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We find a correlation between the dynamics of fluorescence intermittency and spectral diffusion in the spectroscopy of single CdSe nanocrystal quantum dots (QD). A statistical analysis of the data suggests two populations of blinking events: blinking followed by large spectral diffusion shifts and blinking with small or no spectral shifts. Although unexpected from earlier studies, the correlation between blinking and spectral shifting is consistent with a model of QD ionization as the mechanism for the blinking event, followed by a redistribution of local electric fields that results in spectral shifting.

We review some recent results in the spectroscopy of single CdSe nanocrystal quantum dots. By eliminating the effects of inhomogeneous broadening and ensemble averaging, single nanocrystal spectroscopy has revealed many new and previously unexpected physical phenomena. Among those discussed in this review are ultra-narrow emission lineshapes (~600´narrower than ensemble spectra), a highly polarizable emitting state in the presence of strong local electric fields, line broadening as a result of environmental fluctuations, and shifting of the emission spectra over a wide range of energies (from less than 300 meV to 80 meV). In addition, polarization spectroscopy of single nanocrystals has revealed the presence of a theoretically predicted two-dimensional transition dipole moment oriented in the xy plane of the nanocrystals. As a result, it is, in principle, possible to use polarization spectroscopy to determine the three-dimensional orientation of individual nanocrystals. These and other studies of single quantum dots have provided us with significant insight into the detailed physics and dynamics of this unique and fascinating physical system.

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We review some recent results in the spectroscopy of single CdSe nanocrystal quantum dots. By eliminating the effects of inhomogeneous broadening and ensemble averaging, single nanocrystal spectroscopy has revealed many new and previously unexpected physical phenomena. Among those discussed in this review are ultra-narrow emission lineshapes (~600´narrower than ensemble spectra), a highly polarizable emitting state in the presence of strong local electric fields, line broadening as a result of environmental fluctuations, and shifting of the emission spectra over a wide range of energies (from less than 300 meV to 80 meV). In addition, polarization spectroscopy of single nanocrystals has revealed the presence of a theoretically predicted two-dimensional transition dipole moment oriented in the xy plane of the nanocrystals. As a result, it is, in principle, possible to use polarization spectroscopy to determine the three-dimensional orientation of individual nanocrystals. These and other studies of single quantum dots have provided us with significant insight into the detailed physics and dynamics of this unique and fascinating physical system.

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Rotationally resolved spectra of two bands in the S1 → S0 transition of the p-difluorobenzene molecule and its van der Waals complex with Ar have been measured by mass-selective resonance-enhanced two-photon ionization. The rotational structure of the 000 and the 2710 bands in the monomer as well as in the complex differ from each other. They can be theoretically reproduced assuming a transition moment oriented along the short in-plane axis of the molecule in the case of the 000 transition and the long in-plane axis in the case of the 2710 transition. Since the magnitude of the moments of inertia is changed in the complex by adding an Ar atom, complexation leads to a change of rotational structures of the same band. The analysis of the rotational structure points to Herzberg–Teller coupling by vibronic interaction with the S2 (1B1u) state as the mechanism responsible for the appearance of the 2710 band. The rotational constants determined from a fit of the spectra yield an effective van der Waals distance of 3.55 (2) Å (1 Å = 10−10 m) of the Ar atom from the p-difluorobenzene plane, which decreases by 0.06 Å on electronic excitation to the S1 state.