Long-lived Dark States Research Articles

Microheterogeneity in Dye-Doped Silicate and Polymer Films

Single molecule confocal microspectroscopic methods are used to characterize individual molecular-scale environments in silicate thin films for the first time. Rhodamine dyes doped into the materials at nanomolar levels are used as probes of the physicochemical environment in which each molecule is entrapped. The results are compared to those obtained from dye-doped organic polymer films. Static fluorescence spectra and time-dependent fluorescence signals recorded for a large number of single molecules show the silicate materials to be highly inhomogeneous in comparison to the polymer films. Histograms of the fluorescence maxima for encapsulated Rhodamine B show a full width at half-maximum of 13.3 nm for the silicate host framework and 6.7 nm for the polymer film. The integrated fluorescence signal from single molecules, recorded with millisecond time resolution, under continuous illumination conditions, is also sensitive to the local environment. The time-dependent signal traces show dramatic intensity fluctuations for some molecules and none for others. The fluctuations occur most frequently for the silicate-entrapped dye. In the present work, the signal fluctuations are proposed to result from time-dependent variations in the molecular environment, which in turn cause changes in the excitation and emission characteristics of the molecules. The photophysical phenomena behind these fluctuations include quantum yield variations, intersystem crossing to a long-lived dark state, and, to a lesser extent, spectral diffusion. All such effects are highly dependent upon the physicochemical properties of the molecular-scale environment, as shown by comparison to results obtained for the polymer samples. The results are used as further evidence for the heterogeneous nature of entrapment in silicate host structures. The dynamic nature of many of the molecular-scale environments in these materials is demonstrated as well.

High-velocity dark states in velocity-selective coherent population trapping.

We have observed long-lived dark states in velocity-selective coherent population trapping (VSCPT) in a one-dimensional lin-angle-lin configuration with metastable He atoms using the $J=1\ensuremath{\Rightarrow}1$ transition at $\ensuremath{\lambda}=1.083$ \ensuremath{\mu}m. These states are characterized by two peaks in the atomic momentum distribution at $\ifmmode\pm\else\textpm\fi{}Q\ensuremath{\hbar}k(Q=\mathrm{positive}\mathrm{integer})$, where $k$ is the magnitude of the optical wave vector. VSCPT states previously observed are characterized by $Q=1$, but we have observed long-lived states with $Q=2$. Optical Bloch equation calculations show that these high-velocity dark states appear at higher $Q$ for greater light intensity.

Time-resolved molecular chemiluminescence from reactions of Ca(3 P,1 D) atoms with N2O

Chemiluminescence from electronically excited CaO generated by reaction of metastable Ca(4s4p(3 P J )) and Ca(4s3d(1 D 2)) atoms with N2O is investigated in the time-resolved mode under bulk conditions. By monitoring molecular emission from some 41 transitions, the time dependence of chemiluminescence from excited CaO states is compared with that for decay of emission from the atomic precursor states from which they are derived, enabling formation of specific product states to be ascribed to either direct generation from chemical reaction between Ca(3 P) and Ca(1 D) atoms with N2O or to an energy transfer mechanism involving the metastable atoms and long-lived dark states of CaO. The different routes leading to specific molecular states are found to be in accord with simple energetic considerations and correlation rules governing the conservation of spin and spatial symmetry. Measurements of vibrational temperatures in one particular product state, CaO(A′1II), indicate some degree of vibrational excitation accompanying these processes. Relative populations in all emitting states of CaO are found to be in agreement with the results of molecular beam experiments. In addition to the study of electronic chemiluminescence from CaO, effective secondorder quenching rate constants are determined for collisional removal of Ca(3 P) and Ca(1 D) atoms via all available physical and chemical pathways. Measurements over a wide range of temperature lead to estimates of activation energies for quenching of both Ca states by N2O.