The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(111) surface were investigated by surface vibrational spectroscopy via sum frequency generation (SFG) both under ultrahigh vacuum (UHV) and high pressure conditions with 10 Torr cyclohexene and various hydrogen pressures up to 590 Torr. Under UHV, cyclohexene on Pt(111) undergoes a change from πr/σ-bonded, σ-bonded, and c-C₆H₉ surface species to adsorbed benzene when the surface was heated. A site-blocking effect was observed at saturation coverage of cyclohexene and caused the dehydrogenation to shift to higher surface temperature. At high pressures, however, none of the species observed in UHV condition were seen. 1,4-cyclohexadiene (CHD) was found to be the major species on the surface at 295 K even in the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable reaction at the temperature range between 300-400 K with 1,3-CHD on the surface as revealed by SFG. Further increasing surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation reaction rate with both 1,3-CHD and 1,4-CHD detectable on the surface simultaneously. Monitoring the reaction kinetics and the chemical nature of surface species together allows us to postulate a reaction mechanism: cyclohexene hydrogenates to cyclohexane via a 1,3-CHD intermediate, and dehydrogenates to benzene through both 1,4-CHD and 1,3-CHD intermediates. Both 1,3- and 1,4- CHD dehydrogenate to benzene at sufficiently high temperature on Pt(111).

Xingcai Su;Kyle Y. Kung;Jouko Lahtinen;元壤 沈;Gabor A. Somorjai

Journal of Molecular Catalysis A: Chemical

1999-5-6

Recently electroreflectance (at 300 K) and thermoreflectanee (at 77 K) of the layer compound GaSe have been reported. For comparison we have measured the wavelength modulation spectra (1/R) dR/dE of GaSe, GaS, GaSe_0.2S_0.8, and GaSe_0.8S_0.2 at 5, 77, and 300 K between 2 and 6 eV. A large number of new structures are reported.

S. Kohn;Y. Petroff;元壤 沈

Surface Science

1973-6

Ultraviolet (UV) polymerizable discotic liquid-crystalline (DLC) molecules (2,3,6,7,10,ll-hexakis(4′-acryloy-m-alkyloxyben- zoyoxy)triphenylene [HAHBT-m, where m was the number of methylene units, and here m = 6 (HAHBT-6)]) were assembled to form a negative retardation film with an oblique optical axis on a specifically designed rubbing-aligned polyimide layer surface [6FDA-11CBBP (where 11 is the number of methylene units in the side chains)]. The side chains of this polyimide were terminated by cyanobiphenyl groups. Surface-enhanced Raman scattering (SERS) and optical second harmonic generation results showed that rubbing caused a surface structural re-arrangement in the alignment layer resulting in a negative pre-tilt angle (θ_s) of -8.5° (which was in the direction opposite to the rubbing direction). The molecular topology at the rubbed surface was governed by a stable fold-like bent structure of the cyanobiphenyl side chains, in which the CN groups preferentially pointed down towards the surface. When the DLC molecules were deposited onto the alignment surface and polymerized via UV irradiation to generate a new optical film, an oblique optical axis with an average tilt angle of -18.6° with respect to the film normal was detected using ellipsometric measurements. This tilted optical axis was developed by the DLC molecules being wedged on top of the cyanobiphenyl groups when in the bent conformation. Furthermore, the tilt angle difference between the θ_sat the alignment surface and at the air interface of the DLC molecules was attributed to a splay deformation of the DLC molecules along the film surface normal. Optical modeling has also confirmed our experimental observations.

Jason J. Ge;Seok Choel Hong;Benjamin Y. Tang;Christopher Y. Li;Dong Zhang;Feng Bai;Bart Mansdorf;Frank W. Harris;Denke Yang;元壤 沈;Stephen Z.D. Cheng

Advanced Functional Materials

2003-9

An optical Kerr medium in a pulsed laser cavity can appreciably affect the pulsed laser output via self-induced ellipse rotation and self-focusing. Stretching of a Q-switched ruby laser pulse has been observed with either of these two effects. Transverse mode selection by self-focusing has also been seen. The results appear in good agreement with theoretical prediction.

R. Hsu;元壤 沈

Applied Optics

1982-7-15

We describe in detail a technique using optical second-harmonic (SH) diffraction from a one-dimensional laser-induced monolayer grating to probe surface diffusion of adsorbates and its anisotropy on a solid surface. The case of CO on Ni(110) is used as a demonstration. The two orthogonal and independent diffusion tensor components along [11̄0] and [001] are measured, exhibiting a strong anisotropy in both the activation energy Ediff and the preexponential factor D0 in the diffusion coefficients. A compensation effect between Ediff and D0 is observed. In comparison with CO/Ni(111) and CO/Ni(100), our result suggests that the Ni(110) surface seen by CO is much smoother than Ni(111) and Ni(100). Both advantages and limitations of the present technique are mentioned and possible complications in the data analysis are discussed.

旭东 肖;X. D. Zhu;W. Daum;元壤 沈

Physical Review B

1992

The surface plasmon technique is employed to measure the refractive indices and to probe the nematic-isotropic phase transition of 4-cyano-4 prime -n-pentylbiphenyl. Coexistence of isotropic and nematic phases in a 60 mK range and a hysteresis effect of the phase transition have been observed. A diffuse droplet model is used to estimate the supercooling and superheating range.

L. C. Chu;C. K. Chen;元壤 沈

Molecular Crystals and Liquid Crystals

1980

Using the wavelength-modulation technique, we have measured the stress dependence of the E₀¹ peak of Si under uniaxial compression. The results suggest that the peak is a composite peak, with one part arising from transitions in the region around Δ (4-5) and the other part from transitions over an extended region around Γ.

Jackson Koo;元壤 沈;Ricardo R.L. Zucca

Solid State Communications

1971-12-15

Surface enhanced Raman scattering of pyridine adsorbed to weakly roughened silver films is studied using a well-characterized extended surface plasmon excitation. We show that the additional increase in the Raman signal associated with the extended surface plasmon is fully accounted for by the enhancement in the average macroscopic field.

H. W.K. Tom;C. K. Chen;A. R.B. de Castro;元壤 沈

Solid State Communications

1982-1

Sum-frequency generation (SFG) vibrational spectroscopy and scanning force microscopy (SFM) were used to probe structural changes of atactic polypropylene (APP) and isotactic polypropylene (IPP) surfaces in the glass transition. The results indicate that below the glass transition, the surface polymer chains are better oriented with the CH₂ groups pointing outward and the surface elastic modulus is higher. The SFG and SFM results correlate well in the transition towards the glass phase. No discernible difference between the glass transition temperatures of the bulk and surface was observed.

D. H. Gracias;D. Zhang;L. Lianos;W. Ibach;元壤 沈;G. A. Somorjai

Chemical Physics

1999-7-1

FT-SFG with femtosecond light pulses for surface vibrational spectroscopy is demonstrated. Comparison of the measured vibrational spectra of an OTS monolayer on fused silica with those obtained by conventional SFG measurement shows excellent agreement. The main advantages of FT-SFG over the conventional technique are the source-independent spectral resolution and absolute frequency calibration. The technique can be extended to other nonlinear optical spectroscopic studies.

J. A. McGuire;W. Beck;X. Wei;元壤 沈

2000