized donor-acceptor system [tetramethoxypyrene (TMP)]-[tetracyanoquinodimethane (TCNQ)]. One is of chemical nature by substituting the acceptor TCNQ molecules by F4TCNQ molecules. The second consists in simulating the application of uniaxial pressure along the stacking axis of the system. In order to test the chemical substitution, we have grown single crystals of the TMP-F4TCNQ complex and analyzed its electronic structure via electronic transport measurements, ab initio density functional theory (DFT) calculations and UV/VIS/IR absorption spectroscopy. This system shows an almost ideal geometrical overlap of nearly planar molecules stacked alternately (mixed stack) and this arrangement is echoed by a semiconductor-like transport behavior with an increased conductivity along the stacking direction. This is in contrast to TMP-TCNQ which shows a less pronounced anisotropy and a smaller conductivity response. Our band structure calculations confirm the one-dimensional behavior of TMP-F4TCNQ with pronounced dispersion only along the stacking axis. Infrared measurements illustrating the C≡N vibration frequency shift in F4TCNQ suggest however no improvement in the degree of charge transfer in TMP-F4TCNQ with respect to TMP-TCNQ. In both complexes about 0.1e is transferred from TMP to the acceptor. Concerning the pressure effect, our DFT calculations on the designed TMP-TCNQ and TMP-F4TCNQ structures under different pressure conditions show that application of uniaxial pressure along the stacking axis of TMP-TCNQ may be the route to follow in order to obtain a much more pronounced charge transfer.