The puzzling infrared spectra of the nitric oxide dimer radical cation: a systematic application of Brueckner methods

Since symmetry breaking occurs in the ONNO⁺ cation with the self-consistent field (SCF) method and inverse symmetry breaking occurs when density functional theory (DFT) methods are used, Brueckner double excitation coupled cluster methods (BD) and BD plus perturbative triple-excitation contributions [BD(T)]have been used to study the geometries and vibrational frequencies for the trans and cis structures of the ONNO⁺ cation. Double-ζ plus polarization (DZP) basis sets and triple-ζ plus double polarization with f functions (TZ2Pf) basis sets were utilized. The ground state of the trans-ONNO cation is of ²A_g symmetry, which has a slightly (0.36 kcalmol^-1) lower energy than the cis conformer (²A₁). The controversial vibrational frequency corresponding to the asymmetric N–O stretching mode for both trans and cis structures is predicted as about 1600cm^-1. This value is discussed in the context of the many (sometimes variant) experimental assignments.     

Multireference calculation of lifetime of A~2Π_u state in nitrogen-molecule cation

This paper utilizes multireference configuration interaction theory to calculate the lifetime of A2Πu state for nitrogen molecular ion N+2.It obtains the transition moment function for A2Πu→X2Σ+ g,Franck-Condon factors between vibrational levels of the two states.The calculated lifetimes are 16.81,14.62,13.10,12.18,11.40,and 11.64 μs forv'= 0,1,2,3,4,5 vibrational levels of A2Πu state,respectively,which are in excellent agreement with available experimental values.     

Resonance de-enhancement in the 21 A g state oftrans-azobenzene

We analyze the origin of de-enhancement for a number of vibrational modes in the 2¹ A _g excited state oftrans-azobenzene. We have used the time-dependent wave packet analysis of the RR intensities by including the multimode damping effects in the calculation. This avoids the use of unrealistically large values for the damping parameter. It is concluded that the de-enhancement is caused by the interference between the two uncoupled electronic states, and that the intensities observed under the so-called symmetry forbidden 2¹ A _g ← 1¹ A _g transition are purely due to resonance excitation. It is also observed that the use of the time-dependent approach to study the de-enhancement effects caused by multiple electronic states on the RR intensities is not necessarily useful if one is interested in the structural dynamics.     

The luminescence spectrum of ReCl6 2- doped in various host lattices with antifluorite-type structure

The vibrational structure derived from the Γ₇(² T _2g ) → Γ₈(⁴ A _2g ) phosphorescence spectrum at liquid nitrogen temperature of a number of cubic host crystals A₂BX₆ (A = K, Rb, Cs; B = Pt, Sn, Pb, Te; X = Cl, Br) doped with ReCl₆ ^2- has been investigated. The best resolved spectra were obtained for those host crystals absorbing appreciably more of the exciting light than ReCl₆ ^2-. This suggests an energy transfer mechanism from host molecules to the chromophore. The spectral bands are assigned to normal frequencies of the octahedral chromophore, to lattice vibrations and to various combinations. For a complete assignment, vibrational states must be considered which belong to special points in the Brillouin zone other than the zone centre. The zero phonon transitions vary in a definite pattern for host crystals which have closed electron shells. The shifts, which depend more on the cation A⁺ than on the host complex BX₆ ^2-, are discussed by means of ligand field theory.     

Studies on the full vibrational energy spectra for some electronic states of diatomic molecular ions XY+

The first accurate studies on the vibrational spectroscopic constants and the corresponding full vibrational energy spectra of some electronic states of diatomic molecular ions XY⁺ were performed using algebraic method(AM). The AM is applied on the X¹Σ⁺ state of BeH⁺, the X²Σ⁺ state of CO⁺, the X²Π_g state of F ₂ ⁺ , the A²Π_u state of O ₂ ⁺ and the X²Σ _g ⁺ state of Li ₂ ⁺ . The results show that AM can generate accurate vibrational spectroscopic constants as well as accurate full vibrational energy spectra by using some accurate experimental vibrational energies, and that the AM vibrational energies are better than other theoretical data. __________ Translated from Chinese Journal of Atomic and Molecular Physics, 2005, 22 (4) (in Chinese)     

The triplet spectrum of tetramethyl-1,3-cyclobutanedione

The low-temperature single-crystal spectrum of TMCBD has revealed a low-lying triplet state at 25 718 cm^-1 (72·5 kcal/mol). The observed band is ascribed to a ³ A_u (³ A ₂) ← ¹ A_g nπ* transition on the basis of its small singlet-triplet splitting, its polarization behaviour, and its vibrational structure. The ³ A_u state is first-order spin-orbit coupled to either or both of the ¹ B _2u or ¹ B _3u states. No evidence for two or more nπ* transitions was found. Several multi-membered progressions in the carbonyl wagging mode indicate the presence of a distorted excited state, in which the carbonyl carbons attain a pyramidal conformation. Analysis of the site symmetry shows that TMCBD is distorted to a boat-shaped C _2v structure in its triplet nπ* state.     

Ab initio calculation of the He2 A 1Σ u +└X 1Σ g + absorption spectrum

The absorption spectrum of helium gas near 600 Å, assigned to the A ¹Σ _u ⁺└X ¹Σ _g ⁺ transition in He₂, is calculated at 77 K. The excited state potential is taken from a recent ab initio calculation. The theoretical spectrum shows well-defined bands, corresponding to different vibration levels of the A state, with diffuse rotational structure, in agreement with experiment. Fairly good quantitative agreement with Tanaka and Yoshino's measured spectrum is obtained, except for the somewhat too large separation between calculated vibrational bands, probably due to the theoretical potential for the A state rising too steeply at small inter-nuclear separation. Rotational constants derived from the calculated and experimental spectra are in good agreement. It is shown that they are significantly smaller than actual rotational constants of the upper state.     

Mode-selective internal conversion of perylene

We observed fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation of jet-cooled perylene-h ₁₂ and perylene-d ₁₂, and carefully examined the vibrational structures of the S₀ ¹ A _g and S₁ ¹ B _2u states. We performed vibronic assignments on the basis of the results of ab initio calculation, and found that the vibrational energies in the S₁ state are very similar to those in the S₀ state, indicating that the potential energy curves are not changed much upon electronic excitation. We conclude that the small structural change is the main cause of its slow radiationless transition and high fluorescence quantum yield at the zero-vibrational level in the S₁ state. It has been already reported that the lifetime of perylene is remarkably short at specific vibrational levels in the S₁ state. Here, we show that the mode-selective nonradiative process is internal conversion (IC) to the S₀ state, and the ν₁₆(a _g ) in-plane ring deforming vibration is the promoting (doorway) mode in the S₁ state which enhances vibronic coupling with the high-vibrational level (b _2u ) of the S₀ state.     

Electron affinities of cyano-substituted ethylenes

Electron affinities of ethylene and six cyano-substituted ethylenes (cyanoethylene, 1, 1-dicyanoethylene, cis-1, 2-dicyanoethylene, trans-1, 2-dicyanothylene, tricyanoethylene, and tetracyanoethylene) were determined using six different density functional or hybrid Hartree-Fock/density functional methods. Equilibrium geometries and harmonic vibrational frequencies for each species were determined with each density functional method. Experimental electron affinities exist for three of the six systems studied (cyanoethylene, trans-1, 2-dicyanoethylene, and tetracyanoethylene); for the three systems, the absolute average EA errors for the different methods are 0.10eV (BLYP), 0.19ev (BHLYP), 0.22eV (B3LYP), 0.20eV (BP86), 0.78eV (B3P86), and 0.81eV (LSDA). The electron affinities of gem-dicyanoethylene, cis-discyanoethylene, and tricyanoethylene are not known from experiment but are predicted here to be 1.23eV (gem-dicyanoethylene), 1.32eV (cis-dicyanoethylene), and 2.41eV (tricyanoethylene). Contrary to earlier suggestions, tetracyanoethylene is predicted to be planar, rather than twisted. Density functional theory predicts that the ²B_1u state of the ethylene anion lies lower than the ²B_2g state, which is reported by experimentalists as the (transient) ground state, and lower than the ²A_g state. Coupled-cluster results indicate that the ²A_g state is lower than either the ²B_2g or ²B_1u states. The energetic stabilization of cyano substitution on ethylene results from the π and π? conjugation of multiple cyano groups. The HOMO-LUMO gap in ethylene decreases with increasing cyano substitution, from 7.2eV in C₂H₄ to 3.8eV in C₂(CN)₄, explaining the extreme difference between the electron affinities of ethylene (negative) and tetracyanoethylene (～T3.0eV).     

Solvent solute interactions probed by picosecond transient Raman spectroscopy:S 1 1,4-diphenyl-1,3-butadiene in the linear alkanes

We present theS ₁ Raman spectra of 1,4-DiPhenyl-1,3-Butadiene (DPB) in a series of linear alkanes (pentane, hexane, heptane, octane, decane, and dodecane). Bands assignable to both the 1¹ B _u and 2¹ A _g states are present, suggesting that the state we are observing in solution is a mixed state with both 1¹ B _u and 2¹ A _g character. The relative intensities of several bands associated with C_oC_o stretching motions in the 2¹ A _g and 1¹ B _u states change systematically through the solvent series. The relative intensity changes reflect a changing distribution ofs-trans conformers inS ₁ DPB as the solvent is varied. We suggest that the distribution ofs-trans conformers inS ₁ DPB controls the nature of the mixing between the 2¹ A _g and 1¹ B _u states and that the distribution of conformers is controlled by the solvent viscosity. Changes in the peak position and bandwidth of the phenyl C=C stretch with delay reflect vibrational relaxation processes inS ¹ DPB. We observe anomolous behavior in pentane that we attribute to the effect of the solvent structure on the ability of DPB to exchange energy with pentane.     

Application of CI expansion techniques to the computation of two-photon absorptivities

Summary We show how conventional linear expansion techniques for both electronic and vibrational wave functions can be used for the computation of matrix elements governing the two-photon absorption probability in molecules. In particular, the²Σ⁺ A←²∏X transition matrix elements of OH and the¹Σ _g ⁺ E,F←¹Σ _g ⁺ X of H₂are computed using 1) the sum over states in the Born-Oppenheimer approximation with inclusion of vibrational wave functions, 2) solution of perturbation theory equations by expansion in the CI basis at fixed nuclei, followed by a kind of vibrational averaging. The results are compared with experiment and discussed. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Optimal laser pulse design for transferring the coherent nuclear wave packet of H+2

Within the Franck–Condon approximation, the single ionisation of H₂ leaves H⁺₂ in a coherent superposition of 19 nuclear vibrational states. We numerically design an optimal laser pulse train to transfer such a coherent nuclear wave packet to the ground vibrational state of H⁺₂. Frequency analysis of the designed optimal pulse reveals that the transfer principle is mainly an anti-Stokes transition, i.e. the H⁺₂ in 1sσ_g with excited nuclear vibrational states is first pumped to 2pσ_g state by the pulse at an appropriate time, and then dumped back to 1sσ_g with lower excited or ground vibrational states. The simulation results show that the population of the ground state after the transfer is more than 91%. To the best of our knowledge, this is the highest transition probability when the driving laser field is dozens of femtoseconds.     

Cr3+−Cr3+ excited state Raman scattering from molecular vibrations in binuclear Cs3Cr2Cl9 induced by exchange striction

On increasing the temperature, several new bands appear at higher energy from their parent vibrational modes in the Raman spectrum of the molecular complex Cr₂Cl^3?₉. The parent and new bands have intensities that follow the thermal population factors of various ⁴A_2g⁴A_2g Cr³⁺ pair states. This behaviour is attributed to exchange striction which produces a change in vibrational energy with excited state for those normal modes with large net axial Cr³⁺?Cr³⁺ displacements.     

The absorption and magnetic circular dichroism of spectra of Cs2NaPrCl6

The room temperature absorption and magnetic circular dichroism spectra and the absorption spectrum at liquid helium (liquid He) temperature have been measured for Cs₂NaPrCl₆. At room temperature the crystal is cubic and the Pr³⁺ sites have O _h symmetry. All terms above 15 000 cm^-1, except ¹S₀, have been assigned and a previous assignment in PrCl₆ ^3- has been shown to be incorrect. The transition at 20 631 cm^-1 is assigned to ³H₄(A _1g ) →³P₁(T _1g ), in contradiction to previous assignments of Pr³⁺ spectra in other systems. A rich vibrational structure was observed in every transition. Vibrations have been assigned using the site group approximation and there is substantial agreement with the vibrational assignments in Cs₂NaEuCl₆. A crystal phase transition takes place between room temperature and liquid nitrogen temperature and the O _h forbidden transitions, A _1g →E_g and T _2g , are observed. At lower temperatures many additional lines are observed but it is unclear presently whether they are due to lower symmetry or a breakdown of the site group approximation.     

High resolution MCD spectroscopy of transition metal ions in fluoride crystals

Measurements of the MCD spectra of Ni²⁺ in KMgF₃ and KZnF₃ have confirmed and extended previous assignments of spin-orbit structure based on similarities of vibrational patterns. The measurements also reveal the presence of a ¹ A _1g (¹ G) state which interacts strongly with the ³ T _1g ^b (³ P) state. The MCD spectra of the tetragonal crystals K₃Ni₂F₇ and K₂NiF₄ have been measured for the transitions ³ A _2g →³ T _1g ^a , ¹ E_g . The spectrum of K₃Ni₂F₇ is characterized by the appearance of electric dipole zero phonon lines, consistent with the C _4v site symmetry of the Ni²⁺ in the paramagnetic phase. An analysis of the MCD spectrum allows a positive assignment of six of the seven states (from ³ T _1g ^a and ¹ E_g ) to which transitions are allowed by symmetry in the axial spectrum. A complete tetragonal crystal-field analysis has been made which shows that the distortion is an axial compression of the fluoride octahedron. The absorption spectrum of K₂NiF₄ shows much less vibrational structure, but the MCD spectrum allows a definitive assignment of six states (from ³ T _1g ^a and ¹ E_g ) through a _2u vibrations in the usual vibronic mechanism. False origins based on e_u phonons have not been identified. The tetragonal field is larger than for K₃Ni₂F₇ and corresponds to an axial compression, in agreement with the known crystal structure.     

Analytical potential energy functions for the ground and some excited states of N2

The analytical potential energy functions have been calculated for the ground state X¹Σ⁺_g and four excited electronic states a¹Π_g, A³Σ⁺_u, B′³Σ^?_u and B³Π_g of N₂ molecule using the algebraic and energy-consistent methods (AM-ECM). Based on our previously published full AM vibrational energies and spectroscopic constants, the low-lying force constants f_n, the expansion coefficients a_n and the variational parameters λ in the AM–ECM potentials are determined for these states. The computed AM–ECM potential energy curve of each state is in excellent agreement with the experimental data and better than other analytical potentials.     

The Raman Spectrum and Full Vibrational Assignment of Ethylenediamne Dihydrochloride

A full vibrational analysis was recently made for the H³N⁺CH₂CH₂N⁺H₃ ion, on the basis of infrared measurements, and a partial vibrational assignment proposed¹. The observed fundamentals were interpreted In terms of a C_2h, symmetry for the ion. There are thirty-six fundamentals (r - lla_g + 8A_u + 7B_g + 10B_u; A_g and B_g Raman-active only; A_u and B_u infrared-active only). In addition to the eighteen infrared-active vibrations, a number of bands was observed which were assigned as formally forbidden Raman-active modes, excited by strong hydrogen bonding causing departure from the exact site symmetry of the species.     

Radiative lifetime measurements of vibrational levels of Cl2 + A 2Π u

The radiative lifetimes of several vibrational levels of Cl₂ ⁺ A ²Π _u are measured by a novel technique. A uniform electric field extracts the ions from the observation region of a spectrometer, and the decrease of fluorescence signal as a function of the electric field strength is measured to obtain the radiative lifetime. Measurements are made on 20 different vibronic bands of the Cl₂ ⁺ A ²Π _u -X ²Π _g system, and the results are compared with other methods. An attempt is made to correlate the results with the highly perturbed spectroscopic nature of the A ²Π _u electronic state.     

The magnetic circular dichroism spectrum of the 1 B 2u ←1 A 1g transition of benzene in the vapour phase

The magnetic circular dichroism (MCD) spectrum of the ¹ B _2u ←¹ A _1g transition of benzene has been measured in the vapour phase. Many of the bands due to transitions between single vibronic levels display A terms. It has been shown that the angular momentum arises by vibronic mixing both of the ¹ E _1u state with the ¹ B _1u state and of the ¹ E _2g states with the ¹ A _1g ground state by e _2g vibrations. The magnitudes and signs of the experimental and calculated ratios, A/D, for the A ₀ ⁰ vibronic origin are in excellent agreement. Two strong MCD progressions of opposite sign with B-term dispersion have been observed in regions of low absorption. These are identified with vibronic origins due to the v ₈ and v ₉ e _2g modes. By contrast the MCD spectrum of hexadeuterobenzene vapour has a much lower magneto-rotational strength and displays none of the striking features of the benzene MCD spectrum.