Vibrational energies of LiH2(+) and LiD2(+) in the Ã1Σ+ electronic state

In connection with the recent study of the ground electronic state of the LiH2(+) molecular ion (Kraemer, W. P.; Spirko, V. Chem. Phys. 2006, 330, 190), the adiabatic three-dimensional double-minimum potential enery surface of the first excited electronic state was evaluated, including its two lowest atom-diatom dissociation channels as well as the three-atom complete fragmentation asymptote. Applying the Sutcliffe-Tennyson Hamiltonian for triatomic molecules, the levels of all bound vibrational states and the levels of the states localized in the two energy minimum regions were separately determined. The validity of statistical methods such as the density of states approach and the nearest-neighbor level spacing distribution (NNSD) was tested for the light LiH2(+) ion. Special effort was put into investigating possible effects of a tunnelling motion across the proton-transfer barrier on the vibrational level pattern using the NNSD approach.     

B1(1)Π state of KCs: high-resolution spectroscopy and description of low-lying energy levels

The diode laser induced B(1)(1)Π → X(1)Σ(+) fluorescence spectra of the KCs molecule were recorded by Fourier-transform spectrometer with resolution of 0.03 cm(-1). Buffer gas Ar was used to facilitate appearance of rotation relaxation lines in the spectra, thus enlarging the B(1)(1)Π dataset, allowing one to determine the Λ-splitting constants and to reveal numerous local perturbations. A dataset was systematically obtained for B(1)(1)Π vibrational levels ν(') ∈ [0; 5] nonuniformly covering rotational levels with J(') ∈ [7; 233]. For ν(') ∈ [0; 3], the less-perturbed data of f-components were included in the fit. A pointwise potential energy curve (PEC) based on the inverted perturbation approach, as well as the Dunham coefficients, was obtained and compared with ab initio calculations; in particular, the energy of the PEC's minimum T(e) = 14,044.918(6) cm(-1) was determined. Both approaches allowed us to reproduce the vast majority of data used in the fit with accuracy of 0.02 cm(-1). Tentative molecular constants for several vibrational levels of the near lying C(3)(1)Σ(+) state were estimated.     

Ab initio MRD CI calculation of the zero-field splitting of the 2Π ground state of the CBr molecule

The zero-field spin-orbit splitting of the ²Π ground state of CBr is computed by means of an ab initio MRD CI treatment employing the Breit-Pauli formalism. The choice of the one-electron basis is found to be important, but the results are seen to be relatively insensitive to the number of core electrons employed in the CI computations.     

Fourier transform emission spectroscopy of YH and YD: observation of new A1Δ and B1Π electronic states

The emission spectra of YH and YD molecules have been investigated in the 3600-12,000 cm(-1) region using a Fourier transform spectrometer. Molecules were formed in an yttrium hollow cathode lamp operated with a continuous flow of a mixture of Ne and Ar gases, and YH and YD were observed together in the same spectra. A group of bands observed near 1 μm have been identified as 0-0 and 1-1 bands of the A(1)Δ-X(1)Σ(+) and B(1)Π-X(1)Σ(+) transitions of YH and the 0-0 bands of the same two transitions for YD. The A(1)Δ and B(1)Π states of YH are separated by only about 12 cm(-1) and are involved in strong interactions. A perturbation analysis has been performed using the PGOPHER program to fit the two interacting electronic states and spectroscopic parameters for the A(1)Δ and B(1)Π states, including the interaction matrix elements, have been obtained for the first time.     

Vibronic coupling in the A2Π and B2Σ+ electronic states of the NCS radical

The spin-rovibronic energy levels of the A(2)Π and B(2)Σ(+) electronic states of thiocyanate radical have been calculated variationally, using high-level ab initio coupled diabatic potential energy surfaces. Computations up to J = 7∕2 have been performed, obtaining all levels with K ≤ 3 (Σ(1/2),Π(1/2,3/2),Δ(3/2,5/2),Φ(5/2,7/2)), for energies up to 2000 cm(-1) above the A(000)(2)Π(3∕2) level. The available experimental data have been critically reviewed in the light of the theoretical findings.     

Predissociation and hyperfine structure of the B3Πou+ state of Br2

Using molecular-beam laser spectroscopy, the hyperfine structure of the B-X system of ⁷⁹Br₂ was measured to obtain reliable data on hyperfine and rotational predissociation for the levels B(0_u⁺) ν = 11, 13 and 16. We report the first measurement of the hyperfine predissociation with no rotational predissociation on P(1) transitions. The unexpected strongly J-dependent variation of the predissociation parameters leads to the conclusion that the familiar theory describing all known data for the B state of iodine may be inadequate for bromine.     

Atoms-in-molecules calculation of dipole moments of the lowest Σ and Π states of LiH

AIM calculations of dipole and transition moment functions are reported for the lowest Σ and Π states of LiH using a relatively small basis. The functions obtained agree quite satisfactorily with the best ab initio calculations. The results seem to indicate that the AIM approach might become useful in producing missing input data to the DIM calculations of dipole and transition moment surfaces for molecular collision systems interacting with a laser field.     

Ab initio/empirical potential energy functions and stretching vibrational frequencies of the X̃ 2Π and à 2Π states of the chloroacetylene cation

Anharmonic potential energy functions for the stretching vibrational motions of the two lowest doublet states of chloro-acetylene cation have been constructed using ab initio RHF SCF calculations and some experimental information. Stretching vibrational frequencies are calculated variationally for four different isotopomers. The ν₁ vibrations of H¹²C₂³⁵Cl⁺ are predicted to occur at 3176 (X̃ ²Π) and 3231 (à ²Π) and the ν₂ vibration of the à state at 2081 cm⁻¹.     

Ab initio rovibronic states of the thioketenyl radical in its X2Π electronic ground state

We present an ab initio study of the thioketenyl (HCCS) radical in its degenerate X(2)Π electronic ground state. All rotational and vibrational degrees of freedom are taken into account including the electronic orbital and spin angular momenta. The structure of the rovibronic levels and the nature of the corresponding wave functions show resonances even at very low energies due to strong couplings between the bending, rotation, and spin terms in the Hamiltonian. Assignments from the dominant contributions of the eigenvectors are discussed in parallel with previously published data. The rotational structures for the first excited vibronic states are computed as well as transition intensities for the fundamental one.     

A restricted active space (RAS) SCF study of the lifetime of theA 3Π state of OH+

Summary Restricted Active Space (RAS) SCF calculations have been performed of the potential curves for theX ^3– andA ³ states of the OH⁺ ion and on the lifetime of thev=0–2 vibrational levels of theA state. The convergence of the transition moment integral as a function of the size of the active orbital space was used to select the active orbitals. The calculated value of thev=0 lifetime is 2.4 µs. An estimate of the errors remaining in the calculation leads to a final theoretical value of 2.7±0.1 µs. Computed bond distances and bond energies are 1.031 (1.029) Å and 5.05 (5.01) eV, respectively, for theX state, and 1.137 (1.135) Å and 1.57 eV, respectively, for theA state (experimental values within parenthesis).On leave from Departamento de Quimica Fisica, Universitat de València, Dr. Moliner 50, Burjassot, 46 100 València, Spain     

Theoretical evaluation of the radiative lifetimes of LiCs and NaCs in the A<Superscript>1</Superscript>Σ<Superscript>+</Superscript> state

Calculations of the adiabatic potential energy curves and the transition dipole moments between the ground (A¹Σ⁺) and the first excited (A¹Σ⁺) states have been determined for the LiCs and NaCs molecules. The calculations are performed using an ab initio approach based on non-empirical pseudopotentials for Cs⁺, Li⁺ and Na⁺ cores, parameterized l-dependent polarization potentials and full configuration interaction calculations. The potential energy curves and the transition dipole moment are used to estimate the radiative lifetimes of the vibrational levels of the A⁺Σ⁺ state using the Franck–Condon (FC) approximation and the approximate sum rule method. The radiative lifetimes associated with the A⁺Σ⁺ state are presented here for the first time. These data can help experimentalists to optimize photoassociative formation of ultracold molecules and their longevity in a trap or in an optical lattice.     

Density and temperature dependence of the relaxation of the 1Δg state of highly compressed O2 fluid

The electronic relaxation of O₂ is investigated by an absorption excitation and fluorescence detection technique. The relaxation rate constant of O₂(¹Δ_g) is measured in the density range from 10²¹ to 3 × 10²² cm⁻³ at temperatures between 90 and 295 K. The experimental results are compared with theoretical models based on the pair distribution functions of the fluid. The effects of intermolecular potentials with hard or soft cores are discussed.     

Photodissociation dynamics of D2O via the B̃(1A1) electronic state

Photodissociation dynamics of D(2)O in the B?((1)A(1)) state at different photolysis wavelengths have been investigated using the D-atom Rydberg "tagging" time-of-flight (TOF) technique, in combination with a tunable vacuum ultraviolet photolysis light source. TOF spectra of the D-atom product from the D(2)O photodissociation in both parallel and perpendicular polarizations have been measured. Product kinetic energy distributions and angular distributions have been derived from these TOF spectra. From these distributions, internal state distributions of the OD product as well as the OD quantum state specific angular anisotropy parameters have been derived. Two product channels governed by distinct dissociation dynamics have been clearly observed in the B?((1)A(1)) state photodissociation: ground electronic state radical product OD(X (2)Π) + D and excited electronic state OD(A (2)Σ(+)) + D. The OD(A) + D channel proceeds via adiabatic pathway on the B?((1)A(1)) state surface, producing rovibrational excitation in the OD(A) product, while the OD(X) + D channel is generated through nonadiabatic pathway mainly via conical intersections between the B?((1)A(1)) and the X?((1)A(1)) state surfaces. Due to strong angular force induced by the conical intersections, the OD(X) product is extremely hot in the rotational excitation close to the energy limit (N ～ 50 for v = 0). However, the vibrational excitation is cold in the OD(X) product with dominant population in the ground vibrational state v = 0. Detailed experimental results at different photolysis wavelengths show that at higher energy the unstable periodic orbit, from which dissociation starts, on the B? state has stronger excitation degree of the OD internal state. The negative angular anisotropy parameters of the OD(A) products suggest that the angular forces in this adiabatic dissociation pathway from these periodic orbits have changed the original angular distribution of the D(2)O molecule excited by the B?((1)A(1))←X?((1)A(1)) parallel transition.     

An ab initio study of the rotation—vibration energy levels of GeH2 in the ā3B1 state

Thirty-five points on the potential energy surface of the ā³B₁ first excited state for the GeH₂ radical have been calculated using the (ab initio) MRD CI technique. Thirteen parameters in an analytic expression for the potential have been adjusted (by least-squares optimization) so that the surface fits these points. The rotation-vibration energy levels of GeH₂. GeD₂ and GeHD have been calculated using the non-rigid bender Hamiltonian, and we determine for GeH₂ that ν₁ = 1991 cm^-1. ν₂ = 763 cm^-1, and ν₃ = 2012 cm^-1. The equilibrium structure is found to be r_e = 1.545 Å and α_z = 119.8°, and the singlet-triplet splitting is calculated to be 22.8 kcal/mole (7975 cm^-1).     

Spectroscopy of molecular ions: Laser-induced fragmentation spectra of COS+, A2Π ← X2Π and B2Σ ← X2Π

A new method for ion spectroscopy is introduced which is particularly useful for polyatomic ions. The ions are mass selected, and spectroscopically analyzed by a pulsed dye laser employing photofragment detection subsequent to absorption. The sensitivity of this apparatus permits the measurement of photofragmentation spectra with a conventional, broadly tunable pulsed dye laser system in conjunction with a novel reflecting ion mass analyzer. Vibronic spectra of the transitions to both the A and B states of COS⁺ are given, together with rotationally resolved spectra of the vibrationless, and of one vibrationally active, transition to the A state.     

Pseudo-Jahn-Teller effect as the origin of the exalted frequency of the b2u Kekulé mode in the 1(1)B2u excited state of benzene

In this paper we show that a pseudo-Jahn-Teller (PJT) coupling between the (1)A1g ground state and the (1)B2u excited states along the Kekulé mode of b2u symmetry is responsible for the surprisingly low frequency of this mode in the ground state and its remarkable upward shift of 261 cm(-1) upon excitation to the first (1)B2u excited state.