Dr. ERNEST BEHRINGER
FLUORESCENCE SPECTROSCOPY
Background
Ever wonder why highlighting pens are advertised as "fluorescent"? It's because they can absorb ultraviolet light and emit visible light. This effect has been exploited to make dorm room decorations using the cores of worn out highlighters, clear glass bottles, and a black light. Fluorescent materials have many uses beyond dorm room decorations and the highlighter, and in particular have become extremely important in the study of biochemistry and developmental biology. For example, one may observe cellular processes by incorporating a fluorescent molecule (for example, green fluorescent protein, or GFP for short) into DNA molecules in the cell to determine how the DNA is used in the cell. Consequently, characterizing and understanding fluorescent materials can be very useful.
Experiments
Fluorescence spectroscopy experiments are carried out in the Modern Optics Lab. The organization of the experiment is as is shown in the figure below [insert jpg]. Light from a white light source is wavelength-selected using a monochromator. The nearly monochromatic light (i.e., light of a nearly pure spectral color) is directed towards a liquid sample that is housed in a small sample chamber. Two detectors are mounted on the chamber: one to monitor the amount of light that is transmitted straight through the sample, and one that detects the fluorescent light (which is emitted randomly into any angle). The measurement can be repeated with an empty sample holder to obtain the effect of the sample by subtraction. It is possible to then determine the transmittance of the sample, i.e., the ratio of light transmitted by the sample to the light transmitted by no sample (i.e., by air). On the other hand, the emitted fluorescence is collected by a lens and transported optically to a grating spectrometer. Together with a sensitive detector, the spectrometer enables the amount of fluorescence to be measured as a function of its wavelength. An example of a fluorescence spectrum is shown below. This was the first fairly optimized spectrum obtained by an undergraduate student who was using a helium-neon laser to excite a gallium arsenide sample. [insert jpg]
Interested?
If you have approximately 8-10 hours a week to work in the laboratory, you might consider doing an independent study project in which you measure the fluorescence spectra of various materials. It will take a significant portion of a semester to learn the necessary laboratory techniques and experimental procedures. On the other hand, you can expect to obtain significant insight into what it takes to succeed.
