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Wanquan Zheng

Publications and source records attributed to Wanquan Zheng.

5 recordsLinked to original sources
PubMed2005

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation.

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.

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PubMed2005

Novel cyanoterphenyl self-assembly monolayers on Au(111) studied by ellipsometry, x-ray photoelectron spectroscopy, and vibrational spectroscopies.

The self-assembled monolayers (SAMs) of two asymmetric disulfides derivatives (namely, LC1 and LC2) were prepared on Au(111). The disulfides contain a pure alkyl chain and an alkyl chain terminated by a cyanoterphenyl group. LC1 and LC2 differ by the way the cyanoterphenyl group is attached onto the alkyl chain: it is expected to be aligned with the alkyl chain in the case of LC1 and perpendicular to it in the case of LC2 (T shape). The consequences in terms of surface coverage, chemical composition, and molecular conformation of the two SAMs are studied using ellipsometry, x-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and broadband femtosecond sum-frequency generation (SFG). A model of coverage and tilt angle based on ellipsometry and XPS results shows that the SAM "manages" the large size of the terphenyl group by lowering the terphenyl containing chain coverage and by increasing the tilt. In the case of LC2, the disulfide breaks during molecular assembly, less terphenyl chains adsorb than pure alkyl chains, and the overall chain coverage is smaller than for LC1. RAIRS and SFG results show that these differences in surface coverage correspond to a drastically different orientation of the terphenyl axis, which lies nearly parallel to the surface for LC2, while it is tilted by approximately 28 degrees for LC1. This shows that the terphenyl group takes much more space on the surface in the case of LC2 and explains why the terphenyl coverage is found smaller for LC2. The anomalous SFG relative intensities observed in the region of CH stretch between CH2 and CH3 modes, and symmetric and antisymmetric modes, show that the chains are not in the fully stretched, all-trans conformation, LC2 being probably more distorted than LC1. These distorsions allow the molecules to occupy the space available below the large terphenyl group. The relative intensities of symmetric and antisymmetric modes are discussed qualitatively for some typical molecular conformations and orientations of the alkyl chain.

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PubMed2004

Vibrational dynamics of adsorbed molecules under conditions of photodesorption: pump-probe SFG spectra of CO/Pt(111).

Interaction of CO adsorbed on Pt(111) with electrons and phonons is studied experimentally by means of a pump-probe experiment where CO is probed by IR + visible sum frequency generation under a pump laser intensity that allows photodesorption. Vibrational spectra of CO internal stretch are obtained as a function of pump-probe delay. A two-temperature and anharmonic coupling model is used to extract from the spectra the real time variations of CO peak frequency and dephasing time. The main conclusions are the following: (i) The CO stretch is perturbed by two low-frequency modes, assigned to frustrated rotation and frustrated translation. (ii) The frustrated rotation is directly coupled to electrons photoexcited in Pt(111) by the pump laser. (iii) There is no evidence of Pt-CO stretch excitation in the spectra. The implications for the photodesorption dynamics are discussed.

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PubMed2004

Ultrafast laser excitation of CO/Pt(111) probed by sum frequency generation: coverage dependent desorption efficiency.

CO photodesorption from Pt(111) induced by femtosecond laser pulses is probed by IR+visible sum frequency generation (SFG). Steady state analysis of SFG spectra at varying CO pressure and laser fluence allows one to measure a approximately 5 orders of magnitude decrease of the photodesorption rate constant when CO coverage decreases from 0.37 to 0.07 monolayer. We ascribe this effect in the framework of the Menzel-Gomer-Redhead mechanism to electron delocalization in the CO layer. The lifetime of electronic excitation decreases when coverage decreases.

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