J.P. Simons

The stereochemistry of the O(1D)+N2O→NO+NO reaction via velocity‐aligned photofragment dynamics

Velocity‐aligned, superthermal O(1D) atoms generated via the photodissociation of N2O have been employed to investigate the stereodynamics of the title reaction. The power of this experimental technique, when coupled with Doppler‐resolved, polarized laser‐induced fluorescence probing of the reaction products, is demonstrated by reference to the specific reaction channel leading to NO(υ’=0)+NO(υ’=16,17), which is shown to proceed via direct stripping dynamics. Furthermore, the observed product‐state selective linear and angular momenta disposals imply that the reaction is stereodynamically constrained to occur via collinear collision geometries.

Quantum state-selected photodissociation dynamics in H2O and D2O

Two photon excitation, tunable near 248·5 nm, has been used to dissociate H2O/D2O via the [Ctilde] 1 B 1 and [Btilde] 1 A 1 states. Rotationally resolved OH/OD(A 2Σ+) photofragment excitation spectra are reported, following excitation to predissociated levels of [Ctilde] 1 B 1. Rotational resolution of the OH/OD(A 2Σ+ → X 2Π) fluorescence, generated from individual J′ K a K c levels of [Ctilde] 1 B 1, allows full quantum state selection in both the entry and exit channels. The OH/OD(A 2Σ+) fragment is formed rotationally hot as a result of the large change in bond angle in going from [Xtilde] 1 A 1 (or [Ctilde] 1 B 1) to the linear dissociative [Btilde] 1 A 1 surface. Product alignment measurements allow assignment of the two photon continuum absorption to [Btilde] 1 A 1: a-axis rotation in [Ctilde] 1 B 1 destroys product alignment from these levels. Electronic branching from ⪷B 1 A 1 to A 1 B 1 (and/or [Xtilde] 1 A 1) during the dissociation forms ground state OH/OD(X 2Π). Relative branching ratios are o...

Photodissociation of H2O2 at 248 nm: translational anisotropy and oh product state distributions

Abstract Ground-state OH(X 2 Π) fragments, from the photodissociation of H 2 O 2 at 248 nm, have been probed by laser-induced fluorescence. Nascent product rotational distributions and polarisation-dependent Doppler lineshapes are reported. The high translational anisotropy (β = −1 at low N″ ) indicates a prompt repulsion along the O-O axis. Polarisation-dependent Doppler lineshapes are interpreted as a correlation between the fragment recoil velocity v and angular momentum J OH , the product rotation being aligned along v . Product rotation is generated by an axial torsion about the O-O axis, consistent with a change in dihedral angle in the upper state.

Molecular emission from H2O/D2O [Ctilde] 1 B 1 and photodissociation dynamics on the [Btilde] 1 A 1 surface

Bound to free emission centred near 420nm is reported from heavily predissociated levels of H2O/D2O [Ctilde](1 B 1)000 (τ < 6ps) to the dissociative A 1 B 1 state. The quantum yield for fluorescence is small (∼ 10-4) and provides an estimate for the radiative lifetime of the D2O ([Ctilde] → A) emission. The ([Ctilde] → A) fluorescence excitation spectrum has been modelled to obtain refined heterogenous predissociation rates for D2O ([Ctilde] 1 B 1 → [Btilde] 1 A 1) transfer. This has allowed the OD(A 2Σ+) photofragment fluorescence excitation spectrum to be modelled to obtain the J′a dependent Renner-Teller crossing probability for [Btilde] 1 A 1 → A 1 B 1 transfer during dissociation, over a wide range of a-axis rotation. The results are in good, qualitative agreement with theory and provide insight into the dynamics of dissociation on the [Btilde] 1 A 1 surface. Resolved photofragment fluorescence excitation spectra for selected rotational levels N′, in OD (A 2Σ+)v′ = 0 show a correlation between N′ and...

Quantum-state-selected photodissociation of H2O(C̃1 B1)

Abstract Tunable, Line-narrowed KrF laser radiation has been used to populate individual rovibronic levels in the C 1 B 1 state of H 2 O, via two-photon absorption at pressures ≥5 mTorr. Predissociation into OH(A 2 v + ) ν . =0 , detected by scanning its rotationally resolved fluorescence, only occurs for levels with ( J a 2 ) ‡ 0. The homogeneous predissociation channel must generate alternative products. Thus the branching into the accessible electronic dissociation channels is influenced by rotation of the parent molecule. The rotational distribution in the OH(A) fragments peaks at lower values, N ′ pk = 14, than observed using single-photon excitation at 123.6 nm, where N ′ pk = 20.

Measurements of vector correlations in bimolecular reactions by laser-pump and probe techniques

Abstract Velocity-aligned, superthermal atoms generated via polarised molecular photodissociation have been used to investigate vector correlations of bimolecular reactions. Doppler-resolved, polarised laser-probe techniques can measure correlations between k (the reagents relative-velocity vectors), k ′ (the reaction products velocity vectors) and J ′ (the products rotational angular momenta using a simple laboratory to centre-of-mass frame transformation. This approach to the study of the dynamical stereo-chemistry of chemical reactions is illustrated by two examples, O( 1 D)+N 2 O→NO+NO and O( 3 P)+CS→CO+S.

Peripheral chemical reactions

Abstract Forward scattering of the products of direct exoergic atom transfer reactions is proposed as an indication for a peripheral attraction: a reaction in which an atom at the periphery of the reactants is abstracted. Model computations for the O( 1 D) + N 2 O reaction are used to illustrate the proposed mechanism. The opacity function has a peak at higher impact parameters which is correlated with the forward scattering. Potential energy surfaces which do not manifest peripheral attraction lead to backwards scattering of the products.

Vibrational relaxation of KrF* and XeCl* by rare gases

A steady-state, chemiluminescence technique has been used to measure effective rate constants for vibrational relaxation as a function of vibrational level for KrF* in collisions with He, Ne, and Ar and XeCl* with Ar. The effective rate constants reported include contributions to relaxation due to intersystem crossing between theB andC states, in addition to direct relaxation within theB state. The relevance of these results to the understanding of previous measurements in KrF and XeCl lasers is discussed.

Chemiluminescence polarization measurements under supersonic beam-gas conditions

This paper develops the feasibility of extracting useful dynamical inforamtion from chemiluminescence polarization measurements carried out under beam-gas conditions rather than crossed beam conditions in order to achieve higher signal levels. It extends the previous work of Prisant et al., 1981, J. chem. Phys., 55, 2222, to include supersonic beam conditions. Classical analysis gives a simple relationship between the polarization index, R, and the product rotational alignment, . The cylindrical symmetry of the beam-gas configuration allows factorization of the alignment into dynamical and kinematic factors. The kinematic factor takes into account the fluorescence depolarization which arises from the spread of relative velocity vectors about the beam axis. Its dependence on the mass and velocity of the collision partners is derived as a function of two dimensionless parameters, θ and ω, which characterize all beam-gas configurations. The results show that for much of the parameter space,...

Near threshold stereo-dynamics of molecular photodissociation: The visible and near ultraviolet photodissociation of H2O2

Abstract Sub-Doppler LIF probing of OH fragments generated from polarised laser photodissociation of H2O2 at wavelengths ranging from 308 to 465 nm, has established contributions from (at least) two single-photon electronic transitions in the very weak continuum absorption lying in this spectral range. The opportunity of studying the photodissociation at energies fairly close to threshold allows the stereo-dynamics to be probed in the long range parts of the repulsive electronically excited potentials, so providing a simple complementary strategy to resonance Raman luminescence and femtosecond kinetic spectroscopy.