Probing the 1²Σ_u⁺ cation state by low-energy dissociative ionization of Na₂

We have measured highly resolved time-of-flight spectra and concomitant angular distributions of slow Na⁺ ions produced by near-threshold dissociative ionization of Na₂. Using double-resonance excitation via the A¹Σ_u⁺ state, Na₂ molecules are promoted to rotational levels J=14 or 16 and vibrational levels between v=50 and 69 of the 4¹Σ_g⁺ state. Then, a third photon—from the second of two tunable dye lasers employed—causes the transition into the double continuum associated with the 1²Σ_u⁺ potential of the Na₂⁺ cation. A detailed Monte Carlo simulation is employed to study the dynamics of this dissociative ionization process in detail. Based on this approach, we were able to subject recent calculations of this 1²Σ_u⁺ potential to experimental scrutiny. We find our data to be compatible with a zero-energy crossing near 8.8 Å and a well depth of about 70 cm^-1, which agrees with most of the published calculations. Angular distributions for the vibrational level v=60 are also reproduced well using the same potential parameters and an anisotropy parameter β=2 modified by a semiclassical argument to account for parent molecule rotation.