Photoelectron-photofragment Coincidence Spectroscopy 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.

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 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.

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.

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.

State-resolved predissociation dynamics of the formyloxyl radical

Dissociation dynamics of the deuterated formyloxyl radical, DCO2, were studied by photoelectron–photofragment coincidence spectroscopy of DCO2-. Photodetachment at 4.27eV accesses the three lowest-lying electronic states (2A1, 2B2, and 2A2) of DCO2, leading to vibrational excitation dominated by the 2A1 state O−C−O bending mode. Predissociation leads to bending excitation in the CO2 products as observed in the product translational energy distribution. Gating the coincidence spectra by electron kinetic energy for each predissociated vibrational mode in the radical provides a direct measure of the CO2 bending vibration (ν2) product state distribution, revealing substantially more excitation than ab initio quantum-dynamics calculations predict.

Photoelectron-photofragment coincidence spectroscopy in a cryogenically cooled linear electrostatic ion beam trap

A cryogenically cooled linear electrostatic ion beam trap for use in photoelectron-photofragment coincidence (PPC) spectroscopy is described. Using this instrument, anions created in cold, low-duty-cycle sources can be stored for many seconds in a ~20 K environment to cool radiatively, removing energetic uncertainties due to vibrationally excited precursor anions. This apparatus maintains a well-collimated beam necessary for high-resolution fragment imaging and the high experimental duty cycle needed for coincidence experiments. Ion oscillation is bunched and phase-locked to a modelocked laser, ensuring temporal overlap between ion bunches and laser pulses and that ions are intersected by the laser only when travelling in one direction. An electron detector is housed in the field-free center of the trap, allowing PPC experiments to be carried out on ions while they are stored and permitting efficient detection of 3-dimensional electron and neutral recoil trajectories. The effects of trapping parameters on the center-of-mass trajectories in the laser-ion interaction region are explored to optimize neutral particle resolution, and the impact of bunching on ion oscillation is established. Finally, an initial demonstration of radiative cooling is presented.

Photoelectron–photofragment coincidence studies of NO−-X clusters (X = H2O, CD4)

The dissociative photodetachment of NO-(H2O)and NO-(CD4) anion clusters was studied at 775 nm (1.60 eV) using photoelectron-photofragment coincidence spectroscopy. The correlation between the photoelectron and photodetached neutral spectra indicates vibrational excitation in the recoiling NO neutral fragments from NO-(H2O), with a progression consistent with vibrational excitation up to upsilon(NO) = 3 in the products. The correlation remains when D2O is substituted for H2O, implying the NO vibrational mode plays a role in the dissociation coordinate of the complex. In contrast, no correlation was observed between photoelectron kinetic energy and kinetic energy release from NO-(CD4). Consideration of the maximum available kinetic energy allows the binding energies to be detemined as 0.57 and 0.07 eV for NO-(H2O) and NO-(CD4), respectively.

Dissociative photodetachment dynamics of the iodide-aniline cluster

The photodetachment dynamics of the iodide-aniline cluster, I-(C6H5NH2), were investigated using photoelectron-photofragment coincidence spectroscopy at several photon energies between 3.60 and 4.82 eV in concert with density functional theory calculations. Direct photodetachment from the solvated I- chromophore and a wavelength-independent autodetachment process were observed. Autodetachment is attributed to a charge-transfer-to-solvent reaction in which incipient continuum electrons photodetached from I- are temporarily captured by the nascent neutral iodine-aniline cluster configured in the anion geometry. Subsequent dissociation of the neutral cluster removes the stabilization, leading to autodetachment of the excess electron. The dependence of the dissociative photodetachment (DPD) and autodetachment dynamics on the final spin-orbit electronic state of the iodine fragment is characterized. The dissociation dynamics of the neutral fragments correlated with autodetached electrons were found to be identical to the DPD dynamics of the I atom product spin-orbit state closest to threshold at a given photon energy, lending support to the proposed sequential mechanism.

Dissociative Photodetachment Dynamics of Solvated Iodine Cluster Anions

Photoelectron-photofragment coincidence spectroscopy of I- (CO2), I- (NH3), I- (H2O), I- (C6H5NH2), and I- (C6H5OH) clusters was used to study the dissociative photodetachment (DPD) dynamics at 257 nm. Photodetachment from all five clusters was observed to yield bound neutral clusters as well as the DPD products of the iodine atom and the molecular solvent. Photoelectron images and kinetic energy spectra were recorded in coincidence with both the translational energy released between dissociating neutral products and stable neutral clusters. The variation of the photoelectron angular distributions in the clusters was measured, revealing significant perturbations relative to I- for I- (H2O) and I- (C6H5NH2). Product branching ratios for stable versus dissociative photodetachment and photodetachment to the I(2P(3/2)) and I(2P(1/2)) states are reported. The measurements reveal a dependence of the DPD dynamics on the final spin-orbit state of iodine in the cases of I- (C6H5NH2) and I- (CO2) and a threshold detachment process in I- (C6H5NH2).

Dynamics of the Acetyloxyl Radical Studied by Dissociative Photodetachment of the Acetate Anion

Photodetachment of the acetate anion, CH3CO2 - , and the subsequent dissociation dynamics of the CH3CO2 ¥ radical were studied using photoelectron-photofragment coincidence spectroscopy at 355 and 257 nm. An upper limit to the adiabatic electron affinity (EA) of CH 3CO2 ¥ ,E A) 3.47 ( 0.01 eV was determined. Evidence for several low-lying electronic states of the CH 3CO2 ¥ radical were observed in the photoelectron spectra. Most CH3CO2 ¥ radicals dissociated to CH3 ¥ + CO2 products with a large kinetic energy release (〈ET〉/Eavl ) 0.72 for 355-nm excitation) and an anisotropic angular distribution ( ‚ 1.2), while 10% of CH3CO2 ¥ radicals were stable on the microsecond time scale at both wavelengths. Structured near-threshold photoelectron spectra at 355 nm were similar when photoelectrons were recorded in coincidence with either stable radicals or dissociation products, indicating a sequential dissociative photodetachment. Experiments were also carried out on CD3CO2 - to aid in the interpretation of the photoelectron spectra and deduction of the dissociation mechanism.

Three-body dissociation dynamics of (SO 2) 3 studied through dissociative photodetachment of (SO 2) 3−

The dissociative photodetachment dynamics of (SO 2) 3 − were studied by photoelectron–photofragment coincidence spectroscopy at 258 nm. Correlation between the photoelectron and photofragment translational energies was observed as previously seen in the dimer system, implying the presence of a dimer core. The three-body dissociation dynamics of (SO 2) 3 after photodetachment are consistent with a dimer core solvated by a spectator SO 2 molecule with a broad distribution in initial geometry.

Photoelectron–photofragment coincidence spectroscopy of NO 2−(NO) 1,2: solvation effects of NO on NO 2−

A study of the dissociative photodetachment dynamics of N 2O 3 − and N 3O 4 − at 258 nm is reported. Using photoelectron–photofragment coincidence (PPC) spectroscopy, information on the energetics and dissociation dynamics is recorded for both N x O y − species. The results indicate that these anions exist as NO 2 −(NO) 1,2 clusters, and dissociation occurs to form NO and NO 2 products upon photodetachment of the extra electron. It is shown that the energy released in the solvation of NO 2 − with one and two NO molecules is 0.23±0.05 and 0.38±0.05 eV, respectively. The photoelectron spectra show that solvation of NO 2 − by one NO does not significantly affect the electronic structure of the anion. The addition of a second NO to NO 2 −(NO), however, leads to a suppression of photodetachment to the A ̃ 2 B 2 excited state of NO 2 in the NO 2(NO) 2 cluster. The data also suggests that the three-body dissociation process of the larger cluster may occur via a sequential decay mechanism.

Dissociative Photodetachment Dynamics of S2O2-

Photoelectron-photofragment coincidence spectroscopy is employed to study the dissociative photodetachment (DPD) dynamics of S2O2- at 258 nm. Experimental data and theoretical calculations show evidence for photodetachment from a trigonal form of S2O2-. The vertical detachment energy of this isomer was determined to be 3.73 ± 0.02 eV. An upper bound of 2.41 ± 0.14 eV is determined for the enthalpy of the reaction S2O2- → S + SO2 + e- at 0 K. The observed dynamics are interpreted in terms of dissociative photodetachment of S2O2- to S(3P) + SO2(1A1) + e-, S(1D) + SO2(1A1) + e-, and S2(3Σg-) + O2(3Σg-) + e- product channels. The S-atom channels are characterized by a large photofragment kinetic energy release and an anisotropic photofragment angular distribution peaked along the electric vector of the laser. The S2 channel has a low kinetic energy release consistent with elimination of highly vibrationally excited O2 from a strained form of the trigonal isomer.

Dissociative photodetachment studies of O−(H2O)2, OH−(H2O)2, and the deuterated isotopomers: Energetics and three-body dissociation dynamics

Photoelectron–photofragment coincidence spectroscopy was used to study dissociative photodetachment of the doubly hydrated clusters of oxide and hydroxide, M−(H2O)2→M+2H2O+e− (M=O, OH). These experiments yield information on the energetics of the parent anion and the dissociation dynamics of the photodetached neutral species. Photoelectron spectra and photoelectron–photofragment coincidence spectra are presented and compared to data for O−(H2O) and OH−(H2O). Unlike the singly hydrated species, no evidence of vibrationally resolved product translational energy distributions is observed. The second hydration energy of O− with both H2O and D2O was also measured to be 0.80±0.08 and 0.81±0.08 eV, respectively. The three-body dissociation dynamics of the neutral clusters produced by photodetachment were studied by measuring the velocities and recoil angles of all the particles in coincidence. The observed partitioning of momentum is consistent with a two-step mechanism or dissociation from a wide range of starting geometries.

Transition state dynamics of the OH + H2O hydrogen exchange reaction studied by dissociative photodetachment of H3O2-.

Dynamics in the transition state region of the bimolecular OH + H2O-->H2O + OH hydrogen exchange reaction have been studied by photoelectron-photofragment coincidence spectroscopy of the H3O2- negative ion and its deuterated analog D3O2-. The data reveal vibrationally resolved product translational energy distributions. The total translational energy distribution shows a vibrational progression indicating excitation of the antisymmetric stretch of the water product. Electronic structure calculations at the QCISD level of theory support this analysis. Examination of the translational energy release between the neutral products reveals a dependence on the product vibrational state. These data should provide a critical test of ab initio potential energy surfaces and dynamics calculations.