by Anion Photoelectron Imaging and Theoretical Calculations

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Probing Chemical Bonding and Electronic Structures in ThO− by Anion Photoelectron Imaging and Theoretical Calculations Yanli Li,† Jinghan Zou,‡ Xiao-Gen Xiong,*,† Jing Su,† Hua Xie,‡ Zejie Fei,† Zichao Tang,*,§ and Hongtao Liu*,† †

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China § State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡

ABSTRACT: Because of renewed research on thorium-based molten salt reactors, there is growing demand and interest in enhancing the knowledge of thorium chemistry both experimentally and theoretically. Compared with uranium, thorium has few chemical studies reported up to the present. Here we report the vibrationally resolved photoelectron imaging of the thorium monoxide anion. The electron affinity of ThO is first reported to be 0.707 ± 0.020 eV. Vibrational frequencies of the ThO molecule and its anion are determined from Franck−Condon simulation. Spectroscopic evidence is obtained for the two-electron transition in ThO−, indicating the strong electron correlation among the (7sσ)2(6dδ)1 electrons in ThO− and the (7sσ)2 electrons in ThO. These findings are explained by using quantum-chemical calculations including spin−orbit coupling, and the chemical bonding of gaseous ThO molecules is analyzed. The present work will enrich our understanding of bonding capacities with the 6d valence shell.

imaging is a very powerful technique to probe the fine electronic structures of 5f electrons.13−17 Thorium is one of the early actinide elements without 5f electron, the ground state configuration of Th atom is [Rn]6d27s2. The chemical properties of thorium are making sense in comparison with properties of lanthanide and group IV transition metals. Significant relativistic effects and strong electron correlation may lead many unique properties of thorium contained species.18,19 Thorium monoxide is known to be 1Σ+ ground state with a (7sσ)2 electron configuration.20,21 The vibration frequency, ionization energy, electric dipole moment, and other molecular constants have been well studied;22−31 the electronic structures are also reported both experimentally and theoretically.20,21 In comparison with the ThO neutral molecule, ThO− has received much less attention and the limited theoretical studies suggest the ground state of the anion to be 2Σ.20 Here we report the first experimental study of the ThO− anion using photoelectron velocity map imaging (VMI); still, vibrationally resolved PE imaging of ThO− is obtained and discussed in detail with theoretical calculations.

1. INTRODUCTION Actinide chemistry has received significant attention in recent years due to the increasingly valued nuclear industry and environmental pollution.1 In the nuclear power industry, uranium and uranium−plutonium oxide materials are widely used as the nuclear fuel.2−6 Despite the challenges for applying these materials, thorium oxides have been considered as potential nuclear fuels due to their distinctive advantages over uranium and plutonium compounds.7−10 Because of strong relativistic effects, thorium species exhibit many unique chemical bonding properties, so they are of great potential as catalysts.11,12 Owing to radioactivity and toxicity, the predictive power of theoretical calculations is of immense help in the field of actinide chemistry. Highly accurate calculations of actinidecontaining molecules are still very challenging in themselves on account of electron correlations and relativistic effects; therefore, the experimental data are essential to approve theoretical methods conducted to treating actinide-containing species. Recently, the Wang group reported a series of joint studies on uranium oxides and fluorides by anion photoelectron spectroscopy (PES) and theoretical calculation,13−17 PES reveals very complicated electronic structures even for the simple UO2 molecule.14,15 Strong electron correlation effects were observed from photodetachment of UO2−, which breaks down the one-electron MO picture and produces many unexpected excited state transitions in normal anion PES experiments. It has shown high resolution photoelectron © XXXX American Chemical Society

Received: November 16, 2016 Revised: February 21, 2017 Published: February 21, 2017 A

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2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2A. Photoelectron Velocity-Map Imaging. The experiment is carried out using an instrument with a laser vaporization source, a time-of-flight (TOF) mass spectrometer, and a collinear velocity-map photoelectron imaging analyzer. Details of the apparatus have been described elsewhere,32 and only a brief outline is given below. ThOn− (n = 1, 2) complexes are generated by laser vaporization of a freshly cleaned thorium dioxide target in the presence of a helium carrier gas. The cluster anions are cooled and expanded into the source chamber. The typical stagnation pressure of the carrier gas is about 1−3 atm. These anionic clusters are then extracted perpendicularly by a high pulsed voltage and analyzed by the TOF mass spectrometer. After separation in space, the anionic clusters of interest are introduced into the photodetachment region and crossed with a laser beam. Two photon energies, 1064 nm (1.165 eV) and 532 nm (2.331 eV) are used for photodetachment of these anionic clusters. Photoelectrons are analyzed by the orthogonal-extraction velocity-map photoelectron imaging analyzer. The electrons are mapped onto an image detector consisting of a microchannel plate (MCP) assembly and a phosphor screen. The positions of photoelectrons are recorded by a charge-coupled device (CCD) camera and one typical image is obtained by accumulating 50000−100000 laser shots at 10 Hz repetition rate. The obtained raw image reveals the projection of the photoelectron density in the three-dimensional (3D) laboratory frame onto the two-dimensional (2D) imaging detector. The original 3D distribution is reconstructed using the basis set expansion (BASEX) inverse Abel transform method and the photoelectron spectrum is acquired by integrating one central slice of the 3D distribution. The spectrometer is calibrated using the spectra of Bi− at 1064 and 532 nm. The energy resolution is better than 5%, corresponding to 50 meV at an electron kinetic energy (eKE) of 1 eV. The uncertainties of the experimental detachment energies are mainly caused by the restriction of this resolution. They are estimated by the product of the energy resolution and the kinetic energy of corresponding photoelectrons. 2B. Computational Methods. Both the density functional theory (DFT) and wave function theory (WFT) methods were used in this study. The geometries of the molecules were optimized at the generalized gradient approximation (GGA) level with PBE exchange−correlation functional33 as implemented in the Amsterdam Density Functional (ADF 2013.01) program.34 The scalar-relativistic (SR) effects are taken into account by the zero-order approximation (ZORA).35 The Slater basis sets with the quality of triple-ζ plus two polarization function (TZ2P) 36 were used, with the frozen core approximation applied to the [1s2-5d10] for Th and [1s2] for O. A series of different functionals including PBE, PBE0,29 B3LYP,37,38 TPSS, and TPSSh39 were also used to evaluate the performance of the DFT in the actinide molecules with the Gaussian 09 program.40 In the Gaussian calculations, for considering the SR effects of heavy element, we used Stuttgart energy-consistent relativistic pseudopotentials ECP60MDF41 and the corresponding valence triple-ζ basis sets cc-pVTZ-PP were for Th.42 All-electron basis sets cc-pVTZ were used for O.43 To further investigate the effects of electron correlation in the electronic structure of the anion and neutral molecule, we performed the WFT calculations using sophisticated electron

correlation methods implemented in the MOLPRO 2012.1 package.44 The coupled cluster singles and doubles plus perturbative triples (CCSD(T))45,46 and complete-activespace second-order perturbation theory (CASPT2)47−50 were used. The same basis sets used in previously mentioned Gaussian calculations were used in WFT calculations. The geometries of ThO and ThO− were optimized at the level of CCSD(T) with SR effects included through the ECP approach. Single-point calculation of the neutral molecule at the optimized anion geometry will produce the first vertical detachment energy (VDE1). To determine the spin−orbit effects of the anion and the neutral molecule at the anion geometry, which substantially influence the VDEs in ThO−, we performed the CASSCF/CASPT2/SO calculations for both the anion and neutral molecule at the CCSD(T) optimized anion geometry. The active spaces include 16 orbitals from Th 7s, 7p, 6d, and 5f shells, i.e., 5σ, 1δ, 3π, 6σ, 1ϕ, 2δ, 4π, 7σ, 8σ, and 5π, as shown in Figure 4. SO-averaged CASPT2 calculations were carried out on all the singlet and triplet states generated by this active space. In the CASPT2 calculations, the ionization potential electron affinity (IPEA)-corrected zeroth-order Hamiltonian51 was used with and IPEA shift of 0.25 au to reduce the systematic error which can lead to a relative overestimation of the correlation energy for open shell molecules. The level shift technique with 0.2 au was also used to avoid intruder states and improve the convergence. The SO coupling effect was included by using a state-interacting method with SO pseudopotentials,52,53 and the SO splittings were treated as a perturbation to the scalar relativistic state energies and were calculated with the diagonal matrix elements replaced by the individual CASPT2 state energies.54 This CASSCF/CASPT2/SO approach has been used previously to study the strong electron correlation in UO2−.14

3. RESULTS AND DISCUSSION 3A. Photoelectron Spectra of ThO−. Figure 1 shows the photoelectron imaging results of ThO− taken at 1064 and 532 nm, respectively. At 1064 nm, a well-resolved band labeled as X corresponds to the detachment transition from the ground state of ThO− to that of neutral ThO molecule. The EA of ThO or ADE of ThO− was measured with a value of 0.707 ± 0.020 eV. A weak vibrational progression was resolved and Franck−

Figure 1. Photoelectron images and spectra of ThO− obtained at (a) 1064 nm and (b) 532 nm. The double arrow indicates the laser polarization. B

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ThO at the optimized geometry, the geometry optimization parameters are shown in Table 2. With the help of Franck− Table 2. Geometry Optimized Parameters for the Ground State of ThO and ThO− at Different Levelsa R(Th−O)/Å method

ThO

ThO−

PBE(ADF) PBE PBE0 B3LYP TPSS TPSSh CCSD(T)

1.848 1.844 1.828 1.844 1.846 1.839 1.845

1.890 1.890 1.873 1.892 1.891 1.884 1.890

a

ADF/PBE/TZ2P/SR-ZORA; Gaussian09/DFT/cc-PVTZ; Molpro/ CCSD(T)/cc-PVTZ.

Condon simulation, the bond length of the ground state of ThO− is 1.87 Å, and the optimized bond length of ThO− is 1.890 Å at the CCSD(T) level and the ground state of ThO− is predicted as the 2Δ state in all theoretical calculation levels. The calculated EA is 0.68 eV, which is in good agreement with the experimental value 0.707 eV. The low lying excited states of ThO− were determined by performing CASSCF/CASPT2/SO calculations, and the details are listed in Table 3. ThO− 2Δ state

Figure 2. Franck−Condon simulation for the PES measured at 1064 and 532 nm. The red line is the simulated spectrum; the vertical blue lines are the calculated Franck−Condon factors.

Condon simulation (the details shown in Figure 2) using PESCAL program55 revealed a frequency of about 895 cm−1 for the 1Σ+ ground state of ThO. At 532 nm, two excited states were revealed for ThO. The assignment of these bands was aided by the comparison with the literature22,25,56−58 and is given in detail in Table 1. For convenient comparison with

Table 3. Ground State and Low Lying Excited States of ThO−

Table 1. Observed and Calculated Vertical Detachment Energies (VDE) and Final State Assignments for ThOa final state X A B C D E F

Σ Δ1 3 Δ2 3 Δ3 3 Π0 3 Π1 1 Δ2 3 Π2 1 + 3

expb

calcb

composition of SR states

0.707 1.37 1.47 1.57 2.04

0.68 1.23 1.35 1.56 2.09 2.21 2.12 2.43 2.61

100%1Σ+ 99%3Δ + 1%3Π 94%3Δ + 5%1Δ + 1%3Π 100%3Δ 100%3Π 87% 3Π + 13%1Π 89%1Δ + 11%3Δ 95% 3Π + 5%1Δ 87% 1Π + 13%3Π

2.14 2.25

CASSCF/CASPT2/SO

expc

calcd

1.36 1.47 1.76 2.02 2.09 2.17

1.42 1.54 1.77 2.05 2.18 2.25 2.50

SO state

composition of SR states

ΔE/eV

E3/2 E5/2 E1/2 E3/2

99.5% Δ + 0.5% Π 100%2Δ 100%2Π 99.5%2Π + 0.5%2Δ

0.00 0.27 0.31 0.50

2

2

has a 0.27 eV spin−orbital (SO) splitting energy between 2Δ3/2 and 2Δ5/2. The calculated term energies of two ThO− 2Π excited states are 0.31 eV for 2Π1/2 and 0.50 eV for 2Π3/2 above the 2Δ3/2 ground state, respectively. Those excited states have very low populations in our experimental conditions with supersonic cooling. So the two small peaks less than 0.707 eV are vibrational hot bands (label “hb” in Figure 1a), which yield a vibrational frequency of 810 cm−1 from the Franck−Condon simulation (Figure 2) for the ground state of ThO−. Figure 3 shows the qualitative Kohn−Sham valence molecular orbital (MO) energy-level scheme for the ThO− ground state, and the corresponding contour plots of the valence MOs of ThO− are displayed in Figure 4. The O 2Pz orbital interacts with the Th 6dz2 and 7s orbitals to form the bonding 4σ (HOMO−1), and the O 2Px/y orbital interacts with the Th 6dxz/yz orbital to form the bonding 2π (HOMO−2). The bonding interactions 2σ (HOMO−5) between O 2s-2Pz and Th 6Pz are canceled with antibonding 3σ (HOMO−3) MOs. On the basis of the MO configuration, the ThO− anion and ThO neutral molecule possess a single σ bond derived from the 4σ bonding MO and a single π bond derived from the 2π bonding MO. The singly occupied 1δ orbital is nonbonding MO from the Th 6d orbital, and detachment of one electron from the Th(6d)-based 1δ orbital will produce the ThO 1Σ+ ground state. Small bond length variation attributed to the reduction of

a The calculated results are based on CASSCF/CASPT2/SO calculations and the previously reported experimental and theoretical values are also listed. All energies are in eV. bPresent work. cAvailable experimental data. References 22, 25, 56, and 57. dReference 58.

anion PES, all the literature ThO excited state term energies were added with 0.707 eV (EA of ThO). The first excited state of ThO is 3Δ. Due to SO splitting, 3Δ splits into band A (3Δ1) at 1.37 ± 0.03 eV, band B (3Δ2) at 1.47 ± 0.03 eV, and band C (3Δ3) at 1.57 ± 0.03 eV, respectively. Only band C displayed weak vibrational progression, probably because vibrational progression of bands A and B overlapped with band C due to the limited resolution. The peak labeled E at binding energy of 2.14 ± 0.03 eV is the 1Δ2 state. The next excited state of ThO is 3 Π; the bands D and F at 2.04 ± 0.03 and 2.25 ± 0.03 eV are assigned to the 3Π0 and 3Π2 states, respectively. 3B. Discussion. To better understand and explain the PES of ThO−, we performed plenty of high level ab initio calculations on the ground and excited states for ThO− and C

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electron from Th(7s)-based 5σ orbital will produce ThO excited states 3Δ or 1Δ. Considering the significant spin−orbit coupling in actinide elements, the 3Δ of ThO will split into 3Δ1, 3 Δ2, and 3Δ3, corresponding to bands A, B, and C in Figure 1b, respectively. Vibrational progression of bands A and B was buried by the limited spectrum resolution, and only band C (3Δ3) displayed a short vibrational progression. The calculated VDE (1.56 eV) of band C (3Δ3) is in approximate agreement with the experimental result (1.57 ± 0.03 eV), whereas the computed VDEs for bands A and B (1.23 and 1.35 eV, respectively) are slightly underestimated in comparison with experimental values (1.37 ± 0.03 and 1.47 ± 0.03 eV, respectively), as displayed in Table 1, probably because there is slight configuration mixing in states 3Δ1 and 3Δ2. However, band C is completely in the 3Δ3 state and has no mixing of other SO-free states. The peak labeled E at a binding energy of 2.14 ± 0.03 eV is assigned as the ThO 1Δ2 state. Our calculation shows detachment from the 4σ (HOMO−1) orbital of ThO− requests 5.42 eV and would produce an energetically much higher 3Δ state of ThO, which is far beyond the 532 nm laser (photon energy 2.33 eV) detachment limit. Energetically, bands D and F could be assigned as the 3Π state, this detachment channel is a two-electron transition (detaching a 6d electron from the 1δ orbital and at the same time exciting another 7s electron from the 5σ orbital to the 3π orbital, resulting in the 3Π and 1Π final states). Due to the spin−orbit coupling, 3Π will split into 3Π0, 3Π1, and 3Π2. Because of the limitation of experimental resolution, we only observed band D (3Π0) and band F (3Π2) whereas the 3Π1 state was covered between bands D and E. The calculated VDE (2.09 eV) for band D is in excellent agreement with the experimental value (2.04 ± 0.03 eV), whereas the calculated VDE (2.43 eV) for band F is slightly overestimated compared to the experimental result (2.25 ± 0.03 eV), probably because the 3Π1 state has a mixing of 13% from a 1Π state. Generally, two-electron transitions exhibit a very low cross section, and the enhancement by the electron correlation effect usually occurs for the involved two electrons populated at the same orbital, such as the copper 5s and gold 7s orbitals.61,62 Our experimental and theoretical studies indicate there is very strong correlation between the 7s and 6d electrons in ThO−, and the intensity of 3 Π state observed here is almost comparable to those of the main transitions. Such strong electron correlation effects has been reported for the photodetachment of UO2−,14 where strong electron correlation effects happen between 7s and 5f electrons and directly reflects configuration mixing in the ground state of UO2−, whereas the magnitude of two-electron transitions are weaker than those in ThO−, which can be seen from the intensity of two-electron excited states, probably arising from the cross section between s and f orbitals being much smaller than that between s and d orbitals. Different from ThO− and UO2−, there is very weak electron correlation effect in the photodetachment of CeO−,63 in which all are single electron transition, even though the ground state of CeO− has the 4f16s2 Ce+ superconfiguration.63 The reasons for this phenomenon are perhaps the 4f and 6s orbital energy difference is bigger and 4f orbital contracts while the 6s orbital expands resulted in the difference of electronic cloud distribution spatially. Another important piece of information gained from photoelectron imaging is the anisotropy parameter (β), which reflects structures of the parent orbital and the angular distributions of outgoing photoelectrons. In a single-photon

Figure 3. Kohn−Sham valence molecular orbital energy-level scheme for ThO−.

Figure 4. Contour plots of the valence MOs of ThO− at DFT/PBE level (isosurface = 0.05 au) via 2σ21π43σ*22π44σ25σ21δ1 electron configuration.

the static repulsion between Th and O is expected for the detachment of an electron from the nonbonding MO, which can be verified through the charge population analysis (Table 4), hence explaining the observed short vibrational progression in X (1Σ+) state. Our theoretical calculations at the CCSD(T) level suggest the bond length of the ThO ground state is 0.065 Å shorter than ThO−, which is consistent with our expectation. The highest occupied 5σ orbital is another nonbonding MO mainly from the Th 7s atomic orbital, and detachment of one Table 4. Theoretical Atomic Charges on Th and O in ThO− and ThO ThO− a

Voronoi MDC-qb a

ThO

Th

O

Th

O

−0.442 −0.052

−0.558 −0.947

0.306 0.904

−0.306 −0.904

Reference 59. bReference 60. D

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Liu acknowledges support of Hundred Talents Program (CAS) and Shanghai Pujiang Program. We are very grateful for Professor Michael C. Heaven for valuable discussions in preparation of the manuscript.

process with linear polarized light, the selection rule of oneelectron atomic transition is Δl = ±1, where l is the angular momentum quantum number. Although the Cooper−Zare model64,65 fits well for the atomic cases where l is no longer a good quantum number in the molecular orbital, the SOMO orbital (1δ molecular orbital) in ThO− is almost all made up of the Th-6d orbital and the HOMO orbital (5σ molecular orbital) in ThO− is nearly all composed of the Th-5s orbital. Hence, the SOMO and HOMO orbitals can be described as dlike and s-like orbitals, respectively. According to the Cooper− Zare formula,64,65 for l = 2 the asymmetry parameter will be positive at small eKE before becoming negative and detachment from a pure s orbital will produce a p wave whose β value is more than zero. All are consistent with the observed experimental phenomenon. Generally, detachment from the 3d orbital in a transition-metal always produces an outgoing p wave domain; thus, the β values are more than zero but the ring of peak X (Figure 1) reveals a β value of less than zero. A similar phenomenon had been observed in photoelectron imaging of O2− and S2− in which the HOMO orbital is a d-like orbital.66,67 Meanwhile, detachment from a π orbital in UO− also delivers an abnormal β value of more than zero.15 These different appearances reveal there is a strong need to develop theoretical model to interpret photoelectron angular distribution when sharp interest is increasing in photoelectron imaging.

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4. CONCLUSION In conclusion, we have obtained vibrational-resolved photoelectron spectroscopy of ThO− at different photon energies using anion photoelectron imaging and interpreted it on the basis of quantum-chemistry calculation. The electron affinity of 0.707 ± 0.020 eV has been determined for ThO. Specifically, a two-electron transition was viewed in the spectrum of ThO− due to the shakeup process, suggesting strong electron correlation effects in thorium. High level ab initio calculations were performed to aid the interpretation of the experimental spectra, and the computational data permit a thorough understanding of the electronic structures and chemical bonding in ThO. Both strong spin−orbit splitting and electron correlation effects were discovered to be important to achieve good consistence between the calculated values and experimental data. Our study offers accurate experimental electronic and vibrational information, which will be helpful to affirm further theoretical development.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*X.-G. Xiong. E-mail: [email protected]. *Z. Tang. E-mail: [email protected]. *H. Liu. E-mal: [email protected]. ORCID

Hongtao Liu: 0000-0001-6450-2585 Author Contributions

Yanli Li and Jinghan Zou contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573272, 21501189, 21571185, 11605273) and “Strategic Priority Research Program” of the Chinese Academy of Sciences, Grant No. XDA02020000. H. T. E

DOI: 10.1021/acs.jpca.6b11554 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpca.6b11554 J. Phys. Chem. A XXXX, XXX, XXX−XXX