Photodissociation dynamics of phosgene: New observations by applying a three-dimensional imaging technique

Article abstract of DOI:10.1063/1.1427072

The three-dimensional (3D) momentum vector of single Cl atoms in the uv photodissociation of phosgene was directly observed using the recently developed 3D imaging technique. The previous results as there is the generation of chlorine in the ²P_/3/2 electronic ground state as well as in the electronically excited ²P_1/2 state, the highly spin selective process with bimodal kinetic energy distribution and the overall decay mechanism as well as the branching ratios were confirmed. The anisotropy parameter β and its speed dependence were observed for the first time.

Full text of DOI:10.1063/1.1427072

Photodissociation dynamics of phosgene: New observations by applying a three-
dimensional imaging technique
Tina Einfeld, Alexei Chichinin, Christof Maul, and Karl-Heinz Gericke
Citation: The Journal of Chemical Physics 116, 2803 (2002); doi: 10.1063/1.1427072
View online: http://dx.doi.org/10.1063/1.1427072
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 116, NUMBER 7
15 FEBRUARY 2002
Photodissociation dynamics of phosgene: New observations
by applying a three-dimensional imaging technique
Tina Einfeld, Alexei Chichinin,a) Christof Maul, and Karl-Heinz Gericke
¨
¨
Institut fur Physikalische und Theoretische Chemie, Technische Universitat Braunschweig,
Hans–Sommer–Straße 10, D-38106 Braunschweig, Germany
͑Received 7 September 2001; accepted 19 October 2001͒
The photodissociation dynamics of COCl2 has been studied by monitoring ground Cl(2P3/2) and
2
*
spin–orbit excited Cl ( P1/2) fragments by applying a novel technique where the three-dimensional
momentum vector of a single reaction product is directly determined. The photodissociation at 235
nm produces exclusively three fragments: COCl2ϩh␯→COϩ2Cl. The kinetic energy distributions
*
of Cl and Cl are bimodal and exhibit a different behavior for the different spin–orbit states. Our
attention was turned to the dependence of the anisotropy parameter ␤ on the fragment velocity
which was observed for the first time. For both spin–orbit states the anisotropy parameter differs
clearly for slow and fast chlorine atoms, where a pronounced change from the value ϳ0.7 to zero
at about 20 kJ/mol is observed. Slow chlorine atoms are released isotropically and predominantly in
the ground state Cl whereas fast chlorine atoms have an anisotropy parameter close to the
theoretically limiting value and are distributed between ground and excited state Cl. These
observations can be explained by a sequential decay where the first Cl fragment is released in a fast
process characterized by the nonvanishing positive ␤ parameter and a lifetime of р210 fs, whereas
the second Cl fragment is released after a period which is long on a rotational time scale. A
significant contribution of a symmetric mechanism can be excluded. © 2002 American Institute of
Physics. ͓DOI: 10.1063/1.1427072͔
I. INTRODUCTION
plied to the detection of atomic chlorine fragments from the
photoinduced three-body decay of phosgene (COCl2).
Phosgene is a planar molecule belonging to the C2v
symmetry group and exhibits a broad first absorption con-
tinuum from 220 nm to 280 nm which has been assigned to
Carbonyl compounds are known to exhibit a complex
fragmentation scheme upon irradiation by ultraviolet light
because of their small threshold energy with respect to three-
body decay. The decay dynamics of acetone,1 carbonyl
cyanides,2–5 acetyl halides,6,7 and phosgene8–10 have been
investigated by almost any technique offered by modern re-
action dynamics, among them nonresonant and resonance
enhanced photofragment translational spectroscopy, photo-
fragment imaging, velocity-map imaging and ultrafast spec-
troscopy. Earlier work is compiled in a review paper about
photoinduced three-body decay.11 Generally, when decaying
into three fragments, in their first absorption bands all carbo-
nyl compounds are observed to follow a step-wise decay
mechanism where the two bonds break in two separate ki-
netic events.12
13
1
the dipole-forbidden A2 1A1 transition. The weak struc-
ture is evidence of vibrational predissociation of the excited
1
electronic A2 state. According to molecular orbital ͑MO͒
theory the ground state configuration is found to be
The characterization of three-body decays requires to ex-
perimentally obtain more detailed information than the in-
vestigation of the corresponding two-body process due to the
increased number of degrees of freedom. In this paper a
newly developed method is presented which combines the
standard resonance enhanced multiphoton ionization and
time-of-flight method ͑REMPI–TOF͒ with a position sensi-
tive detector ͑PSD͒, thus enabling for the first time the ob-
servation of the full three-dimensional momentum vector of
a single dissociation product along with a spectroscopic de-
termination of its quantum state. The method has been ap-
*
At 235 nm a ␲CO(4b2) nCl(8b1) excitation is expected.
This leads to an increase of the bond length of C–O to be
accompanied with the loss of the planar symmetry from C2v
to Cs .
Phosgene is known to decay into three fragments
COCl2ϩh␯→COϩ2Cl
͑1͒
a͒
when irradiated by light at 235 nm in the peak of the first
absorption band.9 The alternative two-body channels produce
Permanent address: Institute of Chemical Kinetics and Combustion,
630090 Novosibirsk, Russia.
0021-9606/2002/116(7)/2803/8/$19.00
2803
© 2002 American Institute of Physics
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2804
J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Einfeld et al.
FIG. 1. A schematic REMPI/TOF/
DLD 3D imaging experimental setup.
BBO, frequency doubling crystal; PP,
Pellin–Broca prism; ␭/2, half wave
plate; f.1., focal length; ϩU1, accel-
eration voltage ͑ϳ16 kV/m͒; ϩU2, ex-
traction voltage ͑ϳ300 V͒; MCP, mul-
tichannel plates; DLD, delay-line
detector. The photolysis and probe la-
ser is a frequency doubled dye laser
pumped by a Nd:YAG laser.
a chlorine atom and a COCl radical or a CO and a Cl2 mol-
ecule
II. EXPERIMENT
A more detailed description of the experimental setup
and the novel position sensitive detector ͑PSD͒ will be pub-
lished elsewhere.16 It consists of a combination of a home-
built single-field time-of-flight ͑TOF͒ mass-spectrometer and
a position sensitive detector.17–19 The spectrometer was
evacuated to a base pressure of ϳ10Ϫ8 mbar by a turbo mo-
lecular pump system. A mixture of 5% phosgene in helium
was fed into the spectrometer via a continuous supersonic
beam. With a nozzle diameter of 20 ␮m and a stagnation
pressure of ca. 3 bar typical working pressures were in the
order of 10Ϫ7 mbar. Under these conditions the beam is
characterized by a rotational temperature of 8 K, determined
by a rotationally resolved spectrum of the NO (A 2⌺ϩ
X 2⌸) transition.20
Simultaneous dissociation and state-selective detection
of chlorine atoms were performed using a single dye laser
pumped by a Nd:YAG laser ͑Coherent, Infinity 40 100͒. The
dye laser ͑Lamba Physik, Scanmate͒ was operated with Cou-
marin 47 at a repetition rate of 100 Hz, its light was fre-
quency doubled by a BBO crystal and focussed by a 20 cm
lens in order to decrease the reaction volume to 5
ϫ10Ϫ4 mm3. The output window was arranged in a Brewster
angle configuration of 54° to reduce the internally reflected
light. The energy of the frequency-doubled light amounted to
5–10 ␮J per pulse. The energy was kept low to keep the ratio
of approximately one fragment signal per 10 laser pulses to
avoid kinetic energy transfer to the fragments due to space
charge effects and saturation of the dissociation step. The
light beam, the molecular beam, and the detector axes were
mutually orthogonal in the interaction region, as shown in
Fig. 1. Ultimate care was taken to overlap the light and the
molecular beam which was checked frequently by monitor-
ing of NO via (1ϩ1) REMPI at 226 nm.20 The polarization
of the laser was changed by a half wave plate in order to
investigate the spatial fragment distribution. Typically the ac-
celeration voltage was 800 V in the acceleration tube of the
COCl2ϩh␯→COClϩCl,
COCl2ϩh␯→COϩCl2.
͑2͒
͑3͒
At this dissociation wavelength channel ͑2͒ is not accessed
and the contribution of channel ͑3͒ is small, which was de-
rived from the analysis of the kinetic energy distributions
͑KED͒ of the partner fragments Cl and CO, respectively.9,10
For higher dissociation energies accessing different excited
electronic states emission from electronically excited Cl2 was
observed.14 At 235 nm CO molecules are produced with a
high rotational excitation resulting from the nonplanar geom-
1
etry of the excited A2 state and a nonsymmetric two-step
decay mechanism.9 Cl atoms have been monitored in both
2
2
*
and Cl P1/2 ͒
͓ ͔
spin–orbit fine-structure states ͑Cl P
͓
͔
3/2
with a strong preference for producing ground state
Cl(2P3/2) atoms.10,15 The KEDs of Cl and Cl are bimodal,
*
but differ markedly in the contribution of slow and fast
fragments.10 These results are in accord with a nonsymmetric
two-step mechanism for the three-body break-up of phos-
gene. A small contribution of a symmetric decay mechanism
was postulated in order to fully explain the observed KEDs.
Due to the lack of experimental information about the
spatial fragment distribution and about the symmetry of the
excited electronic state the identity of the potential energy
surface on which the dissociation proceeds has remained ob-
scured. With our new three-dimensional imaging setup the
previously observed results are fully confirmed. Additionally,
the three-dimensional momentum distribution of the chlorine
fragments directly yields the anisotropy parameter ␤, which
characterizes the spatial distribution of the photofragments.
For the first time the speed dependence of the ␤ parameter is
observed, allowing us to draw new conclusions on the decay
mechanism and the involved potential energy surfaces.
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J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Photodissociation of phosgene
2805
FIG. 2. Absorption cross section of COCl2 based on the
publication of Moule and Foo ͑Ref. 13͒. The wave-
length used in the present experiment is marked by an
arrow.
TOF spectrometer corresponding to an acceleration field of
The output charge from the MCP resulting from every
single incoming ion ͑‘‘event’’͒ produces altogether four dif-
ferential signals, two on each delay line pair. These signals
are decoupled from the DC voltages on the wires, amplified,
and transmitted to constant fraction discriminators and to the
TDCs. Finally, one event produces two pairs of times, X1 ,
X2 and Y1 , Y2 , on the delay lines that are wound along the
X axis and the Y axis, respectively. The X and Y coordinates
of a single event in time units may be calculated as
(X1 –X2 ,Y1 –Y2); and the time of the event ͑corresponding
to the Z coordinate͒ may be calculated as either (X1
ϩX2)/2 or (Y1ϩY2)/2. Thus the DLD yields the 3D coordi-
nates of each single event which contain the complete infor-
mation about the dissociation process. 2D images, kinetic
energy distributions ͑KEDs͒, and the ␤ anisotropy parameter
can easily be extracted by appropriate averaging techniques.
2
16 kV/m. The PJ state of the chlorine atom is split by 882
cmϪ1 due to spin–orbit coupling into Cl(2P3/2
) and
2
*
Cl ( P1/2). Both states were detected by a (2ϩ1) REMPI
process. The ground state was probed via the (2D3/2
2P3/2) transition at 235.336 nm, the excited state by
)
the (2P1/2 2P1/2 transition at 235.205 nm.21,22 The
CO(X 1⌺ϩ) fragments were ionized via (2ϩ1) REMPI us-
ing the B 1⌺ϩ state as resonant intermediate.10,12 The em-
ployed wavelength around 230.15 nm for realizing the (2
ϩ1) detection is almost at the maximum of the phosgene
absorption spectrum ͑see Fig. 2͒.13 Typically the wavelength
of the Cl atom transitions was scanned over a range of
Ϯ0.003 nm due to the Doppler broadening. Signals were
processed by the electronic setup described below, digitized
by time-to-digital converters ͑TDCs͒, accumulated over 2
ϫ105 laser shots and saved on-line by a personal computer.
The TOF spectrometer has an effective length ratio of
the acceleration to the drift tube of 1:2 at a total length of 15
cm. The PSD includes a delay-line anode ͑DLA͒18 of the size
of 100 mmϫ100 mm introduced into the spectrometer cham-
ber right behind the double stage multichannel plates ͑MCP͒
with an active diameter of 80 mm, as shown in Fig. 1. The
DLA consists of two sets of wire pairs orthogonal to each
other, one monitoring the X component and the other the Y
component of the fragment momentum.16 The individual
lines of each wire pair itself are wound parallel to each other
with a small inner-wire potential difference of about 30 V.
An extraction voltage of about 300 V provides a collection of
the charge cloud on the anode with specific broadening over
several wire distances. The broad impact of the secondary
electrons from the MCPs is focussed by the inner-wire po-
tential difference. The induced signal propagates in both di-
rections towards the ends where impedance adjusted circuits
pick it up for further processing. Four differential amplifiers
at the two ends of each line pair amplify only transient po-
tential differences between the lines of a line pair, thus re-
jecting potential variations simultaneously present on both
lines. The spatial resolution of the delay-line anode is better
than 400 ␮m which corresponds to an energy resolution of
ϳ1% under optimal conditions.
III. ANALYZING PROCEDURE
From the observed 3D distributions not only the speed
distribution can be extracted, but it is also possible to deter-
mine the angular distribution of the photofragments for any
given speed value v. A general approach to study angular
distributions of photodissociation products will be consid-
ered in the following. The normalized angular distribution of
photofragments with a single given speed with respect to the
polarization vector E of the electromagnetic field of the pho-
tolysis laser is usually written as
1
P ␪,␸͒ϭ
͑
1ϩ␤P cos ␪͒ .
͑4͒
͓
͑
͔
2
4␲
Here ␪ and ␸ are the polar and the azimuthal angle of the
photofragment recoil velocity v with the polarization vector
E, ␤ is the anisotropy parameter which characterizes the an-
gular distribution, and P2(x) is the second-order Legendre
polynomial: P2(x)ϭ 12(3x2Ϫ1). P(␪,␸) is normalized such
that
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2806
J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Einfeld et al.
TABLE I. Limiting kinetic energies of the fragments via different decay
channels.
␲
2␲
sin ␪ d␪
d␸ P ␪,␸͒ϭ1.
͑
͵ ͵
0
0
Ekin ͑kJ/mol͒ via
Ekin ͑kJ/mol͒ via three-body
If the integration over ␸ is performed, then a distribution
Fragments
Cl
two-body decay
decay
Pth(␪) with respect to the polar angle may be defined
Pth ␪͒ϭ 1 sin ␪ 1ϩ␤P cos ␪͒
Ͻ150
Ͼ105
Ͻ105 ͑sequential͒
Ͻ80 ͑symmetric concerted͒
͑5͒
͑
͓
͑
2
͔
2
CO
Cl2
Ͻ290
Ͼ120
Ͻ120
such that
␲
Ͻ115
P
␪͒d␪ϭ1.
͑
th
͵
0
The ␤ parameter ranges from Ϫ1 to ϩ2 with an isotropic
distribution of the fragments corresponding to ␤ϭ0. Most
often, the ␤ parameter is considered to be independent of the
fragment velocity. However, this need not to be the case.
Especially, the broader the kinetic energy distribution be-
comes, the more important becomes the velocity dependence
of ␤, and the more information on the dynamics of a reaction
is contained in this velocity dependence. An interesting ap-
plication of the 3D technique is the study of this dependence.
It means that we must calculate the ␤ parameter for a group
The procedure was calibrated against the photolysis of
Cl2 at 355 nm where the perpendicular optical transition
1
1
ϩ
g
⌸
⌺
contributes to more than 90% of the light
1u
0
absorption.23–26 The excited state is repulsive, so the disso-
ciation is fast and the ␤ parameter must be close to its lim-
iting value of Ϫ1. The value determined from our experi-
mental data is ␤ϭϪ1.00Ϯ0.05 and agrees excellently with
previous measurements and theoretical calculations.23
of photofragments that have a velocity within the range
v0
Ͻ Ͻ 1 . The experimental distribution of the photofrag-
IV. RESULTS
v
v
ments over the angle ␪ is given by the function Pexp(␪). The
␤ parameter may be determined from the condition
A. Decay channels
The one-photon dissociation of phosgene can in prin-
ciple proceed along one or several of the pathways ͑1͒–͑3͒.
For a dissociation wavelength of 235 nm the following reac-
tion enthalpies are obtained for the spin–orbit states with the
lowest energy:
␲
ץ
P
␪͒ϪP ␪͒ 2 d␪ϭ0.
͑6͒
͓
͑
͑
th
͔
͵
exp
ץ␤
0
The normalized experimental distribution Pexp(␪ ) of the
photofragments over the angle ␪ may be calculated as
COCl2ϩh␯→COϩ2Cl,
⌬H 0 K͒ϭ344Ϯ3.5 kJ/mol,
͑
N
1
Pexp ␪͒ϭ
␦ ␪Ϫ␪ ͒,
͑7͒
COCl2ϩh␯→COClϩCl, ⌬H 0 K͒ϭ274Ϯ45 kJ/mol,
͑
͑
͑
͚
i
N iϭ1
COCl2ϩh␯→COϩCl2,
⌬H 0 K͒ϭ105Ϯ3.5 kJ/mol.
͑
where ␦(x) is a delta function, ␪ is the polar angle of the ith
i
The dissociation enthalpies were calculated from the
standard enthalpies of formation (⌬H0f ) ͑Ref. 26͒ of the mol-
ecules and radicals involved in the process and the errors
were calculated as the square root of the sum of individual
squared errors. The ⌬H(0 K) required for loss of one or two
photofragment, and the summation is performed over par-
ticles with velocities in the range of consideration. N is the
number of these particles. Inserting Eq. ͑7͒ into Eq. ͑6͒ gives
the final result
␤ϭ 45 32B /␲ϩ1͒,
͑8͒
͑
1
*
Cl atoms is higher than the values given for one or two Cl
atoms in reactions ͑2͒ and ͑1͒ by 10.6 kJ/mol and 21.2 kJ/
mol, respectively. The maximal kinetic energies for the frag-
ments via the different decay channels are shown in Table I.
From the following considerations it is concluded that
the radical channel ͑2͒ does not contribute to the fragmenta-
tion process of phosgene. If a stable COCl radical would be
generated in the dissociation together with an atomic Cl frag-
ment this radical would have to carry an internal energy less
than its very low dissociation energy of ϳ70 kJ/mol.10 Since
the available energy Eav for the radical channel is 234 kJ/mol
(Eavϭh␯Ϫ⌬H(0 K)), the total fragment kinetic energy
must be in the range from 164 to 234 kJ/mol, corresponding
to a Cl kinetic energy range from 105 to 150 kJ/mol. Clearly,
the Cl KED shown in Fig. 3 does not extend into this region,
therefore the participation of the radical channel ͑2͒ can be
ruled out confirming previous results.10 We conclude that the
dissociation process produces two chlorine atoms or molecu-
lar chlorine and one carbon monoxide molecule.
where
B ϭ 1/N͚͒ sin ␪ P cos ␪ ͒ j.
͑
͓
͑
͔
i
j
i
i
2
The ␤ parameter can also be obtained from a subset of
the observed photofragments if the range of the angle ␪ is
accordingly constrained. Let us determine the ␤ parameter
for a group of the photofragments that are situated in the
range aр␪рb. The same approach gives the final result
A2B1ϪA0 A B ϩC ͒ϩA C
͑
1
0
1
1
0
0
␤ϭϪ
,
͑9͒
A0 A B ϪC ͒ϪA A B ϪC ͒
͑
͑
1
1
2
1
1
0
1
where
Ajϭ
Cjϭ
b
a
b
a
͐
͐
P
cos ␪͒ j sin ␪ d␪,
͑ ͔
2
͓
͓
P (cos ␪) j sin2 ␪ d␪.
͔
2
Applying Eq. ͑8͒ or Eq. ͑9͒ to experimental results one
can find the ␤( ) dependence.
v
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J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Photodissociation of phosgene
2807
ecules that was obtained via the B 1⌺ϩ X 1⌺ϩ transition at
ϳ230.15 nm using (2ϩ1) REMPI is shown in Fig. 4. Due to
the maximal kinetic energies shown in Table I, it is con-
cluded that only the CO fragments with higher kinetic ener-
gies than 120 kJ/mol can be released via the molecular chan-
nel. This leads to a contribution of the molecular channel of
less than 20% to the CO ( ϭ1) yield.
v
B. Spin–orbit state branching ratio
Scanning the laser over the two resonance transitions of
Cl yielded Doppler profiles which were used for calculating
the spin–orbital branching ratio. The measurements were re-
peated at different laser light intensities. Integrating the area
*
S under the profiles results in a signal ratio S(Cl )/S(Cl) of
0.27Ϯ0.04. Taking the ratio of transition probabilities B of
*
1.06 ͑Ref. 27͒ into account we determined a Cl yield
*
␾(Cl )ϭ0.22Ϯ0.03 where ␾ is defined as the ratio of the
*
number of excited state atoms P(Cl ) to the total number of
*
*
*
͔
chlorine atoms: ␾(Cl )ϭP(Cl )/ P(Cl)ϩP(Cl ) . This
͓
value agrees well with the previously determined value of
*
␾(Cl )ϭ0.15Ϯ0.03, when the following two aspects are
*
taken into account. Since no excited Cl is produced within
the experimental uncertainty of 5% at a dissociation wave-
length of 248 nm,15 it is concluded that the Cl exit channel
*
from the upper potential energy surface is opened when
changing the dissociation wavelength from 248 to 235 nm.
*
As Cl was probed by 237.8 nm in the previous measure-
FIG. 3. Kinetic energy distribution for ground state Cl(2P3/2) ͑a͒ and ex-
*
ments, a smaller amount of Cl is expected. And although
2
*
cited state Cl ( P1/2) ͑b͒ atoms produced in the photodissociation of COCl2
the effect of ion-flyout was taken into account in the analysis
of the former measurements, experimental inaccuracies as,
e.g., the small size of the ionization volume lead to underes-
timating the contribution of fast particles. Ion-flyout occurs
at 235 nm. The dependence of the ␤ parameter on the Cl fragment kinetic
energy ͑right scale͒ is shown by open circles with error bars. This 1D pre-
sentation is obtained via integration of the 3D data. Details of the respective
high and low energy components of the bimodal distributions are listed in
Table II.
due to particles with slow velocity components
towards
perpen-
vz
vx
the detector and large velocity components
or
vy
dicular to the detector so that these particles do not reach the
The molecular channel cannot be observed directly in
our experiments. Whether it exists or not can only be decided
by monitoring the energetics of the CO molecule or by direct
observation of the molecule chlorine fragment as nascent
*
detector inside its active radius. As Cl in the spin–orbital
excited state is preferentially released with high kinetic en-
*
ergies the total amount of Cl could be slightly underesti-
mated in the previous measurements.
particle. The kinetic energy distribution of CO ( ϭ1) mol-
v
C. Speed dependent anisotropy parameter ␤ Õ
velocity distribution
Previously, experiments were performed under condi-
tions where the dissociation step was saturated.10 Whereas
state specific KED were determined and taken for a detailed
discussion of the dissociation dynamics, the experimental
observed fragment distribution was found to be independent
of the polarization geometry and information about the ␤
parameter could not be extracted. This does not mean, how-
ever, that the dissociation process itself is isotropic. As we
are able to observe the three-dimensional ͑3D͒ momentum
vector of a single reaction product we turned our attention to
the observation of the speed dependent anisotropy parameter
␤. Experimental conditions were carefully chosen such that
saturation of the dissociation did not occur.
FIG. 4. Kinetic energy distribution for CO ( ϭ1) produced in the photo-
v
dissociation of COCl2 at ϳ230.15 nm. The dependence of the ␤ parameter
on the CO fragment kinetic energy ͑right scale͒ is shown by open circles
with error bars. The energetic threshold of the three-body decay channel is
marked.
In Fig. 3 the kinetic energy distributions and the kinetic
*
energy dependent ␤ parameters for Cl and Cl are presented.
This 1D presentation is obtained via integration of the 3D
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2808
J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Einfeld et al.
TABLE II. Characteristic data describing the kinetic energy contributions of
Here ␩ is the angle between the velocity vector that
would result if the molecules were not rotating and the ve-
locity vector that actually results, ␻ is the angular rotational
frequency, ␤exp the experimentally observed anisotropy
˜
the Cl fragments in the COCl2 dissociation. Ekin describes the averaged
¯
kinetic energy in the two energy regions separately, whereas Ekin is the
averaged value over the whole energy distribution. EG is the center of the
Gaussian.
*
parameter and ␤ the theoretical limit. A molecular beam
temperature of 8 K has been assumed. As the lifetime differ-
ences are due to the experimental inaccuracy in the ␤ param-
eter measurement, a mean lifetime of 210Ϯ20 fs is obtained.
These small limiting lifetimes are a consequence of the large
moments of inertia for the COCl2 molecule leading to a fast
loss of anisotropy due to molecule rotation. Under molecular
˜
¯
Ekin
Contribution
EG
␤
Ekin
Fragments
Cl
͑kJ/mol͒ ͑kJ/mol͒
͑%͒
͑kJ/mol͒ parameter
Slow
Fast
12
25
42
47
31
10
36
0
0.7Ϯ0.1
*
Cl
Slow
Fast
9
46
50
3
19
10
46
0
0.6Ϯ0.1
*
beam conditions, in the case of Cl and Cl with high kinetic
energy a ␤ value close to the limiting value is evidence of
fast bond cleavage for the first step of the decay where one
data. Although both KED are bimodal, a remarkably differ-
ent behavior of the two spin components with respect to their
kinetic energy acquisition in the photodissociation is obvi-
ous. Cl in the electronic ground state is produced over the
whole available energy range up to the calculated limit of
105 kJ/mol for the three-body decay. The bimodal character
divides the energy distribution in two parts. Although the two
*
Cl or Cl atom with high kinetic energy is released.
Taking the bimodal structure and the ␤ parameter speed
dependence into account, the existence of a sequential decay
from the originally prepared excited state of the parent mol-
*
ecule is postulated. Fast Cl and Cl atoms with a nonvanish-
ing ␤ parameter are released in the first step, and slow, pre-
dominantly ground state Cl atoms with an almost isotropic
distribution are released in the second step.
*
parts are less clearly separated than in the case of Cl , a
limit between the fast and slow atoms of 25Ϯ5 kJ/mol can
be estimated. The peak of the slow Cl atom distribution is at
4 kJ/mol, whereas the peak for the fast atoms is at 30 kJ/mol.
With this picture one can divide the kinetic energy or
velocity distribution in two parts. Hence, two overlapping
Gaussians were fitted to the distributions, one describing the
distribution of fast Cl atoms released in the first step with a ␤
parameter of 0.65 and the second one describing the distri-
bution of slow Cl atoms with a ␤ parameter of zero released
in the second dissociation step. The ␤ value of 0.65 is taken
as the mean experimental value observed for fast ground Cl
*
Cl in the excited spin–orbit state is preferentially produced
with large kinetic energies above 20 kJ/mol showing only a
tail in the low energy region. The distributions for slow and
*
fast Cl atoms are clearly separated with a peak at 7 and 42
*
kJ/mol for slow and fast Cl , respectively. Noteworthy is the
kinetic energy dependence of the ␤ parameter. In the case of
particles with low kinetic energy the ␤ parameter is about
zero which means that the particles are isotropically distrib-
uted whereas in the region of high kinetic energy the ␤ pa-
rameter increased to 0.6Ϯ0.1 and 0.7Ϯ0.1 for ground-state
*
and excited Cl atoms. The velocity distributions were used
for the fit procedure taking into account the branching ratios
given in Table II. The centers of the Gaussians are at 750 m/s
corresponding to 10 kJ/mol and 1440 m/s ͑36 kJ/mol͒ with
widths of 1500 m/s ͑40 kJ/mol͒ and 1100 m/s ͑20 kJ/mol͒ for
*
chlorine and spin–orbit excited Cl , respectively. For Cl the
increase happens smoothly starting at a kinetic energy value
of 20 kJ/mol whereas for excited chlorine the increase of the
␤ value happens rapidly at the same kinetic energy. Detailed
*
information of the bimodal distributions for Cl and Cl is
given in Table II.
The fast chlorine atoms in both electronic states are pref-
erentially released through an electronic transition with a
positive ␤ parameter. These values are close to the theoreti-
cal limit of 1.04 for the ␤ parameter for instantaneous decay
from the ground state geometry with a Cl–C–Cl bond angle
of 111.3° if the nonplanar geometry of the excited state and
the corresponding symmetry change from C2v to CS are con-
sidered. Hence the A2 A1 excitation becomes an optically
allowed A
A excitation with the transition dipole mo-
Ј
Љ
ment ␮ oriented parallel to the line connecting the two Cl
atoms.
If only rotation28 of the parent molecule is responsible
for the reduction of the ␤ parameter for ground and excited
state chlorine, then lifetimes of ␶ϭ230 fs and 190 fs are
estimated, respectively, based on the relationship
FIG. 5. Fit of the kinetic energy dependent ␤ parameter ͑open circles͒ for
the ground state Cl(2P3/2) to the experimentally observed data ͑solid circles͒
on the assumption that the decay proceeds via a sequential three-body decay.
Two Gaussian were fitted to the bimodal velocity distribution, one describ-
ing the slow chlorine fragments with a ␤ parameter of zero ͑dotted curve͒
and the other the fast chlorine fragments with a ␤ parameter of 0.65 ͑dashed
curve͒. The best-fit functions are shown in the graph. The asymmetry is due
to the presentation of the Gaussian velocity fit in the energy domain.
␩
␶ϭ
,
͑10͒
␻
ͱ
*
where ␤expϭP2(␩)␤ and ␻ϭ ␲kT/2I.
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J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Photodissociation of phosgene
2809
proceeds in two steps, but whether the second decay:
*
COCl →COϩCl proceeds late within or after a rotational
period cannot be distinguished in our experiment. The reduc-
tion of the ␤ parameter to a value below 25% of its original
value indicates a transition of the COCl2 molecule to a non-
planar geometry in the upper state, in accordance with con-
siderations based on MO calculations.
The postulation of a sequential decay releasing the sec-
ond Cl atom with an isotropic distribution agrees with the ␤
dependence found for the CO molecule at ϳ230.15 nm,
which is formed in the second step together with the slow
component of the Cl atoms. The averaged value of the ␤
parameter of 0.07 as shown in Fig. 4, confirms the isotropic
nature of the second step in the sequential decay mechanism.
Therefore it is assumed that predominantly the sequential
decay takes place in the photodissociation at 235 nm which
is in good agreement with former measurements.9,10,30
FIG. 6. The dependence of the observable ␤ parameter on the lifetime of the
*
*
intermediate COCl2 or COCl is clarified for the first and the second decay
step, respectively. The observed time range of the first step is displayed as
well as the estimated time range of the second step and the time of a
rotational period. It can be seen that a slow asynchronous concerted decay
mechanism cannot be distinguished from a pure sequential decay with cer-
tainty.
V. CONCLUSIONS
We report the direct observation of the 3D momentum
vector of single Cl atoms in the uv photodissociation of
phosgene as the first application of the recently developed
3D imaging technique. The previous results as there is the
slow and fast Cl atoms, respectively. Considering these re-
sults, from the overlap one can calculate the theoretical ␤
speed dependence of the ground state chlorine for a pure
sequential decay. The obtained simulation is shown in Fig. 5.
It is noteworthy that the two curves shown in the energy
distribution in Fig. 5 are distorted due to the effect that the
original Gaussians were fitted to the velocity distribution.
The simulated ␤ speed dependence is in very good agree-
ment with the experimentally observed one. The small devia-
tions are likely to be due to a small contribution of the syn-
chronously concerted three-body decay. For the ground state
Cl, the smooth transition from a vanishing ␤ parameter to the
maximum value is a consequence of the large contribution of
slow ground state Cl compared to fast ground state Cl. Since
the fit functions overlap in the medium energy range, the ␤
parameter is accordingly reduced for ground state Cl,
2
generation of chlorine in the P3/2 electronic ground state as
2
well as in the electronically excited P1/2 state, the highly
spin selective process with bimodal kinetic energy distribu-
tion and the overall decay mechanism as well as the branch-
ing ratios were confirmed. The anisotropy parameter ␤ and
its speed dependence were observed for the first time. A sud-
den change of the ␤ parameter value at intermediate kinetic
*
energy for both ground and excited state Cl and Cl frag-
ments is evidence of a stepwise, predominantly sequential
process. The first bond cleavage produces fast Cl fragments
in both spin–orbit states, the second bond cleavage generates
slow Cl fragments preferentially in the electronic ground
state. Products from the first step are distributed highly an-
isotropically which is evidence for a fast fragmentation in
1
less than 210 fs on the originally accessed A (A ) surface
Љ
2
*
whereas for Cl this effect is negligible due to the small
and an effective energy flow from the excited C–O bond to
the C–Cl dissociation coordinate.
*
contribution of slow excited state Cl .
It is noteworthy that there is no suitable way to distin-
guish precisely between the pure sequential decay where the
ACKNOWLEDGMENTS
*
lifetime of the intermediate COCl is longer than its rota-
tional period and the very slow but still asynchronously con-
The authors are greatly indebted to Dr. Uwe Titt and the
*
certed decay where the intermediate COCl lives only
¨
Schmidt–Bocking group who strongly supported the elec-
slightly shorter than its rotational period. Figure 6 clarifies
the issue. Theoretically even very slow fragmentations, ␩
→ϱ, do not destroy the anisotropy of the fragments com-
pletely and therefore the ␤ parameter is only reduced to 25%
of its original value. Under assumption of a first-order decay
which means that the lifetime distribution is described by a
Poisson distribution, ␤ is characterized29 by
tronic and experimental design and to Dr. Melanie Roth for
her support and her continuous encouragement during the
time of this work. These studies were generously supported
by the Fonds der Chemischen Industrie and the Alexander
von Humboldt foundation.
1 E. W. G. Diau, C. Kotting, and A. H. Zewail, Phys. Chem. Chem. Phys. 2,
273 ͑2001͒, and references therein.
2
1ϩ␩
2 Q. Li, R. T. Carter, and J. R. Huber, Chem. Phys. Lett. 323, 105 ͑2000͒.
3 A. Furlan, H. A. Scheld, and J. R. Huber, J. Phys. Chem. A 104, 1920
͑2000͒.
␤ϭ2P2 cos ␪ ͒
,
͑11͒
͑
t
2
1ϩ4␩
where ␪ is the original angle between ␮ and . The tangen-
v
t
4 J. C. Owrutsky and A. P. Baronavski, J. Chem. Phys. 111, 7329 ͑1999͒.
5 R. J. Horwitz, J. S. Francisco, and J. A. Guest, J. Phys. Chem. A 101, 1231
͑1997͒.
tial velocity of Cl in the rotating parent can be neglected as
the recoil velocity of Cl is high in comparison to the rota-
tional velocity. As Fig. 6 shows, the photodissociation clearly
6 S. Deshmukh and W. P. Hess, J. Chem. Phys. 100, 6429 ͑1994͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
128.59.222.12 On: Sun, 30 Nov 2014 23:05:49

2810
J. Chem. Phys., Vol. 116, No. 7, 15 February 2002
Einfeld et al.
7 S. Deshmukh, J. D. Myers, S. S. Xantheas, and W. P. Hess, J. Phys. Chem.
98, 12535 ͑1994͒.
20 J. Danielak, U. Domin, R. Kepa, M. Rytel, and M. Zachwieja, J. Mol.
Spectrosc. 181, 394 ͑1997͒.
8 C. Maul, C. Dietrich, T. Haas, and K.-H. Gericke, Phys. Chem. Chem.
Phys. 1, 1441 ͑1999͒.
21 D. Ascenzi, P. M. Regan, and A. J. Orr-Ewing, Chem. Phys. Lett. 310, 477
͑1999͒.
9 C. Maul, T. Haas, and K.-H. Gericke, J. Phys. Chem. A 101, 6619 ͑1997͒.
10 C. Maul, T. Haas, K.-H. Gericke, and F. J. Comes, J. Chem. Phys. 102,
3238 ͑1995͒.
22 S.Arepalli, N. Presser, D. Robie, and R. J. Gordon, Chem. Phys. Lett. 118,
88 ͑1985͒.
23 T. Ishiwata, A. Ishiguro, and K. Obi, J. Mol. Spectrosc. 147, 300 ͑1991͒.
24 Y. Matsumi, K. Tonokura, and M. Kawasaki, J. Chem. Phys. 97, 1065
͑1992͒.
11 C. Maul and K.-H. Gericke, Int. Rev. Phys. Chem. 16, 1 ͑1997͒.
12 C. Maul and K.-H. Gericke, J. Phys. Chem. A 104, 2531 ͑2000͒.
13 D. C. Moule and P. D. Foo, J. Chem. Phys. 55, 1262 ͑1971͒.
14 H. Okabe, A. H. Laufer, and J. J. Ball, J. Chem. Phys. 55, 373 ͑1971͒.
15 A. Chichinin, Chem. Phys. Lett. 209, 459 ͑1993͒.
16 A. Chichinin, T. Einfeld, C. Maul, and K.-H. Gericke, Rev. Sci. Instr. 73
͑in press͒.
25 P. C. Samartzis, I. Sakellariou, Th. Gougousi, and Th. N. Kitsopoulos, J.
Chem. Phys. 107, 43 ͑1997͒.
26 M. W. Chase, NIST-JANAF, Thermochemical Tables, fourth ed. ͓J. Phys.
Chem. Ref. Data, Monograph 9, Parts IϩII ͑1998͔͒.
27 P. M. Regan, S. R. Langfold, D. Ascenzi, P. A. Cook, A. J. Orr-Ewing, and
M. N. R. Ashfold, Phys. Chem. Chem. Phys. 1, 3247 ͑1999͒.
17 S. E. Sobottka and M. B. Williams, IEEE Trans. Nucl. Sci. 35, 348 ͑1988͒.
18 O. Jarutzki, V. Mergel, K. Ullmann-Pfleger, L. Spielberger, U. Meyer,
28 G. E. Busch and K. R. Wilson, J. Chem. Phys. 56, 3638 ͑1972͒.
¨
¨
R. Dorner, and H. Schmidt-Bocking, Proceedings in Imaging Spectrom-
etry IV, San Diego, California ͑1998͒.
29
¨
K.-H. Gericke, Photodissoziationsdynamik kleiner Molekule, habilitation
19 M. Lampton, O. Siegmund, and R. Raffanti, Rev. Sci. Instrum. 58, 2298
͑1987͒.
thesis, Frankfurt a. M., 1990.
30 M. H. Wijnen, J. Am. Chem. Soc. 83, 3014 ͑1961͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
128.59.222.12 On: Sun, 30 Nov 2014 23:05:49