Zoltan Harman

High-precision measurement of the atomic mass of the electron

The quest for the value of the electron’s atomic mass has been the subject of continuing efforts over the past few decades. Among the seemingly fundamental constants that parameterize the Standard Model of physics and which are thus responsible for its predictive power, the electron mass me is prominent, being responsible for the structure and properties of atoms and molecules. It is closely linked to other fundamental constants, such as the Rydberg constant R∞ and the fine-structure constant α (ref. 6). However, the low mass of the electron considerably complicates its precise determination. Here we combine a very precise measurement of the magnetic moment of a single electron bound to a carbon nucleus with a state-of-the-art calculation in the framework of bound-state quantum electrodynamics. The precision of the resulting value for the atomic mass of the electron surpasses the current literature value of the Committee on Data for Science and Technology (CODATA) by a factor of 13. This result lays the foundation for future fundamental physics experiments and precision tests of the Standard Model.

Direct High-Power Laser Acceleration of Ions for Medical Applications

Theoretical investigations show that linearly and radially polarized multiterawatt and petawatt laser beams, focused to subwavelength waist radii, can directly accelerate protons and carbon nuclei, over micron-size distances, to the energies required for hadron cancer therapy. Ions accelerated by radially polarized lasers have generally a more favorable energy spread than those accelerated by linearly polarized lasers of the same intensity.

An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem

Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects, including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 ångströms. The Fe xvii spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe xvii line is generally weaker than predicted. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions, fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser; our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6σ from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.

X-Ray Resonant Photoexcitation: Linewidths and Energies of Kα Transitions in Highly Charged Fe Ions

Photoabsorption by and fluorescence of the Kα transitions in highly charged iron ions are essential mechanisms for x-ray radiation transfer in astrophysical environments. We study photoabsorption due to the main Kα transitions in highly charged iron ions from heliumlike to fluorinelike (Fe24+ to Fe17+) using monochromatic x rays around 6.6 keV at the PETRA III synchrotron photon source. Natural linewidths were determined with hitherto unattained accuracy. The observed transitions are of particular interest for the understanding of photoexcited plasmas found in x-ray binary stars and active galactic nuclei.

X-ray quantum optics

Quantum optics with X-rays has long been a somewhat exotic activity, but it is now rapidly becoming relevant as precision x-ray optics and novel X-ray light sources, and high-intensity lasers are becoming available. This article gives an overview of the current state of the field and an outlook to future prospects.

Dense monoenergetic proton beams from chirped laser-plasma interaction.

Interaction of a frequency-chirped laser pulse with single protons and a hydrogen gas target is studied analytically and by means of particle-in-cell simulations, respectively. The feasibility of generating ultraintense (10(7) particles per bunch) and phase-space collimated beams of protons (energy spread of about 1%) is demonstrated. Phase synchronization of the protons and the laser field, guaranteed by the appropriate chirping of the laser pulse, allows the particles to gain sufficient kinetic energy (around 250 MeV) required for such applications as hadron cancer therapy, from state-of-the-art laser systems of intensities of the order of 10(21) W/cm(2).

Broadband high-resolution X-ray frequency combs

An optical-frequency-comb laser manipulating a dipole response can imprint the comb on an excited transition with a high photon energy. The concept can be implemented using existing X-ray technology.

Nuclear Shape Effect on the g Factor of Hydrogenlike Ions

The nuclear shape correction to the g factor of a bound electron in the 1S state is calculated for a number of nuclei in the range of charge numbers from Z=6 up to Z=92. The leading relativistic deformation correction has been derived analytically, and also its influence on one-loop quantum electrodynamic terms has been evaluated. We show the leading corrections to become significant for mid-Z ions and for very heavy elements to even reach the 10(-6) level.

Nuclear excitation by electron capture followed by fast x-ray emission

Abstract The resonance strength of the two-step process of nuclear excitation by electron capture followed by γ decay of the nucleus can be significantly increased in highly charged ions if the resonant capture proceeds via an excited electronic state with subsequent fast x-ray emission. For fully ionized 238 92 U and 232 90 Th, the x-ray decay stabilizes the system against internal conversion of the captured electron, with an increase of both nuclear lifetimes and resonance strengths of up to two orders of magnitude compared with the case when occupied atomic orbitals prevent the x-ray de-excitation. Applications of this effect to the measurement of the not yet experimentally observed nuclear excitation by electron capture and to dense astrophysical plasmas are discussed.

QED Theory of the Nuclear Magnetic Shielding in Hydrogenlike Ions

The shielding of the nuclear magnetic moment by the bound electron in hydrogenlike ions is calculated ab initio with inclusion of relativistic, nuclear, and quantum electrodynamics (QED) effects. The QED correction is evaluated to all orders in the nuclear binding strength parameter and, independently, to the first order in the expansion in this parameter. The results obtained lay the basis for the high-precision determination of nuclear magnetic dipole moments from measurements of the g factor of hydrogenlike ions.