Electronically excited-state properties and predissociation mechanisms of phosphorus monofluoride: a theoretical study including spin-orbit coupling

The 51 Ω states generated from the 22 Λ - S states of phosphors monofluoride have been investigated using the valence internally contracted multireference configuration interaction method with the Davidson correction and the entirely uncontracted aug-cc-pV5Z basis set. The spin-orbit coupling is computed using the state interaction approach with the Breit-Pauli Hamiltonian. Based on the calculated potential energy curves, the spectroscopic constants of the bound and quasibound Λ - S and Ω states are obtained, and very good agreement with experiment is achieved. Several quasibound states caused by avoided crossings are found. Various curve crossings and avoided crossings are revealed, and with the help of our computed spin-orbit coupling matrix elements, the predissociation mechanisms of the a(1)Δ, b(1)Σ(+), e(3)Π, g(1)Π, and (3)(3)Π states are analyzed. The intricate couplings among different electronic states are investigated. We propose that the avoided crossing between the A(3)Π(0 +) and b(1)Σ(0+) (+) states may be responsible for the fact that the A(3)Π ν' ≥ 12 vibrational levels can not be observed in experiment. The transition properties of the A(3)Π - X(3)Σ(-) transition are studied, and our computed Franck-Condon factors and radiative lifetimes match the experimental results very well.     

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.     

Extensive theoretical study on electronically excited states and predissociation mechanisms of sulfur monoxide including spin-orbit coupling

The potential energy curves of the 69 Ω states generated from the 24 Λ-S states of sulfur monoxide are calculated for the first time using the internally contracted multireference configuration interaction method with the Davidson correction and the entirely uncontracted aug-cc-pV5Z basis set. Spin-orbit coupling is taken into account by the state interaction approach with the full Breit-Pauli Hamiltonian. Very good agreement is achieved between our computed spectroscopic properties and the available experimental data. The transition properties of the B(3)Σ(-) -X(3)Σ(-) and (4)1-X0(+) transitions are predicted, and our computed Franck-Condon factors and radiative lifetimes match the experimental results very well. The predissociation mechanisms are investigated, and various new predissociation channels are located. We present a new interpretation on the breaking-off of the rotational levels of the B(3)Σ(-) lower vibrational states observed in experiment, and propose that the predissociation is induced by the Coriolis coupling between the B(3)Σ(-) rovibrational levels and the A(3)Π state. Our calculations indicate that, at ν' = 9, the B(3)Σ(-) state predissociates via the C(3)Π state; around ν' = 14, three spin-orbit-induced predissociation pathways via (1)(5)Σ(+) , (2)(5)Π, and e(1)Π would be open; around ν' = 17, the pathways via (2)(1)Σ(+) , (2)(3)Σ(+) and (2)(5)Σ(+) would contribute. These satisfactorily explain the experimental results about the diffuseness of the B(3)Σ(-) bands. Furthermore, various predissociation pathways of the C'(3)Π state are predicted, through which the C'(3)Π state could predissociate rapidly.     

Steric effect in the energy transfer reaction of oriented CO (a 3Π, v'=0, Ω=1 and 2) + NO (X 2Π) → NO (A 2Σ+, B 2Π) + CO (X 1Σ+)

The oriented CO (a (3)Π, v' = 0, Ω = 1 and 2) beam has been prepared by using an electric hexapole and applied to the energy transfer reaction of CO (a (3)Π, v' = 0, Ω = 1 and 2) + NO (X (2)Π) → NO (A (2)Σ(+), B (2)Π) + CO (X (1)Σ(+)). The emission spectra of NO (A (2)Σ(+), B(2)Π) have been measured at three orientation configurations (C-end, O-end, random). The shape of the emission spectra (and/or the internal excitation of products) turns out to be insensitive to the molecular orientation. The vibrational distributions of NO (A (2)Σ(+), v' = 0-2) and NO (B (2)Π, v' = 0-2) are determined to be N(v'=0):N(v'=1):N(v'=2) = 1:0.40 ± 0.05:0.10 ± 0.05 and N(v'=0):N(v'=1):N(v'= 2) = 1:0.6 ± 0.1:0.7 ± 0.1, respectively, and the branching ratio γ/β [=NO (A (2)Σ(+))/NO (B (2)Π)] is estimated to be γ/β ～ 0.3 ± 0.1 by means of spectral simulation. These vibrational distributions of NO (A, B) can be essentially attributed to the product-pair correlations between CO (X, v″) and NO (A (2)Σ(+), v' = 0-2), NO (B (2)Π, v' = 0-2) due to energetic restriction under the vibrational distribution of CO (X, v″) produced from the vertical transition of CO (a (3)Π, v' = 0) → CO (X, v″) in the course of energy transfer. The steric opacity function has been determined at two wavelength regions: 220 < λ < 290 nm [NO (A → X) is dominant]; 320 < λ < 400 nm [NO (B → X) is dominant]. For both channels NO (A (2)Σ(+), B(2)Π), a significant CO (a (3)Π) alignment effect is recognized; the largest reactivity at the sideways direction with the small reactivity at the molecular axis direction is observed. These CO (a (3)Π) alignment effects can be essentially attributed to the steric asymmetry on two sets of molecular orbital overlap, [CO (2π) + NO (6σ (2π))] and [CO (5σ) + NO (1π (2π))]. All experimental observations support the electron exchange mechanism that is operative through the formation of a weakly bound complex OCNO.     

A vacuum-ultraviolet laser pulsed field ionization-photoelectron study of sulfur monoxide (SO) and its cation (SO+)

Vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectroscopy has been applied to the study of the sulfur monoxide radical (SO) prepared by using a supersonically cooled radical beam source based on the 193 nm excimer laser photodissociation of SO(2). The vibronic VUV-PFI-PE bands for the photoionization transitions SO(+)(X(2)Π(1∕2); v(+) = 0) ← SO(X(3)Σ(-); v = 0); and SO(+)((2)Π(3∕2); v(+) = 0) ← SO(X(3)Σ(-); v = 0) have been recorded. On the basis of the semiempirical simulation of rotational branch contours observed in these PFI-PE bands, we have obtained highly precise ionization energies (IEs) of 83,034.2 ± 1.7 cm(-1) (10.2949 ± 0.0002 eV) and 83,400.4 ± 1.7 cm(-1) (10.3403 ± 0.0002 eV) for the formation of SO(+)(X(2)Π(1∕2); v(+) = 0) and SO(+)((2)Π(3∕2); v(+) = 0), respectively. The present VUV-PFI-PE measurement has enabled the direct determination of the spin-orbit coupling constant (A(0)) for SO(+)(X(2)Π(1∕2,3∕2)) to be 365.36 ± 0.12 cm(-1). We have also performed high-level ab initio quantum chemical calculations at the coupled-cluster level up to full quadruple excitations and complete basis set (CBS) extrapolation. The zero-point vibrational energy correction, the core-valence electronic correction, the spin-orbit coupling, and the high-level correction are included in the calculation. The IE[SO(+)(X(2)Π(1∕2,3∕2))] and A(0) predictions thus obtained are found to be in remarkable agreement with the experimental determinations.     

Ab initio study on the ground and low-lying excited states of cesium iodide (CsI)

Potential energy curves (PECs) for the ground and low-lying excited states of the cesium iodide (CsI) molecule have been calculated using the internally contracted multireference configuration interaction calculation with single and double excitation method with the relativistic pseudopotentials. PECs for seven Lambda-S states, X 1Sigma+, 2 1Sigma+, 3Sigma+, 1Pi, and 3Pi are first calculated and then those for 13 Omega states are obtained by diagonalizing the matrix of the electronic Hamiltonian H(el) plus the effective one-electron spin-orbit (SO) Hamiltonian H(SO). Spectroscopic constants for the calculated ground X 0+-state PEC with the Davidson correction are found to agree well with the experiment. Transition dipole moments (TDMs) between X 0 and the other Omega states are also obtained and the TDM between X 0+ and A 0+ is predicted to be the largest and that between X 0+ and B 0+ is the second largest around the equilibrium internuclear distance. The TDMs between X 0+ and the Omega=1 states are estimated to be nonzero, but they are notably small as compared with those between the 0+ states. Finally, vibrational levels of the X 0+ PEC for the two isotopic analogs, (133)CsI and (135)CsI, are numerically obtained to investigate the isotope effect on the vibrational-level shift. It has been found that the maximized available isotope shift is approximately 30 cm(-1) around nu=136.     

Structural and spectroscopic study of the LiRb molecule beyond the Born-Oppenheimer approximation

Adiabatic and diabatic potential energy curves and the permanent and transition dipole moments of the low-lying electronic states of the LiRb molecule dissociating into Rb(5s, 5p, 4d, 6s, 6p, 5d, 7s, 6d) + Li(2s, 2p) have been investigated. The molecular calculations are performed with an ab initio approach based on nonempirical pseudopotentials for Rb(+) and Li(+) cores, parametrized l-dependent core polarization potentials and full configuration interaction calculations. The derived spectroscopic constants (R(e), D(e), T(e), ω(e), ω(e)x(e), and B(e)) of the ground state and lower excited states are in good agreement with the available theoretical works. However, the 8-10(1)Σ(+), 8-10(3)Σ(+), 6(1,3)Π, and 3(1,3)Δ excited states are studied for the first time. In addition, to the potential energy, accurate permanent and transition dipole moments have been determined for a wide interval of internuclear distances. The permanent dipole moment of LiRb has revealed ionic characters both relating to electron transfer and yielding Li(-)Rb(+) and Li(+)Rb(-) arrangements. The diabatic potential energy for the (1,3)Σ(+), (1,3)Π, and (1,3)Δ symmetries has been performed for this molecule for the first time. The diabatization method is based on variational effective Hamiltonian theory and effective metric, where the adiabatic and diabatic states are connected by an appropriate unitary transformation.     

Spectroscopic Parameters of X3∑-, a1△, and A'3△ Electronic States of SO Radical

The potential energy curves (PECs) of three low-lying electronic states (X³∑^-, a¹△, and A^'3△) of SO radical have been studied by ab initio quantum chemical method. The calcula-tions were carried out with the full valence complete active space self-consistent field method followed by the highly accurate valence internally contracted multireference configuration in-teraction (MRCI) approach in combination with correlation-consistent basis sets. Effects of the core-valence correlation and relativistic corrections on the PECs are taken into account. The core-valence correlation correction is carried out with the cc-pCVDZ basis set. The way to consider the relativistic correction is to use the second-order Douglas-Kroll Hamiltonian approximation, and the correction is performed at the level of cc-pV5Z basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size-extensivity errors by means of the Davidson modification (MRCI+Q). These PECs are extrapolated to the complete basis set limit by the two-point energy extrapolation scheme. With these PECs, the spectroscopic parameters are determined.     

New view of the ICN A continuum using photoelectron spectroscopy of ICN-

Negative-ion photoelectron spectroscopy of ICN(-) (X??(2)Σ(+)) reveals transitions to the ground electronic state (X??(1)Σ(+)) of ICN as well as the first five excited states ((3)Π(2), (3)Π(1), Π(0(-) ) (3), Π(0(+) ) (3), and (1)Π(1)) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I-C bond length of 2.65 A?. Because of this bond elongation, the lowest three excited states of ICN ((3)Π(2), (3)Π(1), and Π(0(-) ) (3)) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the Π(0(+) ) (3) and (1)Π(1) states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34(5) (+0.04∕-0.02) eV and the anion dissociation energy, D(0)(X??(2)Σ(+) I-CN(-)), to be 0.83 (+0.04/-0.02) eV.     

MRDCI study of the low-lying electronic states of PbSi

Electronic states of the PbSi molecule up to 4 eV have been studied by carrying out ab initio based MRDCI calculations which include relativistic effective core potentials (RECPs) of both the atoms. The use of semicore RECPs of Pb produces better dissociation limits than the full-core one. However, the (3)P(0)-(3)P(1) splitting due to Pb is underestimated by about 4000 cm(-1). At least 25 bound electronic states of the Λ-S symmetry are predicted for PbSi. The computed zero-field-splitting in the ground state is about 544 cm(-1). A strong spin-orbit mixing changes the nature of the potential energy curves of many Ω states. The overall splitting among the spin components of A(3)Π is computed to be 4067 cm(-1). However, the largest spin-orbit splitting is reported for the (3)Δ state. A number of spin-allowed and spin-forbidden transitions are predicted. The partial radiative lifetime for the A(3)Π-X(3)Σ(-) transition is of the order of milliseconds. The computed bond energy in the ground state is 1.68 eV, considering the spin-orbit coupling. The vertical ionization energy for the ionization to the X(4)Σ(-) ground state of PbSi(+) is about 6.93 eV computed at the same level of calculations.     

Accurate theoretical study of PS(q) (q = 0,+1,-1) in the gas phase

Highly correlated ab initio methods were used in order to generate the potential energy curves and spin-orbit couplings of electronic ground and excited states of PS and PS(+). We also computed those of the bound parts of the electronic states of the PS(-) anion. We used standard coupled cluster CCSD(T) level with augmented correlation-consistent basis sets, internally contacted multi-reference configuration interaction, and the newly developed CCSD(T)-F12 methods in connection with the explicitly correlated basis sets. Core-valence correction and scalar relativistic effects were examined. Our data consist of a set of spectroscopic parameters (equilibrium geometries, harmonic vibrational frequencies, rotational constants, spin-orbit, and spin-spin constants), adiabatic ionization energies, and electron affinities. For the low laying electronic states, our calculations are consistent with previous works whereas the high excited states present rather different shapes. Based on these new computations, the earlier ultraviolet bands of PS and PS(+) were reassigned. For PS(-) and in addition to the already known anionic three bound electronic states (i.e., X(3)Σ(-), (1)Δ, and 1(1)Σ(+)), our calculations show that the (1)Σ(-), (3)Σ(+), and the (3)Δ states are energetically below their quartet parent neutral state (a(4)Π). The depletion of the J = 3 component of PS(-)((3)Δ) will mainly occur via weak interactions with the electron continuum wave.     

Electronic states and spin-forbidden cooling transitions of AlH and AlF

The feasibility of laser cooling AlH and AlF is investigated using ab initio quantum chemistry. All the electronic states corresponding to the ground and lowest two excited states of the Al atom are calculated using multi-reference configuration interaction (MRCI) and the large AV6Z basis set for AlH. The smaller AVQZ basis set is used to calculate the valence electronic states of AlF. Theoretical Franck-Condon factors are determined for the A(1)Π→ X(1)Σ(+) transitions in both radicals and found to agree with the highly diagonal factors found experimentally, suggesting computational chemistry is an effective method for screening suitable laser cooling candidates. AlH does not appear to have a transition quite as diagonal as that in SrF (which has been laser cooled) but the A(1)Π→ X(1)Σ(+) transition transition of AlF is a strong candidate for cooling with just a single laser, though the cooling frequency is deep in the UV. Furthermore, the a(3)Π→ X(1)Σ(+) transitions are also strongly diagonal and in AlF is a practical method for obtaining very low final temperatures around 3 μK.     

Spectroscopic constants and molecular properties of the X2Π and a4Σ− electronic states of the SiF radical

The potential energy curves (PECs) of the X²Π and a⁴Σ^? electronic states of the SiF radical have been studied by an ab initio quantum chemical method. The calculations have been made using the complete active space self‐consistent field (CASSCF) method, which is followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with several correlation‐consistent basis sets. The effects on the PECs by the core‐valence correlation and relativistic corrections are included. The way to consider the relativistic correction is to use the third‐order Douglas–Kroll Hamiltonian approximation. The relativistic corrections are made at the level of cc‐pV5Z basis set. The core‐valence correlation corrections are performed using the cc‐pCV5Z basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size‐extensivity errors by means of the Davidson modification (MRCI+Q). These PECs are extrapolated to the complete basis set limit by the total‐energy extrapolation scheme. Using these PECs, the spectroscopic parameters are determined and compared with those reported in the literature. With these PECs obtained by the MRCI+Q/CV+DK+56 calculations, the vibrational levels, inertial rotation, and centrifugal distortion constants of the first 20 vibrational state of each electronic state are calculated when the rotational quantum number J equals zero. Comparison with the Rydberg‐Klein‐Rees (RKR) data shows that the present results are reliable and accurate. The molecular constants of the X²Π and a⁴Σ^? electronic states determined by the MRCI+Q/CV+DK+56 calculations should be good prediction for future laboratory experiment. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

MRSDCI STUDIES OF LOW-LYING ELECTRONIC STATES OF THE N_2F~+ MOLECULE

Ab initio electronic structure calculations are reported for low-lying electronic states,X ~1Σ~+andA ~1Π of the N_2F~+ molecule.Geometric parameters for the ground state X ~1Σ~+ are predicted by means of mul-tireference single and double excitation configuration interaction(MRSDCI)calculations with a double zeta pluspolarization(DZ+P)basis set.Vertical excitation energy for these two electronic states is determined usingMRSDCI/DZ+P calculations at the ground state equilibrium geometry.The oscillator strength for the X~1Σ+→A ~1Π transition and the radiative lifetime for the A~1Π state are calculated based on the MRSDCI wavefunc-tions.     

Collisional quenching of OD A 2Σ+ by H2: experimental and theoretical studies of the state-resolved OD X 2Π product distribution and branching fraction

We report joint experimental and theoretical studies of outcomes resulting from the nonreactive quenching of electronically excited OD?A (2)Σ(+) by H(2). The experiments utilize a pump-probe technique to detect the OD?X (2)Π product state distribution under single collision conditions. The OD?X (2)Π products are observed primarily in their lowest vibrational state (v(") = 0) with substantially less population in v(") = 1. The OD?X (2)Π products are generated with a high degree of rotational excitation, peaking at N(") = 21 with an average rotational energy of 4600 cm(-1), and a strong propensity for populating the Π(A(')) Λ-doublet component indicative of alignment of the half-filled pπ orbital in the plane of OD rotation. Branching fraction measurements show that the nonreactive channel accounts for less than 20% of quenching outcomes. Complementary classical trajectory calculations of the postquenching dynamics are initiated from representative points along seams of conical intersections between the ground and excited-state potentials of OD(A (2)Σ(+),X (2)Π) + H(2). Diabatic modeling of the initial momenta in the dynamical calculations captures the key experimental trends: OD?X (2)Π products released primarily in their ground vibrational state with extensive rotational excitation and a branching ratio that strongly favors reactive quenching. The OD?A (2)Σ(+) + H(2) results are also compared with previous studies on the quenching of OH?A (2)Σ(+) + H(2); the two experimental studies show remarkably similar rotational energy distributions for the OH and OD?X (2)Π radical products.     

Communication: spectroscopic measurements for HfF+ of relevance to the investigation of fundamental constants

The properties of the HfF(+) cation are thought to be well-suited for investigations of the electron electric dipole moment (eEDM) and temporal variations of the fine structure constant. Precision spectroscopic measurements involving the X(1)Σ(+) and low-lying (3)Δ(1) states have been proposed to measure both. Due to the lack of data for HfF(+), the design of these experiments has relied entirely on the predictions of electronic structure calculations. Spectroscopic characterizations of the X(1)Σ(+), (3)Δ(1), (3)Δ(2) and (3)Δ(3) states are reported. The results further support the contention that HfF(+) is a viable candidate for eEDM measurements. The spacings between adjacent X(1)Σ(+) and (3)Δ(1) levels are found to be less favorable for the proposed studies of the fine structure constant.     

Spectroscopic constants and molecular properties of A3Σu+, B3Πg,W3Δu,and B′3Σu− electronic states of the N2 molecule

The potential energy curves (PECs) of A³Σ, B³Π_g, W³Δ_u, and B^′3Σ electronic states of the N₂ molecule have been studied for internuclear separations from 0.05 to 2.0 nm using the full valence complete active space self‐consistent‐field method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in conjunction with the correlation‐consistent basis sets. Effects on the PECs by the core–valence correlation and relativistic corrections are taken into account. The way to consider the relativistic correction is to use the second‐order Douglas‐Kroll Hamiltonian approximation. The core–valence correlation correction is made with the cc‐pCV5Z basis set. And the relativistic correction is performed at the level of cc‐pV5Z basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size‐extensivity errors by the Davidson modification (MRCI+Q). These PECs are extrapolated to the complete basis set limit. The spectroscopic parameters of ¹⁴N₂, ¹⁴N¹⁵N, and ¹⁵N₂ isotopologs have been evaluated and compared with those reported in the literature. Excellent agreement has been found between the present results and the Rydberg‐Klein‐Rees (RKR) data. With the PECs obtained by the MRCI+Q/CV+DK+56 calculations, the first 30 vibrational states for three species are computed for each electronic state. And for each electronic state of each species, the vibrational level G(ν), inertial rotation constant B_ν, and centrifugal distortion constant D_ν have been determined, which agree well with the RKR data. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Electronic spectroscopy of the previously unknown arsenic carbide (AsC) free radical

The previously unknown arsenic carbide (AsC) free radical has been identified in the gas phase through a combination of laser-induced fluorescence (LIF), single vibronic level emission, and stimulated emission pumping (SEP) spectroscopy in a supersonic expansion. The As(12)C and As(13)C isotopologues have been detected as products of an electric discharge in mixtures of arsine (AsH(3)) and carbon dioxide ((12)CO(2) or (13)CO(2)) in high pressure argon. The B (2)Σ(+)-X (2)Σ(+) band system was recorded by LIF spectroscopy and emission transitions from the B state down to the ground state and to the low-lying A (2)Π(i) state were observed. High resolution studies of the B-X 0-0 band by LIF and the B-A 0-0 band by SEP spectroscopy enabled a determination of the molecular structures in the three states. Although CN, CP, and AsC have similar X (2)Σ(+) and A (2)Π(i) states, the B (2)Σ(+) state molecular orbital configuration of CP and AsC differs from that of the CN free radical.     

Ion-pair dissociation dynamics of H2S in the photon energy range 15.26-15.55 eV

The H(+) velocity map images from the ion-pair dissociation of H(2)S + hν → SH(-)(X(1)Σ(+), υ = 0, 1) + H(+) have been measured at the excitation energies 15.259, 15.395, and 15.547 eV, respectively. The experimental results show that most of the available energies are transformed into the translational energies. The angular distributions of the fragments SH(-)(X(1)Σ(+), υ = 0) indicate that the dissociation occurs via pure parallel transition with limiting anisotropy parameter of +2. Because the ion-pair dissociation usually occurs via the predissociation of Rydberg states, this suggests that the ion cores of the excited Rydberg states have linear geometries. The geometries and electronic structures of the linear H(2)S(+) have been calculated employing the quantum chemistry calculation method at the CASPT2/avqz level. The electronic structures for the ion-pair states have been calculated at the CASSCF/avtz level, which indicates that the equilibrium geometries of the ion-pair states have bent geometries.     

Electronic spectroscopy and electronic structure of diatomic TiFe

Diatomic TiFe, a 12 valence electron molecule that is isoelectronic with Cr(2), has been spectroscopically investigated for the first time. In addition, the first computational study that includes the ground and excited electronic states is reported. Like Cr(2), TiFe has a (1)Σ(+) ground state that is dominated by the 1σ(2) 2σ(2) 1π(4) 1δ(4) configuration. Rotationally resolved spectroscopy has established a ground state bond length of 1.7024(3) A?, quite similar to that found for Cr(2) (r(0) = 1.6858 A?). Evidently, TiFe exhibits a high degree of multiple bonding. The vibronic spectrum is highly congested and intense to the blue of 20?000 cm(-1), while two extremely weak band systems, the [15.9](3)Π(1) ← X (1)Σ(+) and [16.2](3)Π(0+) ← X (1)Σ(+) systems, are found in the 16?000-18?500 cm(-1) region. The bond lengths, obtained by inversion of the B(e) (') values, and vibrational frequencies of the two upper states are nearly identical: 1.886?A? and 344 cm(-1) for [15.9](3)Π(1) and 1.884 A? and 349 cm(-1) for [16.2](3)Π(0+). The measured spin-orbit splitting of the (3)Π state is consistent with its assignment to the 1σ(2) 2σ(2) 1π(4) 1δ(3) 2π(1) configuration, as is also found in the ab initio calculations.