Robert E. Continetti

Femtosecond time-resolved photoelectron–photoion coincidence imaging studies of dissociation dynamics

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(C 2Π)+O(3P) followed by one-photon ionization to NO+(X 1Σ+). Dissociation along this surface is followed on a femtosecond timescale.

Photodissociation dynamics of the N3 radical

The dissociation dynamics of the B 2Σu+ state of N3 were investigated using fast radical beam photodissociation coupled with a new coincidence wedge‐and‐strip‐anode particle detector. With this detector, detailed photofragment kinetic energy and angular distributions can be measured as a function of excitation energy. Calibration of the detector by studies of the predissociation of the O2 B 3Σu− state is discussed. Methods of extracting the center‐of‐mass kinetic energy and angular distributions are presented. The photodissociation results for N3 show that from the vibrationless level of the B 2Σu+ state in N3, both spin‐allowed [N3→N(2D)+N2(1Σg+)] and spin‐forbidden [N3→N(4S)+N2(1Σg+)] dissociation processes occur. Bend excitation in the B 2Σu+ state, however, enhances the spin‐allowed dissociation process considerably. The kinetic energy distributions reveal partially resolved vibrational structure in the N2 fragment, and indicate substantial rotational excitation of the N2. This implies that bent ge...

Photodissociation dynamics of the N[sub 3] radical

The dissociation dynamics of the B 2Σu+ state of N3 were investigated using fast radical beam photodissociation coupled with a new coincidence wedge‐and‐strip‐anode particle detector. With this detector, detailed photofragment kinetic energy and angular distributions can be measured as a function of excitation energy. Calibration of the detector by studies of the predissociation of the O2 B 3Σu− state is discussed. Methods of extracting the center‐of‐mass kinetic energy and angular distributions are presented. The photodissociation results for N3 show that from the vibrationless level of the B 2Σu+ state in N3, both spin‐allowed [N3→N(2D)+N2(1Σg+)] and spin‐forbidden [N3→N(4S)+N2(1Σg+)] dissociation processes occur. Bend excitation in the B 2Σu+ state, however, enhances the spin‐allowed dissociation process considerably. The kinetic energy distributions reveal partially resolved vibrational structure in the N2 fragment, and indicate substantial rotational excitation of the N2. This implies that bent ge...

Molecular beam studies of the photodissociation of benzene at 193 and 248 nm

The photodissociation processes of benzene following excitation at 193 and 248 nm have been studied by molecular beam photofragmentation translational spectroscopy. When benzene was excited to the 1 B1u state by absorption at 193 nm, dissociation occurred through three primary channels, C6H5+H (80%), C6H4+ H2 (16%), and C5H3+CH3 (4%), following internal conversion to the vibrationally excited ground state. When benzene was excited to the 1 B2u state at 248 nm, two primary dissociation channels, C6H4+H2 (96%), and C5H3+CH3 (4%), were observed. Photodissociation to produce two C3H3 was induced by two photon absorption of benzene at both 193 and 248 nm. Numerous secondary photodissociation processes of the primary photoproducts were also observed at both 193 and 248 nm.

Photodissociation of H2S and the HS radical at 193.3 nm

Abstract Photodissociation of H 2 S has been studied at 193.3 nm using H atom photofragment-translational spectroscopy with mass-spectrometric detection. H + HS( v ) product branching ratios are reported which are not in quantitative agreement with other recent experimental results. Secondary photodissociation of HS radicals has also been observed, showing that both S( 3 P) and S( 1 D) are produced via perpendicular transition. The HS bond energy was found to be 3.62 ± 0.03 eV.

Crossed molecular beams study of the reaction D+H2→DH+H at collision energies of 0.53 and 1.01 eV

This paper reports the first product differential cross section (DCS) measurements for the D+H2→DH+H reaction as a function of laboratory (LAB) scattering angle with sufficient resolution to resolve product DH vibrational states. Using a D‐atom beam produced by the photodissociation of DI at 248 nm, product velocity and angular distributions were measured at 12 LAB angles at a nominal collision energy of 0.53 eV and at 22 LAB angles at a nominal collision energy of 1.01 eV with a crossed molecular beams apparatus. After correction of the raw product time‐of‐flight (TOF) spectra for modulated background, a comparison with recent exact quantum mechanical scattering calculations was made using a Monte Carlo simulation of the experimental conditions. The simulation showed that although the theoretical predictions agree qualitatively with the measurements, some significant discrepancies exist. Using the Monte Carlo simulation, a best‐fit set of DH(v,j) DCS’s which showed good agreement with the measurements wa...

Symmetric stretch excitation of CH3 in the 193.3 nm photolysis of CH3I

New high resolution photofragment translational spectroscopy measurements on the 193.3 nm photolysis of CH3I reveal up to four quanta of ν1 C–H symmetric stretch excitation in the C–I bond fission channel. In addition these experiments show for the first time that C–H bond fission occurs at wavelengths longer than 185 nm with a quantum yield of ∼3%. The data yields upper bounds to the C–H bond dissociation energy in CH3I of 101±1 kcal/mol and for the C–I bond energy, 55.0±0.3 kcal/mol.

Fast beam studies of N3 photodissociation

Abstract A new fast radical beam apparatus has been used to study the photodissociation cross section of N3 as a function of wavelength. Neutralization of a fast negative ion beam by photodetachment is used to create a beam of cold, mass-selected radicals, with subsequent photodissociation and efficient detection of the neutral fragments. N3 was observed to predissociate throughout the first electronic absorption near 270 nm. High-resolution predissociation spectra were obtained which show rotationally resolved structure. Time-of-flight measurements of the kinetics energy release confirm that the lowest energy dissociation pathway which occurs is the first spin-allowed channel producing (N(2D ) + N2(1Σ+g). The photoelectron spectrum of N−3 was also obtained, yielding an electron affinity of 2.68±0.01 eV for N3.

Imaging Dynamics on the F + H2O → HF + OH Potential Energy Surfaces from Wells to Barriers

The study of gas-phase reaction dynamics has advanced to a point where four-atom reactions are the proving ground for detailed comparisons between experiment and theory. Here, a combined experimental and theoretical study of the dissociation dynamics of the tetra-atomic FH2O system is presented, providing snapshots of the F + H2O → HF + OH reaction. Photoelectron-photofragment coincidence measurements of the dissociative photodetachment (DPD) of the F¯(H2O) anion revealed various dissociation pathways along different electronic states. A distinct photoelectron spectrum of stable FH–OH complexes was also measured and attributed to long-lived Feshbach resonances. Comparison to full-dimensional quantum calculations confirms the sensitivity of the DPD measurements to the subtle dynamics on the low-lying FH2O potential energy surfaces over a wide range of nuclear configurations and energies. A reaction is studied in fine detail by electron removal from a charged precursor to unveil and track a neutral intermediate. A View from the Middle The intuitive way to study a bimolecular reaction is to induce a collision between separate reagents and then track the ensuing events. Crossed molecular beam studies have revealed the quantum mechanical details of numerous systems in this fashion. Otto et al. (p. 396, published online 9 January) applied a more recent approach of starting in the middle of the F + H2O → HF + OH reaction trajectory, postcollision, by photodetaching an electron from a stabilized complex of water and a fluoride ion, and then tracking the fate of the neutral fragments.

Photoelectron-multiple-photofragment coincidence spectrometer

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 O2−. 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 O6−.