Probing the Geometric and Electronic Structures of the

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Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Probing the Geometric and Electronic Structures of the Monogadolinium Oxide GdOn−1/0 (n = 1−4) Clusters Jing Chen,†,‡ Huan Yang,§ Jing Wang,† and Shi-Bo Cheng*,† †

School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China Suzhou Institute of Shandong University, Suzhou, Jiangsu 215123, China § School of Physics, Shandong University, Jinan 250100, China

J. Phys. Chem. A Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 10/27/18. For personal use only.

‡

ABSTRACT: The existence of abundant 4f electrons significantly increases the complexity and difficulty in precisely determining the geometric and electronic structures of the lanthanide oxide clusters. Herein, by combining the photoelectron imaging spectroscopy and density functional theory (DFT) calculations, the electronic structure of GdO was investigated. An electron affinity (EA) of 1.16 ± 0.09 eV is obtained, and the measured anisotropy parameter (β) provides direct experimental evidence about the orbital symmetry of the detached electron in GdO−. DFT calculations have been employed to acquire the optimized geometries of the GdOn−1/0 (n = 2−4) clusters, and multiple activated oxygen species, which are radical, peroxide, superoxide, triradical, and ozonide radical, are found in these oxide clusters. Simulated photoelectron spectra (PES) of the GdOn−1/0 (n = 2−4) clusters are examined, which may stimulate further experimental investigations on the gadolinium oxide clusters. In addition, the valence molecular orbitals (MOs) of these clusters are also discussed to reveal the interaction between the lanthanide metal (Gd) and O atoms.

1. INTRODUCTION Owing to their potential applications in catalysts, optics, electronics, biomaterials, and magnetics, etc., lanthanide oxides have stimulated considerable interest during the past several decades.1−9 The most distinctive feature of lanthanide metals is their variable 4f occupation. This results in complicated electronic structures of lanthanide oxides, posing extreme challenges to both theoretical and experimental investigations. Among the lanthanide group elements, gadolinium (Gd) possesses the highest spin multiplicity with a [Xe]4f75d16s2 atomic configuration. The existence of various valence atomic orbitals (AOs), namely, s, d, and f, will increase the complexity of the electronic structures and spectra of the Gd-doped oxide clusters. Note that the study of atomic clusters could provide valuable molecular models and insights into examining how atomic or molecular properties evolve into those of the condensed phases. Therefore, exploring the electronic structures of gadolinium oxide clusters is particularly important, which has, however, not been well-understood so far. Diatomic GdO, as the simplest gadolinium oxide cluster, has been investigated previously. By employing the electron spin resonance (ESR) technique, Van Zee et al. proposed a 9Σ− ground-state for GdO.10 The emission spectra of GdO were obtained by Yadav et al., and the bond length of Gd−O was suggested to be 1.812 Å.11 In addition, DeKock and Weltner performed infrared spectroscopy (IR) in a matrix at 4 K, and found a vibrational frequency of 824 cm−1 for GdO.12 The photoelectron spectrum of GdO− was also recorded by Klingeler et al., and the EA of GdO was measured to be 1.19 eV by using the magnetic-bottle photoelectron spectrometer.13 They argued © XXXX American Chemical Society

that the additional electron in the anion occupies the 6s orbital according to the theoretical electron configuration, which could not be detected experimentally by the conventional magneticbottle technique. The photoelectron imaging spectroscopic technique, however, can undoubtfully resolve this issue since it can simultaneously acquire the PES and the photoelectron angular distribution (PAD) of clusters, which will be discussed below. Apart from experimental efforts, numerous theoretical methods have been attempted to account for the geometry and electronic states of GdO.14−19 It is necessary to note that the previous theoretical study using B3LYP with the 6-311++G(df) basis set for O and SDD basis set for Gd estimated an EA of 0.689 eV for GdO,17 which is much smaller than the experimental measurement (1.19 eV).13 This discrepancy demonstrates that there is indeed a challenge in describing the electronic structures of the 4f-rich gadolinium oxide clusters precisely, which also motivates us to conduct the present investigations. In this study, with the aim of determining the orbital symmetry of the attached electron in GdO− and resolving the discrepancy in previous experiment and theory about the EA of GdO, we have visited the PES of GdO− by employing advanced photoelectron imaging spectroscopy. The current experimental findings offer solid evidence that the photodetachment stems from the σ-type molecular orbital (MO) of GdO−, verifying the previous theoretical assumption. The EA of GdO calculated here Received: September 16, 2018 Revised: October 16, 2018 Published: October 17, 2018 A

DOI: 10.1021/acs.jpca.8b09058 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

predict theoretical VDEs in line with experimental PES results,35−38 and has also been applied to calculate VDEs in many theoretical studies recently.39−41

is in good agreement with the experimental value, showing the accuracy of the present theoretical level in predicting the electronic structures of Gd-doped oxide clusters. In addition, the geometries, electronic structures, and simulated PES of the GdOn−1/0 (n = 2−4) clusters were also investigated theoretically, which may stimulate further experimental efforts on these gadolinium oxide clusters.

4. RESULTS AND DISCUSSION 4.1. Photoelectron Imaging Spectrum of GdO−. The photoelectron imaging spectrum of GdO− taken at 532 nm (2.33 eV) is shown in Figure 1. As shown in the figure, three

2. EXPERIMENTAL SECTION The details of the experimental apparatus used in the current investigation have been described previously.20,21 Briefly, in the laser vaporization (LaVa) source, the second harmonic (532 nm) output of a Nd:YAG laser with a frequency of 10 Hz was focused to ablate a pure Gd rod (0.25 in. diameter). A mixture of 5% N2O seeded in a high-purity helium gas with a total pressure of about 50 psi was delivered by a pulsed valve to provide the oxygen source in the present experiment. The formed negative ions were extracted and subsequently analyzed in a Wiley− McLaren time-of-flight mass spectrometer.22 The mass selected anionic cluster of interest, GdO−, was subjected to another pulsed 532 nm Nd:YAG laser in the photodetachment region, and photodetached electrons were directed toward a detector including a 40 mm diameter microchannel plate (MCP) and a phosphor screen. A charge-coupled device (CCD) camera was applied to record the images generated on the phosphor screen. Last, the photoelectron images were reconstructed to obtain the photoelectron kinetic energy spectra, which were calibrated on the basis of the known spectrum of Bi−.23

Figure 1. Anion photoelectron spectrum and the corresponding photoelectron image of the GdO− cluster obtained using 532 nm photon energy.

transition bands, labeled X′, X, and A, are identified from the PES of GdO−. Among three bands, X represents the transition from the ground-state GdO− to its corresponding neutral ground-state, from which a VDE of 1.16 ± 0.09 eV is determined from the band maximum. Note that this result is in good agreement with the previously reported value of 1.19 eV measured from the conventional magnetic-bottle experiment.13 Moreover, X features a vibrational progression, yielding a vibrational frequency of about 871 ± 60 cm−1 for GdO. Therefore, the ADE of GdO− is 1.16 ± 0.09 eV based on the vibrationally resolved PES, representing the EA of GdO as well. As mentioned earlier, compared with the conventional magnetic-bottle PES, advanced photoelectron imaging spectroscopy could not only measure the kinetic energy distributions of photoelectrons but also draw pictures about the MOs from which the photodetachments occur by mapping the PAD.21,42−44 In the present experiment, as displayed in Figure 1, the PAD of X is preferably oriented parallel to the laser polarization. The β for this band is measured to be 1.67 ± 0.01. Such a high β value is always an indicator for a photodetachment process involving mainly the σ-type MO,45,46 which provides direct experimental evidence for the previous theoretical assumption.13 A lower-intensity band (A), whose VDE is 1.39 ± 0.05 eV, lies about 0.23 eV (1855 cm−1) higher in energy than X. This could be assigned as the transition between the groundstate of GdO− and the 7Σ− neutral excited-state. This VDE difference (1855 cm−1) represents the excitation energy between the 9Σ− and 7Σ− of GdO, which agrees well with the previous experimental result (1837.6 cm−1).47 Similarly, a β value of 1.61 ± 0.01 is determined for band A, which implies a σtype photodetachment channel as well. In addition, a hot band transition X′ located at around 1.06 eV has also been disclosed in Figure 1. This peak most likely comes from the transition between the excited-state of GdO− and the ground-state of GdO.

3. COMPUTATIONAL METHOD The Gaussian 09 program package24 was used to perform the calculations in the present study. The geometric and electronic structures of the GdOn−1/0 (n = 1−4) clusters were calculated using DFT with the B3LYP hybrid functional,25,26 which has been proven to be an accurate theoretical method to predict the physicochemical properties of lanthanide or lanthanide oxide clusters.27−30 Global minima and low-lying isomers of the GdOn−1/0 (n = 1−4) clusters were searched by considering various initial geometries and spin states. In order to account for the relativistic effects, the Gd AO was described by a 28-electron pseudopotential with atomic natural orbital (ANO) basis, in which the ANO contracted Gaussian valence basis set consists of (14s13p10d8f6g)/[6s6p5d4f3g] functions centered on Gd.31 Additionally, the augmented correlation consistent triple-ζ (aug-cc-pvtz) basis set was applied for the O atom.32,33 In the following parts of this paper, the term B3LYP/ANOTZ will be used as an abbreviation of the above-mentioned theoretical level. Moreover, vibrational frequency calculations were accomplished to verify the nature of the stationary points, and all geometric structures discussed herein are true minima. The adiabatic detachment energy (ADE) of GdO− was calculated as an energy difference between the neutral species and anion at each optimized structure including zero-point energy (ZPE) corrections, while the vertical detachment energies (VDEs) were obtained employing the generalized Koopmans’ theorem by adding a correction term (δE) to the eigenvalues of the anion.34 The correction term was calculated as δE = E1 − E2 − εHOMO, where E1 and E2 are the total energies of the anion and the neutral cluster with the anionic equilibrium geometry, and εHOMO represents the eigenvalue of the highest occupied molecular orbital (HOMO) of the anion. Note that the generalized Koopmans’ theorem has been demonstrated to B


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optimized structures, the ADE and VDEs of GdO− were computed to compare with our experimentally measured values, which are listed in Table 1. The previous theoretical result about the ADE of GdO− is also included. As shown in the table, the deviations between the present theoretical and experimental ADE and VDEs of GdO− are around 0.1 eV, representing a significant improvement with respect to the previous theoretical deviation of 0.471 eV.17 These findings demonstrate that the theoretical level (B3LYP/ANOTZ) used here is more appropriate to describe the electronic structures of gadolinium oxide clusters than the prior one. Meanwhile, the computed energy gap between bands X and A is 0.22 eV, which is in excellent agreement with the previously reported experimental result of 0.23 eV (1837.6 cm−1).47 In addition, the frequency of the symmetric Gd−O stretching mode of structure 1A is computed to be 843 cm−1, which is in line with the measured value of 871 ± 60 cm−1 obtained in the present experiment. 4.2. Optimized Structures of the GdOn0/−1 (n = 2−4) Clusters. In this part, we want to turn our attention to shedding light on the geometric structures of O-rich gadolinium oxides since the present B3LYP/ANOTZ theoretical level has been proven to be a reliable method to describe the properties of gadolinium oxide clusters. The optimized global minima and low-lying isomers of the GdOn0/−1 (n = 2−4) clusters are depicted in Figure 2. The lowest-energy structure (2A) of GdO2 possesses a bent structure with septet spin multiplicity, in which the GdO bond length is 1.911 Å and the ∠OGdO bond angle is around 122°. The nonet GdO2 (2B) is calculated to be 0.18 eV higher in energy than that of 2A. Structure 2B features an elongated Gd O• single bond (2.077 Å) and a typical GdO double bond (1.833 Å). As for anionic GdO2, the structure of the global minimum of GdO2 (2a) is similar to that of the neutral 2A with a slightly elongated GdO bond distance (1.938 Å) and a decreased ∠OGdO bond angle (110°). The anionic structure 2b is a 0.77 eV higher-energy isomer with respect to 2a, which features a GdO double bond (1.883 Å) and an elongated GdO• single bond (2.143 Å). As shown in Figure 2, the lowest-energy structure of GdO3 is 3A (7A′), which is 0.03 eV lower in energy than its nonet isomer 3B (9A). Structures 3A and 3B are almost identical in geometry, both featuring a GdO double bond (1.821 or 1.819 Å) and an elongated O2 unit (1.331 Å), typical for a superoxide species.48−54 Another higher-energy isomer of GdO3 is 3C with a 9A″ electronic state, whose ΔE is 1.89 eV with respect to 3A. Interestingly, structure 3C possesses three elongated Gd O• single bonds, which may be considered as a triradical species. The global minimum of GdO3− is 3a according to the present calculations, featuring a typical peroxide unit (1.517 Å).48−54 Additionally, another two isomers 3b and 3c are also optimized, which are 0.22 and 0.36 eV higher in energy than that of the ground-state of GdO3− (3a).

To explicate the present experimental PES, calculations about the optimized geometries of GdO−1/0, the theoretical ADE and VDEs of GdO−, were explored, which are listed in Figure 2 and

Figure 2. Theoretical ground-states and low-lying isomers of the neutral and anionic GdOn (n = 1−4) clusters calculated at the B3LYP/ ANOTZ level of theory. Electronic states, relative energies (ΔE), and geometric parameters including bond lengths (in Å) and bond angles (in deg) of different clusters are shown. The dark red and light blue spheres represent Gd and O atoms, respectively.

Table 1, respectively. As shown in Figure 2, the global minima of GdO (1A) and GdO− (1a) have nonet and octet spin multiplicities, respectively. The bond length for GdO is calculated to be 1.802 Å, which is consistent with the previously reported value of 1.812 Å determined by the emission spectra of GdO.11 As mentioned earlier, one motivation of the present study is to discover a precise theoretical level that can better predict the electronic structures of gadolinium oxide clusters. Therefore, to further diagnose the accuracy of the theoretical level (B3LYP/ANOTZ) used here and the theoretically

Table 1. Experimental and Theoretical ADE, VDEs, and Experimental β Parameter of 8GdO− ADE (eV) theor a

species

bands

exptl

GdO−

X A

1.16(9)

8

VDE (eV)

this work

previous

1.05

0.689

b

exptl

theor this work

β

1.16(9) 1.39(5)

1.07 1.29

1.67(1) 1.61(1)

a

Numbers in the parentheses represent experimental uncertainties in the last digit, which are obtained by calculating the standard deviation of multimeasurements. bThe previous result about the ADE of 8GdO− comes from ref 17. C


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The Journal of Physical Chemistry A Four isomers of neutral GdO4 are located within 0.50 eV, which are labeled 4A−4D. Structure 4A, the global minimum of GdO4, is calculated to be a septet, which features one peroxide (1.504 Å) and one superoxide (1.335 Å) unit. The following two isomers, 4B and 4C, have geometries similar to that of 4A, both possessing one peroxide and one superoxide unit, respectively. The total energies of these two isomers are almost degenerate, which are 0.03 and 0.04 eV higher in energy than that of 4A. Located 0.44 eV above the global minimum, structure 4D (7A) differs apparently from the above-mentioned three isomers. This isomer (4D) has a GdO double bond (1.817 Å) and an ozonide radical (O3•) unit. In the case of GdO4−, as shown in Figure 2, the first three low-lying isomers with different spin multiplicities (4a, 4b, and 4c) are calculated to have similar geometry and symmetry. The global minimum of GdO4− (4a) is a sextet, which possesses a superoxide (1.335 Å) unit. The octet and dectet isomers, 4b and 4c, are estimated to be 0.02 and 0.17 eV less stable than 4a, respectively. Structure 4d located 0.67 eV above the global minimum 4a is an octet and possesses two peroxide (∼1.517 Å) units. As can be seen from Figure 2, some isomers of different oxide clusters are almost energetically degenerate, such as 4a and 4b of GdO4−. Considering the complicated nature of lanthanide elements, the challenge in calculating the absolute total energies of species containing lanthanide metals, and the quantitative accuracy of the present theoretical level, the ΔE arrangements for these degenerate clusters should be considered tentatively solely based on the theoretical prediction. Future highresolution experiments, therefore, are urgently needed to provide evidence in the isomer assignment. Here, we will attempt to simulate the PES of the corresponding oxide clusters, which may be helpful in the isomer identification in future experiments. 4.3. Simulated Photoelectron Spectra and Molecular Orbital Analyses. It is well-accepted that the photoelectron spectroscopic technique serves as an electronic fingerprint for a given species, which could assist in the isomer identification. Specifically, the simulated PES can provide valuable information about electronic structures of clusters, allowing comparison with future experiments. Herein, the theoretical VDEs of the groundstates and low-lying isomers (within 0.3 eV) of GdOn− (n = 1− 4) have been calculated using the generalized Koopmans' theorem at the B3LYP/ANOTZ level of theory, and these are collected in Tables 2 and 3, respectively. Meanwhile, the simulated PES for GdOn− (n = 2−4) are depicted in Figures 3 and 4, and three-dimensional plots of the corresponding valence MOs of clusters are displayed in Figure 5. As shown in Figure 2, all clusters studied here are open-shell. Thus, photodetachment from a fully occupied MO would lead to two transitions due to the removal of either the α or β spin electron. As can be seen from Table 2, the theoretical VDEs of GdO− (8Σ−) for bands X and A are 1.07 and 1.29 eV, respectively, which are in good agreement with the present experimental measurements with the values of 1.16 and 1.39 eV (Table 1). Moreover, these transitions are calculated to originate from the 10σ MO of GdO−, consistent with the experimental β values (1.67 and 1.61) mentioned above. This finding offers direct evidence demonstrating the previous theoretical assumption that the photodetachment occurs from the σ-type MO of GdO−. In the following part, we will attempt to qualitatively describe the features of the simulated PES of the GdOn− (n = 2−4) clusters. The ground-state of GdO 2 − is an octet with a ...f 7 (11a1)2(8b2)2(3a2)2(6b1)2(12a1)2(9b2)2 valence electron con-

Table 2. Theoretical VDEs of the Ground-States and LowLying Isomers of the GdOn− (n = 1−3) Clusters Calculated at the B3LYP/ANOTZ Level of Theorya species GdO− (8Σ−) GdO2− (8A2)

feature X A X A B C D E

MO

VDE (eV)

10σ (α) 9b2 (α)

1.29 1.79

12a1 (α) 6b1 (α)

2.42 2.66

3a2 (α) 8b2 (α)

3.00 3.04

F

GdO3− (8A″ 0.00 eV)

G X A B

GdO3− (8A″ 0.22 eV)

C D E X A B C

11a1 (α) 12a″ (α)

3.42 2.39

11a″ (α) 22a′ (α) 21a′ (α)

3.13 3.16 3.43

10a″ (α) 24a′ (α)

3.81 3.42

23a′ (α) 10a″ (α) 22a′ (α) 9a″ (α)

3.71 3.79 3.95 4.29

8a″ (α)

5.00

D

E a

MO

VDE (eV)

10σ (β)

1.07

9b2 (β)

1.99

12a1 (β) 6b1 (β)

2.62 2.83

3a2 (β) 8b2 (β) 11a1 (β)

3.16 3.17 3.43

12a″ (β) 11a″ (β) 22a′ (β)

2.50 3.25 3.26

21a′ (β) 10a″ (β)

3.59 3.87

22a′ (β)

3.77

10a″ (β) 20a′ (β) 8a″ (β) 19a′ (β) 9a″ (β) 21a′ (β)

4.15 4.16 4.55 4.64 4.69 4.86

The majority and minority spins are labeled as “α” and “β”.

figuration, where f7 represents seven single-occupied MOs with a primary contribution from seven delocalized f electrons of the Gd atom, respectively. According to the present calculations, detachment from the HOMO (9b2) of GdO2− (Figure 5b) gives rise to the first two bands (X and A in Figure 3a) with the calculated VDEs of 1.79 and 1.99 eV, respectively. The next detachment channel (B in Figure 3a) located at around 2.42 eV stems from the removal of an electron of 12a1(α) (Figure 5b), a σ-type MO consisting of the 2py AO from O atoms and the 6s AO from Gd. Another three higher-energy bands, C, E, and F in Figure 3a, come from the detachments of the combination of the σ- and π-type MOs, and the calculated VDEs for these peaks are 2.62, 3.02, and 3.16 eV, respectively. For instance, the removal of electrons from 12a1(β) (a σ-type MO as mentioned above) and 6b1(α) (a π-type MO involving the interaction between the 2px AOs from O atoms and the 5dxz AO from Gd) MOs of GdO2− (Figure 5b) yields the C band. The remaining two detachment channels, D and G in Figure 3a, are from MOs 6b1(β) and 11a1(α, β) with the theoretical VDEs of 2.83 and 3.43 eV, respectively. The simulated PES of the ground-state and low-lying isomer of the GdO3− clusters are depicted in Figure 3b,c. The groundstate of GdO3− is an octet, which possesses a ...f7 (18a′)2(19a′)2(20a′)2(10a″)2(21a′)2(22a′)2(11a″)2(12a″)2 valence electron configuration. As shown in Figure 3b, the first broadened band D


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The Journal of Physical Chemistry A Table 3. Theoretical VDEs of the Ground-States and LowLying Isomers of the GdO4− Clusters Calculated at the B3LYP/ANOTZ Level of Theorya species

feature

MO

VDE (eV)

GdO4− (6B1)

X

16a1 (α) 9b2 (α) 15a1 (α) 8b2 (α)

4.10 4.13 4.30 4.58

9b1 (α) 3a2 (α) 16a1 (α) 9b2 (α) 15a1 (α)

4.86 4.95 4.23 4.38 4.44

A B C GdO4− (8B1)

GdO4− (10B1)

X A B X A B

9b2 (α) 16a1 (α)

VDE (eV)

9b2 (β) 15a1 (β) 10b1 (β)

4.67 4.71 4.90

9b2 (β)

4.46

16a1 (β) 15a1 (β) 9b2 (β) 16a1 (β)

4.85 4.05 4.32 4.38

8b2 (β) 9b1 (β) 4a2 (β)

4.79 4.90 4.97

4.61 4.62

C D 10b1 (α)

MO

4.99

Figure 4. Simulated PES of the low-lying GdO4− clusters calculated at the B3LYP/ANOTZ level of theory. The PES are plotted by fitting the distribution of computed VDEs with Gaussian functions of 0.05 eV width.

The majority and minority spins are labeled as “α” and “β”.

a

Figure 3. Simulated PES of the low-lying GdO2− and GdO3− clusters calculated at the B3LYP/ANOTZ level of theory. The PES are plotted by fitting the distribution of computed VDEs with Gaussian functions of 0.05 eV width.

Figure 5. Representative valence MOs of the ground-states and lowlying isomers of the GdOn− (n = 1−4) clusters. The isosurface value of the MOs is 0.03 au.

3b is ...f6 (19a′)2(20a′)2(21a′)2(8a″)2(9a″)2(22a′)2(10a″)2(23a′)1(24a′)2. As shown in Table 2, the first VDE of structure 3b is estimated to be 3.42 eV, coming from the detachment of the 24a′(α) MO. This value is 1.03 eV higher in energy than the first VDE of the ground-state of GdO3−, which may be considered a strong criterion in identifying different isomers in future PES experiments. Additionally, Figure 4a−c displays the simulated PES of the ground-state (4a in Figure 2) and low-lying isomers (4b and 4c in Figure 2) of the GdO4− clusters. The electron configurations of structures 4a−4c are calculated to be ...f7 (3a2)2(9b1)2(8b 2 ) 2 (15a 1 ) 2 (9b 2 ) 2 (16a 1 ) 2 (10b 1 ) 1 ( β ) (4a 2 ) 1 ( β ) , ...f 7 (3a2) 1(α)(4a 2)2 (9b1) 2(8b2 )2(15a 1) 2(9b2 )2(16a 1) 2(10b1) 1(β), and ...f7 (15a1)2(3a2)1(α)(9b1)2(4a2)2(8b2)2(10b1)1(α)(16a1)2 (9b2)2, respectively. The first PES band (X in Figure 4a) of the ground-state GdO4− located at around 4.11 eV is formed by removing electrons from 16a1(α) and 9b2(α) MOs. Another

can be considered as the overlap of two transitions (X and A), which originates from the photodetachment of the O 2p based HOMO (12a″) of GdO3−. Theoretical VDEs for these two transitions are estimated to be about 2.39 and 2.50 eV, respectively. Band B in Figure 3b corresponds to electron detachments from the HOMO−1 (11a″) and HOMO−2 (22a′), and the peak maximum for this band is 3.25 eV. In addition, photodetachments from 21a′(α) and 21a′(β) MOs yield bands C and D with the calculated VDE values of 3.43 and 3.59 eV, respectively. As shown in Figure 5c, 21a′ is a σ-type MO formed through the overlap between the 2py AO from an O atom and the 5dz2 AO from Gd. The last band in the PES (E in Figure 3b) is from MO 10a″ with the theoretical VDE of 3.86 eV. Furthermore, Figure 3c displays the theoretical PES of the lowlying isomer of GdO3− (structure 3b in Figure 2). The present calculation predicts that the electron configuration of structure E


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Foundation of Jiangsu Province (BK20170396), the Young Scholars Program of Shandong University (YSPSDU) (2018WLJH48) and the Fundamental Research Funds of Shandong University (2017TB003). S.-B.C. gratefully acknowledges supports from Dr. A. W. Castleman, Jr., during his stay in Penn State.

transition band A, which is next to X, has a calculated VDE of 4.30 eV, and is derived from 15a1(α), an MO that mainly consists of the 2px AOs from two O atoms (Figure 5e). In addition, as shown in Table 3, the calculated VDEs for the third and fourth detachment channels (B and C in Figure 4a) are 4.69 and 4.90 eV, respectively. The enhanced intensity for these two bands is due to the overlap of three different detachment MOs in each band (B or C), as can be seen from Table 3. Compared with the ground-state GdO4−, the octet GdO4− cluster (structure 4b in Figure 2) is calculated to have different spectroscopic features. Three main transition bands are found below 5 eV binding energy in the PES (Figure 4b). Photodetachment from the HOMO 16a1(α), an MO involving the interaction between the 2pz AOs from O atoms and the 5dxz AO from Gd, yields the first peak (X in Figure 4b) with the calculated VDE of 4.23 eV. Correspondingly, photodetachment from the β electron in the 16a1 MO gives rise to the third transition band (B) with a VDE value of 4.85 eV. Moreover, the VDE for band A in Figure 4b is computed to be about 4.45 eV, corresponding to the detachments from 9b2(α, β) and 15a1(α) MOs. Finally, it is necessary to note that the noticeable difference of spectroscopic features with different VDE values in three isomers of GdO4− (Figure 4 and Table 3) may be used as experimental evidence in identifying diverse isomers in future PES experiments.

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5. CONCLUSIONS In conclusion, a joint photoelectron imaging spectroscopy and DFT study has been constructed to investigate the geometric and electronic structure of GdO. The EA of GdO is measured to be 1.16 ± 0.09 eV from the vibrationally resolved PES. The orbital symmetry of the detached electron in GdO− is verified to possess σ-type characteristics based on the experimentally measured β parameter of the EA defined peak. Meanwhile, the theoretical level and basis set used here, namely, B3LYP/ ANOTZ, nicely reproduce the current experimental EA of GdO, resolving an apparent contradiction in previous theory and experiment. Moreover, various activated oxygen species including peroxide, superoxide, radical, triradical, and ozonide radical have been identified from the optimized structures of the ground-states and low-lying isomers of the GdOn−1/0 (n = 2−4) clusters, which may play important roles in their surface chemistry and photochemistry. In addition, the generalized Koopmans' theorem is employed to simulate the theoretical PES of the GdOn− (n = 2−4) clusters, and the corresponding MOs from which the photodetachements occur are also examined. We wish these theoretical findings could stimulate further experimental efforts on these gadolinium oxide clusters.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shi-Bo Cheng: 0000-0002-4131-2540 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Natural Science Foundation of China (NSFC) (21603119, 21705093), the Taishan Scholars project of Shandong Province (ts201712011), the Natural Science Foundation of Shandong Province (ZR2016BQ09, ZR2017BB061), the Natural Science F


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