Dissociative ionization of Na2 via repulsive Rydberg states:: Elucidating femtosecond dynamics with nanosecond lasers

We have studied the dissociative ionization behavior of Na-2 molecules using two-color, three photon optical-optical double resonance enhanced excitation via the A(1)Sigma(+)(u) and the 2(1)Pi(g). states. Excess energy ranges from about 150 to about 1500 cm(-1) above threshold for dissociative ionization into ground-state Na and Na+. Slow atomic Na+ fragments and Na-2(+) molecular ions are detected using a linear time-of-flight spectrometer operated in low field extraction, core sampling mode. To explain the observed energy dependence of the Na+/Na-2(+) branching ratio, we introduce a semiclassical model for the underlying decay dynamics. Franck-Condon overlap densities for bound-free transitions starting in 2(1)Pi(g), vibrational levels indicate that atomic Na+ fragments are primarily produced via Rydberg states, with principal quantum number n between 5 and 12, converging to the repulsive l(2)Sigma(+)(u) first excited-state potential of Na-2(+). Dynamics along these Rydberg curves involves competition between electronic (autoionizing) and nuclear (dissociative) degrees of freedom. Within the model, the autoionization lifetime tau(auto) is the only one free parameter available to fit calculated Na+/Na-2(+) branching ratios as a function of excess energy to the observed values. The lifetime is assumed to be the same multiple c of the Bohr period of each Rydberg potential. A chi(2)-minimization procedure yields, for the range of principal quantum numbers involved, a most likely value of c = 1.5 +/- 0.3, implying that on average the Rydberg electron completes only 1 to 2 orbits before interaction with the excited core electron leads to autoionization.