The spectroscopy of Rydberg to Rydberg transitions in I2 and Br2 investigated by vibrationally induced autoionization

Dipole-allowed transitions from vibrationally selected members of the lowest nd Rydberg state cluster in I2 (n = 5) and Br2 (n = 4) to autoionizing Rydberg series based on vibrationally excited states of the core are investigated. The photoelectron production spectrum is recorded using delayed field extraction to detect free electrons released by autoionization with up to a few hundred cm−1 of energy that are subsequently trapped for ∼1μs in the plasma formed at the laser focus. The final state principal quantum numbers typically range from n ∼ 20–35 and both p and f series can be identified. When one of these core-excited Rydberg state energies coincides with the ionization threshold of a lower vibrational state to within a few cm−1, a marked increase in the ZEKE-PFI signal is observed, leading to non-Franck–Condon behaviour. Probing the same Rydberg states based on the spin–orbit excited core reveals preferential autoionization by electron exchange rather than by vibronic coupling in the p series.