Laser-Induced Fluorescence (LIF)
Introduction
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.
Instrumentation
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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