Double Tunneling: An Overlooked Quantum Effect in Anionic Molecular Clusters

Simultaneous tunneling of an electron and nuclei in hydrogen-bonded molecular clusters which support a dipole bound electron is reported. A whole class of systems including hydrogen halide and water dimer anions is predicted to exhibit this effect. A measurable signature of double tunneling is a strong reduction of the tunneling splitting compared to the neutral cluster. Quantum mechanical calculations give for the hydrogen fluoride dimer anion a ground-state tunneling splitting of 114 MHz, while for the first excited vibrational state the tunneling splitting reaches 4.20 GHz.