Ozone Photodissociation in the Singlet Channel at 226 nm

Abstract

We report the rotational state distribution and vector correlations of the O2(a 1Δg, v = 0) fragments arising from the 226 nm photodissociation of jet-cooled O3. Consistent with previously reported trends, the rotational distribution is shifted to higher rotational states with decreasing wavelength. We observe highly suppressed odd rotational state populations due to a strong Λ-doublet propensity. The measured rotational distribution is in agreement with classical trajectory calculations for the v = 0 products, although the distribution is slightly narrower than predicted. The spatial anisotropy follows the previously observed trend of decreasing β with increasing photon energy with β = 0.72 ± 0.14 for v = 0, j = 38. As expected for a triatomic molecule, the v–j correlation is consistent with v perpendicular to j, but the measured correlation is nonlimiting due, in part, to rotational and translational depolarization. The j-dependent line width of the O2(a 1Δg) REMPI spectrum is also discussed in connection with the lifetime of the resonant O2(d 1Πg) state due to predissociation via the II 1Πg valence state.

Virtual Issue on Photodissociation: From Fundamental Dynamics and Spectroscopy to Photochemistry in Planetary Atmospheres and in Space

“…Experimental strategies to study the intricate mechanisms of photodissociation often focus on isolated gas-phase molecules to avoid the complexities introduced by a surrounding environment such as a solvent. The preferred design for experiments of this type uses a molecular beam of internally cooled molecules, intersected by two pulsed laser beams to excite the molecules and probe the consequences of the excitation, and information-rich detection methods such as velocity map imaging (VMI), ,,,− H Rydberg atom photofragment translational spectroscopy (HRA PTS), , or photoelectron spectroscopy. , These approaches can be divided into two broad categories. Ultrafast time-resolved methods use femtosecond laser spectroscopy methods to observe the real-time changes in molecular structures and electronic states following the initial photoexcitation step .…”

“…Notably, the James Webb Space Telescope is now providing IR spectroscopic data on the compositions of the atmospheres of exoplanets outside our solar system. In addition to wavelength-dependent absorption cross-sections derived from spectroscopic studies, laboratory measurements can quantify the branching between competing photochemical pathways together with the associated photodissociation quantum yields, for which accurate new methods of determination have recently been proposed. , A combination of such data is needed in order to calculate rates of photolytic loss and production of reactive intermediates in environments such as the Earth’s atmosphere and the interstellar medium and, hence, to establish the influence of photochemical reactions on the chemistry of these regions. , …”

“…38,39 A combination of such data is needed in order to calculate rates of photolytic loss and production of reactive intermediates in environments such as the Earth's atmosphere and the interstellar medium and, hence, to establish the influence of photochemical reactions on the chemistry of these regions. 1,23 The photochemistry occurring in the Earth's troposphere and much of the stratosphere is driven by photodissociation at UV or visible wavelengths. However, photochemistry initiated by VUV photons becomes important in the upper regions of planetary atmospheres and further out into space, involving a variety of otherwise unreactive molecules including CO, H 2 O, OCS, and complex organic compounds such as nitriles and alcohols.…”

“…For example, ozone is produced photochemically in the stratosphere and troposphere by release of O atoms from O 2 or NO 2 . Subsequent photodissociation of ozone makes reactive species such as O­( 1 D) atoms or OH radicals . Indeed, most of the oxidation pathways for biogenic and anthropogenic volatile organic compounds in the troposphere are initiated by these or other photodissociation processes.…”

“…Subsequent photodissociation of ozone makes reactive species such as O( 1 D) atoms or OH radicals. 1 Indeed, most of the oxidation pathways for biogenic and anthropogenic volatile organic compounds in the troposphere are initiated by these or other photodissociation processes. Shorter wavelength vacuum UV radiation does not penetrate to low altitudes in the Earth's atmosphere, but VUV light emitted by the sun and other stars does play a role in the photochemistry of planetary atmospheres, cold molecular clouds, and other regions of interstellar space.…”