We have measured the ratio of the electric quadrupole ( E 2 ) to magnetic dipole ( M 1 ) transition amplitude within the 6 P 1 / 2 → 6 P 3 / 2 transition in atomic thallium. Defining χ to be this E 2 to M 1 amplitude ratio, we find that χ = 0.2387 ( 10 ) ( 38 ) , where the first error is statistical and the second represents a combined systematic error. In addition to providing a stringent test of theoretical wave-function calculations in thallium, accurate knowledge of this quantity is essential for existing and future measurements of parity nonconserving optical rotation on this same 1283-nm line in thallium.
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