Y. R. Shen

Infrared predissociation spectra of water dimer in a supersonic molecular-beam

The infrared absorption spectrum of the hydrogen-bonded water dimer, (H2O)2, formed in a supersonic expansion, was recorded in the region of th

MULTIPHOTON DISSOCIATION OF SF6 BY A MOLECULAR BEAM METHOD

The dynamics of infrared multiphoton excitation and dissociation of SF6 has been investigated under collision‐free conditions by a crossed laser–molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments have been carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF6, the pulse duration of the CO2 laser, and the frequency in both one and two laser experiments. Translational energy distributions of the primary dissociation product SF5, measured by time‐of‐flight and angular distributions and the dissociation lifetime of excited SF6 as inferred from the observation of secondary dissociation of SF5 into SF4 and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer and can be adequately described by RRKM theory. An improved phenom...

Studies of surface structures and compositions of polyethylene and polypropylene by IR+visible sum frequency vibrational spectroscopy

Abstract IR+visible sum-frequency generation (SFG) was used to obtain surface vibrational spectra of polyethylene and polypropylene. It was found that low density and ultrahigh-molecular-weight polyethylene have different surface structures. The latter has a surface mainly composed of orderly packed gauche conformers while the former has a surface with a more random distribution of polymer chains. The SFG spectra also show that surface structures of atactic and isotactic polypropylenes are different. Segregation of additives at surfaces of industrial polyethylene and polypropylene, that cannot be detected by IR and Raman spectroscopies, can be readily observed by SFG.

Competition between atomic and molecular chlorine elimination in the infrared multiphoton dissociation of CF2Cl2

Infrared multiphoton dissociation of CF2Cl2 has been reinvestigated by the crossed laser‐molecular beam technique using a high repetition rate CO2 TEA laser. Both the atomic and molecular chlorine elimination channels were observed: (1) CF2Cl2→CF2Cl+Cl and (2) CF2Cl2→CF2+Cl2. No evidence was found for secondary dissociation of CF2Cl at laser energy fluences up to 8 J/cm2. Center‐of‐mass product translational energy distributions were obtained for both dissociation channels. In agreement with previous work, the products of reaction (1) are found to have a statistical translational energy distribution. The products of reaction (2) are formed with a mean translational energy of 8 kcal/mol, and the distribution peaks rather sharply about this value, indicating a sizeable exit barrier to molecular elimination. The product branching ratio was directly determined. Reaction (2) accounts for roughly 10% of the total dissociation yield in the fluence range 0.3–8 J/cm2. These results provide an additional test of th...

A novel method for measurements of second-order non-linearities of organic molecules

Abstract The second-order non-linear polarizabilities of a number of organic molecules have been determined by measuring the optical second-harmonic generation from monolayers of such molecules spread on a water surface.

Interference between second-harmonic generation from a substrate and from an adsorbate layer

Several experiments on surface second-harmonic generation are presented to show how the contribution from a transparent substrate can interfere with that from an adsorbate monolayer. The interference depends on the relative phase of the two contributions, which varies with the molecular orientation, the laser frequency, the polarization, and the optical geometry.

Probing the proton excess at interfaces by second harmonic generation

Abstract Optical second harmonic generation from dye molecules that can be protonated at a lipid/water interface are used to determine the proton excess at interfaces. The surface pH value at a stearic acid monolayer/water interface has been measured in response to variations of the bulk pH value and the salt concentration.

Second-order nonlinear polarizability of various biphenyl derivatives

The second-order nonlinear polarizabilities of several organic molecules derived from the cyanobiphenyl structure are compared by using second-harmonic generation from molecular monolayers. Changes in optical nonlinearity with changes in molecular structure are discussed.

Vibrational and electronic spectroscopy of pyridine and benzene adsorbed on the Rh(111) crystal surface

We report the vibrational and electronic spectra for pyridine and benzene adsorbed on the Rh(111) crystal surface obtained by high‐resolution electron energy loss spectroscopy (HREELS). Low‐energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and optical second harmonic generation (SHG) have also been used to provide complementary information. Pyridine adsorption on Rh(111) was studied over the 77–450 K temperature range. At 77 K, multilayers of pyridine are observed with a vibrational spectrum similar to that of liquid pyridine. Between 185 and 230 K, HREELS and TDS indicate that both physisorbed and chemisorbed pyridine species are present on the surface. The physisorbed species desorbs at 295 K, while the chemisorbed species is stable until it decomposes on the surface at 400 K. We propose that the chemisorbed species is an α‐pyridyl complex as thermal desorption spectroscopy indicates partial dehydrogenation of this pyridine surface species. Electronic energy loss spectra for bot...

MEASUREMENT OF REFRACTIVE INDICES AND STUDY OF ISOTROPIC-NEMATIC PHASE TRANSITION BY THE SURFACE PLASMON TECHNIQUE

Abstract The surface plasmon technique is employed to measure the refractive indices and to probe the nematic-isotropic phase transition of 4-cyano-4′-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.