Cornelis A. de Lange

NMR of ordered liquids

I Basics.- 1 Basics of NMR of molecules in uniaxial anisotropic environments.- 2 Density matrix methods in NMR.- 3 Coherent averaging and correlation of anisotropic spin interactions in oriented molecules.- 4 Multiple Quantum NMR Spectroscopy in Orientationally Ordered Fluids.- 5 Spectral Analysis of Orientationally Ordered Molecules.- II NMR of solute atoms and molecules.- 6 NMR of Noble Gases Dissolved in Liquid Crystals.- 7 NMR of partially ordered solutes with emphasis on structure determination.- 8 Observation and interpretation of residual dipolar couplings in biomolecules.- 9 The search for high-resolution NMR methods for membrane peptide structure.- III Theory, models, and simulations.- 10 Solutes as probes of simplified models of orientational order.- 11 Molecular Models of Orientational Order.- 12 Molecular theory of orientational order.- 13 Very Flexible Solutes: Alkyl Chains and Derivatives.- 14 NMR Studies of Solutes in Liquid Crystals: Small Flexible Molecules.- 15 Simulations of Orientational Order of Solutes in Liquid Crystals.- IV Dynamic aspects and relaxation.- 16 Spin relaxation in orientationally ordered molecules.- 17 Low-frequency NMR relaxometry of spatially constrained liquid crystals.- 18 NMR on macroscopically oriented lyotropic systems.- 19 Dynamic NMR in liquid crystals and liquid crystalline solutions.

Resonance enhanced multiphoton ionization spectroscopy of carbon disulphide

Rydberg excited states of the CS2 molecule in the energy range 56 000–81 000 cm−1 have been further investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet‐cooled sample of the parent molecule. Spectral interpretation has been aided by parallel measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. Thus we have been able to extend, and clarify, previous analyses of the tangled spin–orbit split vibronic structure associated with the 3Πu and 1Πu states derived from the configuration [2Πg]4pσu and the 3Δu, 1Δu, and 1Σ+u states resulting from the configuration [2Πg]4pπu, and to deduce an approximate wave number for the origin of the hitherto unidentified 3Σ+u state derived from this same configuration. Moving to higher energies we are able to locate, unambiguously, the origins of the next (n=5) members of four of these [2Πg]np Rydberg series, and to identify extensive series...

Resonance enhanced multiphoton ionization spectroscopy of carbonyl sulphide

Rydberg excited states of the OCS molecule in the energy range 70500–86000 cm−1 have been investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet‐cooled sample of the parent molecule. Spectral interpretation has been assisted by companion measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. The present study supports the earlier conclusions of Weinkauf and Boesl [J. Chem. Phys. 98, 4459 (1993)] regarding five Rydberg origins in the 70500–73000 cm−1 energy range, attributable to, respectively, states of 3Π, 1Π, 3Δ, 1Δ and 1Σ+ symmetry arising from the 4pλ←3π orbital promotion. We also identify a further 21 Rydberg origins at higher energies. These partition into clumps with quantum defects ca. 3.5 and 4.5, which we associate with the orbital promotions npλ←3π (n=5,6), and others with near integer quantum defect which are interpretable in terms of excitation to s,d and (...

The spectroscopy of high Rydberg states of ammonia

This paper extends our knowledge of the higher excited states of the ammonia molecule by presenting detailed measurements of the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum of both NH3 and ND3 obtained following excitation in the wavelength range 298–242 nm, i.e., at energies up to the first ionization energy. Complementary analyses of the wavelength resolved REMPI spectrum and the accompanying REMPI-photoelectron spectra leads to the identification of ten new Rydberg origins of NH3 (four for ND3) with principal quantum numbers n⩽8 and, in most cases, of the accompanying out-of-plane bending vibrational progression. Symmetry assignments for the various newly identified excited states are offered, based on band contour simulation and/or quantum defect considerations. Dominant amongst these are the Ẽ″ 1A2″ (5sa1′←1a2″) state: ν0=74 118(2) cm−1 [NH3], ν0=74 258(2) cm−1 [ND3], the F″ 1E″ (5pe′←1a2″) state: ν0=76 220(50) cm−1 [NH3], ν0=76 240(50) cm−1 [ND3], the F′ 1A1′ (5pa2″←1a2″) state:...

Femtosecond time-resolved charged particle imaging studies of the ultraviolet photodissociation of the NO dimer

Ultraviolet photodissociation dynamics of the NO dimer into NO(Α)+NO(Χ) in a supersonic molecular beam is studied by femtosecond pump-probe charged particle imaging. Time-resolved photoelectron imaging reveals that photoionization from the Franck–Condon region of the excited state(s) reaches vibrationally excited states of the NO dimer cation, resulting in spontaneous fragmentation into NO+NO+ even at the shortest pump-probe time delays. The corresponding photoelectron energy distribution extending up to more than 1 eV above the ionization threshold indicates a large structural difference between the dimer cation and the photoexcited neutral state. From the isotropic photoelectron distribution, this state is assigned to a valence state rather than a Rydberg state. The characteristic photoelectron distribution and also the small NO dimer ion signal vanish within 200 fs. In the same time the photoelectron image rapidly evolves from an almost isotropic distribution into an anisotropic one characteristic for ...

Rotationally resolved photoelectron spectroscopy of hot N2 formed in the photofragmentation of N2O.

The photoionizationdynamics of rotationally hot molecular nitrogen are studied employing resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy.Photodissociation of N_2O at ∼203 nm results in highly rotationally excited N2 fragments in X ^1∑^+_g (N″,v″=0,1) states and O atoms in the excited ^1D_2 state. Photoelectron detection of the rotationally hot N_2 states is performed by a two-photon excitation to the lowest a″ ^1∑^+_g Rydberg state followed by one-photon ionization. The large number of observed rotational levels, from N′=49 up to N′=94, results in improved rotational parameters for a″ ^1∑^+_g (v′=0). In addition, experimental and theoretical rotationally resolved photoelectron spectra of the a″ ^1∑^+_g (v′=0,1;N′) state are presented. In these spectra only ΔN = N^+ − N′ = even transitions are observed, with a dominant ΔN=0 peak and rather weak ΔN = ±2 peaks. The one-photon ionization is dominated by ejection of electrons in p and f partial waves. The agreement between experimental and calculated spectra is excellent.

Communication: Molecular dynamics and 1H NMR of n-hexane in liquid crystals

The NMR spectrum of n-hexane orientationally ordered in the nematic liquid crystal ZLI-1132 is analysed using covariance matrix adaptation evolution strategy (CMA-ES). The spectrum contains over 150 000 transitions, with many sharp features appearing above a broad, underlying background signal that results from the plethora of overlapping transitions from the n-hexane as well as from the liquid crystal. The CMA-ES requires initial search ranges for NMR spectral parameters, notably the direct dipolar couplings. Several sets of such ranges were utilized, including three from MD simulations and others from the modified chord model that is specifically designed to predict hydrocarbon-chain dipolar couplings. In the end, only inaccurate dipolar couplings from an earlier study utilizing proton-proton double quantum 2D-NMR techniques on partially deuterated n-hexane provided the necessary estimates. The precise set of dipolar couplings obtained can now be used to investigate conformational averaging of n-hexane in a nematic environment.

One- and two-color photoelectron imaging of the CO molecule via the B 1Σ+ state

This paper is concerned with photoelectron imaging following one-color (2+1) and two-color (2+1′) resonance enhanced multiphoton ionization in the CO molecule. After the two-photon absorption step B 1Σ+ (v′=0)←←X 1Σ+ (v″=0) or B 1Σ+ (v′=1)←←X 1Σ+ (v″=0), the subsequent one-photon ionization X 2Σ+(v+)←B 1Σ+ (v′=0,1) shows deviations from the expected Δv=0 Franck–Condon propensity rule. The results are in good agreement with a previous study using time-of-flight photoelectron spectroscopy [Sha et al., J. Chem. Phys. 99, 4334 (1993)]. The experimental photoelectron kinetic energy spectra and their angular distributions are analyzed, and the essential role played by “superexcited” Rydberg states with an A 2Π ion core in this process is examined. Moreover, photoelectron imaging methods appear to be useful in extracting information about superexcited states.

Communication: Molecular gears

The (1)H nuclear magnetic resonance spectrum of hexamethylbenzene orientationally ordered in the nematic liquid crystal ZLI-1132 is analysed using covariance matrix adaptation evolution strategy. The spectrum contains over 350 000 lines with many overlapping transitions, from which four independent direct dipolar couplings are obtained. The rotations of the six methyl groups appear to be correlated due to mutual steric hindrance. Adjacent methyl groups show counter-rotating or geared motion. Hexamethylbenzene thus behaves as a molecular hexagonal gear.

Resonance-enhanced multiphoton ionisation photoelectron spectroscopy of the ClO radical: the C 2Σ− state

A (2 + 1) one-colour resonance-enhanced multiphoton ionisation study is carried out on the C 2Σ− state of the ClO radical in the one-photon energy range 29500–31250 cm−1. The ClO radical is produced by one-photon photolysis of ClO2 employing 359.2 nm photons derived from a separate laser. In this way a significant concentration of vibrationally excited ClO in its spin–orbit split X 2ΠΩ (Ω=3/2 or 1/2) electronic ground state is produced. In addition to mass-resolved excitation spectra, kinetic-energy resolved photoelectron spectra for the X3Σ−(+)←C 2Σ−(′=3–5) transitions are measured. These transitions are not completely Franck–Condon diagonal, and indicate a decrease in bond length on removal of the Rydberg electron from the C 2Σ− state. In addition to an unambiguous assignment of the C 2Σ− state, valuable information is obtained on the degree of vibrational excitation with which the nascent ClO radical is formed in the photolysis of ClO2 . Analysis of the photoelectron spectra is supported by Franck–Condon calculations based on potential energy curves either from experimental spectroscopic parameters, or obtained by theoretical ab initio methods.