- Email: [email protected]
Dicyanoacetylene photodissociation at 193 nm and at 248 nm studied by transient absorption spectroscopy. Production of CN, C2 and C3N
Volume 180, number 1,2
CHEMICAL PHYSICS LETTERS
IOMay 1991
Dicyanoacetylene photodissociation at 193 nm and at 248 nm studied by transient absorption spectroscopy. Production of CN, C, and C3N * Robert Kolos, Zbigniew Zielieski, Zbigniew R. Grabowski Institute qfPhysica1 Chemistry
ofthePolish Academy
q/Sciences, Kasprzaka 44, 01-224 Warsaw, Poland
and Tadeusz Mizerski Technical University, Institute of Basic Chemistry, Noakowskicgo 3. 00-666 Warsaw, Poland
Received 25 January 199 1;in final form 13 February 1991
Laser photolysis of dicyanoacetylene at 193 nm and at 248 nm, leading to the production of CN, C, and (probably) &N, has been investigated. CN violet bands and Cz Swan bands were observed in absorption. Photolysis at 193 nm produces CN in both the X %+ and in the A ‘II, manifolds, the latter - probably populated via a two-photon process - being revealed by a delayed (collision-induced) population of higher vibrational levels of the ground electronic state. Photolysis at 248 nm is an efficient twophoton process. It is still not clear what mechanism is responsible for the formation of the Cz molecule at 193 nm. Throughout the paper, the theoretical results of Sadlej and Roos helped to choose the optimal interpretation of the experiments.
1. Introduction Dicyanoacetylene (hereafter DCA) gained some attention as it has the interesting linear conjugated structure and relatively easy to follow UV photodissociation channels. The postulated abundance of DCA in the atmosphere of Titan [ 1] makes its photochemistry interesting from astrophysical point of view. DCA, just as cyanoacetylene and other cyanopolyynes, may probably be present also in the interstellar gas clouds, although its zero dipole moment excludes the radiospectroscopic detection. Photodissociation of DCA seems to be a convenient laboratory source of CjN radical. This species is also detected in the interstellar space [ 21 but its electronic spectrum remains unknown. Photolysis of DCA has been studied in the vacuum-UV (Ar flash lamp) by Sabety-Dzvonik et al.
[ 3 ] and Halpern et al. [4] and also at 193 nm (ArF excimer laser) by Halpern et al. [5 1. Both groups employed the laser-induced fluorescence (LIF) from the B ‘C+ manifold of the CN radical to monitor the mechanism of photodissociation. To our knowledge no experiments with photolysis of DCA at 248 nm have been performed. According to Halpern et al. the following dissociation scheme of DCA irradiated at 193 nm is expected: C.,N2K3N+CN,
(1)
after which the C3N radical may undergo the secondary photolysis: &N-C2
+CN
(2)
This Letter reports the direct observations of the electronic absorption spectra of CN (B %+ tX *E+ violet bands) and C2 (d 311gta311u Swan bands) produced during the laser photolysis of DCA.
* Dedicated to Professor Kurt Schaffner on the occasion of his 60th birthday. 0009-26 14/9 I/$ 03.50 0 I99 I - Etsevier Science Publishers B.V. (North-Holland )
73
Volume 180, number I,2
CHEMICAL PHYSICS LETTERS
2. Experimental DCA was synthesized by the method of Moreau and Bongrand [ 6 ] starting with dimethyl-acetylenedicarboxylate (Aldrich). Sample has been purified by repeated fractional bulb-to-bulb sublimation. Purity has been checked by IR spectrometry and gas chromatography. The UV absorption spectra of DCA were measured with the Specord M40 (Zeiss Jena) spectrometer in a 10 cm long gas cell. Sample has been stored at the dry ice temperature under vacuum. A 2.5 dm3, 38 cm long static Pyrex reaction cell with Suprasil quartz windows has been tilled with DCA to the pressure in the range from 0.1 up to several Torr controlled with an MKS Baratron capacitance manometer. Optionally, for quantitative measurements at long delay times, several Torr of Ar were added to prevent the diffusion out of the reaction zone. An unfocused beam from the Lambda Physik EMG 10 1 excimer laser ( ArF: 193 nm, fwhm 14 ns, energy flux = 100 mJ/cm2 or K_rF: 248 nm, 16 ns, % 100 mJ/cm2) has been used to photolyse the sample. A part of the beam was reflected onto the pyroelectric laser energy meter (PEM 521; ZWG, GDR); thus the energy of each pulse could be measured. The amplified spontaneous emission (ASE) generator [7] pumped with the N2 laser served as a monitoring light source for transient absorption, Briefly, it consists of two quartz cells filled with proper laser dye solutions, one of which is transversely pumped with the UV light pulse. The ASE light from this source has the bandwidth of some 3070 nm and is almost free from spectral structure. The typical value of its energy (with a 300 kW Nz laser used for pumping) was 50 to 100 pJ. The ASE beam was making either one or two passages through the cell being confined within the channel originally excited by the excimer laser beam. Thus the optical path for absorption amounted to 38 or 76 cm. After leaving the cell the light was analyzed by the Jarrell-Ash 3.4 m Ebert spectrograph equipped with a 300 grooves/mm grating. For the 10th order of interference the resolving power ti/n was 150000. Spectra were registered on the ORWO WP-1 spectroscopic plates. The timing of excimer and Nz laser pulses has been controlled by the master trigger unit with a 10 MHz 74
lOMay 1991
quartz clock. This device could also trigger the storage oscilloscope Iwatsu TS 8123. For delay times At > 1 ps the settled sequence of flashes was repeated up to 20 times to expose a single spectrum. This was done to average some random spectral structure of the ASE thus enhancing the signal-to-noise ratio. The large time jitter of Nz laser resulted in the poor reproducibility of At < 1 ps delay times. Hence, for the time range 20 ns to 1 ps only one ASE pulse has been used to expose the spectrum and the exact value of delay time has been measured with the photomultiplier+storage oscilloscope. Such spectra, however, could not be used for the quantitative analysis. Spectroscopic plates were digitally processed to convert the optical density of photographic emulsion into the intensity of light originally used for the exposure. On the spectra thus derived (reaction cell transmission versus wavelength) the equivalent widths [ 81 of rotational lines have been measured. Equivalent widths, being the measure of energy absorbed, are useful when the instrumental broadening of lines is present - which is the case in our experiment. To transform these values into the concentrations of absorbing species in given rovibrational levels the numerical procedure based on the theory developed by Mitchell and Zeemansky [ 91 has been applied. This method of data reduction, however, works only for spectra with well-defined translational temperature. Only then the natural profile of spectral lines (determined mainly by the Doppler effect; the pressure broadening can be neglected at our experimental conditions) is known. Hence, the concentration measurements could be performed only for the spectra of both rotationally and vibrationally relaxed species for which the translational temperature should be equal to the rotational one (which in turn was found to be the room temperature). Oscillator strengths of vibrational bands for CN and Cz were taken from refs. [ lo] and [ 111, respectively. HBnl-London factors were derived using the formulae reproduced in ref. [ 12 1. 3. Results and discussion 3.1. UV absorption of DCA The absorption spectrum of DCA has been studied earlier, both in the vacuum-UV [ 131 and in the
Volume 180, number I ,2
CHEMICAL PHYSICS LETTERS
10May 1991
finding of C3N absorption within our detection range (roughly 360-I 300 nm, limited by the emissive properties of laser dyes and the sensitivity of spectroscopic plates) is improbable. The first two excited levels of C3N, A 21-1and B ‘II, are predicted to lie at 0.5 and 3.9 eV (2500 and 310 nm), respectively.
KTF I
3.3. CN radical produced at 248 nm g:
!
$2
&I
4’a
64
WAVENUMBER (units
d2
d0
a’6
al 10 ’ cm-’
3/6
)
Fig. 1, Electronic absorption spectrum of DCA. Path length of 10 cm, pressure 10.5 Torr, roomtemperature.
quartz-UV [ 141 regions. However, no quantitative data regarding the absorption cross sections in the “intermediate” region 190-200 nm can be extracted from the published spectra. Hence, we have measured the low-resolution spectra at room temperature for low pressures of several Torr and for the saturated vapour pressure. In the spectrum reproduced in fig. 1 the absorption cross section at 193 nm is 0.8 x lo-l9 cm’. Independently, the cross section for the absorption of ArF laser radiation has been measured in the photolysis cell with the aid of a laser energy meter. The value 1X lo-l9 cm2 has been found (identical to that reported by Halpern et al. [ 51). The IQ-Flaser (bandwidth x 1 nm, maximum of intensity in the region 248.1-248.3 nm) overlaps one of the sharp, strong vibronic bands in the DCA spectrum. The absorption cross section for this radiation amounts to 8 x 1O-l9 cm’. 3.2. C,N radical Originally we expected to find - apart from CN and C2 species - also the C3N radical as a product of the DCA photolysis. The preliminary search within the 377-408 nm and 480-580 nm regions revealed no other spectral lines than those of CN and Cz. As quantum-chemical calculations of small molecules permit us to predict spectra with fairly good accuracy, we asked B.O. Roos and J. Sadlej to perform such calculations on C,N in parallel with our experiments. Their results [ 151 indicate indeed that the
The B 2Et tX 2C+ Au=0 progression could be traced up to u= 5 instantaneously after the laser flash. If the CN radical characterized by such a high initial excitation ( z 10000 cm-‘) were formed by a onephoton reaction analogous to (1) then a very low value of A@(C,N) would have to be adopted - 463 kJ/mol (standard heats of formation of DCA and CN taken from refs. [ 16] and [ 17 1, respectively). As this does not look probable (see table 1 ), a twophoton dissociation of DCA should be considered, C,N,+=+
(C,N,)*-+?B
CN+C3N.
(3)
The two-step excitation mechanism presented above is possible if both absorption cross sections 0 and dc are large enough (which seems to be the case at least for q see fig. 1) and if the lifetime of the excited species (C4N2)* is not substantially shorter than the duration of the photolysing pulse. A direct two-photon absorption process seems to be less probable, as (see ref. [ 131) there are no calculated g states in the energy region of two quanta. To estimate the dc value a simple experiment has been done in which the energy of a 248 nm laser beam has been measured both ahead and behind the cell filled with the DCA gas. In this way the cell transmission could be studied as a function of the laser photon flux (varied within the range of three orders of magnitude). For a two-step absorption model one can expect that: Table I Standard heats of formation for the C,N radical AH; (kJ/mol)
Method
Ref.
710+20 629+ 17 622 0’
CASSCF calculations VUV photolysis of DCA dissociation of DCA with excited Ar atoms mass spectrometry
[I51
548
141 [I91 [I81
75
Volume 180, number I,2
l/T=
l/T, +dcF,( l-?-,)/T,
CHEMICAL PHYSICS LETTERS
)
where F0 is the laser flux ahead of the cell and T, denotes the cell transmission which should be observed if there were no two-photon absorption present. T, =exp( - NLa), where L and N are the cell length and the DCA number density (concentration), respectively; 0 is taken from the measured absorption spectrum of DCA. The experiment (fig. 2) was not conclusive since no dependence of the cell transmission on the light intensity (i.e. no deviation from the Lambert-Beer law ) has been found. However, it supplies a rough upper limit for the value of a*: (2 !I 1) x lo-‘* cm2/molecule. For reasons discussed in section 2, the quantitative analysis of CN radicals produced during the photolysis is possible only on the spectra of translationally relaxed species, i.e. those observed at the long delay times. Modeling of the CN decay process must take into account both the radical recombination and their pseudo-first-order reaction with the parent (DCA) molecules. Diffusion of radicals out of the photolysis zone can be neglected due to the presence of the buffer gas (Ar) at the properly chosen pressure. It was assumed that the three possible recombi-
IOMay 1991
nation reactions among the CN and CJN radicals are zero activation energy processes with rates determined by the number of “close” collisions. These rates were calculated for the attractive potential V(r) = - a/r6 using the formulae given in ref. [20]. Values of the parameter a were derived, by the method of Pitzer [ 211, from the estimated polarizabilities of reactants. The bimolecular recombination rates found are 6.4x 10-l’ and 1.0x lo-” cm3 molecule-’ s-l for CN +CN and C3N+ C,N, respectively. Further, it was assumed that both radicals arc identical with respect to the occurring reactions. This reduces the kinetics to the following scheme (R denotes radicals): RtDCAA
P, R+R*
(R)I,
(4)
where k2, having the meaning of some average recombination rate, is likely to be in the range between the two values derived above. Recombination of radicals P with R (which may produce tetracyanoethylene, for example) should have no significant influence on the decay of R as k, -K k2 and since (due to the steric reasons) it is expected to be slower than R t R reaction. Integration of kinetic equations derived from (4)
” 1Fo(l-T,)/T, 1 016
ph/cm’
Fig. 2. Deviation from the Lambert-Beer law for transmission of the 248 nm laser radiation through the DCA gas sample (measurements made with the laser energy meter) as a function of the photon flux. rI denotes the transmission value deduced from the spectrophotometric measurements. Linear relations expected within the coordinate scales chosen, for different values of the d’ parameter (see text), are drawn.
76
Volume 180, number I,2
10 May 1991
CHEMICAL PHYSICS LETTERS
gives the volume concentration of CN radicals as a function of time, or photolysing photon flux and of the distance from the entrance window of the cell. This function has been integrated over the cell length, which gave the CN column concentration (i.e. number of radicals per unit surface in the direction of observation) - dependent on time and photon flux. The least-squares fitting of this function, with a fixed value of k, (fig. 3) to the experimental data (it is the column concentration which is extracted from our spectra) supplies the value of k, (k, = 9x lo- I3 cm3 molecule-l s-‘; it is practically independent of k2 parameter chosen) and the value of No - the initial column density of CN (determined by both the experimental points and the choice of k,). When the k,, k2 and No parameters are known then the CN column concentration at a fixed delay time can be expressed as a function of the photon flux and compared with experimental data. Fig. 3 (insert), which shows the result of this comparison, gives the proof for the two-photon model of CN production. Within the framework of this model, the No found from the CN decay studies should be connected to the dc parameterby the formula: No =n@F,a*( 1-T:), where
n denotes the number of passages of the analyzing light through the cell and @, the quantum yield, is defined as the probability of (DCA)* dissociation upon absorption of a second photon. Since Cp< I, this formula makes possible to derive the lower limit for the value of 8 once No is known. This, together with the upper limit estimated before, gives: (0.7~0.1)x10-‘8
‘T-
400
-------
time/
ps
Fig. 3. Time decay of CN radicals produced during the photolysis of DCA at 248 nm (DCA pressure0.19Torr, Ar pressure 5 Torr, photon flux; F0=8.8 x 10’6photon/cm2). Curves A and B represent the results of fitting the theoretical functions (see text), with radical recombination rate constant kz equal 1.0~ lo-lo and 0.64~ 1O-‘o cm3 molecule-’ s-l, respectively. Insert: Dependence of CN column density on the photolysing photon flux. Pressure conditions as above, fixed delay time; t= 250 ks. Thin and thick curves represent the result of fitting within the one- and two-photon photolysis models, respectively. A and B denote different values of k2 used, as above. 17
Volume 180, number I,2
CHEMICAL PHYSICS LETTERS
3860
3640
10May 1991
3880 WAVELENGTH
/
A
Fig. 4. Example ofthe time evolution ofCN violet bands produced during the DCA photolysis at 193 n’m.DCA pressure 1.2 Torr. Vertical scale is the cell absorption (same for all spectra). Spectrum observed at t= 20 ns, being the product ofonly one exposition, exhibits a low
signal-to-noise ratio (see section 2). Origins of the B %* CX %+ (Au=O) rovibrational branches are marked with the corresponding U’=u” numbers.
rotational
excitation
corresponding
to the room tem-
perature has been derived from the spectra of vibrationally relaxed species, i.e. those observed some 150 ps after the laser flash. The one-photon photolysis of DCA at 193 nm postulated by Halpern et al. (reaction ( 1) ) should leave 77OOi.200 or 90012500 cm-’ of excess energy in the created molecular fragments depending on whether the Halpern et al. or Sadlej and Roos value of the AH,”(C3N) is adopted (cf. table 1). Either way the observed initial vibrational excitation within the X 2Z+ manifold of CN (x2000 cm-‘; corresponding to U= 1) is consistent with the one-photon process mentioned above. The time evolution of the vibrational excitation within the X *Z+ manifold, such as illustrated by fig. 4, can be interpreted in terms of the two electronic states of CN being originally populated: X 2Z+ and A ‘II,. CN radicals originally created in the first three vibrational levels of A ‘l-Ii undergo (during their radiative lifetime of 49 ps [ lo] ) the collisionally induced transfer to the v=6, u=5 and v=4 levels of the ground electronic state (fig. 5). This process should compete with the 78
X'C'
A"lYIi
Fig. 5. Vibrational levels of CN radical in the X ‘C+ and A *lI, manifolds.
radiative transition to the corresponding low-lying
vibrational levels of the ground state. Indeed, at pressures several times lower than that used in the
Vohme 180, number I,2
CHEMICAL PHYSICS LETTERS
experiment illustrated in fig. 4, the radiative channel predominates and practically no vitiational excitation higher than to u= 2 is observed. However, even in this case transitions from the A *ni manifold can still be traced, due to the delayed enhancement of intensity of the absorption bands corresponding to the u=O, 1, 2 levels of the ground state. An attempt to observe, shortly after photolysing pulse, the B *C+tA *l-l,absorption of CN at 532 nm failed, even though the (1,0) band that we have looked for is the strongest in the system. This is probably because of the too low sensitivity of our detection. Oscillator strength here is 270 times lower than for the violet (0,O) band [lo]. To interpret the generation of CN radials excited at least to the A 211iv=2 level (i.e. not less than 12700 cm- ’ ) within the one-photon photolysis model, one would have to adopt 569? 10 kJ/moi as an upper limit for the C3N heat of formation. Such value, although improbable in the light of Sadlej and Roos calculations, would be consistent with the one derived by Dibeler et al. (table I). If this were the case then one of the possibilities would be the production of CN via the following two channels: GN, =C,N(A*II)+CN(X*Z+;v=O,u=l),
(5)
(6)
which would satisfy the orbital symmetry correlation rules. Then reaction (5) would be the only one observed by Halpern et al. who maintained the collision-free conditions in their LIF experiment. A more reasonable explanation, perhaps is the production of CN (A *Iii ) via a two-photon process. Reaction (2 ) , however, has to be excluded here, as it would imply 790 kJ/mol as a lower limit for AH,“(C,N) (standard heat of formation of Cz taken from ref. [ 17 ] ). This value is unlikely since it would not permit the one-photon reaction ( 1) observed by Halpern et al. to take place. Just as pointed out earlier, the quantitative concentration data can be extracted only from the spectra of completely relaxed species. Unfortunately, such spectra (i.e. those registered at the long delay times) lack information on the two originally present elec-
10May 1991
tronic states of CN. Additional difficulty is caused by the presence of C2 (see section 3.4) which makes the decay of CN much more complex than discussed in section 3.3. Hence neither can the full decay curve be constructed nor can the dependence of the CN concentration (at a fixed long delay time) on the photolysing photon flux be used to distinguish between the one- and two-photon processes. 3.5. C, molecule The region of d 3lI,ta 311, Au=0 Swan bands of C2 was observed. Photolysis at 248 nm produces C2 in extremely low quantities, just above the threshold of our detection capabilities. At 193 nm, on the other hand, the prominent (0,O) P, Q and R branches were found together with some traces of the ( 1,l) band. However, to obtain a good signal, the pressure of DCA has to be several times higher than during the experiments aimed at CN. No rotational relaxation has been observed and the rotational temperature = 300 K has been inferred from the spectra. This suggests a small amount of excess energy taken away by the products of the photolysis. The dependence of C2 concentration on the excimer laser intensity at a fixed delay time is hard to interpret in terms of a oneor two-photon DCA dissociation, since the concentration of CN, which is expected to react easily with C2, is not known (see section 3.3). Experiments with the short (submicrosecond) delay times should be conclusive, but these are difficult to perform on our present setup due to the time jitter of the monitoring light source (see section 2). Preliminary estimations, however, favour the one-photon production of C1 with the quantum yield of the order of lo-*. This will be the subject of a further study. A priori one can postulate the production of C2 in a two-photon-overall process. This can be either a direct absorption of two 193 nm quanta or reaction (2 ), preceded by ( 1). Thresholds for the photolysis reaction (2 ), leading to the observed &(a’&) u= 1 (highest excitation observed; x 2250 cm-’ above the ground level), as derived from Halpern et al. and Sadlej and Roos valuesofA?I,“(C3N),are181+7nmand207f12nm, respectively.
19
Volume 180, number I.2
CHEMICAL PHYSICS LETTERS
Acknowledgement The authors are indebted to J. Sadlej and B.O. Roos for the fruitful and inspiring cooperation and for the permission to use their unpublished results. We also thank J.B. Halpem for sending a copy of his work [ 51 prior to publication, J. Jasny for numerous advices together with support in form of materials and instrumentation and K. Kamienska-Trela who furnished the reagent for DCA synthesis. The work was supported by the CPBP 01.19. project.
References [I] Y.L. Yung, Icarus 72 ( 1987) 468. [2] M. Guelinand P. Thaddeus, Astrophys. J. 212 (1977) L81. [3] M.J. Sabety-Dzvonik, R.J. Cody and W.M. Jackson, Chem. Phys. Letters 44 ( 1976) I3 1. [4] J.B. Halpem, G.E. Miller, H. Okabeand W. Nottingham, I. Photochem. Photobiol. A 42 ( 1988) 63. [5] J.B. Halpem, L. Petway, R. Lu, W.M. Jackson, V.R. McCrary and W. Nottingham, J. Phys. Chem. 94 (1990) 1869. [6]C. Moreau and J.C. Bongrand, Ann. Chim. (Paris) 14 (1920) 5. [ 71 R. Kolos and J. Sepiol, Optics Commun. 69 (1989) 308.
80
10May 1991
[ 81 K.R. Lang, Astrophysical formulae (Springer, Berlin, 1980) p. 180. [ 91 A.C.G. Mitchell and M.W. Zeemansky, Resonance radiation and excited atoms (Cambridge Univ. Press, Cambridge, 1961). [ IO] P.J. Knowles, H.-J. Werner, P.J. Hay and DC. Cartwright, J. Chem. Phys. 89 (1988) 7334. [ 111D.L. Lambert, Month. Not. Roy. Astron. Sot. 182 ( 1978) 249. [ 12] G. Herzberg Molecular spectra and molecular structure, Vol. 1.Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). [ 13] R.E. Connors, J.L. Roebber and K. Weiss, J. Chem. Phys. 60 (1974) 501 I. [ II] EA. Miller and R.B. Hannan, Spectrochim. Acta 12 (1958) 321. [ 151J. Sadlej and B.O. Roos, Chem. Phys. Letters 180 ( 1991) 81. [ 161G.T. Armstrong and S. Marantz, J. Phys. Chem. 64 ( 1960) 1776. [ 171D.R. Stull and H. Prophet, eds., JANAF thermochemical tables, 2nd Ed., NSRDS-NBS (US) 37 (1971). [ 181 V.H. Dibeler, R.M. Reese and J.L. Franklin, 3. Am. Chem. Sot. 83 (1961) 1816. [ 191 J.A. Mayer and D.W. Setser, J. Phys. Chem. 74 ( 1970) 3452. [ 201 H.S. Johnstone and P. Goldfinger, J. Chem. Phys. 37 ( 1962) 700. [21] K.S.Pitzer, J. Am. Chem. Sot. 78 (1956) 4565.
CHEMICAL PHYSICS LETTERS
IOMay 1991
Dicyanoacetylene photodissociation at 193 nm and at 248 nm studied by transient absorption spectroscopy. Production of CN, C, and C3N * Robert Kolos, Zbigniew Zielieski, Zbigniew R. Grabowski Institute qfPhysica1 Chemistry
ofthePolish Academy
q/Sciences, Kasprzaka 44, 01-224 Warsaw, Poland
and Tadeusz Mizerski Technical University, Institute of Basic Chemistry, Noakowskicgo 3. 00-666 Warsaw, Poland
Received 25 January 199 1;in final form 13 February 1991
Laser photolysis of dicyanoacetylene at 193 nm and at 248 nm, leading to the production of CN, C, and (probably) &N, has been investigated. CN violet bands and Cz Swan bands were observed in absorption. Photolysis at 193 nm produces CN in both the X %+ and in the A ‘II, manifolds, the latter - probably populated via a two-photon process - being revealed by a delayed (collision-induced) population of higher vibrational levels of the ground electronic state. Photolysis at 248 nm is an efficient twophoton process. It is still not clear what mechanism is responsible for the formation of the Cz molecule at 193 nm. Throughout the paper, the theoretical results of Sadlej and Roos helped to choose the optimal interpretation of the experiments.
1. Introduction Dicyanoacetylene (hereafter DCA) gained some attention as it has the interesting linear conjugated structure and relatively easy to follow UV photodissociation channels. The postulated abundance of DCA in the atmosphere of Titan [ 1] makes its photochemistry interesting from astrophysical point of view. DCA, just as cyanoacetylene and other cyanopolyynes, may probably be present also in the interstellar gas clouds, although its zero dipole moment excludes the radiospectroscopic detection. Photodissociation of DCA seems to be a convenient laboratory source of CjN radical. This species is also detected in the interstellar space [ 21 but its electronic spectrum remains unknown. Photolysis of DCA has been studied in the vacuum-UV (Ar flash lamp) by Sabety-Dzvonik et al.
[ 3 ] and Halpern et al. [4] and also at 193 nm (ArF excimer laser) by Halpern et al. [5 1. Both groups employed the laser-induced fluorescence (LIF) from the B ‘C+ manifold of the CN radical to monitor the mechanism of photodissociation. To our knowledge no experiments with photolysis of DCA at 248 nm have been performed. According to Halpern et al. the following dissociation scheme of DCA irradiated at 193 nm is expected: C.,N2K3N+CN,
(1)
after which the C3N radical may undergo the secondary photolysis: &N-C2
+CN
(2)
This Letter reports the direct observations of the electronic absorption spectra of CN (B %+ tX *E+ violet bands) and C2 (d 311gta311u Swan bands) produced during the laser photolysis of DCA.
* Dedicated to Professor Kurt Schaffner on the occasion of his 60th birthday. 0009-26 14/9 I/$ 03.50 0 I99 I - Etsevier Science Publishers B.V. (North-Holland )
73
Volume 180, number I,2
CHEMICAL PHYSICS LETTERS
2. Experimental DCA was synthesized by the method of Moreau and Bongrand [ 6 ] starting with dimethyl-acetylenedicarboxylate (Aldrich). Sample has been purified by repeated fractional bulb-to-bulb sublimation. Purity has been checked by IR spectrometry and gas chromatography. The UV absorption spectra of DCA were measured with the Specord M40 (Zeiss Jena) spectrometer in a 10 cm long gas cell. Sample has been stored at the dry ice temperature under vacuum. A 2.5 dm3, 38 cm long static Pyrex reaction cell with Suprasil quartz windows has been tilled with DCA to the pressure in the range from 0.1 up to several Torr controlled with an MKS Baratron capacitance manometer. Optionally, for quantitative measurements at long delay times, several Torr of Ar were added to prevent the diffusion out of the reaction zone. An unfocused beam from the Lambda Physik EMG 10 1 excimer laser ( ArF: 193 nm, fwhm 14 ns, energy flux = 100 mJ/cm2 or K_rF: 248 nm, 16 ns, % 100 mJ/cm2) has been used to photolyse the sample. A part of the beam was reflected onto the pyroelectric laser energy meter (PEM 521; ZWG, GDR); thus the energy of each pulse could be measured. The amplified spontaneous emission (ASE) generator [7] pumped with the N2 laser served as a monitoring light source for transient absorption, Briefly, it consists of two quartz cells filled with proper laser dye solutions, one of which is transversely pumped with the UV light pulse. The ASE light from this source has the bandwidth of some 3070 nm and is almost free from spectral structure. The typical value of its energy (with a 300 kW Nz laser used for pumping) was 50 to 100 pJ. The ASE beam was making either one or two passages through the cell being confined within the channel originally excited by the excimer laser beam. Thus the optical path for absorption amounted to 38 or 76 cm. After leaving the cell the light was analyzed by the Jarrell-Ash 3.4 m Ebert spectrograph equipped with a 300 grooves/mm grating. For the 10th order of interference the resolving power ti/n was 150000. Spectra were registered on the ORWO WP-1 spectroscopic plates. The timing of excimer and Nz laser pulses has been controlled by the master trigger unit with a 10 MHz 74
lOMay 1991
quartz clock. This device could also trigger the storage oscilloscope Iwatsu TS 8123. For delay times At > 1 ps the settled sequence of flashes was repeated up to 20 times to expose a single spectrum. This was done to average some random spectral structure of the ASE thus enhancing the signal-to-noise ratio. The large time jitter of Nz laser resulted in the poor reproducibility of At < 1 ps delay times. Hence, for the time range 20 ns to 1 ps only one ASE pulse has been used to expose the spectrum and the exact value of delay time has been measured with the photomultiplier+storage oscilloscope. Such spectra, however, could not be used for the quantitative analysis. Spectroscopic plates were digitally processed to convert the optical density of photographic emulsion into the intensity of light originally used for the exposure. On the spectra thus derived (reaction cell transmission versus wavelength) the equivalent widths [ 81 of rotational lines have been measured. Equivalent widths, being the measure of energy absorbed, are useful when the instrumental broadening of lines is present - which is the case in our experiment. To transform these values into the concentrations of absorbing species in given rovibrational levels the numerical procedure based on the theory developed by Mitchell and Zeemansky [ 91 has been applied. This method of data reduction, however, works only for spectra with well-defined translational temperature. Only then the natural profile of spectral lines (determined mainly by the Doppler effect; the pressure broadening can be neglected at our experimental conditions) is known. Hence, the concentration measurements could be performed only for the spectra of both rotationally and vibrationally relaxed species for which the translational temperature should be equal to the rotational one (which in turn was found to be the room temperature). Oscillator strengths of vibrational bands for CN and Cz were taken from refs. [ lo] and [ 111, respectively. HBnl-London factors were derived using the formulae reproduced in ref. [ 12 1. 3. Results and discussion 3.1. UV absorption of DCA The absorption spectrum of DCA has been studied earlier, both in the vacuum-UV [ 131 and in the
Volume 180, number I ,2
CHEMICAL PHYSICS LETTERS
10May 1991
finding of C3N absorption within our detection range (roughly 360-I 300 nm, limited by the emissive properties of laser dyes and the sensitivity of spectroscopic plates) is improbable. The first two excited levels of C3N, A 21-1and B ‘II, are predicted to lie at 0.5 and 3.9 eV (2500 and 310 nm), respectively.
KTF I
3.3. CN radical produced at 248 nm g:
!
$2
&I
4’a
64
WAVENUMBER (units
d2
d0
a’6
al 10 ’ cm-’
3/6
)
Fig. 1, Electronic absorption spectrum of DCA. Path length of 10 cm, pressure 10.5 Torr, roomtemperature.
quartz-UV [ 141 regions. However, no quantitative data regarding the absorption cross sections in the “intermediate” region 190-200 nm can be extracted from the published spectra. Hence, we have measured the low-resolution spectra at room temperature for low pressures of several Torr and for the saturated vapour pressure. In the spectrum reproduced in fig. 1 the absorption cross section at 193 nm is 0.8 x lo-l9 cm’. Independently, the cross section for the absorption of ArF laser radiation has been measured in the photolysis cell with the aid of a laser energy meter. The value 1X lo-l9 cm2 has been found (identical to that reported by Halpern et al. [ 51). The IQ-Flaser (bandwidth x 1 nm, maximum of intensity in the region 248.1-248.3 nm) overlaps one of the sharp, strong vibronic bands in the DCA spectrum. The absorption cross section for this radiation amounts to 8 x 1O-l9 cm’. 3.2. C,N radical Originally we expected to find - apart from CN and C2 species - also the C3N radical as a product of the DCA photolysis. The preliminary search within the 377-408 nm and 480-580 nm regions revealed no other spectral lines than those of CN and Cz. As quantum-chemical calculations of small molecules permit us to predict spectra with fairly good accuracy, we asked B.O. Roos and J. Sadlej to perform such calculations on C,N in parallel with our experiments. Their results [ 151 indicate indeed that the
The B 2Et tX 2C+ Au=0 progression could be traced up to u= 5 instantaneously after the laser flash. If the CN radical characterized by such a high initial excitation ( z 10000 cm-‘) were formed by a onephoton reaction analogous to (1) then a very low value of A@(C,N) would have to be adopted - 463 kJ/mol (standard heats of formation of DCA and CN taken from refs. [ 16] and [ 17 1, respectively). As this does not look probable (see table 1 ), a twophoton dissociation of DCA should be considered, C,N,+=+
(C,N,)*-+?B
CN+C3N.
(3)
The two-step excitation mechanism presented above is possible if both absorption cross sections 0 and dc are large enough (which seems to be the case at least for q see fig. 1) and if the lifetime of the excited species (C4N2)* is not substantially shorter than the duration of the photolysing pulse. A direct two-photon absorption process seems to be less probable, as (see ref. [ 131) there are no calculated g states in the energy region of two quanta. To estimate the dc value a simple experiment has been done in which the energy of a 248 nm laser beam has been measured both ahead and behind the cell filled with the DCA gas. In this way the cell transmission could be studied as a function of the laser photon flux (varied within the range of three orders of magnitude). For a two-step absorption model one can expect that: Table I Standard heats of formation for the C,N radical AH; (kJ/mol)
Method
Ref.
710+20 629+ 17 622 0’
CASSCF calculations VUV photolysis of DCA dissociation of DCA with excited Ar atoms mass spectrometry
[I51
548
141 [I91 [I81
75
Volume 180, number I,2
l/T=
l/T, +dcF,( l-?-,)/T,
CHEMICAL PHYSICS LETTERS
)
where F0 is the laser flux ahead of the cell and T, denotes the cell transmission which should be observed if there were no two-photon absorption present. T, =exp( - NLa), where L and N are the cell length and the DCA number density (concentration), respectively; 0 is taken from the measured absorption spectrum of DCA. The experiment (fig. 2) was not conclusive since no dependence of the cell transmission on the light intensity (i.e. no deviation from the Lambert-Beer law ) has been found. However, it supplies a rough upper limit for the value of a*: (2 !I 1) x lo-‘* cm2/molecule. For reasons discussed in section 2, the quantitative analysis of CN radicals produced during the photolysis is possible only on the spectra of translationally relaxed species, i.e. those observed at the long delay times. Modeling of the CN decay process must take into account both the radical recombination and their pseudo-first-order reaction with the parent (DCA) molecules. Diffusion of radicals out of the photolysis zone can be neglected due to the presence of the buffer gas (Ar) at the properly chosen pressure. It was assumed that the three possible recombi-
IOMay 1991
nation reactions among the CN and CJN radicals are zero activation energy processes with rates determined by the number of “close” collisions. These rates were calculated for the attractive potential V(r) = - a/r6 using the formulae given in ref. [20]. Values of the parameter a were derived, by the method of Pitzer [ 211, from the estimated polarizabilities of reactants. The bimolecular recombination rates found are 6.4x 10-l’ and 1.0x lo-” cm3 molecule-’ s-l for CN +CN and C3N+ C,N, respectively. Further, it was assumed that both radicals arc identical with respect to the occurring reactions. This reduces the kinetics to the following scheme (R denotes radicals): RtDCAA
P, R+R*
(R)I,
(4)
where k2, having the meaning of some average recombination rate, is likely to be in the range between the two values derived above. Recombination of radicals P with R (which may produce tetracyanoethylene, for example) should have no significant influence on the decay of R as k, -K k2 and since (due to the steric reasons) it is expected to be slower than R t R reaction. Integration of kinetic equations derived from (4)
” 1Fo(l-T,)/T, 1 016
ph/cm’
Fig. 2. Deviation from the Lambert-Beer law for transmission of the 248 nm laser radiation through the DCA gas sample (measurements made with the laser energy meter) as a function of the photon flux. rI denotes the transmission value deduced from the spectrophotometric measurements. Linear relations expected within the coordinate scales chosen, for different values of the d’ parameter (see text), are drawn.
76
Volume 180, number I,2
10 May 1991
CHEMICAL PHYSICS LETTERS
gives the volume concentration of CN radicals as a function of time, or photolysing photon flux and of the distance from the entrance window of the cell. This function has been integrated over the cell length, which gave the CN column concentration (i.e. number of radicals per unit surface in the direction of observation) - dependent on time and photon flux. The least-squares fitting of this function, with a fixed value of k, (fig. 3) to the experimental data (it is the column concentration which is extracted from our spectra) supplies the value of k, (k, = 9x lo- I3 cm3 molecule-l s-‘; it is practically independent of k2 parameter chosen) and the value of No - the initial column density of CN (determined by both the experimental points and the choice of k,). When the k,, k2 and No parameters are known then the CN column concentration at a fixed delay time can be expressed as a function of the photon flux and compared with experimental data. Fig. 3 (insert), which shows the result of this comparison, gives the proof for the two-photon model of CN production. Within the framework of this model, the No found from the CN decay studies should be connected to the dc parameterby the formula: No =n@F,a*( 1-T:), where
n denotes the number of passages of the analyzing light through the cell and @, the quantum yield, is defined as the probability of (DCA)* dissociation upon absorption of a second photon. Since Cp< I, this formula makes possible to derive the lower limit for the value of 8 once No is known. This, together with the upper limit estimated before, gives: (0.7~0.1)x10-‘8
‘T-
400
-------
time/
ps
Fig. 3. Time decay of CN radicals produced during the photolysis of DCA at 248 nm (DCA pressure0.19Torr, Ar pressure 5 Torr, photon flux; F0=8.8 x 10’6photon/cm2). Curves A and B represent the results of fitting the theoretical functions (see text), with radical recombination rate constant kz equal 1.0~ lo-lo and 0.64~ 1O-‘o cm3 molecule-’ s-l, respectively. Insert: Dependence of CN column density on the photolysing photon flux. Pressure conditions as above, fixed delay time; t= 250 ks. Thin and thick curves represent the result of fitting within the one- and two-photon photolysis models, respectively. A and B denote different values of k2 used, as above. 17
Volume 180, number I,2
CHEMICAL PHYSICS LETTERS
3860
3640
10May 1991
3880 WAVELENGTH
/
A
Fig. 4. Example ofthe time evolution ofCN violet bands produced during the DCA photolysis at 193 n’m.DCA pressure 1.2 Torr. Vertical scale is the cell absorption (same for all spectra). Spectrum observed at t= 20 ns, being the product ofonly one exposition, exhibits a low
signal-to-noise ratio (see section 2). Origins of the B %* CX %+ (Au=O) rovibrational branches are marked with the corresponding U’=u” numbers.
rotational
excitation
corresponding
to the room tem-
perature has been derived from the spectra of vibrationally relaxed species, i.e. those observed some 150 ps after the laser flash. The one-photon photolysis of DCA at 193 nm postulated by Halpern et al. (reaction ( 1) ) should leave 77OOi.200 or 90012500 cm-’ of excess energy in the created molecular fragments depending on whether the Halpern et al. or Sadlej and Roos value of the AH,”(C3N) is adopted (cf. table 1). Either way the observed initial vibrational excitation within the X 2Z+ manifold of CN (x2000 cm-‘; corresponding to U= 1) is consistent with the one-photon process mentioned above. The time evolution of the vibrational excitation within the X *Z+ manifold, such as illustrated by fig. 4, can be interpreted in terms of the two electronic states of CN being originally populated: X 2Z+ and A ‘II,. CN radicals originally created in the first three vibrational levels of A ‘l-Ii undergo (during their radiative lifetime of 49 ps [ lo] ) the collisionally induced transfer to the v=6, u=5 and v=4 levels of the ground electronic state (fig. 5). This process should compete with the 78
X'C'
A"lYIi
Fig. 5. Vibrational levels of CN radical in the X ‘C+ and A *lI, manifolds.
radiative transition to the corresponding low-lying
vibrational levels of the ground state. Indeed, at pressures several times lower than that used in the
Vohme 180, number I,2
CHEMICAL PHYSICS LETTERS
experiment illustrated in fig. 4, the radiative channel predominates and practically no vitiational excitation higher than to u= 2 is observed. However, even in this case transitions from the A *ni manifold can still be traced, due to the delayed enhancement of intensity of the absorption bands corresponding to the u=O, 1, 2 levels of the ground state. An attempt to observe, shortly after photolysing pulse, the B *C+tA *l-l,absorption of CN at 532 nm failed, even though the (1,0) band that we have looked for is the strongest in the system. This is probably because of the too low sensitivity of our detection. Oscillator strength here is 270 times lower than for the violet (0,O) band [lo]. To interpret the generation of CN radials excited at least to the A 211iv=2 level (i.e. not less than 12700 cm- ’ ) within the one-photon photolysis model, one would have to adopt 569? 10 kJ/moi as an upper limit for the C3N heat of formation. Such value, although improbable in the light of Sadlej and Roos calculations, would be consistent with the one derived by Dibeler et al. (table I). If this were the case then one of the possibilities would be the production of CN via the following two channels: GN, =C,N(A*II)+CN(X*Z+;v=O,u=l),
(5)
(6)
which would satisfy the orbital symmetry correlation rules. Then reaction (5) would be the only one observed by Halpern et al. who maintained the collision-free conditions in their LIF experiment. A more reasonable explanation, perhaps is the production of CN (A *Iii ) via a two-photon process. Reaction (2 ) , however, has to be excluded here, as it would imply 790 kJ/mol as a lower limit for AH,“(C,N) (standard heat of formation of Cz taken from ref. [ 17 ] ). This value is unlikely since it would not permit the one-photon reaction ( 1) observed by Halpern et al. to take place. Just as pointed out earlier, the quantitative concentration data can be extracted only from the spectra of completely relaxed species. Unfortunately, such spectra (i.e. those registered at the long delay times) lack information on the two originally present elec-
10May 1991
tronic states of CN. Additional difficulty is caused by the presence of C2 (see section 3.4) which makes the decay of CN much more complex than discussed in section 3.3. Hence neither can the full decay curve be constructed nor can the dependence of the CN concentration (at a fixed long delay time) on the photolysing photon flux be used to distinguish between the one- and two-photon processes. 3.5. C, molecule The region of d 3lI,ta 311, Au=0 Swan bands of C2 was observed. Photolysis at 248 nm produces C2 in extremely low quantities, just above the threshold of our detection capabilities. At 193 nm, on the other hand, the prominent (0,O) P, Q and R branches were found together with some traces of the ( 1,l) band. However, to obtain a good signal, the pressure of DCA has to be several times higher than during the experiments aimed at CN. No rotational relaxation has been observed and the rotational temperature = 300 K has been inferred from the spectra. This suggests a small amount of excess energy taken away by the products of the photolysis. The dependence of C2 concentration on the excimer laser intensity at a fixed delay time is hard to interpret in terms of a oneor two-photon DCA dissociation, since the concentration of CN, which is expected to react easily with C2, is not known (see section 3.3). Experiments with the short (submicrosecond) delay times should be conclusive, but these are difficult to perform on our present setup due to the time jitter of the monitoring light source (see section 2). Preliminary estimations, however, favour the one-photon production of C1 with the quantum yield of the order of lo-*. This will be the subject of a further study. A priori one can postulate the production of C2 in a two-photon-overall process. This can be either a direct absorption of two 193 nm quanta or reaction (2 ), preceded by ( 1). Thresholds for the photolysis reaction (2 ), leading to the observed &(a’&) u= 1 (highest excitation observed; x 2250 cm-’ above the ground level), as derived from Halpern et al. and Sadlej and Roos valuesofA?I,“(C3N),are181+7nmand207f12nm, respectively.
19
Volume 180, number I.2
CHEMICAL PHYSICS LETTERS
Acknowledgement The authors are indebted to J. Sadlej and B.O. Roos for the fruitful and inspiring cooperation and for the permission to use their unpublished results. We also thank J.B. Halpem for sending a copy of his work [ 51 prior to publication, J. Jasny for numerous advices together with support in form of materials and instrumentation and K. Kamienska-Trela who furnished the reagent for DCA synthesis. The work was supported by the CPBP 01.19. project.
References [I] Y.L. Yung, Icarus 72 ( 1987) 468. [2] M. Guelinand P. Thaddeus, Astrophys. J. 212 (1977) L81. [3] M.J. Sabety-Dzvonik, R.J. Cody and W.M. Jackson, Chem. Phys. Letters 44 ( 1976) I3 1. [4] J.B. Halpem, G.E. Miller, H. Okabeand W. Nottingham, I. Photochem. Photobiol. A 42 ( 1988) 63. [5] J.B. Halpem, L. Petway, R. Lu, W.M. Jackson, V.R. McCrary and W. Nottingham, J. Phys. Chem. 94 (1990) 1869. [6]C. Moreau and J.C. Bongrand, Ann. Chim. (Paris) 14 (1920) 5. [ 71 R. Kolos and J. Sepiol, Optics Commun. 69 (1989) 308.
80
10May 1991
[ 81 K.R. Lang, Astrophysical formulae (Springer, Berlin, 1980) p. 180. [ 91 A.C.G. Mitchell and M.W. Zeemansky, Resonance radiation and excited atoms (Cambridge Univ. Press, Cambridge, 1961). [ IO] P.J. Knowles, H.-J. Werner, P.J. Hay and DC. Cartwright, J. Chem. Phys. 89 (1988) 7334. [ 111D.L. Lambert, Month. Not. Roy. Astron. Sot. 182 ( 1978) 249. [ 12] G. Herzberg Molecular spectra and molecular structure, Vol. 1.Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). [ 13] R.E. Connors, J.L. Roebber and K. Weiss, J. Chem. Phys. 60 (1974) 501 I. [ II] EA. Miller and R.B. Hannan, Spectrochim. Acta 12 (1958) 321. [ 151J. Sadlej and B.O. Roos, Chem. Phys. Letters 180 ( 1991) 81. [ 161G.T. Armstrong and S. Marantz, J. Phys. Chem. 64 ( 1960) 1776. [ 171D.R. Stull and H. Prophet, eds., JANAF thermochemical tables, 2nd Ed., NSRDS-NBS (US) 37 (1971). [ 181 V.H. Dibeler, R.M. Reese and J.L. Franklin, 3. Am. Chem. Sot. 83 (1961) 1816. [ 191 J.A. Mayer and D.W. Setser, J. Phys. Chem. 74 ( 1970) 3452. [ 201 H.S. Johnstone and P. Goldfinger, J. Chem. Phys. 37 ( 1962) 700. [21] K.S.Pitzer, J. Am. Chem. Sot. 78 (1956) 4565.