After completing this unit the student will be able to: Describe the difference between a singlet and triplet state. Draw an energy level diagram and identify the transitions that correspond to absorption, fluorescence, internal conversion, radiationless decay, intersystem crossing and phosphorescence. Explain why phosphorescence emission is weak in most substances. Draw a diagram that shows the layout of the components of a fluorescence spectrophotometer. Describe the difference between a fluorescence excitation and emission spectrum. Draw representative examples of fluorescence excitation and emission spectra. Describe a procedure for measuring phosphorescence free of any interference from fluorescence. Justify why fluorescence measurements are often more sensitive than absorption measurements. Describe the meaning and consequences of self-absorption. Identify variables including the effect of pH that can influence the intensity of fluorescence. Identify the features that occur in organic molecules that are likely to have high fluorescent quantum yields. Compare two molecules and determine which one will undergo more collisional deactivation.