VACUUM ULTRAVIOLET PHOTOIONIZATION CROSS SECTION OF THE HYDROXYL RADICAL
3.3 Observed kinetics
Examples of the time-dependent O3, OH, and O(3P) signals are shown in Figures 1–3.
Photolysis led to an instantaneous depletion of typically 10–20% in O3. The ozone signal continued to drop as secondary chemistry (reaction (6)) removed additional O3. OH radicals formed rapidly after the reaction of the photolysis product O(1D) with H2O, reaching peak concentrations of approximately 0.2–1.5×1013 cm−3 in the first 0.5 ms. O(3P) atoms, formed from photolysis of O3 (reaction (1b)) or quenching by O2 (reaction (3)) were also present in relatively high concentrations (0.2–4×1012 cm−3) at early times.
3.3.a Ozone Photolysis
Figure 1(a) shows the observed O3 signal measured at m/z 48 during Experiment 4 (Table I).
The instantaneous depletion from photolysis was 23.3±0.3% (indicated by the dashed black line in Figure 1(b)), in good agreement with the O3 depletion of 24% estimated from the measured laser fluence, O3 concentration, and the O3 absorption cross section (1.08×10−17 cm2) at 248 nm.43 As seen in Figure 1(b), subsequent removal of O3 occurs through reaction with H atoms (reaction (6)) at early times and reaction with remaining OH later. The model slightly overpredicts the initial loss and underpredicts the loss rate at longer times (t > 10 ms).
3.3.b O(1D) Atoms
In order to model [OH](t), we needed to determine the absolute O(1D) atom concentrations.
O(1D) atoms could not be observed directly because their expected lifetime (~1 μs) was much shorter than the instrument temporal resolution. Instead, we estimated the initial O(1D) atom concentrations from the measured O3 concentration, observed fractional depletion of O3
caused by photolysis, and known O(1D) quantum yield (reaction (1a)).
Table II lists the measured [O(1D)]0 atom concentration for each experiment, measured from the initial O3 depletion. The measured depletion was consistent for experiments with the same laser settings (Experiments 1–12) and ranged from 21.0±0.1 % to 23.8±0.2 % (experiments using an attenuator to achieve lower radical concentrations (experiments 13–
15) had smaller, self-consistent depletions around 14%). O(1D) atom concentrations ranged from 1–13×1012 cm−3.
3.3.c OH Radicals
OH radicals were formed from reaction of photolytically-generated O(1D) atoms with H2O.
Figure 2 shows the time-dependent behavior of the OH+ signal from Experiment 4 (13.436 eV). There is a sharp rise in OH upon O3 photolysis reaching a peak at t < 1 ms, followed by a decay that approaches baseline after 60 ms. The modeled OH time profile shown in Figure 2 (red dashed line) agrees reasonably with the observed trace with two notable differences.
First, the observed OH decay appears to have two components, with a fast component with
~2 ms lifetime that is not present in the kinetics model (the lifetime of OH with respect to O(3P), reaction (3), at the maximum O(3P) concentration is 8 ms). We attribute this feature to the decay of vibrationally-excited OH radicals formed in the flow tube from reaction (2).
Experiments on isotopically-labeled H218O revealed that two OH radicals are formed from reaction (2); the heavier OH radical product is formed cold, with nearly all of its population in OH(v=0).45-50 At the same time, a significant fraction of the lighter OH radical is in OH(v=1) (29%) and OH(v≥2) (30%).50 In our experiments, we did not isotopically label the water, meaning that all OH products would appear at the same mass regardless of their vibrational states. Vibrationally-excited OH radicals would exhibit a different dependence on photoionization energy than OH(v=0), as well as dissimilar kinetics. OH(v≥1) would primarily decay to form ground state OH via vibrational quenching with all the other collision partners (e.g., the lifetime of OH(v=1) with respect to collisional deactivation51 by O2 is 1.5–
11 ms for the range of [O2] used here). However, the spectroscopic evidence to assign this fast component in the OH decay to OH(v≥1) is inconclusive.
The other difference between model and observation is in the second component of the decay, which should be equal to the modeled OH decay if the model is complete. The measured lifetime is about 16 ms—slightly longer than the modeled lifetime of about 13 ms.
Similar to our observations of the HO2 radical,8 it is expected that some of the OH radicals are removed through heterogeneous interactions with the wall of the flow tube. Under this assumption, we conducted several experiments where we varied the initial conditions of the reaction system (in particular, varying the initial OH radical density over a range 0.2–
1.5×1013 OH radicals cm−3) and independently fit the model to these data to quantify the OH wall loss rate. The average heterogeneous wall loss rate across all experiments was 28±6 s−1. The random error in this measurement was quite small, but the reported uncertainty in this wall loss rate is on the order of 20% due to the uncertainty in the kinetics simulation.
Figure 2 (green line) shows the modeled decay of OH when incorporating a fitted wall loss rate. We did not attempt to fit the sharp spike in signal at early times (the component assumed to be from OH(v=1)), but used the model with the added wall loss term in our determination of the photoionization cross section of OH. The OH wall loss rate described above is not large, and is of a similar magnitude as the HO2 wall loss rate we measured previously,8 but it significantly perturbs the kinetics and must be included in order to obtain an OH cross section.
The model including OH wall loss predicts slightly less depletion of O3 at long times (Fig.
1b, green line) than the model without OH wall loss (red dashed line). Modeling of the secondary depletion of O3 plays only a minor role in constraining [OH](t), so discrepancies between the modeled and observed O3 traces do not have a large effect on the reported OH cross section.
3.3.d O(3P) Atoms
O(3P) atoms, formed from initial photolysis and rapid collisional deactivation of O(1D) were also observed during these experiments. The O(3P) signal rises quickly, and then decays rapidly primarily due to reaction with OH. Figure 3 shows the measured O(3P) signal, scaled relative to the kinetics model for Experiment 11 (14.193 eV). The agreement between the
observed and modeled [O(3P)](t) is good; both show O(3P) decay on the order of 4 ms.
Due to the rapid reaction of O(3P) atoms with OH, heterogeneous decay of O(3P) on the reactor walls makes a negligible contribution to O(3P) loss.
3.4 Experimental cross section and spectrum