Matrix-isolated NF: Radiative rates of b 1Σ+→a 1Δ, b 1Σ+→X 3Σ−, and a 1Δ→X 3Σ−, in solid argon

Lifetimes of NF b ¹Σ⁺ (2.33 ms) and a ¹Δ (2.85 s) in solid argon have been determined by pulsed laser excitation of the b state. The integrated (0,0) band intensities of the b→X, b→a, and a→X transitions are 1.00, 2.9±0.6, and 2.7±0.6, in relative units. It is concluded that cascading from b ¹Σ⁺ to a ¹Δ is purely radiative within these error limits. While the b→X and a→X transition rates compare favorably with gas-phase data and theoretical calculations, the b→a transition rate is enhanced by four orders of magnitude in the matrix.     

Determination of the radiative lifetimes of the b1Σ+ and a1Δ states in NH by ab initio methods

Probabilities for the spin-forbidden transitions from the b¹Σ⁺ and a¹Δ states to the X³Σ^? ground state of NH have been evaluated by a first-order perturbation expansion into S-eigenfunctions Nine ³Π and ¹Π, five ¹Σ⁺ and three ³Σ^? states have been calculated by the MRD CI method at the experimental equilibrium distance of the X³Σ^? state (1.0362 Å) which cover a vertical spectral region of = 100000 cm^?1. The expansion terms of the perturbation sum are spin-orbit coupling coefficients obtained by using the Breit-Pauli one- and two-electron spin-orbit operator. The radiative lifetime of b¹Σ⁺ has been determined in the Franck-Condon approximation to be 72 ms from ab initio data and 97 ms if experimental excitation energies for the low-lying valence states are employed. Recent experiments give a somewhat shorter lifetime for the corresponding 0-0 transition of 53 ms. The lifetime is governed by the transition to the ³Σ^?_±1 level of the non-rotating molecule, borrowing its intensity mainly from the A³Π → X³Σ^? dipole transition. The second possible transition to the Ω = 0 level of the ground state is found to be weak. A similar relation of μ₁/μ₀ is expected for all the hydrogen containing isovalent molecules such as PH and AsH. The radiative lifetime of the a¹Δ state has been calculated to be = 1.7 s. Recent matrix experiments predict a gas-phase lifetime of at least 3 s. Further experimental and theoretical investigations are in progress to clarify this unusual finding that the experimentally determined lifetime is longer than that calculated theoretically.     

Theoretical study of electronic structure of rhodium mononitride and interpretation of experimental spectra

Potential energy curves of 22 electronic states of RhN have been calculated by the complete active space second‐order perturbation theory method. The X¹Σ₀⁺ is assigned as the ground state, and the first excited state a³Π₀+ is 978 cm^?1 higher. The ¹Δ(I) and B¹Σ⁺ states are located at 9521 and 13,046 cm^?1 above the ground state, respectively. The B¹Σ⁺ state should be the excited state located 12,300 cm^?1 above the ground state in the experimental study. Moreover, two excited states, C¹Π and b³Σ⁺, are found 14,963 and 15,082 cm^?1 above the X¹Σ⁺ state, respectively. The transition C¹Π₁–X¹Σ₀⁺ may contribute to the experimentally observed bands headed at 15,071 cm^?1. There are two excited states, D¹Δ and E¹Σ⁺, situate at 20,715 and 23,145 cm^?1 above the X¹Σ⁺ state. The visible bands near 20,000 cm^?1 could be generated by the electronic transitions D¹Δ₂–a³Π₁ and E¹Σ⁺₀–X¹Σ⁺₀ because of the spin–orbit coupling effect. © 2013 Wiley Periodicals, Inc.     

Laser photoelectron spectrometry of CH−

Fixed-frequency laser photoelectron spectrometry has been utilized to study photodetachment of CH^? ions by 4880 A laser radiation. Transitions involving low-lying excited states of ¹Δ and ⁴Σ^? symmetry have been observed for CH^? and CH, respectively. The electron affinity of CH was determined to be (1.238 ± 0.008) eV. The a ⁴Σ^? CH term energy was found to be (0.742 ± 0.008) eV and other spectroscopic constants for X³Σ^?CH^?, a ¹ΔCH^?, and a ⁴Σ^?CH have been determined.     

Spectroscopic properties and potential energy curves of some low-lying electronic states of AlO,AlO+, LaO,and LaO+: An ab initio CASSCF study

Results of CASSCF state-averaged calculations on the lowest electronic states of LaO and LaO⁺ are reported in this work. For comparison, some low-lying electronic states of AlO and AlO⁺ are also reported. The ground state of LaO was found to be the X²Σ⁺ (R_e = 1.987 Å, ω_e = 794 cm^?1) with a low-lying A²Δ excited state. Five more excited states below 26000 cm^?1 were found. The first ionization potential (IP ) is found at 5.16 eV, resulting in an X¹Σ⁺ ground state for the LaO⁺ diatom, in opposition to AlO⁺ for which an X³ Π ground state has been found. Analysis of the wave functions, dipole moments, and Mulliken populations reveal that the bonding is quite ionic in both systems. © 1994 John Wiley & Sons, Inc.     

Theoretical investigations of the low-lying electronic states of the HeC2+ dication

Extended MC SCF computations of the CAS SCF type have been performed on four energetically low-lying electronic states of HeC²⁺ dications. The X ¹Σ⁺ ground state is predicted to be thermodynamically stable by 0.72 eV, while the a ³Π and A ¹Π excited states represent metastable species with barrier heights of 2.19 and 0.20 eV, respectively. The b ³Σ⁺ state exhibits merely a very shallow potential dip with a well depth of only 0.06 eV. The HeC²⁺ dication is therefore predicted to be experimentally observable in the gas phase. Bonding in these unusual dications is discussed and compared to the isoelectronic CH⁺ cation.     

Crossed beam study of charge transfer of Ar+ with NO

The charge transfer reaction Ar⁺ (NO, Ar) NO⁺ was studied in a crossed beam experiment at collision energies above and below 1 eV (c.m.). Two mechanisms could be distinguished, an electron exchange process, and an intermediate complex formation; the latter involves moderate momentum exchange between the collision partners and is increasingly more important at low collision energies. Distributions of product translational energies showed that both mechanisms give rise to NO⁺ in the excited a ³Σ⁺ state in a quasi-resonant process. NO⁺ which originated via the complex mechanism was formed in Franck-Condon transitions in the a ³Σ⁺ state and, at higher collision energy, also in the b ³Π and w ³Δ states, in much less probable inelastic processes. These transitions are presumably obeyed because the NO bond distance is net perturbed during Ar⁺ approach.     

The kinetics of free radicals generated by IR laser photolysis. II. Reactions of C2(X 1Σ+g), C2(a 3Πu), C3(X̄ 1Σ+g) and CN(X 2Σ+) with O2

The 300 K reactions of O₂ with C₂(X ¹Σ⁺_g), C₂(a ³ Π_u), C₃(X? ¹Σ⁺_g) and CN(X ²Σ⁺), which are generated via IR multiple photon dissociation (MPD), are reported. From the spectrally resolved chemiluminescence produced via the IR MPD of C₂H₃CN in the presence of O₂, CO molecules in the a ³Σ⁺, d ³Δ_i, and e ³Σ^? states were identified, as well as CH(A ²Δ) and CN(B ²Σ⁺) radicals. Observation of time resolved chemiluminescence reveals that the electronically excited CO molecules are formed via the single-step reactions C₂(X ¹Σ⁺_g, a ³Π_u) + O₂ → CO(X ¹Σ⁺ + CO(T), where T denotes are electronically excited triplet state of CO. The rate coefficients for the removal of C₂(X ¹Σ⁺_g) and C₂(a ³Π_u) by O₂ were determined both from laser induced fluorescence of C₂(X ¹Σ⁺_g) and C₂(a ³Π_u), and from the time resolved chemiluminescence from excited CO molecules, and are both (3.0 ± 0.2)10^?12 cm³ molec^?1 s^?1. The rate coefficient of the reaction of C₃ with O₂, which was determined using the IR MPD of allene as the source of C₃ molecules, is <2 × 10^?14 cm³ molec^?1 s^?1. In addition, we find that rate coefficients for C₃ reactions with N₂, NO, CH₄, and C₃H₆ are all < × 10^?14 cm³ molec^?1 s^?1. Excited CH molecules are produced in a reaction which proceeds with a rate coefficient of (2.6 ± 0.2)10^?11 cm³ molec^?1 s^?1. Possible reactions which may be the source of these radicals are discussed. The reaction of CN with O₂ produces NCO in vibrationally excited states. Radiative lifetime of the ā ²Σ state of NCo and the ā ¹Π_u(000) state of C₃ are reported.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

The electronic spectrum of hydrogen fluoride (7–46 eV)

The electron energy loss spectrum of HF between 7 and 46 eV has been studied with forward, inelastic scattering of 2.5 keV electrons at resolutions up to 60 meV fwhm. The (1π^?t, 4σ)A ¹Π state of HF has been identified at 10.35 eV (vertical). An apparently prominent progression, observed in optical studies and assigned to the (3σ^?1, 4σ)B ¹Σ⁺ state, was not detected in this work. A reasonably complete assignment of the Rydberg structure of HF is proposed.     

O‐loss photodissociation of the OCS+ ion in the low‐lying electronic states studied using multiconfiguration second‐order perturbation theory

The CASPT2 potential energy curves (PECs) for O‐loss dissociation from the X²Π, A²Π, B²Σ⁺, C²Σ⁺, 1⁴Σ^?, 1²Σ^?, and 1⁴Π states of the OCS⁺ ion were calculated. The PEC calculations indicate that X²Π, 1⁴Σ^?, 1²Σ^?, and 1⁴Π correlate with CS⁺(X²Σ⁺) + O(³P_g); A²Π and B²Σ⁺ correlate with CS⁺(A²Π) + O(³P_g); and C²Σ⁺ probably correlates with CS⁺(X²Σ⁺) + O(¹D_g). The CASSCF minimum energy crossing point (MECP) calculations were performed for the C²Σ⁺/1⁴Σ^?, C²Σ⁺/1⁴Π, A²Π/1⁴Σ^?, A²Π/1²Σ^?, A²Π/1⁴Π, and B²Σ⁺/1²Σ^? state pairs and the spin‐obit couplings were calculated at the located MECPs. A conical intersection point between the B²Σ⁺ and C²Σ⁺ potential energy surfaces was found at the CASSCF level. Based on our calculations, seven O‐loss predissociation processes of the C²Σ⁺ state are suggested and an appearance potential value of 7.13 eV for the CS⁺ + O product group is predicted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A theoretical study of the bound electronic states of the C−2 negative ion

Configuration interaction calculation are employed to study the X ²Σ⁺_g, A ²Π_u, B ²Σ⁺_u, ⁴Σ⁺_u and ⁴Δ_u states of the C^?₂ ion. The results are in good quantitative agreement with experimental findings for the Herzberg—Lagerquist (²Σ⁺_u-²Σ⁺_g) bands and predict a T_e value for the ²Π_u state of only 0.40 eV; corresponding transition moment results are obtained as a function of CC distance. The Cl electron affinity is 3.43 eV, in good agreement with the most recent experimental estimate for this quantity.     

Laser spectroscopic investigation of the X 2Σ1/2 state of the van der Waals molecule NaAr

The laser-excitation spectrum of the transition X ²Σ⁺ → A ²Π of NaAr has been investigated using a supersonic expansion of a mixture of sodium vapor and argon gas for production of the molecules. In comparison to preyous investigations the rotational constants of the vibrational levels ν″ = 2, 3 and 4 of the X ²Σ⁺ state could in addition be determined. From our results we deduce a value of R_e = 5.008(5) × 10^?10 m for the equilibrium internuclear distance and of D_e = 41.7(δ) cm^?1 for the well depth of the X ²Σ⁺ state.     

Nature of the resonant states of NO−

The stabilization method of Taylor has been used to calculate the nature of the states of NO^? corresponding to the lowest resonances observed recently by Sanche and Schulz. It is confirmed that they consist of two Rydberg electrons in the core of the X ¹Σ⁺ ground state of the ion. The proposed assignments are the (Rsσ)²¹Σ⁺, (Rsσ)(Rpπ) ³Π, (Rsσ)(Rpσ) ³Σ⁺ states. The fourth resonance is attributed to an (Rpπ)²³Σ⁺ state. The Rydberg or of these states of the negative ion are more diffuse than the corresponding ones of the neutral molecule.     

Spectroscopy of Ethylenedione

The long sought‐after, intrinsically short‐lived molecule ethylenedione (OCCO) was observed and investigated by anion photoelectron spectroscopy. The adiabatic electron affinity of its quasi‐bound ³Σ_g^? state is 1.936(8) eV. The vibrational progression with a 417(15) cm^?1 frequency observed within the triplet band corresponds to a trans‐bending mode. Several dissociative singlet states are also observed, corresponding to two components of the ¹Δ_g state and the ¹Σ_g⁺ state. The experimental results are in agreement with theoretical predictions and constitute the first spectroscopic observation and characterization of this elusive compound.     

Theoretical evidence for multiple 3d bondig in the V2 and Cr2 molecules

Calculated CAS SCF potential curves are reported for the ³Σ_g^? state of V₂ and the ¹Σ_g⁺ state of Cr₂. At the CAS SCF level the ³Σ_g^? state of V₂ is calculated to be bound (R_c = 1.77 Å ω_c = 593.6 cm^?1, D_e 0.33 eV) and to involve a triple 3d bond; while the Cr₂ potential curve is not bound but shows a shoulder near the experimental R_e and the wave function shows multiple 3d bonding in this region.     

Investigation of the molecular chemiluminescence SrCl(A2Π1/2,3/2, B2Σ+ → X2Σ+) and the atomic resonance fluorescence Sr(53P1 → 51S0) in the time-domain following the pulsed dye laser generation of Sr(53 PJ) in the presence of CH2Cl2

Time-resolved investigations of the atomic resonance fluorescence Sr(5³P₁ → 5¹S₀) and the molecular chemiluminescence from SrCl(A²Π_1/2,3/2, B²Σ⁺ → X²Σ⁺) are reported following the reaction of the electronically excited strontium atom, Sr(5s5p(³P_J)), 1.807 eV above its 5s²(¹S₀) electronic ground state, with CH₂Cl₂. The optically metastable strontium atom was generated by pulsed dye-laser excitation of ground state strontium vapor to the Sr(5³P₁) state at λ = 689.3 nm (Sr(5³P₁ ← 5¹S₀)) at elevated temperature (850 K) in the presence of excess helium buffer gas in which rapid Boltzmann equilibration within the 5³P_J manifold takes place. Sr(5³P_J) was then monitored by time-resolved atomic fluorescence from Sr(5³P₁) at the resonance wavelength together with chemiluminescence from electronically excited SrCl resulting from reaction of the excited atom with CH₂Cl₂. The molecular systems recorded in the time-domain were SrCl(A²Π_1/2 → X²Σ⁺) (Δν = 0, λ = 674 nm), SrCl(A²Π_3/2 → X²Σ⁺) (Δν = 0, λ = 660 nm), and SrCl(B²Σ⁺ → X²Σ⁺) (Δν = 0, λ = 636 nm). Both the A²Π (179.0 kJ mol^?1) and (B²Σ⁺(188.0) kJ mol^?1) states of SrCl are energetically accessible on collision between Sr(³P) and CH₂Cl₂. Exponential decay profiles for both the atomic and molecular (A,B – X) chemiluminescence emission are observed and the first-order decay coefficients characterized in each case. These are found to be equal under identical conditions and hence SrCl(A²Π, B²Σ⁺) are shown to arise from direct Cl-atom abstractions on reaction with this halogenated species. The combination of integrated molecular and atomic intensity measurements, coupled with optical sensitivity calibration, yields estimations of the branching ratios into the A_1/2,3/2, B, and X states arising from Sr(5³ P_J) + CH₂Cl₂ which are found to be as follows: A_1/2, 3.0 × 10^?3; A_3/2, 1.7 × 10^?3; B, 4.4 × 10^?4 yielding ΣSrCl(A_1/2 + A_3/2 + B) = 5.1 × 10^?3. As only the X, A and B states of SrCl are accessible on reaction, this indicates an upper limit for the branching ratio into the ground state of 0.995. The present results are compared with previous time-resolved measurements on SrF, Cl, Br(A²Π,B²Σ⁺ ? X²Σ⁺) that we have reported on various halogenated species and with analogous chemiluminescence studies on Sr(³P) with other halides obtained from molecular beam measurements. The results are further compared with those from a series of previous analogous investigations in the time-domain we have presented of molecular emissions from CaF, Cl, Br, I (A,B – X) arising from the collisions of Ca(4³P_J) with appropriate halides and with branching ratio data for Ca(4³P_J) obtained in beam measurements. © 1995 John Wiley & Sons, Inc.     

CAS SCF/CI calculations of potential energy surfaces of He3+ and He2+

Complete active space MC SCF (CAS SCF) calculations followed by second-order configuration interaction (SOCI) calculations are carried out on the potential energy surfaces (bending surface, linear surfaces) of the ²Σ_g⁺ ground state of He₃⁺. The potential minimum for the ²Σ_g⁺ state occurs at a linear geometry with HeHe bond length of 1.248 Å. The binding energy of He₃⁺ with respect to He + He⁺ + He was calculated to be 2.47 eV at the SOCI level. The energy required to dissociate He₃⁺ (²Σ_g⁺) into He₂⁺ (²Σ_u⁺) and He(¹S) is calculated to be 0.14 eV. The same level of SOCI calculations of He₂⁺ yield a D_e value of 2.36 eV.     

A Valence Bond Study of the Low‐Lying States of the NF Molecule

The electronic structures of the three lowest‐lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self‐consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9–12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O₂ molecule, the ground state ³Σ^? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet ¹Δ and ¹Σ⁺ excited states the π‐bonding component is classically covalent, and it contributes 28 % and 37 % to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that π bonding contributes at least 24, 35 and 42 kcal mol^?1 to the total bonding energies of the ³Σ^?, ¹Δ and ¹Σ⁺ states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.     

Vibrational structure in kinetic energy spectra of O+ ions from electron impact dissociative ionization of O2: Predissociation of the B 2Σg− state of O2+

Structure has been observed in the ≈ 0.8 eV kinetic energy O⁺ ion peak produced by electron impact dissociative ionization of O₂. The spacing between the features observed is approximately 0.08 eV which is consistent with their originating through predissociation of the B ²Σ_g^? state of O⁺.