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Reactive formation of dicarbon from the reactions of electronically excited radicals, CH(A2Δ) and CCl(A2Δ)
Chemical Physics Letters 464 (2008) 26–30
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Reactive formation of dicarbon from the reactions of electronically excited radicals, CH(A2D) and CCl(A2D) Ken J. McDonald, Andrew D. Buettner, Benjamin J. Petro, Robert W. Quandt * Department of Chemistry, Illinois State University, Normal, IL 61790-4160, United States
a r t i c l e
i n f o
Article history: Received 14 March 2008 In final form 28 August 2008 Available online 3 September 2008
a b s t r a c t The 2 193 nm photodissociations of CHCl3 and CCl4 have been examined using dispersed emission. It was found that the initial photodissociation of CHCl3 forms large amounts of CH(A2D) while the photodissociation of CCl4, forms lesser but still significant amounts of CCl(A2D). Emission rise time measurements show that photoproducts quickly react to form C2(d3Pg). Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years the formation of molecular photoproducts from the high-energy dissociation of halocarbons has received renewed interest [1–8]. In a series of recent papers [9,10] we have investigated the 2 193 nm photodissociation of a group of halogenated methanes (CX4 and CHX4 where X = Br or I) via photoproduct emission. Depending on the particular halogen the emission was found to be from either the molecular halogen or the Swan system (d3Pg ? a3Pu) of C2. The high-energy dissociation of CHnCl4n molecules has been studied somewhat more extensively than their iodated and brominated counterparts. Seccombe et al. have observed emission from a series of CCl3X (where X = H, Cl or Br) molecules after vacuum– ultraviolet (VUV) excitation [1,2]. Ibuki et al. have observed emission from the VUV photolysis of CCl4, CBrCl3, CCl3F and CCl2F2 [5,6]. Other techniques used include radiofrequency-excited gas discharges, electron-impact and multiphoton photodissociation [4,7,8]. All of the aforementioned studies, with the notable exception of the electron-impact study, reported broad emission bands from 400 to 700 nm. These bands were variously attributed to emission from CCl2, CCl3 and Cl2. In this study, previous work on the photodissociation of CX4 and CHX3 (X = Br or I) has been extended to the 2 193 nm photodissociation of CHCl3 and CCl4. In addition, an alternative identity for the source of the emission between 400 and 700 nm observed in other works will be discussed. 2. Experimental The experimental technique has been discussed in detail elsewhere [9,10] therefore only a brief description will be given here. For all experiments approximately 50–250 mTorr of CHCl3 * Corresponding author. E-mail address: [email protected] (R.W. Quandt). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.08.099
(Fischer) or CCl4 (J.T. Baker) was flowed through an 8 cm cubic cell with fused silica windows. The back of the cell was offset by an additional 8 cm with an aluminum tube. All chemicals were used without further purification and were stored in opaque containers in a freezer when not in use. The photolysis source was an ArF excimer laser (Lambda Physik, Compex 110) operating at 5 Hz and 50–100 mJ. The excimer beam was focused into the aluminum tube by a quartz lens (f = 100 mm) some 4–5 cm away from the collection optics. This ensured that there was no direct line of site between the focal point and the probe, the stainless steel cell is physically between the two, and minimizes the chance of observing three or more photon processes. Emission signals were collected at right angles to the excimer beam, with both a photomultiplier tube (Electron Tubes Limited model 9129b) and a collimating lens coupled to a fiber optic. The fiber optic then transmitted the collected light to an asymmetric crossed Czerny–Turner monochromator (Ocean Optics model S2000) and the resulting signal stored on a personal computer for later analysis. The emission signals were not corrected for the sensitivity of detection system. 3. Results and discussion 3.1. Power dependence In order to determine the number of photons involved in the photodissociation, the photoproduct emission intensity as a function of photolysis laser power was measured. Excimer power in the cell typically ranged from 0.15 to 25 mJ. For reasons to be discussed in detail below, the power dependence was measured at 431 nm (CHCl3) and 278 nm (CCl4) as well as for total emission. A least-squares fit of the data for CHCl3 emission gave power dependence values of 2.09 ± 0.24 for the peak at 431 nm and 2.07 ± 0.14 for total emission. Due to experimental limitations attempts to determine the power dependence of the emission peak
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K.J. McDonald et al. / Chemical Physics Letters 464 (2008) 26–30
at 278 nm from the dissociation of CCl4 were unsuccessful. However, the total emission was found to have a value of 1.95 ± 0.18. 3.2. Photoproduct spectra Dispersed emission spectra for CCl4 and CHCl3 are shown in Figs. 1 and 2, respectively. A quick perusal of both figures shows a lack of an obvious molecular chlorine channel which is surprising because Cl2 should be a major photoproduct. Indeed, in previous work on CHI3 and CI4 we have detected large amounts of I2* emission under similar conditions [10]. In order to further study this dark channel, an ab initio study was conducted and will be published in a separate paper. Several of the common features of the emission spectra are of interest. Both spectra contain bands with origins at 619, 563, 516, 473 and 438 nm which are degraded to the violet. As mentioned above, previous studies [4–8] of these systems have attributed emission in this range to CCl2, ð1 B1 ! X1 A1 Þ; 0 0 1 þ Cl2 A3 Pð0þ u Þ ! X Rg Þ, or CCl3 ð2A1 ! 2A2 Þ. However, based on previous work in this lab [9], these bands are assigned to the well known Swan system (d3Pg ? a3Pu) of C2 [11]. The spectrum in Fig. 1 is virtually identical to the dispersed emission of CCl4 taken by Tiee et al. who also used ArF dissociation. This indicates that the very tentative assignment of these peaks to Cl2, CCl, CCl2 and CCl3 by Tiee et al. is incorrect [8]. Because C2 emission appears in well under single collision conditions (1 ls/collision at 298 K) it is not surprising that the authors did not consider collisional processes as a source for their spectra. Several other emission bands from the photodissociation of CHCl3 are of interest: specifically, the bands with origins at 744, 656, 495, 313, 277 and 246 nm as well as bands at 431 and 389 nm. The 431 and 389 nm bands arise from the formation of excited state CH. The former is the A2D ? X2P Dv = 0 sequence, while the latter is the B2R ? X2P (0,0) band. It should be noted that the analogous A2D ? X2P band of CCl at 278 nm is present, albeit weak, in both spectra. The feature at 656 nm observed from the photolysis of CHCl3 is the well known Balmer alpha line of
hydrogen atoms. The peak at ca. 246 nm is assigned to the 2s2 2p3sð1 P1 Þ ! 2s2 2p2 ð1 S0 Þ (n = 3 ? n = 2) transition of C(I) [12]. The lines at ca. 495 nm and 744 nm are experimental artifacts due to the second and third order diffractions of the 246 nm line. Finally, the peak at 313 nm could be from a couple of sources as well. The previous studies of these systems have identified molecular chlorine bands at 306–308 nm. While the shoulders of this peak may be from Cl2 the sharp peak at ca. 313 nm seems more likely to be from an atomic ion such as the CH (C2R+ ? X2P) Dv = 0 sequence [13]. An explanation for the formation of C2 (d3Pg) is not intuitively obvious. Using the heats of formation from the NIST-JANAF thermochemical tables [14] the energy required to form C2, that is breaking 7 C–Cl bonds (8 for CCl4) and one C–H bond, is 27.28 eV. However, 2 193 nm photons only have 12.85 eV of energy. Therefore, secondary reactions such as CHCl2 + CHCl3, CHCl + CHCl3 or even CHCl2 + CHCl2 can be ruled out because any reaction forming C2 from these photofragments would be endothermic. For reasons discussed below we propose the following mechanism for the formation electronically excited C2 from the photodissociation of CHCl3 and CCl4. Photolysis:
CHCl3 þ 2hmð193 nmÞ ! CHðA2 DÞ þ Cl2 þ Cl
DHo ¼ 2:62 eV
ð1Þ 2
CCl4 þ 2hmð193 nmÞ ! CClðA DÞ þ Cl2 þ Cl
DHo ¼ 0:95 eV
ð2Þ
Reaction: CHCl3:
CHðX2 PÞ þ CHðX2 PÞ ! C2 ða3 Pu Þ þ H þ H
DHo ¼ þ0:97 eV 2
2
ð3Þ 3
CHðA DÞ þ CHðX PÞ ! C2 ða Pu Þ þ H þ H
DHo ¼ 1:90 eV
ð4Þ
Fig. 1. Dispersed emission spectra resulting from the 2 193 nm photodissociation of CCl4 at a resolution of 1 nm. The data shown are an average of several spectra and were corrected by subtracting an averaged background sinal that was obtained in the absence of CCl4. The peak at 246 nm is the 2s22p3s(1P1) ? 2s22p(1S0) (n = 3 ? n = 2) transition of C(l). The lines a 495 nm ad 744 nm are experimental art facts due to the second and third order diffractions of the 246 nm line.
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K.J. McDonald et al. / Chemical Physics Letters 464 (2008) 26–30
Fig. 2. Dispersed emission spectra resulting from the 2 193 nm photodissociation of CHCI3 at a resolution of 1 nm. The data shown are an average of several spectra and were corrected by subtracting an averaged background signal that was obtained in the absence of CHCI3. The peak at 246 nm is the 2s22p3s(1P1) ? 2s22p2(1S0) (n = 3 ? n = 2) transition of C(I). The line at 495 nm is an experimental artifact due to the second order diffraction of this line.
CHðA2 DÞ þ CHðA2 DÞ ! C2 ða3 Pu Þ þ H þ H
DHo ¼ 4:77 eV
ð5aÞ 3
! C2 ðd Pg Þ þ H þ H o
DH ¼ 1:94 eVðDm ¼ þ2 bandÞ DHo ¼ 2:77 eVðDm ¼ 2 bandÞ
ð5bÞ
CCl4:
CCl4 þ 2hmð193 nmÞ ! CClðA2 DÞ þ Cl2 þ Cl
DHo ¼ 0:96 eV
ð6Þ
CClðX2 PÞ þ CClðX2 PÞ ! C2 ða3 Pu Þ þ Cl þ Cl
DHo ¼ þ0:88 eV
ð7Þ
CClðA2 DÞ þ CClðX2 PÞ ! C2 ða3 Pu Þ þ Cl þ Cl
DHo ¼ 3:57 eV
ð8aÞ 3
! C2 ðd Pg Þ þ Cl þ Cl
DHo ¼ 0:74 eVðDm ¼ þ2 bandÞ DHo ¼ 1:56 eVðDm ¼ 2 bandÞ CClðA2 DÞ þ CClðA2 DÞ ! C2 ða3 Pu Þ þ Cl þ Cl DHo ¼ 8:02 eV o
ð8bÞ ð9Þ
Note that in Eqs. (5) and (8) the DH value given is after emission from C2(d3Pg). The photolysis steps are proposed for several reasons. First, the photodissociation of both species is a two photon process which adds 12.85 eV to the system. It should be noted that the CH(A2D) + 3Cl channel is endothermic at this energy and therefore was excluded. In addition, Seccombe et al. [1] have looked at the VUV photodissociation of CCl3X (X = F, H and Cl). In their work they found that due to a large density of states near the dissociation threshold, state-specific or isolated-state dissociation is less likely. This favors formation of the higher energy channel CX* + Cl2 + Cl as opposed to the lower energy production of CCl2 + Cl2. As will be discussed below, in their paper on a similar molecule, namely CHBr3, Chikan et al. [15] found that at energies comparable to this work (13 eV versus 12.85 eV) the CH + Br2 + Br channel was favored. Finally, as can be seen in Fig. 2, large amounts of CH(A2D) and even
some CH(B2R) emission were directly detected from the photodissociation of CHCl3. In a previous study there was a lack of CBr(A2D) emission that was explained by several factors [9]. Since the monochromator used is even less sensitive at the emission wavelength of CCl(A2D), 278 nm, than it is for CBr(A2D), yet CCl(A2D) was detected, we therefore conclude that a relatively large amount of CCl(A2D) is present although we cannot quantify it. These results are in keeping with previous work on the high-energy dissociation of CCl3X molecules which have reported CCl(A2D) emission [4,7–8]. Once formed, the CH(A2D) or CCl(A2D) intermediates form C2(d3Pg) through reactions (5b) and (8b). These mechanisms are proposed for several reasons. The first is that weak C2(d3Pg) chemiluminescence, was observed in the K + CHBr3 reaction by Bergeat et al. [16]. They attributed at least some of this chemiluminescence to the CH + CH reaction. Next, is the plot of CH(A2D) emission intensity at 431 nm versus excimer power for the photodissociation of CHCl3 shown in Fig. 3 (similar plots for the total emission of CHCl3 and CCl4 are virtually identical and are, therefore, not shown). It is readily apparent from the figure that even at low excimer powers (ca. 5–7 mJ) the photodissociation is saturated. This is due to the fact that CCl4, and CHCl3, have significant absorbances at 193 nm which would lead to a resonance enhancement for a twophoton absorption. Since experimental photolysis powers were in the range of 50–100 mJ this means that under experimental conditions a significant fraction of the CHCl3 or CCl4 is undergoing photodissociation. Third, several groups [1,7,9,10,17] have noted the formation of CH(A2D) and CH(B2R) as primary photoproducts in both the single and multiphoton dissociation of CHCl3 as well as from CHBr3 and CHI3. Analogously, CCl(A2D) has also been noted a photoproduct in the dissociation of CCl4 [2,4,6,8]. Fourth, of the myriad of possible secondary reactions only reactions (5b) and (8b) (as well as (9)) are exothermic under experimental conditions. Fifth, if C2(d3Pg) is formed via reaction its emission would be expected to have a two-photon power dependence (as opposed to a four-photon dependence for a secondary absorption/dissociation mechanism). This, of course, is because as a reaction product the final amount of C2(d3Pg) would be proportional to the initial con-
K.J. McDonald et al. / Chemical Physics Letters 464 (2008) 26–30
29
Fig. 3. Power dependence data for the luminescence of CH(A2D) resulting from the 2 193 am photodissociation of CHCl3.
centration of CH(A2D) (or CCl(A2D)). The power dependence measurements, discussed above, show that not only does CH(A2D) emission show a two-photon dependence, the total emission, which is dominated by C2(d3Pg), does as well. Finally, in order for reactions such as 3, 4 or 6 to form C2(d3Pg) the ground state CH or CCl fragments would need a minimum of 3.10 eV total, or 1.55 eV per fragment, of translational, rotational, and vibrational energy. Leone and co-workers recently measured the energy distribution into CH after the photodissociation of CHBr3 [15]. They found that at 13 eV there is a slightly higher than statistical distribution of energy into translation (79% of Eavl). While assuming that CHCl3 dissociates in a similar manner would help explain the fast rise time of CH emission, all or a large fraction of the energy in each degree of freedom would then have to be converted into electronic energy in order to form the C2(d3Pg) product which is highly unlikely. It should be noted that the presence of carbon atom lines in Fig. 1 opens another possible channel for C2 formation, namely as a product of the C + C reaction. This is unlikely for three reasons. First, 12.85 eV is not enough to form carbon atoms directly. The C atom signal is most likely due to an impurity in the precursors or a surface process on the windows making it a minor product channel. Second, the C2 signal from CHCl3 was as strong as that from CCl4, however, the carbon atom lines in the dispersed emission from CCl4 are significantly weaker. In addition, no carbon atom lines appeared in previous work on CBr4 and CHBr3 yet large amounts of C2 were observed [9,10]. Finally, Bergeat et al. observed C2 from the self-reaction of carbon atoms [16]. Their spectra was composed almost entirely of Dm = 1 while Fig. 1 contains Dm = 2, Dm = 0, Dm = 1 and Dm = 2. As noted above ours is the only work to explicitly note the formation of C2(d3Pg). Other groups have observed large broad bands in the 400–700 nm region that have been attributed to emission from CCl2, CCl3 and Cl2. For example, Tiee et al. [8] noted a series of emissions from 380 to 650 nm when an ArF excimer laser was focused into a sample of CCl4. They strongly suggested that these bands were due, at least in part, to the known band system of CCl2 radicals. However, their emission spectrum is virtually identi-
cal to Fig. 1 indicating that they were instead observing C2 emission. In addition Breitbarth et al. [4], Biehl et al. [2] and Ibuki et al. [6] also noted emission over the same range, peaking at 490 nm, from the high-energy dissociation of CCl4 while Seecombe et al. [1] noted a similar band from CHCl3. While none of these spectra are identical to either this work or that of Tiee et al. it is possible that they are due to C2 emission as well. In all of these works the energy spread of the photolysis source was relatively large and the resolution of the dispersed emission low compared to this work. This could result in the formation of highly rotationally and vibrationally excited C2 which would broaden the emission enough that individual bands are lost. The only work that did not note emission in this range was the electron-impact study of Tokue et al. [7]. However, in their experiment the pressure of the CHCl3 and CCl4 precursors were over 1000 times lower than in any other works. Obviously, this would severely attenuate any signals resulting from a collisional process. Acknowledgments The authors are grateful to the Petroleum Research Fund (PRF) for their financial support of this work under Grant #39759-B6. The authors would also like to thank Dr. D.V. Jean Standard for a critical review of this Letter prior to publication. References [1] D.P. Seccombe, R.P. Tuckett, H. Baumgärtel, H.W. Jochims, Phys. Chem. Chem. Phys. 1 (1999) 773. [2] H. Biehl, K.J. Boyle, D.P. Seccombe, D.M. Smith, R.P. Tuckett, H. Baumgärtel, H.W. Jochims, J. Electr. Spectrosc. Relat. Phenom. 97 (1998) 89. [3] R.A. Brownsword, M. Hillenkamp, T. Laurent, R.K. Vatsa, H.-R. Volpp, J. Wolfrum, J. Chem. Phys. 106 (1997) 1359. [4] F.-W. Breitbarth, D. Berg, Chem. Phys. Lett. 149 (1988) 334. [5] T. Ibuki, A. Hiraya, K. Shobatake, J. Chem. Phys. 90 (1989) 6290. [6] T. Ibuki, N. Takahashi, A. Hiraya, K. Shobatake, J. Chem. Phys. 85 (1986) 5717. [7] I. Tokue, T. Honda, Y. Ito, Chem. Phys. 140 (1990) 157. [8] J.J. Tiee, F.B. Wampler, W.W. Rice, J. Chem. Phys. 72 (1980) 2925. [9] B.J. Petro, E.D. Tweeten, R.W. Quandt, J. Phys. Chem. A. 108 (2004) 384. [10] E.D. Tweeten, B.J. Petro, R.W. Quandt, J. Phys. Chem. A. 107 (2003) 19. [11] E.A. Ballik, D.A. Ramsay, Astrophys. J. 137 (1963) 61.
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[12] V. Kaufman, J.F. Ward, J. Opt. Soc. Am. 56 (1966) 1591. [13] L.J. Radziemski Jr., V. Kaufman, J. Opt. Soc. Am. 64 (1974) 366. [14] M.W. Chase Jr., J. Phys. Chem. 9 (1998) 1. Ref. Data Monograph, NIST-JANAF Themochemical Tables, fourth edn., pp. 1–1951.
[15] V. Chikan, F. Fournier, B. Nizamov, S.R. Leone, J. Phys. Chem. A 110 (2006) 2850. [16] A. Bergeat, T. Calvo, G. Dorthe, J.-C. Loison, J. Phys. Chem. A 103 (1999) 6360. [17] C. Chen, Q. Ran, S. Liu, S. Yu, X. Ma, Huaxue Wuli Xuebao 6 (1993) 299.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Reactive formation of dicarbon from the reactions of electronically excited radicals, CH(A2D) and CCl(A2D) Ken J. McDonald, Andrew D. Buettner, Benjamin J. Petro, Robert W. Quandt * Department of Chemistry, Illinois State University, Normal, IL 61790-4160, United States
a r t i c l e
i n f o
Article history: Received 14 March 2008 In final form 28 August 2008 Available online 3 September 2008
a b s t r a c t The 2 193 nm photodissociations of CHCl3 and CCl4 have been examined using dispersed emission. It was found that the initial photodissociation of CHCl3 forms large amounts of CH(A2D) while the photodissociation of CCl4, forms lesser but still significant amounts of CCl(A2D). Emission rise time measurements show that photoproducts quickly react to form C2(d3Pg). Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years the formation of molecular photoproducts from the high-energy dissociation of halocarbons has received renewed interest [1–8]. In a series of recent papers [9,10] we have investigated the 2 193 nm photodissociation of a group of halogenated methanes (CX4 and CHX4 where X = Br or I) via photoproduct emission. Depending on the particular halogen the emission was found to be from either the molecular halogen or the Swan system (d3Pg ? a3Pu) of C2. The high-energy dissociation of CHnCl4n molecules has been studied somewhat more extensively than their iodated and brominated counterparts. Seccombe et al. have observed emission from a series of CCl3X (where X = H, Cl or Br) molecules after vacuum– ultraviolet (VUV) excitation [1,2]. Ibuki et al. have observed emission from the VUV photolysis of CCl4, CBrCl3, CCl3F and CCl2F2 [5,6]. Other techniques used include radiofrequency-excited gas discharges, electron-impact and multiphoton photodissociation [4,7,8]. All of the aforementioned studies, with the notable exception of the electron-impact study, reported broad emission bands from 400 to 700 nm. These bands were variously attributed to emission from CCl2, CCl3 and Cl2. In this study, previous work on the photodissociation of CX4 and CHX3 (X = Br or I) has been extended to the 2 193 nm photodissociation of CHCl3 and CCl4. In addition, an alternative identity for the source of the emission between 400 and 700 nm observed in other works will be discussed. 2. Experimental The experimental technique has been discussed in detail elsewhere [9,10] therefore only a brief description will be given here. For all experiments approximately 50–250 mTorr of CHCl3 * Corresponding author. E-mail address: [email protected] (R.W. Quandt). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.08.099
(Fischer) or CCl4 (J.T. Baker) was flowed through an 8 cm cubic cell with fused silica windows. The back of the cell was offset by an additional 8 cm with an aluminum tube. All chemicals were used without further purification and were stored in opaque containers in a freezer when not in use. The photolysis source was an ArF excimer laser (Lambda Physik, Compex 110) operating at 5 Hz and 50–100 mJ. The excimer beam was focused into the aluminum tube by a quartz lens (f = 100 mm) some 4–5 cm away from the collection optics. This ensured that there was no direct line of site between the focal point and the probe, the stainless steel cell is physically between the two, and minimizes the chance of observing three or more photon processes. Emission signals were collected at right angles to the excimer beam, with both a photomultiplier tube (Electron Tubes Limited model 9129b) and a collimating lens coupled to a fiber optic. The fiber optic then transmitted the collected light to an asymmetric crossed Czerny–Turner monochromator (Ocean Optics model S2000) and the resulting signal stored on a personal computer for later analysis. The emission signals were not corrected for the sensitivity of detection system. 3. Results and discussion 3.1. Power dependence In order to determine the number of photons involved in the photodissociation, the photoproduct emission intensity as a function of photolysis laser power was measured. Excimer power in the cell typically ranged from 0.15 to 25 mJ. For reasons to be discussed in detail below, the power dependence was measured at 431 nm (CHCl3) and 278 nm (CCl4) as well as for total emission. A least-squares fit of the data for CHCl3 emission gave power dependence values of 2.09 ± 0.24 for the peak at 431 nm and 2.07 ± 0.14 for total emission. Due to experimental limitations attempts to determine the power dependence of the emission peak
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K.J. McDonald et al. / Chemical Physics Letters 464 (2008) 26–30
at 278 nm from the dissociation of CCl4 were unsuccessful. However, the total emission was found to have a value of 1.95 ± 0.18. 3.2. Photoproduct spectra Dispersed emission spectra for CCl4 and CHCl3 are shown in Figs. 1 and 2, respectively. A quick perusal of both figures shows a lack of an obvious molecular chlorine channel which is surprising because Cl2 should be a major photoproduct. Indeed, in previous work on CHI3 and CI4 we have detected large amounts of I2* emission under similar conditions [10]. In order to further study this dark channel, an ab initio study was conducted and will be published in a separate paper. Several of the common features of the emission spectra are of interest. Both spectra contain bands with origins at 619, 563, 516, 473 and 438 nm which are degraded to the violet. As mentioned above, previous studies [4–8] of these systems have attributed emission in this range to CCl2, ð1 B1 ! X1 A1 Þ; 0 0 1 þ Cl2 A3 Pð0þ u Þ ! X Rg Þ, or CCl3 ð2A1 ! 2A2 Þ. However, based on previous work in this lab [9], these bands are assigned to the well known Swan system (d3Pg ? a3Pu) of C2 [11]. The spectrum in Fig. 1 is virtually identical to the dispersed emission of CCl4 taken by Tiee et al. who also used ArF dissociation. This indicates that the very tentative assignment of these peaks to Cl2, CCl, CCl2 and CCl3 by Tiee et al. is incorrect [8]. Because C2 emission appears in well under single collision conditions (1 ls/collision at 298 K) it is not surprising that the authors did not consider collisional processes as a source for their spectra. Several other emission bands from the photodissociation of CHCl3 are of interest: specifically, the bands with origins at 744, 656, 495, 313, 277 and 246 nm as well as bands at 431 and 389 nm. The 431 and 389 nm bands arise from the formation of excited state CH. The former is the A2D ? X2P Dv = 0 sequence, while the latter is the B2R ? X2P (0,0) band. It should be noted that the analogous A2D ? X2P band of CCl at 278 nm is present, albeit weak, in both spectra. The feature at 656 nm observed from the photolysis of CHCl3 is the well known Balmer alpha line of
hydrogen atoms. The peak at ca. 246 nm is assigned to the 2s2 2p3sð1 P1 Þ ! 2s2 2p2 ð1 S0 Þ (n = 3 ? n = 2) transition of C(I) [12]. The lines at ca. 495 nm and 744 nm are experimental artifacts due to the second and third order diffractions of the 246 nm line. Finally, the peak at 313 nm could be from a couple of sources as well. The previous studies of these systems have identified molecular chlorine bands at 306–308 nm. While the shoulders of this peak may be from Cl2 the sharp peak at ca. 313 nm seems more likely to be from an atomic ion such as the CH (C2R+ ? X2P) Dv = 0 sequence [13]. An explanation for the formation of C2 (d3Pg) is not intuitively obvious. Using the heats of formation from the NIST-JANAF thermochemical tables [14] the energy required to form C2, that is breaking 7 C–Cl bonds (8 for CCl4) and one C–H bond, is 27.28 eV. However, 2 193 nm photons only have 12.85 eV of energy. Therefore, secondary reactions such as CHCl2 + CHCl3, CHCl + CHCl3 or even CHCl2 + CHCl2 can be ruled out because any reaction forming C2 from these photofragments would be endothermic. For reasons discussed below we propose the following mechanism for the formation electronically excited C2 from the photodissociation of CHCl3 and CCl4. Photolysis:
CHCl3 þ 2hmð193 nmÞ ! CHðA2 DÞ þ Cl2 þ Cl
DHo ¼ 2:62 eV
ð1Þ 2
CCl4 þ 2hmð193 nmÞ ! CClðA DÞ þ Cl2 þ Cl
DHo ¼ 0:95 eV
ð2Þ
Reaction: CHCl3:
CHðX2 PÞ þ CHðX2 PÞ ! C2 ða3 Pu Þ þ H þ H
DHo ¼ þ0:97 eV 2
2
ð3Þ 3
CHðA DÞ þ CHðX PÞ ! C2 ða Pu Þ þ H þ H
DHo ¼ 1:90 eV
ð4Þ
Fig. 1. Dispersed emission spectra resulting from the 2 193 nm photodissociation of CCl4 at a resolution of 1 nm. The data shown are an average of several spectra and were corrected by subtracting an averaged background sinal that was obtained in the absence of CCl4. The peak at 246 nm is the 2s22p3s(1P1) ? 2s22p(1S0) (n = 3 ? n = 2) transition of C(l). The lines a 495 nm ad 744 nm are experimental art facts due to the second and third order diffractions of the 246 nm line.
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K.J. McDonald et al. / Chemical Physics Letters 464 (2008) 26–30
Fig. 2. Dispersed emission spectra resulting from the 2 193 nm photodissociation of CHCI3 at a resolution of 1 nm. The data shown are an average of several spectra and were corrected by subtracting an averaged background signal that was obtained in the absence of CHCI3. The peak at 246 nm is the 2s22p3s(1P1) ? 2s22p2(1S0) (n = 3 ? n = 2) transition of C(I). The line at 495 nm is an experimental artifact due to the second order diffraction of this line.
CHðA2 DÞ þ CHðA2 DÞ ! C2 ða3 Pu Þ þ H þ H
DHo ¼ 4:77 eV
ð5aÞ 3
! C2 ðd Pg Þ þ H þ H o
DH ¼ 1:94 eVðDm ¼ þ2 bandÞ DHo ¼ 2:77 eVðDm ¼ 2 bandÞ
ð5bÞ
CCl4:
CCl4 þ 2hmð193 nmÞ ! CClðA2 DÞ þ Cl2 þ Cl
DHo ¼ 0:96 eV
ð6Þ
CClðX2 PÞ þ CClðX2 PÞ ! C2 ða3 Pu Þ þ Cl þ Cl
DHo ¼ þ0:88 eV
ð7Þ
CClðA2 DÞ þ CClðX2 PÞ ! C2 ða3 Pu Þ þ Cl þ Cl
DHo ¼ 3:57 eV
ð8aÞ 3
! C2 ðd Pg Þ þ Cl þ Cl
DHo ¼ 0:74 eVðDm ¼ þ2 bandÞ DHo ¼ 1:56 eVðDm ¼ 2 bandÞ CClðA2 DÞ þ CClðA2 DÞ ! C2 ða3 Pu Þ þ Cl þ Cl DHo ¼ 8:02 eV o
ð8bÞ ð9Þ
Note that in Eqs. (5) and (8) the DH value given is after emission from C2(d3Pg). The photolysis steps are proposed for several reasons. First, the photodissociation of both species is a two photon process which adds 12.85 eV to the system. It should be noted that the CH(A2D) + 3Cl channel is endothermic at this energy and therefore was excluded. In addition, Seccombe et al. [1] have looked at the VUV photodissociation of CCl3X (X = F, H and Cl). In their work they found that due to a large density of states near the dissociation threshold, state-specific or isolated-state dissociation is less likely. This favors formation of the higher energy channel CX* + Cl2 + Cl as opposed to the lower energy production of CCl2 + Cl2. As will be discussed below, in their paper on a similar molecule, namely CHBr3, Chikan et al. [15] found that at energies comparable to this work (13 eV versus 12.85 eV) the CH + Br2 + Br channel was favored. Finally, as can be seen in Fig. 2, large amounts of CH(A2D) and even
some CH(B2R) emission were directly detected from the photodissociation of CHCl3. In a previous study there was a lack of CBr(A2D) emission that was explained by several factors [9]. Since the monochromator used is even less sensitive at the emission wavelength of CCl(A2D), 278 nm, than it is for CBr(A2D), yet CCl(A2D) was detected, we therefore conclude that a relatively large amount of CCl(A2D) is present although we cannot quantify it. These results are in keeping with previous work on the high-energy dissociation of CCl3X molecules which have reported CCl(A2D) emission [4,7–8]. Once formed, the CH(A2D) or CCl(A2D) intermediates form C2(d3Pg) through reactions (5b) and (8b). These mechanisms are proposed for several reasons. The first is that weak C2(d3Pg) chemiluminescence, was observed in the K + CHBr3 reaction by Bergeat et al. [16]. They attributed at least some of this chemiluminescence to the CH + CH reaction. Next, is the plot of CH(A2D) emission intensity at 431 nm versus excimer power for the photodissociation of CHCl3 shown in Fig. 3 (similar plots for the total emission of CHCl3 and CCl4 are virtually identical and are, therefore, not shown). It is readily apparent from the figure that even at low excimer powers (ca. 5–7 mJ) the photodissociation is saturated. This is due to the fact that CCl4, and CHCl3, have significant absorbances at 193 nm which would lead to a resonance enhancement for a twophoton absorption. Since experimental photolysis powers were in the range of 50–100 mJ this means that under experimental conditions a significant fraction of the CHCl3 or CCl4 is undergoing photodissociation. Third, several groups [1,7,9,10,17] have noted the formation of CH(A2D) and CH(B2R) as primary photoproducts in both the single and multiphoton dissociation of CHCl3 as well as from CHBr3 and CHI3. Analogously, CCl(A2D) has also been noted a photoproduct in the dissociation of CCl4 [2,4,6,8]. Fourth, of the myriad of possible secondary reactions only reactions (5b) and (8b) (as well as (9)) are exothermic under experimental conditions. Fifth, if C2(d3Pg) is formed via reaction its emission would be expected to have a two-photon power dependence (as opposed to a four-photon dependence for a secondary absorption/dissociation mechanism). This, of course, is because as a reaction product the final amount of C2(d3Pg) would be proportional to the initial con-
K.J. McDonald et al. / Chemical Physics Letters 464 (2008) 26–30
29
Fig. 3. Power dependence data for the luminescence of CH(A2D) resulting from the 2 193 am photodissociation of CHCl3.
centration of CH(A2D) (or CCl(A2D)). The power dependence measurements, discussed above, show that not only does CH(A2D) emission show a two-photon dependence, the total emission, which is dominated by C2(d3Pg), does as well. Finally, in order for reactions such as 3, 4 or 6 to form C2(d3Pg) the ground state CH or CCl fragments would need a minimum of 3.10 eV total, or 1.55 eV per fragment, of translational, rotational, and vibrational energy. Leone and co-workers recently measured the energy distribution into CH after the photodissociation of CHBr3 [15]. They found that at 13 eV there is a slightly higher than statistical distribution of energy into translation (79% of Eavl). While assuming that CHCl3 dissociates in a similar manner would help explain the fast rise time of CH emission, all or a large fraction of the energy in each degree of freedom would then have to be converted into electronic energy in order to form the C2(d3Pg) product which is highly unlikely. It should be noted that the presence of carbon atom lines in Fig. 1 opens another possible channel for C2 formation, namely as a product of the C + C reaction. This is unlikely for three reasons. First, 12.85 eV is not enough to form carbon atoms directly. The C atom signal is most likely due to an impurity in the precursors or a surface process on the windows making it a minor product channel. Second, the C2 signal from CHCl3 was as strong as that from CCl4, however, the carbon atom lines in the dispersed emission from CCl4 are significantly weaker. In addition, no carbon atom lines appeared in previous work on CBr4 and CHBr3 yet large amounts of C2 were observed [9,10]. Finally, Bergeat et al. observed C2 from the self-reaction of carbon atoms [16]. Their spectra was composed almost entirely of Dm = 1 while Fig. 1 contains Dm = 2, Dm = 0, Dm = 1 and Dm = 2. As noted above ours is the only work to explicitly note the formation of C2(d3Pg). Other groups have observed large broad bands in the 400–700 nm region that have been attributed to emission from CCl2, CCl3 and Cl2. For example, Tiee et al. [8] noted a series of emissions from 380 to 650 nm when an ArF excimer laser was focused into a sample of CCl4. They strongly suggested that these bands were due, at least in part, to the known band system of CCl2 radicals. However, their emission spectrum is virtually identi-
cal to Fig. 1 indicating that they were instead observing C2 emission. In addition Breitbarth et al. [4], Biehl et al. [2] and Ibuki et al. [6] also noted emission over the same range, peaking at 490 nm, from the high-energy dissociation of CCl4 while Seecombe et al. [1] noted a similar band from CHCl3. While none of these spectra are identical to either this work or that of Tiee et al. it is possible that they are due to C2 emission as well. In all of these works the energy spread of the photolysis source was relatively large and the resolution of the dispersed emission low compared to this work. This could result in the formation of highly rotationally and vibrationally excited C2 which would broaden the emission enough that individual bands are lost. The only work that did not note emission in this range was the electron-impact study of Tokue et al. [7]. However, in their experiment the pressure of the CHCl3 and CCl4 precursors were over 1000 times lower than in any other works. Obviously, this would severely attenuate any signals resulting from a collisional process. Acknowledgments The authors are grateful to the Petroleum Research Fund (PRF) for their financial support of this work under Grant #39759-B6. The authors would also like to thank Dr. D.V. Jean Standard for a critical review of this Letter prior to publication. References [1] D.P. Seccombe, R.P. Tuckett, H. Baumgärtel, H.W. Jochims, Phys. Chem. Chem. Phys. 1 (1999) 773. [2] H. Biehl, K.J. Boyle, D.P. Seccombe, D.M. Smith, R.P. Tuckett, H. Baumgärtel, H.W. Jochims, J. Electr. Spectrosc. Relat. Phenom. 97 (1998) 89. [3] R.A. Brownsword, M. Hillenkamp, T. Laurent, R.K. Vatsa, H.-R. Volpp, J. Wolfrum, J. Chem. Phys. 106 (1997) 1359. [4] F.-W. Breitbarth, D. Berg, Chem. Phys. Lett. 149 (1988) 334. [5] T. Ibuki, A. Hiraya, K. Shobatake, J. Chem. Phys. 90 (1989) 6290. [6] T. Ibuki, N. Takahashi, A. Hiraya, K. Shobatake, J. Chem. Phys. 85 (1986) 5717. [7] I. Tokue, T. Honda, Y. Ito, Chem. Phys. 140 (1990) 157. [8] J.J. Tiee, F.B. Wampler, W.W. Rice, J. Chem. Phys. 72 (1980) 2925. [9] B.J. Petro, E.D. Tweeten, R.W. Quandt, J. Phys. Chem. A. 108 (2004) 384. [10] E.D. Tweeten, B.J. Petro, R.W. Quandt, J. Phys. Chem. A. 107 (2003) 19. [11] E.A. Ballik, D.A. Ramsay, Astrophys. J. 137 (1963) 61.
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[12] V. Kaufman, J.F. Ward, J. Opt. Soc. Am. 56 (1966) 1591. [13] L.J. Radziemski Jr., V. Kaufman, J. Opt. Soc. Am. 64 (1974) 366. [14] M.W. Chase Jr., J. Phys. Chem. 9 (1998) 1. Ref. Data Monograph, NIST-JANAF Themochemical Tables, fourth edn., pp. 1–1951.
[15] V. Chikan, F. Fournier, B. Nizamov, S.R. Leone, J. Phys. Chem. A 110 (2006) 2850. [16] A. Bergeat, T. Calvo, G. Dorthe, J.-C. Loison, J. Phys. Chem. A 103 (1999) 6360. [17] C. Chen, Q. Ran, S. Liu, S. Yu, X. Ma, Huaxue Wuli Xuebao 6 (1993) 299.