Photochemistry of cyanoacetylene at 193.3 nm

Article abstract of DOI:10.1021/jp952787z

Cyanoacetylene (CA) is an important minor constituent in the Titan atmosphere and is present in the interstellar medium. The absorption cross section of CA has been measured in the region from 190 to 255 nm with a resolution of 1 nm. The photochemistry of CA at 193.3 nm has been studied using a quadrupole mass spectrometer and a Fourier transform infrared spectrometer for product analysis. From the photolysis of HC₃N-D₂ and HC₃N-CD₄ mixtures and a plateau value of 0.3 for the quantum yield (QY) of DC₃N (C₃N + D₂ → DC₃N + D), it is concluded that the main dissociation process is HC₃N + hv → H + C₃N with a QY of 0.30 ± 0.05 and a minor process is HC₃N + hv → C₂H + CN with a QY equal to or less than 0.02. The remaining process is the formation of metastable CA (a triplet or carbene). The photolysis of CA induces a noticeable pressure decrease and a concomitant formation of a mist. The QY of CA disappearance is 4.5 ± 0.5, which is much higher than that of diacetylene (QY = 2.0 ± 0.5) and of acetylene (QY = 2.3). The rapid mist formation in CA may explain a haze observed in the Titan atmosphere. A detailed mist formation process is not known. The C₃N radical disappears partially by C₃N + HC₃N → C₆N₂ + H and 2C₃N → C₆N₂. To explain the formation of minor products, HCN, C₂H₂, HC₅N, and C₄N₂, two processes involving an unspecified CA metastable state or states may be proposed: HC₃N* + HC₃N → HC₅N + HCN and C₄N₂ + C₂H₂.

Full text of DOI:10.1021/jp952787z

J. Phys. Chem. 1996, 100, 5349-5353
5349
Photochemistry of Cyanoacetylene at 193.3 nm
Kanekazu Seki,† Maoqi He, Renzhang Liu, and Hideo Okabe*
Department of Chemistry, Howard UniVersity, Washington, DC 20059
ReceiVed: September 21, 1995; In Final Form: January 16, 1996X
Cyanoacetylene (CA) is an important minor constituent in the Titan atmosphere and is present in the interstellar
medium. The absorption cross section of CA has been measured in the region from 190 to 255 nm with a
resolution of 1 nm. The photochemistry of CA at 193.3 nm has been studied using a quadrupole mass
spectrometer and a Fourier transform infrared spectrometer for product analysis. From the photolysis of
HC3N-D2 and HC3N-CD4 mixtures and a plateau value of 0.3 for the quantum yield (QY) of DC3N (C3N
+ D2 f DC3N + D), it is concluded that the main dissociation process is HC3N + hν f H + C3N with a
QY of 0.30 ( 0.05 and a minor process is HC3N + hν f C2H + CN with a QY equal to or less than 0.02.
The remaining process is the formation of metastable CA (a triplet or carbene). The photolysis of CA induces
a noticeable pressure decrease and a concomitant formation of a mist. The QY of CA disappearance is 4.5
( 0.5, which is much higher than that of diacetylene (QY ) 2.0 ( 0.5) and of acetylene (QY ) 2.3). The
rapid mist formation in CA may explain a haze observed in the Titan atmosphere. A detailed mist formation
process is not known. The C3N radical disappears partially by C3N + HC3N f C6N2 + H and 2C3N f
C6N2. To explain the formation of minor products, HCN, C2H2, HC5N, and C4N2, two processes involving
an unspecified CA metastable state or states may be proposed: HC3N* + HC3N f HC5N + HCN and C4N2
+ C2H2.
Introduction
dissociation process. Apparently, the reaction rate of C3N with
hydrocarbons has not been measured before. Kolos et al.10 have
The atmosphere of Titan, the largest moon of Saturn, consists
mainly of N2 and CH4, while substantial amounts of H2 have
escaped from the atmosphere because of the low gravitational
field of Titan. Titan is the only solar system object besides the
Earth to have a substantial nitrogen atmosphere. The minor
constituents are C2H6, C3H8, C2H2, HCN, CH3C2H, C4H2, HC3N,
and C2N2. These products are most likely formed as a result
of complex reactions initiated by the photolysis of methane and
dissociation of N2 by cosmic rays.1,2a
not found the UV absorption of C3N in the 377-580 nm region.
Sadlej and Roos11 conclude from quantum mechanical calcula-
tions that transition intensities to the excited states, B2Π (3.9
eV) and C2Σ (5.7 eV), may be too weak. Nevertheless,
Cherchneff and Glassgold7 believe that the reactions of the C3N
radical with cyanopolyynes and acetylenes proceed with almost
every collision. The UV absorption spectrum of CA has been
measured and analyzed by Job and King.12a,b The spectrum in
the region 230-270 nm has been assigned to the transition from
Similar photochemical processes may have occurred in the
prebiotic Earth atmosphere.
1
1
the ground state Σ+ to a nonlinear planar excited state, A′′.
The stronger absorption in the region 200-230 nm is ascribed
to the transition from the ground state to the linear excited state
The surface of Titan is covered by orange-colored haze which
is a polymer formed probably by chemical reactions involving
unsaturated hydrocarbons and nitriles. The photochemistry of
unsaturated hydrocarbons and nitriles has been studied by many
workers and is summerized in ref 1. Recently, the photochem-
istry of diacetylene, another constituent in the Titan atmosphere,
has been studied by Glicker and Okabe3 and by Bandy et al.4 It
has been shown tht the quantum yield (QY) of primary
dissociation is much less than 1, and reaction of metastable
diacetylene is important. Similar results have been found for
acetylene photolysis at 193 nm, where the QY of dissociation
is only 0.3 and the reaction of metastable acetylene to form
diacetylene is 0.6.5a On the other hand, the QY of dissociation
of unity is obtained for methylacetylene at 193 nm,6 in which
the most important process is CH3C2H + hν f CH3C2 + H.
Cyanoacetylene (CA) and its radical, C3N, have been found
in the interstellar medium1,2,7,8 by radioastronomy. Cherchneff
and Glassgold7a,b have postulated that the C3N radical reacts
rapidly with C2H2, C3N + C2H2 f HC5N + H, similar to the
reaction CN + C2H2 f HC3N + H.
1
1Σ- (or ∆).
Fluorescence from CA was observed from absorption in both
1
1
the A-X (1A′′ r Σ+) and B-X (1∆ r Σ+) region.5b The
fluorescence yield excited in the A-X region is 150 times
stronger than that in the B-X region. The fluorescence
spectrum excited in the A region lies in the 300 nm region with
a lifetime of 1.6 ns and has no structure, while the fluorescence
produced in the B region has a double-exponential decay of 1
and 20 ns in the 300-400 nm region.
The purpose of this study is to find the QY of the
photochemical primary process of CA at 193.3 nm and the QY
of disappearance of CA. The QY measurements are based on
reactant and product analysis by a quadrupole mass spectrometer
and the Fourier transform infrared (FT-IR) method. Because
of a rapid buildup of deposit on the window surface and of
mist inside the reaction cell, the number of photons absorbed
in the cell by CA has to be corrected accordingly.
Halpern et al.9 conclude that the QY of HC3N + hν f C2H
+ CN is about 0.05, and HC3N + hν f H + C3N is the main
Experimental Details
The reaction cell was directly connected with the mass
spectrometer leak into the ionization chamber. It was found
that the intensity of the parent peak of CA in the mass
spectrometer was linearly proportional to the cell pressure of
† Present address: Yokohama National University, Hodogaya, Yokohama
240, Japan.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5349$12.00/0 © 1996 American Chemical Society

5350 J. Phys. Chem., Vol. 100, No. 13, 1996
Seki et al.
CA in Torr. The yield of a product is expressed as the ratio of
the peak intensity of the product to that of CA (which was about
20% less after photolysis), using a commercial quadrupole mass
spectrometer (AMETEK, MA200M, mass range 1-200, 30 eV).
A light source was a 193.3 nm ArF excimer laser (AQX-150,
MPB Technologies). Mass spectrometric intensities of CA and
hydrogen are calibrated using pure samples. FT-IR intensities
of HCN and C2H2 are calibrated at 712 and 730 cm-1
,
respectively, with pure samples by a Perkin-Elmer Model 1600
spectrometer. The QY of products is given as the ratio of
product yield (the number of molecules produced by photolysis)
to the number of photons absorbed by CA. The absorbed photon
number is obtained from the yield of CO ()0.95) produced from
the photolysis of acetone at 193.3 nm13 contained in the cell
(15 cm path, 3.8 cm diameter) placed behind the reaction cell.
The 193 nm laser beam was simultaneously directed to one
acetone cell to measure the integrated incident light intensity
and to another placed behind the sample cell (30 cm, 3.8 cm
diameter) using a beam splitter as described before.5a,6 Acetone
pressures of 7 Torr or more absorbed 99.9% of the 193 nm
light in the 15 cm pass. The pressure of CO produced was
measured by a capacitance manometer (MKS Baratron 220C).
Typically 300 laser shots of about 3 mJ cm-2 (3 × 1015 photons
cm-2) or less were used to decompose 20% of CA at a typical
pressure of 1.5 Torr. The decomposition was accompanied by
a decrease of gas pressure from 1.5 to 1.3 Torr and a
concomitant formation of films on windows and the mist in
the gas phase, suggesting a rapid polymerization of CA gas.
To obtain the correct number of photons absorbed by CA, it
was necessary to consider the transmission decrease by the
windows (about 20%) and the decrease of transmission by the
scattering of the beam by the mist formed in the gas phase. It
was found that while the absorption cross section was constant
below a pulse rate of 0.25 Hz, it started to increase almost
linearly above 0.25 Hz. The concomitant gas phase mist
formation was observed. It is likely that the increase is caused
by the scattering of the laser beam by the mist. It takes longer
than 4 s before the mist diffuses out of the laser beam. Thus,
the QY obtained is based on pulse intervals longer than 4 s.
The CA sample, obtained commercially, was purified by bulb-
to-bulb distillation at about -77 °C to remove impurities (mostly
acetone2b) which contribute to lines at 195 nm and those above
200 nm in the UV and 1247 and 1776 cm-1 in the IR region.
Figure 1. Absorption cross sections of HC3N In the 190-250 nm
region (resolution, 1 nm). The peak assignments and values of the
absorption cross section are given in Table 1.
TABLE 1: Absorption Cross Sectiona of Cyanoacetylene in
the 190-260 nm Region
cross section (10-19 cm2
peak (P)
or valley (V)
λ (nm)
molecules-1 base e)
peak assigntb
248.0
227.4
226.4
224.3
222.9
221.6
220.1
217.2
216.1
214.2
212.9
211.8
210.5
209.8
207.9
205.1
203.6
203.1
201.4
201.1
199.0
196.6
194.4
194.1
193.3c
190.6
0.067
1.0
0.6
4.6
0.6
1.6
0.9
1.6
1.0
8.6
1.0
3.5
1.6
2.3
1.8
7.5
2.0
3.4
1.8
2.3
1.8
4.5
2.5
3.7
3.3
2.1
511
P
V
P
V
P
V
P
V
P
V
P
V
P
V
P
V
P
V
P
V
P
V
P
501
610, 720
210, 511
210, 510
210, 610, 720
220, 511
220, 510
220, 610, 720
230, 511(d)
230, 510(d)
230, 610, 720
V
V
Results and Discussion
a Error limit ) (0.2 nm. b Reference 12b unless otherwise noted.
c (3.3 ( 0.2) × 10-19 at 193.3 nm laser line; see text. d Reference 2a.
Absorption Cross Section. The absorption of CA was
measured by a Cary 2390 (Varian) spectrophotometer with a
resolution of 0.2 nm over the range from 190 to 280 nm. The
absorption cell (15 cm path) with two quartz windows attached
was used. CA samples (∼10 Torr) were employed. Figure 1
shows the absorption spectrum. Cross section values are given
in Table 1. The cross section of CA at 193.3 nm was measured
by use of both the spectrophotometer with 1 nm resolution and
the ArF light source and an acetone actinometer. The average
the bond energies by ab initio molecular orbital theory and
obtained D0(H-C3N) ) 138 ( 2 and D0(HC2-CN) ) 146.9
( 2, both in kcal mol-1 14 On the other hand, Mains15 calculated
.
D0(H-C3N) ) 119.9 kcal mol-1, in agreement with an
experimental threshold of 240 nm or 119 kcal mol-1 for the H
+ C3N production.16 From the heats of formation of CN (101),
HCC (127), HC3N (85) in kcal mol-1, D0(HC2-CN) ) 143
kcal mol-1 can be calculated,17 in good agreement with ref 14.
Since process 2 requires more energy than process 1, it is likely
that process 1 is the main dissociation process.
cross section is (3.3 ( 0.2) × 10-19 cm2 molecule-1
.
Photochemical Processes in Cyanoacetylene. Photodisso-
ciation Processes. Two dissociation processes may be consid-
ered at 193.3 nm photolysis
To show that process 1 is more important, we used CA-D2
and CA-CD4 mixtures to find products DC3N (from process
1) nd C2HD and DCN (from process 2).
Photolysis of CA-D2 and CA-CD4 Mixtures. The QY of
process 1, HC3N + hν f H + C3N, may be obtained from the
yield of HCl in the photolysis of HC3N + Cl2 mixtures, similar
to the case of methylacetylene photolysis.6 However, CA
disappears rapidly in the FT-IR spectrum, as soon as Cl2 is
added, probably because of the formation of an addition product.
HC3N + hν f H + C3N
HC3N + hν f C2H + CN
(1)
(2)
Bond dissociation energies in CA have not been accurately
known. Fransisco and Richardson14 have recently calculated

Photochemistry of Cyanoacetylene
J. Phys. Chem., Vol. 100, No. 13, 1996 5351
TABLE 2: Photolysis of HC3N at 193.3 nm and Mass
Spectrometric Analsysi of Products without and with Added
D2 (HC3N Pressure, 1.5 Torr; Ionization Voltage, 30 eV;
Laser Intensity, e3 mJ cm-2; Total Shots, 300)
converted to DC3N in the presence of D2 (or CD4) may be
complete above 200 Torr of D2 (and 50 Torr of CD4).
The DCN (m/e ) 28) peak intensity does not change even in
the presence of 600 Torr of D2 and 50 Torr of CD4 (Table 2).
No DCN peak was produced in FT-IR at 569 cm-1. If CN is
produced in process 2, HC3N + hν f C2H + CN, a fraction of
CN may be converted to DCN in the presence of D2 by the
reaction
peak ratio of product to CA (%)
after photolysisa at
a D2 press. (Torr) of
mass to
type
before
charge ratio
of peak
photolysis
0
100 200 400 600
1
26
27
28
50
51
52
H
2.2
4.4
0.53
1.02
71.0
100.0
5.0
0
2.8
4.6
0.86 5.8
1.02 1.02 1.02 1.02 1.02
51.0 50.0 49.0 48.0 49.0
78.0 78.0 78.0 78.0 78.0
5.0
0
3.4
7.2
2.9
6.0
7.5
2.8
5.6
7.1
2.8
5.4
6.3
CN + D2 f DCN + D
(3)
CN, C2H2
HCN, C2HDb
N2, DCN
C3N
HC3N
CA isotopes
DC3N
HC5N
C4N2
C6N2
In the presence of 600 Torr of D2, the fraction of CN converted
to DCN can be calculated to be 0.13 from k3 ) 7.2 × 10-15 19a
and k4 ) 1.7 × 10-11 cm3 molecule-1 s-1 20
5.0
4.0
0.8
1.9
0.1
5.0
5.7
0.3
0.5
5.0
5.4
0.2
0.4
5.0
4.6
0.1
0.3
0.04
CN + HC3N f C4N2 + H
(4)
75
76
100
0
0
0
0.4
0.5
0.5
c
d
while in 50 Torr of CD4, 0.24 of CN may be converted to DCN
by
0.05 0.1
a Peak intensity ratios of products to the remaining CA (78%).
b C2HD is detected by FT-IR, when D2 is added; an estimated QY of
C2HD is less than 0.02. c Contribution of the parent peak, C4N2 (m/e
) 76), to fragment peaks, C3N, C2N (m/e ) 50, 38), is less than 20%
(ref 18), that is, less than 0.1. Likewise, the contribution of C6N2 to
fragment peaks (C5N, C3N, m/e ) 74, 50) would be less than 0.1. d The
addition of D2 removes most C6N2 product.
CN + CD4 f DCN + CD3
(5)
since k5 ) 1.6 × 10-13 cm3 molecule-1 s-1 21
.
Little change in the DCN intensity (Table 2) shown by the
addition of D2 or CD4 indicates that the QY of process 2 may
be equal to or less than 0.02, the experimental error limit.
The FT-IR spectra of photolysis products at 193.3 nm of 1.5
Torr HC3N and 300 Torr D2 mixtures show a small C2HD peak
at 677 cm-1 and much larger DC3N peaks at 494 and 522 cm-1
after 300 shots of the laser beam.
Table 2 shows that m/e ) 27 (C2HD) increases to a plateau
value, when D2 above 100 Torr is added. The estimated QY
of C2HD from a mass spectrometric analysis, assuming an equal
sensitivity for C2H2 and C2HD, is equal to or less than 0.02.
The C2HD (m/e ) 27) contributes to the peak from the reaction
C2H + D2 f C2HD + D
(6)
Since k6 is 2.3 × 10-13 cm3 molecule-1 s-1 19b
,
600 Torr of D2
should remove at least 50% of C2H radicals, assuming that the
C2H radical reacts with 1.5 Torr of HC3N in every collision.
In the photolysis of CA-C2D6 mixtures, the FT-IR spectrum
shows the QY less than 0.03 for C2HD (677 cm-1) and for DCN
(569 cm-1).
Figure 2. QY of DC3N from the photolysis of HC3N-D2 (or CD4)
mixtures at 193.3 nm as a function of added gas pressure of D2 (or
CD4); HC3N, 1.5 Torr. The QY of DC3N increases and levels off at
0.3 ( 0.05, as the D2 pressure increases over 200 Torr. The QY
increases to 0.3 above 50 Torr for CD4.
Photolysis Products and Secondary Processes Involving
Radicals and Metastable State. Photolysis Products. Peak
values of photolysis products of CA at 193.3 nm obtained by
the mass spectrometer are given in Table 2 with respect to that
of the remaining CA in arbitrary units. They are HCN, C2H2,
HC5N (monocyanodiacetylene), C4N2 (dicyanoacetylene), and
C6N2 (dicyanodiacetylene). Products observed by FT-IR are
HCN (1712), C2H2 (730), and C6N2 (dicaynodiacetylene, 717),
Therefore, we use the reaction C3N + D2 f DC3N + D to
obtain the QY of process 1 from the yield of DC3N.16
Table 2 presents the mass peak values of photolysis products
of CA with and without D2 additions, where the CA pressure
is 1.5 Torr and D2 pressures are 100, 200, 400, and 600 Torr.
The peak values were measured after pumping out D2 at -195
°C or CD4 at -100 °C.
in cm-1 22
C6N2 is also observed in the UV absorption at about
.
272, 289, and 309 nm.23 The approximate QYs of 1.5 Torr
CA photolysis products at 193.3 nm are H2 e 0.01, HCN )
0.04, C2H2 ) 0.03, and small yields (about 0.03) of HC5N, C4N2,
and C6N2, where the H2 mass spectrometric yield is calibrated
with pure H2. The HCN and C2H2 FT-IR yields are calibrated
with pure samples.
Figure 3 shows the QYs of two minor products, HCN and
C2H2, at 193.3 nm from the photolysis of 2.5 Torr of CA as a
function of N2 pressure. The yields of the products are obtained
from calibrated FT-IR yields of HCN and C2H2 at 712 and 730
cm-1, respectively. The QY without N2 is 0.04 and 0.03 for
HCN and C2H2, respectively, with an error limit of (0.01. The
yield of HCN increases with an increase of N2, while C2H2
decreases slightly with an increase of N2.
A large increase of DC3N (m/e ) 52) signal in the mass
spectrometric analysis when D2 is added (Table 2) shows that
process 1, HC3N + hν f H + C3N, is the major process. We
may obtain the QY of process 1 on the assumption that the
DC3N signal has the same intensity as the HC3N signal. In the
case of C6H6 and C6H5D the same peak sensitivity (111 vs 112)
is observed,18 where the fragmentation of the parent peak is
relatively small. Since HC3N shows also a small fragmentation
(Table 2), it may be justified to assume that DC3N has
approximately the same sensitivity as HC3N.
Figure 2 shows the QY of DC3N (m/e ) 52) as a function of
D2 pressure. The QY of DC3N increases with an increase of
pressure and reaches a plateau of 0.30 ( 0.05 above 200 Torr
of D2 and for CD4 above 50 Torr. The fraction of C3N

5352 J. Phys. Chem., Vol. 100, No. 13, 1996
Seki et al.
TABLE 3: QY of Disappearance in Acetylenes
cyanoacetylene at 193.3 nma
4.3,b 4.0,c 4.0,b 4.8,b 5.5,d 4.2b
average 4.5 ( 0.5
diacetylene in the UVe
2.0 ( 0.5
acetylene at 193.3 nmf
2.3
a This work; 300 shots of 3 mJ laser pulse. b 1.5 Torr of CA. c 1.5
Torr of CA + 100 Torr of D2. d 1.5 Torr of CA + 400 Torr of D2.
f
e Reference 3. Reference 5a. The QY is obtained from the mechanism
C2H2+hν f C2H + H, C2H2 + hν f C2H2*, C2H2* + C2H2 f C4H2
+ H2, C2H + C2H2 f C4H2 + H, H + C2H2 f C2H3, 2C2H3 f C4H6.
the QY of CA disappearance, indicating that the C3N radical is
not involved in polymer formation.
To explain the products HC5N (m/e ) 75), HCN (m/e ) 27),
C4N2 (m/e ) 76), and C2H2 (m/e ) 26, see Table 2), processes
10 and 11 are proposed,
Figure 3. QYs of products HCN and C2H2 from the photolysis of 2.5
Torr of CA, as a function of N2 pressure. The yields of C2H2 and
HCN are obtained from intensities of FT-IR (730 and 712 cm-1
respectively) calibrated with standard samples.
,
Products at the 248 nm photolysis of 5 Torr of CA are also
HCN and C2H2 in comparable amounts with those at 193.3 nm.
Secondary Processes by Radicals. The rate constants of
reactions 7 and 8
HC3N* + HC3N f HC5N + HCN
HC3N* + HC3N f C4N2 + C2H2
(10)
(11)
C3N + D2(CD4) f DC3N + D(CD3)
C3N + HC3N f C6N2 + H
(7)
(8)
The products HCN and C2H2 are produced even from the
248 nm excitation, corresponding to 115 kcal mol-1, which is
much less than the dissociation limit of CA (about 138 kcal
mol-1), a finding which supports that processes 10 and 11 are
molecular processes. As shown in Figure 3, the HCN yield
increases with an increase of N2, while C2H2 decreases with an
increase of N2. The result may indicate that HCN and C2H2
are produced from the metastable state with different vibrational
energies or from different excited states. The QYs of these
products are less than 5%, and a majority of HC3N* leads to
the formation of a polymer, process 12,
have apparently not been measured before, because of the
difficulty in detecting C3N. The reaction rate of C3N with CD4
is several times faster than that with D2 (Figure 2.). Subsequent
reactions of C3N formed in process 1, HC3N + hν f H + C3N,
must be process 8 in analogy with CN + C2H2 f HC3N + H,
proposed by Yang et al.,24 and CN + HC3N f C4N2 + H,
suggested by Halpern et al.25
The H atoms formed by process 1, HC3N + hν f H + C3N,
may be consumed partially by the addition to CA, H + HC3N
f products. Dicyanodiacetylene is formed partially by the
combination of C3N, process 9
HC3N* + nHC3N f polymer
(12)
where n is 4.
C3N + C3N f C6N2
(9)
Conclusion
and mainly by process 8. By the addition of D2 to CA, C6N2
molecules formed by processes 8 and 9 disappear, as shown in
Table 2, indicating that C6N2 is formed from the C3N radical.
Formation of Metastable State and Its Reactions. A large
fraction (70%) of photoexcited CA do not dissociate im-
mediately and remain in an unspecified metastable state (or in
an isomeric form). Although the detailed mechanism is not
clearly understood, an average of four molecules disappear per
one photon absorbed to produce a polymer. Ferris and
Guillemin26 found that tricyanobenzenes are minor products of
UV photolysis, and the major product is an unspecified polymer.
Clarke and Ferris16 found that CA is more likely to form a
polymer than to form acetylene.
A decrease of pressure from 1.5 to 1.2 Torr suggests the
formation of a polymer rather than gas phase photochemical
reactions. The QY of CA disappearance is on average 4.5 (
0.5, which is shown in Table 3. The QY of disappearance in
the gas phase is about 0.9 (processes 1, 8, and H + HC3N f
product), and the QY of mist formatioin process is 3.6. That
is, for one photon absorbed about four molecules disappear by
forming a polymer, and only one molecule of CA disappears
in the gas phase. Since H2 and C2N2 are not found in the
products (see Table 2), processes such as HC3N* + HC3N f
C6N2 + H2 and HC3N* + HC3N f C4H2 + C2N2 may be
excluded, where HC3N* means an unspecified metastable state
of CA. As shown in Table 3, the addition of D2 does not change
The photochemical processes of CA, an important minor
constituent in the Titan atmosphere, have been studied at 193.3
nm. The major dissociation process is HC3N + hν f H +
C3N with a QY of 0.30 ( 0.05 and a minor process, HC3N +
hν f C2H + CN, with a QY equal to or less than 0.02. The
QY is obtained from the yield of DC3N plateau or C2HD as a
function of D2 pressure in the photolysis of HC3N-D2 mixtures.
Minor photolysis products are HCN, C2H2, HC5N, C4N2, and
C6N2. Secondary processes involving radicals and metastable
state to form these products are discussed.
The major photochemical process is the formation of a
metastable CA, HC3N*, which leads to the formation of a
polymer, resulting in a pressure decrease. The QY of CA
disappearance is 4.5 ( 0.5. The efficient formation of a polymer
in CA is probably a source of haze observed in the Titan
atmosphere. A similar study will be extended to the photolysis
of dicyanoacetylene.
Acknowledgment. We acknowledge the support of this work
by the NASA Planetary Atmospheres Program. We thank Prof.
J. Halpern for helpful discussions.
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