Photodetachment spectroscopy and resonant photoelectron imaging of cryogenically cooled 1-pyrenolate

Photoelectron Imaging and Photodetachment Spectroscopy for the Cryogenically Cooled Cyanocyclopentadienide Anion

The cyano-cyclopentadiene molecule (CN-C5H5) has attracted significant interest since its detection in the interstellar medium, but the radical (CN-C5H4) and anionic (CN-C5H4-) forms of cyano-cyclopentadiene have not been studied. The cyano-cyclopentadienyl radical (CN-Cp) has a strong dipole moment, rendering it an ideal system for vibrational and rotational spectroscopy. We report an investigation of the cryogenically cooled cyano-cyclopentadienide anion (CN-Cp-) using high-resolution photoelectron imaging, photodetachment spectroscopy, and resonant photoelectron imaging. The electron affinity of the CN-Cp radical is measured accurately to be 2.7741 ± 0.0003 eV (22,375 ± 2 cm-1). A low-lying excited state is observed for the CN-Cp neutral radical at 151 cm-1 above the ground state. The overlap and vibronic coupling of the ground and low-lying electronic states give rise to complicated and congested photoelectron spectra. A dipole-bound state is observed for the CN-Cp- anion with a binding energy of 94 cm-1, along with 15 vibrational Feshbach resonances. Resonant photoelectron spectra via the vibrational resonances yield well-resolved spectra, allowing 26 vibronic levels to be identified for CN-Cp. The rich spectroscopic information will be valuable to compare with theoretical studies to unravel the vibronic coupling and nonadiabatic effects in the CN-Cp radical.

Dynamics of Anions: From Bound to Unbound States and Everything In Between

Gas-phase anions present an ideal playground for the exploration of excited-state dynamics. They offer control in terms of the mass, extent of solvation, internal temperature, and conformation. The application of a range of ion sources has opened the field to a vast array of anionic systems whose dynamics are important in areas ranging from biology to star formation. Here, we review recent experimental developments in the field of anion photodynamics, demonstrating the detailed insight into photodynamical and electron-capture processes that can be uncovered. We consider the electronic and nuclear ultrafast dynamics of electronically bound excited states along entire reaction coordinates; electronically unbound states showing that photochemical concepts, such as chromophores and Kasha's rule, are transferable to electron-driven chemistry; and nonvalence states that straddle the interface between bound and unbound states. Finally, we consider likely developments that are sure to keep the field of anion dynamics buoyant and impactful.

Electron-Binding Dynamics of the Dipole-Bound State: Correlation Effect on the Autodetachment Dynamics

The nature of the electron-binding forces in the dipole-bound states (DBS) of anions is interrogated through experimental and theoretical means by investigating the autodetachment dynamics from DBS Feshbach resonances of ortho-, meta-, and para-bromophenoxide (BrPhO-). Though the charge-dipole electrostatic potential has been widely regarded to be mainly responsible for the electron binding in DBS, the effect of nonclassical electron correlation has been conceived to be quite significant in terms of its static and/or dynamic contributions toward the binding of the excess electron to the neutral core. State-specific real-time autodetachment dynamics observed by picosecond time-resolved photoelectron velocity-map imaging spectroscopy reveal that the autodetachment processes from the DBS Feshbach resonances of BrPhO- anions cannot indeed be rationalized by the conventional charge-dipole potential. Specifically, the autodetachment lifetime is drastically lengthened depending on differently positioned Br-substitution, and this rate change cannot be explained within the framework of Fermi's golden rule based on the charge-dipole assumption. High-level ab initio quantum chemical calculations with EOM-EA-CCSD, which intrinsically takes into account electron correlations, generate more reasonable predictions on the binding energies than density functional theory (DFT) calculations, and semiclassical quantum dynamics simulations based on the EOM-EA-CCSD data excellently predict the trend in the autodetachment rates. These findings illustrate that static and dynamic properties of the excess electron in the DBS are strongly influenced by correlation interactions among electrons in the nonvalence orbital of the dipole-bound electron and highly polarizable valence orbitals of the bromine atom, which, in turn, dictate the interesting chemical fate of exotic anion species.

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.

Role of Polarization Interactions in the Formation of Dipole-Bound States

Even though there is a critical dipole moment required to support a dipole-bound state (DBS), how molecular polarizability may influence the formation of DBSs is not well understood. Pyrrolide, indolide, and carbazolide provide an ideal set of anions to systematically examine the role of polarization interactions in the formation of DBSs. Here, we report an investigation of carbazolide using cryogenic photodetachment spectroscopy and high-resolution photoelectron spectroscopy (PES). A polarization-assisted DBS is observed at 20 cm^-1 below the detachment threshold for carbazolide, even though the carbazolyl neutral core has a dipole moment (2.2 D) smaller than the empirical critical value (2.5 D) to support a dipole-bound state. Photodetachment spectroscopy reveals nine vibrational Feshbach resonances of the DBS, as well as three intense and broad shape resonances. The electron affinity of carbazolyl is measured accurately to be 2.5653 ± 0.0004 eV (20,691 ± 3 cm^-1). The combination of photodetachment spectroscopy and resonant PES allows fundamental frequencies for 14 vibrational modes of carbazolyl to be measured. The three shape resonances are due to above-threshold excitation to the three low-lying electronic states (S₁-S₃) of carbazolide. Resonant PES of the shape resonances is dominated by autodetachment processes. Ultrafast relaxation from the S₂ and S₃ states to S₁ is observed, resulting in constant kinetic energy features in the resonant PES. The current study provides decisive information about the role that polarization plays in the formation of DBSs, as well as rich spectroscopic information about the carbazolide anion and the carbazolyl radical.

Photon energy dependence of the photoelectron spectra of the anthracene anion: On the influence of autodetaching states

Vibrationally resolved photoelectron spectra of anthracene anions have been measured for photon energies between 1.13 and 4.96 eV. In this energy range, photoemission mostly occurs via autodetaching electronically excited states of the anion, which strongly modifies the vibrational excitation of the neutral molecule after electron emission. Based on the observed vibrational patterns, eight different excited states could be identified, seven of which are resonances known from absorption spectroscopy. Distinctly different photon energy dependencies of vibrational excitations have been obtained for different excited states, hinting at strongly different photoemission lifetimes. Unexpectedly, some resonances seem to exhibit bimodal distributions of emission lifetimes, possibly due to electronic relaxation processes induced by the excitation of specific vibrational modes.

Observation of a Polarization-Assisted Dipole-Bound State

The critical dipole moment to bind an electron was empirically determined to be 2.5 debye, even though smaller values were predicted theoretically. Herein, we report the first observation of a polarization-assisted dipole-bound state (DBS) for a molecule with a dipole moment below 2.5 debye. Photoelectron and photodetachment spectroscopies are conducted for cryogenically cooled indolide anions, where the neutral indolyl radical has a dipole moment of 2.4 debye. The photodetachment experiment reveals a DBS only 6 cm^-1 below the detachment threshold along with sharp vibrational Feshbach resonances. Rotational profiles are observed for all of the Feshbach resonances, which are found to have surprisingly narrow linewidths and long autodetachment lifetimes attributed to weak coupling between vibrational motions and the nearly free dipole-bound electron. Calculations suggest that the observed DBS has π-symmetry stabilized by the strong anisotropic polarizability of indolyl.

Investigation of the Electronic and Vibrational Structures of the 2-Furanyloxy Radical Using Photoelectron Imaging and Photodetachment Spectroscopy via the Dipole-Bound State of the 2-Furanyloxide Anion

The 2-furanyloxy radical is an important chemical reaction intermediate in the combustion of biofuels and aromatic compounds. We report an investigation of its electronic and vibrational structures using photoelectron and photodetachment spectroscopy and resonant photoelectron imaging (PEI) of cryogenically cooled 2-furanyloxide anion. The electron affinity of 2-furanyloxy is measured to be 1.7573(8) eV. Two excited electronic states are observed at excitation energies of 2.14 and 2.82 eV above the ground state. Photodetachment spectroscopy reveals a dipole-bound state 0.0143 eV below the detachment threshold and 25 vibrational Feshbach resonances for the 2-furanyloxide anion. The combination of photodetachment spectroscopy and resonant PEI yields frequencies for 18 out of a total of 21 vibrational modes for the 2-furanyloxy radical, including all six of its bending modes. The rich electronic and vibrational information will be valuable for further understanding the role of 2-furanyloxy as a key reaction intermediate of combustion and atmospheric interests.

Spectroscopy and Theoretical Modeling of Tetracene Anion Resonances

The positions and widths of the optically allowed electronic states of the tetracene radical anion located above the detachment threshold energy (i.e anion resonances) are mapped out using total photodetachment yield spectroscopy of cryogenically cooled ions. The presence of these states is detected via the sharp increase in the photodetachment yield compared to that of the monotonic nonresonant direct photodetachment background. The resolution of the resulting spectrum is limited by the ∼5 cm^-1 line width of the tunable laser and thus provides a stringent benchmark for computations of the energies and autodetachment lifetimes of these resonance states. The experimental results are compared to high-level electronic structure computations and line width modeling using the orbital stabilization method. These theoretical results are found to be in near quantitative agreement with the experimental data, highlighting their capability to accurately describe the energies and lifetimes of anion resonances for relatively large molecules.

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 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.

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...

Nonadiabatic Dynamics between Valence, Nonvalence, and Continuum Electronic States in a Heteropolycyclic Aromatic Hydrocarbon

Internal conversion between valence-localized and dipole-bound states is thought to be a ubiquitous process in polar molecular anions, yet there is limited direct evidence. Here, photodetachment action spectroscopy and time-resolved photoelectron imaging with a heteropolycyclic aromatic hydrocarbon (hetero-PAH) anion, deprotonated 1-pyrenol, is used to demonstrate a subpicosecond (τ₁ = 160 ± 20 fs) valence to dipole-bound state internal conversion following excitation of the origin transition of the first valence-localized excited state. The internal conversion dynamics are evident in the photoelectron spectra and in the photoelectron angular distributions (β₂ values) as the electronic character of the excited state population changes from valence to nonvalence. The dipole-bound state subsequently decays through mode-specific vibrational autodetachment with a lifetime τ₂ = 11 ± 2 ps. These internal conversion and autodetachment dynamics are likely common in molecular anions but difficult to fingerprint due to the transient existence of the dipole-bound state. Potential implications of the present excited state dynamics for interstellar hetero-PAH anion formation are discussed.

Observation of a dipole-bound excited state in 4-ethynylphenoxide and comparison with the quadrupole-bound excited state in the isoelectronic 4-cyanophenoxide

Negative ions do not possess Rydberg states but can have Rydberg-like nonvalence excited states near the electron detachment threshold, including dipole-bound states (DBSs) and quadrupole-bound states (QBSs). While DBSs have been studied extensively, quadrupole-bound excited states have been more rarely observed. 4-cyanophenoxide (4CP^-) was the first anion observed to possess a quadrupole-bound exited state 20 cm^-1 below its detachment threshold. Here, we report the observation of a DBS in the isoelectronic 4-ethynylphenoxide anion (4EP^-), providing a rare opportunity to compare the behaviors of a dipole-bound and a quadrupole-bound excited state in a pair of very similar anions. Photodetachment spectroscopy (PDS) of cryogenically cooled 4EP^- reveals a DBS 76 cm^-1 below its detachment threshold. Photoelectron spectroscopy (PES) at 266 nm shows that the electronic structure of 4EP^- and 4CP^- is nearly identical. The observed vibrational features in both the PDS and PES, as well as autodetachment from the nonvalence excited states, are also found to be similar for both anions. However, resonant two-photon detachment (R2PD) from the bound vibrational ground state is observed to be very different for the DBS in 4EP^- and the QBS in 4CP^-. The R2PD spectra reveal that decays take place from both the DBS and QBS to the respective anion ground electronic states within the 5 ns detachment laser pulse due to internal conversion followed by intramolecular vibrational redistribution and relaxation, but the decay mechanisms appear to be very different. In the R2PD spectrum of 4EP^-, we observe strong threshold electron signals, which are due to detachment, by the second photon, of highly rotationally excited anions resulted from the decay of the DBS. On the other hand, in the R2PD spectrum of 4CP^-, we observe well-resolved vibrational peaks due to the three lowest-frequency vibrational modes of 4CP^-, which are populated from the decay of the QBS. The different behaviors of the R2PD spectra suggest unexpected differences between the relaxation mechanisms of the dipole-bound and quadrupole-bound excited states.