Understanding the Effect of an Amorphous Surface on the Ultrafast Dynamics of a Heterogeneous Photoinduced Reaction: CD₃I Photoinduced Reaction on Amorphous Cerium Oxide Films

Abstract

In this work, to understand how an amorphous surface influences the dynamics of surface photoinduced reactions, pump–probe spectroscopy in conjunction with mass spectrometry is employed to track the ultrafast evolution of intermediates and final products with time, mass, and energy resolution. As a model system, the photoinduced reaction of CD3I adsorbed on amorphous cerium oxide films is investigated. A fraction of the first intermediates produced on a freshly prepared surface is trapped to passivate the surface. After the A-band excitation, the minimum dissociation time of CD3I indicates that CD3I adsorption geometries with either CD3 or I facing the gas phase exist; however, the transient data suggest that most molecules are adsorbed with the I atom facing the surface. CD3 and I are consumed to form I2 and reform CD3I, which are produced at a steady rate only after the intermediates have lost the excess translational energy released from photodissociation.

Simultaneous tracking of ultrafast surface and gas-phase dynamics in solid-gas interfacial reactions

“…Temperature programmed desorption (TPD) is employed to estimate the local pressure in front of the surface at the location in which the surface is positioned when pump-probe molecular dynamics investigations are performed using the TOF-MS. TPD is a technique that is routinely employed in our laboratory to investigate the interaction between adsorbed molecules and the surface. 34,37,38 First, a series of TPD spectra are recorded from a Si(111) surface on which CH3I molecules are dosed with the gas tube while the surface sample is positioned close to the ion gauge detector, which is far from the gas tube so that the pressure can be accurately measured in the vicinity of the surface. Figure 2.…”

“…For the mass spectra displayed in Figure 3, to restrict the amount of CD3I molecules on the surface at sub-monolayer (ML) coverage and maintain this coverage constant during the surface reaction, the cerium oxide sample temperature is kept at 160 K as demonstrated in one of our recent publications. 34 The mass spectra in Figure 3 display four peaks corresponding to CD3 + , I + , I2 + , and CD3I + . The CD3 + , I + , and CD3I + appear as narrow peaks when CD3I is dosed at pressures below 5×10 -9 Torr.…”

“…The sharp peaks resemble the mass spectra obtained from surface chemical reactions reported in our previous publications in which molecules are dosed with a pulsed valve that is synchronized with a laser shutter, which stops the laser irradiation while molecules are dosed to avoid the detection of species from gas-phase, 9,30,31,33,39,40 or when molecules are 5 continuously dosed using a leak valve that maintains a low gas-phase pressure during pumpprobe surface molecular dynamics investigations. 34 Several factors suggest that the broad shoulders originate from gas-phase reactions: (i) the broad shoulders of CD3 + , I + , and CD3I + are only visible when the CD3I gas-phase pressure is increased, which enhances the probability of detecting ions originating from the gas-phase; (ii) I2 + signal appears only as a sharp peak in the mass spectra independent of the CD3I gas-phase pressure. This indicates that I2 is only formed at the surface, because bimolecular reactions are not possible in the gas-phase at the pressures employed in this experiment, due to (a) the low density of I atoms in the gas-phase, which leads to a low probability of collisions between I atoms above the surface and hence a low probability of a bimolecular reaction in the gas-phase, and (b) low density of gas-phase molecules, which reduce the possibility of I atoms produced in the gas-phase to lose their excess translational energy released through the photodissociation process, and hence a low probability of I atoms that collide to form stable bonds in the gas-phase.…”

“…The methyl iodide molecule is selected as a model system for photoinduced interfacial reactions, because its gas-phase photodissociation has been studied both experimentally [10][11][12][13][14][15][16] and theoretically. [17][18][19][20][21][22][23][24] Furthermore, the photoinduced reaction dynamics of methyl iodide has been investigated with femtosecond resolution in gas-phase [25][26][27][28][29] as well as on insulating, [30][31][32] metallic, 33 and semiconducting 34 surface. The photoinduced reaction dynamics of methyl iodide was also investigated with time, mass, and energy resolution on cerium oxide films like those used in this investigation, providing a base of comparison of the current and previously reported results.…”

“…The photoinduced reaction dynamics of methyl iodide was also investigated with time, mass, and energy resolution on cerium oxide films like those used in this investigation, providing a base of comparison of the current and previously reported results. 34 In this work, a detailed account of experimental setup employed to monitor photoinduced reactions at the gas-solid interface is provided. The ultrafast molecular dynamics experiments are performed using deuterated methyl iodide (CD3I) dosed on a cerium oxide surface.…”