Vacuum Ultraviolet Photodissociation Dynamics of Isocyanic Acid: The

Article pubs.acs.org/JPCA

Vacuum Ultraviolet Photodissociation Dynamics of Isocyanic Acid: The Hydrogen Elimination Channel Shengrui Yu,† Shu Su,† 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 ABSTRACT: Photodissociation dynamics of the H-atom channel from HNCO photolysis between 124 and 137 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. Two dissociation channels, H + NCO (X2Π) and H + NCO(A2Σ+), have been observed. The former channel involves two different dissociation pathways; one is a slow predissociation pathway through internal conversion from the excited state to the S0 state, and the other is a fast predissociation pathway through internal conversion from the excited state to the S1 state. The latter channel dominates by a prompt dissociation via coupling to the S2 state. As the photon energy increases, dissociation on the ground state S0 becomes dominant. Vibrational structures are observed in both the NCO(X) and NCO(A) channels, which can be assigned to the bending mode excitation with some stretching vibrational excitation.

1. INTRODUCTION

D0 = 30060 ± 25 cm−1 (1)

Recently, many aspects of HNCO decomposition in this wavelength region have become fully understood.21−27 Detailed discussions can be found by Reisler et al.,14 Schinke et al.,28 and Yu et al.29. The S1 state correlates with channels 2 and 3. However, due to a large barrier of ∼8710 cm−111 in the H + NCO(X2Π) dissociation channel, direct dissociation on the S1 potential energy surface (PES) to yield channel 2 products is not possible except for with a sufficiently high excitation energy. Thus, for energies below the channel 3 threshold, dissociation should be in the ground electronic state S0, following internal conversion (IC) from S1 to S0. On the other side, there is only a low barrier of 400−600 cm−1 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(X2Π) channel, both channels 2 and 3 can be reached by direct dissociation on the S1 PES. Such competitive mechanism changing is directly observed by recent experimental investigations.29 The spin-forbidden channel 1 has also recently been observed by direct detection of NH(3Σ−) in 260−217 nm photolysis. Zyrianov et al.30 suggested that channel 1 requires intersystem conversion (ISC) by which HNCO eventually reaches the triplet state T1 and then dissociates. Theoretical studies31 showed that the dissociation of this channel follows

D0 = 38370 ± 25 cm−1 (2)

Special Issue: Terry A. Miller Festschrift

Isocyanic acid (HNCO) plays an important role in the removal of nitrogen oxides in combustion and in energetic materials combustion.1−3 It is also a possible intermediate in the reaction H + NCO → NH + CO and a convenient photolytic source of the NCO and NH radicals. In addition to these practical significances, it can serve as a benchmark of the rich photochemistry for more complex systems. Therefore, the mechanisms of its decomposition and chemical reactions are of interest. 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 Kirby4 and by Rabalais et al.5,6 and assigned to an S1(1A″) ← S0(1A′) transition to the first singlet state, (9a′)2(2a″)1(10a′)1 ← (9a′)2(2a″)2(10a′)0. Photodissociation dynamics of HNCO on the S1 state have been studied in great detail using various experimental techniques and theoretical calculations.7−20 In the wavelength range of 260− 193.3 nm, HNCO photodissociation is dominated by the following competitive dissociation channels, for which only channels 2 and 3 are spin-allowed: HNCO(S1) → NH(X3Σ−) + CO(X1Σ+) H(2S) + NCO(X2Π) 1

NH(a Δ) + CO(X1Σ+)

D0 = 42750 ± 25 cm

© 2013 American Chemical Society

−1

(3)

Received: July 29, 2013 Revised: August 31, 2013 Published: September 16, 2013 13564

dx.doi.org/10.1021/jp407556k | J. Phys. Chem. A 2013, 117, 13564−13571

The Journal of Physical Chemistry A

Article

Figure 1. Absorption spectrum of HNCO in the region of 1200−2000 Å with a resolution of 0.8 Å (adapted from ref 33). The positions of the photolysis excitation wavelengths used in this work are indicated by downward-pointing arrows.

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. Compared to the number of UV photolysis experiments, rather little work has been done on the dynamics of the HNCO dissociation after excitation in the vacuum ultraviolet (VUV). Figure 1 shows the VUV absorption spectrum reported by Okabe et al.33 It is composed of two weak, continuous features with peaks at about 166.5 and 157 nm, a few sharp absorption bands at around 133−140 nm, and one rising continuum superimposed by several diffuse bands. The sharp absorption bands may belong to parallel transitions from the ground state to a bound Rydberg state with some vibrational excitation, ns(1A′) ← S0(1A′). The energy separation between peaks is regular and consistent with NCO bending progression of the HNCO molecule, while the broad band shorter than 132.5 nm may be associated with transitions to higher Rydberg states. In the VUV region, more dissociation channels will be energetically possible, which is shown in Figure 2. Milligan et al. measured the spectroscopy of the NCO(B2Π) free radical for the VUV photolysis of HNCO in the low-temperature matrix.32 In gas-phase photolysis study by Okabe,33 strong emission bands originating from electronically excited NCO(A2Σ+), NH(c1Π), and NH(A3Π) were observed in the photodissociation of HNCO in the VUV. From the analysis of the NCO(A2Σ+) emission spectrum, it was deduced that the photoexcited HNCO molecule has a bent NCO configuration. Okabe et al. also suggested that NH(A3Π) was generated mostly by the secondary process CO(a3Π) + NH(X1Σ−) → CO(X1Σ+) + NH(A3Π). Hikida et al.,34 however, could not find an indication for such a process in time-resolved NH(A3Π) fluorescence studies in the Lyman-α photolysis of HNCO and proposed that NH(A3Π) was a direct product formed from a “spin-forbidden” HNCO dissociation channel. Uno et al.35 investigated HNCO dissociation in the wavelength region of 107−180 nm using synchrotron radiation as a light source. Fluorescence excitation spectra of the photofragments, NH(c1Π,A3Π) and NCO(A2Σ+) were measured under collisionfree conditions. The NCO(B2Π→X2Σ+) emission was too weak to be observable. They also found that the NCO(A2Σ+) quantum yield exhibited a pronounced maximum at 146 nm, then abruptly decreased to about 10% in going from 146 to 140 nm, and then gradually decreased to about 1.2% in the region of 140−110 nm. Recently, Volpp et al.36 studied the

Figure 2. Energy level diagram for the HNCO dissociation processes (adapted from ref 36). The red arrow covers a range of wavelengths investigated in this VUV photolysis study.

dissociation dynamics of HNCO(X1A′) + 121.6 → H + NCO. The observed quantum yields indicated that H-atom production is a major channel in the VUV dissociation process. Moreover, a dissociation mechanism was suggested in which Hatom formation proceeded via a statistical unimolecular decay of a hot HNCO intermediate formed by a radiationless transition of the optically excited bound HNCO state to a lower-lying dissociative state. Though electronically excited radicals NCO and NH from HNCO photodissociation in the VUV region have been reported by fluorescence emission measurements, the dissociation processes involving multiple PES couplings are still unclear. Information regarding the predissociation pathways to produce the H + NCO(X) and H + NCO(A) is not yet available. In the present work, the H-atom Rydberg tagging time-of-flight (HRTOF) technique is used to characterize Hatom elimination channels and mechanisms involved in the 13565



Article

3. RESULTS (i). Product Translational Energy Distribution. Figure 3 shows TOF spectra of H-atom products obtained following

decomposition of expansion-cooled HNCO following VUV excitation.

2. EXPERIMENTAL METHOD The high-n HRTOF technique was first introduced by Schnieder et al. in 199037,38 and has been successfully employed in translational energy distribution measurements of many molecular photodissociation processes, like H2O,39−42 CH4,43 and C2H2.44 In brief, a pulsed molecular beam was produced by expanding ∼3% HNCO in argon at a total pressures of ∼700 Torr. We choose argon as a carrier gas because a better cooling effect will be obtained in the adiabatic expansion when the carrier gas and the sample gas have similar weights. Mixtures were expanded into the source chamber through a 0.5 mm diameter pulsed nozzle. The molecular beam was differentially pumped and collimated by a 1.5 mm diameter skimmer. At 3 cm downstream from the skimmer, the molecular beam was crossed with the VUV photolysis radiation generated by using the difference four-wave mixing (DFWM) method of 212.5 nm and another tunable source from 470.25 to 730.01 nm in a pure Kr cell.45,46 The nascent H-atom products formed in the interaction region were doubleresonantly excited sequentially, via the 2p state, by 121.6 nm radiation and then to a high Rydberg state (n = 46) using a ∼366 nm photon. The 121.6 nm light was also generated by using DFWM of the same 212.5 nm light for tunable VUV photolysis light and 845 nm in the same Kr cell. Any accidental prompt ions formed within the interaction region were extracted using a biased deflector plate that straddles the interaction region. The tagged H-atoms then flew a distance of about 740 mm to reach a multichannel plate (MCP) detector with a grounded fine metal grid in the front. After passing through the grid, the Rydberg H-atoms were efficiently fieldionized by the strong electric field applied between the front plate of the Z-stack MCP detector and the fine metal grid. The signal received by the MCP was amplified by a fast preamplifier and counted by a multichannel scaler (P7888-2(E) FASTCOMTEC). The detector in this experiment was fixed in a direction perpendicular to the molecular beam. The molecular beam and the photolysis laser beam were perpendicular to each other. To make angular anisotropy measurements, the polarization direction of the VUV photolysis light was varied by rotating the polarization of the tunable source from 470.25 to 730.01 nm using the zero-order half wave plates. Because 121.6 and 212.5 nm also generate H-atom signals, background subtraction was achieved by alternating the VUV photolysis laser on and off. To reduce the errors caused by background subtraction, the 121.6 nm generated background signal was suppressed as much as possible relative to the tunable VUV photolysis signal. Great efforts have been made on the optimization of the performance of the pulsed valve. A short beam pulse with a fast rise time (∼80 μs) is very important to minimize the HNCO clusters in the molecular expansion. HNCO was prepared by heating potassium cyanate (KOCN) in excess stearic acid under vacuum for ∼3 h by using a water bath at ∼90 °C.18,47 Impurities were removed by distilling from −60 to −196 °C. HNCO samples were stored in a stainless steel container cooled to the liquid nitrogen temperature to prevent polymerization. During the experiments, pure argon gas passed through the HNCO bubbler cooled by lowtemperature baths at −50 °C to carry out the HNCO molecule. The purity was verified by mass spectrometry (SRS, RGA200).

Figure 3. TOF 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) 137.26, (b) 135.25, (c) 133.75, (d) 128.02, (e) 124.35 nm.

photolysis of jet-cooled HNCO molecules at five specific wavelengths, (a) 137.26, (b) 135.25, (c) 133.75, (d) 128.02, and (e) 124.35 nm (indicated by downward-pointing arrows in Figure 1). Each spectrum was recorded with the polarization vector of the photolysis laser, εphot, aligned parallel (θ = 0°) and perpendicular (θ = 90°) to the TOF axis. The TOF spectrum at the magic angle (θ = 54.7°) was also measured in order to check the accuracy of the polarization direction. It is clear that three distinct features have been observed in the parallel direction at photolysis wavelengths between 133 and 137 nm. One is a strong peak with partially resolved structures at the earlier arriving time, another is well-resolved progression extending up to the arriving time limit, and the third is a broad component underlying the two resolved progressions. The first two structures are less pronounced when the photon energy increases, while the spectra in the perpendicular direction at five wavelengths all present a broad continuum distribution. It seems that multiple pathways are involved in the VUV photodissociation of HNCO. These TOF spectra can be converted into the total translational energy distribution spectra of the photodissociation products (H and NCO) in the center-of-mass frame through conservation of momentum by using 13566



Article

Figure 4. The total translational energy release spectra with photolysis laser polarization parallel and perpendicular to the detection axis and the anisotropy parameter β as a function of the total translational energy for the photodissociation of HNCO at 137.26 (a,b), 135.25 (c,d), 133.75 (e,f), 128.02 (g,h), and 124.35 nm (i,j).

ET =

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

mainly from the measurement error. (We can get the experimental resolution of 1% in the translation energy distribution.) A relatively small fraction of the available energy deposited into the translational energy suggests that an indirect dissociation pathway dominants in the VUV photolysis of HNCO, compared with the value of 0.7 obtained from direct dissociation of HNCO at 193.3 nm.21 (ii). Product Angular 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

(4)

where mH and mR are the masses of the H-atom and the cofragment NCO, d is the path length from the interaction region to the detector, and t is the measured TOF. The TOF to translational energy spectra in three-body dissociations will give slightly different energy scales and will depend on whether the fragmentation occurs simultaneously or sequentially. However, given the mass combinations involved, the differences are generally too small to affect the subsequent discussions. Figure 4 (left-hand column) shows the translational energy spectra from HNCO photodissociation at five photolysis wavelengths. Three components have been clearly observed in the parallel direction at five excitation wavelengths. It is clear that the fraction of the two structural components decreases as the photoexcitation wavelength decreases. From these translational energy distribution spectra, the fraction of the available energy deposited into the translational energy, f T, is determined to be about 0.33, 0.33, 0.32, 0.27, and 0.25, corresponding to the photolysis wavelengths at 137.26, 135.25, 133.75, 128.02, and 124.35 nm, respectively. The fitting is direct, and the errors are

f (E , θ ) = φ(E)[1 + β(E)P2(cos θ )]

(5)

where θ = 0° is for the parallel detection scheme and θ = 90° for the perpendicular detection scheme. Figure 4 (right-hand column) displays the anisotropy parameter β(E) as a function of the total translational energy ET for five photolysis wavelengths. It is obvious that the anisotropy distributions are quite complicated at the wavelengths between 133 and 137 nm. The anisotropy parameter β is about 0.5 in the low translational energy region, then changes to near 0 at the 13567



Article

time is long enough to allow for complete internal energy randomization. The dissociation process is more complicated at a photolysis wavelength of 133−137 nm. It is obvious that the product translational energy distributions in the perpendicular direction are similar to that observed at 124.35 nm photolysis, which means that the similar indirect dissociation pathway is dominant at photolysis wavelengths of 133−137 nm, while the structure components in the parallel direction should come from other dissociation pathways with large angular anisotropy. We use the translational energy profile P1 (shown in Figure 5),

middle translational energy region, and then reaches the maximum of 1.5 at the high translational energy region. As the wavelength decreases, however, the translational energy distribution looks almost isotropic, and the anisotropy parameter is near zero. Similar behavior has also been found in the case of HN348 and CH449 photodissociation, in which they proposed that there are several distinct dissociation pathways with different β parameters involved in the photolysis process, and the finally observed translational-energy-dependent anisotropy parameters arise from the mixture of anticipated pathways. Therefore, multiple dissociation pathways should be involved in HNCO photodissociation in the VUV region.

4. DISCUSSION We now seek to reveal the complicated VUV photodissociation dynamics. Clearly, several dissociation channels are energetically accessible in the VUV photolysis of HNCO (Figure 2). Considering that only H-atom products are detected in this experiment, channels 2 and 4−7 (Figure 2) can be observed if they present. Due to the high threshold energy, the dissociation channel 7 should not be possible. Though the dissociation channel 6 is open at excitation wavelengths used in this work, no notable indication of H + NCO(B2Σ+) formation is observed in the translational energy spectra. From previous studies, although NCO(B2Σ+) emission was observed in the 123.6 nm33 and Lyman-α photolysis35 of HNCO, the low band intensity reported (0.01 of that of NCO(A2Σ+)) suggested that the H + NCO(B2Σ+) channel should not account for the observed H-atom yield. Therefore, the H-atom products from dissociation channel 6 should be a very minor process. There is no evidence to exclude the contribution of channel 4. However, Volpp et al.36 suggested that channel 4 could be regarded as a sequential three-body dissociation process, HNCO + hν → H(2S) + NCO(X2Π) and then NCO(X2Π) → N(4S) + CO(X1Σ+). This means that the dissociation dynamics of Hatom elimination for channel 4 is similar to that for channel 2. Thus, the most possible H-atom elimination channels are channel 2 (H + NCO(X 2 Π)) and channel 5 (H + NCO(A2Σ+)) in the VUV photolysis of HNCO. This is quite reasonable because the starting point of well-resolved structures at low translational energy and partially resolved structures at high translational energy are consistent with the energetic limit of channels 5 and 2, respectively. Previous fluorescence studies33 showed that the NCO(A2Σ+) formation quantum yield ΦNCO(A2Σ+) was found to exhibit a pronounced maximum of 0.8 at 146 nm, then decrease to a value of about 0.10 in going from 146 to 140 nm, and finally decrease to 0.012 at 110 nm. This suggests that the H + NCO(A2Σ+) channel is less important in the VUV photolysis experiment in this work. Especially at 124.35 nm photolysis, most of the H-atom products should come from the H + NCO(X2Π) channel. It is well-known that the electronically excited state S1 and the ground state S0 both correlate with the H + NCO(X2Π) channel. It is noted that the product translational energy distribution is nearly statistical, and the angular distribution is isotropic at 124.35 nm photolysis. This is quite similar to that observed at photolysis wavelengths of 215−240 nm,29 in which the dissociation predominantly proceeds in the ground state S0, following IC from the S1 to S0 state. In our case, the HNCO molecules are excited to the high Rydberg state with a bound PES at 124.35 nm excitation; then, the fragmentation process is dominated by the IC to S0, followed by dissociation on the S0 surface. Such a dissociation

Figure 5. The translational energy release spectra for photodissociation of HNCO at 137.26 nm (θ = 54.7°). The three curves fitted to the spectra represent signals from three distinct dissociation pathways (P1: IC to the S0 surface and then dissociation; P2: IC to the S1 surface and then dissociation; P3: IC to the S2 surface and then dissociation). The observed structures can be assigned to the NCO vibrational excitation (ν1, ν2, ν3), ν1 symmetric stretch, ν2 bend, and ν3 asymmetric stretch.

similar to that observed at 124.35 and 215−240 nm photolysis, to simulate the indirect dissociation pathway with an isotropic distribution in the dissociation process at photolysis wavelengths of 133−137 nm. Subtracting the P1 profile from the total translational energy spectra, we can get the other two components P2 and P3 with resolved progressions. The starting point of the P2 component is consistent with the energetic limit of the H + NCO(X2Π) channel. It is clear that the average translational energy is quite high, and the angular distribution is anisotropic with the maximum value of 1.5 (see the anisotropy spectra at high translational energy in Figure 4b, d, and f). All of these features are quite similar to that observed by Zhang et al.21 at the 193.3 nm photolysis of HNCO, in which a prompt dissociation process on the repulsive S1 surface has been suggested. Thus, we have the conclusion that the P2 component comes from the prompt dissociation pathway on the S1 surface, following IC from the ns Rydberg state to the S1 state. The P3 component with well-resolved progression should be responsible for the H + NCO(A2Σ+) channel because the starting point of P3 is exactly the same as the threshold of the H + NCO(A2Σ+) channel. There would be the second electronically singlet state S2, which correlates with the H + NCO(A2Σ+) channel, as shown in Figure 2. Though the exact energy level of S2 is not known, it should be located just above the energy level of channel 5. The S2 state is a dissociative state and repulsive in the RH−NCO coordinate. The P3 component may come from the fast dissociation pathway on the S2 surface, following IC from the ns Rydberg state to the S2 state. The 13568



Article

5. SUMMARY Photodissociation dynamics of HNCO in the VUV region have been investigated using the HRTOF technique. TOF 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 also determined. Three competitive dissociation pathways have been observed when HNCO molecules are initially excited to certain Rydberg states. Nonadiabatic dissociation from the electronically excited state to the ground state S0 is the dominant pathway, while fast dissociation on the S2 state to produce the NCO(A2Σ+) radical is also important at photolysis wavelengths of 133−137 nm. The structures observed in the translational energy spectra can be assigned to the NCO(A2Σ+) and NCO(X2Π) bending excitation with a little stretching excitation. The present study would help us to understand the complicated dissociation mechanism from a highly electronically excited state.

positive anisotropy parameter of about 0.5 (see the anisotropy spectra at low translational energy in Figure 4b, d, and f) suggests that the initial excitation is from the 1A′ ← 1A′ parallel transition. Figure 5 shows the translational energy profiles from three distinct dissociation pathways P1, P2, and P3 used to simulate the total translational energy spectrum for photodissociation of HNCO at 137.26 nm. The dissociation mechanism can be described as

where ** indicates a hot (vibrationally excited) HNCO molecular state prepared in an ns Rydberg state (excitation at 133−137 nm). It is interesting that more than 70% of the molecules dissociate on the ground state S0 through nonadabatic coupling, while only about 10% of the molecules dissociate on the repulsive state S2 at 137.26 nm. Similar results also can be obtained at 135.25 and 133.75 nm. It is obvious that the photodissociation dynamics of HNCO are quite similar for photolysis wavelengths between 133 and 137 nm, which means that the three peaks between 133 and 137 nm come from the same electronic transition with different vibrational excitation, while for photodissociation of HNCO at 128.02 and 124.35 nm, predissociation through coupling to the ground electronic state S0 becomes the dominant pathway. Such a mechanism should be quite normal in the photodissociation of polyatomic molecules at high electronically excited states.50 Now, we discuss the nature of the peaks in the spectrum (Figure 5). The partially resolved structures in the P2 component corresponding to the H + NCO(X2Π) channel can be assigned to a progression in the ν2 bend mode and a progression of ν2 levels with one quantum of the ν3 asymmetry stretch vibration. Such a high bend excitation with a mount of stretch anticipated in the dissociation process is in reasonable accord with a previous experimental result by H. Okabe,33 implying that the Rydberg state of HNCO from which NCO predissociates has a bent NCO configuration with larger average lengths of the N−C and C−O bonds compared to that of HNCO in the ground state. On the other hand, the positive β distribution being found for this component suggests that the overall dissociation processes must be quite fast compared to that of the P1 pathway. The well-resolved structures in the P3 component corresponding to the H + NCO(A2Σ+) channel also can be assigned to the ν2 bend excitation with a little symmetry stretch excitation. Up to ν2 = 15 of the bending vibration in the NCO(A2Σ+) product has been observed. Such high vibrational excitation also suggests a large angular change for the NCO radical on the S2 surface. The large positive β means that the dissociation process should be much faster than the time scale of internal energy randomization. This study helps us to understand the nature of complicated dissociation pathways of HNCO following VUV excitation. However, knowledge about the nature of the Rydberg states, the electronically excited state S2, and the couplings between these states is still lacking. Further experimental and theoretical investigations are clearly needed.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.Y.). *E-mail: [email protected] (X.Y.). Author Contributions †

S.Y. and S.S. made similar contributions to this work.

Notes

The authors declare no competing financial interest.

■

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.

■

REFERENCES

(1) Perry, R. A.; Siebers, D. L. Rapid Reduction of Nitrogen Oxides in Exhaust Gas Streams. Nature 1986, 324, 657−658. (2) Haynes, B. S. The Oxidation of Hydrogen Cyanide in Fuel-Rich Flames. Combust. Flame 1977, 28, 113−121. (3) Miller, J. A.; Bowman, C. T. Kinetic Modeling of the Reduction of Nitric Oxide in Combustion Products by Isocyanic Acid. Int. J. Chem. Kinet. 1991, 23, 289−313. (4) Dixon, R. N.; Kirby, G. H. Ultra-violet Absorption Spectrum of Isocyanic Acid. Trans. Faraday Soc. 1968, 64, 2002−2012. (5) Rabalais, J. W.; McDonald, J. R.; McGlynn, S. P. Electronic States of HNCO, Cyanate Salts, and Organic Isocyanates. II. Absorption Studies. J. Chem. Phys. 1969, 51, 5103−5111. (6) McGlynn, S. P.; Rabalais, J. W.; McDonald, J. M.; Scherr, V. M. Electronic Spectoscopy of Isoelectronic Molecules. II. Linear Triatomic Groupings Containing Sixteen Valence Electrons. Chem. Rev. 1971, 71, 73−108. (7) Nakamura, H.; Truhlar, D. G. Extension of the Fourfold Way for Calculation of Global Diabatic Potential Energy Surfaces of Complex, Multi arrangement, non-Born-Oppenheimer Application to Systems: HNCO(S0, S1). J. Chem. Phys. 2003, 118, 6816−6829. (8) Zyrianov, M.; DrozGeorget, T.; Sanov, A.; Reisler, H. Competitive Photodissociation Channels in Jet Cooled HNCO: Thermochemistry and Near Threshold Predissociation. J. Chem. Phys. 1996, 105, 8111−8116. (9) Brown, S. S.; Cheatum, C. M.; Fitzwater, D. A.; Crim, F. F. A Simple Model of the HNCO (1A′) Excited State Potential Energy Surface and a Classical Trajectory Analysis of the Vibrationally

13569



Article

Directed Bondselected Photodissociation. J. Chem. Phys. 1996, 105, 10911−10918. (10) Klossika, J. J.; Flothmann, H.; Schinke, R.; Bittererova, M. On the S1→S0 Internal Conversion in the Photodissociation of HNCO. Chem. Phys. Lett. 1999, 314, 182−188. (11) Klossika, J. J.; Flothmann, H.; Beck, C.; Schinke, R.; Yamashita, K. The Topography of the HNCO(S1) Potential Energy Surface and Its Implications for Photodissociation Dynamics. Chem. Phys. Lett. 1997, 276, 325−333. (12) Stevens, J. E.; Cui, Q.; Morokuma, K. J. An Ab Initio Study of the Dissociation of HNCO in the S1 Electronic State. Chem. Phys. 1998, 108, 1452−1458. (13) Zyrianov, M.; Droz-Georget, T.; Reisler, H. Fragment Recoil Anisotropies in the Photoinitiated Decomposition of HNCO. J. Chem. Phys. 1999, 110, 2059−2068. (14) Conroy, D.; Aristov, V.; Feng, L.; Sanov, A.; Reisler, H. Competitive Pathways via Nonadiabatic Transitions in Photodissociation. Acc. Chem. Res. 2001, 34, 625−632. (15) Sanov, A.; Droz-Georget, Th.; Zyrianov, M.; Reisler, H. Photofragment Imaging of HNCO Decomposition: Angular Anisotropy and Correlated Distributions. J. Chem. Phys. 1997, 106, 7013− 7022. (16) DrozGeorget, T.; Zyrianov, M.; Reisler, H.; Chandler, D. W. Correlated Distributions in the Photodissociation of HNCO to NH(X1Σ−, a1Δ)+CO(X1Σ+) near the Barrier on S1. Chem. Phys. Lett. 1997, 276, 316−324. (17) DrozGeorget, T.; Zyrianov, M.; Sanov, A.; Reisler, H. Photodissocation of HNCO: Three Competing Pathways. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 469−477. (18) Drozdoski, W. S.; Baronavski, A. P.; McDonald, J. R. Photodissociation of HNCO at 193 nm: NH(a1Δ) Internal Energy Distribution and Reaction Rate with O2. Chem. Phys. Lett. 1979, 64, 421−425. (19) Spiglanin, T. A.; Perry, R. A.; Chandler, D. W. Photodissociation Studies of HNCO: Heat of Formation and Product Branching Ratios. J. Phys. Chem. 1986, 90, 6184−6189. (20) Woods, E.; Berghout, H. L.; Cheatum, C. M.; Crim, F. F. Controlling the Bimolecular Reaction and Photodissociation of HNCO through Selective Excitation of Perturbed Vibrational States. J. Phys. Chem. A 2000, 104, 10356−10361. (21) Zhang, J. S.; Dulligan, M.; Wittig, C. HNCO + hν(193.3 nm) → H + NCO: Center-of-Mass Translational Energy Distribution, Reaction Dynamics, and Do(H−NCO). J. Phys. Chem. 1995, 99, 7446−7452. (22) Yi, W. K.; Bersohn, R. The Hydrogen Atom Channel in the Photodissociation of HNCO. Chem. Phys. Lett. 1993, 206, 365−368. (23) Brown, S. S.; Berghout, H. L.; Crim, F. F. Vibrational State Controlled Bond Cleavage in the Photodissociation of Isocyanic Acid (HNCO). J. Chem. Phys. 1995, 102, 8440−8447. (24) Brown, S. S.; Berghout, H. L.; Crim, F. F. Internal Energy Distribution of the NCO Fragment from Near-Threshold Photolysis of Isocyanic Acid, HNCO. J. Phys. Chem. 1996, 100, 7948−7955. (25) Brown, S. S.; Metz, R. B.; Berghout, H. L.; Crim, F. F. Vibrationally Mediated Photodissociation of Isocyanic Acid (HNCO): Preferential N−H Bond Fission by Excitation of the Reaction Coordinate. J. Chem. Phys. 1996, 105, 6293−6303. (26) Berghout, H. L.; Brown, S. S.; Delgado, R.; Crim, F. F. Nonadiabatic Effects in the Photodissociation of Vibrationally Excited HNCO: the Branching between Singlet (a1Δ) and Triplet (X3Σ−) NH. J. Chem. Phys. 1998, 109, 2257−2263. (27) Wang, H.; Liu, S.-L.; Liu, J.; Wang, F.-Y.; Jiang, B.; Yang, X.-M. Photofragment Imaging of HNCO Decomposition at 210 nm: the Primary NH(a1Δ)+CO(X1Σ+) Channel. Chin. J. Chem. Phys. 2007, 20, 388−394. (28) Klossika, J. J.; Schinke, R. The Photodissociation of HNCO in the S1 Band: A Five-Dimensional Classical Trajectory Study. J. Chem. Phys. 1999, 111, 5882−5896. (29) Yu, S. R.; Su, S.; Dorenkamp, Y.; Wodtke, A. M.; Dai, D. X.; Yuan, K. J.; Yang, X. M. Competition between Direct and Indirect

Dissociation Pathways in Ultraviolet Photodissociation of HNCO. J. Phys. Chem. A 2013, DOI: 10.1021/jp312793k. (30) Zyrianov, M.; DrozGeorget, T. H.; Reisler, H. Competition between Singlet and Triplet Channels in the Photoinitiated Decomposition of HNCO. J. Chem. Phys. 1997, 106, 7454−7457. (31) Kaledin, A. L.; Cui, Q.; Heaven, M. C.; Morokuma, K. Ab Initio Theoretical Studies on Photodissociation of HNCO upon S1(1A″)← S0(1A′) Excitation: The Role of Internal Conversion and Intersystem Crossing. J. Chem. Phys. 1999, 111, 5004−5016. (32) Milligan, D. E.; Jacox, M. E. Matrix-Isolation Study of the Infrared and Ultraviolet Spectra of the Free Radical NCO. J. Chem. Phys. 1967, 47, 5157−5168. (33) Okabe, H. Photodissociation of HNCO in the Vacuum Ultraviolet; Production of NCOA2Σ and NH(A3π, π1c). J. Chem. Phys. 1970, 53, 3507−3515. (34) Hikida, T.; Maruyma, Y.; Sato, Y.; Mori, Y. Formation of NH(c1Π) and NH(A3Π) in the Photolysis of NH3, HN3 and HNCO at 121.6 nm. Chem. Phys. 1988, 121, 63−71. (35) Uno, K.; Hikida, T.; Hiraya, A.; Shobatake, K. Formation of NH(c1Π), NH(A 3Π) and NCO(A2Σ) in the VUV Photolysis of HNCO. Chem. Phys. Lett. 1990, 166, 475−479. (36) Brownsword, R. A.; Laurent, T.; Hillenkamp, M.; Vatsa, R. K.; Volpp, H.-R. Dissociation Dynamics of HNCO and DNCO after Laser Photoexcitation in the Vacuum Ultraviolet. J. Chem. Phys. 1997, 106, 9563−9569. (37) Schneider, L.; Meier, W.; Welge, K. H.; Ashfold, M. N. R.; Western, C. Photodissociation Dynamics of H2S at 121.6 nm and A Determination of the Potential Energy Function of SH(A2Σ+). J. Chem. Phys. 1990, 92, 7027−7037. (38) Schnieder, L.; Seekamp-Rahn, K.; Wrede, E.; Welge, K. H. Experimental Determination of Quantum State Resolved Differential Cross Sections for the Hydrogen Exchange Reaction H+D2→HD+D. J. Chem. Phys. 1997, 107, 6175−6195. (39) Mordant, D. H.; Ashfold, M. N. R.; Dixon, R. N. Dissociation Dynamics of H2O (D2O) following Photoexcitation at the Lyman-α Wavelength (121.6 nm). J. Chem. Phys. 1994, 100, 7360−7375. (40) Yuan, K. J.; Cheng, Y.; Cheng, L. N.; Guo, Q.; Dai, D. X.; Yang, X. M. Quantum State-Selected Photodissociation Dynamics of H2O: Two-Photon Dissociation via the C Electronic State. J. Chem. Phys. 2010, 133, 134301. (41) Yuan, K. J.; Dixon, R. N.; Yang, X. M. Photochemistry of the Water Molecule: Adiabatic versus Nonadiabatic Dynamics. Acc. Chem. Res. 2011, 44, 369−378. (42) Cheng, Y.; Yuan, K. J.; Cheng, L. N.; Guo, Q.; Dai, D. X.; Yang, X. M. Photodissociation Dynamics of H2O: Effect of Unstable Resonances on the B(1A1) Electronic State. J. Chem. Phys. 2011, 134, 064301. (43) Zhang, Y. W.; Yuan, K. J.; Yu, S. R.; Yang, X. M. Highly Rotationally Excited CH3 from Methane Photodissociation through Conical Intersection Pathway. J. Phys. Chem. Lett. 2010, 1, 475−479. (44) Zhang, Y. W.; Yuan, K. J.; Yu, S. R.; Parker, D. H.; Yang, X. M. Photodissociation Dynamics of Acetylene via the C̃ 1Πu Electronic State. J. Chem. Phys. 2010, 133, 014307. (45) Yuan, K. J.; Cheng, L. N.; Cheng, Y.; Guo, Q.; Dai, D. X.; Yang, X. M. Tunable VUV Photochemistry Using Rydberg H-Atom Time-ofFlight Spectroscopy. Rev. Sci. Instrum. 2008, 79, 124101. (46) Yuan, K. J.; Cheng, Y.; Cheng, L. N.; Guo, Q.; Dai, D. X.; Yang, X. M.; Dixon, R. N. Nonadiabatic Dissociation Dynamics in H2O: Competition between Rotationally and Nonrotationally Mediated Pathways. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 19148−19153. (47) Fujimoto, G. T.; Umstead, M. E.; Lin, M. C. Dynamics of CO Formation in the Photodissociation of HNCO and CH2CO at 193 nm. Chem. Phys. 1982, 65, 197−203. (48) Yuan, K. J.; Cheng, Y.; Wang, F. Y.; Yang, X. M. Photodissociation Dynamics of HN3 and DN3 at 157nm. J. Phys. Chem. A 2008, 112, 5332−5337. (49) Cook, P. A.; Ashfold, M. N. R.; Jee, Y. J.; Jung, K. H.; Harich, S.; Yang, X. M. Vacuum Ultraviolet Photochemistry of Methane, Silane and Germane. Phys. Chem. Chem. Phys. 2001, 3, 1848−1860. 13570



Article

(50) Ashfold, M. N. R.; King, G. A.; Murdock, D.; Nix, M. G. D.; Oliver, T. A. A.; Sage, A. G. πσ* Excited States in Molecular Photochemistry. Phys. Chem. Chem. Phys. 2010, 12, 1218−1238.

13571


Vacuum Ultraviolet Photodissociation Dynamics of Isocyanic Acid: The

Recommend Documents