High-resolution diode-laser spectroscopy on a fast beam of metastable atoms for detecting very rare krypton isotopes

A fast krypton ion beam with an energy of 10 keV is transferred through a mass filter and neutralized in a Rb- (or Cs-) vapour charge exchange cell. The emerging beam of metastable Kr atoms of a selected Kr isotope is collinearly irradiated with a tunable cw GaAlAs diode laser at 811 nm. The spectrum of the 1s 5–2p 9 transition covers ∼ 10 GHz and consists of one line each for the five stable isotopes of Kr with even mass number and 15 hyperfine structure lines of 83Kr. The individual lines are recorded by detecting the fluorescence signal perpendicular to the beams. Photons are guided to the PM tube by diffuse reflection from a high reflectance thermoplastics light collector. Up to 20 photons per atom are emitted by cycling between the two states during the flight time of 1.2 µs through the detector. The observed absorption linewidth of 100 MHz is a combination of laser linewidth and energy uncertainty in the fast atomic beam. In order to achieve maximum isotope selectivity the beam energy has to be adjusted in such a way that the Doppler shifted lines of all isotopes form an optimum pattern for detecting the very rare isotopes 81Kr and 85Kr in natural environmental samples