Showing papers by "Yutaka Matsumi published in 1998"

Photochemistry of Ozone: Surprises and Recent Lessons

Abstract: The hydroxyl radical OH is nature9s detergent--it oxidizes pollutants and removes them from the atmosphere. Hydroxyl is created in the photolysis of atmospheric ozone, and the mechanisms and rates of production of OH are therefore very important. In their commentary, Ravishankara et al. discuss recent research into the details of ozone photolysis, particularly the role of electronically excited oxygen atoms. Surprisingly, long wavelength ultraviolet solar radiation has a greater influence on ozone photodissociation than previously believed.

Wavelength and temperature dependence of the absolute O(1D) production yield from the 305–329 nm photodissociation of ozone

Abstract: O(1D) and O(3Pj) photofragments produced in the photodissociation of ozone in the wavelength range 305–329 nm both at 295 and 227 K have been detected directly using a technique of laser induced fluorescence (LIF) in the vacuum ultraviolet (vuv). Photofragment excitation (PHOFEX) spectra for both species have been measured by scanning the photodissociation laser wavelength whilst monitoring vuv-LIF at 115 nm [O(1D)] and 130 nm [O(3Pj)]. After applying suitable corrections for the relative detection sensitivities, suitably weighted combinations of these PHOFEX spectra were found to provide a quantitative match to the parent O3 absorption spectrum both at 295 and 227 K, thereby providing a method of determining both the wavelength and temperature dependence of the absolute O(1D) quantum yield, Φ1D(λ,T). Hot band excitation of internally excited O3 molecules and dissociation via the spin-allowed channel yielding O(1D)+O2(a 1Δg) products makes the dominant contribution to the quantum yield Φ1D(λ,T) in the wav...

The ultraviolet photodissociation of Cl2O at 235 nm and of HOCl at 235 and 266 nm

Abstract: The primary photochemistry of gas phase dichlorine monoxide (Cl2O) and of hypochlorous acid (HOCl) following excitation at 235 nm has been investigated using photofragment ion imaging to obtain the recoil velocity and angular distributions of the ground (2P3/2) and spin-orbit excited (2P1/2) atomic chlorine products. In the case of Cl2O, both Cl spin-orbit products exhibit angular distributions characterized by an anisotropy parameter, β=1.2±0.2, consistent with previous interpretations of the ultraviolet (UV) absorption spectrum of Cl2O which associate the broad intense absorption feature peaking at λ∼255 nm with excitation to a (bent) dissociative state of 1B2(C2v) symmetry. The recoil velocity distributions of the two Cl spin-orbit products are markedly different. The ground state atoms (which constitute >90% of the total Cl atom yield) are partnered by ClO fragments carrying significantly higher average levels of internal excitation. The slowest Cl atoms are most readily understood in terms of three body fragmentation of Cl2O to its constituent atoms. These findings are rationalized in terms of a model potential energy surface for the 1 1B2 state, which correlates diabatically with ClO(X) radicals together with a spin-orbit excited Cl atom, with efficient radiationless transfer to one (or more) lower energy surfaces at extended Cl-O bond lengths accounting for the dominance of ground state Cl atom fragments. The image of the ground state Cl atoms resulting from photolysis of HOCl at 235 nm is consistent with parent excitation via a transition for which the dipole moment is closely aligned with the Cl-O bond, followed by prompt dissociation (β=1.7±0.2) with the bulk of the excess energy partitioned into product recoil. Such conclusions are consistent with the results of laser induced fluorescence measurements of the OH(X) products resulting from 266 nm photodissociation of HOCl which reveal OH(X) products in both spin-orbit states, exclusively in their zero-point vibrational level, and carrying only modest levels of rotational excitation (well described by a Boltzmann distribution with Trot∼750±50 K).

Ion Fragment Imaging of the Photodissociation of Methyl Iodide Small Clusters at 266 nm.

Abstract: Photofragment iodine atoms and methyl radicals from the photodissociation of small clusters of methyl iodide at 266 nm were detected, using ion photofragment imaging spectroscopy. Three different components of the I+ image were observed with average laboratory translational energies of the I fragments, = 2, 16, and 25 kcal mol−1 and anisotropy parameters, β = 0, 2.0 ± 0.6, and 2.1 ± 0.5, respectively. Similarly, for CH3+three different components in the CH3+ image correspond to average laboratory translational energies, = 5, 25, and 38 kcal mol−1 and β = 0, 2.0 ± 0.2, and 2.2 ± 0.6, respectively. These results suggest the following reaction mechanisms; a) a CH3I chromophore in small clusters is excited to the A band (n,σ*) system of free CH3I, survives intact within the clusters and dissociates, b) dissociation of the CH3I moiety in the small clusters that have two adjacent iodine atoms results in the formation of slow photofragment iodine atoms and fast methyl radicals, and c) dissociatio...