Solvent interaction with the (2p3s) Rydberg state of hexamethylenetetramine: Energetics and relaxation dynamics

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Abstract
The (1+1) mass resolved excitation spectra are reported for the (2p3s)←(2p)2Rydberg transition of the tricyclic, high symmetry molecule hexamethylenetetramine [HMT (C6H12N4)] and its van der Waals clusters. The solvent molecules employed include both nonpolar (Ar, CH4) and polar (NH3, CH3OCH3) species. HMT and its clusters are generated and cooled in a supersonic expansion. The observed electronic transition is assigned as T 2←A 1 within the T d print group. A transition blue shift of 52 cm−1 for each Ar atom and 65 cm−1 for each methane molecule in the HMT van der Waals cluster is characterized. These shifts are caused by an excited state repulsive interaction between the excited Rydberg electron and the closed shell solvent which reduces the attractive dispersion interaction between the HMT and nonpolar solvent species in the van der Waals cluster. A transition red shift of more than 600 cm−1 for NH3 and CH3OCH3 solvent/HMT clusters is observed; this large increase in interaction energy for the excited Rydberg state of HMT with respect to the ground state of HMT is associated with the delocalization of the excited electron into available (virtual) Rydberg orbitals of the solvent molecules. The interaction is characterized as an electron transfer interaction. These results and assignments are consistent with previously reported ones for DABCO/solvent clusters. Relaxation dynamics of excited HMT and its clusters are investigated via a pump/probe (ionization) technique. Relaxation of the clusters is dominated by an intersystem crossing mechanism resulting in an excited state singlet lifetime of ∼5 ns compared to a bare molecule HMT excited state lifetime of ∼1.0 μs. A triplet state of HMT lies 255 cm−1 below the first excited singlet Rydberg state as determined by two‐color threshold ionization studies.