A method is described which allows the detection of atomic quantum beats or wave-packet evolution in low-lying states, phenomena which are usually difficult to observe. The excitation method consists of a phase-sensitive two-pulse technique. Detection of the excited-state population after the two pulses is achieved by monitoring rubidium dimer ions which are formed in the associative ionization reaction, Rb*(n)p+Rb ÆRb+2+e-. The dimer ion yield as a function of the wavelength showed strong features, which coincided with the 8p to 15p atomic resonances. The two-pulse technique revealed atomic fine-structure quantum beats due to the coherent excitation of the np2P1/2,3/2 fine-structure states of the atom.

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Journal Phys. Rev. A
Christian, J. F, Snoek, L. C, Clement, S. G, & van der Zande, W. J. (1996). Application of associative ionization to the observation of quantum beats in low-lying atomic resonances. Phys. Rev. A, 53, 1894–1899.