Winfried Storck

Ultrafast dynamics of light-induced electron injection from a molecular donor into the wide conduction band of a semiconductor as acceptor

Abstract Coherent vibrational wave packet motion is shown here to survive the heterogeneous light-induced electron transfer reaction and to continue even in the product state M + . Thus, the reaction does not start from a thermally equilibrated occupation of the vibrational modes in the donor molecule M*, in contrast to the hitherto standard model applied in photoelectrochemistry. Whereas the rise of the transient absorption signals of the product states are probing the forward electron injection reaction their decay does not simply probe the recombination reaction to the ground state dye and is complicated due to several additional effects. It is postulated here that the injection of hot electrons is in general faster than the capture of hot electrons by an adsorbed molecule.

Vibrational Davydov splitting in 7-(2-anthryl)-1-heptanoic acid Langmuir-Blodgett films☆

The IR absorption spectrum of Langmuir-Blodgett multilayers of 7-(2-anthryl)-1-heptanoic acid (I) deposited onto CaF2 substrates was recorded between 1800 cm−1 and 1400 cm−1 using normally incident polarized light. For multilayers deposited from a pH 4.9 subphase the Cue5f8O stretch at approximately 1700 cm−1 shows a polarization similar to that observed for the monoclinic forms of single-crystal fatty acids. This band is split by 14 cm−1 into two components that are polarized parallel and perpendicular to the transfer direction. This polarization effect is explained by vibrational Davydov splitting and indicates local order, or island formation, with a preferred, macroscopic orientation induced by the transfer process.

Femtosecond electron injection from optically populated donor states into the conduction band of semiconductors

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.

Ultrafast electron transfer via a bridge-extended donor orbital

Electron transfer from the excited aromatic donor perylene to TiO2 occurred with 10 fs time constant via the conjugated -CH=CH- bridge unit compared to 57 fs in the presence of the saturated -CH2-CH2- bridge unit.

Measurements and Theoretical Modelling of Ultrafast Heterogeneous Spacer-Controlled Electron Transfer

Femtosecond interfacial electron transfer from a chemically modified perylene molecule into empty electronic states of a sponge-like TiO2 semiconductor was measured as transient absorption signal. A CH2 -spacer group between anchor group and chromophore ensured weak electronic coupling with the electronic and optical properties of the chromophore retained.

Ultrafast molecule to semiconductor electron transfer via different anchor groups in ultra-high vacuum

Electron transfer in the wide band limit occurs from the donor perylene to TiO2 via the carboxyl group in 13 fs and via the phosphonate group in 28 fs.