High-resolution photoelectron imaging of cold C₆₀⁻ anions and accurate determination of the electron affinity of C₆₀

Structures, fundamental properties, and potential applications of low-dimensional C60 polymers and other nanocarbons: a review

Since carbon (C) atom has a variety of chemical bonds via hybridization between s and p atomic orbitals, it is well known that there are robust carbon materials. In particular, discovery of C60 has been an epoch making to cultivate nanocarbon fields. Since then, nanocarbon materials such as nanotube and graphene have been reported. It is interesting to note that C60 is soluble and volatile unlike nanotube and graphene. This indicates that C60 film is easy to be produced on any kinds of substrates, which is advantage for device fabrication. In particular, electron-/photo-induced C60 polymerization finally results in formation of one-dimensional (1D) metallic peanut-shaped and 2D dumbbell-shaped semiconducting C60 polymers, respectively. This enables us to control the physicochemical properties of C60 films using electron-/photo-lithography techniques. In this review, we focused on the structures, fundamental properties, and potential applications of the low-dimensional C60 polymers and other nanocarbons such as C60 peapods, wavy-structured graphene, and penta-nanotubes with topological defects. We hope this review will provide new insights for producing new novel nanocarbon materials and inspire broad readers to cultivate new further research in carbon materials.

Probing Dipole-Bound States Using Photodetachment Spectroscopy and Resonant Photoelectron Imaging of Cryogenically Cooled Anions

Molecular anions with polar neutral cores can support highly diffuse dipole-bound states below their detachment thresholds due to the long-range charge-dipole interaction. Such nonvalence states constitute a special class of excited electronic states for anions and were observed in early photodetachment experiments to measure the electron affinities of organic radicals. Recent experimental advances, in particular, the ability to create cold anions using a cryogenically cooled Paul trap, have allowed the investigation of dipole-bound excited states at a new level. For the first time, the zero-point level of dipole-bound excited states can be observed via resonant two-photon detachment, and resonant photoelectron spectroscopy can be performed via the above-threshold vibrational levels (Feshbach resonances) of the dipole-bound states. This Perspective describes recent progress in the investigation of dipole-bound states in the authors' lab using an electrospray photoelectron spectroscopy apparatus equipped with a cryogenically cooled Paul trap and high-resolution photoelectron imaging.

The Spontaneous Electron-Mediated Redox Processes on Sprayed Water Microdroplets

Water is considered as an inert environment for the dispersion of many chemical systems. However, by simply spraying bulk water into microsized droplets, the water microdroplets have been shown to possess a large plethora of unique properties, including the ability to accelerate chemical reactions by several orders of magnitude compared to the same reactions in bulk water, and/or to trigger spontaneous reactions that cannot occur in bulk water. A high electric field (∼10⁹ V/m) at the air-water interface of microdroplets has been postulated to be the probable cause of the unique chemistries. This high field can even oxidize electrons out of hydroxide ions or other closed-shell molecules dissolved in water, forming radicals and electrons. Subsequently, the electrons can trigger further reduction processes. In this Perspective, by showing a large number of such electron-mediated redox reactions, and by studying the kinetics of these reactions, we opine that the redox reactions on sprayed water microdroplets are essentially processes using electrons as the charge carriers. The potential impacts of the redox capability of microdroplets are also discussed in a larger context of synthetic chemistry and atmospheric chemistry.

Two-Photon Photoemission Spectroscopy and Microscopy for Electronic and Plasmonic Characterizations of Molecularly Designed Organic Surfaces

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C₆₀) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C₆₀ can be visualized by Ag_n NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n ≥ 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Ag_n NCs on C₆₀ layers.

Rigorous Negative Ion Binding Energies in Low-Energy Electron Elastic Collisions with Heavy Multi-Electron Atoms and Fullerene Molecules: Validation of Electron Affinities

Dramatically sharp resonances manifesting stable negative ion formation characterize Regge pole-calculated low-energy electron elastic total cross sections (TCSs) of heavy multi-electron systems. The novelty of the Regge pole analysis is in the extraction of rigorous and unambiguous negative ion binding energies (BEs), corresponding to the measured electron affinities (EAs) of the investigated multi-electron systems. The measured EAs have engendered the crucial question: is the EA of multi-electron atoms and fullerene molecules identified with the BE of the attached electron in the ground, metastable or excited state of the formed negative ion during a collision? Inconsistencies in the meaning of the measured EAs are elucidated and new EA values for Bk, Cf, Fm, and Lr are presented.

An investigation into the structural, electronic, and non-linear optical properties in CN (N = 20, 24, 26, 28, 30, 32, 34, 36, and 38) fullerene cages

The present study attempts to investigate the structural, electronic, and non-linear optical properties of C_N (N = 20, 24, 26, 28, 30, 32, 34, 36, and 38) fullerene cages based on Density Functional Theory (DFT). In the DFT calculations, the B3LYP/6-311G(d,p) and CAM-B3LYP/6-311 +  + G(d,p) level of theories were used. The isomers of each fullerene have been received from the Fullerene Structure Library. These isomers have optimized using the B3LYP/6-311G(d,p). The results included optimization of the neutral and ionic state structures according to their multiplicity. Geometries, optimization energies, relative energies, frequencies, HOMO, LUMO, and HOMO-LUMO gap of these stable fullerene cages have been predicted by B3LYP/6-311G(d,p). Afterwards, the most stable structures have been re-optimized using the CAM-B3LYP /6-311 +  + G(d,p). Finally, non-linear optical properties, Fukui functions, density of state, electron affinity, and ionization potential values of the most stable fullerene cages have been found out by the DFT/ CAM-B3LYP /6-311 +  + G(d,p) level of theory. All calculation results have been compared with both C60 fullerene and the relevant literature on corresponding fullerenes.

Theoretical Study on the Diels–Alder Reaction of Fullerenes: Analysis of Isomerism, Aromaticity, and Solvation

Fullerenes are reactive as dienophiles in Diels–Alder reactions. Their distinctive molecular shape and properties result in interesting and sometimes elusive reaction patterns. Herein, to contribute to the understanding of fullerene reactivity, we evaluate the energies of reactions for Diels–Alder cycloadditions of C60, C70, and IC60MA with anthracene (Ant), by means of DFT computational analysis in vacuum and solution. The methods used showed little differentiation between the reactivity of the different fullerenes. The C70-Ant adducts where addition takes place near the edge of the fullerene were found to be the most stable regioisomers. For the IC60MA-Ant adducts, the calculated energies of reaction increase in the order: equatorial > trans-3 > trans-2 ≈ trans-4 ≈ trans-1 > cis-3 > cis-2. The change in the functional suggests the existence of stabilizing dispersive interactions between the surface of the fullerene and the addends. HOMA (harmonic oscillator model of aromaticity) analysis indicated an increase in aromaticity in the fullerene hexagons adjacent to the bonded addend. This increase is bigger in the rings of bisadduct isomers that are simultaneously adjacent to both addends, which helps explain the extra stability of the equatorial isomers. Solvation by m-xylene decreases the exothermicity of the reactions studied but has little distinguishing effect on the possible isomers. Thermal corrections reduce the exothermicity of the reactions by ~10 kJ∙mol−1.

Probing the Strong Nonadiabatic Interactions in the Triazolyl Radical Using Photodetachment Spectroscopy and Resonant Photoelectron Imaging of Cryogenically Cooled Anions

Although the adiabatic potential energy surfaces defined by the Born-Oppenheimer approximation are the cornerstones for understanding the electronic structure and spectroscopy of molecular systems, nonadiabatic effects due to the coupling of electronic states by nuclear motions are common in complex molecular systems. The nonadiabatic effects were so strong in the 1,2,3-triazolyl radical (C₂H₂N₃) that the photoelectron spectrum of the triazolide anion was rendered unassignable and could only be understood using nonadiabatic calculations, involving the four low-lying electronic states of triazolyl. Using photodetachment spectroscopy and resonant photoelectron imaging of cryogenically cooled anions, we are able to completely unravel the complex vibronic levels of the triazolyl radical. Photodetachment spectroscopy reveals a dipole-bound state for the triazolide anion at 172 cm^-1 below the detachment threshold and 32 vibrational Feshbach resonances. Resonant photoelectron imaging is conducted by tuning the detachment laser to each of the Feshbach resonances. Combining the photodetachment spectrum and the resonant photoelectron spectra, we are able to assign all 28 vibronic peaks resolved for the triazolyl radical. Fundamental frequencies for 12 vibrational modes of the ground state of the triazolyl radical are measured experimentally. The current study provides unprecedented experimental vibronic information, which will be valuable to verify theoretical models to treat nonadiabatic effects involving multiple electronic states.

Recent Progress in Low-Energy Electron Elastic-Collisions with Multi-Electron Atoms and Fullerene Molecules

We briefly review recent applications of the Regge pole analysis to low-energy 0.0 ≤ E ≤ 10.0 eV electron elastic collisions with large multi-electron atoms and fullerene molecules. We then conclude with a demonstration of the sensitivity of the Regge pole-calculated Ramsauer–Townsend minima and shape resonances to the electronic structure and dynamics of the Bk and Cf actinide atoms, and their first time ever use as novel and rigorous validation of the recent experimental observation that identified Cf as a transitional element in the actinide series (A. Müller, et al., Nat. Commun. 12, 948 (2021)).

Al13− and B@Al12− superatoms on a molecularly decorated substrate

Aluminum nanoclusters (Al_n NCs), particularly Al₁₃⁻ (n = 13), exhibit superatomic behavior with interplay between electron shell closure and geometrical packing in an anionic state. To fabricate superatom (SA) assemblies, substrates decorated with organic molecules can facilitate the optimization of cluster–surface interactions, because the molecularly local interactions for SAs govern the electronic properties via molecular complexation. In this study, Al_n NCs are soft-landed on organic substrates pre-deposited with n-type fullerene (C₆₀) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C₆₆H₆₆), and the electronic states of Al_n are characterized by X-ray photoelectron spectroscopy and chemical oxidative measurements. On the C₆₀ substrate, Al_n is fixed to be cationic but highly oxidative; however, on the HB-HBC substrate, they are stably fixed as anionic Al_n⁻ without any oxidations. The results reveal that the careful selection of organic molecules controls the design of assembled materials containing both Al₁₃⁻ and boron-doped B@Al₁₂⁻ SAs through optimizing the cluster–surface interactions.

Anionic aluminium clusters are promising candidates for the fabrication of superatom-assembled nanomaterials. Here, the authors report enhanced stability for Al₁₃⁻ and boron-doped B@Al₁₂⁻ on a molecularly decorated p-type organic substrate.

Probing the Electronic Structure and Spectroscopy of the Pyrrolyl and Imidazolyl Radicals using High-Resolution Photoelectron Imaging of Cryogenically-Cooled Anions

High-resolution photoelectron imaging and photodetachment spectroscopy of cryogenically-cooled pyrrolide and imidazolide anions are used to probe the electronic structure and spectroscopy of the pyrrolyl and imidazolyl radicals. The high-resolution data...

Low-Energy Electron Elastic Collisions with Actinide Atoms Am, Cm, Bk, Es, No and Lr: Negative-Ion Formation

The rigorous Regge-pole method is used to investigate negative-ion formation in actinide atoms through electron elastic total cross sections (TCSs) calculation. The TCSs are found to be characterized generally by negative-ion formations, shape resonances and Ramsauer-Townsend(R-T) minima, and they exhibit both atomic and fullerene molecular behavior near the threshold. Additionally, a polarization-induced metastable cross section with a deep R-T minimum is identified near the threshold in the Am, Cm and Bk TCSs, which flips over to a shape resonance appearing very close to the threshold in the TCSs for Es, No and Lr. We attribute these new manifestations to size effects and orbital collapse significantly impacting the polarization interaction. From the TCSs unambiguous and reliable ground, metastable and excited states negative-ion binding energies (BEs) for Am−, Cm−, Bk−, Es−, No− and Lr− anions formed during the collisions are extracted and compared with existing electron affinities (EAs) of the atoms. The novelty of the Regge-pole approach is in the extraction of the negative-ion BEs from the TCSs. We conclude that the existing theoretical EAs of the actinide atoms and the recently measured EA of Th correspond to excited anionic BEs.

Ultrafast Transfer and Transient Entrapment of Photoexcited Mg Electron in Mg@C_{60}

Electron relaxation is studied in endofullerene Mg@C_{60} after an initial localized photoexcitation in Mg by nonadiabatic molecular dynamics simulations. Two approaches to the electronic structure of the excited electronic states are used: (i) an independent particle approximation based on a density-functional theory description of molecular orbitals and (ii) a configuration-interaction description of the many-body effects. Both methods exhibit similar relaxation times, leading to an ultrafast decay and charge transfer from Mg to C_{60} within tens of femtoseconds. Method (i) further elicits a transient trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption or photoelectron spectroscopy in gas phase, in solution, or as thin films.

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y _x Sc_3-x N@C₈₀ (x = 0-3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁-T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin-lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

2D-Molybdenum Disulfide-Derived Ion Source for Mass Spectrometry

Generation of current or potential at nanostructures using appropriate stimuli is one of the futuristic methods of energy generation. We developed an ambient soft ionization method for mass spectrometry using 2D-MoS₂, termed streaming ionization, which eliminates the use of traditional energy sources needed for ion formation. The ionic dissociation-induced electrokinetic effect at the liquid-solid interface is the reason for energy generation. We report the highest figure of merit of current generation of 1.3 A/m² by flowing protic solvents at 22 μL/min over a 1 × 1 mm² surface coated with 2D-MoS₂, which is adequate to produce continuous ionization of an array of analytes, making mass spectrometry possible. Weakly bound ion clusters and uric acid in urine have been detected. Further, the methodology was used as a self-energized breath alcohol sensor capable of detecting 3% alcohol in the breath.

Confined Hot Electron Relaxation at the Molecular Heterointerface of the Size-Selected Plasmonic Noble Metal Nanocluster and Layered C60

The plasmonic response of metallic nanostructures plays a key role in amplifying photocatalytic and photoelectric conversion. Since the plasmonic behavior of noble metal nanoparticles is known to generate energetic charge carriers such as hot electrons, it is expected that the hot electrons can enhance conversion efficiency if they are transferred into a neighboring molecule or semiconductor. However, the method of transferring the energized charge carriers from the plasmonically generated hot electrons to the neighboring species remains controversial. Herein, we fabricated a molecularly well-defined heterointerface between the size-selected plasmonic noble-metal nanoclusters (NCs) of Ag_n (n = 3-55)/Au_n (n = 21) and the organic C₆₀ film to investigate hot electron generation and relaxation dynamics using time-resolved two-photon photoemission (2PPE) spectroscopy. By tuning the NC size and the polarization of the femtosecond excitation photons, the plasmonic behavior is characterized by 2PPE intensity enhancement by 10-100 times magnitude, which emerge at n ≥ 9 for Ag_n NCs. The 2PPE spectra exhibit contributions from low-energy electrons forming coherent plasmonic currents and hot electrons with an excitation energy up to photon energy owing to two-photon excitation of an occupied state of the Ag_n NC below the Fermi level. The time-resolved pump-probe measurements demonstrate that plasmon dephasing generates hot electrons which undergo electron-electron scattering. However, no photoemission occurs via the charge transfer state forming Ag_n⁺C₆₀^- located in the vicinity of the Fermi level. Thus, this study reveals the mechanism of ultrafast confined hot electron relaxation within plasmonic Ag_n NCs at the molecular heterointerface.

Structure of C60F36: A Gas-Phase Electron Diffraction and Quantum Chemical Computational Study of a Remarkably Distorted Fluorofullerene

The equilibrium molecular structure of the gaseous fluorofullerene C₆₀F₃₆ has been determined for the first time by the electron diffraction method with the use of quantum chemical calculations up to the RI-MP2/def2-TZVPP level of theory. Vibrational amplitudes and quadratic and cubic force constants were calculated by density functional theory methods. It was found that the sample under study consists of the isomer of C₁ symmetry, 81(4)%, with a small amount of the isomer of C₃ symmetry, in good accordance with HPLC-MS (atmospheric pressure photoionization), HPLC-UV/vis, and NMR spectroscopic data. The presence of the isomer of T symmetry, up to 5%, cannot be completely excluded. Theoretical structural parameters of the C₆₀F₃₆ molecule were compared with those of the C₆₀F₄₈ molecule. Relative to C₆₀, the C₆₀F₃₆ molecule has a remarkably distorted carbon cage because of steric, electrostatic, and orbital interactions. This results in the longest carbon-carbon bond (1.671 Å) found in free molecules. In particular, about the longest FC-CF bond, the dihedral angle is only around 20°, which leads to the very short nonbonded distance between electronegative vicinal fluorine atoms (2.531 Å) that is much shorter than the sum of van der Waals radii of fluorine atoms (2.94 Å). A natural bond orbital analysis revealed that strong n_π(F) → σ*(FC-CF) interactions delocalize the lone pair of π-type at the fluorine atoms into the antibonding orbital of the FC-CF bond. This hyperconjugation results in additional elongation of FC-CF bonds.

Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

We report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB^-) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment spectroscopy of TCNB^- reveals selected symmetry-allowed vibronic transitions to the QBS, but the ground vibrational state was not observed because the transition from the ground state of TCNB^- (A_u symmetry) to the QBS (A_g symmetry) is triply forbidden by the electric and magnetic dipoles and the electric quadrupole. The binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to strong correlation and polarization effects. A centrifugal barrier is observed for near-threshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB^-. The current study shows a rare example where symmetry selection rules, rather than the Franck-Condon principle, govern vibronic transitions to a nonvalence state in an anion.

Doubly-Charged Negative Ions as Novel Tunable Catalysts: Graphene and Fullerene Molecules Versus Atomic Metals

The fundamental mechanism underlying negative-ion catalysis involves bond-strength breaking in the transition state (TS). Doubly-charged atomic/molecular anions are proposed as novel dynamic tunable catalysts, as demonstrated in water oxidation into peroxide. Density Functional Theory TS calculations have found a tunable energy activation barrier reduction ranging from 0.030 eV to 2.070 eV, with Si2-, Pu2-, Pa2- and Sn2- being the best catalysts; the radioactive elements usher in new application opportunities. C602- significantly reduces the standard C60- TS energy barrier, while graphene increases it, behaving like cationic systems. According to their reaction barrier reduction efficiency, variation across charge states and systems, rank-ordered catalysts reveal their tunable and wide applications, ranging from water purification to biocompatible antiviral and antibacterial sanitation systems.

Low-Energy Electron Elastic Total Cross Sections for Ho, Er, Tm, Yb, Lu, and Hf Atoms

The robust Regge-pole methodology wherein is fully embedded the essential electron-electron correlation effects and the vital core polarization interaction has been used to explore negative ion formation in the large lanthanide Ho, Er, Tm, Yb, Lu, and Hf atoms through the electron elastic total cross sections (TCSs) calculations. These TCSs are characterized generally by dramatically sharp resonances manifesting ground, metastable, and excited negative ion formation during the collisions, Ramsauer-Townsend minima, and shape resonances. The novelty and generality of the Regge-pole approach is in the extraction of the negative ion binding energies (BEs) of complex heavy systems from the calculated electron TCSs. The extracted anionic BEs from the ground state TCSs for Ho, Er, Tm, Yb, Lu, and Hf atoms are 3.51 eV, 3.53 eV, 3.36 eV, 3.49 eV, 4.09 eV and 1.68 eV, respectively. The TCSs are presented and the extracted from the ground; metastable and excited anionic states BEs are compared with the available measured and/or calculated electron affinities. We conclude with a remark on the existing inconsistencies in the meaning of the electron affinity among the various measurements and/or calculations in the investigated atoms and make a recommendation to resolve the ambiguity.

Fullerene Negative Ions: Formation and Catalysis

We first explore negative-ion formation in fullerenes C₄₄ to C₁₃₆ through low-energy electron elastic scattering total cross sections calculations using our Regge-pole methodology. Then, the formed negative ions C₄₄ˉ to C₁₃₆ˉ are used to investigate the catalysis of water oxidation to peroxide and water synthesis from H₂ and O₂. The exploited fundamental mechanism underlying negative-ion catalysis involves hydrogen bond strength-weakening/breaking in the transition state. Density Functional Theory transition state calculations found C₆₀ˉ optimal for both water and peroxide synthesis, C₁₀₀ˉ increases the energy barrier the most, and C₁₃₆ˉ the most effective catalyst in both water synthesis and oxidation to H₂O₂.

High-resolution photoelectron imaging and resonant photoelectron spectroscopy via noncovalently bound excited states of cryogenically cooled anions

Valence-bound anions with polar neutral cores (μ > ∼2.5 D) can support dipole-bound excited states below the detachment threshold. These dipole-bound states (DBSs) are highly diffuse and the weakly bound electron in the DBS can be readily autodetached via vibronic coupling. Excited DBSs can be observed in photodetachment spectroscopy using a tunable laser. Tuning the detachment laser to above-threshold vibrational resonances yields vibrationally enhanced resonant photoelectron spectra, which are highly non-Franck-Condon with much richer vibrational information. This perspective describes recent advances in the studies of excited DBSs of cryogenically cooled anions using high-resolution photoelectron imaging (PEI) and resonant photoelectron spectroscopy (rPES). The basic features of dipole-bound excited states and highly non-Franck-Condon resonant photoelectron spectra will be discussed. The power of rPES to yield rich vibrational information beyond conventional PES will be highlighted, especially for low-frequency and Franck-Condon-inactive vibrational modes, which are otherwise not accessible from non-resonant conventional PES. Mode-selectivity and intra-molecular rescattering have been observed during the vibrationally induced autodetachment. Conformer-specific rPES is possible due to the different dipole-bound excited states of molecular conformers with polar neutral cores. For molecules with μ ≪ 2.5 D or without dipole moments, but large quadrupole moments, excited quadrupole-bound states can exist, which can also be used to conduct rPES.

Charged Clusters of C60 and Au or Cu: Evidence for Stable Sizes and Specific Dissociation Channels

We have doped helium nanodroplets with C60 and either gold or copper. Positively or negatively charged (C60) mM n± ions (M = Au or Cu) containing up to ≈10 fullerenes and ≈20 metal atoms are formed by electron ionization. The abundance distributions extracted from high-resolution mass spectra reveal several local anomalies. The sizes of the four most stable (C60) mAu n± ions identified in previous calculations for small values of m and n ( m ≤ 2 and n ≤ 2, or m = 1 and n = 3) agree with local maxima in the abundance distributions. Our data suggest the existence of several other relatively stable ions including (C60)2Au3± and (C60)3Au4-. Another feature, namely the absence of bare (C60)2±, confirms the prediction that (C60)2M± dissociates by loss of C60± rather than loss of M. The experimental data also reveal the preference for loss of (charged or neutral) C60 over loss of a metal atom from some larger species such as (C60)3M3+. In contrast to these similarities between Au and Cu, the abundance distributions of (C60)3Au n- and (C60)3Cu n- are markedly different. In this discussion, we emphasize the similarities and differences between anions and cations, and between gold and copper. Also noteworthy is the observation of dianions (C60) mAu n2- for m = 2, 4, and 6.

Rovibrational quantum state resolution of the C60 fullerene

The unique physical properties of buckminsterfullerene, C₆₀, have attracted intense research activity since its original discovery. Total quantum state-resolved spectroscopy of isolated C₆₀ molecules has been of particularly long-standing interest. Such observations have, to date, been unsuccessful owing to the difficulty in preparing cold, gas-phase C₆₀ in sufficiently high densities. Here we report high-resolution infrared absorption spectroscopy of C₆₀ in the 8.5-micron spectral region (1180 to 1190 wave number). A combination of cryogenic buffer-gas cooling and cavity-enhanced direct frequency comb spectroscopy has enabled the observation of quantum state-resolved rovibrational transitions. Characteristic nuclear spin statistical intensity patterns confirm the indistinguishability of the 60 carbon-12 atoms, while rovibrational fine structure encodes further details of the molecule's rare icosahedral symmetry.

Probing the interaction between the encapsulated water molecule and the fullerene cages in H2O@C60- and H2O@C59N. Chem Sci 2018;9:5666-5671. [PMID: 30062000 PMCID: PMC6050629 DOI: 10.1039/c8sc01031e]

We report a high-resolution photoelectron imaging study of cryogenically-cooled H2O@C60- and H2O@C59N- endohedral fullerene anions. The electron affinity (EA) of H2O@C60 is measured to be 2.6923 ± 0.0008 eV, which is 0.0088 eV higher than the EA of C60, while the EA of H2O@C59N is measured to be 3.0058 eV ± 0.0007 eV, which is 0.0092 eV lower than the EA of C59N. The opposite shifts are found to be due to the different electrostatic interactions between the encapsulated water molecule and the fullerene cages in the two systems. There is a net coulombic attraction between the guest and host in H2O@C60-, but a repulsive interaction in H2O@C59N-. We have also observed low-frequency features in the photoelectron spectra tentatively attributed to the hindered rotational excitations of the encapsulated H2O molecule, providing further insights into the guest-host interactions in H2O@C60- and H2O@C59N-.

Bound electronic states of the smallest fullerene C20- anion

We report on high-level coupled-cluster calculations for the anion states of the smallest fullerene C20. Using the state-of-the-art EA-EOM-CCSD method we revealed that the C20- anion has five bound electronic states at the C20 neutral ground-state D3d equilibrium configuration. These are two pairs of 2Eu and 2A2u states and one 2A1g state. The binding energies vary from 2.05 eV for the most bound 2Eu state to <1 meV for the 2A1g state. An analysis in terms of radial and angular density distribution of the excess electron revealed that the two 2Eu/2A2u pairs are valence-like states while 2A1g corresponds to a super-atomic-like (SAMO) state. The valence states of the first 2Eu/2A2u pair were found to be of p-type whereas those of the second pair are of hybrid sp-type. We have also applied a simple model to understand the binding of the excess electron in C20-. The model confirms the existence of only one SAMO state of the s-type for C20-. At the same time, it overestimates the number of the bound valence states of C20-.

Highly Stable [C60AuC60]+/- Dumbbells

Ionic complexes between gold and C₆₀ have been observed for the first time. Cations and anions of the type [Au(C₆₀)₂]^+/- are shown to have particular stability. Calculations suggest that these ions adopt a C₆₀-Au-C₆₀ sandwich-like (dumbbell) structure, which is reminiscent of [XAuX]^+/- ions previously observed for much smaller ligands. The [Au(C₆₀)₂]^+/- ions can be regarded as Au(I) complexes, regardless of whether the net charge is positive or negative, but in both cases, the charge transfer between the Au and C₆₀ is incomplete, most likely because of a covalent contribution to the Au-C₆₀ binding. The C₆₀-Au-C₆₀ dumbbell structure represents a new architecture in fullerene chemistry that might be replicable in synthetic nanostructures.

Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc

Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C₆₀ and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ₁ between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ₁ may provide alternative strategies to optimize the electronic properties of organic semiconductors.

Nitric oxide oxidation of a Ta encapsulating Si cage nanocluster superatom (Ta@Si16) deposited on an organic substrate; a Si cage collapse indicator

Stepwise oxidative reaction of a Ta-encapsulating Si₁₆ caged nanocluster superatom upon exposure to nitric oxide is investigated by monitoring N 1s core level signals.

High-Resolution Photoelectron Imaging of Cryogenically-Cooled C59N- and (C59N)22- Azafullerene Anions

We report a photoelectron imaging study of cryogenically cooled C₅₉N^- and (C₅₉N)₂^2- anions produced from electrospray ionization. High-resolution photoelectron spectra are obtained for C₅₉N^- for the first time, allowing seven vibrational frequencies of the C₅₉N azafullerene to be measured. The electron affinity of C₅₉N is determined accurately to be 3.0150 ± 0.0007 eV. The observed vibrational features are understood on the basis of calculated frequencies and compared with those of C₆₀ and C₅₉HN. The photoelectron image of (C₅₉N)₂^2-, which has the same mass/charge ratio as C₅₉N^-, is also observed, allowing the second electron affinity of the (C₅₉N)₂ azafullerene dimer to be measured as 1.20 ± 0.05 eV. The intramolecular Coulomb repulsion of the (C₅₉N)₂^2- dianion is estimated to be 1.96 eV and is investigated theoretically using the electron density difference between (C₅₉N)₂^2- and (C₅₉N)₂.

Ab Initio Confirmation of a Harpoon-Type Electron Transfer in a Helium Droplet

An ab initio study of a long-range electron transfer or "harpoon"-type process from Cs and Cs₂ to C₆₀ in a superfluid helium droplet is presented. The heliophobic Cs or Cs₂ species are initially located at the droplet surface, while the heliophilic C₆₀ molecule is fully immersed in the droplet. First, probabilities for the electron transfer in the gas phase are calculated for reactants with velocities below the critical Landau velocity of 57 m/s to account for the superfluid helium environment. Next, reaction pathways are derived that also include the repulsive contribution from the extrusion of helium upon the approach of the two reactants. Our results are in perfect agreement with recent experimental measurements of electron ionization mass spectroscopy [ Renzler , M. ; et al., J. Chem. Phys. 2016 , 145 , 181101 ], showing a high possibility for the formation of a Cs₂-C₆₀ complex inside of the droplet through a direct harpoon-type electron transfer involving the rotation of the molecule but a negligibly low reactivity for atomic Cs.

A detailed-balance model for thermionic emission from polyanions: The case of fullerene dianions

A detailed-balance model for thermionic emission from polyanions has been developed and applied to fullerene dianions. The specificity of this delayed decay process is electron tunneling through the repulsive Coulomb barrier (RCB). An analytical expression of the RCB is derived from electrostatic modeling of the fullerene cage. The reverse process, namely, electron attachment to the singly charged anion, is described by a hard sphere cross section weighted by the Wentzel-Kramers-Brillouin tunneling probability. This simple expression leads to a very good agreement with a measured time-resolved kinetic energy distribution of C₈₄^2-. Electron binding energy is reduced when the fullerene cage size decreases, leading to an almost zero one for C₇₀^2- and a negative one for C₆₀^2-. Extension of the model to these systems of interest is discussed, and model outputs are compared with the experimental data from the literature.

How many bound valence states does the C₆₀⁻ anion have?

We report on unprecedentedly large coupled cluster calculations for the C60(-) anion, and on a heuristic model uncovering the valence states of C60(-) that allow the resolution of the headlined question. Our results convincingly demonstrate that C60(-) possesses as many as four bound valence states: (2)T1u, (2)T1g, (2)T2u and (2)Hg. Our findings reconcile previous controversies regarding the existence of the bound (2)T2u and (2)Hg states. For all bound states of C60(-) we present an analysis of the radial and angular distributions of the excess electron, which reveals some unique properties of the valence states. Some interesting features of the introduced model are analyzed and discussed.