Tunable laser electronic spectroscopy

Abstract

The crucial difference between classical and laser spectroscopies is the capability with the latter of performing separately the usual components of a spectroscopic experiment: sample preparation, signal detection, frequency resolution and measurement, and spectral line assignment. Incompatibilities, such as intensity vs. resolution, are eliminated by this separation. Various laser experiments will be reviewed as illustrations of solutions to tactical spectroscopic problems.Excitation spectroscopy separates frequency measurement from signal detection, thus facilitating the recording of relatively sparse (<5 lines per cm–1) spectra at a limiting precision ( 10–4 cm–1) far beyond what is possible with presently available and convenient standards. Selective fluorescence detection eliminates “unnecessary” inter-band and inter-branch blends and simplifies rotational assignment. Doppler-width-free techniques, such as intermodulation and optical-optical double resonance (OODR) spectroscopy, provide additional resolution at small cost in signal strength.OODR combines near-perfect prior state selection, the ability to access “special” levels, and a strong probe in which all prepared molecules participate at a selectable time, rate and on a single probe transition. Optically pumped lasers (OPL) yield spectroscopic information analogous to OODR, but with one crucial advantage and disadvantage. These are, respectively: the OPL signal appears in a diffraction-limited beam, not over 4π steradians; certain OODR-observable transitions have insufficient gain for OPL oscillation.Spectra acquired with tunable lasers are ideally suited to computer control over the recording, frequency calibration, and line assignment processes. All of the tedious measurement, trial-and-error and mass data-handling aspects of classical spectroscopy and the every-experiment-is-a-special-case character of high-resolution laser spectroscopy are eliminated.