Fig. 1: Optical-excitation ATTA for ⁸¹Kr and ⁸⁵Kr analysis of 1-kg ice samples.

a Schematic of the system. The resonant 124 nm light needed for optical excitation is generated by a krypton discharge lamp. After undergoing several laser cooling stages, the individual ⁸¹Kr and ⁸⁵Kr atoms are captured and detected in a magneto-optical trap. The turbo-molecular pumps (TMP) are configured to recirculate the sample in the vacuum system. b Atomic transitions and energy levels of krypton relevant for optical excitation, laser cooling and detection. See Method section for a more detailed level scheme.