Multiconfigurational study on the synchronous mechanisms of the ClO self-reaction leading to Cl or Cl<Subscript>2</Subscript>

For studying the adiabatic and nonadiabatic mechanisms of the ClO (X ²Π) + ClO (X ²Π) → ClOOCl → 2Cl (² P _u) + O₂ (X ³Σ _g ⁻) reaction (1) and the ClO (X ²Π) + ClO (X ²Π) → ClOOCl → Cl₂ (X ¹Σ _g ⁺) + O₂ (X ³Σ _g ⁻) reaction (2), we calculated, by partial geometry optimizations under the C₂ constraint, the O–O and O–Cl dissociation potential energy curves (PECs) from the five low-lying states of ClOOCl at the CASPT2 level. The CASSCF minimum-energy crossing point (MECP) between the potential energy surfaces of the 1 ¹A ground state [correlating with the product of reaction (1)] and the 1 ³B state [correlating with the product of reaction (2)] states was also determined. Based on the CAS calculation results (PECs, energies, and spin–orbit coupling at the MECP), we predict that reaction (1) occurs along pathway 1: ClO (X ²Π) + ClO (X ²Π) → ClOOCl (1 ¹A) → 2Cl (² P _u) + O₂ (X ³Σ _g ⁻) and that reaction (2) occurs along pathway 2: ClO (X ²Π) + ClO (X ²Π) → ClOOCl (1 ¹A) → 1 ¹A/1 ³B MECP (142.4 cm⁻¹) → ClOOCl (1 ³B) → Cl₂ (X ¹Σ _g ⁺) + O₂ (X ³Σ _g ⁻). The needed energies (relative to the reactant) for pathways 1 and 2 are predicted to be 5.3 and 11.1 kcal/mol, respectively, which indicates that reaction (1) is more favorable than reaction (2). The present work supports the traditional photochemical model for ozone degradation: ClOOCl (1 ¹A), formed by two ClO (X ²Π), can directly produce O₂ plus two Cl atoms.     

Calculating the molecular constants for the B1Πu, C1Πu, (1)1Πg, and (2) Σ g+ electronic states of the rubidium dimer

The vibrational, rotational, and centrifugal constants are calculated for the B ¹Π _u , C ¹Π _u , (1) ¹Π _g , and (2) electronic states of a ⁸⁵Rb₂ molecule. The calculations are based on the semi-empirical potential curves obtained in this work. The results from calculating the molecular constants are compared with experimental data. The Franck-Condon factors and R _v′v″ centroids are calculated for the electronic transitions B ¹Π _u -X ¹Σ _g ⁺, C ¹Π _u -X ¹Σ _g ⁺, C ¹Π _u -(1) ¹Π _g , and C ₁Π _u -(2) ¹Σ _g ⁺.     

Spectroscopic Analysis of a Pulsed-Laser Deposition System for Fullerene-like Cn x Film Production

Plasma produced by a (1064 nm) Nd:YAG laser focused onto a graphite target at different nitrogen pressures in the range of 1–90 mTorr, was studied spectroscopically. In the spectral range of 350–600 nm, emission lines of CI neutral carbon (501.12, and 505.21 nm), NI neutral nitrogen (493.5 nm), CII (426.72, 463.7, 515.11 nm), and CIII ions (465.02 and 569.59 nm), and NII ions (501.06, and 500.73 nm), were dominating. Bands of C₂ Swan (d³Π_g → a³Π_u, Δ ν=2, 1, 0, −1), and CN Violet (B²Σ ⁺→ X²Σ⁺, Δ ν=1, 0, −1) systems, and ionic emissions from the First Negative system N2⁺ (band head at 391.44 nm), were faintly observed under our specific experimental conditions. From the band intensities, vibrational temperature for CN and C₂ was calculated to be 1.25 and 0.31 eV at 90 mTorr, respectively. The electron density and temperature, measured by Stark broadening, assuming a local thermodynamic equilibrium (LTE), were found to be 2.1× 10¹⁷ cm⁻³ and 0.33 eV at 1mTorr, respectively. The validity of the LTE is discussed according to the results discussed. Pressure dependence shows a decrease in the vibrational temperature when nitrogen pressure increases, while the electron density and temperature increase.     

Should a franck-condon or a curve-crossing picture be applied to ion-target collisional activation? A study of keV CO<Stack><Subscript>2</Subscript><Superscript>+·</Superscript></Stack>/He collisions by emission spectroscopy

Collision-induced photon emissions (CIE) were observed for keV CO₂^+·/He collisions from 190 to 1020 nm. The emissions were assigned to the Δν=0 band of the CO₂^+· B ²Σ_u⁺ → X ²Π_g electronic transition and the Δν = +3, +2, +1, 0, −1, −2, −3 vibrational transition progression in the CO₂^+· A ²Π_u → X ²Π_g electronic transition. The other peaks arise from the emissions of excited O^· fragment atoms and the target gas. The relative intensities of the CO₂^+· and O^· emissions are independent of the ion translational energy above 3 keV, supporting the curve-crossing mechanism for collisional excitation. Investigation of the relative intensities within the A ²Π_u → X ²Π_g emission of CO₂^+· indicates that the vibrational distribution is well described by the Franck-Condon principle at high collision energy, a consequence of short collision time but not necessarily an indication of vertical transitions. Below 3 keV ion translational energy, vibrational excitation in the A ²Π_u electronic state was observed. The observation is consistent with the explanation that the reaction occurs at small impact parameters, in which short-range, repulsive interactions between the projectile and the target result in direct translational-vibrational excitation.     

The QCT calculation of the rate constants for the N(4S)+O2(X3Σ g ? ) →NO(X2Π)+O(3P) reaction

A quasi-classical trajectory (QCT) calculation with the fourth-order explicit symplectic algorithm for the N(⁴S) + O₂(X³Σ_g⁻) → NO(X²Π) + O(³P) reaction has been performed by employing the ground and first-excited potential energy surfaces (PESs). Since the translational temperature considered is up to 5000 K, the larger relative translational energy and the higher vibrational and rotational level of O₂ molecule have been taken into account. The affect of the relative translational energy, the vibrational and rotational level of O₂ molecule in the reaction cross-sections of the ground and first-excited PESs has been discussed in a extensive range. And we exhibit the dependence of microscopic rate constants on the vibrational and rotational level of O₂ molecule at T = 4000 K. The thermal rate constants at the translational temperature betweem 300 and 5000 K have been evaluated and the corresponding Arrhenius curve has been fitted for reaction (1). It is found by comparison that the thermal rate constants determined in this work have a better agreement with the experimental data and provide a more valid theoretical reference.     

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.     

Franck-Condon factors and r -centroids for the E 1Πu- X 1Σ g+ system of Ag2

An array of Franck-Condon factors and r-centroids is reported for the bands of the E ¹Π_u-X ¹Σ _g ⁺ system of the Ag₂ molecule. Both parameters were calculated using the Morse and Rydberg-Klein-Rees potentials. The results showed a reasonable agreement between the two sets of data for transitions, including lower vibrational levels (v ≤ 6). Differences appear with increasing v, but two sets of calculated Franck-Condon factors follow the same pattern. The predicted intensity distribution was compared with the estimated band intensity in the experimental spectrum. The text was submitted by the authors in English.     

Spin-orbit and spin-spin couplings in He2 and He2−

The spin-orbit and the spin-spin coupling constants of the ⁴Π_g state of the He₂ ⁻ ion, of the parent a³Σ_u ⁺, and of the b³Π_g states of He₂ have been evaluated by a multireference configuration interaction method. The theoretical spin-spin splittings of the a³Σ_u ⁺ state and the R-dependent spin-spin function are found to be in excellent agreement with experiment, with deviations in the range of a few MHz. The theoretical spin-orbit constants and splittings of the b³Π_g state are larger than the experimental values by about 370 MHz. The spin-orbit coupling constant of the ⁴Π_g state of He₂ ⁻ is␣estimated to be three times smaller than in the b³Π_g state, but one of the intramultiplet off-diagonal spin-spin interactions is predicted to give a large contribution to the fine structure of the metastable ion. The theoretical fine structure constants for the He₂ ⁻ ion are expected to␣aid future spectroscopic investigations of the fine structure splittings of the negative ion. Received: 14 April 1998 / Accepted: 27 July 1998 / Published online: 19 October 1998     

Evidence for charge exchange between N and N2(A Σu) in a low-temperature nitrogen plasma

The emission from the first negative system, N₂⁺(B ²Σ⁺_u)→N₂⁺(X ²Σ⁺_g)+hν, is studied in the flowing nitrogen afterglow of a DC arc plasma. Investigation of the spectrum shows overpopulation of the vibrational levels 6 and 7 of the excited molecular ion, N₂⁺(B ²Σ⁺_u). Selective excitation of these levels is explained by a charge exchange reaction between atomic ions in the ground state and metastable molecules in the N₂(A ³Σ⁺_u) state. The emitted intensity of the first negative system is shown to be linear with electron density n_e for n_e>2×10¹⁶ m⁻³, a higher-order dependence exists below this value. This is consistent with population of N₂⁺(B ²Σ⁺_u) by atomic ions, N⁺.     

Recombination energy of N22+

The recombination energy of N₂²⁺ has been computed using N₂²⁺, N₂²⁺ and N₂ potential curves from the literature. Vibrational overlaps and energies liberated in the various N₂²⁺³∑^?_g,¹∑_g⁺, ³Π_u, ¹Π_u → N₂⁺(X²∑⁺_g, A ²∑⁺_g, A ²Π_u, B²∑_u⁺,C²∑_u⁺) vibronic transitions have been computed and used as input for determination of the N₂⁺ recombination energy.     

Computation of electronic transition moments: the length versus the velocity representation

Summary We have compared transition moments (TMs) obtained using the length and velocity representations for transitions from the ground state of H₂ to the lowest two¹ _u and two¹ _u ⁺ Rydberg states, theA ¹–X ¹⁺ transition in BH, and theA ¹ _u –X ¹ _g ⁺ transition in C₂. For H₂, the TMs in the length and velocity representations agree well even in cases where the one-particle basis is incomplete and the TM has not converged. For BH and C₂ the TM in the length representation converges rapidly with improvements in the one-particle basis set and is insensitive to inner-shell correlation. In contrast, in the velocity representation convergence with improvements in the one-particle basis is much slower, especially for C₂, and the TMs are significantly changed by inner-shell correlation. Thus the difference between the TMs in the length and velocity representations would not appear to be a viable diagnostic of TM convergence.     

Dissociative recombination of HeH+. I. Rovibrational spectrum of HeH Rydberg states

The repulsive ground electronic state X²Σ⁺ of HeH is strongly coupled to the Rydberg states at small interatomic distances. Such large couplings also occur between some of the Rydberg states. HeH⁺ ions that capture an electron in a Rydberg state end up in separated He and H atoms by indirect predissociation. This paper presents a study of potential functions and pertinent matrix elements involving the lowest electronic states: the ²Σ⁺ states, X, A, C, and D, and the ²Π states B and E. Individual transition rates as well as total radiative and non-radiative lifetimes have been computed for the lowest vibrational and rotational levels. Received: 22 June 1998 / Accepted: 21 August 1998 / Published online: 12 October 1998     

Mechanism of vibrational relaxation and intersystem crossing within excited ion-pair states of I2

Visible and ultraviolet fluorescence of I₂, following excitation by ArF/193nm excimer laser pulses, was recorded for different pressures of argon buffer gas in a flow system. Dispersed fluorescence spectra due to the transitionsD’(2_g) → A’(2_y andD(0 _n ⁺ )→X0 _g ⁺ ) were analysed by inversion and spectral simulations. Thus vibrational distributions in the emitting states were obtained as a function of pressure to determine the mechanism of relaxation to populate the lowest quantum levels of theD’ state, which are the emitting states in the iodine laser. Fast intersystem crossing is found to occur from initially populated vibrational levels of theD state to other ion-pair states correlating with the ground state ions, followed by rapid relaxation, involving both direct vibrational relaxation within individual states and intersystem crossing between states.     

High-Resolution Experimental Study on Photodissocaition of N2O

We study the photodissociation dynamics of nitrous oxide using the time-sliced ion velocity imaging technique at three photolysis wavelengths of 134.20, 135.30, and 136.43 nm. The O(¹S_J=0)+N₂(X¹∑_g⁺) product channels were investigated by measuring images of the O(¹S_J=0) products. Vibrational states of N₂(X¹∑_g⁺) products were fully resolved in the images. Product total kinetic energy releases (TKER) and the branching ratios of vibrational states of N₂ products were determined. It is found that the most populated vibrational states of N₂ products are v=2 and v=3. The angular anisotropy parameters (β values) were also derived. The β values are very close to 2 at low vibrational states of the correlated N₂(X¹∑_g⁺) products at all three photolysis wavelengths, and gradually decrease to about 1.4 at v=7. This indicates the dissociation is mainly through a parallel transition state to form products at lower vibrational states, and the highly vibrational exited products are from a more bent configuration. This is consistent with the observed shift of the most intense rotational structure in the TKER as the vibrational quantum number increases.     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

Theoretical treatment of predissociation of the CO (3sσ) B and (3pσ) C1Σ+ Rydberg states based on a rigorous adiabatic representation

Ab initio multireference configuration interaction calculations for adiabatic potential curves, nonadiabatic couplings 〈φ _i (R,r)|d/dR|φ _j (R,r)〉 and 〈φ _i (R,r)|d²/dR ²|φ _j (R,r)〉, and nuclear kinetic energy corrections 〈dφ _i (R,r)/dR|dφ _i (R,r)/dR〉 for the (3sσ) B and (3pσ) C¹Σ⁺ Rydberg states of the CO molecule have been carried out. The energy positions and predissociation linewidths for the observed vibrational levels of these two states have been determined in a rigorous adiabatic representation by the complex scaling method employing a basis of complex scaled harmonic vibrational functions in conjunction with the Gauss-Hermite quadrature method to evaluate the complex Hamiltonian matrix elements. The present treatment correctly reproduces the observed trends in energies and line broadening for vibrational levels of the B¹Σ⁺ state and represents an improvement over the previous treatment in literature. The errors in the determined spacings of the v = 0–4 vibrational levels of the C¹Σ⁺ state are less than 2% compared with measured data. The predissociation linewidths for the v=3,4 levels of the C¹Σ⁺ state are found to be 4.9 and 8.9 cm⁻¹, respectively, in good agreement with the observed values. Received: 23 March 1998 / Accepted: 27 July 1998 / Published online: 9 October 1998     

Vacuum UV emission by electronically-excited N2: The radicative lifetime of the N2(a′1Σ−u state

A measurement of the electronic transition moment variation for the N₂(a'¹Σ^?_uX¹Σ^+g) band system has allowed a reassessment of the radiative lifetime of N₂(a′). Relaxation to N₂(a′,υ=0) is established as the major channel for quenching of N₂(a¹Π_g, υ = 0) molecules by Ar.     

Vibrational energy transfer from N2 to CO on electron excitation of N2-CO solids at 4.2 K

Spectra emitted from 0.1% CO-N₂ solids excited with high energy electrons at 4 K show evidence for resonant transfer of vibrational energy from highly excited vibrational levels of N₂ to CO in the process N₂(X¹Σ_g⁺, ν) + CO(ν = 0) → N₂(X¹Σ_g⁺, ν - 1) + CO(ν = 1) + phonons. Energy transfer from levels with ν ? 9 has been observed.     

Low-lying electronic states of HNCS and its ions: a CASSCF/CASPT2 study

Complete active space self-consistent-field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) calculations in conjunction with the ANO-L basis set were performed to investigate systematically the low-lying electronic states of HNCS and its ions in C _s symmetry. Our highly accurate calculation indicated that theoretically determined geometric parameters and harmonic vibrational frequencies for the ground-state X ¹A′ are in good agreement with observed experimental data. The geometry of triplet HNCS is clearly favored C ₁ symmetry, and the relative energy is predicted to be 3.000 eV (69.2 kcal/mol). The vertical transition energies for the selected excited states of HNCS were calculated at CASSCF/CASPT2/ANO-L level of theory based on CASSCF optimized geometry. Except for a few linear states of X ²Π (1²A′, 1²A″), 1⁴Σ⁻ (1⁴A″), and 1²Σ⁺ (3²A′) states of HNCS⁺, our results confirmed that the majority of excited states are twisted trans-bend structures. The existence of bound excited anion states has been found for the first time in HNCS⁻. A more elaborate examination of ionization potential of HNCS (AIP, VIP) than previous reports has been presented.