Robert Neuhauser

Photoluminescence from Single Semiconductor Nanostructures

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 μeV 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.

Three-dimensional orientation measurements of symmetric single chromophores using polarization microscopy

A complete understanding of any complex molecular system generally requires a knowledge of the three-dimensional (3D) orientation of its components relative both to each other, and to directional perturbations such as interfaces and electromagnetic fields. Far-field polarization microscopy is a convenient and widespread technique for detecting and measuring the orientation of single chromophores. But because the polarized electromagnetic field that is used to probe the system lacks a significant longitudinal component, it was thought that, in general, only 2D orientation information could be obtained. Here we demonstrate that far-field polarization microscopy can yield the 3D orientation of certain highly symmetric single chromophores (CdSe nanocrystal quantum dots in the present case). The key requirement is that the chromophores must have a degenerate transition dipole oriented isotropically in two dimensions, which gives rise to a perpendicular ‘dark axis’ that does not couple to the light field. By measuring the fluorescence intensity from the dipole as a function of polarization angle, it is possible to calculate both the tilt angle between the dark axis and the sample plane, as well as the in-plane orientation, and hence obtain the 3D orientation of the chromophore

High n Rydberg spectroscopy of benzene: Dynamics, ionization energy and rotational constants of the cation

In a high resolution uv-uv double resonance experiment with two Fourier-transform limited nanosecond laser pulses high Rydberg states up to n>100 were selectively excited and detected by ionization in a pulsed electric field. We were able to identify 64 Rydberg series in benzene C6D6 and 20 series in benzene C6H6 converging to different rotational levels of the ionic molecular core. Their assignment by application of a crosscorrelation analysis yields accurate rotational constants and a precise value for the lowest rotationless ionization energy of 74556.57(5) cm−1 (C6H6) and 74583.51(5) cm−1 (C6H6). All observed series show surprisingly low quantum defects below 0.01 and no strong local perturbations at their crossings points. This points to a weak intramolecular coupling of the series in the absence of an external electric field. In a second series of experiments the influence of external fields on the intensity, the position and dynamics of single n Rydberg peaks is investigated. It is shown that the p...

Resolved high Rydberg spectroscopy of benzene⋅rare gas van der Waals clusters: Enhancement of spin–orbit coupling in the radical cation by an external heavy atom

Individual high n Rydberg states in van der Waals dimers of benzene and noble gas atoms are resolved after double resonance excitation with two Fourier-transform limited narrow band UV laser pulses. For a selected rovibronic intermediate state several Rydberg series appear converging to different rotational states of the cation. Their position is determined by an automated cross correlation (CRIES) of the experimental with a theoretical Rydberg spectrum with an accuracy of 0.02 cm−1. Analysis of the resolved rotational states of the cationic clusters in their vibrational ground state yields precise information on the ionization energies, the structure and average van der Waals distances. Rotational analysis provides clear spectroscopic evidence for an enhancement of spin–orbit coupling between the orbital angular momentum and the spin of the remaining unpaired electron in the ionic benzene by the external heavy atoms Ar and Kr. The resulting effect of the spin–orbit coupling on the rotational energy level...

ROTATIONAL ANALYSIS OF THE GROUND STATE OF THE BENZENE - NE VAN DER WAALS CLUSTER CATION BY RESOLVED HIGH RYDBERG SPECTROSCOPY

Series of resolved high n (40<n<110) Rydberg states of the benzene·Ne van der Waals cluster are measured by delayed field ionization after double resonance excitation with Fourier-transform limited UV laser pulses. A cross–correlation analysis yields the first rotationally resolved spectrum of an aromatic molecule noble gas cluster ion. Its analysis leads to accurate values for the adiabatic ionization energy (AIE=74497.512(30) cm−1) of the benzene·Ne cluster and the rotational constants of the cluster ion. The average van der Waals bonding length of the Ne atom from the (benzene)+-plane is 3.320(2) A.