Time-resolved fluorescence is a direct measure for excited states lifetimes, decay channels and corresponding
rates. Hitherto, investigations on systems exhibiting fluorescence lifetimes below approximately 10 ps have
been restricted to ensemble measurement. Ensemble measurements bear the disadvantage of averaging sample
inhomogeneities and complex distributions. However, the latter problem can be circumvented by single-molecule
experiments, without the restriction to special, typically simple systems that can be prepare with very high
homogeneity. Time-resolved single-molecule microscopy is especially powerful as it allows one to probe the
spatial, temporal and spectral inhomogeneities. At present, its most common implementation, the scanning
confocal time correlated single photon counting (TCSPC), is limited to a time resolution of 20 ps. In the
wide-field epifluorescence microscopy temporal resolution is achieved by the use of intensified CCD cameras, the
fastest of which reach resolution of 80 ps. Here we present a Kerr-gated microscope setup capable of collecting
diffraction limited 2D fluorescence images with approximately 100 fs time resolution. The concept is based on the
insertion of an optical Kerr gate into a standard wide-field microscope. In addition to the considerably improved
temporal resolution, the wide-field design will allow simultaneous tracking of several molecules or nanoparticles
and ultrafast fluorescence lifetime imaging of doped and heterogeneous surfaces. Preliminary measurements to
demonstrate the performance of the setup are presented.
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.
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.