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Volume 81, Issue 9, 01 November 1984

Detection of the NH_{2}O radical by far infrared LMR
View Description Hide DescriptionNew far infrared lasermagnetic resonancespectra have been detected at 11 laser wavelengths using the reaction of fluorine atoms with NH_{2}OH as source. Weaker but identical spectra were found in the reaction NH_{2}+O_{3}. On the basis of their chemical behavior and hyperfine structure they are assigned to the NH_{2}O radical. Rotational transitions calculated using a planar symmetric structure from a b i n i t i o calculations are in good agreement with the frequencies of the observed LMR spectra.

The infrared spectrum of KH
View Description Hide DescriptionReported here is the first observation of the infrared spectrum of KH. Fundamental bands and first hotbands of the ^{3} ^{9}KH and ^{4} ^{1}KH isotopic forms were measured at high resolution using a diode laser based spectrometer. An ac glow discharge through potassium vapor and hydrogen gas was used to produce KH. A combined isotopic Dunham coefficient analysis was used to fit all the spectra, with the Dunham coefficients for ^{3} ^{9}KH coming out as Y _{1} _{0}=985.6714(30) cm^{−} ^{1}, Y _{2} _{0}=−14.9013(10) cm^{−} ^{1}, Y _{0} _{1}=3.416 40(10) cm^{−} ^{1}, Y _{1} _{1}=−0.085 313(26) cm^{−} ^{1}, Y _{2} _{1}=5.41(60)×10^{−} ^{4} cm^{−} ^{1}, Y _{0} _{2}=−1.6354(36)×10^{−} ^{4} cm^{−} ^{1}, Y _{1} _{2}=1.13(10)×10^{−} ^{6} cm^{−} ^{1}, Y _{0} _{3}=7.6(8)×10^{−} ^{9} cm^{−} ^{1}, (quoted at 2σ error limits). A bond length of 2.241 152(16) Å and a Dunham corrected value for ω_{ e } of 986.0505(30) cm^{−} ^{1} are obtained. The pressure broadening of KH by H_{2} and the chemistry of KH formation in the glow discharge plasma are also discussed.

The interdependence of small linewidth and frequency changes of a vibration in liquid mixtures of CDCl_{3} and CCl_{4}
View Description Hide DescriptionThe isotropic Raman linewidths and frequency shifts of the ν_{1} (C–D) vibration of CDCl_{3} in CCl_{4} have been measured precisely as a function of the concentration of CDCl_{3} using the recently developed technique of four‐channel Raman difference spectroscopy. The results obtained reveal that even the small variation of the linewidths and frequency shifts with concentration have a strong interdependence. A rigorous analysis of the results with the analytical model [E. W. Knapp and S. F. Fischer, J. Chem. Phys. 7 6, 4730 (1982)] which takes into account simultaneously, the static and dynamic aspects of the concentration fluctuations at the reference molecule, yields that a reasonable agreement with the experimental data is obtained only if the dynamics is included (R>0).

Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CF_{3}H
View Description Hide DescriptionThe absorptionspectrum of trifluoromethane has been recorded between 900 and 14 000 cm^{−} ^{1} with resolutions between 0.004 and 0.5 cm^{−} ^{1} (pressure broadened). 22 bands were assigned as arising from the interacting CH stretching and bending manifolds, which account for most of the absorption in the overtone region. The results can be understood quantitatively with an effective, tridiagonal many‐level Fermi resonance Hamiltonian. The experimental and theoretical results are summarized in Table II. The Hamiltonian is given in Table III and shows a very large stretching–bending interaction constant ‖k _{ s b b }‖=106 cm^{−} ^{1}, which is even larger than the diagonal anharmonic constant for the stretching vibration ‖x ^{′} _{ s s }‖=62 cm^{−} ^{1}. This leads to extensive vibrational redistribution between stretching and bending motions at high levels of excitation. The time dependent redistribution is calculated with the spectroscopic Hamiltonian. A rotational analysis is presented for some of the bands involved in the Fermi resonance. The effect of the Fermi resonance on hot bands is investigated using the same Hamiltonian in comparison with experiment. The results are discussed in relation to the universal local dynamics of the isolated alkyl CH‐stretching chromophore and in relation to the vibrational dynamics of highly excited polyatomic molecules as a function of certain elements of molecular structure.

Electron diffraction studies of hot molecules. IV. Asymmetries of nonbonded distribution functions of SF_{6}, SiF_{4}, and CF_{4}
View Description Hide DescriptionHighly significant improvements in the agreement between observed and calculated intensities of electrons diffracted by hot molecules were obtained by optimizing the skew parameter â for the nonbonded distributions. Derived â values (±2σ) were 2.75(11), 0.72(13), and 2.0(4) Å^{−} ^{1} for SF_{6}, SiF_{4}, and CF_{4}, respectively. These measured skew parameters are approximately 2 Å^{−} ^{1} higher in each case than values previously proposed on the basis of Morse asymmetry factors and the nonlinear transformation between curvilinear and normal coordinates of molecules. The principal factor responsible for the increase is the previously unknown intrinsic anharmonicity in bending deformations. Silicon tetrafluoride has a lower â value than the other molecules studied primarily because its bending force constant is lower, relative to stretching. Practical as well as theoretical implications of present findings are discussed.

Photoexcitation of NO at 1576 Å
View Description Hide DescriptionAn accidental resonance between the 1576.299 Å line of the F_{2} laser and the Q _{1} _{1} (7.5) line in the B′ ^{2}Δ−X ^{2} Π 3–0 band of NO provides a means of generating intense emission in three NO transitions—B′ ^{2}Δ–X ^{2}Π, and B′ ^{2}Δ–B ^{2}Π, and B ^{2}Π–X ^{2}Π. By accessing a particular B′ ^{2}Δ level in this manner, state‐to‐state kinetics involving a variety of high electronic states of NO can be easily studied. These include the H′ E, D, C, B, and A states. The specific nature of the process makes it possible to use the emission as an unambiguous measure of laser power and also as a technique for detecting small amounts of NO. Observations can be made at wavelengths from the vacuum UV to the visible, and intercalibration between UV and visible bands result in a branching ratio of 155 for emission from NO(B′ ^{2}Δ) to the X ^{2}Π and B ^{2}Π states.

Diode laser spectroscopy of the ν_{3} band of carbon tetrachloride (C^{3} ^{5}Cl_{4}): Stark modulation and cold jet infrared absorption spectrum
View Description Hide DescriptionThe infrared vibration‐rotation spectrum of the ν_{3} band of C^{3} ^{5}Cl_{4} was measured by using a tunable diode laser. Heavily congested Q‐branch spectrum was analyzed on the basis of the Stark effect, which extract transitions of tetrahedral species among various isotopic species. Low‐J lines were measured in a supercooled molecular jet, which supressed hot bands and high‐J rotational lines. The following constants of the ν_{3} band were determined: ν_{0}=798.7489(4) cm^{−} ^{1}, B _{3}−B _{0}=−0.39(5) MHz, α_{2} _{2} _{0}=−3.33(6) MHz, and α_{2} _{2} _{4}=−0.185(9) MHz. The Fermi resonance between the ν_{3} and ν_{1}+ν_{4} states was analyzed with the aid of an empirical anharmonic force field.

ESR studies of the formyl radical in a CO matrix: Magnetophotoselective photolysis and thermally activated rotations
View Description Hide DescriptionPhotolysis of the formyl radical (HCO) in a CO matrix at 13 K by yellow light (λ>500 nm) polarized with its electric vector parallel to an external magnetic field (H _{DC}) is orientationally selective, as shown by changes in the powderESRspectrum of the radical. The depleted orientation in the cyclic photolysis [HCO→^{ hν} H+CO; H+ (a different CO)→(randomly oriented HCO)] is (HCO plane) ⊥ H _{DC}, showing that the largest component of the optical transition moment is perpendicular to the HCO plane, as predicted by theory for a nonrotating HCO. Around 25 K part of the photoinduced orientational anisotropy is lost due to incipient rotation about an axis close to the CO bond, but the remaining orientational anisotropy persists to higher temperatures (∼40 K), where it decays only slowly. Around 35 K this rotation becomes fast enough to yield a pseudoaxially symmetric powderESRspectrum with g _{∥}=1.9973,g _{⊥}=2.0021, and A _{∥}=A _{⊥}=383.0 MHz. The rotation axis is closest to but significantly different from the minimum inertial axis.

Photoabsorption cross section of OD at 115–180 nm
View Description Hide DescriptionThe photoabsorption cross sections of OD in the 115–180 nm region were measured. The OD radicals were produced from a pulsed discharge in a mixture containing a trace of D_{2}O in a few Torr of argon. Results are compared with the photoabsorption of OH previously measured.

C_{2}N_{2} photodissociation at 1576 Å. I. CN(A ^{2}Π) radiative lifetimes, nascent vibrational distribution, and C_{2}N_{2} quenching
View Description Hide DescriptionPhotodissociation of C_{2}N_{2} at the 1576 Å F_{2} laser line generates CN(A ^{2}Π) up to the thermodynamic limit of v=5, with a bimodal vibrational distribution peaking at v=0 and v=2. Radiative lifetimes for the six observed levels have been obtained by extrapolating time decays to zero pressure, and the data show rapidly decreasing lifetimes with increasing v, in accord with recent theoretical predictions, and contrary to previous experimental determinations. The range of lifetimes observed is 4.3–8.5 μs. Quenching of these levels by the parent molecule shows a factor of 12 increase in rate coefficient in going from v=0 to v=5, correlated to the energy gap between the A ^{2}Π state and the closest lower X ^{2}Σ^{+} level, suggesting that the quenching mechanism is one involving cross relaxation between CN(A) and CN(X).

Nuclear magnetic relaxation rate dispersion in supercooled heavy water under high pressure
View Description Hide DescriptionSpin‐lattice (T _{1}) and spin–spin (T _{2}) relaxation times of the deuterons in supercooled D_{2}O at 225 MPa, measured at two frequencies: 55.54 and 39.14 MHz down to 188 K are reported. The results show that T _{1} and T _{2} become frequency dependent in supercooled liquid water under high hydrostaticpressure at temperatures below ∼220 K. Theoretical expressions for the relaxation rates are deduced under the assumption that the orientational fluctuations of the water molecules are composed of fast librational oscillations and slower diffusional motions. The effect of the librations is to reduce the size of the deuterium quadrupole coupling constant. The diffusional motions are nearly isotropic and dominate the T dependence of the relaxation times. The autocorrelation function of the slow orientational fluctuations was assumed to be exponential at long times with a single time constant, the orientational correlation time τ_{2}. The T dependence of the latter is well described by the VTF equation. The parameters obtained by least squares fitting the experimental spin‐lattice relaxation times to an isotropic motional model correctly predict the temperature and frequency dependence of the spin–spin relaxation times.

Angle‐resolved photoelectron cross section of CF_{4}
View Description Hide DescriptionPartial photoelectron cross sections σ and angular distribution parameters β were obtained for the first five valence orbitals in CF_{4}: 1t _{1}, 4t _{2}, 1e, 3t _{2}, and 4a _{1}, as a function of photon energy from 17 to 70 eV. These data were taken with the aid of angle‐resolved photoelectron spectroscopy and synchrotron radiation. The results were compared with earlier data on CCl_{4}. Substantial differences were found. These are explained partly in terms of the absence of a Cooper minimum with a fluorine compound as opposed to the presence of a Cooper minimum with chlorine compounds and partly in terms of the position of shape resonances. Data on CF_{4} were also compared with recent calculations of Stephens e t a l., who used the multiple‐scattering Xα method. Structure in the photoelectron spectrum of CF_{4} lying on the low energy side of the third band was identified as due to autoionization and evidence is given as to its specific nature.

Anharmonic potential functions as derived from Raman intensities: Methane
View Description Hide DescriptionThe cubic force field of methane has been investigated on the basis of the analysis of Raman scattering cross sections of binary overtones and combination bands. Seven absolute cross sections, ten symmetry cubic force constants, and the second derivative of the mean CH bond polarizability are the original results of present work. Some of these cubic force constants are determined with high accuracy. Most Ĥ_{2} _{2} rovibrational constants calculated by means of present anharmonic force field are in good agreement with the experimental values reported by various authors.

Water–hydrocarbon interactions: Rotational spectroscopy and structure of the water–acetylene complex
View Description Hide DescriptionThe radiofrequency and microwave spectra of C_{2}H_{2}–H_{2}O, C_{2}H_{2}–D_{2}O, C_{2}D_{2}–H_{2}O, and C_{2}D_{2}–D_{2}O have been measured by molecular beam electric resonance spectroscopy.Rotational constants and dipole moments are reported. The structure is effectively planar with the acetylene hydrogen bonded to the oxygen of the water. The hydrogen‐bond length and stretching force constant are calculated to be. 2.229 Å and 0.065 mdyn/Å, respectively. Considering the lone pair orbital structure of the water molecule, an equilibrium structure having the water plane tilted away from the a axis of the complex is expected. Even so, a comparison of the dipole moments for the different isotopically substituted species shows that the height of the barrier hindering an inversion motion is low enough that no vibrational levels lie below the top of the barrier in the double‐minimum potential well.

Photoionization spectra and electronic structure of small iron clusters
View Description Hide DescriptionLaser photoionizationspectra over the range 4.5–6.5 eV have been taken for iron clusters of from 2 to 25 atoms. From the observed ionization thresholds, the cluster ionization potentials are determined and trends in I.P. vs cluster size are examined. In the case of the iron dimer, a sharp, vertical threshold is seen which places the I.P. of Fe_{2} at 6.30±0.01 eV. Finally, SCF Xα scattered wave molecular orbital calculations have been carried out on the iron dimer and iron trimer. These results show the importance of ferromagnetic spin polarization in the electronic structure of Fe_{2} and Fe_{3}.

Nascent internal energy distributions of MgH produced in the reaction of Mg(3s3p ^{1} P _{1}) with a variety of polyatomic molecules
View Description Hide DescriptionNascent rotational quantum state distributions of MgH(v=0,1) produced in the reactions of excited Mg(3s3p ^{1} P _{1}) with a variety of polyatomic molecules have been determined using the laser pump‐and‐probe technique. The distributions are generally bimodal, with one component peaked at low rotational energy and another component at high values of the rotational quantum number N. Except for (CH_{3})_{2}O, and (C_{2}H_{5})_{2}O, which produced very little rotationally excited MgH, all compounds with C–H, N–H, and or O–H bonds showed microscopic branching ratios for the ‘‘high‐N’’ component of 0.5±0.1. For the reactants SiH_{4}, GeH_{4}, and PH_{3}, the high‐N branching ratio was ≥0.9. The results are interpreted in terms of competition between side‐on, insertive, and end‐on, abstractive attack of the reagent bonds. Little vibrational energy was found in MgH product for any molecule, consistent with extremely ‘‘late’’ energy release for these heavy–light–heavy kinematic conditions. The results are compared to those from similar studies of the reactions of O(^{3} P), O(^{1} D), and F(^{2} P).

Semiclassical treatment of rotational excitation of NH_{3} colliding with ^{4}He
View Description Hide DescriptionThe semiclassical (classical path) theory is formulated for an atom colliding with a symmetric top molecule. Cross sections for rotational excitation of NH_{3} colliding with ^{4}He are calculated and compared with numbers obtained from exact quantum, coupled states, and other semiclassical calculations. The validity of the semiclassical coupled states approximation is investigated.

Semiclassical multistate curve‐crossing models: Reduction of the transition amplitudes to diabatic and adiabatic phase integrals
View Description Hide DescriptionA systematic procedure for deriving semiclassical transition amplitudes in multistate curve‐crossing situations is used to prove that, apart from a phase factor, this amplitude can be expressed entirely with the action angles defining the diabatic and adiabatic levels. This formulation is useful in intermediate coupling regimes where coincidences between diabatic and adiabatic levels are responsible for the structure in cross sections. The way to use this transition amplitude when the initial or final (or both) channels are closed is indicated. A numerical example concerning resonance Raman scattering is worked out.

The use of exponential perturbation theory in the determination of scattering wave functions
View Description Hide DescriptionExponential perturbation theory (EPT) is used as the basis for generatingmultichannel scatteringwave functions. The wave functions are constructed so that at each order of perturbation theory, the corresponding order of EPT scattering matrix is generated asymptotically. The relationship between this analysis and earlier work is examined. The usefulness of the present approach is illustrated by application to collinear single channel scattering. Perturbative solutions are generated for the test problem by separating a zeroth order interaction potential from the complete interactions. Finally, these developments are briefly reviewed and further applications discussed.

Infinite‐order sudden calculations of pressure broadening cross sections for noble gas–oxygen binary mixtures
View Description Hide DescriptionA detailed computational study is presented of both the self‐broadening and foreign gas broadening of the magnetic dipole and rotational Raman lines of O_{2}. The calculations are based on a recent extension of the quantum mechanical line shape functiontheory to paramagnetic molecules. Quantal infinite‐order sudden calculations are compared with the results of classical and semiclassical trajectory calculations and with experimental measurements where available. For the O_{2}–He system, a number of different anisotropicpotential energy surfaces have been tested and a recent surface determined from molecular beamscattering data has been found to give by far the best overall agreement with the line broadening data.