State selected unimolecular dissociation of HOCl

Article abstract of DOI:10.1063/1.474729

The unimolecular decomposition of HOCl is investigated by exciting the molecule to the region of the sixth overtone of its OH stretching vibration (7ν_OH) using overtone-overtone double resonance.The excitation scheme is sufficiently selective to allow preparation of a single angular momentum quantum state within the 7ν_OH vibrational manifold lying ca. 2500 cm^-1 above the dissociation limit.From the measured linewidths associated with the rotational features appearing in the action spectrum, we obtain an upper limit estimate for the dissociation rate of k_uni<9.4*10⁹ s^-1 which is substantially slower than that expected on the basis of RRKM theory.The nascent OH fragment product state distribution exhibit strong oscillations which depend on the rotational quantum numbers of the parent HOCl molecule.From the measured energy release associated with the OH fragment, the heat of formation of HOCl is estimated to be ΔH⁰₀(0K)=-16.7+/-0.6 kcal/mol.

Full text of DOI:10.1063/1.474729

State selected unimolecular dissociation of HOCl
Rhett James Barnes and Amitabha Sinha
Citation: The Journal of Chemical Physics 107, 3730 (1997); doi: 10.1063/1.474729
View online: http://dx.doi.org/10.1063/1.474729
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State selected unimolecular dissociation of HOCl
Rhett James Barnes and Amitabha Sinha
Department of Chemistry and Biochemistry, University of California-San Diego, 9500 Gilman Drive,
La Jolla, California 92093-0314
͑Received 8 May 1997; accepted 27 June 1997͒
The unimolecular decomposition of HOCl is investigated by exciting the molecule to the region of
the sixth overtone of its OH stretching vibration (7␯OH) using overtone–overtone double resonance.
The excitation scheme is sufficiently selective to allow preparation of a single angular momentum
quantum state within the 7␯OH vibrational manifold lying ϳ2500 cmϪ1 above the dissociation limit.
From the measured linewidths associated with the rotational features appearing in the action
spectrum, we obtain an upper limit estimate for the dissociation rate of kuniϽ9.4ϫ109 sϪ1 which is
substantially slower than that expected on the basis of RRKM theory. The nascent OH fragment
product state distribution exhibit strong oscillations which depend on the rotational quantum
numbers of the parent HOCl molecule. From the measured energy release associated with the OH
fragment, the heat of formation of HOCl is estimated to be ⌬H00(0 K)ϭϪ16.7Ϯ0.6 kcal/mol.
© 1997 American Institute of Physics. ͓S0021-9606͑97͒02633-0͔
INTRODUCTION
example, to study the unimolecular dissociation of
HCO͑DCO͒ and NO2, respectively. For HOCl, whose low
lying excited electronic states are all dissociative, we have
used the technique of vibrational overtone–overtone double
resonance to access its vibrationally excited levels. This ap-
proach is similar to the method developed by Rizzo and
co-workers8 and involves accessing a high energy vibrational
overtone state by breaking up the overall excitation process
into two sequential steps. In our case we use two sequential
lower order overtone transitions, where as in their original
work Rizzo and co-workers used the combination of a fun-
damental excitation followed by an overtone transition. We
have recently demonstrated the utility and feasibility of the
overtone–overtone double resonance technique by accessing
highly vibrationally excited states of water in the energy
range of ϳ25 200 cmϪ1.9 In the present case application of
the double resonance technique to HOCl involves using the
idler beam from an OPO to excite ground state molecules to
the first overtone level of the OH stretch (2␯OH) around
7050 cmϪ1 and then subsequently using a dye laser to further
excite the selected molecules to the target vibrational level
Hypochlorous acid, HOCl, is an important species in at-
mospheric chemistry.1 Consequently, there has been great
interest in characterizing the photochemistry2 and dissocia-
tion dynamics3,4 of this molecule from its electronic excited
states. Of course, bond cleavage in HOCl can also occur on
its ground electronic surface, however, to the best of our
knowledge there has been no prior experimental study re-
ported on this phenomena. In this communication we present
the first results on the state-selected unimolecular decompo-
sition of HOCl on its ground electronic surface.
As a covalently bonded closed shell triatomic molecule,
HOCl offers several attractive features for investigating uni-
molecular decomposition. In particular, the relatively low
density of vibrational states, a readily detectable product
fragment, and the presence of a convenient vibrational chro-
mophore ͑i.e., the high frequency O–H stretch͒ all make
HOCl an attractive system for experimental study. In addi-
tion, the large frequency mismatch between the O–H stretch-
ing coordinate and the other vibrations of the molecules sug-
gests that vibrational state mixing is likely to be restricted in
this system, thus providing an opportunity for observing pos-
sible nonstatistical behavior with regards to its dissociation
dynamics. Previous state selected unimolecular dissociation
studies on other triatomic systems such as NO2, HCO, and
DCO,5–7 where the density of vibrational states is also fairly
low, have observed a range of dissociation dynamics with
HCO being highly mode specific,6 NO2 being statistical on
average5 and DCO being somewhere in between.7
corresponding to the 7␯
state which lies ϳ2500 cmϪ1
OH
above the dissociation limit. Monitoring the yield of OH
fragments resulting from the unimolecular decomposition as
a function of the excitation laser frequency, provides the
means for obtaining information about the predissociative
final vibrational state and for probing the dissociation dy-
namics ͑see Fig. 1͒.
EXPERIMENT
Experimentally the challenge of studying unimolecular
reactions on the ground electronic surface lies in the diffi-
culty associated with preparing high energy vibrationaly ex-
cited states lying above the dissociation limit. This is particu-
larly true for molecular systems that do not possess
accessible bound excited electronic states for use in state
preparation schemes such as stimulated emission pumping or
internal conversion, the two methods that have been used, for
The details of the experimental apparatus have been de-
scribed in our earlier work on H2O and HOBr.9,10 We syn-
thesize HOCl by simultaneously flowing water vapor and Cl2
through two separate ports at one end of a glass column
(1.5 cm diameterϫ35 cm length) packed with a mixture of
yellow HgO and 6 mm diameter glass beads. The beads al-
low for a loose packing of the HgO powder and thus insure
3730
J. Chem. Phys. 107 (9), 1 September 1997
0021-9606/97/107(9)/3730/4/$10.00
© 1997 American Institute of Physics
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3731
FIG. 1. Schematic diagram illustrating the vibrational overtone-overtone
double resonance excitation scheme used to initiate unimolecular decompo-
sition of HOCl. The vibrational quantum numbers shown are for the O–H
stretching coordinate. The HO–Cl bond dissociation energy is estimated to
be ϳ 19 210 cmϪ1 ͑Ref. 14͒.
FIG. 2. ͑a͒ Action spectrum of the HOCl 2␯OH vibrational state obtained by
photodissociating HOCl͑2␯ ͒ through electronic excitation using 355 nm
OH
uniform flow of reagents through the column. The other end
of the column is connected to the glass photolysis cell and
the mixture of reactants and HOCl products continuously
flows into the cell. The cell is evacuated by a partially
throttled mechanical vacuum pump resulting in a total pres-
sure in the cell between ϳ60–100 mTorr. Light (ϳ11 mJ)
from an OPO ͑Spectra-Physics MOPO 730͒ which is
pumped by the third harmonic of a pulsed Nd:Yag laser ex-
cites the HOCl molecules in the region of the first overtone
level of the OH stretching vibration (2␯OH). We pick off a
small portion of infrared light from the OPO and pass it
through a photoacoustic cell containing water vapor for
wavelength calibration. After a delay of ϳ30 ns a separate
Nd:Yag-pumped dye laser, operating in the region of 680
nm, further excites the HOCl molecules from the intermedi-
light while monitoring the yield of OH͑ ϭ2, Nϭ2͒ fragments as the vi-
vЉ
brational excitation laser frequency is scanned. ͑b͒ Action spectrum of the
2␯OH→7␯ transition with the IR laser frequency set on the PQ2 stack
OH
shown in ͑a͒ while probing the OH͑ ϭ0, Nϭ8, 2⌸ ͒ state resulting from
vЉ
3/2
unimolecular dissociation. ͑c͒ Action spectrum of the 2␯OH→7␯OH transition
with the IR laser parked on the PQ3 stack shown in ͑a͒ while probing the
2
OH͑ ϭ0, Nϭ8,
vЉ
⌸ ͒ state resulting from unimolecular dissociation.
3/2
dissociation.11 In these experiments HOCl molecules at room
temperature are first vibrationally excited to the 2␯ level
OH
using infrared light from the OPO and then subsequently
photodissociated using UV light at 355 nm from a Nd:Yag
laser. Monitoring the yield of a particular quantum state of
the OH fragment resulting from the photodissociation, as the
frequency of the vibrational excitation laser is varied, gener-
ates the action spectrum shown in Fig. 2͑a͒. The spectrum
consists of both perpendicular and parallel transitions char-
acteristic of a hybrid band of a near prolate symmetric top
and the assignments shown in Fig. 2͑a͒ are based on the
earlier work of Cavazza et al. using high resolution absorp-
tion spectroscopy.12͑a͒ Having determined the location of
these spectral features, we are able to implement the second
step in the double resonance scheme.
ate state to the 7␯
vibrational level. The frequency
OH
doubled output of a third Nd:Yag pumped dye laser system
is used to detect the OH fragments resulting from the unimo-
lecular decomposition by laser induced fluorescence ͑LIF͒
through the A–X transition at ϳ308 nm.
RESULTS AND DISCUSSION
The first requirement for implementing the double-
resonance scheme to investigate unimolecular reaction, in-
volves characterization of the intermediate vibrational state.
As is evident from Fig. 2͑a͒, the resolution of our OPO
(ϳ0.3 cmϪ1) is insufficient to resolve individual rotational
transitions of the 2␯ band in HOCl. However, if we park
OH
For HOCl, we have examined the 2␯
using the technique of vibrationally mediated photo-
vibrational level
the OPO on one of the isolated Ka stacks associated with a
perpendicular component of this band, for example the Q2
OH
P
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3732
Letters to the Editor
ening at ⌬␯Ͻ0.05 cmϪ1. This value allows us to place a
stack shown in Fig. 2͑a͒ and then scan the dye laser corre-
sponding to the second step of the double resonance excita-
tion process, we obtain the spectrum shown in Fig. 2͑b͒. This
action spectrum corresponds to excitation to the 7␯OH vibra-
tional level from the selected intermediate state͑s͒ and is ob-
tained by monitoring the yield of OH(Nϭ8,2⌸3/2) fragments
from the unimolecular dissociation of HOCl as the frequency
of the 2␯OH→7␯OH excitation laser is varied. As is clear from
the figure, we are able to resolve individual rotational tran-
sitions of this parallel band with our ϳ0.1 cmϪ1 resolution
dye laser. Thus, even though the resolution of our OPO pre-
vents complete state selection in the 2␯OH intermediate level,
lower limit on the dissociation lifetime of HOCl from the
7␯
level at ⌬tϾ1.06ϫ10Ϫ10 s (⌬tϭ1/2␲⌬␯). We can
OH
compare this result with predictions of RRKM theory where
the dissociation rate is given by k(E)ϭN](EϪE0)/h␳(E).
In the above expression N] corresponds to the number of
open channels at the transition state, h is Plank’s constant, ␳
is the density of states of the reactant molecule and E0 is the
threshold energy for reaction. As is well known, application
of this expression to a barrierless potential, such as in HOCl,
is difficult due to the problems associated with locating the
transition state and thus calculating N](EϪE0). Typically in
these situations the transition state is determined by applying
variational methods to locate the region of the potential en-
ergy surface where the reactive flux is a minimum. For our
current purposes, however, it suffices to take N](EϪE0)
ϭ1, which gives a lower limit for the RRKM rate. As we
show below, even this lower limit RRKM rate is substan-
tially larger than that obtained from the measurements. Apart
from N](EϪE0), the other quantity required for the rate
calculation is the density of states, which using currently
available spectroscopic data,12͑b͒ we determine to be ␳
ϭ0.2 states/cmϪ1 at the excitation energy. Using these val-
ues in the RRKM expression gives a lower limit estimate for
the unimolecular rate of: kRRKM(E)Ͼ1/h␳(E)ϭ1.5
the final rotational states of the 7␯
band are cleanly re-
OH
solved in these experiments allowing a state selective study
of the dissociation dynamics. By parking the OPO on various
other Ka stacks of the 2␯OH spectrum shown in Fig. 2͑a͒ and
then scanning the dye laser, we can effectively decompose
2␯OH→7␯
excitation spectrum into its Ka components.
Figure 2͑c͒ shows the results of selecting transitions belong-
OH
P
ing to the Q3 stack in the intermediate state.
Although extensive spectroscopic analysis exist for the
lower vibrational levels of HOCl,12 the 7␯ band has not
OH
been previously analyzed. The rotational assignment shown
in Fig. 2͑b͒, is based on a analysis using an asymmetric rotor
fitting program13 and has been subsequently verified using
combination differences of the well characterized 2␯ in-
ϫ1011 sϪ1
, which corresponds to a lifetime of 6.7
OH
termediate state. In these experiments for confirming spectral
assignment, we fix the dye laser ͑corresponding to the sec-
ond step of the double resonance scheme͒ on a particular
ϫ10Ϫ12 s. The difference between this lifetime and that es-
timated from the linewidth measurements suggests that the
fundamental assumption of RRKM theory, that energy ran-
domizes on a time scale much faster than the rate of disso-
feature of the 7␯ band that we are interested in assigning,
OH
and then scan the wavelength of the IR laser to determine
ciation, appears not to hold for the 7␯ vibrational state of
OH
which states of the 2␯ level are connected to it through
dipole selection rules. The lines appearing in the resulting
spectra can easily be assigned using known spectroscopic
HOCl. Incomplete vibrational mode mixing in the molecule
is consistent with the fairly regular spectrum observed. We
have also tried to determine the dissociation rate of the
OH
constants for the 2␯ level and from this information, the
HOCl͑7␯ ͒ level directly by varying the time delay be-
OH
OH
assignment of the spectral feature in the 2␯OH→7␯OH spectra
can then be deduced. From a preliminary analysis of the
action spectra, such as the ones shown in Figs. 2͑b͒–2͑c͒, we
have obtained a set of approximate spectroscopic constants
for the band origin and rotational parameters of the HOCl35
tween the excitation and probe lasers, but have found the rate
to be faster than the time resolution afforded by our nano-
second laser system.
In addition to measuring the linewidths, we have also
examined the reaction dynamics by monitoring the OH frag-
ment internal state distributions resulting from the unimo-
lecular dissociation. Figure 3, shows the product rotational
(7␯
)
OH
band. These are ␯ϭ21 709.07 cmϪ1
ϭ15.156 cmϪ1, Bϭ0.503 cmϪ1, and Cϭ0.487 cmϪ1
,
A
.
2
An examination of the linewidths associated with the
resonances appearing in Kaϭ1 subband ͓i.e., Figs. 2͑b͔͒ in-
dicates that they are limited by the linewidths of our dye
laser (ϳ0.1 cmϪ1). For example, the feature indicated by the
arrow in Fig. 2͑b͒, which corresponds to the Jϭ12, Kaϭ1,
Kcϭ11, 12(7␯OH)←Jϭ13, Kaϭ1, Kcϭ12, 13(2␯OH) reso-
nance, has a linewidth of 0.11 cmϪ1 ͑FWHM͒. We are un-
able to resolve the asymmetry splitting associated with this
transition or the splitting due to the Cl35/Cl37 isotopes.
Within our resolution, the widths of the resonances in the
Kaϭ1 manifold appear to be independent of the J quantum
number of the parent molecule spanning a narrow range of
values around 0.11 Ϯ 0.01 cmϪ1 ͑FWHM͒. Taking into ac-
count the influence of Doppler broadening and the finite
spectral width of the excitation laser, we estimate an upper
limit contribution to the linewidth from homogeneous broad-
state distribution for the OH ⌸3/2 manifold obtained from
dissociation of the Jϭ12 state of HOCl͑7␯ ͒ for various
OH
values of Ka . These measurements clearly reveal that the
position and amplitude of the oscillations appearing in the
OH product state distribution are strongly dependent on the
rotational quantum numbers of the initial state. From the
Doppler widths associated with the laser induced fluores-
cence spectra of the OH fragment, it is possible to obtain an
estimate for the heat of formation, ⌬H00 of HOCl. So far, we
have not determined the branching ratio between the two
possible spin–orbit states of the chlorine fragment which are
separated by ϳ 882 cmϪ1. However, if we assume that the
highest energy rotational state of the OH fragment that is
detected, Nϭ11, correlates exclusively with the production
2
of chlorine atoms in their lowest spin–orbit state, the P3/2
state, then the linewidth associated with transitions appearing
J. Chem. Phys., Vol. 107, No. 9, 1 September 1997
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3733
with the OH(Nϭ11,2⌸3/2) state, we estimate ⌬H00 for HOCl
to be Ϫ16.7Ϯ0.6 kcal/mol in fair agreement with the value
reported in the JANAF data base (Ϫ17.1Ϯ0.5 kcal/mol).15
In addition, we note that the small translational energy re-
lease observed for the OH(Nϭ11) state, suggests that if
there is a barrier along the OHϩCl dissociation coordinate
of HOCl, it must be fairly modest as we would expect a
substantial fraction of the energy associated with the barrier
height to be channeled into relative translation. We are cur-
rently examining dissociation of HOCl from the 6␯OH vibra-
tional level, which due to its lower energy, will facilitate a
more critical examination of the threshold region for reac-
tion.
ACKNOWLEDGMENTS
We thank the National Science Foundation for support
of this work and Professor Fleming Crim for useful discus-
sion.
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FIG. 3. OH fragment product rotational state distribution resulting from
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OH
angular momentum state for various values of the Ka quantum number. The
data is obtained by probing rotational levels in the ground vibrational state
of OH using transitions belonging to the Q1(N) branch.
8
in the OH LIF spectra allows an estimate of the heat of
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Ϫ
2
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D0ϭErot OH(N) ϩEtran , with D0ϭ⌬H0(OH)ϩ⌬H0(Cl)
͓
͔
0
0
Ϫ⌬H00(HOCl). In the above equations Ei is the energy of
the initial rotational state of HOCl from which the double
resonance excitation is initiated, h␯ and h␯ are the photon
12
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1
2
energies used in the double resonance scheme, Erot OH(N)
͓
͔
is the rotational energy of the probed OH fragment, Etran is
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this equation and using the Doppler linewidths associated
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J. Chem. Phys., Vol. 107, No. 9, 1 September 1997
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