Infrared Laser Spectroscopy of Short-Lived Atoms and Molecules

Abstract

The infrared spectroscopy of free radicals and ions at Doppler limited resolution has produced impressive results over the past two decades. Initial successes in measuring the absorption spectra of transient species, primarily neutral free radicals, was due to highly sensitive techniques like Laser Magnetic Resonance (LMR), (Evenson, 1981). In infrared LMR a ro-vibrational or fine structure transition is tuned through coincidence with a fixed frequency laser line using a magnetic field. The laser sources are gas lasers using different isotopic forms of CO and CO₂. The sensitivity of the technique primarily derives from an intracavity absorption configuration. Amongst the notable discoveries with LMR has been, for example, the first gas phase spectroscopy of the OF free radical, (McKellar, 1979). LMR has undergone continual development in the meantime with particular emphasis on extending the wavelength coverage of the laser. The CO laser is now line tunable from 1200 to 2100 cm ⁻¹ and from 2500 to 3800 cm⁻¹, (Bachem et al., 1993). Nevertheless two drawbacks remain with LMR. Firstly, the transition must lie close to a laser line even at low J when the Zeeman effect is largest. Secondly, the magnetic field effects can lead to complicated Zeeman patterns — even for diatomic free radicals.