Rydberg states of the K2 molecule studied by laser spectroscopy in a supersonic beam

Rydberg states of potassium dimer have been studied in a crossed laser-molecular beam experiment. The K₂ molecules were formed in a supersonic expansion and excited by low-power cw dye laser. Two different excitation schemes have been used: The first scheme uses a single mode ring dye laser to induce near resonant two-photon transitions while in the second scheme stepwise excitation with two dye lasers is used. In each case excitation of Rydberg levels was detected by monitoring the ionization signal resulting from three-photon absorption. We report a detailed study of 700 two-photon resonances between 625 nm and 650 nm. Most of these signals can be assigned to transitions from the X¹σ _g ⁺ to¹σ _g ⁺ ,¹Π _g , and^1Δ _g states, which are all enhanced by the B¹Π _u intermediate state. Accurate rotational constants are given for the populated vibrational levels of these states. By stepwise excitation of Rydberg levels via theB ¹Π _u state we identify 3 series of Rydberg states as¹Δ _g (4S+nD),¹Σ _g ⁺ (4S+nD), and¹Σ _g ⁺ (4S+nS) with principal quantum numbers 7≦n≦20. Molecular constants of these and other observed but as yet unidentified states are given; quantum defects and dissociation energies are discussed.     

Analytical photoion spectroscopy applied to dissociative single and double photoionization of diatomic molecules (H2, N2, O2, CO,and NO)

This work reports the principle, advantage, and limitations of analytical photoion spectroscopy which has been applied to dissociative photoionization processes for diatomic molecules such as H₂, N₂, CO, and NO. Characteristic features observed in the differential photoion spectra are summarized with a focus on (pre)dissociation of(i) multielectron excitation states commonly observed in the inner valence regions,(ii) shape resonances, and(iii) doubly charged parent ions. Possible origins for negative peaks in the differential spectra are discussed. This spectroscopy is applied to the reported photoion branching ratios for D₂ (and H₂ at high energies). The main findings are as follows: (1) The direct dissociation of theX ²Σ _g ⁺ (1sσ _g ) state of D ₂ ⁺ , the two-electron excited state¹Σ _u ⁺ (2pσ _u 2sσ _g ) of D₂, and the²Σ _u ⁺ (2pσ _u ) state of D ₂ ⁺ appear clearly in the differential spectrum, as previously observed for H₂. (2) Decay of H ₂ ⁺ (D ₂ ⁺ ) to H⁺ (D⁺) above 38 eV is due to the direct dissociation of highly excited states of H ₂ ⁺ (D ₂ ⁺ ) such as the²Σ _g ⁺ (2sσ _g ) and high-lying Rydberg states converging on H ₂ ²⁺ (D ₂ ²⁺ ). (3) In the ionization continuum of H ₂ ²⁺ (D ₂ ²⁺ ) peculiar dissociation pathways are observed. The differential photoion spectra for O₂ derived from the reported photoion branching ratios are also presented. The (pre)dissociation of theb ⁴Σ _g ^? ,B ²Σ _g ^? , III²Π _u ,²Σ _u ^? , and^2,4Σ _g ^? states of O ₂ ⁺ appears as the corresponding positive values in the spectra in accord with previous observations. Some other dissociation pathways possibly contributing to the spectra are discussed including dissociative double ionization.     

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.     

Multi-reference configuration interaction calculations on the systems Xe 2 + and Xe 3 +

Potential curves for the ground (²Σ _u ⁺ ) and the three lowest excited states of the Xe ₂ ⁺ dimer ion (²Π _g ,²Π _u ,²Σ _g ⁺ ) have been calculated using pseudopotentials in MRD-CI (multi-reference single anddouble excitationconfigurationinteraction) calculations. Spin-orbit interaction — leading to the six states 1.(1/2) _u , 1.(3/2) _g , 1.(3/2) _u , 1.(1/2) _g , 2.(1/2) _u , 2.(1/2) _g — has been taken into account using a semiempirical technique [1]. Subsequently, starting with a relaxed Xe ₂ ⁺ ion in its ground state, the potential energy surface for the system Xe-Xe ₂ ⁺ was studied. We found that the collinear approach of the Xe atom leads to the most stable geometry. This is a linear symmetric molecule with bond lengths of 6.38 bohr. In the bestT-shaped structure, the Xe atom is 7.83 bohr away from the midpoint of the Xe ₂ ⁺ (r=6.1 bohr) dimer. The calculated binding energy of 0.25 eV for the equilibrium structure of the Xe ₃ ⁺ molecule (i.e. the linear symmetric geometry), is in very good agreement with experimental results of 0.27 ± 0.02 eV [2].     

The predissociation of Li2 b3Πu by a3Σ+u

⁶Li₂ 1³Δ_g(F₁) → b³Π_u(F₁v = 0–11) rotationally resolved fluorescence spectra are recorded following perturbation-facilitated optical—optical double resonance excitation of 1³Δ_g via spin—orbit mixed A¹Σ⁺_u ∼ b³Π_u(F₁e) intermediate levels. The f-symmetry Λ-components of b³Π_u(F₁) are broadened above the 0.05 cm⁻¹ detection threshold owing to predissociation by the vibrational continuum of the a³Σ⁺_u state. The observed v = 0–11, N = 31f level widths were used to determine the potential energy curve for the Li₂ a³Σ⁺_u state in the region 2.35 < R < 2.60 Å and 11200 < E < 14900 cm⁻¹ (relative to E = 0 at the minimum of X¹Σ⁺_g). The a³Σ⁺_u ∼ b³Π_u curve crossing is at R = 2.57 Å and E = 11246 cm⁻¹ and the electronic part of the − BN·LL-uncoupling matrix element is 〈b Π¦L⁺ ¦aΣ〉 = 1.216H at an R-centroid Rv_bϵ_a = 2.61Å.     

Time and spectrally resolved fluorescence of Cl*2 and ArCl* in Cl2 doped Ar Under state selective pulsed photoexcitation with synchrotron radiation

Synchrotron Radiation is used to selectively excite chlorine and Cl₂ doped argon in the VUV region. Stationary fluorescence and excitation spectra of the 1¹Σ _u ⁺ , 2¹Σ _u ⁺ and 2³Πg Cl ₂ ^* states and of the ArCl*(B?X) transition are obtained. The excitation threshold of ArCl*(B) in Ar/Cl₂ system is found to be 1,285±5 Å and that of ArCl(C) at ～1,260 Å. The formation of ArCl* and Cl*₂(2³Πg) is discussed in terms of recent potential curves data. A detailed time resolved study is reported which allows us to determine precisely the radiative lifetime of ArCl*(B) state (5.2 ns) and numerous kinetic parameters of this system, to estimate theC state energy and to discuss the relaxation and mixing process of the ArCl*(B) and (C) states. A two ladder multilevel kinetic model is described which accounts for the experimental results and shows the difficulty of studying this particular ArCl* system as compared to the closely related XeCl* and KrCl* ones.     

Laser-induced fluorescence in N2 and N+2 by multiple-photon excitation at 266 nm

The fluorescence transitions corresponding to the second positive system of N₂ (C³Π_u → B³Π_g) for Δv = 0, 1 and the first negative system of N⁺₂(B²Σ⁺_u → X²Σ⁺_g) for Δv = 0, 1, 2 have been observed following laser-induced mul excitation of N₂.     

cw Two-photon transitions in K2 molecular beams

We report on the two-photon visible excitation spectra of potassium molecules in a supersonic beam. A dye laser excites K₂ to different (7s ¹Σ_g, 9s ¹Σ_g, 8d ¹Δ_g) Rydberg states. Two-photon transitions are detected by monitoring the resulting (third-photon) ionization. We have made detailed studies, including rotational assignment of 92 such transitions, ¹Δ_g(v¹, J¹) ← X ¹Σ_g(v = 0, J), where the B ¹Π_u state forms the intermediate enhancing state.     

Theoretical spectroscopy and metastability of BeS and its cation

Multiconfiguration self-consistent field and multiconfiguration reference interaction including the Davidson’s correction techniques were employed to calculate the potential energy curves (PECs) of the BeS/BeS⁺ electronic states correlating to the 4/5 lowest dissociation limits. After nuclear motion treatment, we deduced reliable spectroscopic data for the neutral and cationic bound states. For BeS, the transition moments and spin-orbit couplings were also evaluated and used later with the PECs to deduce the rovibronic transition probabilities and the radiative lifetimes in the low-lying states, and to investigate the unimolecular decomposition processes of BeS (X¹Σ⁺, A¹Π, ³Σ⁺ and B¹Σ⁺) leading to Be(¹S_g) + S(³P_g). The prominent mechanism is a spin-orbit induced predissociation via the repulsive BeS(1³Σ⁻) state. Finally, we give the single ionization spectrum of BeS (X¹Σ⁺) populating the BeS⁺ (X²Π, 1²Σ⁻, 1²Σ⁺, 1²Δ, 2²Σ⁺, 2²Π and 3²Π) electronic states. The adiabatic ionisation energy of BeS is estimated to be ∼9.15 eV.     

CAS SCF Cl calculations for the 3Σ−g, 1Σ+g, 3Σ+u, and 5Δu states of Sc2

CAS SCF CI (SD) calculations have been carried out for the ³Σ^?_g, ¹Σ⁺_g, ³Σ⁺_u, and ⁵Δ_u states of Sc₂ using large gaussian basis sets. The ³Σ^?_g, ¹Σ⁺_g, and ³Σ⁺_u states arise from the ²D(4s² 3d¹) + ²D(4s² 3d¹) limit of Sc₂ and are found to be only weakly bound (D_c ≈ 0.06 eV and R_c ≈ 8.0a₀). The ⁵Δ_u state arises from the ²D(4s² 3d¹) + ⁴F(4s¹ 3d¹ 4p¹) atomic limit. This state is found to be strongly bound relative to its limits (D_c ≈ 0.8 eV and R_c ≈ 7.0a₀).     

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.     

Electronic assignments of the violet bands of sodium

The puzzling violet bands of sodium ( ≈ 425-460 nm), known since 1932, are shown conclusively to arise from the superposition of two distinct continuum emission bands - one singlet (2 ¹Σ⁺_u → X ¹Σ⁺_g) and one triplet (primarily 2 ³Π_g → 1 ³Π_u⁺). Each continuum emission system shows complex interference structure arising from multiple branches of the Mulliken difference potential.     

The structure of BO2 in its ground and low-lying excited states from ab initio SCF RHF AND CI calculations

Potential curves for the X²Π_g, A²Π_u, B²Σ⁺_u and C²Σ_g⁺ electronic states of BO₂ were calculated at ab initio SCF RHF and configuration interaction (CI) level. The results obtained are consistent with a linear molecular model for all states considered. The calculated structural parameters and transition energies are in good agreement with relevant experimental data.     

Potential curves of Li2+ by a local potential method

Local potential calculations have been carried out for the first eight ²Σ_g, ²Σ_u and the first five ²Π_g, ²Π_u states of Li₂⁺. The results indicate the usefulness of calculating highly excited potential curves by a local potential method.     

Calculating constants of the rates of the reactions of excitation,ionization, and atomic exchange: A model of a shock oscillator with a change of the Hamiltonian of the system

A new model for calculating the rates of reactions of excitation, ionization, and atomic exchange is proposed. Diatomic molecule AB is an unstructured particle M upon the exchange of elastic-vibrational (VT) energy, i.e., a model of a shock forceful oscillator with a change in Hamiltonian (SFOH). The SFOH model is based on the quantum theory of strong perturbations. The SFOH model allows generalization in simulating the rates of the reactions of excitation, ionization, and atomic exchange in the vibrational-vibrational (VV) energy exchange of diatomic molecules, and the exchange of VV- and VT-energy of polyatomic molecules. The rate constants of the excitation of metastables A ³Σ _u ⁺, B ³Π _g , W ³Δ _u , B′³Σ _u ^-, a′³Σ _u ^-, and the ionization of a nitrogen molecules from ground state X²Σ _g ⁺ upon a collision with a heavy structureless particle (a nitrogen molecule), are found as examples.     

Electronic states of Al2

Ab initio multi-configuration self-consistent field and first-order configuration interaction (FOCI) calculations in an extended basis set have been carried out for the lower energy electronic states of Al₂. The ten core electrons of each Al atom were replaced by an accurate compact effective core potential. The FOCI calculated T_o value for the ³Σ_g^?-³Σ_u^? transition agrees with the experimentally observed emission band to within 90 cm^?1. ³Π_u is calculated to be the electronic ground state of Al₂. Based on FOCI energies and qualitative intensity arguments, the reported optical absorption spectrum of matrix isolated Al₂ also agrees best with a ³Π_u ground state. The ³Σ_g^?1 state is calculated (T_e) at only 324 cm^?1 above the ³Π_u state, and the ¹ΣE_g⁺ state is predicted to lie higher.     

Potential energy and dipole moment of the Na2+ ionic molecule

The potential energy curves of 26 electronic states of ²Σ⁺_{g, u}, ²Π_{g, u}, and ²Δ_{g, u} symmetries of the alkali dimer Na₂⁺, dissociating up to Na(4d) + Na⁺, are investigated using an ab initio approach involving a nonempirical pseudopotential for the Na⁺(1s²2s²2p⁶) core and core‐valence correlation corrections. Furthermore, the transition dipole functions between many electronic states and vibrational energy spacings are presented. The spectroscopic constants of these electronic states are extracted and compared with the available theoretical and experimental results. A very good agreement is observed, especially, for the ground and the first excited states. However, the comparison between our study and the model potential (MP) calculations (Magnier and Masnnou‐Seeuws Mol. Phys. 1996, 89, 711) for several states has shown a clear disagreement. The MP well depths of the 3‐4²Σ⁺_g, 1²Π_g, 3‐4²Π_g, and 2²Π_u electronic states are largely underestimated. In addition, the 5‐7²Σ⁺_g, 3‐7²Σ⁺_u, 2²Π_g, 4²Π_g, and 1‐2²Δ_u MP electronic states are repulsive, although in this work, they are attractive with potential well depths of some hundreds of cm^?1. The data presented in this study are very useful for studies on ion–atom interaction and cold collision in the presence of electromagnetic fields. © 2013 Wiley Periodicals, Inc.     

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.