The photodissociation of carbonyl cyanide CO(CN)2 at 193 nm studied by photofragment translational energy spectroscopy

Article abstract of DOI:10.1063/1.479376

The photodissociation of carbonyl cyanide CO(CN)₂ at 193 nm was investigated by photofragment translational energy spectroscopy. For all the fragments created (CO, CN, OCCN, NCCN), the kinetic energy distributions were measured and two decay channels identified. The radical decay, CO(CN)₂ + hν→OCCN+CN, dominates with a yield of 94%±2% and shows the available energy mainly (82%) channeled into the internal degrees of freedom of the fragments. A fraction of 18%±6% of the nascent OCCN radicals has sufficient energy to spontaneously decay to CO+CN involving a barrier ≤160 kJ/mol. With a yield of 6%±2% the molecular decay produces the fragments CO+NCCN. These fragments acquire a high available energy owing to the formation of the new C-C bond in NCCN. An average fraction of 70% is partitioned into internal fragment energy. Even the fastest fragments are still internally hot, indicating that with the high barrier expected, a substantial exit channel interaction is operative. The isotropic recoil distribution found for the products CN, OCCN, and NCCN further suggests that both the radical and the molecular decay are, on the time scale of a parent rotation, slow and probably indirect.

Full text of DOI:10.1063/1.479376

The photodissociation of carbonyl cyanide CO(CN) 2 at 193 nm studied by
photofragment translational energy spectroscopy
Heiner A. Scheld, Alan Furlan, and J. Robert Huber
Citation: The Journal of Chemical Physics 111, 923 (1999); doi: 10.1063/1.479376
View online: http://dx.doi.org/10.1063/1.479376
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/111/3?ver=pdfcov
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 3
15 JULY 1999
The photodissociation of carbonyl cyanide CO„CN…2 at 193 nm studied
by photofragment translational energy spectroscopy
Heiner A. Scheld, Alan Furlan, and J. Robert Hubera)
¨
¨
Physikalisch-Chemisches Institut der Universitat Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland
¨
͑Received 15 March 1999; accepted 22 April 1999͒
The photodissociation of carbonyl cyanide CO͑CN͒2 at 193 nm was investigated by photofragment
translational energy spectroscopy. For all the fragments created ͑CO, CN, OCCN, NCCN͒, the
kinetic energy distributions were measured and two decay channels identified. The radical decay,
CO͑CN͒2ϩh␯→OCCNϩCN, dominates with a yield of 94%Ϯ2% and shows the available energy
mainly ͑82%͒ channeled into the internal degrees of freedom of the fragments. A fraction of
18%Ϯ6% of the nascent OCCN radicals has sufficient energy to spontaneously decay to COϩCN
involving a barrier р160 kJ/mol. With a yield of 6%Ϯ2% the molecular decay produces the
fragments COϩNCCN. These fragments acquire a high available energy owing to the formation of
the new C–C bond in NCCN. An average fraction of 70% is partitioned into internal fragment
energy. Even the fastest fragments are still internally hot, indicating that with the high barrier
expected, a substantial exit channel interaction is operative. The isotropic recoil distribution found
for the products CN, OCCN, and NCCN further suggests that both the radical and the molecular
decay are, on the time scale of a parent rotation, slow and probably indirect. © 1999 American
Institute of Physics. ͓S0021-9606͑99͒00927-7͔
I. INTRODUCTION
nm. The molecule consists of three strongly bound groups, a
CO and two CN moieties, connected by two comparatively
weak C–C single bonds. Beside the simple radical decay
with an ␣-cleavage ͑1a͒, a molecular decay
Carbonyl cyanide CO͑CN͒ ͑Fig. 1, insert͒ has recently
2
attracted attention as precursor for the OCCN radical.1 As
various carbonyl and cyano-containing polycarbon mol-
ecules have been identified in the interstellar medium2,3 the
existence of this radical is of interest in astrophysics and it
may also play a role in combustion processes. In a prelimi-
nary study1 we focused on the preparation and stability of the
CO͑CN͒2→COϩNCCN,
͑2͒
favored by a symmetric substitution of the parent molecule,
may be operative involving scission of the two ␣–C–C
bonds and the simultaneous formation of the new C–C bond
of the NCCN product. Molecular decays have been observed
for a number of molecules7–10 and probably most extensively
studied in the case of formaldehyde H2CO.7 To our knowl-
edge, the systems reported so far were limited to the associa-
photofragment OCCN. Following photolysis of CO͑CN͒ in
2
the gas phase at 193 nm, it was shown that dissociation pro-
ceeds primarily as
CO͑CN͒2ϩh␯→OCCNϩCN.
͑1a͒
tion of two atoms to a diatomic, whereas CO͑CN͒ would
Only about 25% of the nascent OCCN fragments have suf-
2
decay to a four atomic molecule requiring a complex transi-
tion state with substantial changes of bond angles and dis-
tances. The equivalence of the ␣–C–C bonds also makes
carbonyl cyanide a candidate for a three-body decay,11,12
ficient internal energy for a spontaneous decay according to
OCCN→COϩCN.
͑1b͒
The simple generation of a large fraction of stable OCCN
radicals was thus demonstrated, allowing this species to be
studied by spectroscopic methods or used as a reactant for
synthetic purposes. Prior to this study, OCCN has not been
prepared by photolysis and the only previous observation
was in a neutralization-reionization experiment by reduction
of the OCCNϩ cation.4 Francisco and Liu5 have recently
reported results of ab initio calculations on stability, struc-
ture and vibrational frequencies, and an experimental inves-
tigation on the spectroscopy of this interesting radical is in
progress.6
CO͑CN͒2→COϩCNϩCN,
͑3͒
where the two bond cleavages occur in a single kinetic step.
For such a dissociation process one distinguishes between a
synchronous concerted and an asynchronous concerted reac-
tion depending whether the dissociation dynamics follows a
synchronous or an asynchronous bond breaking of the two
identical bonds.11,12
To investigate the full picture of the photochemical de-
cay paths of CO͑CN͒ we used photofragment translational
2
energy spectroscopy ͑PTS͒.13,14 We were able to detect all
the generated fragments ͑CO, CN, OCCN, and NCCN͒ and
to measure their kinetic energy distributions as well as the
pertinent fragment recoil anisotropies.
The aim of the present work was to establish the decay
mechanism of isolated CO͑CN͒ following excitation at 193
2
a͒
Electronic mail: jrhuber@pci.unizh.ch
0021-9606/99/111(3)/923/8/$15.00
923
© 1999 American Institute of Physics
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924
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Scheld, Furlan, and Huber
angles ͑as defined between the electric field vector of the
laser and the detection axis͒. For this purpose, the excimer
laser beam was directed through a stack of ten quartz plates
at Brewster’s angle generating linearly polarized light with a
polarization degree of 92%Ϯ1%.
Carbonyl cyanide CO͑CN͒2 was synthesized and purified
according to a published procedure16 by reaction of tetracya-
noethylene oxide with n-butyl sulfide at 50 °C. The yellow-
ish liquid with a boiling point of 66 °C was purified by mul-
tiple distillation at reduced pressure. Premixed gas samples
of 2% CO͑CN͒ in 350 mbar He were expanded through a
2
piezoelectrically actuated valve ͑orifice 0.5 mm and opening
angle 140°͒ providing molecular pulses of 100 ␮s. The con-
centration of clusters CO͑CN͒
in the molecular beam
͔
2 n
͓
pulses was found to be negligible under these expansion con-
ditions. The onset of clustering occurs at He stagnation pres-
sure у800 mbar.17 The delay between the opening of the
nozzle and the photolysis pulse was kept fixed to probe the
molecular beam pulse Ϸ30 ␮s after its onset. The stream
velocity of the pulses was determined before and after each
photodissociation measurement using a chopper wheel syn-
chronized with the pulsed valve. The width of the velocity
distribution corresponds to a translational temperature of
Ϸ4 K.
FIG. 1. Gas phase absorption spectrum of carbonyl cyanide CO͑CN͒2 in the
wavelength range 190–410 nm. The spectrum was recorded at room tem-
perature using a vapor pressure of 2 mbar and a resolution of about 0.2 nm
͑FWHM͒.
II. EXPERIMENT
The experiments were carried out with a photofragment
translational energy spectrometer described in detail
elsewhere.15 A pulsed laser beam is directed along the rota-
tion axis of a pulsed molecular beam source which rotates in
a plane with the fixed detection axis. The variable angle be-
tween the molecular beam and the detection axis is denoted
as the scattering angle ⌰. Following a flight path of 34.5 cm,
the neutral fragments created in the intersection region of the
laser and the molecular beam are ionized by electron bom-
bardment and mass-filtered by a quadrupole mass spectrom-
eter ͑Balzers QMA 160͒. The ion signal from a secondary
electron multiplier was fed into a multichannel scaler ͑Stan-
ford Research SR 430, 1.28 ␮s bin width, 1024 channels͒.
The experiments were performed at 193 nm with an ArF
excimer laser ͑Lambda Physik Compex 102, repetition rate
20 Hz͒. The photolysis laser crossed the molecular beam at a
distance of 65 mm from the pulsed nozzle and 45 mm from
the skimmer. The laser beam was slightly focused to an el-
liptical spot, Ϸ2 mm along the detection axis and Ϸ6 mm
perpendicular to the detection axis, at the intersection with
the molecular beam. The time-of-flight ͑TOF͒ spectra were
recorded with laser fluences ranging from 10 to 100 mJ/cm2.
The probability for multiphoton absorption at fluences р20
mJ/cm2 was found to be very low, as expected from the
absorption cross section at 193 nm and the corresponding
saturation factor. A distinctive effect of multiphoton pro-
cesses was observed only at у50 mJ/cm2.
III. RESULTS
A. UV spectrum
The absorption spectrum of CO͑CN͒ in the range 190–
2
400 nm, shown in Fig. 1, exhibits a weak band system with
a maximum at around 330 nm and a strong absorption below
230 nm. The weak band system, which is due to the S0
*
→S1(n␲ ) transition localized on the CO group, is highly
structured. At least ten evenly spaced vibronic bands can be
distinguished in the range 270–400 nm. The spacing is ini-
tially about 1230 cmϪ1 and decreases with increasing exci-
tation. Prochorow et al.18 have analyzed the spectrum in
some detail and assigned this progression to the CϭO
stretching vibration in the S1 state guided by the results of
acetone. Each of these bands is further split into eight dis-
tinct, narrow peaks spaced by Ϸ128 cmϪ1. They are most
likely due to the excited state in-plane ␦͑N-C-C-C-N͒ bend-
ing mode (a1), whose corresponding ground state frequency
is 127 cmϪ1
.
*
Besides the n␲ transition, a strong, presently unas-
signed absorption starts at around 230 and extends to 190
nm, the lower limit of our ultraviolet ͑UV͒ measurement.
This absorption very probably involves a Rydberg transition
to the 1(n,3s) state by analogy with other carbonyl
compounds.19–21 Our photolysis experiments were carried
out at 193 nm, where the absorption cross section ⑀
Ϸ2300 dm3 molϪ1 cmϪ1 (␴ϭ8.7ϫ10Ϫ18 cm2), about 1000
The TOF distributions were recorded with unpolarized
laser light at scattering angles ⌰ϭ9, 30 and 60°. Particularly
severe at ⌰ϭ9°, the interfering background signal from par-
ent molecules cracked in the ionizer, was subtracted on a
shot-to-shot basis by leaving every second molecular pulse
unphotolyzed. The TOF distributions presented in this article
were corrected for the ion flight times through the mass filter.
The recoil anisotropy ␤ of the fragments13,15 CN,
OCCN, and (CN)2 was measured at six different polarization
*
times greater than at the maximum of the n␲ transition.
B. TOF spectra of „CN…2 and OCCN
Photofragments were collected at m/eϭ26(CNϩ),
28(COϩ), 52͓(CN)ϩ2 ͔, and 54(OCCNϩ). At the mass filter
setting m/eϭ54 and 52, the species OCCN and (CN)2 from
the primary dissociation steps ͑1a͒ and ͑2͒, respectively, con-
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925
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Carbonyl cyanide CO(CN)2 at 193 nm
FIG. 2. TOF distributions of NCCN fragments (m/eϭ52) measured at scat-
tering angles 9 and 60° after unpolarized excitation of CO͑CN͒ at 193 nm.
2
The solid lines through the data points were calculated from the translational
energy distribution P(ET) shown in Fig. 3͑a͒ as a dashed line which coin-
cides with the solid line above 80 kJ/mol.
FIG. 3. Center-of-mass translational energy distributions P(ET) for the
photoinduced decay of CO͑CN͒ to ͑a͒ COϩNCCN ͑molecular channel͒,
2
and ͑b͒ CNϩOCCN ͑radical channel͒. The dashed lines which coincide at
higher ET with the solid lines represent truncated P(ET)’s used to fit by
forward convolution the TOF profiles of NCCNϩ and OCCNϩ, respectively.
The arrows mark the available energy of the fragment pairs.
tribute exclusively to the signal. The signals at m/eϭ26 and
28, however, may originate from primary dissociation ͑1a͒
and ͑2͒, secondary dissociation of OCCN ͑1b͒, or cracking of
OCCN and (CN)2 in the ionizer of the detection system. In
the fit procedure, the unambiguous TOF spectra of OCCNϩ
and (CN)ϩ2 were therefore analyzed first. The center-of-mass
kinetic energy distributions P(ET) obtained from these fits22
were then used to calculate the TOF profiles of the counter-
fragments CN and CO, respectively, based on the strict
momentum-correlation of the fragment pairs.
(CN)2 to be 0.97, as calculated from the sum of atomic
polarizabilities,23 and correcting for the dependence of the
detection efficiency on kinematic factors,22 we obtain a ratio
OCCN/͑CN͒2Ϸ15. This estimate has, however, a substantial
uncertainty due to the neglect of the secondary dissociation
In the TOF spectra monitored at m/eϭ52 and shown in
Fig. 2, the signal stems exclusively from the fragment
(CN)2 . The spectra were recorded at scattering angles ⌰
ϭ9 and 60°. The solid lines represent the best fit obtained by
forward convolution22 of the center-of-mass kinetic energy
distribution P(ET). The latter is displayed in Fig. 3͑a͒ as a
dashed curve. As will be discussed below, this P(ET) repre-
sents not the full distribution which is given by the solid line,
but only a truncated part of it. The missing part below ET
ϳ80 kJ/mol is due to the slow (CN)2 fragments which pos-
sess sufficient internal energy to decay spontaneously into
CNϩCN before reaching the detector.
The same fitting procedure was applied to the TOF spec-
trum recorded at m/eϭ54 shown in Fig. 4 where only
OCCN fragments contribute to the signal. The solid line was
obtained from the dashed P(ET) curve in Fig. 3͑b͒. Similar
to the result for (CN)2 above, this P(ET) represents the
stable OCCN fragments which reach the detector without
undergoing secondary dissociation. A rough first estimate of
the branching ratio between the primary channels leading to
OCCNϩCN ͑1a͒ and to ͑CN͒2ϩCO ͑2͒ is obtained by com-
parison of the relative signal strengths at m/eϭ54 and 52.
Assuming the ratio of the ionization efficiencies of OCCN to
FIG. 4. TOF distributions of OCCN fragments (m/eϭ54) measured at ␪
ϭ9 and 30° after unpolarized excitation of CO͑CN͒ at 193 nm. The solid
2
lines are forward convolution fits using the truncated P(ET) shown in Fig.
3͑b͒.
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926
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Scheld, Furlan, and Huber
Furthermore, we assumed a forward-backward symmetric
angular distribution of these fragments and a statistical
decay24,25 implying a lifetime of the unstable OCCN longer
than one rotational period. The unstructured TOF spectra of
CNϩ do not justify a refined fit of the secondary decay pro-
cess.
The third contribution to the m/eϭ26 signal, which
stems from dissociative ionization of OCCN in the detector
and indicated by a dash-dotted line in Fig. 5, can be deter-
mined straightforwardly since the cracking pattern of OCCN
is known. Under our experimental conditions the relative sig-
nals were measured to be OCCNϩ/CNϩ/COϩϳ55/30/15.
͑We determined this ratio in a separate experiment using
methyl cyanoformate, NCCOOCH3, which yields 98% of
OCCNϩOCH3 after photolysis at 193 nm. Consequently, the
CNϩ and COϩ signals measured in this case originate almost
exclusively from cracking. The OCCN fragments from
CO͑CN͒ and NCCOOCH3 are expected to have essentially
2
the same cracking patterns since their average internal ener-
gies, about 170 and 150 kJ/mol, are not much different.͒
Furthermore, the OCCNϩ→CNϩ cracking contribution was
considered to have the same shape as the OCCNϩ profile in
Fig. 4 except for a small delay owing to the different ion
flight times.15 This is supported by the findings that the de-
pendence of the cracking on the internal energy of OCCN is
negligible; TOF spectra measured at ionization energies of
70, 90, and 120 eV, spanning a much larger range than the
internal energies of the OCCN fragments ͑Ϸ0.5 eV͒, were
identical. It is noted that the OCCN cracking contribution to
the CNϩ signal in Fig. 5 appears only at ⌰ϭ9° but vanishes
at ⌰ϭ60° because the OCCN fragments are too slow to
reach the detector at this angle.
FIG. 5. Unpolarized TOF spectra of m/eϭ26(CNϩ) at ⌰ϭ9 and 60°. The
contributions from primary dissociation to CNϩOCCN ͑thin solid line͒,
secondary dissociation of OCCN ͑dotted line͒, and OCCN cracking in the
ionizer ͑dash-dotted line͒ are shown. The primary CN coincident with stable
OCCN is indicated with a thin dashed line in the ⌰ϭ9° spectrum.
and the differences in the amount of cracking of OCCN and
(CN)2 . A refined calculation of this branching ratio is pre-
sented later in the text.
C. TOF spectra of CN
The TOF profiles of the CNϩ fragments are displayed in
Fig. 5. These spectra contain three different contributions,
namely the CN from the primary process ͑1a͒, the CN from
the secondary decay of OCCN ͑1b͒ and the CN from crack-
ing of OCCN in the detector. In this order the kinetic energy
of CN is expected to decrease. The dashed line in the ⌰
ϭ9° spectrum was calculated with the P(ET) obtained from
channel ͑1a͒ for the counterfragment OCCN ͓dashed line in
Fig. 3͑a͔͒. It is evident that this P(ET) curve only reproduces
the fast part of the CNϩ spectrum while a slow contribution
is missing. With the total P(ET) that also considers the in-
ternally hot OCCN fragments which decay before reaching
the detector ͓given as a solid curve in Fig. 3͑a͔͒, we obtain
the total signal contribution from the primary CN fragment.
This is displayed by the thin solid lines in Fig. 5.
The dotted line represents the CN contribution from the
secondary decay of OCCN to COϩCN ͑1b͒. The P(ET) of
these unstable OCCN radicals prior to their secondary decay
was obtained by subtracting the truncated P(ET) fitted to the
OCCN TOF spectra from the total primary P(ET) in Fig.
3͑b͒. The process following the secondary bond cleavage
was calculated using the procedure given in Refs. 24–26 and
a kinetic energy distribution of the COϩCN pairs which is
symmetrically centered at 30 kJ/mol ͓full width at half maxi-
mum (FWHM)ϭ20 kJ/mol͔. This value is a rough estimate
based on the dissociation energies of 41027,28 and 126
kJ/mol5,27 for ͑1a͒ and ͑1b͒, respectively, leaving a maxi-
mum available energy for secondary COϩCN of 93 kJ/mol.
A last, but negligible source of CNϩ, is dissociative ion-
ization of (CN)2 . In a separate experiment we used a pure
cyanogen ͑NCCN͒ expansion and determined the ratio of the
ion signals CNϩ/͑CN͒ϩ to be Ϸ0.2. The photoproduct (CN)2
2
has a yield of about 6%, as estimated in the previous section.
The relative contribution to m/eϭ26 from (CN)2 cracking is
therefore two orders of magnitude smaller than the contribu-
tion from primary CN fragments.
Thus the best fit to the TOF spectrum of CNϩ given by
the strong solid line in Fig. 5 was calculated iteratively by
changing the total primary P(ET) in small steps, evaluating
the resulting contributions from ͑1a͒ and ͑1b͒ by forward-
convolution, and adding the cracking contribution. The fi-
nally obtained total TOF profile represents then the sum of
all the contributions indicated by the thin solid line, the dot-
ted line and the dash-dotted line shown in the upper part of
Fig. 5.
Given the negligible contribution of (CN)2 cracking to
the m/eϭ26 signal, our preliminary analysis of the CNϩ
spectra is found to be basically correct.1 The fit is now
slightly modified with regard to the relative weight of the
CNϩ contribution from secondary dissociation of OCCN
͑1b͒ and dissociative ionization of OCCN. Having deter-
mined the latter we can now quantify the OCCN cracking
contribution to the CNϩ signal and in turn the fraction of
unstable OCCN radicals which undergoes secondary frag-
mentation following ͑1a͒. Accordingly, this fraction is esti-
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927
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Carbonyl cyanide CO(CN)2 at 193 nm
fragments is kinematically unfavorable so that the OCCN
cracking contribution is weak at ⌰ϭ30 and vanishes at ⌰
ϭ60°.
The bold solid line in Fig. 6 represents the sum of the
contributions from the molecular channel ͑2͒, the spontane-
ous decay ͑1b͒ and from the cracking of primary OCCN
products. The contributions from spontaneous decay and
cracking of OCCN were calculated without further adjust-
ments, as described in Sec. III C and added with relative
weights of 2:1, respectively, which are given by the OCCN
cracking ratio and the branching of channel ͑1a͒ and ͑2͒. The
thin solid line in Fig. 6 represents the forward convolution of
the P(ET) shown by the bold line in Fig. 3͑a͒. On the other
hand, the TOF spectrum Fig. 2 which depicts all (CN)2 frag-
ments reaching the detector, is fit by the kinetic energy dis-
tribution P(ET) indicated by the dashed line in Fig. 3͑a͒. The
discrepancy of the dashed and the solid P(ET) below ϳ80
kJ/mol is reminiscent of the findings for the OCCN fragment
͓see Fig. 3͑b͔͒. It is therefore likely that this effect is due to
the decay of slow (CN)2 fragments which possess sufficient
internal energy to dissociate spontaneously to CNϩCN be-
fore reaching the detector ͑see below͒.
As mentioned above, the relative yield of the radical
channel ͑1a͒ and the molecular channel ͑2͒ can be estimated
from the relative signal intensities at m/eϭ54 and 52, re-
spectively. This estimate is now examined using the fit of the
COϩ spectra in Fig. 6 by comparing the contribution from
the molecular channel to that from the secondary decay of
OCCN. The relative weight of the two contributions in Fig. 6
is ϳ1:3. With a fraction of 18% of nascent OCCN undergo-
FIG. 6. Unpolarized TOF spectra of m/eϭ28(COϩ) at ⌰ϭ30 and 60°. The
COϩ contributions from the molecular channel ͑thin solid line͒, secondary
dissociation of OCCN ͑dotted line͒ and from OCCN cracked in the ionizer
͑dashed line͒ are shown. The sum of the three components is indicated by a
bold solid line reproducing the measured spectrum.
mated from the ratio of the integrated areas of the truncated
curve to the total P(ET) in Fig. 3͑b͒ and found to be 18%
Ϯ6%. The relatively large error of Ϯ6% is due to the un-
certainty in the overlapping contributions of the CNϩ from
the secondary decay and the cracking of OCCN.
ing secondary decay, we obtain
a branching ratio
͑radical channel͒/͑molecular channel͒Ϸ17, consistent with
the rough estimation given in Sec. III B. We believe the ac-
curacy of this ratio to be within about 30%, which mainly
results from the uncertainty in the fraction of the secondary
dissociation of OCCN.
Photodissociation measurements with a polarized laser
beam13,14 were carried out at m/eϭ26, 52, and 54. At all
these mass settings the TOF spectra were found to be inde-
pendent of the polarization angle and hence the fragment
recoil anisotropy ␤ is zero. In the TOF spectra of COϩ and
CNϩ, the overlap of the components could potentially hide
the anisotropy of fragments from a particular decay process
but this is definitely not the case for (CN)ϩ2 and OCCNϩ.
D. TOF spectra of CO
The m/eϭ28 spectra obtained at ⌰ϭ30 and 60° are
displayed in Fig. 6. By recording these spectra ͑and also
those at m/eϭ26), it was important to lower the laser flu-
ence to р20 mJ/cm2 to avoid two-photon absorption contri-
butions to the signal. At higher fluences the intensity of a fast
signal component appeared between 50 and 150 ␮s flight
time, growing much faster with fluence than the rest of the
signal. This was attributed to laser-induced secondary decay
of OCCN, since it only appeared at the ion masses of CN and
CO, but not at those of OCCN or (CN)2 .1 With 10 mJ/cm2,
the spectra in Fig. 6 were found to have three components.
The fastest COϩ part, centered at around 100 ␮s, stems from
the molecular channel ͑2͒. The P(ET) obtained from the fit
of the corresponding counterfragment (CN)2 was used to
calculate this contribution ͑thin solid lines in Fig. 6͒. The fit
is in good agreement with the rising edge and the fast peak
of the m/eϭ28 spectra. The second contribution ͑dotted
line͒ is from the spontaneous secondary decay of OCCN ͑1b͒
and the third contribution ͑dashed line͒ is from dissociative
ionization of OCCN. Similarly to the analysis of the CNϩ
spectrum above, the contributions from spontaneous disso-
ciation and cracking of OCCN strongly overlap in the COϩ
spectrum recorded at small scattering angle. The ratio of
COϩ from ͑1b͒ to OCCN cracking ͑ϳ18 to ϳ16%͒ is close
to unity. It is noted that the detection of the COϩ cracking
IV. DISCUSSION
Following excitation at 193 nm, CO͑CN͒ is found to
2
dissociate into the products CN, CO, OCCN, and (CN)2 .
Using the PTS method, the kinetic energy distributions of all
these fragments could be measured and the analysis of this
complete TOF data set is shown to be consistent with two
distinct decay pathways. The major fraction (94%Ϯ2%) of
the parent molecules decays via a radical channel to
CNϩOCCN ͑1a͒, while a small fraction (6%Ϯ2%) under-
goes a molecular decay to COϩ͑CN͒ ͑2͒. The excellent fit
2
of all measured TOF spectra required no further primary pro-
cesses, such as a three-body decay to COϩCNϩCN, to be
included in the reaction scheme. Thus based on these two
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928
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Scheld, Furlan, and Huber
primary decay channels we discuss the energetic and mecha-
nistic features, including secondary decay processes, of the
photofragmentation of CO͑CN͒2.
A. Radical channel
The simplest decay involves a single C–C bond cleav-
age. The energy available to be partitioned among the pri-
mary photoproducts CNϩOCCN following this reaction ͑1a͒
is given by
Eavl CNϩOCCN͒ϭh␯ϩE CO͑CN͒ ͒
͑
͑
int
2
ϪD0 NCϪCOCN͒.
͑4͒
The photon energy h␯ is 619 kJ/mol and the internal energy
int of CO͑CN͒2 can be neglected owing to the efficient cool-
͑
E
ing in the supersonic expansion (EintϽ50 KϷ0.4 kJ/mol).
The bond dissociation energy D0(NC-COCN) has previ-
ously been estimated to be 430 kJ/mol1 based on an ab initio
calculation of the related molecule acetyl cyanide
CH3COCN.27 Recent ab initio calculations carried out in our
FIG. 7. Energy level diagram of CO͑CN͒ and its possible photofragment
species emerging from the molecular or the radical decay following excita-
2
group showed that the elimination of ϪCN from CO͑CN͒
2
requires 20 kJ/mol less than the ϪCN elimination from
CH3COCN.28 Therefore we now adopt 410 kJ/mol as the
best estimate for D0(NCϪCOCN), which yields Eavl
ϭ210 kJ/mol. Experimentally, an upper bound for the bond
dissociation energy is obtained from the high-energy thresh-
old of the P(ET) fitted to the OCCN and CN spectra
tion at 193 nm ͑619 kJ/mol͒.
ure to fit the OCCN signal and its counterfragment CN with
the same P(ET) distribution after momentum-matching as
shown in Fig. 3͑b͒. The slow, internally hot OCCN frag-
ments are missing in the OCCN spectra, which results in the
truncated P(ET) displayed as dashed curve. Below a trans-
lational energy of ETϷ50 kJ/mol where this P(ET) starts to
separate from the total P(ET) ͓marked by a thin line in Fig.
3͑b͔͒, the barrier to dissociation of OCCN is overcome and
decomposition to COϩCN occurs. Although subject to a
relatively large error, the bifurcation energy provides an up-
per limit for the barrier height given by Eavl
ϪET(bifurcation)Ϸ160 kJ/mol, which is consistent with the
calculated dissociation energy of 126 kJ/mol.27
D0 NCϪCOCN͒рh␯ϪE max͒,
͑5͒
͑
͑
T
where the equality sign holds if all available energy is chan-
neled into fragment recoil. The high-energy threshold in Fig.
3͑b͒ of ET(max)ϭ140Ϯ20 kJ/mol yields D0(NCϪCOCN)
р480Ϯ20 kJ/mol. Taking the true value as D0
ϭ410 kJ/mol then even the internally coldest fragments have
partitioned a substantial part of Ϸ70 kJ/mol ͑Ϸ33%͒ of the
available energy into internal degrees of freedom. The uncer-
tainty in ET(max) is, however, relatively large due to the
long flat tail of the P(ET) curve at high kinetic energies. The
average kinetic energy of the fragment pair is
E
͘
T
͗
ϭ37 kJ/mol, which corresponds to an average internal en-
B. Molecular channel
ergy E ϭE Ϫ E ϭ173 kJ/mol. The latter being 82%
͗
͘
int
͗
͘
T
avl
of Eavl is quite large for a direct dissociation of a small
molecule where the kinetic energy release is usually found to
dominate the impulsive process. This would, however, be
changed if one of the nascent fragment species is electroni-
cally excited. The only excited electronic state accessible in
the range Ͻ210 kJ/mol is the A 2⌸ state of CN lying 110
The photodissociation product detected at m/eϭ52 can
be assigned to one of three possible isomers of (CN)2
emerging from reaction ͑2͒. While cyanogen ͑NCCN͒ and
isocyanogen ͑CNCN͒ are well-known and stable species, di-
isocyanogen ͑CNNC͒ has not yet been observed and is pre-
dicted to be rather unstable.30 The energies required to dis-
sociate NCCN and CNCN to 2⌺ CNϩ2⌺ CN are displayed in
Fig. 7. The bond dissociation energy of NCCN has been
determined experimentally from fragment Doppler profiles,31
while the value for D0(CNCN) was derived using the differ-
ence in stability from ab initio calculations of NCCN and
CNCN30 combined with D0(NCCN) from experiment. Ob-
viously our mass spectroscopic detection does not allow a
distinction between NCCN and CNCN,30 but we believe that
formation of NCCN is much more likely since NCCN is
more stable than CNCN by about 134 kJ/mol and requires a
much smaller conformational change along the reaction path
͑see below͒.
kJ/mol ͑9245 cmϪ1 29 above the ground state X 2⌺. Since the
͒
P(ET) distribution shows no discernible structure in this re-
gion, evidence for the formation of CN(A 2⌸) is lacking.
The internal energy deposited into OCCN can be sub-
stantial, in principle up to 210 kJ/mol if the CN counterfrag-
ment is formed with negligible internal ͑rovibrational and
electronic͒ excitation. Thus a substantial part of nascent
OCCN will have Eint exceeding the dissociation energy for
the secondary decay of OCCN to COϩCN, which has been
predicted to be 126 kJ/mol by an ab initio calculation.27 Our
experimental data are consistent with the partial decomposi-
tion of primary OCCN fragments as manifested by the fail-
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929
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Carbonyl cyanide CO(CN)2 at 193 nm
The generation of NCCN releases a large available en-
ergy of 638 kJ/mol owing to the exothermal formation of the
strong new C–C bond (D0ϭ555 kJ/mol,31 see Fig. 7͒. A
remarkable feature of the P(ET) distribution found for the
COϩNCCN pair is its high energy end being 170
Ϯ15 kJ/mol below the maximum possible value ET(max) in-
dicated by the arrow ͓Fig. 3͑a͔͒. This implies that the lowest
internal energy ͑rovibrational and electronic͒ of the
COϩNCCN pairs is Ϸ170 kJ/mol ͑14 200 cmϪ1͒ or, in other
words, that even the fragment pairs with the highest kinetic
energy possess still such a substantial amount of internal
energy. The average kinetic energy E of the COϩNCCN
contrast to the initial B2 state, the A1 ground state is de-
scribed by a wave function without a nodal plane between
the two CN groups, this state would satisfy the requirement
for the molecular decay path proceeding over a transition
state under retention of the C2v symmetry. While the two old
C–C bonds are elongated, the new C–C bond between the
two cyano groups is formed. Considering the substantial
change in the distance between the two cyano C atoms ͓from
256 pm34 in CO͑CN͒2 to 139 pm35 in NCCN͔, the excitation
of the C–C stretching mode in the nascent NCCN product is
expected to be high, as well as the excitation of the cis-
bending mode ͑233 cmϪ1͒ due to the simultaneous change
from the bent to the linear geometry. This expectation is
supported by the large measured average internal energy of
430 kJ/mol which also indicates a high-lying transition state
in agreement with results from a preliminary ab initio
calculation.28 On the other hand, a distortion of the initially
excited molecule along the reaction path resulting in a break
of the C2v symmetry, such as by an asymmetric C–CN
stretching motion, will preferentially create the product
CNϩOCCN. The radical channel which strongly dominates
the decay is somewhat surprisingly also characterized by a
large internal energy release. A prior distribution predicts a
fragment partitioning of 14% into translation and 86% into
internal energy; this is indeed close to the observed partition-
͗
͘
T
pair is 190Ϯ10 kJ/mol and corresponds to an average inter-
nal excitation of 430 kJ/mol ͑35 900 cmϪ1͒. The only excited
state accessible within Eavlϭ638 kJ/mol is the ˜
a 3⌺ state of
NCCN at 395 kJ/mol32 which would require intersystem
crossing to occur. Since a partial production of such a spe-
cies would lead to an additional P(ET), structure in the dis-
tribution curve of Fig. 3͑a͒ should become discernible, which
is, however, not evident. The P(ET) in Fig. 3͑a͒ shows a
bifurcation at ETϳ80 kJ/mol where the fragment pairs have
acquired an internal energy (EavlϪET) of 560Ϯ20 kJ/mol.
This value is close to D0(NCϪCN)ϭ555 kJ/mol, in support
of our suggestion that at ETϽ80 kJ/mol the nascent NCCN
product can decay spontaneously into CNϩCN.
Finally, we briefly address the one-step ͑synchronous or
asynchronous concerted͒ three-body decay11,12 to
COϩCNϩCN ͑3͒ which is also energetically feasible at 193
nm. The available energy is, however, merely ϳ80 kJ/mol
ing of E ϭ18% and E ϭ82%. In view of this parti-
͗ ͘ ͗ ͘
T int
tioning, an exit channel barrier in the direct C–C fission
seems likely, which would then suggest that the formation of
OCCNϩCN proceeds via an intermediate state, such as the
1
3
* *
(n␲ ) or even (n␲ ) state, rather than the ground state, in
͑Fig. 7͒. The average kinetic energy E ϭE Ϫ E of
͘
int
͗
͘
͗
T
avl
each fragment is therefore expected to be below 20 kJ/mol.
Examination of the P(ET) curves for CNϩ and COϩ at the
corresponding low energy side in Fig. 3 reveals that a pos-
sible contribution from such a decay would be small. More-
over, since the analysis of the experimental data set was very
satisfactory without inclusion of an additional decay mode, a
one-step three-body decay can be considered negligible un-
der our excitation condition.
analogy to the mechanisms proposed for other carbonyl
compounds.21,36,37 In conclusion, the absence of fragment an-
isotropy and an energy partitioning conforming to a statisti-
cal distribution is consistent with a relatively slow and indi-
rect dissociation process ͑1a͒, and similar findings suggest
the same features for the molecular decay ͑2͒, which in-
volves, in addition, a rather complex transition state.
V. CONCLUSIONS
C. Mechanistic considerations
Excitation at 193 nm is assumed to prepare the 3s Ryd-
The collision-free, photoinduced decay of CO͑CN͒ at
2
berg state of CO͑CN͒2. This state has B2 symmetry in C2v
,
193 nm ͑ϳ620 kJ/mol͒ has been investigated using photo-
fragment translational energy spectroscopy. We monitored
the TOF distributions of the photofragments CO, CN,
OCCN, and NCCN. Excellent agreement with the complete
set of spectra measured at different masses and scattering
angles was obtained with the following scheme of decay
pathways.
The radical channel ͑1a͒ with a yield of 94%Ϯ2% pro-
duces the primary dissociation products CNϩOCCN with
the average internal energy ͑82%͒ strongly exceeding the
translation energy ͑18%͒. A fraction of 18%Ϯ6% of the
OCCN radicals possesses sufficient internal energy to un-
dergo secondary decay to COϩCN over a barrier of approxi-
mately р160 kJ/mol. The low average kinetic energy of the
primary products, together with an isotropic recoil distribu-
tion of the fragments OCCN and CN, indicate that the dis-
sociation mechanism is indirect and slow on a time scale of
a parent rotation.
which implies a transition dipole moment for the B2←A1
transition lying in the molecular plane perpendicular to the
CϭO bond.19,21 In the case of the radical decay ͑1a͒ with an
instantaneous separation of the fragments OCCN and CN
and with a recoil direction of CN along the breaking C–C
bond, an anisotropy ␤ϭ1.25 is expected.33 The angle
Є(CNϪCϪCN)ϳ120° is taken from the geometry of the
molecule in the ground state.34 On the other hand, for the
molecular decay involving a collinear recoil of CO and
NCCN, one would expect ␤ϭϪ1.0 if the decay again is
instantaneous, i.e., within a time much shorter than a rota-
tional period of the parent molecule.33 The lack of any an-
isotropy (␤ϭ0Ϯ0.1) on the fragments of the two channels
may therefore indicate that the two decay processes are rela-
tively slow ͑у1 ps͒ or the initially excited potential surface
is deactivated by, e.g., internal conversion, to the dissociative
ground state surface losing the initial alignment. Since in
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930
J. Chem. Phys., Vol. 111, No. 3, 15 July 1999
Scheld, Furlan, and Huber
The molecular decay channel ͑2͒ produces the fragments
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2
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decay which involves the cleavage of two bonds and the
formation of a new combination product with more than two
atoms. From the isotropic recoil distribution of (CN)2 we
conclude that this decay process is slow relative to the parent
rotation, not unexpected in view of the complex transition
state involved. The fragment species (CN)2 is assigned to
cyanogen ͑NCCN͒ rather than isocyanogen ͑CNCN͒ based
on mechanistic and energetic arguments. The fragments
COϩNCCN have an excess energy of 638 kJ/mol, essen-
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to the strongly exothermal formation of a new C–C bond.
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͑ϳ7%͒ of nascent NCCN to dissociate into CNϩCN. Since
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This work was supported by the Swiss National Science
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