Andrew J. Alexander

Measurements of Cl-atom photofragment angular momentum distributions in the photodissociation of Cl2 and ICl

We have studied the complete Cl-atom molecular-frame photofragment angular momentum distributions from the photodissociation of Cl2 and ICl in the 320–560 nm region using time-of-flight mass spectroscopy with laser detection. The experimental signals were analyzed using the polarization-parameter formalism described in the preceding paper. These experiments study three distinct cases. The first case is the 470 nm dissociation of Cl2 through the B 3Π0+u state accessed via a parallel transition, yielding Cl-atom photofragments with polarizations described by the single parameter a0(2)(∥)=−0.7±0.2. The second case is the 320 nm dissociation of Cl2 through the C 1Π1u state accessed via a perpendicular transition, yielding Cl-atom photofragments with polarizations described by the two parameters a0(2)(⊥)=−0.50±0.10 and a2(2)(⊥)=−0.32±0.06. The third case is the dissociation of ICl in the 490–560 nm region in which dissociative states of both parallel and perpendicular character are accessed. In this wavelength...

Spatial Control of Crystal Nucleation in Agarose Gel

Spatial and temporal control of crystal nucleation is demonstrated by nonphotochemical laser-induced nucleation of an aqueous agarose gel prepared with supersaturated potassium chloride. The location of nucleation was controlled by means of an optical mask; crystals were only observed in the area exposed to near-infrared laser radiation. The dependence of nucleation on laser power was measured, and the results suggest that the agarose gel reduces the effective supersaturation of the aqueous potassium chloride.

Cl+HD(v=1; J=1,2) reaction dynamics: Comparison between theory and experiment

Vibrationally state-resolved differential cross sections (DCS) and product rotational distributions have been measured for the Cl+HD(v=1, J=1)→HCl(DCl)+D(H) reaction at a mean collision energy of 0.065 eV using a photoinitiated reaction (“photoloc”) technique. The effect of HD reagent rotational alignment in the Cl+HD(v=1, J=2) reaction has also been investigated. The experimental results have been compared with exact quantum mechanical and quasiclassical trajectory calculations performed on the G3 potential energy surface of Allison et al. [J. Phys. Chem. 100, 13575 (1996)]. The experimental measurements reveal that the products are predominantly backward and sideways scattered for HCl(v′=0) and HCl(v′=1), with no forward scattering at the collision energies studied, in quantitative agreement with theoretical predictions. The experimental product rotational distribution for HCl(v′=1) also shows excellent agreement with quantum-mechanical calculations, but the measured DCl+H to HCl+D branching ratio is ne...

Oriented chlorine atoms as a probe of the nonadiabatic photodissociation dynamics of molecular chlorine

Molecular chlorine was photolyzed using circularly polarized radiation at 310 and 330 nm, and orientation moments of the chlorine-atom Cl(2Pj) photofragment distributions were measured by resonance enhanced multiphoton ionization using circularly polarized light with Doppler resolution. The product atoms were found to be strongly oriented in the laboratory as a result of both incoherent and coherent dissociation mechanisms, and the orientation moments contributed by each of these mechanisms have been separately measured. The experimental results can be explained by nonadiabatic transitions from the C 1Π1u state to higher states of Ω=1u symmetry, induced by radial derivative coupling. Ab initio calculations indicate strong Rosen–Zener–Demkov noncrossing-type radial derivative couplings between states of 1u symmetry. The observed angular distribution (β parameter) indicates that 88% of Cl*(2P1/2) fragments produced at 310 nm originate from a perpendicular transition to the C state. The orientation measureme...

Product state-resolved stereodynamics: quasiclassical study of the reaction O(1D) + HD → OH(OD) (ν′, j′) + D(H)

Abstract State-resolved integral and differential cross sections and product energy partitioning have been calculated for the reaction O ( 1 D )+ HD (ν = 0, j = 0–1) → OH(OD) + D(H) by means of quasi-classical trajectory (QCT) calculations at a collision energy of 0.197 eV (19 kJ mol −1 ) using the potential energy surface of Schinke and Lester. The results indicate the dominance of an insertion mechanism producing HOD intermediates and reveal a strong dependence of the differential scattering cross section on the product quantum state. This dependence has been related to the product state-resolved opacity functions and the estimated lifetimes of the reactive collisional trajectories.

Photofragment angular momentum polarization from dissociation of hydrogen peroxide near 355 nm

Hydrogen peroxide (H2O2) was photodissociated at around 355 nm, using both linearly and circularly polarized light. OH(2Π) rotational distributions, spin–orbit branching ratios, lambda doublet populations, and angular momentum J polarization in the laboratory and molecule frames were measured by polarized laser probing of the products using laser induced fluorescence. The effects of dynamical torsion and parent molecule bending vibrations on product rotational alignment is discussed, and evidence supporting preferential dissociation of ground-state molecules far from the equilibrium configuration is presented. Possible mechanisms for orientation of product angular momentum in the molecule frame are discussed, and evidence is presented that interference occurs between OH molecules dissociating via the A and B electronic states of H2O2.

An experimental and quasiclassical study of the product state resolved stereodynamics of the reaction O(1D2) + H2(υ = 0) → OH (X2Π32; υ = 0, N, f) + H

The product state resolved dynamics of the photon initiated reaction N2O+hν(193 nm) → O(1D2)+N2, O(1D2)+H2(υ = 0) → OH(X2Π32; υ = 0, N = 5, f) + H, conducted at 300 K and at a mean collision energy of 10 kJ mol−1, have been studied experimentally and computationally. Product state-resolved differential cross-sections of the scattered OH fragments have been probed through analysis of their polarised, Doppler resolved laser induced fluoresence spectra and compared with the predictions of quasiclassical trajectory (QCT) computations conducted on the Schinke-Lester ab initio ground state surface (SL1). The experiments confirm the QCT prediction of strongly peaked backward scattering in the channels generating OH(υ = 0, N = 5). The two individually probed Λ-doublet components display identical angular distributions. Although most collision complexes are estimated to have lifetimes less than their mean rotational periods, the total differential cross-section, summed over all product channels, is predicted to display forward and backward peaks with a moderate bias in favour of backward scattering.

Structure of monolayer dye films studied by Brewster angle cavity ringdown spectroscopy

Cavity ringdown spectroscopy of films of oxazine and malachite green dyes coated on thin borosilicate substrates is described. The method involves insertion of the substrate into the cavity at Brewsters angle to reduce reflection losses. Measured absorption features of the adsorbed dyes in the visible region 580 to 700 nm show the presence of dye aggregates. The measurement of sub-monolayers of oxazine 1 as low as 0.032 monolayers with an absorption loss per pass of 5.79 × 10−5 is demonstrated.

Comparison of near-threshold reactivity of ground-state and spin-orbit excited chlorine atoms with methane

A 4:1 mixture of CH4 and BrCl diluted in He are coexpanded into a vacuum chamber and the reaction of methane with atomic chlorine is initiated by photolysis of BrCl. Near 420 nm, the resulting mixture of ground- and excited-state chlorine atoms have spatial anisotropies of βphot=−0.7 for the Cl(2P3/2)+Br channel and βphot=+1.8 for the Cl*(2P1/2)+Br channel. The speed-dependent spatial anisotropy βrxn(ν) of the CH3(ν=0) reaction product is detected by 2+1 resonance-enhanced multiphoton ionization. Our results indicate that the Cl*+CH4 reaction is unimportant in the near-threshold collision energy range of 0.13–0.16 eV, whereas the reaction with ground-state Cl atoms with CH4 excited with one quantum in the ν2 (torsion) or ν4 (bending) mode is dominant.

Photofragment angular momentum distributions in the molecular frame. II. Single state dissociation, multiple state interference, and nonaxial recoil in photodissociation of polyatomic molecules

We present an a(q) (k)(s) polarization-parameter model to describe product angular momentum polarization from the one-photon photodissociation of polyatomic molecules in the molecular frame. We make the approximation that the final photofragment recoil direction is unique and described by the molecular frame polar coordinates (alpha,phi(i)), for which the axial recoil approximation is a special case (e.g., alpha=0). This approximation allows the separation of geometrical and dynamical factors, in particular, the expression of the experimental sensitivities to each of the a(q) (k)(s) in terms of the molecular frame polar angles (chi(i),phi(i)) of the transition dipole moment mu(i). This separation is applied to the linearly polarized photodissociation of polyatomic molecules (asymmetric, symmetric, and spherical top molecules are discussed) and to all dissociation mechanisms that satisfy our recoil approximation, including those with nonaxial recoil and multiple state interference, giving important insight into the geometrical properties of the photodissociation mechanism. For example, we demonstrate that the ratio of polarization parameters A(0) (k)(aniso)/A(0) (k)(iso)=beta (where beta is the spatial anisotropy parameter) is an indication that the dynamics can be explained by a single dissociative state. We also show that for asymmetric top photodissociation, the sensitivity to the a(1) (k)(s) parameters, which can arise either from single-surface or multiple-surface interference mechanisms, is nonzero only for components of the transition dipole moments within the v-d plane of the recoil frame.