Abstracts numbered to match full list of publications in the bibliography.
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Molecular fragmentation into three products poses an analytical challenge to theory and experiment alike. We used translational spectroscopy and high-level ab initio calculations to explore the highly debated three-body dissociation of sym-triazine to three hydrogen cyanide molecules. Dissociation was induced by charge exchange between the sym-triazine radical cation and cesium. Calculated state energies and electronic couplings suggest that reduction initially produces a population of sym-triazine partitioned between the 3s Rydberg and π* ← n electronically excited manifolds. Analysis of the topology of these manifolds, along with momentum correlation in the dissociation products, suggests that a conical intersection of two potential energy surfaces in the 3s Rydberg manifold leads to stepwise dissociation, whereas a four-fold glancing intersection in the π* ← n manifold leads to a symmetric concerted reaction.
A coincidence study of the photoelectron-photofragment angular distributions in the one- and two-photon dissociative photodetachment (DPD) of HOCO- is reported. The photoelectron angular distributions in the one-photon DPD channels HOCO- + hv → OH + CO + e- and HOCO- + hv → H + CO2 + e- at Ehv3.21 eV are examined in both the laboratory and neutral photofragment recoil frames, while the angular distributions of the OH + CO products in the laboratory frame are also investigated. The results show a parallel dipole transition for the DPD of HOCO-. The radical lifetime was examined as a function of the orientation of the transition dipole, with upper limits of 9 × 10-13s and 1.3 × 10-12s estimated for HOCO and DOCO lifetime for dissociation into OH/OD + CO, respectively. The photoelectron-photofragment angular correlation in the two-photon DPD of HOCO- is also presented, showing that when the photon energy is near the photodetachment threshold, a molecular anion like HOCO- can be effectively aligned through a continuum shape resonance.
One-color?two-photon photodetachment of HOCO- at Ehv = 1.60 eV accompanied by a measurement of the photoelectron angular distribution (PAD) is used to illustrate a novel approach to the alignment of a molecular anion. A quantitative analysis of the PAD reveals this alignment process is associated with a temporary anion formed by a p-wave shape resonance and the PAD in the two-photon signal is a result of interfering s- and d-partial waves within the atomic approximation. The extracted intensity and phase shift of the partial waves are consistent with the Wigner threshold law for photodetachment.
A continuous-flow rapid expansion of supercritical solution (RESS) apparatus is used to grow thin iron oxide thin films under ambient and vacuum conditions. The magnetic thin films are produced by expanding a supercritical solution of ferric acetylacetonate (Fe(acac)3) and CO2 and directing the resulting supersonic jet onto both hot and cold silicon wafers. The concentration of the expanding solution is monitored in-line with a UV?vis high pressure view cell which is also used to perform solubility measurements. The resulting films contain nano- and sub-micronic particles in the 13?700 nm size range and show magnetic order. Structural and magnetic data for these thin particle films have been obtained by SQUID and SEM measurements and compared as a function of substrate surface temperature, growth times, and initial solute concentrations. Experimental and theoretical analysis of the thermodynamics and fluid mechanics appropriate for this RESS process is discussed.
An experimental study of the dissociative photodetachment (DPD) dynamics of HOCO- and DOCO- at a photon energy of 3.21 eV has been carried out to probe the potential energy surface of the HOCO free radical and the dynamics of the OH + CO → H + CO2 reaction. These photoelectron-photofragment coincidence experiments allow the identification of photodetachment processes leading to the production of stable HOCO free radicals and both the H + CO2 and OH + CO dissociation channels on the neutral surface. Isotopic substitution by deuterium in the parent ion is observed to reduce the product branching ratio for the D + CO2 channel, consistent with tunneling playing a role in this dissociation pathway. Other isotope effects on the detailed partitioning of kinetic energy between photoelectrons and photofragments are also discussed. The results are compared to recent theoretical predictions of this DPD process, and evidence for the involvement of vibrationally excited HOCO- anions is discussed.
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.
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(2P3/2) and I(2P1/2) 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).
Charge-exchange neutralization of H3+ with Cs allows preparation of the low-lying Rydberg states of H3. These states are predissociated by the repulsive ground state and may play roles as intermediates in the dissociative recombination of H3+ + e-. Translational spectroscopy and measurements of product momentum partitioning in three-body dissociative charge exchange of fast (12 keV) H3+ and D3+ with Cs yields insights into the nuclear motion during dissociation for the three lowest-lying 2s 2A1′, 2p 2A2″ and 3p 2E′ bound Rydberg states of H3 and the two 2s 2A1′ and 2p 2A2″ states for D3. This data provide an empirical benchmark for the refinement of theoretical models involving non-adiabatic interactions and dynamics for H3.
A photoelectron?photofragment coincidence (PPC) study of the dissociative photodetachment of OHF- at a photon energy of 4.80 eV is presented. The correlated electron kinetic energy (eKE) and translational energy release (ET) into the O + HF + e- products yield information on the potential energy surface close to the transition state of the neutral reaction OH + F → O + HF. The correlation spectrum shows two different features in the energetically allowed O + HF product channel: (a) diagonal ridges, resulting from direct dissociative photodetachment (DPD) and (b) areas with higher ET in the neutral fragments from nonadiabatic dissociation. The total translational energy spectrum (ETOT = eKE + ET) reveals a vibrationally resolved product state distribution. These results are discussed in the context of recent theoretical studies of the dissociative photodetachment of OHF-.
The dynamics of the three-body dissociative charge exchange of fast (12 keV) and with Cs have been studied using multiparticle translational spectroscopy. The observed partitioning of product momenta was found to be state-specific and yields insights into the nuclear motion during dissociation for the three lowest-lying , , and metastable Rydberg states of H3 and the and states for D3. These results provide direct empirical information on the nonadiabatic couplings that govern the three-body dissociation of the lowest-lying Rydberg states of H3 and D3.
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 CH3CO2•, EA = 3.47 ± 0.01 eV was determined. Evidence for several low-lying electronic states of the CH3CO2• radical were observed in the photoelectron spectra. Most CH3CO2• radicals dissociated to CH3• + CO2 products with a large kinetic energy release (〈ET〉/Eav1 = 0.72 for 355-nm excitation) and an anisotropic angular distribution (β ∼ 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.
A photodetachment imaging study of the photoelectron angular distributions produced by the 354.8-nm photodetachment of the vinoxide anion, H2C=CHO-, is reported. The photoelectron angular distributions for both the X ?2 A″ ground state and Ã 2A′ excited state of the neutral vinoxy radicals were measured. Energy-dependent photoelectron anisotropy parameters are reported for both electronic states. Photodetachment from the HOMO of the anion (the CCO nonbonding π(a″) orbital) yields ground-state vinoxy radicals and exhibits a sin2? angular dependence (? (X ?2A″)=-0.7 ± 0.1) relative to the electric vector of the laser. Photodetachment from the HOMO-1 of the anion (the oxygen ?2px(a′) orbital) yields excited-state Ã 2A′ radicals and exhibits a cos2? angular dependence (? (A ?2A′) = 0.6 ± 0.1) of the photoelectrons. These results are qualitatively interpreted in terms of the electronic structure of the anion-neutral system.
The dissociative photodetachment dynamics of (SO2)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 (SO2)3 after photodetachment are consistent with a dimer core solvated by a spectator SO2 molecule with a broad distribution in initial geometry.
Cyclopentoxide c-C5H9O- undergoes photodetachment to stable cyclopentoxy or the ring-opened 5-oxo-pentan-1-yl radical and dissociative photodetachment, yielding C3H5O and C2H4 photofragments, at both 532 and 355 nm. The adiabatic electron affinity of c-C5H9O- is estimated from the experimental results and ab intio calculations to be 1.5 ± 0.1 eV. The results show that c-C5H9O- is stable relative to dissociation into C3H5O- and C2H4 by 1.23 ± 0.07 eV, whereas c-C5H9O is unstable relative to C3H5O and C2H4 by -0.12 ± 0.12 eV. These results are discussed in terms of the factors affecting the stability of cyclic alkoxides and the corresponding alkoxy radicals.
Dissociative photodetachment (DPD!) of the molecular anion HOCO- is used to probe the potential energy surface for the OH+CO→H+CO2 reaction. The HOCO- anion, formed by electron impact on an expansion of CH4+N2O+CO, is characterized for the first time in these experiments by photoelectron spectroscopy and photoelectron angular distribution measurements. Photodetachment of HOCO- is found to produce H+CO2+e- and OH+CO+e- products in addition to stable HOCO radicals. Ab initio calculations of the energetics and structure of HOCO- and HOCO are consistent with the experimental results and show that photodetachment to the ground electronic HOCO surface samples the vicinity of the HOCO well. The product translational energy distributions observed on the ground state surface are consistent with unimolecular decomposition out of the HOCO well. In addition, direct DPD to a repulsive excited state of HOCO, correlating to ground state OH+CO products is observed.
The four-body dissociative photodetachment (DPD) dynamics of O-8 were studied using photoelectron photofragment coincidence (PPC) spectroscopy. All four neutral photofragments were measured in coincidence with the photodetached electron, yielding a five-body kinematically complete experiment. Velocity and angular correlations for DPD of O-8 are presented and compared to those for O-6. The DPD dynamics and energetics of O-8 are found to be similar to those of O-4 and O-6 implying that the additional solvating O2 molecules act essentially as spectators, but exhibit inequivalent kinematic behavior implying asymmetric solvation.
A study of the dissociative photodetachment dynamics of N2O3? and N3O4? at 258 nm is reported. Using photoelectron?photofragment coincidence (PPC) spectroscopy, information on the energetics and dissociation dynamics is recorded for both NxOy? species. The results indicate that these anions exist as NO2?(NO)1,2 clusters, and dissociation occurs to form NO and NO2 products upon photodetachment of the extra electron. It is shown that the energy released in the solvation of NO2? 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 NO2? by one NO does not significantly affect the electronic structure of the anion. The addition of a second NO to NO2?(NO), however, leads to a suppression of photodetachment to the Ã2B2 excited state of NO2 in the NO2(NO)2 cluster. The data also suggests that the three-body dissociation process of the larger cluster may occur via a sequential decay mechanism.
The excited states of N2O2 have been characterized by studies of the dissociative photodetachment of N2O2- at 266 nm. Photoelectron-photofragment coincidence spectroscopy reveals the correlation between features observed in the photoelectron spectrum and different neutral dissociation pathways. Evidence for at least two isomers of N2O2- is presented and upper limits for their stabilities are determined. One isomer, ONNO-, is stable relative to NO + NO + e- by < 1.70 ± 0.05 eV, while the second isomer, trigonal N2O2-, is stable relative to O- + N2O by < 0.57 ± 0.05 eV. The observed dissociation channels are assigned to ONNO → NO + NO, N2O2 → O (3P) + N2O, and either O(1D) + N2O or N(4S) + NO2. No evidence for stable N2O2 was found, and the dissociation dynamics of the excited states of N2O2 are discussed.
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(3∑g-) + O2(3∑g-) + 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.
The dissociative photodetachment (DPD) of HCO-2 and DCO-2 was studied at 258 nm. State-resolved translational energy distributions were observed correlated to bending excitation in the CO2 product for the channel producing H/D + CO2, indicating very low rotational excitation in the products consistent with predissociation of a C2v HCO2 molecule. No evidence was found for dissociation into OH + CO. All three low-lying electronic states (2A1, 2B2, and 2A2) were found to dissociate, but resolved progressions were only observed from photodetachment to the 2A1 and 2B2 states. Photoelectron-photofragment coincidence spectra for DCO-2 show resolved vertical bands and indicate that multiple CO2 vibrational states are accessible from each vibrational level in the predissociating DCO2 molecule. The resolved structure is assigned to vibrational predissociation sequence bands, observable in this DPD process owing to the dissociation dynamics and the near degeneracy of the vibrational levels in the 2A1 and 2B2 states of HCO2 and the bending mode of the CO2 products.
The photoelectron spectra of the structural isomers of the three- and four-carbon enolate anions, n-C3H5O-, i-C3H5O-, n-C4H7O-, s-C4H7O-, and i-C4H7O- have been measured at 355 nm. Both the X(2A″) ground and A(2A′) first excited states of the corresponding radicals were accessed from the X(1A′) ground state of the enolate anions. The separation energies of the ground and first excited states (T0) were determined: T0[(E)-n-C3H5O] = 1.19 ± 0.02 eV, T0[(Z)-n-C3H5O] = 0.99 ± 0.02 eV, T0[i-C3H5O] = 1.01 ± 0.02 eV, T0[n-C4H7O] = 1.19 ± 0.02 eV, T0[(2,3)-s-C4H7O] = 1.25 ± 0.02 eV, T0[(1,2)-s-C4H7O] = 0.98 ± 0.02 eV, and T0[i-C4H7O] = 1.36 ± 0.02 eV. The effects of alkyl substitution on the vibronic structure and energetics previously observed in the vinoxy radical are discussed. The X(1A′)-X(2A″) relative stability is strongly influenced by substitution whereas the X(1A′)-A(2A′) relative stability remains nearly constant for all of the observed structural isomers. Alkyl substitution at the carbonyl carbon affects vibronic structure more profoundly than the energetics, while the converse is observed upon alkyl substitution at the alpha carbon.
Photoelectron-photofragment coincidence (PPC) spectroscopy has been used to study the dissociative photodetachment of H2O-2 and D2O-2. The observed partitioning of photoelectron and photofragment translational energies provides information on the dynamics in the transition state region of the reaction between two hydroxyl radicals: OH + OH → O(3P) + H2O. The data reveal vibrationally resolved product translational energy distributions for both the entrance channel OH + OH and the exit channel O(3P) + H2O upon photodetachment. The total translational energy distribution shows a convoluted vibrational progression consistent with antisymmetric stretch excitation of H2O in the exit channel and OH stretch in the entrance channel. The photoelectron spectra are compared to two-dimensional time-dependent wave packet dynamics simulations based on an anharmonic potential in the anion and a model collinear potential energy surface for the neutral complex. The PPC spectra also yield the dissociation energies D0(H2O-2 → H2O + O-) = 1.15 ± 0.08 eV and D0(D2O-2 → D2O + O-) = 1.05 ± 0.08 eV.
An innovative approach to increase the throughput of mass spectrometric analyses using a multiple-ion-beam mass spectrometer is described. Two sample spots were applied onto a laser desorption/ionization target and each spot was simultaneously irradiated by a beam of quadrupled Nd:YLF laser radiation (261 nm) to produce ions by laser-desorption ionization. Acceleration of the ions in an electric field created parallel ion beams that were focused by two parallel einzel lens systems. After a flight path of 2.34 m, the ions were detected with a microchannel plate-phosphor screen assembly coupled with a charge coupled device camera that showed two resolved ion beams. Time-of-flight mass spectra were also obtained with this detector. Experiments were performed using both metal atom cations (Ti+ and Cr+) produced by laser desorption/ionization and the molecular ions of two different proteins (myoglobin and lysozyme), created by matrix assisted laser desorption/ionization using an excess of nicotinic acid as matrix.
The application of coincidence techniques to the study of the reaction dynamics of isolated molecules is reviewed. Coincidence spectroscopy is a powerful approach for carrying out a number of measurements. At its most basic level, coincidence techniques can identify the source of a specific signal, as in the well-known photoelectron-photoion coincidence approach used for several years. By carrying out coincidence experiments in an increasingly differential manner, correlated energy and angular distributions of reaction products may be recorded. Completely energy- and angle-resolved measurements of photoelectrons and ionic or neutral products can reveal molecular-frame photoelectron and photofragment angular distributions and aid in the characterization of dissociative states of molecules and ions. Recent work in this area is reviewed, including examples from studies of dissociative photodetachment, dissociative photoionization, time-resolved studies of dissociative photoionization. and three-body dissociation processes.
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.
Photodetachment and dissociative photodetachment processes of cyclopropoxide, c-C3H5O-, and cyclobutoxide have been studied at 532 nm. Photodetachment of c-C3H5O- produces both the ground X(2A″) state and the first excited A(2A′) state of cyclopropoxy radical, c-C3H5O. The X(2A″) state is stable at lower levels of excitation, but with increasing internal energy, dissociation into HCO + C2H4 is observed. The A(2A′) state completely dissociates into HCO + C2H4. Correlated measurements of photoelectron and photofragment kinetic energies provide dissociation energies c-C3H5O- and c-C3H5O into HCO- + C2H4 and HCO + C2H4 of 0.85 ± 0.07 and -0.26 ± 0.07 eV, respectively. Ab initio calculations have been performed to aid the interpretation of the dissociation mechanism. Cyclobutoxide, c-C4H7O-, undergoes only dissociative photodetachment to ground-state vinery radical and ethylene. The adiabatic electron affinity (AEA) of c-C4H7O is estimated to be 1.7 ± 0.1 eV. c-C4H7O- and c-C4H7O are both found to be thermodynamically unstable relative to dissociation into C2H3O- + C2H4 and C2H3O + C2H4 by -0.52 ± 0.07 and -0.45 ± 0.07 eV, respectively. Factors affecting the relative stability of the c-C3H5O and c-C4H7O radicals and the corresponding alkoxide anions are discussed on the basis of the observed differences in the dissociative photodetachment dynamics.
Studies of the dynamics of the dissociative photodetachment of (SO2)-2 reveal a strong preference in translational energy partitioning that can be related to the structure of (SO2)-2 using a simple impulse model. An anisotropic product angular distribution was also observed, indicative of a rapid dissociation on a repulsive potential energy surface. Over a wide range of available energies the translational energy distributions for this DPD process peak at approximate to 36% of the available energy, consistent with the soft-bond impulse model prediction that 33% of the available energy would appear in translation for an OSO-SO2 geometry (S-O bound) dimer anion.
The photoelectron spectra of the trifluoromethyl anion, CF3-, at 355 and 258 nm are reported, Simulation of the partially resolved vibrational structure is used to extract the adiabatic electron affinity, AEA[CF3] = 1.82 ± 0.05 eV. The heat of formation for the trifluoromethyl anion derived from the adiabatic electron affinity (Δ [CF3-] = -153.4 ± 1.5 kcal/mol) is compared to the high-accuracy "isodesmic bond additivity corrected" (BAC) complete basis set (CBS-Q) theory prediction (Δ [CF3-] = -152.6 kcal/mol). We find the CBS-Q prediction of (Δ [CF3] = -112.1 kcal/mol, after BAC, to be in excellent agreement with the most recent experimental determination of the radical heat of formation. The photoelectron angular distribution at 355 nm was also extracted from the photoelectron image, revealing p wave photodetachment with an energy-averaged anisotropy parameter of β = 1.5 ± 0.1.
The dissociative photodetachment of O-2(H2O)n=1-6 was studied at 388 and 258 nm using photoelectron-multiple-photofragment coincidence spectroscopy. Photoelectron spectra for the series indicate a significant change in the energetics of sequential solvation beyond the fourth water of hydration. Photoelectron-photofragment kinetic energy correlation spectra were also obtained for O-2(H2O)1-2, permitting a determination of the first and second energies of hydration for O-2 to be 0.85 ± 0.05 and 0.70 ± 0.05 eV, respectively. The correlation spectra show that the peak photofragment kinetic energy release in the dissociative photodetachment of O-2(H2O) and O-2(H2O)2 are 0.12 and 0.25 eV, respectively, independent of the photon and photoelectron kinetic energies. The molecular frame differential cross section for the three-body dissociative photodetachment: O-2(H2O)2 + hv →O2 + 2H2O + e- is also reported. The observed partitioning of momentum is consistent with either a sequential dissociation or dissociation from a range of initial geometries.
Femtosecond time-resolved photoelectron angular distributions (PADs) are measured for the first time in the molecular frame of a dissociating molecule. Various stages of the dissociation process, NO2 → NO(C2Π) + O(3P), are probed using ionization of the NO(C2Π) fragment to NO+(X1∑+). The PADs evolve from forward-backward asymmetric with respect to the dissociation axis at short time delays (?500 fs) to symmetric at long time delays (?1 ps). Changes in the PADs directly reflect the time-dependent separation and reorientation of the dissociating photofragments.
Photoelectron spectra of SO3- were recorded at 266 and 355 nm to study photodetachment of the SO3- anion (2A1) to the ground state of neutral SO3 (1A′1). A long vibrational progression in the 355 nm spectrum is attributed to excitation of the umbrella mode, v2, consistent with predictions that C3v symmetry SO3- yields D3h symmetry SO3 upon photodetachment. At 266 nm, photodissociation of SO3- to SO2 + O- was also observed. The geometry and normal-mode frequencies of SO3- and SO3 as well as the adiabatic electron affinity (AEA) and vertical detachment energy (VDE) of SO3 have also been calculated with ab initio (MP2 and CCSD(T)) and DFT methods. Using theoretical predictions and experimental data, Franck-Condon simulations of the photoelectron spectra were found to be in good agreement with experiment. The calculated AEA agreed well with experiment, but the VDE was found to be less accurate, presumably because of the large geometry change between anion and neutral.
The oxides TiO, CrO, and CoO, formed by reaction of the laser-ablated metal atoms and O2 in excess argon during condensation at 10 K, have been laser desorbed/ionized from solid argon with 308 nm radiation for observation by TOF mass spectrometry. Mass peaks for Ti+, Cr+, Co+, and particularly TiO+ and CrO+ were enhanced by adding to the copper support a thin film of organic acid typically used as a matrix in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Adding the C6H5Br chromophore to the Ar/O2 gas mixture also enhanced the metal and oxide ion signals. The laser desorption/ionization (LDI) process in these cryogenic experiments with low ionization energy subject atoms and molecules is assisted by the organic acid and bromobenzene chromophores, suggesting that charge exchange plays an important role in this process.
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.
In these experiments, the dynamics of the three-body dissociative photodetachment of O3-(D2O) at 258 nm are directly probed for the first time. Photodetachment of a negatively charged precursor, with coincident energy analysis of the photoelectron, allows production of energy-selected excited-state O3(D2O) complexes. By measurement of the laboratory velocities and recoil angles of the O + O2 + D2O products simultaneously, a kinematically complete description of the three-body dissociation dynamics of O3(D2O) is obtained. The results show that clustering of D2O to O3- stabilizes the system by 0.75 ± 0.09 eV. Photodetachment to the triplet states of O3 in the complex results in three-body dissociation, with no clear evidence observed for quenching or intracluster reaction in the complex. The molecular-frame differential cross section (MF-DCS), showing how the three products scatter in the molecular frame, is presented and discussed in light of DFT calculations of possible equilibrium geometries of the parent O3-(D2O) anion.
The photoelectron spectrum of vinoxide, C2H3O-, at 355 nm is reported, showing photodetachment to both the X(2A″) ground and first excited A(2A′) states of the vinoxy radical. Both direct interpretations and Franck-Condon simulations of the photoelectron spectrum of this simple enolate anion have been used to obtain insights into the energetics and structures of the anion and the ground and first excited state of the neutral radical. Franck-Condon simulations were generated from ab initio geometry and frequency calculations using the CASSCF method and showed good agreement with the vibrational structure visible in the experimental spectrum. The electron affinity (E.A.exp = 1.795 ± 0.015 eV; E.A.calc = 1.82 eV) and separation energy of the ground and first excited states (T0,exp = 1.015 ± 0.015 eV; T0,calc = 0.92 eV) obtained from the ab initio calculations are in good accord with the experimental values.
We present the first results using a new technique that combines femtosecond pump—probe methods with energy- and angle-resolved photoelectron—photoion coincidence imaging. The dominant dissociative multiphoton ionization (DMI) pathway for NO2 at 375.3 nm is identified as three-photon excitation to a repulsive potential surface correlating to NO(C2Π) + O(3P) followed by one-photon ionization to NO+(X1∑+). Dissociation along this surface is followed on a femtosecond timescale.
A new photoelectron-photofragment-coincidence spectrometer is described. Using a multiparticle time- and position-sensitive detector, this apparatus allows the study of dissociation processes of negative ions yielding three photofragments in coincidence with a photoelectron. The photoelectron spectrometer uses two detectors and works in time of flight mode, detecting 10% of the photoelectrons with an energy resolution of 5% at 1.3 eV as shown in studies of the photodetachment of . A third detector is used for collection of multiple photofragments (up to 8) in coincidence. This multiparticle detector uses a crossed-delay-line anode and fast timing signals to encode the time- and position-of-arrival of multiple photofragments. The detector was demonstrated to record all three particles produced in a single three-body dissociation event, yielding an energy resolution of ≈15% ΔE/E at 0.7 eV in experiments on the three-body dissociative photodetachment of .
The dissociative photodetachment dynamics of ( + hv → O2 + O2 + e- have been studied at 532, 355, and 266 nm by triple-coincidence measurements of the energy and angular distributions of the photoelectron and photofragments. The data reveal vibrationally resolved product translational energy distributions and a strong angular correlation between the photoelectron and the photofragments for this direct process. The translational energy distributions show that photodetachment of over this photon energy range occurs to several low-lying repulsive states of O4, producing O2 in the ground and low-lying electronically excited states (O2(X3 ), O2(a1Δg), and O2(b1 )). The partitioning of energy into vibration and rotation of the O2 products is analyzed in terms of a Franck?Condon model, indicating that the excess electron in is delocalized over two identical O2 moieties in a symmetric species. A qualitative analysis of the product angular distributions in terms of the electronic structure of is consistent with recent ab initio calculations.
Measurement of the translational energy partitioning in the three-body dissociative photodetachment of ( + hv → O2 + O2 + O2 + e-) at 532 nm is reported. Using photoelectron and photofragment translational energy spectroscopies in coincidence, a complete kinematic measurement of the three-body dissociation of neutral O/sub 6/ is obtained. Vibrationally resolved product translational energy distributions are observed. The results provide insights into the structure, binding energy, and dissociation dynamics of and O6 and illustrate a new approach to the study of three-body reaction dynamics.
Important insights into the energetics and dynamics of transient molecules and clusters have been obtained from studies of the photodetachment of negative ions. When photodetachment produces a neutral in a metastable or dissociative state, further information can be obtained by studying the subsequent dissociation dynamics of the molecule. This can be done by measuring the photoelectron kinetic energy and the photofragment translational energy release in coincidence. Energy and angle resolved measurements of this type are now possible in photoelectron-photofragment coincidence experiments. The photoelectron-photofragment coincidence technique is reviewed and the DPD dynamics of , , NO-(N2O) and N3 are discussed. These systems provide examples of direct and sequential dissociative photodetachment processes, along with the application to the study of isomeric forms of molecular and cluster anions.
The dissociative photodetachment of N3O2- at 532, 355 and 266 nm has been studied using coincident photoelectron and photofragment translational spectroscopy in a fast ion beam. The photoelectron spectra confirm previous experimental evidence for a weakly perturbed NO-(N2O) complex and show for the first time a broad continuum at large electron binding energies. Translational energy release spectra exhibit both low and high energy release channels, indicating that two regions of the NO — N2O neutral potential energy surface with very different intermolecular repulsion are accessed. Photoelectron—photofragment coincidence data confirm that the high translational energy release events are correlated with the continuum in the photoelectron spectrum. These dissociative photodetachment dynamics are consistent with the existence of two isomeric species, an ion—dipole complex with the charge localized on the NO moiety, and a molecular anion with an estimated stability of 2.0 ± 0.2 eV relative to NO + N2O + e-.
Considerable insights into the dynamics of both ionic (photodissociation) and neutral (dissociative photodetachment) decomposition pathways of O4- and O6- have been gained using photoelectron and photofragment translational spectroscopy in a fast-ion-beam. The O4- data at 532 nm reveal a novel process involving sequential photodetachment of an electron with a near-zero binding energy from photodissociating O4-. Studies of O6- at 532 nm reveal that addition of a third O2 to the O4- core leads to a dramatic change in the photodissociation dynamics, producing highly vibrationally excited O2- photofragments not observed in the case of O4-. At 355 nm, both O4- and O6- yield vibrationally excited O2- photofragments, as observed by autodetachment of the nascent O4-(ν⩾5) → O2 + e-. At 266 nm, photofragment TOF measurements on O6- and O4- show that the dynamics of dissociative photodetachment in O6- are only slightly perturbed relative to O4-. The anisotropic product angular distribution previously observed in O4- is observed to persist in the three-body neutral decomposition O6- + hν → O2 + O2 + O2 + e-. The origins of these diverse phenomena in O4- and O6- are discussed.
This paper reports an investigation of the stability and dissociation dynamics of the low-lying singlet and triplet states of ozone (O3) using coincident photoelectron and photofragment translational spectroscopy. These experiments provide an upper limit of 0.5 μs for the lifetime of the low-lying excited states of O3. The 3A2 and 3B2 states dissociate with nearly all available energy appearing in translation. Dissociation of the higher-lying 3B1 and 1A2 states produces rotationally and possibly vibrationally excited O2 products. The product translational energy distributions indicate that the 3B1 state dissociates non-adiabatically to ground state O(3P) + O2(3∑g-) products.
An investigation of the photodissociation dynamics of the dimer anion at 523.6, 349.0 and 261.8 nm is reported. Product translational energy and angular distributions have been obtained using photofragment translational spectroscopy in a fast ion beam. At all wavelengths photodissociation ( + hv → O2 + O2 + e-) is observed to proceed via a rapid parallel electronic transition, with the photofragment angular distribution strongly peaked along the laser electric vector. The lowest energy photodissociation channel produces O2 (a1Δg) and ground state (X2Πg), indicating that is a doublet anion. The partitioning of energy in the dissociation reveals a complicated wavelength dependence.
The low-lying electronic states of CCO have been investigated by photoelectron spectroscopy of CCO- at wavelengths of 266 and 355 nm in conjunction with ab initio calculations. Photodetachment is observed to occur to the 3∑-, 3Π, 1Δ and 1∑+ electronic states of CCO. This marks the first observation of the low-lying singlet states. A revised value for the electron affinity of CCO is found to be 2.289 ± 0.018 eV. These results are compared with CASPT2 ab initio calculations of the energetics and structure of the ground and excited states of CCO and CCO-. Using the measured electron affinity of CCO, the heats of formation Δ (CCO) = 3.99±0.20 eV and Δ (CCO-) = 1.67±0.20 eV are determined. In addition, the C—C bond dissociation energies in CCO and CCO- are determined, as well as the H—CCO bond energy in HCCO.
Measurements of photoelectron-photofragment angular correlation and energy partitioning in the dissociative photodetachment of O4- (O4- + hv → O2+O2+e-) at 532 nm are reported. Using photofragment translational energy and photoelectron spectroscopy in coincidence, vibrationally resolved product translational energy distributions and a pronounced angular correlation between the photoelectron and photofragment recoil directions are observed. These results provide insights into the molecule-fixed photoelectron angular distribution of a negative ion and structural information on O4- in the gas phase.
The dynamics of dissociative photodetachment in O2-.(H2O) were studied using the technique of fast-beam photofragment translational spectroscopy. The kinetic energy and angular distributions of the neutral molecules produced by dissociative photodetachment (O2-.(H2O) + hv → O2 + H2O + e- ) were recorded at 523.6, 349.0 and 261.8 nm. Dissociative photodetachment is observed to occur at all of these wavelengths with nearly identical kinetic energy release. The product angular distributions from dissociative photodetachment are slightly anisotropic and exhibit a small wavelength dependence. These results suggest that dissociative photodetachment of O2-.(H2O) occurs via Franck-Condon excitation to a weakly repulsive region of the O2-.(H2O) potential energy surface.
The photodissociation dynamics of O3- at 523 nm have been studied using fast-ion-beam translational energy spectroscopy. Translational energy and angular distributions of coincident O- + O2 products from the process O3- + hv → O- + O2 were measured. O3- was generated by electron-impact in a pulsed beam from two precursors - neat O2 and a seeded beam of O3. The observed photodissociation dynamics are very different in the two cases, indicating a great difference in internal excitation of ozonide in the two sources.
Photoelectron-neutral-neutral coincidence spectra have been measured for the dissociative photodetachment of O4- ( O4- + hv → O2 + O2 + e- ) at 523 and 349 nm. The neutral photofragment translational energy spectrum, the photoelectron spectrum and the correlations of the translational energy and photoelectron energy are presented here. The correlation spectra reveal phenomena that are not discernable in either one-dimensional measurement. Features are observed which indicate that non-Franck-Condon processes play a role in the dissociative photodetachment of O4- at 349 nm.
A high collection efficiency fast-ion beam photoelectron spectrometer is described. In a straight time-of-flight mode, the spectrometer collects ~1% of the photoelectrons and achieves an energy resolution (delta) E / E of ~5%. For coincidence experiments requiring greater collection efficiency, a paraboloidal electrostatic mirror is used. The mirror collects ~40% of the photoelectrons while maintaining (delta) E/E <= 35%. In both modes of operation, a time- and position-sensitive electron detector allows conversion of the photoelectron laboratory energy to center-of-mass energy. The fast-ion beam photoelectron spectrometer is used to create mass- and energy-selected neutral molecules which are used in molecular dissociation studies.
A preliminary report is presented on experiments using fast ion-beam translational energy spectroscopy to study dissociative photodetachment and photodissociation dynamics in the small cluster ions O4- and O2-. (H2O). Translational energy and angular distributions of coincident molecular fragments were recorded from the photodestruction of O4- and O2-.(H2O) at 523, 349 and 262 nm. At each wavelength, the O4- results confirm the existence of at least two distinct channels: dissociative photodetachment ( O4- + hv → O2 + O2 + e-) and photodissociation ( O4- + hv → O2 + O2-).
Observation of strongly anisotropic angular distributions shows that dissociation occurs on the time-scale of molecular rotation in both processes. The photodissociation of O4- at 523 nm gives a new value for the O2-O2- bond energy, D0 = 0.39 +/- 0.05 eV. In O2-.(H2O), a single dissociative photodetachment channel ( O2-.(H2O) + hv → O2 + H2O + e-) is observed at all wavelengths. Angular distributions from this process are slightly anisotropic and exhibit a small wavelength dependence.
An anisotropic product angular distribution has been observed in the dissociative photodetachment of O4- at 523 nm. Energy and angular distributions of coincident O2 products from the process O4- + hv → O2 + O2 + e- were measured using translational energy spectroscopy in a fast ion beam. The angular distribution peaks perpendicular to the electric vector of the laser beam.