Trapped Ion Oscillation Frequencies as Sensors for Spectroscopy

Abstract

The oscillation frequencies of charged particles in a Penning trap can serve as sensors for spectroscopy when additional field components are introduced to the magnetic and electric fields used for confinement. The presence of so-called “magnetic bottles” and specific electric anharmonicities creates calculable energy-dependences of the oscillation frequencies in the radiofrequency domain which may be used to detect the absorption or emission of photons both in the microwave and optical frequency domains. The precise electronic measurement of these oscillation frequencies therefore represents an optical sensor for spectroscopy. We discuss possible applications for precision laser and microwave spectroscopy and their role in the determination of magnetic moments and excited state life-times. Also, the trap-assisted measurement of radiative nuclear de-excitations in the X-ray domain is discussed. This way, the different applications range over more than 12 orders of magnitude in the detectable photon energies, from below μeV in the microwave domain to beyond MeV in the X-ray domain.

Resistive and sympathetic cooling of highly-charged-ion clouds in a Penning trap

“…These effects have been carefully discussed in [20][21][22]. In traps like the present one, the dominant contributions to an energydependent shift of the axial frequency come from higher-order dependences of the axial trapping potential on the axial and radial coordinates (including mixed terms), measured by the coefficients C 4 and C 6 as defined in [20,21].…”

“…If terms of orders other than z 2 are present, the oscillation frequency becomes energy-dependent, as has been described in detail in [20,22]. In real traps, this is always the case and usually efforts are undertaken to minimize these effects by appropriate choice of the trap geometry and the applied voltages [21].…”

Switchable magnetic bottles and field gradients for particle traps

“…The other important aspect is the geometry of the field itself which can give rise to desired effects, for example magnetic gradient forces on dipoles [32] as used in time orbiting potential (TOP) traps [33], in Ioffe-Pritchard traps [13], and various magnetic microtraps [14,15,16,17]. Quadratic distortions of the magnetic field in the shape of a so-called 'magnetic bottle' are valuable to Penning-trap experiments, for example when the continuous Stern-Gerlach effect [34] is used for measurements of magnetic moments of unbound electrons [35,36], unbound protons and antiprotons [37], and bound electrons in highly-charged ions [38,39,40,41,42,43,44], or when defined couplings amongst oscillatory degrees of freedom in the Penning trap are used for spectroscopic purposes [45,46]. Nested Penning traps [47] can benefit from introduced magnetic field gradients, regarding for example Penning-Ioffe traps as used in antihydrogen research [48].…”

“…Hence, electronic detection of the ion motions tends to become more difficult in low fields. This can be an issue for experiments like the ones of Stern-Gerlach type [38,39,40,41,42,43,44], but not necessarily for optical experiments like the ones discussed in [45,46,68,69].…”

“…An artificial electric asymmetry U a of a trap as presented in the previous section can be combined with a magnetic bottle to allow a measurement of optical transition rates, using a purely electronic measurement, i.e. without optical detection [45]. This can find application in spectroscopy experiments like the ones discussed in [45,46], in [68,69] and [76], and would be an interesting addition to the Stern-Gerlach experiments of the kind [38,39,40,41,42,43,44] when an ion with a suitable optical transition is considered.…”