CO_2-laser sideband spectroscopy at ultrahigh resolution

A transit-limited linewidth of 1.46 kHz (half-width at half-maximum) on SiF4 transitions has been obtained in the 9.6-μm region by saturation spectroscopy with sidebands generated by the mixing of a CO2-laser line with a microwave source in the range 8–18 GHz. This resolution is an improvement of 2 orders of magnitude with respect to that obtained with this technique by several other groups. This laser sideband technique gives, with isotopic CO2 lasers, complete tunability throughout the range 9–12 μm and, when used in ultrahigh-resolution conditions, opens new fields of study in atomic and molecular spectroscopy. As an illustration we show hyperfine and superhyperfine structures of SiF4 and list absolute frequencies with an accuracy better than 5 kHz.