Laser Optogalvanic Analysis in a Radiofrequency Plasma: Detection of Iodine Atoms and Molecules

The optogalvanic effect (OGE) may be used to detect specific species in a plasma (ions, atoms, radicals, or molecules) by selective laser excitation of the plasma. The plasma itself is merely the reservoir of electronically excited, ionized, and atomized species. Compared with conventional ICP-AES, OGE has many advantages: no external detector, zero background, no interferences, and greater versatility. Since the photoacoustic (PA) and ionization rate change (IRC) components of the OGE signal can be separated, optogalvanic spectroscopy can exhibit a further selectivity based on this discriminatory ability. The OGE method has been applied to iodine analysis. By careful selection of the operating conditions, pure atomic and pure molecular iodine signals constitute the PA and IRC profiles, respectively. The best detection limit, in a somewhat primitive experiment, is 10⁻⁷μg/ml of I₂, which is comparable to laser-induced fluorescence. This detection limit can be improved by at least three orders of magnitude by optimizing the experimental conditions.