Jeremy Randell

Low energy electron scattering in H2, N2 and O2

Electron beam transmission experiments have been performed, in the energy range 10 meV to 175 meV, with a magnetically collimated electron beam formed in a synchrotron radiation photoionization source using SuperACO, LURE. The apparatus has been calibrated with He and absolute backward scattering cross sections have been measured for the target gases H2, N2 and O2. A relationship, involving s and p partial waves, has been established between the backward scattering cross sections ( sigma B) and the momentum transfer cross sections ( sigma M). This has been used to check the accuracy of experimental data and the consistency of values of sigma B, sigma M and total scattering cross sections. Experimental data and theory for H2 are in good agreement, whereas for N2 experimental values of sigma B and sigma M conflict below 80 meV and agreement with theories is mixed. For O2, discrepancies are greater than for N2 both in experimental data and between theory and experiment, which may differ by up to a factor of 5 for the total scattering cross section at the lowest energies.

The microwave spectrum and potential function of propanal

The potential function for internal rotation of the aldehyde group in propanal has been determined. All available microwave and far-infrared data are fitted to the potential V(α)=½ [graphic omitted] Vn(1 – cos nα) and where (in cm–1)V1= 251(2), V2= 736(8), V3= 550(4), V4=–179(5) and V5=–7(4). The energy difference between the two stable conformers has been redetermined from microwave relative intensities to be ΔW(gauche–cis)= 420(27) cm–1. Torsional fundamentals measured in the infrared have also been used in the potential function calculation. Isotopic data have been measured providing insight into the torsional motion in CH3CH2CH18O and CH3CH2CDO. The resulting potential is compared with some isoelectronic molecules. The dipole moment of gauche propanal has been measured as 2.864(8) D orientated within 0.6° of the CO bond.

Very low energy electron scattering in some hydrocarbons and perfluorocarbons

Low-energy scattering has been studied in , , , , , and in a transmission experiment using a synchrotron radiation photoionization source. Backward scattering cross-sections have been determined over the energy range 10-175 meV. The variation of these cross-sections with electron impact energy has been analysed for non-polar and weakly polar species using modified effective range theory, yielding scattering lengths and low-energy limiting cross-sections. Rotationally inelastic scattering cross-sections have been calculated for and using the first Born point-dipole approximation. Results suggest that rotationally inelastic events contribute strongly to low energy scattering in but only weakly in , reflecting the larger dipole moment in .

Low energy electron scattering by CF3Cl, CF2Cl2, CFCl3 and CCl4

We report the results of electron transmission experiments in the chlorofluoromethanes and CCl4 in the energy range from less than 10 meV to a few hundred meV, using a magnetically confined electron beam, generated from a high resolution, synchrotron photoionization source on SuperACO, LURE. Scattering spectra show evidence of electron attachment, Ramsauer-Townsend effects, and pure rotational and rovibrational inelastic scattering. For CF3Cl and CF2Cl2 we analyse the inelastic scattering phenomena in terms of the first Born approximation, showing that this gives a good fit to the form of the variation of the pure rotationally inelastic cross section with electron kinetic energy.

Low-energy negative-ion states of NO at high electron energy resolution

Abstract Low-energy resonances involving the 3 Σ − , 1 Δ and 1 Σ + states of NO − have been studied by high-resolution electron scattering integrated over the angular range 60–120° and in the energy range 0.07–1.6 eV. The resonant vibrational energies of the 3 Σ − state yield agreement with the NO electron affinity of 24 meV determined from photodetachment studies. The widths of the vibrational series 3 Σ − and 1 Δ increase slowly with the vibrational quantum number at variance with calculations.

Dicyanomethane : microwave spectrum, quadrupole coupling and structure

The microwave spectra of seven isotopic species of CH2(CN)2 have been studied in the ground vibrational state. The zero-point average structure has been determined to be: r(C—C)= 1.459(2)A, r(CN)= 1.160(2)A and r(C—H)= 1.109(2)A with ∠ CCN = 180 – 1.4(3)°, ∠ CCC/2 = 56.3(2)° and ∠ HCH/2 = 53.5(2)°. A near-equilibrium structure has also been estimated using an rρm method tested by a similar calculation for formaldehyde. Microwave Fourier-transform spectroscopy has been used to investigate the main species and single-nitrogen-15 species under high resolution. For CH2(CN)C15N accurate 14N quadrupole coupling constants have been determined as χaa=–2.364(9) MHz, χbb= 0.313(6) MHz and χcc= 2.051(6) MHz. These results have been combined with those previously obtained for the main species to show that the quadrupole tensor is almost symmetrical about the CN bond direction. There is some disagreement between the constants for the two isotopic molecules close to experimental error, possibly indicating zero-point vibrational effects on the coupling tensor. The quadrupole tensor is of especial interest in CH2(CN)2 in view of the bent CCN moieties.

Microwave spectrum and rotational isomerism of gaseous nitrosoethane, CH3CH2NO

The microwave spectrum of gaseous nitrosoethane has been measured in the 7–40 GHz region. The monomeric vapour is shown to exist as two rotational isomers, one the cis form with a planar heavy-atom structure, the other a staggered gauche conformer. The microwave spectrum of gauche-nitrosoethane shows large centrifugal distortion shifts and additional splitting arising from tunnelling through the trans barrier. Relative intensity measurements show the cis to be more stable than the gauche form by 2.1 ± 0.4 kJ mol–1. The dipole moments were determined through the Stark effect: µa= 2.316 (2), µb= 0.623 (4) and µt= 2.398 (2) D (1 D ≈ 3.33564 × 10–30 C m) for the 15N cis, and µa= 2.288 (4), µb= 0.814 (5), µc= 0.460 (9) and µt= 2.471 (4) D for the 15N gauche form. Nitrogen-14 quadrupole coupling constants have been determined to be χaa=–2.525 (56) and (χbb–χcc)=–7.311 (97) MHz for the cis, and (χbb–χcc)= 3.518 (40) MHz for the gauche conformer.Molecular structures have been calculated for both conformers on the basis of the oxygen-18, nitrogen-15 and main species data. The ∠ CCN angle opens up by 8° on going from gauche- to cis-nitrosoethane. The barrier hindering internal rotation of the methyl group is the same for both conformers within experimental error, V3= 10.9 ± 0.3 kJ mol–1 for the cis form and 10.9 ± 0.3 kJ mol–1 for the gauche conformer.

Nitrogen-14 quadrupole coupling constants in N2O3 by isotopic labelling

Abstract Several microwave transitions of O 15 N…NO 2 and ON… 15 NO 2 have been measured under high resolution using a pulsed-nozzle Fourier transform spectrometer. The 14 N quadrupole coupling constants in the inertial axis system have been determined for both nitrogen atoms. The quadrupole coupling constants for ON … 15 NO 2 are eQq aa = −0.5203(20) and eQq bb = −4.1981(19) MHz, and for O 15 N…NO 2 are eQq aa = −1.7999(13) and eQq bb = 0.0808(17) MHz. The 14 N quadrpole coupling constants determined by Kukolich for the main species ON…NO 2 should be reversed with respect to the NO and NO 2 groups. Combining the present data with the main species constants allows the complete quadrupole coupling tensor to be estimated for the NO 2 group; the tensor in N 2 O 3 lies within 2° of the N…N bond direction and within 0.4° of the bisector of the ONO angle. Spin-rotation effects are significant for nitrogen in the NO group; C aa is determined to be 8 ± 2 kHz for 14 N in ON… 15 NO 2 and −15 ± 3 kHz for 15 N obtained directly from splittings in the spectrum of O 15 N… 15 NO 2 .

High‐resolution electron‐molecule scattering using synchrotron‐generated electron beams

A synchrotron photoionisation technique has been used in an electron spectrometer at the SERC‐Daresbury Laboratory, to study electron‐molecule scattering. We have shown that it is possible to achieve electron scattering at photon resolution down to an energy resolution of 3.5 meV for energies in a few hundred meV range. Electron scattering data in O2 suggest an important contribution of the partial waves l=4 and 6 which should give rise to rotational excitation of the molecule. Work in progress on electron scattering in NO and on vibrational excitation of the (100) and (001) modes near threshold in CO2 benefits from the improved resolution.

Cis and Gauche Propanal: Microwave Spectra and Molecular Structures

The microwave spectra of twelve isotopic species of cis propanal (CH3CH2CHO) and six isotopic forms of the less stable gauche rotamer have been studied to determine accurate structural parameters for both conformers. The following bond lengths (Å) and angles (°) were derived: