Interfacial electron transfer (IET) dynamics have been measured in a series of dye-sensitized metal oxides commonly using in water splitting dye-sensitized photoelectrochemical cells using optical-pump terahertz-probe spectroscopy. The IET dynamics were compared using a model that allows for the relative contributions of injection from the singlet and triplet excited states of the dye to be distinguished. In addition, we present a newly developed method for performing terahertz (THz) measurements under potentiostatic control (i.e., THz spectroelectrochemistry).
Much can be learned about charge and magnetization dynamics at surfaces and in nanometer thickness films through terahertz emission spectroscopy (TES). In some respects, TES is the difference-frequency analog of second harmonic generation (SHG). As such, interface-specific properties contribute to the generation of a THz pulse upon ultrafast optical excitation. In addition, there can be bulk contributions as well, as is also true in SHG. The dependence of THz pulse emission on surface orientation has been used to study carrier dynamics, both real and virtual, in GaAs(111). We find that the dependence on the angle of linearly polarized excitation is well described by known theory. Magnetization dynamics in polycrystalline nickel films ranging in thickness from 5 nm to 60 nm have also been characterized with TES. Distinct bulk and surface contributions each play a role, and their origins are discussed.
The transient photoconductivity of dye-sensitized nanocrystalline colloidal TiO<sub>2</sub> has been measured time-resolved THz spectroscopy (TRTS), a non-contact electrical probe with sub-picosecond temporal resolution. The photoconductivity deviates strongly from Drude behavior and is explained by disorder-induced carrier localization and/or backscattering of the photogenerated carriers. In addition, the carriers are found to thermally equilibrate with the lattice in roughly 300 femtoseconds. The size-dependent photoconductivity of CdSe nanoparticles ranging from 2.54 nm up to >25 nm has also been measured using TRTS. The measured change in the frequency-dependent optical density and change in phase of the transmitted THz pulse fall into three distinct groupings as a function of size and can be classified for diameters smaller than the Bohr exciton radius, diameters greater than the Bohr exciton radius but smaller than the bulk mean free path, and diameters greater than the bulk mean free path. The underlying cause of the grouping is a size-dependent mobility (or carrier scattering rate).
We present a detailed description of ultrafast electron diffraction and its applications to study photoinduced molecular dynamics at single crystal surfaces. Experimental investigations for a new design of an ultrashort pulsed laser activated electron gun ((tau) < 5 ps) for time- resolved surface analysis are described. In addition, a novel electron detection and image analysis system, as it applies specifically to time-resolved reflection high-energy electron diffraction in the multiple-shot operation, are reviewed. The total experimental temporal resolution is discussed in terms of the electron pulse width and the time difference between an electron scattered at the front edge of the sample to an electron scattered at the trailing edge of the sample.
Femtosecond time resolved two photon photoemission and above- threshold photoemission (ATP) have been used to characterize the dynamics of photoexcited electrons at single crystal Cu surfaces. The two photon photoemission studies measure nonradiative relaxation pathways of electrons near the surface, and the ATP studies demonstrate that photoemission occurs even when using light that is far below the work function. These studies provide important information regarding the extent and duration of the interaction of photoexcited electrons with surface adsorbates.
Conference Committee Involvement (1)
Physical Chemistry of Interfaces and Nanomaterials IV
2 August 2005 | San Diego, California, United States