Potential energy curves of diatomic molecular ions from high-resolution photoelectron spectra. II. The first six electronic states of Xe2 +

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe(2) has been measured between 90 000 and 109 000 cm(-1) following single-photon excitation from the ground neutral state. Transitions to five of the six low-lying electronic states of Xe(2) (+) could be observed. Whereas extensive vibrational progressions were observed for the X0(g) (+)-->I(1/2u), I(3/2g), and II(1/2u) photoelectron transitions, only the lowest vibrational levels of the I(3/2u) and II(1/2g) states could be detected. Unambiguous assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts of the vibrational bands and of the intensity distribution and from the modeling of the potential energy curves. Analytical potential energy curves of spectroscopic accuracy (i.e., approximately 1 meV) were determined for all six low-lying electronic states using a global model, which includes the first (charge-induced dipole, proportional to 1/R(4)) member of the long-range interaction series and treats the spin-orbit interaction explicitly. The assumption of an R-independent spin-orbit coupling constant was tested and found to be an excellent approximation.     

Luminescence measurements of Xe(+) + N2 and Xe(2+) + N2 hyperthermal charge transfer collisions

Luminescence spectra are recorded for collisions between Xe(+)/Xe(2+) and molecular nitrogen at energies ranging from 4.5 to 316 eV in the center-of-mass frame. In the Xe(+) + N(2) collision system, evidence for luminescent charge-transfer products is only found through Xe I emission lines. The most intense features of the luminescence spectra are attributed to atomic N emissions observed above ～20 eV. Intense N(2)(+) A (2)Π(u) - X(2)Σ(g)(+) and B(2)Σ(u)(+) - X(2)Σ(g)(+) radiance is observed from Xe(2+) + N(2) collisions. The B state formation cross section decreases with collision energy until 20 eV, after which it becomes independent of impact energy with an approximate value of 3 ?(2). The cross section for N(2) (+) A (ν > 0) formation increases with energy until 20 eV, after which it remains nearly constant at ～1 ?(2). The N(2)(+) product vibrational distributions extracted from the spectra are non-Franck-Condon for both electronic product states at low collision energies. The distributions resemble a Franck-Condon distribution at the highest energies investigated in this work.     

Electronic states and spectroscopic properties of SiTe and SiTe+

Ab initio based configuration interaction calculations have been carried out to study the low-lying electronic states and spectroscopic properties of the heaviest nonradioactive silicon chalcogenide molecule and its monopositive ion. Spectroscopic constants and potential energy curves of states of both SiTe and SiTe+ within 5 eV are reported. The calculated dissociation energies of SiTe and SiTe+ are 4.41 and 3.52 eV, respectively. Effects of the spin-orbit coupling on the electronic spectrum of both the species are studied in detail. The spin-orbit splitting between the two components of the ground state of SiTe+ is estimated to be 1880 cm(-1). Transitions such as 0+ (II)-X1Sigma(+)0+, 0+ (III)-X1Sigma(+)0+, E1Sigma(+)0+ -X1Sigma(+)0+, and A1Pi1-X1Sigma(+)0+ are predicted to be strong in SiTe. The radiative lifetime of the A1Pi state is less than a microsecond. The X(2)2Pi(1/2)-X(1)2Pi(3/2) transition in SiTe+ is allowed due to spin-orbit mixing. However, it is weak in intensity with a partial lifetime for the X2 state of about 108 ms. The electric dipole moments of both SiTe and SiTe+ in their low-lying states are calculated. The vertical ionization energies for the ionization of the ground-state SiTe to different ionic states are also reported.     

A theoretical study of spectroscopy and metastability of the CN dication

Potential energy curves of low-lying electronic states of the CN²⁺ dication and of the electronic ground states of CN⁺ and the neutral CN molecule were calculated using internally contracted multireference CI and the coupled cluster RCCSD(T) methods. Spectroscopic constants and adiabatic excitation energies of 13 quasibound electronic states of the dication were obtained and the energy of charge stripping of CN⁺ and double ionization energy of CN were predicted. Tunneling and spin-orbit induced predissociation lifetimes for the vibrational levels in the low-lying electronic states are presented and the metastability of the dication is discussed.     

Communication: imaging wavefunctions in dissociative photoionization

The dissociative ionization dynamics of excited electronic states of the xenon dimer, Xe(2), have been studied using velocity map ion imaging (VMI). A one-colour, (2+1) resonant excitation scheme was employed to first excite and then ionize selected vibrational levels of the Xe(2) 6p (2)[(1)∕(2)](0) 0(g)(+) Rydberg state. Cationic fragments were then detected by the VMI. The data provide an outstanding example of the reflection principle in photodissociation with the full nodal structure of the Rydberg state wavefunctions clearly observed in the final Xe(+) kinetic energy distributions without the need for scanning the excitation energy. Fitting of the observed distributions provides detailed and precise information on the form of the Xe(2)(+) I((1)/(2)g) potential energy curve involved which is in excellent agreement with the results of photoelectron imaging studies [Shubert and Pratt, J. Chem. Phys. 134, 044315 (2011)]. Furthermore, the anisotropy of the product angular distributions yields information on the evolution of the electronic character of the ionic state with internuclear separation, R. The combination of the nature of dissociative ionization and the extent of the bound state wavefunctions provide information over an unusually wide range of internuclear separation R (ΔR > 0.75 ?). This would normally require scanning over a considerable energy region but is obtained in these studies at a fixed excitation energy.     

Structure of low-lying electronic states of NdO: quantum chemical calculations

Low-lying states of the NdO molecule have been predicted from quantum mechanical complete active-space self-consistent field/multireference configuration interaction/spin-orbit calculations. 54 states labeled through the quantum number Omega(+/-) have been determined in the excitation energy range of approximately 1 eV. For each state molecular constants T(e), T(v), omega(e), deltaG(v), R(e), B(e), and B(v) have been calculated. All these states display nearly identical principal structural characteristics: equilibrium internuclear distance and vibrational frequency. Calculated values of T(v), deltaG(v), and B(v) agree satisfactorily with experimental values available for nine electronic states among the 54 considered. The feasibility of a statistical representation of the low-lying states of NdO is considered.     

Photoinitiated predissociation of the NO dimer in the region of the second and third NO stretch overtones

Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.     

Potential energy curves of diatomic molecular ions from high-resolution photoelectron spectroscopy. I. The first six electronic states of Ar2+

High-resolution photoelectron spectroscopic data have been used to determine the potential energy curves of the first six electronic states of Ar2+. The potential energy functions properly include the effects of the long-range interactions and of the spin-orbit interaction and are of spectroscopic accuracy (1-2 cm(-1)) over a wide range of internuclear distances. The total number of adjustable parameters could be reduced to only 12 by truncating the long-range interaction series after the R(-6) term and assuming an R-independent spin-orbit coupling constant. This assumption was verified to be valid to an accuracy of +/-2 cm(-1) over the range of internuclear distances between 3.0 and 4.6 A. The interaction potential proposed by Siska [P. E. Siska, J. Chem. Phys. 85, 7497 (1986)] was generalized to a form that is expected to be sufficiently flexible to describe chemical bonding in other diatomic molecular ions. The potential energy curves are more accurate than the best available ab initio curves by two orders of magnitude and provide quantitative information on dissociation energies and equilibrium internuclear distances. The local maximum between the two potential wells of the I(1/2g) state was determined to lie 62 cm(-1) below the Ar(1S0)+Ar(+)(2P(3/2)) dissociation limit, and the II(1/2g) state is found to be significantly more bound (De=177 cm(-1)) than previously assumed.     

Spin-orbit ab initio investigation of the photolysis of o-, m-, and p-bromotoluene

The photodissociations of o-, m-, and p-bromotoluene were investigated by ab initio and spin-orbit ab initio calculations. The possible photodissociation mechanisms at 266 and 193 nm were clarified by multistate second order multiconfigurational perturbation theory (MS-CASPT2) calculated potential energy curves, vertical excitation energies, and oscillator strengths of low-lying states. The dissociation products with spin-orbit-coupled states of Br(*)((2)P(12)) and Br((2)P(32)) were identified by the MS-CASPT2 method in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) calculations. The effects of methyl rotation and substituent on the photodissociation mechanism were discussed.     

Theoretical investigation of the SO(2+) dication and the photo-double ionization spectrum of SO

Highly correlated ab initio methods were used in order to generate the potential energy curves of the electronic states of the SO(2+) dication and of the electronic ground state of the neutral SO molecule. These curves were used to predict the spectroscopic properties of this dication and to perform forward calculations of the double photoionization spectrum of SO. In light of spin-orbit calculations, the metastability of this doubly charged ion is discussed: for instance, the rovibrational levels of the X (1)Sigma(+) and A (3)Sigma(+) states are found to present relatively long lifetimes. In contrast, the other electronic excited states should predissociate to form S(+) and O(+) in their electronic ground states. The simulated spectrum shows structures due to transitions between the v=0 vibrational level of SO (X (3)Sigma(-)) and the vibrational levels below the barrier maximum of 11 of the calculated electronic states. The 2 (1)Sigma(+) electronic state of SO(2+) received further treatment: in addition to vibrational bands due to the below barrier energy levels of this electronic state, at least nine continuum resonances were predicted which are responsible for the special shape of the spectrum in this energy region. This work is predictive in nature and should stimulate future experimental investigations dealing with this dication.     

Accurate ab initio potential for the Na+...I* complex

High-level ab initio calculations employing the multireference configuration interaction and coupled clusters methods with a correlation-consistent sequence of basis sets have been used to obtain accurate potential energy curves for the complex of the sodium cation with the iodine atom. Potential curves for the first two electronic Lambda-S states have very different characters: the potential for the 2pi state has a well depth of approximately 10 kcal/mol, while the 2sigma state is essentially unbound. This difference is rationalized in terms of the anisotropic interaction of the quadrupole moment of the iodine atom with the sodium cation, which is stabilizing in the case of the 2pi state and destabilizing in the case of the 2sigma state. The effects of spin-orbit coupling have been accounted for with both ab initio and semiempirical approaches, which have been found to give practically the same results. Inclusion of spin-orbit interactions does not affect the X(omega = 32) ground state, which retains its 2pi character, but it results in two omega = 12 spin-orbit states, with mixed 2sigma and 2pi characters and binding energies roughly half of that of the ground spin-orbit state. Complete basis set (CBS) extrapolations of potential curves, binding energies, and equilibrium geometries were also performed, and used to calculate a number of rovibronic parameters for the Na+...I* complex and to parameterize model potentials. The final CBS-extrapolated and zero-point vibrational energy-corrected binding energy is 10.2 kcal/mol. Applications of the present results for simulations of NaI photodissociation femtosecond spectroscopy are discussed.     

An ab initio study on the ground and low-lying doublet electronic states of SbO2

Geometry optimization and harmonic vibrational frequency calculations have been carried out on the low-lying doublet electronic states of antimony dioxide (SbO(2)) employing a variety of ab initio methods, including the complete active space self-consistent field/multireference configuration interaction and the RCCSD(T) methods. Both large and small core relativistic effective core potentials were used for Sb in these calculations, together with valence basis sets of up to aug-cc-pV5Z quality. Contributions from outer core correlation and off-diagonal spin-orbit interaction to relative electronic energies have been calculated. The ground electronic state of SbO(2) is determined to be the X (2)A(1) state, as is the case for dioxides of other lighter group 15 p-block (or group VA) elements. However, the A (2)B(2) and B (2)A(2) states are estimated to be only 4.1 and 10.7 kcalmole above the X (2)A(1) state, respectively, at the complete basis set limit. Reliable vertical excitation energies from the X (2)A(1) state to low-lying excited states of SbO(2) have been computed with a view to assist future spectral assignments of the absorption and/or laser-induced fluorescence spectra of SbO(2), when they become available.     

Spectroscopic analysis of the coupled 1(1)Π, 2(3)Σ+ (Ω = 0-, 1), and b(3)Π (Ω = 0±, 1, 2) states of the KRb molecule using both ultracold molecules and molecular beam experiments

We report the spectroscopic characterization of excited electronic states of KRb by combining spectra from molecular beam (MB) experiments with those from ultracold molecules (UM) formed by photoassociation (PA) of ultracold atoms. Spectra involving the 1(1)Π, 2(3)Σ(+), and b(3)Π states in a strongly perturbed region have been identified. This approach provides a powerful method to identify the vibrational levels of the excited electronic states perturbed globally by neighboring electronic states. This is because the two sets of spectra from the UM and the MB experiments probe the same energy region from very different initial electronic states. The UM experiments utilize high v' levels of the a(3)Σ(+) state with large internuclear separations, while the MB experiments utilize low v' levels of the ground X(1)Σ(+) state with near-equilibrium internuclear separations. Only the Ω = 1 levels of the 2(3)Σ(+) and b(3)Π states are observed in the MB spectra, while the Ω = 0(-), 1 levels of the 2(3)Σ(+) state and the Ω = 0(±), 1, 2 levels of the b(3)Π state are observed in the UM spectra.     

Ab initio and quantum-defect calculations for the Rydberg states of ArH

Potential energy curves were evaluated for the ground and thirteen low-lying excited electronic states of the ArH molecule over a wide range of internuclear distances by the multi-reference averaged quadratic coupled cluster method. The ab initio energy differences and transition dipole moments were used to estimate Einstein emission coefficients, absorption oscillator strengths and radiative lifetimes. Diagonal and off-diagonal quantum defects, as functions of internuclear distance, were extracted from ab initio potentials of the lowest Rydberg states of the neutral ArH molecule by taking account of configuration interaction between Rydberg series converging to the ground and two electronic excited states of the ArH(+) cation. The derived quantum-defect functions were used to generate manifolds of higher excited Rydberg states. The agreement between experimental and calculated energies and radiative transition probabilities was found to be as good as or better than that obtained by earlier calculations.     

Femtosecond Time-Resolved Stimulated Raman Spectroscopy of the S(2) (1B(u)) Excited State of beta-Carotene

The electronic and vibrational structure of beta-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived ( approximately 160 fs) second excited singlet state (S(2),1B(u) (+))of beta-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at approximately 1100, 1300, and 1650 cm(-1) that decay with a time constant corresponding to the electronic lifetime of S(2). The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1B(u) (+) and 2A(g) (-)) in the ultrafast relaxation pathway of beta-carotene.     

Multiphoton ion pair spectroscopy (MPIPS) with ultrashort laser pulses for the H2 molecule

Time-dependent Schr?dinger equation, TDSE, simulations have been performed in order to prepare and study via MPIPS the evolution of vibrational wave packets on the ion pair electronic state potentials B'B1Sigma(u)(+) and Hh1Sigma(g)(+) of the H2 molecule. Using ab initio potential surfaces and transition moments, we present two- and three-photon excitation schemes with ultrashort pulses (tau 相似文献   

Probing the electronic structure of UO+ with high-resolution photoelectron spectroscopy

The pulsed field ionization-zero kinetic energy photoelectron technique has been used to observe the low-lying energy levels of UO+. Rotationally resolved spectra were recorded for the ground state and the first nine electronically excited states. Extensive vibrational progressions were characterized. Omega+ assignments were unambiguously determined from the first rotational lines identified in each vibronic band. Term energies, vibrational frequencies, and anharmonicity constants for low-lying energy levels of UO+ are reported. In addition, accurate values for the ionization energies for UO [48,643.8(2) cm(-1)] and U [49,957.6(2) cm(-1)] were determined. The pattern of low-lying electronic states for UO+ indicates that they originate from the U3+(5f3)O2- configuration, where the uranium ion-centered interactions between the 5f electrons are significantly stronger than interactions with the intramolecular electric field. The latter lifts the degeneracy of U3+ ion-core states, but the atomic angular momentum quantum numbers remain reasonably well defined.     

An ab initio study of the CH3I photodissociation. I. Potential energy surfaces

The multireference spin-orbit (SO) configuration interaction (CI) method in its Lambda-S contracted SO-CI version is employed to calculate two-dimensional potential energy surfaces for the ground and low-lying excited states of CH3I relevant to the photodissociation process in its A absorption band. The computed equilibrium geometry for the X A1 ground state, as well as vibrational frequencies for the nu2 umbrella and nu3 symmetric stretch modes, are found to be in good agreement with available experimental data. The 3Q0+ state converging to the excited I(2P1/2o) limit is found to possess a shallow minimum of 850 cm(-1) strongly shifted to larger internuclear distances (RC-I approximately 6.5a0) relative to the ground state. This makes a commonly employed single-exponent approximation for analysis of the CH3I fragmentation dynamics unsuitable. The 4E(3A1) state dissociating to the same atomic limit is calculated to lie too high in the Franck-Condon region to have any significant impact on the A-band absorption. The computed vertical excitation energies for the 3Q1, 3Q0+, and 1Q states indicate that the A-band spectrum must lie approximately between 33,000 and 44,300 cm(-1), i.e., between 225 and 300 nm. This result is in very good agreement with the experimental findings. The lowest Rydberg states are computed to lie at >or=49,000 cm(-1) and correspond to the ...a(1)2n3a1(6sI) leading configuration. They are responsible for the vacuum ultraviolet absorption lines found experimentally beyond the A-band spectrum at 201.1 nm (49,722 cm(-1)) and higher.