Exploring the electronic states of the hydroxyl dication OH2+: thermodynamic (meta)stability, bound-free emission spectra, and charge transfer processes.

Accurate potential energy curves were constructed for a manifold of electronic states of the hydroxyl dication using a highly correlated electronic structure approach (SA-CASSCF/MRCI+Q/aug-cc-pV5Z). The existence of a bound (meta)stable ground state and bound low-lying states for OH2+ are ruled out, but do not exclude the possibility of its transient formation and dissociation along the repulsive ground state potential energy curve. Our results do not support the conclusion reported for the observation of OH2+ by electron ionization from ground state OH+. Despite the repulsive character of the low-lying states, thermodynamic stability was indeed verified for the states 2 4Π and 3 4Σ- along with a series of metastable high-lying doublet states. For the (quasi)bound states, we obtained vibrational levels, spectroscopic parameters, and dipole moment functions. Using accurate transition dipole moment functions, we also evaluated bound-free emission transition probabilities and radiative lifetimes. For transitions from v'= 0, our estimates of 92.8 ns (4Π) and 9.3 ns (4Σ-) indicate that the ones obtained by a multichannel theory of predissociating states are too short (2-60 ps). Landau-Zener cross sections averaged over the Maxwellian distribution of relative velocities, and rate coefficients for the reaction O2+ + H → O+ + H+ were obtained using the potential energy curves of the states 4Π and 4Σ- associated with the channel O2+ + H and the repulsive ones dissociating into O+ + H+ leading to good results for the rate constant thus supporting its importance to explain the distribution of O+ in astrophysical plasmas.

Thermodynamically stable diatomic dications: The cases of SrO2+ and SrH2.

A high level theoretical investigation of the low-lying electronic states of the diatomic dications SrO2+ and SrH2+ is presented for the first time along with experimental results of their mass spectra where they were detected. A global and reliable picture of the potential energy curves of the electronic states and the associated spectroscopic parameters provide quantitative results attesting to the thermodynamic stability of both species. Inclusion of spin-orbit interactions does not significantly change the energetic characterization. For SrO2+, the ground (X 3Σ-) and first excited (A 3Π, Te = 3971 cm-1) states are bound (De) by 15.94 kcal mol-1 and 4.71 kcal mol-1, respectively. Transition probabilities (Av'v″) have been evaluated and radiative lifetimes estimated for the vibrational states of A 3Π (v'), and transition probabilities are expected to be diagonally dominant and fall in the far-IR region of the spectrum. For the singlet states a 1Δ, b 1Π, c 1Σ+, and d 1Σ+, transition probabilities have also been calculated for all symmetry allowed transitions and the radiative lifetimes evaluated for selected vibrational states of the upper levels. The transitions associated with the band systems d 1Σ+-b 1Π and d 1Σ+-c 1Σ+, although falling in the yellow region of the spectrum, with overlapping bands, are expected to show quite distinct intensities since the transition moment associated with d 1Σ+-c 1Σ+ is much larger. For singlet transitions, the prediction of relative intensities using the Franck-Condon approximation fails in most of the cases. For SrH2+, only the ground state is bound (De = 6.54 kcal mol-1); with an equilibrium distance of 5.117 a0, the associated spectroscopic parameters (ωe, ωexe, Be) turned out to be (518.9, 32.77, 2.3227) in cm-1. For both species, dipole moment functions illustrate the variation of the molecular polarity with the internuclear distance.

A theoretical study of SnF2+, SnCl2+, and SnO2+ and their experimental search.

We present a detailed theoretical study of the stability of the gas-phase diatomic dications SnF(2+), SnCl(2+), and SnO(2+) using ab initio computer calculations. The ground states of SnF(2+), SnCl(2+), and SnO(2+) are thermodynamically stable, respectively, with dissociation energies of 0.45, 0.30, and 0.42 eV. Whereas SnF(2+) dissociates into Sn(2+) + F, the long range behaviour of the potential energy curves of SnCl(2+) and SnO(2+) is repulsive and wide barrier heights due to avoided crossing act as a kind of effective dissociation energy. Their equilibrium internuclear distances are 4.855, 5.201, and 4.852 a(0), respectively. The double ionisation energies (T(e)) to form SnF(2+), SnCl(2+), and SnO(2+) from their respective neutral parents are 25.87, 23.71, and 25.97 eV. We combine our theoretical work with the experimental results of a search for these doubly positively charged diatomic molecules in the gas phase. SnO(2+) and SnF(2+) have been observed for prolonged oxygen ((16)O(-)) ion beam sputtering of a tin metal foil and of tin (II) fluoride (SnF(2)) powder, respectively, for ion flight times of about 10(-5) s through a magnetic-sector mass spectrometer. In addition, SnCl(2+) has been detected for (16)O(-) ion surface bombardment of stannous (tin (II)) chloride (SnCl(2)) powder. To our knowledge, SnF(2+) is a novel gas-phase molecule, whereas SnCl(2+) had been detected previously by electron-impact ionization mass spectrometry, and SnO(2+) had been observed before by spark source mass spectrometry as well as by atom probe mass spectrometry. We are not aware of any previous theoretical studies of these molecular systems.

The diatomic dication CuZn2+ in the gas phase.

In the present combined experimental and theoretical study we report the observation of the novel gas-phase dication CuZn(2+) and provide some theoretical insight into the electronic binding of this exotic metastable molecule and its formation mechanism. Using mass spectrometry we have detected four isotopomer signals of CuZn(2+) at half-integer m/z values for ion flight times of about 14 µs. CuZn(2+) was unambiguously identified by its isotopic abundance. High-current energetic Ar(+) ion bombardment of a brass surface was used for its production. Subsequent dication formation was found to take place in the ion extraction region of our mass spectrometer several tens of microns in front of the sputtered brass surface. The dication formation mechanism appears to be resonant electron transfer in soft gas-phase collisions between sputter-ejected singly charged CuZn(+) molecular ions and incoming Ar(+) projectiles. This conclusion is supported by our theoretical study that obtained an ionization energy of CuZn(+) of 15.75 eV, in excellent agreement with both the experimental and calculated ionization energy of Ar (15.76 and 15.67 eV, respectively). The ground state of CuZn(2+) is found to be a metastable one with a very shallow potential well at an internuclear equilibrium distance of about 2.7 Å the dissociation energy being very difficult to estimate. Interestingly, spin-orbit corrections are found to be necessary to get an adequate description of the metastable state of CuZn(2+), whereas relativistic corrections have no effects on neutral CuZn nor on CuZn(+).

Calcium-containing diatomic dications in the gas phase.

Sputtering (ion surface bombardment) of various calcium-containing powder samples with an energetic (17 keV), high-current (16)O(-) beam has produced the diatomic dications of CaSi(2+), CaP(2+), CaF(2+), CaH(2+), CaCl(2+), CaBr(2+) and CaI(2+). These molecular gas-phase species have been identified in positive ion mass spectra at half-integer m/z values; their ion flight times through a magnetic-sector mass spectrometer were roughly 10(-5) s. Most of them appear to be novel molecular ions; the stability of the latter four (CaH(2+), CaCl(2+), CaBr(2+) and CaI(2+)) had been demonstrated in previous theoretical studies, whereas only CaF(2+) and CaBr(2+) had been observed before. Here we combine the results of our experimental search with a detailed theoretical study of the remaining three systems CaSi(2+), CaP(2+) and CaF(2+). All electronic states correlating with the first dissociation channel are characterized using high level ab initio electronic structure calculations. In their ground states, we find CaSi(2+) to be a long-lived metastable molecule, whereas CaF(2+) and CaP(2+) are thermodynamically stable, with respective equilibrium internuclear distances of 6.253, 4.740, and 5.731 a(0). CaSi(2+) has a well depth of 7116 (0.88) cm(-1) (eV) and a dissociation asymptote 7956 (0.99) cm(-1) (eV) below the ground state minimum. The dissociation energy of CaF(2+) is estimated to be 3404 (0.42) cm(-1) (eV), whereas for CaP(2+) we found 2547 (0.32) cm(-1) (eV), and a barrier height of 8118 (1.01) cm(-1) (eV). Their adiabatic double ionisation energies are 22.87, 16.91, and 17.32 eV, respectively, for the F, Si, and P containing dications.

Metastable BrO2+ and NBr2+ molecules in the gas phase.

The doubly positively charged gas-phase molecules BrO(2+) and NBr(2+) have been produced by prolonged high-current energetic oxygen (17 keV (16)O(-)) ion surface bombardment (ion beam sputtering) of rubidium bromide (RbBr) and of ammonium bromide (NH(4)Br) powdered ionic salt samples, respectively, pressed into indium foil. These novel species were observed at half-integer m∕z values in positive ion mass spectra for ion flight times of roughly ∼12 µs through a magnetic-sector secondary ion mass spectrometer. Here we present these experimental results and combine them with a detailed theoretical investigation using high level ab initio calculations of the ground states of BrO(2+) and NBr(2+), and a manifold of excited electronic states. NBr(2+) and BrO(2+), in their ground states, are long-lived metastable gas-phase molecules with well depths of 2.73 × 10(4) cm(-1) (3.38 eV) and 1.62 × 10(4) cm(-1) (2.01 eV); their fragmentation channels into two monocations lie 2.31 × 10(3) cm(-1) (0.29 eV) and 2.14 × 10(4) cm(-1) (2.65 eV) below the ground state minimum. The calculated lifetimes for NBr(2+) (v(") < 35) and BrO(2+) (v(") < 18) are large enough to be considered stable against tunneling. For NBr(2+), we predicted R(e) = 3.051 a(0) and ω(e) = 984 cm(-1); for BrO(2+), we obtained 3.033 a(0) and 916 cm(-1), respectively. The adiabatic double ionization energies of BrO and NBr to form metastable BrO(2+) and NBr(2+) are calculated to be 30.73 and 29.08 eV, respectively. The effect of spin-orbit interactions on the low-lying (Λ + S) states is also discussed.