Zero kinetic energy electron-induced reaction of formaldehyde on Ag(111) and its significance for surface photochemistry

Fleck, L. E.; Kim, Jung-Soo; Dai, Hai‐Lung

doi:10.1016/0039-6028(96)00668-1

Cited by 12 publications

(22 citation statements)

References 19 publications

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“…The peak at 943 cm -1 , for which there is no corresponding mode in molecular H 2 CO, is assigned to the ν s (OCO) mode of polymer formed during acquisition of the EELS spectrum. Previous studies of H 2 CO adsorption on Ag(111) have shown that polymerization of molecular formaldehyde can be initiated by low-energy electrons and that even exposure to the low fluxes of electrons encountered in EELS can initiate polymerization of multilayers of H 2 CO. , …”

Section: Resultsmentioning

confidence: 97%

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Adsorption and Polymerization of Formaldehyde on Cu(100)

Bryden

Garrett

1999

J. Phys. Chem. B

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The adsorption of formaldehyde (H2CO) on clean Cu(100) at 85 K has been studied using electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For coverages up to (1.06 ± 0.22) × 1015 H2CO molecules/cm2, formaldehyde spontaneously polymerized to form a monolayer of disordered poly(oxymethylene) (POM), arranged with the chain directions parallel to the surface plane. Thermal decomposition/desorption of the polymer monolayer occurred by three routes, producing peaks in temperature-programmed desorption (TPD) at approximately 177, 200, and 215 K. The lowest temperature peak was exclusively associated with production of H2 and CO in approximately equal proportions. The two higher temperature peaks were produced by molecular H2CO generated via depolymerization of the polymer. The 200 and 215 K features displayed zero- and first-order desorption kinetics, corresponding to estimated activation energies for depolymerization of 75 ± 10 and 53.9 ± 0.5 kJ/mol, respectively. The presence of two polymer desorption peaks is attributed to chain conformational differences present within the monolayer and has not been previously observed in studies of formaldehyde adsorption on metal surfaces. Large exposures of H2CO on this surface formed multilayers of molecular formaldehyde on top of the first polymer layer. The second layer desorbed at 105 K and subsequent layers at ∼100 K.

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Section: Resultsmentioning

confidence: 97%

“…Thermal polymerization of acetaldehyde on O/Ag(111) has also been observed . Ultraviolet photons or low-energy electrons have been shown to initiate polymerization of TCNQ, dinitrobenzene, thiophene, , formaldehyde, , and styrene on metal surfaces.…”

Section: Introductionmentioning

confidence: 94%

Adsorption and Polymerization of Formaldehyde on Cu(100)

Bryden

Garrett

1999

J. Phys. Chem. B

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“…The high average kinetic energy and small laser-induced temperature jump suggest an electronic desorption mechanism, i.e., desorption induced by electronic transitions (DIET), which is often invoked to explain the origin of high translational energies in photon-stimulated desorption . Specifically, the Antoniewicz−DIET model , involving the formation and decay of a temporary negative ion provides a reasonable description since Fleck et al have shown that polymerization most likely proceeds via a DEA process involving the H 2 C anion . − In the Antoniewicz model, “hot” subsurface electrons produced by UV excitation of the metal substrate resonantly scatter into the lowest unoccupied level (2π*) of the adsorbate (formaldehyde). The 2π* level is estimated to lie ∼1 eV below the vacuum level. , Electron attachment to the adsorbate results in additional attractive image−charge interactions, and the anion moves toward the surface.…”

Section: Resultsmentioning

confidence: 99%

“…This picture is based on the results of Fleck et al who found that the cross section for polymerization, which is also attributed to H 2 CO (a) -formation, increases rapidly above the measured threshold of 3.1 ( 0.2 eV. 14,15 The tunneling probability will increase with increasing photon energy up to the vacuum level at 4.4-4.5 eV, 15,16 beyond which free electrons can be generated which can directly scatter and attach to an adsorbed formaldehyde molecule. A qualitative measure of the internal energy of the desorbing formaldehyde molecules can be obtained from Figure 5, where the ion time-of-flight mass spectrum for formaldehyde molecules desorbed by 266 nm light (140 µJ/cm 2 ) is compared to a roomtemperature gas-phase sample.…”

Section: Methodsmentioning

confidence: 94%

State-Selective Laser Photochemistry of Formaldehyde on Ag(111)

et al. 1998

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State-selective (VUV + UV) resonant multiphoton ionization is used to measure the state-and energydistributions of the gas-phase products which accompany the UV photoinduced polymerization of formaldehyde on Ag(111). At 355 nm, only direct desorption of H 2 CO is observed, whereas both CO product formation and H 2 CO desorption are found at 266 nm. The rotational state and translational energies of the desorbed CO products exhibit a "fast" channel associated with a prompt fragmentation process and a "slow" channel in near-thermal equilibrium with the Ag surface. No gas-phase hydrogen products (H atoms, H 2 ) were detected during UV exposure, but H 2 desorption is observed during breakup of the polymer above 200 K. The timedependent desorption yields for both slow CO and H 2 CO are strongly temperature dependent and reflect the rate of polymerization. A simple kinetic model is used to describe the competing processes of desorption, fragmentation, and polymerization which ultimately result from the decay or fragmentation of an H 2 CO -(a) or other H 2 CO (a) excited-state species. In combination with depletion rates measured over the temperature range 30-95 K, the kinetic model results in an activation energy for polymerization of 0.90 ( 0.06 kcal/mol.

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“…The adsorption structure and energetics of vinyl chloride on the Ag(111) surface, one of the few systems of molecules physisorbed on metal surfaces that have demonstrated substrate-mediated photochemical activity, [1][2][3] have been characterized 4 using Thermal Desorption Spectroscopy (TDS) and High Resolution Electron Energy…”

Section: Introductionmentioning

confidence: 99%