A New Insight on Stereo-Dynamics of Penning Ionization Reactions.

Recent developments in the experimental study of Penning ionization reactions are presented here to cast light on basic aspects of the stereo-dynamics of the microscopic mechanisms involved. They concern the dependence of the reaction probability on the relative orientation of the atomic and molecular orbitals of reagents and products. The focus is on collisions between metastable Ne*(3P2, 0) atoms with other noble gas atoms or molecules, for which play a crucial role both the inner open-shell structure of Ne* and the HOMO orbitals of the partner. Their mutual orientation with respect to the intermolecular axis controls the characteristics of the intermolecular potential, which drives the collision dynamics and the reaction probability. The investigation of ionization processes of water, the prototype of hydrogenated molecules, suggested that the ground state of water ion is produced when Ne* approaches H2O perpendicularly to its plane. Conversely, collisions addressed toward the lone pair, aligned along the water C2v symmetry axis, generates electronically excited water ions. However, obtained results refer to a statistical/random orientation of the open shell ionic core of Ne*. Recently, the attention focused on the ionization of Kr or Xe by Ne*, for which we have been able to characterize the dependence on the collision energy of the branching ratio between probabilities of spin orbit resolved elementary processes. The combined analysis of measured PIES spectra suggested the occurrence of contributions from four different reaction channels, assigned to two distinct spin-orbit states of the Ne*(3P2, 0) reagent and two different spin-orbit states of the ionic M+(2P3/2, 1/2) products (M = Kr, Xe). The obtained results emphasized the reactivity change of 3P0 atoms with respect to 3P2, in producing ions in 2P3/2 and 2P1/2 sublevels, as a function of the collision energy. These findings have been assumed to arise from a critical balance of adiabatic and non-adiabatic effects that control formation and electronic rearrangement of the collision complex, respectively. From these results we are able to characterize for the first time, according to our knowledge, the state to state reaction probability for the ionization of Kr and Xe by Ne* in both 3P2 and 3P0 sublevels.

Penning ionization electron spectroscopy of hydrogen sulfide by metastable helium and neon atoms.

The dynamics of the Penning ionization of hydrogen sulfide molecules by collision with helium and metastable neon atoms, occurring in the thermal energy range, has been studied by analyzing the energy spectra of the emitted electrons obtained in our laboratory in a crossed beam experiment. These spectra are compared with the photoelectron spectra measured by using He(I) and Ne(I) photons under the same experimental conditions. In this way we obtained the negative energy shifts for the formation of H2S(+) ions in the first three accessible electronic states by He*(2(3,1)S1,0) and Ne*((3)P2,0) Penning ionization collisions: the 2b1 (X̃(2)B1) fundamental one, the first 5a1 (Ã(2)A1), and the second 2b2 (B̃(2)B2) excited states, respectively. The recorded energy shifts indicate that in the case of He* and Ne*-H2S the autoionization dynamics depends on the features of the collision complex and is mainly driven by an effective global attraction that comes from a balance among several non covalent intermolecular interaction components. This suggests that the Penning ionization should take place, in a specific range of intermolecular distances, as we have already observed in the case of Penning ionization of water molecules [Brunetti, B. G.; Candori, P.; Falcinelli, S.; Pirani, F.; Vecchiocattivi, F. J. Chem. Phys. 2013, 139, 164305-1-164305-8].

The stereodynamics of the Penning ionization of water by metastable neon atoms.

The stereodynamics of the Penning ionization of water molecules by collision with metastable neon atoms, occurring in the thermal energy range, is of great relevance for the understanding of fundamental aspects of the physical chemistry of water. This process has been studied by analyzing the energy spectrum of the emitted electrons previously obtained in our laboratory in a crossed beam experiment [B. G. Brunetti, P. Candori, D. Cappelletti, S. Falcinelli, F. Pirani, D. Stranges, and F. Vecchiocattivi, Chem. Phys. Lett. 539-540, 19 (2012)]. For the spectrum analysis, a novel semiclassical method is proposed, that assumes ionization events as mostly occurring in the vicinities of the collision turning points. The potential energy driving the system in the relevant configurations of the entrance and exit channels, used in the spectrum simulation, has been formulated by the use of a semiempirical method. The analysis puts clearly in evidence how different approaches of the metastable atom to the water molecule lead to ions in different electronic states. In particular, it provides the angular acceptance cones where the selectivity of the process leading to the specific formation of each one of the two energetically possible ionic product states of H2O(+) emerges. It is shown how the ground state ion is formed when neon metastable atoms approach water mainly perpendicularly to the molecular plane, while the first excited electronic state is formed when the approach occurs preferentially along the C2v axis, on the oxygen side. An explanation is proposed for the observed vibrational excitation of the product ions.

Dissociative double photoionization of benzene molecules in the 26-33 eV energy range.

A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH(3)(+) + C(5)H(3)(+), C(2)H(3)(+) + C(4)H(3)(+) and C(2)H(2)(+) + C(4)H(4)(+) ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 ± 0.1, 29.5 ± 0.1, and 30.2 ± 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C(6)H(6)(2+) dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.

Experimental and theoretical evidence of charge transfer in weakly bound complexes of water.

We report the definitive assessment of a measurable (half a kJ/mol) charge-transfer (CT) component in the interaction of water with the noble gases. The CT is found to be strongly stereoselective and is mediated by a highly asymmetric, donor/acceptor concerted role of the two hydrogen atoms, which can in fact dictate the equilibrium geometry of the adducts. This finding may be of general relevance in explaining the known peculiar orientational features of water's interactions. By using an original method of partial integration of the change in electron density occurring upon the formation of Ng-water complexes (computed with accurate ab initio methods and basis sets), we map the local charge displacement along the whole intermolecular region and are thus able to appraise CT effects free of the inherent ambiguity of charge decomposition models. With this, it was possible to prove that a small CT (of the order of a couple of millielectrons at most) takes place from Ng to water. Most importantly, this CT correlates quantitatively with some unexpected "glory" quantum interference effects observed in high-resolution molecular-beam Ng-water scattering experiments that indicate a stronger bond than a pure van der Waals force. The energy stabilization associated with the observed CT does not exceed half a kJ mol(-1). The Ng-water CT is found to be strongly stereoselective and is mediated by a highly asymmetric, donor-acceptor concerted role of the two hydrogen atoms, which can in fact dictate the equilibrium geometry of the adducts. This finding may be of general relevance in explaining the known peculiar orientational features of water's interactions.

Penning ionization of N2O molecules by He*(2(3,1)S) and Ne*(3P2,0) metastable atoms: a crossed beam study.

The energetics of [Rg... N2O]* autoionizing collision complexes (where Rg=He or Ne) and their dynamical evolution have been studied in a crossed beam apparatus, respectively, by Penning ionization electron spectroscopy (PIES) and by mass spectrometry (MS) techniques in the thermal energy range. The PIES spectra, detected by an electron energy analyzer, were recorded for both complexes at four different collision energies. Such spectra allowed the determination of the energy shifts for Penning electron energy distributions, and the branching ratios for the population of different electronic states and for the vibrational population in the molecular nascent ions. For the [Ne...N2O]* collision complex it was found, by MS, that the autoionization leads to the formation of N2O+, NO+, O+, and NeN2O+ product ions whose total and partial cross sections were measured in the collision energy range between 0.03 and 0.2 eV. The results are analyzed exploiting current models for the Penning ionization process: the observed collision energy dependence in the PIES spectra as well as in the cross sections are correlated with the nature of the N2O molecule orbitals involved in the ionization and are discussed in term of the Rg-N2O interaction potentials, which are estimated by using a semiempirical method developed in our laboratory.

Penning ionization of N2O molecules by He*(2(3,1)S) and Ne*(3P2,0) metastable atoms: theoretical considerations about the intermolecular interactions.

A theoretical investigation of the intermolecular interaction, operative in collision complexes of He*(2 3S1), He*(2 1S0), and Ne*(3P2,0) with N2O, is carried out to explain the main results of the experimental study reported in the preceding paper. The analysis is carried out by means of a semiempirical method based on the identification, modeling, and combination of the leading interaction components, including the effect of the selective polarization of the more external electronic cloud of the metastable atom in the intermolecular electric field. These and other crucial aspects of our approach have been quantitatively verified by ab initio calculations. The proposed method permits to evaluate the interaction at any configuration of the complexes and provides a useful and inexpensive representation of the intermolecular potential energy for dynamics studies. The main experimental findings can be rationalized taking into account the critical balancing between molecular orientation effects in the intermolecular interaction field and the ionization probability. These orientation effects tend to become less pronounced with increasing collision energy.

Threshold-photoelectron-spectroscopy-coincidence study of the double photoionization of HBr.

A threshold-photoelectron-coincidence spectrum of HBr has been recorded in the 32.2-35.8 eV photon energy range, with a resolution of approximately 0.01 eV, using a synchrotron radiation source. The X (3)Sigma(-) and a (1)Delta(2) states of the HBr(2+) dication are clearly observed in the spectrum, while there is no clear evidence for the formation of the b (1)Sigma(+) electronic state. For the first two states, the vibrational states v=0-3 have been resolved, while for the ground X (3)Sigma(-) state also spin-orbit splitting has been detected. The results appear in good agreement with previous experimental observations. A comparison with theoretical predictions indicates the role of "noncovalent" contributions to the interaction between the two atomic partners for the formation of metastable states.

Low-lying electronic states of HBr2+.

The present study describes the characterization of energy and structure of HBr(2+) in its low-lying electronic states, achieved through an extension of a new empirical method [Chem. Phys. Lett. 379, 139 (2003)] recently introduced to evaluate the interatomic interaction in the HX(2+) (X=F,Cl,Br,I) molecular dications. The method is based on identification of the main components of the interaction and their evaluation through some simple correlation formulas. Potential energy curves, given in a simple, natural, and analytical form, made possible the calculations of some important properties, such as double-photoionization energy thresholds, vibrational spacing, average lifetime, and Franck-Condon factors. The predictions, compared with data available in the literature, are of great interest for the analysis and interpretation of some new experimental results.