Surface photochemistry.  4.  Quenching of methyl iodide on platinum(111)

Liu, Z. M.; Akhter, Sohail; Roop, B.; White, John

doi:10.1021/ja00234a029

Cited by 55 publications

(28 citation statements)

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“…The photodissociation and the desorption of the molecules from the surface was subsequently monitored by means of X-ray photoemission spectroscopy. White and coworkers reported that only 10% of the first CH 3 I layer adsorbed on the Pt(111) substrate dissociated when the sample was irradiated for 120 min with a 100 W Hg arc lamp [ 29 ]. In contrast, a facile cleavage of the second layer was observed.…”

Section: Resultsmentioning

confidence: 99%

“…In contrast, the excited state is strongly bound to the metal surface, which facilitates the recapturing of the ground state before the C–I bond stretches considerably. As a consequence, the molecule will almost instantaneously relax back to the ground state after excitation, and the molecular dissociation will be quenched [ 29 ]. Also on an Ag(111) surface the methyl iodide molecules adsorbed at monolayer coverage are photodissociated at a slower rate compared to those adsorbed at multilayer coverages, which also indicates a substantial quenching of the photodissociation when the molecules are in contact with the metal substrate [ 33 – 34 ].…”

Section: Resultsmentioning

confidence: 99%

“…3 indicates a change in the nature of the surface during laser irradiation. As discussed in the previous section, just a small amount of C–I bond cleavage of CH 3 I at monolayer coverage on Pt(111) was observed after a long period (120 min) of irradiation with a 100 W Hg arc lamp [ 29 ]. In the present experiment a much more intense light source, i.e., a fs-laser, was employed.…”

Section: Resultsmentioning

confidence: 99%

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Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films

Vaida¹,

Tchitnga²,

Bernhardt³

2011

Beilstein J. Nanotechnol.

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SummaryThe photodissociation of small organic molecules, namely methyl iodide, methyl bromide, and methyl chloride, adsorbed on a metal surface was investigated in real time by means of femtosecond-laser pump–probe mass spectrometry. A weakly interacting gold surface was employed as substrate because the intact adsorption of the methyl halide molecules was desired prior to photoexcitation. The gold surface was prepared as an ultrathin film on Mo(100). The molecular adsorption behavior was characterized by coverage dependent temperature programmed desorption spectroscopy. Submonolayer preparations were irradiated with UV light of 266 nm wavelength and the subsequently emerging methyl fragments were probed by photoionization and mass spectrometric detection. A strong dependence of the excitation mechanism and the light-induced dynamics on the type of molecule was observed. Possible photoexcitation mechanisms included direct photoexcitation to the dissociative A-band of the methyl halide molecules as well as the attachment of surface-emitted electrons with transient negative ion formation and subsequent molecular fragmentation. Both reaction pathways were energetically possible in the case of methyl iodide, yet, no methyl fragments were observed. As a likely explanation, the rapid quenching of the excited states prior to fragmentation is proposed. This quenching mechanism could be prevented by modification of the gold surface through pre-adsorption of iodine atoms. In contrast, the A-band of methyl bromide was not energetically directly accessible through 266 nm excitation. Nevertheless, the one-photon-induced dissociation was observed in the case of methyl bromide. This was interpreted as being due to a considerable energetic down-shift of the electronic A-band states of methyl bromide by about 1.5 eV through interaction with the gold substrate. Finally, for methyl chloride no photofragmentation could be detected at all.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films

Vaida¹,

Tchitnga²,

Bernhardt³

2011

Beilstein J. Nanotechnol.

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“…Experimental methods for studying CH x fragments have used methyl halides (especially CH 3 I) [94,95], which decompose upon adsorption on Pt(1 1 1); hyperthermal methane beams to induce the direct adsorption of methyl radicals [43]; and the pyrolysis of azomethane, which leads to the formation of gas-phase CH 3 radicals [96]. In several RAIRS studies only the symmetric C-H stretch is identified at 2885 cm À1 [43,96,97].…”

Section: Nitrosyl Hydride (Noh Hno)mentioning

confidence: 99%

Atomic and molecular adsorption on Pt(111)

Ford¹,

Xu²,

Mavrikakis³

2005

Surface Science

260

211

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The adsorption of several atomic (H, O, N, S, and C) and molecular (N 2 , HCN, CO, NO, and NH 3 ) species and molecular fragments (CN, CNH 2 , NH 2, NH, CH 3 , CH 2 , CH, HNO, NOH, and OH) on the (1 1 1) facet of platinum, an important industrial and fuel cell catalyst, was studied using self-consistent periodic density functional theory (DFT-GGA) calculations at a coverage of 1/4 ML. The best binding site, energy, and position, as well as an estimated diffusion barrier, of each species were determined. The binding strength for all the species can be ordered as follows: N 2 < NH 3 < HCN < NO < CO < CH 3 < OH < NH 2 < H < CN < NH < O < HNO < CH 2 < NOH < CNH 2 < N < S < CH < C. Although the atomic species generally preferred fcc sites, there was no clear trend in site preference by the molecular species or molecular fragments. The vibrational frequencies of all the stable adsorbates in their best and second best adsorption sites were calculated and found to be in good agreement with experimental values reported in the literature. Finally, the decomposition thermochemistry of NOH, HNO, NO, NH 3 , N 2 , CO, and CH 3 was analyzed.

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“…Most studies to date have concentrated on CH 3a , due to the simplicity in preparing pure adlayers, by either thermal decomposition of methyl halide precursors [3][4][5][6][7] or using a CH 3 radical source. 8,9 The rehydrogenation and exchange of CH 3a has been studied by many groups.…”

Section: Introductionmentioning

confidence: 99%

Facile H–D exchange in adsorbed methylidyne on Pt{110}–(1×2) and deuteration to gaseous methane

Watson

King

2001

The Journal of Chemical Physics

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Microcanonical unimolecular rate theory at surfaces. III. Thermal dissociative chemisorption of methane on Pt(111) and detailed balance Hydrogen-deuterium exchange in adsorbed methylidyne, CH a , on Pt͕110͖ -(1ϫ2) has been studied for the first time using supersonic D 2 /H 2 molecular beams, which provides new insights into the reversible hydrogenation of adsorbed hydrocarbon fragments. The exchange reaction is extremely facile at surface temperatures of 350-450 K and proceeds via a Langmuir-Hinshelwood reaction between D a and a CH a fragment to produce gas phase H 2 and HD. The CD a ϩH a ͑i.e., reverse͒ reaction was also studied and was found to proceed more slowly. Both exchange reactions were successfully modeled and the difference in reaction rates is explained using zero point energy differences alone. Finally, we demonstrate that with high incident D 2 fluxes CH a can be completely deuterated to produce gaseous CHD 3 and CD 4 .

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Surface photochemistry. 4. Quenching of methyl iodide on platinum(111)

Cited by 55 publications

References 0 publications

Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films

Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films

Atomic and molecular adsorption on Pt(111)

Facile H–D exchange in adsorbed methylidyne on Pt{110}–(1×2) and deuteration to gaseous methane

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