Speaker
Description
Spectroscopy of atoms and molecules have played a central role in our understanding of physics. It has also become increasingly important to measure the fundamental physical constants such as the fine-structure constant, the Rydberg constant, the proton-electron mass ratio, or the charge radius of the proton, deuteron and the alpha particle. In order to do so, a single type of measurement is typically not enough. Instead, measurements are performed (either with spectroscopy or other methods) in many different systems to construct a consistent picture of the laws of physics, and the fundamental constants that are required for them.
We pursue several targets for this purpose, 1S-2S spectroscopy of trapped singly-ionized helium, a determination of the ionization potential of molecular hydrogen, and spectroscopy of ultra-cold metastable neutral helium. In particular with He+ one can test the value for the Rydberg constant, the charge radius of the alpha particle, or test higher-order QED. Both He+ and molecular hydrogen require light sources at deep-ultraviolet or shorter wavelengths for excitation from the ground state. Frequency comb generation in the deep- and extreme-UV has been demonstrated based on enhancement cavities, which can be used for direct frequency comb excitation. However, we use and developed a different method, Ramsey-comb spectroscopy (RCS), that has been highly successful for precision spectroscopy at short wavelengths. It is based on direct excitation with only two amplified and upconverted frequency comb laser pulses to generate a form of Ramsey fringes. The required phase and timing control of the light pulses is provided by an ultra-stable fiber frequency comb, and the short pulses enable amplification to high peak power for efficient frequency upconversion. Ramsey signals recorded at two or more inter-pulse delays (spaced at multiples of the comb repetition time) are used to record the phase evolution of the excitation signal as a function of time, from which the transition frequency can be accurately determined.
In the talk I will introduce our goal of precision spectroscopy of the 1S-2S two-photon transition (using 32 nm and 790 nm) of trapped and sympathetically cooled He+ ions. I will discuss the principle of RCS and harmonic upconversion to generate the wavelengths required for He+ excitation, and illustrate it with our latest progress of spectroscopy of the X-EF transition in para-hydrogen at 202 nm, and xenon at 110 nm using high-harmonic generation (HHG). In the last part I will present the current status of the He+ experiment, which involves setting up a new vacuum system, a planer ion trap with electronics and control software, and a new low phase-noise Ramsey-comb laser system specifically for the He+ experiment.
The work is funded by an ERC Advanced grant no. 695677 and a NWO Program grant 16MYSTP.
