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Chinese Physics B

Characterization of cold atoms based on photoionization momentum spectra

Zhixian 志贤 Wu 吴, S. N. Ruan, Zhenjie Shen, Jie Liu, Xinglong Yu, Lifeng Chen, Bing Zhu, Xincheng 新成 Wang 王, Yuhai Jiang

Published Apr 24, 2025• 0 citations

Abstract

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 5s5p3P2 populated via the cascade transition 5s5p1P1 → 5s4d1D2 → 5s5p3P2 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.

DOI: 10.1088/1674-1056/add00d

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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 5s5p3P2 populated via the cascade transition 5s5p1P1 → 5s4d1D2 → 5s5p3P2 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.

We developed a magneto-optical trap reaction microscope (MOTREMI) for strontium atoms by combining multiparticle coincident detection with the laser cooling technique. The present compact injection system enables the production of cold Sr atoms in three modes of two-dimensional (2D) magneto-optical trap (MOT) beam, molasses, and three-dimensional (3D) MOT, providing adjustable densities and ratios of the ground state

5{s}^{2}

(

^{1}S_{0}

) and the excited states

5s5p{\phantom{\rule{4pt}{0ex}}}^{1}{P}_{1}

and

^{3}P_{J}

. The target profiles including temperature, density, and size of the 3D MOT as well as the 2D MOT beam cold atomic flux were characterized in detail. Using the present setup, we investigate the single photoionization of Sr atoms with molasses through few-photons absorption in 800 nm, where

{\mathrm{Sr}}^{+}

and ${e}^{\ensuremath{-}}$ are detected in coincidence and most of the ionization channels are identified. The momentum resolution for coincident

{\mathrm{Sr}}^{+}

and ${e}^{\ensuremath{-}}$ along time-of-flight direction are achieved up to 0.12 a.u. and 0.02 a.u., respectively. The spectra of photoelectron ionized from the ground state as well as several excited states show distinct structures manifesting prominent contributions from multiphoton absorption. Detecting complete vector momenta of electrons and recoil ions in coincidence paves a way for further investigation of two-electron correlation dynamics and multielectron effects with alkaline-earth atoms.

We study photoionization of cold rubidium atoms in a strong infrared laser field using a magneto-optical trap (MOT) recoil ion momentum spectrometer. Three types of cold rubidium target are provided, operating in two-dimension (2D) MOT, 2D molasses, and 3D MOT with densities in the orders of

10^7

atoms/cm

^3

,

10^8

atoms/cm

^3

, and

10^9

atoms/cm

^3

, respectively. The density profile and the temperature of 3D MOT are characterized using the absorption imaging and photoionization. The momentum distributions of Rb

^+

created by absorption of two- or three-photon illuminate a dipole-like double-peak structure, in good agreement with the results in the strong field approximation. The yielding momentum resolution of

0.12 \pm 0.03

a.u. is achieved in comparison with theoretical calculations, exhibiting the great prospects for the study of electron correlations in alkali metal atoms through interaction with strong laser pulses.

Photoionization of a cold atomic sample offers intriguing possibilities for observing collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap (MOT) with laser pulses ionizing through one-photon and two-photon absorption processes was performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample were examined as functions of the ionizing laser parameters. Ionization cross sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.

The results of characterization of cold cloud of cesium (Cs) atoms trapped in a magneto-optical trap (MOT) have been presented. The MOT is a part of the Physics package of the Cs Fountain Clock being developed at National Physical Laboratory (NPL), India. The number of atoms, size and temperature of the cloud have been measured and calculated. It is also been investigated how the number of trapped atoms changes with the trapping laser’s beam intensity, detuning and magnetic-field gradient of the trap.

The newly developed method of time-of-flight (TOF) momentum microscopy was used to analyze the cold electron emission from a Cs three-dimensional magneto-optical trap (MOT). Three-step resonant photoionization was implemented via two intermediate states (

6{P}_{3/2}

pumped with an 852-nm laser and

7{S}_{1/2}

with a 1470-nm laser) and a tunable femtosecond (fs) Ti:sapphire laser for the final ionization step. The magnetic field of the MOT is switched off during the photoionization step. The natural bandwidth of the fs laser is reduced to 4 meV using optical spectral filters. Precise tuning of the photon energy makes it possible to observe the transition regime between direct photoemission into the open continuum and field-induced ionization of highly excited Rydberg states. The paths can be identified by their characteristic dependency on the extraction field and on the Ti:sapphire polarization. TOF analysis allowed us to disentangle the ionization paths and the dependence of the spatiotemporal distribution of the cold electrons on the polarization of the ionizing laser.

Pulsed photoionization of laser-cooled atoms in a magneto-optical trap (MOT) has the potential to create cold electron beams of few meV bandwidths and few ps pulse lengths. Such a source would be highly attractive for the study of fast low-energy processes like coherent phonon excitation. To study the suitability of MOT-based sources for the production of simultaneously cold and fast electrons, we study the photoionization dynamics of trapped Cs atoms. A momentum-microscope-like setup with a delay-line detector allows for the simultaneous measurement of spatial and temporal electron distributions. The measured patterns are complex, due to the Lorentz force inducing spiral trajectories. Ray-tracing simulations reproduce the main features. We find that the production of electron bunches with bandwidths of a few meV is straightforward; however, pulses in the ps-range are more demanding and require beam blanking or partial blocking.