The direct access to atomic transitions between close by quantum states employing standard spectroscopic methods is often limited by the size of the necessary radio-frequency cavities. Here we report on a new tool for fundamental spectroscopy measurements that can overcome this shortcoming. For this, a Sona transition unit was used, i.e., two opposed solenoidal coils that provide an oscillating field in the rest frame of the through-going atomic beam. In this way, we were able to control the induced photon energy down to 10 neV or $$f \sim $$
f
∼
MHz. The tuneable parameter is the velocity of the atomic beam. For illustration of the method, we report a measurement of the hyperfine splitting energies between the substates with $$F=1$$
F
=
1
and $$m_F = -1, 0, +1$$
m
F
=
-
1
,
0
,
+
1
of $$2S_{1/2}$$
2
S
1
/
2
metastable hydrogen atoms as function of a magnetic field.
Graphic Abstract