Ã2Σ+g → X̃2Πu emission band system of dicyanoacetylene radical cation in the gaseous phase

The emission spectrum of the dicyanoacetylene radical cation has been observed in the 580–720 nm wavelength region as a result of low energy electron impact excitation in a crossed-beam arrangement. The band system is attributed to the òΣ⁺_g → X?²Π_u electronic transition by comparison with the photoelectron spectroscopic and calculated data on the states of dicyanoacetylene cation. The frequencies of the three Σ⁺_g vibrational fundamentals in the ground cation state have been deduced from the emission spectrum. The lifetime of the lowest vibrational level of the òΣ⁺_g state of dicyanoacetylene cation was determined to be 13 ± 2 ns. Emission could not be detected from the corresponding states, òΣ⁺, of fluorocyanoacetylene and cyanoacetylene cations, and these results are discussed.     

Interference effects in the predissociation of H2 and HD

Several absorption lines in the B? ¹Σ⁺_u ← X ¹Σ⁺_g system at 83 nm have been observed with sub-Doppler resolution in H₂ and HD. A detailed analysis of the lineshapes arising from the homogeneously predissociaied B ¹Σ⁺_u states shows that the linewidths are strongly affected by the Franck-Condon factors associated with coupling to the B' ¹Σ⁺_u continuum.     

Dynamics of reactions of O(3P) atoms with CS,CS2 and OCS

The reactions of CS(X ¹Σ⁺), CS₂(X ¹Σ⁺_g) and OCS(X ¹Σ⁺) with O(³P) were studied at 298 K by means of a CO laser resonance absorption technique. The CO(ν) population distribution produced from the reaction O(³P) + CS(X ¹Σ⁺) studied in a quartz flash photolysis tube (λ>/ 200 nm) is similar to distributions observed previously for ν> 7. For ν < 7 an energetically colder vibrational population was observed which is attributed to the reaction of O(³P) atoms with undissociated CS₂(X ¹Σ⁺_g). Subsequent experiments carried out in a Pyrex flash photolysis tube (λ>/ 300 nm) in which the O(³P) + CS₂(X ¹Σ⁺_g) reaction is the only one which can occur confirmed that the colder population observed is attributable to this process. The branching ratio for the reaction channel O(³P) + CS₂(X ¹Σ⁺_g) → CO(X ¹Σ⁺) + S₂(³Σ^?_g) has been measured. We find that 1.4 ± 0.2% of the O + CS₂ reaction proceeds through this channel, and that the rate constant for this reaction channel is, k = 3.5 (±0.5) × 10¹⁰ cm³/mole s. Isotope labeled experiments using ¹⁸O atoms show that the O(³P) + OCS(X ¹Σ⁺) reaction takes place by a direct stripping mechanism, wherein CO(ν) is produced exclusively from the parent OCS molecule. The CO(ν) formed in this reaction carries about 9% of the total available energy.     

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.     

Absolute emission cross sections for the calibration of optical detection systems in the 120–250 nm range

Absolute emission cross sections have been determined for electron impact on CO, NO and N₂. For the CO(A ¹ΠX ¹Σ⁺) and N₂(a ¹ΠX ¹Σ_g) radiation our data is in good agreement with that of other groups. For CO⁺ (B²Σ⁺X²Σ⁺) the values of the emission cross sections are different from those measured previously. This discrepancy is explained in terms of an inadequate straylight correction in the former experiments. For the NO(A²Σ⁺X²Π) emission no previous σ_em values are known to the authors. Furthermore the electronic transition moments of the NO(A²Σ⁺X²Π) and CO⁺(B²ΣX²Σ⁺) systems have been measured and are found to be independent of the internuclear distance.     

De-excitation rate constants of He(2 3S) by atoms and molecules as studied by the pulse radiolysis method

Time-resolved measurement of He (2 ³S) concentration by its optical absorption after electron pulse irradiation of HeN₂ mixtures confirms that the optical emission of N⁺₂(B ²Σ⁺_u → X ²Σ⁺_g) is based on the energy transfer (Penning ionization) from He (2 ³S) to N₂. The addition of other atoms and molecules to HeN₂ mixtures changes the decay rate of the optical emission N⁺₂(B ²Σ⁺_u → X ²Σ⁺_g), which is a detector of He (2 ³S), and gives the rate constant of He (2 ³S) de-excitation by various atoms and molecules. Our results are discussed from the viewpoint of a gas-kinetic collision model.     

A CAS SCF CI study of the 1Σ+g and 3IIu states of the C2 molecule and the 4Σ−g and 2IIu states of the C+2 ion

Ab initio calculations of the X ¹Σ⁺_g and a ³II_u states of C₂ and the X⁴Σ⁻_g and a²II_u states of the C⁻₂ molecular ion are performed to determine the corresponding potential curves around the potential minima and at the dissociation limits. A large Gaussian basis set augmented by three d-type polarization functions on each carbon center is used to approximate the molecular orbits. The calculations are done at the complete-active-space SCF and multi-reference configuration interactions level. Spectroscopic constants and rotation—vibration energies are derived from the ab initio calculated potentials. Good agreement between theory and experiment is obtained for the X¹Σ⁺_g and a ³II_u states of C₂. In the earlier tentative assignment of the observed electronic transition around 2490 Å to the ²Σ⁻_g ← ²II_u system in C⁺₂, the lower state is confirmed by the present calculations to be C⁺ ₂ (²II_u).     

Diabatic molecular states and the shielded diatomic orbital method. Evolution of two-interacting state systems for molecular hydrogen

Within Lichten's method, shielded diatomic orbitals (SDO's) are proposed as a basis for building-up diabatic molecular states in the single configurati approximation. The determination of SDO's i.e. eigenfunctions in prolate spheroidal coordinates of the two-center problem with a parametric shielding potential is extended to mono-excited states of many-electron diatomic systems, the shielding potential being obtained from simple electrostatic considerations. Four diabatic molecular states of H₂ are investigated i.e. ¹Σ_g⁺ (2pσ², 1Σ_g⁺ (1s,σ 2sσ), ¹Σ_u⁺(1sσ,4Pσ), ¹Σ_u⁺ (1sσ, 4fσ) using a minimal basis of SDO'S. The dynamical evolution of the nuclei for the two sets ¹Σ_g and ¹Σ_u⁺ of two interacting states is described in both the diabatic and the corresponding adiabatic representation.     

On xenon molecule excited state lifetimes

Time-resolved excitation spectra of xenon vapor in the 150 nm region are analysed in terms of four main fluorescence lifetimes corresponding to decays of four stable excited electronic states of the Xe dimer. The two shortest decay times, ≈ 2 ns and ≈ 60 ns, are assigned to the direct radiative relaxation of the two lowest excited ungerade states, (¹Σ⁺_u)0⁺_u and (³Σ⁺_u))1 u respectively. The two longest decay times, ≈ 150 ns and ≈ 500 ns, must correspond to the overall depopulation rates of the two lowest excited gerade states, (³Σ⁺_g)1g and (¹Σ⁺_g)0⁺_g, decaying into the gerade ground state by cascading down through the intermediate ungerade states.     

The effect of internal excitation on the collision induced dissociation of I+2(2Πg,υ) ions

Cross sections for collision induced dissociation of 0.65 to 3.2 keV I⁺₂(²Π_g, υ) ions in I⁺₂(²Π_g, υ) + N₂(X ¹Σ⁺_g, υ = 0) interactions have been determined. Reaction cross sections for I⁺₂(²Π_{$\text{3}\text{2}$,g}, υ) ions in low vibrational levels vary smoothly from 6 to 10 A² with increasing kinetic energy. Dissociation cross sections for I⁺₂(²Π_{$\text{1}\text{2}$,g}, υ) ions are larger than those involving ground state ions. Processes involving highly excited metastable states of I⁺₂ are not observed in this investigation.     

The photodissociation of Li2

The absorption of photons by Li₂ from the X ¹Σ⁺_g state to the A ¹Σ⁺_u and B ¹Π_u states is considered and the mechanisms that lead to dissociation are studied quantitatively. Calculations are reported on the direct predissociation of the b ³Π_u state. The significance of accidental predissociation of the A ¹Σ⁺_u state is discussed and a quantal theory of the process is presented.     

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.     

Lifetimes of C-2 in rotational levels of the B̃2Σ+u state in the gas phase

Lifetimes of C^-₂ in rotational levels of the B?²Σ⁺_u:ν′ = 0, ν′ = 1 states have been measured. C^-₂ was produced from bromoacetylene and rare-gas metastables and the B?²Σ⁺_u—X?²Σ⁺_g transition was laser excited. The lifetimes are constant within a vibrational level, 77 = 8 ns for ν′= 0 and 73 = 7 ns for ν′ = 1. The oscillator strength f_νo = 0.044 ± 0.004.     

Theoretical study of the 2Σu+ and 2Σg+ states of Li2− and Na2−

Ab initio calculations are performed to obtain potential energy curves for the X¹Σ_g⁺ state of Li₂ and Na₂ and the X²Σ_g⁺ and A²Σ_g⁺ states of their anions. The A²Σ_g⁺ M₂^? curves are found to intersect the X¹Σ_g⁺M₂ curves at low energies and are expected to play a major role in the e^? + M₂ → M^? + M process.     

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.     

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 study of the N+2 molecular ion

The Equations of Motion method has been applied in the calculation of potential energy curves for the X²Σ⁺_g, A²Π_u and B²Σ⁺_u states of N⁺₂. Results are also reported for a new dissociative ²Σ⁺_g state. The theoretical curves are directly compared with the experimental ones as well as in terms of spectroscopic constants. The applicability of the Equations of Motion method to this type of problem is critically examined and discussed with regard to the choice of basis set, numerical effort and agreement with experiment.     

The observation of predissociations in the oxygen molecular ion by low-energy electron impact

The kinetic energy distribution of the O⁺ ions formed by dissociative ionization of O₂ has been carefully examined in the range of 0.0 to 1.3 eV. In the peak at 600 meV fine structure is observed and interpreted by predissociation. The high energy side of the peak has been ascribed to the predissociation of the ²Δ_g, B²Σ^?_g and the C⁴Σ^?_u states of O⁺₂. The low energy side of the peak is presumably due to the predissociation of the b⁴Σ^?_g state or the ²Φ_u state of O⁺₂ or to autoionizing predissociation.     

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⁺.