Probing Torsional Tunneling in Al ₂ H ₄ via Anion Photoelectron Spectroscopy

This study investigates the tunneling dynamics associated with the torsional transition in the Al 2 H 4 molecule by photodetaching its anionic counterpart, Al 2 H 4 – . The stable structure of Al 2 H 4 – exhibits D 2 h symmetry, whereas neutral Al 2 H 4 adopts a D 2 d symmetric configuration. According to the Franck–Condon principle, the tunneling dynamics across the torsional barrier in Al 2 H 4 can be directly probed through the high-resolution photoelectron spectroscopy of Al 2 H 4 – anions. Our Franck–Condon factor simulations, based on solutions to the Mathieu equation that incorporate tunneling effects, successfully reproduce the experimental observations. The approach presented here is also applicable to other molecular systems involving intramolecular torsion.