Robert L. Swofford

Molecular conformers in gas-phase ethanol: A temperature study of vibrational overtones

Abstract Overtone absorption spectra are reported for ethanol vapor (10150–19900 cm −1 ) measured by intracavity photoacoustic spectroscopy. The OH overtones are composed of two sub-bands which are assigned as the transitions of two conformers of the OH bond in the trans or gauche position with respect to the methyl group. From the temperature dependence of the OH overtone intensity we determine the enthalpy difference between the conformers to be 0.7 ± 0.1 kcal/mole.

Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. I. CHCl3, CHBr3, CH2Cl2, CH2Br2

Visible absorption spectra (14 700–19 100 cm−1) measured with the thermal lensing spectrometer and near‐infrared absorption (5 600–14 000 cm−1) measured with a conventional spectrophotometer are reported for CHCl3, CHBr3, CH2Cl2, and CH2Br2. Relatively strong absorption peaks are identified as overtones of the C–H stretching vibrations in these compounds. The overtone spectra are analyzed in terms of the local mode (LM) model, which treats the molecule as a set of loosely coupled anharmonic oscillators localized on individual C–H bonds. Relatively less intense peaks are observed and are assigned as combinations of a local mode C–H vibration and some lower frequency (normal) mode of the molecule. Fermi resonance is seen to occur in the spectra of CHCl3 and CHBr3, resulting in anomalously high intensities for the combination bands involved. Significant anharmonic local–normal coupling constants are seen for these same molecules. These coupling constants appear to indicate the presence of strong physical cou...

Local mode overtone intensities of C–H stretching modes in alkanes and methyl substituted benzenes

A thermo‐optical spectrometer is used to resolve absorption peaks due to nonequivalent C–H local mode vibrations and combination bands in several molecules containing aromatic, primary, and secondary hydrogens. Although the bandwidths vary, a constant absorption cross section per C–H is observed for the transitions to the v=3,4,5, and 6 overtone levels. This result stands in contrast to the behavior of the fundamental transitions, where C–H stretching intensities are seen to vary by as much as a factor of four between aromatic and aliphatic C–H bonds.

The effect of spatial and temporal laser beam characteristics on two-photon absorption

Abstract We present calculations which indicate possible sources of error in the determination of two-photon absorptivity introduced by the choice of experimental conditions. Both the direct absorption experiment and the two-photon excitation experiment are considered. We include the effects of beam alignment, diffraction-limited focusing, and pulse width variation on the observed two-photon absorptivity.

Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. II. Normal, branched, and cycloalkanes

The C–H stretching overtones of n‐pentane, n‐hexane, n‐heptane, n‐octane, n‐nonane, n‐decane, n‐undecane, n‐dodecane, 2‐methylbutane, 2‐methylpentane, 3‐methylpentane, 2,2,4‐trimethylpentane, cyclopentane, cyclohexane, methylcyclohexane, cycloheptane, and cyclooctane in the liquid phase are reported as seen by conventional absorption (5600–15 800 cm−1) and thermal lensing (14 700–18 500 cm−1) spectroscopy. A local mode (LM) model based on uncoupled, anharmonic C–H oscillators is used to analyze the observed spectra. Relatively strong, well‐resolved absorption peaks are observed and are assigned within the LM model as the ’’pure’’ vibrational overtones of nonequivalent C–H oscillators in the molecule. The relative intensities of the absorption bands assigned to primary and secondary C–H oscillators at a given level of excitation reflect the relative numbers of primary and secondary hydrogens in the molecule. Both LM anharmonicity constants and LM mechanical frequencies are obtained from the overtone analys...

Photoacoustic spectroscopy of vibrational overtones in polyatomic molecules

Intracavity gas-phase photoacoustic spectroscopy is used to study the near IR and visible overtone spectra of propylene, 2-butene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, acetone, 2-butanone, and 3-pentanone. The spectra are described in terms of the local-mode theory of vibrations as the absorption of loosely coupled anharmonic C-H oscillators within the molecule.

Two‐photon absorption photothermal spectroscopy using a synchronously pumped picosecond dye laser. Thermal lensing spectra of naphthalene and diphenylbutadiene

We demonstrate the use of a synchronously pumped picosecond dye laser as the excitation source for two‐photon photothermal (thermal lensing) spectroscopy. The two‐photon absorption spectrum of naphthalene in CCl4 is recorded in the range 31 450–35 200 cm−1, and the measurement is in agreement with the previously reported two‐photon excitation spectrum. Similar agreement is found for the spectrum of diphenylbutadiene in the range 30 000–35 200 cm−1. It is demonstrated how the technique can provide accurate values for two‐photon absorption cross sections by comparison of the thermal lens signals from one‐ and two‐photon absorption. The two‐photon absorption measurement in naphthalene indicates a symmetry‐forbidden, vibronically induced transition, while the measurement in diphenylbutadiene indicates a fully symmetry‐allowed two‐photon transition. Both cross section measurements are in agreement with previously reported results.

Overtone absorption spectra of gaseous amines: CH3NH2, (CH3)2NH, Et2NH, and N2H4. Conformationally non-equivalent CH and NH local mode oscillators

Abstract Overtone absorption spectra are resorted for gas-phase methylamine, dimethylamine, diethy lamine, and hydrazine as measured by intracavity photoacoustic and FT IR spectroscopy. Prominent features in the spectra are assigned as NH and CH stretching overtones within the local mode model of loosely coupled, anharmonic vibrations. In dimethylamine, CH overtone bands arising from more than one conformation are observed. In dimethylamine, CH stretching frequencies are observed that correspond to the methyl CH bond trans to the other methyl, trans to the hydrogen, or trans to the electron lone pair on nitrogen. The NH overtones in hydrazine also show distinct bands arising from conformationally non-equivalent oscillators.

New two−photon absorption spectrometer and its application to diphenylbutadiene

We describe a new spectrometer which promises to increase by several orders of magnitude the sensitivity for measurement of aboslute two−photon absorptivities. The spectrometer employs two TEM00 laser beams propagating in opposite directions through the sample. A tunable pulsed laser generates the powerful beam and a continuous krypton ion laser provides a constant frequency probe beam for the two−photon spectrometer. We report the two−photon absorptivity of 1−chloronaphthalene at 35 400 cm−1 to be 1.3×10−50 cm4 sec photon−1 molecule−1, in good agreement with previously reported values. We also report the two−photon absorptivity of 1,4−diphenyl−1,3−butadiene at 28 259 cm−1 to be 1.0×10−50 cm4 sec photon−1 molecule−1. This relatively high value indicates the presence of a previously undetected 1Ag excited state as the lowest excited singlet state of this molecule.

Peak Height Measurement System for Pulsed Laser Experiments

A peak height measurement system (PHMS) is described which detects and measures the peak height of several simultaneous pulses with peaks as narrow as 1 nsec. Jitter in the input pulses produces no error in the measured heights. A detailed description of the peak detector circuit is given, and an experimental setup is presented for determining two‐photon absorption cross‐sections with PHMS. It is expected that this system will see application to numerous experiments which use pulsed lasers.