Single molecule data, interpretation

Single-molecule detection in confocal spectroscopy is characterized by an excellent signal-to-noise ratio, but the detection efficiency is in general very low because the excitation volume is very small with respect to the whole sample volume, and most molecules do not pass through the excitation volume. Moreover, the same molecule may re-enter this volume several times, which complicates data interpretation. Better detection efficiencies can be obtained by using microcapillaries and micro structures to force the molecules to enter the excitation volume. A nice example of the application of single-molecule detection with confocal microscopy is... 

ISS data have been recorded in many pure and mixed molecular liquids [34,49, 75, 83, 83-85], In most cases, the data are not described precisely by Eq. (27). Rather, an additional decay component appears at intermediate times (decay times 500 fs). This has been interpreted [49, 84] in terms of higher order polarizability contributions to C (t) which represent translational motions, an interpretation supported by observations in CCI4 (whose single-molecule polarizability anisotropy vanishes by symmetry). This interpretation is not consistent with several molecular dynamics simulations of CSj [71, 86]. An alternative analysis has been presented [82] that incorporates theoretical results showing that even the single-molecule orientational correlation function C (t) should in fact show decay on the 0.5-ps time scale of cage fluctuations [87, 88]. 

Here we report on the second approach. To conduct an experiment as the one sketched in Figure 12.12a would be the dream of every experimentalist in the field. Indeed, many mechanical break junction experiments have been reported which come quite close to this ideal [93-95]. However, a major unsolved problem of all these experiments is that there are currently no robust methods to image and determine the precise adsorption site and conformation of the molecule in the break junction [96]. But at the same time it is known that the connection between molecule and electrode greatly affects the current-voltage characteristics, sometimes even more than the properties of the molecule itself [97]. This poses a serious problem for the interpretation of single molecule transport data. 

It is important to emphasize that this comparison of molecular orbital calculations and experimental spectra is qualitative, as the limitation to a single molecule and the intrinsic precision of the computational approach used do not allow a quantitative comparison. Nevertheless, the characteristics of the orbitals, such as the electron density distribution, are very similar for different density functional and semiempirical calculations, indicating that the calculations are useful for a qualitative interpretation of the spectra. Detailed electronic structure calculations have been recently applied to the understanding of intermolecular magnetic interaction pathways in nitroxide radicals [43], underlining the importance of comparisons between computational data and spectroscopic results which provide a quantitative test for the theoretical models. 

In a typical experiment, the frequency of the laser is slowly increased in steps while the electric field is scanned at a fast rate ( 45scans/s). Fig. 12 reflects the way the data were recorded the x-axis represents the laser frequency whereas the y-axis represents the applied electric field strength. The x-axis can also be interpreted as a time axis of the experiment running from right to left. It took 2.1 s to complete the 100 electric field scans for one frequency position. One can see several single-molecule traces shifting linearly as a function of the applied electric field. 

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