C. von Borczyskowski

Kinetics of optically detected magnetic resonance of single molecules

Various time‐resolved experiments on optically detected magnetic resonance (ODMR) of single pentacene molecules in p‐terphenyl at 1.8 K are described and discussed with the help of a model based on the optical‐microwave Bloch equations. Intersystem crossing rates for this system are deduced from the ODMR spectra, from the fluorescence correlation function, which varies nonmonotonically with the microwave power, and from the analysis of the fluorescence recovery transients. We also discuss the enhancement of the ODMR effect on using fluorescence photons from single molecules as a time base to trigger the microwaves in fluorescence recovery experiments.

Blinking of single molecules in various environments

The fluorescence intermittency of various dye molecules in different environments was studied by wide-field fluorescence microscopy. The present work focuses on the analysis of long dark periods that are not due to triplet states. It is shown that the distributions of the length of dark periods follow power laws for all systems studied here. Furthermore, blinking kinetics is strongly influenced by the gas atmosphere to which the molecules are exposed. The presence of oxygen is of crucial importance.

ODMR and Proton-ENDOR investigations of the symmetry and geometry of the lowest excited triplet state of quinoxaline and p-dichlorobenzene at high magnetic fields

Abstract ODMR and ODENDOR spectra were obtained for quinoxaline and p-dichlorobenzene in their excited triplet states. These molecules were studied in mixed crystals using heavy atom containing hosts. For protons the complete hyperfine tensors could be evaluated, yielding information about the symmetry and geometry of these molecules in their triplet states. For quinoxaline no deviations from planarity have been observed whereas triplet p-dichlorobenzene turned out to be non-planar. The measured π-spin densities in p-dichlorobenzene confirm the 3B1u symmetry of this molecule in its first excited triplet state.

Competition between energy transfer and charge separation in covalently linked quinone-porphyrin dimers on a picosecond timescale

Abstract Fast electron transfer is observed for tetraphenylporphyrin dimers and monomers substituted with the electron acceptor quinone. The transfer rate for the monomer as compared to the dimer is faster by about a factor of 2, which is due to competition between energy transfer among the porphyrin subunits and charge separation.

Gaussian quadrature approach to the calculation of the optical constants in the vicinity of inhomogeneously broadened absorption lines

A fast numerical procedure to calculate the convolution of Gaussian and Lorentzian functions is introduced as well as its application to the approximation of experimental spectra. It is shown that these spectra can be well approximated with the method, especially in the case when the inhomogeneous and homogeneous linewidth contributions as parameters of a Voigt lineshape are of the same order of magnitude. Particularly, the method is applicable to refractive index modelling in the vicinity of inhomogeneously broadened absorption lines.

A distance regulation scheme for scanning near‐field optical microscopy

In this letter we present a detection scheme for shear force distance regulation in scanning near‐field optical microscopy. Instead of an optical detection of the amplitude of tip vibrations and damping due to shear forces, a piezoelectric pickup has been developed. It is shown that the signal obtained with this pick up is of comparable or even superior sensitivity than the conventional optical scheme and can be easily incorporated into every near‐field microscope. Because of its easy implementation and adjust free performance, the method has the potential to considerably simplify the usage of near‐field microscopes.

Magnetic resonance on single nuclei

Abstract The magnetic resonance signal of individual hydrogen nuclei has been detected. The experiments have been performed on a single pentacene molecule with the aid of optically detected electron nuclear double resonance. Two nuclear magnetic resonance lines of 30 kHz width have been observed as a 3% change in the fluorescence intensity of a single molecule. The results can be reproduced by a spin Hamilton operator describing the interaction of a single electron spin with two hydrogen nuclei.

Simultaneous optical detection of chlorine NQR of p-dichlorobenzene in its non-radiative So and phosphorescent T1 state in a mixed single crystal

Abstract By a combined optical and microwave pumping cycle electron spin alignment is transferred to the diluted chlorine nuclear spins. Because of the conservation of nuclear spin alignment during the electronic excitation and phosphorescence decay, NQR transitions within the excited triplet and electronic ground state of p -dichloro-benzene ( p -DCB) can be detected optically in a dilute mixed single crystal. A detailied analysis of NQR transition frequencies and intensities in the T 1 state has been performed. For the first time the change of the molecular field gradient under electronic excitation could be observed at the chlorine site. The NQR frequency change Δν q (S o , T 1 ) was found to be matrix dependent and varied from 2.52 MHz ( p -dibromobenzene) to 3.75 MHz (xylene).

Electron transfer in self-organized porphyrin-quinone compounds on a picosecond time scale

Abstract Electron and excitation transfer is observed in self-organized porphyrin-quinone tetrades. Self-organization is established via a complexation of the zinc porphyrin component. Energy transfer is faster than 10 ps and is thus competitive with electron transfer which occurs on a 500 ps time scale and is slowed down in the non-polar solvent cyclohexane.

Formation of energy funnels for triplet energy transport in a chemically mixed molecular crystal

Abstract Dichlorobenzene guest molecules form energy traps and additionally induce next-neighbour X-traps in a dibromobenzene host crystal. This shows up as a funnel for triplet energy transport. The width of the funnel is less than four lattice constants. Energy transport within and out of this funnel can be described by a thermally activated process.