1. Vibrational state-dependent predissociation dynamics of ClO (A 23/2): Insight from final correlated state branching ratios
Kristin S. Dooley1, Justine N. Geidosch1, Hahkjoon Kim1, Gerrit C. Groenenboom2, and Simon W. North1
1. Department of Chemistry, Texas A&M University, P. O. Box 30012, College Station, Texas 77842, USA.
2. Institute of Theoretical Chemistry, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
OVERALL BRANCHING RATIO
RECENT IO BDE DETERMINATION
INTRODUCTION
THEORETICAL TREATMENT
It is well established that chlorine plays an important role in the destruction cycle of stratospheric ozone. One important intermediate species in this cycle is ClO which is the focus of our current work. ClO is also a benchmark system for comparison of experiment and theory for open-shelled halogen oxides. Our work focuses on characterizing the role of the low-lying electronic states in predissociating the A 23/2 state using Velocity Map Ion Imaging to analyze the nascent photofragments. There have been several theoretical investigations of the ClO excited states: notably Toniolo et al. (Toniolo, A., Persico, M, and Pitea, D., J. Chem. Phys., 2000, 112, 2790) and Orr-Ewing and co-workers (I. C. Lane, W. H. Howie, and A. J. Orr-Ewing, Phys. Chem. Chem. Phys., 1999, 1, 3087). Understanding the predissociation dynamics of small open-shell species is a challenge for modern theory, and our measurements provide a rigorous test for these calculations.
Orr-Ewing and co-workers narrowed the 17 candidates for predissociation to those whose dominant configuration differed by only one spin orbital from the dominant configuration of the A(2Ω) state. The authors then used Fermi’s Golden Rule calculations to calculate the v´-dependent rates of predissociation, and adjusted their weightings to best fit experimental predissociation lifetimes.
The study by Orr-Ewing and co-workers suggests that three of the states (1 4+, - , and 3 2) are dominant in the predissociation of v´= At higher vibrational levels, extension of this model is less accurate as there are several excited states that show similar v´-dependencies in this region and the lifetime data is not enough to distinguish between these states as the linewidth data is only affected by couplings inside the Franck-Condon region. Data such as product state distributions can provide insight into both the couplings within the Franck-Condon region, but also reflect the diabatic interactions of the repulsive states at large internuclear distances. Hence, our data would supplement the linewidth data and help determine which states are contributing to the predissociation, thus providing a rigorous test of theory.
The v´-dependent branching ratios were calculated in the limits of the diabatic and adiabatic models. The adiabatic limit is appropriate for systems exibiting large coupling and slow recoil velocities. The opposite limit, known as the diabatic or sudden model, corresponds to the non-Born Oppenheimer limit.
Overall Cl (2P1/2) Branching
We recently studied the photodissociation dynamics of the IO radical at nm, corresponding to the A23/2 - X23/2 (1-0) bandhead, using velocity map ion imaging. The radicals were generated using a late-mixing dual valve photolytic reactor source, shown below. I2 and O3, both in He, flowed through separate pulsed valves. They mixed just before being injected into a quartz tube where they were irradiated with the 248 nm output of an excimer laser.
The figure on the right shows the total branching ratio of the Cl (2P1/2) species as a function of vibrational band. As you can see from the figure, the diabatic model follows the experimental overall Cl branching much more accurately than the adiabatic model. The model is very good especially at the higher vibrational bands (v´ = 5-11), but deviates at v´= 3 and 4.
ClO UV SPECTROSCOPY
FRONT VIEW
BACK VIEW
The spectroscopy of ClO, in particular the characterization of the bound X 23/2 and A 23/2 states, has been well studied. The transitions between the vibrational levels of the X 2 state to the bound vibrational levels of the A 2 state are responsible for the resolved bands at wavelengths between 316 nm and the dissociation threshold at ±.01 nm. Below 263 nm, the spectrum is characterized by a broad continuum that terminates near 220 nm. In this system, there are 17 dissociative electronic states that lead to predissociation of the A 2 state.
Adiabatic Correlation Diagram
EXPERIMENTAL RESULTS
Both sets of images below were produced with the photodissociation laser at nm and the probe laser at nm on. The image on the right shows a typical raw and reconstructed I(2P3/2) image arising when the 248 nm laser off. Only features consistent with I2 photodissociation are observed in the image. The image on the right shows I(2P3/2) images with the 248 nm source laser on. An additional ring, corresponding to nm photodissociation of IO to yield the I(2P3/2) + O(3PJ) channel is clearly observed.
(4-0)
(6-0)
SUMMARY OF RESULTS
3 2P ... (s)2(p)4(p*)1 (s*)2
At v´ < 6, Orr=Ewing and co-workers predict that the 3 2 state, which crosses at v´=6, ceases to affect the predissociation of the A 23/2 state leaving only 2 states (1 4+ and 2 4-) to be involved in the predissociation at these lower levels. At very low vibrational levels, v´ = 1 and 0, it is predicted that there is only one repulsive state (1 4+) that is involved in the predissociation. The figure above shows that in this simple case the diabatic model predicts quite accurately the final correlated branching ratio for the 1-0 and 0-0 bands.
Cl(2PJ) + O(1D2)
A 2P (s)2(p)3(p*)4
I nm  I(2P3/2) + I(2P3/2)
I nm  I(2P1/2) + I(2P3/2)
I nm  I(2P1/2) + I(2P3/2)
IO nm  I(2P3/2) + O(3PJ)
Cl(2P1/2) + O(3P0)
Cl(2P1/2) + O(3P1)
Cl(2P1/2) + O(3P2)
Cl(2P3/2) + O(3P0)
Cl(2P3/2) + O(3P1)
ClO X 2P (s)2(p)4(p*)3
Cl(2P3/2) + O(3P2)
ClO
Cl + O
I nm  I(2P3/2) + I(2P3/2)
INSTRUMENTATION
BOND DISSOCIATION ENERGY
(BDE) DETERMINATION:
Source
Region
Ionization
Field-free
Detection
Detector
Assembly
CCD
Camera
To D.P.
To T.P.
Relevant sections of the speed distributions derived from I(2P3/2) images with the source laser on. Based on these, we have directly determined the ground state bond energy of IO to be:
I nm  I(2P3/2) + I(2P1/2)
EXPERIMENTAL RESULTS
The correlated branching ratios were determined in two steps. First, images of each of the spin-orbit states of oxygen were measured and the coincident Cl/Cl* branching ratios were obtained from the speed distributions. Next, these ratios were weighted using integrated Doppler profiles of the REMPI transitions for each O state.
IO nm  I(2P3/2) + O(3PJ)
O (3P2) Image from 2-0 Band
Cl (2P3/2)
Cl (2P1/2)
CONCLUSIONS
Phosphor
Screen
MCPs
CCD
Camera
TOF tube
Laser beams
Molecular
beam
Accelerated
Ions
We have measured the v´ dependent correlated branching ratios for the v´= 0-11 bands of ClO. Results of the comparison suggest that the dynamics fit more closely to the diabatic limit as this limit is in good agreement with the experimental data for the overall Cl(2PJ) branching. For the v´= 1 and 0,there is only one state involved in the predissociation, and the diabatic limit accurately predicts the branching on the correlated state level. However, at larger v´ values, the diabatic limit shows significant differences to experimental branching ratios suggesting a significant role of exit channel coupling in this system and highlighting a need for more sophisticated quantum dynamical calculations to describe these branching ratios.
AKNOWLEDGEMENTS
National Science Foundation
Environmental Protection Agency
Robert A. Welch Foundation
2 + 1 Oxygen REMPI for 2-0 Band
O(3P2)
O(3P1)
O(3P0)