Trapping-desorption and direct-scattering of formaldehyde at Au(111)

Abstract

Nonreactive surface scattering of atoms, molecules and clusters can be almost universally described by two mechanisms: trapping-desorption and direct-scattering. A hard cube model with an attractive square well provides a zeroth order description of the branching ratio between these two mechanisms as a function of the incidence energy. However, the trapping process is likely to be enhanced by excitation of internal degrees of freedom during the collision. In this molecular beam surface scattering study, we characterize formaldehyde/Au(111) scattering using angle resolved time-of-flight techniques. The two mechanisms are found to compete in the range of the investigated normal incidence energies between 0.1 and 1.3 eV. Whereas at low incidence energies trapping-desorption dominates, direct-scattering becomes more likely at incidence energies above 0.37 eV. This incidence energy is slightly higher than the desorption energy which is found to be 0.32 ± 0.03 eV by temperature programmed desorption techniques. A simple hard cube model underestimates the observed trapping probabilities indicating the importance of trapping induced by excitation of internal molecular degrees of freedom.

An axis-specific rotational rainbow in the direct scatter of formaldehyde from Au(111) and its influence on trapping probability

“…Trapping/desorption channel: dependence of the rotational distribution on T s At the lowest incidence kinetic energy employed in our study (E i = 0.11 eV), the trapping/desorption mechanism dominates. 30 We have recorded REMPI spectra of molecules scattered in this regime for surface temperatures ranging from 140-480 K. An analysis of the spectra (see Section SIII of the ESI †) reveals no significant departure from a thermal Boltzmann distribution of rotational state population and indicates nearly complete equilibration of rotational degrees of freedom to the surface temperature.…”

“…38 The UFF van der Waals interaction underestimates the measured 30 and calculated [39][40][41] binding energy of formaldehyde on Au(111) by more than an order of magnitude, so we scale the potential by a factor of 41 in order to bring the optimized binding energy into agreement with the measured value (0.31 eV). 30 The trajectories were calculated at normal incidence angle and random incidence orientation, over a range of incidence kinetic energies (E i = 0.33-1.2 eV). The surface was initially at rest and the molecule was initially at a distance of z 0 = 10 Å from the surface with zero incident angular momentum.…”

“…The essential details of the experiment are described in ref. 30. A pulsed beam of formaldehyde was generated by cracking solid paraformaldehyde (97% purity) in a sample holder inside the nozzle of a home-built pulsed solenoid valve, described in ref.…”

“…These channels have been characterized in detail in ref. 30. At incidence energies between 0.33-0.39 eV we probe the scattered molecules at a distance of 9 mm from the surface, 3 mm above the incoming beam.…”

“…At high E i , the direct scattering channel can also be resolved from the trapping/desorption channel and from the incoming beam via ion time-of-flight (TOF) to the detector. 30 The scattering products in the E i = 0.48-1.2 eV range were thus monitored 3 mm from the surface, such that a wide range of scattering angles is probed. No significant variation of rotational distribution was found over the range of distances and angles probed in our experiment.…”