Optical second-harmonic generation and the related technique of infrared – visible light sum-frequency generation are extremely versatile tools for studies of many kinds of surfaces and interfaces. With the help of ultra-short laser pulses, they can be used to monitor surface dynamics and reactions with sub-picosecond time resolution.

Surface properties probed by second-harmonic and sum-frequency generation

On the basis of different types of experiments, the authors develop implicitly the model of surface-enhanced Raman scattering (SERS) of adsorbates on metal surfaces. The long-range enhancement by resonances of the macroscopic laser and Stokes field is separated quantitatively from the metal electron-mediated resonance Raman effect. The latter mechanism proceeds by increased electron-photon coupling at an atomically rough surface and by temporary charge transfer to orbitals of the adsorbates. This model can account for the chemical specificity and vibrational selectivity of SERS and (partly) for the SERS specificity of the various metals.

https://iopscience.iop.org/article/10.1088/0953-8984/4/5/001/pdf

Surface-enhanced Raman scattering

Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network within the tropics.

/pdf/towards-robust-regional-estimates-of-co2-sources-and-sinks-5zu9uccwzd.pdf

Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models.

The modes of formation of carbonaceous deposits (“coke”) during the transformation of organic compounds over acid and over bifunctional noble metal-acid catalysts are described. At low reaction temperatures, ( 350°C), the coke components are polyaromatic. Their formation involves hydrogen transfer (acid catalysts) and dehydrogenation (bifunctional catalysts) steps in addition to condensation and rearrangement steps. On microporous catalysts, the retention of coke molecules is due to their steric blockage within the micropores.

Organic chemistry of coke formation

Surface chemistry of transition metal carbides.

Benzene adsorption on the Rh(111) crystal surface has been studied by HREELS (high-resolution electron energy loss spectroscopy), LEED (electron diffraction), and TPD (temperature programmed desorption). The vibrational spectra indicate that benzene adsorbs molecularly at 300 K and is ..pi..-bonded to the surface with the ring plane parallel to the surface plane. Recent dynamic LEED calculations together with the angle-dependent HREELS studies reported here establish a C/sub 3v/(sigma/sub d/) bonding symmetry for the c(2..sqrt..3 x 4)rect-C/sub 6/H/sub 6/ structure. Several other ordered benzene overlayers can be formed between 300 and 400 K depending on the benzene coverage. No large changes occur in the chemisorption bonding mode or geometry coincident with the two-dimensional ordering phase transitions in this temperature range. The vibrational spectra show that two molecular adsorption sites can be populated. Benzene adsorption is only partially reversible; less than 20% of the adsorbed benzene desorbs molecularly upon heating. The remaining benzene irreversibly decomposes, evolving hydrogen and leaving a carbon-covered surface. The TPD and HREELS data on Rh(111) and other single-crystal surfaces show correlations between the metal-benzene bond strength, the work function of the clean surface, and the frequency shifts of some of the molecular benzene vibrational modes. 44 references, 10 figures,more » 4 tables.« less

A high-resolution electron energy loss spectroscopy study of the surface structure of benzene adsorbed on the rhodium(111) crystal face

Benzene decomposition on the Rh(111) crystal surface has been studied over the temperature range of 300-800 K by high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). Our results show that benzene decomposition begins at 400 K, forming a mixture of CH and C/sub 2/H species. The relative amounts of these two species vary with temperature; at 470 K, the concentration ratio of CH to C/sub 2/H is 0.4. Above 500 K, these fragments dehydrogenate and condense to form C/sub n/H polymers. By 800 K, the adsorbed monolayer is completely dehydrogenated, forming a polymeric carbon monolayer with a vibrational spectrum similar to that of an ordered graphite monolayer. The authors propose that benzene decomposition on Rh(111) proceeds by decyclotrimerization to form three acetylenes which immediately decompose to CH and C/sub 2/H species.

https://escholarship.org/content/qt0mh1g8tp/qt0mh1g8tp.pdf?t=p0wtyz

Thermal decomposition of benzene on the rhodium(111) crystal surface

The reactions of propylene, propadiene, and methylacetylene with Rh(111) crystal faces in ultrahigh vacuum (UHV) from 80 to 800 K have been studied by low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). All three hydrocarbons adsorb intact at 80 K. At this temperature, methylacetylene forms a p(2 x 2) LEED pattern while propylene and propadiene adsorb as disordered monolayers. Above 200 K, propylene and propadiene also form p(2 x 2) LEED patterns which are stable up to 270 K. The propylene fragment in this temperature range is propylidyne (CCH/sub 2/CH/sub 3/). By room temperature all the C/sub 3/ hydrocarbons have decomposed to ethylidyne (CCH/sub 3/) and polymerized C/sub x/H fragments. Surface hydrogen is instrumental in the C-C bond breaking, and partial deuteriation studies were performed to determine which C-C bonds break first. The ethylidyne species are stable up to 400 K, above which they decompose to C/sub x/H fragments. The bonding and the thermal decomposition pathways for propylene on Rh(111) are compared to Pt(111) surface chemistry.

https://escholarship.org/content/qt7k52t854/qt7k52t854.pdf?t=p0jv95

Bonding and thermal decomposition of propylene, propadiene, and methylacetylene on the Rh(111) single-crystal surface

http://www.physics.ucdavis.edu/xdzhu/publication%20in%20pdf/SurfSci_SHG_Na_K_Rh(111).pdf

Studies of alkali adsorption on Rh(111) using optical second-harmonic generation

We report the vibrational and electronic spectra for pyridine and benzene adsorbed on the Rh(111) crystal surface obtained by high‐resolution electron energy loss spectroscopy (HREELS). Low‐energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and optical second harmonic generation (SHG) have also been used to provide complementary information. Pyridine adsorption on Rh(111) was studied over the 77–450 K temperature range. At 77 K, multilayers of pyridine are observed with a vibrational spectrum similar to that of liquid pyridine. Between 185 and 230 K, HREELS and TDS indicate that both physisorbed and chemisorbed pyridine species are present on the surface. The physisorbed species desorbs at 295 K, while the chemisorbed species is stable until it decomposes on the surface at 400 K. We propose that the chemisorbed species is an α‐pyridyl complex as thermal desorption spectroscopy indicates partial dehydrogenation of this pyridine surface species. Electronic energy loss spectra for bot...

/pdf/vibrational-and-electronic-spectroscopy-of-pyridine-and-1z80a6b8ut.pdf

C. M. Mate

Papers

A high-resolution electron energy loss spectroscopy study of the surface structure of benzene adsorbed on the rhodium(111) crystal face

Thermal decomposition of benzene on the rhodium(111) crystal surface

Bonding and thermal decomposition of propylene, propadiene, and methylacetylene on the Rh(111) single-crystal surface

Studies of alkali adsorption on Rh(111) using optical second-harmonic generation

Vibrational and electronic spectroscopy of pyridine and benzene adsorbed on the Rh(111) crystal surface