We discuss experiments that address the ultrafast dynamics inherent to the photoemission process in condensed
matter. In our experimental approach, an extreme ultraviolet attosecond light pulse launches photoelectron wave
packets inside a solid. The subsequent emission dynamics of these photoelectrons is probed with the light field of
a phase-stabilized near-infrared laser pulse. This technique is capable of resolving subtle emission delays of only
a few attoseconds between electron wave packets that are released from different energy levels of the crystal. For
simple metals, we show that these time shifts may be interpreted as the real-time observation of photoelectrons
propagating through the crystal lattice prior to their escape into vacuum. The impact of adsorbates on the
observed emission dynamics is also investigated.
We observe the electric fields caused by charge distributions during femtosecond laser ablation from a silicon
(100) surface. Femtosecond electron pulses passing near the ablation site serve as a probe of the electric field
generated by the emitted charges and countercharges on the sample surface. The density map of the electron pulse
downstream from the sample contains information about the charge distributions. We invert this information
by fitting the beam maps using a simple charge distribution model. Under the present excitation conditions
(390 nm, 150 fs, 5.6 J/cm2), we observe the emission of 5.3×1011 electrons/cm2 within 3 ps of the excitation
pulse, leading to self-acceleration of the emitted electrons to 2% of the speed of light. Preliminary experiments
on a metal sample display even faster dynamics.
Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the organic chromophore perylene
into the inorganic semiconductor rutile TiO2 was investigated with femtosecond time-resolved two-photon
photoemission (2PPE). With 2PPE one can address adsorbates at coverages far below a monolayer on single
crystal surfaces. With the same chromophore perylene fixed with different anchor and bridge groups at the surface
of rutile TiO2(110) the corresponding 2PPE transients revealed the relevant parameters that characterize
the contributing processes. Instantaneous optical injection on one hand and slow injection over a long distance
on the other hand were realized. Direct optical charge transfer was realized with the chromophore catechol that
is known to form a charge transfer complex with Ti atoms on the surface of TiO2. The slow injection cases
were realized by inserting rigid molecular bridges. Comparison of the different 2PPE signals with corresponding
transient absorption (TA) signals for the identical systems revealed the physical processes and time scales that
control the 2PPE transients. On the surface of the single crystals only one long time constant was measured via
2PPE also in the case of a long rigid bridge/anchor group in contrast to a broad distribution of time constants
observed for the same molecules anchored in the nm-size cavities of an anatase TiO2 film measured via TA.
The broad distribution of time constants in the latter measurements can be attributed to different microscopic
environments giving rise to different distances between the chromophore and the nearest TiO2 wall.
Hot electron injection from the excited electronic singlet state of perylene chromophores into the (110) surface of rutile TiO2 single crystals was measured with femtosecond two-photon photoemission (2PPE) for different anchor/bridge groups attached to the perylene chromophore. Femtosecond 2PPE probes the time and energy dependence of the population of firstly the excited state of the chromophore and secondly of the hot electrons injected into the surface layer of the semiconductor. Measuring both these contributions gives a complete picture of the ultrafast photo-induced injection process and bridges the gap to conventional measurements of the rise time of the corresponding photocurrent. Studying the system in ultra-high-vacuum (UHV) makes all the tools of surface science available. Impurities on the surface were studied with XPS, the alignment of the occupied and unoccupied electronic levels at the interface with UPS and with 2PPE, respectively. The orientation of the elongated chromophores with respect to the crystal surface was deduced from angle and polarization dependent 2PPE signals making use of the known orientation of the dipole moment for the optical transition, the energy distribution of the injected hot electrons was determined with 2PPE from the energy distribution of the photoemitted electrons, and finally the escape of the injected electrons from the surface to bulk states of the semiconductor was obtained from femtosecond 2PPE transients.
The ability to watch atoms move in real time - to directly observe transition states - has been referred to as "making the molecular movie". Femtosecond electron diffraction is ideally suited for this purpose since it records the atomic structure of the sample with sub-Angstrom spatial resolution and femtosecond temporal resolution. Many-body simulations of ultrashort electron pulse propagation dynamics allowed the development of sources for femtosecond electron pulses with sufficient number density to perform near single shot structure determinations, a requirement for studies of irreversible processes. We have obtained atomic level views of melting of thin films of aluminum and gold under strongly driven conditions. The results are consistent with a thermally driven phase transition and the observed time scales reflect the different electron-phonon coupling constants for these metals.
Recent technical advances in electron gun design have further improved the temporal resolution of femtosecond electron diffraction. New electron pulse characterization techniques use direct laser-electron interaction and electron-electron interaction to determine the temporal overlap of the pump and probe pulses as well as the time resolution of the system. These advances have made femtosecond electron diffraction capable of observing transition states in molecular systems. The camera for "making the molecular movie" is now in hand.
Hot electron injection from the aromatic chromophore perylene into TiO2 was measured with transient absorption signals for different rigid anchor-cum-spacer groups revealing 15 fs as the shortest and 4 ps as the longest injection time. The energetic position of the donor orbital of the chromophore with respect to the conduction band edge was determined at about 0.8 eV employing ultraviolet photoelectron spectroscopy (UPS)and simple absorption
spectroscopy.It is not clear in the case of rutile or anatase TiO2 whether unoccupied surface states are involved
in the electron injection process as acceptor states. Since the surface reconstruction of TiO2 is difficult to control
electron scattering between a well-defined surface state and isoenergetic unoccupied bulk states was studied with InP(100). Electron scattering was time-resolved employing two-color two-photon-photoemission (2PPE). Scattering from isoenergetic bulk states to the empty C1 surface state was found to occur with a 35 fs time
constant, and the reverse process showed a time constant in the range of 200 fs. The latter was controlled by energy relaxation in bulk states, i.e. via the emission of longitudinal optical phonons in InP. In general, the injection of a hot electron from a molecular donor into electronic states of a semiconductor as to be distinguished
from consecutive electron scattering processes between surface states and bulk states. Distinguishing between the different processes may become difficult, however,if the electronic interaction becomes large for a small chromophore directly attached to the semiconductor.