[time-nuts] Why Cesium and Rubidium only

[Dr Bruce Griffiths]

Don Collie wrote:
> Please excuse my ignorance [I marvel/wonder at some of the essotevric 
> comments on this group], but why are atomic clocks reliant on these two rare 
> elements? - why not mercury, or water vapor, they are a lot easier to find.
> Ignorantly yours,...................................Don Collie jnr. 
>
>   
As far as water is concerned hydrogen masers are in use, but they are 
not beams standards and the frequency is affected by cavity pulling and 
energy wall shifts when the hydrogen atoms collide with the walls.

Water masers exist in interstellar space and have strong emissions at 
22GHz, but as far as I am aware no one has tried to construct a 
laboratory equivalent using this emission line as a frequency standard.

Hydroxyl ion masers (15GHz) also exist interstellar space but this 
transition has not been exploited as frequency standard.

Mercury ion standards have been built but are not available off the 
shelf, they exhibit very high stability but the frequency is a lot 
higher (~40.5 GHz) than either the caesium (9.192GHz) or rubidium 
(6.8GHz) resonances in use.
Natural mercury is a mixture of the isotopes Hg(196), Hg(198), Hg(199), 
Hg(200), Hg(201), Hg(202), and Hg(204).
Only the isotope Hg(199) has been used in frequency standards.
http://horology.jpl.nasa.gov/papers/mercuryrpt.pdf

Historically the alkali metal microwave transitions (Caesium, Rubidium) 
were discovered in a readily accessible region of the microwave spectrum 
and found to be useful
as frequency standards, Caesium atom state selection is readily done 
with a magnet and simple hot wire detectors can be used to detect the 
atoms. Optical pumping using a noncoherent discharge lamp can be used to 
pump Rubidium atoms into the upper energy state, allowing the 
development of Rubidium standards well before suitable laser sources 
were available.

The atomic transition has to have an adequately high Q, its frequency 
has to lie with a readily accessible region of the spectrum, a 
relatively inexpensive means of pumping or state selection has to be 
available before commercial development is feasible. Size weight and 
power consumption are also issues.

Bruce