Long-lived metastable states of
Yb+ ions are used for atomic frequency standards, precision measurements, and quantum information research. The effect of population trapping and transfer in these states must be well understood. We report here the transfer of
Yb+ ions into the long-lived
2F7/2 state by means of collisions between He buffer gas and
Yb+ ions held in a linear Paul trap. Transfer rates were measured as functions of buffer-gas pressure and repump-laser power, and the collisional population transfer rates were extracted. The measured transfer rate coefficients are $8.32(75)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ and $8.65(33)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }{\text{cm}}^{3}/\text{s}$ for the collisional processes ${^{2}P}_{1/2}\ensuremath{\rightarrow}{^{2}D}_{5/2}$ and ${^{2}D}_{3/2}\ensuremath{\rightarrow}{^{2}F}_{7/2}$, respectively.
Resonant laser excitation of Yb ions confined in an rf ion trap is found to result in perfect population trapping in the highly metastable
2F7/2 state. The lifetime of this level is measured to be greater than 8 days. Population and depopulation of this level under narrow- and broad-band laser excitation conditions is investigated and its potentiality for ultimate-resolution laser spectroscopy indicated.
The storage time of Yb+ ions trapped in an r.f. trap was determined by the rf resonance absorption method in the presence of buffer gas, without and with laser radiation of a frequency equal to the transition from the S1/2 ground state to the excited P1/2 state. The storage time of the ions with the buffer gas (presure between 1 × 10−4 and 4·2 × 10−4 Pa) was several hours without the laser radiation; this period was shortened to several hundred seconds by irradiation with a 500 nW laser source. This phenomenon shows that the trapped Yb+ ions disappeared from the trap as a result of irradiation with the resonance laser light.
The approaches to realize superslow relaxation receive growing attention for its correspondence to sharp optical resonance in frequency domain and to long term of information storage in time domain. Dephasing time as long as 2.6 mS was observed in the condensed matter of crystal under the temperature of 1.4K [1]. In RF ion trap Yb ion has been kept in its metastable excited state for 8 days [2]. Population trapping is one of the interested approaches recently. While most papers discussed it for Λ- or V-type three-level systems, Agarwarl et al has theoretically predicted the possibility of population trapping been established by frequency modulated laser field in two-level system [3]. The authors has demonstrated population trapping in high-lying state doublet and in intermediate state doublet via quantum interferences between two sum-frequency two-photon excitations in Y- and in rhomb-type four-level systems respectively [4].
It is known that fluorescence of trapped
Yb+ ions fades with continuous driving of the
2 S1/2${\mathrm{\ensuremath{-}}}^{2}$
P1/2 transition in the presence of buffer gases. This phenomenon has been interpreted as being caused by population trapping in the
2 F7/2 state of
Yb+ . As another cause of the fluorescence disappearance, we found the production of
YbH+ by chemical reaction of
Yb+ in excited states with
H2 gas. We found that the wavelengths of 369.482, 369.202, and 368.947 nm, formerly assigned as wavelengths for deexcitation of
Yb+ in the
2 F7/2 state to the
2 S1/2 ground state, are actually photodissociation lines of
YbH+ . The change of the resonance point of rf resonance absorption signals indicates the production and dissociation of
YbH+ ions. The production of
YbH+ may be a problem in establishing frequency standards using buffer-gas-cooled
Yb+ , owing to difficulty in removing
H2 gas from vacuum systems.