Perylene in Nonane

In the system perylene in n-nonane, the main 0-0 absorption band at liquid helium temperatures is located around 443.8 nm with a few weaker sites. To work in this spectral region a cw Ti Sapphire autoscan laser is frequency-doubled by means of a LiI03-crystal [18]. For the following experiments a tiny drop of a 10 to 10 moll solution of perylene in n-nonane is squeezed between two thin glass plates, covered with a conducting sheet, and placed after a 5 pm pinhole. The spacing between the electrodes is 300 pm. A microscope objective immersed in the liquid helium is used to collect the fluorescence [19]. An overview of the results obtained for this system is given in Fig. 12. 

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. 

4 Quadratic Stark effect of single pentacene molecules in /nterphenyl 

Pentacene in p-terphenyl was the first system where single molecules were detected by optical excitation spectroscopy and was also the first system in which the effect of external electric fields on single molecules [20] was studied. 

The pinhole setup previously described was modified for Stark shift experiments. Fig. 13 shows a close-up and indicates the proper dimensions of the components. A 

---