Abstract
The caesium 6S <sub>1/2</sub> → 8P <sub>1/2.3/2</sub> transitions at 387.6 nm and 388.8 nm have been observed with a linewidth of < 5 MHz by laser spectroscopy. The absolute frequency of the 6S <sub>1/2</sub> → 8P <sub>1/2</sub> transition was determined to an uncertainty of ±1 MHz using the calibrated rubidium 5S <sub>1/2</sub> → 5D <sub>5/2</sub> two-photon transition and accurate interferometry. The potential of this caesium transition as a useful secondary frequency standard in blue-UV region is discussed.