利用激光冷却原子束测量氦原子精密光谱

⁴He原子2³S₁→2³P_0,1,2跃迁的精细结构分裂,目前在理论和实验上都能够达到10^-8水平的精度,并可被应用于测定精细结构常数α, 和对量子电动力学进行检验.该方面实验研究的关键, 是需要提高测量信噪比,并消除各种可能的系统偏差, 将这一精细结构分裂测量到亚kHz水平.在设计的这套实验方案中, 首次结合激光冷却原子技术,通过激光横向冷却来提高亚稳态氦原子束的束流强度,并对三态亚稳态氦原子进行偏折, 将其从原子束中分离,从而大幅降低测量背景,并利用频率锁定激光器的边带扫描的方式来进行光谱测量,以使得扫描测量中保持足够的频率精度. 在目前基本搭建成的实验装置上,实验方法的可行性已经获得验证,分析表明有望实现亚千赫兹水平的测量准确度.     

Precision spectroscopy with individual ions

Individual charged atomic or molecular particles can be confined by electromagnetic fields for nearly unlimited times under well controlled conditions in a small volume in space. This allows performing spectroscopic experiments with unprecedented accuracy. We discuss a project to determine the magnetic moment of the electron bound in hydrogen-like ions with different nuclear charges. This serves for testing bound-state quantum-electrodynamics calculations with high precision. Previous results on C⁵⁺ and O⁷⁺ as well as the present status of the project with Si¹³⁺ and Ca¹⁹⁺ are presented in this contribution and future possibilities are discussed.     

Precision spectroscopy on trapped atomic ions

Recent examples of lifetime measurements, ground state hyperfine splittings and spectroscopy on minute samples of radioactive ions illustrate the extension of classical methods of spectroscopy to new regions of interest by the use of ion storage techniques. Estimates are given on the sensitivity of the method with respect to be possible investigation of the Bohr-Weisskopf effect in isotopic chains of elements.     

Precision laser spectroscopy of highly charged ions

Recently, intense beams of highly charged ions have become available at heavy ion cooler rings. The obstacle for producing these highly interesting candidates is the large binding energy of K-shell electrons in heavy systems in excess of 100 keV. One way to remove these electrons is to strip them off by passing the ion through material. In the cooler ring, the ions are cooled to a well defined velocity. At the SIS/ESR complex it is possible to produce, store, and cool highly charged ions up to bare uranium with intensities exceeding 10⁸ atoms in the ring. This opens the door for precision laser spectroscopy of hydrogenlike-heavy ions, e.g.²⁰⁹Bi⁸²⁺, and allows to examine the interaction of the single electron with the large fields of the heavy nucleus, exceeding any artificially produced electric and magnetic fields by orders of magnitude. In the electron cooler the interaction of electrons and highly charged ions otherwise only present in the hottest plasmas can be studied.     

Photoassociation spectroscopy of laser-cooled ytterbium atoms

We report the photoassociation spectroscopy of laser-cooled ytterbium atoms in an optical trap. We observed more than 90 photoassociation resonances of vibrational levels in the (1)Sigma(+)(u) state, including 80 consecutive series, up to 490 GHz detuning with respect to the atomic resonance. From the resonance frequencies we derived the atomic radiative lifetime of the (6s6p) 1P1 state to be 5.464+/-0.005 ns, which is about 2 orders of magnitude improvement over previous results. We also observed line broadening of resonances, which is ascribed to the predissociation to the triplet states, and estimated the transition probability to be 0.2. Furthermore, we observed the decrease of the photoassociation signal intensity, from which the scattering length is estimated to be equal to or less than 3 nm.     

High-resolution spectroscopy with laser-cooled and trapped calcium atoms

We report on high-resolution spectroscopy with two different samples of calcium atoms, in a laser-cooled and deflected beam and in a magneto-optical trap. The atomic beam was excited by spatially separated laser fields. For spectroscopy with stored atoms in a magneto-optical trap we used a multiple-pulse excitation scheme. The resolution as low as 2.5 kHz was limited by residual frequency fluctuations of our dye-laser spectrometer. The results should allow to establish a frequency standard with a relative uncertainty below 10^–14.     

Cold Reactive Collisions between laser-cooled ions and velocity-selected neutral molecules

We report a new experimental method to study reactive ion-molecule collisions at very low temperatures. A source of laser-cooled ions in a linear Paul trap has been combined with a quadrupole-guide velocity selector to investigate the reaction of Ca+ with CH3F at collision energies E[over](coll)/k(B)> or =1 K with single-particle sensitivity. The technique represents a general approach to study reactive collisions between ions and polar molecules over a wide temperature range down to the cold regime.     

Laser spectroscopy on forbidden transitions in trapped highly charged Ar(13+) ions

We demonstrate resonant fluorescence laser spectroscopy in highly charged ions (HCI) stored in an electron beam ion trap by investigating the dipole-forbidden 1s(2)2s(2)2p (2)P(3/2)-(2)P(1/2) transition in boronlike Ar(13+) ions. Forced evaporative cooling yielded a high resolving power, resulting in an accurate wavelength determination to λ=441.255?68(26) nm. By applying stronger cooling and two-photon excitation, new optical frequency standards based upon ultrastable transitions in such HCI could be realized in the future, e.g., for the search of time variations of the fine-structure constant.     

The dynamic Kingdon trap: A novel design for the storage and crystallization of laser-cooled ions

On the basis of analytical pseudo-potential calculations and extensive numerical simulations it is proved that the dynamic Kingdon trap is a working design for the permanent storage and the crystallization of laser-cooled ions.Dedicated to H. Walther on the occasion of his 60th birthday     

Toward a transportable microwave frequency standard based on laser-cooled 113Cd+ ions

A transportable microwave frequency standard based on laser-cooled ¹¹³Cd⁺ ions is introduced, and its working principle and potential performance are discussed. Based on the experimental setup developed at JMI, the frequency of the ground-state hyperfine splitting of ¹¹³Cd⁺ is measured to within milliHertz uncertainty and its frequency stability is obtained.     

Dynamics of laser-cooled Ca+ ions in a Penning trap with a rotating wall

We have performed systematic measurements of the dynamics of laser-cooled ⁴⁰Ca⁺ ions confined in a Penning trap and driven by a rotating dipole field (‘rotating wall’). The trap used is a copy of the one used in the SPECTRAP experiment located at the HITRAP facility at GSI, Germany. The size and shape of the ion cloud has been monitored using a CCD camera to image the fluorescence light resulting from excitation by the cooling laser. We have varied the experimental conditions such as amplitude and frequency of the rotating wall drive as well as the trapping parameters. The rotating wall can be used for a radial compression of the ion cloud thus increasing the ion density in the trap. We have also observed plasma mode excitations in agreement with theoretical expectations. This work will allow us to define the optimum parameters for high compression of the ions as needed for precision spectroscopy of forbidden transitions.     

Auger transitions in Li-like and Be-like ions

Measurements of the Auger decay of beam-foil excited Be II and Be I levels are reported along with a proposed assignment of the experimental spectra. The Li I, Be II and Be III (1s2s²)²S → (1s²2s)²S Auger transitions as presented in this letter represents the first observation of such states in positive ions with Z?5.     

Radiative transitions in highly ionised silicon-like ions

Transition energies, oscillator strengths and transition probability values for radiative transitions have been calculated for the highly ionised atoms of Si isoelectronic sequence from Mn¹¹⁺ to Kr²²⁺ for the singly excited states up to principal quantum number n = 7. Time-dependent coupled Hartree-Fock (TDCHF) theory has been used to estimate such transition properties. Most of the results for the oscillator strengths and transition probabilities are new. Transition energies agree reasonably well with available spectroscopic values. Received 25 January 2000 and Received in final form 24 October 2000