Capabilities to structure X-ray light are developing fast and now allow to generate and characterize light carrying orbital angular momentum (OAM). The use of X-rays to design novel spectroscopic signals sensitive to molecular chirality will be presented. Their differences and similarities with optical techniques will be briefly outlined and we then introduce X-ray helical dichroism, the dichroic measurement of absorption spectra with positive and negative OAM. An experimental implementation of helical dichroism developed at the Swiss Light Source (SLS) synchrotron using spiral Fresnel zoneplates on iron chiral complexes will be discussed.
We present our studies of femtosecond photoluminescence of colloidal solutions of CdSe and PbSe nanocrystals, using
polychromatic fluorescence upconversion. Ultrafast relaxation processes are observed in both cases upon excitation at
400 nm and 800 nm respectively. Under moderately high excitation densities we studied the formation and dynamics of
biexitons and triexcitons in CdSe nanocrystals. Contrary to earlier reports, all results could be understood without
invoking the presence of charged particles. The dynamics of single excitons in PbSe nanocrystals is found to be similar
to the case of CdSe, despite the high confinement in the former., The early-time spectra are characterized by emission
from several low lying excited states. The kinetics point to a fast sequential cascade process between excited states,
governed by energy gaps, and indicate the presence of additional dark states. The first direct measurement of the lowest
excited states Huang-Rhys factors excludes that strong electron-phonon coupling mediates the intraband relaxation.
Time-resolved x-ray absorption fine structure (XAFS) spectroscopy with picosecond temporal resolution is a
new method to observe electronic and geometric structures of short-lived reaction intermediates. It combines an intense
femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light
Source (SLS). We present key experiments on charge transfer reactions as well as spin-crossover processes in
coordination chemistry compounds next to solvation dynamics studies of photogenerated atomic radicals.
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