Energetic Ion-Stimulated Desorption of Physisorbed Molecules

Abstract

We have conducted an experimental investigation into molecular desorption stimulated by 8 keV Ar + ions. The investigated systems are comprised of aromatic molecules (benzene and phenol) adsorbed to an Ag(111) surface. Resonance-enhanced laser ionization coupled with time-of-flight mass spectrometry provide the ability to obtain quantum state resolved kinetic energy distributions of the desorbed molecules. Our results indicate that the desorption mechanisms for the molecules are dictated by the molecular coverage and determine if the molecules are desorbed in an internally excited state. We use specific mechanisms observed in molecular dynamics simulations reported in the literature to describe our experimental observations. In the low coverage regime, ballistic collisions between dislocated silver substrate atoms and the adsorbed molecules lead to the emission of energetic molecules. A collision between a single substrate atom and an adsorbed molecule leads to the emission of translationally and internally hotter molecules. A collision between an adsorbed molecule and several substrate atoms with similar momentum leads to the emission of slower, internally cooler molecules. In multilayer systems, a gentler mechanism, such as a molecular collision cascade or localized heating, generates the emission of translationally slow molecules. Temperature-programmed desorption studies allow characterization of the benzene film structure and show how the emission characteristics of the desorbed molecules change concomitantly with changes in the film structure. In general, we provide a framework describing the collision events responsible for stimulated molecular desorption.

SERS Spectra of the Pesticide Chlorpyrifos Adsorbed on Silver Nanosurface: The Ag20 Cluster Model

“…When the aromatic ring is pushed further away from the surface at the distance of ∼4.5 Å and out of the range for van der Waals interaction, − the CPF adsorbs on Ag(111) only via the S–Ag bond (Figure c) and this configuration is 11.2 kcal/mol less stable than when the aromatic ring is involved. The value of −11.2 kcal/mol for the physisorption energy of aromatic ring coordinating to Ag(111) surface observed herein is also in good agreement with the data reported from previous experiments lying in a range of −9.2 to −13.2 kcal/mol. − …”

Structure and energetics of benzene adsorbed on transition-metal surfaces: density-functional theory with van der Waals interactions including collective substrate response

“…The only experimentally deduced adsorption height, to the best of our knowledge, was determined for the disordered Bz chemisorbed on the Pt(111) surface at a coverage close to or less than one [36].The binding energy, which reflects the strength of the interaction between an adsorbate and the substrate, is another key parameter for the description of HIOS. Experimental binding energies are mainly obtained by temperature-programmed desorption (TPD) [22][23][24][25]27] and microcalorimetry measurements [32,[37][38][39]. TPD is the most extensively used method for determining the kinetic and thermodynamic parameters of desorption processes and decomposition reactions.…”

Organic surfaces excited by low-energy ions: atomic collisions, molecular desorption and buckminsterfullerenes

“…Experimental results from SNMS 74,75 and SIMS 76 show that the bombardment of thick organic films gives rise to the emission of intact molecules. In many cases, however, they are not observed in SIMS because there is no efficient ionization mechanism (for bulk PS oligomer sample, no intact molecule is observed in the 15 keV Ga 1 SIMS spectrum).…”

“…Experimentally, the reduction of the KED width when going from a monolayer coverage on metal to a multilayered sample has been elegantly demonstrated by Meserole et al for benzene and phenol molecules under 8 keV; 451 bombardment. 75 Using state-selective ionization, the same authors show that vibrationally excited benzene molecules are selectively emitted from the topmost layers of the sample, in contrast to ground-state molecules, which can originate either from the surface or from buried layers. 78 A closer look at the microscopic level reveals the effect of the metallic substrate on the emission process.…”