Study on the photodissociation spectra of CS2+ via B2Sigma u+ and C2Sigma g+ electronic states

The photodissociation spectra of CS(2)(+) ions via B(2)Sigma(u)(+) and C(2)Sigma(g)(+) electronic states have been studied by using two-photon excitation, where the parent CS(2)(+) ions were prepared by [3 + 1] REMPI (resonance-enhanced multiphoton ionization) at 483.2 nm from the jet-cooled CS(2) molecules. The [1 + 1] photodissociation spectrum of CS(2)(+) via the B(2)Sigma(u)(+)(upsilon(1)upsilon(2)0) <-- X(2)Pi(g,3/2)(000) transition was obtained by scanning the dissociation laser in the wavelength range of 270-285 nm and detecting the signal of both S(+) and CS(+). The [1 + 1'] photodissociation spectra of CS(2)(+) were obtained by fixing the first dissociation laser at 281.94 or 277.15 nm to excite the B(2)Sigma(u)(+) (000 or 100) <-- X(2)Pi(g,3/2)(000) transitions and scanning the second dissociation laser in the range of 606-763 nm to excite C(2)Sigma(g)(+)(upsilon(1)upsilon(2)0) <-- B(2)Sigma(u)(+)(000,100) transitions. New spectroscopic constants of nu(1) = 666.2 +/- 2.5 cm(-1), nu(2) = 363.2 +/- 1.9 cm(-1), chi(11) = -5.5 +/- 0.1 cm(-1), chi(22) = 1.6 +/- 0.1 cm(-1), chi(12) = -8.6 +/- 0.2 cm(-1), and k(122) = 44.9 +/- 2.5 cm(-1) (Fermi resonance constant) for the C(2)Sigma(g)(+) state are deduced from the [1 + 1'] photodissociation spectra. On the basis of the [1 + 1] and [1 + 1'] photodissociation spectra, the wavelength and level dependence of the product branching ratios CS(+)/S(+) has been found and the dissociation dynamics of CS(2)(+) ions via B(2)Sigma(u)(+) and C(2)Sigma(g)(+) electronic states are discussed.     

Observation and analysis of the 2g(1D) ion-pair state of I2: the g/u mixing between the 1u(1D) and 2g(1D) states

We report the analysis of the 2g(1D) ion-pair state of I2 by perturbation-facilitated optical-optical double resonance. The present study began with the observation of the 2g(1D)-A' 3Pi(2u) emission at around 230 nm during the analysis of the ultraviolet emissions originating form the 1u(1D) ion-pair state. The identification of this new transition helped us to specify the wavelengths for detecting the 2g(1D) state by emission, and also to estimate its absolute position. The intermediate states used to observe the 2g(1D) state were the B 3Pi(0u(+))-b' 2u mixed states by the hyperfine interaction, which allowed us to combine the X 1Sigmag(+) ground state with the 2g(1D) state in the (1+1) photon excitation following the optical selection rules for one-photon transitions: 2g(1D)<--b' 2u-B 3Pi(0u(+))<--X 1Sigmag(+). Our analysis covered the 2g(1D) state in the 0< or =v< or =12 and 9< or =J< or =40 ranges. The molecular constants and Rydberg-Klein-Rees (RKR) potential of the 2g(1D) state were reported. We discussed the occurrence of the 2g(1D)-A' 3Pi(2u) emission, when exciting to the 1u(1D) v=0 state, and attributed it to the g/u mixing between the 2g(1D) and 1u(1D) states by the hyperfine interaction. The effect of the perturbation on measured line intensities and lifetimes was evident.     

Electric-field-induced g/u mixing of the E0g+(3P2) and D0u+(3P2) ion-pair states of jet-cooled I2 observed using optical triple resonance

Electric-field-induced electronic state g/u mixing of nearly isoenergetic rovibrational levels of the E0g+(3P2) and D0u+(3P2) ion-pair states of I2 has been observed using optical triple resonance combined with resonance ionization. Detectable mixing with applied fields of 1 kV/cm occurs over a range of energy level separations of < or = 0.3 cm(-1).     

Electric properties of the water molecule in 1A1, 1B1, and 3B1 electronic states: Refined CASSCF and CASPT2 calculations

Summary The dipole moments and dipole polarizabilities of the ¹A₁, ¹B₁, and ³B₁ electronic states of the water molecule have been calculated by using the CASSCF approach followed by the evaluation of the dynamic electron correlation contribution by the second-order perturbation scheme CASPT2. All calculations have been carried out in a specifically extended ANO basis set which accounts for the Rydberg character of the two excited states. In order to estimate the correctness and accuracy of the present data a scan over a variety of different active spaces for the CASSCF wave function has been made. The present results are superior to earlier CASSCF calculations, although their qualitative features remain essentially the same. The dipole moments in ¹B₁ and ³B₁ states are predicted to be about 0.49 a.u. and 0.33 a.u., respectively, and have the opposite orientation with respect to the ground state dipole moment. The dipole polarizability tensors of the excited states are characterized by high anisotropy and are dominated by the in-plane component perpendicular to the symmetry axis. All their components are found to be about an order of magnitude larger than those of the ground state polarizability tensor. The excitation energy dependence on the choice of the active orbital space in the CASSCF reference function is also considered and the analysis of the present data concludes in the concept of what is called the mutually compatible active spaces for the two states involved in excitation. All CASPT2 results are in good agreement with the results of recent calculations carried out in the framework of the open-shell coupled cluster formalism. This agreement confirms the high efficiency of the CASSCF/CASPT2 approach to the treatment of the electron correlation effects.     

Relative stability of the2 A 1g and2 E g states of the C2H 6 + ion

Anab initio study of the relative stability for the states² A _1g and² E _g of C₂H ₆ ⁺ has been carried out. The results of the Open Shell Restricted Hartree-Fock calculations lead to assign the² A _{1 g} as the ground state of the molecule in agreement with previous SCF calculations.The correlation energy associated to both states has been calculated within the correlation hole model and the results, contrary to those obtained from Configuration Interaction calculations, do not alter qualitatively the conclusions from SCF.     

A CASPT2 study of the valence and lowest Rydberg electronic states of benzene and phenol

Summary The valence excited states and the 3s, 3p, and 3d (united atom) Rydberg states of benzene and phenol have been obtained by the CASPT2 method, which computes a second-order perturbation correction to complete active space self-consistent field (CASSCF) energies. All non-zero dipole oscillator strengths are also computed, at the CASSCF level. For benzene, 16 singlet and 16 triplet states with excitation energies up to ca. 7.86 eV (63 400 cm^–1) are obtained. Of these, 12 singlet and three triplet energies are experimentally known well enough to allow meaningful comparison. The average error is around 0.1 eV. The highest of these singlet states (2¹ E_2g) is the highest valence * state predicted by elementary -electron theory. Its energy is then considerably lower than has been suggested from laser flash experiments, but in perfect agreement with a reinterpretation of that experiment. For phenol, 27 singlet states are obtained, in the range 4.53–7.84 eV (63 300 cm^–1). Only the lowest has a well-known experimental energy, which agrees with the computed result within 0.03 eV. The ionization energy is in error by 0.05 eV.     

The X˜1A1, a˜3B1, a˜1B˜1, and B˜1A1 electronic states of SiH2

Four electronically low-lying states of silylene (SiH₂) have been studied systematically using high level ab initio electronic structure theory. Self-consistent field (SCF), two-configuration (TC) SCF, complete active space (CAS) SCF, configuration interaction with single and double excitations (CISD), and CASSCF second-order (SO) CI levels of theory were employed with eight distinct basis sets. The zeroth-order wave functions of the ground (X˜ ¹A₁ or 1 ¹A₁) and B˜ ¹A₁ (or 2 ¹A₁) excited states are appropriately described by the first and second eigenvectors of the TCSCF secular equations. The TCSCF-CISD, CASSCF, and CASSCF-SOCI wave functions for the B˜ ¹A₁ (or 2 ¹A₁) state were obtained by following the second root of the CISD, CASSCF, and SOCI Hamiltonian matrices. At the highest level of theory, the CASSCF-SOCI method with the triple zeta plus triple polarization augmented with two sets of higher angular momentum functions and two sets of diffuse functions basis set [TZ3P(2f,2d)+2diff], the energy separation (T₀) between the ground (X˜ ¹A₁) and first excited (a˜ ³B₁) states is determined to be 20.5 kcal/mol (0.890eV,7180cm⁻¹), which is in excellent agreement with the experimental T₀ value of 21.0 kcal/mol (0.910eV,7340cm⁻¹). With the same method the T₀ value for the a˜ ¹B₁−X˜ ¹A₁ separation is predicted to be 45.1 kcal/mol (1.957 eV,15780 cm⁻¹), which is also in fine agreement with the experimental value of 44.4 kcal/mol (1.925 eV,15530 cm⁻¹). The T₀ value for the B˜ ¹A₁−X˜ ¹A₁ separation is determined to be 79.6 kcal/mol (3.452 eV,27 840 cm⁻¹). After comparison of theoretical and experimental T₀ values for the a˜ ³B₁ and a˜ ¹B₁ states and previous studies, error bars for the B˜ ¹A₁ state are estimated to be ±1.5 kcal/mol (±525 cm⁻¹). The predicted geometry of the B˜ ¹A₁ state is r_e(SiH)=1.458 and θ_e=162.3^∘. The physical properties including harmonic vibrational frequencies of the B˜ ¹A₁ state are newly determined. Received: 10 March 1997 / Accepted: 2 April 1997     

Observation and calculation of the Cs2 2 3Delta(1g) and b 3Pi(0u) states

The Cs(2) 2 (3)Delta(1g) and b (3)Pi(0u) states have been observed by infrared-infrared double resonance spectroscopy for the first time. 221 2 (3)Delta(1g)<--A (1)Sigma(u) (+)<--X (1)Sigma(g) (+) double resonance lines have been assigned to transitions into the 2 (3)Delta(1g) v=6-13 vibrational levels. Resolved fluorescence into the b (3)Pi(0u) v(')=0-48 levels has been recorded. Molecular constants and potential energy curves are determined by the global fit of the entire set of the experimental data. Theoretical potential energy curves of the 2 (3)Delta(g) and b (3)Pi(u) states have been determined in the framework of the pseudopotential method and are compared with the experimental results.     

A laser spectroscopic study of the X2pi(g), A2pi(u), and B2sigma+ states of BS2: Renner-Teller, spin-orbit, and K-resonance effects

The lowest-lying vibronic levels of the X, A, and B states of BS2 have been investigated at high resolution using a combination of room-temperature absorption and supersonic jet data. In both cases, the BS2 radical was prepared in an electric discharge using a precursor gas mixture of BCl3,CS2, and either helium or argon. Extensive absorption spectra were obtained for the 0(0)0 and 2(1)1 bands of the A2pi(u)-X2pi(g) electronic transition in the visible. The A-X 2(1)1 and B2sigma(u)(+)-X2pi(g) 2(1) bands of jet-cooled BS2 were also studied with laser-induced fluorescence techniques. By fitting the 0(0) bands of both electronic transitions simultaneously, we were able to precisely determine the spin-orbit splittings in both the A and X states. Similarly, the 21 bands were fitted in a merged analysis in order to determine the relative separations of the vibronic components of the ground and first excited state bending levels as accurately as possible. Due to a large spin-orbit splitting and small Renner-Teller interaction, the A state bending level shows small but definite K-resonance effects, which were fitted using a full matrix for the four components of upsilon2' = 1. The resulting parameters were used along with previously published data to refine the Renner-Teller analyses in both the A2pi(u), and X2pi(g) electronic states. Where possible, the fitted constants and observed boron isotope splittings have been shown to be in accord with theoretical estimates of their sign and magnitude.     

Spectroscopic constants and molecular properties of A3Σu+, B3Πg,W3Δu,and B′3Σu− electronic states of the N2 molecule

The potential energy curves (PECs) of A³Σ, B³Π_g, W³Δ_u, and B^′3Σ electronic states of the N₂ molecule have been studied for internuclear separations from 0.05 to 2.0 nm using the full valence complete active space self‐consistent‐field method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in conjunction with the correlation‐consistent basis sets. Effects on the PECs by the core–valence correlation and relativistic corrections are taken into account. The way to consider the relativistic correction is to use the second‐order Douglas‐Kroll Hamiltonian approximation. The core–valence correlation correction is made with the cc‐pCV5Z basis set. And the relativistic correction is performed at the level of cc‐pV5Z basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size‐extensivity errors by the Davidson modification (MRCI+Q). These PECs are extrapolated to the complete basis set limit. The spectroscopic parameters of ¹⁴N₂, ¹⁴N¹⁵N, and ¹⁵N₂ isotopologs have been evaluated and compared with those reported in the literature. Excellent agreement has been found between the present results and the Rydberg‐Klein‐Rees (RKR) data. With the PECs obtained by the MRCI+Q/CV+DK+56 calculations, the first 30 vibrational states for three species are computed for each electronic state. And for each electronic state of each species, the vibrational level G(ν), inertial rotation constant B_ν, and centrifugal distortion constant D_ν have been determined, which agree well with the RKR data. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Simple perturbation and perturbation-variation treatments of the 1sσg and 2pσu states of H

First-order wave functions and binding energies of the 1sσ_g and 2pσ_u states of H are calculated by simple methods for internuclear separations between 0.2 and 10_a0. An exact perturbation treatment of the lsσ_g state with a zeroth-order function of the form N exp (-sRλ/2) exp (sR μ2) yields only fair results. An alternative method starts with zeroth-order functions of the form N exp (-sRλ/2)[exp (-sRμ/2) ± exp (sRμ/2)] for the 1 and 2 states, respectively. An approximate first-order trial function is set up and the energies are determined variationally. For both states the results are comparable to those obtained by exact perturbation treatments of the same order.     

Electron scattering in Pt(PF3)4: elastic scattering, vibrational, and electronic excitation

Experimental absolute differential cross sections for elastic scattering, and for vibrational and electronic excitation of Pt(PF(3))(4) by low-energy electrons are presented. The elastic cross sections have a deep angle-dependent Ramsauer-Townsend minimum (E(min) = 0.26 eV at θ = 135°). The angular distributions of the elastic cross section at and above 6.5 eV show an unusually narrow peak at an angle which decreases with increasing energy (it is at 40° at 20 eV). Wavy structure is observed at higher angles at 15 and 20 eV. Vibrational excitation cross sections reveal five shape resonances, at 0.84, 1.75, 3.3, 6.6, and 8.5 eV. The angular distributions of the vibrational cross sections have a strong forward peak and are nearly isotropic above about 60°. Electronically excited states are characterized by electron energy-loss spectra. They show a number of unstructured bands, the lowest at 5.8 eV. They are assigned to Rydberg states converging to the 1st and 2nd ionization energies. The cross sections for electronic excitation have very high forward peaks, reaching the value of 50 A?(2) at 50 eV and 0° scattering angle. Purity of the sample was monitored by the very low frequency (26 meV) Pt-P stretch vibration in the energy-loss spectra.     

A correlated ab initio study of the X2A1 and A2E states of MgCH3

The X2A1 and A2E states of the MgCH3 radical have been studied with correlation consistent basis sets and the coupled cluster method RCCSD(T) in order to compare with two recent experimental efforts [M. A. Anderson and L. M. Ziurys, Astrophys. J. 452, L157 (1995); R. Rubino, J. M. Williamson, and T. A. Miller, J. Chem. Phys. 103, 5964 (1995)]. The best computed values [RCCSD(T)/cc-pCVTZ] for the X2A1 state are (experimental results in parentheses): Ae = 160.433 GHz, Be = 10.948 GHz (B0 = 11.008 GHz), and Mue = 1.011 D. The Mg-CH3 bond is weak, 26.3 kcal/mol. Values for the A2E state are Ae = 154.648 GHz (A0 = 149.666 GHz), Be = 10.87 GHz (B0 = 10.932 GHz), and Mue = 1.022 D. The excitation energy (Te) for the A2E <-- X2A1 transition is 19 999 cm-1 (T00 = 20 030 cm-1). A brief discussion of bonding trends in Mg-containing radials is included.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

Evidence of a weak dipole transition moment between the X(1)0g+ and (3)1u(5(3)P1) electronic energy states in Cd2

Theoretical and experimental evidence of a weak M(z)(R) dipole transition moment between the X(1)0g+ ground and (3)1u(5(3)P1) excited states in Cd2 is presented. Two independent attempts at recording an excitation spectrum of the (3)1u <-- X(1)0g+ transition using a laser beam crossed with a supersonic free-jet expansion beam are reported. The measurements were performed in a spectral range predicted as a result of both ab initio calculations of the electronic energy-state potentials involved in the transition and a simulation of the excitation spectrum. Unfortunately it was impossible to provide unambiguous experimental support for the calculated (3)1u-state potential, due to the very poor signal to noise ratio. However, the experimental results corroborate the very small values of the <(3)1u|M(z)|X(1)0g+> elements obtained in the calculations. This work provides as a reliable starting point for further analysis of the (3)1u-state characteristics.