An instrument used to measure the intensity and the @W06659@ distribution of the light emitted as @F02453@ from a molecule excited at a specific @W06659@ or wavelengths within the absorption band of a particular compound. Characteristic @F02453@ bands may be used to identify specific pollutants such as the polynuclear aromatic @H02889@. Excitation spectra of impurities can be observed by scanning the @W06659@ of the excitation light which is incident on the sample over a range of wavelengths and observing the relative intensity of the @F02453@ emitted at a given @W06659@. These spectra are also characteristic of the impurity.