Thomas A. Stephenson

Quantum Calculations On The Vibrational Predissociation Of NeBr2: Evidence For Continuum Resonances

Quantum mechanical calculations on the vibrational predissociation dynamics of NeBr2 in the B electronic state have been performed and the results compared with both experimental data and other computational studies. For vibrational levels with v⩽20 we find that the vibrational state dependence of the predissociation lifetimes is in qualitative agreement with experimental measurements, as are the calculated Br2 fragment rotational distributions. For higher vibrational levels, the B←X excitation profiles are well represented by a sum of two Lorentzian line shapes. We attribute this result to the presence of long-lived resonances in the dissociative continuum that are reminiscent of long-lived dissociative trajectories in previous classical studies of NeBr2.

Fragment Rotational Distributions From The Dissociation Of NeBr2: Experimental And Classical Trajectory Studies

The Br2 fragment rotational distributions that result from the vibrational predissociation of NeBr2 in the B electronic state have been measured for several initial vibrational levels. In each case, the rotational distributions extend to the effective energetic limit determined by the amount of energy available (Eavl) for disposal into the fragment rotational and translational degrees of freedom. Analysis of the data allows refinement of the NeBr2 dissociation energy; we find that D0=70.0±1.1 cm−1 for the X electronic state, v=0. Both Δv=−1 and −2 dissociation events have been examined. For dissociation pathways with approximately the same value of Eavl the Δv=−2 pathways are observed to have a higher fraction of the fragment energy in rotational excitation. The overall shape of the Δv=−1 distributions are insensitive to the value of Eavl, suggesting that a Franck–Condon model for the dissociation may have some validity, though quantitative quantum mechanical calculations demonstrate that this model does ...

Collision-induced electronic energy transfer from v=0 of the E(0g+) ion-pair state in I2: Collisions with I2(X)

The collision-induced electronic energy transfer that occurs when I 2 in the E(0 g ϩ) ion-pair electronic state collides with ground electronic state I 2 has been investigated. We prepare I 2 in single rotational levels in vϭ0 of the E state using two-color double resonance laser excitation. The resulting emission spectrum shows that the nearby (⌬T e ϭϪ385 cm Ϫ1) D(0 u ϩ) electronic state is populated. The cross section for collision-induced E→D energy transfer is found to be 18Ϯ3 Å 2. A range of D state vibrational levels are populated, consistent with a model in which overlap between the initial and final vibrational wave functions is important, but modulated by propensities for small vibrational energy gaps and those energy gaps that are closely matched to the vϭ0→vϭ1 energy separation in the I 2 (X) collision partner.

C–Cl bond fission, HCl elimination, and secondary radical decomposition in the 193 nm photodissociation of allyl chloride

The primary photodissociation dynamics of allyl chloride upon excitation at 193 nm is investigated in a crossed laser-molecular beam scattering apparatus. Tunable vacuum ultraviolet (VUV) photoionization of the products provides a unique ability to learn about the secondary reaction products of the nascent photoproducts formed. The data show evidence for four significant primary reaction channels: a previously unidentified low kinetic energy C–Cl bond fission channel producing unstable allyl radicals, an excited state C–Cl bond fission channel producing Cl atoms with high translational energy, an HCl elimination pathway releasing significant energy to product translation to HCl and its momentum-matched mass 40 partner, and an HCl elimination channel producing low kinetic energy HCl products and predominantly unstable mass 40 products. The measured branching of these primary reaction channels of [all C–Cl] : [fast C–Cl] : [slow C–Cl] : [fast HCl] : [slow HCl] : [all HCl] is 1.00: 0.971: 0.029: 0.291: 0.167...

Nonadiabatic electronic interactions in the ion‐pair states of NeICl

Nonadiabatic interactions in the NeICl van der Waals complex have been explored in the lowest energy triad of ICl ion‐pair states (∼39 000 cm−1). Dispersed fluorescence measurements reveal emission characteristic of multiple ion‐pair electronic states, with the relative contributions from the E(0+), β(1), and D’(2) states changing with the initial ICl vibrational excitation (vICl). Emission directly from NeICl (vICl=0) complexes indicates that the initially prepared NeICl levels have mixed electronic character and that the ICl electronic parentage changes with the initial van der Waals vibrational level selected. NeICl complexes prepared with 1–4 quanta of ICl stretch undergo rapid vibrational predissociation with a strong propensity for ΔvICl=−1 relaxation. The electronic state(s) populated in the ICl fragments differ from the mixed electronic character of the initially prepared level, demonstrating that vibrational predissociation is accompanied by nonadiabatic electronic state changing processes. The o...

Fragment rotational state distributions from the dissociation of NeIBr: Experimental and theoretical results

The IBr fragment rotational state distributions that result when the NeIBr van der Waals molecule undergoes vibrational predissociation have been measured in a pump–probe laser‐induced fluorescence experiment. Independent of initial vibrational state and the number of quanta of vibrational energy lost from the I–Br coordinate, the rotational distributions extend over the full range of energetically accessible states. From the observation of energetic constraints on the rotational distribution, the dissociation energy (D0) is calculated to be 65.5±1.2 cm−1 for the A electronic state, v=16. For the X electronic state, v=0, D0=71.8±1.2 cm−1. Quantum mechanical bound state calculations carried out on a model A electronic state potential energy surface are in quantitative agreement with this result. The rotational distributions are broader than that predicted by either a Franck–Condon or classical impulsive model for the dissociation. The distributions are qualitatively in accord with classical trajectory calc...

Vibrational Branching Ratios From The Dissociation Of The NeIBr Van Der Waals Molecule

The degree of vibrational excitation in the IBr fragment from the vibrational predissociation of NeIBr (A 3Π1) has been measured using two‐color pump–probe laser‐induced fluorescence spectroscopy. We find that for the lowest initial vibrational states examined, Δv=−1 dissociation pathways dominate the dynamics, while this channel is closed for v≥17. From this result, the A state binding energy (D0) of the complex is determined to be 67±4 cm−1, while that in the X electronic state is found to be 73±4 cm−1. The X state binding energy is identical to that for NeI2 and NeBr2, suggesting that the potential energy surface for NeIBr can be constructed from a summation of atom–atom pair potentials; we present such a model potential energy surface. The variations in the vibrational branching ratios, when combined with the trends in the predissociation rates, point to the importance of fragment rotational excitation in the dynamics of the dissociation.

Reactive quenching of OD A 2Σ+ by H2: translational energy distributions for H- and D-atom product channels.

The H- and D-atom products from collisional quenching of OD A (2)Σ(+) by H(2) are characterized through Doppler spectroscopy using two-photon (2 (2)S ←← 1 (2)S) laser-induced fluorescence. Partial deuteration enables separation of the channel forming H + HOD products, which accounts for 75% of reactive quenching events, from the D + H(2)O product channel. The Doppler profiles, along with those reported previously for other isotopic variants, are transformed into product translational energy distributions using a robust fitting procedure based on discrete velocity basis functions. The product translational energy distribution for the H-atom channel is strongly peaked at low energy (below 0.5 eV) with a long tail extending to the energetic limit. By contrast, the D-atom channel exhibits a small peak at low translational energy with a distinctive secondary peak at higher translational energy (approximately 1.8 eV) before falling off to higher energy. In both cases, most of the available energy flows into internal excitation of the water products. Similar distributions are obtained upon reanalysis of D- and H-atom Doppler profiles, respectively, from reactive quenching of OH A (2)Σ(+) by D(2). The sum of the translational energy distributions for H- and D-atom channels is remarkably similar to that obtained for OH A (2)Σ(+) + H(2), where the two channels cannot be distinguished from one another. The product translational energy distributions from reactive quenching are compared with those obtained from a previous experiment performed at higher collision energy, quasiclassical trajectory calculations of the post-quenching dynamics, and a statistical model.

The spectroscopy and A state dynamics of the NeIBr van der Waals complex

The A 3Π1←X 1Σ+ laser‐induced fluorescence excitation spectrum of the NeIBr van der Waals complex is reported and analyzed to extract information regarding the structure and vibrational predissociation dynamics of the complex. While no definitive geometric information regarding NeIBr is obtained, our data indicate that a linear geometry is at least plausible. The vibrational predissociation lifetimes are a strong function of A state vibrational level and range from 2.6 to 23 ps. The variation in lifetime with vibrational level is consistent with the results of previous measurements on rare gas–halogen complexes, particularly NeBr2.

Experimental characterization of the weakly anisotropic CN X2Σ+ + Ne potential from IR-UV double resonance studies of the CN-Ne complex

IR-UV double resonance spectroscopy has been used to characterize hindered internal rotor states (n(K) = 0(0), 1(1), and 1(0)) of the CN-Ne complex in its ground electronic state with various degrees of CN stretch (ν(CN)) excitation. Rotationally resolved infrared overtone spectra of the CN-Ne complex exhibit perturbations arising from Coriolis coupling between the closely spaced hindered rotor states (1(1) and 1(0)) with two quanta of CN stretch (ν(CN) = 2). A deperturbation analysis is used to obtain accurate rotational constants and associated average CN center-of-mass to Ne separation distances as well as the coupling strength. The energetic ordering and spacings of the hindered internal rotor states provide a direct reflection of the weakly anisotropic intermolecular potential between CN X (2)Σ(+) and Ne, with only an 8 cm(-1) barrier to CN internal rotation, from which radially averaged anisotropy parameters (V(10) and V(20)) are extracted that are consistent for ν(CN) = 0-3. Complementary ab initio calculation of the CN X (2)Σ(+) + Ne potential using MRCI+Q extrapolated to the complete one-electron basis set limit is compared with the experimentally derived anisotropy by optimizing the radial potential at each angle. Experiment and theory are in excellent accord, both indicating a bent minimum energy configuration and nearly free rotor behavior. Analogous experimental and theoretical studies of the CN-Ne complex upon electronic excitation to the CN B (2)Σ(+) state indicate a slightly more anisotropic potential with a linear CN-Ne minimum energy configuration. The results from these IR-UV double resonance studies are compared with prior electronic spectroscopy and theoretical studies of the CN-Ne system.