S.R. Andrews

Dipolar relaxation mechanisms in the vitreous state, in the glass transition region and in the mesophase, of a side chain polysiloxane liquid crystal

Abstract The dipolar relaxation mechanisms in a side chain liquid crystalline polysiloxane have been studied by Thermally Stimulated Discharge Currents (t.s.d.c.) and by Dielectric Relaxation Spectroscopy (d.r.s.). The study was carried out in a wide temperature range covering the vitreous phase, the glass transition region and the liquid crystalline phase. Different discharges were observed in the t.s.d.c. spectrum of this polymer which were attributed, in the order of increasing temperature, to local non-cooperative motions probably involving internal rotations in the spacer and in the alkyl group of the mesogenic moiety, to the Brownian motions of the main chain associated with the glass transition and to motions involving reorientations of the components of the dipole moment of the mesogenic side group in the liquid crystalline phase. The dielectric relaxation spectrum, on the other hand, is dominated by two relaxation processes both of which are above the measured glass transition temperature and sho...

Double ionization of the ethyne molecule

When previously measured double-ionization energies of the ethyne (C2H2) molecule to singlet and triplet electronic states of its dication are compared with previously calculated values, it becomes apparent that some of the predicted values are not matched with equivalent experimental data. In the present investigation, the results of ADC(2) Greens function calculations indicate that some of the transitions are to satellite states and so too weak to allow double-ionization energies to those states to be measured. The 1Δg and 1Σ+g states are, however, main states and transitions to them should be sufficiently strong to give two separate double-ionization energies. Only one peak was seen in the appropriate region using both Auger-electron spectroscopy and double-charge-transfer spectroscopy. This may be due to limitations in resolving power since the states are predicted to lie within 0.8 eV of one another. To investigate this possibility, the double-ionization energies of C2H2 to singlet states of C2H2+2 have been measured using a double-charge-transfer spectrometer which is capable of resolving spectral peaks 0.7 eV apart. Four peaks were observed, the first two corresponding to double-ionization energies of 33.6±0.3 eV and 34.3±0.2 eV. These are identifiable with transitions to the 1Δg and 1Σ+g states, corresponding to calculated energies of 33.8 eV and 34.6 eV, respectively. The two other peaks correspond to double-ionization energies of 38.7±0.4 eV and 40.7±0.5 eV. They identify with transitions to the 1Πu state and 1±g state, the double-ionization energies to which are predicted to be 39.0 eV and 41.0 eV, respectively.

Double ionization to singlet and triplet electronic states of CCl42

Abstract The semiempirical MSXα computational method has been applied to calculate the double-ionization energies to singlet and triplet electronic states of CCl42+. In addition, a high resolution double-charge-transfer spectroscopy study of CCl4 was undertaken in which the double-ionization energies to singlet and triplet states of the dication were measured. These are in good agreement with the calculated values, thus giving confidence in the overall predictions of the computational study.

Bulk thermal polymerisation of diethylene glycol bis(allyl carbonate) as studied by dielectric relaxation spectroscopy

Broad-band dielectric relaxation spectroscopy (DRS) has been used to study the changes in molecular relaxation behaviour when diethylene glycol bis(allyl carbonate) (CR39 monomer) is polymerised thermally in its bulk state. As polymerisation proceeds the dielectric α-relaxation process broadens markedly and moves to higher temperatures while, at the same time, a small low-temperature β-relaxation process increases in its intensity and moves to higher temperatures. The dielectric loss spectra are shown to provide a convenient fingerprint that indicates the extent of cure in partially cured materials.

Electronic-state energies of the CH2I2+2, CHI2+3 and CI2+4 dications

The double-ionization energies of CH2I2, CHI3 and CI4 have been measured and calculated. Double-charge-transfer spectrometry was used in the experimental part of the investigation, and a modified multiple scattering Xα method was used to calculate the energies. In order to test the accuracy of the calculations, the computational method was initially applied to CH3I; the values obtained were found to be in good agreement with those measured previously. Calculated data for CH2I2 indicate that a large number of electronic states of the dication exist. By forming appropriate groups of states, mean calculated double-ionization energies were obtained which agree with the measured data. For CHI3 and CI4, the number of states calculated to exist is considerably less. When groups need to be formed, the numbers in them are much less than those for CH2I2. Good agreement between grouped calculated data and those measured was obtained for CHI3 and CI4. The exception was the four lowest states of CI42+ which appear not to be populated experimentally. A probable explanation is that the endoergicities of the relevant double-electron-capture reactions are too low, lying outside the operative reaction window of endoergicities.

Double-ionization energies of the chloroethane molecules CH3CH2Cl, CH3CHCl2, CH3CCl3, CH2ClCH2Cl, CH2ClCHCl2, CH2ClCCl3, CHCl2CHCl2, CHCl2CCl3 and CCl3CCl3

Abstract Double-ionization energies of the molecules listed in the title were measured by double-charge-transfer spectroscopy. Since spin is expected to be conserved in double-electron-capture reactions, and OH + was used as the projective ion, the lowest energies measured should correspond to ground triplet states of the dications. Double-ionization energies to these states were calculated using the GAUSSIAN 86 ab initio program package at the Hartree—Fock level with second-order Moller—Plesset perturbation theory. The measured double-ionization energy of CH 3 CH 2 Cl is 29.8 ± 0.5 eV but the corresponding energies for all the other molecules are lower, lying within ± 0.7 eV of 28.1 eV. A similar pattern is seen in the calculated data. The measured energies are in general higher than those calculated but in all cases by not more than 5%. Double-ionization energies for the fluorine analogues were also calculated and are compared with previously published measured data.

Combined experimental and computational investigation of the double ionization of (CH3)4M molecules (M = Si, Ge, Sn and Pb)

The double-ionization energies of four (CH3)4M molecules (M = Si, Ge, Sn and Pb) to triplet electronic states of their dications have been calculated using a modified MSXα method. The results are used to interpret the peak positions in experimental spectra obtained by the application of double-charge-transfer spectroscopy to the molecules. Although the density of states is high, the main features of the spectra correlate well with the calculated average double-ionization energies to selected groups of states.

The allene dication: calculated and measured singlet-electronic-state energies

Vertical double-ionization energies of the allene molecule to singlet electronic states of the dication have been measured by double-charge-transfer spectroscopy, and are in excellent agreement with values predicated using a semi-empirical MSXα-based method. The present data, when combined with those of a previous investigation in which energies to triplet states were determined, provide comprehensive electronic-state information for the allene dication in a range up to ca. 12 eV above that for the triplet ground state.

Double ionization of the n-alkyl iodides C2H5I, C3H7I and C4H9I

Double ionization of C2H5I, n-C3H7I and n-C4H9I has been investigated experimentally using double-charge-transfer spectroscopy. On the assumption that spin is conserved in the double-electron-capture reactions, and because of the use of OH+ as the projectile ion, it is probable that triplet states of the dications were populated. Peaks in the spectra were interpreted in terms of the electronic transitions giving rise to double-ionization energies calculated using a semi-empirical version of the multiple scattering Xα(MSXα) method of molecular orbital calculations. The density of states was found to increase as the alkyl chain-length increased, and most of the peaks observed were interpreted as being due to transitions to groups of close-lying states. In addition to the use of the MSXα method, ab initio double-ionization energies to the lowest triplet states of the dications were calculated.

Calculated and measured double-ionization energies to triplet electronic states of the doubly charged ions of methylamine, dimethylamine and trimethylamine

Abstract Double ionization of methylamine, dimethylamine and trimethylamine was studied experimentally by double-charge-transfer spectroscopy and theoretically both using an ab initio method with different basis sets and by the application of a semi-empirical form of the multiple scattering Xα (MSXα) method. The use of OH+ and F+ projectile ions in the experimental work ensured that dications in triplet electronic states were produced. Also, the high velocity of the projectile ions ensured that vertical transitions from the molecules to the dications took place. Ab initio calculations predict quite well the observed reduction in the lowest double-ionization energies as the methyl content of the amine molecule increases. Application of the MSXα method allowed double-ionization energies to higher-lying triplet states of the dications to be predicted, providing a valuable insight into the electronic transitions which give rise to the spectra recorded for the three amines.