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Laser-induced fluorescence (LIF) is the optical emission from molecules that have been excited to higher energy levels by absorption of electromagnetic radiation. The main advantage of fluorescence detection compared to absorption measurements is the greater sensitivity achievable because the fluorescence signal has a very low background. For molecules that can be resonant excitated, LIF provides selective excitation of the analyte to avoid interferences. LIF is useful to study the electronic structure of molecules and to make quantitative measurements of analyte concentrations. Analytical applications include monitoring gas-phase concentrations in the atmosphere, flames, and plasmas; and remote sensing using light detection and ranging (LIDAR). Because of the differences in the nature of the energy-level structure between atoms and molecules, the discussion on atomic fluorescence spectroscopy is in a separate document.
The excitation source for molecular LIF is typically a tunable dye laser in the visible spectral region. Studies in the near-ultraviolet and near-infrared are becoming more common as near-infrared lasers and frequency-doubling methods improve. High-resolution studies require cooling of the molecules to remove spectral congestion and to reduce the Doppler width of the transitions. A separate document on high-resolution spectroscopy describes cooling methods such as molecular beams, free-jet expansions, and cryogenic glass or crystalline matrices.
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