High resolution vacuum ultraviolet emission spectrum of D(2) from 78 to 103 nm: The D (1)Pi(u)-->X (1)Sigma(g) (+) and D(') (1)Pi(u) (-)-->X (1)Sigma(g) (+) band systems

The emission spectrum of the D(2) molecule has been studied at high resolution in the vacuum ultraviolet region 78.5-102.7 nm. A detailed analysis of the two D (1)Pi(u)-->X (1)Sigma(g) (+) and D(') (1)Pi(u) (-)-->X (1)Sigma(g) (+) electronic band systems is reported. New and improved values of the level energies of the two upper states have been derived with the help of the program IDEN [V. I. Azarov, Phys. Scr. 44, 528 (1991); 48, 656 (1993)], originally developed for atomic spectral analysis. A detailed comparison is made between the observed energy levels and solutions of coupled equations using the newest ab initio potentials by Wolniewicz and co-workers [J. Chem. Phys. 103, 1792 (1995); 99, 1851 (1993); J. Mol. Spectros. 212, 208 (2002); 220, 45 (2003)] taking into account the nonadiabatic coupling terms for the D (1)Pi(u) state with the lowest electronic states B (1)Sigma(u) (+), C (1)Pi(u), and B(') (1)Sigma(u) (+). A satisfactory agreement has been found for most of the level energies belonging to the D and D(') states. The remaining differences between observation and theory are probably due to nonadiabatic couplings with other higher electronic states which were neglected in the calculations.     

Hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2)

The hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2) have been resolved with sub-Doppler continuous wave perturbation facilitated optical-optical double resonance spectroscopy via A (1)Sigma(u) (+) approximately b (3)Pi(u) mixed intermediate levels. The hyperfine patterns of these three states are similar. The hyperfine splittings of the low rotational levels are all very close to the case b(betaS) limit. As the rotational quantum number increases, the hyperfine splittings become more complicated and the coupling cases become intermediate between cases b(betaS) and b(beta J) due to spin-rotation interaction. We present a detailed analysis of the hyperfine structures of these three (3)Sigma(g) (+) states, employing both case b(betaS) and b(beta J) coupling basis sets. The results show that the hyperfine splittings of the (3)Sigma(g) (+) states are mainly due to the Fermi-contact interaction. The Fermi contact constants for the two d sigma Rydberg states, the 2 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+), are 245+/-5 MHz and 225+/-5 MHz, respectively, while the Fermi contact constant of the s sigma 3 (3)Sigma(g) (+) Rydberg state is 210+/-5 MHz. The diagonal spin-spin and spin-rotation constants, and nuclear spin-electronic spin dipolar interaction parameters of the 3 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+) states are also obtained.     

Predissociation mechanism for the lowest 1 Pi u states of N2

Separate coupled-channel Schr?dinger-equation (CSE) models of the interacting (1)Pi(u) (b,c,o) and (3)Pi(u) (C,C(')) states of N(2) are combined, through the inclusion of spin-orbit interactions, to produce a five-channel CSE model of the N(2) predissociation. Comparison of the model calculations with an experimental database, consisting principally of detailed new measurements of the vibrational and isotopic dependence of the (1)Pi(u) linewidths and lifetimes, provides convincing evidence that the predissociation of the lowest (1)Pi(u) levels in N(2) is primarily an indirect process, involving spin-orbit coupling between the b (1)Pi(u)- and C (3)Pi(u)-state levels, the latter levels themselves heavily predissociated electrostatically by the C(') (3)Pi(u) continuum. The well-known large width of the b(v=3) level in (14)N(2) is caused by an accidental degeneracy with C(v=9). This CSE model provides the first quantitative explanation of the predissociation mechanism for the dipole-accessible (1)Pi(u) states of N(2), and is thus likely to prove useful in the construction of realistic radiative-transfer and photochemical models for nitrogen-rich planetary atmospheres.     

Interactions of the 3ppi u c1Pi u(v=2) Rydberg-complex member in isotopic N2

The 3ppi u c1Pi u-X 1Sigmag+(2,0) Rydberg and b' 1Sigmau+-X 1Sigmag+(7,0) valence transitions of 14N2, 14N15N, and 15N2 are studied using laser-based 1 extreme ultraviolet (XUV)+1' UV two-photon-ionization spectroscopy, supplemented by synchrotron-based hotoabsorption measurements in the case of 14N2. For each isotopomer, effective rotational interactions between the c(v=2) and b'(v=7) levels are found to cause strong Lambda-doubling in c(v=2) and dramatic P/R-branch intensity anomalies in the b'-X(7,0) band due to the effects of quantum interference. Local perturbations in energy and predissociation line width for the c(v=2) Rydberg level are observed and attributed to a spin-orbit interaction with the crossing, short-lived C 3Pi u(v=17) valence level.     

Full configuration interaction potential energy curves for the X (1)Sigma(g) (+), B (1)Delta(g), and B(') (1)Sigma(g) (+) states of C(2): a challenge for approximate methods

The C(2) molecule exhibits unusual bonding and several low-lying excited electronic states, making the prediction of its potential energy curves a challenging test for quantum chemical methods. We report full configuration interaction results for the X (1)Sigma(g) (+), B (1)Delta(g), and B(') (1)Sigma(g) (+) states of C(2), which exactly solve the electronic Schrodinger equation within the space spanned by a 6-31G( *) basis set. Within the D(2h) subgroup used by most electronic structure programs, these states all have the same symmetry ((1)A(g)), and all three states become energetically close for interatomic distances beyond 1.5 A. The quality of several single-reference ab initio methods is assessed by comparison to the benchmark results. Unfortunately, even coupled-cluster theory through perturbative triples using an unrestricted Hartree-Fock reference exhibits large nonparallelity errors (>20 kcal mol(-1)) for the ground state. The excited states are not accurately modeled by any commonly used single-reference method, nor by configuration interaction including full quadruple substitutions. The present benchmarks will be helpful in assessing theoretical methods designed to break bonds in ground and excited electronic states.     

Theoretical spectroscopy of the calcium dimer in the A 1Sigma(u)+, c3Pi(u), and a3Sigma(u)+ manifolds: an ab initio nonadiabatic treatment

Nonadiabatic theory of molecular spectra of diatomic molecules is presented. It is shown that in the fully nonadiabatic framework, the rovibrational wave functions describing the nuclear motions in diatomic molecules can be obtained from a system of coupled differential equations. The rovibrational wave functions corresponding to various electronic states are coupled through the relativistic spin-orbit coupling interaction and through different radial and angular coupling terms, while the transition intensities can be written in terms of the ground state rovibrational wave function and bound rovibrational wave functions of all excited electronic states that are electric dipole connected with the ground state. This theory was applied in the nearly exact nonadiabatic calculations of energy levels, line positions, and intensities of the calcium dimer in the A (1)Sigma(u) (+)(1 (1)S+1 (1)D), c (3)Pi(u)(1 (3)P+1 (1)S), and a (3)Sigma(u) (+)(1 (3)P+1 (1)S) manifolds of states. The excited state potentials were computed using a combination of the linear response theory within the coupled-cluster singles and doubles framework for the core-core and core-valence electronic correlations and of the full configuration interaction for the valence-valence correlation, and corrected for the one-electron relativistic terms resulting from the first-order many-electron Breit theory. The electric transition dipole moment governing the A (1)Sigma(u) (+)<--X (1)Sigma(g) (+) transitions was obtained as the first residue of the frequency-dependent polarization propagator computed with the coupled-cluster method restricted to single and double excitations, while the spin-orbit and nonadiabatic coupling matrix elements were computed with the multireference configuration interaction wave functions restricted to single and double excitations. Our theoretical results explain semiquantitatively all the features of the observed Ca(2) spectrum in the A (1)Sigma(u) (+)(1 (1)S+1 (1)D), c (3)Pi(u)(1 (3)P+1 (1)S), and a (3)Sigma(u) (+)(1 (3)P+1 (1)S) manifolds of states.     

The d (3)Pi(g)-c (3)Sigma(u) (+) band system of C2

A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the d (3)Pi(g)<--c (3)Sigma(u) (+) and d (3)Pi(g)<--a (3)Pi(u) excitations. Rotationally resolved excitation spectra of the (4<--1), (5<--1), (5<--2), and (7<--3) bands in the d (3)Pi(g)<--c (3)Sigma(u) (+) system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the c (3)Sigma(u) (+) state. The principal molecular constants for the c (3)Sigma(u) (+) state are B(e)=1.9319(19) cm(-1), alpha(e)=0.018 55(69) cm(-1), omega(e)=2061.9 cm(-1), omega(e)x(e)=14.84 cm(-1), and T(0)(c-a)=8662.925(3) cm(-1). We report also the first experimental observations of dispersed fluorescence from the d (3)Pi(g) state to the c (3)Sigma(u) (+) state, namely, d (3)Pi(g)(v=3)-->c (3)Sigma(u) (+)(v=0,1).     

Observation and calculation of the Cs2 2 3Delta(1g) and b 3Pi(0u) states

The Cs(2) 2 (3)Delta(1g) and b (3)Pi(0u) states have been observed by infrared-infrared double resonance spectroscopy for the first time. 221 2 (3)Delta(1g)<--A (1)Sigma(u) (+)<--X (1)Sigma(g) (+) double resonance lines have been assigned to transitions into the 2 (3)Delta(1g) v=6-13 vibrational levels. Resolved fluorescence into the b (3)Pi(0u) v(')=0-48 levels has been recorded. Molecular constants and potential energy curves are determined by the global fit of the entire set of the experimental data. Theoretical potential energy curves of the 2 (3)Delta(g) and b (3)Pi(u) states have been determined in the framework of the pseudopotential method and are compared with the experimental results.     

Isotopic variation of experimental lifetimes for the lowest 1 Pi u states of N2

Lifetimes of several (1)Pi(u) states of the three natural isotopomers of molecular nitrogen, (14)N(2), (14)N(15)N, and (15)N(2), are determined via linewidth measurements in the frequency domain. Extreme ultraviolet (XUV)+UV two-photon ionization spectra of the b (1)Pi(u)(v=0-1,5-7) and c(3) (1)Pi(u)(v=0) states of (14)N(2), b (1)Pi(u)(v=0-1,5-6) and c(3) (1)Pi(u)(v=0) states of (14)N(15)N, and b (1)Pi(u)(v=0-7), c(3) (1)Pi(u)(v=0), and o (1)Pi(u)(v=0) states of (15)N(2) are recorded at ultrahigh resolution, using a narrow band tunable XUV-laser source. Lifetimes are derived from the linewidths of single rotationally resolved spectral lines after deconvolution of the instrument function. The observed lifetimes depend on the vibrational quantum number and are found to be strongly isotope dependent.     

Infrared spectra of the water-nitrogen complexes (H2O)2-(N2)n(n = 1-4) in argon matrices

The infrared spectra of the water-nitrogen complexes trapped in argon matrices have been studied with Fourier transform infrared absorption spectroscopy. The absorption lines of the H20-N2 1:1, 1:2, 1:n, and 2:1 complexes have been confirmed on the basis of the concentration effects. In addition, we have observed a few lines and propose the assignments for the 2:2, 2:3, and 2:4 complexes in the nu1 symmetric stretching and nu2 bending regions of the proton-acceptor molecule, and in the bonded OH stretching region of the proton-donor molecule. The redshifts in the bonded OH stretching mode and blueshifts in the OH bending mode suggest that the hydrogen bonds in the (H2O)2-(N2)n complexes with n = 1-4 are strengthened by the cooperative effects compared to the pure H2O dimer. Two absorption bands due to the 3:n complexes are also observed near the bonded OH stretching region of the H2O trimer.     

Solvation of UCl6(2-) anionic complex by MeBu3N(+), BuMe2Im(+), and BuMeIm(+) cations

The complexes [MeBu 3N] 2[UCl 6] and [BuMe 2Im] 2[UCl 6] were characterized in the solid state and in solution of [MeBu 3N][Tf 2N], [BuMe 2Im][Tf 2N], and [BuMeIm][Tf 2N] room-temperature ionic liquids using single-crystal XRD, EXAFS, electrochemistry, UV-visible absorption spectroscopy, and NMR. In the solid state and in solution, the existence of hydrogen bonding between the UCl 6 (2-) complex and the ionic liquid cations was revealed by these techniques. The MeBu 3N (+) cation interacts with UCl 6 (2-) via the protons on the alpha-carbon atoms of nitrogen. The protons of the imidazolium ring account for the interaction between the BuMe 2Im (+) cation and the UCl 6 (2-) anion. For the BuMeIm (+) cation the major interaction was confirmed between the most acidic proton on C(2) and the chlorides of UCl 6 (2-). The experimental results also show that the intensity of the interaction between the UCl 6 (2-) anion and the cation varies with the ionic liquid cation in the following order: MeBu 3N (+) approximately BuMe 2Im (+) < BuMeIm (+).     

Application of direct potential fitting to line position data for the X (1)Sigma(+) and A (1)Sigma(+) states of LiH

A collection of 9089 spectroscopic LiH line positions, of widely varying precision, which sample 84.9% and 98.6% of the A and X state well depths, respectively, have been employed in a direct least-squares fit of the effective potential energy and Born-Oppenheimer breakdown functions for the two states. For the four isotopomers (6)LiH, (7)LiH, (6)LiD, and (7)LiD, the data comprise both pure rotational and vibration-rotational transitions within the ground state, as well as rotationally resolved transitions in the A-X system. Despite the unusual shape and associated anomalous properties of the A state potential, no special features or considerations were required in the direct potential fitting approach. The reduced standard deviation of the fit was close to unity, indicating that the quantum mechanical eigenvalues calculated from the fully analytical functions of the Hamiltonians of the two states, which are characterized by a total of only 53 fitted parameters, represent the line positions, on average, to within the estimated uncertainties. A quantum mechanical calculation of the molecular constants G(nu), B(nu), D(nu), H(nu), L(nu), M(nu), N(nu), and O(nu) from the fitted potential for the A state of (7)LiH confirms that the usual polynomial expansion in J(J+1) is an unsatisfactory representation for the rotational terms of the lowest vibrational levels.     

The NaK 1(b) 3Pi(Omega=0) state hyperfine structure and the 1(b) 3Pi(Omega=0) approximately 2(A) 1Sigma+ spin-orbit interaction

We have measured the hyperfine structure of mutually perturbing rovibrational levels of the 1(b) 3Pi0 and 2(A) 1Sigma+ states of the NaK molecule, using the perturbation-facilitated optical-optical double resonance method with copropagating lasers. The unperturbed 1(b) 3Pi0 levels are split into four hyperfine components by the Fermi contact interaction bFIS. Mixing between the 1(b) 3Pi0 and 2(A) 1Sigma+ levels imparts hyperfine structure to the nominally singlet component of the perturbed levels and reduces the hyperfine splitting of the nominally triplet component. Theoretical analysis relates these observations to the hyperfine splitting that each 1(b) 3Pi0 level would have if it were not perturbed by a 2(A) 1Sigma+ level. Using this analysis, we demonstrate that significant hyperfine splitting arises because the 1(b) 3Pi0 state cannot be described as pure Hund's case (a). We determine bF for the 1(b) 3Pi0 levels and also a more accurate value for the magnitude of the singlet-triplet spin-orbit coupling HSO=[1(b) 3Pi0(vb,J)(H(SO))2(A) 1Sigma+(vA,J). Using the known spectroscopic constants of the 1(b) 3Pi state, we obtain bF=0.009 89+/-0.000 27 cm(-1). The values of (H(SO)) are found to be between 2 and 3 cm(-1), depending on vb, vA, and J. Dividing (H(SO)) by calculated vibrational overlap integrals, and taking account of the 1(b) 3Pi(Omega) rotational mixing, we can determine the magnitude of the electronic part H(el) of H(SO). Our results yield (H(el))=(16.33+/-0.15) cm(-1), consistent with our previous determinations using different techniques.     

Infrared spectra of the Li+-(H2)n (n=1-3) cation complexes

The Li+-(H2)n n=1-3 complexes are investigated through infrared spectra recorded in the H-H stretch region (3980-4120 cm-1) and through ab initio calculations at the MP2/aug-cc-pVQZ level. The rotationally resolved H-H stretch band of Li+-H2 is centered at 4053.4 cm-1 [a -108 cm-1 shift from the Q1(0) transition of H2]. The spectrum exhibits rotational substructure consistent with the complex possessing a T-shaped equilibrium geometry, with the Li+ ion attached to a slightly perturbed H2 molecule. Around 100 rovibrational transitions belonging to parallel Ka=0-0, 1-1, 2-2, and 3-3 subbands are observed. The Ka=0-0 and 1-1 transitions are fitted by a Watson A-reduced Hamiltonian yielding effective molecular parameters. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.056 A increasing by 0.004 A when the H2 subunit is vibrationally excited. The spectroscopic data are compared to results from rovibrational calculations using recent three dimensional Li+-H2 potential energy surfaces [Martinazzo et al., J. Chem. Phys. 119, 11241 (2003); Kraemer and Spirko, Chem. Phys. 330, 190 (2006)]. The H-H stretch band of Li+-(H2)2, which is centered at 4055.5 cm-1 also exhibits resolved rovibrational structure. The spectroscopic data along with ab initio calculations support a H2-Li+-H2 geometry, in which the two H2 molecules are disposed on opposite sides of the central Li+ ion. The two equivalent Li+...H2 bonds have approximately the same length as the intermolecular bond in Li+-H2. The Li+-(H2)3 cluster is predicted to possess a trigonal structure in which a central Li+ ion is surrounded by three equivalent H2 molecules. Its infrared spectrum features a broad unresolved band centered at 4060 cm-1.     

Electron correlation in the 3 (1)Sigma(g)+ and 2 (1)Sigma(u)+ excited state lithium molecule

Electron correlation effects in the two excited states of Li(2), 3 (1)Sigma(g) (+) and 2 (1)Sigma(u) (+), one with a shelf shape and another with double minima in their potential energy curves, have been studied with the aid of the calculated electron pair density distribution as a function of the internuclear distance and the analysis of the natural orbitals. Both states show increased electron pair densities at intermediate interelectronic distances around the second minimum of their potential energy curves. Since the bond breaks homolitically this observation runs contrary to regular expectations. Analysis of the electron pair density distributions and the natural orbitals provides mechanisms to account for this abnormal behavior.     

The 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) states of the SO(2) (+) ion studied using multiconfiguration second-order perturbation theory

The 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) electronic states of the SO(2) (+) ion have been studied using multiconfiguration second-order perturbation theory (CASPT2) and two contracted atomic natural orbital basis sets, S[6s4p3d1f]/O[5s3p2d1f] (ANO-L) and S[4s3p2d]/O[3s2p1d] (ANO-S), and the three states were considered to correspond to the observed X, B, and A states, respectively, in the previous experimental and theoretical studies. Based on the CASPT2/ANO-L adiabatic excitation energy calculations, the X, A, and B states of SO(2) (+) are assigned to 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2), respectively, and our assignments of the A and B states are contrary to the previous assignments (A to (2)A(2) and B to (2)B(2)). The CASPT2/ANO-L energetic calculations also indicate that the 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) states are, respectively, the ground, first excited, and second excited states at the ground-state (1 (2)A(1)) geometry of the ion and at the geometry of the ground-state SO(2) molecule. Based on the CASPT2/ANO-L results for the geometries, we realize that the experimental geometries (determined by assuming the bond lengths to be the same as the neutral ground state of SO(2)) were not accurate. The CASPT2/ANO-S calculations for the potential energy curves as functions of the OSO angle confirm that the 1 (2)B(2) and 1 (2)A(2) states are the results of the Renner-Teller effect in the degenerate (2)Pi(g) state at the linear geometry, and it is clearly shown that the 1 (2)B(2) curve, as the lower component of the Renner splitting, lies below the 1 (2)A(2) curve. The UB3LYP/cc-pVTZ adiabatic excitation energy calculations support the assignments (A to (2)B(2) and B to (2)A(2)) based on the CASPT2/ANO-L calculations.     

Identification and analysis of the perturbed c3Pi(v=1)-X 1Sigma+ and k 3Pi(v=5)-X 1Sigma+ absorption bands of carbon monoxide

Two new red-degraded bands in the room-temperature vacuum-ultraviolet absorption spectrum of carbon monoxide have been identified in the 94,000-94,500 cm(-1) energy region and analyzed. One of the bands at approximately 94,225 cm(-1) (106.1 nm) has three observable bandheads and is partially overlapped with the strong C 1Sigma+-X 1Sigma+ (1-0) transition at lower energy. It is assigned to the c 3Pi-X 1Sigma+ (1-0) transition. The other band at approximately 94,437 cm(-1) (105.9 nm) with one clear bandhead is assigned to the k 3Pi-X 1Sigma+ (5-0) transition. A strong homogeneous perturbation was found to exist between the two upper states that strongly influences the line positions and shapes of these bands. A rotational deperturbation analysis was performed and molecular rotational constants for both upper states were determined. These deperturbed molecular constants are entirely consistent with the expected values for the k 3Pi valence and c 3Pi Rydberg states. The Hamiltonian interaction term between these two states is found to be separable into vibrational and electronic factors and the electronic factor is determined to be H(e)=323+/-40 cm(-1). A discrepancy in the literature regarding the location of the c 3Pi (v=1) state is identified and discussed.     

Electronic excitation spectra of the [Ir(ppy)2(bpy)]+ photosensitizer bound to small silver clusters Ag(n) (n = 1-6)

The changes in nature and order of the excited electronic states of the photosensitizer [Ir(ppy)(2)(bpy)](+) upon binding to small silver clusters, Ag(n) (n = 1-6), were studied theoretically using the linear response TDDFT method with the range-separated LC-BLYP functional. Binding energies and localization of HOMO and LUMO orbitals are found to oscillate with the number of silver atoms. Special emphasis is put on the discussion of long-range charge transfer transitions between the photosensitizer and the silver cluster. The energies of these transitions were found to be only slightly dependent on the relative orientations of both fragments, but strongly dependent on the intermolecular distance. The absorption spectrum of the combined system does not show a systematic trend with respect to cluster size, but it is strongly modified by the charge transfer transitions. Possible photophysical processes of the systems containing larger clusters are discussed.     

A regularized inverted perturbation approach method: potential energy curve of the 4 (1)Sigma(u) (+) state in Na(2)

We describe a modification of the inverted perturbation approach method allowing to construct physically sensible potential energy curves for electronic states of diatomic molecules even when some parts of the potential are not adequately characterized by the experimental data. The method is based on a simple regularization procedure, imposing an additional constraint on the constructed potential curve. In the present work it is applied to the double minimum 4 (1)Sigma(u) (+) state of Na(2), observed experimentally by polarization labeling spectroscopy technique.