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Light from the 87Rb (rf
discharge) lamp passes through the 85Rb filter cell and into the
absorption cell, which contains 87Rb gas plus a buffer gas. The 87Rb lamp emits wavelengths
corresponding to both the 87Rb F = 1 and F = 2 transitions. The 85Rb filter cell absorbs
more of the F=2 transition light. The light which passes through the filter
cell is absorbed by the 87Rb
F=1 state, the excited atoms relax to both the F=1 and F=2 states, but
the F=1 states are excited again; the F=2 state is overpopulated; the applied
6.8 GHz microwave converts F=2 back to F=1, which provides more atoms to
absorb light. The microwave at the
correct resonance frequency causes increased light absorption, i.e., a (<
1%) dip, in the light detected by the photocell. The microwave frequency is locked to
photocell detection dip, thus the atomic transition frequency controls the
microwave frequency, i.e., the frequency of the crystal oscillator.
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The absorption cell contains Rb gas at ~10-6
torr and an inert buffer gas at ~1 torr. The Rb atom oscillation lifetime is
limited by collisions to ~10-2 s; the atomic resonance linewidth
~100 Hz; Q ~5 x 107. The
buffer gas, a mixture of positive (e.g., N2) and negative (e.g.,
Ar) pressure-shift gases, provides zero temperature coefficient at some
temperature in the operating temperature range, and confines Rb atoms to a
small region to reduce wall-collisions and first order Doppler effects.
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F. G. Major, The
Quantum Beat - The Physical Principles of Atomic Clocks, Springer-Verlag,
1998.
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J. Vanier and C.
Audoin, The Quantum Physics of Atomic Frequency Standards, ISBN
0-85274-434-X, Adam Hilger, 1978.
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