Multiphoton ionization of cesium through resonant dissociative states of Cs₂

The multiphoton excitation of cesium through potentially resonant continuum molecular states is investigated over the 6200-5000-Å wavelength region with a tunable dye-laser source having a 0.06-0.08-Å linewidth and a space-charge ionization detector sensitive to a few ions per second. Conditions are established under which the lifetime of the resonant intermediate state against photoexcitation exceeds the lifetime against dissociation. Single- and double-photon resonances occur for the same wavelength and give absorption maxima corresponding to the line spectrum from the intermediate state following dissociation. These lines are modulated in amplitude as a function of wavelength by the more slowly varying absorption resonance from the initial to the intermediate continuum state. As a consequence, the resulting dispersion curve for two-photon absorption in cesium shows what appear to be resonant intermediate dπ(³Π_g) terms dissociating to give a 5²D atom and resulting in the strong development of features corresponding to the fundamental series (5²D→n²F) of atomic cesium in absorption. Components to n=50 were observed and recorded to a precision sufficient to determine the average quantum defect for F states to be 〈n-n^*〉=0.033 in the limit of large n. More complex structure is attributed to pπ(³Π_g) and pσ(¹Σ_u) terms dissociating to give a 6²P_3/2 atom and resulting in the development of the n=8 to 32 components of the 6²P_3/2→n²D_3/2,5/2 part of the diffuse series in absorption. Terms dissociating into 6²P_1/2 were found only at higher photon energies (∼2.27 eV) corresponding to the n≥10 members of the 6²P_1/2→n²D_3/2 series.