Lifetime distribution of multiexponential recovery processes

Ultrafast dynamics of transient photobleaching and recovery of cadmium sulphide (CdS) nanoparticles of different size ranging from less than 3.5 nm to greater than 5.0 nm, dispersed in Nafion films, in self-assembled films, and in aqueous solutions were studied upon excitation at 400 nm with a femtosecond laser. All samples gave transient bleaching in less than 1 ps and a multi-exponential recovery in the wavelength region above about 410 nm. The peak wavelength of the transient bleaching and its time dependence together with the recovery behavior were affected by the particle size, surface treatment and the supporting medium. The peak of the transient bleaching spectrum of larger CdS nanoparticles showed a red-shift during the recovery process, while for the smaller particles no such change was observed. CdS nanoparticles in self-assembled films showed an extremely long-lived bleaching recovery process. Fluorescence-activated CdS nanoparticles in aqueous solutions, which showed very strong and sharp fluorescence near the exciton absorption, showed very fast recovery. These results were explained by the decay of excitons and the recombination of surface trapped electron-hole pairs with different energies depending on the particle size.

The molecular motor, Myo1c, a member of the myosin family, is widely expressed in vertebrate tissues. Its presence at strategic places in the stereocilia of the hair cells in the inner ear and studies using transgenic mice expressing a mutant Myo1c that can be selectively inhibited implicate it as the mediator of slow adaptation of mechanoelectrical transduction, which is required for balance. Here, we have studied the structural, mechanical and biochemical properties of Myo1c to gain an insight into how this molecular motor works. Our results support a model in which Myo1c possesses a strain-sensing ADP-release mechanism, which allows it to adapt to mechanical load.

We investigate the fluorescence intensity of rhodamine 6G in poly(vinyl alcohol) as a function of excitation intensity, illumination time, the presence of oxygen, and temperature. The variations in emissivity (or fluorescence brightness) are attributed to a dark state, which shows populating kinetics resembling those of the triplet state, but a much longer lifetime. We simulate the observed kinetics by a four-level model, in which a long-lived dark state is formed through the triplet as an intermediate state. The weak temperature dependence of the lifetime of the dark state points to electron tunneling as the main recovery process. This intermolecular mechanism also explains the observed broad distribution of lifetimes. An electron-spin-resonance experiment confirms the assignment of the dark state to a radical. For the first time, photoinduced charge transfer is identified as a source of blinking in single-molecule measurements.