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Journal of the Physical Society of Japan

Detection of Ultracold Ground-State Molecules by One- and Two-Color Resonance-Enhanced Two-Photon Ionization

Zhonghao Li, Zhonghua Ji, Xiang Zhang, Jinpeng Yuan, Yanting Zhao, Liantuan Xiao, Suotang Jia

Published Jul 7, 2016• 3 citations
Publisher

Abstract

One- and two-color resonance-enhanced two-photon ionization (RETPI) is used to detect ultracold ground-state RbCs molecules which are formed via short-range photoassociation from laser-cooled atoms. The transition from the X1Σ+(v = 0) state to the 21Π(v = 10) state of ultracold RbCs molecules shows the consistence of one- and two-color RETPI. A multi-photon photoionization rate model is introduced to interpret the dependence of molecular ion intensity on photoionized laser energy, and can be used to verify the photoionization scheme. This photoionization rate model can be expanded to multi-color photoionization for all kinds of atoms and molecules, which is a powerful method of determining the photoionization scheme.

DOI
10.7566/jpsj.85.084301

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One- and two-color resonance-enhanced two-photon ionization (RETPI) is used to detect ultracold ground-state RbCs molecules which are formed via short-range photoassociation from laser-cooled atoms. The transition from the X1Σ+(v = 0) state to the 21Π(v = 10) state of ultracold RbCs molecules shows the consistence of one- and two-color RETPI. A multi-photon photoionization rate model is introduced to interpret the dependence of molecular ion intensity on photoionized laser energy, and can be used to verify the photoionization scheme. This photoionization rate model can be expanded to multi-color photoionization for all kinds of atoms and molecules, which is a powerful method of determining the photoionization scheme.
Resonance-enhanced two-photon ionization (RETPI) is a powerful tool in ultracold molecular photoassociation spectroscopy measurement. Using the RETPI, the line shape of ultracold RbCs molecular photoassociation spectroscopy, which contains intensity and full width at half maximum, is measured and analyzed systematically with varied photoassociation laser intensity and ionization laser energy. The saturation of photoassociation spectroscopy at a high ionization laser energy is observed and the suppression phenomenon is observed at high photoassociation laser intensity. The saturation intensity of RbCs molecular photoassociation, the linewidths of excited state molecules in different rovibrational levels at zero photoassociation laser intensity, and two-photon ionization efficiency have been derived from the line shape of photoassociation spectroscopy. The line shape analysis of photoassociation spectroscopy is important for precision spectroscopic analysis.
Using resonance-enhanced two-photon ionization, we detect ultracold, metastable RbCs molecules formed in their lowest triplet state $a\text{ }^{3}\ensuremath{\Sigma}^{+}$ via photoassociation in a laser-cooled mixture of 85Rb^{85}\mathrm{Rb}85Rb and 133Cs^{133}\mathrm{Cs}133Cs atoms. We obtain extensive bound-bound excitation spectra of these molecules, which provide detailed information about their vibrational distribution, as well as spectroscopic data on several RbCs molecular states including $a\text{ }^{3}\ensuremath{\Sigma}^{+}$, $(2)\text{ }^{3}\ensuremath{\Sigma}^{+}$, and $(1)\text{ }^{1}\ensuremath{\Pi}$. Analysis of this data allows us to predict strong transitions from observed levels to the absolute vibronic ground state of RbCs, potentially allowing the production of stable, ultracold polar molecules at rates in excess of ${10}^{6}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.
We have investigated photoassociative formation of RbCs molecules in the ${(2)}^{\phantom{\rule{0.16em}{0ex}}3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ excited state correlated to v=8v=8v=8, (5)0+(5){0}^{+}(5)0+ vibrational level in detail. The metastable ground-state RbCs molecules formed by spontaneous decay are ionized by pulsed dye laser through resonance-enhanced two-photon ionization. A rate equation of the photoionization process is introduced to explain the dependence of RbCs+{}^{+}+ molecular ion intensity on the ionization laser intensity. The saturation effect of molecular ion intensity appears as the photoassociation laser intensity increases. The rotational constant and centrifugal distortion constant are derived to be 0.01304 cm${}^{\ensuremath{-}1}$ and 0.000015 cm${}^{\ensuremath{-}1}$ from the photoassociation spectrum with a high sensitivity, respectively. The measured electric dipole moment of the observed $(2){\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ state RbCs molecules is 4.7(6) D by Stark effect in static electric field.
We have produced ultracold polar RbCs molecules via photoassociation starting from laser-cooled 85 Rb and 133 Cs atoms in a dual-species, forced dark magneto-optical trap. The formed electronically excited RbCs* molecules correlated to the Rb(5S 1/2 ) + Cs(6P 1/2 ) dissociation limit are observed by trap loss spectroscopy. Following the decay of these excited RbCs* molecules, the formed ground state molecules are directly ionized by a two-photon single-color pulse dye laser, which is a new ionization mechanism for ground state RbCs molecules and thence detected by time-of-flight mass spectroscopy.
Ultracold polar RbCs molecules are produced via photoassociation in a laser-cooled mixture of 85 Rb and 133 Cs atoms. The a 3 Σ + state molecules which decay from electronically excited (2)0 − state RbCs molecules are detected by resonance-enhanced two-photon ionization. The new rovibrational levels ( v = 189, 190) in the (2)0 − state are also observed, which exist in theory and have not been observed in experiments yet. The corresponding rotational constants are measured by photoassociation spectroscopy, which are consistent with theoretical calculations using a nonrigid rotor model.
We investigate the production of ultracold ground state X 1 σ + (v = 0) RbCs molecules in the lowest vibrational level via short-range photoassociation followed by spontaneous emission. The starting point is the laser cooled 85 Rb and 133 Cs atoms in a dual species, forced dark magneto-optical trap. The special intermediate level (5)0 + (v = 10) correlated to the (2) 3 π electric state is achieved by the photoassociation process. The formed ground state X 1 σ + (v = 0) molecule is resonantly excited to the 2 1 π intermediate state by a 651 nm pulse laser and is ionized by a 532 nm pulse laser and then detected by the time-of-flight mass spectrum. Saturation of the photoionization spectroscopy at large ionization laser energy is observed and the ionization efficiency is obtained from the fitting. The production of ultracold ground state 85 Rb 133 Cs molecules is facilitative for the further research about the manipulation of ultracold molecules in the rovibrational ground state.