Atomic or molecular beam frequency standard with optical pumping and open resonator

DETAILED DESCRIPTION Having described the invention I claim: 1.
An atomic (molecular) resonance apparatus, particularly for the generation of a standard frequency, including: a source capable of generating a slow atomic beam of atoms (molecules) of one type, directed almost vertically upward in a fountain configuration, a single electromagnetic open resonator through which said atomic (molecular) beam passes a first time going up and a second time coming back down, optical pumping means to produce transitions of the beam atoms (molecules) into a first selected level or energetic state, before said beam passes through said resonator, an electromagnetic field generator device, connected to said resonator capable of generating inside said resonator a spatial distribution of oscillating electromagnetic field at a frequency substantially correspondent to the energy difference between said first level or energetic state and a second level or energetic state of the beam atoms (molecules), detecting means capable of providing an electric signal indicative of the number of beam atoms (molecules) which inside said resonator have undergone a transition from said first to said second level or energetic state, and, frequency control means connected to said detecting means and said generator, and capable of steering the frequency of said generator in such a way as to maximize the number of atoms (molecules) which undergo a transition from said first to said second level or energetic state while passing through said resonator.
2.
Apparatus according to claim 1 including also selecting means capable of allowing through said resonator only the beam atoms (molecules) which have an initial velocity between a first and a second selected value.
3.
Apparatus according to claim 2, in which said selecting means are also capable of determining the cross-section of the beam crossing said resonator, and the positioning of said beam inside the resonator.
4.
Apparatus according to claim 1, in which said electromagnetic resonator is positioned at the height at which a selected velocity group of said atomic (molecular) beam reaches the top of its parabolic trajectory