A scanning tunneling microscopy study and ab initio total energy calculations have identified the atomic-level structure of novel sp;{3}-bonded carbon nanoscale domains formed dynamically from graphite by femtosecond-laser excitation. The structure is characterized by a pseudo sp;{3}-bonding configuration that results in inward displacement of surface carbon atoms by 0.5 A in every third row along the [11[over ]00] orientation. This structural periodicity is unique and differs from conventional phases of diamond.

No abstract

Two-photon photoemission (2PPE) spectroscopy is used to reveal dynamic relaxation of highly excited electrons generated by 3 eV photons in Si. Monochromatic 2PPE at probe-photon energies ranging from 3.2 to 3.5 eV reveals a coherent 2PPE peak from the valence band maximum, and a transiently populated photoemission peak near L 1 in the conduction band. Time-resolved 2PPE measurements show that electrons injected into the L valley undergo L-to-X intervalley scattering with a time constant of 180 fs at 293 K. A deformation potential constant of 1.2 eVÅ −1 is obtained for the scattering. The highly excited electrons are relaxed down around the conduction band minimum to form a quasi-equilibrated hot electron distribution; the highest density is accumulated at 660 fs after excitation. The hot electron distribution is equilibrated with the lattice within 2 ps after excitation. These results are compared to relaxation processes of electrons injected directly into the X valley by photons with energies below 2.2 eV.

We study ultrafast scattering dynamics of hot electrons photoinjected with high excess energies in the valley of the conduction band of GaAs, using time-and angle-resolved photoemission spectroscopy and ab initio calculations. At ultrafast rates of the order of 10 fs, the packets in the valley are transformed into hot-electron ensembles (HEEs) quasiequilibrated in momentum space but not in energy space. The energy relaxation of the HEEs takes place as a whole on a longer time scale with rates dependent only on the excess energy, irrespective of the momenta of hot electrons. Both momentum scattering and energy relaxation are ruled by the electron-phonon interaction.

Laser-induced structural changes on the Si(111)-͑7 3 7͒ surface were studied for laser fluences below thresholds of melting and ablation. The adatoms of the reconstructed structure are removed selectively by an electronic process, and Si atoms in the electronic ground state are ejected with a peak translational energy of 0.15 eV. The electronic process of this bond breaking of adatoms exhibits the site-sensitive efficiency which shows a resonant wavelength dependence and is highly superlinear with respect to the excitation intensity. [S0031-9007(98)05872-4]