XUV frequency comb metrology on the ground state of helium

Abstract

The operation of a frequency comb at extreme ultraviolet (XUV) wavelengths based on pair-wise amplification and nonlinear upconversion to the 15th harmonic of pulses from a frequency comb laser in the near-infrared range is reported. Following a first account of the experiment [Kandula et al., Phys. Rev. Lett. 105, 063001 (2010)], an extensive review is given of the demonstration that the resulting spectrum at 51 nm is fully phase coherent and can be applied to precision metrology. The pulses are used in a scheme of direct-frequency-comb excitation of helium atoms from the ground state to the 1s4p and 1s5p 1P_1 states. Laser ionization by auxiliary 1064 nm pulses is used to detect the excited state population, resulting in a cosine-like signal as a function of the repetition rate of the frequency comb with a modulation contrast of up to 55%. Analysis of the visibility of this comb structure yields an estimated timing jitter between the two upconverted comb laser pulses of 50 attoseconds, whch indicates that extension to even shorter wavelengths should be feasible. The helium metrology investigation results in transition frequencies of 5740806993(10) MHz and 5814248672(6) MHz for excitation of the 1s4p and 1s5p 1P_1 states, respectively. This constitutes the first absolute frequency measurement in the XUV, attaining unprecedented accuracy in this windowless part of the electromagnetic spectrum. From the measured transition frequencies an eight-fold improved 4He ionization energy of 5945204212(6) MHz is derived. Also a new value for the 4He ground state Lamb shift is found of 41247(6) MHz. This experimental value is in agreement with recent theoretical calculations up to order mα^6 and m^2/(Mα^5), but with a six times higher precision, therewith providing a stringent test of quantum electrodynamics in bound two-electron systems.

18 pages, 13 figures

References in corpus (13)

• Improved Measurement of the Hydrogen 1S - 2S Transition Frequency
• Extreme ultraviolet frequency comb metrology
• Frequency metrology in quantum degenerate helium: Direct measurement of the 2 3S1 - 2 1S0 transition
• Theoretical energies of low-lying states of light helium-like ions
• ${\alpha }^4$ Ry corrections to singlet states of helium
• Proton size anomaly
• Two-Loop Bethe Logarithms
• QED effects in molecules: test on rotational quantum states of H${}_2$
• Direct frequency comb spectroscopy of trapped ions
• Relativistic corrections of mα^6(m/M) order to the hyperfine structure of the H_2^+ molecular ion
• Third Zemach Moment of the Proton
• QED confronts the radius of the proton
• Proton Radius, Darwin-Foldy Term and Radiative Corrections

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• Laser Probing of Neutron-Rich Nuclei in Light Atoms
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• Hydrogen molecular ions for improved determination of fundamental constants
• Theory of the Lamb shift and Fine Structure in muonic ${}^4\mathrm{He}$ ions and the muonic ${}^3\mathrm{He}{-}^4\mathrm{He}$ Isotope Shift
• High-precision measurements of $n=2\to n=1$ transition energies and level widths in He- and Be-like Argon Ions
• All-solid-state VUV frequency comb at 160 nm using multi-harmonic generation in a non-linear femtosecond enhancement cavity
• Experiments towards resolving the proton charge radius puzzle
• Atomic structure calculations of helium with correlated exponential functions
• Numerical evaluations of the isotopic shifts and lowest-order QED corrections for the ground $1^{1}S-$states of the ${}^3$He and ${}^4$He atoms
• Deep survey of millimeter RRLs towards the planetary nebulae IC 418 and NGC 7027