Thermal and hyperthermal collision-energy depositions of transition metal-benzene sandwich complexes onto a self-assembled n-octadecanethiol monolayer

Gas-phase synthesized transition metal-benzene sandwich complexes of M(benzene)₂ (M = Ti, V, and Cr) are soft-landed onto a self-assembled monolayer of n-octadecanethiol (C₁₈-SAM) at a collision energy of 10-20 eV. The resulting adsorption states and thermal desorption kinetics of the soft-landed complexes are studied with infrared reflection absorption spectroscopy and temperature-programmed desorption. The complexes keep their native sandwich structure intact on the SAM substrate even after the "hyperthermal" deposition event. The soft-landed complexes are oriented with their molecular axes largely tilted off the surface normal of the SAM substrate and exhibit unusually large desorption activation energies (E _d = ∼130 kJ/mol). For comparison, thermal deposition (∼25 meV) of Cr(benzene)₂ vapor onto the C₁₈-SAM, carried out using a physical vapor deposition technique, showed that the complexes are weakly physisorbed (E_d = ∼70 kJ/mol) on the SAM with a random orientation. Only a hyperthermal collision event allows the incident complexes to penetrate into the SAM matrix. The desorption of the embedded complexes in the SAM is then suppressed to around room temperature and may be associated with the crystal-rotator phase transitions of the SAM matrix.