Measurement of the caesium 6S1/2?8P1/2 transition frequency

. The caesium 6 S 1/2 ? 8 P 1/2,3/2 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 6 S 1/2 ?8 P 1/2 transition was determined to an uncertainty of ±1 MHz using the calibrated rubidium 5 S 1/2 ?5 D 5/2 two-photon transition and accurate interferometry. The potential of this caesium transition as a useful secondary frequency standard in blue–UV region is discussed.