Time Resolved Single Molecule Spectroscopy

Abstract

A new method based on the calculation of autocorrelation functions for spectra measured at a high acquisition rate is developed to study spectral dynamics of single molecules. The technique allows for spectroscopy with time resolutions down to the luminescence lifetime. The method is used to study spectral diffusion in two-photon excitation spectra of diphenyloctatetraene molecules doped in an n-tetradecane crystal matrix. The diffusion is light induced, and is absent in one-photon excitation spectra. It has a "steplike" time behavior, different from gradual diffusion observed in glasses.[ S0031-9007(98) Time resolution in SM spectroscopy is also desirable, for example, to study SM interactions with an environment which cause time dependent resonance frequency changes or so-called spectral diffusion (SD). According to the two-level systems (TLSs) model, which was suggested for amorphous solids [13,14] to explain their acoustic and thermodynamic properties, expanded later to include optical phenomena [15], and confirmed in many experiments [16,17], the environment is imitated by a set of TLSs with flip rates distributed from microhertz to gigahertz. A study of fast dynamics requires high time resolution, but it has been considered impossible to implement high time resolution spectroscopy for SMs so far, because the number of detected photons emitted by a SM is small and to first approximation fluctuates according to a Poisson distribution. Even when the exciting laser power saturates the transition and the emission rate is at maximum, a SM signal rarely reaches 10 5 counts͞s (a strong emitter is terrylene [18]). To measure the SM linewidth, the laser intensity should be well below the saturation value. So, only 10 4 counts͞s are detected. To determine the line shape, the recording time should be on the order of 10 ms in the best case. For molecules with 100 times lower emission rate [pentacene, diphenyloctatetraene (DPOT), and many others], this time can be as long as a few seconds. A conventional photon correlation technique [19,20] allows one to gain insight into fast SM dynamics but does not provide spectral information.In this paper, a new approach to SM spectroscopy is reported which pushes the time resolution far below one second, even for molecules with poor emission rates. This technique, which we call intensity-time-frequency correlation (ITFC) SM spectroscopy, can yield microsecond or even better time resolution with an intrinsic theoretical limit at the luminescence lifetime. The ITFC technique is used for studying dynamics in two-photon excitation (TPE) spectra of DPOT molecules in an n-tetradecane matrix [21], where a significant difference between the linewidths in one-photon excitation (OPE) and TPE spectra was observed and tentatively explained by SD induced by the powerful laser illumination required for TPE [22,23]. This explanation agrees with the significant line broadening observed for OPE data in the presence of infrared illumination [24].ITFC spectroscopy works as follows. Instead of record...

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