We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the molecule-substrate interface for manipulating the magnetic properties of adsorbates.

Photofragment excitation (PHOFEX) spectra of NO2 are observed by monitoring the specific quantum states of a fragment NO (2Π1/2;v=0, J=0.5–6.5) in the energy region 0–160 cm−1 above the dissociation limit to NO (2Π1/2) and O (3P2). Preparation of NO2 in a quasibound eigenstate above the dissociation limit is attained by the combination of extremely cooled (∼1 K) parent NO2 in a supersonic jet and a high resolution (∼0.05 cm−1) photolysis laser. The dissociation rate constants are obtained from the peak width of PHOFEX spectra and the smallest rate constant is k=8.5×109 s−1, in the energy region where only J=0.5 of NO (2Π1/2; v=0) is produced. The observation that the rate constant increases stepwise when a new product channel J=1.5 opens implies that the transition state is a loose complex. This behavior of the rate constant is direct experimental proof of the statistical theory of the unimolecular reaction process. The product state distribution of NO fluctuates depending on the quasibound state of NO2, though the average value is consistent with the calculation by phase space theory. This state specificity of the rate constant is interpreted in terms of quantum fluctuations associated with individual quasibound eigenstates.

We investigate the polaronic ground state of anatase TiO2 by bulk-sensitive resonant inelastic x-ray spectroscopy (RIXS) at the Ti L3 edge. We find that the formation of the polaron cloud involves a single 95 meV phonon along the c axis, in addition to the 108 meV ab-plane mode previously identified by photoemission. The coupling strength to both modes is the same within error bars, and it is unaffected by the carrier density. These data establish RIXS as a directional bulk-sensitive probe of electron-phonon coupling in solids.

Time-resolved hard x-ray photoelectron spectroscopy (trHAXPES) is established using the x-ray free-electron laser SACLA. The technique extends timeresolved photoemission into the hard x-ray regime and, as a core-level spectroscopy, combines element and atomic-site specificity and sensitivity to the chemical environment with femtosecond time resolution and bulk (sub-surface) sensitivity. The viability of trHAXPES using 8 keV x-ray free-electron-laser radiation is demonstrated by a systematic investigation of probe and pump pulseinduced vacuum space-charge effects on the V 1s emission of VO 2 and the Ti 1s emission of SrTiO 3 . The time and excitation energy dependencies of the measured spectral shifts and broadenings are compared to the results of N-body numerical simulations and simple analytic (mean-field) models. Good agreement between the experimental and calculated results is obtained. In particular, the 9 Present address: characteristic temporal evolution of the pump pulse-induced spectral shift is shown to provide an effective means to determine the temporal overlap of pump and probe pulses. trHAXPES opens a new avenue in the study of ultrafast atomic-site specific electron and chemical dynamics in materials and at buried interfaces.Keywords: time-resolved photoelectron spectroscopy, x-ray free-electron laser, space-charge effects IntroductionSub-picosecond time-resolved solid-state photoemission spectroscopy has recently emerged as a powerful novel technique for studying the electronic properties of condensed matter. The power of the technique is that it provides direct access to the electronic structure dynamics in materials and at their surfaces on the time scales of the underlying elementary electronic and lattice processes, such as electron-electron scattering, electron screening and thermalization, coherent phonon vibrations, electron-phonon and phonon-phonon coupling, as well as substrate-adsorbate charge transfer or the buildup of surface photovoltages. Time-resolved photoemission spectroscopy generally combines frequency-domain information with subpicosecond time resolution through a pump-probe scheme in which typically an infrared (IR) pump pulse is used to excite the system whose dynamics is then probed at different time delays by detecting the photoelectrons emitted by ultrashort pulses in the ultraviolet (UV) to soft x-ray regime. When probe pulses in the UV to extreme ultraviolet (XUV) range are used and angular resolution is added, the technique is referred to as time-and angle-resolved photoemission spectroscopy (trARPES) and provides direct information on the momentum-resolved dynamics of valence electrons, including the temporal evolution of electronic populations, band structures, Fermi surfaces, and energy gaps [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. When the probe pulses have higher photon energies, in the extreme ultraviolet to soft x-ray range, the technique becomes time-resolved x-ray photoemission spectroscopy (trXPS), with element specificity, sensitivi...