Nori Taniguchi

Determination of the heat of formation of O3 using vacuum ultraviolet laser-induced fluorescence spectroscopy and two-dimensional product imaging techniques

Two different techniques, vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy and two-dimensional (2D) ion counting product imaging, have been used to determine the bond energy for the dissociation of jet-cooled O3 into O(1D)+O2(a 1Δg). The photofragment excitation (PHOFEX) spectrum for O(1D) products is recorded by detecting the VUV-LIF signal associated with the 3s 1D0–2p 1D transition at 115.22 nm while scanning the photolysis laser wavelength between 305 and 313 nm. A clear cut-off corresponding to the appearance threshold into O(1D)+O2(a 1Δg) is observed in this PHOFEX spectrum. The 2D image of the O(1D) products from the O3 photolysis near 305 nm is measured using an ion-counting method, with the detection of O(1D) atoms by [2+1] resonance enhanced multiphoton ionization (REMPI) at 205.47 nm. The kinetic-energy distribution obtained from the 2D image shows rotational structure due to the O2(a 1Δg,v″=0) fragment. The bond energy into O(1D)+O2(a 1Δg) has been obtained from the rotatio...

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

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...

Photofragment excitation spectrum for O(1D) from the photodissociation of jet-cooled ozone in the wavelength range 305–329 nm

The photofragment excitation (PHOFEX) spectrum for O(1D) production from the photolysis of ozone under supersonic free-jet conditions was measured, scanning the photodissociation wavelength in the region of 305–329 nm and probing the O(1D) atoms by vacuum ultraviolet laser induced fluorescence at 115.2 nm. The bond dissociation energy D00(O2–O) was determined to be 101.53±0.25 kJ mol−1 from the cut-off wavelength in the PHOFEX spectrum for the photodissociation of jet-cooled ozone (Trot≈5 K) to O(1D)+O2(a 1Δg). The cut-off wavelength for vibrationally hot band excitation to the dissociative continuum of O(1D)+O2(a 1Δg) was also observed in the PHOFEX spectrum. It was found that the active mode for the hot band excitation was the antisymmetric stretching mode ν3 in the ground electronic state of ozone. Sharp peaks corresponding to vibrational bands in the Huggins system were also observed in the PHOFEX spectrum of the O(1D) atoms produced via the spin-forbidden dissociation process, O(1D)+O2(X 3∑g−). The s...

Near ultraviolet photodissociation of allene and propyne

The fragmentation dynamics of allene and propyne molecules following photoexcitation at 203.3, 209.0 and 213.3 nm have been investigated by H (Rydberg) atom photofragment translational spectroscopy methods. Contrary to conclusions reached in previous photochemical studies of these molecules, at a photolysis wavelength of 193 nm, we find the translational energy spectra associated with the H atom product forming channel in both molecules to be essentially identical, and to have a form that is reproduced well by an approximate statistical model that assumes population of all possible vibrational states of the H 2 CCCH partner. Such behavior can be most readily accommodated by assuming that, for both molecules, at the excitation energies used in the present work, internal conversion to, and isomerization on, the ground state potential energy surface precedes fragmentation.

TRANSLATIONAL ENERGY AND ANGULAR DISTRIBUTIONS OF O(1D) AND O(3PJ) FRAGMENTS IN THE UV PHOTODISSOCIATION OF OZONE

Abstract Nascent O( 1 D ) and O( 3 P ) photoproducts from the photodissociation of O 3 are state-selectively detected in a flow cell, using a technique of vacuum-ultraviolet laser-induced fluorescence. Doppler profiles of the O( 1 D ) atoms are measured in the photodissociation of O 3 at 193, 230, and 266 nm, and those of the O( 3 P ) atoms at 193, 266 and 308 nm. The translational energy and the speed-resolved spatial angular distributions are derived from the Doppler profiles, from which photodissociation dynamics at each wavelength is discussed. The photodissociation dynamics in the Hartley band is characterized by the spin-allowed photodissociation processes from the O 3 ( 1 B 2 ) state. Below the blue end of the Hartley band, the photoexcitation process is mainly perpendicular transition. Multiple peaks observed in the translational energy distribution of O( 1 D ) and O( 3 P ) photoproducts from the photodissociation of O 3 at 193 nm are attributed to multiple dissociation processes accessible energetically.

Electronic quenching of O(1D) by collisions with O2: A theoretical study in a collinear case

Potential energy curves of triplet states for collinear O–O2 were calculated by ab initio CASSCF and MRSDCI methods. We found a pseudocrossing between 1 3Σ− (6 3A″) and 2 3Σ− (7 3A″) states at long O–O2 separation. The electronic quenching reaction, O(1D)+O2(X 3Σg−)→O(3P)+O2(b 1Σg+), is dominated by the nonadiabatic transition via the pseudocrossing. The collision energy dependence of the quenching reaction probability, which is evaluated by Zhu and Nakamura’s formula, is found to be in good agreement with experiment.