Absorption of UV radiation by a molecule excites it from a vibrational level in the electronic ground state to one of the many vibrational levels in the electronic excited state. This excited state is usually the first excited singlet state. A molecule in a high vibrational level of the excited state will quickly fall to the lowest vibrational level of this state by losing energy to other molecules through collision. The molecule will also partition the excess energy to other possible modes of vibration and rotation. Fluorescence occurs when the molecule returns to the electronic ground state, from the excited singlet state, by emission of a photon
Perkin Elmer Luminescence Spectrometer
The LS55B is a computer controlled ratioing luminescence spectrometer with the capability of measuring fluorescence or phosphorescence, as well as a range of other processes including electro-, chemi- and bio-luminescence. Excitation is provided by a pulsed Xenon discharge lamp, of pulse width at half peak height of <10 micro seconds and pulse power 20kW. The source is monochromated using a Monk- Gillieson type monochrometer and can be scanned over the range 200-800nm.
The luminescence is passed through a similar monochromator, which can be scanned over the range 200-900nm. Holographic gratings are incorporated on both monochromators to reduce stray light. Synchronous scanning is available with constant wavelength or frequency difference. Excitation spectra are automatically corrected and sensitivity is specified as a signal to noise ratio of 500:1 rms using the Raman band of water with the excitation at 350nm and 10nm excitation and emmission bandpass. The excitation slits (2.5 - 15nm) and the emmission slits (2.5 - 20nm) can be varied in and selected in 0.1nm increments. In the phosphorescence mode, delay and gate times can be varied with a minimum total period of 13.0msecs. Excitation and emmission polarizers consisting of two filter wheels each with horizontal and vertical polarizer elements are also included.