Articles you may be interested inThe intermolecular potential energy surface for CO2-Ar: Fitting to highresolution spectroscopy of Van der Waals complexes and second virial coefficients High resolution infrared spectra of H2-Ar, HD-Ar, and D2-Ar van der Waals complexes between 160 and 8620 cm−1The structure, dissociation dynamics, and intermolecular potential energy surfaces of the He 79 Br 2 van der Waals molecule have been studied using high resolution, two color, pump-probe laser induced fluorescence spectroscopy and three dimensional quantum mechanical calculations. A conical nozzle produces higher centerline cluster densities than a standard nozzle, and allows data collection further downstream from the nozzle. This yields improved signal to noise ratios and lower Doppler widths. He 79 Br 2 is found to have a T-shaped average geometry with He to Br 2 center-of-mass distances of 3.98 Å and 4.11 Å for the X and B states, respectively, somewhat longer than previously reported. Spectra were also obtained for excitation to excited bending levels of the van der Waals coordinate. However, these spectra have yet to be rotationally assigned. Vibrational predissociation line widths for the B state of He 79 Br 2 have been measured for three new vibrational levels and range from 0.036 cm Ϫ1 for B, vЈϭ8 to 0.062 cm Ϫ1 for B, vЈϭ12. These values are somewhat larger than was expected based on previous HeBr 2 linewidth measurements for higher vibrational levels. Forms for the potential energy surface that have previously been used to simulate the spectra of HeCl 2 have been applied to the HeBr 2 data. For the HeBr 2 X state, two potentials are tested. First, a slightly anisotropic, one center Morse-Spline-van der Waals potential with angle dependent parameters is used. Second a much more anisotropic potential obtained from ab initio calculations is tested. The more anisotropic potential produces a significantly better fit to the data. The B state potential is constructed using Morse atom-atom potentials for the short range part of the He-Br interaction. This simple potential is sufficient to fit the main excitation band, but does not yield a fit to spectra involving vibrationally excited van der Waals modes.

Implementation of SERS as an analytical technique is limited because the factors that govern the enhancement of individual vibrational modes are not well understood. Although the chemical effect only accounts for up to two orders of magnitude enhancement, it can still have a significant impact on the consistency of chemical spectral signatures. We report on a combined theoretical and experimental study on the benzenethiol on silver and 4-mercaptophenol on silver systems. The primary and unique finding was that for the benzenethiol on silver system the inclusion of interaction between multiple benzenethiol analyte molecules was essential to account for the relative enhancements observed experimentally. An examination of the molecular orbitals showed sharing of electron density across the entire model of multiple benzenethiol molecules mediated by the metal atoms. The addition of multiple 4-mercaptophenol molecules to the theoretical model had little effect on the predicted spectra, and we attribute this to the fact that a much larger model is necessary to replicate the networks of hydrogen bonds. Molecular orientation was also found to affect the predicted spectra, and it was found that an upright position improved agreement between theoretical and experimental spectra. An analysis of the vibrational frequency shifts between the normal Raman spectrum of the neat compound and the SERS spectrum also suggests that both benzenethiol and 4-mercaptophenol are in an upright position.

The first results are reported from a new single-particle two-color laser time-of-flight mass spectrometer, incorporating a combination of infrared (CO(2)) and UV (excimer) laser irradiation. This combination of lasers has the capability to effectively separate the desorption or evaporation step from the ionization step, thereby greatly improving the analytical capabilities of such an instrument. The results on liquid aerosols, such as aniline, show that prior evaporation of the aerosol particle with the IR laser increases the ion signal produced by the excimer laser by more than 2 orders of magnitude. In the case of nitrobenzene aerosols, the excimer laser alone produces no ions, while a very large signal is observed when the aerosol is first irradiated with the CO(2) laser. A simple model, based on the Coulomb explosion of the ionized aerosol, is used to estimate the number of ions generated by the excimer laser (∼10(5) ions). Experimental evidence based on the observed time delay of protonated aniline parent ions indicates that the laser irradiation of the liquid aerosol results in a stable neutral plasma which separates into positive and negative charges only after a 100-500-ns delay.

ABSTRACT. The rst results are reported from an investigation into the evaporation dynamics of liquid aerosols us ing a newly constructed two color laser time-of-( ) ight mass spectrometer, which utilizes a CO laser IR for vaporization and an