Specific Molecular Vibrations Research Articles

FEL induced dynamics of small molecules on surfaces: N2O on NaCl(1 0 0)

The infrared laser-induced desorption of N2O condensed on the single crystal surface of NaCl(100) is studied. In investigations of the desorption process of molecules from surfaces as well as of the dynamics of vibrationally excited adsorbate systems, a selective excitation of specific molecular vibrations is desirable. The Free Electron Laser for Infrared eXperiments “FELIX” is providing rapidly tunable infrared radiation that is well suited for this kind of investigations. A comparison of the wavelength dependent desorption yield to the linear absorption spectrum shows that desorption of intact, neutral molecules occurs when FELIX is resonant with an infrared active mode of the molecules. An analysis of the time-of-flight spectra as well as the dependence of the desorption yield on the layer thickness, laser fluence and substrate temperature point towards a resonant heating mechanism. First measurements on the dynamics of the vibrationally excited adsorbate using pump–probe spectroscopy are presented.

Plasma characterization during laser ablation of graphite in nitrogen for the growth of fullerene-like CNx films

Chemistry, energy, and spatial distributions of species in carbon–nitrogen plasma plumes were investigated to define plasma conditions for growth of carbon nitride CNx films with a fullerene-like structure. Plumes were generated by ablation of graphite using a 248 nm excimer laser in the presence of low-pressure nitrogen. The plumes were investigated using element specific imaging, time-of-flight experiments, fluorescence spectroscopy, and molecular vibration sequence analyses. Studies showed the importance of plume/substrate interaction in causing secondary excitation phenomena. For N2 pressures within the 5–50 mTorr range, plasmas at the substrate vicinity were found to consist mostly of atomic carbon, CN and C2 molecules. Kinetic energies were calculated within 10–20 eV for mono atomic carbon, 30–55 eV for CN, and 20–40 eV for C2. Excited CN and C2 molecules were generated by laser ablation and by collisions of the plume with the substrate surface. Their vibrational energies were strongly influenced by nitrogen pressure and time after a laser pulse. For pressures below 30 mTorr, vibrational energy was as high as 4.0 eV at 2–4 μs for CN and 2.5 eV at 8–10 μs for C2. This low pressure was suggested for the growth of fullerene-like CNx films based on correlations between plasma parameters and film composition and bonding. Synthesis of the fullerene-like structure required high molecular temperatures at the condensation surface. High concentrations of CN radicals in the plasma promoted nitrogen incorporation into the films. Correlations among CNx film composition/bonding, excitation maximums, and kinetic/vibrational energies of atomic carbon, CN and C2 species located near the condensation surface are discussed.

Resonant infrared laser-induced desorption of CD3F condensed on NaCl(100)

In investigations of the dynamics of the desorption of molecules from surfaces, a selective excitation of specific molecular vibrations is desirable. We here report studies on the infrared laser-induced desorption of CD3F condensed on the NaCl(100) single crystal surface, in which the intramolecular vibrations of the CD3F molecules are directly excited. Desorption of the intact molecules is monitored using electron impact ionization followed by mass-selective detection in a quadrupole mass spectrometer. Desorption only occurs when the excitation laser is at 9.0, 10.5, 9.4, or 11.0 µm, corresponding to excitation on the ν2, ν3, ν5, and ν6 modes of CD3F. An analysis of the time-of-flight spectra as a function of the laser fluence is presented. Different desorption mechanisms are discussed.

Anomalous Raman scattering from the surface of conjugated polymer melts

Confocal Raman spectroscopy from the surface of a polymer melt is shown to produce anomalous results for polarized Raman scattering of specific molecular vibrations. Values of the depolarization ratio that cannot exist in normal liquids are interpreted as due to surface-induced molecular orientation of the polymer chains. The technique provides one of the few direct experimental evidences of molecular-orientation effects predicted in lattice models of polymer melts.

Time-resolved infrared spectroscopy of molecule/binding site reorientation during ferroelectric liquid crystal electro-optic switching

Polarized Fourier transform infrared (IR) absorption is used to probe molecular conformation in a ferroelectric liquid crystal during the large-scale collective reorientation induced by external applied electric field. Spectra of planar-aligned cells of the ferroelectric liquid crystal (FLC) W314 ((S)-4(')-(decyloxy)4-[(1-methylheptyl)oxy]-2-nitrophenyl-[1, 1'-biphenyl]-4-carboxylic acid ester) are measured as functions of IR polarizer orientation and time following the reversal of the electric field applied to the FLC. The time evolution of the dichroism of the absorbance due to the specific molecular vibration modes, particularly from the biphenyl core and alkyl tail, is observed. Static IR dichroism experiments show a W314 IR dichroism structure in which the principal axis of the dielectric tensor from molecular core vibrations are tilted further from the smectic layer normal than those of the tail. This structure indicates that the effective binding site in which the molecules are confined in the Sm-C phase has, on average, a "zig-zag" shape. The dynamic experiments show that this zig-zag binding site structure is rigidly maintained while the molecular axis rotates about the layer normal during field-induced switching.

Fluorinated NLO polymers with improved optical transparency in the near infrared

AbstractThe absorption from vibrational overtones in the near infrared and specifically at 1.55 μm, is a major contribution to the optical loss of NLO polymer waveguides. In an effort to reduce this value, the specific molecular vibrations responsible, which had previously been predicted by theoretical means, were identified by solvent spectra analysis. A scheme was proposed to reduce the intrinsic absorption through synthetic modification of the polymer by replacement of appropriate aliphatic CH bonds with CF bonds. Specific bonds were chosen for replacement based on a consideration of both contribution to intrinsic absorption as well as ease of synthetic modification. The target polymer was synthesized and its absorption measured by a solution technique. A reduction in optical loss at 1.55 µ from 0.75 to 0.35 dB cm−1 was achieved. © 1995 John Wiley & Sons, Inc.

Hyper-Raman vibrational spectroscopy with circularly polarized light

This paper presents a new method for the analysis of hyper-Raman vibrational spectra. It is shown that, by irradiating the sample with circularly polarized light, the measurement of two polarization ratios suffices for a complete analysis of vibrational symmetry. For this purpose a simple graphical method is introduced which can be applied for any convenient scattering geometry. Using this method, it is possible to test experimentally some of the assumptions in earlier hyper-Raman theories. These principles are illustrated by references to several specific molecular vibrations.

Infrared picosecond pulses and applications

By using the three-photon parametric process, tunable infrared pulses between 2500 and 7000 cm -1 can be generated with LiNbO 3 crystals. By varying the time delay between the pump pulse and a signal input pulse the duration of the i.r. pulses may be changed. I.r. pulses as short as 0.5 ps have been experimentally observed. The dynamics of vibrational modes of polyatomic molecules was studied by using a two-pulse system. A first i.r. pulse excites a specific molecular vibration while a second properly delayed pulse monitors the instantaneous state of the vibrational system. Two types of experiments are discussed, those where the anti-Stokes Raman scattering or a fluorescence signal is observed.

Odor and molecular vibration: Neural coding of olfactory information

Starting from an established correlation between specific molecular vibration patterns and the olfactory responses of organisms, a method is described by which equivalently specific patterns of neural excitation may be identified electrophysiologically. A molecular mechanism for the interaction is proposed which also takes into account such related matters as olfactory thresholds, and the possibility of both “specialist” and “generalist” receptors.