We use a diode laser locked to a Rb transition as the frequency reference, along with a scanning Michelson interferometer, to make precise frequency measurements of atomic energy levels. The high precision is possible because the Rb reference frequency is known with submegahertz accuracy. We measured the frequencies of the D lines in K and obtained the following values: 12 985.169 60(20) cm-1 for the state line), 13 042.875 95(21) cm-1 for the state line), and 57.706 35(24) cm-1 for the fine-structure interval. The errors represent approximately an order-of-magnitude improvement over tabulated values.
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