Intermolecular potential energy surfaces for the interaction between H2X (XO, S) and a metastable Ne*(3P2,0) atom

Chemical Physics Letters 614 (2014) 171–175

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Intermolecular potential energy surfaces for the interaction between H2 X (X O, S) and a metastable Ne* (3 P2,0 ) atom Stefano Falcinelli a,∗ , Alessio Bartocci b , Pietro Candori a , Fernando Pirani b , Franco Vecchiocattivi a a b

Dipartimento d’Ingegneria Civile ed Ambientale, Università di Perugia, 06125 Perugia, Italy Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, 06123 Perugia, Italy

a r t i c l e

i n f o

Article history: Received 29 July 2014 In final form 5 September 2014 Available online 16 September 2014

a b s t r a c t Potential energy surfaces for the interaction of a Ne* (3 P2,0 ) atom with H2 O and H2 S molecules are obtained on the basis of a semi empirical method that has been previously used for some specific orientations in Ne* (3 P2,0 )–H2 O system. The method is now suitable for all orientations and also for Ne* (3 P2,0 )–H2 S system. Interesting features emerge by comparing the two systems and appear related to different characteristics of the two molecular partners. The potential energy surface for Ne* (3 P2,0 )–H2 S has been also successfully tested for reproducing recent experimental Penning ionization electron spectroscopy data, sensible to some specific orientations of the two colliding partners. © 2014 Elsevier B.V. All rights reserved.

1. Introduction We have recently discussed the importance of Penning ionization reactions in several non equilibrium systems, including atmospheres of planets and low energy plasmas [1,2]. In this contest, we have recently studied the ionization of some hydrogenated molecules (H2 O, NH3 , and H2 S) by Penning Ionization coupled with both mass spectrometry and Electron Spectroscopy (PIES) techniques [3–7]. The target of such experiments is obtaining detailed information about the energetics and the stereo-dynamics of the collision complex. In particular, the analysis of the obtained results shows the evidence of a quite pronounced anisotropy of attraction that controls the stereo-dynamics in the entrance channels of the analyzed systems. In the case of He* – and Ne* –H2 O autoionizing collisions, we were able to confirm by a preliminary analysis the presence of a strong anisotropic interaction, after previous experimental and theoretical results by Ohno et al. [8], Haug et al. [9], Bentley [10], and Ishida [11]. In particular, we rationalized our experimental findings taking into account the critical balancing between molecular orientation effects in the intermolecular interaction field and the ionization probability. Finally, in analyzing our PIES spectra, a novel semiclassical method was proposed that assumes ionization events as mostly occurring in the vicinities of the collision turning points. Moreover, the potential energy

∗ Corresponding author. E-mail addresses: [email protected] (S. Falcinelli), [email protected] (F. Vecchiocattivi). http://dx.doi.org/10.1016/j.cplett.2014.09.017 0009-2614/© 2014 Elsevier B.V. All rights reserved.

driving the system in the relevant configurations of the entrance and exit channels, employed in the spectra simulation, has been formulated by the use of a semiempirical method [6]. This procedure was able to clearly point out how different orientations of approach between the metastable atom and water molecules selectively lead to the formation of ions in different electronic states. In particular, it provides an estimation of the angular acceptance cones where the dynamics of the process leading to a specific state formation exhibits the probability of occurring at most. The two energetically accessible electronic states of H2 O+ final product ions ˜ 2 A1 ) excited one. ˜ 2 B1 ) ground state and the first A( [7] are the X( Our recent results about the He* – and Ne* –H2 S Penning ionization experiments indicate a similar anisotropic behavior for the potential energy surface (PES) describing the Rg* –H2 S incoming channel with a smaller attractive interaction respect to the Rg* –H2 O case [5]. In the present work we analyze our recent PIES spectra, for the case of Ne* –H2 S collisions, by using the same procedure already applied to water molecules, in order to compare the two sets of data, obtained under the same experimental conditions, and to identify relevant differences in the Penning ionization stereo-dynamics of the two hydrogenated molecules. PES’s that we present and exploit here, are not for only some specific orientations of the two interacting partners, but for all possible orientations and therefore are more suitable for dynamical studies, as we show here for the Ne* –H2 S system. The experimental data that we analyze here have been already published and a description of the experimental set up has been already given [5]. However, those data have been analyzed only in

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terms of the shifts of the PIES peaks, showing a negative value with respect to the expected nominal energy, defined as the difference between the excitation energy of the metastable Ne* atoms and the ionization potential of the relevant molecular state that is affected by the energetics of collision complex. In particular, such a negative shift is the indication that a phenomenological global attractive interaction affects the collision dynamics of the approaching partners [12,13]. 2. The potential energy surfaces In this section, we identify the nature of the intermolecular interaction and we model its leading components in order to control the most relevant dynamical aspects of the collision processes. This is also important in order to emphasize differences and analogies between the two homologous Ne* –H2 O and Ne* –H2 S systems. As reported in previous papers [12–14], the optical potential controlling ionization processes in the thermal energy range, depends on the combination of a real V and an imaginary  part, both depending on the intermolecular distance R and on the relative orientation of two partners, as defined by the polar and azimuthal angles,  and  respectively, as shown in Figure 1a, where both Cartesian and polar coordinates are defined for the interaction of H2 X (X O,S) and the metastable atom Rg* . For the systems we are interested to in this letter, V(R,,) controls the collision dynamics, while  (R,,) defines the ionization probability. In a recent paper [5], we were interested to the energy shifts in the PIES, which depend on the behavior of V(R,,), both in the entrance and exit channels of the ionization process. Here we plan to extend the same semi-empirical method, discussed and recently applied for Ne* –H2 O [6,7], to all possible configurations and distances of the collision complex and we use the same formulation to completely characterize the PES in the entrance Ne* –H2 S and exit Ne–H2 S+ channels. This combined study, carried out with the same methodology than before, is suitable to discover similarities and to emphasize differences between the two systems, since the proposed PES’s are formulated in a consistent way. Briefly, such a method assumes V(R,,) as due to the balance of few leading effective interaction components, all represented by semi-empirical analytical functions. The involved parameters have been defined in terms of fundamental physical

properties of interacting partners. At large distance R, the dispersion (induced dipole-induced dipole) and the induction (permanent dipole-induced dipole) attractions combine with the repulsion, due to the sizes of the interacting partners, providing a global component, usually called van der Waals interaction (VvdW ). All such contributions depend only on the polarizability of interacting partners. In the same R range a weak charge transfer contribution, Vct , due to the electron exchange effects between the outermost occupied orbital of the metastable atom and the lowest unoccupied orbital of the molecule, can be also operative and it is related to the low ionization potential of Ne* . Because of its high polarizability (27.8 A˚ 3 ), mostly determined by the ‘floppy sphere’ nature of its more external electronic cloud, the metastable neon atom at shorter R can disclose its ionic core toward the molecule. This mostly occurs when the Ne* atom approaches the X side of the molecule. This behavior generates an additional effective charge (q)-induced multipole (m) attraction component [6,7,14]. Therefore, Vqm represents the balancing of a modified size repulsion with an additional induction attraction. Under these conditions, also an electrostatic contribution, Velectr , arises at short R, being dependent on the interaction between the charge q on the polarized metastable atom and the charge distribution responsible for the permanent dipole and quadrupole moments of the H2 X molecule. We have adopted a point charge distribution, as depicted in Figure 1b, obtained by exploiting the dipole moment of both molecules and the quadrupole component values taken from the literature about H2 O [6], while those about H2 S have been calculated at the CCSD ab initio level of the theory, throughout the MOLPRO program and by using the aug-cc-pvqz basis set. Note that the adoption of the point charge representation leads often to a non coincidence of the charge positions with those of the atoms in the molecules. All the quantities necessary to define the H2 X charge distribution are given in Table 1. Therefore, the term Velectr , represented as sum of coulomb contributions, plays a crucial role in defining the interaction anisotropy, and therefore its dependence on  and , being strongly affected by the molecular orientation. According to the above considerations and following the same guidelines recently applied to Ne* –H2 O, it has been possible to formulate the full interaction V(R,,) as a combination of few effective components, whose few parameters have been defined in terms of atomic and molecular polarizability, ionization potential,

Figure 1. (a) Cartesian and polar coordinates that are used for defining the interaction between H2 X (X O,S) and a metastable atom Rg* . (b) Point charge distribution around the H2 X molecule as defined to reproduce the permanent dipole and quadrupole moments and exploited for the potential energy evaluation. The values of related parameters are listed in Table 1.

S. Falcinelli et al. / Chemical Physics Letters 614 (2014) 171–175 Table 1 The charge distribution parameters reproducing permanent electric dipole and quadrupole components of the H2 X molecule, as defined in Figure 1. H2 O q rq (Å) dq (Å) ˛

H2 S

0.33 0.60 0.97 105◦

0.17 1.34 1.00 105◦

permanent charge and charge distribution. As it has been done in previous works from this laboratory [6,7,14], the values of these parameters have been anticipated by the use of correlation formulas, and for a more complete discussion about the leading interaction components in anisotropic systems see ref. [15]. The adopted representation of V(R,,) has been found to be suitable for obtaining all basic features of the complete PES’s in the neutral entrance channel of Ne* –H2 X systems. Specifically, V(R,,) has been formulated as

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Table 2 Potential parameters used in the formulation of the various interaction components in the entrance, determining the real part of the optical potential of Ne* interacting with H2 X (X O,S). Component

Parameters

Ne* –H2 O system VvdW Vqm Vct f(R,,)

ε = 9.29 meV ε = 69.13 meV A = 4277 meV R0 = 4.5 A˚

Rm = 5.00 A˚ Rm = 2.99 A˚  = 1.1 A˚ −1 d = 0.5 A˚

ˇ = 7.0 ˇ = 9.0

Ne* –H2 S system VvdW Vqm Vct f(R,,)

ε = 13.59 meV ε = 83.24 meV A = 3500 meV R0 = 5.0 A˚

Rm = 5.19 A˚ Rm = 3.55 A˚  = 1.1 A˚ −1 d = 0.5 A˚

ˇ = 7.0 ˇ = 9.0

V (R, , ϕ) = (VvdW + Vct )f (R, , ϕ) + (Velectr + Vqm )(1 − f (R, , ϕ)) where VvdW and Vqm have been represented by improved Lennard–Jones (ILJ) functions [16] with long range attraction depending on ∼R−6 and ∼R−4 , respectively. The general form of ILJ model is



VILJ (R) = ε where

m n(R) − m

 R n(R) m

R

−

n(R) n(R) − m

 R m  m

R

 R 2

n(R) = ˇ + 4

Rm

The first term of the VILJ (R) function describes the repulsion, while the second one the attraction, both defined by ε and Rm , that are the potential well depth and its location. ˇ is a parameter representing the ‘hardness’ of the two partners. For the neutral–neutral and ion–neutral interactions, m = 6 and m = 4 are respectively used. Extending the results of previous investigations [6,7,14], the Vct (R,,) component has been represented as Vct (R, , ϕ) = −Ae−R sin  sin ϕ. The switching function f(R,,), modulating the transition from neutral–neutral to ion–neutral behavior, has been defined according to f (R, , ϕ) =

1 1 + e(R0 −R/d) (1 − sin2  sin ϕ)

.

Where the R0 value, related to the size of the neutral partners, represents the distance where the combined potential forms have equal weight, and d describes how fast occurs such passage. As usual, in these formulas  varies, by definition (see also Figure 1a), in the 0– range, while  varies between 0 and , but outside this range sin is assumed to vanish. All the parameters for the two H2 X systems are listed in Table 2, while the contour maps for some specific cuts of the PES’s are shown in Figures 2 and 3, in order to compare the behavior and to emphasize differences in the interaction of the Ne* –H2 O and Ne* –H2 S systems. The potential values so obtained are in the right scale of the early ab initio calculations by Bentley [10]. The comparison clearly shows that Ne* –H2 O is more attractive than Ne* –H2 S, and this feature is expected to substantially affect at microscopic level the precursor state driving the Penning ionization collision dynamics. Looking at the contour maps in Figures 2 and 3, it is interesting to note that in both systems two potential wells are present, one above and the other below the molecular plane, along the z

Figure 2. Contour maps for some specific cuts of the potential energy surface of the Ne* –H2 O system. The (y,x), (y,z) and (x,z) planes are defined as in Figure 1a. Only the negative energies are plotted and the separation between contour lines is of 40 meV.

direction, reflecting the presence of a pz orbital on X atom. However, while in H2 O they are two relative minima, joint by a long and deep valley, in H2 S they are separated by a saddle. These features are reflecting the partial sp2 hybridation in H2 O, which is practically not existing in H2 S. Furthermore, the removal of one electron ˜ 2 B1 ) from the pz orbital is producing the final ion in the ground X( state, while the ionization occurring when the metastable atom is approaching along the C2v direction, by the X atom side, leads to ˜ 2 A1 ) state. Therefore, the the formation of the H2 X+ ion in the A( features of the PES are expected to be crucial in determining the stereo-dynamics and the selectivity of the ionization reaction.

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Figure 3. Contour maps for some specific cuts of the potential energy surface of the Ne* –H2 S system. The (y,x), (y,z) and (x,z) planes are defined as in Figure 1a. Only the negative energies are plotted and the separation between contour lines is of 40 meV.

polarizability and size much smaller than that in the metastable state (0.40 A˚ 3 and 27.8 A˚ 3 , respectively) and, on the other hand, H2 X+ is a rather compact molecular ion. Therefore, for the present purpose, its behavior has been simply approximated by an isotropic-radial ILJ function with m = 4, whose parameters can be obtained by appropriately exploiting the basic features of the interacting partners. As it has been done previously [6,7], the ˚ have values of the ε and Rm parameters (ε = 26.34 meV, Rm = 3.36 A) been again anticipated by the correlation formulas, by assuming the H2 X+ molecular ion as a spherical partner (with an estimated average polarizability of 2.40 A˚ 3 for H2 S+ ) interacting with a compact Ne atom, whose polarizability amounts to only 0.40 A˚ 3 . The guidelines of the semiclassical method used for the spectrum simulation start from the consideration that the ionization, during the collision, mainly occurs in the vicinities of the classical turning point, because at those distances the system spends a longer time and the coupling for the ionization is higher. Therefore, we calculated the energy peak position by simply assuming the mutual orientation of the two colliding partners as ‘frozen’, being the average rotational period much longer than the ionization time. The peak position is shifted with respect the nominal energy by the critical balance of the interaction potential in the entrance and exit channels, at those distances where the ionization mainly occurs. Details of this methodology are already given and discussed in a recent paper [7]. The spectrum is therefore build up by using the energy levels of the ionic states available in the literature for the H2 S+ product ions, appropriately shifted because the effect of the intermolecular interaction driving the collision complex in the specific orientations promoting the selective formation of product ions in the final electronic states. A Gaussian shape, related to the energy resolution of the experiment, is then given to the levels. The H2 S+ ion levels are taken from the photoionization experiment of Hochlaf et al. [17], ˜ 2 A1 ) state. ˜ 2 B1 ) state, and from Baltzer et al. [18] for the A( for the X( ˜ 2 B2 ) Unfortunately we did not find suitable data available for the B( state. The electron energy spectrum of Ne* –H2 S is reported in Figure 4, where in the lower panel the experimental data from this

In order to test the reliability of these PES’s on a quantitative ground, we have performed a molecular dynamics calculation of the PIES spectrum of Ne* + H2 S, as we have recently done in the case of Ne* + H2 O [4,7], that exhibited the relative importance of strength and anisotropy of the intermolecular interaction in determining the measured spectral features. 3. The electron spectrum simulation The simulation of the PIES spectrum for Ne* + H2 S ionization has been carried out by using the semiclassical method that we have recently developed and applied to Ne* + H2 O [7]. To do such a calculation it is necessary also to define the imaginary component  of the optical potential and the interaction in the Ne–H2 S+ exit channel. In the case of Ne* + H2 O, the imaginary component  was estimated by the analysis of the collision energy dependence of the total ionization cross section measured in the thermal energy range. Since in the Ne* + H2 S system such kind of data is not available, and considering that we are here interested only to the relative effects in the spectral features, we have reasonably used here the same  function exploited for Ne* + H2 O [4,7]. In the exit channels, asymptotically leading to Ne–H2 X+ products, the interaction appears to be simpler to be described than in the entrance channels, since here the balance is between two components only, size repulsion and dispersion plus induction attraction. Moreover, the ground state Ne atom exhibits a

Figure 4. In the lower panel the experimental Penning ionization electron spectrum for Ne* + H2 S system at 55 meV collision energy is reported, while the simulation is plotted in the upper panel. Shift = 0 refers to a spectrum simulation without effect because the intermolecular potential, while a shift of −0.18 eV and −0.12 eV is ˜ 2 A1 ) state, ˜ 2 B1 ) and A( obtained for the two limiting orientations leading to the X( ˜ 2 B1 ) state is given respectively (see text). However a better fit of the data for the X( by −0.12 eV.

S. Falcinelli et al. / Chemical Physics Letters 614 (2014) 171–175

laboratory are plotted. In the upper panel the simulation is displayed, where the dotted line is the spectrum without any energy shift (shift = 0), that is by assuming no effects because the intermolecular interaction. The calculation for a Ne* approach along ˜ 2 A1 ) state ˜ 2 B1 ) state formation) and along the C2v (A( the z-axis (X( formation) provides a shift of −0.18 eV and −0.12 eV, respectively. While such a simulation satisfactorily reproduces the position and ˜ 2 A1 ) state band, that of the X( ˜ 2 B1 ) the shape of the experimental A( state is too shifted with respect the experimental band [5]. A shift of −0.12 eV appears to be more appropriate, and in reasonable agreement with the results that we have previously estimated on the basis of empirical methods [5]. 4. Conclusions In this letter we have exploited a semi empirical formulation of the intermolecular potential in the entrance and exit channels of Penning ionization processes promoted by the Ne* + H2 X (X O,S) collisions in the thermal energy range. This formulation is relevant to cast light on the basic features of the full PES, that control the selectivity in the collision dynamics, and to rationalize the changes observed when the behavior of Ne* + H2 O is compared with that of Ne* + H2 S. The analytical formulation of the PES’s, defined in terms of few parameters (without any adjustment of their predicted values), allows the evaluation of the interaction energy for all accessible (stable and unstable) configurations of the Ne* –H2 X colliding system, including both entrance and exit channels, and therefore it is suitable for any type of molecular dynamics calculation. As general comment, it is interesting to note that, while the absolute scale of the interaction on both sides of H2 X depends on ε and Rm values, and on the point charge distribution of the molecule, its anisotropy is affected by the value and angular dependence of the switching function. The phenomena analyzed here must depend on the intermolecular interaction averaged over the relative configurations mainly probed by the collision complex, promoting autoionization under the used conditions. It must be also taken into account that during each collision the molecular partner H2 X is not motionless, but rotating. Therefore, the ionization reaction selectivity emerges in a defined angular cone of the PES, whose borders are those regions mainly controlling the energy shift of emitted electrons. Differences in the topography of the full PES’s justify the change experimentally observed in the formation probability of the product ions in the ground and in the first exited state. In particular, ˜ 2 A1 )/X( ˜ 2 B1 ) branching ratio is much higher for water with the A( respect to the one for hydrogen sulfide (about 3.7 and 1.0, respectively, as measured at an average collision energy of 55 meV). Since the first excited state of the ion product is mostly formed in both the cases when the metastable neon atom is approaching the H2 X molecule along the C2v axis on the X side (confined on the (xy) plane of Figure 1), the observed change in the propensity confirms that

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the stronger interaction in water provides a sort of appreciable orientation effects that the weaker interaction driving the hydrogen sulfide collisions is unable to promote (compare the contour plots in Figures 2 and 3). Moreover, the best simulation of the measured PIES spectra has been obtained in all cases with an energy negative peak shifting, suggesting a stronger attraction in the entrance channels with respect to the exit channels. Also this experimental finding is fully justified by the semiclassical model adopted to describe the collision dynamics on the proposed PES’s. However, the absolute value of the peak shifting’s, predicted by the model for the two limiting configurations (along the pz and C2v directions) promoting the formation of the product ions in the ground and in the first excited state, when compared with those used for the best simulation, appears to be significantly larger for H2 O+ , while for H2 S+ the shif˜ 2 B1 ) state (see Figure 4). ting is larger only for the formation of the X( This confirms that each electronic state of the final ion is formed with a given ‘effective’ acceptance cone. The features of the PES’s plotted in Figures 2 and 3 and the evaluated peak shifting’s suggest that for the ionization of H2 O the two acceptance cones exhibit a width that appears in agreement with previous estimates [7]. For ˜ 2 B1 ) state the ionization of H2 S the acceptance cone leading to the X( is comparable with that of water (the width is about ±30◦ ), while ˜ 2 A1 ) state is much narrower, that promoting the formation of the A( determining a less efficient reaction probability. Acknowledgement Financial contributions from the Italian MIUR (Ministero dell’Istruzione, dell’Università e della Ricerca) (grant no. 2009W2W4YF 002) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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