Probing the O2 (aΔg1) 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(2) (a (1)Delta(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 ((1)D(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 (1)B(2)<--(1)A(1) excitation responsible for the Hartley band [for example, beta=1.40+/-0.12 for the O(2) (a (1)Delta(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 approximately 1/3 of this to be from excitation in the nu(3) asymmetric stretching mode. These observations are substantiated by the values of the beta(0) (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(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(2) (a (1)Delta(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(2) (a (1)Delta(g)) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O ((1)D(2)) atomic fragment with data for the O(2) (a (1)Delta(g); J=16-20) fragments.

Ab Initio Quantum Mechanical Study of the O(1D) Formation in the Photolysis of Ozone between 300 and 330 nm

“…In this region, the above mechanism is superseded by the cold band production of O( 1 D) in the spin-forbidden channel 4. Spin-forbidden dissociation in the Huggins band is well-characterized experimentally. − ,,, These studies, focused on the “tail of the tail” region of 325 nm ≤ λ ≤ 340 nm, detected pronounced peaks in the high resolution yields and bounded the average yield between 0.06 , and 0.08 or 0.12 . Yields of about 0.1 were also observed for the spin-forbidden channels 2 and 3, producing O( 3 P). , Dynamical aspects of the spin-forbidden dissociation remain poorly understood.…”

“…The mechanism, dating back to the model calculations of Adler-Golden et al, suggests that tiny Boltzmann populations of some states (υ 1 , υ 2 , υ 3 ) X are counterbalanced by a strong increase in the absorption from these states. Photofragment vector correlations measured in channel 5 indicate that transitions from vibrationally excited states are responsible for ∼30% of Φ O1D at 308 nm and for ∼100% at 311 nm . The temperature-independent component of Φ O1D at λ > 320 nm is sharply structured at high resolution, as demonstrated by the jet-cooled photofragment excitation spectra .…”

Photodissociation of Ozone from 321 to 329 nm: The Relative Yields of O(3P2) with O2(X 3Σg–), O2(a 1Δg) and O2(b 1Σg+)

“…While many investigators have studied the fall-off of the quantum yield for O( 1 D)+ O 2 ( a 1 Δ g ) starting at 310 nm, − , O’Keeffe et al − were the first to show the complexity of other product channels in the ozone dissociation wavelength range above 320 nm. These include: .25ex2ex normalO 3 → normalO 2 ( X Σ − g 3 ) + normalO ( P 3 ) ( 1 ) infix→ normalO 2 ( a 1 Δ g ) + normalO ( P 3 ) ( 2 ) infix→ normalO 2 ( b Σ + g 1 ) + normalO ( P 3 ...…”

Frequency modulated circular dichroism spectroscopy: application to ICN photolysis

“…This velocity dependence was confirmed and characterized for a wide range of N in v = 0, for both 248 and 266 nm photolysis, by Costen and Hall using FM Doppler spectroscopy [7], These experiments demonstrated that single rotational levels of CN produced in the I and I* channels did indeed have opposite orientations, and that both the integral and Doppler-resolved orientations could be described by a single parameter. [38][39][40]. The molecular frame orientation in these systems integrates to zero in the laboratory frame, and is undetectable with one-photon Doppler absorption spectroscopy, although it can be observed with REMPI time-of-flight, Doppler-resolved LIF with polarization analyzed emission, or ion-imaging methods.…”