Renner Teller Splitting Research Articles

Threshold photoelectron spectroscopy of iminoborane, HBNH.

We report the mass-selected threshold photoelectron spectrum (ms-TPES) of iminoborane (HBNH), generated by pyrolysis of borazine. The adiabatic ionization energy (IE) of the X+ 2Π ← X 1Σ+ transition was determined to be 11.31 ± 0.02 eV and the wavenumber of the B-N stretching vibration in the cation was measured to be 1550 cm-1. The Renner-Teller splitting in the X+ 2Π state gives rise to two sets of vibrational progressions, separated by 70 meV.

Theoretical rovibronic spectroscopy of the calcium monohydroxide radical (CaOH).

The rovibronic (rotation-vibration-electronic) spectrum of the calcium monohydroxide radical (CaOH) is of interest to studies of exoplanet atmospheres and ultracold molecules. Here, we theoretically investigate the Ã2Π-X̃2Σ+ band system of CaOH using high-level ab initio theory and variational nuclear motion calculations. New potential energy surfaces (PESs) are constructed for the X̃2Σ+ and Ã2Π electronic states along with Ã-X̃ transition dipole moment surfaces (DMSs). For the ground X̃2Σ+ state, a published high-level ab initio PES is empirically refined to all available experimental rovibrational energy levels up to J = 15.5, reproducing the observed term values with a root-mean-square error of 0.06 cm-1. Large-scale multireference configuration interaction calculations using quintuple-zeta quality basis sets are employed to generate the Ã2Π state PESs and Ã-X̃ DMSs. Variational calculations consider both Renner-Teller and spin-orbit coupling effects, which are essential for a correct description of the spectrum of CaOH. Computed rovibronic energy levels of the Ã2Π state, line list calculations up to J = 125.5, and an analysis of Renner-Teller splittings in the ν2 bending mode of CaOH are discussed.

Diborene: Generation and Photoelectron Spectroscopy of an Inorganic Biradical.

Diborenes, R-BB-R', are of current interest in inorganic chemistry because they offer the opportunity to tune the properties of a biradical by modifying the substituents of the diborene parent, HBBH. Here we synthesize the elusive diborene by H atom abstraction from diborane, B2H6, using fluorine atoms and report a vibrationally resolved photoelectron spectrum of the HBBH biradical. The spectrum is interpreted by comparison with high-level ab initio computations, taking into account the Renner-Teller splitting in the X+2Π ionic ground state, which show an excellent agreement with the experimental spectrum. An adiabatic ionization energy of 9.080 ± 0.015 eV was determined, and a vibrational progression in the boron-boron stretching vibration of 0.14 eV is visible. This is due to the reduction of bond order upon ionization, accompanied by an increase of the computed boron-boron bond length, RBB, from 1.514 to 1.606 Å.

On the Electronic and Geometric Structures of FeO2(-/0) and the Assignment of the Anion Photoelectron Spectrum.

The photoelectron spectrum of FeO2(-) has been assigned by performing geometry optimizations at the CASPT2 and RCCSD(T) levels of computation. All relevant states are found to possess floppy C2v geometrical structures as the Renner-Teller splittings of the linear states are extremely small and the corresponding energy barriers for the OFeO bond angle inversions are calculated in the range of a few hundred wavenumbers. In this sense, the description of the electronic structure in terms of the D∞h point group is acceptable, and the experimentally proposed linear structure for FeO2(-) is theoretically confirmed. High accuracy single-point multireference RASPT2 and single-reference RCCSD(T) calculations support a (2)Δg as the ground state of the anion, even though the energy differences between the (4)Πg and (6)Σg(+) states are smaller than 0.2 eV. After this identification of the doublet ground state, the photoelectron spectra of FeO2(-) could be assigned in all aspects. The (2)Δg→(3)Δg ionization appears to be at the origin of the X band at 2.36 eV, while the A band at 3.31 eV should be ascribed to the (2)Δg→(3)Σg(+) ionization. This assignment is substantiated by Franck-Condon factors for which BP86 optimized geometries and harmonic vibrational frequencies were employed. Indeed, no pronounced vibrational progression should be observed since both bands involve electron detachments out of nonbonding mainly 3d iron molecular orbitals.

ChemInform Abstract: Two-Color Double Resonance Spectroscopy via A 1Au State of Acetylene: 3p Rydberg State and Its Renner-Teller Effect.

The two‐color double resonance multiphoton ionization spectra due to the transitions from various rovibrational levels of the A 1Au state of acetylene to the 3p Rydberg state have been measured. The observed spectra showed large dependence on the quantum number K. It was concluded from the spectral analysis that the geometry of the 3p Rydberg state is linear. The spectra also showed a long progression of the trans‐bending vibration, supporting the linear structure. The trans‐bending progression belonging to the 1Δg electronic state showed the Renner–Teller splittings. The observed splittings were reproduced by the calculation using the formulas for a linear triatomic molecule in a Δ electronic state.

Giant Renner–Teller vibronic coupling in the BF2 radical: An ab initio study of the X̃ A21 and à Π2 electronic states

The potential energy surfaces (PESs) of the ground X (2)A(1) and the first excited A (2)Pi (1 (2)B(1), 2 (2)A(1)) electronic states of the BF(2) radical have been studied ab initio, using a large basis set and CCSD(T) and EOM-CCSD techniques. The calculated PESs were used to variationally calculate the energy levels up to approximately 36,000 cm(-1) above the ground state. The Renner-Teller splitting parameter (epsilon=0.928) found for the A (2)Pi state of this radical is very large which results in an unusual excited state energy level structure.

Selective vibrational excitation in the resonant Auger decay following core-to- [formula omitted] transitions in [formula omitted

In N 2 O a detailed study of the vibrational distribution of the X ˜ state reached after decay of core-to- π ∗ excitation of N terminal, N central and O 1 s core levels is reported. We observe a change in the relative intensity of bending versus stretching modes while scanning the photon energy across all three resonances. While this effect is known to be due to the Renner–Teller splitting in the core-excited states, we could derive that the antisymmetric stretching is excited mainly in the decay of the N terminal 1 s-to- π ∗ excitation. An explanation for such selectivity is provided in terms of interplay of vibrational structure on potential energy surfaces of different electronic states involved in the process.

Theoretical Study on the Predissociation Mechanism of CO2+ (C 2Σg+)

The main purpose of the present theoretical work was to study predissociation mechanism of the C (2)Sigma(g)(+) state of the CO(2)(+) ion using the CAS methods. Since the X (2)Pi(g), A (2)Pi(u), B (2)Sigma(u)(+), 1 (4)Sigma(g)(-), and 1 (4)Pi(u) states are involved in the predissociation, we also studied these five states. The CASPT2 calculations indicate that Renner-Teller splitting in 1 (4)Pi(u) leads to two C(2v) states, 1 (4)A(1) and 1 (4)B(1). For the X (2)Pi(g), A (2)Pi(u), B (2)Sigma(u)(+), and C (2)Sigma(g)(+) states, the CASPT2 T(0) values and geometries are in good agreement with experiment. The CASPT2 calculations for the O-loss dissociation potential energy curves indicate that the 1 (4)Sigma(g)(-), X (2)Pi(g), 1 (4)Pi(u), A (2)Pi(u), B (2)Sigma(u)(+), and C (2)Sigma(g)(+) states correlate with the first, second, second, third, third, and fourth dissociation limits, respectively. The CASSCF minimum energy crossing point (MECP) calculations (in the C(infinityv), C(s), and C(2v) symmetries) were performed for selected state/state pairs, and the spin-orbit couplings were calculated at the MECPs. All the MECPs (including the C (2)Sigma(g)(+)/1 (4)Pi(u) (1 (4)B(1)) MECP), involved in the proposed predissociation mechanism of Praet et al. ( J. Chem. Phys. 1982 , 77 , 4611 - 4618 ), were found and the calculated spin-orbit couplings at these MECPs are not small. Our calculations support the mechanism of Praet et al. and indicate that an energy value of 8.9 eV from CO(2)(+) (X (2)Pi(g)) is needed. The C (2)Sigma(g)(+) state in the previous [1 + 1] photodissociation experiments ( J. Chem. Phys. 2008 , 128 , 164308. ) could predissociate through the mechanism of Praet et al. since the two-photon energy was around 8.9 eV, while the C (2)Sigma(g)(+) state in the previous VUV-PFI-PE experiments ( J. Chem. Phys. 2003 , 118 , 149 - 163 ) would predissociate through another mechanism via A (2)Pi(u).

Ab Initio Treatment of the Chemical Reaction Precursor Complex Br(2P)−HCN. 2. Bound-State Calculations and Infrared Spectra

Rovibronic energy levels and properties of the Br(2P)-HCN complex were obtained from three-dimensional calculations, with HCN kept linear and the CN bond frozen. All diabatic states that correlate to the 2P3/2 and 2P1/2 states of the Br atom were included and spin-orbit coupling was taken into account. The 3 x 3 matrix of diabatic potential surfaces was taken from the preceding paper (paper 1). In agreement with experiment, we found two linear isomers, Br-NCH and Br-HCN. The calculated binding energies are very similar: D0 = 352.4 cm(-1) and D0 = 349.1 cm(-1), respectively. We established, also in agreement with experiment, that the ground electronic state of Br-NCH has |Omega| = (1/2) and that Br-HCN has a ground state with |Omega| = (3/2), where the quantum number, Omega, is the projection of the total angular momentum, J, of the complex on the intermolecular axis R. This picture can be understood as being caused by the electrostatic interaction between the quadrupole of the Br(2P) atom and the dipole of HCN, combined with the very strong spin-orbit coupling in Br. We predicted the frequencies of the van der Waals modes of both isomers and found a direct Renner-Teller splitting of the bend mode in Br-HCN and a smaller, indirect, splitting in Br-NCH. The red shift of the CH stretch frequency in the complex, relative to free HCN, was calculated to be 1.98 cm(-1) for Br-NCH and 23.11 cm(-1) for Br-HCN, in good agreement with the values measured in helium nanodroplets. Finally, with the use of the same potential surfaces, we modeled the Cl(2P)-HCN complex and found that the experimentally observed linear Cl-NCH isomer is considerably more stable than the (not observed) Cl-HCN isomer. This was explained mainly as an effect of the substantially smaller spin-orbit coupling in Cl, relative to Br.

Experimental and ab initio study of a new D̃Δg1 state of the C3 radical

We report here the first observation of the D (1)Delta(g) state of the C(3) radical, which provides the first comprehensively analyzed example of the dynamic Renner-Teller splitting in Delta symmetry. Two color double resonance spectroscopy via the A (1)Pi(u) state was employed to experimentally probe an extensive range of vibronic levels in this D (1)Delta(g) state, covering all three modes of vibration of C(3). The analysis was supported by ab initio potential energy surface calculations on the C(3) radical to outline the lowest eight singlet electronic states. Two methods were used to analyze the Renner-Teller effect. The first method is an empirical Hamiltonian based on normal modes, using harmonic oscillator functions as a basis, with Renner-Teller and other terms added as required, which allows conventional vibrational parameters to be determined. The second is a much larger program that uses the exact kinetic energy operator for a triatomic molecule to calculate vibronic energy levels directly from the Renner-Teller pair of potential energy surfaces. Both methods give a good fit to the experimental results, with only a small adjustment to the ab initio surfaces required for the latter. One of the overall conclusions is that the Renner-Teller effect is rather smaller in the D (1)Delta(g) state than in the A (1)Pi(u) state.

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: Dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.

CHARACTERIZATION OF THE $\tilde{X}\,^2 \Pi$ AND Ã2Σ+ELECTRONIC STATES OF THE PHOSPHAETHYNE CATION (HCP+)

The electronic ground state [Formula: see text] and first excited state (Ã2Σ+) of phosphaethyne cation (HCP+) have been systematically investigated using ab initio electronic structure theory. The total energies, geometries, rotational constants, dipole moments, harmonic vibrational frequencies, and parameters for Renner–Teller splittings were determined using self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster (CC) with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], CC with single, double, and iterative partial triple excitations (CCSDT-3), and CC with single, double, and full triple excitations (CCSDT) methods and eight different basis sets. Some of the largest full triples coupled cluster computations to date are reported. Degenerate bending frequencies for the Ã2Σ+state were determined using the equation-of-motion (EOM)-CCSD technique. The two states have been confirmed to have linear equilibrium structures. At the full CCSDT level of theory with the correlation-consistent polarized valence quadruple zeta (cc-pVQZ) basis set, the classical [Formula: see text] splitting ( Tevalue) is predicted to be 47.7 kcal/mol (2.07 eV, 16,700 cm-1) and the quantum mechanical splitting (T0value) to be 48.1 kcal/mol (2.08 eV, 16,800 cm-1), which are in excellent agreement with the experimental values of Te= 47.77 kcal/mol (2.072 eV , 16,708 cm-1) and T0= 47.94 kcal/mol (2.079 eV, 16,766 cm-1). The excitation energies predicted by the CCSDT-3 and CCSD(T) methods differ from the full triples CCSDT result by 0.38 and 0.45 kcal/mol, respectively. With the aug-cc-pVQZ CCSDT-3 method the Renner parameter and the averaged harmonic bending vibrational frequency are determined to be ∊= -0.0390 and [Formula: see text] for the ground state of HCP+, which are reasonably consistent with the experimental values of ∊=-0.0415 and [Formula: see text]. The predicted dipole moments are 1.30 Debye ([Formula: see text] state, polarity-hydrogen atom positive) and 0.06 Debye (Ã2Σ+state, polarity-phosphorus atom positive).

Laser-assisted (1+1′)-photon ionization-detected absorption spectrum of the 3pπ 2Π state of HCO and DCO

We re-examine the ionization-detected ultraviolet absorption spectrum of the 3pπ 2Π←X 2A′ transition in HCO and DCO using a high-power visible laser to enhance the observation of first-photon resonant features. This technique, which we term here, assisted REMPI, significantly improves the signal-to-noise ratio of the spectrum, making many weak vibronic sub-bands visible for the first time. A comprehensive fit to the structure evident in a progression of bending levels from (000) to (040) refines the assignment of Song and Cool [X. M. Song and T. A. Cool, J. Chem. Phys. 96, 8664 (1992)] to yield a set of rotational constants that vary with K in relation to v2, together with a higher-order contribution to the Renner–Teller splitting in HCO, which is mirrored in DCO for all levels but (040). The (040) band falls at a frequency that is commensurate with that of CD stretch, and Fermi resonance between 3pπ 2Π(1000)Π and the higher-energy (040) K=1(Π) component gives rise to an added splitting that increases the energy of this (040) component and causes an apparent increase in the Renner parameter.

Coupled cluster study of the X̃ 2Π and à 2Σ+ electronic states of the HCGe radical: Renner–Teller splitting and the effects of relativistic corrections

The X̃ 2Π and à 2Σ+ states of the germanium methylidyne radical (HCGe) have been investigated at the SCF, CISD, CCSD, and CCSD(T) levels of theory. The total energies, equilibrium geometries, dipole moments, harmonic vibrational frequencies, infrared intensities, and Renner–Teller splitting are reported. The relativistic one-electron Darwin and mass-velocity terms are calculated using first-order perturbation theory and the effects of these corrections on energetics, harmonic vibrational frequencies, and Renner–Teller splitting are discussed. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state (X̃ 2Π) has a linear geometry with re(CH)=1.079 Å and re(CGe)=1.769 Å, in good agreement with the experimental values of r0(CH)=1.067 Å and r0(CGe)=1.776 Å. In the electronically excited à 2Σ+ state, HCGe is also found to have a linear geometry with re(CH)=1.074 Å and a much shorter re(CGe)=1.669 Å at the [CCSD(T)/cc-pVQZ] level, in agreement with experimental values of r0(CH)=1.059 Å, r0(CGe)=1.674 Å, and the observation of a C–Ge triple bond character. The à 2Σ+ state C–Ge stretching vibrational frequency is determined to be ω3=990.2 cm−1 at the CCSD(T)/cc-pVTZ level with the inclusion of relativistic effects, which is in essentially perfect agreement with the experimental value of ν3=990 cm−1. With the same method, the X̃ 2Π state harmonic vibrational frequencies are predicted to be 846 cm−1 for the C–Ge stretching, 443 and 506 cm−1 for the two nondegenerate bending Renner–Teller components, and 3249 cm−1 for the C–H stretching modes for which experimental values are not available. The quantum mechanical splitting (T0 value) was determined to be 38.0 kcal/mol at the nonrelativistic CCSD(T)/TZ3P(2 f,2d)+2diff level, while it is found to be 39.9 kcal/mol with the inclusion of relativity, in very good agreement with the experimental value of 39.8 kcal/mol. The Renner parameter (ε) is determined to be −0.1386, and the effects of relativity were seen to produce a smaller Renner parameter of −0.1329.

Vibrational Branching Ratios and Asymmetry Parameters in the Photoionization of CO2 in the Region Between 650 Å and 840 Å.

The vibrational branching ratios and asymmetry parameters for CO2 have been determined in the wavelength region of 650 Å to near the ionization onset at about 840 Å. The study was performed using synchrotron radiation from the Daresbury storage ring that was dispersed with a 5 m grating monochomator that afforded resolution of 0.1 Å to 0.2 Å. This resolution allowed the study of the branching ratios and asymmetry parameters with enough detail to see the changes in the parameters within the pronounced autoionization structure in CO2 in this wavelength region. While the electron spectrometer resolution was not sufficient to resolve the spin orbit and Renner-Teller splitting in the photoelectron spectra, we are able to fit the data with a model that identifies the major structure in terms of the symmetric stretch and elements of the asymmetric stretch and bending modes. A calculation of the expected relative vibrational excitations based upon the Franck-Condon principle clearly showed non-Franck-Condon behavior in some of the vibrational-electronic transitions.

The X̃ 2Π and à 2Σ+ electronic states of the HCSi radical: Characterization of the Renner–Teller effect in the ground state

The electronic structures of the ground and lowest lying excited state of the silicon methylidyne radical (HCSi) have been investigated at the self-consistent field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD including a perturbative expansion for connected triples CCSD(T) levels of theory with a wide range of basis sets. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and Renner–Teller splitting are reported. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state (X̃ 2Π) has a linear geometry with re(CH)=1.0781 Å and re(CSi)=1.6956 Å. This is in good agreement with the experimental values of r0(CH)=1.0677 Å and r0(CSi)=1.6925 Å, respectively. In the à 2Σ+ state, HCSi is also found to have a linear geometry with re(CH)=1.0737 Å and re(CSi)=1.6130 Å at the [CCSD(T)/cc-pVQZ] level, confirming experimental values of r0(CH)=1.0625 Å, r0(CSi)=1.6118 Å, and the observation of C–Si triple bond character. With the same method, the X̃ 2Π and à 2Σ+ state C–H stretching vibrational frequencies are predicted to be 3271 and 3319 cm−1, respectively, for which experimental values are not available. The classical X̃–à splitting (Te value) was determined to be 32.6 kcal/mol (1.41 eV, 11 730 cm−1) and quantum mechanical splitting (T0 value) to be 33.5 kcal/mol (1.45 eV, 11726 cm−1), which are in excellent agreement with the experimental T0 value of 11 766.721 cm−1 (33.64 kcal/mol, 1.459 eV). The linear excited à 2Σ+ state of the molecule has a real degenerate bending vibrational frequency, whereas the ground state (X̃ 2Π) is subject to the Renner–Teller effect and presents two distinct real vibrational frequencies. The Renner parameter (ε) and average harmonic bending frequency (ω2) of the X̃ 2Π state are predicted to be ε=−0.114 and ω2=518 cm−1 at the CCSD(T)/cc-pVQZ level of theory. The electronic structure analysis of the ground state showed that the HCSi radical is an A-type Renner–Teller molecule.

Excited electronic states of carbon disulphide

The near-ultraviolet spectrum of CS2 is studied thoroughly using high level CISTDQ and EOM-CCSD theory with DZP and TZ2P(f) basis sets. Potential energy curves of the first seven singly excited states with respect to the SCS bond angle have been predicted. The relative energy ordering of the excited states as established at the equilibrium geometries using EOM-CCSD theory is: X˜1Σ+g, < a3B2 < b˜3A2(R) < A1A2 < c˜3B2 < B˜1B2 (V) < d˜3A2 < C˜1A2, which is at odds with previous theoretical work. The bent B˜1B2 state arises from the Renner—Teller splitting of a linear 1Δu state into 1B2 and 1A2 states. In disagreement with previous experimental predictions that the 1A2 state is the lower lying component of this splitting, it is firmly established from the high level results in this research that the C˜1A2 state is the higher lying component of this Renner—Teller pair. Equilibrium geometries are determined and shown to agree qualitatively with experiment, save for the B˜1B2 state. From an analysis of the complex rotational structure of the V band the experimental geometry of the B˜1B2 state was determined to be slightly bent (re = 1.544 Å, θe = 160°), whereas the methods employed in this research predict a significantly bent geometry (re = 1.632 Å, θe = 132.1° at EOM-CCSD/TZ2P(f)). Relative transition energies, Te, are predicted, and EOM-CCSD calculations are shown to agree well with available experimental data, and show the assignment of the V and R bands to be correct while the T band is correctly assigned to the A1A2 state and not the lower lying Renner—Teller component of the 1Δu state. The high level CI methods, although yielding qualitatively correct potential energy curves, do not provide quantitatively accurate transition energies, and it is clear that even an exhaustive treatment of the eight valence π electrons is not entirely satisfactory, while EOM-CCSD is established as an accurate method for the prediction of excited state potential curves.

Excited electronic states of carbon disulphide

The near-ultraviolet spectrum of CS2 is studied thoroughly using high level CISTDQ and EOM-CCSD theory with DZP and TZ2P(f) basis sets. Potential energy curves of the first seven singly excited states with respect to the SCS bond angle have been predicted. The relative energy ordering of the excited states as established at the equilibrium geometries using EOM-CCSD theory is: X˜1Σ+g, < a3B2 < b˜3A2(R) < A1A2 < c˜3B2 < B˜1B2 (V) < d˜3A2 < C˜1A2, which is at odds with previous theoretical work. The bent B˜1B2 state arises from the Renner—Teller splitting of a linear 1Δu state into 1B2 and 1A2 states. In disagreement with previous experimental predictions that the 1A2 state is the lower lying component of this splitting, it is firmly established from the high level results in this research that the C˜1A2 state is the higher lying component of this Renner—Teller pair. Equilibrium geometries are determined and shown to agree qualitatively with experiment, save for the B˜1B2 state. From an analysis of the complex rotational structure of the V band the experimental geometry of the B˜1B2 state was determined to be slightly bent (re = 1.544 Å, θe = 160°), whereas the methods employed in this research predict a significantly bent geometry (re = 1.632 Å, θe = 132.1° at EOM-CCSD/TZ2P(f)). Relative transition energies, Te, are predicted, and EOM-CCSD calculations are shown to agree well with available experimental data, and show the assignment of the V and R bands to be correct while the T band is correctly assigned to the A1A2 state and not the lower lying Renner—Teller component of the 1Δu state. The high level CI methods, although yielding qualitatively correct potential energy curves, do not provide quantitatively accurate transition energies, and it is clear that even an exhaustive treatment of the eight valence π electrons is not entirely satisfactory, while EOM-CCSD is established as an accurate method for the prediction of excited state potential curves.

A tentative multiphoton ionization of CS2 molecule: Spectroscopy of Ã2πu state of CS 2 + ion

The spectroscopic study of excited molecular cationCS2+ (A2πu ) by a different method is presented. In this technique the decay of excited states is monitored by measuring the photons emitted. The peaks in the photon spectrum would correspond to the energy levels of the ion. The vibrational-vibrational, vibrational-electronic interactions have been observed. Symmetry forbidden excitation of one quantum of bending vibration is observed which gives unperturbed value for the bending vibration. The Renner-Teller splitting for the δg and Σg- components of the bending mode (v2′ = 1) in the upper excited A2πu state has been observed. A tentative vibrational analysis of the A→ X system has been made.

A, C, and D electronic states of the Ar–NO van der Waals molecule revisited: Experiment and theory

The A–X transition of ArNO has been reinvestigated by laser induced fluorescence (LIF) both in the bound–free and bound–bound region. The discrete part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry from the ground state. This very different configuration, both from the ground state and from the C and D states, can only be explained by strong interactions, induced by the perturbing argon atom, between the excited states of the van der Waals complex converging to the 3sσ,A, 3pπ,C, and 3pσ,D Rydberg states of NO. In order to quantitatively understand the observed structure of the A–X, C–X, and D–X excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar A′ and A″ symmetries, including a configuration interaction between the states of same symmetry. Small adjustments of the diabatic energy surfaces lead to a satisfactory agreement between the observed and calculated spectra. In contrast to the ground state, the Renner–Teller splitting of the 3pπ,C state into two A′ and A″ components is very large, of the order of 4000 cm−1. This effect is complicated by further mixing between the states of A′ symmetry induced by the argon atom. The A state is anisotropic and weakly bound with a small potential well at the linear configuration (the argon atom being on the side of the oxygen). The C(A″) and the bound electronic component of the strongly mixed C+D(A′) states exhibit a vibrational structure close to that of the ion and, consequently, present some Rydberg character even if the Coulomb field central symmetry (s-p) is broken by the perturbing argon atom.