H Elimination and Metastable Lifetimes in the UV Photoexcitation Of

H elimination and metastable lifetimes in the UV SPECIAL FEATURE photoexcitation of diacetylene

R. Silva†, W. K. Gichuhi†, C. Huang†, M. B. Doyle†, V. V. Kislov‡, A. M. Mebel‡, and A. G. Suits†§

†Department of Chemistry, Wayne State University, Detroit, MI 48202; and ‡Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199

Edited by F. Fleming Crim, University of Wisconsin, Madison, WI, and approved May 12, 2008 (received for review February 11, 2008)

We present an experimental investigation of the UV photochem- ever, at the time, the published thermochemical thresholds were istry of diacetylene under collisionless conditions. The H loss in error, and it is now known that the threshold for H elimination channel is studied using DC slice ion imaging with two-color is 5.77 eV (215 nm), significantly higher than the energy assumed reduced-Doppler detection at 243 nm and 212 nm. The photochem- by Glicker and Okabe. The diacetylene dissociation quantum istry is further studied deep in the vacuum UV, that is, at Lyman- yield was ascribed to reactivity of a long-lived metastable form, ,alpha (121.6 nm). Translational energy distributions for the H ؉ C4H universally assumed to be the lowest triplet state. Subsequently product arising from dissociation of C4H2 after excitation at 243, Zwier and coworkers (10, 13) extensively investigated the UV 212, and 121.6 nm show an isotropic angular distribution and photoinduced chemistry of diacetylene through reactions in a characteristic translational energy proﬁle suggesting statistical ceramic nozzle with a VUV probe of the products downstream. 1 dissociation from the ground state or possibly from a low-lying After excitation of the ⌬u excited state, secondary reactions triplet state. From these distributions, a two-photon dissociation were found to lead to the formation of various larger hydrocar- process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a bons (12, 14, 15). The laser-based studies, principally at 231 and one-photon dissociation process prevails. The results are inter- 243 nm, also found no evidence for radical products proceeding preted with the aid of ab initio calculations on the reaction from primary photodissociation of diacetylene (1, 11, 16–18). pathways and statistical calculations of the dissociation rates and Although triplet diacetylene reactions invoked to account for the product branching. In a second series of experiments, nanosecond observed chemistry are now often incorporated in models of time-resolved phototionization measurements yield a direct de- Titan’s atmosphere, with an assumed metastable lifetime of 1 ms termination of the lifetime of metastable triplet diacetylene under or more, to clarify their role, the triplet lifetime must be collisionless conditions, as well as its dependence on excitation measured directly and as a function of excitation energy (19). We energy. The observed submicrosecond lifetimes suggest that reac- present such measurements here. tions of metastable diacetylene are likely to be less important in Several other theoretical models and experiments have exam- Titan’s atmosphere than previously believed. ined the secondary photochemistry of C4H2 (2, 20, 21). However, for a clear understanding of the role of diacetylene in Titan’s ion imaging ͉ photochemistry ͉ Titan CHEMISTRY atmosphere it is essential to have a better knowledge of its primary photochemistry (product branching and energy depen- aturn’s moon, Titan, is the only solar system body besides dence), in addition to the electronic decay pathways and rates. SEarth and Venus with a dense atmosphere (1, 2). It is widely In this article, we report the experimental results for primary considered as a natural laboratory on the planetary scale in C4H2 photodissociation and metastable lifetimes under colli- understanding the prebiotic chemistry on proto-Earth. Diacety- sionless conditions. The experiments are supported by a series of lene is believed to play a key role in the formation of polyynes ab initio and Rice–Ramsperger–Kassel–Marcus (RRKM) calcu- and polycyclic aromatic hydrocarbons (PAHs) that partially lations to assist in interpretation of the results (22). comprise the haze layer in Titan’s upper atmosphere (2–4). It is well established that the formation of diacetylene is initiated by Results and Discussion photodissociation of acetylene below 217 nm (2, 5–8) according To facilitate the following discussion, computed stationary to the following reaction mechanism: points and dissociation asymptotes for ground-state diacetylene C H ϩ hv ¡ C H ϩ H͑␭ Ͻ 217 nm͒ are shown in Fig. 1. In the first series of experiments, DC-sliced 2 2 2 ion images of H atoms from diacetylene photodissociation at ϩ ¡ ϩ C2H C2H2 C4H2 H three different wavelengths were recorded (Fig. 2). Background signals were subtracted from the raw images and total center- The importance ascribed to diacetylene arises in part because it of-mass translational energy distributions were derived from the absorbs light at longer wavelengths, where the solar flux is refined data (Fig. 3). The images all show isotropic angular higher, than any other major constituents of Titan’s atmosphere; distributions. The distributions at all three wavelengths studied moreover, experimental results suggest it is still photochemically (243 nm, 212 nm, and 121.6 nm) have peaks Ϸ0.45 eV and decay reactive even well below the threshold for dissociation (9–12). to higher recoil energies, extending to 3–4 eV for the 243 and Understanding the dynamics of diacetylene photoexcitation is 121.6 nm results and to Ϸ5 eV for the 212 nm dissociation. The thus key to revealing the factors driving the chemistry of Titan’s isotropic angular distributions and structureless translational atmosphere. energy distributions peaking at low energy are typical of the To date, no experiments on the photochemistry of diacetylene have been performed under collisionless conditions. In a pio- neering study, Glicker and Okabe (9) determined a quantum Author contributions: A.M.M. and A.G.S. designed research; R.S., W.K.G., C.H., M.B.D., yield of 2.0 Ϯ 0.5 for diacetylene photodissociation in the V.V.K., and A.M.M. performed research; and R.S., W.K.G., M.B.D., A.M.M., and A.G.S. wrote wavelength region of 147–254 nm. Between 184 and 254 nm, no the paper. free-radical products were detected and polymeric material was The authors declare no conﬂict of interest. found to coat the inside of the reaction cell. The upper limit for This article is a PNAS Direct Submission. § the quantum yield of C4H formation was then determined to be To whom correspondence should be addressed. E-mail: [email protected]. only 0.06 at 228 nm based on experimental uncertainty. How- © 2008 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0801180105 PNAS ͉ September 2, 2008 ͉ vol. 105 ͉ no. 35 ͉ 12713–12718 Downloaded by guest on September 28, 2021 Fig. 1. Proﬁle of the ground-state potential energy surface of diacetylene calculated at the CCSD(T)/CBSϩ ZPE(B3LYP/6-311G**) level of theory.

statistical, barrierless hydrogen elimination process (23) and species. In the introduction we mentioned the importance suggest dissociation on the ground electronic state or possibly ascribed to metastable diacetylene in Titan’s atmosphere. If from the lowest triplet. As shown in Fig. 1, the C–H bond in reactive C4H2* is very long-lived, its contribution to the chem- diacetylene is very strong, with a dissociation energy of 133 istry in Titan’s stratosphere will clearly be much greater than if kcal/mol. The threshold wavelength for single-photon dissocia- intersystem crossing (ISC) takes it to the unreactive ground state tion of diacetylene is thus Ϸ215 nm, whereas at 243 nm the before it has an opportunity to encounter a suitable reaction single-photon energy is only 118 kcal/mol. Single-photon disso- partner (e.g., some other unsaturated hydrocarbon.) ciation at 243 nm is clearly not possible. However, if C4H2 To examine these issues, first, we consider the possible excited absorbs two photons at 243 nm, dissociation to C4H ϩ His states involved. Vila et al. (22) have calculated energies and possible with a total excess energy of 103 kcal/mol. Such a geometries for a range of low-lying excited states of diacetylene process is consistent with the translational energy distribution in by using CASSCF and CASMP2 methods and we draw on their Fig. 3, which extends nearly to this limit. results for this discussion. If we consider first the linear geometry We can directly compare this result with that obtained at the of the ground state, the initial excitation is to the second singlet same two-photon energy by considering dissociation (and probe) state. This is the only low-lying excited state that is linear. at 121.6 nm. The kinetic energy distribution recorded at this Internal conversion (IC) may then populate the first singlet state, wavelength is also shown in Fig. 3 and is nearly superimposable or ISC may take the system to one of several triplet states. IC in on that obtained at 243 nm. We thus conclude that 243 nm the triplet manifold will then result in formation of the lowest production of H atom from diacetylene likely results from triplet (T1), generally regarded as the identity of the long-lived absorption of two photons, but at 121.6 nm, it is a single-photon metastable species. We should note that these other excited dissociation. states split into cis and trans and even nonplanar isomers fairly We next consider 212 nm, which is several kilocalories per close in energy, but with significant associated relaxation energy mole above the threshold for H loss in diacetylene, the lowest in some cases. Zwier and coworkers (24) have shown by line- photochemical channel. The distribution in Fig. 3 shows a width analysis that the initially excited state must have a subpi- translational energy release similar in shape but extending to cosecond lifetime, and our spectra are in excellent agreement even higher energy than at 243 or 121.6 nm. However, the with their results. Relative rates for IC and ISC leading to the distribution is entirely confined within the available energy (138 lowest triplet, such as could be obtained by femtosecond time- kcal/mol) of the two-photon excitation at 212 nm. We thus resolved photoelectron spectroscopy, would be very interesting conclude that two-photon dissociation still dominates, even for this system, but these measurements are not yet available. In though we are now certainly above the single-photon dissocia- any case, we may assume that these processes are very rapid tion threshold. relative to the final step, ISC for T1-S0. This is the decay rate that All of the observed dissociation processes likely come from the represents the metastable lifetime for an isolated molecule, and ground state after some intramolecular electronic relaxation one that we can measure by using a UV-pump, VUV-probe processes. This ground-state decomposition is only a part of the strategy. In this approach, analogous to early studies by Smalley picture for diacetylene. We also need to understand the elec- and coworkers (25, 26), the nanosecond UV laser excites C4H2, tronic decay dynamics leading to the ground state and the after which it relaxes rapidly to T1. A 7.9-eV (157 nm, F2 excimer) possible time spent as a potentially reactive metastable triplet probe laser can then ionize the electronically excited states of

12714 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0801180105 Silva et al. Downloaded by guest on September 28, 2021 SPECIAL FEATURE

Fig. 3. Total translational energy release spectra derived from images in Fig. 2.

energy. Results for a typical scan at 231.5 nm are shown in Fig. 4A. Single exponential decays are readily seen and fitted after accounting for the laser pulse duration. Experimental decay rates were determined at 231.5, 243.11, and 247.6 nm and plotted in Fig. 4B. Great care was taken in these measurements to ensure that fly-out effects did not contribute to the decays. This conclusion is supported by the fact that the measured decays varied strongly with pump wavelength, but were insensitive to the position of the unfocused lasers. This strong dependence of lifetime on excitation energy is a manifestation of the dependence of the T1-S0 ISC rate on vibrational excitation in the triplet molecule. This behavior is CHEMISTRY commonly seen, and may be ascribed to a barrier on the triplet surface leading to the crossing region. The barrier may be the actual crossing seam of T1 and S0, or it may simply be a region of T1 that must be passed through to access a lower-energy T1/S0 crossing. If we fit the experimental points to an Arrhenius rate expression based on excess energy in the lowest triplet, we can extrapolate this determination to higher excitation energies (Fig. 4B). We use our theoretical value of 3.41 eV for the origin of T1. An experimental value of 3.27 eV was determined by Vuitton et al. (8) based on emission in the matrix; however, it is possible that poor Franck–Condon factors preclude emission from the vibra- tionless level. Based on the fit shown in Fig. 4B, we obtain a triplet lifetime of 36 ns at 212 nm. Given the uncertainty in the origin energy for T1, and variations in our decay measurements, we estimate an uncertainty of, at most, an order of magnitude in this extrapolated lifetime. This result indicates that at 212 nm, just above the lowest dissociation threshold, ISC to S0 is likely to occur for a significant fraction of the excited molecules within the duration of our laser pulse. Fig. 2. DC sliced H atom images from diacetylene excitation at 243 nm (Top), The only other measurement of the triplet lifetime is the study 121.6 nm (Middle), and 212 nm (Bottom). by Vuitton et al. (8) in argon and krypton matrices. They monitored phosphorescence after excitation at 249 nm and

C4H2 [I.P. 10.30 eV vertical, 10.17 adiabatic (27)], but not the extrapolated the results to the gas phase after accounting for the ground state. Our own CCSD(T) calculations for the lowest dielectric effect of the matrix on the lifetime. They obtained a 3 3 Ϸ triplet states, Bu (trans) and B2 (cis), give adiabatic energies of value of 70 ms. It is difficult to compare this thermalized value 3.41 and 3.43 eV relative to the singlet ground state, and vertical in a matrix at 5–30 K with our microcanonical determinations at ionization energies of 7.71 and 7.67 eV, respectively. The latter 2–3 eV vibrational energy, but we may simply note that the trend values confirm our ability to induce efficient single-photon we observe is not inconsistent with their measurement. ionization at 157 nm for the triplets. By monitoring the parent The most important practical issue arising from our lifetime ion yield as a function of pump-probe delay, we determine the measurement is the relevant value for Titan. In attempting to lifetime of T1, and we can do this for any initial UV excitation incorporate metastable reactions into atmospheric models, a

Silva et al. PNAS ͉ September 2, 2008 ͉ vol. 105 ͉ no. 35 ͉ 12715 Downloaded by guest on September 28, 2021 have more directly examined the triplet lifetime in acetylene after ISC from specific rovibrational levels of S1 and obtained a value on the order of 80–100 ␮s that depends on excitation energy (31), just as we have seen here for diacetylene. For diacetylene, our measurement clearly shows a lifetime that is many orders of magnitude lower than what has been assumed at the energies relevant for UV photoexcitation on Titan. However, if collisions result in vibrational cooling of the triplet to the ambient temperature, this lifetime will be extended, perhaps to the point that triplet reactions can contribute significantly to the chemistry. However, in Titan’s upper atmosphere where the UV solar flux is significant, the pressure is too low for vibrational cooling of the triplet to dominate over T1-S0 ISC given these submicrosecond lifetimes. We now return to considering the possible H loss products and pathways on the ground state, guided by the ab initio calculations as shown in Fig. 1. H elimination from diacetylene can occur without a barrier to the product channel P1 giving linear C4H (32, 33). This dissociation pathway can also be accessed via an intermediate (IS1). In this case, H atom migration first occurs from one terminal carbon atom to the other terminal carbon atom with a 101 kcal/mol barrier. The intermediate product will also undergo H elimination without a barrier. At high energy (e.g., Lyman-␣ wavelength), all three channels will be accessible. Starting from the diacetylene ground state, one way of producing P2 and P3 products is through the ring-closing reaction that occurs with a 91.8 kcal/mol barrier to form IS2. Finally, IS2 can undergo H elimination without a barrier to produce P3. Alter- natively, IS2 can rearrange via ring contraction to IS3 with a small barrier of 15.1 kcal/mol. The IS3 intermediate finally undergoes barrierless H elimination to produce P2. The other alternative route of forming P2 products is through intermediate IS1 followed by cyclization with a 59.9 kcal/mol barrier to IS4. The IS4 intermediate dissociates to P2 products. Finally, it is also possible to have rearrangement of IS4 to IS3 through H migration. This reaction occurs with a barrier of 28.6 kcal/mol. To gain a sense of the relative importance of these different pathways, we have performed RRKM calculations of the disso- Fig. 4. Lifetime measurements for triplet diacetylene. (A) Typical pump- ciation rates and branching ratios to the H loss pathways, as well probe decay profile obtained after excitation at 231.5 nm. Points are exper- as all other possible dissociation channels, at several energies of imental result and line is single exponential fit after convolution over laser interest. The results are shown in Table 1. First, we examine the pulse duration. (B) Triplet decay rate plotted vs. excess energy in T1. Solid line total rates for H elimination summed over all C4H product is Arrhenius fit to the points. channels. Just above threshold, at 212 nm, we see a H loss rate of 5.2 ϫ 104 sϪ1. This very low rate readily accounts for the absence of any dissociation signal in our experiment, for which lifetime of 1 ms has generally been used (10, 12, 28), and this has the detection window is only the 10-ns duration of the laser pulse. been cited as the lifetime of triplet acetylene determined indi- If the fluence at 212 nm were sufficiently low, single-photon rectly by Klemperer and coworkers (29, 30). Recent experiments dissociation at 212 nm should be seen. However, the time scale

Table 1. Computed branching ratios (%) and dissociation rates for indicated product channels Wavelength

Product 212 nm 193 nm 157 nm 121.6 nm 2x212 nm

Energy, eV 5.85 6.4 7.9 10.2 11.7 HCCCC ϩ H 100.0 88.4 79.6 74.6 68.3 1 ϩ C4( ⌺g )ϩH2 0.0 0.0 1.5 5.3 7.3 1 C4( AЈ)ϩH2 0.0 0.0 0.0 0.0 0.1 1 C4( Ag)ϩH2 0.0 0 0.0 0.0 0.0 C2ϩC2H2 0.0 11.6 15.8 6.4 4.7 C2H ϩ C2H 0.0 0.0 3.2 13.7 19.6

HCCCC ϩ H (from R1) 81.5 55.0 41.9 40.1 40.3 HCCCC ϩ H (from IS1) 18.5 33.4 37.7 34.5 28.0 k(H loss), sϪ1 5.23 ϫ 104 5.26 ϫ 107 2.39 ϫ 1010 6.88 ϫ 1011 1.98 ϫ 1012 Ϫ1 8 11 11 k(C2H), s 0 0 8.24 ϫ 10 1.14 ϫ 10 5.12 ϫ 10

12716 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0801180105 Silva et al. Downloaded by guest on September 28, 2021 is such that very little will be formed within the 10-ns duration 212-nm light alone. The 243-nm beam is produced by frequency doubling of of the laser pulse, so that it drops below the sensitivity limits of the output of a dye laser pumped by a 308-nm XeCl excimer laser. Lyman-␣ SPECIAL FEATURE our experiment. At 193 nm, the rate rises rapidly to 5.3 ϫ 107 sϪ1. radiation is generated by frequency tripling of 364.7 nm laser light in a VUV Although we have not yet studied dissociation at this energy, we cell containing 30% xenon gas, phase matched with argon at a total pressure see there should be reasonable probability of decomposition of 900 torr. The 364.7-nm beam is focused to the center of the VUV cell by using a tight-focusing quartz lens. The resultant Lyman-␣ light is then loosely within the duration of the laser pulse, with some minor branching ϩ focused to the center of the interaction region by a MgF2 lens. After ioniza- to C2 C2H2. At 157 nm, we begin to see a small contribution tion, the resulting protons are accelerated along the 46-cm flight tube onto a from C2H ϩ C2H, which has the highest threshold energy of any position-sensitive 75-mm diameter microchannel plate (MCP) detector cou- of these channels. At a total energy of 9–10 eV, H elimination pled to a P-47 phosphor screen. Application of a narrow gate (43 ns) to the still dominates (68%) and approximately half is formed via R1 MCP assembly is used to implement the experimental slicing of the equatorial and half via IS1. It is interesting, however, that despite the high region of the H atom recoiling velocity distribution in this particular experiment. The images are recorded by using a CCD camera with our IMACQ threshold, the larger A factor for the C2H ϩ C2H channel causes megapixel software. Negligible Hϩ signal is seen when the TCRD or Lyman-␣ it to win out over the other minor channels, C4 ϩ H2 and C2 ϩ C H . Branching to the lower energy rhombic isomer of C is resonant condition is not met. 2 2 4 In the lifetime measurements, we use a tunable, narrow-linewidth (0.07 negligible in the calculations except at the highest energy stud- cmϪ1) OPO laser that is frequency doubled to provide the pump light at ied, where it is still 100-fold lower than that to the linear isomer. 230–250 nm. The probe light is an F2 excimer laser beam at 157 nm. Both lasers These results suggest the following scenario for the observed are unfocused and directed at right angles mutually perpendicular to the H atom signals: the first photon excites the C4H2 molecule from diacetylene beam. The power in each beam is attenuated to the point that 1 1 ⌺g ground state, which has linear Dϱh symmetry, to ⌬u with the negligible signal is seen from either laser alone. For the UV beam this corre- same symmetry. Very rapid electronic relaxation likely precedes sponds to 0.3 mJ in a spot of 1-cm diameter (Ϸ150 ␮J/cm2), whereas the VUV absorption of a second photon, which then excites the molecule probe is estimated to be roughly half this value. Total ion yield at the parent to one of many high-lying Rydberg states, likely now with C4H2 mass is then recorded as a function of delay between the two lasers. considerable vibrational excitation (34). This highly excited diacetylene can then undergo efficient electronic relaxation, Computational. Molecular geometries and vibrational frequencies of various ultimately to the ground state where dissociation takes place. C4H2 and C4H local minima and transition states were calculated at the hybrid density functional B3LYP/6-311G** level of theory (39, 40) with the only This second photon absorption may be attributed to the presence exception being the linear HCCCC(2⌺ϩ) product, for which this method gives of low-lying Rydberg states of C H Ϸ9.4 eV and below (34). 4 2 one imaginary frequency. When the Cϱv symmetry constraints were lifted for Excited-state calculations at CIS(D) and EOM-CCSD levels of HCCCC, the B3LYP/6-311G** geometry optimization converged to a slightly theory have confirmed the presence of a multitude of such states bent structure, in contradiction to experiment. Alternatively, calculations at in the range of 8.0–10.2 eV with significant oscillator strength. the quadratic configuration interaction QCISD/6-311G** level (41) gave a In Okabe’s work, discharge lamps were used with fluences perfectly linear HCCCC geometry with all real frequencies. The B3LYP and lower by many orders of magnitude. Two-photon processes are QCISD calculations were carried out by using the GAUSSIAN 98 package (42). thus unlikely in that work, and in Titan as well, we should note. Relative energies of the reactant, products, intermediates, and transition However, in the previous work by Zwier and coworkers, it seems states on the C4H2 ground-state potential energy surface were refined at the highest theoretical level feasible by using the coupled cluster CCSD(T) method

laser fluences greater than ours were used, so it is interesting that CHEMISTRY as implemented in the MOLPRO program package (43) with extrapolation to no radical processes were detected. In those experiments, it may the complete basis set (CBS) limit. To achieve this, we computed CCSD(T) total be the number density in the irradiated nozzle extension that is energies for each stationary point with Dunning’s correlation-consistent cc- key, so that the rate of metastable reaction could exceed the rate pVDZ, cc-pVTZ, cc-pVQZ, and cc-pV5Z basis sets (44) and projected them to of additional photoexcitation and decomposition to radicals. In CCSD(T)/CBS total energies by fitting to the following equation (45) comparing and reconciling all of the various experiments per- ͑ ͒ ϭ ͑ϱ͒ ϩ ϪCx formed under a wide range of conditions, we see many compet- Etot x Etot Be ing factors come into play. This underscores the importance of understanding time scales and the complex interactions between where x is the cardinal number of the basis set (2, 3, 4, and 5, respectively) and Etot(ϱ) is the CCSD(T)/CBS total energy. electronic and vibrational relaxation, secondary photoabsorp- Adiabatic excitation energies to the lowest triplet triplet electronic states tion, and metastable reaction to determine the processes truly 3 3 of C4H2, B2 (cis) and Bu (trans) were calculated at the CCSD(T)/CBS level with relevant to Titan’s atmosphere. their geometries optimized and vibrational frequencies computed at B3LYP/ 6-311G**. Vertical ionization energies of the triplet structures were also Materials and Methods calculated by using the CCSD(T)/CBS method. Experimental. The detailed DC slice imaging experimental set-up has been To compute rate constants for individual reaction steps dependent on the reported elsewhere (35, 36). Here, we review the essential components of the energy of absorbed photons, we used conventional microcanonical RRKM present configuration. A pulsed supersonic molecular beam containing Ϸ40% theory (46) with ab initio calculated relative energies and molecular param- diacetylene seeded in argon is expanded from 1000 torr via a piezoelectric eters. The computational procedure has been described in detail previously pulsed valve into the source chamber held at Ϸ10Ϫ6 torr. The beam, collimated (47). The harmonic approximation was used in calculations of numbers and after passing through a skimmer, enters an interaction chamber (Ϸ10Ϫ8 torr) densities of state. For reaction steps occurring without distinct transition between the repeller and extractor electrodes. The laser and molecular beam states, which include H eliminations from various C4H2 intermediates and the delay is adjusted to access the earlier portion of the molecular beam pulse to C–C bond fission to produce C2H ϩ C2H, we used microcanonical variational eliminate the contribution from diacetylene dimer or clusters. Two counter- transition state theory (VTST) (47, 48). With all rate constants in hand, we propagating laser beams of different wavelengths are focused on the molec- calculated product-branching ratios by solving first-order kinetic equations ular beam in the interaction region. The C4H2 is excited and dissociated to for unimolecular reactions on the C4H2 surface according to the reaction produce H atoms, which are then probed by a two-color reduced-Doppler scheme shown in Fig. 1. Only a single total-energy level was considered (TCRD) REMPI (37, 38) scheme, or 1 ϩ 1Ј ionization in the case of the Lyman-␣ throughout, as for collisionless conditions. We used the fourth-order Runge– dissociation. Kutta method to solve the equations; the product concentrations at the time In this application of the TCRD scheme, one laser beam is used to excite the when they converged were used to compute branching ratios. The calculated diacetylene, and a second laser is used in combination with the first to detect concentration profile of the C4H ϩ H product vs. time was then fit to a the product. For example, laser light at 212 nm is focused onto the molecular first-order kinetic law to deduce the overall rate constant for H elimination beam, giving rise to H atom products. Counterpropagating 285-m light then from diacetylene. combines with the 212-m light to drive the H atom 1s–2s two-photon transition with a significantly reduced Doppler width. Ionization is then achieved by ACKNOWLEDGMENTS. A.G.S. thanks J. Martinez and R. I. Kaiser for helpful absorption of a third photon from either laser beam. Furthermore, the di- discussions. This work was supported by National Science Foundation Award acetylene is transparent at 285 nm, so the photodissociation is induced by the CHE-0627854.

Silva et al. PNAS ͉ September 2, 2008 ͉ vol. 105 ͉ no. 35 ͉ 12717 Downloaded by guest on September 28, 2021 1. Wilson EH, Atreya SK (2004) Current state of modeling the photochemistry of Titan’s 24. Bandy RE, Lakshminarayan C, Zwier TS (1992) Spectroscopy and photophysics of the 1 1 mutually dependent atmosphere and ionosphere. J Geophys Res Planets 109:E06002- ⌬u4 ⌺gϩ transition of jet-cooled C4H2,C4HD, and C4D2. J Phys Chem 96:5337–5343. 1-39. 25. Dietz TG, Duncan MA, Smalley RE (1982) Time evolution studies of triplet toluene by 2. Yung YL, Allen M, Pinto JP (1984) Photochemistry of the atmosphere of Titan: Com- 2-color photo-ionization. J Chem Phys 76:1227–1232. parison between model and observations. Astrophys J Suppl S 55:465–506. 26. Duncan MA, Dietz TG, Liverman MG, Smalley RE (1981) Photo-ionization measurement 3. West RA, Strobel DF, Tomasko MG (1986) Clouds, aerosols, and photochemistry in the of the triplet lifetime of benzene. J Phys Chem 85:7–9. Jovian atmosphere. Icarus 65:161–217. 27. Lias, S. G (2005) Ionization Energy Evaluation, eds Linstrom PJ, Mallard WG (National 4. Thompson WR, Singh SK, Khare BN, Sagan C (1989) Triton—Stratospheric molecules Institute of Standards and Technology, Gaithersburg, MD). and organic sediments. Geophys Res Lett 16:981–984. 28. Lebonnois S, Toublanc D, Hourdin F, Rannou P (2001) Seasonal variations of Titan’s 5. Nizamov B, Leone SR (2004) Kinetics of C2H reactions with hydrocarbons and nitriles in atmospheric composition. Icarus 152:384–406. the 104–296 K temperature range. J Phys Chem A 108:1746–1752. 29. Hemminger JC, Wicke BG, Klemperer W (1976) Delocalization of electronic energy in 6. Vakhtin AB, Heard DE, Smith IWM, Leone SR (2001) Kinetics of reactions of C2H radical the lowest triplet states of molecules. J Chem Phys 65:2798. with acetylene, O-2, methylacetylene, and allene in a pulsed Laval nozzle apparatus at 30. Lisy JM, Klemperer W (1980) Electric deflection studies of metastable acetylene. J Chem Tϭ103 K. Chem Phys Lett 344:317–324. Phys 72:3880–3883. 7. Pedersen JOP, Opansky BJ, Leone SR (1993) Laboratory studies of low-temperature 31. Suzuki T, Shi Y, Kohguchi H (1997) Detection of metastable triplet acetylene produced reactions of C2h with C2h2 and implications for atmospheric models of Titan. J Phys by intersystem crossing from the excited A((1)A( )) state. J Chem Phys 106:5292–5295. Chem 97:6822–6829. u 32. Mebel AM, Kislov VV, Kaiser RI (2006) Ab initio/Rice-Ramsperger-Kassel-Marcus study 8. Vuitton V, et al. (2003) Intrinsic lifetime of metastable excited C4H2: implications for the photochemistry of C4H2 in Titan’s atmosphere. Planet Space Sci 51:847–852. of the singlet C4H4 potential energy surface and of the reactions of C2(X ϩ ϩ 9. Glicker S, Okabe H (1987) Photochemistry of diacetylene. J Phys Chem 91:437–440. (1)Sigma( )(g)) with C2H4(X (1)A(1g)( )) and C(D-1) with C3H4 (allene and methylac- 10. Bandy RE, Lakshminarayan C, Frost RK, Zwier TS (1992) Direct detection of c4h2 etylene). J Chem Phys 125:133113. photochemical products—Possible routes to complex hydrocarbons in planetary- 33. Kaiser RI, Balucani N, Charkin DO, Mebel AM (2003) A crossed beam and ab initio study ϩ ϩ ϩ atmospheres. Science 258:1630–1633. of the C2(X-1 Sigma( )(g)/a(3)Pi (u)) C2H2(X-1 Sigma( ) (g)) reactions. Chem Phys Lett 11. Bandy RE, Lakshminarayan C, Frost RK, Zwier TS (1993) The ultraviolet photochemistry 382, 112–119. of diacetylene—Direct detection of primary products of the metastable C4H2* ϩ C4H2 34. Allan M (1984) The excited-states of 1,3-butadiyne determined by electron-energy loss reaction. J Chem Phys 98:5362–5374. spectroscopy. J Chem Phys 80:6020–6024. 12. Zwier TS, Allen M (1996) Metastable diacetylene reactions as routes to large hydro- 35. Townsend D, Minitti MP, Suits AG (2003) Direct current slice imaging. Rev Sci Instrum carbons in Titan’s atmosphere. Icarus 123:578–583. 74:2530–2539. 13. Arrington CA, Ramos C, Robinson AD, Zwier TS (1999) Ultraviolet photochemistry of 36. Townsend D, Lee SK, Suits AG (2004) Orbital polarization from DC slice imaging: diacetylene with alkynes and alkenes: Spectroscopic characterization of the products. S(D-1(2)) alignment in the photodissociation of ethylene sulfide. Chem Phys 301:197– J Phys Chem A 103:1294–1299. 208. 14. Arrington CA, Ramos C, Robinson AD, Zwier TS (1998) Aromatic ring-forming reactions 37. Huang CS, Lahankar SA, Kim MH, Zhang BL, Suits AG (2006) Doppler-free/Doppler- of metastable diacetylene with 1,3-butadiene. J Phys Chem A 102:3315–3322. sliced ion imaging. Phys Chem Chem Phys 8:4652–4654. 15. Frost RK, Arrington CA, Ramos C, Zwier TS (1996) Ultraviolet photochemistry of 38. Huang CS, Li W, Kim MH, Suits AG (2006) Two-color reduced-Doppler ion imaging. diacetylene: The metastable C4H2* reaction with ethene, propene, and propyne. JAm J Chem Phys 125:121101. Chem Soc 118:4451–4461. 39. Lee CT, Yang WT, Parr RG (1988) Development of the Colle-Salvetti correlation-energy 16. Frost RK, Zavarin GS, Zwier TS (1995) Ultraviolet photochemistry of diacetylene— formula into a functional of the electron-density. Phys Rev B 37:785–789. ϩ metastable C4H2* C2H2 reaction in helium and nitrogen. J Phys Chem 99:9408–9415. 40. Becke AD (1993) Density-functional thermochemistry. 3. The role of exact exchange. 17. Robinson AG, Winter PR, Zwier TS (2002) The ultraviolet photochemistry of diacetylene J Chem Phys 98:5648–5652. with styrene. J Phys Chem A 106:5789–5796. 41. Pople JA, Headgordon M, Raghavachari K (1987) Quadratic configuration- 18. Robinson AG, Winter PR, Ramos C, Zwier TS (2000) Ultraviolet photochemistry of interaction—A general technique for determining electron correlation energies. diacetylene: Reactions with benzene and toluene. J Phys Chem A 104:10312–10320. J Chem Phys 87:5968–5975. 19. Hebrard E, Dobrijevic M, Benilan Y, Raulin F (2006) Photochemical kinetics uncertain- 42. Frisch MJ, et al. (1998) Gaussian 98 (Gaussian, Inc., Wallingford, CT), Revision A. 9. ties in modeling Titan’s atmosphere: A review. J Photochem Photobiol C Photochem 43. Werner HJ, Knowles PJ (2003) MOLPRO (University of Birmingham, Birmingham, Rev 7:211–230. United Kingdom). 20. Stearns JA, Zwier TS, Kraka E, Cremer D (2006) Experimental and computational study of the ultraviolet photolysis of vinylacetylene. Part II. Phys Chem Chem Phys 8:5317– 44. Dunning TH (1989) Gaussian-Basis sets for use in correlated molecular calculations. 1. 5327. The atoms boron through neon and hydrogen. J Chem Phys 90:1007–1023. 21. Kaiser RI, Stahl F, Schleyer PV, Schaefer HF (2002) Atomic and molecular hydrogen 45. Peterson KA, Dunning TH (1995) Intrinsic errors in several ab-initio methods—The dissociation-energy of N . J Phys Chem 99:3898–3901. elimination in the crossed beam reaction of d1-ethinyl radicals C2D(X (2)Sigma(ϩ)) with 2 acetylene, C2H2(X (1)Sigma(ϩ)(g)): Dynamics of d1-diacetylene (HCCCCD) and dl- 46. Eyring H, Lin SH, Lin SM (1980) Basic Chemical Kinetics (Wiley, New York). butadiynyl (DCCCC) formation. Phys Chem Chem Phys 4:2950–2958. 47. Kislov VV, Nguyen TL, Mebel AM, Lin SH, Smith SC (2004) Photodissociation of benzene 22. Vila F, Borowski P, Jordan KD (2000) Theoretical study of the low-lying electronically under collision-free conditions: An ab initio/Rice-Ramsperger-Kassel-Marcus study. excited states of diacetylene. J Phys Chem A 104:9009–9016. J Chem Phys 120:7008–7017. 23. Feltham EJ, et al. (2003) Ketene photodissociation in the wavelength range 193–215 48. Steinfeld JI, Francisco JS, Hase WL (1999) Chemical Kinetics Dynamics (Prentice-Hall, nm: The H atom production channel. J Chem Phys 119:6017–6031. Upper Saddle River, NJ), 2nd Ed.

12718 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0801180105 Silva et al. Downloaded by guest on September 28, 2021