Optimization of geometry
The interatomic distances (Re), the dipole moments (μ) and the vibration frequencies (ωe) of ZnTe system and its ions ZnTe+ and ZnTe- are given in (Table 1). The analysis for this table shows that all calculation models used underestimate the internuclear distance (Re) from ZnTe. The MRCI method gives the smallest value of the internuclear distance Re = 2.414 Å of ZnTe. The CCSD(T) method provides a distance that approximates the experimental value (the difference is of 4.13%)[41].The internuclear distance of the ZnTe+cation decreases in this order: Re (UHF)> Re (UCCSD (T))> Re (UMP2)>Re (MRCI).The order for the ZnTe-anion is: Re (UCCSD (T))> Re (UHF)> Re (UMP2)>Re (MRCI).For ZnTe, MRCI and CCSD (T) methods give a vibration frequency in good agreement with the experimental value (the difference is about 3%)[42].The values of the dipole moment increase by passing from the anion (ZnTe-) to the neutral (ZnTe) towards the cation (ZnTe+).For ZnTe, ZnTe+, and ZnTe-the large value of the dipole moment is obtained at the level of the MRCI method.
Ionization energy and electronic affinity of ZnTe and its ions
The ionization potential (IP) and the electronic affinity (EA) are calculated with Koopmans' theorem: IP (X system) = - EHOMO of X, EA (X system) = - ELUMO of X where X = ZnTe, ZnTe+, or ZnTe-.
The values of the HOMO and the LUMO energies and the energy gap, the ionization potential and the electronic affinity of the ZnTe system and its ions are represented in (Table 2). For ZnTe, the table indicates similar values of IP for the different methods with a small difference of 0.01 eV. For the anion, the largest energy gap is obtained at the CCSD level. The negative electronic affinity values of ZnTe- mean that it would be necessary to supply energy to this molecule to attach an electron to it. Regarding the ionization potentials and the electronic affinities of the studied systems, the order of evolution is as follows: IPcation˃ IPneutral˃I Panion
Dissociation energies of ZnTe and its ions
Experimentally, the values of the dissociation energy are based essentially on thermochemical data in the case for the ZnTe neutral system [43].In theory, only a few studies have been done to determine the dissociation energy (De) of ZnTe. RenginPekoz, SakirErkoç[26] and Mustafa Kurban[27] calculated De and showed that ZnTe dissociates according to the relation below:
ZnTe (X 1Σ+) → Zn (1S) + Te (1D)
For ZnTe+, we give the dissociation following this relation:
ZnTe+ (X2Π) → Zn + (2S) + Te (3P)
For ZnTe-, the dissociation is:
ZnTe- (X 2Σ+) → Zn (1S) + Te- (2P)
The dissociation energies of the studied systems are obtained from the potential energy curves plotted with the different models used. These curves are shown respectively for ZnTe, ZnTe + and ZnTe- in Fig.1, Fig.2, Fig.3, Fig.4 and Fig.5.
The dissociation energy values (De) are collated in (Table 3).We note that the dissociation energy of the neutral system is greater than those of the ions for MP2, CCSD(T), and MRCI methods. For these methods, the increasing order of energies is as follows: Anion<Cation<Neutral. For ZnTe and ZnTe+, the dissociation energy is the same at MRCI level. The largest dissociation energy is obtained by MRCI method for both ZnTe and ZnTe+ systems. Concerning ZnTe-, the higher dissociation energy is obtained at CCSD (T) level. The De value of ZnTe estimated by the previous method is more or less in agreement with the experiment with a difference of19-44%.
Fig 2 represents the potential energy curves of ZnTe, ZnTe+, and ZnTe-molecules in their ground states obtained by the MRCI and CASSCF methods. The calculation was made over a distance of 1.4-11.38 Å for a step of 0.02 Å. It is noted that the MRCI methods minimizes the potential energy of the molecule better than the CASSCF.
Curves of potential energy and spectroscopic constants
The potential energy curves of the ground state and the different excited states for the ZnTe and its ions are studied using the program Molpro. Theses curves are represented respectively for ZnTe, ZnTe+ and ZnTe- in Figures 2, 3, 4 and 5.Table 4 summarizes the spectroscopic constants of the ground state and the excited states of the studied systems.
The potential energy curves (see Fig 3) are shown for ground state X1Σ+ and the three singlet excited states: B1Σ+, A1Π and C1Π, as well as the four triplet states: d3Σ+, b3Σ+, a3Π and d3Π.The electron states correlate adiabatically to these dissociation limits: Zn (1S) + Te (3P), Zn (1S) + Te (1D), Zn (1S) + Te (1S).We note from Fig 3 that several avoided crossings are remarkable between the ground state X1Σ+ of ZnTe and the excited states a3Π and A1Π.
These electronic states intersect in the energy range (8.8-8.6 a.u), which implies the presence of rotational and spin-orbit interactions.
The potential energy curves show that the energy of the state a3Πis less than that of the singlet state, this can be interpreted with the notion of the hole Fermi. This suggests that two electrons of opposite spins are more likely to repel each other if they have parallel spins. By analyzing the results of the spectroscopic constants reported in Table 4, we observe that the internuclear distance Re of the ground state is less than the experimental values [38], the deviation is of 19%.
The same value of Re is recorded for the ground state X1Σ+ and the excited state B1Σ+ (R "e = Re ') which means that Re stay invariant during the transition and ZnTe molecule describes a vertical path without variation of the value of Re: this is the first case of Franck Condon principle.
The distance Re of the excited state A1Π differs from that of the ground state X1Σ+, so that Re '>Re": this is the second case of Franck Condon principle. For the state C1Π, Re '> ˃Re", this is the third case of Franck Condon principle.
The vibration frequency ωe of the ground state is in agreement with the experimental value [42], the deviations are of the order of 4.39 %. We record that most of the vibration frequencies of the excited states are greater than that of the ground state. The majority of the energies of dissociation of the excited states are lower than that of the ground state apart from the states b3Σ+ and c3Π.
The calculated value of the dissociation energy of the ground state is in good agreement with the experimental result [43] compared to the calculations performed with the Gaussian program. The difference is of 17%. The electronic state X1Σ+ has the small value of the rotational constant Be. Our results are compared to the work of Fancher et al [44] done on zinc oxide and its anion.
For ZnTe+ cation, the potential energy curves (Fig 4) are presented for the ground state X2Πand the three doublet electronic states: A2Σ+, C2Π, and B2Σ + as well as the four quartet states: b4Π, c4Π, a4Σ+ and d4Σ +.
These electron states converge to the following lowest dissociation limits: Zn + (2S) + Te (3P), Zn + (2S) + Te (1D), Zn + (2S) + Te (1S). Our results are compared to the work of Maatouk et al [45] carried out on the MgO+ cation. We note the absence of potential wells for the states c4Π, d4Σ+ and a4Σ+ (table 4), this complicates the calculation of the spectroscopic constants.
These states have very large internuclear distances and tend towards ionization. The distances Re of the excited states are greater than that of the ground state. The excited states give frequencies of vibration lower than that of the ground state. Concerning the electronic state C2Π, it has a very low dissociation energy, which confirms its instability. The internuclear distance of this state is so greater than that of the ground state (Re'> ˃Re''), thus we note the presence of the third case of Franck Condon principle. About ZnTe- anion (Fig.5), the potential energy curves are presented for the ground state X2Σ + and the two doublet electronic states: B2Σ+ and A2Π. The calculation did not give results for quartet excited states. All electron states converge adiabatically towards the lowest dissociation limit Zn (1S) + Te- (2P).A crossing between the X2Σ+ and A2Π states is recorded, this results reveals the presence of rotational and spin-orbit interactions. The comparison between internuclear distances indicates that the first transition X2Σ+ B2Σ+is placed within the framework of the second case of Franck Condon principle (Re '> R "e).The highest vibration frequency is given to the ground state X2Σ+.
The electronic state B2Σ+ gives the highest dissociation energy, although the state A2Π indicates the lowest dissociation energy. Our results are compared to the work of Fancher et al [44] about zinc oxide and its anion.
Evolution of transition moments
The evolution of the transition moments for ZnTe and its ions ZnTe+ and ZnTe- are reported respectively in Figs.6(a), 6(b) and 6(c).We only considered the allowed transitions. These curves show changes at different internuclear distances.
Sudden jumps are noted for these moments, this can be explained by the existence of avoided crosses between potential energy curves at these particular distances. On the other hand, these changes are often related to the strong interactions (responsible for the transfer of electrons) and the change of sign of electronic wave functions. For the dipole-allowed transition between the lowest states for the same spin. Our theoretical results will be useful in the design of future experiments (fluorescence, luminescence, etc.) for the molecule ZnTe and its ions. We bring to mind that zinc was detected in the interstellar medium [46] the same thing for tellurium [47]. Advanced research in this field will soon be used to detect our diatomic systems.
Properties of excited states of ZnTe and its ions
The excitation energies, the oscillator forces, the wavelengths of the system ZnTe and its ions are calculated using the TDDFT method with B3LYP functional and shown in (Table 5).The examination of this table indicate that the oscillator force increases in this order ZnTe→ZnTe-→ZnTe+. ZnTe system has wavelengths greater than those of ZnTe- and ZnTe+ (Fig.7). The excitation energies of ZnTe+ are very higher compared to the other studied systems. The majoritar transitions for the intense band absorption are: HOMO-2→LUMO (48.02%) for ZnTe, HOMO-1→LUMO + 1 (98%) for ZnTe + and HOMO-1→LUMO + 1 (50%) for ZnTe-.