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High-resolution Fourier-transform infrared spectra of Al(BH4)3 and Al(BD4)3 were obtained for low-pressure samples at room temperature. Results indicated only wide vibrational bands with no resolved rotational structure. The BH bridge stretching and the BH2 deformation areas were also analyzed at 0.001/cm resolution through tunable infrared diode laser spectroscopy for samples cooled to about 10 K in a multipass, slit expansion. Results showed near continua, with much more crowding than predicted by nonrigid rotor simulations based on boron isotopic shifts.

Undergraduate computational chemistry can be significantly enhanced through the use of Gaussian for Windows, allowing students to explore ab initio computations and the effects of varied calculational levels and orbital choices. Free-standing personal computers have advantages over mainframes in that they do not require computer accounts, have quick processors and do not have batch queues. An experiment using Gaussian for Windows to calculate the diatomic molecule HCl's electronic and structural properties is provided.

Discusses electron spin resonance, a form of magnetic resonance spectroscopy, used to study electronic structure of molecules with unpaired electrons. Studies benzosemiquinone radical anions which are long-lived and inexpensive. Uses a Project SERAPHIM computer program to perform a Huckel molecular orbital calculation. (MVL)

A laboratory experiment involving the isotope exchange reaction between DCl and HBr reveals a close relation of quantum theory with statistical mechanics and measurements at the microscopic level. A gas-phase NMR experiment with sensitive, pulsed NMR instruments efficiently measures the equilibrium constant, K(sub p), for the reaction. The reaction between D2O and benzoyl chloride produces DCl.

High-resolution coherent anti-stokes Raman spectroscopy (CARS) is used to study the Nu(sub 1) symmetric CO stretching mode of the quasi-linear molecule carbon suboxide, C3O2. The results obtained indicate that the ground-state barrier to linearity increases by only 0.6 cm(super -1) when the CO symmetric stretch is excited.

A study of the fluorescent lifetime of exited iodide and its variation with pressure when different quenching gases are added, are experimented. This experiment gives students valuable experience with lasers and computerized instrumentation.

A new method for measuring state-to-state rotational energy transfer in crossed supersonic beams is described. The method is based on direct absorption of tunable, high-resolution infrared laser light by target molecules collisionally excited into final rotational states. The direct IR absorption approach offers high sensitivity, full quantum state resolution, a Doppler probe of final velocity components, and is applicable to any target molecule that absorbs in the near IR. Preliminary results are presented for Ar+CH4 scattering in crossed supersonic beams at a mean center-of-mass collision energy of 41 meV. Because of the high spectral resolution, the method can readily distinguish rotational fine structure states of A, F, and E symmetry in the tetrahedral group, as well as the much more energetically separated final j states. The results are compared with full quantum close-coupling calculations on two different Ar+CH4 potential energy surfaces. The state-to-state scattering results provide a sensitive measure of the potential anisotropy, and in particular probe the relative magnitudes of the different anisotropic terms in the potential (V3 and V4). [ABSTRACT FROM AUTHOR]

Double resonance spectra have been obtained for nitrogen and benzene molecules in the low density region of a molecular beam by use of stimulated Raman pumping followed by ionization probing. For N2 the photoionization step was via a 2+2 resonantly enhanced multiphoton ionization process using the a1Πg(v=2)←X1Σg(v=1) two-photon resonance. The detection limit is 103–104 times lower than for the direct optical measurement of a stimulated Raman signal. For benzene, a more efficient 1+1 ionization step permitted measurements at state densities less than 1010 cm-3 and the Raman step yielded linewidths of 30 MHz for the Q-branch lines of the weak Raman active ν16 fundamental. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High resolution coherent anti-Stokes Raman spectroscopy has been used to study the symmetric CH stretching mode of methyl radical formed by UV laser photolysis of CH3I cooled in a free jet expansion. Spectra obtained under near-nascent conditions (∼3 to ∼6 collisions) show that most of the CH3 product is formed in the ground vibrational state, with little excitation seen in the ν2 out-of-plane bending coordinate [v2=1/v2=0, population ratio 0.27(10)]. This is in accord with recent theoretical calculations favoring slow, adiabatic CH3 relaxation from a pyramidal to planar structure as the C–I bond breaks. Extensive N,K rotational structure is resolved and the distributions obtained lend support to those deduced for nascent CH3 by Chandler and co-workers from modeling of unresolved resonance enhanced multiphoton ionization spectra. The results are consistent with conservation during dissociation of CH3 ortho-para nuclear spin forms and of K spinning angular momentum about the symmetry axis. CH3 product tumbling motion is slightly greater (1–2 units of angular momentum) than predicted theoretically. Rapid collisional excitation of higher rotational levels is seen and the rich spectrum observed after ∼50 collisions has been analyzed to give improved or new vibrational-rotational parameters for 1000 and 1100 states. The 1100 band origin is determined to be 2996.21(4) cm-1, yielding -8.23(5) cm-1 for the x12 anharmonicity constant. [ABSTRACT FROM AUTHOR]

Coherent anti-Stokes Raman spectroscopy has been used to study C2H6 in the rotational and ν3 CC stretching regions. Pure rotational transitions for torsionally excited molecules are seen for the first time and direct analysis of the peak maxima gives B values of 0.663 06 and 0.660 37 cm-1 for the υ4=0 and 1 torsional levels. The change in B is combined with ab initio calculations to show that the primary response of the molecule to the increased torsional amplitude is a CC extension of 0.0031 Å, plus a slight increase of 0.10 degrees in the CCH angle. For the ν3 vibrational Q branch, a complex mixture of four band systems is seen, a consequence of Fermi resonance interactions causing the predicted four torsional sublevels of this state to separate. Spectra of jet-cooled samples aid in the assignment of the spectrum and the four torsional components are determined as 994.973 (E3s), 994.878 (E3d), 994.864 (A1s), and 993.791 cm-1 (A3d). The A and B rotational constants decrease by 0.003 27 and 0.006 21 cm-1, respectively, when ethane is excited in the ν3 mode. These differences are used with the ab initio results to deduce that this transition results in a net increase of 0.010 Å in the average CC distance, accompanied by a decrease of 0.12 degrees in the CCH angle and a very small CH decrease of 0.0002 Å. The calculations also suggest that the V3 torsional barrier for the υ3=1 state is about 3% smaller than for the ground state. [ABSTRACT FROM AUTHOR]

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational–rotational Q-branch structure of the ν1 symmetric stretch of methyl radicals produced by 266 nm laser photolysis of methyl iodide. Spectra were recorded in both flow cells and free jet expansions at instrumental resolutions ranging from 0.25 to 0.005 cm-1. Due to the high recoil velocity of the CH3 fragment, Doppler and collisional broadening of the transitions is appreciable. Even at the highest resolution such broadening of the transitions leads to interference effects among the closely spaced Raman transitions that influence both the line positions and intensities in the observed CARS spectra. The molecular parameters (cm-1) obtained from the analysis are ν1=3004.42(4), αB1=0.0851(8), αC1=0.0475(7), DN1-DN0=-0.000 046(8), DNK1-DNK0=0.000 083(20), and, with assumptions, DK1-DK0=-0.000 039. These results and infrared data in the literature yield a CH bond length of 1.08378(5) Å for the (1000) state and, with some assumptions, an equilibrium bond length Re of 1.076 Å for this prototypic case of sp2 bonding. [ABSTRACT FROM AUTHOR]

High resolution stimulated Raman spectroscopy (SRS) has been used to examine N2 and N2/He free jet expansions and also equilibrium samples of N2 from 15 to 110 K. The jet spectra show the formation of large liquid clusters which supercool and subsequently freeze to form crystalline β-N2 solid and, in He expansions, undergo a further transformation to a partially annealed α-N2 form. CW-SRS frequency and linewidth data obtained for equilibrium samples of the condensed phases of N2 yielded frequency–temperature relations used in deducing internal temperatures for the clusters produced in the expansion experiments. Analysis of the cooling curves indicates a mean cluster diameter of 35 nm and favors a prompt freezing process rather than a gradual conversion of liquid to solid in a single cluster on the microsecond time scale of the experiments. Supercooling limits of 34 to 44 K are deduced for the liquid, far below the triple point temperature of 63.2 K at which equilibrium samples freeze. Some evidence for surface versus bulk contributions to the spectra is seen in the asymmetric line shapes observed for liquid clusters in the condensation region. The results show that the high spectral and spatial resolution of nonlinear Raman methods such as SRS and CARS provide a unique probe of the condensation processes in free jet expansions. [ABSTRACT FROM AUTHOR]

Photothermal lensing effects are observed in the early expansion stages (X/D=1–7) of a supersonic free jet and narrow P and R branch rotational lines have been recorded at 100 MHz resolution for the ν2 +5ν3 band of acetylene. Rotational Raman loss spectra were also recorded and, from the collision-broadened linewidths of these, it is estimated that about 100 collisions occur in the beam transit from pump to probe positions. This is sufficient to produce rotational and some vibrational relaxation in forming the lens. The photothermal intensities are consistent with a Boltzmann rotational distribution in the ground state and a rotational temperature of 70 K is deduced for 100 PSI expansions of 20% C2 H2 in helium. Charac- teristics of the photothermal signal in the jet are described and the time and spatial dependence of the signal is used to measure a flow velocity 625 m/s for the jet. This photothermal method may prove useful as a diagnostic probe of beam properties and as a spectroscopic measure of weak one- and two-photon transitions of molecules cooled in jets. [ABSTRACT FROM AUTHOR]

Describes a nonlinear optical experiment demonstrating stimulated Raman scattering in benzene and deuterated benzene. Experimental setup for stimulated Raman measurements; Force constant displays for benzene vibrations; Loci of solutions of the quadratic equation; Enthusiastic student response to the experiment.