D. R. Cyr

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...

Photodissociation spectroscopy and dynamics of the N2O−2 anion

The spectroscopy and dissociation dynamics of the N2O−2(C2v) anion have been investigated using the technique of fast ion beam translational spectroscopy. A newly developed pulsed supersonic discharge source is described for the production of internally cold N2O−2. A structured absorption band beginning near 580 nm is observed, and is assigned to the B(2A2)←X(2B2) transition with the aid of ab initio calculations. Two dissociation channels from the upper state are observed: (1) O−+N2O and (2) NO−+NO. Translational energy and angular distributions are measured for both channels at several excitation energies. The translational energy distribution for channel (1) at 570 nm shows resolved structure corresponding to N2O vibrational excitation. The translational energy distributions for channel (1) are reasonably well described by prior distributions, indicating this channel results from dissociation from the N2O−2 ground electronic state. In contrast, channel (2) appears to result from dissociation on a rep...

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.

Fast beam studies of NCO free radical photodissociation

The spectroscopy and dissociation dynamics of the NCO radical have been investigated by applying fast radical beam photodissociation spectroscopy to the B 2Π←X 2 Π electronic transition. Measurements of the photodissociation cross section as a function of dissociation wavelength show that even the lowest vibrational levels of the B 2Π state predissociate. Analysis of fragment kinetic energy release reveals that the spin‐forbidden N(4S)+CO(1Σ+) products are produced exclusively until 20.3 kcal/mol above the origin, at which point, the spin‐allowed N(2D)+CO product channel becomes energetically accessible. The spin‐allowed channel dominates above this threshold. By determining the location of this threshold, we obtain a new ΔHf0 for NCO of 30.5±1 kcal/mol, several kcal/mol lower than the previously accepted value.

Predissociation dynamics of the O2 B 3Σ−u state: Vibrational state dependence of the product fine‐structure distribution

The predissociation of the O2B 3Σ−u state (υ=0–11) is investigated using fast beam photofragment translational spectroscopy. The energy resolution of the experiment, 7–10 meV, is sufficient to yield the correlated fine structure distribution P(j1,j2) for the two O(3Pj) fragments. These spin–orbit branching ratios depend markedly on the vibrational quantum number, providing detailed insight into a relatively unexplored facet of molecular dissociation dynamics. No less than four repulsive states are expected to mediate the predissociation of the B 3Σ−u state, primarily via spin–orbit coupling, and the couplings among these states at long range (R∼5–7 A) determine the final spin–orbit distributions P(j1,j2). We have attempted to model these distributions in both the adiabatic and diabatic limits, with neither limit proving very successful. A more phenomenological approach to fitting our data suggests that products with j1=j2=2 result from single transitions between adiabatic potentials at long range, whereas...

Fast beam photodissociation of the CH2NO2 radical

The photodissociation of the nitromethyl radical, CH2NO2, has been studied using a fast beam photofragment translational spectrometer. In these experiments, a fast beam of mass selected, internally cold nitromethyl radicals is formed via negative ion photodetachment of CH2NO−2 and subsequently dissociated. The recoiling photofragments are detected in coincidence using a microchannel plate detector equipped with a time‐ and position‐sensing anode. Two dissociation product channels are observed at each of three dissociation wavelengths investigated in the range 240–270 nm and are identified as (I) CH2NO2→CH2NO+O and (II) CH2NO2→H2CO+NO. In marked contrast to the ultraviolet photodissociation of CH3NO2, no evidence is found for simple C–N bond fission to give (III) CH2NO2→CH2+NO2. Translational energy and angular distributions were obtained for the two observed channels. The translational energy distribution of channel (I) peaks at only 5–8 kcal/mol, while the distribution for channel (II) peaks at ∼60 kcal/...

Observation of the correlated O 3Pj1, 3Pj2 state distribution from the predissociation of O2 B 3Σ−u

Abstract A fast beam photofragment translational spectroscopy study of the predissociation of the O 2 B 3 Σ − u state is presented. A 5 keV beam of vibrationally excited O 2 X 3 Σ − g is prepared via photodetachment of O − 2 . The B 3 Σ − u (gn′=7)←X 3 Σ − g (ν″=4) transition of the Schumann-Runge band is then excited, resulting in predissociation to two O 3 P j atoms. We determine the photofragment kinetic energy angular distribution using a time- and position-sensitive detector. Our energy resolution (10 meV) is sufficient to resolve the energy splittings of the O atom spin-orbit levels, enabling us to determine the correlated ( j 1 , j 2 ) fine structure distribution for the photofragments. These results do not appear to be consistent with predictions based on a recent model of O 2 B 3 Σ − u state predissociation.

Fast-beam studies of free-radical photodissociation: the CH2NO2 radical

We report a study of the photodissociation of the nitromethyl radical utilizing a fast beam photofragment translational spectrometer. A fast radical beam is prepared via the synthesis, acceleration and subsequent photodetachment of mass-selected, internally cold nitromethyl anions. Following ultraviolet photodissociation, the recoiling photofragments are detected in coincidence by a microchannel plate detector with a time- and position-sensitive wedge-and- strip anode. The data reveal that the electronically excited nitromethyl radicals dissociate into two mass channels: (I) CH2NO2

Study of the 2B1 and 2A2 states of nitromethyl free radical via ultraviolet photoelectron spectroscopy of the nitromethyl(1-) anion

DAG yields CH2NO + O, and (II) CH2NO2