Photon-induced reactions of aromatics adsorbed on rough and smooth silver surfaces

Abstract

We have investigated the thermal and photon-induced chemistry of several aromatics adsorbed on smooth and roughened Ag( 11 1) surfaces at low temperatures (1 10 K). Pyrazine, pyridine, 3-chloropyridine, and chlorobenzene quantitatively desorb from Ag( 11 1) near 200 K. After roughening the surface with 2 kV Ar ion bombardment, the breadth of the molecular desorption curves increases, and there is a high-temperature tail due to desorption from defect sites produced by the roughening process. Upon UV photolysis of 1 monolayer, chlorobenzene and 3-chloropyridine photodissociate on smooth and rough Ag surfaces, whereas no detectable photoreactions were observed for pyrazine or pyridine adsorbed on either surface. For the molecules that did undergo photodissociation, a shift to lower energy in the photodissociation threshold was observed on the rough surface relative to the smooth: 3.3 eV versus 3.5 eV for chlorobenzene and 3.5 eV versus 3.9 eV for 3-chloropyridine. We postulate that the decrease in the photodissociation threshold is due to defect sites produced by surface roughening, through either excitation of the surface plasmon resonance, which is allowed on the rough but not the smooth surface, or a more localized excitation near defect sites. Excitation in the molecular absorption band shows that the photodissociation yield is enhanced for 3-chloropyridine and quenched for chlorobenzene on the rough surface relative to the smooth. Theoretical calculations suggest that there is a decrease in the decay rate for 3-chloropyridine and an increase in the decay rate for chlorobenzene on the rough surface compared to the smooth. Differences between the quenching rates for 3-chloropyridine and chlorobenzene may be related to the different molecular orientations of these two molecules on the surface.

State-resolved investigation of the photodesorption dynamics of NO from (NO)2 on Ag nanoparticles of various sizes in comparison with Ag(111)

“…Interesting photochemical effects of NPs do not only derive from their increased surface/volume ratio, but also from their unique optical, electronic, and catalytic properties which can be tuned by size, shape, and/or support material. Plasmon-induced enhancement of the photoreactivity of adsorbates via field enhancement on roughened metal surfaces 10,11 and on MNPs (Refs. [7][8][9] Nevertheless, the number of experimental investigations that address the effects of size and/or plasmon excitation on the photochemistry of adsorbates on MNP surfaces is very limited.…”

Modelling the influence of nonthermal electron dynamics in thin and ultrathin gold films

“…Following the characterisation of the mechanism of adsorbate photochemistry on metal surfaces, there has been increasing interest in enhancing or controlling its rate. Strategies proposed have included modification of the substrate morphology to promote photochemistry at defect sites [18] or through excitation of particle plasmon resonances [19][20][21][22][23]. Indeed a number of plasmon enhanced substrate excited processes have been reported.…”

Surface plasmon enhanced substrate mediated photochemistry on roughened silver

“…An ion sputter gun ͑Phi Model 04-161͒, operating at a beam energy of 2.0 kV, with the beam defocused, produced a beam current at the crystal of 5 A cm Ϫ2 , as measured by a picoammeter ͑Keithley͒. Although the electron microscope employed in the present experiments lacked the resolution required to see these nanometer scale features, the conditions used are identical to these earlier studies, 7,9 and it is assumed that the present surface had a similar morphology. This was confirmed by separate measurements of the beam profile using a 2 mm diam platinum Faraday cup mounted on a translation stage.…”

“…15 The roughened surface was prepared following exactly the procedure described by Goncher et al 7 The Ag͑111͒ crystal was argon ion sputtered at room temperature for 90 min by backfilling the UHV chamber with argon ͑Messer Grieshiem, Ͼ99.999% purity͒ to a pressure of 3ϫ10 Ϫ5 Torr. 9 This revealed features of around 75 nm as well as smaller ͑tens of nm͒ features, superimposed upon the larger bumps. Analysis of the damage induced by the beam on a silicon target confirmed that the Ar ϩ beam profile was uniform over the dimensions of the Ag͑111͒ crystal.…”

“…[1][2][3][4][5][6] For irradiation with photons in the visible and near ultraviolet region of the spectrum two distinct excitation mechanisms have been identified. Examples are the early work of Goncher et al, 7 and some more recent measurements from Kidd et al 8 and Myli et al 9 All of these studies indicated that there were significant modifications to the photochemical cross section on roughened surfaces, relative to that observed on atomically smooth surfaces. The second involves absorption of the incident radiation by the substrate to generate energetic ''hot'' electrons between the Fermi edge and the vacuum level.…”