Investigation of the 2B2 state of H2O+ using valence-bond techniques

Ab initio valence-bond calculations have been performed on the low-lying states of H₂O⁺, with special attention being focused on the

²B₂ state of the ion. The calculated potential energy surface for the

²B₂ state is in qualitative agreement with several previously published molecular orbital calculations in predicting an equilibrium angle of about 60°. This prediction is, however, inconsistent with the most recent interpretation of the high-resolution photoelectron spectrum of H₂O. Examination of the potential energy surfaces for geometries which have been distorted from C_2v symmetry indicates that the

²B₂ and òA₁ states are strongly coupled by the asymmetric stretching motion of the molecular ion. The presence of such a coupling supports the interpretation of the H₂O photoelectron spectrum which invokes excitation of the asymmetric stretching vibration of the ion.     

Laser induced fluorescence of CH2 (ã1A1) produced in the photodissociation of ketene at 337 nm. The CH2 (ã1A1-X̃3B1)energy separation

The photodissociation of ketene, CH₂CO(X?¹A₁) → CH₂ (ã¹A₁) + CO(X ¹Σ⁺) has been observed at 337 nm, using a pulsed nitrogen laser. The CH₂ (ã ¹A₁) radical has been detected by laser induced fluorescence with a tunable dye laser. A laser excitation spectrum has been obtained from CH₂(ã ¹A₁) over the wavelength interval from 588.9 to 595.6 nm in the Σ ← Π vibronic subband of the CH₂ (ã ¹A₁); υ″ = 0, 0, 0?b? ¹B₁; υ′ = 0, 14, 0) transition. For the CH₂(ã ¹A₁ ; υ′= 0, 0, 0?X? ³B₁; υ′' = 0, 0, 0) energy separation an upper limit of (6.3 ^± 0.8) kcal/mole has been found. The radiative lifetime τ and the rate constant k for the removal of the 0₀₀ rotational level of the Σ(0, 14, 0) vibronic state have been measured directly. The values are τ = (4.2 ^± 0.2) μs and k = (7.4 ± 0.3) × 10^?10 cm³ molecule^?1 s^?1, respectively.     

Excited states of gaseous ions. V. The predissociation of the h2o+ ion

The B?² state of H₂O⁺ is predissociated twice. First, by the ã⁴B₁ state, giving OH⁺ + H fragments via spinorbit coupling interaction. Secondly, by a²A state, giving H ⁺ OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B? state of H₂O⁺ and D₂O⁺ is carried out. This provides the vibrational frequencies of the H₂O⁺, D₂O⁺ and HDO⁺ ions, as well as a vibrational assignment of the peaks. The H₂O⁺ ion in its B?²B₂ state is found to have a OH bond length of 1.12 A and a valence angie of 78°.In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH⁺ and OD⁺ ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O⁺ ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H₂O⁺ ions, and not to that of the OH⁺ fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.     

Threshold energies for production of CH2(3B1) and CH2(1A1) from ketene photolysis. The CH2(3B1) ↔ CH2(1A1) energy splitting

By measuring the relative CO quantum yields from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25° for CH₂CO(¹A₁) → CH₂(³B₁) + CO(¹Σ⁺) to be 75.7 ± 1.0 kcal/mole. This corresponds to a value of 90.7 ± 1.0 kcal/mole for ΔH_f298⁰[CH₂(³B₁)]. By measuring the relative ratio of CH₂(¹A₁)/CH₂(³B₁) from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25°C for CH₂CO(¹A₁) → CH₂(¹A₁) + CO(¹Σ⁺) to be 84.0 ± 0.6 kcal/mole. This corresponds to a value of 99.0 ± 0.6 kcal/mole for ΔH_f298⁰[CH₂(¹A₁)]. Thus a value for the CH₂(³B₁) ? CH₂(¹A₁) energy splitting of 8.3 ± 1 kcal/mole is determined, which agrees with three other recent independent experimental estimates and the most recent quantum theoretical calculations.     

Potential curves and molecular properties of Na2+

A model potential method in which a molecule is described as a single electron moving in the field of two polarizable cores is used to calculate the potential energy curves and the wavefunctions of the lowest six electronic states of the molecular ion Na₂⁺. The ground X²Σ_g state has a dissociation energy of 0.98 eV at an equilibrium separation of 3.3 Å and the excited ²Π_u state has a dissociation energy of 0.23 eV at an equilibrium separation of 5.2 Å. Various molecular properties of these two bound states are calculated. An analysis of the long range behaviour of all the six states is presented.     

Singlet methylene kinetics: Direct measurements of removal rates of ã1A1 and b̃1B1 CH2 and CD2

Rate constants for collisional removal of ã¹A₁ and b?¹B₁ CH₂ and CD₂ have been directly measured, using IR laser induced multiple photon dissociation to prepare the radicals, and time resolved laser induced fluorescence to observe them. For CH₂(ã¹A₁) removal by He, Ne, Ar, Kr, Xe, N₂, H₂, O₂, CO and CH₄, rate constants of 3.1, 4.2, 6.0, 7.0, 16, 8.8, 130, 30, 56 and 73 × 10^?12 cm³ molecule^?1 s^?1 were found respectively. These represent significant increases over the previously accepted values. Essentially no isotope effect is observed in the removal of CD₂(ã¹A₁) by the rare gases. The rate determining step in removal by the rare gases and N₂ is thought to be singlet—triplet intersystem crossing controlled by long range attractive forces, and the results are discussed in terms of both isolated and mixed state theoretical models of these processes. For the other molecular collision partners, bimolecular chemical removal channels are possible, and may account for the relatively fast rates observed. Radiative lifetimes of five Σ vibronic levels of CH₂(b?¹B₁) and three Σ vibronic levels of CD₂(b?¹B₁) have been measured and found to lie in the range 2.5–6.0 μs, and collisional quenching rates for CH₂(b?¹B₁) are found to be of the order of the gas kinetic collisional frequency.     

Observation of X1A1 vinylidene by photoelectron spectroscopy of the C2H2− ion

Photoelectron spectra of the vinylidene anion (C₂H₂^?) show vibrational structure in X¹A₁ vinylidene up 12 kcal/ mol above the vibrational ground state. Analysis yields an EA(C₂H₂$\text{X}$¹ A₁) of 0.47 ± 0.02 eV, and frequencies for the CC stretch and HCH bend. Absence of the ³B₂ state in the photoelectron spectra indicates the ¹A₁-³B₂ splitting in vinylidene is ? 1.7 eV.     

Investigation of the B2Σ and test of the A2II interaction potential for Na(32P)-Ar from high-resolution differential scattering measurements

We have measured differential cross sections for scattering of laser-excited Na(3²P_{$\text{3}\text{2}$}) by Ar(¹S₀) at thermal collision energies with high angular resolution (0.1°). In the investigated range of scattering angles (1°–15°) the cross sections contain contributions from scattering along the excited state B²Σ potential (rainbow scattering) and the A²II potential (supernumerary rainbows). By performing fit calculations in which the spectroscopically determined A²II potential was adopted we were able to obtain information about the B²Σ potential. With the assumption of a Lennard-Jones (12.6) potential shape we obtain a well depth ?^BΣ = (0.14±0.02)×10^?3 au and an equilibrium distance r_m^BΣ = 10.4±1.0 au. This work presents the first experimental determination of the B²Σ potential well parameters.     

à 2A1 → X̃ 2B1 emission spectrum of cis-1,2-difluoroethylene cation and the non-radiation decay of the à states of the haloethylene cations in the gaseous phase

The results of the search for the à → X? radiative relaxation of haloethylene cations in the gaseous phase are reported. Only in the case of cis-1,2-difloroethylene cation was an emission spectrum detected. It is identified as the à ²A₁ → X? ²B ₁ band system on the basis of photoelectron spectroscopic measurements. An assignment of the emission bands yields the vibrational frequencies of four of the A₁ fundamentals (under C₂, symmetry) for the X? state and one for the à state. Well resolved Ne(I) photoelectron spectra of cis- and trans-1,2-difluoroethylene are presented, from which some vibrational frequencies for these cations in the X? and à states are also obtained. The lifetimes of cis-1,2-difluoroethylene cation in the lowest vibrational levels of the à ²A₁ state have been measured. The decay of this cation is unusual as these levels are depleted both by, radiative, and pathways leading to fragment ions (C₂HF⁺). The lack of detectable emissions with other fluoro-, chloro- and bromo-ethylene cations is discussed and the likely symmetries of the à states are proposed.     

Lifetimes of the vibronic Ã2A1 states of H2O+ and of the 3Πi (υ′ = 0) state of OH+

Using the delayed coincidence technique, lifetimes have been measured for some Σ and Π vibronic òA₁ states of H₂O⁺ and for the ³Π_i (υ′ = 0) state of OH⁺ by analysing the decay curves of the òA₁(0, υ′₂, 0) ? X?²B₁ (0, υ″₂, 0) and the ³Π_i(υ′ = 0) ? ³Σ^?(υ″ = 0) emission intensities respectively. The excited molecular ionic states are produced via excitation of H₂O molecules by 200 eV electrons. For H₂O⁺(òA₁) the vibronic Σ levels with υ′₂ = 13 and 15 and the vibronic Π levels with υ′₂ = 12 and 14 have been considered. The radiative lifetimes obtained for these levels have about the same value, namely 10.5(±1) × 10^?6 s. The radiative lifetime for the OH⁺(³Π_iυ′= 0) state is 2.5(±0.3) × 10^?6 s. The lifetimes found in this work for H₂O⁺(òA₁) and OH⁺(³Π_i,υ′= 0) are about ten and three times longer respectively than the corresponding lifetimes given by other investigators [1,2]. The probable reason for this discrepancy is that in the other experiments no attention has been paid to the presence of a large space charge effect. This effect is caused by the positive ions which are created by the primary electron beam.     

1B2u1A1g fluorescence from benzene produced by electron impact (30–1000 eV)

The ¹B_2u → ¹A_1g fluorescence resulting from electron impact (30–1000 eV) on benzene has been studied in the pressure range 10^?4 ?2 × 10^?3 torr. The fluorescence spectrum is compared with the spectrum obtained by other methods. The energy dependence of the absolute emission cross section indicates a small probability for internal conversion from higher singlet states to the ¹B_2u state.     

1B2u1A1g fluorescence from benzene produced by electron impact (0–30 eV)

The ¹B_2u¹A_1g fluorescence of benzene resulting from the impact of low energy electrons (0–30 eV) has been studied in the pressure range 10^?4 ?2 × 10^?3 torr. It is found that the apparent emission cross section near threshold varies linearly with the pressure. A reaction scheme explaining this behaviour is given. From the absolute value of the apparent emission cross section it follows that excitation of the ³E_1u state is by far dominant over excitation of the ¹B_2u state at low electron impact energies.     

Lifetimes of the vibronic Ã2A1 states of H2S+

Lifetimes have been measured for the Σ and Π vibronic òA₁ states of H₂S⁺ by studying the decay curves of the òA₁ (0, υ′₂, 0) → X? ²B₁ (0, υ″₂, 0) emission bands. The vibronic òA₁ states are produced via excitation of H₂S molecules by 150 eV electrons. The Σ sublevels 1 ? υ′₂ ? 7 and the Π sublevels 3 ? υ′₂ ? 6 have been considered. Predissociation occurs in the Σ sublevels for υ′₂ ? 7 and in the Π sublevels for υ′₂ ? 6. The obtained radiative lifetimes for the non-predissociated Σ and Π sublevels are around 4.2(±0.4) × 10^?6 s and 5.6(±0.5) × 10^?6 s respectively. For the predissociated Σ(0, 7, 0) and Π(0, 6, 0) levels the corresponding lifetimes are 2.3(±0.3) × 10^?6 s and 1.6(±0.3) × 10^?6 s respectively. The rate constant for collisional deactivation (quenching) of the vibronic òA₁ states by H₂S molecules was found to equal 2.3(±0.3) × 10^?9 cm³ mol^?1 s^?1.     

The B̃2B1 ← X̃2A1 absorption system of NO2 in Xe matrices: evidence for Renner—Teller interaction

A tentative vibrational assignment of the B?²B₁ ← X?²A₁ absorption system of NO₂ in solid Xe is reported. About 65 bands were analysed, yielding normal vibration energies of ν₁ = 1230, ν₂ = 450 and ν₃ = 2040 cm^?1. The electronic transition energy can be estimated to be T₀₁₀ = 14160 cm^?1 (14220 cm^?1 for the gaseous phase). These observations are in good agreement with predictions made using ab initio calculations. Evidence for Renner—Teller interaction is documented by a systematic staggering of frequency intervals between successive bands in the ν₂ progression of the B? state.     

Crystal field effects on the magnetic behavior of Yb2V2O7 and Tm2V2O7

The bulk magnetic behaviors of the pyrochlores Yb₂V₂O₇ and Tm₂V₂O₇ were investigated. Calculated susceptibilities were adjusted to obtain the best fit to experimental data. A cubic crystal field Hamiltonian was used with B°₄ = ?0.633 and B°₆ = 0.000705 K for Yb³⁺ and B°₄ = 0.0297 and B°₆ = 0.000339 K for Tm³⁺. The calculated susceptibility for Yb³⁺ was found to be insensitive to the addition of an axial B°₂ parameter to the cubic Hamiltonian.     

New vibronic bands of CH2 observed in a pulsed supersonic jet

Spectroscopic studies of the methylene radical in a supersonic expansion have resulted in the observation of previously unreported CH₂ B?¹ B₁ ← ã¹A₁, hot band transitions. These ã state levels are populated by radiative cascade following multiphoton dissociation of ketene. Multiphoton excitation of ketene is also found to produce a diffuse luminescence with a lifetime longer than 50 μs. The conclusion of Lengel and Zare are singlet methylene is not produced one-photon nitrogen laser photolysis of cold ketene is confirmed.     

Ground electronic state and geometry of SF3

Ab initio LCAO MO SCF calculations with DZ + 3d(S) basis functions show that the sulphur trifluoride radical is a planar π-radical having a ²B₁ ground state. Like ClF₃, it has an umbrella-structure. However, it becomes Y-shaped in its first ²A₁ excited state which has been calculated to lie only ≈ 2.4 eV above the ²B₁ ground state.     

Dissociation of hydrogen molecules in collisions with light alkali ions. II. Li+-H2

The dissociation of a ground state H₂ molecule in single collisions with a Li⁺ ion has been studied using a time of flight technique over a large range of center of mass scattering angles (30° ? υ ? 180°) and collision energies (16 eV < E_cm < 55.5 ev).The results have been transformed into the center of mass system to obtain inelastic differential cross sections (contour maps). In contrast to most other scattering experiments on collision induced dissociation, the results at high energies (E_cm > 40 eV) cannot be explained by a two-step mechanism. Instead dissociation appears to occur in a time comparable to the collision time. The results are consistent with several collision models. Of these the spectator model in which only one of the atoms of the molecule is struck by the incident ion is favored since it is in good agreement with the differential cross sections for backward scattering.     

Laser-induced fluorescence of CsH: The X1Σ+ state dissociation energy

Laser-induced fluorescence of CsH from the A¹Σ⁺ electronic state to the X¹Σ⁺ state was recorded using high-resolution Fourier transform spectrometry. Ground-state vibrational levels were observed from ν″ = 1 to the dissociation limit. These measurements showed anomalies in the X¹Σ⁺ potential energy curve due to the avoided crossing of ionic and covalent potential curves. Accurate molecular constants were derived for the lower X¹Σ⁺ vibrational levels. The observation of a quasibound level gave the first experimental determination of the dissociation energy (in cm^?1): 14802 ? D_c ? 14813.     

Analysis of the stability of finite subspaces in density functional theory

We have studied photodissociation of the A state of the H₂S⁺ ion using the quantum-chemical CAS methods, and the 1² A″ (X ² B ₁) and 1⁴ A″ states are involved in photodissociation of the 1² A′ (A ² A ₁) state (the electronic states in dissociation were studied in the C _s symmetry). The CASPT2 S-loss dissociation potential energy curve (PEC) calculations indicate that the 1² A″ and 1² A′ states correlate with the second limit [H₂ + S⁺(² D)] while the 1⁴ A″ state correlates with the first limit [H₂ + S⁺(⁴S)] and that there are a transition state and a local minimum along the 1² A′ PEC and the repulsive 1⁴ A″ PEC crosses the 1² A″ and 1² A′ PECs. The CASPT2 H-loss dissociation PEC calculations indicate that the 1² A″ and 1⁴ A″ states correlate with the first limit [HS⁺(X ³Σ^?) + H] while the 1² A′ state correlates with the second limit [HS⁺(a ¹Δ) + H] and that the repulsive 1⁴ A″ PEC crosses the 1² A′ PEC. For the crossing doublet and quartet states in pairs, we performed CASSCF minimum energy crossing point (MECP) calculations, and the CASSCF spin-orbit couplings and CASPT2 energies at the MECP geometries were calculated. We examined the two previously proposed mechanisms (mechanisms I and II) for dissociation of the A state to the S⁺ ion, based on our calculation results. We suggest processes for dissociation of the A state to the S⁺ ion (processes I and II, based on mechanisms I and II, respectively) and to the SH⁺ ion (process III) and conclude that photodissociation of the A state mainly leads to the S⁺ ion via the most energetically favorable process II: A ² A ₁ (1² A′) (2.38 eV) → barrier at the linearity (2.96 eV) → X ² B ₁ (1² A″) (0.0 eV) → the 1² A″/1⁴ A″ MECP (3.50 eV, large spin-orbit coupling) → H₂ $ (X^{ 1} \Upsigma_{\text{g}}^{ + } ) $  + S⁺(⁴S) (2.92 eV) (the CASPT2 relative energy values to X ² B ₁ are given in parentheses and the largest value is 3.50 eV at the MECP).