Penning and associative ionization of mercury by collision with metastable rare gas atoms

Probing anisotropic interaction potentials of unsaturated hydrocarbons with He*(2 3S) metastable atom: attractive-site preference of sigma-direction in C2H2 and pi-direction in C2H4

State-resolved collision energy dependence of Penning ionization cross sections of acetylene (C2H2) and ethylene (C2H4) with He*(2 3S) metastable atoms was observed in a wide collision energy range from 20 to 350 meV. A recently developed discharge nozzle source with a liquid N2 circulator was employed for the measurements in the low-energy range from 20 to 80 meV. Based on classical trajectory calculations for the energy dependence of the partial ionization cross sections, anisotropic potential energy surfaces for the present systems were obtained by optimizing ab initio model potentials for the chemically related systems Li+C2H2 and C2H4. In the case of C2H2, the global minimum was found to be located around the H atom along the molecular axis with a well depth of 48 meV (ca. 1.1 kcal/mol). On the other hand, a dominant attractive well with a depth of 62 meV (ca. 1.4 kcal/mol) was found in the piCC electron region of C2H4. These findings were discussed in connection with orbital interactions between molecular orbitals of the target molecules and atomic orbitals of the metastable atom. It is concluded that sigma-type unoccupied molecular orbitals of C2H2 and a piCC-type highest occupied molecular orbital of C2H4 play a significant role for the attractive-site preference of sigma direction in C2H2 and pi direction in C2H4, respectively.

Collision-Energy-Resolved Penning Ionization Electron Spectroscopy of Phenylacetylene and Diphenylacetylene by Collision with He*(23S) Metastable Atoms

Penning ionization of phenylacetylene and diphenylacetylene upon collision with metastable He*(2(3)S) atoms was studied by collision-energy-/electron-energy-resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES). On the basis of the collision energy dependence of partial ionization cross-sections (CEDPICS) obtained from 2D-PIES as well as ab initio molecular orbital calculations for the approach of a metastable atom to the target molecule, anisotropy of interaction between the target molecule and He*(2(3)S) was investigated. For the calculations of interaction potential, a Li(2(2)S) atom was used in place of He*(2(3)S) metastable atom because of its well-known interaction behavior with various targets. The results indicate that attractive potentials localize in the pi regions of the phenyl groups as well as in the pi-conjugated regions of the acetylene group. Although similar attractive interactions were also found by the observation of CEDPICS for ionization of all pi MOs localized at the C[triple bond]C bond, the in-plane regions have repulsive potentials. Rotation of the phenyl groups about the C[triple bond]C bond can be observed for diphenylacetylene because of a low torsion barrier. So the examination of measured PIES was performed taking into consideration the change of ionization energies for conjugated molecular orbitals.

Development of a cooled He*(23S) beam source for measurements of state-resolved collision energy dependence of Penning ionization cross sections: Evidence for a stereospecific attractive well around methyl group in CH3CN

A low-temperature discharge nozzle source with a liquid-N(2) circulator for He*(2(3)S) metastable atoms has been developed in order to obtain the state-resolved collision energy dependence of Penning ionization cross sections in a low collision energy range from 20 to 80 meV. By controlling the discharge condition, we have made it possible to measure the collision energy dependence of partial ionization cross sections (CEDPICS) for a well-studied system of CH(3)CN+He*(2(3)S) in a wide energy range from 20 to 350 meV. The anisotropic interaction potential energy surface for the present system was obtained starting from an ab initio model potential via an optimization procedure based on classical trajectory calculations for the observed CEDPICS. A dominant attractive well depth was found to be 423 meV (ca. 10 kcal/mol) at a distance of 3.20 A from the center of mass of CH(3)CN in the N-atom side along the CCN axis. In addition, a weak attractive well (ca. 0.9 kcal/mol) surrounding the methyl group (-CH(3)) has been found and ascribed to the interaction between an unoccupied molecular orbital of CH(3)CN and 2s atomic orbital of He*(2(3)S).

Probing the Shape and Stereochemistry of Molecular Orbitals in Locally Flexible Aromatic Chains: A Penning Ionization Electron Spectroscopy and Green's Function Study of the Electronic Structure of Biphenyl

We report on the results of an exhaustive study of the interplay between the valence electronic structure, the topology and reactivity of orbitals, and the molecular structure of biphenyl by means of Penning ionization electron spectroscopy in the gas phase upon collision with metastable He*(2(3)S) atoms. The measurements are compared with one-particle Green's function calculations of one-electron and shake-up valence ionization spectra employing the third-order algebraic diagrammatic construction scheme [ADC(3)]. Penning ionization intensities are also analyzed by means of the exterior electron-density model and comparison with photoelectron spectra: in contrast with the lines originating from sigma orbitals, ionization lines belonging to the pi-band system have large Penning ionization cross sections due to their greater extent outside the molecular van der Waals surface. The involved chemi-ionization processes are further experimentally investigated using collision-energy-resolved Penning ionization electron spectroscopy. The cross sections of pi-ionization bands exhibit a markedly negative collision-energy dependence and indicate that the interaction potential that prevails between the molecule and the He*(2(3)S) atom is strongly attractive in the pi-orbital region. On the other hand, the partial ionization cross sections pertaining to sigma-ionization channels are characterized by more limited collision-energy dependencies, as a consequence of rather repulsive interactions within the sigma-orbital region. A comparison of ADC(3) simulations with the Penning ionization electron spectra and UV photoelectron spectra measured by Kubota et al. [Chem. Phys. Lett. 1980, 74, 409] on thin films of biphenyl deposited at 170 and 109 K on copper demonstrates that biphenyl molecules lying at the surface of polycrystalline layers adopt predominantly a planar configuration, whereas within an amorphous sample most molecules have twisted structures similar to those prevailing in the gas phase.

Collision-energy-resolved penning ionization electron spectroscopy of HCOOH, CH3COOH, and HCOOCH3 by collision with He*(2(3)S) metastable atoms

Penning ionization of formic acid (HCOOH), acetic acid (CH3COOH), and methyl formate (HCOOCH3) upon collision with metastable He*(2(3)S) atoms was studied by collision-energy/electron-energy-resolved two-dimensional Penning ionization electron spectroscopy (2D-PIES). Anisotropy of interaction between the target molecule and He*(2(3)S) was investigated based on the collision energy dependence of partial ionization cross sections (CEDPICS) obtained from 2D-PIES as well as ab initio molecular orbital calculations for the access of a metastable atom to the target molecule. For the interaction potential calculations, a Li atom was used in place of He*(2(3)S) metastable atom because of its well-known similarity in interaction with targets. The results indicate that in the studied collision energy range the attractive potential localizes around the oxygen atoms and that the potential well at the carbonyl oxygen atom is at least twice as much as that at the hydroxyl oxygen. Moreover we can notice that attractive potential is highly anisotropic. Repulsive interactions can be found around carbon atoms and the methyl group.

Ab initio investigation of the autoionization process Ar*(4s3P2, 3P0)+Hg --> (Ar-Hg)+ + e-: potential energy curves and autoionization widths, ionization cross sections, and electron energy spectra

Multireference configuration interaction (MRCI) calculations have been performed for the Ar*(4s3P2,0) + Hg collision complex. Feshbach projection based on orbital occupancy defines the entrance channel resonance states and provides their potential energy curves as well as resonance-continuum coupling matrix elements, which are turned into an autoionization width function by Stieltjes imaging. Coupled cluster calculations with singles, doubles, and pertubative triples [CCSD(T)] give the exit channel potential of ArHg+. The Hg20+ core is treated by a scalar-relativistic effective core potential, reparametrized to reproduce experimental excitation and ionization energies. Spin-orbit interaction is included for the Ar* open 3p shell. The nuclear motion is treated within the local complex potential approximation. Ionization occurs for 85% (3P0) and 98% (3P2) of the symmetry allowed close collisions. Calculated ionization cross sections show good agreement with experimental data. The difference potential of the collision complex is remarkably flat down to internuclear separations of 8a0 and leads to very sharp peaks in theoretical electron energy spectra for single collision energies. After accounting for the experimental energy distribution and the resolution function of the spectrometer, a very satisfying agreement with experimental electron energy spectra is found, including subtle differences due to spin-orbit coupling. Theoretical input appears indispensable for an analysis of the measured data in terms of potential energy curves and autoionization width functions.

Collision-energy-resolved Penning ionization electron spectroscopy of bromomethanes (CH3Br, CH2Br2, and CHBr3) by collision with He*(23S) metastable atoms

Ionization of bromomethanes (CH3Br, CH2Br2, and CHBr3) upon collision with metastable He*(2(3)S) atoms has been studied by means of collision-energy-resolved Penning ionization electron spectroscopy. Lone-pair (nBr) orbitals of Br4p characters have larger ionization cross sections than sigma(C-Br) orbitals. The collision-energy dependence of the partial ionization cross sections shows that the interaction potential between the molecule and the He*(2(3)S) atom is highly anisotropic around CH3Br or CH2Br2, while isotropic attractive interactions are found for CHBr3. Bands observed at electron energies of approximately 2 eV in the He*(2(3)S) Penning ionization electron spectra (PIES) of CH2Br2 and CHBr3 have no counterpart in ultraviolet (He I) photoionization spectra and theoretical (third-order algebraic diagrammatic construction) one-electron and shake-up ionization spectra. Energy analysis of the processes involved demonstrates that these bands and further bands overlapping with sigma(C-Br) or piCH2 levels are related to autoionization of dissociating (He+ - Br-) pairs. Similarly, a band at an electron energy of approximately 1 eV in the He*(2(3)S) PIES spectra of CH3Br has been ascribed to autoionizing Br** atoms released by dissociation of (unidentified) excited states of the target molecule. A further autoionization (S) band can be discerned at approximately 1 eV below the lone-pair nBr bands in the He*(2(3)S) PIES spectrum of CHBr3. This band has been ascribed to the decay of autoionizing Rydberg states of the target molecule (M**) into vibrationally excited states of the molecular ion. It was found that for this transition, the interaction potential that prevails in the entrance channel is merely attractive.

Collision-energy-resolved Penning ionization electron spectroscopy ofp-benzoquinone: Study of electronic structure and anisotropic interaction with He*(2 3S) metastable atoms

Collision energy dependence of partial ionization cross sections (CEDPICS) of p-benzoquinone with He(*)(2 (3)S) metastable atoms indicates that interaction potentials between p-benzoquinone and He(*)(2 (3)S) are highly anisotropic in the studied collision energy range (100-250 meV). Attractive interactions were found around the C==O groups for in-plane and out-of-plane directions, while repulsive interactions were found around CH bonds and the benzenoid ring. Assignment of the first four ionic states of p-benzoquinone and an analogous methyl-substituted compound was examined with CEDPICS and anisotropic distributions of the corresponding two nonbonding oxygen orbitals (n(O) (+),n(O) (-)) and two pi(CC) orbitals (pi(CC) (+),pi(CC) (-)). An extra band that shows negative CEDPICS was observed at ca. 7.2 eV in Penning ionization electron spectrum.

EXPERIMENTS ON THE DYNAMICS OF MOLECULAR PROCESSES: A Chronicle of Fifty Years

[Figure: see text] [Figure: see text] [Figure: see text] [Figure: see text]

▪ Abstract  This paper reviews the way in which, in the Italy of the years immediately after World War II, interest in the dynamics of molecular processes was awakened. The narrative begins with the work of a small number of chemists and physicists who, in the initial stage, interacted closely. In the course of the years, their interests diverged and younger people joined the newly formed groups. Even now, after half a century, a common approach can still to be seen regarding how to attack problems and perform experiments. Experimental work is discussed, bringing out the common viewpoint of fields as diverse as mass spectrometry, isotope effects, chemical kinetics, molecular beams, molecule-molecule interactions, molecule-ion interactions, molecule-surface interactions, and plasma chemistry.