Fourier spectrometry investigation of the 1 3Σg+ →a 3Σu+ transition of the 7Li2 molecule

The 1 ³Σ_g⁺ → a ³Σ_u⁺ transition in the ⁷Li₂ molecule has been observed in the 8200–10 000 cm⁻¹ region with a high resolution Fourier spectrometer. Rotational analysis of 1 ⩽ υ′ ⩽ 7 of 1 ³Σ_g⁺ and 0 ⩽ υ″ ⩽ 7 of a ³Σ_u⁺ has been carried out. We found D_e(a ³Σ_u⁺) = 332.5 ± 1.0 cm⁻¹ that gives T_e(a ³Σ_u⁺) = 8184.3 ± 1.5 cm⁻¹ and D_e(1 ³Σ_g⁺) = 7090.4 ± 1.5 cm⁻¹ with T_e = 16330 ± 2 cm⁻¹.     

The first two excited 1Σg+ states of Cs2

Laser-induced fluorescence Of Cs₂ molecules in the infrared region (4000–9000 cm^?1) has been observed using several exciting wavelengths from an argon-ion laser and from a ring dye laser. Accurate molecular constants for the first two excited ¹Σ_g⁺ electronic states are derived from spectra recorded at high resolution by Fourier transform spectroscopy. Main molecular constants are: (2)¹Σ_g⁺: T_c = 12114.090 cm^?1, ω_e = 23.350 cm^?1, B_c = 7.4.5 × 10^?3 cm^?1, R_c = 5.8316 Å; (3)¹Σ_g⁺: T_e = 15975.450 cm^?1, ω_e = 22.423 cm^?1 , B_e = 8.23 × 10^?3 cm^?1, R_c = 5.5569 Å.     

Resonance-enhanced multiphoton ionization of molecular hydrogen via the E,F1Σg+ state: Photoelectron energy and angular distributions

Photoelectron energy and angular distributions are measured for the 2+1 multiphoton ionization process H₂ X¹Σ_g⁺ (ν = 0,J) + 2hv → E,F¹Σ_g⁺(ν_E,J_E = J) + hν → H₂⁺X²Σ_g⁺ (ν⁺) + e^?, for ν_E = 0, 1, or 2 and for J_E = 0 or 1 of the inner well of the double-minimum E,F state. Although a strong preference is found for ν⁺ = ν_E, the detailed H₂⁺ vibrational distribution does not exhibit Franck-Condon behavior, and the photoelectron angular distributions vary markedly with both the J_E value of the intermediate state and the ν⁺ value of the ion.     

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.     

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.     

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.     

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.     

The pluvinage method for alkali dimers: Calculations for the Σ excited states of Na2 up to the (3p + 3p) dissociation limit

Twenty-six Σ potential curves of Na₂ are computed using a correlated orbital method. Rydberg series are seen converging to a bonding or antibonding Na₂⁺ curve. Crossings with the Na₂⁺ X ²Σ_g⁺ curve occur for the 8 ¹Σ_g⁺, 6 ¹Σ_u⁺, 6 ³Σ_u⁺, 7 ³Σ_u⁺, 1 ¹Σ_u⁻, 8 ³Σ_u⁺ curves at internuclear distances R = 5.4, 6.5, 6.6, 8.2, 9.0 and 9.4 au, respectively.     

Case studies in multiphoton ionisation and dissociation of Na2

Dissociative ionisation of Na₂ via the 3s 3d ¹Σ _g and¹Π _g states has been studied in the near threshold energy regime up to 120 meV above the three particle (Na⁺ + Na(3s) +e ^?) break up limit. A pulsed, cold molecular beam, pulsed laser 2 colour 3 photon resonantly enhanced multiphoton ionisation, and kinetic energy analysis of the fragments by a time of flight method (KETOF) is used. As series of vibrational levels in the two intermediate 3s 3d Rydberg states are excited, slow Na⁺ fragments are observed with a maximum kinetic energy given by the excess energy of the 2 + 1 photon process above threshold, thus confirming a direct dissociative ionisation process. The intensity distribution of the Na⁺ fragments shows a very pronounced maximum at zero kinetic energy, its shape differing somewhat for the¹Σ _g and¹Π _g intermediate states. Also observed is a strong signal of fast fragments arising from a typical 4 photon process which leads to dissociation of Na ₂ ⁺ molecules in their electronic ground state.     

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.     

A joint vibro-electronic mechanism in the dissociation of molecular hydrogen in nonequilibrium plasmas

A new mechanism Of H₂ dissociation in electrical discharges (10¹¹ ? n_e ? 10¹² cm^?3, 2.10^?16 ? E/N ? 3.10^?16 V cm², 300 ? T_g ? 1000 K, 3 ? p ? 30 torr) is presented and discussed. In this mechanism, called joint vibro-electronic mechanism (JVE), the electrons of the discharge create a strong vibrational disequilibrium with respect to the gas temperature (T_g) and promote electronic transitions from all vibrational levels of ¹Σ_g H₂ state to the repulsive ³Σ_u one. Moreover the V-V (vibration-vibration) and V-T (vibration-translation) energy exchanges are considered for building up the vibrational distribution of ¹Σ_g state. A complete set of e - D cross sections (e + H₂(¹Σ_g,ν) → e + H₂ (³Σ_u) → + 2H, ν = 0,14) is calculated by using an extension of the semiclassical Gryzinski theory in combination with the Franck-Condon principle. Dissociation rates calculated according to JVE are larger either than those obtained by the pure vibrational mechanism (PVM) discussed in our previous work or than those from the direct electronic impact mechanism (DEM) from the ground vibrational level. The behaviour of JVE rates as a function of gas temperature (T_g), of E/N, of electron density (n_e) and of pressure is then reported. The results show strong differences as compared, with the corresponding values obtained, with PVM. Finally the influence of the atoms as well as their recombination on the dissociation rates is discussed. The results have been obtained by solving a system of vibrational master equations.     

Laser-induced fluorescence in supersonic nozzle beams predissociation in the Rb2 C1πu and D 1πu states

By exciting Rb₂ in a supersonic nozzle beam with a pulsed dye laser in the C ¹Π_u-X ¹Σ⁺_g and the D ¹Π_u-X ¹Σ⁺_g band system, we find evidence tor different predissociation processes The products appear as follows from the C state, Rb^* (5 ²P_{$\text{3}\text{2}$}) exclusively, and from the D state Rb*(4²D_{$\text{3}\text{2}$}) predominantly, followcd by Rb^*(5 ²P_i-5²S) cascade radiation In addition, a lower bound of D_e(Rb₂X¹Σ⁺_g)? 3939± 10 cm^?1 is obtained.     

Geometric and electronic structure of ground and excited states of groupVA diatomics. A theoretical LCGTO-MP-LSD study

LCGTO-MP-LSD calculation was performed for the ground and several low-lying excited states of homo- (N₂, P₂, As₂, and Sb₂) and hetero-nuclear (PN, AsN, AsP, AsSb, SbN, and SbP) groupVA diatomics. For all the systems the ground state is found to be¹Σ⁺. For N₂ and P₂, the¹Σ _g ⁺ ground state is followed by the³Σ _u ⁺ ,³Π _g ,³Δ _u ,¹Π _g , and¹Δ _u low-lying exited states while for As₂ the order is found to be³Σ _u ⁺ ,³Δ _u ,³Π _g ,¹Δ _u ,¹Π _g . Finally for Sb₂ the relative stability of excited states is³Σ _u ⁺ ,³Δ _u ,¹Δ _u ,³Π _g ,¹Π _g . For the hetero-nuclear diatomics the¹Σ⁺ ground state is, in the case of PN, AsN, AsP, SbN, and SbP, followed by the³Σ⁺,³Δ,³Π,¹Π and¹Δ low-lying excited states while for the AsSb diatomic an inversion of stability of the two last singlets occurs. The calculated spectroscopic parameters (Re, ωe, andDe) are in good agreement with all the available experimental results while, theTe values are overestimated by about 0.5 eV. Mulliken population analysis shows that both homo- and hetero-nuclear groupVA diatomics are essentially triple bonded systems.     

Theoretical Determination of the X1Σ+g potential of Cs2 using relativistic effective core potentials

Theoretical potential energy curves are computed for the X¹Σ⁺_g state of Cs₂ using relativistic effective core potential and a large valence gaussian basis set. Eighteen electrons are correlated by a four-reference MC SCF Cl(SD) procedure. Our best calculation (with experimental values in parenthesis) gave R_e = 9.05 (8.78) bohr and D_e = 3141 (3648 ± 8) cm?1.     

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.     

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.     

Analysis of bound-free fluorescence and improved characterization of the electronic and spectroscopic properties of the 11Σ u + state of Cl2

Synchrotron radiation is used to selectively excite the chlorine molecule in the VUV spectral range. Stationary fluorescence spectra of the 1¹Σ _u ⁺ state are observed following primary excitation of 1¹Σ _u ⁺ and 2¹Σ _u ⁺ . The bound-free part of the spectra is analysed with the aid of quantum mechanical computer simulations. A potential energy curve is constructed which is an approximation of the adiabatic double well potential energy curve of the 1¹Σ _u ⁺ state. The inner well is characterized byT _e =(73428±50) cm^?1,r _e =(1.85 ± 0.05) Å; for the outer well holdT _e =(64631±50) cm^?1,r _e =(2.57±0.05) Å, ω _e =(261±5) cm^?1, ω _e x _e =(0.668±0.01) cm^?1 (³⁵Cl₂;v′<30). The potential energy curve is successfully checked with fluorescence excitation spectra. Within the error limits, the results of a former synchrotron radiation study are verified. It is ruled out, that the Cl₂ “γ-state” recently observed with laser spectroscopic methods, can be attributed to the outer well of 1¹Σ _u ⁺ .     

The 6Liz A1Σu+ ∼ b3Πu spin—orbit perturbations: Sub-Doppler spectra and steady state kinetic lineshape model

The ⁶Li₂ A¹Σ_u⁺ υ_A = 2, J = 33 and υ_A = 9, J = 20 levels are shown to be spin—orbit perturbed by the b³Π_u υ_b = 9, F₁e N = 32 and υ_b = 15, F₁e N = 19 levels from which an electronic matrix element of <b³Π_oc|H^SO|A¹Σ⁺ > = 0.114±0.006 cm^?1 is determined. Previous estimates of this quantity are shown to be incorrect. Although the main and extra levels are separated by less than the 900 K Doppler width of A¹Σ_u⁺ ? X¹Σ_g⁺ rotational lines, sub-Doppler intermodulated fluorescence and perturbation-facilitated optical—optical double resonance spectra allow direct observation of the separation of main and extra levels. The mixing coefficients and other perturbation parameters are inferred from a steady state kinetic model of the composite main plus extra lineshape.