Photoexcited processes at metal and alloy surfaces: electronic structure and adsorption site

Photoexcited processes of NO and CO at photon energies ranging from 2.3 to 6.4 eV are investigated on Pt(111), Ni(111) and Pt(111)---Ge surface alloys by reflection-absorption infrared spectroscopy and resonance-enhanced multiphoton ionization. The branching between three competitive processes of desorption, recapture and dissociation upon laser irradiation is dramatically changed on the three surfaces. On Pt(111), NO is either photodesorbed or photodissociated depending on the coverage, while NO is exclusively photodissociated on Ni(111). UV-photon irradiation of NO on Pt(111)---Ge, on the other hand, induces only desorption of NO. Desorption of CO bound at the on-top site of Pt(111) is induced by laser irradiation. The electronic mechanism for photodesorption and competitive branching is discussed in terms of the electronic structure of the substrate and the adsorbate.     

Infrared-visible sum-frequency generation spectra of CO on Ni(100)

Infrared-visible sum-frequency generation (SFG) spectra of the stretching region of carbon monoxide adsorbed on Ni(100) are measured as a function of overlayer coverage and substrate temperature. The frequency and linewidth of the CO vibrational resonance in SFG have a similar dependence on coverage and temperature to that observed in linear absorption and emission spectroscopies. The lineshape of the SFG resonance, however, is influenced by the nonlinear response of the underlying metal surface. A simple model indicates that the nonresonant response of the substrate is out of phase with respect to that of the resonant CO response.     

Photodesorption of weakly bound molecules

A recently reported lineshape function which specifically applies to the problem of one-photon photodesorption is used to perform model calculations. The expected efficiency and timescale for such a process is obtained. Molecules physisorbed on solid surfaces may desorb when vibrational energy from laser excitation leaks into the surface-adsorbate bond. Using phenomenological rate constants for desorption, quenching and dephasing mechanisms, a simple expression for surface coverage as a function of laser and molecular parameters is obtained. Analogy is drawn to the vibrational predissociation of Van der Waals molecules, to which this model has been previously applied. Sample experimental sytems are examined and calculations are made to explore the feasibility and range of the photodesorption technique. Observable effects are calculated, even for low-power laser parameters and fast quenching rates. Conversely, the lineshape formula provides a method for extracting desorption and quenching rates from experimental data. Extension of the model to other systems is discussed.     

CO stretching vibrations on Pt(111) and Pt(110) studied by sumfrequency generation

Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm⁻¹ for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom.     

Sum frequency generation and density functional studies of CO-H interaction and hydrogen bulk dissolution on Pd(1 1 1)

CO-H interaction and H bulk dissolution on Pd(1 1 1) were studied by sum frequency generation (SFG) vibrational spectroscopy and density functional theory (DFT). The theoretical findings are particularly important to rationalize the experimentally observed mutual site blocking of CO and H and the effect of H dissolution on coadsorbate structures. Dissociative hydrogen adsorption on CO-precovered Pd(1 1 1) is impeded due to an activation barrier of ∼2.5 eV for a CO coverage of 0.75 ML, an effect which is maintained down to 0.33 ML CO. Preadsorbed hydrogen prevented CO adsorption at 100 K, while hydrogen was replaced from the surface by CO above 125 K. The temperature-dependent site blocking of hydrogen originates from the onset of hydrogen diffusion into the Pd bulk around 125 K, as shown by SFG and theoretical calculations using various approaches. When Pd(1 1 1) was exposed to 1:1 CO/H₂ mixtures at 100 K, on-top CO was absent in the SFG spectra although hydrogen occupies only threefold hollow sites on Pd(1 1 1). DFT attributes the absence of on-top CO to H atoms diffusing between hollow sites via bridge sites, thereby destabilizing neighboring on-top CO molecules. According to the calculations, the stretching frequency of bridge-bonded CO with a neighboring bridge-bonded hydrogen atom is redshifted by 16 cm⁻¹ when compared to bridging CO on the clean surface. Implications of the observed effects on hydrogenation reactions are discussed and compared to the C₂H₄-H coadsorption system.     

Desorption induced by hot electrons: wave packet calculation of CO on Cu surfaces

A quantum mechanical photodesorption model, valid for metallic substrates and sub-picosecond laser pulses, is presented. It takes into consideration the photodesorption coordinate and models the metal hot-electron mediated desorption by a three electronic states: an ionic state of the adsorbate and two effective states representing the continuum of the metal. This multiple-state picture allows the sharing of the flow of energy injected by the laser between the adsorbate and the substrate. For the first time, the present modeling introduces the hot electrons of the metal through an optical potential based on the kinetic model developed earlier by the authors. This potential, and the resulting desorption yield, depend on the laser fluence. For CO on Cu(1 0 0) or Cu(1 1 1), the results are in fair agreement with the experimental findings.     

高效频率转换下双波长外腔共振和频技术研究

基于外腔的高效频率转换, 尤其是当系统运行在抽运不消耗近似机理下, 信号光可实现大于90%的转换, 因此无法通过信号光直接获得其到腔模频率锁定的误差信号. 本文通过对信号光调制、和频光解调的方法获得了该误差信号, 实现了双波长激光到外腔腔模的级联锁定. 实验中外部环形腔将1.3 W的1064 nm抽运光放大到约14.3 W. 当1583 nm信号光从10 μW变化到50 mW, 其到636 nm和频光的转化效率约为73%; 当从50 mW变化到295 mW时, 转换效率呈线性降低到60%, 最终获得了440 mW的636 nm激光.     

Enhanced photoinduced desorption from metal nanoparticles by photoexcitation of confined hot electrons using femtosecond laser pulses

Strong fluence dependence of photodesorption cross sections is observed in femtosecond laser photodesorption of NO from (NO)2 on silver nanoparticles, in contrast to femtosecond photodesorption on bulk metals. The time scale of excitation buildup is found to be equal or less than the pulse duration of ～100 fs; NO translational energies are independent of fluence and pulse duration. We propose a nanoparticle-specific nonlinear mechanism in which, due to confinement, strongly nonthermal hot-electron distributions are maintained during the femtosecond pulses, enhancing the normal desorption pathway.     

Femtosecond laser induced desorption of water from silver nanoparticles

The photodesorption mechanism of H₂O from quartz-supported silver nanoparticles has been studied by femtosecond laser two-pulse correlation and fluence dependence measurements. With the laser wavelength close to the maximum of the (1,1) plasmon resonance of the nanoparticles, the desorption was found to be purely thermal, i.e., induced by coupling of the desorption coordinate to the nanoparticle lattice temperature, both in the low- and the high-coverage regimes. The lattice cooling times of the nanoparticles are in the range of several hundred ps, in accordance with recent time-resolved X-ray measurements. Also observed is a reversible red-shift of the nanoparticle plasmon modes with increasing H₂O coverage which is attributed to dielectric screening. PACS 78.67.-n; 82.53.-k; 65.80.+n     

Desorption of an organic conducting polymer by soft X-ray radiation created by a femtosecond laser

The possibility of the desorption of complicated molecular complexes by soft X rays resulting from a solid target irradiated by a single sharply focused femtosecond laser pulse with an energy of several millijoules has been experimentally demonstrated for polyaniline, which is an organic conducting polymer. X-ray desorption and photodesorption of polyaniline by femtosecond laser pulses have been compared using a time-of-flight mass spectrometer. The results provide the possibility of studying surfaces with spatial nanoresolution and high elemental (chemical) selectivity, as well as observing the photodesorption with a high temporal resolution.     

Chemisorption,photodesorption and conductivity measurements on ZnO surfaces

Experimental results of a mass-spectro metric analysis of photodesorption from ZnO single crystals at different temperatures are reported. They provide direct evidence that CO₂ is the only photodesorbed species from both the single crystal and powder samples studied. The CO₂ photodesorption occurs only when the incident photon energy exceeds the ZnO band gap energy. Excellent agreement between the illumination time dependence of the CO₂ photodesorption and surface conductivity data in both single crystals and powder samples strongly suggests a substrate dependent mechanism in which photodesorption occurs by the neutralization of chemisorbed CO₂^? “ion-molecules” by photogenerated holes. In addition, measurements of the chemisorption kinetics of oxygen on ZnO single crystal and powder surfaces are reported. The results are compared with CO₂ and CO chemisorptiun experiments to show that, of these gases, only oxygen chemisorbs from the gas phase. Auger analysis of oxygen saturated and photodesorbed surfaces of ZnO show a significant relation between the carbon content and the photo-desorptive and conductive activities of those surfaces. These observations indicate that impurity carbon atoms on ZnO surfaces can be oxidized by electron capture to produce chemisorbed CO₂^? “ion-molecules’ which will then readily photodesorb by bandgap radiation. This proposed process is discussed together with further supporting evidence.     

宽带钕玻璃激光的高效三次谐波转换

提出了一种新颖的宽频带钕玻璃激光三倍频技术组合激光工作模式（宽带激光与窄带激光混频）方案。数值模拟研究表明:利用窄线宽钕玻璃激光脉冲,可以缓解宽频带激光谐波转换过程中群速度失配对转换效率的影响,从而提高宽带钕玻璃激光的转换效率;并且该方案可以与目前使用成熟的双和频晶体方案结合,从而能支持目前钕玻璃激光装置能达到的最大带宽约5 nm的高效三倍频（理论上效率达约80%）。     

CO2激光作用下运动石英玻璃的温度分布

 研究了运动石英玻璃板在CO₂激光作用下的热效应，在考虑表面辐射和空气对流的情况下建立了数学模型，采用有限元软件ANSYS进行数值计算，得到了运动情况下石英玻璃的温度分布。比较了激光功率、光斑半径和运动速度对温度分布的影响，得到了温度分布与运动速度和激光参数之间的关系。结果显示石英玻璃板的表面温度随激光功率的增加而增加，随光斑半径、运动速度的增加而减小。     

Photochemistry on thin metal films: probe of electron dynamics in metal-semiconductor heterosystems

The surface photochemistry of on ultrathin epitaxial Ag films on Si(100) substrates has been studied with the goal to employ it as a tool to unravel the electron dynamics in such films. An increase of the photodesorption cross section is observed--a factor of 5 for 266 nm light and 12 nm film thickness--when the film thickness is decreased, despite the fact that the optical absorbtivity decreases. The increased photodesorption cross section is interpreted to result from photon absorption in the Si substrate producing electrons at energies and parallel momenta which are not allowed in Ag. These electrons penetrate through the Ag film despite the gap in the surface projected band structure utilizing quantum resonances.     

Photodesorption from powdered zinc oxide

Mass spectrometric measurements of photodesorption from powdered ZnO under controlled radiation are reported. Neutral carbon dioxide and atomic oxygen prevail as the desorbing species. From photodesorption rate decay curves we obtain cross sections for photodesorption of ~ 2 × 10^?17 cm² for O and ~4 × 10^?19 cm² for CO₂. Both species desorb only when the incoming photon energy exceeds the ZnO band gap energy (3.2 eV). The desorption rate is linear with incoming photon flux. All the data seem to support a substrate dependent process of photodesorption by neutralization of chemisorbed species by photogenerated holes. The temperature dependence of the photodesorption rate for CO₂ yields a value of 0.26 eV for the apparent binding energy of the physisorbed CO₂ molecule.     

Understanding surface photochemistry from first principles: the case of CO-NiO(100)

The excitation mechanism in the CO-NiO(100) system induced by a uv-laser pulse has been investigated from first principles. For the laser-driven process, the relevant electronically excited states are identified, and it is shown that a transition within the CO molecule is the crucial excitation step rather than substrate mediated processes. A new mechanism is proposed, in which the formation of a genuine C-Ni bond in the excited state is the driving force for photodesorption rather than electrostatic interactions, as has been found in similar systems. This results in very high velocities of CO molecules desorbing from the NiO(100) surface after electronic relaxation.     

无热池的11μm附近腔内光强增强型热带连续波CO_2激光器(英文)

介绍了一种腔内光强增强型凹面光栅选线法的热带连续波CO2激光器的设计及实验参数。在腔内不加CO2热吸收池,采用总气压为1 074.9 Pa、体积分数为13%CO2,15%N2和72%He的混合气体,放电电流为25 mA的情况下,除了在常规带0001-[1000,0200]Ⅰ,Ⅱ内观察到从R(56)到P(60)的100多条谱线以外,还得到11μm附近的其它谱线18条,其中0111-0310谱带9条,0111-1110谱带9条。实验发现,当激光器中氦气的比例升高时,激光的输出功率随之下降。每条谱线的腔内单程功率都超过35 W,足够满足大多数腔内光化学研究方面的需要。     

Adsorption and desorption of CO and H2 on Rh(111): Laser-induced desorption

Isosteric heats of adsorption ΔH_ad of CO and sticking coefficients S for CO and H₂ on Rh(111) are determined by laser-induced thermal desorption (LITD) in which a pulsed laser beam is focused onto the surface, and rapid local heating yields a desorption signal that is proportional to the adsorbate coverage θ. ΔH_ad for CO falls from 32.0±2 kcal/mol at low coverage to 14 kcal/mol at saturation, and the desorption pre-exponential factor v_d decreases from 10^14±0.5 to 10¹⁰ s^-1. ΔH_ad, v_d, and S of CO all decline sharply above θ = 0.2, corresponding to the occupation of a second binding state. Sticking coefficients for CO and hydrogen indicate precursor intermediates in adsorption.     

Role of adsorption on surface composition of Pd–Cu nanoparticles

Monte-Carlo (MC) simulation is used to study the role of adsorption of hydrogen, oxygen and carbon monoxide (CO) on the surface composition and surface bond geometry of Pd–Cu nanoparticles. For clean particles the surface is found to be enriched in Cu. But in the presence of adsorbed hydrogen and CO there is a segregation reversal from Cu segregation at low coverage to Pd segregation at high coverage. In the presence of adsorbed oxygen, on the contrary, the extent of Cu segregation increases with coverage. For a 586-atom nanoparticle with 50% Pd in the bulk the corner sites are found to be occupied by Cu atoms up to one monolayer adsorption. But, while the occupancy of 7, 8 and 9-coordinated sites by Cu atoms decreases with increase of H and CO coverage, for oxygen adsorption this occupancy increases with coverage. The relevance of such results in catalysis studies is discussed.     

基于单块周期极化铌酸锂晶体级联三倍频的440 nm蓝光固体激光器

阐述了一种基于单块周期极化铌酸锂晶体级联三倍频实现440 nm蓝光输出的实验方案。根据周期极化铌酸锂晶体的Sellmeier方程以及倍频与和频的相位匹配条件,在一块周期极化铌酸锂晶体上设计了两段不同的极化周期,使其在同一工作温度下能分别实现倍频与和频,在先后经过倍频与和频后,实现级联三倍频输出。实验采用Nd: YAG产生的1319 nm光作为基频光,重频400 Hz,脉宽110 ns,横向和纵向光束质量因子分别为1.81和2.65。耦合进周期极化铌酸锂晶体后,出射光中检测到660 nm的红光和440 nm的蓝光。通过调整工作温度和入射基频光功率,得到2.4 mW的最大蓝光输出,此时工作温度55.5 ℃,基频光功率530 mW。实验结果验证了单块晶体实现级联三倍频440 nm蓝光输出的可行性。