J. Phys. Chem. 19!J3,97,5284-5290
5284
Photochemistry of Acetylene at 193.3 nm Kanekam &kit and Hidm Okabe' Department of Chemistry, Howard University, Washington, D. C. 20059 Received: October 8, 1992; In Final Form: January 3, 1993
The quantum yield (QY) of diacetylene in the 193.3-nm photolysis of acetylene has been measured as a function of pressure, decomposition, and added gases. The QY of diacetylene is near unity (0.9 f 0.1) when the decomposition is 1% or less. The QY of CZHZ hv CZH H is 0.3 f 0.1, which is determined by the C2HD yield from the photolysis of C2H2 C2D6 (or Dz) mixtures. The yield is in good agreement with the QY of H atoms photodissociated from C2Hz; C2H reacts with C2H2 to produce an equivalent amount of diacetylene. The remaining 60-70% diacetylene arises from the metastable acetylene (C2H2**) reacting with ground-state acetylene at 193.3 nm and above 0.1 Torr of acetylene. The quenching of diacetylene formed via C2H2** by various foreign gases at various wavelengths is compared. The quenching order at 193.3 nm is N2 C D2 C H2 C CzD6 C n-C4Hlo. It is postulated that an adduct from CzH2** C2H2 is initially formed before it dissociates into C ~ H Z Hz or C2H C2H3. Yields of C4H2, CZHD,and C2H2 were measured by FT-IR calibrated with pure samples. The C4H2 yield at 193 nmdoes not change with the addition of N2 up to 600 Torr. The absorption cross section of C2H2 has been measured in the 190-230-nm region. The production of C4H2 was examined at wavelengths below and above thedissociation threshold. The metastable acetylene reactions may be important in haze formation in Titan's atmosphere.
+
+
+
-
+
+
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Introduction Acetylene and the ethynyl radical play important roles in combustion. The catalytic reaction of acetylene for the decomposition of methane is important in assessing the photochemistry of Titan's atmosphere.' The spectroscopy of acetylene in the vacuum233 and near-ultraviolet regions4v5have been studied by many workers. The 193-247-nm region is assigned to a very weak A-X transition with the 0-0 band at 237.0 nm. The A state is a nonlinear trans planar IAU(C2h)species. The system consists of a long progression of the trans bending vj' and C-C stretching vi modes. The 165-1 95-nm region is assigned by Foo and Innes2 to the BlB,-X transition. The 193.3-nm laser line corresponds to the v3' = 10 and v2' = 1, vj' = 8 transitions of the A state. The Hg 185-nm line coincides with the v3' = 0 band of the E X transition,2v6band the 147-nm line falls into the strong C-X transition.3 Fluorescence from the A state of acetylene has been observed in the 21 5-240-nm region of A sudden decrease of the fluorescence yield occurs between 214.3 and 215.8 nm? corresponding to a photon energy of 132.5-133.4 kcal mol-I. The decrease is attributed to the predissociation of C2H2 to C2H He8 A value of 126.6 kcal mol-' is obtained for Do(H-C2H) by Green et a1.108 from Stark anticrossing spectroscopy and 127 f 1.5 kcal mol-' by a Doppler shift of photodissociated H atoms.Iob More recently, however, a value of 131.4 f 0.5 kcal mol-' has been obtained from the photofragment translational energy of H atoms.'la.b Several consistent values near 131 kcal mol-' have recently been obtained experimentallyll*' and by computation.I2 The value of 131.4 f 0.5 kcal mol-l is slightly lower than the fluorescence threshold energy but is much closer to the threshold valueof 132.5-133.4kcalmol-I 8athan127kcalmol-I lomentioned before. A high-resolution laser-induced fluorescence (LIF) and the lifetime have been measured by Ochi and Tsuchiya9in the vi = 2-4 region. They found some singlet excited levels couple strongly with triplet levels. Two different lifetimes and self-quenching constants for vj' = 0-4 fluorescence were detected by Wolff and Zacharia~,~ about 0.4 p s and 5 X 10-10 cm3 molecule-' s-1 for the
+
Present address: National Institute for Environmental Studies, Tsukuba, Ibaraki 305 Japan. +
0022-3654/93/2097-5284sO4.00/0
short-lived states7J3and about 2 p s and 3 X 1O-Io cm3molecule-' s-I for the long-lived state^.^ Stephenson et al.I3 have measured quenching constants for the A state by C2H2, N2, 0 2 , He, and Ar. Absorption cross sections of acetylene in the 105-200-nm region6bJ4 and 147-203.4-nm regionI4Js have been measured. The transient absorption at 137.4 nm in the vacuum ultraviolet photolysis of acetylene is attributed by LauferI6J7to the triplet vinylidene. Its quantum yield is estimated to be 0.4.18 The photochemical process hasbeenstudiedaat 126,I9v2O147:a.C 185,6a-cand 193 nm.23b3-24 Satyapal and Bersohn23 and Shin and Michael22ahave determined the primary quantum yield of dissociationat 193nm, C2H H, to be 0.26 and 0.21, respectively, from the yield of H atoms. The remaining undissociated metastable acetylene is attributed to vinylidene as described beforeI6J7or to a triplet acetylene.6a*bSeki et al.21bhave found by nanosecond time-resolved absorption spectroscopy that C4H2 is formed by a fast (within laser pulse) process, C2H C2H2 C4H2 + H, as well as a much slower (100 times) process attributed to triplet vinylidene radical H2CC or a metastable acetylene, C2H2**, C2H2** + C2H2 -w C4H2 H2. In this report, we show that the total quantum yield (QY) of C4H2 production at 193 nm is near unity and the yield of C4H2 from the C2H radical is only 0.3. (The QY of C2H is determined from the C2HD yield in the photolysis of C2H2 and C2D6 mixtures.) The remaining 60-70% of diacetylene arises from reactions of C2H2** with C2H2. C2H2**thus far has eluded detection in the molecular beam mass spectrometer system,1° while at higher acetylene pressures (>On 1Torr) C2H2** reacts with ground-state C2H2 to form C4H2 and H2.21b
+
+
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+
Experimental Procedure The quantum yield of products has been determined as previously described25 using IT-IR. Briefly, a 193-nm ArF excimer laser (Questek Model 2420) was used as a light source. The quantum yield of products is based on the yield of CO (=0.95) from the 193-nmphotolysisof acetone.26 The 193-nmlaser beam was simultaneously directed to an acetone cell ( 10-cm path) and an acetylene cell (30 cm) using a beam splitter to determine the total quanta of incident light absorbed. Acetone pressures of 7 Torr or more absorbed 99.9% of the 193-nm light in the 10-cm 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 20, 1993 5285
Photochemistry of Acetylene at 193.3 nm Mirror
TABLE I: Absorption Crow Sections of C f i 2 in the 190-220-nm Redon (Peaks (D) a d Valleys (v)) ~~
1
wavelength, nm
cross section, cm2 molecule-’ base e
218.8 217.7 216.9 216.3 215.9 214.9 214.1 213.1 211.5 21 1.0 209.7 208.9 207.8 206.8 205.5 204.7 203.5 202.7 201.5 200.9 199.7 198.9 197.9 197.0 196.1 195.6 194.5 193.3 192.4 191.8
5.9 3.5 6.0 4.7 8.3 5.0 10.3 4.9 14.7 7.9 19.1 8.8 26.7 16.5 36.8 19.1 57.8 32.2 67.7 38.6 97.2 50.0 117.7 72.7 141.3 103.4 330.1 134.7 175.9 129.5
2
Figure 1. Measurement of the absorption cross section of acetylene at the ArF line. The cross section can be obtained from the ratio of the intensity measured by reference cell 2 with the cell empty, 10, and that,
I,, with acetylene at a given pressure in the sample cell. Reference cell 1 is used to calibrate the incident intensity.
~ a t h . 2The ~ amount of CO produced was obtained from the pressure in Torr (MKS Baratron 220C) and the volume of the system including the cell and gauge. The CO gas was separated from other gases by a liquid nitrogen trap. A few hundred laser shots were used to decompose 1%or less of acetylene. The quantum yield of diacetylene was found to decrease stronglywith an increase of exposure. Hence, acetylene decomposition was limited to 1%or less. The laser fluence was always maintained at 0.5 mJ cm-2 or less to avoid a multiple photon process. The amount of the reactant, C2H2, and of photolysis products, C2HDand C4H2, was measured from the intensity of the infrared absorption bands at 730,678, and 630 cm-I, respectively for C2H2, C2HD,and C4H2.28 A calibration chart for each compound was needed because the absorbance/pressurevaluesof FT-IR increase with a decrease of pressure;25that is, an apparent absorption cross section increases with decreasing pressure. The amount of photoproduct was determined from the calibration chart. The detection limit of diacetylene by IR analysis was 5 mTorr. The IR measurement was performed by a Perkin-Elmer Model 1600 FT-IR spectrometer using a 13.3-cm length glass cell with KBr windows. A few photolysis wavelengths other than 193.3 nm were tried to see if diacetylene is formed even above the dissociation wavelength, 217.6 nm, that is, 218.8 nm and below the wavelength of dissociation, 216.9 and 215.9 nm (see Figure 2), which were generated by dye lasers pumped by the 193-nm excimer laser. At all three wavelengths, diacetylene was found after 30 000 shots. The preparation of diacetylene was previously described.29 Commercialresearch-grade acetylene,acetone, and n-butane were purified by bulb-to-bulb distillation at low temperatures. The amount of acetone in purified acetylene was less than 0.01%, since no peak was found at 1731cm-1 2* by IR absorption. Highpurity nitrogen (99.99%) was used without further purification. Acetylene-dl (stated purity 63.4 atom% D) and ethane4 (stated purity 99 atom % D) were obtained in a 1-L glass bulb from MSD Isotopes and were used without further purification. Diacetylene and acetylene4 samples were used only for IR calibration. The UV absorption cross section of acetylene was measured in the 191-240-nm range by a Cary 2390 (Varian) spectrophotometer with a bandwidth of 0.5 or 1 nm using a quartz cell of 14.7 cm in length. The cross section at 193.3 nm may not be exactly the same as that by the ArF laser line, because (1) the absorption cross section of acetylene near 193.3 nm changes drastically near this wavelength14b and (2) the ArF line has a half-width (FWHM) of 0.5 nmz7 and is modified by an absorption of oxygen in air. Therefore, we directly measured the absorption cross section by the ArF line using actinometry as shown in Figure 1. Here, acetonewas used as an actinometer.26 The incident light intensity of the ArF laser beam was measured with reference cell 2 for the
a
peak assignment# p or v P
YZ’
1
uj’
K,’
2
1
4
1
3
1
5
1
4
1
6
1
5
1
7
1
6
1
8
1
7
1
9
1
8
1
10
1
V
P V
P V
P
1
V
P V
P
1
V
P V
P
1
V
P V
P
1
V
P V
P
1
V
P V
P
1
V
P V
Reference 4. Absorption Cross Section of Acetylene
0
1.5 5
d k 8 1.0 =. 0 E
.-
H
v)
u)
0.5
2i0 230 Wavelengthhm Figure 2. Absorption cross sections of acetylene in the 19G23O-nm region with a resolution of about 1 nm. The peak assignment and absorption cross section are given in Table I. The dissociation and fluorescence threshold are designated as (a) and (b), respectively. 190
empty sample cell (Zo) and the cell with acetylene (Z,) at a given pressure, respectively. From Zo/Z,, we obtain the cross section of acetylene at the ArF laser line. Reference cell 1 was used to calibrate the total number of photons of the incident light, because the laser intensity fluctuated for each shot. Results and Discussion Table I gives absorption cross sectionvalues of peaks and valleys and peak assignments of acetylene in the 191-230-nm region. The absorption cross section of acetylene in the 190-230-nm region is shown in Figure 2. The cross section at 193.3 nm is (1.34 f 0.05) X 10-19cm2 molecule-1 (Table I), while the directly measured cross section at the ArF line is (1.40 f 0.05) X cm2 molecule-’, which is slightly higher than that measured by
Seki and Okabe
5286 The Journal of Physical Chemistry, Vol. 97, No. 20, 1993
I
1 ,
.lo
-f Acetylene 0.5 Torr
0.8
4
I
Acetylene 1 Torr +
:ca2 :CdD
1
1
0
j;
0.4
'I E
4 0.2
0
0
m
It
tl
00 1
Figure 3. Quantum yield of diacetylene from 0.5 Torr of acetylene at 193 nm as a function of % decomposition of acetylene.
1
20
1
1
1
1
1
1
1
1
1
1
1
1
1
60 80 100 120 140 160 180 Ethane-& Pressure I Torr Figure 5. Quantum yields of C2HD and C4Hz production in C2H2 and C2D6 mixtures a t 193-nm photolysis; acetylene, 1 Torr. The C2HD yield tends to approach 0.3 f 0.1 above 100 Torr of C2D6. 0
Acetylene Decomposition / %
1
40
and chlorine mixtures, however, the technique could not be used, since the FT-IRpeak of acetylene started to decrease by the formation of chlorinated acetylenes, as soon as chlorinewas added to acetylene. Alternatively, the QY of CZHhas successfully been measured by the photolysis of CzH2 and C2D6 mixtures. The CZHradical formed in reaction 1 abstracts D atoms from CzD6 C,H
C2HD
+ C2D,
(2)
and reacts competitively with CZHZ
5 0.2j 0
+ CZH2
C,H
4 6 8 10 Acetylene Pressure / Torr Figure 4. Quantum yield of diacetylene from the 193-nm photolysis of acetylene as a function of acetylene pressure. The decomposition is 1% or less. 0
-
+ C2D6
2
the spectrophotometer. Absorption cross sections of acetone and acetylene at 193.3 nm are, respectively, 2.88 X 25 and 1.34 X lW9cm2molecule-'. Thus, less than O.Ol%acetonecorresponds to an absorption of less than 0.21% and can be neglected within an experimental error of f4%. The dissociation of CzH2 into C2H H occurs at 218.2 nm, indicated as (a) in Figure 2, corresponding to Do(H-C,H) = 131 kcal The fluorescencethreshold,givenas(b), isbetween214.2and215.8 nm,8a which is slightly shorter than the dissociation wavelength. Figure 3 shows the dependence of the QY of diacetylene formation on the % decomposition of acetylene. The QY of diacetylene formation is near unity only at the lowest d e " position of acetylene (I%), and it decreases as the decomposition of acetylene increases, probably because the absorption cross section of diacetylene is higherz9at 193 nm (4.0 X cm2 molecule-') than that of acetylene, and reaction products may further react with diacetylene. The QY of diacetylene is plotted as a function of acetylene pressure in Figure 4. The QY is 0.9 f 0.1, independent of acetylenepressure, when the decomposition is 1% or less. A. Quantum Yield of C2H Production. An attempt was made to measure the QY of H atoms produced from the photolysis of CZHZat 193 nm,
+
C,H,
+ hu
C,H
+H
(1) by measuring the yield of HCl expected toarise from thephotolysis of CZHZ+ C12 mixtures, since H + Clz HCl + C1 is 100 times faster than the reaction H C2Hz --c CzH3.30 A similar technique was successfully used in measuring the yield of H atoms from methylacetylene using C H J C ~ H and C12mixtures.25 In acetylene
+
+
-
C4Hz
+H
(3) Reaction 3 is about 10 times faster than reaction 2 from a rate constant (k3) of 3.1 X 10-l' 3 1 a for reaction 3 and k2 of 3.1 X 31b for reaction 2 in cm3 molecule-' s-I. Alternatively, the k3/kz ratio may be obtained from directly measured k3 and kz' values and the k2/k2/ ratio, where k3 is 1.5 X 10-10 based on three recent direct measurements32c and k2' = 3.6 X 10-l' obtained directly from the decay of C2H absorption32bfor reaction 2'. C,H
--c
-
+ C2H6
C2H,
+ C,H,
(Incidentally, the k3 value here is about 5 times larger than Laufer's Since the ratio kz/k$ is 0.48,31bk2 becomes 1.7 X 10-l' and k3/k2 = 8.8, which is close to 10 obtained above. Thus, reaction 2 becomes over 90% of the total when more than 100 Torr of CzD6 is added to 1 Torr of CzH2. The results of the photolysis of 1 Torr of acetylene and various amounts of C2D6 mixtures at 193nm are shown in Figure 5. When more than 100 Torr of ethane-d6is added, the CzHDyield tends to approach 0.3 f 0.1, while the yield of C4H2 decreasesto almost 0.1. This value is much smaller than 0.6 expected if added C2D6 docs not quench C4H2 yields. Figure 6 shows the QY of C2HD and C4H2 as a function of D2 pressure. The CzHD yield approaches a plateau of 0.3f 0.1 with DZpressures over 600 Torr. On the other hand, from k3 = (1.5 f 0.3) X 10-10 32 and k4 = (2.0 f 0.5) X 10-13,32b,c we have k,/k4 = 750 f 300 C,H C,H
-
+ C2H, + D,
C4H,
C,HD
+H
+D
(3) (4)
indicating that, even at 750 Torr of Dz, the calculated QY of C2HDis 50% of that at much higher pressures (>7500 Torr) of
The Journal of Physical Chemistry, Vol. 97, No. 20, 1993 5287
Photochemistry of Acetylene at 193.3 nm
I+ 4'
I
Acetylene 1 Torr
!
0.8
n
-55
6
:C&
It
HI
0.6-
a
:C#D
I+)
Acetylene 1 Torr
-
I*
0.4-
-
0.2 -
Q 0
0
0
0
0
0
200
800
600
400
D2Pressure / Torr Figure 6. Quantum yields of C4H2 and C2HD from the photolysis of C2H2 and D2 mixtures at 193 nm as a function of D2 pressure. The C2HD yield approaches 0.3 f 0.1 above 600 Torr of Dz. 1 ,
0
400
200
600
N, Pressure I Torr Figure 8. Quantum yield of diacetylene from the photolysis of 1 Torr of acetylene at 193 nm as a function of N2 pressure. 0.61
I
1
I
Acetylene 1Torr
3 0.2
0
-1 0
0 0
400
200
H2 Pressure I Torr
Figure 7. Quantum yield of diacetylene production from the photolysis of 1 Torr of acetylene at 193 nm as a function of hydrogen pressure.
D2. The discrepancy between the experimental and calculated yield may arise from a large error limit found both in the QY and rate constant measurements. It is also possible that there is an additional source of C2HD other than (4) such as D
- - + + + + - +
+ C2H2
C2H2D*
C2HD
H
where C2H2D*is the internally excited radical. This source is unlikely, because the excited radical would be deactivated by collisions with D2 molecules at high pressures of D230 C2H,D*
D2
C2H2D D2
Still another route to produce C2HD would be a disproportionation of vinyl radicals (1.8 X lo-" cm3 molecule-I s - I ) , ~ ~ C2H2D C2H2D
C2HD
+
-
+
C2H2D2 D (-
20 30 C, H,gressure/Torr
cm3 molecule-' s-1)34
The QY of C2H obtained from this study is in agreement with 0.3 f 0.1 obtained from C2H2 and C2D6 mixtures. This value agrees well with the direct measurement of the yield of H atoms dissociated from C2H2 photolysis by Shin and Michael22and Satyapal and Bersohn.23 The QY of C4H2 becomes somewhat lower than 0.6 at higher pressures of D2, indicating that quenching of C4H2 yields by D2 occurs. Figure 7 shows the QY of C4H2 as a function of H2 in the photolysis of C2H2 and H2 mixtures. The C2H radical photodissociated from C2H2 with a QY of 0.3 reacts with H2 to form
40
50
Figure 9. Quantum yield of diacetylene production from 193-nm photolysis of 1 Torr of acetylene as a function of n-C4Hlo pressure.
C2H2. The C4H2 yield then should decrease to 0.6 at higher pressures of H2 if H2 does not quench C4H2 yields. The gradual decrease of C4H2 yields indicates some quenching of H2 on C4H2 yields at higher H2 pressures. Figure 8 shows the effect of N2 on the QY of C4H2 in the photolysis of C2H2 and N2 mixtures. The addition of N2 up to 600 Torr has almost no effect on the C4H2 yield. Figure 9 shows the QY of diacetyleneformation when n-C4Hlo is added. The rate constant of C2H
-
+ n-C4Hl,
+
C2H2 C,H,
(5) has apparently not been directly measured. Since the ratio of k i to k3 is 0.246b.32
C2H
C2H3D
However, since the laser intensity used is weak (0.1 Tom), however, the C2H2**molecule reacts with the ground-state molecule to produce C4H2.2’b At 50 Torr of acetylene, the rate of C4H2 formation becomes 0.6 X 106 s-I; that is, diacetylene is formed within 1 fis, as observed. The ratio of C4H2producedfromthefast reactiontothatfromtheslow reaction is 1-2.21b This ratio agrees well with that of QY(C4H2) = 0.3 from the fast reaction (eq 3) to the QY(C4H2)= 0.6 from the
TABLE Ik Quantum Yields of Photochemical Processes in Acetvlene at Various Wavelengths process wavelength, nm
193.3 193.3 193.3 184.9 147.0 123.6
C2H + H 0.3 0.1 0.26 f 0.04 0.21 f 0.04 0.06 0.3 0.1
C2H2**
*
ref this work
0.6 f 0.1
a
b 0.7 0.6 0.9 0.4 (vinylidene)
vacuum UV br band excitation
C
d e
f
Reference 23. From the yield of the primary H atoms in acetylene photolysis. Reference 22a. From the yield of total H atoms after the C2H C2H2 = C4H2 H reaction. Reference 6b. Reference 6a. e Reference 19.fReference 18.
+
*
+
slow reaction (eq 3‘ ). A similar reaction of C4H2** withgroundstate diacetylene was proposed before,29
-
+
C4H2** C4H2
C,H2
+ H2
(8) The metastable diacetylene was not quenched by the addition of N2.29 It has been shown that C4H2 was produced by absorption of light both above and below the threshold wavelength of C2H2 dissociation, supporting the conclusion that C4H2 arises from reaction 3 as well as reaction 3’, that is, from C2H and C2H2** reactions with C2H2. C. Quenching of the Metastable Acetylene. It has been proposed by Laufer et a1.16-18,36J7 that the metastable acetylene produced by vacuum UV flash photolysis is probably the triplet vinylidene, H2CC(jB2),which has a strong absorption at 137.4 nm.I6 The QY of formation is 0.4.18 The reactivity of C2H2** with other gases, however, is not well understood except that it reacts with C2H2 to form C4H2 H2 at 193 nm, reaction 3’. Reaction 3’ is based on the results by Irion and K ~ m p that a ~ the ~ amount of C4H2 is about equal to the H2 amount produced. Below 193 nm, it has been found that the reduction rate of C4H2 by the addition of H2 or CH4 is independent of C2H2 pressureasa To explain the results, reaction 3” is proposed. Laufer et al. conclude
+
+
-
C2H2** C2H2
C,H
+ C2H3
that H2CC does not react chemically withN2, H2, CO, and CH4,35 indicating that the triplet H2CC is deactivated to ground-state acetylene by collision-induced intersystem crossing. The H2CC radical, however, appears to add to C2H4, although the reaction product is unknown.18 At wavelengths shorter than 193nm, the QY of C4H2becomes near unity only for acetylene pressures over 5-10 Torr.6a.b If we assume the same rate constant of 3.5 X lO-I3 found by Seki et a1.21bat 193 nm, the decay rate of CzH2** below 193 nm becomes about 1 X los SKI,which is about 100 times faster than that formed at 193 nm.21b The rate constant found by LauferI7 for vinylidene with acetylene is close to the values found by Seki et aLZ1bforreaction 3’, that is, 3.5 X It is not clear, however, whether the triplet H2CC also reacts with acetylene to form diacetylene* We have compared at various wavelengths quenching of the metastable acetylene by various gases. The results are listed in and IV* At 193 nm*no quenching Of C2H2** by N2 is found up to 6oo The quenching constant, by may be estimated from a half quenching pressure, ‘/2(M), in k(M)y
W4)[’/2(M)/~(C,H,)I = ‘ / 2 where k(C2H2) = 3.5 X lO-’3 cm3 molecule-’ s-I 21b and (CzH2) is 1 Torr. Quenching constants for N2, D2, H2, C2D6, andn-C~Hlo at 193 nm are 10-'4
30
+
[C4H4It C,H2
quenching state halfquenching wavelength, initially quenching pressure," const: cm3 nm formed gas Torr molecule-' s-'
c
thiswork thiswork d d e e
Pressure of a quenching gas to reduce the amount of C4H2 to onehalf; CzH2 pressure, 1 Torr. Quenching constant is calculated on the cm3 molecule-l s-I for C2H2** C2Hz. Reference basis of 3.5 X 21b. Reference 6b. e Reference 6a. Reference 19.
+
TABLE Iv: Quenching of Vinylidene and A'A. of Acetylene by Various Cases wavelength, nm
quenching gas
state
quenching mnst, cm3 molecule-' s-I
Vinylidene (D2CC) vacuum UV flash photolysis (no chem quenching)
3B2
He Ar
N2 H2 Vinylidene (H2CC) 'B2 He
C2Hz C2H4
C2H2* He
AIA, vj'-0
237.0
1.9 x 10-15 7 x 10-15 8.5 x 10-15 3.4 x 10-14 1 x 10-14 3.5 X l W 3 3.5 X 1OW3
1 x 10-11 1 x 10-11 1 x lo-" 5 x 10-11 1.47 X 1O-Io
Ar
N2 0 2
C2H2
ref a (I
a a
b, c b
d e e e e
10-13 obtained from quenching half-pressures of >600,200,100, 30, and 2 Torr, respectively. On the other hand, at wavelengths shorter than 193 nm (185, 147, and 123.6 nm), both N2 and He quench C4H2. N2 and He also quench vinylidene. We propose that an adduct is formed initially before it dissociates into products. At 193 nm,
+ C2H2
-
[C4HJt
+
C4H2
+ H2
(3' )
where [C4H4]tisthevibrationally (and rotationally)excitedtriplet stateor statesoftheadduct. It relaxesto thegroundstate (physical quenching) of acetylene by collisions with M,
+
[C4H4It M
-
+
C2H2 C2H2
(9)
where M means He or N2 or Hz, Dz. At 193 nm,
+
C2H2** C,H2
-
[C4H4It
(3a'
(3c'
1
-
-
+
C2H2** C2H2 [C4H4Itt C,H C2H3 (3" ) which is 96 kcal mol-' endothermic for ground-state acetylene.38 [C4H4]ttis quenched to ground-state acetylene by collisionswith
M,
+
[C4H4Itt M
-
+
C2H2 C2H2
(9' ) Quenching of [C4H4]tt by Nz formed below 193 nm is about 10 timeslargerthanthatof [C4H4]tformedat 193nm. Itisexpected that [C4H4]tand [C4H4]tthave a different quenching efficiency by a foreign gas, because the electronic states of acetyleneleading to the formation of respective adducts are wavelength dependent and dissociation products from the adducts are also different, although it is not clear why N2 is a more efficient quencher below 193 nm. More data are needed on the states of the adducts, [C4H4]t and [C.+H,]tt,and the nature of quenching, that is, whether it is physical or chemical by foreign gases. Quenching of C2H2*(A1A,) by He, Ar, N2, and CzH2 (Table IV) is very efficient, and it may involve the collisionally induced transition to the triplet state. It is entirely different from that of vinylidene or C2H2**. Conclusion
e
Reference 37. Reference 17. Reference 36. Reference 18. Reference 13.
C2H2**
3C2H2
If [C4H4]t is quenched by acetylene for every collision (kJd = 10-10cm3 molecule-' s-I), k3b' becomes 3.5 X 108 s-1 at 100 Torr of acetylene; that is, the lifetime of [C4H4]t is equal to 3 ns. Quenching constants for [C4H# by He, Nz, H2, and D2 are small, about On the other hand, quenching by C2D6 and n-C4Hlo is efficient (10-13-10-14) and it may be chemical. Reaction 3' is only 5 kcal mol-I endothermic for ground-state C2H2.6s29J8For 193-nm excitation, reaction 3' is 142 kcal mol-l exothermic. Below 193 nm, the reaction would be6a-b
+
f
-
The QY of C2H production from the 193.3-nm photolysis of CzH2,0.3f 0.1, is obtained from the C2HD yield in the photolysis of C2H2 and C Z D(or ~ D2) mixtures, in agreement with the QY of H atoms directly measured from the photolysis of C2H2. The QY of C4H2 above 0.1 Torr of CzH2 is near unity, indicating 60-70% of C4H2 arises from the metastable acetylene (C2H2**) reacting with ground-state acetylene at 193.3 nm. Up to 600 Torr of Nz, addition does not reduce the production of diacetylene, while n-butane, ethane-d6, hydrogen, and deuterium reduce the production of C4H2 in the order n-C4H1o> C2D6 > HZ> D2. Quenching of C4H2 production and the triplet vinylidene is compared and found to be similar for He and NZat wavelengths below 193 nm. The yield of C4H2 formed by the fast reaction to that of C4H2 from the slow reaction is 1-2, which agrees with the yield of C4H2 from C2H + C2H2 C4H2 H to that from C2H2** C2H2 C4H2 H2. The different quenching behavior by N2 for C4H2 production formed at the 193.3-nm photolysis of acetylene from those at shorter wavelengths may be ascribed to the different adducts, [C4H4]t and [C,H#t, dissociating into C4Hz H2 and C2H C2H3, respectively. The metastable acetylene and diacetylene reactions may be important contributors to Titan's haze.
+
-
+
-
+
+
+
1
At very high pressures of acetylene, Seki et a1.2Ib found the QY of diacetylene decreases with acetylene pressure. The decrease
Acknowledgment. We acknowledge the support of this study by the NASA Planetary Atmospheres Program under Grant No. NAGW-785 and the NASA Center Grant NAGW-2950 for the study of terrestrial and extraterrestrial atmospheres. We thank Dr. W. Braun for providing ethane-d6. We also thank Dr. L. Stief for helpful comments and Prof. J. Halpern for discussions.
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