Dissociation of vibrational state-selected O2(+) Ions in the B(2)Σ(g)¯ state using threshold photoelectron-photoion coincidence velocity imaging

Using the recently developed threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging mass spectrometer (Tang et al. Rev. Sci. Instrum.2009, 80, 113101), dissociation of vibrational state-selected O(2)(+)(B(2)Σ(g)(ˉ), v(+) = 0-6) ions was investigated. Both the speed and angular distributions of the O(+) fragments dissociated from individually vibronic levels of the B(2)Σ(g)(ˉ) state were obtained directly from the three-dimensional time-sliced TPEPICO velocity images. Two dissociation channels, O(+)((4)S) + O((3)P) and O(+)((4)S) + O((1)D), were respectively observed, and their branching ratios were found to be heavily dependent on the vibrational states. A new intersection mechanism was suggested for the predissociation of O(2)(+)(B(2)Σ(g)(ˉ)) ions, especially for dissociation at the energy of the v(+) = 4 level. In addition, the anisotropic parameters for O(+) fragments from different dissociative pathways were determined to be close to zero, indicating that the v(+) = 0-6 levels of B(2)Σ(g)(ˉ) predissociate on a time scale that is much slower than that of molecular rotation.     

The X1Σ+ ground state of kh near the dissociation limit

Fluorescence excited in the A¹Σ⁺ -X¹Σ⁺ system of ³⁹KH by the 4880 Å argon-ion laser line gives information about the ground state as far as the last bound rovibrational level. This is identified as J = 6 in v = 23, and, assuming a limit midway between J = 6 and J = 7, D_e(KH) = 14776 ± 4 cm⁻¹.     

(2+1) REMPI of NO via the D 2Σ+ state: rotational branching ratios

Recent photoelectron spectroscopic studies in a (2 + 1) REMPI of NO via the Rydberg D²Σ⁺ state have revealed anomalous ionic rotational branching ratios. We have performed ab initio calculations of these branching ratios and find that the molecular nature of the ionization continuum plays an essential role in the dynamics. Even though the bound orbital is very atomic-like (⪢ 98% p-like), the photoelectron continuum wavefunction is quite sensitive to the non-spherical nature of the molecular ionic potential and causes a strong persistence of the p-partial wave which, in turn, leads to a large ΔN = 0 peak.     

Prediction of the rovibrational emission spectroscopy of B2Σ(+)-X2Σ(+) system in 12C17O+

An analytical formula based on the Herzberg's conventional rovibrational energy levels for diatomic system is proposed by taking multiple differences of spectral lines to predict the R-branch high-lying rovibrational emission spectroscopy, where only 15 accurate known transition lines and rotational constants D(v'), D(v') are needed. Using the formula, the R(11ee) and R(22ff) branches of (0, 2) and (0, 3) transition bands in the B(2)Σ(+)-X(2)Σ(+) system of (12)C(17)O(+) are studied. The results show that not only the relatively lower order rovibrational transition lines given by experiments are reproduced but also the higher and the absent spectral lines are correctly predicted for each band.     

Excited state N2+(B̃) fragments from the photodissociation of N4+ at 270–290 nm

The photodissociation spectrum of N₄⁺ from 270 to 310 nm is broad and structureless at 300 K, with the cross section diminishing by a factor of 10 towards shorter wavelengths. We have detected fluorescence from N₂⁺(B̃) excited state fragments having quantum yield < 0.01 and risetime < 10 ns. The threshold at λ ≈ 300 nm indicates no exit channel barrier.     

Alignment effect of N2(A3Σu+) in the energy transfer reaction of aligned N2(A3Σu+) + NO(X2Π) → NO(A2Σ+) + N2(X1Σg+)

Steric effect in the energy transfer reaction of N(2)(A(3)Σ(u)(+)) + NO(X(2)Π) → NO(A(2)Σ(+)) + N(2)(X(1)Σ(g)(+)) has been studied under crossed beam conditions at a collision energy of ~0.07 eV by using an aligned N(2)(A(3)Σ(u)(+)) beam prepared by a magnetic hexapole. The emission intensity of NO(A(2)Σ(+)) has been measured as a function of the magnetic orientation field direction (i.e., alignment of N(2)(A(3)Σ(u)(+))) in the collision frame. A significant alignment effect on the energy transfer probability is observed. The shape of the steric opacity function turns out to be most reactive at the oblique configuration of N(2)(A(3)Σ(u)(+)) with an orientation angle of γ(v(R)) ~ 45° with respect to the relative velocity vector (v(R)), which has a good correlation with the spatial distribution of the 2pπ(g)* molecular orbital of N(2)(A(3)Σ(u)(+)). We propose the electron exchange mechanism in which the energy transfer probability is dominantly controlled by the orbital overlap between N(2)(2pπ(g)*) and NO(6σ).     

Quantum state-selected photodissociation dynamics of H2O: two-photon dissociation via the C̃ electronic state

The photodissociation dynamics of H(2)O via the C? state by two-photon excitation has been investigated using the H atom Rydberg tagging time-of-flight technique. The rotational resolved action spectrum of the C?←X? transition band has been measured. The line widths show a pronounced dependence on the parent rotational excitation in the C? state. The quantum state resolved OH product translational energy distributions and angular distributions have also been obtained. By carefully simulating these distributions, quantum state distributions of the OH product as well as the state-resolved angular anisotropy parameters were determined. The experimental results confirm the variation of two competitive predissociation pathways. A heterogeneous predissociation channel is mediated by rotational coupling to the B??(1)A(1) state associated with the a-axis (k(a)(')), and a homogeneous pathway arises from purely electronic coupling to the A??(1)B(1) state. We have also obtained the branching ratios of the OH(X) and OH(A) products, and related these to the C?→A? and C?→B? pathways. The branching ratios display a strong k(a)(') dependence.     

On the inhibition of positronium formation by NO3− ions in different solvents

In previous papers [1, 2] the inhibition of the Ps formation by certain solutes was investigated and a new equation based on a hot Ps reaction model was proposed for describing the dependence of the Ps survival probability on the inhibitor concentration. In this equation a constant K appears which depends both on the reaction cross section of the solute and on the moderating power of the solvent for hot Ps atoms. It was also hypothesized that the moderating power of the solvent molecule should be linearly correlated with the number of its vibrational modes. In the present paper it is reported that the experimental results agree with this hypothesis. The results are also discussed in the framework of the model mentioned above.     

The ν2 bending vibrational structure of the X̃2Σ+ state of MgNC

We have generated MgNC in supersonic free jet expansions and observed the laser induced fluorescence (LIF) of the A?(2)Π-X?(2)Σ(+) transition. We measured the LIF dispersed spectra from the single vibronic levels of the A?(2)Π electronic state of MgNC, following excitation of each ν(2) bending vibronic band observed, i.e., the κ series of the (0,v(2)('),0)-(0,0,0), v(2)(') = 0, 1, 2, 4, and 6 vibronic bands. In the vibrational structure in the dispersed fluorescence spectra measured, the long progression of the ν(2) bending mode in the X?(2)Σ(+) state is identified, e.g., up to v(2)(')=14 in the (0,6,0)-(0,v(2)('),0) spectrum. This enables us to derive the potential curve of the ν(2) bending mode in the X?(2)Σ(+) state. We used two kinds of models to obtain the potential curve; (I) the customary formula expressed in the polynomial series of the (v(2)(')+(d(2)/2)) term and (II) the internal rotation model. The potential curve derived from model (I) indicates the convergence of the bending vibrational levels at about 800 cm(-1) from the vibrationless level of MgNC, which may correspond to the barrier height of the isomerization reaction, MgNC ? MgCN, in the X?(2)Σ(+) state. Model (II) gives a simple picture for the isomerization reaction pathway with a barrier height of about 630 cm(-1) from the vibrationless level of the more stable species, MgNC. This shows that the v(2)(')=8 bending vibrational level of MgNC is already contaminated by the v(2)(')=2 bending vibrational level of the isomer, MgCN, and implies that the isomerization reaction begins at the v(2) (')=8 level. The bending potential surface and the isomerization reaction pathway, MgNC ? MgCN, in the X?(2)Σ(+) state are discussed by comparing the potential derived in this study with the surface obtained by quantum chemical calculation.     

Vibrational energies of LiH2(+) and LiD2(+) in the Ã1Σ+ electronic state

In connection with the recent study of the ground electronic state of the LiH2(+) molecular ion (Kraemer, W. P.; Spirko, V. Chem. Phys. 2006, 330, 190), the adiabatic three-dimensional double-minimum potential enery surface of the first excited electronic state was evaluated, including its two lowest atom-diatom dissociation channels as well as the three-atom complete fragmentation asymptote. Applying the Sutcliffe-Tennyson Hamiltonian for triatomic molecules, the levels of all bound vibrational states and the levels of the states localized in the two energy minimum regions were separately determined. The validity of statistical methods such as the density of states approach and the nearest-neighbor level spacing distribution (NNSD) was tested for the light LiH2(+) ion. Special effort was put into investigating possible effects of a tunnelling motion across the proton-transfer barrier on the vibrational level pattern using the NNSD approach.     

Nodal surfaces of the wave functions of the hydrogen molecule in the triplet state 3Σu +

For molecular hydrogen in the triplet state ³Σ_u ⁺, the nodal surfaces of the wave function corresponding to the minimum basis set of Slater orbitals in the Hartree—Fock approximation and those of the wave function used in calculations by the diffusion quantum Monte Carlo method were plotted and analyzed. Taking account of the condition for antisymmetrical wave function of the triplet state ³ S of He atom, the Hartree-Fock approximation in the minimum basis set of one-electron orbitals is inappropriate for a priori determination of the nodal surfaces of many-electron wave functions (MWF). An MWF quantum chemical method developed by the authors is outlined. The alternative nodal surfaces for H₂ (³Σ_u ⁺) a priori specified in this method are presented.     

Non-adiabatic effects on oxygen atom fine structure populations in the predissociation of the A2Σ+ state of OH

Multichannel scattering calculations are used to evaluate the role of non-adiabatic interactions and interference effect on the predissociation linewidths of several vibrational levels of the A²Σ⁺ state of OH due to coupling to three different repulsive electronic states. Oxygen atom fine structure branching ratios are calculated as a possible probe of the predissociation pathway. Non-adiabatic effects are found to be very strong for the lower vibrational predissociative A²Σ⁺ levels and to remain non-negligible even for the higher vibrational states of A ²Σ⁺.     

Time-sliced ion-velocity imaging study of the reaction Y + O2 → YO + O

The oxidation reaction dynamics of the gas-phase yttrium atoms by oxygen molecules was studied under crossed-beam conditions. The product YO was detected using a time-of-flight mass spectrometer combined with laser single-photon ionization. An acceleration lens system designed for the ion-velocity mapping conditions, a two-dimensional (2-D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products. Two ionization wavelengths were used for the internal (vibrational and/or electronic) energy selective detection of YO. The single photon of the shorter wavelength (202.0 nm) can ionize all states of YO(X?(2)Σ, A'?(2)Δ, and A?(2)Π), while electronically excited YO(A' and A) are dominantly ionized at a longer wavelength (285.0 nm). Time-sliced images were converted to the velocity and angular distributions in the center-of-mass frame. The general features of the velocity distributions of YO, determined at two wavelengths, were well represented by those expected from the statistical energy disposal model. The forward-backward symmetry was also observed for two images. These results suggest that the reaction proceeds via long-lived intermediates, and that this mechanism is consistent with previous chemiluminescence/LIF studies.     

Wavelength dependence of the BaO* product chemiluminescence on the N2O reactant orientation in the Ba + N2O → BaO* + N2 reaction

Coarse wavelength analysis of chemiluminescence from the crossed-beam reaction of Ba with oriented N₂O reveals a distinct dependence of the BaO final product internal energy distribution on the initial collision geometry. Favorable (Ba approaches the “O” end of N₂O) and unfavorable (Ba approaches the “N” end of N₂O) orientations are compared at two collision energies. Reactions taking place in the unfavorable orientation produce BaO with relatively higher internal energy than reactions via the favorable orientation. The steric effect depends strongly on the wavelength and it is more pronounced at a higher collision energy.     

The influence of the internal state and translational energy of the molecular reactant upon the chemiluminescent reaction Ba + N2O → BaO* + N2

Employing a hexapole focuser to produce a beam of state-selected N₂O molecules (n₂ = 1, J = M = 1 or 2, l = 1) of variable translational energy, E_tr (0.08–0.16 eV), we have found the chemiluminescent cross section, σ_hν, to be enhanced strongly, up to a factor of 4, upon excitation (n₂ = 1) of the bending mode of the N₂O molecule. Revoking the commonly accepted ion-pair harpooning model we propose a revised mechanism in which the ν₂-bending vibration provides the reaction-promoting coordinate for overcoming the symmetry barrier to the breaking of the NNO bond and the release of ground state O atoms to form the BaO product. A weak enhancement of σ_hν(n₂ = 1) is observed for the J = 2 rotational state compared to the J = 1 state of the N₂O reactant. Our results have been utilized successfully to explain the temperature dependence of the chemiluminescent rate observed earlier. Absolute cross sections have been extracted using a recent calibration.     

Clustering of nonvalently bonded NO2…O2N fragments at C(sp3) atoms

Russian Chemical Bulletin - Based on the "invariom" approach and the "Atoms in Molecules" theory, a simplified method is proposed to reveal and analyze bonding interatomic...     

RG+ formation following photolysis of NO-RG via the Ã-X transition: a velocity map imaging study

Kr(+) and Xe(+) formation following photodissociation of NO-RG (RG = Kr or Xe) molecules via the ?-X electronic transition in the 44,150-44,350 cm(-1) region has been investigated using velocity map imaging. Nuclear kinetic energy release (nKER) spectra indicate that the NO cofragment is produced in multiple vibrational states of the electronic ground state, with a high degree of rotational excitation. Photofragment angular distributions and nKERs are consistent with photo-induced charge transfer at the two-photon level followed by dissociative ionization at the three-photon level. RG(+) angular distributions showing highly parallel character relative to the laser polarization axis are indicative of a high degree of molecular alignment in the dissociating species.     

Laser-induced fluorescence detection of N+2(X 2Σ+g) resulting from the He(23S) + N2 and He+, He+2 + N2 reactions in a flowing afterglow

The formation processes of N⁺₂ (X ²Σ⁺_g) resulting from the He(2 ³S) + N₂ Penning ionization and the thermal energy He⁺, He⁺₂ + N₂ charge transfer reaction are studied by observing the N⁺₂ (B ²Σ⁺_u ← X ²Σ⁺_g) laser-induced fluorescence (LIF) in a flowing afterglow. In both reactions, the vibrational population) decrease monotonically with increasing vibrational quantum number from υ″ = 0 to 3, and a population inversion with a peak at υ″ = 4 is seen. In the He(2 ³S) + N₂ Penning ionization, the vibrational populations of N⁺₂ (X, υ″ = 0–2) are explained by a direct channel and the B —X radiative cascade, while those of N⁺₂ (X, υ″ = 4–6) are ascribed to the collision-induced electronic energy transfer between the A ²Π_u and X ²Σ⁺_g states. In the He⁺, He⁺₂ + N₂ reaction, the N⁺₂ (X, υ″ = 0–3) is interpreted as the B−X radiative cascade and the collisional quenching of the unidentified states produced from the He⁺ + N₂ reaction, while the collision-induced electronic energy transfer from the N⁺₂ (A) state produced through the He⁺₂ + N₂ reaction is probably important for the formation of N⁺₂ (X, υ″ = 4–6).