Competition between Direct and Indirect Dissociation Pathways in

Mar 15, 2013 - atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determine...
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Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO Shengrui Yu,† Shu Su,† Yvonne Dorenkamp,‡ Alec M. Wodtke,‡ Dongxu Dai,† Kaijun Yuan,*,† and Xueming Yang*,† †

State key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ‡ Institut fur Physikalische Chemie, Georg-August-Universitat and Max-Planck-Institut fur biophysikalische Chemie, Gottingen, Germany ABSTRACT: Photodissociation dynamics of HNCO at photolysis wavelengths between 200 and 240 nm have been studied using the Hatom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. At low photon energy excitation, the product translational energy distribution is nearly statistical and the angular distribution is isotropic, which is consistent with an indirect dissociation mechanism, i.e., internal conversion from S1 to S0 surface and dissociation on S0 surface. As the photon energy increases, a direct dissociation pathway on S1 surface opens up. The product translational energy distribution appears to be quite nonstatistical and the product angular distribution is anisotropic. The fraction of direct dissociation pathway is determined to be 36 ± 5% at 202.67 nm photolysis. Vibrational structures are observed in both direct and indirect dissociation pathways, which can be assigned to the NCO bending mode excitation with some stretching excitation.

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al.7 suggested that channel 1 requires ISC by which HNCO eventually reaches the triplet state T1 and then dissociates. Theoretical studies12 showed that the dissociation of this channel follows both the IC and ISC processes from S1 to S0 and then from S0 to T1. Although its exact quantum yield has not been determined, channel 1 appears to be the major channel in the region just above the threshold of channel 2.13,14 The dynamics of channel 3 have been extensively studied at several different wavelengths.15−20 Drozdoski et al.15 measured the 1NH state distribution by using laser-induced fluorescence (LIF) following 193.3 nm photodissociation. They found that NH(a1Δ) was formed to be predominantly in v = 0, with rotational excitation accounting for about 8% of the available energy. Fujimoto et al.16 probed CO(v) following 193.3 nm photolysis and levels up to v = 4 were observed. Chandler and co-workers21 determined the energy disposal in 1NH and CO produced from HNCO photodissociation at several wavelengths between 230 and 193 nm. They found cold 1NH rotational distribution with hot CO distribution. Such a trend has also been reported by Wang et al.22 following the excitation at 210 nm by using ion velocity slice imaging technique. They suggested impulsive direct and vertical dissociation process of

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Special Issue: Curt Wittig Festschrift

INTRODUCTION Photodissociation of isocyanic acid (HNCO) has attracted much attention over the last thirty years because of its importance in combustion and atmospheric chemistry. In addition, HNCO can serve as a benchmark system for understanding the multiple decomposition pathways in a simple four-atom molecule, such as internal conversion (IC), intersystem crossing (ISC), and direct dissociation processes. The first UV absorption band of HNCO is known to extend from 280 nm to wavelengths shorter than 200 nm. It has been analyzed by Dixon and Kirby1 and by Rabalais et al.2,3 and assigned to an S1(1A″) ← S0(1A′) transition to the first singlet state. Photodissociation dynamics of HNCO in the S1 state have been revealed by dozens of experimental and theoretical studies and are found to be quite complicated, because at least three electronic states (S0, S1, and T1) participate in the dissociation.4−11 There are three thermodynamically allowed dissociation channels from the S1 state: 1

NH(X3Σ−) + CO(X Σ+)

D0 = 30060 ± 25 cm−1 (1)

H(2S) + NCO(X2Π) 1

NH(a1Δ) + CO(X Σ+)

D0 = 38370 ± 25 cm−1 D0 = 42750 ± 25 cm−1

The energetically lowest energy channel 1 is a spin-forbidden dissociation pathway and has only recently been observed by direct detection of 3NH in 260−217 nm photolysis. Zyrianov et © 2013 American Chemical Society

Received: December 28, 2012 Revised: March 7, 2013 Published: March 15, 2013 11673

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been successfully employed in translational energy distribution measurements of many molecular photodissociation processes, like H2O,35−38 CH4,39 and C2H2.40 In brief, a skimmed pulsed molecular beam of HNCO, seeded in argon (mixing ratio ∼3%, total pressure ∼760 Torr), is crossed perpendicularly with the photolysis laser beam. In the wavelength range 220−240 nm, a frequency doubled dye laser (Radiant Dye Laser-Jaguar, D90MA) pumped by a Nd:YAG (Spectra Physics Pro-290) is used. Laser wavelengths in the range 200−220 nm are generated by frequency tripling of the dye laser output. The H products from photodissociation were then excited to a highn Rydberg level using a two-step excitation scheme: coherent VUV excitation at the Lyman-α wavelength (121.6 nm) followed by UV photon excitation at about 366 nm. VUV coherent radiation at the Lyman-α wavelength is generated by four-wave mixing of two 212.5 nm photons and one 845 nm photon in a cell filled with a 3:1 ratio Ar/Kr mixture. Photons at 212.5 nm are produced by doubling the output of a tunable dye laser (Sirah, PESC-G-24) operating at ∼425 nm, pumped by the third harmonic output of a Nd:YAG laser (Spectra Physics Pro-290). A portion of the 532 nm output of the YAG is used to pump another dye laser (Continuum ND6000) operating at ∼845 nm. These beams are then focused into a cell with Kr/Ar mixing gas where four-wave mixing at 121.6 nm is generated. The remainder of the 532 nm source is used to pump a third dye laser (Radiant Dye Laser-Jaguar, D90MA), operating at ∼555 nm. The output from this dye laser was mixed with the fundamental output of the YAG (1064 nm) to generate light at about 366 nm and used to excite the H atoms from the n = 2 level to a Rydberg state with n = 30−80, lying just below the ionization threshold. Any charged species formed at the tagging region by initial laser excitation are extracted away from the TOF axis by a small electric field (∼30 V/cm) placed across the interaction region. The neutral Rydberg H atoms then fly a certain distance (∼333 mm) to reach a MCP detector with a fine metal grid (grounded) in the front. After passing through the grid, the Rydberg tagged atoms are then immediately field-ionized by the electric field applied between the front plate of the Z-stack MCP detector and the fine metal grid. The signal detected by the MCP is then amplified by a fast preamplifier, and counted by a multichannel scaler. HNCO was prepared by heating potassium cyanate (KOCN) in excess stearic acid under vacuum for 3−4 h at ∼90 °C.15,16 Volatile substances were collected in a liquid nitrogen trap at −196 °C. Impurities were removed by trap-to-trap distillations at −60 and −120 °C. The HNCO sample was stored in a stainless steel container cooled to the liquid nitrogen temperature to prevent polymerization. During the experiments, pure Ar gas passed through the HNCO bubbler cooled by low-temperature baths at −50 °C to carry out the HNCO molecule. Purity was checked by mass spectrometry (SRS, RGA200).

HNCO on the singlet state at 210 nm. More recently, Reisler and co-workers11,23 performed a series of experiments to examine the different pathways leading to dissociation following photoexcitation to S1. They suggested that channel 3 dissociation evolves initially on S0, but after exceeding a small barrier on S1, estimated at 400−600 cm−1, direct dissociation on this surface quickly dominates. Channel 2 has also received a great deal of experimental attention in recent years. Drozdoski et al.15 and Spiglanin et al.17 detected NCO via LIF technique, though little dynamical information was observed. Yi and Bersohn24 investigated this channel at 193.3 nm through detecting H atoms via LIF. The average translation energy was found to be 32% of the total available energy. This value contrasted sharply with the value of 70% measured by Zhang et al.25 using the high-Rydberg H atom time-of-flight method. The NCO fragment was observed with substantial bending excitation, and anisotropy β = −0.85 was reported. The dynamics was characterized as direct dissociation on a repulsive surface. However, Crim and coworkers,26−30 using 300 K HNCO samples, looked at this channel near its threshold and deduced a predissociation mechanism with nonstatistical behavior in the NCO vibrational distribution based on the electronic absorption spectrum. Zyrianov et al.10 examined the dissociation of jet-cooled HNCO to channel 2 in the photolysis wavelength range 243−215 nm and found that NCO was rotationally cold with isotropic angular distribution. They favored a mechanism involving IC followed by predissociation on the S1 state. A lower limit of 8140 cm−1 on the barrier to direct dissociation on S1 has also been reported. Recently, several theoretical studies of electronically excited state of HNCO have been reported.8,12,31 Schinke and coworkers32 have calculated a large portion of the S1 surface in five dimensions. The S1 electronic state correlates with channels 2 and 3. However, due to a large barrier of 8710 cm−1 8 or 11200 cm−1 9 in the H + NCO dissociation channel, direct dissociation on the S1 potential energy surface (PES) to yield channel 2 products is not possible except for sufficiently high excitation energy. Thus, for energies below the channel 3 threshold, dissociation should be in the ground electronic state S0, following IC from S1 to S0. On the other side, there is only a low barrier for channel 3 and therefore direct dissociation on the S1 PES becomes possible almost immediately when channel 3 is open. At much higher excitation energies, when the energy exceeds the large barrier in the H + NCO channel, both channels 2 and 3 can be reached by direct dissociation on the S1 PES. Theoretical calculations are in agreement with previous experimental findings. A transition from a mechanism dominated by radiationless decay followed by decomposition on the lower surface S0 to a mechanism dominated by direct dissociation on excited surface S1 is predicted. However, such a competitive mechanism changing is presently the least well understood experimentally. To shed some light on this issue, we reinvestigate the photodissociation dynamics of HNCO in the region 240−200 nm by using high resolution HRTOF technique. The competitive pathways for channel 2 and stateresolved NCO internal energy distributions have been observed and assigned.

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RESULTS AND DISCUSSIONS 3.1. Product Translational Energy Distribution. Figure 1 shows the time-of-flight spectra of the H atom product from photodissociation of HNCO at 202.67, 210.02, 219.53, 229.53, and 239.55 nm using two different polarization schemes (parallel and perpendicular to the detection axis). TOF spectra of the H atom product at the magic angle polarization were also measured to ensure that the intensity ratio of the TOF spectra obtained at parallel and perpendicular polarization is correct. It

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EXPERIMENTAL METHODS The high-n Rydberg H-atom time-of−flight (HRTOF) technique was first introduced by Schnieder et al.33,34 and has 11674

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Figure 2. Translational energy distributions with photolysis laser polarization parallel and perpendicular to the detection axis, and anisotropy parameter β as a function of the total translational energy for the photodissociation of HNCO at 202.67 nm (a, b); 210.02 nm (c, d); 219.53 nm (e, f); 229.53 nm (g, h); 239.55 nm (i, j). Figure 1. Time-of-flight spectra of the H atom product from the photodissociation of HNCO at different wavelengths with the rotating detector direction perpendicular (black line) and parallel (red line) to the photolysis laser polarization: (a) 202.67 nm; (b) 210.02 nm; (c) 219.53 nm; (d) 229.53 nm; (e) 239.55 nm.

from S1 to S0 state. The other feature is clearly nonstatistical distribution with some structures in the high translational energy, which can be only seen at the high excitation energies. This feature is similar to that observed by Zhang et al. at 193.3 nm photolysis.25 These products should come from direct dissociation on the S1 state. This is consistent with the theoretical prediction, in which channel 2, the direct dissociation pathway, would open once the photon energy exceeds the energy barrier in the H + NCO dissociation channel. An additional spectrum, not shown, is taken at 212.5 nm. A little portion of nonstatistical distribution is also present. This places an estimated value of 8674 cm−1 on the barrier to the direct dissociation on S1 to channel 2. From the translational energy distributions shown in Figure 2, the fraction of the available energy released into the translational energy, f T, is determined to be about 0.47, 0.35, 0.33, 0.34, and 0.38, corresponding to the photolysis wavelengths 202.67, 210.02, 219.53, 229.53, and 239.55 nm, respectively. It is noted that the average translational energy increases significantly when the direct dissociation pathway is open. Such a trend is consistent with that reported by Zhang et al.25 at 193.3 nm photolysis, where a value of 0.70 was determined. 3.2. Product Anisotropy Distribution. The product angular distribution has also been determined in this experiment by measuring signals for both parallel and perpendicular directions. The anisotropy distribution β(E) for the dissociation process can be determined from eq 5:

is clear that a well-resolved progression with little anisotropy has been observed at the photolysis wavelengths between 219.53 and 239.55 nm. These structures are less pronounced when the photon energy increases. For wavelengths at 210.02 and 202.67 nm, an additional group of structures with large anisotropy presents at the earlier arriving time. The bimodal TOF distributions at these two wavelengths suggest two competitive pathways should exist in the photodissociation of HNCO. From the TOF spectra, the center-of-mass translational energy distributions can be obtained using ET =

m ⎞⎛ d ⎞ 2 1 ⎛ mH⎜1 + H ⎟⎜ ⎟ 2 ⎝ mR ⎠⎝ t ⎠

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where mH is the mass of the H atom, mR is the mass of the fragment NCO, d is the path length from the interaction region to the detector, and t is the H atom TOF measured over this distance. Figure 2 shows the product translational energy distributions for all five photolysis wavelengths. There are two kinds of features: one is nearly statistical distribution which peaks at rather low translational energy; this can be seen clearly at low excitation energies. This result is quite similar to that observed by Reisler and co-workers.10 These photoproducts should be generated by dissociation on S0 state following IC 11675

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excitation energy (Eavail = 10955 cm−1), the S0 pathway still makes a major contribution. Only 36 ± 5% products come from S1 pathway, whereas for the photolysis wavelength 210.02 nm, less than 10% products come from direct dissociation pathway on S1. Some sharp structures are observed in the spectrum from the direct dissociation pathway, which can be assigned to the vibrational levels of the ground electronic state of NCO(X2Π) products. The peaks in the Eint(NCO) distribution in the range 0−5000 cm−1, shown in Figure 3, are separated by ∼560 cm−1 and have widths of ∼320 cm−1. The two spin−orbit states, X̃ 2Π3/2 and X̃ 2Π1/2, of the NCO products are only separated by 95 cm−1, which are not resolvable in this spectrum. The electronic transition from X̃ 2Π to the low-lying excited 2 + Σ state of the NCO products was studied using LIF,41−43 and the recommended vibrational frequencies of symmetric stretching ν1, bending ν2, and asymmetric stretching ν3 are 1266, 535, and 1921 cm−1, respectively. According to these vibrational frequencies, the observed vibrational structure in the translational energy distribution could be assigned to a progression involving the NCO bending mode ν2. The previous theoretical results showed that the first excited state of HNCO is bending with an N−C−O angle near 120°, which is quite different from the linear ground state geometry of HNCO with an N−C−O angle near 180°.8 In addition, the average lengths of N−C and C−O bond are larger in the excited state than that in the ground state. Thus, bending excitation with a mount of stretching anticipates in the dissociation process. One possible assignment has been made in Figure 3 by using a progression of bending mode ν2 with one quanta of symmetry stretching ν1 or one quanta of asymmetry stretching ν3. Due to the Fermi resonance occurring between the ν1 vibration and the doubly excited bend 2ν2, however, such assignment is not unique. It is also rationalized that most of peaks are assigned to bending excitation, because the large angular changing of NCO radical involved in the dissociation. Figure 4 shows the NCO internal energy distribution at the photolysis wavelength 239.55 nm. Because this photon energy is lower than the channel 2 barrier, the dominate dissociation pathway is thought to occur via IC to S0. It is clear that the

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where θ = 0° for the parallel detection scheme and θ = 90° for the perpendicular detection scheme. Figure 2 presents the anisotropy parameter β(E) as a function of the total translational energy for five wavelengths. Nearly isotropic angular distributions with the value of −0.15 ± 0.05 have been observed at photolysis wavelengths 219.53−239.55 nm. These results are very close to the previous experimental values obtained by Reisler et al. (−0.13 ± 0.05 at 243.1 nm).23 Such small anisotropic values suggest a relatively slow time scale (maybe a few picoseconds) involved in the dissociation. While at wavelengths 210.02 and 202.67 nm, the anisotropy parameter changes from about −0.2 to −0.8 as the translational energy increases. The anisotropy value of the fast component is also close to that obtained by Zhang et al. (−0.85 ± 0.1 at 193 nm),25 which means that the fast direct dissociation pathway is open at relatively short photolysis wavelength. The change in recoil anisotropies around 210.02 nm shows a transition region from a mechanism dominated by IC followed by decomposition via the lower surface S0 to a mechanism dominated by direct dissociation on the S1 surface. 3.3. Competitive Pathways. From the translational energy distribution obtained in this experiment, we can determine the internal energy distribution of the NCO radical. Figure 3 shows

Figure 3. NCO internal energy spectra for photodissociation of HNCO at 202.67 nm (θ = 54.7°). The dotted curves fitted to the spectra represent signals from two competitive dissociation pathways: direct dissociation on the S1 surface, and internal conversion from the S1 to S0 surface and then dissociates on the S0 surface. The observed structures can be assigned to the NCO vibrational excitation (ν1, ν2, ν3), where ν1 is symmetric stretching, ν2 is bending, and ν3 is asymmetric stretching.

the NCO internal energy distribution from the photodissociation of HNCO at 202.67 nm. It is clear that there are two types of distributions: the lower internal energy distribution with sharp structures peaking at ∼1000 cm−1, and the higher internal energy distribution peaking at ∼9000 cm−1. From the previous theoretical and experimental results, the former distribution of the NCO radical should come from the direct dissociation on S1 surface, whereas the latter one should come from the predissociation on S0 surface through S1/ S0 coupling. Because the two pathways have markedly different anisotropy parameters, deconvolution of the spectrum could be achieved, which is also shown in Figure 3. We can find that although direct dissociation on S1 is already open at this

Figure 4. NCO internal energy spectrum for photodissociation of HNCO at 239.55 nm (θ = 54.7°). All the (ν1ν20) levels split into different components by the combination of spin−orbit and Renner− Teller interactions. 11676

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pronounced, which could be due to a more statistical population of the NCO vibrational levels. If the photon energy is high enough to access the barrier, a nonstatistical dissociation pathway would be present and will compete with the statistical distribution. The study of the appearance of the direct dissociation channel as photon energy increases will help us to understand the nature of the competition of the two dissociation pathways.

NCO internal energy distribution from IC pathway is quite different from that obtained from direct dissociation pathway. The experimental value of internal energy (f int = 0.62 for 239.55 nm) is quite similar to the corresponding result of a simple statistical calculation.44 Though a considerable loss of anisotropy in the internal energy distribution would be consistent with decomposition of a predissociative state with a lifetime of the order of its rotational period. The NCO internal energy spectrum also reveals many sharp structures, which are quite different to that observed in Figure 3. Vibrational assignments are not immediately clear, because the spaces between peaks are not regular. The space between first two peaks is 94 cm−1, which is consistent with the two spin−orbit states, X̃ 2Π3/2 and X̃ 2Π1/2, of the NCO (000) products. This implies fine structures of NCO vibrational levels can be obtained in this spectrum. The electron configurations of lower-lying electronic states of NCO are ...(1π)4(σ)2(2π)3, X(2Πi). The degenerate 2Π electronic state splits into Ω = 3/2 and Ω = 1/2 fine structure components. Because the NCO radical has linear structure, the Renner−Teller effect should be involved in degenerate 2Π electronic states. Thus, all the (ν100) levels split into two components by spin−orbit coupling whereas the (ν1ν20) levels (ν2 ≠ 0) split into four components by the combination of spin−orbit and Renner−Teller interaction. Fortunately, the term values for a number of NCO (X2Π) vibronic levels have been reported.41 Most of peaks in Figure 4 can be assigned in accordance with those term values, which has been displayed in Table 1 and also in Figure 4. It is clear that the dominant vibrational excitation is still the bending mode, which is determined by the geometry difference between the HNCO excited state and the NCO ground state. As the photon energy increases, the sharp structures in the NCO internal energy spectra are less

4. CONCLUSIONS The UV photodissociation of HNCO has been reinvestigated using HRTOF technique. Time-of-flight spectra of the H atom product were measured at the detection direction both perpendicular and parallel to the photolysis laser polarization. Total product translational energy distributions and the product angular anisotropy parameters were determined. Two competitive dissociation pathways have been observed. One is the indirect dissociation pathway through S1/S0 coupling, the other is the direct dissociation pathway on the S1 surface. The direct dissociation pathway is more and more important when the photon energy increases. Sharp structures have been observed both in the direct dissociation pathway and in the indirect dissociation pathway. These structures can be assigned to the NCO bending excitation with a little stretching excitation. The present study of the competition between the indirect and direct dissociation pathways would help us to understand the relative roles of S0 and S1 state involving in the HNCO dissociation following the S1(1A″) ← S0(1A′) transition at different photon energies.

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Corresponding Author

*Email: X.Y., [email protected]; K.Y., [email protected]. Notes

Table 1. Peak Positions and Assignments for NCO Vibrational Levelsa peak position (cm−1)

assignment: vibronic term (ν1 ν2 ν3)

0 94 487 530 663 987 1061 1213 1272 1357 1493 1576 1643 1762 1950 2006 2314 2451 2532 2780* 2951 3033

Π3/2 (0 0 0) 2 Π1/2 (0 0 0) μ2Σ+ (0 1 0) 2 Δ5/2 (0 1 0) κ2Σ− (0 1 0) μ2Π(b) (0 2 0) 2 Φ7/2 (0 2 0) κ2Π(b) (0 2 0) 2 Π3/2 (1 0 0) 2 Π1/2 (1 0 0) μ2Δ(b) (0 3 0) 2 Γ9/2 (0 3 0) 2 Γ7/2 (0 3 0) κ2Δ(b) (0 3 0) or μ2Σ+ (1 1 0) μ2Π(b) (0 4 0) or κ2Σ− (1 1 0) μ2Φ(b) (0 4 0) κ2Π(b) (0 4 0) 2 Δ5/2 (0 1 1) 2 Δ3/2 (0 1 1)

AUTHOR INFORMATION

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very grateful for the support of this work by the National Natural Science Foundation of China (No. 21133006), the Chinese Academy of Sciences, and the Ministry of Science and Technology.

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