RESUMEN
We studied the dynamics in ultrathin subsurface layers of an amorphous polymer by the spectra of single fluorescent molecules embedded into the layer by vapor deposition and subsequent controlled diffusion to the desired depth in ≈0.5 nm steps. The spectral trails of single molecules were recorded at 4.5 K as a function of diffusion depth. In depths shallower than 20 nm, the spectral dynamics deviate from those deep in the bulk. Less than 5 nm deep, the linewidths increase rapidly, whereas the number of detected molecules decreases. No zero-phonon lines were observed closer than 0.5 nm to the polymer surface. Possible physical reasons of the observed phenomena are discussed.
RESUMEN
A single donor-acceptor (D-A) pair with a fluctuating FRET rate F is studied theoretically in the frame of a "two-state model" in which the FRET rate F fluctuates taking two values, F1 and F2, with average rates B for the forward and b for the backward transitions. Theoretical expressions are derived for the autocorrelation function g((2))(DA)(τ) and for the Mandel parameter Q(D,A)(τ) allowing for background emission. Fluctuating intensities ID,A(t) and FRET efficiency E(t) = IA(t)/[IA(t) + ID(t)] are calculated with the help of the Monte Carlo technique. The probability w(D,A)(N)(T) of finding N photons in a time interval T, and the distribution of the FRET efficiency P(E) are found by statistical treating of the fluctuating intensities ID,A(t). The shape of the distribution w(D,A)(N)(T) enables one to find the values of the parameters: F1, F2, and b/B. The influence of the background light on g((2))(DA)(τ), Q(D,A)(τ) and w(D,A)(N)(T) is studied. It is shown how the background light influences the ratio b/B found from the analysis of w(D,A)(N)(T) and P(E).