Penning ionization electron spectroscopy of water molecules by metastable neon atoms

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Abstract

The Penning ionization electron spectra in metastable Ne(3P0,2) colliding with H2O molecules at 0.040, 0.075, 0.130, 0.200 and 0.300 eV collision energies have been measured and compared with the photoelectron spectrum with Ne(I) photons. The results show the formation of the ground X̃(2B1) and the first excited state Ã(2A1) of the H2O+ product ion, with a X̃/Ã branching ratio slightly increasing with the collision energy, and with an average value of 0.29 ± 0.03. The spectra have been analyzed in order to obtain some information and some basic features of the potential energy interaction between the two reacting particles.

Highlights

► In Ne–H2O collisions, water ions are formed in the first two electronic states. ► Electron spectra have been measured at several collision energies. ► Results contain information about the potential energy surface for the system.

Introduction

Reactions between metastable rare gas atoms and molecules are rather interesting because of their importance in a variety of applications, like electronic energy transfer, molecular dissociation, excimer formation, and Penning ionization. As an example, the reactions between metastable argon, krypton and xenon atoms with halogen-containing molecules have been extensively studied because their applications to the development of rare-gas halide excimer lasers. On the other hand, the role of metastable rare gas atoms in the atmospheric reactions is of interest for several reasons [1].

The content of rare gas atoms in the atmosphere is not negligible: actually argon is the third component of the dry air, after nitrogen and oxygen molecules, and some studies have shown that the exosphere is rich in He(21S0) [1]. Moreover, the rate constant for ionization processes induced by metastable rare gas atoms is generally larger than those of common bimolecular chemical reactions of atmospheric interest, involving ground state neutral species.

Furthermore, the metastable rare gas atoms at high interatomic separation behave as alkaline metals because of their weakly bound external electron, whereas at shorter distances becomes prominent the role of their core looking like halogen atoms. Therefore, their phenomenological behaviour, when interacting with hydrogenated molecules (H2O, NH3, H2S), is very interesting for fundamental and applied science, as shown by the many discussions in the literature about the role of hydrogen and halogen bonds [2].

The Penning ionization is a widely studied elementary process, because collisional autoionization of intermediate complexes represents one of the most fundamental processes for chemi-ionization [3], [4]. Several experimental techniques are used to study the microscopic dynamics of these collisional autoionization processes. It is well established that the most valuable information about the dynamics of a collisional process is provided by molecular beam scattering experiments [3], [4]. In fact, in these cases it is possible to study single collision events and also to define the translational and internal energy of the two colliding partners, thus avoiding statistical averaging. In some systems, molecular orientation or orbital polarization during the collision can also be achieved by using appropriate external fields or laser light as it is discussed, for instance, in Ref. [5]. The most detailed studies on collisional autoionization are those where the molecular beam technique is coupled with an appropriate detection method of product particles such as electrons, ions, neutral atoms or molecules and, in some cases, photons [3], [4].

Penning ionization of water molecules is an important subject and has been studied in several previous experimental and theoretical works. Early studies reported by Yee et al. [6] and by Cermák and Yencha [7] have been performed exploiting the electron spectroscopy in collisions with He(21S, 23S) metastable atoms, while Sanders and Muschlitz [8] have reported a mass spectrometric investigation of the final ionization products in this system. Such studies demonstrated that the ionization of H2O molecules by metastable helium atoms leads to the formation of nascent H2O+ ions in the ground, X̃(2B1), and in the first two excited states, Ã(2A1) and B̃(2B2). Moreover, a dissociation of the molecular ion takes place with the formation of OH+ in addition to molecular H2O+ ions, beside 4.2% of other ionic products. Later on, Ohno et al. [9], Mitsuke et al. [10] and Haug et al. [11] reported electron energy spectra for the same system, but measured with metatable helium in the pure 23S state. More recently, an ab initio potential energy surface for He(23S)–H2O system was calculated by Ishida [12] and used for a quasi-classical trajectory calculation of the Penning ionization process [13]. These investigations have shown how the formation of each electronic state of the nascent ion is correlated with specific directions of approach of the metastable atom to the molecule and demonstrate that the overall interaction between He and H2O is characterized by a strong attraction, being more intense for the approach direction along the C2v axis of the molecule with respect to the perpendicular case [9], [10], [11], [12], [13]. In 1994 Ben Arfa et al. [14] measured the electron energy for the three observed H2O+ states, X̃(2B1), Ã(2A1) and B̃(2B2), in a Penning ionization electron spectroscopy (PIES) study of He(21S)–H2O collisions, also obtaining evidences for a strong anisotropy of the He–H2O interaction potential energy surface.

The ionization of H2O by thermal energy collisions with metastable neon atoms involves the formation of only two states of the nascent H2O+ ion, X̃(2B1) and Ã(2A1), since the two spin–orbit states (3P2 and 3P0) of metastable Ne atoms have not enough energy to produce upper molecular ionic states. For such a system, only an early low resolution electron spectroscopy experiment [7] and a theoretical study of the potential energy surface [15] have been published so far.

In the present letter we report on an experimental investigation of the ionization of H2O molecules by collision with Ne(3P2,0) atoms, in the 0.040–0.300 eV energy range, by the measurement of the energy distribution of emitted electrons with a resolution higher than in the early experiment [7]. Several PIES spectra have been here recorded at different collision energies and compared with photo-electron spectroscopy (PES) data obtained with Ne(I) photons. Our results are then discussed and compared with experimental and theoretical information previously reported. The analysis of the present PIES and PES spectra provides interesting features about the dynamics of the Ne–H2O collisional autoionization process and makes possible to characterize some aspects of the interaction potential between the two colliding partners. Also the Ne–H2O interaction appears to be strongly anisotropic and attractive with a more attractive approach of the Ne atom towards the C2v molecular axis of H2O when compared with that along the perpendicular direction, as it has been already found for He–H2O system [9], [10], [11], [12], [13], [14].

The present work aims to complete and integrate previous studies on the same system with particular attention to underline possible effects resulting from the collision energy dependence.

Section snippets

Experimental

The Perugia crossed molecular beam set up has been already used previously for ionization cross section studies [16], [17], [18], and has been recently modified to allow also electron energy analysis of emitted electrons [19], [20], which represents a crucial aspect of the present investigation. Basically, it consists of a metastable rare gas atom beam, which crosses at right angles an effusive secondary beam of molecules produced by a microcapillary array source. When an ionization cross

Results and discussion

The energy spectra of the electrons emitted during the ionization of water molecules by mestastable neon atoms are reported in Figure 3 for five different collision energies: 0.040, 0.075, 0.130, 0.200, and 0.300 eV. For each energy several PIES spectra are recorded and in the Figure 3 we report only a representative set of the experimental data. These data appear to be in good agreement with previous unpublished results from another laboratory [23]. In the PIES studies, it is well known that

Conclusions

The present experimental results show the evidence of a strong attraction that dominates the potential energy surface describing the interaction and the ionization in metastable neon atoms with H2O molecule, for those orientations and distances where the ionization mainly occurs. A similar situation for metastable helium and metastable neon atoms interacting with N2O molecules [29] has been recently explained by the use of a semiempirical method that accounts for the metastable atom orbital

Acknowledgements

Financial contributions from the MIUR (Ministero dell’Istruzione, dell’Università e della Ricerca) are gratefully acknowledged. P.C. gratefully acknowledge financial support by Regione Umbria, project POR UMBRIA FSE 2007-2013 Asse IIOccupabilità, Obiettivo specificoe– Asse IVCapitale Umano, Obiettivo specificoI’.

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