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Title: Probing the O{sub 2} (a {sup 1}{delta}{sub g}) photofragment following ozone dissociation within the long wavelength tail of the Hartley band

Abstract

The technique of resonance enhanced multiphoton ionization (REMPI) has been used in conjunction with time-of-flight mass spectrometry (TOFMS), to investigate the dynamics of ozone photolysis in the long wavelength region of the Hartley band (301-311 nm). Specifically, both the translational anisotropy and the rotational angular momentum orientation of the O{sub 2} (a {sup 1}{delta}{sub g}; {nu}=0, J=16-20) fragments have been measured as a function of photolysis wavelength. Within this region, the thermodynamic thresholds for the formation of these products in combination with O ({sup 1}D{sub 2}) are approached and passed, and consequently these studies have allowed an investigation into the effects on the dynamics of slowing fragment recoil velocities and the increasing importance of vibrationally mediated photolysis. The determined {beta} parameters for all the J states probed follow a similar trend, decreasing from a value typical for the initial {sup 1}B{sub 2}(leftarrow){sup 1}A{sub 1} excitation responsible for the Hartley band [for example, {beta}=1.40{+-}0.12 for the O{sub 2} (a {sup 1}{delta}{sub g}; J=18) fragment], to a much lower value beyond the thermodynamic threshold for the fragment's production (for example, {beta}=0.63{+-}0.19 for the J=18 fragment following photolysis at 311 nm). This trend, similar to that observed when probing the atomic fragment inmore » a previous set of experiments, [Horrocks et al., J. Chem. Phys. 125, 133313 (2006); Denzer et al., Phys. Chem. Chem. Phys. 16, 1954 (2006)] is consistent with the photodissociation of vibrationally excited ozone molecules beyond the threshold wavelengths and we estimate {approx}1/3 of this to be from excitation in the {nu}{sub 3} asymmetric stretching mode. These observations are substantiated by the values of the {beta}{sub 0}{sup 2}(2,1) orientation moment measured, which for photolysis at 301 nm are negative, indicating that a bond opening mechanism provides the key torque for the departing O{sub 2} fragment. The orientation moment becomes positive again for photolysis beyond threshold, however, as the increasing impulsive dissociation again begins to dominate the nature of the rotation of the departing molecular fragment. In addition, a (2+2) REMPI scheme has been utilized to probe the O{sub 2} (a {sup 1}{delta}{sub g}) 'low' J fragments, where the majority of the population resides following photolysis within this region. The REMPI-TOFMS technique has been used to confirm the rotational character of a spectral feature through examination of the signal line shapes obtained using different experimental geometries. The dynamical information subsequently obtained, probing the 'low' J O{sub 2} (a {sup 1}{delta}{sub g}) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O ({sup 1}D{sub 2}) atomic fragment with data for the O{sub 2} (a {sup 1}{delta}{sub g}; J=16-20) fragments.« less

Authors:
; ;  [1]
  1. Physical and Theoretical Chemistry Laboratory, The University of Oxford, South Parks Road, Oxford, OX1 3QZ (United Kingdom)
Publication Date:
OSTI Identifier:
20991210
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 126; Journal Issue: 4; Other Information: DOI: 10.1063/1.2429656; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY; ANISOTROPY; DISSOCIATION; MASS SPECTRA; MASS SPECTROSCOPY; OXYGEN; OZONE; PHOTOCHEMISTRY; PHOTOIONIZATION; PHOTOLYSIS; PHOTON-MOLECULE COLLISIONS; TIME-OF-FLIGHT METHOD

Citation Formats

Horrocks, S. J., Ritchie, G. A. D., and Sharples, T. R. Probing the O{sub 2} (a {sup 1}{delta}{sub g}) photofragment following ozone dissociation within the long wavelength tail of the Hartley band. United States: N. p., 2007. Web. doi:10.1063/1.2429656.
Horrocks, S. J., Ritchie, G. A. D., & Sharples, T. R. Probing the O{sub 2} (a {sup 1}{delta}{sub g}) photofragment following ozone dissociation within the long wavelength tail of the Hartley band. United States. doi:10.1063/1.2429656.
Horrocks, S. J., Ritchie, G. A. D., and Sharples, T. R. Sun . "Probing the O{sub 2} (a {sup 1}{delta}{sub g}) photofragment following ozone dissociation within the long wavelength tail of the Hartley band". United States. doi:10.1063/1.2429656.
@article{osti_20991210,
title = {Probing the O{sub 2} (a {sup 1}{delta}{sub g}) photofragment following ozone dissociation within the long wavelength tail of the Hartley band},
author = {Horrocks, S. J. and Ritchie, G. A. D. and Sharples, T. R.},
abstractNote = {The technique of resonance enhanced multiphoton ionization (REMPI) has been used in conjunction with time-of-flight mass spectrometry (TOFMS), to investigate the dynamics of ozone photolysis in the long wavelength region of the Hartley band (301-311 nm). Specifically, both the translational anisotropy and the rotational angular momentum orientation of the O{sub 2} (a {sup 1}{delta}{sub g}; {nu}=0, J=16-20) fragments have been measured as a function of photolysis wavelength. Within this region, the thermodynamic thresholds for the formation of these products in combination with O ({sup 1}D{sub 2}) are approached and passed, and consequently these studies have allowed an investigation into the effects on the dynamics of slowing fragment recoil velocities and the increasing importance of vibrationally mediated photolysis. The determined {beta} parameters for all the J states probed follow a similar trend, decreasing from a value typical for the initial {sup 1}B{sub 2}(leftarrow){sup 1}A{sub 1} excitation responsible for the Hartley band [for example, {beta}=1.40{+-}0.12 for the O{sub 2} (a {sup 1}{delta}{sub g}; J=18) fragment], to a much lower value beyond the thermodynamic threshold for the fragment's production (for example, {beta}=0.63{+-}0.19 for the J=18 fragment following photolysis at 311 nm). This trend, similar to that observed when probing the atomic fragment in a previous set of experiments, [Horrocks et al., J. Chem. Phys. 125, 133313 (2006); Denzer et al., Phys. Chem. Chem. Phys. 16, 1954 (2006)] is consistent with the photodissociation of vibrationally excited ozone molecules beyond the threshold wavelengths and we estimate {approx}1/3 of this to be from excitation in the {nu}{sub 3} asymmetric stretching mode. These observations are substantiated by the values of the {beta}{sub 0}{sup 2}(2,1) orientation moment measured, which for photolysis at 301 nm are negative, indicating that a bond opening mechanism provides the key torque for the departing O{sub 2} fragment. The orientation moment becomes positive again for photolysis beyond threshold, however, as the increasing impulsive dissociation again begins to dominate the nature of the rotation of the departing molecular fragment. In addition, a (2+2) REMPI scheme has been utilized to probe the O{sub 2} (a {sup 1}{delta}{sub g}) 'low' J fragments, where the majority of the population resides following photolysis within this region. The REMPI-TOFMS technique has been used to confirm the rotational character of a spectral feature through examination of the signal line shapes obtained using different experimental geometries. The dynamical information subsequently obtained, probing the 'low' J O{sub 2} (a {sup 1}{delta}{sub g}) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O ({sup 1}D{sub 2}) atomic fragment with data for the O{sub 2} (a {sup 1}{delta}{sub g}; J=16-20) fragments.},
doi = {10.1063/1.2429656},
journal = {Journal of Chemical Physics},
number = 4,
volume = 126,
place = {United States},
year = {Sun Jan 28 00:00:00 EST 2007},
month = {Sun Jan 28 00:00:00 EST 2007}
}
  • The translational anisotropy and rotational angular momentum polarization of a selection of rotational states of the O{sub 2} (a {sup 1}{delta}{sub g}; v=0) photofragment formed from ozone photolysis at 248, 260, and 265 nm have been determined using the technique of resonance enhanced multiphoton ionization in combination with time of flight mass spectrometry. At 248 nm, the dissociation is well described as impulsive in nature with all rotational states exhibiting similarly large, near-limiting values for the bipolar moments describing their angular momentum alignment and orientation. At 265 nm, however, the angular momentum polarization parameters determined for consecutive odd and evenmore » rotational states exhibit clear differences. Studies at the intermediate wavelength of 260 nm strongly suggest that such a difference in the angular momentum polarization is speed dependent and this proposal is consistent with the angular momentum polarization parameters extracted and reported previously for longer photolysis wavelengths [G. Hancock et al., Phys. Chem. Chem. Phys. 5, 5386 (2003); S. J. Horrocks et al., J. Chem. Phys. 126, 044308 (2007)]. The alternation of angular momentum polarization for successive odd and even J states may be a consequence of the different mechanisms leading to the formation of the two O{sub 2} (a {sup 1}{delta}{sub g}) {lambda} doublets. Specifically, the involvement of out of plane parent rotational motion is proposed as the origin for the observed depolarization for the {delta}{sup -} relative to the {delta}{sup +} state.« less
  • The ability to routinely acquire measurements of the ozone density profile in the mesosphere and lower thermosphere is important for use in chemical-dynamical models of the middle atmosphere. Zenith measurements of the O[sub 2]([sup 1][Delta][sub g]) (0,1) band emission rate in the evening twilight were acquired near the spring equinox of 1982 with a Michelson interferometer from London, Ontario, Canada. Knowledge of the change of the O[sub 2]([sup 1][Delta][sub g])(0,1) band emission rate at twilight can be related directly to ozone density, since ozone destruction by sunlight is the primary source of O[sub 2]([sup 1][Delta][sub g]) in the atmosphere. Measurementsmore » and calculations have shown that a secondary peak in the ozone density often exists in the middle atmosphere. A model has been developed to infer the ozone profile in the middle atmosphere by simultaneously solving the time-dependent chemistry of the molecular oxygen atmospheric and atmospheric-IR bands and O(ID) during twilight. Calculations are presented which show the effect of a secondary peak in the ozone density at various heights on the O[sub 2]([sup 1][Delta][sub g]) (0,1) band emission rate during twilight. The model is used to demonstrate that the London measurements are consistent with an ozone profile with a secondary peak at 85-90 km. 17 refs., 7 figs.« less
  • The authors describe the use of CARS spectroscopy of photofragments as a means of obtaining extensive spectroscopic constants and accurate potential curves. UV photodissociation of ozone yields O/sub 2/(a/sup 1/..delta../sub g/) in nu = 0-6 and J = 0-50. CARS spectra of the a/sup 1/..delta../sub g/ photofragment under collision-free conditions are analyzed to yield vibrational constants through omega/sub e/z/sub e/ and rotational constants through ..gamma../sub e/. The constants are used to compute classical turning points for the vibrational motion, via the RKR analysis. The RKR points are fit to a simple analytical potential function, the extended Rydberg function, that providesmore » an excellent description of the O/sub 2/(a/sup 1/..delta../sub g/) potential energy curve.« less
  • It is well-known that the presence of molecular oxygen (/sup 3/..sigma../sub g//sup -/O/sub 2/) in a variety of organic solvents causes an often substantial red shift in the solvent absorption spectrum. This extra, broad absorption feature is reversibly removed by purging the solvent with nitrogen gas. Mulliken and Tsubomura assigned the oxygen-dependent absorption band to a transition from a ground state solvent-oxygen complex to a solvent-oxygen charge transfer (CT) state (sol/sup .+/O/sub 2//sup .-/). In addition to the broad Mulliken CT band, there are, often in the same spectral region, distinct singlet-triplet transitions (T/sub 1/ reverse arrow S/sub 0/) whichmore » are enhanced by molecular oxygen (/sup 3/..sigma../sub g//sup -/O/sub 2/). Since both of these solvent-oxygen cooperative transitions may result in the formation of reactive oxygenating species, singlet molecular oxygen (/sup 1/..delta../sub g/O/sub 2/) and/or the superoxide ion (O/sub 2//sup .-/), it follows that recent studies have focused on unsaturated hydrocarbon oxygenation subsequent to the irradiation of the oxygen-induced absorption bands in both the solution phase and cryogenic (10 K) glasses. In these particular experiments, oxygenated products characteristic of both /sup 1/..delta../sub g/O/sub 2/ and O/sub 2//sub .-/ were obtained, although the systems studied appeared to involve the participation of one intermediate at the exclusion of the other. In this communication, the authors provide, for the first time, direct spectroscopic evidence for the formation of /sup 1/..delta../sub g/O/sub 2/ following a solvent-oxygen (/sup 3/..sigma../sub g//sup -/O/sub 2/) cooperative absorption. They have observed, in a time-resolved experiment, a near-IR luminescence subsequent to laser excitation of the oxygen-induced absorption bands of mesitylene, p-xylene, o-xylene, toluene, and benzene at 355 nm and 1,4-dioxane at 266 nm. They suggest that this signal is due to /sup 1/..delta../sub g/O/sub 2/ phosphorescence.« less
  • Singlet molecular oxygen ({sup 1}{Delta}{sub g}O{sub 2}) phosphorescence ({sup 3}{Sigma}{sub g}{sup {minus}}O{sub 2} {l arrow} {sup 1}{Delta}{sub g}O{sub 2}: 1270 nm) has been observed in a time-resolved experiment subsequent to pulsed UV laser irradiation of the oxygen ({sup 3}{Sigma}{sub g}{sup {minus}}O{sub 2})-organic molecule charge-transfer bands of liquid aromatic hydrocarbons (mesitylene, p-xylene, o-xylene, toluene, benzene), ethers (tetrahydrofuran, 1,4-dioxane, glyme, diglyme, triglyme), alcohols (methanol, propanol), and aliphatic hydrocarbons (cyclohexane, cyclooctane, decahydronaphthalene). Although {sup 1}{Delta}{sub g}O{sub 2} could originate from a variety of different processes in these oxygenated solvent systems, we have used the results of several independent experiments to indicate that anmore » oxygen-solvent charge-transfer (CT) state is the {sup 1}{Delta}{sub g}O{sub 2} precursor. Other transient species have also been observed in time-resolved absorption experiments subsequent to pulsed UV irradiation of the oxygen-solvent CT bands. Some of these molecular transients, or species derived from these intermediates, may be responsible for an observed increase in the rate of {sup 1}{Delta}{sub g}O{sub 2} decay under certain conditions.« less