Kazumoto Hosaka

Trapped ion optical frequency standards

Optical frequency standards based on narrow absorptions in laser-cooled single trapped ions have recently begun to demonstrate stabilities that are competitive with cold atom fountain microwave standards. This paper presents a short review of the wider state-of-the-art development of these single cold trapped ion frequency standards, coupled with a more detailed account of recent results achieved at National Physical Laboratory in respect of single ion systems based on 88Sr+, 87Sr+ and 171Yb+. Narrow linewidth data for the optical clock quadrupole and octupole transitions respectively at 674 nm in 88Sr+ and 467 nm in 171Yb+, are presented, together with a discussion of current systematics and future projections. The potential for optical clock operation is outlined.

An optical frequency standard based on the electric octupole transition in /sup 171/Yb/sup +/

The frequency of the /sup 2/S/sub 1/2/(F=0,m/sub F/=0)-/sup 2/F/sub 7/2/(F=3,m/sub F/=0) transition in a single, trapped, laser cooled ion of /sup 171/Yb/sup +/ has been measured with an improved narrow probe laser and a femtosecond laser frequency comb generator. Our best estimate of the frequency is 642 121 496 772.3 /spl plusmn/ 0.6 kHz by comparison with earlier measurements. The uncertainty is limited by measurement statistics and by the ac Stark shift.

A scaling law of cross sections for multiple electron transfer in slow collisions between highly charged ions and atoms

A simple scaling relation is derived for the partial and total multiple-electron capture cross sections in slow collisions of highly charged ions with atoms based on the extended classical over-barrier model. It is shown that the currently available experimental cross sections are reproduced quite satisfactorily by this relation.

Systematic frequency shifts of the 467 nm electric octupole transition in 171Yb

The 2S 1/2 (F = 0) ?2F 7/2 (F = 3, mF = 0) transition at 467 nm in a single trapped, laser-cooled ion of 171Yb+ has the potential to be an extremely accurate optical frequency reference. The systematic shifts of the transition frequency currently make a significant contribution to the uncertainty on absolute frequency measurements of the transition. Here the systematic shifts of a recent absolute frequency measurement using a femtosecond laser frequency comb generator are evaluated, and realistic limits for the ultimate performance of the standard are estimated.

SECONDARY CHARGED PARTICLES EMITTED FROM NON-METALLIC SURFACES UNDER LOW-ENERGY ION IMPACT

The secondary electron emission yields, γ−, and the secondary positive ion emission yields, γ+, from various non-metallic (perovskite type) targets induced by low-energy ion impact have been measured under ultra-high vacuum (∼10−11 Torr) conditions. It has been found that the observed γ− from the non-metallic surfaces is proportional to the electronic stopping power over the whole impact energy range investigated. But the proportional coefficients in γ− are found to be 2–3 times larger than those for metal targets. We have observed, for the first time, that γ− strongly depends on the incident ion beam flux and becomes zero above a critical ion flux for some targets. We have also found that the observed γ+ from non-metallic surfaces under Ar+ ion impact is far large compared with the calculated results and, unexpectedly, is proportional to the electronic stopping power but not to the nuclear stopping power. Furthermore, it is shown that, under high incident ion fluxes, these emitted ions have large kinetic energy, almost comparable to the incident ion energy, which can be due to local charge-accumulation formed through the interactions with the incident ions on the non-metallic surfaces.

Trapped Ion Optical Clocks at NPL

Forbidden transitions in single laser‐cooled trapped ions provide highly stable and accurate references for optical frequency standards. This paper describes recent progress on strontium and ytterbium ion optical frequency standards under development at NPL.

Ion flux-dependence of secondary charged particle emissions from non-metallic surfaces

We have measured the total secondary charged particle emission (SCPE) yields by H+ and H− impact from some non-metallic surfaces such as oxidized Be. It is found that the SCPE yields by H+ impact remarkably depend on the current (flux) of the incident ions, while those by H− ion impact are nearly constant. In particular, it is noted that when the current of the incident H+ ions exceeds a critical value, the SCPE changes from negative (electrons) to positive (ions).

Projectile charge dependence of multi-electron transfer processes in highly charged ion-atom collisions

We have measured the absolute multi-electron transfer cross sections and branching ratios for the decay of multiply excited Rydberg ions in slow Iq+-Xe, Ar (10 ≤ q ≤ 20) collisions combining the initial growth rate method with coincidence method. We report the incident ion charge dependence of transfer cross sections and branching ratios.

Multiple electron capture processes between highly charged ions and molecules

We have determined the absolute cross sections for one electron capture and total electron capture between slow, highly charged ions and multi-electron molecule targets (H2, N2, CO, CO2, CH4) under single collision conditions. It is found that the cross sections increase as the charge of the projectile ions increases and as the ionization potential of the target molecules decreases, as previously observed in rare gas targets. Furthermore, we have found that a scaling law previously proposed for atoms, which is based on the extended classical over barrier model, can also reproduce our data for molecular targets.

Multielectron transfer processes of highly charged ion Iq+ (6 ≤ q ≤ 30) atom collisions

Abstract We have determined the absolute cross-sections σq and σq,q−1 both total and for one-electron capture for the processes Iq+ (6 ≤ q ≤ 30) + B → I(q−i)+ + Bj+ + (j − i)e− (B = Ne, Ar, Kr, Xe) at a collision energy of 1.5q keV. The absolute electron capture cross-sections were measured by the initial growth rate method. The experimental results for the initial charge state dependence of the total electron capture cross-sections were compared with the predictions of the extended classical overbarrier model.