An experimental and theoretical characterization of the electronic structure of doubly ionised disulfur.

Using time-of-flight multiple electron and ion coincidence techniques in combination with a helium gas discharge lamp and synchrotron radiation, the double ionisation spectrum of disulfur (S[Formula: see text]) and the subsequent fragmentation dynamics of its dication are investigated. The S[Formula: see text] sample was produced by heating mercury sulfide (HgS), whose vapour at a suitably chosen temperature consists primarily of two constituents: S[Formula: see text] and atomic Hg. A multi-particle-coincidence technique is thus particularly useful for retrieving spectra of S[Formula: see text] from ionisation of the mixed vapour. The results obtained are compared with detailed calculations of the electronic structure and potential energy curves of S[Formula: see text] which are also presented. These computations are carried out using configuration interaction methodology. The experimental results are interpreted with and strongly supported by the computational results.

S2O2q+ (q = 0, 1, and 2) Molecular Systems: Characterization and Atmospheric Planetary Implications.

We use accurate ab initio methodologies at the coupled cluster level ((R)CCSD(T)) and its explicitly correlated version ((R)CCSD(T)-F12) to investigate the electronic structure, relative stability, and spectroscopy of the stable isomers of the [S2O2] system and of some of its cations and dications, with a special focus on the most relevant isomers that could be involved in terrestrial and planetary atmospheres. This work identifies several stable isomers (10 neutral, 8 cationic, and 5 dicationic), including trigonal-OSSO, cis-OSSO, and cyc-OSSO. For all these isomers, we calculated geometric parameters, fragmentation energies, and simple and double ionization energies of the neutral species. Several structures are identified for the first time, especially for the ionic species. Computations show that in addition to cis-OSSO and trans-OSSO proposed for the absorption in the near-UV spectrum of the Venusian atmosphere other S2O2, S2O2+, and S2O22+ species may contribute. Moreover, the characterization of the stability of singly and doubly charged S2O2 entities can also be used for their identification by mass spectrometry and UV spectroscopy in the laboratory or in planetary atmospheres. In sum, the quest for the main UV absorber in Venus' atmosphere is not over, since the physical chemistry of sulfur oxides in Venus' atmosphere is far from being understood.

Characterization of the electronic states of the biological relevant SSNO molecule.

Using configuration interaction ab initio methods, we investigate the lowest electronic states of doublet and quartet spin multiplicities of SSNO where the one-dimensional cuts of the six-dimensional potential energy surfaces of these electronic states along the stretching and bending coordinates are computed. Mainly, these electronic states are found to be repulsive along the central SN distance. A high density of electronic states is computed even at low excitation energies that may favor their couplings. Therefore, the dynamics of the SSNO electronic states is expected to be very complex. We also characterized the bound electronic states spectroscopically where we derived their equilibrium structures and vibrational frequencies. Our calculations show the importance of taking into account of dynamical correlation, in addition to static correlation, for the accurate description of SSNO electronic excited states and more generally for those of R-NO molecular species. Finally, we highlighted the potential role of SSNO in light-induced NO delivery from SSNO related species in biological media.

Toward the laboratory identification of [O,N,S,S] isomers: Implications for biological NO chemistry.

Benchmark ab initio calculations are performed to investigate the stable isomers of [O,N,S,S]. These computations are carried out using coupled cluster (RCCSD(T)) and explicitly correlated coupled cluster methods (RCCSD(T)-F12). In addition to the already known cis isomer of SSNO, nine other stable forms are predicted. The most stable isomer is cis-OSNS. Nine structures are chain bent-bent with relatively large dipole moments which make them detectable, as cis-SSNO, by infrared, far-infrared, and microwave spectroscopies. We found also a C2v isomer (NS2O). Since these species are strongly suggested to play an important role as intermediates during the bioactive reaction products of the NO/H2S interaction, the rotational and vibrational spectroscopic parameters are presented to help aid the in vivo identification and assignment of these spectra. Results from this work show that [O,N,S,S] may play key roles during nitric oxide transport and deliver in biological media, as well as, provide an explanation for the weak characteristic of disulfide bridges within proteins.

State-to-state vacuum ultraviolet photodissociation study of CO2 on the formation of state-correlated CO(X(1)Σ(+); v) with O((1)D) and O((1)S) photoproducts at 11.95-12.22 eV.

The state-to-state photodissociation of CO2 is investigated in the VUV range of 11.94-12.20 eV by using two independently tunable vacuum ultraviolet (VUV) lasers and the time-sliced velocity-map-imaging-photoion (VMI-PI) method. The spin-allowed CO(X(1)Σ(+); v = 0-18) + O((1)D) and CO(X(1)Σ(+); v = 0-9) + O((1)S) photoproduct channels are directly observed from the measurement of time-sliced VMI-PI images of O((1)D) and O((1)S). The total kinetic energy release (TKER) spectra obtained based on these VMI-PI images shows that the observed energetic thresholds for both the O((1)D) and O((1)S) channels are consistent with the thermochemical thresholds. Furthermore, the nascent vibrational distributions of CO(X(1)Σ(+); v) photoproducts formed in correlation with O((1)D) differ significantly from that produced in correlation with O((1)S), indicating that the dissociation pathways for the O((1)D) and O((1)S) channels are distinctly different. For the O((1)S) channel, CO(X(1)Σ(+); v) photoproducts are formed mostly in low vibrational states (v = 0-2), whereas for the O((1)D) channel, CO(X(1)Σ(+); v) photoproducts are found to have significant populations in high vibrationally excited states (v = 10-16). The anisotropy ß parameters for the O((1)D) + CO(X(1)Σ(+); v = 0-18) and O((1)S) + CO(X(1)Σ(+); v = 0-9) channels have also been determined from the VMI-PI measurements, indicating that CO2 dissociation to form the O((1)D) and O((1)S) channels is faster than the rotational periods of the VUV excited CO2 molecules. We have also calculated the excited singlet potential energy surfaces (PESs) of CO2, which are directly accessible by VUV excitation, at the ab initio quantum multi-reference configuration interaction level of theory. These calculated PESs suggest that the formation of CO(X(1)Σ(+)) + O((1)S) photoproducts occurs nearly exclusively on the 4(1)A' PES, which is generally repulsive with minor potential energy ripples along the OC-O stretching coordinate. The formation of CO(X(1)Σ(+)) + O((1)D) photofragments can proceed by non-adiabatic transitions from the 4(1)A' PES to the lower 3(1)A' PES of CO2via the seam of conical intersections at a near linear OCO configuration, followed by the direct dissociation on the 3(1)A' PES. The theoretical PES calculations are consistent with the experimental observation of prompt CO2 dissociation and high rotational and vibrational excitations for CO(X(1)Σ(+)) photoproducts.