Spectroscopic and spin-orbit calculations on the SO+ radical cation

Highly correlated ab initio methods were used in order to generate the potential-energy curves of the SO+ electronic states correlating to S+(4Su)+O(3Pg) and S+(2Du)+O(3Pg). These curves were used for deducing accurate spectroscopic properties for these electronic states. Our calculations predict the existence of a 2Phi state lying close in energy to the well-characterized b 4Sigma- state and several weakly bound quartet and doublet states located in the 6-9 eV internal energy range not identified yet. The spin-orbit integrals between these electronic states were evaluated using these highly correlated wave functions, allowing the discussion of the metastability and the predissociation processes forming S+ +O in their electronic ground states. Multistep spin-orbit-induced predissociation pathways are suggested. More specifically, the experimentally determined dissociative potential-energy curve [H. Bissantz et al., Z. Phys. D 22, 727 (1992)] proposed to explain the rapid SO+(b 4Sigma-, v> or =13)-->S+(4Su)+O(3Pg) reaction is found to coincide with the 2 4Pi potential-energy curve for short internuclear distances and with the repulsive 1 6Pi state for longer internuclear separations.     

A new reaction path for the C + NO reaction: dynamics on the 4A' potential-energy surface

We present a new reaction path without significant barriers for the C + NO reaction, forming ground state N((4)S) and CO. Electronic structure (CASPT2) calculations have been performed for the two lowest (4)A' states of the CNO system. The lowest of these states shows no significant barriers against reaction in the C + NO or O + CN channels. This surface has been fitted to an analytical function using a many-body expansion. Using this surface, and the previously published (2)A' and (2)A' surfaces [Andersson et al., Phys. Chem. Chem. Phys., 2000, 2, 613; Andersson et al., Chem. Phys., 2000, 259, 99], we have performed quasiclassical trajectory (QCT) calculations, obtaining rate coefficients for the C((3)P) + NO(X(2)Pi) --> CO(X(1)Sigma(+)) + N((4)S,(2)D) and C((3)P) + NO(X(2)Pi) --> O((3)P) + CN(X(2)Sigma(+)) reactions. We have also simulated the crossed molecular beam experiments of Naulin et al. [Chem. Phys., 1991, 153, 519] The inclusion of the (4)A' surface in the QCT calculations gives excellent agreement with experiments. This is the first time an adiabatic pathway from C((3)P) + NO(X(2)Pi) to CO(X(1)Sigma(+))+N((4)S) has been reported.     

Electronic structure and bonding of the 3d transition metal borides, MB, M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu through all electron ab initio calculations

The electronic structure and bonding of the ground and some low-lying states of all first row transition metal borides (MB), ScB, TiB, VB, CrB, MnB, FeB, CoB, NiB, and CuB have been studied by multireference configuration interaction (MRCI) methods employing a correlation consistent basis set of quintuple cardinality (5Z). It should be stressed that for all the above nine molecules, experimental results are essentially absent, whereas with the exception of ScB and CuB the remaining seven species are studied theoretically for the first time. We have constructed full potential energy curves at the MRCI/5Z level for a total of 27 low-lying states, subsequently used to extract binding energies, spectroscopic parameters, and bonding schemes. In addition, some 20 or more states for every MB species have been examined at the MRCI/4Z level of theory. The ground state symmetries and corresponding binding energies (in kcal/mol) are 5Sigma-(ScB), 76; 6Delta(TiB), 65; 7Sigma+(VB), 55; 6Sigma+(CrB), 31; 5Pi(MnB), 20; 4Sigma-(FeB), 54; 3Delta(CoB), 66; 2Sigma+(NiB), 79; and 1Sigma+(CuB), 49.     

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.     

Ab initio calculation of (2+1) resonance enhanced multiphoton ionization spectra and lifetimes of the (D,3)2Sigma- states of OH and OD

High-level ab initio potential-energy curves and transition dipole moments for the OH X 2Pi, 2 2Pi, 1 2Sigma-, D 2Sigma-, 3 2Sigma-, A 2Sigma+, B 2Sigma+, 1 2Delta, 1 4Sigma-, and 1 4Pi states are computed. The results are used to estimate the (2+1) resonance enhanced multiphoton ionization spectrum for the (D,3)2Sigma-(upsilon')<--2hnuX 2Piupsilon") transitions, which are compared with experiments by Greenslade et al. [see M. E. Greenslade, M. I. Lester, D. C. Radenovic, J. A. van Roij, and D. H. Parker, J. Chem. Phys. 123, 074309 (2005), preceeding paper]. We use the discrete variable representation-absorbing boundary condition method to incorporate the effect of the dissociative intermediate 1 2Sigma- state. We obtain qualitative agreement with experiment for the line strengths. Radiative and predissociative decay rates of the Rydberg (D,3)2Sigma- states of OH and OD were computed, including spin-orbit coupling effects and the effect of spin-electronic and gyroscopic coupling. We show that the lifetime of the Rydberg 2Sigma- states for rotationally cold molecules is limited mainly by predissociation caused by spin-orbit coupling.     

On the ground and some low-lying excited states of ScB: a multiconfigurational study

The electronic structure of a series of low-lying excited triplet and quintet states of scandium boride (ScB) was examined using multireference configuration interaction (including Davidson's correction for quadruple excitations) and single-reference coupled cluster (CC) methods with averaged natural orbital (ANO) basis sets. The CC approach was used only for the lowest quintet state. The authors have analyzed eight low-lying triplets 3Sigma-(2), 3Sigma+, 3Pi(3), and 3Delta(2) dissociating to Sc(2D)/B(2P) atoms and eight low-lying quintet states 5Sigma-, 5Sigma+, 5Pi(2), 5Phi, and 5Delta(3) dissociating to Sc(4F)/B(2P) atoms. They report the potential energy curves and spectroscopic parameters of ScB obtained with the multireference configuration interaction (MRCI) technique including all singly and doubly excited configurations obtained with the ANO-S basis set. For the two lowest states they obtained also improved ANO-L spectroscopic constants, dipole and quadrupole moments as well as scalar relativistic effects based on the Douglas-Kroll-Hess Hamiltonian. They provide the analysis of the bonding based on Mulliken populations and occupation numbers. Since the two lowest states, 3Sigma- and 5Sigma-, lie energetically very close, their principal goal was to resolve the nature of the ground state of ScB. Their nonrelativistic MRCI(Q) (including Davidson correction) results indicate that the quintet is more stable than the triplet by about 800 cm(-1). Inclusion of scalar relativistic effects reduces this difference to about 240 cm(-1). The dissociation energies for 5Sigma- ScB range from 3.20 to 3.30 eV while those for the 3Sigma- range from 1.70 to 1.80 eV.     

Theoretical study on the electronic states of NaLi

Configuration interaction calculations have been carried out on electronic states of the NaLi molecule and the cation NaLi(+). Potential energy curves are presented for the lowest nine (1)Sigma(+), seven (1)Pi, four (1)Delta, eight (3)Sigma(+), seven (3)Pi, and four (3)Delta states of NaLi as well as for the lowest ten (2)Sigma(+), six (2)Pi, and two (2)Delta states of NaLi(+). The results of the present many-electron configuration interaction calculations on the cation are in support of previous core-polarization effective potential calculations. The present calculations on the NaLi molecule are complementary to previous theoretical work on this system, including recently observed electronic states that had not been calculated previously as well as an investigation of nonadiabatic effects leading to spectral perturbations. Furthermore, ab initio potential energy curves of the neutral and the ground state of the cation are employed to determine quantum defect that may be employed to generate potential energy curves for nd and (n+1)p (for n>3) Rydberg states of NaLi. The present results on the 3 (1)Pi and 4 (1)Pi states are in good agreement with recent experimental work, whereas on the basis of theoretical data, the recently observed state 5 (1)Pi is better described as 6 (1)Pi.     

Multireference configuration interaction study of the electronic states of ZrC

The potential energy curves and spectroscopic constants of the ground and 32 low-lying electronic states of ZrC have been studied by employing multireference configuration interaction methods, in conjunction with relativistic effective core potentials and 5s3p3d1f, 3s3p1d basis sets con Zr and C, respectively. We have determined that the ground state is (3)Sigma(+). However there are two low-lying (1)Sigma(+) states (below 5000 cm(-1)) which strongly interact resulting in avoided crossings. The lowest (1)Sigma(+) state corresponds to a combination of 1sigma(2) Xsigma(2) 1pi(4) configurations whereas the second is an open shell singlet 1sigma(2) 2sigma(1) 3sigma(1) 1pi(4). Several avoided crossings were observed, for (1)Pi, (3)Pi, (1)Delta, (3)Sigma(+), and (3)Delta states. We have identified (3)Pi and (1)Pi lying at 4367 and 5797 cm(-1), respectively. The results are in good agreement with the recent experimental findings of Rixon et al. [J. Mol. Spectrosc. 228, 554 (2004)], and indicate that the (3)Pi-(3)Sigma(+), and (1)Pi-(1)Sigma(+), bands located between 16 000-19 000 cm(-1) are extremely complex due to near degeneracy of several (1)Pi and (3)Pi states. We also have identified a (1)Sigma(+) state in the same region that may interfere with the (1)Pi emission bands. The present results not only shed further light into the spectra of ZrC but also predict yet to be observed systems.     

Theoretical investigation on the electronic and geometric structure of GaN2+ and GaN4+

The electronic and geometric structures of gallium dinitride cation, GaN2+ and gallium tetranitride cation, GaN4+ were systematically studied by employing density functional theory (DFT-B3LYP) and perturbation theory (MP2, MP4) in conjunction with large basis sets, (aug-)cc-pVxZ, x = T, Q. A total of 7 structures for GaN2+ and 24 for GaN4+ were identified, corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces, and bonding mechanisms for some low-lying electronic states. The calculated dissociation energy (De) of the ground state of GaN2+, X1Sigma+, is 5.6 kcal/mol with respect to Ga+(1S) + N2(X1Sigmag+) and that of the excited state, ?3Pi, is 24.8 kcal/mol with respect to Ga+(3P) + N2(X1Sigmag+). The ground state and the first excited minimum of GaN4+ are of 1A1(C2v) and 3B1(C2v) symmetry with corresponding De of 11.0 and 43.7 kcal/mol with respect to Ga+(1S) + 2N2(X1Sigmag+) for X1A1 and Ga+(3P) + 2N2(X1Sigmag+) for 3B1.     

The low-lying electronic excited states of NiCO

Highly correlated coupled cluster methods with single and double excitations (CSSD) and CCSD with perturbative triple excitations were used to predict molecular structures and harmonic vibrational frequencies for the electronic ground state X 1Sigma+, and for the 3Delta, 3Sigma+, 3Phi, 1 3Pi, 2 3Pi, 1Sigma+, 1Delta, and 1Pi excited states of NiCO. The X 1Sigma+ ground state's geometry is for the first time compared with the recently determined experimental structure. The adiabatic excitation energies, vertical excitation energies, and dissociation energies of these excited states are predicted. The importance of pi and sigma bonding for the Ni-C bond is discussed based on the structures of excited states.     

Dissociative photoionization of N(2)O in the region of the N(2)O(+)(B (2)Pi) state studied by ion-electron velocity vector correlation

Dissociative direct photoionization of the N2O(X 1Sigma+) linear molecule via the N2O+(B 2Pi) ionic state induced by linearly polarized synchrotron radiation P in the 18-22 eV photon energy range is investigated using the (VA+,Ve,P) vector correlation method, where VA+ is the nascent velocity vector of the NO+, N2+, or O+ ionic fragment and Ve that of the photoelectron. The DPI processes are identified by the ion-electron kinetic energy correlation, and the IchiA+(thetae,phie) molecular frame photoelectron angular distributions (MFPADs) are reported for the dominant reaction leading to NO+ (X 1Sigma+,v) + N(2D)+ e. The measured MFPADs are found in satisfactory agreement with the reported multichannel Schwinger configuration interaction calculations, when bending of the N2O+(B 2Pi) molecular ion prior to dissociation is taken into account. A significant evolution of the electron scattering anisotropies is observed, in particular in the azimuthal dependence of the MFPADs, characteristic of a photoionization transition between a neutral state of Sigma symmetry and an ionic state of Pi symmetry. This interpretation is supported by a simple model describing the photoionization transition by the coherent superposition of two ssigma and ddelta partial waves and the associated Coulomb phases.     

Ab initio study of the electronic structure and bonding of aluminum nitride

For the diatomic aluminum nitride (AlN), we have constructed potential energy curves for 45 states employing multi-reference variational methods and quantitative basis sets. Thirty-six states are relatively strongly bound, five present local minima, and four are of repulsive nature. Almost all states are of intense multi-reference character rendering their calculation and interpretation quite problematic. Our tentative assignment of the ground state is 3Pi, while a 3Sigma- state is above by less than 1 kcal/mol. Our best estimate for the binding energy of the X3Pi state is D0 = 56.0 +/- 0.5 kcal/mol at re = 1.783 A, in good agreement with the experimental values of D = 66 +/- 9 kcal/mol and re = 1.7864 A. The binding energy of the A3Sigma- state is very similar to the X state because they both correlate to the ground-state atoms, but the bond distance of the former is 0.13 A longer. The first seven states can be tagged as follows: X3Pi, A3Sigma-, a1Sigma+, b1Pi, c1Delta, B3Sigma+, and d1Sigma+, a rather definitive order with the exception of X and A states.     

Electronic and geometric structure of the 3d-transition metal monocarbonyls MCO, M=Sc, Ti, V, and Cr

The electronic and geometric structure of the 3d-transition metal monocarbonyls MCO, M=Sc, Ti, V, and Cr was investigated through coupled cluster (CC) and multireference variational methods (MRCI) combined with large basis sets. For the ground and a few low-lying excited states complete potential energy profiles were constructed at the CC-level of theory. The M-CO dissociation energies of the ground states X 4Sigma-,X 5Delta,X 6Sigma+, and X 7A' are calculated to be 36, 27, 18, and 2 kcal/mol for ScCO, TiCO, VCO, and CrCO, with respect to Sc(4F),Ti(5F),V(6D),Cr(7S)+CO(X 1Sigma+). The bonding is rather complicated and could be attributed mainly to pi-conjugation effects between the M and CO pi-electrons, along with weak sigma-charge transfer from CO to M atoms. Almost in all cases the metal atoms appear to be slightly positively charged, at least according to the direction of the dipole moment vectors and the MRCI population densities.     

Electronic states and spectroscopic properties of SiTe and SiTe+

Ab initio based configuration interaction calculations have been carried out to study the low-lying electronic states and spectroscopic properties of the heaviest nonradioactive silicon chalcogenide molecule and its monopositive ion. Spectroscopic constants and potential energy curves of states of both SiTe and SiTe+ within 5 eV are reported. The calculated dissociation energies of SiTe and SiTe+ are 4.41 and 3.52 eV, respectively. Effects of the spin-orbit coupling on the electronic spectrum of both the species are studied in detail. The spin-orbit splitting between the two components of the ground state of SiTe+ is estimated to be 1880 cm(-1). Transitions such as 0+ (II)-X1Sigma(+)0+, 0+ (III)-X1Sigma(+)0+, E1Sigma(+)0+ -X1Sigma(+)0+, and A1Pi1-X1Sigma(+)0+ are predicted to be strong in SiTe. The radiative lifetime of the A1Pi state is less than a microsecond. The X(2)2Pi(1/2)-X(1)2Pi(3/2) transition in SiTe+ is allowed due to spin-orbit mixing. However, it is weak in intensity with a partial lifetime for the X2 state of about 108 ms. The electric dipole moments of both SiTe and SiTe+ in their low-lying states are calculated. The vertical ionization energies for the ionization of the ground-state SiTe to different ionic states are also reported.     

Autoionization and neutral dissociation of superexcited HI studied by two-dimensional photoelectron spectroscopy

Two-dimensional photoelectron spectroscopy of hydrogen iodide (HI) has been performed in the photon energy region of 11.10-14.85 eV, in order to investigate dynamical properties on autoionization and neutral dissociation of Rydberg states HI*(RA) converging to HI+(A 2Sigma1/2(+)). A two-dimensional photoelectron spectrum exhibits strong vibrational excitation of HI+(X 2Pi) over a photon energy region from approximately 12 to 13.7 eV, which is attributable to the autoionizing feature of the 5 dpi HI*(RA) state. A noticeable set of stripes in the photon energy region of 13.5-14.5 eV is assigned as resulting from autoionization of the atomic Rydberg states of I* converging to I+ (3P0 or 3P1). The formation of I* is understood in terms of predissociation of multiple HI*(RA) states by way of the repulsive Rydberg potential curves converging to HI+(4Pi1/2).     

Wavelength Dependent Photodissociation of OCS via F 31Π Rydberg State:CO(X1Σ+)+S(1D2)Product Channel

The vacuum ultraviolet photodissociation of OCS via the $F$ $3^1\Pi$ Rydberg states was investigated in the range of 134$-$140 nm by means of the time-sliced velocity map ion imaging technique. The images of S($^1$D$_2$) products from the CO($X^1\Sigma^+$)+S($^1$D$_2$) dissociation channel were acquired at five photolysis wavelengths, corresponding to a series of symmetric stretching vibrational excitations in OCS($F$ $3^1\Pi$, $v_1$=0$-$4). The total translational energy distributions, vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) coproducts were derived. The analysis of experimental results suggests that the excited OCS molecules dissociate to CO($X^1\Sigma^+$) and S($^1$D$_2$) products via non-adiabatic couplings between the upper $F$ $3^1\Pi$ states and the lower-lying states both in the C$_{\infty \textrm{v}}$ and C$_{\rm{s}}$ symmetry. Furthermore, strong wavelength dependent behavior has been observed: the greatly distinct vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) products from the lower ($v_1$=0$-$2) and higher ($v_1$=3, 4) vibrational states of the excited OCS($F$ $3^1\Pi$, $v_1$) demonstrate that very different mechanisms are involved in the dissociation processes. This study provides evidence for the possible contribution of vibronic coupling and the crucial role of vibronic coupling on the vacuum ultraviolet photodissociation dynamics.     

Insight into the Rydberg states of CH

Ab initio electronic structure calculations of a relatively large number of Rydberg states of the CH radical were carried out employing the multireference single and double excitation configuration interaction (MRD-CI) method. A Gaussian basis set of cc-pV5Z quality augmented with 12 diffuse functions was used together with an extensive treatment of electron correlation. The main focus of this contribution is to investigate the 3d Rydberg complex assigned by Watson [Astrophys. J. 555, 472 (2001)] to three unidentified interstellar bands. The authors' calculations reproduce quite well the absolute excitation energies of the three components of the 3d complex, i.e., 2Sigma+(3dsigma), 2Pi(3dpi), and 2Delta(3ddelta), but not the energy ordering inferred from a rotational assignment of the 3d<--X 2Pi laboratory spectrum. The computation of the 4d complex is reported for the first time along with a number of other higher lying Rydberg species with an X 1Sigma+ core. The lowest Rydberg states belonging to series converging to the a 3Pi and A 1Pi excited states of CH+ are also calculated.     

Laser induced fluorescence spectroscopy of NC3O

Laser induced fluorescence spectra of the NC(3)O radical in a supersonic jet have been observed. The radical was produced in a pulsed electric discharge of HC(3)N and O(2) diluted to 0.3% with Ar. A total of 17 vibronic bands with a radiative lifetime of approximately 30 ns have been observed in a region from 27 000 to 27 500 cm(-1). The observed vibronic bands are classified as (2)Pi(12)-(2)Pi(12), (2)Pi(32)-(2)Pi(32), and (2)Sigma-(2)Sigma types. The upper states of the (2)Sigma-(2)Sigma bands have large spin-rotation constants, which should be denoted as Sigma((+)) and Sigma((-)). From high-level ab initio calculations and rotational analyses, the observed transition was assigned to the B (2)Pi-X (2)A(") transition. Dispersed fluorescence spectra from the upper (2)Sigma and (2)Pi vibronic levels have also been observed, yielding fundamental vibrational frequencies for the nu(1), nu(2), nu(3), and nu(7) modes of the ground state.     

A study of the reaction of N+ with O2: experimental quantification of NO+(a 3Sigma+) production (298-500 K) and computational study of the overall reaction pathways

The product branching ratios for NO+(X 1Sigma+) and NO+(a 3Sigma+) produced from the reaction of N+ with O2 have been measured at 298 and 500 K in a selected ion flow tube. Approximately 0.5% of the total products are in NO+(a) at both temperatures, despite the fact that the reaction to form NO+(a) is 0.3 eV exothermic. High-level ab initio calculations of the potential energy surfaces for the N+ + O2 reaction show that the reaction from N+(3P) + O2(3Sigma(g)) reactants starts with an efficient early stage charge transfer to the N(2D) + O2+(X 2Pi) channel, which gives rise to the O2+(X 2Pi) product and, at the same time, serves as the starting point for all of the reaction channels leading to NO+ and O+ products. Pathways to produce NO+(a 3Sigma+) are found to be less favorable than pathways leading to the major product NO+(X 1Sigma+). Production of N(2D) has implications for the concentration of NO in the mesosphere.