Photoelectron Imaging of Doubly Charged Anions, −O2C(CH2)nCO2− (n = 2−8): Observation of Near 0 eV Electrons Due to Secondary Dissociative Autodetachment

Spectroscopic observation and ultrafast coherent vibrational dynamics of the aqueous phenylalanine radical

The phenylalanine radical (Phe˙) has been proposed to mediate biological electron transport (ET) and exhibit long-lived electronic coherences following attosecond photoionization. However, the coupling of ultrafast structural reorganization to the oxidation/ionization of biomolecules such as phenylalanine remains unexplored. Moreover, studies of ET involving Phe˙ are hindered by its hitherto unobserved electronic spectrum. Here, we report the spectroscopic observation and coherent vibrational dynamics of aqueous Phe˙, prepared by sub-6 fs photodetachment of phenylalaninate anions. Sub-picosecond transient absorption spectroscopy reveals the ultraviolet absorption signature of Phe˙. Ultrafast structural reorganization drives coherent vibrational motion involving nine fundamental frequencies and one overtone. DFT calculations rationalize the absence of the decarboxylation reaction, a photodegradation pathway previously identified for Phe˙. Our findings guide the interpretation of future attosecond experiments aimed at elucidating coherent electron motion in photoionized aqueous biomolecules and pave way for the spectroscopic identification of Phe˙ in studies of biological ET.

Photoelectron spectroscopy of the protoporphyrin IX dianion

Two-dimensional photoelectron spectroscopy using nanosecond and femtosecond lasers has been used to study the protopophyrin IX dianion at photon energies between 1.8-4.1 eV. The photoelectron spectra indicated the presence of two direct detachment channels, tunnelling through the repulsive Coulomb barrier (RCB) and thermionic emission from monoanions. A direct detachment feature suggested a near 0 eV electron affinity, which may be attributable to the repulsive through space interaction of the unshielded carboxylate groups. The minimum height of the repulsive Coulomb barrier (RCB) was found to be between 1.4-1.9 eV. Adiabatic tunnelling through the RCB was seen to occur on a timescale faster than rotational dephasing of the molecule. The observation of thermionic emission below the RCB in the nanosecond spectra originated from monoanions, which were produced via photon-cycling of the dianion.

Effects of Hydration on the Conformational Behavior of Flexible Molecules with Two Charge Centers

The hydration behavior of alkyl-diammonium di-cations and alkyl-dicarboxylate di-anions, of varying alkyl chain length, was examined using basin-hopping (BH) global optimization techniques. For every di-ion investigated, a conformational transition from linear to folded is observed at a critical hydration number, n*, specific to each di-ion. A stepwise hydration study has been undertaken for alkyl-dicarboxylate di-anions in finite water clusters containing 1-12 water molecules, and low-energy structures have been examined for larger water clusters. An even number of carbons in the alkyl chain gives rise to more stable conformations in unhydrated, implicitly solvated, and explicitly solvated conditions. This work provides valuable information on how the hydration of ammonium and carboxylate ions influence larger biomolecules' conformations.

A high beam energy photoelectron-photofragment coincidence spectrometer for complex anions

A new high beam energy photoelectron-photofragment coincidence (PPC) spectrometer is described that allows acceleration of heavy anions (>100 amu) to energies in the tens of keV using a linear accelerator (LINAC). High beam energies result in more efficient detection of the neutral photofragments produced via dissociative photodetachment (DPD) of the parent anion and increase the mass range that can be studied with PPC spectroscopy. The novel experimental setup couples an electrospray ionization (ESI) source and a hexapole accumulation trap with a 10-stage LINAC to give a kinematically complete measurement of the dissociation dynamics for heavier anions. ESI dramatically increases the range of anions that can be studied by PPC spectroscopy to include multiply charged anions and larger, more complex molecular ions important in biological, atmospheric, and combustion processes. A radiofrequency buffer-gas-cooled hexapole trap is used to accumulate sufficient ion density for single-shot coincidence measurements and thermalize the anions to room temperature. The photoelectron and up to three neutral fragments resulting from DPD are recorded in coincidence using time and position sensitive detectors. This novel experimental setup is characterized by studying the photodetachment of I^-, and the DPD of I 2 - and the oxalate anion C₂O₄H^- at beam energies of 11 keV, 16 keV, and 21 keV.

Probing the early stages of solvation of cis-pinate dianions by water, acetonitrile, and methanol: a photoelectron spectroscopy and theoretical study

Photoelectron spectroscopy and theoretical studies indicate the coexistence of symmetric and asymmetric solvated clusters for cis-pinate dianions.

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

High-resolution photoelectron imaging and spectroscopy of cold C₆₀⁻ anions are reported using a newly built photoelectron imaging apparatus coupled with an electrospray ionization source and a temperature-controlled cryogenic ion trap. Vibrationally resolved photoelectron spectra are obtained for the detachment transition from the ground state of C₆₀⁻ to that of C60 at various detachment wavelengths from 354.84 nm to 461.35 nm. The electron affinity of C60 is accurately measured to be 2.6835 ± 0.0006 eV. Numerous unexpected vibrational excitations are observed in the photoelectron spectra due to the Jahn-Teller effect in C₆₀⁻ and Hertzberg-Teller vibronic coupling in both C₆₀⁻ and C60. Both the relative intensities of vibrational peaks and their photoelectron angular distributions provide evidence for the vibronic couplings. The observed p-wave-like behavior in the angular distribution of the 0₀⁰ transition suggests that the electron is detached from an s-type orbital.

Anion resonances of para-benzoquinone probed by frequency-resolved photoelectron imaging

The resonant attachment of a free electron to a closed shell neutral molecule and the interplay between the following electron detachment and electronic relaxation channels represents a fundamental but common process throughout chemical and biochemical systems. The new methodology of anion frequency-resolved photoelectron imaging is detailed and used to map out molecular excited state dynamics of gas-phase para-benzoquinone, which is the electron accepting moiety in many biological electron-transfer chains. Three-dimensional spectra of excitation energy, electron kinetic energy, and electron ejection anisotropy reveal clear fingerprints of excited and intermediate state dynamics. The results show that many of the excited states are strongly coupled, providing a route to forming the ground state radical anion, despite the fact that the electron is formally unbound in the excited states. The relation of our method to electron impact attachment studies and the key advantages, including the extension to time-resolved dynamics and to larger molecular systems, are discussed.

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

Recent progress towards understanding the repulsive Coulomb barrier in multiply-charged anion using photoelectron spectroscopy is discussed.

Photoelectron spectroscopy of 1-nitropropane and 1-nitrobutane anions

We present low-energy velocity map photoelectron imaging results for bare and Ar-solvated 1-nitropropane and 1-nitrobutane anions. We report the adiabatic electron affinity of 1-nitropropane as (223 ± 6) meV and that of 1-nitrobutane as (240 ± 6 meV). The vertical detachment energies of these two species are found to be (0.92 ± 0.05) and (0.88 ± 0.05) eV, respectively. The photoelectron spectra are discussed in the framework of Franck-Condon simulations based on density functional theory. We observe unusual resonances in the photoelectron spectra of both ions under study, whose kinetic energy is independent of the photon energy of the detaching radiation. We discuss possible origins of these resonances as rescattering phenomena, consistent with the experimental photoelectron angular distributions.