Photoionization of oriented molecules in a gas phase

Use of a coincidence technique for registration of fragment ions and photoelectroms from dissociative ionization of molecules opens the possibility of studying the photoionization of oriented molecules in a gas phase. The first results obtained by this technique for O₂ molecules are presented. The angular distribution of photoelectrons as a function of an angle between molecular axis and a photoelectron momentum is measured for theB ² _g ^– final ionic state using the HeI resonance radiation. From measured data it follows that the ratio of channel cross sections is /=0.67±0.08.     

Elastic electron scattering cross sections of oriented and aligned molecules

Elastic differential electron scattering cross sections of oriented methyl iodide are calculated using the independent atom model. Results are presented for two specific orientations of the ICH₃ molecule for the purpose of comparison with the fictitious molecule IC, similarly oriented, at electron energies of 600 eV and 40 keV. Cross sections are also calculated for IC with a large angular momentum. In a comparison of the results for different orientations of the angular momentum vector, including random orientation, large differences between the cross sections are evident. This sensitivity to the plane of rotation of the molecule suggests the possibility of determining the degree of alignment of the angular momenta of a beam of such molecules by electron diffraction.     

State- and conformer-selected beams of aligned and oriented molecules for ultrafast diffraction studies

The manipulation of the motion of neutral molecules with electric or magnetic fields has seen tremendous progress over the last decade. Recently, these techniques have been extended to the manipulation of large and complex molecules. In this article we introduce experimental approaches to the manipulation of large molecules, i.e., the deflection, focusing and deceleration using electric fields. We detail how these methods can be exploited to spatially separate quantum states and how to select individual conformers of complex molecules. We briefly describe mixed-field orientation experiments made possible by the quantum-state selection. Moreover, we provide an outlook on ultrafast diffraction experiments using these highly controlled samples.     

Interaction of the trimethylsilyl cation with molecules of aliphatic nitro compounds in the gas phase

Interaction of mononitroalkanes with the trimethylsilyl cation in the gas phase under chemical ionization (CI) conditions results in the formation of [M+SiMe₃]⁺ ions, which are more stable than the corresponding protonated molecular ions. In the case of 2-nitro-2-methylpropane and 2-nitropentane, fragmentation of the [M+SiMe₃]⁺ ions occurs with the formation of C₄H₉ ⁺ and C₅H₁₁ ⁺ carbocations, respectively. In the case of 1,1-dinitroethane and 1-halo-1,1-dinitroethane, fragmentation of the [M+SiMe₃]⁺ ions occurs with splitting off of a NO₂ ^. radical or an XNO₂ molecule (X=H, F, or Cl). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1232–1234, June, 1997.     

Spectroscopy of complex molecules in the gas phase

A brief review is presented of theoretical and experimental research into the role of statistical factors in the formation of the characteristics of absorption and emission processes of light, observed for low-density and rarefied vapours of complex molecules. It is shown, in particular, that the average energy of molecules excited per unit time differs from the mean energy of molecules in the ground state, not by the energy of the exciting photon hv, but by the sum of hv and the selective energy which is the result of different absorption probabilities for molecules of different energy. This correction is the most important in rotational—vibrational absorption bands. This was established when the selective energy was calculated using the experimental data with the help of the formulae obtained. Average energies of the initial and final combining states reduced by the average energy of molecules in the ground state are calculated. Correlation curves similar to those of Condon are plotted according to the calculated data. The electron transition frequencies and the identities of absorption and emission transitions are determined through these curves; whereas for rotational—vibrational bands the value of the rotational constant and the variation of the latter upon excitation are estimated.     

Laser-induced alignment and anti-alignment of rotationally excited molecules

We numerically investigate the post-pulse alignment of rotationally excited diatomic molecules upon nonresonant interaction with a linearly polarized laser pulse. In addition to the simulations, we develop a simple model which qualitatively describes the shape and amplitude of post-pulse alignment induced by a laser pulse of moderate power density. In our treatment we take into account that molecules in rotationally excited states can interact with a laser pulse not only by absorbing energy but also by stimulated emission. The extent to which these processes are present in the interaction depends, on the one hand, on the directionality of the molecular angular momentum (given by the M quantum number), and on the other hand on the ratio of transition frequencies and pulse duration (determined by the J number). A rotational wave packet created by a strong pulse from an initially pure state contains a broad range of rotational levels, over which the character of the interaction can change from non-adiabatic to adiabatic. Depending on the laser pulse duration and amplitude, the transition from the non-adiabatic to the adiabatic limit proceeds through a region with dominant rotational heating, or alignment, for short pulses and a large region with rotational cooling, and correspondingly preferred anti-alignment, for longer pulses.     

Collision spectroscopy with aligned and oriented atoms

Neutralisation processes in 0.15–1.5 keV collisions of H^? with Na atoms in the 3s ground state or in the excited 3p state have been investigated by means of time-of-flight analysis of the neutral H atoms produced. The H^? - Na(3p) system, investigated here for the first time, is particularly interesting since the entrance channel is embedded in the [H - Na(3s)] +e ^? continuum, enabling Penning detachment to occur. The measured relative neutralisation cross section ratios σ(3p)/σ(3s) decrease from 3 to 1.6 with increasing energy. Based on earlier published results for σ(3s), σ(3p) total cross sections exceeding 100 Ų are estimated.     

Collision spectroscopy with aligned and oriented atoms

Electron capture processes in the H⁺?Na(3s) and H⁺?Na(3p) collisions are experimentally investigated in the 0.3–3 keV energy range using a crossed beam experiment. The excited Na(3p) target is produced with a well-defined alignment using laser pumping. The time of flight technique enables the identification of all the H(n)+Na⁺ channels populated in the collision. Total cross section ratios σ_3p (n=2)/σ_3s (n=2),σ_3p (n=3)/σ_3s (n=2) and σ_3s (n=3)/σ_3s (n=2) for the production of H(n=2) and H(n=3) are measured in the H⁺?Na (3s) and H⁺?Na (3p) collisions. They reveal a strong dominance of the production of H(n=2) in the H⁺?Na(3p) collision, especially for energies below 1 keV.     

Self-assembly of organic molecules at metal surfaces

Self-assembly represents a promising strategy for surface functionalisation as well as creating nanostructures with well-controlled, tailor-made properties and functionality. Molecular self-assembly at solid surfaces is governed by the subtle interplay between molecule–molecule and molecule–substrate interactions that can be tuned by varying molecular building blocks, surface chemistry and structure as well as substrate temperature.In this review, basic principles behind molecular self-assembly of organic molecules on metal surfaces will be discussed. Controlling these formation principles allows for creating a wide variety of different molecular surface structures ranging from well-defined clusters, quasi one-dimensional rows to ordered, two-dimensional overlayers. An impressive number of studies exist, demonstrating the ability of molecular self-assembly to create these different structural motifs in a predictable manner by tuning the molecular building blocks as well as the metallic substrate.Here, the multitude of different surface structures of the natural amino acid cysteine on two different gold surfaces observed with scanning tunnelling microscopy will be reviewed. Cysteine on Au(110)-(1×2) represents a model system illustrating the formation of all the above mentioned structural motifs without changing the molecular building blocks or the substrate surface. The only parameters in this system are substrate temperature and molecular coverage, controlling both the molecular adsorption state (physisorption versus chemisorption) and molecular surface mobility. By tuning the adsorption state and the molecular mobility, distinctly different molecular structures are formed, exemplifying the variety of structural motifs that can be achieved by molecular self-assembly.     

Reactions of ions with molecules in the gas phase

Existing knowledge concerning the gas phase reactions of ions with molecules is summarized in terms of the identification of the reactions, the rate constants of the reactions, and the energetic properties of the ions observed to be formed and of those inferred as intermediates.     

Chirality recognition between neutral molecules in the gas phase

Noncovalent interactions are particularly intriguing when they involve chiral molecules, because the interactions change in a subtle way upon replacing one of the partners by its mirror image. The resulting phenomena involving chirality recognition are relevant in the biosphere, in organic synthesis, and in polymer design. They may be classified according to the permanent or transient chirality of the interacting partners, leading to chirality discrimination, chirality induction, and chirality synchronization processes. For small molecules, high-level quantum chemical calculations for such processes are feasible. To provide reliable connections between theory and experiment, such phenomena are best studied in vacuum isolation at low temperature, using rotational, vibrational, electronic, and photoionization spectroscopy. We review these techniques and the results which have become available in recent years, with special emphasis on dimers of permanently chiral molecules and on the influence of conformational flexibility. Analogies between the microscopic mechanisms and macroscopic phenomena and between intra- and intermolecular cases are drawn.     

Photoionization of oriented molecules

A general expression for the angular distribution of photoelectrons with defined spin polarization ejected from oriented molecules is derived in the electric-dipole approximation in the limit of a weak radiation field. An analysis of its geometrical part permits to draw definite conclusions without calculating matrix elements. For linear molecules it is shown that in Hund's cases (a) and (b) photoelectrons may be polarized only parallel to the molecular axis, while in Hund's case (c) they may be polarized in any direction. Appearance of a circular dichroism for nonchiral oriented molecules of relatively low symmetry is predicted. Dependence of a circular dichroism in the angular distribution of photoelectrons on the symmetry of molecules is demonstrated. The results may serve as the framework for studying molecules oriented on surfaces, in liquid crystals, or by molecular beam techniques.     

On the dissociation of diatomic molecules at metal surfaces

The role of activation barriers in the process of dissociative adsorption of diatomic molecules on metal surfaces is considered in terms of the topology of the diabatic potential energy surfaces of the interacting system. The location of possible barriers on the potential energy surface determines the dependence of the dissociative threshold on either the normal translational or total energy content of the incident molecules.     

Non-statistical energy partitioning in the gas phase reactions of polyatomic molecules

The internal energy distributions in the HF products from the reactions of F atoms with H₂CO and HFCO have been determined via arrested-relaxation infrared chemiluminescence experiments. The results show that the HF vib-rotational levels are strongly excited and suggest that the HCO and FCO radical products are created with excitation in the ν₁ (asymmetric stretch) and ν₂ (bend) modes respectively.     

Solvation of propanediol ions by water molecules in the gas phase

The thermochemical properties of protonated hydrates of 1,2- and 1,3-propanediols have been investigated using electrospray ionization-high pressure mass spectrometry. The binding enthalpies, entropies, and free energies of the stepwise hydration of protonated propanediols with one to three waters are reported. The observed negative entropy change [ΔΔS_1,3^o for the addition of the third water to 1,3-propanediol·H⁺(H₂O)₂ suggests a stable structure due to an increased number of hydrogen bonds and the loss of the intramolecular hydrogen bond in the water cluster ion. The thermochemical properties of two isomers of butanediol were also investigated in order to further elucidate the structures of the protonated propanediols.     

Stern-Gerlach deflection of field-free aligned paramagnetic molecules

The effects of laser-induced pre-alignment on the deflection of paramagnetic molecules by inhomogeneous static magnetic field are studied. Depending on the relevant Hund's coupling case of the molecule, two different effects were identified: either suppression of the deflection by laser pulses (Hund's coupling case (a) molecules, such as ClO), or a dramatic reconstruction of the broad distribution of the scattering angles into several narrow peaks (for Hund's coupling case (b) molecules, such as O(2) or NH). These findings are important for various applications using molecular guiding, focusing and trapping with the help of magnetic fields.     

Laser-induced low energy electron diffraction in aligned molecules

We measured the photoelectron spectra and angular distributions of partially aligned N(2), O(2), and CO(2) in the rescattering plateau of above threshold ionization (ATI). The measured ATI electrons have relatively low collision energies (<15 eV). The photoelectron angular distributions (PAD) show clearly species and energy dependence. A simple two-center interference model was not able to consistently retrieve structural properties. We conclude that due to the interplay between the electrons and rescattering potential, the molecular structural information is obscured and cannot be extracted conveniently. However, the sensitivity of the PAD to the scattering potential in laser-induced electron diffraction promises a practical tool for studying electron-ion scattering dynamics.