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High-Resolution Laser Spectroscopy

Introduction

The electronic spectra of atoms and molecules are broadened by various mechanisms. This broadening obscures features in the spectra, and can be especially problematic in trying to make quantitative measurements in mixtures of molecules.


Instrumentation

High-resolution spectroscopy requires the narrow-bandwidth excitation sources that are only achievable with lasers. Studies in the visible spectral region typically use a tunable dye laser and studies in the near-ultraviolet and near-infrared are becoming more common as frequency-doubling and wave-mixing methods improve. Near-infrared diode lasers are also used for high-resolution vibrational spectroscopy.


Atomic Methods

Gas-phase atoms are inhomogeneously broadened due to the random motion of the atoms. Narrowed lines can be obtained with counterpropogating beams, as described in a separate document on Doppler-free laser spectroscopy. Removing Doppler broadening allows measurement of the natural linewidth, and shows any underlying fine structure such as isotope shifts, hyperfine splitting, and Zeeman splitting.

Atoms or atomic ions in solids will also be broadened due to the random variations of the environments they occupy. This broadening can be reduced by fluorescence-line narrowing (FLN) or hole-burning methods.


Molecular Methods

High-resolution studies require cooling of the molecules to remove spectral congestion and to reduce the Doppler width of the transitions. Gas-phase studies use free-jet expansions or molecular beams to cool molecules to very low temperatures. Large molecules are commonly dissolved in a suitable solvent and cooled to cryogenic temperatures to form a glass or crystalline matrix.

Collimated beam

Preparing molecules in a collimated beam greatly reduces the distribution of velocities perpendicular to the beam direction. The beam is produced by effusion through a pinhole and a skimmer, and the laser direction must be perpendicular to the direction of the beam. Since the velocity component parallel to the laser is greatly reduced the Doppler width is likewise reduced.

Low-temperature matrices

Samples that can be doped into high-quality crystals can be cooled to very low temperatures in liquid nitrogen (77 K) or liquid helium (4.2 K). Samples that are frozen in some glasses (Sphol'skii matrices) will also show very narrow spectral lines.


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