C.A. de Lange

Molecular solutes in nematic liquid crystals: Orientational order and electric field gradients

Abstract The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D 2 and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for hydrogen and several other solutes.

High-resolution UV photoelectron spectroscopy of diatomic halogens

Abstract Diatomic halogens are studied with UV photoelectron spectroscopy using new techniques to preserve high resolution even for reactive species. For the first time vibrational structure is observed on the 2 Π u, i ( i = 1/2 , 3/2 ) states (F 2 + , Cl 2 + ), the 2 Σ g + states (F 2 + , Cl 2 + ) and the Br 2 + ( 2 Π u, 3 2 ) state. On the 2 Π u, i states (F 2 + , Cl 2 + , Br 2 + ) spin-orbit splitting is resolved. Indications for a small potential barrier on the F 2 + ( 2 Π u, i ) state for large internuclear distances are found. A new value for the spin-orbit splitting of the Cl 2 + ( 2 Π g ) state is presented (= −725 cm −1 ). The complementary nature of optical emission and photoelectron spectroscopy for small ions is demonstrated leading to a more complete picture of the F 2 + ( 2 Π u, i ) and Cl 2 + ( 2 Π u, i ) ionic states.

Effects of interaction between molecular internal motion and reorientation on NMR of anisotropic liquids

Abstract NMR spectra and order parameters of nonrigid partially oriented molecules are expected to show effects due to molecular reorientation following a configurational change. The order parameters are functions of configuration lifetime τv and reorientation rate k. The process is best described in an axis system in which internal motion and molecular reorientation are decoupled. The theory is developed assuming that the configurational change is a stationary Markov process. For slow change, a separate order matrix is required for each configuration; for rapid interconversion, the order parameters for the above axis system are the average of the order parameters for all configurations weighted by the product kτv. Effects of small-amplitude vibrations and large-amplitude conformational changes are discussed. NMR splittings observed in tetrahedral molecules are explained without having to invoke distortion by the liquid-crystal solvent.

NMR of deuterium in liquid crystal mixtures

Deuterium NMR spectra of D 2 dissolved in mixtures of the nematic liquid crystals N-( p -ethoxybenzylidene)- p ′- n -butyl-aniline and Merck ZLI 1132 are discussed in terms of a theory based on the interaction of the molecular quadrupole moment with the average electric field gradient. The average field gradient depends upon the liquid crystal composition and hence can be adjusted to various values (including zero) between those of the pure liquid crystals. It is shown that for deuterium dissolved in these phases the molecular quadrupole—average field gradient mechanism can account for most but not all of the orientational ordering.

He(l) Photoelectron spectroscopy of transient species: The SF2 Molecule

Abstract The He(I) photoelectron spectrum of SF 2, formed in the reaction between fluorine atoms and flowers of sulphur, is presented. The spectral interpretation is based on the results of Hartree-Fock-Slater calculations, using STO basis sets of double zeta quality with and without a 3d-polarization function in the sulphur basis set. For comparison similar computations are performed for SH 2. Calculated dipole moments and population analyses are presented. The SF 2 photoelectron spectrum is compared with spectra of similar molecules, i.e. SCl 2 , OCl 12 and OF 2 . A discussion in terms of Walshs predictions for AB 2 molecules with twenty valence electrons is given. Sum rule considerations are found to be especially useful for the assignment of π-type orbitals, and the perfluoro effect is illustrated.

Resonance enhanced multiphoton ionization photoelectron spectroscopy on nanosecond and picosecond time scales of Rydberg states of methyl iodide

Rydberg states of methyl iodide have been investigated using resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy with nanosecond and picosecond laser pulses. The study of the ns (6≤n≤10) Rydberg states in two‐, three‐, and four‐photon excitations has resulted in an unambiguous identification of state [1] in the 7s and 8s Rydberg states. As a consequence, it is concluded that the transition to 6s[1] in two‐ and three‐photon excitations is anomalously weak. The application of photoelectron spectroscopy to identify the electronic and vibrational nature of a resonance has led to a major reinterpretation of the excitation spectrum of the 6p Rydberg state in two‐photon excitation. In many of the recorded photoelectron spectra anomalous electrons are observed, which derive from a one‐photon ionization process. This process is suggested to find its origin in the mixing of 6p and 7s character into higher‐lying Rydberg states. The major difference between resonance enhanced mult...

On the average orientation of molecules undergoing large-amplitude conformational changes in anisotropic liquids

Abstract Description of the average orientation of non-rigid molecules in anisotropic fluids is non-trivial. The assumption that molecular orientation is independent of conformation is discussed. For such a case the Euler angles relating each conformation to the common order-tensor axes must still be known. These common axesare normally not fixed to any rigid segment of the molecule.

Rotationally resolved photoelectron spectroscopy of the 2Σ- Rydberg states of OH: the role of Cooper minima

We have measured rotationally resolved photoelectron spectra of the OH radical using (2+1) resonance enhanced multiphoton ionizationspectroscopy via the D  ^2Σ^−(3pσ) and 3 ^2Σ^−(4sσ) Rydberg states. For the D  ^2Σ^−(3pσ) state, we observe primarily ΔN=even distributions of ionic rotational states, in contrast to the ΔN=odd distribution expected for ionization of a 3pσ Rydberg electron. The observations are described quantitatively by ab initio calculations which predict a Cooper minimum in the 3pσ→kπ(l=2) channel, whose occurrence determines the ΔN=even ion rotational distribution. In contrast, the 3 ^2Σ^−(4sσ) photoelectron spectra reveal a broad distribution in rotational levels, arising from greater l mixing in the higher Rydberg orbital and much weaker Cooper minima in the continuum.

Resonance enhanced multiphoton ionization photoelectron spectroscopy and pulsed field ionization via the F1Δ2(v'=0) and f3Δ2(v'=0) Rydberg states of HCl

In this paper, we report the first rotationally resolved one‐ and two‐color resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI‐PES) study of the HCl molecule. The agreement between our experimental branching ratios and theoretical investigations is excellent. We also report the first zero kinetic energy pulsed field ionization (ZEKE‐PFI) experiments carried out in a ‘‘magnetic bottle’’ electron spectrometer. A direct comparison is made between ZEKE‐PFI and REMPI‐PES spectra for ionization via several rotational levels of the F 1Δ2(v’=0) and f 3Δ2(v’=0) Rydberg states of HCl. Large differences in both the spin–orbit and rotational branching ratios are found between the ZEKE‐PFI and REMPI‐PES spectra. These differences can be understood qualitatively on the basis of rotational and spin–orbit autoionization mechanisms.

Vibration‐rotation coupling in anisotropic environments: NMR of methanes in liquid crystals

It is shown that the interaction potential which describes the orientation of methane and its isotopic derivatives in a liquid–crystal environment has a simple second‐order tensorial form. The dipolar couplings previously observed in the methanes arise from a vibration‐rotation coupling mechanism and a rigid‐molecule effect present in the nonsymmetrically substituted methanes. These effects are calculated from the methane force field. A large part of the interaction can be identified with the coupling between the anisotropic liquid–crystal electric field and the (vibrationally induced) solute anisotropic polarizability. Relative sign information is obtained on the derivatives of the methane polarizability with respect to the symmetry modes, quantities which are crucial in the determination of absolute Raman intensities.