A phase-sensitive sum-frequency vibrational spectroscopic technique is developed to study interfacial water structure of water/quartz interfaces. Measurements allow deduction of both real and imaginary parts of the surface nonlinear spectral response, revealing an unprecedentedly detailed picture of the net polar orientations of the water species at the interface. The orientations of the icelike and liquidlike species appear to respond very differently to the bulk pH change indicating the existence of different surface sites on quartz with different deprotonation pK values.

Victor Ostroverkhov;Glenn A. Waychunas;元壤 沈

Physical Review Letters

2005-2-4

Second-harmonic generation is shown to be sensitive enough to detect molecular monolayers adsorbed on a silver surface. Adsorption of AgCl and pyridine on silver during and after an electrolytic cycle can be easily observed.

C. K. Chen;T. F. Heinz;D. Ricard;元壤 沈

Physical Review Letters

1981

It is shown that the small-scale filaments generated by laser pulses in nonlinear media are the natural consequence of moving foci. Results of many controlled experiments performed with single-mode Q-switched laser pulses agree very well with the predictions of the moving focus model. The earlier experimental results on filaments reported in the literature are also shown to be consistent with the moving focus model. In the picosecond case, this model appropriately modified, gives essentially the same description as the so-called dynamic trapping model, which explains qualitatively the observed filament characteristics in that limit.

Michael M.T. Loy;元壤 沈

IEEE Journal of Quantum Electronics

1973-3

Rotational cooling by supersonic expansion and laser labeling spectroscopy were used to obtain high-resolution fundamental and first- and second-overtone spectra of C-H stretches of benzene in a molecular beam. Rich fine structure in the spectra has been observed. Transformation from the normal-mode picture to the local-mode picture seems to occur between v=2 and v=3. Vibrational lifetimes in local-mode bands exceed f0.5 psec.

Ralph H. Page;元壤 沈;And Y.T. Lee

Physical Review Letters

1987

Results of Raman measurements on I₂ complexes in various solutions are presented. Emphasis is placed on the variation of the Raman spectrum of I₂ in mixtures of n-hexane and benzene or methylated benzenes. Our results indicate that each I₂ molecule can probably interact with more than one donor and the effect of inert molecules in the solution should be taken into account.

H. Rosen;元壤 沈;F. Stenman

Molecular Physics

1971

In this paper, it is shown that important structural information of liquids can be deduced by use of third-harmonic generation (THG) and infrared-visible sum-frequency generation (IVSFG). A simple molecular model based on the hydrogen bonding strength will be proposed to explain the features observed in the measured THG susceptibility. A different behavior from that of CH₃OH/H₂O was observed from the study of CH₃CN/H₂O with THG technique.

J. Y. Huang;M. H. Wu;元壤 沈

1994

IR-visible sum-frequency generation as a versatile surface vibrational spectroscopic tool is described. Applications to neat water interfaces and hydrogen adsorption on diamond are used as illustrative examples.

元壤 沈

1994

We have observed far-infrared radiation generated by picosecond pulses in LiNbO₃ with several different phase-matching conditions. The output spectra, analyzed by a far-infrared Michelson interferometer and by a Fabry-Perot interferometer, agree well with theoretical calculations. The laser pulsewidth deduced from these measurements was about 2 psec in comparison with 5 psec obtained from two-photon fluorescence measurements.

K. H. Yang;P. L. Richards;元壤 沈

Applied Physics Letters

1971

The paper demonstrated the possibility of using sum-frequency generation (SFG) to obtain vibrational spectra of water at hydrophobic interfaces, and the results show that hydrophobicity is characterized by the appearance of dangling OH bonds on 25% of the surface water molecules. Moreover, the results show that structure of water molecules depends on the interface rigidity.

E. Freysz;Q. Du;元壤 沈

1994

It is well known that second harmonic generation (SHG) in atomic vapor is highly forbidden by symmetry. Experimentally, however, with input laser pulses tuned to either one or two-photon resonances, appreciable amount of second harmonic generation in alkali vapor can be detected. In nearly all cases, nanosecond pulses were used. It has been proposed that multiphoton ionization of the vapor followed by the electron separation from ions may lead to the creation of macroscopic dc electric field that breaks the inversion symmetry and gives rise to the second harmonic generation. Some experimental results obtained with nanosecond lasers seem to support this proposed mechanism. To our knowledge, there exists no report on SHG by picosecond pulses with resonance excitation in atomic vapor. There is also the question whether the above mentioned dc-field-induced SHG mechanism can operate on the picosecond time scale. We have studied SHG in potassium vapor using tunable picosecond resonant pulse excitations. Each pump pulse, derived from an amplified mode-locked dye laser pulse, had an energy of 200 μJ and a pulsewidth of 2 ps. Focusing the pump beam to confocal dimensions of 20 cm × 200 μm in a heat-pipe containing approx.1 torr potassium vapor and tuning the laser to two-photon resonance of 4s-9d or 4s-11s generated a readily detectable SH signal. The signal strength as a function of input laser pulse energy is shown in Fig. 2. In comparison with the nanosecond cases, however, the process is much less efficient with a reduction factor of about 10^-5. It also has a higher input intensity dependence before saturation. Other characteristics of SHG were also probed. The SH output showed a two-lobe beam pattern peaked at the maximum of the input intensity gradient. As the density of the potassium vapor was increased, output exhibited a dip resulting from phase mismatch. A time-delayed pump-probe experiment was carried out to investigate the time variation of the SHG process. The results can be explained by the dc-field-induced SHG model mentioned earlier. However, in contrast to the nanosecond cases, the transient coherence established between the ground (4s) and the excited (e.g., 9d) states and the time development of the dc field originated from multiphoton ionization are important.

Doseok Kim;Christopher S. Mullin;元壤 沈

1994