Dissociative Ionization of Na₂ via Repulsive Rydberg States: Elucidating Femtosecond Dynamics with Nanosecond Lasers

Abstract

We have studied the dissociative ionization behavior of Na2 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 Na2(+) 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(+)/Na2(+) 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 1(2)Sigma(u)(+) first excited-state potential of Na2(+). 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(+)/Na2(+) 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.

Trends in Time-of-Flight Parameters with Application to Autoionization Lifetime Measurements

“…The second half of the thesis describes a specific experiment that is sensitive to the autoionization lifetime of repulsive Rydberg states of neutral sodium dimer that asymptotically approach excited neutral atomic states. These Rydberg states have been shown previously to contribute significantly in the production of Na + ions through dissociative autoionization processes [7][8][9]. Specifically, the Rydberg state whose separated-atom limit is the neutral 3s + 5d atomic state will be studied.…”

“…The heart of this experiment is the autoionization event that can occur after photo-excitation into these Rydberg states by either two blue photons (355 nm) or three green photons (532 nm) from a pulsed nanosecond nd:YAG laser. The Rydberg states are repulsive, and as the molecule falls apart there is a probability of spontaneously ejecting an electron [7].…”

“…The experimental apparatus has been discussed in detail in several previous theses and papers [7,10,11]. Therefore, only a rough description of the experiment is provided.…”