Extending Studies of Ozone Photodissociation to Near and Far UV Wavelengths in the Huggins and Hartley Bands
Abstract
Understanding ozone photodissociation dynamics is a key component to accurately modeling atmospheric chemistry, and the energy partitioning into the fragments and its wave-length dependence can be used to assess the potential energy surfaces used in calculations. Experimental REMPI spectra and ion images of the O2 fragment were measured following photodissociation of O3 in the near and far UV wavelengths. The O2(a^1∆g) fragment was probed following 226 nm dissociation to extend previous studies on the Λ-doublet model to shorter wavelengths in the Hartley band and to extend studies of the j-dependent predisso-ciation lifetime of the resonant O2(d^1Πg) state used in the REMPI scheme. Following 226 nm dissociation, the O2(a^1∆g) fragment is highly rotationally excited and the odd rotational states are suppressed. The rotational distribution is consistent with previous measurements at longer dissociation wavelengths, with the distribution shifting to higher rotational states and the suppression of the odd states increasing with increasing dissociation energy. The vector correlations support a parallel µ-v correlation as expected for a parallel transition from the X state of O3 to the B state, and the v-j correlation is perpendicular as expected for the dissociation of a triatomic molecule. The µ-v correlation at 226 nm is weaker than the correlation measured at longer wavelengths, which is consistent with the previously reported trend and attributed to a faster recoil velocity and less influence from the restoring force in the bending potential.
In the Huggins band, longer wavelengths excite O3 to the bound region of the B state and spin-forbidden dissociation processes dominate. Huggins band dissociation has not been studied in as much detail as the Hartley band. Both the O2(a^1∆g) and O2(b^1Σ+) fragments were probed following photodissociation in the Huggins band. The REMPI spectrum of O2(a^1∆g) following 320 nm dissociation exhibits enhanced odd rotational states which is the opposite of the trend observed in the Hartley band and is attributed to greater coupling between the B state and ^3A′′ states correlating to odd rotational states of O2 and weaker coupling between the B state and ^3A′ states leading to even rotational states of O2.
Ion image speed distributions at 320 nm and the REMPI spectrum at 330 nm both support a broad rotational state distribution of O2(a^1∆g, v = 0). A 2D-REMPI spectrum collected near 330 nm allowed the simultaneous measurement of the rotational distributions of v=0-4 of the O2(b^1Σ+) fragment and the high rotational states of O2(a^1∆g, v = 0). In contrast to the broad rotational distribution of O2(a^1∆g), the O2(b^1Σ+) distribution is narrow and largely unchanging with vibrational state. The simultaneous measurements of five vibrational levels of O2(b^1Σ+) allowed the vibrational state distribution to be obtained as well. The distribution peaks at v=0 and decreases monotonically with increasing vibrational state. New measurements of the O2 fragment following dissociation at 320 and 330 nm will provide a stringent test of the potential energy surfaces as calculations are performed on Huggins band dissociation dynamics.
In the REMPI schemes utilized, O2(a^1∆g) and O2(b^1Σ+) are resonantly excited to the O2(d^1Πg) Rydberg state which is predissociated by the O2(II^1Πg) valence state. The spectral linewidth reflects the lifetime of the O2(d^1Πg) state which varies significantly with ro-vibrational state. The predissociative lifetimes and corresponding linewidths were calculated and the potentials used in the calculations were optimized so the calculated linewidths are consistent with experimental measurements. The j-dependence of the linewidths for several vibrational levels of the O2(d^1Πg) state are discussed.
Citation
Aardema, Megan Nicole (2023). Extending Studies of Ozone Photodissociation to Near and Far UV Wavelengths in the Huggins and Hartley Bands. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /199864.