Dynamics within the Exciton Fine Structure of Colloidal CdSe Quantum Dots

Evidence for an interaction between the quantum dot exciton fine structure states F = ±1 is obtained by measuring the dynamics of transitions among those states, exciton spin relaxation or flipping. An ultrafast transient grating experiment based on a crossed-linear polarization grating is reported. By using the quantum dot selection rules for absorption of circularly polarized light, it is demonstrated that it is possible to detect transitions between nominally degenerate fine structure states, even in a rotationally isotropic system. The results for colloidal CdSe quantum dots reveal a strong size dependence for the exciton spin relaxation rate from one bright exciton state (F = ±1) to the other in CdSe colloidal quantum dots at 293 K, on a time scale ranging from femtoseconds to picoseconds, depending on the quantum dot size. The results are consistent with an interaction between those states attributed to a long-range contribution to the electron−hole exchange interaction

Measurement of Electron−Electron Interactions and Correlations Using Two-Dimensional Electronic Double-Quantum Coherence Spectroscopy

A two-dimensional (2D) optical coherent spectroscopy that correlates the double excited electronic states to constituent single excited states is described. The technique, termed two-dimensional double-quantum coherence spectroscopy (2D-DQCS), makes use of multiple, time-ordered ultrashort coherent optical pulses to create double and single quantum coherences over the time intervals between the pulses. The resulting 2D electronic spectra map out the energy correlation between the first excited state and two-photon-allowed double-quantum states. Measurements of organic dye molecules show that the near-resonant energy offset for adding a second electronic excitation to the system relative to the first excitation is on the order of tens of millielectronvolts. Simulations of DQC spectra show that vibronic transitions add rich features to the 2D spectra. The results of quantum chemical calculations on model systems provide insight into the many-body origin of the energy shift measured in the experiment. These results demonstrate the potential of 2D-DQCS for elucidating quantitative information about electron−electron interactions, many-electron wave functions, and electron correlation in electronic excited states and excitons

CdSe Nanoparticle Elasticity and Surface Energy

The acoustic phonon modes of colloidal CdSe nanoparticles in solution (293 K) are passively measured by a third order ultrafast heterodyne cross-polarized transient grating measurement. Using the observed size-dependence of the acoustic phonon frequency, the elastic properties of the nanoparticles are determined. The size-dependence of the elastic modulus is then used to ascertain information about the relative surface energies of the nanocrystals and suggests the extent and depth of surface reconstruction

Nanocrystal Shape and the Mechanism of Exciton Spin Relaxation

The rate of exciton spin relaxation (flips) between the bright exciton states (F = ±1) of CdSe nanocrystals is reported as a function of shape, for dots and nanorods. The spin relaxation is measured using an ultrafast transient grating method with a crossed linearly polarization sequence. It is found that the spin relaxation rate depends on the radius, not length, of the nanocrystals. That observation is explained by deriving an expression for the electronic coupling matrix element that mixes the bright exciton states

Insights into the Structural Changes Occurring upon Photoconversion in the Orange Carotenoid Protein from Broadband Two-Dimensional Electronic Spectroscopy

Carotenoids play an essential role in photo­protection, interacting with other pigments to safely dissipate excess absorbed energy as heat. In cyano­bacteria, the short time scale photo­protective mechanisms involve the photo­active orange carotenoid protein (OCP), which binds a single carbonyl carotenoid. Blue-green light induces the photoswitching of OCP from its ground state form (OCPO) to a metastable photoproduct (OCPR). OCPR can bind to the phyco­bilisome antenna and induce fluorescence quenching. The photoswitching is accompanied by structural and functional changes at the level of the protein and of the bound carotenoid. Here, we use broadband two-dimensional electronic spectroscopy to study the differences in excited state dynamics of the carotenoid in the two forms of OCP. Our results provide insight into the origin of the pronounced vibrational lineshape and oscillatory dynamics observed in linear absorption and 2D electronic spectroscopy of OCPO and the large inhomo­geneous broadening in OCPR, with consequences for the chemical function of the two forms