Photodissociation dynamics of H2O: effect of unstable resonances on the B̃(1)A1 electronic state

We report a tunable vacuum ultraviolet photodissociation study of H(2)O from different unstable resonances in the B?(1)A(1) electronic state, using the H-atom Rydberg tagging technique. The quantum state resolved OH product translational energy distributions and angular distributions have been measured. Experimental results illustrate, for the first time, that excitation to the different unstable resonances has very different effect on the OH(X) and OH(A) product channels. The OH(X) product rotational distributions vary only slightly, while the OH(A) product rotational distributions and state-resolved angular distributions change dramatically as the photolysis energy increases. Effect of parent rotational excitation on the OH(A) product has also been observed. Through careful simulations to the experimental spectra, OH(A)∕OH(X) branching ratios have been determined at five photolysis wavelengths. The general agreement between theory and experiment in the branching ratios is good. The branching ratios for the OH(A) product from different parent rotational levels are close to the nuclear spin-statistics value, which is also consistent with the extremely low rotational temperature of the H(2)O beam in the current experiment.     

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.     

Photodissociation of CS2+( 2 Πg) via (1+1) Excitation

Photodissociation dynamics of CS2+molecular ions has been investigated by (1+two-photon resonance technique. CS2+were prepared by (3+1) resonance-enhanced multi-photon ionization (REMPI) of CS2molecules at 483. 2nm. The photofragment S+excitati (PHOFEX) spectra were recorded by scanning another laser in the 424~482nm region, and we assigned essentially to CS2+(~A2Πu,3/2(v′=0~4)←~X2Πg,3/2(0,0,0)) and (~A2Πu,1/2(v′=0,4)←~X2Πg,1/2(0,0,0)) (herev′=v1′+(1/2)v2′) transitions. The S+production channel wpreliminarily attributed to, (i) one-photon excitation CS2+from the ground state~X2Πgto texcited state~A2Πu; (ii) vibronic coupling between the~A2Πustate and the high vibrational lev in the~X2Πgstate; (iii) second photon excitation from the coupling vibrational levels to the excied state~B2Σu+and dissociation to produce S++ CS via the repulsive4Σ-state through spin-orb interaction between the~B2Σu+and4Σ-states.     

Optical Stark spectroscopy of the B̃(1)A(')(000)←X̃(1)A(')(000) system of copper hydroxide

The B?(1)A(')(000)←X?(1)A(')(000) band system of a cold beam of CuOH has been studied field-free and in the presence of a static electric field. The Stark tuning of the low-J levels of the X?(1)A(')(000) state were analyzed to give a value of 3.968(32) D for the a-component of the permanent electric dipole moment, μ(a). An upper limit of 0.3 D for μ(a)(B?(1)A(')) is established from the lack of observable Stark tuning for the low-J levels of the B?(1)A(')(000) state. The experimental value for μ(a)(X?(1)A(')) is compared to theoretical predictions and other Cu-containing molecules. A molecular orbital correlation diagram is used to rationalize the large change in μ(a) upon excitation. The electronegativity of OH was determined to be 2.81 from a comparison of the determined μ(a) with the experimental μ values for CuF, CuO, and CuS.     

Rotational state specific dissociation dynamics of HOD → H + OD via two-photon excitation to the C̃ electronic state

The dissociation dynamics of HOD via two-photon excitation to the C? state have been investigated using the H-atom Rydberg tagging time-of-flight (TOF) technique. The H-atom action spectrum for the C? ← X? transition shows resolved rotational structure. Product translational energy distributions and angular distributions have also been recorded for the H + OD channel for three excited levels each with k(a)′ = 2. From these distributions, quantum state distributions and angular anisotropy parameters (β2 and β4) for the OD product were determined. These results are consistent with the nonadiabatic predissociation picture illustrated in the one-photon dissociation process for H2O. The heterogeneous dissociation pathway via Coriolis coupling is the dominant dissociation process in the present study. A high proportion of the total available energy is deposited into the rotational energy of the OD product. The anisotropic recoil distributions reveal the distinctive contributions from the alignment of the excited states and the dissociation process. Comparisons are also made between the results for HOD and H2O via the equivalent rotational transitions. The OH bond energy, D(o)(H?OD), of the HOD molecule is also determined to be 41283.0 ± 5 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.     

Rotationally resolved (3 + 1) rempi of H 2 via the B1Σ+u state

In the paper, we compare the results of our ab initio calculations for the ro-vibrational branching ratios resulting from a (3+1) REMPI of H₂ via the B¹Σ⁺_u state with the experimental data of Pratt, Poliakoff, Dehmer and Dehmer. These results indicate that non-Franck-Condon effects are less important here than in the (3+1) REMPI of H₂ via the C¹Π_u state. We observe that the ΔJ = ±3 peaks in the photoelectron spectrum are of negligible strength and that the ratio of ΔJ = +1 to ΔJ = −1 peak is independent of the ionic vibrational state. A detailed analysis indicates that these features arise as a result of a dynamic interference between the do and dμ ionization channels and do not imply either the smallness of the d-wave or the smallness of the j_t = 3 angular momentum coupling terms.     

Photodissociation dynamics of acetaldehyde at 267 nm: A computational study of the CO-forming channels

A recent experiment on the 267 nm photolysis of gaseous acetaldehyde reveals that the molecular products (CO + CH₄) present product state distributions with computation –experiment discrepancy. This work aims to provide solution to such issue. The direct dynamics simulation of classical trajectories and the generalized multi-center impulsive model (GMCIM) are both utilized to predict the product state distributions of CO and CH₄ from 267 nm photolysis of acetaldehyde. Comparison with the experimental observation provides an assessment of the conventional approach of minimum energy path within the framework of three-center transition state (TS) and CH₃-roaming pathways. The result shows that the high-speed products observed in experiment are likely formed via the three-center TS pathway. The reasons that lead to the discrepancy between computation and experiments in previous studies are also investigated. A better agreement between computational and state-resolved experimental results can be achieved by selecting specific internal states of CO product from the computational data.     

The (1) 3Σg+ electronic state of Cs2

Laser-induced fluorescence of Cs₂ molecules, recorded by high-resolution Fourier spectroscopy, has been used for the first spectroscopic identification of the lowest gerade triplet (1) ³Σ_g⁺ electronic state. This state can be described by the molecular parameters: T_e = 11602.10 cm⁻¹, B_c = 8.258×10⁻³ cm⁻¹, D_c = 2.56×10⁻⁹ cm⁻¹ and R_c = 5.5425 Å. Determination of the absolute vibrational numbering will require further experiments.     

Adiabatic and non-adiabatic quantum dynamics calculation of O(1D) + D2 → OD + D reaction

Adiabatic (1A' or 1A' state) and non-adiabatic (2A'/1A' states) quantum dynamics calculations have been carried out for the title reaction (O((1)D) + D(2) → OD + D) to obtain the initial state-specified (v(i) = 0, j(i) = 0) integral cross section and rate constant using the potential energy surfaces of Dobbyn and Knowles. A total of 50 partial wave contributions have been calculated using the Chebyshev wave packet method with full Coriolis coupling to achieve convergence up to the collision energy of 0.28 eV. The total integral cross section and rate constant are in excellent agreement with experimental as well as quasi-classical trajectory results. Contributions from the adiabatic pathway of the 1A' state and the non-adiabatic pathway of the 2A'/1A' states, increase significantly with the collision energy. Compared to the O((1)D) + H(2) system, the kinetic isotope effect (k(D)/k(H)) is found to be nearly temperature independent above 100 K and its value of 0.77 ± 0.01 shows excellent agreement with the experimental result of 0.81.     

Effect of electronic state of ions on the electrochemical properties of layered cathode materials LiNi1−2x Co x Mn x O2

The cathode materials of the composition LiNi_{1 − 2x} Co _x Mn _x O₂ (x = 0.1, 0.2. 0.33) synthesized from the Ni, Co, Mn mixed hydroxides and LiOH by using mechanical activation method are studied. It is shown that all synthesized compounds have layered structure described by the space group R-3m. With the decreasing of the nickel content the cell volume and the degree of structure disordering decrease. According to XPS data, the electronic main state of d-ions at the prepared samples’ surfaces corresponds to Ni²⁺, Co³⁺, and Mn⁴⁺. An increase in the nickel content leads to the increase of the Ni2p _3/2 and Co2p _3/2 binding energy, which points to the change in the Me-O bond covalence. According to magnetic susceptibility measurements data, the nickel ions in LiNi_0.6Co_0.2Mn_0.2O₂ exist in the two oxidation states: Ni²⁺ and Ni³⁺. It is shown that this sample has the highest specific discharge capacity (∼170 mAh/g). The positions of redox peaks in the differential capacitance curves depend on the sample composition: with the increasing of nickel content they are shifted toward lower voltages. Based on the paper presented in the IX International Conference “Basic Problems of Energy Conversion in Lithium Electrochemical Systems” (Ufa, 2006).     

A comparison between the potential energy curves for the X1Σg+ state of H2 and D2

The electronic potential for the ground state of H₂ and D₂, molecules has been calculated from spectroscopic molecular constants. Numerical integration of the radial wave equation gives accurate self-consistent values (an eigenvalue mean deviation of about 1 cm⁻¹). A comparison between different potentials is reported.     

Vibronic coupling in the A2Π and B2Σ+ electronic states of the NCS radical

The spin-rovibronic energy levels of the A(2)Π and B(2)Σ(+) electronic states of thiocyanate radical have been calculated variationally, using high-level ab initio coupled diabatic potential energy surfaces. Computations up to J = 7∕2 have been performed, obtaining all levels with K ≤ 3 (Σ(1/2),Π(1/2,3/2),Δ(3/2,5/2),Φ(5/2,7/2)), for energies up to 2000 cm(-1) above the A(000)(2)Π(3∕2) level. The available experimental data have been critically reviewed in the light of the theoretical findings.     

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.     

Synchrotron vacuum ultraviolet radiation studies of the D (1)Π(u) state of H(2)

The 3pπD?(1)Π(u) state of the H(2) molecule was reinvestigated with different techniques at two synchrotron installations. The Fourier transform spectrometer in the vacuum ultraviolet wavelength range of the DESIRS beamline at the SOLEIL synchrotron was used for recording absorption spectra of the D?(1)Π(u) state at high resolution and high absolute accuracy, limited only by the Doppler contribution at 100 K. From these measurements, line positions were extracted, in particular, for the narrow resonances involving (1)Π(u) (-) states, with an accuracy estimated at 0.06?cm(-1). The new data also closely match multichannel quantum defect calculations performed for the Π(-) components observed via the narrow Q-lines. The Λ-doubling in the D?(1)Π(u) state was determined up to v=17. The 10 m normal incidence scanning monochromator at the beamline U125/2 of the BESSY II synchrotron, combined with a home-built target chamber and equipped with a variety of detectors, was used to unravel information on ionization, dissociation, and intramolecular fluorescence decay for the D?(1)Π(u) vibrational series. The combined results yield accurate information on the characteristic Beutler-Fano profiles associated with the strongly predissociated Π(u) (+) parity components of the D?(1)Π(u) levels. Values for the parameters describing the predissociation width as well as the Fano-q line shape parameters for the J=1 and J=2 rotational states were determined for the sequence of vibrational quantum numbers up to v=17.     

The emission spectrum of AsH2(Ã2A1 → X̃2B1) via uv photolysis of AsH3

The AsH₂(òA₁ → X̃²B₁) emission spectrum (402–650 nm) following ArF laser photolysis of AsH₃ is reported. Seven bands are assigned and the relation ν = 19928 + (868v'₂-3v'₂²)-(987v″₂-6v″₂²) is obtained. The AsH₂ (òA₁) radiative lifetime is 130 ± 20 ns. Emission spectra from nascent As and a multiphoton ionization signal were also obtained.     

Density and temperature dependence of the relaxation of the 1Δg state of highly compressed O2 fluid

The electronic relaxation of O₂ is investigated by an absorption excitation and fluorescence detection technique. The relaxation rate constant of O₂(¹Δ_g) is measured in the density range from 10²¹ to 3 × 10²² cm⁻³ at temperatures between 90 and 295 K. The experimental results are compared with theoretical models based on the pair distribution functions of the fluid. The effects of intermolecular potentials with hard or soft cores are discussed.     

Wavelength Dependent Photodissociation of OCS via F 31Π Rydberg State:CO(X1Σ+)+S(1D2)Product Channel

The vacuum ultraviolet photodissociation of OCS via the $F$ $3^1\Pi$ Rydberg states was investigated in the range of 134$-$140 nm by means of the time-sliced velocity map ion imaging technique. The images of S($^1$D$_2$) products from the CO($X^1\Sigma^+$)+S($^1$D$_2$) dissociation channel were acquired at five photolysis wavelengths, corresponding to a series of symmetric stretching vibrational excitations in OCS($F$ $3^1\Pi$, $v_1$=0$-$4). The total translational energy distributions, vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) coproducts were derived. The analysis of experimental results suggests that the excited OCS molecules dissociate to CO($X^1\Sigma^+$) and S($^1$D$_2$) products via non-adiabatic couplings between the upper $F$ $3^1\Pi$ states and the lower-lying states both in the C$_{\infty \textrm{v}}$ and C$_{\rm{s}}$ symmetry. Furthermore, strong wavelength dependent behavior has been observed: the greatly distinct vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) products from the lower ($v_1$=0$-$2) and higher ($v_1$=3, 4) vibrational states of the excited OCS($F$ $3^1\Pi$, $v_1$) demonstrate that very different mechanisms are involved in the dissociation processes. This study provides evidence for the possible contribution of vibronic coupling and the crucial role of vibronic coupling on the vacuum ultraviolet photodissociation dynamics.