Vibrationally Assisted Below-Threshold Ionization

We perform time-resolved UV pump (4.77 eV) and VUV probe (7.94 eV) measurements of internal conversion of 1,3-cyclohexadiene (CHD). Our measurements reveal a substantial ionization of the "hot" ground state following internal conversion despite the fact that our probe photon energy is below the ionization potential (8.21 eV). With the aid of electronic structure calculations, we interpret our results in terms of vibrationally assisted below-threshold ionization, where vibrational energy is converted to electronic energy. The effect relies on both having vibrational modes which allow for this vibrational-electronic coupling and exciting these modes during the internal conversion. We contrast our measurements in CHD with another similar molecule, cis, cis-1,3-cyclooctadiene (cis, cis-COD), for which we do not see the effect.