Low-Energy Photoelectron Imaging Spectroscopy of Lan(benzene) (n

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Low-Energy Photoelectron Imaging Spectroscopy of La(Benzene) (n = 1 and 2) W.Ruchira Silva, Wenjin Cao, and Dong-Sheng Yang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09750 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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The Journal of Physical Chemistry

Low-Energy Photoelectron Imaging Spectroscopy of Lan(benzene) (n = 1 and 2) W. Ruchira Silva, Wenjin Cao, and Dong-Sheng Yanga Department of Chemistry, University of Kentucky, Lexington, KY, 40506-0055

a) Corresponding authors: [email protected] Friday, October 20, 2017

1

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Abstract Lan(benzene) (n = 1 and 2) are formed in a pulsed laser-ablation molecular beam source and characterized by low-energy photoelectron imaging spectroscopy. The photoelectron spectrum of La2(benzene) displays a strong origin band, a short metal-ligand stretching progression, and a weak ring deformation band. Four isomers are considered for La2(benzene), and the preferred structure is an inverse sandwich with two La atoms residing on the opposite sides of the benzene ring. The ground electronic state of the inverse sandwich is 1A1g (D3d) with (5d xy , x2 − y 2 + π *) 4 6 s 2 electron configuration. Ionization removes a La-based 6s electron and yields a 2A1g ion. The spectrum of La(benzene) is similar to the zero electron kinetic energy spectrum reported previously by our group, although the spectral linewidth is somewhat broader. The measurement of the photoelectron angular distribution of La(benzene) confirms that the ejected electron has largely a p wave character. The metal-ligand bonding of La2(benzene) is considerably stronger than that of La(benzene) due to the three-fold binding of each La atom in the di-lanthanum species and the two-fold binding in the mono-lanthanum complex.

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1. INTRODUCTION Benzene is an important organic compound, a nature component of crude oil, and one of the most basic petrochemicals. Transition metal complexes with benzene or its derivatives are true classics among organometallic compounds, and many of benzene-containing metal complexes have been identified as crucial intermediates in homogeneous catalysis of organic reactions.1,2 Metalbenzene complexes have thus attracted considerable interest in gas phase chemistry and spectroscopy. Some of these studies include laser photodissociation,3-7 photoionization,8-13 photoelectron dissociation,19-21

detachment,8,14-17 ion

mobility,22

photoelectron-photoion magnetic23,24

and

coincidence,18 dipole

collision

moments25,

and

induced reactivity

measurements.26,27 A large number of computational studies have also been reported on structures and energetics of metal-benzene systems.28-41 In spite of the extensive studies, electronic spectroscopy with resolved metal-ligand vibrational bands has lagged behind for transition metal-benzene systems containing multiple metal atoms or benzene molecules. Our group has recently studied metal-benzene complexes using pulsedfield ionization zero electron kinetic energy (ZEKE) spectroscopy.42-48 The ZEKE experiment is a two-step process, which begins with photoexcitation of a neutral species to high-lying Rydberg levels near its ionization threshold and proceeds with pulsed electric field ionization of the excited Rydberg states. The ZEKE technique has been successful for mono- and di-benzene complexes, but it has met challenges for larger systems where the lifetime of the high-lying Rydberg states may be diminished and the ZEKE signal becomes weak. To attempt to expand our studies to larger metalbenzene and other organometallic systems, we have recently implemented a low-energy electron velocity-map imaging method49-51 in our metal cluster beam instrument. Here we report the photoelectron spectra of Lan(benzene) (n = 1 and 2) complexes obtained using the imaging method. Compared to ZEKE, the electron imaging method employs a single-step ionization process and does 3



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not rely on long-lived Rydberg states. Although it has been used to study anions for more than a decade,51 this work appears to be the first low-energy electron velocity-map imaging study of neutral polyatomic transition metal-organic radicals.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS A newly developed low-energy electron velocity-map imaging device is installed in one of our metal-cluster beam instruments that has been used for ZEKE or mass-analyzed threshold ionization measurements.52 The imaging device is equipped with a 60 cm-long flight tube, a 75 mm diameter two-dimensional position-sensitive microchannel plate detector (MCP) coupled with a p-47 phosphor screen (Beam Imaging Solution), and a charge-coupled device (CCD) camera with a native resolution of 768 x 768 (IDS Imaging Development System). The velocity mapping electrode assembly is based on the Suits group’s design for ion imaging53 and is modified for low-energy photoelectron imaging as shown in Figure 1. The assembly consists of a repeller (R), two electrostatic lenses (L1 and L2), and seven rings made of polished, non-magnetic stainless steel. The stainless steel rings are designed to minimize the effects of stray fields from surrounding electric connections and to improve the quality of the electric fields applied to the repeller and lenses. Electron velocity focusing is achieved by optimizing the nonlinear electric fields on R, L1, and L2 using the SIMION program, and the optimal ratios of the applied DC voltages are VL1 = 0.87 VR and VL2 = -0.78 VR with VR at -300 V. Stray fields from high voltage connections of the MCP detector and phosphor screen are blocked by mounting a stack of grounded stainless steel disks in front of the MCP detector. External magnetic fields are isolated from the imaging device by shielding the full length of the extraction-detection region with a double-layer µ metal cylinder. Lan(C6H6) (n = 1 and 2) were formed in a metal cluster beam source. La atoms were produced by pulsed laser (Nd:YAG, 532 nm, Continuum Minilite II) ablation of a La rod (99.9%, Alfa Aesar). 4


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The metal atoms were entrained in a He gas (60 psi) delivered by a home-made piezoelectric pulsed value. Benzene vapor was introduced, perpendicularly to the La-seeded He gas beam, to a small collision chamber through a stainless steel capillary, and the amount of the vapor was regulated by a metering valve. The amount of benzene vapor, the power of the ablation laser (~ 1-2 mJ/pulse), and the length of the collision tube that was attached to the collision chamber were optimized to produce preferentially La(C6H6) or La2(C6H6). The resultant metal-benzene complexes were then supersonically expanded into the source chamber, collimated by a cone-shaped 2 mm dimeter skimmer, and passed through a pair of electric deflection plates. Ionic species in the molecular beam that were formed by laser ablation were removed by an electric filed (100 V cm-1) applied on a pair of deflection plates, and neutral products were identified with photoionization time-of-flight mass spectrometry. Single-photon photoionization efficiency spectra were recorded to locate the approximate ionization threshold of each species to guide electron imaging measurements. In the electron imaging experiments, the collimated neutral molecular beams entered the velocity mapping electrode assembly, intersected at 90o by a linear polarized ionization laser, and were ionized by laser photoionization with the frequency doubled output of a tunable dye laser (Lumonics HD-500) pumped by the third harmonic output (355 nm) of a Nd:YAG laser (Continuum Surelite II). The resultant electrons were accelerated and focused on to the position-sensitive MCP detector coupled with the phosphor screen. Two-dimension electron images were captured by the CCD camera and accumulated 60,000-100,000 laser pulses using the NuACQ acquisition program.54 The two-dimension images were symmetrized by dividing the circular image into four quarters and inverse-Abel transformed to three-dimensional images.55 In the Abel-inverted image, electron speed is proportional to the distance from the image center. Integration over all angles at each particular distance yields a radial distribution or a photoelectron spectrum in a velocity space, which is then converted to the conventional photoelectron energy spectrum via Jacobian transformation.54 Laser 5



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wavelengths were calibrated against vanadium atomic transitions in the spectral region after recording the electron images.56 The Stark shift (∆) on the adiabatic ionization energy (AIE) induced by the dc field (Ef) was calculated using the relation of ∆ΑIE = 6.1Ef1/2, where Ef is in V cm-1 and ∆ΑΙE is in cm-1.57 To show the spectral resolution of the imaging device, Figure 2 displays the photoelectron image and spectrum of V atom recorded through a two-photon ionization process. At the electron kinetic energy of 100 cm-1, the spectral linewidth at the half maximum is ~ 4 cm-1. Because the energy resolution of the technique increases with the decrease of the electron kinetic energy, the spectral linewidth may be further narrowed for slower electrons. Computationally, density functional theory (DFT)/B3LYP was used to calculate the equilibrium geometries and vibrational frequencies of the neutral and cationic complexes. The basis sets used in these calculations were 6-311+G(d,p) for C and H and the Stuttgart/Dresden (SDD) effective-corepotential basis set with a 28 electron core for La atom. We have extensively used the DFT/B3LYP method and found that it generally produced adequate results for the spectral and structural assignments of organometallic radicals.42,58-64 For each optimized stationary point, a vibrational analysis was performed to identify the nature of the stationary point (minimum or saddle point). To refine the energies of various spin states, single-point energy calculations were carried out using the coupled cluster method with single, double, and perturbative triple excitations (CCSD(T)) involving the third-order Douglas-Kroll-Hess scalar relativistic correction, and at the DFT/B3LYP optimized geometries. Basis sets used in the CCSD(T) calculations were aug-cc-pVTZ-DK65,66 for C and H and cc-pVTZ-DK367 for La. The DFT/B3LYP calculations were performed with Gaussian 09 software package,68 whereas the CCSD(T) calculations were carried out with MOLPRO 2010.1.69 To compare with the experimental spectra, multi-dimensional Franck-Condon (FC) factors were calculated from the equilibrium geometries, harmonic vibrational frequencies, and normal coordinates of the neutral and ionized complexes.70 In these calculations, the recursion relations 6


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from Doktorov et al.71 were employed, and the Duschinsky effect72 was considered to account for a possible axis rotation from the neutral complex to the cation. Spectral simulations were obtained using the experimental linewidth and Lorentzian or Gaussian line shapes. Both Lorentzian and Gaussian functions yield similar simulations. Transitions from excited vibrational levels of the neutral complex were considered by assuming thermal excitation at specific temperatures. In these simulations, the calculated 0-0 transition is aligned with the experimental origin band, and the vibrational frequencies are unscaled in order to directly compare with the experimental data.

3. RESULTS AND DISCUSSION 3.1. La(C6H6) Figure 3a presents the photoelectron spectrum of La(C6H6) converted from the electron image. The left side of the image is the raw image, while the right side of the image is the reconstructed one. The four rings in the electron image correspond to the four major bands in the spectrum, with the smallest ring related to the lowest-energy band. The spectrum displays the origin band at 36822 (10) cm-1 and a short vibrational progression of ~ 295 cm-1 intervals with up to three quanta. It is similar to the previously reported ZEKE spectrum, though the spectral linewidth is somewhat broader.73 The linewidth is measured to be ~ 30 cm-1 in the photoelectron spectrum and ~ 5 cm-1 in the ZEKE spectrum. The broader linewidth is due to the unresolved vibrational sequence transitions. Following the previous ZEKE study, the energy of the origin band at 36822 cm-1 corresponds to the AIE of the complex, and the 295 cm-1 progression is assigned to the excitation of La-benzene stretching mode. The electron imaging measurement yields not only the metal-ligand vibrationally resolved spectrum, but also information about the character of the highest occupied molecular orbital (HOMO). Figure 4 shows the photoelectron intensity as a function of the angle between the direction

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of the ejected electron and the polarization vector of the incident photon. By fitting the experimental data to the equation below74

I (θ ) =

σ β [1 + (3cos 2 θ − 1)] 4π 2

(1)

where I(θ) is the photoelectron intensity, θ is the angle between the direction of the ejected electron and polarization vector of the incident photon, and σ is the total cross-section integrated over all angles, the anisotropy parameter β can be determined. The β value depends almost entirely on the nature of the orbital from which the electron is ejected and is determined to be + 1.6 for the ionization of La(C6H6). From the measured β value, the HOMO of La(C6H6) has predominantly a s character or the outgoing electron has largely a p wave, which confirms the theoretical prediction (insert in Figure 4). The β value deviates from +2 because the HOMO is not a pure La 6s orbital.

3.2. La2(C6H6) A. Photoelectron spectrum. Figure 5a shows the photoelectron spectrum and image of La2(C6H6). The spectrum displays the origin band at 32141 (10) cm-1; a vibrational progression of 180 cm-1 with up to three quanta, a weak band at 307 cm-1, two even weaker bands at 285 cm-1 (marked with “*”) and (307+180) cm-1 (marked with “#”) on the higher energy side of the origin band; and a weak band at 176 cm-1 on the lower energy side of the origin band. Compared to La(C6H6), the La2(C6H6) complex shows a similar spectral profile, but a lower AIE (by 4681 cm-1), a smaller interval progression, and a narrower linewidth (~ 15 cm-1). B. Structural isomers. Figure 6 presents four structural isomers of La2(C6H6) predicted by the DFT/B3LYP calculations. Iso A has two isolated La atoms residing on the opposite sides of the benzene ring and may be considered an inverse sandwich structure, while other three isomers (Iso B, C, and D) have two La atoms on the same side of the ring, which may be considered La2-bound 8


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species. The main differences among the La2-bound three isomers are the relative position of the La-La bond and the degree of bending of the benzene ring. The top-down projection of the La-La bond overlaps with a C atom in Iso B but bisects one or two C-C bonds in Iso C and D, respectively. Moreover, one of the C atoms bends away from a La atom in Iso B and two C atoms bend toward the La-La bond in Iso D, while the carbon ring remains almost planar in Iso C. In searching for La2bound isomers, we started with structures having the La-La bond parallel and perpendicular to or tilted from the benzene ring but obtained only the four isomers. For Iso A, we calculated singlet and triplet states for the neutral species by considering three outmost valence electrons of the La atom in the ground configuration (i.e., 5d16s2). Benzene in the singlet ground state (a2u2e1g4, 1A1g, D6h)75 does not contribute to the overall spin multiplicity of the complex. Because the two isolated La atoms in the inverse sandwich are far apart, the relative orientation of the electrons on the two metal atoms should have a small effect on the electron energy. The singlet and triplet states of the inverse sandwich are thus expected to be close in energy. For the other three isomers, we considered singlet, triplet, and quintet states because La2 has the ground electron

configuration

(5dπu46sσg2)

(1Σg+)

and

a

very

low-energy

configuration

6sσg25dπu25dσg16sσu1 (5Σu-) at about 0.1 eV.73 However, a quintet state was only located for Iso D. Table 1 lists energies of various spin states of the four isomers predicted by DFT/B3LYP calculations and CCSD(T) single-point energy calculations at the DFT/B3LYP geometries. In the table, the singlet, triplet or quintet states are for the neutral species and the doublet and quartet states for the corresponding ion. Iso A is predicted to be the most stable isomer, and the other three are in the energy order of Iso B ~ Iso C < Iso D. For each isomer, both B3LYP and CCSD(T) methods yield consistent energy order for the various spin states, except for the singlet and triplet of Iso A. For the two spin states of Iso A, the B3LYP method predicts the 3A2u to be more stable than 1A1g by 1534 cm-1, while the CCSD(T) method calculates the triplet to be less stable by 304 cm-1. It is well 9



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known that hybrid DFT methods (such as B3LYP) include Hartree-Fock (HF) exchange and thus favor high-spin states.76,77 The HF method involves electron-exchange interactions between electrons of the same spins but no exchange interactions between unlike electron spins. Electronic states in higher spins are thus corrected with more exchange interactions than those in lower spins and tend to be more stable under the HF treatment. Therefore, cautions should be taken wherever higher spin states are predicted to be more stable than lower spin states by HF or hybrid DFT methods. We carried out the CCSD(T) single-point energy calculations because the post-HF method includes electron correlations between unlike and like spins and treat the exchange term equally for all spin states. With the better treatment of the electron correlations in CCSD(T), the 1A2g state becomes slightly more stable than the 3A2u state, and the energy separation between 4A1g and 2A1g of Iso A becomes larger. Similarly, energy differences between the triplet and singlet and between the quartet and doublet of other isomers are larger from the CCSD(T) calculations than those from B3LYP. C. Observed structure and electronic transition. The most likely assignment for the observed spectrum is the 2A1g ← 1A1g transition of the inverse sandwich (D3d). This assignment is supported by three pieces of evidence: the 1A1g state is predicted to be the ground state, the 2A1g ← 1

A1g transition energy is close to the experimental AIE, and the 2A1g ← 1A1g FC profile (Figure 5b)

matches nicely with the observed spectrum (Figure 5a). Although it has a similar FC profile to the experimental spectrum, the 2A1g ← 3A2u simulation exhibits sequence transitions (associated with a La-C6H6-La bending) superimposed on the main vibrational progression (Figure 5c), which are absent from the measurement. Moreover, because the 3A2u state is predicted to be an excited state by the CCSD(T) calculations, the ionization of 3A2u would have yielded a second band system in the photoelectron spectrum. However, the experimental spectrum shows only a single band system. Therefore, the 2A1g ← 3A2u transition is unlikely a spectral carrier. A third possible transition of Iso 10


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A is 4Ag ← 3A1u. This transition has a much higher energy and a mismatched FC profile (Figure 5d). Thus, it can be safely excluded from the observation as well. Moreover, the other three isomers can be omitted from consideration because they are much less stable and their simulated spectra are inconsistent with the experimental spectrum (Figures S1-S3). By comparing the 2A1g ← 1A1g simulation of the inverse sandwich isomer (Figure 5b) with the experimental spectrum (Figure 5a), the 180 cm-1 progression in the measured spectrum is attributed to excitations of the La+-(C6H6)-La+ symmetric stretching mode (ν5+) and the weak transition at 307 cm-1 above the origin band to a benzene ring deformation (ν4+) in the 2A1g ion state. The weak transition at 176 cm-1 below the origin band is assigned to the La-(C6H6)-La symmetric stretching (ν5) in the 1A1g neutral state. The calculated frequencies of ν5+, ν5, and ν4+ are 180, 177, and 301 cm-1, respectively, which are in excellent agreement with the measured values (Table 2). The weak band at (180 + 307) cm-1 (#) above the origin band is the combination band of the first quantum of the metal-ligand stretching (ν5+) and the benzene ring deformation (ν4+). The other weak band at 285 cm-1 (*) is not associated with any of the vibrational modes of the complex and may be contamination from two-photon ionization of La atom or three-photon ionization of benzene. The narrower linewidth of the photoelectron bands of this complex than that of La(C6H6) is due to the absence of unresolved vibrational hot bands. The 1A1g neutral state of La2(C6H6) has the electron configuration (eu)4(a1g)2 (Figure 7). The HOMO (a1g)2 is the in-phase combination of two La 6s1 orbitals. Because the two La atoms are well separated (by 4.328 Å at the B3LYP level), the 6s1 orbitals on the two La atoms have no overlap. The degenerate (eu)4 orbital is the bonding combination of the La ( d xy )1 ( d x2 − y 2 )1 orbitals with the benzene empty π* (e.g., π4 and π5) orbitals, where x and y axes are on the benzene plane and the z

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axis is in collinear with the threefold rotational axis. In this bonding combination, the benzene π* orbitals are stabilized by electron back donation from the metal d orbitals.

3.2. Metal-ligand bonding in La(C6H6) and La2(C6H6) Table 3 lists the calculated geometries and bond dissociation energies of La(C6H6) and La2(C6H6). Unlike the free benzene molecule where six C-C bonds are of equal distances (1.395 Å), four C-C bonds (1.463 Å) in La(C6H6) are close to single bonds and other two (1.371 Å) to double bonds. The four C atoms involved in the double bonds remain planar, while the two C atoms in the single bonds bend toward La because of the change of the orbital hybridization from sp2 to sp3. The distinct C-C bonds and distortion of the benzene planarity is caused by the differential interactions of La with the sp3 and sp2 C atoms. The La-C(sp3) bonds are predicted to be shorter (2.530 Å) than the La-C(sp2) distances (2.759 Å). Because the four C(sp2) atoms are already coordinately saturated and cannot accommodate additional coordination, La atom may be considered two-fold binding with the two C(sp3) atoms in La(C6H6). Ionization has basically no effect on the C-C distances, but it shrinks the La-C distances because of the additional charge-multipole interaction. As a result, the Labenzene bond dissociation energy (D0) is considerably larger in the 1A1 ion state than in the 2A1 neutral state. The bond energy difference (∆D0) between D0(La+-C6H6) and D0(La-C6H6), which equals ∆AIE between AIE(La) and AIE [La(C6H6)], is measured to be 23.33 kcal mol-1 (or 8160 cm-1). The bond energy of the inverse sandwich La2(C6H6) is predicted to be more than twice as that of La(C6H6) (Table 3). In La2(C6H6), all six C-C bonds are predicted to be equal (1.476 Å) and close to single bonds. Because they are coordinately unsaturated, the six C(sp3) atoms each are able to accommodate an additional La-C bond (2.512 Å), three of which are formed by each La atom. Therefore, each of the La atoms possesses three-fold binding in the inverse sandwich structure, 12


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rather than the two-fold binding as in La(C6H6). The increased number of the metal coordination explains why the dissociation energy of La2(C6H6) is more than twice as that of La(C6H6). The La-C distances in the 2A1g ion (2.505/2669 Å) are slightly shorter than those in the 1A1g neutral state (2.512/2.707 Å). The measured ∆D0 between the ionic and neutral complexes (i.e., ∆AIE between AIE (La) and AIE [La2(C6H6)] is 36.68 kcal mol-1 (or 12839 cm-1), which is considerably larger than that of La(C6H6). The larger bonding difference between the ionic and neutral states is caused by a stronger charge-multipole interaction in La2(C6H6) than in La(C6H6). For example, by Mulliken analyses La carries a positive charge of 0.23e in the 2A1 neutral state and 1.10 e in the 1A1 ion state of La(C6H6) , while the overall charges on the two La atoms are 0.37e in the 1A1g neutral state and 1.67e in the 2A1g state of La2(C6H6). These analyses yield charge increases of 0.87e in La(C6H6) and 1.30e in La2(C6H6) upon ionization.

4. CONCLUSIONS We have reported the photoelectron spectra of Lan(C6H6) (n =1 and 2) from low-electron velocity-map imaging. Although the spectral resolution is not as high as that of ZEKE measurements, the imaging technique is able to resolve low-frequency metal-ligand vibrational modes. Adiabatic ionization energies and metal-benzene stretching frequencies are measured for both species. The ground electronic state of La(C6H6) (C2v) is 2A1 with La binding to benzene in a two-fold mode, while the ground state of the inverse sandwich La2(C6H6) (D3d) is 1A1g with a threefold La binding mode. The electron configurations are (5d x2 − y 2 + π *)2 6s1 in the La(C6H6) 2A1 state and (5d xy , x2 − y 2 + π *) 4 6 s 2 in the La2(C6H6) 1A1g state. Ionization removes a La 6s-based electron from the neutral complexes; the resultant ions have similar geometries to those of the neutral states, but stronger metal-ligand bonding due to enhanced charge-multipole interactions. To further explore 13



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the applications of the imaging technique, measurements will be attempted on larger metal-aromatic complexes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge at the ACS publication website at DOI: Spectral simulations of various transitions of the three isomers of La2(C6H6).

AUTHOR INFORMATION Corresponding Authors *D. –S. Yang, Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

We acknowledge financial support from the National Science Foundation Division of Chemistry (Chemical Structure, Dynamics, and Mechanisms, Grant No. CHE-1362102). We also acknowledge additional support from the Kentucky Science and Engineering Foundation. We appreciate Professor Arthur Suits for helpful discussions about the design of the imaging device and the data acquisition software and Professor Ivan Powis for providing us with a copy of the pBsex software.

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15



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1: Relative energies including vibrational zero point corrections (cm-1) and point groups of various electronic states of four La2(C6H6) isomers from DFT/B3LYP

and CCSD(T)//B3LYP

calculations. Isomer

Iso A

State

D3d

1534

0

3

A2u

D3d

0

304

2

A1g

D3d

34112

33942

4

D2h

41732

42252

Ag

1

A′

Cs

6534

7271

3

A′

Cs

9167

10107

2

A′

Cs

42577

43507

A″ a

Cs

47191

48956

1

Cs

7182

6825

A

C1

11369

12424

A′

Cs

44664

45122

A″

Cs

47480

48965

C2v

8737

8290

A′

3 2 4

1

a

ECCSD(T)

A1g

4

Iso D

EB3LYP

1

Iso B

Iso C

Point Group

A1

3

B2

C2v

14286

14965

5

B1

C2v

12293

13272

2

B1

C2v

44538

44620

4

B2

C2v

44751

45333

The 4A′′ state of Iso B has an imaginary frequency of 176i cm-1.

23



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Page 24 of 31

Table 2: Adiabatic ionization energies (AIE, cm-1) and vibrational frequencies (cm-1) of Lan(C6H6) (n = 1 and 2) from photoelectron spectra and DFT/B3LYP calculations. The uncertainty in experimental AIEs is ± 10 cm-1. νn and νn+ are vibrational modes in the neutral and ion states, respectively. The AIEs in parentheses are from CCSD(T)//B3LYP calculations. Experimental

Computational

36820

37472 (36447)

250/295

263/287

32141

32578 (33942)

176/180

177/180

307

301

La(C6H6) (C2v) AIE (1A1 ← 2A1) La-benzene stretch, ν 10 / ν 10+ La2(C6H6) (D3d) AIE (2A1g ← 1A1g) symmetric La-benzene stretch, ν 5 / ν 5+ ring deformation, ν 4+

24


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3: La-C and C-C bond lengths (R, Å), benzene ring dihedral angle (δ, degree), and Labenzene bond dissociation energies (D0, kcal mol-1 ) of La(C6H6) (C2v) and La2(C6H6) (D3d) from DFT/B3LYP calculations.

La-(C6H6)

La2(C6H6)

State

R(La-C)

R(C-C)

δ(C-C-C-C)

Do

2

A1

2.530 / 2.759

1.463 / 1.371

24.5

36.2

1

A1

2.469 /2.691

1.463 / 1.375

23.7

57.2

1

A1g

2.512 / 2.707

1.476

30.9

102.3

2

A1g

2.505 / 2.669

1.473

26.3

137.2

25



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Page 26 of 31

Figure Captions Figure 1. Electrode assembly for low-energy electron velocity mapping. Figure 2. Photoelectron image and spectrum of V atom recorded through a resonant two photoionization process. The spectral linewidth (∆E) is ~ 4 cm-1 at the electron kinetic energy of 100 cm-1. Figure 3. Photoelectron spectrum and image (a) and ZEKE spectrum (b) of La(C6H6). Figure 4. Photoelectron angular distribution and HOMO of La(C6H6). Figure 5. Photoelectron spectrum and image (a) and spectral simulations of the 2A1g ← 1A1g (b), 2

A1g ← 3A2u (c), and 4Ag ← 3A2u (d) transitions of the inverse sandwich La2(C6H6) (Iso A) at 100 K.

Figure 6. Four La2(C6H6) isomers from DFT/B3LYP calculations. Figure 7. Cartesian axes (a) and the HOMO-1 (b, c) and HOMO (d) of the inverse sandwich La2(C6H6) (Iso A).

26


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normal View

Cross View

Figure 1. Electrode assembly for low-energy electron velocity mapping.

Figure 2. Photoelectron image and spectrum of V atom recorded through a resonant two photoionization process. The spectral linewidth (∆E) is ~ 4 cm-1 at the electron kinetic energy of 100 cm-1.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

36820 295

(a)

(b)

36800 37000 37200 37400 37600

Electron Binding Energy (cm-1)

Figure 3. Photoelectron spectrum and image (a) and ZEKE spectrum (b) of La(C6H6).

0

30

60

90

120

150

180

Angle (Degrees) Figure 4. Photoelectron angular distribution and HOMO of La(C6H6).

28


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Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32141 180

176 307 (a)

*

#

(b)

(c)

(d) 32000

32200

32400

32600

32800 -1

Electron Binding Energy (cm ) Figure 5. Photoelectron spectrum and image (a) and spectral simulations of the 2A1g ← 1A1g (b), 2

A1g ← 3A2u (c), and 4Ag ← 3A2u (d) transitions of the inverse sandwich La2(C6H6) (Iso A) at 100 K.

29



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Page 30 of 31

Top View

Side View

(a)

(c)

(b)

(d)

Figure 6. Four La2(C6H6) isomers from DFT/B3LYP calculations.

z

y x

(a)

(b) 5d

x2 − y 2

+π*

(c) 5d xy + π *

(d) 6s

Figure 7. Cartesian axes (a) and the HOMO-1 (b, c) and HOMO (d) of the inverse sandwich La2(C6H6) (Iso A).

30


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TOC Graphic:

31


Low-Energy Photoelectron Imaging Spectroscopy of Lan(benzene) (n

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