Photoelectron-photofragment Coincidence Research Articles

Probing Anion-Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy.

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

Probing the Exit Channel of the OH + CH3OH → H2O + CH3O Reaction by Photodetachment of CH3O-(H2O).

Transition state dynamics of bimolecular reactions can be probed by photodetachment of a precursor anion when the Franck-Condon region of the corresponding neutral potential energy surface is near a saddle point. In this study, photodetachment of anions at m/z = 49 enabled investigation of the exit channel of the OH + CH3OH → H2O + CH3O reaction using photoelectron-photofragment coincidence spectroscopy. High-level coupled-cluster calculations of the stationary points on the anion surface show that the methoxide-water cluster CH3O-(H2O) is the stable minimum on the anion surface. Photodetachment at a 3.20 eV photon energy leads to long-lived H2O(CH3O) complexes and H2O + CH3O products consistent with both direct dissociative photodetachment and resonance mediated processes on the neutral surface. The partitioning of total kinetic energy in the system indicates that water stretch and bend excitation is induced in dissociative photodetachment and evidence for long-lived complexes consistent with vibrational Feshbach resonances is reported.

On-the-Fly Ring-Polymer Molecular Dynamics Calculations of the Dissociative Photodetachment Process of the Oxalate Anion.

The dissociative photodetachment dynamics of the oxalate anion, C2O4H− + hν → CO2 + HOCO + e−, were theoretically studied using the on-the-fly path-integral and ring-polymer molecular dynamics methods, which can account for nuclear quantum effects at the density-functional theory level in order to compare with the recent experimental study using photoelectron–photofragment coincidence spectroscopy. To reduce computational time, the force acting on each bead of ring-polymer was approximately calculated from the first and second derivatives of the potential energy at the centroid position of the nuclei beads. We find that the calculated photoelectron spectrum qualitatively reproduces the experimental spectrum and that nuclear quantum effects are playing a role in determining spectral widths. The calculated coincidence spectrum is found to reasonably reproduce the experimental spectrum, indicating that a relatively large energy is partitioned into the relative kinetic energy between the CO2 and HOCO fragments. This is because photodetachment of the parent anion leads to Franck–Condon transition to the repulsive region of the neutral potential energy surface. We also find that the dissociation dynamics are slightly different between the two isomers of the C2O4H− anion with closed- and open-form structures.

Dissociative Photodetachment Dynamics of the OH-(C2H4) Anion Complex.

Photoelectron-photofragment coincidence (PPC) measurements on OH-(C2H4) anions at a photon energy of 3.20 eV revealed stable and dissociative photodetachment product channels, OH-C2H4 + e- and OH + C2H4 + e-, respectively. The main product channel observed was dissociation to the reactants (>67%), OH + C2H4 (v = 0, 1, 2) + e-, where vibrational excitation in the C-H stretching modes of the C2H4 photofragments corresponds to a minor channel. The low kinetic energy release (KER) of the dissociating fragments is consistent with weak repulsion between the OH + C2H4 reactants near the transition state as well as the partitioning of energy into rotation of the dissociation products. An impulsive model was used to account for rotational energy partitioning in the dissociative photodetachment (DPD) process and showed good agreement with the experimental results. The low KER of the dissociating fragments and the similarities in the photoelectron spectra between stable and dissociative events support a mechanism involving the van der Waals complex formed upon photodetachment of OH-(C2H4) as an intermediate in the dominant OH + C2H4 + e- dissociative channel.

Dissociative photodetachment of H3O2-: a full-dimensional quantum dynamics study.

The transition state is a central concept of chemistry. Photoelectron-photofragment coincidence (PPC) spectroscopy has been proven as an attractive method to study the transition state dynamics. Within a state-of-the-art full-dimensional quantum mechanical model, the dissociative photodetachment dynamics of H3O2- is investigated on accurate anion and neutral potential energy surfaces. The calculated PPC spectrum of H3O2- agrees well with the experimental measurement. The dissociative product OH is exclusively populated on the ground vibrational state, implying the character of the spectator bond. In contrast, the product H2O is predominantly populated in the ground and fundamental states of the symmetric and antisymmetric stretching modes, which is caused by the strong coupling between the antisymmetric motion of the transferred H atom in the transient intermediate [H3O2]* and both stretching modes of the product H2O.

Photoelectron photofragment coincidence spectroscopy of aromatic carboxylates: benzoate and p-coumarate.

Photoelectron-photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p-coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO2 + C6H5 + e-, with a very small amount of stable C6H5CO2 showing that the radical ground state is stable and the excited states are dissociative. For p-coumarate (p-CA-) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO2 + HOC6H4CHCH + e-, characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOC6H5 + HCC- + CO2. The ion beam contained both the phenolate and carboxylate isomers of p-CA-, but DPD only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state.

Photoelectron photofragment coincidence spectroscopy of carboxylates.

Photoelectron–photofragment coincidence (PPC) spectroscopy is a powerful technique for studying the decarboxylation dynamics of carboxyl radicals. Measurement of photoelectron and photofragment kinetic energies in coincidence provides a kinematically complete measure of the dissociative photodetachment (DPD) dynamics of carboxylate anions. PPC spectroscopy studies of methanoate, ethanoate, propanoate, 2-butenoate, benzoate, p-coumarate and the oxalate monoanion are reviewed. All of the systems studied undergo decarboxylation via a two-body DPD channel i.e., driven by the thermodynamic stability of CO2. Additionally, decarboxylation is observed via a three-body ionic photodissociation channel for p-coumarate. In some cases photodetachment also results in a stable carboxyl radical (RCO2). The branching ratio for DPD, the threshold detachment energy and the peak of the kinetic energy release spectrum are compared for different carboxylates, as a probe of the character of the potential energy landscape in the Franck–Condon region.

Photoelectron-photofragment coincidence spectroscopy of the mixed trihalides.

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to study the photodetachment, photodissociation, and dissociative photodetachment (DPD) of I2Br-, IBr2 -, I2Cl-, and ICl2 - at 266 nm. The mixed trihalides are asymmetric analogs of the well-studied I3 - anion, with distinguishable dissociation asymptotes and the potential for selective bond breaking. The high beam energy PPC spectrometer used in this study couples an electrospray ionization source, a hexapole accumulation ion trap, and a linear accelerator to produce a 21 keV beam of a particular trihalide. Total, stable, and dissociative photoelectron spectra have been recorded for all the anions, except ICl2 - that does not photodetach at 266 nm. A bound ground state (X) is observed for all the anions, and a dissociative first excited (A) state is also seen for I2Br- and I2Cl- at low electron kinetic energies (eKE). A 258 nm photoelectron spectrum recorded for I2Br- and I2Cl- rules out autodetachment of a dipole-bound state as the origin of the low eKE feature. The threshold detachment energy (TDE) of I2X- to the X state of the radical is similar to I3 -, whereas the TDE to the radical A state increases with substitution of iodine for a lighter halogen. Two-body DPD is observed for I2Br- and I2Cl-, resulting in IBr/ICl + I + e-. For IBr2 - and ICl2 -, the charge symmetric three-body photodissociation of [Br-I-Br]- and [Cl-I-Cl]- is seen yielding Br + Br and Br + Br*, and Cl + Cl and Cl + Cl* neutral fragments. Evidence for the minimum energy anion structure is observed in all cases, where the iodine atom is located at the center of the trihalide.

Dissociative photodetachment dynamics of the oxalate monoanion.

The dissociative photodetachment (DPD) dynamics of the oxalate monoanion are studied using photoelectron-photofragment coincidence (PPC) spectroscopy. Following photodetachment of C2O4H- at 4.66 eV HOCO + CO2 products are observed, indicating the facile decarboxylation of the radical driven by the thermodynamic stability of CO2. No evidence is seen for photodetachment to stable C2O4H or ionic photodissociation to produce HOCO-. Calculations indicate the stabilizing presence of an intramolecular hydrogen bond in the anion via the formation of a strained five-membered ring. No intramolecular hydrogen bond is predicted in the radical due to the lower charge density on the oxygen atom. The PPC spectrum is consistent with a single direct two-body DPD channel that results in fragments of similar mass and is characterized by a large kinetic energy release (KER) and a broad photoelectron spectrum. The large KER is indicative of substantial repulsion in the radical following photodetachment. The form of the photoelectron spectrum is dominated by the bound to continuum Franck-Condon factors (BCFCF) and is suggestive of photodetachment to a repulsive potential energy surface. A lower bound for the electron affinity of C2O4H is reported as 4 eV. BCFCF calculations allow an approximate functional form of the repulsive surface along the C-C stretch coordinate to be extracted from the experimental photoelectron spectrum. PPC spectroscopy of the deuterated analogue (C2O4D-), at higher anion beam energies is used to increase the detectability of any possible D atom products, but none are observed.

Photoelectron-photofragment coincidence spectroscopy of the dissociative photodetachment of I2- at 258 and 266 nm

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to examine the dissociative photodetachment (DPD) of at 266 and 258 nm in a kinematically complete manner. The resonance enhancement of predissociation in the DPD of is reported for the first time and a previously reported shift to a low KER channel at high photon energy is ascribed to vibrationally excited anions [K.E. Kautzman, P.E. Crider, D.E. Szpunar and D.M. Neumark, J. Phys. Chem. A 111 (49), 12795–12801 (2007)]. Photodetachment to the bound X and A states of results in stable , whereas excitation to a repulsive state results in dissociation of , forming two ground state I atoms. DPD occurs via multiple energetically accessible repulsive states, both directly and via predissociation of the bound state of . At 266 nm direct DPD and resonance enhanced predissociation dominate the DPD dynamics, whereas at 258 nm predissociation is less favoured and direct DPD shifts to preferentially occur via the state, particularly for vibrationally excited anions. Bound-continuum Franck–Condon factors are calculated to model the individual contributions to the dissociative photoelectron spectra from three energetically accessible repulsive states.

Photoelectron-Photofragment Coincidence Studies on the Dissociation Dynamics of the OH-CH4 Complex.

Photoelectron-photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH4 → H2O + CH3 reaction initiated by photodetachment of the OH-(CH4) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable (OH-CH4 + e-) and dissociative (OH + CH4 (ν1 or ν3, v = 0, 1) + e-) channels. The main channel is dissociation to OH + CH4 + e- with a low kinetic energy release (KER), peaking at 0.04 eV. Interpretation of the experimental results was supported by quantum chemistry and quasiclassical trajectory calculations. The anion potential energy surface was constructed at the correlated coupled cluster singles, doubles, and perturbative triples level with augmented correlation consistent polarized valence triple-ζ basis set, and previously calculated neutral potential energy surfaces were used. Quasiclassical simulation of the dynamics of the OH-CH4 complex was carried out by selecting the momenta and coordinates from the Wigner distribution for the anion, providing the starting point for 4000 trajectories on the neutral potential energy surface. In agreement with the experimental results, most of the trajectories yield slowly recoiling OH + CH4 reactants while some are trapped in the entrance channel van der Waals well.

Photoelectron-Photofragment Coincidence Spectroscopy With Ions Prepared in a Cryogenic Octopole Accumulation Trap: Collisional Excitation and Buffer Gas Cooling.

A cryogenic octopole accumulation trap (COAT) has been coupled to a photoelectron-photofragment coincidence (PPC) spectrometer allowing for improved control over anion vibrational excitation. The anions are heated and cooled via collisions with buffer gas <17 K. Shorter trapping times (500 μs) prevent thermalization and result in anions with high internal excitation while longer trapping times (80 ms) at cryogenic temperatures thermalize the ions to the temperature of the buffer gas. The capabilities of the COAT are demonstrated using PPC spectroscopy of at 388 nm (Ehν = 3.20 eV). Cooling the precursor anions with COAT resulted in the elimination of the autodetachment of vibrationally excited produced by the photodissociation + hν → O + (v ≥ 4). Under heating conditions, a lower limit temperature for the anions was determined to be 1,500 K through Franck-Condon simulations of the photodetachment spectrum of , considering a significant fraction of the ions undergo photodissociation in competition with photodetachment. The ability to cool or heat ions by varying ion injection and trapping duration in COAT provides a new flexibility for studying the spectroscopy of cold ions as well as thermally activated processes.

Quantum dynamical investigation of product state distributions of the F + CH3OH → HF + CH3O reaction via photodetachment of the F-(HOCH3) anion.

The photodetachment of the F-(HOCH3) anion, which sheds light on the post-transition-state dynamics of the F + CH3OH → HF + CH3O reaction, is investigated using a reduced-dimensional quantum wave packet method on ab initio based potential energy surfaces for both the neutral and anionic species. The detachment of an electron in the anion precursor produces both bound and resonance species in a hydrogen-bonded potential well in the product channel, in qualitative agreement with the photoelectron-photofragment coincidence (PPC) spectrum. The measured photoelectron-photofragment coincidence spectroscopy is reproduced by the quantum calculations. Our results indicated that the HF product is vibrationally excited, while the OCH3 product is internally cold, thus providing unambiguous assignments of the experimental spectrum.

Photoelectron-photofragment coincidence studies of I3- using an electrospray ionization source and a linear accelerator.

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to examine the dissociative photodetachment (DPD) of I3-. The high beam energy PPC spectrometer for complex anions couples an electrospray ionization source, a hexapole accumulation ion trap and a linear accelerator to produce fast beams of I3- (M = 381 amu) anions, the heaviest system studied to date. Following photodetachment, the photoelectron and up to three photofragments are recorded in coincidence yielding a kinematically complete picture of the DPD dynamics at beam energies of 11 keV and 21 keV. Photodetachment leads to the production of stable I3, two-body DPD, as well as evidence for two- and three-body photodissociation. DPD is found to occur predominantly via the first excited A state, with some contributions from highly excited vibrational levels in the neutral ground state. With the ions thermalized to 298 K in the hexapole trap, there are significant contributions from vibrational hot bands. Three-body photodissociation at 4.66 eV is found to occur preferentially via a charge-symmetric process to form I + I- + I. In the future this method will be applied to other polyatomic systems with a large molecular mass, including multiply charged anions and complex clusters, in concert with a cryogenically cooled hexapole trap to reduce thermal effects.

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 and the oxalate anion C2O4H- at beam energies of 11 keV, 16 keV, and 21 keV.

Spectroscopy of Ethylenedione and Ethynediolide: A Reinvestigation

AbstractIn an effort to characterize the electronic states of ethylenedione, OCCO, photoelectron‐photofragment coincidence (PPC) spectroscopy was applied to measure anions at m/z 56 and 57 using a pulsed discharge of glyoxal vapor and N2O. PPC measurements at a photon energy of 3.20 eV yield photoelectron spectra in coincidence with either neutral photofragments or stable neutral products. The measurements showed that primarily stable neutral products were formed, with photoelectron spectra consistent with the oxyallyl diradical, C3H4O, and acetone enolate radical, C3H5O. The spectra were also found to have features nearly identical to those reported for OCCO and HOCCO by Sanov and co‐workers. The stability of the neutral products, as well as an examination of spectra reported for the oxyallyl anion and acetone enolate show that the previous assignments of OCCO and HOCCO are in error, and are instead attributed here to the oxyallyl diradical, C3H4O, and the acetone enolate radical, C3H5O.

Internal energy dependence of the photodissociation dynamics of O3- using cryogenic photoelectron-photofragment coincidence spectroscopy.

Photoelectron-photofragment coincidence (PPC) spectra of ozonide, O3-, were measured at 388 nm (Ehν = 3.20 eV) using a newly constructed cryogenic octopole accumulation trap coupled to a PPC spectrometer. The photoelectron spectra reveal three processes consisting of a stable photodetachment channel, and two distinct photodissociation pathways yielding (1) O2 + O- or (2) O + O2-. The first photodissociation pathway is observed in the PPC spectra by photodetachment of the O- product by a second photon, and produces electronically excited O2(1Δg). The O2- product of the second photodissociation pathway undergoes autodetachment for O2-(2Πg, v″ > 4), a process greatly enhanced by vibrational excitation of the precursor O3-. Cooling anions thermalized at 300 K to <17 K in a cryogenic octopole accumulation trap essentially turns off this autodetachment pathway. The product kinetic energy distribution in coincidence with the autodetached electrons from O2-(v″ = 4) exhibits resolved features consistent with bend (ν2), asymmetric stretch (ν3) and a stretching combination band (ν1 + ν3) in the intermediate electronic state, illustrating the insights that can be gained from kinematically complete measurements. These results are discussed in the context of the low-lying excited states of O3-.

Imaging a multidimensional multichannel potential energy surface: Photodetachment of H(-)(NH3) and NH4 (.).

Probes of the Born-Oppenheimer potential energy surfaces governing polyatomic molecules often rely on spectroscopy for the bound regions or collision experiments in the continuum. A combined spectroscopic and half-collision approach to image nuclear dynamics in a multidimensional and multichannel system is reported here. The Rydberg radical NH4 and the double Rydberg anion NH4 (-) represent a polyatomic system for benchmarking electronic structure and nine-dimensional quantum dynamics calculations. Photodetachment of the H(-)(NH3) ion-dipole complex and the NH4 (-) DRA probes different regions on the neutral NH4 PES. Photoelectron energy and angular distributions at photon energies of 1.17, 1.60, and 2.33 eV compare well with quantum dynamics. Photoelectron-photofragment coincidence experiments indicate dissociation of the nascent NH4 Rydberg radical occurs to H + NH3 with a peak kinetic energy of 0.13 eV, showing the ground state of NH4 to be unstable, decaying by tunneling-induced dissociation on a time scale beyond the present scope of multidimensional quantum dynamics.

Energetics and transition-state dynamics of the F + HOCH3 → HF + OCH3 reaction.

The F + HOCH3 → HF + OCH3 reaction is a system with 15 internal degrees of freedom that can provide a benchmark for the development of theory for increasingly complex chemical reactions. The dynamics of this reaction were studied by photoelectron-photofragment coincidence (PPC) spectroscopy carried out on the F-(HOCH3) anion, aided by a computational study of both the anion and neutral potential energy surfaces, with energies extrapolated to the CCSDT(Q)/CBS level of theory. Photodetachment at 4.80 eV accesses both the reactant and product channels for this reaction. In the product channel (HF + OCH3 + e-) of the neutral potential energy surface, vibrationally excited HF products in addition to the stable product-channel hydrogen-bonded complex (FH-OCH3) are observed in the PPC and photoelectron spectra. In addition, experimental evidence is observed for the reactant-channel van der Waals complex (F-HOCH3), in good agreement with the theoretical predictions. The relative stability of these long-lived complexes was probed by reducing the ion beam energy, increasing the product time-of-flight, indicating lifetimes on the microsecond timescale for the reactant- and product-channel complexes as well as providing evidence for long-lived vibrational Feshbach resonances associated with the HF(v > 0) + OCH3 product states. This system will provide a model for extending full-dimensionality quantum dynamics to larger numbers of degrees of freedom.

Photoelectron-photofragment coincidence studies of the tert-butoxide anion (CH3)3CO((-)), the carbanion isomer (CH3)2CH2COH((-)), and corresponding radicals.

A study of the photodetachment and dissociative photodetachment (DPD) of the C(4)H(9)O(-) isomers tert-butoxide, (CH(3))(3)CO(-), and the α-hydroxy carbanion (CH(3))(2)C(CH(2))OH(-) is reported. Photoelectron-photofragment coincidence spectroscopy was used to study these anions at 387, 537, and 600 nm. Supported by CBS-QB3 ab initio calculations, the product mass and translational energy distributions were found to be consistent with dissociation of either highly excited (CH3)(3)CO radicals or (CH(3))(2)C(CH2)OH alkylhydroxy radicals. Vibrationally resolved photoelectron spectra of stable radicals at 537 and 600 nm in conjunction with Franck-Condon simulations were used to assign the dominant channel to tert-butoxide ((CH(3)3)CO(-)) anions thermalized to a vibrational temperature of 550 K. DPD is assigned to highly vibrationally excited radicals produced by photodetachment of unrelaxed tert-butoxide products formed at an effective source temperature of 1400 K. The higher energy carbanion was found to be a minor channel and was not observed to dissociate. Calculated energetics for photodetachment and DPD of (CH(3))(3)CO(-) and (CH(3))(2)C(CH(2))OH(-) are discussed and compared with the experimental results.