Ion imaging studies of ClONO2 photodissociation: Primary branching ratios and secondary dissociation

Article abstract of DOI:10.1016/j.chemphys.2009.09.009

The photodissociation dynamics of ClONO₂ at 235 nm has been reinvestigated using velocity map ion imaging. We report branching ratios for the Cl + NO₃ and ClO + NO₂ channels to be 0.49:0.51 with anisotropy parameters of β

Full text of DOI:10.1016/j.chemphys.2009.09.009

Chemical Physics 364 (2009) 90–97
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Ion imaging studies of ClONO2 photodissociation: Primary branching ratios
and secondary dissociation
1
*
Hahkjoon Kim , Erin E. Greenwald, Simon W. North
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2009
Accepted 4 September 2009
Available online 10 September 2009
The photodissociation dynamics of ClONO2 at 235 nm has been reinvestigated using velocity map ion
imaging. We report branching ratios for the Cl + NO3 and ClO + NO2 channels to be 0.49:0.51 with anisot-
ropy parameters of b = 0.5 0.1 and b = ꢀ0.1 0.3 for the Cl and ClO production channels, respectively.
Photodissociation at 248 nm and 262 nm results in similar branching ratios and dynamics as observed
at 235 nm. Measured O(3P2) images arising from ClONO2 dissociation at 226 nm suggest that oxygen
atoms result from the spontaneous dissociation of metastable NO3. The quantum yield of O atoms arising
from the spontaneous dissociation of NO3 varies from 0.09 at 262 to 0.38 at 235 nm based on the derived
internal energy distributions of the NO3 fragments. We also describe a Monte-Carlo forward-convolution
fitting of imaging data which permits detailed analysis of both spontaneous secondary dissociation and
secondary photodissociation.
Keywords:
Photodissociation
Reaction dynamics
Atmospheric photochemistry
Ion imaging
Ó 2009 Elsevier B.V. All rights reserved.
2
ClONO2 þ h
v
! Clð PJÞ þ NO3
ð1Þ
ð2Þ
ð3Þ
1. Introduction
! ClOðX2PXÞ þ NO2
! ClONO þ Oð PJÞ
Chlorine compounds are known to play a key role in strato-
spheric ozone loss in the Polar Regions. Chlorine nitrate (ClONO2)
is an important temporary reservoir for reactive chlorine in the
atmosphere [1,2]. As a consequence there have been numerous
experimental and theoretical investigations to assess the thermo-
chemistry, kinetics, and photochemistry of this species. Photolysis
is the primary stratospheric loss channel of ClONO2 during the day
[3–5]. Thus, quantification of the wavelength-dependent photoly-
sis product yields is critical to understanding stratospheric chlo-
rine chemistry and modeling atmospheric ozone.
The ClONO2 absorption spectrum exhibits broad structure,
including peaks at 190 nm, 215 nm and a small feature near
370 nm which is thought to involve low lying triplet states [1,6].
Grana et al. have assigned 190 nm peak to n ?
215 nm peak to * transition on the NO2 group based on a
comparison with ab initio vertical excitation energies [7]. The
authors concluded that two of the lowest excited states are corre-
lated to the Cl + NO3 and ClO + NO2 channels. The result suggests
that the quantum yields associated with specific product channels
may exhibit strong wavelength dependence. There are three disso-
ciation pathways that are energetically accessible following ultra-
violet excitation,
3
While channel 1 provides a catalytic route to ozone depletion,
channel 2 is simply the reverse of the ClONO2 formation reaction.
Early studies on the photochemistry of ClONO2 using static-cells
or flow-tubes and atomic resonance fluorescence reported that
channel 1 was the major dissociation pathway [8–11] based on
the observation of O(3P) following photolysis, but could not estab-
lish whether these products originated from a primary channel or a
secondary process. Marinelli and Johnston detected NO3 by absorp-
tion following ClONO2 photolysis at 248 nm and reported a yield of
0.5 0.3 for channel 1 [12]. More recently, Nelson et al. investi-
gated the photodissociation of ClONO2 at 193 nm, 248 nm, and
308 nm using photofragment translational energy spectroscopy
in a molecular beam [13–15]. The relative yields of channels 1
and 2 were calibrated using the photodissociation of Cl2O at
308 nm [16]. The authors reported that ClO + NO2 and Cl + NO3
were the primary photoproducts at these wavelengths and deter-
mined relative branching ratios of ClO:Cl to be 0.66:0.34 at
193 nm, 0.54:0.46 at 248 nm, and 0.33:0.67 at 308 nm. No conclu-
sive evidence for channel 3 was observed and the detection of O
atom fragments was attributed to the spontaneous secondary dis-
sociation of NO3. Photofragment anisotropy indicated that dissoci-
ation was rapid compared to parent rotational motion at all
wavelengths.
p* transition and
p
? p
* Corresponding author. Fax: +1 979 845 2971.
E-mail address: swnorth@tamu.edu (S.W. North).
Ravishankara and co-workers measured quantum yields for the
photoproducts of ClONO2 for wavelengths ranging from 193 nm to
1
Current address: Duksung Women’s University, 419 ssangmun dong, Dobong Gu,
Seoul 132-714, Korea.
0301-0104/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2009.09.009

H. Kim et al. / Chemical Physics 364 (2009) 90–97
91
352.5 nm using both Cl and O atomic resonance fluorescence [17]
and direct absorption of NO3 [18]. The authors reported relative
branching ratios for the Cl + NO3:ClO + NO2 channels along with
NO3 quantum yields at these wavelengths and the results were
consistent with the molecular beam experiments.
A previous study employing one-color ion time-of-flight (TOF)
translational spectroscopy at 235 nm demonstrated the ability to
distinguish between Cl fragments from channel 1 and those arising
from the combination of channel 2 and the photodissociation of
the nascent ClO products [19],
the images using the basis set expansion (BASEX) algorithm devel-
oped by Reisler and co-workers [26].
ClONO2 was prepared by the method of Schmeisser through the
reaction of N2O5 and Cl2O [27]. Cl2O was synthesized by the meth-
od of Cady [28]. Impurities in the ClONO2 sample (mainly Cl2) were
removed by vacuum distillation. The purity of samples was con-
firmed by UV–Vis absorption spectroscopy. Although we found
that minor Cl2 contamination (<5%) was present the absorption
cross section of Cl2 is nearly two orders of magnitude smaller than
that of ClONO2 and the contribution of Cl2 photolysis to the images
is small and easily distinguished from the ClONO2 signal [29].
ClOðX2PXÞ þ h
v
! Clð PJÞ þ Oð PJ; 1D2Þ
ð4Þ
2
3
Since the focussed laser beam, necessary for 2 + 1 REMPI detection
of Cl at 235 nm, ensured complete dissociation of ClO, quantitative
relative branching ratio information for channels 1 and 2 could be
derived from only Cl detection. Based on simulations to the TOF
data relative Cl atom and ClO quantum yields of 0.42 0.1 and
0.58 0.1, respectively, were determined. In addition, the fraction
of nascent NO3 fragments that undergo secondary dissociation
was calculated based on the derived internal energy distribution
yielding a quantum yield of 0.20 for the NO3 ? NO2 + O channel.
In the present experiments, we employ the ion imaging tech-
nique to reinvestigate the photodissociation at 235 nm. We find
that the advantage of ion imaging over ion TOF is the determina-
tion of quantitative speed-dependent anisotropy which is critical
in constraining forward-convolution modeling of the data. Our
new analysis confirms the previous conclusions albeit with minor
differences. We have also extended these studies to the photodis-
sociation at 248 nm and 262 nm. Finally, we have measured the
O(3P2) fragment images arising from ClONO2 photolysis at
225 nm and, based on the forward-convolution modeling, demon-
strate that the oxygen atoms derive from the secondary dissocia-
tion of vibrationally excited NO3 fragments.
3. Results and discussion
Fig. 1 shows raw Cl(2P3/2) and Cl(2P1/2) images arising from the
235 nm photodissociation of ClONO2. No signal was observed
when the laser was detuned from the Cl(2PJ) REMPI transitions.
By inspection there are clear differences between the two images;
(1) the Cl(2P3/2) image shows evidence for a significant contribu-
tion from low speed fragments and (2) the anisotropy of the
Cl(2P3/2) is more sharply peaked than expected. Based on the image
intensity and correcting for the relative detection efficiency we
determine a Cl(2P1/2)/Cl(2P3/2) branching ratio of 0.09 0.01. This
reflects a slightly greater contribution of Cl(2P1/2) than has been
previously reported. Fig. 2 shows the corresponding speed distri-
butions derived from the images in Fig. 1 [19]. The Cl(2PJ) atoms
can be formed from either channel 1 or the photodissociation of
ClO produced via channel 2. However, we have shown previously
that the photodissociation of ClO at 235 nm results in predomi-
2. Experimental
A detailed description of the velocity map ion imaging appara-
tus can be found elsewhere [20,21]. Briefly, a pulsed molecular
beam of 2% ClONO2 in He was collimated by a conical skimmer
and intersected at 90° by either one or two linearly polarized laser
beams. In all cases, the probe beam was the frequency doubled
output of a Nd:YAG (Spectra Physics LAB-150-10) pumped dye la-
ser (LAS LDL). A photoelastic modulator (PEM-80, HINDS) was used
to rotate the polarization of the probe beam. The photolysis beam
was the frequency doubled output of a Nd:YAG (Spectra Physics
LAB-150-10) pumped dye laser (Spectra Physics PDL-1). An assem-
bly consisting of four Pellin-Broca prisms was used to maintain
overlap of the photolysis and probe laser beams during the REMPI
scans. Typical photolysis and probe pulse energies were 800
lJ and
200 J, respectively. The fundamental wavelengths of both lasers
l
were accurately calibrated using a Ne-filled hollow cathode lamp.
The Cl(2P3/2) and Cl(2P1/2) atoms were probed using 2 + 1 REMPI
2
2
transitions at 235.336 nm (4p D3/2 3p P3/2) and 235.205 nm
(4p P1/2 3p P1/2), respectively [22]. The O(3P2) atoms were
probed using 2 + 1 REMPI transitions at 225.572 nm [23]. The
resulting ions were accelerated by velocity mapping ion optics
[24] through a 50 cm long field-free flight tube. The ions were pro-
jected on a position-sensitive detector consisting of a dual micro-
channel plate-phosphor assembly. Images were acquired using a
CCD camera and a frame grabber controlled by commercial soft-
ware (Coda32) which included nearest neighbor centroiding and
event counting [25]. Due to the magnitudes of the fragment veloc-
ities, images were obtained by scanning over the Doppler profiles.
Fragment speed and angular distributions were reconstructed from
2
2
Fig. 1. Raw Cl(2P3/2) and Cl(2P1/2) images arising from ClONO2 photodissociation at
235 nm.

92
H. Kim et al. / Chemical Physics 364 (2009) 90–97
of these channels and a more complete description can be found
1.0
0.8
0.6
0.4
0.2
Cl (2P1/2
)
in the Appendix A. Fig. 3 displays a best-fit (solid line) to the mea-
sured Cl(2P3/2) speed distribution which is the sum of the contribu-
tions from the ClO + NO2 and Cl + NO3 channels (dashed lines).
Based on the best forward-convolution fit, we determine a ClO + -
NO2:Cl(2P3/2) + NO3 branching ratio of 0.52 0.05:0.48 0.05 con-
tributing to the Cl(2P3/2) image. However, the total branching of
channels 1 and 2 requires the overall Cl(2P1/2)/Cl(2P3/2) ratio. Com-
bining the forward-convolution fitting results with the Cl(2P1/2)/
Cl(2P3/2) ratio, we find an overall ClO + NO2:Cl + NO3 branching of
0.51 0.05:0.49 0.05 in good agreement with previous results
[19]. The center-of-mass translational energy distribution for the
ClO + NO2 channel, derived from the forward-convolution fitting
procedure is shown in Fig. 4. The distribution is peaked away from
zero with an average energy of 124 kJ/mol. Nelson et al. have re-
ported slightly lower values for this channel which, interestingly,
decrease with decreasing wavelength although the variation is
not large. The general shape our derived P(ET) distribution is in
good agreement with the P(ET) reported by Nelson et al. at
248 nm and 193 nm [13].
0.0
1.0
Cl (2P3/2
)
0.8
0.6
0.4
0.2
0.0
There are two important observations regarding the spatial
anisotropy of the Cl(2P3/2) fragments. The spatial anisotropy was
fitted using the following form,
PðhÞ / ð1 þ b2ð
v
ÞP2ðcos hÞ þ b4ð
v
ÞP4ðcos hÞÞ
ð5Þ
0
1000
2000
3000
m/s
since the signal is a composite of both 1- and 2-photon processes
Fig. 2. Speed distributions derived from the Cl(2P1/2) (upper panel) and Cl(2P3/2
(lower panel) images in Fig. 1.
)
(see Appendix A). Fig. 5 shows speed-dependent b2( ) and b4(v)
v
terms derived from the Cl(2P3/2) image analysis averaged over
120 m/s intervals. The data exhibit a clear speed-dependence as
do the best-fit Monte-Carlo forward-convolution fits. For compari-
son, forward-convolution fits using three different speed-indepen-
dent anisotropy parameters for the ClO + NO2 channel of ꢀ1, 0,
and 2 are shown. The simulations result in distinctive speed-
dependent anisotropy for each case. Based on the best-fit to the
data we determine a best-fit speed-independent anisotropy param-
eter of b = ꢀ0.1 0.3 for the ClO + NO2 channel based on a compar-
ison to the data. Although inspection shows that relaxing the
constraint of a speed-independent anisotropy for the ClO + NO2
channel provides an improved fit we did not pursue additional
parameterization.
nately Cl(2P3/2) + O(1D2) with an anisotropy parameter of 1.8 0.1
[30]. We assume, therefore, that the Cl(2P1/2) image is dominated
by chlorine atoms originating from channel 1. Converting the
Cl(2P1/2) atom speed distribution to a center-of-mass translational
energy distribution yields an average translational energy for this
channel of 163 kJ/mol with a FWHM of 62 kJ/mol and is consistent,
albeit exhibiting a slightly different shape, with previous lower res-
olution measurements [19]. The asymmetric shape of the distribu-
tion is very similar to the distribution reported by Nelson et al. at
248 nm. In addition, the Cl(2P1/2) fragments are well described by a
single speed-independent anisotropy parameter of 0.5 0.1, provid-
ing further evidence that these products arise exclusively from
channel 1. The measured anisotropy parameter for this channel
is identical to the values reported by Nelson et al. at 193 nm,
248 nm, and 308 nm [13,15].
In order to extend the study to additional wavelengths we per-
formed photodissociation experiments at 248 nm and 262 nm. The
235 nm probe laser power was decreased so that only a small sig-
Unlike the Cl(2P1/2) signal, the Cl(2P3/2) signal consists of contri-
butions from both channel 1 and the combination of channels 2
and 3. In order to analyze the speed distribution and spatial anisot-
ropy of the ground state Cl(2P3/2) atoms produced via secondary
photodissociation in the tightly focused laser beam, we employ a
forward-convolution fitting scheme. We assume that only a single
asymptotic channel, Cl(2P3/2) + O(1D2), contributes in the photodis-
sociation of ClO. In previous work, employing a hot source of ClO
which populates v > 1 states, identical branching ratios and anisot-
ropy parameters for ClO photodissociation were observed [31].
Since the spin orbit energy of Cl(2PJ) is small (ꢁ10.6 kJ/mol), the
energy distributions of Cl(2P3/2) and Cl(2P1/2) are expected to be
similar. We therefore assume that the measured Cl(2P1/2) speed
distribution describes the contribution from channel 1 to the
Cl(2P3/2) image. A fit to the Cl(2P3/2) then involves an iteratively ad-
justed speed and angular distributions associated with the primary
photodissociation channel 2 of chlorine nitrate to form ClO and
NO2, and a relative branching ratio between channels 1 and 2.
The forward-convolution modeling of the data implements a
Monte-Carlo sampling over the speed and angular distributions
1.0
0.8
0.6
0.4
0.2
0.0
0
1000
2000
m/s
3000
Fig. 3. Forward-convolution fit to the Cl(2P3/2) speed distribution. The contributions
due to ClO + NO2 (followed by ClO photodissociation) and Cl + NO3 channels are
shown as dashed lines and the weighted sum as a solid line (see text for details).

H. Kim et al. / Chemical Physics 364 (2009) 90–97
93
248 nm and 262 nm were qualitatively similar. As observed at
235 nm, analysis of the Cl(2P3/2) images revealed speed-dependent
anisotropy while the Cl(2P1/2) data could be described by a single
speed-independent anisotropy of b = 0.5 0.1 at both 248 nm and
262 nm. Based on the Cl(2P1/2) speed distributions we derive aver-
age ET values of 146 kJ/mol and 153 kJ/mol for the Cl + NO3 channel
at 248 nm and 262 nm, respectively. The values are consistent with
a value of 170 kJ/mol reported by Nelson et al. at 248 nm [13]. The
relative insensitivity of the average energy to wavelength is not
surprising given the rather modest change in available energy.
ClONO2 → ClO + NO2
1.0
0.8
0.6
0.4
0.2
0.0
The Cl(2P1/2)/Cl(2P3/2
) branching ratio was determined to be
0.10 0.03 and 0.11 0.03 for 248 nm and 262 nm, respectively.
Fig. 6 shows the derived Cl(2P3/2) speed distribution and the asso-
ciated forward-convolution fits for photolysis at 248 nm (lower pa-
nel). The upper panel of Fig. 6 shows the speed-dependent
anisotropy (b2(v)) and the forward-convolution fit. Forward-convo-
lution fits yield anisotropy parameters of b = 0.0 0.3 and b
= ꢀ0.1 0.4 for the ClO + NO2 channel for photodissociation at
248 nm and 262 nm, respectively. As with the 235 nm analysis
the Cl(2P1/2) speed distribution serves the basis function for the pri-
mary Cl + NO3 channel in fitting the Cl(2P3/2) data. The speed distri-
bution derived from the Cl(2P3/2) images is similar in shape to the
235 nm distribution but shows a decrease in average speed with
increasing wavelength. The results of the analysis for the primary
dissociation channels at all three wavelengths are summarized in
Table 1.
0
50
100
150
200
250
300
ET (kJ/mol)
Fig. 4. Derived center-of-mass translational energy distribution for the ClO + NO2
channel at 235 nm.
3.1. Spontaneous dissociation of nascent NO3
2.0
1.5
1.0
The nascent internal energy distribution of NO3 can be deduced
from the energy conservation of the Cl + NO3 channel. A plot of the
nascent NO3 internal energy distribution at 235 nm derived from
the Cl(2P3/2) speed distribution is shown in Fig. 7. There are two
possible dissociation pathways for energized NO3 (see Table 2),
0.5
β
NO3 ! NO þ O2;
NO3 ! NO2 þ Oð PÞ
ð6Þ
ð7Þ
0.0
-0.5
-1.0
3
and the relative branching of these two channels with respect to
atmospheric photolysis is important for estimating ozone deletion
2.0
1.5
1.0
2
1
β
0
0.5
β
0.0
-1
1.0
0.8
0.6
0.4
0.2
0.0
-0.5
-1.0
0
1000
2000
m/s
3000
Fig. 5. Speed-dependent anisotropy parameters (b2(v) and b4(v )
)) for the Cl(2P3/2
fragments arising from ClONO2 photodissociation at 235 nm along with the
forward-convolution fit using three different anisotropy parameters to describe
the ClO + NO2 channel.
nal was observed in the absence of the dissociation laser. This min-
or contribution from the probe laser was subtracted from the final
images prior to analysis. Although the signal-to-noise of the cor-
rected two-color images was not as good as the one-color experi-
ments at 235 nm it was sufficient for forward-convolution
analysis. Images arising from the photodissociation of ClONO2 at
0
1000
2000
m/s
3000
Fig. 6. Speed-dependent anisotropy parameters, b2(
distribution (lower panel) arising from ClONO2 photodissociation at 248 nm. The
solid and dashed lines are the results of simulations as described in the text.
v
), (upper panel) and speed

94
H. Kim et al. / Chemical Physics 364 (2009) 90–97
Table 1
Summary of primary channels in the photodissociation of ClONO2 at the wavelengths investigated in the study.
235 nm
Cl + NO3
248 nm
Cl + NO3
262 nm
Cl + NO3
ClO + NO2
0.51 0.04
124
ꢀ0.1 0.3
ClO + NO2
ClO + NO2
U
0.49 0.04
163
0.5 0.1
0.44 0.08
146
0.5 0.1
0.56 0.08
96
0.0 0.4
0.51 0.1
153
0.5 0.2
0.49 0.1
104
ꢀ0.1 0.4
a
hETi
b
a
kJ/mol.
of NO3 which dissociates to NO2 + O(3P), the final O(3P) quantum
yield can be calculated by weighting this fraction by the Cl + NO3
branching ratio value. Based on such analysis we derive O(3P) yields
of 0.38 0.04, 0.18 0.03, and 0.09 0.03 at 235 nm, 248 nm, and
262 nm respectively. The value of 0.54 reported by Nelson et al. is
consistent with the increasing trend with decreasing wavelength,
a consequence of increasing available energy and only a modest in-
crease in the average translation energy of the Cl + NO3 channel.
1.0
0.8
0.6
0.4
0.2
0.0
NO3 → NO2 + O
0
100
200
300
Eint (kJ/mol)
Fig. 7. Internal energy distribution of the nascent NO3 radicals arising from 235 nm
photodissociation of ClONO2 based on the Cl(2P1/2) speed distribution. The vertical
line denotes the energetic threshold associated with the secondary dissociation of
NO3 to give NO2 + O products.
Fig. 8. Raw O(3P2) image arising from ClONO2 photodissociation at 226 nm.
Table 2
Fraction of spontaneous NO3 decomposition in the photodissociation of ClONO2.
2.0
1.5
Wavelength (nm)
Oxygen atom yield
Reference
193
193
235
248
262
308
0.4
0.54
0.38 0.04
0.18 0.03
0.09 0.03
0.0
[17]
[13]
This work
This work
This work
[15]
1.0
0.5
β
0.0
-0.5
-1.0
1.0
since NO participates in catalytic cycle whereas the production NO2
participates in a null cycle. Davis et al. studied these two channels
using molecular beam photofragment translational spectroscopy
and determined thresholds for channels 6 and 7 to be 47.3
0.8 kcal/mol and 48.69 0.25 kcal/mol, respectively [32]. Since
channel 7 involves a loose transition state this pathway will domi-
nate for energies above this threshold although recent reports of
‘roaming’ dynamics in formaldehyde dissociation [33] suggest that
the situation may be more complex. The quantum yield of second-
ary O(3P) can be therefore, obtained by integrating the normalized
NO3 internal energy distribution above the threshold corresponding
to channel 7. It should be noted that collisional stabilization of ener-
getic NO3 radicals which exceed the energy of dissociation should
not be significant under stratospheric conditions since the rates of
spontaneous dissociation exceed the collision frequency even for
energies a few kcal/mol above the threshold [19]. Given the fraction
0.8
0.6
0.4
0.2
0.0
0
1000
2000
m/s
3000
Fig. 9. Derived speed-dependent anisotropy parameters (upper panel) and speed
distribution (lower panel) for O(3P2) fragments arising from ClONO2 photodissoci-
ation at 226 nm (symbols) along with forward-convolution fit (solid lines).

H. Kim et al. / Chemical Physics 364 (2009) 90–97
95
We have observed a direct evidence of the spontaneous dissoci-
ation of primary NO3 generated in coincidence with Cl(2PJ). Fig. 8
shows a representative O(3P2) ion image arising from the photodis-
sociation of ClONO2 at 226 nm. In these one-color experiments no
signal was observed when the laser was detuned from the oxygen
REMPI transition. If the oxygen atoms originated from the primary
ClONO + O channel one would expect the O(3P2) image to exhibit a
broad annular feature similar to Cl(2PJ) image arising from Cl + NO3
channel, a result of large recoil velocities. By inspection, the O(3P2)
image is more consistent with secondary dissociation. In order to
confirm this assignment we have modeled this process using
Monte-Carlo forward-convolution (see Appendix A for details)
[32] the fitting procedure employed the P(ET) derived from the
Cl(2P1/2) images at 235 nm to model the translational energy distri-
bution of the primary Cl + NO3 channel. As shown in Fig. 9, the
forward-convolution fits (solid lines) assuming spontaneous sec-
ondary dissociation of NO3 are in excellent agreement with the
cases, the quantitative photodissociation of primary products can
provide beneficial results given proper analysis. In this Appendix
A, we demonstrate that there are clear signatures of secondary
photodissociation in the speed-dependent angular distribution.
Thus, if either photodissociation step is well-characterized then
considerable information regarding the speed and angular distri-
bution of the other photodissociation step, obtained at higher laser
powers, can be extracted. In order to illustrate the sensitivity we
have performed forward-convolution simulations for a simple test
case although the procedure is general. We then provide details on
the Monte-Carlo forward-convolution simulation specific to the
case of ClONO2 photodissociation.
A.1. Modeling secondary photodissociation
Fig. A1 shows a schematic representation of the vector addition
involved in calculating the resultant secondary fragment atom
speed and angular distributions from the combination of two sep-
arate photodissociation steps. The forward-convolution procedure
uses Monte-Carlo sampling over a model velocity distribution for
the primary fragment and sampling over a model second step
velocity distribution, constrained by conservation of energy. As a
test case to demonstrate the forward-convolution fitting procedure
we first consider two identical Gaussian speed distributions,
v = 1000 m/s and FWHM = 200 m/s, characteristic of direct dissoci-
ation on a repulsive potential to describe the two photodissocia-
tion steps. The anisotropy parameter for the second step was
fixed at 2 and the anisotropy parameter for the first step was cho-
sen to be –1, 0, and 2. We assume that the spatial anisotropy for
each step is independent of speed.
speed-dependent anisotropy parameters, b2(v), and speed distribu-
tion of the O(3P2) fragments derived from the image in Fig. 8.
4. Conclusions
We have reinvestigated the photodissociation dynamics of
ClONO2 at 235 nm using velocity map ion imaging. Based on the
forward-convolution fit we have determined relative branching ra-
tio for the Cl + NO3 and ClO + NO2 to be 0.49:0.51 along with
anisotropy parameters of b = 0.5 0.1 and b = ꢀ0.1 0.3 for the Cl
and ClO production channels, respectively. The reported branching
ratios and average translational energy distributions are consistent
with previous measurements. We have also extended our mea-
surements to examine the photodissociation of ClONO2 at
248 nm and 262 nm. Photolysis at these wavelengths results in
similar branching and dynamics as observed at 235 nm. Measured
O(3P2) images arising from ClONO2 dissociation at 226 nm suggest
that the O atom fragments observed in laboratory studies result
from the spontaneous dissociation of metastable NO3. We have
determined that the quantum yield of O atom arising from the
spontaneous dissociation of NO3 varies from 0.09 at 262 nm to
0.38 at 235 nm based on the derived internal energy distribution
of the NO3 products. The trend is consistent with increasing avail-
able energy and little change in the average translation energy re-
lease for the Cl + NO3 channel.
In order to facilitate comparison with experiment we have fit
the resultant speed-dependent anisotropy from the Monte-Carlo
simulation using the following equation [34],
1
Iðh;
v
Þ ¼
½1 þ b2ð
v
ÞP2ðcos hÞ þ b4ð
v
ÞP4ðcos hÞꢂ;
ðA:1Þ
4
p
where P2(cos h) and P4(cos h) are the second and fourth Legendre
polynomial, and h is the angle between the fragment recoil direction
and laser polarization direction. Such analysis mimics the analysis
of experimental ion images.
We present a Monte-Carlo forward-convolution approach to the
analysis of imaging data. The method permits detailed analysis of
both spontaneous secondary dissociation and secondary photodis-
sociation. Specifically, we find that analysis of the velocity-depen-
dent anisotropy can provide additional constraints in model such
processes.
z
θ2
v2
v
Acknowledgments
The authors acknowledge Musa Ahmed and Annalise L. Van
Wyngarden for preliminary imaging measurements on this system.
Support for this project was provided by the Robert A. Welch Foun-
dation (Grant A-1405).
θ
θ1
v1
Appendix A
The advantage of imaging techniques is their multiplexing abil-
ity, through reconstruction of the full center-of-mass velocity dis-
tribution. As a result the correlation between the angular and
speed distributions can be extracted to provide considerable in-
sight into the photodissociation dynamics. Often the laser powers
or absorption cross sections ensure that some secondary photodis-
sociation of the primary photofragments is unavoidable. In other
Fig. A1. Coordinate system for the forward-convolution fitting of secondary
photodissociation. The values v1 and h1 correspond to the primary process
(ClO + NO2 in the case of ClONO2) and the values v2 and h2 correspond to the
second photodissociation process (ClO photodissociation in the case of ClONO2).
P(
v,h) describes the resultant laboratory speed and angular distributions for the
secondary fragment (Cl(2P3/2) atoms in the case of ClONO2).

96
H. Kim et al. / Chemical Physics 364 (2009) 90–97
The upper panel of Fig. A2 shows the resultant product speed
dissociation steps, we first perform Monte-Carlo sampling over the
initial center-of-mass speed ( 1) of the ClO fragment using the fol-
lowing probability distribution [35],
distributions for first step anisotropy parameters of –1, 0, and 2
(dashed, dotted, and solid lines, respectively). All the distributions
are broadened about 1000 m/s and have significant population
v
ꢀ
ꢁ ꢀ
ꢁ
a
b
v1
vmax
v1
near 2000 m/s. The lower panels show the resultant b2(
b4( ) from the simulations. Although both steps are described by
speed-independent anisotropy the resultant fragments exhibit
strongly speed-dependent anisotropy. In addition, the b4( ) terms
v) and
Pðv1Þ ¼
1 ꢀ
;
ðA:2Þ
v
max
v
where vmax is the maximum speed for the ClO fragment and a and b
v
are adjustable parameters. Monte-Carlo sampling then selects the
polar angle (h1) with respect to the polarization axis of the dissoci-
ation laser using a speed-independent probability distribution,
are significant and the sign of these terms can be used as a robust
measure for the signs of the anisotropy parameters for the two indi-
vidual steps. If the photodissociation steps both have positive
anisotropy then the resultant products anisotropy is more sharply
ꢀ
ꢁ
3 cos2 h1 ꢀ 1
Pðh1Þ ¼ 1 þ b ꢃ
:
ðA:3Þ
peaked than cos2h, (i.e. positive b4(
v
)) whereas if the two photodis-
sociation steps have opposite signs the product angular distribu-
tion will exhibit clear fourfold symmetry (i.e. negative b4( )).
2
Since the primary photodissociation is the result of one-photon
absorption using linearly polarized light, there is azimuthal symme-
try about the polarization axis. The joint probability distribution
associated with the velocity of the ClO fragment is then given by,
v
Finally, if either step is isotropic, then the angular distribution of
the final products will be described by cos2h. These conclusions
are most easily observed when the velocity distributions for both
steps are comparable.
It should be noted that this contrasts with a process in which
the parent molecule absorbs two photons sequentially prior to dis-
Pðv1Þ ¼ Pðv1Þ ꢃ Pðh1Þ:
ðA:4Þ
The energy partitioned to the ClO fragment and available fol-
lowing the secondary photodissociation step is determined by en-
ergy conservation,
sociation. Although such a process can give rise to non-zero b4(
is unlikely to produce the identical strong speed-dependence ob-
served in the present case. Often only b2( ) values are reported
for features which are attributed to secondary photodissociation.
We suggest that including the b4( ) term in fits to the fragment
v), it
v
1
2
Ea ¼ h
m
ꢀ DoðClO—NO2Þ ¼
l
g2 þ EintðClOÞ þ EintðNO2Þ;
ðA:5Þ
v
ail
o
v
where h
m
is the photon energy, Do(ClO–NO2) is the bond dissocia-
angular distributions could provide important constraints on such
analysis.
o
tion energy of ClONO2 (Refs. [36,37]) and Eint(ClO) and Eint(NO2)
are the internal energies of the ClO and NO2 fragments, respectively.
The fitting requires an additional parameter to reflect the fraction of
the internal energy partitioned to the ClO fragment, Eint(ClO) as this
ultimately defines the available energy for the second step. Such an
approximate treatment is severe but certainly not unreasonable.
The translational energy of the resultant Cl(2P3/2) atom upon photo-
dissociation of the ClO will include this internal energy,
A.2. Secondary photodissociation in ClONO2
In order to fit the Cl(2P3/2) fragment speed and angular distribu-
tion arising from the combination of channels 2 and 4, each photo-
1.0
0.8
0.6
0.4
0.2
0.0
ET ¼ h
m
þ EintðClOÞ ꢀ DoðCl—OÞ ꢀ ESOðOÞ;
ðA:6Þ
o
where Doo(Cl–O) is the bond dissociation energy of ClO
(38,050 cmꢀ1) [38], and ESO (O) is electronic energy of the O(1D2)
(18,650 cmꢀ1). The secondary angular distribution is given by the
same functional form as (A.3), using an anisotropy parameter of
b = 1.8 determined previously. The probability of the Cl(2P3/2) frag-
ments having particular speed (
and (A.6).
v2) is evaluated according to (A.5)
The vector sum of the two photodissociation steps provides a
resultant vector representing the Cl(2P3/2) atom velocity with re-
spect to the axis of linear polarization of the dissociation laser with
a probability equal to the product of the individual probabilities.
Integration over the azimuthal angle allows one to derive the
speed-dependent angular distribution, P(h), while integration over
both angular coordinates results in the final center-of-mass speed
2
1
β
0
-1
-2
distribution, P(v). The resulting final spatial anisotropy can be de-
scribed by Eq. (A.4). There is no a priori reason to believe that the
speed-dependent anisotropy resulting from a secondary photodis-
sociation process should be described by a second Legendre poly-
nomial (such as given by (A.3)). In fact, for secondary
photodissociation the final angular distribution requires inclusion
of a cos4h term, where the sign of the coefficient is a strong func-
tion of the b values for the two individual steps. Fitting of the sim-
ulated speed-dependent angular distributions provides values for
1
0
β
-1
1000
2000
Speed (m/s)
3000
b2(v) and b4(v) that can be directly compared to experimental
values.
The forward-convolution fitting scheme for ClONO2 ultimately
involves five adjustable parameters in the total fit of the ground
state Cl(2P3/2) experimental data. Our initial estimates for the
parameters a, b, and b were based on the earlier PTS experiments
Fig. A2. The top shows the resultant product speed distributions for first step
anisotropy parameters of –1, 0, and 2 (dashed, dotted, and solid lines, respectively)
given the initial speed distribution described in the text. The lower panels show the
resultant b2(v) and b4(v) values from the simulations.

H. Kim et al. / Chemical Physics 364 (2009) 90–97
97
distribution given in (A.4). The b parameter in this case is an adjust-
able parameter in the fit to the resultant oxygen atom data. It is
important to note that the angular distribution is defined relative
to primary step relative velocity vector. In the case of a long-lived
intermediate, relative to its rotational period, there is forward–
backward symmetry and b = 2.0 would represent the oblate top lim-
it and b = ꢀ1 the prolate top limit. The secondary dissociation has
azimuthal symmetry about the primary velocity vector.
A vector sum of the two dissociation steps provides a resultant
vector representing the O(3P2) atom velocity with respect to the
axis of linear polarization of the dissociation laser with a probabil-
ity equal to the joint probabilities of the individual vectors. Vector
of Nelson and co-workers at 248 nm. [13] As a first approximation
of the fraction of internal energy partitioned into the ClO fragment
for channel 2 we consider two limiting models; a prior, or statisti-
cal, model and an impulsive model. The prior model [39] yielded a
value of 0.21 while the soft-fragment impulsive model yielded
0.41. [40] We found that the best-fit to the data was obtained using
a value between these two cases.
To determine the best-fit to the data, the program was run with
a minimum sampling of 1 ꢃ 106 points. A v2 minimization of the
weighting of the secondary and primary Cl(2P3/2) speed distribu-
tions provided the branching ratio between channels 1 and 2. Once
an adequate fit of the speed distribution was achieved the derived
speed-dependent anisotropy was compared to the experimental
measurement and the optimization repeated.
addition of the two vectors (v1 and v2) was achieved by a coordi-
nate frame transformation via rotation using Euler angles. Once the
speed and angular distributions are obtained for the resultant oxy-
gen atom,
determined.
a
translational energy distribution, PðETÞ, can be
A.3. Spontaneous secondary dissociation in ClONO2
Fig. A3 shows the vector addition involved in evaluating deter-
mining the resultant O(3P2) atom speed and angular distributions.
Arising from a two-step dissociation. The relevant quantities have
been defined above. In the case of ClONO2, measurement of the
Cl(2P1/2) speed distribution yields both the speed distribution for
the momentum matched NO3 fragment, and by energy conserva-
tion the internal energy of the NO3 fragment,
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Fig. A3. Coordinate system for the forward-convolution fitting of spontaneous
dissociation. Parameters for Cl + NO3 dissociation (
measurement. P( ,h) describes the laboratory speed and angular distributions for
O(3P2) arising from spontaneous dissociation of NO3.
v1,h1) are based on the previous
v