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Few recent experiments on surface studies by second harmonic generation

Description: Surface second harmonic generation (SHG) is used to monitor adsorption and desorption of molecules on metal and semiconductor surfaces in ultrahigh vacuum. Surface SHG has also been used to study monolayers of molecules at a liquid/air interface. 13 refs., 6 figs. (WRF)
Date: December 1, 1985
Creator: Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Nonlinear optical techniques for surface studies. [Monolayers]

Description: Recent effort in developing nonlinear optical techniques for surface studies is reviewed. Emphasis is on monolayer detection of adsorbed molecules on surfaces. It is shown that surface coherent antiStokes Raman scattering (CARS) with picosecond pulses has the sensitivity of detecting submonolayer of molecules. On the other hand, second harmonic or sum-frequency generation is also sensitive enough to detect molecular monolayers. Surface-enhanced nonlinear optical effects on some rough metal surfaces have been observed. This facilitates the detection of molecular monolayers on such surfaces, and makes the study of molecular adsorption at a liquid-metal interface feasible. Advantages and disadvantages of the nonlinear optical techniques for surface studies are discussed.
Date: September 1, 1981
Creator: Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Nonlinear optical studies of organic monolayers

Description: Second-order nonlinear optical effects are forbidden in a medium with inversion symmetry, but are necessarily allowed at a surface where the inversion summary is broken. They are often sufficiently strong so that a submonolayer perturbation of the surface can be readily detected. They can therefore be used as effective tools to study monolayers adsorbed at various interfaces. We discuss here a number of recent experiments in which optical second harmonic generation (SHG) and sum-frequency generation (SFG) are employed to probe and characterize organic monolayers. 15 refs., 5 figs.
Date: February 1, 1988
Creator: Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Nonlinear optical studies of surfaces

Description: The possibly of using nonlinear optical processes for surface studies has attracted increasing attention in recent years. Optical second harmonic generation (SHG) and sum frequency generation (SFG), in particular, have been well accepted as viable surface probes. They have many advantages over the conventional techniques. By nature, they are highly surface-specific and has a submonolayer sensitivity. As coherent optical processes, they are capable of in-situ probing of surfaces in hostile environment as well as applicable to all interfaces accessible by light. With ultrafast pump laser pulses, they can be employed to study surface dynamic processes with a subpicosecond time resolution. These advantages have opened the door to many exciting research opportunities in surface science and technology. This paper gives a brief overview of this fast-growing new area of research. Optical SHG from a surface was first studied theoretically and experimentally in the sixties. Even the submonolayer surface sensitivity of the process was noticed fairly early. The success was, however, limited because of difficulties in controlling the experimental conditions. It was not until the early 1980`s that the potential of the process for surface analysis was duly recognized. The first surface study by SHG was actually motivated by the then active search for an understanding of the intriguing surface enhanced Raman scattering (SERS). It had been suspected that the enhancement in SERS mainly came from the local-field enhancement due to local plasmon resonances and pointing rod effect on rough metal surfaces. In our view, Raman scattering is a two-photon process and is therefore a nonlinear optical effect.
Date: July 1, 1994
Creator: Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Optical second harmonic generation from Langmuir-type molecular monolayers

Description: A single molecular layer is generally sufficient to produce observable optical second harmonic generation (SHG). Furthermore, the selection rules governing this process make the SHG from a single monolayer often stronger than that from the medium supporting the monolayer. We have studied SHG from various Langmuir-type monolayers (i.e., monolayers spread on a water surface) in the following contexts: Study of chemical reactions (e.g., polymerization) and two-dimensional phase transitions in molecular monolayers on water. Development of a new technique to evaluate optical nonlinear coefficients of organic molecules, and their relationship to the molecular structure.
Date: January 1, 1987
Creator: Berkovic, G.; Rasing, Th. & Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Nonlinear optical studies of organic liquid interfaces

Description: Infrared-visible sum-frequency generation is the only available technique that can generate surface vibrational spectra for liquid interfaces. Several examples from recent studies in our laboratory are used to show how the surface vibrational spectra can yield useful information about structures of liquid surfaces.
Date: July 1, 1995
Creator: Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Nonlinear optical studies of polymer interfaces

Description: Second-order nonlinear optical processes can be used as effective surface probes. They can provide some unique opportunities for studies of polymer interfaces. Here the author describes two examples to illustrate the potential of the techniques. One is on the formation of metal/polymer interfaces. The other is on the alignment of liquid crystal films by mechanically rubbed polymer surfaces.
Date: November 1, 1993
Creator: Shen, Y. R.
Partner: UNT Libraries Government Documents Department

Laser probing of interfaces

Description: In the past decade, the possibility of using lasers to probe surfaces and interfaces has attracted a great of attention. Such techniques have the advantages of being able to interrogate a surface or interface in a non-detrimental and remote sensing way. Optical second harmonic generation (SHG) and sum frequency generation (SFG) have turned out to be most effective and versatile. By symmetry, these second-order nonlinear optical processes are forbidden (under the electric dipole approximation) in media with an inversion center. They can therefore be highly surface specific. They are also capable of probing with high spatial, temporal, and spectral resolutions, suitable for in-situ studies of surfaces in hostile environment, and applicable to all interfaces accessible by light. Indeed, they have been successful in finding applications to a large variety of surface and interfacial problems: probing adsorption and desorption of molecules from surfaces, measuring average molecular orientation of adsorbates, monitoring surface symmetry and surface phase transitions, conducting surface microscopy and surface spectroscopy, and many others. Here, we discuss a few examples to illustrate the power of the techniques.
Date: March 1, 1994
Creator: Shen, Y. R.
Partner: UNT Libraries Government Documents Department

Second-harmonic generation from sub-monolayer molecular adsorbates using a c-w diode laser: Maui surface experiment

Description: Optical second-harmonic generation (SHG) can be an extremely sensitive tool for surface studies. The technique is capable of probing adsorbed molecules at various interfaces. It is based on the idea that SHG is forbidden in a medium with inversion symmetry, but necessarily allowed at a surface. To see such a surface nonlinear optical effect, high laser intensity is often needed. Thus, in the experiments reported so far, pulsed lasers were used exclusively. From the consideration for practical applications, however, the technique would look much more attractive if the bulky pulsed laser can be replaced by a simple inexpensive c-w diode laser. This paper describes the first demonstration of surface SHG with a c-w laser. 3 refs., 1 fig.
Date: June 1, 1985
Creator: Boyd, G.T.; Shen, Y.R. & Hansch, T.W.
Partner: UNT Libraries Government Documents Department

Second-order sum- and difference-frequency generation via quadrupole transitions in atomic vapors

Description: The inclusion of higher-multipole terms in the interaction of light with matter allows second-order nonlinear processes to be observed in isotropic media. These processes may be useful for both generating new wavelengths of radiation and measuring atomic or molecular quadrupole transition moments otherwise hard to determine. After a brief theoretical introduction, the experimental arrangement used to observe quadrupole sum-frequency generation is shown. Generally, the characteristics of the observed output agree with those predicted: the output shows the sharp two-photon resonance expected from the expression for chi/sup (Q)/ and the expected variation with k vector mismatch. At high densities and input powers, deviations from the simple theory occur: output falls sharply because of absorption associated with D-line resonances, two-photon saturation occurs, and third-order nonlinearities and population redistribution can cause changes in the index of refraction of the vapor which defocus the pump beams and destroy the phase matching of the output radiation. Quadrupole sum-frequency generation allows a novel and accurate measurement of quadrupole transition moments; values were found for the 3s-4d transition moment of Na. The creation of an electric dipole polarization by mixing of two pump fields, one of which is coupled to a quadrupole transition, can be described by the difference frequency polarization; chi/sup (Q/sub 1/)/ = 2.1 x 10/sup -12/ esu was found for Cs. 8 figs. (RWR)
Date: September 1, 1977
Creator: Bethune, D.; Smith, R.W. & Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Second-harmonic and sum-frequency generation for surface studies

Description: Second harmonic generation (SHG) has now been well established as a versatile surface-sensitive probe. It has been used to study electrochemical processes at electrode surfaces, molecular adsorption and desorption at metal and semiconductor surfaces, orientational phase transition of molecular monolayers on water, surface reconstruction and epitaxial growth, and so on. More recently, it has been employed as a tool to monitor monolayer polymerization and other surface reactions, to probe polar order of molecules at interfaces, and to measure molecular nonlinearity. While most surface techniques are restricted to the solid/vacuum environment, SHG is applicable to nearly all interfaces as long as the interfaces are accessible by light. In addition, SHG has the advantages of being capable of in-situ measurements with high temporal, spatial, and spectral resolutions.
Date: July 1, 1987
Creator: Hunt, J.H.; Guyot-Sionnest, P. & Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Basic considerations on surface optical nonlinearities

Description: The origins of the surface nonlinearity in surface second harmonic generation are discussed. It is shown that this second-order nonlinear optical process is characterized by a surface nonlinear susceptibility tensor containing both local and nonlocal contributions.
Date: January 1, 1986
Creator: Guyot-Sionnest, P.; Chen, W. & Shen, Y.R.
Partner: UNT Libraries Government Documents Department

A Note on Two-Phonon Coherent Antistokes Raman Scattering

Description: Difference-frequency mixing of two pump waves can in principle excite two coherent phonon waves via the parametric process. Only when the phonon excitation is small, can the nonlinear susceptibility of two-phonon coherent antiStokes Raman scattering be described as proportional to the product of two Raman tensors.
Date: April 1, 1980
Creator: Shen, Y. R.
Partner: UNT Libraries Government Documents Department

Studies of molecular adsorbates at interfaces by optical second-harmonic generation. [p-nitrobenzoic acid]

Description: Optical second-harmonic generation (SHG) is surface-specific and has the sensitivity to detect submonolayers of molecules. It can be applied to study molecular adsorbates at the interface between any two centrosymmetric media. This has been demonstrated by using dye and p-nitrobenzoic acid (PNBA) molecules adsorbed on smooth fused quartz substrates as examples. With resonant SHG, the S/sub 0/ A..-->.. S/sub 2/ transition of the adsorbed dye molecules was measured. From the polarization dependence of the SHG, the orientation of PNBA at both the air/quartz and the ethanol/quartz interfaces was deduced. By varying the concentration of PNBA in ethanol, an adsorption isotherm for PNBA at the ethanol/quartz interfaces was also obtained.
Date: December 1, 1982
Creator: Tom, H.W.K.; Heinz, T.F. & Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Investigation of the Si(111) surface in uhv: oxidation and the effect of surface phosphorus

Description: We have studied the initial stages of oxidation, the segregation of phosphorus, and the effect of phosphorus on oxidation of the Si(111) 7 x 7 surface using optical second-harmonic generation. We have also observed a (..sqrt..3 x ..sqrt..3)R30/sup 0/ LEED pattern for P on Si(111).
Date: June 1, 1984
Creator: Tom, H.W.K.; Zhu, X.D.; Shen, Y.R. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Molecular spectroscopy by stepwise two-photon ion-pair production at 71 nm

Description: The Rydberg states of H/sub 2/ have been a continuing subject of intensive study by various research groups. However, understanding of the high lying electronic states of this molecule has been inhibited by the lack of spectroscopic data in the region <75 nm. Experimental studies have been difficult because spectroscopic features are generally buried under an intense absorption or photoionization continuum. Intense, high-resolution excitation sources are not easily available. Recent developments on tunable, narrowband, coherent xuv sources provide new means of studying the spectroscopy in this region with high resolution (+- .0005 nm). We have applied the technique of stepwise two-photon excitation to study photoionization of H/sub 2/ in a molecular beam using the two lowest excited states of H/sub 2/ as the intermediate level. This excitation, coupled with the detection of background-free H/sup -/ ions has enabled us to uncover, for the first time, spectroscopic features that are difficult to observe in positive ion detection. These features have been successfully assigned to new Rydberg series converging to the high vibrations of the H/sub 2//sup +/ ground electronic state.
Date: June 1, 1985
Creator: Kung, A.H.; Page, R.H.; Larkin, R.J.; Shen, Y.R. & Lee, Y.T.
Partner: UNT Libraries Government Documents Department

Surface nonlinear optics

Description: Surface electromagnetic waves are waves propagating along the interface of two media. Their existence was predicted by Sommerfield in 1909. In recent years, interesting applications have been found in the study of overlayers and molecular adsorption on surfaces, in probing of phase transitions, and in measurements of refractive indices. In the laboratory, the nonlinear interaction of surface electromagnetic waves were studied. The preliminary results of this recent venture in this area are presented.
Date: January 1, 1980
Creator: Shen, Y.R.; Chen, C.K. & de Castro, A.R.B.
Partner: UNT Libraries Government Documents Department

Surface vibrational spectroscopy of pure liquids

Description: We report the use of infrared visible sum frequency generation (SFG) to obtain the surface vibrational spectra of pure liquid methanol and water. These are the first surface vibrational spectra ever obtained for pure liquids. We have also deduced from the SFG results the absolute orientations of molecules at the pure liquid/vapor interface. The surface methanol molecules appear to have their CH{sub 3} groups projecting out of the liquid in agreement with the theoretical prediction. For the orientation of surface water molecules, however, different calculations have yielded very different predictions. Our SFG measurement provides clear evidence that the molecules are oriented with an unbonded hydrogen projecting out of the liquid. 9 refs., 3 figs.
Date: March 1, 1991
Creator: Superfine, R.; Huang, J.Y.; Du, Q. & Shen, Y.R.
Partner: UNT Libraries Government Documents Department

Effects of reagent translational and vibrational energy on the dynamics of endothermic reactions

Description: The endothermic reactions Br + CH/sub 3/I ..-->.. CH/sub 3/ + IBr (..delta..H/sub 0//sup 0/ = 13 kcal/mole) and Br + CF/sub 3/I ..-->.. CF/sub 3/ + IBr (..delta..H/sub 0//sup 0/ = 11 kcal/mole) have been studied by the crossed molecular beams method. Detailed center-of-mass contour maps of the IBr product flux as a function of recoil velocity and scattering angle are derived. For both systems it is found that the IBr product is sharply backward scattered with respect to the incident Br dirction, and that most of the available energy goes into product translation. Vibrational enhancement of the Br + CF/sub 3/I reaction was investigated by using the infrared multiphoton absorption process to prepare highly vibrationally excited CF/sub 3/I. At a collision energy of 31 kcal/mole (several times the barrier height), reagent vibrational energy appears to be less effective than an equivalent amount of (additional) translational energy in promoting reaction. More forward scattered IBr is produced in reactions of Br with vibrationally hot CF/sub 3/I.
Date: July 1, 1981
Creator: Krajnovich, D.; Zhang, Z.; Huisken, F.; Shen, Y.R. & Lee, Y.T.
Partner: UNT Libraries Government Documents Department