Characterization of cold atoms based on photoionization momentum spectra

Abstract We propose a method to characterize the features of a cold strontium cloud in a magneto-optical trap (MOT) through the photoionization of cold Sr atoms in a custom-designed reaction microscope. Sr atoms in the dark state of 5s5p 3 P 2 populated via the cascade transition 5s5p 1 P 1 → 5s4d 1 D 2 → 5s5p 3 P 2 accumulate a significant fraction, giving a long lifetime of 520 s. These atoms in the dark state are subsequently trapped by the gradient magnetic field of the MOT. By scanning the Sr + momentum distributions ionized with an 800 nm infrared femtosecond laser, we are able to outline the size of ∼0.55 mm in radius and the temperature of ∼0.40 mK for the dark-state atoms, which is significantly cooler than the MOT temperature of 3.3 mK trapped in the 461 nm. The size of MOT exhibits an oblate spheroidal distribution with a radius of approximately 0.35 mm and 0.55 mm, extracted with momenta of photoion and absorption imaging, respectively. The results using the photoion momenta are consistent with the expected results from absorption imaging, which confirms the method’s reliability. The advantage of this method is the ability to simultaneously characterize the distribution information of atoms in different initial states within the cold atomic cloud.