Strongly bound metastable states of B2+2

Production and Characterization of Molecular Dications: Experimental and Theoretical Efforts

Molecular dications are doubly charged cations of importance in flames, plasma chemistry and physics and in the chemistry of the upper atmosphere of Planets. Furthermore, they are exotic species able to store a considerable amount of energy at a molecular level. This high energy content of several eV can be easily released as translational energy of the two fragment monocations generated by their Coulomb explosion. For such a reason, they were proposed as a new kind of alternative propellant. The present topic review paper reports on an overview of the main contributions made by the authors’ research groups in the generation and characterization of simple molecular dications during the last 40 years of coupling experimental and theoretical efforts.

Theoretical Studies on Dications and Trications of FH, ClH, and BrH. Properties of the Bound 15Σ- States. Electron-Spin g-Factors and Fine/Hyperfine Constants of the Metastable X3Σ- States of ClH2+ and BrH2+

This theoretical study reports calculations on the fine and hyperfine structure parameters of the metastable X(3)Sigma(-)(sigma(2)pi(2)) state of ClH(2+) and BrH(2+). Data on the repulsive FH(2+) system are also included for comparison purposes. The hyperfine structure (hfs) coupling constants for magnetic (A(iso), A(dip)) and quadrupole (eQq) interactions are evaluated using B3LYP, MP4SDQ, CCSD, and QCISD methods and several basis sets. The fine structure (fs) constants (zero-field splitting lambda and spin-rotation coupling gamma) and electron-spin magnetic moments (g-factor) are evaluated in 2nd-order perturbation theory using multireference CI (MRCI) wave functions. Our calculations find for (35)Cl of ClH(2+) A(iso)/A(dip) = 110/-86 MHz; eQq(0) = -59 MHz; 2lambda = 20.4 cm(-1); g( perpendicular)(v = 0) = 2.02217; and gamma = -0.31 cm(-1) (to be compared with the available experimental A(iso)/A(dip)= 162/-30 MHz). For (79)BrH(2+), the corresponding values are 300/-400 MHz; 368 MHz; 362.6 cm(-1); 2.07302; and -0.98 cm(-1) (experimental 2lambda = 445(+/-80) cm(-1)). We find g( perpendicular)(ClH(2+)) to increase by about 0.0054 between v = 0 and 2, whereas the experimental effective g( perpendicular) changes drastically with vibrational excitation. Nuclear quadrupole coupling constants for halogen atoms X are found to be as large as corresponding A(dip)(X)'s, indicating that both terms may have to be included in the Hamiltonian used to interpret XH(2+) hyperfine spectra. A novel finding relates to the bound character of the 1(5)Sigma(-)(sigmapi(2)sigma) state in FH(2+), as already known for ClH(2+) and BrH(2+), but having a deeper potential well D(e) approximately 4,000 cm(-1) (versus 1,000 cm(-1) in the heavier radicals). Vertical ionization potentials for formation of XH(3+) trications are also discussed.

Strongly bound doubly excited states of Be2

The triplet and singlet potential curves of Be2 generated by single and double excitations from 2σu into 3σg and (or) 1πu are studied with a multireference configuration interaction (MRD-CI) method. Relative to X1Σg+(2σg2 σu2) with Re = 4.72 bohr and ωe = 258 cm−1 (calculated here), these antibonding MO → bonding MO excitations lead to average decreases in bond distance (in bohr) of 0.55 (2σu → 3σg), 0.88 (2σu → 1πu), 0.93 (2σu2 → 3σg1πu), and 1.22 (2σu2 → 1πu2). The increase in vibrational frequencies ranges from 240 to 600 cm−1. The 3σg MO is found to be less bonding than 1πu, confirming predictions made by Bader et al. The experimental states A1Πu and B1Σu+ correspond to doubly excited 11Πu (2σu2 → 3σg1πu) and singly excited 11Σu+(2σu → 3σg), respectively. The 13Σg− and 11Δg states, both 2σu2 → 1πu2, preserve their doubly excited structure up to dissociation. Within the Franck–Condon region, 13Πu changes from bound (2σu2 → 3σg1πu) to repulsive (mixed 2σu → 1πg/2σg → 1πu), thereby creating the unusual situation of a strongly bound potential (short Re, high ωe) with an adiabatic dissociation energy near zero. The singlet counterpart 11Πu, however, behaves regularly as its doubly excited character is maintained up to large R(Be—Be). Key words: ab initio calculations, beryllium dimer, doubly excited states, electronic transitions, potential curves.