High-resolution Photoelectron Imaging Research Articles

Observation of bound valence excited electronic states of deprotonated 2-hydroxytriphenylene using photoelectron, photodetachment, and resonant two-photon detachment spectroscopy of cryogenically cooled anions.

Polycyclic aromatic hydrocarbons (PAHs) are common atmospheric pollutants, and they are also ubiquitous in the interstellar medium. Here, we report the study of a complex O-containing PAH anion, the deprotonated 2-hydroxytriphenylene (2-OtPh-), using high-resolution photoelectron imaging and photodetachment spectroscopy of cryogenically cooled anions. Vibrationally resolved photoelectron spectra yield the electron affinity of the 2-OtPh radical as 2.629(1) eV and several vibrational frequencies for its ground electronic state. Photodetachment spectroscopy reveals bound valence excited electronic states for the 2-OtPh- anion, with unprecedentedly rich vibronic features. Evidence is presented for a low-lying triplet state (T1) and two singlet states (S1 and S2) below the detachment threshold. Single-color resonant two-photon photoelectron spectroscopy uncovers rich photophysics for the 2-OtPh- anion, including vibrational relaxation in S1, internal conversion to the ground state of 2-OtPh-, intersystem crossing from S2 to T1, and a long-lived autodetaching shape resonance about 1.3eV above the detachment threshold. The rich electronic structure and photophysics afforded by the current study suggest that 2-OtPh- would be an interesting system for pump-probe experiments to unravel the dynamics of the excited states of this complex PAH anion.

High-Resolution Photoelectron Imaging of Cryogenically Cooled BiB2- and BiB3- Bismuth-Boron Clusters.

We report a high-resolution photoelectron imaging study of cryogenically cooled BiB2- and BiB3- clusters. Vibrational features are completely resolved for the ground-state detachment transitions, providing critical information about the structures of the anionic clusters and their corresponding neutrals. The electron affinities of BiB2 and BiB3 are accurately measured to be 2.174(1) and 2.121(1) eV, respectively. The B-B and Bi-B stretching frequencies are measured to be 1262 and 476 cm-1, respectively, in the ground state of BiB2. Three vibrational frequencies are measured for the ground state of BiB3: 1194 cm-1 (B-B stretching), 782 cm-1 (B-B stretching), and 339 cm-1 (Bi-B stretching). Both BiB2- and BiB3- and their neutral ground states are found to have planar C2v structures in which the Bi atom bridges two B atoms. BiB2- is found to have a triplet spin state (3B2), consistent with its complicated photoelectron spectra, whereas BiB3- is a doublet (2B1) and neutral BiB3 is closed shell (1A1). Both BiB2 and BiB3 consist of peripheral localized Bi-B and B-B σ bonds and delocalized π and σ bonds.

On the electronic structure and spin-orbit coupling of BiB from photoelectron imaging of cryogenically-cooled BiB- anion.

We report a study on the electronic structure and chemical bonding of the BiB molecule using high-resolution photoelectron imaging of cryogenically cooled BiB- anion. By eliminating all the vibrational hot bands, we can resolve the complicated detachment transitions due to the open-shell nature of BiB and the strong spin-orbit coupling. The electron affinity of BiB is measured to be 2.010(1) eV. The ground state of BiB- is determined to be 2Π(3/2) with a σ2π3 valence electron configuration, while the ground state of BiB is found to be 3Σ-(0+) with a σ2π2 electron configuration. Eight low-lying spin-orbit excited states [3Σ-(1), 1Δ(2), 1Σ+(0+), 3Π(2), 3Π(1), 1Π(1)], including two forbidden transitions, [3Π(0-) and 3Π(0+)], are observed for BiB as a result of electron detachment from the σ and π orbitals of BiB-. The angular distribution information from the photoelectron imaging is found to be critical to distinguish detachment transitions from the σ or π orbital for the spectral assignment. This study provides a wealth of information about the low-lying electronic states and spin-orbit coupling of BiB, demonstrating the importance of cryogenic cooling for obtaining well-resolved photoelectron spectra for size-selected clusters produced from a laser vaporization cluster source.

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.

Development of High-Resolution Photoelectron Imaging with Dual-Reflection Time of Flight Mass Analyzer

Development of High-Resolution Photoelectron Imaging with Dual-Reflection Time of Flight Mass Analyzer

Photoelectron imaging of cryogenically cooled BiO- and BiO2 - anions.

The advent of ion traps as cooling devices has revolutionized ion spectroscopy as it is now possible to efficiently cool ions vibrationally and rotationally to levels where truly high-resolution experiments are now feasible. Here, we report the first results of a new experimental apparatus that couples a cryogenic 3D Paul trap with a laser vaporization cluster source for high-resolution photoelectron imaging of cold cluster anions. We have demonstrated the ability of the new apparatus to efficiently cool BiO- and BiO2 - to minimize vibrational hot bands and allow high-resolution photoelectron images to be obtained. The electron affinities of BiO and BiO2 are measured accurately for the first time to be 1.492(1) and 3.281(1) eV, respectively. Vibrational frequencies for the ground states of BiO and BiO2, as well as those for the anions determined from temperature-dependent studies, are reported.

Dipole-Bound State, Photodetachment Spectroscopy, and Resonant Photoelectron Imaging of Cryogenically-Cooled 2-Cyanopyrrolide.

Valence-bound anions with polar neutral cores can have diffuse dipole-bound excited states just below the electron detachment threshold. Because of the similarity in geometry and vibrational frequencies between the dipole-bound states (DBSs) and the corresponding neutrals, DBSs have been exploited as intermediate states to conduct resonant photoelectron spectroscopy (PES), resulting in highly non-Franck-Condon photoelectron spectra via vibrational autodetachment and providing much richer vibrational information than conventional PES. Here, we report a photodetachment and high-resolution photoelectron imaging study of the 2-cyanopyrrolide anion, cooled in a cryogenic ion trap. The electron affinity of the 2-cyanopyrrolyl radical is measured to be 3.0981 ± 0.0006 eV (24 988 ± 5 cm-1). A DBS is observed for 2-cyanopyrrolide at 240 cm-1 below its detachment threshold using photodetachment spectroscopy. Twenty-three above-threshold vibrational resonances (Feshbach resonances) of the DBS are observed. Resonant PES is conducted at each Feshbach resonance, yielding a wealth of vibrational information about the 2-cyanopyrrolyl radical. Resonant two-photon PES confirms the s-like dipole-bound orbital and reveals a relatively long lifetime of the bound zero-point level of the DBS. Fundamental frequencies for 19 vibrational modes (out of a total of 24) are obtained for the cyanopyrrolyl radical, including six out-of-plane modes. The current work provides important spectroscopic information about 2-cyanopyrrolyl, which should be valuable for the study of this radical in combustion or astronomical environments.

Probing the electronic structure and spectroscopy of 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 pyrrolyl and imidazolyl radicals. The high-resolution data allow the ground state vibronic structures of the two radicals to be completely resolved, yielding accurate electron affinities of 2.1433 ± 0.0008 eV and 2.6046 ± 0.0006 eV for pyrrolyl and imidazolyl radicals, respectively. Fundamental frequencies for eight vibrational modes of pyrrolyl and ten vibrational modes of imidazolyl are measured, including several nonsymmetric Franck-Condon-forbidden modes. Two electronic excited states are also observed for the two radicals, displaying diffuse spectral features in both systems. The observations of nonsymmetric vibrational modes in the ground states and the diffuse excited state features provide strong evidence for vibronic couplings between the ground state and the two close-by excited states. The 2-pyrrolide isomer is also observed as a minor species from the electrospray ionization source and the electron affinity of 2-pyrrolyl is measured to be 1.6690 ± 0.0030 eV along with five vibrational frequencies. Even though the HOMOs of both pyrrolide and imidazolide anions are p orbitals, photodetachment spectroscopy reveals completely different threshold behaviors for the two anions: a d-wave-dominated spectrum for pyrrolide and an s-wave-dominated spectrum for imidazolide. The current study provides a wealth of electronic and spectroscopic information, which is ideal to compare with more accurate vibronic coupling calculations for these two important radicals, as well as interesting information about the photodetachment dynamics of the two anions.

High-Resolution Photoelectron Imaging and Photodetachment Spectroscopy of Cryogenically Cooled IO–

We report a high-resolution photoelectron imaging and photodetachment spectroscopy study of cryogenically-cooled IO-. The high-resolution photoelectron spectra yield a more accurate electron affinity (EA) of 2.3805(5) eV for IO, as well as a more accurate spin-orbit splitting energy between 2Π3/2 and 2Π1/2 of IO as 2093(5) cm-1. Photodetachment spectroscopy revealed for the first time several excited states for the IO- anion, including two valence-type excited states, the repulsive 3Π state and a shallow bound 1Π state. More interestingly, we have observed two vibrational resonances which are proposed to be due to a dipole-induced resonant state, about 230 cm-1 above the detachment threshold of IO-.

High-resolution photoelectron imaging of MnB3 -: Probing the bonding between the aromatic B3 cluster and 3d transition metals.

The B3 triangular unit is a fundamental bonding motif in all boron compounds and nanostructures. The isolated B3 - cluster has a D3h structure with double σ and π aromaticity. Here, we report an investigation of the bonding between a B3 cluster and a 3d transition metal using high-resolution photoelectron imaging and computational chemistry. Photoelectron spectra of MnB3 - are obtained at six different photon energies, revealing rich vibrational information for the ground state detachment transition. The electron affinity of MnB3 is determined to be 1.6756(8) eV, and the most Franck-Condon-active mode observed has a measured frequency of 415(6) cm-1 due to the Mn-B3 stretch. Theoretical calculations show that MnB3 - has a C2v planar structure, with Mn coordinated to one side of the triangular B3 unit. The ground states of MnB3 - (6B2) and MnB3 (5B2) are found to have high spin multiplicity with a significant decrease in the Mn-B bond distances in the neutral due to the detachment of an Mn-B3 anti-bonding electron. The Mn atom is shown to have weak interactions with the B3 unit, which maintains its double aromaticity with relatively small structural changes from the bare B3 cluster. The bonding in MnB3 is compared with that in 5d MB3 clusters, where the strong metal-B3 interactions strongly change the structures and bonding in the B3 moiety.

Photodetachment spectroscopy and resonant photoelectron imaging of the 2-naphthoxide anion via dipole-bound excited states.

We report a photodetachment spectroscopy and high-resolution resonant photoelectron imaging study of cryogenically cooled 2-naphthoxide anions (C10H7O-). The photodetachment spectrum revealed a dipole-bound state (DBS) 202(4) cm-1 below the detachment threshold and 38 resonances corresponding to the vibrational levels of the DBS. By tuning the detachment laser to these above-threshold resonances, we obtained 38 resonantly enhanced photoelectron spectra, which were highly non-Franck-Condon as a result of mode-selective vibrational autodetachment from the DBS. The resonances were assigned by comparing the resonant and non-resonant photoelectron spectra, assisted by the computed vibrational frequencies. Specifically, vibrational features with low Franck-Condon factors or from Franck-Condon-forbidden vibrational modes were significantly enhanced in the resonant photoelectron spectra, resulting in much richer spectroscopic information. The electron affinity of the 2-naphthoxy radical was measured accurately to be 19 387(4) cm-1 or 2.4037(5) eV. In addition, a total of 17 vibrational frequencies were obtained for the 2-naphthoxy radical. In particular, seven Franck-Condon-forbidden out-of-plane bending modes, including the two lowest frequency modes (ν48 at 102 cm-1 and ν47 at 171 cm-1), were observed, demonstrating the advantages of combining photodetachment spectroscopy and resonant photoelectron spectroscopy in obtaining vibrational information for polar radical species via DBS.

The nature of the chemical bonding in 5d transition-metal diatomic borides MB (M = Ir, Pt, Au).

Boron can form strong bonds with transition metals in diatomic metal borides (MB), but the nature of the chemical bonding has not been well understood. Recently, a quadruple bond was discovered in Rh≣B, consisting of two σ bonds formed between the Rh 4dz2 and B 2s/2p orbitals and two π bonds between the Rh 4dxz/4dyz and the B 2px/2py orbitals. The bonding between the 5d transition metals and boron is expected to be even stronger. Here, we report an investigation on the electronic structure and chemical bonding of the 5d transition metal diatomic borides (IrB, PtB, and AuB) using high-resolution photoelectron imaging on the corresponding anions (MB-) and theoretical calculations. Vibrationally resolved photoelectron spectra are obtained for all three anions, and the electron affinities are measured for IrB, PtB, and AuB to be 1.995(1), 2.153(3), and 0.877(6) eV, respectively. It is found that the weakly anti-bonding 3σ molecular orbital (mainly of M 6s and B sp characters) is singly occupied in IrB (3Δ) and PtB (2Σ+), resulting in a bond order of three and half for these two diatomic borides. The 3σ orbital is doubly occupied in AuB (1Σ+), giving rise to a weak triple bond. Despite the lower bond order, the bonding in IrB and PtB is only slightly weaker than that in RhB due to the more favorable interactions between the M 5d orbitals and the B sp orbitals.

MnB6-: An Open-Shell Metallaboron Analog of 3d Metallabenzenes.

We report a study of the structure and bonding of a transition-metal-doped boron cluster, MnB6-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra indicate a significant geometry change between the anionic and neutral ground states of MnB6. The electron affinity of MnB6 is measured to be 2.4591(5) eV, and vibrational frequencies for five of its vibrational modes were determined. The experimental data are combined with theoretical calculations to determine the structure and bonding of MnB6-, which is found to be planar with a B-centered hexagonal structure (C2v symmetry) and a quintet spin state (5A2). Nuclear-independent chemical shift calculations indicate that MnB6- is aromatic. Molecular orbital analyses reveal that MnB6- contains three π orbitals, one of which is singly occupied. Hence, MnB6- can be considered as an open-shell metallaboron analog of 3d metallabenzenes.

Observation of Möbius Aromatic Planar Metallaborocycles.

Möbius aromaticity was developed for twisted annulenes with electron counting rules opposite to those of Hückel aromaticity. The introduction of transition metals makes it possible for planar cyclic systems to exhibit Möbius aromaticity. Here we report the first planar monocyclic metallaboron systems with Möbius aromaticity. The structures and bonding of two rhenium-boride clusters are studied by high-resolution photoelectron imaging and ab initio calculations. The ReB3- cluster is shown to have a near-pyramidal structure, while ReB4- is found to be a planar pentagonal ring. Chemical bonding analyses show that both ReB4- and ReB4 possess four delocalized π-electrons, including two π-electrons in an orbital of Möbius topology. NICS calculations reveal strong aromatic characters in ReB4- and ReB4, consistent with the 4n electron counting rule for Möbius aromaticity.

The dicarbon bonding puzzle viewed with photoelectron imaging

Bonding in the ground state of C{}_{2} is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of 2, 3, or 4. Here we report on photoelectron spectra of the C{}_{2}^{-} anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground {X}^{1}{Sigma}_{mathrm{g}}^{+} and first-excited {a}^{3}{Pi}_{{mathrm{u}}} electronic states. These measurements yield electron angular anisotropies that identify the character of two orbitals: the diffuse detachment orbital of the anion and the highest occupied molecular orbital of the neutral. This work indicates that electron detachment occurs from predominantly s-like (3{sigma}_{mathrm{g}}) and p-like (1{pi }_{{mathrm{u}}}) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly sp-mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2pi bonds and no accompanying sigma bond.

Candidate for Laser Cooling of a Negative Ion: High-Resolution Photoelectron Imaging of Th^{-}.

Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have La^{-} as the most promising candidate. We report on experimental and theoretical studies supporting Th^{-} as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, 4901.35(48) cm^{-1} and 4832 cm^{-1}, or 0.607 690(60) and 0.599eV, almost a factor of 2 larger than the previous theoretical value of 0.368eV. The ground state of Th^{-} is determined to be 6d^{3}7s^{2} ^{4}F_{3/2}^{e} rather than 6d^{2}7s^{2}7p ^{4}G_{5/2}^{o}. The consequence of this is that there are several strong electric dipole transitions between the bound levels arising from configurations 6d^{3}7s^{2} and 6d^{2}7s^{2}7p in Th^{-}. The potential laser-cooling transition is ^{2}S_{1/2}^{o}↔^{4}F_{3/2}^{e} with a wavelength of 2.6 μm. The zero nuclear spin and hence lack of hyperfine structure in Th^{-} reduces the potential complications in laser cooling as encountered in La^{-}, making Th^{-} a new and exciting candidate for laser cooling.

ReB6-: A Metallaboron Analog of Metallabenzenes.

Metallabenzenes are a class of molecules in which a CH unit in benzene is replaced by a functionalized transition-metal atom. While all-boron analogues of aromatic and antiaromatic hydrocarbons are well-known, there have not been any metallaboron analogs. We have produced and investigated two metal-doped boron clusters, ReB6- and AlB6-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra have been obtained and compared with the theoretical results. The ReB6- cluster is found to be perfectly planar with a B-centered hexagonal structure (C2v, 1A1), while AlB6- is known to have a similar structure, but with a slightly out-of-plane distortion (Cs, 1A'). Chemical bonding analyses show that the closed-shell ReB6- is doubly σ- and π-aromatic, while AlB6- is known to be σ-aromatic and π-antiaromatic. The out-of-plane distortion in AlB6- is due to antiaromaticity, akin to the out-of-plane distortion of the prototypical antiaromatic cyclooctatetraene. The π-bonding in ReB6- is compared with that in both benzene and rhenabenzene [(CO)4ReC5H5], and remarkable similarities are found. Hence, ReB6- can be viewed as the first metallaboron analog of metallabenzenes and it may be viable for syntheses with suitable ligands.

Probing the Critical Dipole Moment To Support Excited Dipole-Bound States in Valence-Bound Anions.

We report photodetachment spectroscopy and high-resolution photoelectron imaging of para-halogen substituted phenoxide anions, p-XC6H4O- (X = F, Cl, Br, I). The dipole moments of the p-XC6H4O neutral radicals increase from 2.56 to 3.19 D for X = F to I, providing a series of similar molecules to allow the examination of charge-dipole interactions by minimizing molecule-dependent effects. Excited DBSs ([XC6H4O]*-) are observed for the four anions with binding energies of 8, 11, 24, and 53 cm-1, respectively, for X = F to I, below their respective detachment thresholds. The binding energies exhibit a linear correlation with the dipole moments of the neutral radicals, extrapolating to a critical dipole moment of 2.5 D for zero binding energy. Because of the small binding energy of the excited DBS of [FC6H4O]*-, rotational autodetachment is observed to compete with vibrational autodetachment in the resonant photoelectron spectra, resulting in electrons with near zero kinetic energies.

High-Resolution Photoelectron Imaging of IrB3 - : Observation of a π-Aromatic B3 + Ring Coordinated to a Transition Metal.

In a high-resolution photoelectron imaging and theoretical study of the IrB3 - cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B3 ring coordinated with the Ir atom. The isomer with the higher EA consists of a B3 ring with a bridge-bonded Ir atom (Cs , 2 A'), and the second isomer features a tetrahedral structure (C3v , 2 A1 ). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C3v isomer involves significant covalent Ir-B bonding and weak ionic bonding with charge transfer from B3 to Ir, and can be viewed as an Ir-(η3 -B3 + ) complex. This study provides the first example of a boron-to-metal charge-transfer complex and evidence of a π-aromatic B3 + ring coordinated to a transition metal.

High resolution photoelectron imaging of boron-bismuth binary clusters: Bi2Bn - (n = 2-4).

Bismuth boride is a heavy member of the III-V semiconductors. Although there have been some theoretical interests in this material, it has not been synthesized experimentally. Here, we report a high-resolution photoelectron imaging study on a series of boron-bismuth binary clusters, Bi2Bn - (n = 2-4), produced by laser vaporization of a B/Bi mixed target. Vibrationally resolved photoelectron spectra are obtained for all three clusters, and the measured vibrational and electronic information is used to compare with theoretical calculations to understand their structures and bonding. Bi2B2 - is found to be linear (D∞h, 2Πg) with a B2 unit and two terminal Bi atoms, while Bi2B3 - is found to be planar (C2v, 1A1), consisting of a B3 triangle with two bridging Bi atoms. Interestingly, the spectra of Bi2B4 - reveal two co-existing isomers; both are found to be planar and contain a rhombus B4 unit with two bridging Bi atoms in a trans (C2h, 2Au) and cis (C2v, 2B1) fashion separated only by 0.03 eV in energy. The interactions between the two Bi atoms and the Bn motifs are understood using chemical bonding analyses. This study shows that the Bi-B bonding is weak enough so that the Bn units maintain their structural integrity with the Bi atoms bonded to the cluster periphery only.