# Laboratory for Particle Physics (LTP)

### LTP Colloquium

##
'Extreme' Spectroscopy on Helium and Helium Ions

Thursday, May 23, 2024, 16:00

WBGB/019

Kjeld Eikema, LaserLaB Vrije Universiteit Amsterdam, The Netherlands

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Abstract:
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Precision spectroscopy of atoms and molecules is widely used for tests
of elements of the Standard Model, such as quantum electrodynamics
(QED), and for determinations of fundamental constants and nuclear
charge radii. This requires 'simple' systems that can be calculated
accurately, and in Amsterdam we perform spectroscopy on several of them.
I will focus on helium and singly-ionized helium; both provide
significant challenges and extremes.

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For the neutral helium experiment we (laser) cool *^{3}He and
^{4}He to quantum degeneracy and trap it in a focused laser beam
at the 'magic' wavelength of 320 nm. The resulting ultra-cold Fermi gas
(^{3}He) and Bose-Einstein condensate (^{4}He) show
widely different quantum behavior and spectroscopy. In both isotopes we
measured the doubly-forbidden 2 ^{3}S_{1} ‒ 2
^{1}S_{0} transition at 1557 nm with better than 200 Hz
accuracy (~1:10^{12}), from which we can deduce a charge radius
difference between the helion and alpha particle with unprecedented
accuracy [1]. Interestingly, an evaluation recently by the CREMA
collaboration of the same charge radius difference from muonic helium
ion measurements [2] leads to a value that deviates by 3.6 combined
sigma.

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The other experiment is based on spectroscopy of the 1S-2S transition in
*^{4}He^{+}, which requires two-photon excitation
involving extreme ultraviolet. This is difficult, but a precision at the
kHz level (1:10^{13}) would enable an independent check of the
Rydberg constant, an improved determination of the absolute alpha
particle charge radius, or a test of higher-order QED. We recently
demonstrated optical excitation of the 1S-2S transition in
He^{+} for the first time by combining an intense ultrashort 790
nm laser pulse with its 25^{th} harmonic at 32 nm [3]. We also
simulated the excitation process with Time-Domain Schrödinger
Equation calculations. The 790 nm intensity of 10^{14}
W/cm^{2} boosts the transition probability strongly, but also
leads to extreme light-induced transition shifts of tens of THz.
Surprisingly, this will not hamper future precision measurements! I will
explain why and give an update on the current status of the experiments.

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[1] Y. van der Werf et al., arXiv:2306.02333
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[2] K. Schuhmann et al., arXiv:2305.11679
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[3] E.L. Gründeman et al., arXiv:2308.13271
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