Schematic representation of some of the energy levels in a molecule. The solid lines indicate the potential of the electrons in the ground and first excited states , and the dotted levels represent the vibrational modes in the molecule. The arrows indicate the stimulated excitation (deexcitation) (I) of the molecule to (from) the excited state by the laser and the spontaneous decay (II) back to the ground state.
Schematic description of the fluorescent response of the molecule upon excitation with two saturating pulses with variable delay times. For zero delay the fluorescence probability is 0.5. For delay times longer than the vibrational redistribution time the fluorescence probability increases to 0.75.
The simulated fluorescence probability of a single molecule with a redistribution time of when varying the delay between two femtosecond pulses. Different conditions have been simulated. The (엯) indicates a pulse duration of with a peak intensity such that it exactly saturates the molecule. For pulses of the intensity was varied. The case where the intensity is such that the molecule is exactly saturated is indicated by (▵). The (◻) and (⋆) show the results when half and double the saturation power is used, respectively. The response of the molecule under illumination of a single pulse is depicted on the left side of the graph. The solid lines show fits through the points using function Eq. (2), yielding in all cases the correct redistribution time of .
SM2P experimental setup used in the experiments. Short laser pulses are split into two different branches using a beam splitter (BS) and are recombined on the same beam splitter after reflecting the two beams by two corner mirrors (CMs). One of the mirrors can be moved, varying the delay between the pulses. The light is made circularly polarized before entering the confocal microscope. After reflection from the dichroic beam splitter (DBS) the pulse train is focused with a high NA objective (Obj.) onto the sample. The fluorescence light from the single molecules (SMs) embedded in the sample is collected with the same objective, passes through the dichroic beam splitter and a long pass (LP) filter. The light is then split in two orthogonal polarizations by a polarizing beam splitter (PBS) and finally focused on two avalanche photodiodes (APDs).
Bulk absorption and emission spectra of DiD. The inset shows the molecular structure of the molecule. The dipole direction is indicated by the arrow.
Three delay traces obtained on single DiD molecules in a PMMA matrix. The fluorescence signal is plotted vs the delay time between the pulses. A clear reduction in the fluorescence at can be observed in all three traces. Differences in the depth and width of the dips can clearly be observed. Taking into account the finite pulse length we recover redistribution times of , , and for traces (a), (b), and (c), respectively.
(a) Histogram of the different redistribution times found for DiD in PMMA at four different excitation wavelengths. (b) Histogram of the times found for perylene in PMMA at two different excitation wavelengths.
The peak value and width from a Lorentzian fit of the distributions shown in Figs. 7(a) and 7(b) vs the energy difference with respect to the lowest excited state for DiD (circles) and perylene (squares) in PMMA (solid symbols) and in zeonex (open symbols).
SM2P traces on Cy3.5 in air (a) and PVA (b) and Atto590 in air (c) and in PVA (d). The solid lines show the fits to the data; the resulting redistribution times are noted for each trace.
Overview of the experimental conditions used for the SM2P experiments on different dyes and the number of photon counts before photodissociation
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