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We report on laser spectroscopic measurements on Li$^+$ ions in the experimental storage ring ESR at the GSI Helmholtz Centre for Heavy Ion Research. Driving the $2s\,^3\!{S}_1\;(F=\frac{3}{2}) \,\leftrightarrow\,2p\,^3\!P_2\;(F=\frac{5}{2}) \leftrightarrow 2s\,^3\!{S}_1\;(F=\frac{5}{2})$ $\Lambda$-transition in $^7$Li$^+$ with two superimposed laser beams it was found that the use of circularly polarized light leads to a disappearance of the resonance structure in the fluorescence signal. This can be explained by optical pumping into a dark state of polarized ions. We present a detailed theoretical analysis of this process that supports the interpretation of optical pumping and demonstrates that the polarization induced by the laser light must then be at least partially maintained during the round trip of the ions in the storage ring. Such polarized ion beams in storage rings will provide opportunities for new experiments, especially on parity violation., Comment: 14 pages, 8 figures, new version with small adjustments, closest to final publication in PRAB

We present preparatory measurements for an improved test of time dilation at the experimental storage ring (ESR) at GSI in Darmstadt. A unique combination of particle accelerator experiments and laser spectroscopy is used to perform this test with the highest precision. 7Li+ ions are accelerated to 34% of the speed of light at the GSI Helmholtzzentrum für Schwerionenforschung and stored in the experimental storage ring. The forward and backward Doppler shifts of an electric dipole transition of these ions are measured with laser spectroscopy techniques. From these Doppler shifts, both the ion velocity β = ν/c and the time dilation factor [Formula: see text] can be derived for testing Special Relativity. Two laser systems have been developed to drive the 3S1→3P2 transition in 7Li+. Moreover, a detector system composed of photomultipliers, both to monitor the exact laser ion beam overlap as well as to optimize fluorescence detection, has been set up and tested. We investigate optical-optical double-resonance spectroscopy on a closed Λ-type three-level system to overcome Doppler broadening. A residual, broadened fluorescence background caused by velocity-changing processes in the ion beam is identified, and a background subtraction scheme implemented. At the present stage the experimental sensitivity, although already comparable with previous measurements on slower ion beams at the TSR storage ring that led to [Formula: see text] –8, suffer from a poor signal-to-noise ratio. Modifications of the ion source as well as the detection system are discussed that promise to improve the sensitivity by one order of magnitude.

We report on the first laser cooling of a bunched beam of multiply charged C3+ ions performed at the ESR (GSI) at a beam energy of E = 1.47 GeV. Moderate bunching provided a force counteracting the decelerating laser force of one counterpropagating laser beam. This versatile type of laser cooling lead to longitudinally space-charge dominated beams with an unprecedented momentum spread of Δp/p ≈ 10−7. Concerning the beam energy and charge state of the ion, the experiment depicts an important intermediate step from the established field of laser cooling of ion beams at low energies toward the unique laser cooling scheme proposed for relativistic beams of highly charged heavy ions at SIS 300 (FAIR).

Spectroscopic results obtained on multiply charged few electron ions by using laser stimulated two-step recombination in ion storage rings are presented in an overview. Line splittings in transitions between Rydberg levels were observed for helium like and beryllium like ions. Emphasis is given to results on the beryllium like system N3+ where information about the splitting between high angular momentum levels (l⩾4) was obtained for the Rydberg levels n=13 and 8 with experimental errors of about ±0.02 meV.

We report on injection locking of optically excited mechanical oscillations of a single, trapped ion. The injection locking dynamics are studied by analyzing the oscillator spectrum with a spatially selective Fourier transform technique and the oscillator phase with stroboscopic imaging. In both cases we find excellent agreement with theory inside and outside the locking range. We attain injection locking with forces as low as 5(1)×10^(-24) N so this system appears promising for the detection of ultraweak oscillating forces.

Red-detuned laser pumping of an atomic resonance will cool the motion of an ion or atom. The complementary regime of blue-detuned pumping is investigated in this work using a single, trapped Mg+ ion interacting with two laser beams, tuned above and below resonance. Widely thought of as a regime of heating, theory and experiment instead show that stimulated emission of centre-of-mass phonons occurs, providing saturable amplification of the motion. A threshold for transition from thermal to coherent oscillating motion has been observed, thus establishing this system as a mechanical analogue to an optical laser -— a phonon laser. Such a system has been sought in many different physical contexts.

Cooling of atoms and ions [1,2] using a red-detuned laser has had a profound impact on science and technology [3–5]. In this work simultaneous laser cooling and blue-detuned laser pumping of a Mg+ ion in a Paul trap is studied. Blue-detuned pumping is conventionally referred to as the heating regime, and in early work, remarkably complex behaviors (bistability and limit cycles) have been associated with this regime [6,7]. These behaviors have so far not been fully explained. Here, it is shown that blue-detuned pumping, as opposed to heating, causes stimulated emission of center-of-mass phonons, leading to coherent oscillatory motion of the ion in analogy with a laser. Mechanical amplification is calculated as well as the threshold pumping condition for oscillation. A single ion in a linear radio-frequency trap is studied to verify these predictions (figure 1 left panel). Blue-detuned pumping of the magnesium D2 transition at 279.6 nm provides amplification along the long axis of the ion trap so as to excite only axial oscillations. A slightly off-axis, red-detuned beam cools the center-of-mass motion to approximately 1 mK. The experimental arrangement is identical to that detailed in ref. [8].

We demonstrate a method for precision spectroscopy on trapped ions in the limit of unresolved motional sidebands. By sympathetic cooling of a chain of crystallized ions, we suppress adverse temperature variations induced by the spectroscopy laser that usually lead to a distorted line profile and obtain a Voigt profile with negligible distortions. We applied the method to measure the absolute frequency of the astrophysically relevant $D2$ transition in single $^{24}\mathrm{Mg}^{+}$ ions and find $1\text{ }072\text{ }082\text{ }934.33(16)\text{ }\text{ }\mathrm{MHz}$, a nearly 400-fold improvement over previous results. Further, we find the excited state lifetime to be 3.84(10) ns.

A trapped Mg+ion is laser cooled within a Paul trap, and is simultaneously excited using a continuous blue-detuned pump wave. The ion subsequently exhibits coherent oscillatory motion with a well-defined threshold.

A cooled, trapped Mg^+ ion is excited using a continuous, blue-detuned pump. Coherent oscillatory motion is observed. It is shown to result from stimulated emission of center-of-mass phonons, constituting the mechanical analog of a laser.

We investigate the laser-assisted photoelectric effect from a solid surface. By illuminating a $\mathrm{Pt}(111)$ sample simultaneously with ultrashort 1.6 and $42\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ pulses, we observe sidebands in the extreme ultraviolet photoemission spectrum, and accurately extract their amplitudes over a wide range of laser intensities. Our results agree with a simple model, in which soft x-ray photoemission is accompanied by the interaction of the photoemitted electron with the laser field. This strong effect can definitively be distinguished from other laser surface interaction phenomena, such as hot electron excitation, above-threshold photoemission, and space-charge acceleration. Thus, laser-assisted photoemission from surfaces promises to extend pulse duration measurements to higher photon energies, as well as opening up measurements of femtosecond-to-attosecond electron dynamics in solid and surface-adsorbate systems.

We observe the laser-assisted photoelectric effect on a surface for the first time. Simultaneously illuminating Pt(111) with the fundamental and 27th harmonic of a Ti:sapphire femtosecond laser results in energy modulation of the photoelectron spectrum.

We report the first observation of the laser-assisted photoelectric effect from a solid surface. By illuminating a Pt(111) sample simultaneously with ultrashort 1.6 eV and 42 eV pulses, we observe sidebands in the extreme ultraviolet photoemission spectrum. The magnitude of these sidebands as a function of time delay between the laser and extreme ultraviolet pulses represents a cross-correlation measurement of the extreme ultraviolet pulse. This effect promises to be useful to extend extreme ultraviolet pulse duration measurements to higher photon energies, as well as opening up femtosecond-to-attosecond time-scale electron dynamics in solid and surface-adsorbate systems.

We report on the experimental demonstration of laser cooling of a C3+ion beam performed at the ESR (GSI) at an energy of E = 1.47 GeV. The decelerating laser force of one Doppler-tuned UV laser beam was counteracted by moderately bunching the beam. This versatile scheme lead to longitudinally ‘space-charge dominated’ beams with an unprecedented momentum spread of Δp/p ≈ 10–7. Concerning beam energy and charge state of the ion, the experiment depicts an important step from the field of laser cooling of ion beams at low energies toward the laser cooling scheme proposed for relativistic beams of highly charged heavy ions at the future GSI facility FAIR.

We report on first laser cooling studies of bunched beams of triply charged carbon ions stored at an energy of 1.46 GeV at the ESR (GSI). Despite for the high beam energy and charge state laser cooling provided a reduction of the momentum spread of one order of magnitude in space‐charge dominated bunches as compared to electron cooling. For ion currents exceeding 10 μA intra‐beam‐scattering losses could not be compensated by the narrow band laser system presently in use. Yet, no unexpected problems occurred encouraging the envisaged extension of the laser cooling to highly relativistic beams. At ESR, especially the combination with modest electron cooling provided three‐dimensionally cold beams in the plasma parameter range of unity, where ordering effects can be expected and a still unexplained signal reduction of the Schottky signal is observed.

Mechanical amplification by stimulated phonon emission is reviewed in two optomechanical systems: an optical resonator system and a trapped-ion system. The physics of these devices and observations of threshold and line narrowing are presented.