Collinear apparatus for laser spectroscopy and applied science
The COALA apparatus (see Fig. 1) has been designed and already successfully tested for accurate laser-based high voltage measurements. In future, the facility is supposed to be used for highly precise measurements of absolute transition frequencies in atoms to test atomic theories and to determine absolute charge radii of atomic nuclei.
By quasi-simultaneous measurement with two lasers, one superposed with the ion beam in collinear and one in anticollinear direction (see Fig. 2), the voltage dependency in the relation between transition frequency f0 and laser frequency
fLcoll = f0 (1 – β) γ,
fLanticoll = f0 (1 + β) γ respectively, with γ=1/(1 – β2)1/2
resulting from excitation with a single laser can be eliminated and the square of the absolute frequency results from the product of both laser frequencies:
fLcoll fLanticoll= f02(1- β2)/(1- β2)= f02.
Combined with a highly precise measurement of the laser frequencies with an optical frequency comb the most precise measurement of the absolute transition frequency of the D1-line of the stable isotope 138Ba was recently performed within a campaign to measure the isotope shifts of stable barium isotopes . The goal of these measurements was to study the ratio of the field shift coefficients in the isotope shift, because in the stable calcium isotopes a discrepancy between theoretical predictions and experiment was recently found .
In a future stage of expansion the apparatus will be equipped with an electron beam ion source (EBIS)  which will produce highly-charged ions of a variety of elements. This will allow experimental tests of atomic ab-initio theories and the determination of absolute nuclear charge radii.
 P. Imgram, Master Thesis, TU Darmstadt (2018), article in preparation
 C. Shi et al., Applied Physics B 123, 2 (2016)