Weitere Messungen zur Anisotropie der intermolekularen Wechselwirkung durch Streuung von Molekülen in definiertem Quantenzustand

Improved measurements of the ratio of scattering cross sections for various molecular rotational states are reported for scattering of TlF in rotational states ¦J, M〉=¦1, 0〉 and ¦1, 1〉, and CsF in rotational states ¦2, 0〉 and ¦2, 2〉 by rare gases. The results are interpreted in terms of an angle dependent attractive potentialV=?2ε(r _m /r) ⁶(1+q ₆ P ₂(cosΘ) in which the repulsive part of the interaction is neglected. The “high energy” approximation is used to calculate the cross section, which contains the velocity dependence and the state dependence as factors. The experiments show for all scattering partners with the exception of He and Ne, that the state dependence is velocity independent. In those cases this result provides a justification for the neglect of the repulsive potential term. The results for the anisotropy parameterq ₆, which to a good approximation depends only on properties of the moleculus, are:q ₆=0.23±0.01 for TlF,q ₆=0.28±0.02 for CsF.     

Analysis of the microwave spectrum of hydrazine

Microwave measurements in the interval from 6 to 133 GHz, consisting of 444 rotational transitions in the vibrational ground state of hydrazine with J ≤ 31 and K_a ≤ 6 were fit to an effective rotational Hamiltonian containing 9 asymmetric rotor constants, 14 NH₂ inversion parameters, and 1 internal rotation parameter, with an overall standard deviation of the fit of 0.40 MHz. This set of parameters contains: (i) the three rotational constants; (ii) tunneling splitting constants for NH₂ inversion at one end of the molecule, for NH₂ inversion at both ends of the molecule, and for internal rotation through the trans barrier; (iii) two K-type doubling constants affecting the K = 1 levels; (iv) an a-type Coriolis interaction with matrix elements linear in K; and (v) various centrifugal distortion corrections to the above parameters. A consistent group theoretical formalism was used to label the energy levels and to select terms in the phenomenological rotational Hamiltonian. The Hamiltonian matrix, which is set up in a tunneling basis set, is of dimension 16×16 and contains only ΔK_a = 0 matrix elements, asymmetric rotor effects being taken into account on the diagonal by terms from a Polo expansion in bⁿ. Hyperfine splittings and barrier heights are not discussed.     

High-Resolution Analysis of the ν6, ν17, and ν21 Bands of Dimethyl Ether

The ν₆, ν₁₇, and ν₂₁ fundamental bands of dimethyl ether have been assigned and rotationally analyzed. The spectra used were recorded at 0.005 cm⁻¹ spectral resolution with a Fourier-transform spectrometer coupled to a supersonic molecular beam leading to a rotational temperature of about 70 K. The ν₆ and ν₂₁ bands do not seem to be perturbed and the analysis of the rotational structure leads to band centers located at 933.906 6(9) and 1 103.951(1) cm⁻¹, respectively, and to accurate rotational and centrifugal distortion constants. For the ν₁₇ band at 2817.385(2) cm⁻¹, only the P and R branches could be assigned.     

Effects of surface corrugation on the molecular rotational dependence of H2 dissociative adsorption dynamics on Cu(100)

We investigate and discuss how surface corrugation affects the molecular rotational dependence of H₂ dissociative adsorption dynamics on Cu(100) by performing six-dimensional (6D) quantum dynamics calculations. We calculate the dissociative adsorption probability as a function of the initial rotational state J and the normal energy E_norm of incident molecules, and compare with the dissociative adsorption results obtained by four-dimensional (4D) quantum dynamics calculations where the surface is treated as flat. In our calculation, for the case of normal incidence, the increase in dissociative adsorption probability with increasing E_norm and the non-monotonic behavior of dissociative adsorption probability with respect to J are suppressed on a corrugated surface as compared to that on a flat surface.     

Diode-laser spectroscopy of supersonic free jets

A supersonic-free-jet infrared spectrometer has been constructed for investigation of molecular vibrational spectra at low rotational and vibrational temperatures. The sensitivity of measurement in a pulsed jet is increased by employing a phase-sensitive detection method synchronized with the pulse frequency. The performance of the spectrometer is examined for the absorption lines of the NH₃ v ₂ band. A rotational temperature as low as 16K is attained when seeded in He. Cold-jet spectra are demonstrated for thev ₃ bands of PF₅,³⁴SF₆, and¹⁸²WF₆.     

Microwave spectra of several molecular isotopes of toluene

The microwave rotational spectra of five further isotopic species of toluene have been measured. Spectra characteristic for the presence of a very small sixfold barrier potential V₆ hindering the internal methyl rotation were observed. It is demonstrated that the sets of rotational constants obtained from the ground state spectra, A′ (frame alone), B, C, can be used for the determination of a (partial) substitution structure also in the case of off-axis substituted isotopes. Attempts to obtain a more complete r₀-structure were less successful. Values of V₆ were derived for all isotopes from spectra in excited states of internal rotation. A 15 percent reduction of V₆ upon methyl deuteration was observed, just as in earlier, similar cases.     

The infrared band ν6 of CD3I

The lowest perpendicular fundamental ν₆ of CD₃I around 650 cm^?1 is studied at a resolution of 0.015 cm^?1. More than 2000 rotational transitions are identified and they are used to get the molecular constants of the level v₆ = 1, including, e.g., the η constants.     

Microwave spectra of thionyl fluoride in vibrationally excited states

The microwave spectrum has been observed and rotational constants obtained for the v₄ = 1 and v₆ = 1 vibrationally excited states of thionyl fluoride. The perturbation of the C rotational constant is explained in terms of a Coriolis resonance between these levels.     

Rotational spectrum, structure and internal rotation in CH3CCl3

The rotational spectra of the v₆ = 1 and v₆ = 2 torsional states of CH₃C³⁵Cl₃ have been measured in the millimeterwave range and accurate spectroscopic constants have been determined. The equilibrium structure, the torsional frequency and the barrier to internal rotation have been calculated ab initio. These results are shown to be compatible with the absence of splittings in the rotational spectra.     

Inelastic collision cross section of excited molecules

The (v′=6,J′=43) level in theB ¹Π _u electronic state of Na₂ has been selectively populated by excitation with the 4 880 Å line of the argon laser. Through collisions with He atoms energy is transferred to neighbouring rotational states in Na₂ and the density of these states is determined by observing fluorescence to electronic ground state. From previous measurement of the lifetime of theB ¹Π _u state and new measurements of the intensities of collision induced spectral lines as a function of He pressure, absolute collision cross sections for all rotational transitions up to ΔJ=±5 have been obtained. The total cross section for all rotational transitions observed is σ _rot ^total =65±15 Ų. Preliminary results about collision induced vibrational transitions are also presented.     

Spin-Hamiltonian parameters and local rotational order parameters for Gd ion in the tetragonal phase of SrTiO3 crystal

Using a diagonalization method, the spin-Hamiltonian parameters (g factors g_||, g_⊥ and zero-field splittings b₂⁰, b₄⁰, b₂⁴, b₆⁰, b₆⁴) for Gd³⁺ ion at the 12-fold coordinated Sr²⁺ site in the tetragonal phase of SrTiO₃ at T≈4.2 and 77 K are calculated in a unified way. The calculated results are in reasonable agreement with the experimental values. The local rotational order parameters ?_l of Gd³⁺ center at the two temperatures are also obtained from the calculations. It is found that the local rotational order parameters ?_l of Gd³⁺ impurity center at both temperatures are different from the corresponding parameters ?_h in the host SrTiO₃ crystal and the changes of spin-Hamiltonian parameters with temperature are caused by the change of local rotational order parameter?_l with temperature. The results are discussed.     

Experimental studies by complementary terahertz techniques and semi-classical calculations of N2- broadening coefficients of CH335Cl

Room-temperature N₂-broadening coefficients of methyl chloride rotational lines are measured over a large interval of quantum numbers (6≤J≤50, 0≤K≤18) by a submillimeter frequency-multiplication chain (J≤31) and a terahertz photomixing continuous-wave spectrometer (J≥31). In order to check the accuracy of both techniques, the measurements of identical lines are compared for J=31. The pressure broadening coefficients are deduced from line fits using mainly a Voigt profile model. The excellent signal-to-noise ratio of the frequency-multiplication scheme highlights some speed dependence effect on the line shape. Theoretical values of these coefficients are calculated by a semi-classical approach with exact trajectories. An intermolecular potential including atom–atom interactions is used for the first time. It is shown that, contrary to the previous theoretical predictions, the contributions of short-range forces are important for all values of the rotational quantum numbers. Additional testing of modifications required in the semi-classical formalism for a correct application of the cumulant expansion is also performed. It is stated that the use of the cumulant average on the rotational states of the perturbing molecule leads, for high J and small K values, to slightly higher line-broadening coefficients, as expected for the relatively strong interacting CH₃Cl–N₂ system. The excellent agreement between the theoretical and the experimental results ensures the reliability of these data.     

High resolution infrared spectroscopic studies of ethylene-1,1-D2

Rotational assignments in the ν₆ + ν₉ type-A band and the ν₉, ν₆ + ν₁₁, and ν₁ + ν₆ type-B bands of ethylene-1,1-D₂, recorded at a resolution of ～0.03 cm^?1, enable the ground state rotational constants to be determined much more accurately than previously. A significant change in the A₀ constant is noted. All upper states suffer perturbations to their rotational structures. Analyses, excluding the areas of perturbation, still enable the excited state constants to be determined with considerable precision.     

The ν2 fundamental vibration-rotation band of T2O

The ν₂ fundamental vibration-rotation band of T₂O vapor has been measured at grating resolution, and the rotational structure has been analyzed. The band center and the values of the rotational constants A, B, and C for the ground state and excited state have been determined. These values are consistent with the data for J through 6, and with extrapolation from H₂O and D₂O.     

Monomer counterrotations and tunneling splitting in CO dimer by data of millimeter wave spectroscopy

Analysis of the rotational spectrum of the molecular dimer (CO)₂ measured in the millimeter wave range has been performed and four new rotational states are revealed. Three of these states are characterized by almost free rotations of both monomers in the dimer. These states have approximately the same first term σ in the expansion of the rotational energy in powers of the rotational angular momentum J for various values of the momentum projections on the dimer axis (K=0, 1, 2) and various rotational constants B. The intrinsic rotational angular momenta of CO dimers, j₁=j₂=1, are determined from the σ value. In addition, a state with K=2 is found which corresponds to one of the known shape isomers of (CO)₂. The values of the tunneling splitting for each of the new states are determined. The results indicate that previous data on the suppressed tunneling are determined by the asymmetry of internal rotations in the CO monomers rather than by the _K value.     

Rotational excitation of HD by collisions with He

The information theoretic surprisal analysis is employed to characterise the recent quantal close-coupling computations of rotational excitation rates by Green. The analysis is based on comparing the actual rates to the priors k⁰ which depend only on the temperature and the reduced “energy defect” ΔE_R/kT. The surprisal representation of the computed rates as k=k⁰ x exp (-I₀ - θ_R6ΔE_R6/kT is accurate throughout the temperature range considered by Green and is found to reduce in the low and high temperature limits to functional forms previously employed to characterise the rotational energy transfer rates in H₂.     

16 μm emission from a chemical hydrogen fluoride TE laser

A number of emission lines within the wavelength range from 12.7 to 17 μm originating from pure rotational transitions in hydrogen fluoride have been observed in a chemical HF-TE laser. The operational conditions have been optimized for the 16.02 μm line which is due to the rotational J(16→15) transition in the vibrational ground state of the HF molecules. This line is in the near vicinity of the v₃ UF₆ absorption band.     

Effective dipole moment of rotational transitions of X 2 Y molecules

Contributions determining the rotational dependence of the effective dipole moment of molecules are calculated for the ground state of H₂S and H₂O molecules. The calculation is carried out in various ordering algorithms of perturbation theory. It is shown that the convergence of the effective dipole moment for the ground state of an H₂O molecule in the polynomial representation is rather slow in the rotational operator J _z (the convergence radius is K*≤17). Nonpolynomial forms of the dipole moment as a function of rotational operators are discussed.     

Temperature dependence of rotational relaxation of methane in the 2ν3 vibrational state by self- and nitrogen-collisions and comparison with line broadening measurements

Depopulation rates of rotational levels in the v₃ = 2 vibrational state of ¹²CH₄ are investigated by a pump-probe technique. Methane molecules are excited into selected rotational levels by tuning the pump laser to 2ν₃ lines. The time evolution in population of the excited level after the pumping pulse is monitored by tuning the probe laser to a (3ν₃ ← 2ν₃) line corresponding to a transition with the excited rotational level as the lower level. Measurements were performed from room temperature down to 100 K in pure CH₄ and in CH₄-N₂ mixtures. The rotational relaxation rate coefficients are given for the J = 1, A₂, J = 1, E, J = 1, F₂ and J = 0, F₂ levels. The results are compared with the available data on line broadening coefficients. The temperature dependence of the data on N₂-broadening is particularly well reproduced by the power law deduced from the results on rotational relaxation.     

Rotational Raman spectrum of cyclopropane

The pure rotational Raman spectrum of cyclopropane was observed up to J = 43. We have taken into account the effects of the unresolved K structure of the lines by assigning an effective value of K to the center of each unresolved line. Methods are developed for calculating effective K values for each value of J that allow a simultaneous fit to the R-and S-branch lines. The rotational constants of cyclopropane derived from this research are, in wavenumber units (cm^?1); B₀ = 0.6702₈, D_J = 1.0 × 10^?6, and D_JK = ?1.3 × 10^?6. We have also tested the validity of the method by using it with recent Raman data for BF₃.