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

The caesium 6S(1/2) --> 8P(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 6S(1/2) --> 8P(1/2) transition was determined to an uncertainty of +/-1 MHz using the calibrated rubidium 5S(1/2) --> 5D(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.