Pietro Candori

Dissociative double photoionization of CO2 molecules in the 36–49 eV energy range: angular and energy distribution of ion products

Dissociative double photoionization of CO(2), producing CO(+) and O(+) ions, has been studied in the 36-49 eV energy range using synchrotron radiation and ion-ion coincidence imaging detection. At low energy, the reaction appears to occur by an indirect mechanism through the formation of CO(+) and an autoionizing state of the oxygen atom. In this energy range the reaction leads to an isotropic distribution of products with respect to the polarization vector of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO(2)(2+) dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO(+) and O(+) product ions separate predominantly with a total kinetic energy between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a kinetic energy between 5 and 6 eV is dominant.

Penning ionization of N2O molecules by He*(2S3,1) and Ne*(P2,03) 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.

Predicted structure and energetics of HCl2

Abstract The potential energy in the low-lying electronic states of the HCl 2+ molecular dication has been investigated through an empirical method, founded on the identification and characterization of the main interaction components. The interaction components, properly represented, have been combined to generate the potential energy curves, for metastable and unstable states, in a simple analytical form. The proposed method provided the structure and the energetics of HCl 2+ in its low-lying electronic states and made also possible the calculation of some important properties, such as double photo-ionization energy thresholds, vibrational spacing, average lifetimes and Franck–Condon factors. The predictions have been compared with data already available in the literature.

Angular and energy distribution of fragment ions in dissociative double photoionization of acetylene molecules at 39 eV.

The two-body dissociation reactions of the dication, C(2)H(2)(2+), produced by 39.0 eV double photoionization of acetylene molecules, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The results provide the kinetic energy and angular distributions of product ions. The analysis of the results indicates that the dissociation leading to C(2)H(+)+H(+) products occurs through a metastable dication with a lifetime of 108±22 ns, and a kinetic energy release (KER) distribution exhibiting a maximum at ∼4.3 eV with a full width at half maximum (FWHM) of about 60%. The reaction leading to CH(2)(+)+C(+) occurs in a time shorter than the typical rotational period of the acetylene molecules (of the order of 10(-12) s). The KER distribution of product ions for this reaction, exhibits a maximum at ∼4.5 eV with a FWHM of about 28%. The symmetric dissociation, leading to CH(+)+CH(+), exhibits a KER distribution with a maximum at ∼5.2 eV with a FWHM of 44%. For the first two reactions the angular distributions of ion products also indicate that the double photoionization of acetylene occurs when the neutral molecule is mainly oriented perpendicularly to the light polarization vector.

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.

Penning ionization of N2O molecules by He*(2S3,1) and Ne*(P2,03) 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.

The double photoionization of HCl: An ion–electron coincidence study

The double photoionization of HCl molecules by synchrotron radiation has been studied in the energy range between 30 and 50 eV. The HCl(2+) and Cl(2+) product ions have been detected by a photoelectron-photoion-coincidence technique, while the H(+)+Cl(+) formation, which follows the double ionization of HCl, has been studied by photoelectron-photoion-photoion coincidence. The photon energy threshold for the production of HCl(2+) ions has been found to be 35.4+/-0.6 eV, while for the dissociative channel leading to H(+)+Cl(+), it has been measured a threshold at 36.4+/-0.6 eV and a change in the slope of the cross-section energy dependence at 38.7+/-0.7 eV. The production of H+Cl(2+) occurs with a threshold photon energy of 42.8+/-1.1 eV. These results appear to be in a good agreement with previous data by different experimental techniques and recent theoretical calculations performed by our laboratory.

Double photoionization of CO2 molecules in the 34-50 eV energy range.

The double photoionization of CO(2) molecules has been studied in the 34-50 eV photon energy range, by the use of synchrotron radiation and detecting electron-ion and electron-ion-ion coincidences. Three processes have been observed: (i) the formation of the CO(2)(2+) molecular dication, (ii) the production of a metastable (CO(2)(2+))* that dissociates, with an apparent lifetime of 3.1 micros, giving rise to CO(+) and O(+) ions, and (iii) the dissociation leading to the same products, but occurring with a lifetime shorter than 0.05 micros. The relative dependence on the photon energy of the cross section for such processes has been measured. While for the production of the molecular dication a threshold is observed, in agreement with the vertical threshold for double ionization of CO(2), for the dissociative processes the threshold appears to be lower than that value, indicating the presence of an indirect dissociation, probably leading to the formation of CO(+) together with a neutral autoionizing oxygen atom.

Anisotropy of the angular distribution of fragment ions in dissociative double photoionization of N2O molecules in the 30–50eV energy range

The double photoionization of N2O molecules by linearly polarized light in the 30-50 eV energy range has been studied by coupling ion imaging technique and electron-ion-ion coincidence. For the two possible dissociative processes, leading to N++NO+ and O++N2+, angular distributions of ionic fragments have been measured, finding an evident anisotropy. This indicates that the molecules ionize when their axis is parallel to the light polarization vector and the fragments are separating in a time shorter than the dication rotational period. The analysis of results provides, in addition to the total kinetic energy of ionic fragments, crucial information about the double photoionization dynamics.

Mass spectrometric study of double photoionization of HBr molecules

The double photoionization of HBr molecules, by synchrotron radiation in the energy range between 25 and 55 eV, has been studied in a mass spectrometric experiment. The HBr2+ and Br2+ product ions have been detected by a photoelectron-photoion-coincidence technique, while the H++Br+ formation, which follows the double ionization of HBr, has been studied by photoelectron-photoion-photoion-coincidence technique. HBr2+ ions are produced with a threshold of 32.4±0.4 eV, while the dissociative channel leading to H++Br+, shows a threshold around 33 eV. The production of H+Br2+ occurs with a threshold of 40.2±0.4 eV. These results appear to be in a fairly good agreement with earlier nonrelativistic calculations of potential energy curves and also with values indirectly obtained from experimental Auger spectra.