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.

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.