Imaging the Pair-Correlated HNCO Photodissociation: The NH(a1Δ) +

Article pubs.acs.org/JPCA

Imaging the Pair-Correlated HNCO Photodissociation: The NH(a1Δ) + CO(X1Σ+) Channel Zhiguo Zhang,†,‡ Zhichao Chen,‡ Cunshun Huang,‡ Yang Chen,*,† Dongxu Dai,‡ David H. Parker,*,§ and Xueming Yang*,†,‡ †

Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China § Department of Molecular and Laser Physics, University of Nijmegen, 6525 ED Nijmegen, The Netherlands ‡

ABSTRACT: The NH(a1Δ) + CO(X1Σ+) product channel for the photodissociation of HNCO at 201 nm was investigated using the sliced velocity map ion imaging technique with the detection of NH(a1Δ) products via (2 + 1) resonance enhanced multiphoton ionization (REMPI). Images were measured for the NH(a1Δ) rotational states in the ground and vibrational excited states (v = 0 and 1). Correlation between the NH(a1Δ) and CO rovibrational state distributions were determined from these images. Experimental results show that the vibrational distribution of the CO fragment in the NH(a1Δ) + CO(X1Σ+) channel peaks at v = 1. The negative anisotropy parameter measured for the NH(a1Δ) (v = 0 and 1|j) products indicates a direct dissociation process for the N−C bond cleavage in the S1 state. A bimodal CO rotational distribution was observed, suggesting that HNCO dissociates in the S1 state in two distinctive pathways.

lifetime of the intermediate state exceeds 5 ps. Zyrianov et al.14 and Kaledin et al.15 suggested that dissociation in this channel follows IC from S1 to S0 and then ISC from S0 to T1. The exact quantum yield of this channel is unknown, but it should still be the primary channel in the region just above the opening of channel (2).16,17 Channel (2) has been widely investigated in the recent years. Spiglanin et al.18 probed NCO via the LIF technique. Zhang et al.19 using the high-Rydberg H atom time-of-flight (HRTOF) method found that the average translation energy was about 70% of the total available energy. The NCO product was observed with substantial bending excitation, and anisotropy parameter of β = −0.85 was reported. The HNCO channel (2) dynamics was characterized as a direct dissociation on a repulsive surface. Analyzing the electronic absorption spectrum, Crim and co-workers20−24 proposed a predissociation mechanism with nonstatistical behavior in the NCO vibrational distribution near the channel (2) threshold. Zyrianov et al.14 investigated channel (2) in the photolysis wavelength range of 217−244 nm (46083−40984 cm−1) and found that NCO was rotationally cold with an isotropic angular distribution. They suggested a mechanism involving IC followed by predissociation in the S1 state. A lower limit of 8140 cm−1 for the barrier to direct dissociation on S1 state has also been reported.

I. INTRODUCTION Isocyanic acid (HNCO) photodissociation has attracted much attention because of its important role in combustion1,2 and interstellar space. In addition to these practical aspects, HNCO serves as a benchmark system for understanding multiple dissociation pathways in a four-atom molecule, such as internal conversion (IC), intersystem crossing (ISC), and direct dissociation processes. The first UV absorption band of HNCO (180−280 nm) has been analyzed and assigned by Dixon and Kirby3 and by Rabalais et al.4,5 as an S1(1A″) ← S0(1A′) transition. Photodissociation dynamics of HNCO in the S1 state has been studied in great detail using various experimental techniques and theoretical methods.6−25 At least three electronic states (S0, S1, and T1) participate in the dissociation processes,6−13 which leads to three dissociation channels: HNCO(S1) → NH(X3Σ−) + CO(X1Σ+) D0 = 30060 ± 30 cm−1

(1)

D0 = 38370 ± 30 cm−1

H(2S) + NCO(X2Π) NH(a1Δ) + CO(X1Σ+)

D0 = 42750 ± 30 cm−1

(2) (3)

NH(a Δ) is denoted by NH in the following paragraphs. Channel (1), energetically the lowest one, is a spin-forbidden dissociation pathway. Zyrianov et al.14 detected CO around 230 nm (∼43500 cm−1) using the ion imaging technique. They found that the angular distribution of CO originating from channel (1) is essentially isotropic, which indicates that the 1

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© 2014 American Chemical Society

Received: January 19, 2014 Revised: March 7, 2014 Published: March 10, 2014 2413

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Recently, Yu et al.25 reinvestigated channel (2) in the photolysis wavelength range 200−240 nm using the high resolution HRTOF technique. Two competitive dissociation pathways have been observed: one is the indirect dissociation pathway through S1/S0 coupling, and the other is the direct dissociation pathway on the S1 surface. Channel (3) is a spin-allowed dissociation pathway and has also been investigated over the recent years. Reisler and coworkers12 performed a series of experiments to investigate channel (3) at the photolysis wavelength range 217−230 nm. Their results indicated that HNCO dissociation through channel (3) evolves initially on S0 via IC from S1 to S0, but after exceeding a small barrier on S1, estimated at 400−600 cm−1, direct dissociation on this surface quickly dominates. At 217.6 nm, energy disposal indicates that dissociation is dominated by decomposition on S1 with some contribution from S0. Wang et al.26 investigated channel (3) at 210 nm using the sliced velocity map ion imaging technique for CO fragment detection. Fujimoto et al.27 detected CO(v) at 193.3 nm and levels up to v = 4 were observed. Chandler and co-workers28,29 studied the internal state distributions of CO and rotational state distributions of 1NH between 230 and 193 nm. They found a cold 1NH rotational distribution together with a hot CO distribution. Vibrationally excited NH(a1Δ) photodissociation products were reported by Bohn et al.30 and Drozdoski et al.31 using LIF detection. At 193 nm, NH(a1Δ, v = 1) and NH(a1Δ, v = 2) were found to be in the ratio of 0.26 and 0.031 relative to NH(a1Δ, v = 0), respectively. Theoretically, the electronically excited states of HNCO have been calculated in various levels of details.10,15,32 Schinke et al.33 have calculated a large portion of the S1 surface in five dimensions. The S1 electronic state correlates with channels (2) and (3). For energies below the threshold of channel (3), dissociation should be in the ground electronic state S0, following IC from S1 to S0. There is a low barrier ∼470 cm−1 for the 1NH + CO dissociation channel and direct dissociation on the S1 potential energy surface (PES) happens immediately as the excitation energy goes above the barrier. However,because of a high barrier of 8710 or 11200 cm−1 in channel (2), direct dissociation on the S1 PES to yield channel (2) products is not possible except for sufficiently high excitation energy. At much higher excitation energies, when the energy exceeds the high barrier in the H + NCO channel, both channels (2) and (3) can be reached by direct dissociation on the S1 PES. In this work, we investigate channel (3) of HNCO photodissociation dynamics at 201 nm using sliced velocity map ion imaging the 1NH fragments. The CO internal state populations, the angular distributions, and the pair-correlation between 1NH and CO products are measured. A bimodal rotational distribution is observed in the CO product. The new results should be helpful in revealing the photodissociation mechanism of HNCO dynamics in the S1 state.

mm downstream from the nozzle, the collimated beam passes through a 2 mm hole in the first electrode plate and propagated further along the axis of the 500 mm long time-of-flight tube of the ion imaging detector. The HNCO molecular beam was intercepted by a 201 nm laser beam, which was generated by the tripled output of a tunable dye laser (Cobra-Stretch, Sirah) pumped by the second harmonic of a Continuum Nd:YAG laser (Continuum PL8000) with 10 Hz repetition rate. In order to avoid multiphoton complications, only about 0.5 mJ/pulse of the 201 nm light was focused onto the HNCO beam with a lens of 300 mm focal length. The resulting 1NH fragment was photoionized about 20 ns later by a probe laser beam produced by doubling the output of a tunable dye laser (Radiant Narrowscan), which was pumped by the third harmonic of a second Continuum Nd:YAG laser (Surelite III), and about 0.8 mJ of the probe laser was focused into the HNCO photolysis region using a 200 mm focal length lens. The 1NH fragments were ionized by the g1Δ (3pπ) ←← a1Δ (2 + 1) REMPI scheme ∼265 nm for 1NH(v = 0|j) and ∼267 nm for 1NH(v = 1|j).38 The two laser beams were spatially overlapped at the HNCO beam at the middle position between the third and the fourth electrode,35 80 mm downstream from the nozzle. The electric field polarization direction of the pump laser was set to be parallel to the detector face, while that of the probe laser was perpendicular to the detector face. The images were collected by an imaging detector with dual 40 mm diameter Chevron multichannel plates (MCP) coupled to a phosphor screen (P47). The transient images on the phosphor screen were captured by a charge-coupled device (CCD) camera (ImagerPro2M 640 × 480 pixels, LaVision) and transferred to a computer on every shot for event counting39 and data analysis. In addition, the fluorescence from the phosphor screen was monitored with a photomultiplier tube to optimize the experimental conditions. Timing of the pulsed valve, dissociation and ionization lasers, and the gate pulse applied to the MCP detector were controlled by a multichannel digital delay pulse generator (BNC Model 555).

III. RESULTS AND DISCUSSION Figure 1 displays the 1NH(v = 0 and 1|j) raw images of HNCO photodissociation at 201 nm, obtained by accumulating the REMPI 1NH+ signals over 40000 laser shots with background subtraction. The background was taken using the pump laser only with the molecular beam on. The dissociation energy of the NH(a1Δ) + CO(X1Σ+) channel is 42750 cm−1,14 ∼234 nm. There is not enough energy for the probe laser, ∼265 nm, to induce NH(a1Δ) fragment. All observed signals are pump− probe and molecular beam dependent. Well-resolved anisotropic rings were measured, and these structures are assigned to the vibrational states of the partner CO product in the 1NH + CO binary dissociation process, channel (3). All the resolved rovibrational 1NH states were detected, a few selected states are shown in Figure 1. To our best knowledge, these results provide the fully correlated information of HNCO photodissociation around 200 nm. As can be seen directly in the left-side images of Figure 1A− D, the relative intensities of the structures in each 1NH(v = 0|j) are changing with the 1NH rotational quantum number. With j increasing, some inner rings disappear and the strongest rings of each image become sharper, indicating clear pair-correlation between the 1NH(v = 0|j) and CO(v) products. For the rightside 1NH(v = 1|j) images of Figure 1E−H, a similar trend was

II. EXPERIMENTAL SECTION HNCO photodissociation at 201 nm was studied using a sliced velocity map ion imaging apparatus,34,35 which has been described previously.36,37 Briefly, a pulsed valve (General valve, Series 9) with a 0.5 mm nozzle produces a ∼100 μs pulsed molecular beam (2% HNCO seeded in He) at 1 bar stagnation pressure behind the nozzle. At a distance of 22 mm downstream from the nozzle, the expanded beam passes through a skimmer with a 1 mm diameter aperture. About 60 2414

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Figure 2. Product total kinetic energy distributions (black empty circles) derived from the speed distributions in Figure 1 for the 1NH(v| j) + CO(v) channels. The red lines are the fitting results and the dashed lines are the individual CO rotational components.

To extract the correlated vibrational distribution of the CO product, a qualitative fitting of the energy distribution was carried out. Each structure was fitted in total kinetic energy space using a Gaussian line shape. All the CO vibrational states correlated with 1NH(v = 0|j) are fitted by a bimodal distribution, which accounts for the obvious two peaks in the distribution. This bimodal feature is only seen for the CO(v = 0) product correlated with 1NH(v = 1|j). The CO(v = 1) component is fitted by a single Gaussian distribution. The simulated results are also shown in Figure 2. The nature of the bimodal distribution will be discussed later in this article. From the fitting, relative CO vibrational populations for each of the 1NH(v = 0|j) production states are obtained (Figure 3A). The CO vibrational excitation distributions peak at v = 1 and extend to v = 3. As the rotational excitation of the 1NH(v = 0|j) increases, the correlated CO vibrational state distribution shifts to lower vibrational state. This shows that the vibrational excitation of the correlated CO product is anticorrelated to the 1 NH(v = 0|j) rotational excitation. Angular distribution of the 1NH(v = 0|j) + CO(v) channel was obtained for all the 1NH(v = 0|j) rotational states by integrating the images over the relevant radius region. The corresponding product anisotropy parameters determined by fitting the angular distributions are shown in Figure 3B. As can be seen, anisotropy parameters of the 1NH(v = 0|j) + CO(v = 0) are near −0.7, and around −0.65 and −0.45 for v = 1 and v = 2, respectively. With j increasing, the angular distribution becomes slightly less anisotropic. The 1NH(v = 0|j) + CO(v = 3) anisotropy parameters become as low as −0.3; however, this

Figure 1. Raw images of 1NH (v = 0 and 1|j) products from photodissociation of HNCO at 201 nm. The double arrow indicates the polarization direction of the dissociation laser. The ring features correspond to the rovibrational states of the coincident CO(v) product.

also observed as that of 1NH(v = 0|j). In addition, resolved ring structures are observed within some of the broad rings. Total kinetic energy distributions in the center-of-mass frame for the different 1NH(v = 0 and 1|j) products were extracted from Figure 1. Results are shown for the individual rovibrational states of 1NH in Figure 2. The CO vibrational distributions are inverted and peak at v = 1 for most of the measured 1NH(v = 0|j) internal states. The vibrational excitation of CO(v) decreases as the NH1 rotational excitation increases. The CO(v = 0) rotational distributions shown in Figure 2 can be consistently described by the sum of two broad distributions, which peak at low-j (j ≈ 0−15) and high-j (j ≈ 20−30) values. The CO(v = 1) curve is also well-described by a similar low-j and high-j distribution; however, the data for CO(v = 1) is not as clear as CO(v = 0) because of the signal overlap of the CO vibrational states. 2415

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Figure 3. (A) Vibrational distribution of the CO coproducts for the NH1(v = 0|j) product; (B) anisotropy parameter for individual CO vibrational state correlated to NH1(v = 0|j); (C) vibrational distribution of the CO coproducts for the NH1(v = 1|j) product; (D) anisotropy parameter for individual CO vibrational state correlated to NH1(v = 1|j).

low value could also be due to the weak signal compared to the roughly isotropic background. Only v = 0 and v = 1 of CO can correspond with vibrational excited 1NH(v = 1|j). As the 1NH(v = 1|j) internal energy increases, the population of CO(v = 1) goes down to zero after j = 7 (Figure 3C). Angular distributions of the 1NH(v = 1|j) + CO(v = 0 and 1) signal were less anisotropic than those of 1NH (v = 0|j) (Figure 3D). At 201 nm excitation energy (∼49751 cm−1), the photon energy is above the barriers to all the three channels by several thousand wave numbers. There is a strong repulsive gradient along the C−N bond on the S1 surface. The CO bond length of the NH(a1Δ) + CO(X1Σ+) dissociation limit is 1.998 Å for the transition state of trans- and 1.988 Å for that of cis-state.11 Comparing with the CO ground state molecule (1.128 Å equilibrium40), the CO products should be vibrationally excited. As can be seen in Figure 3A, the CO(v = 1) relative population for all the detected 1NH(v = 0|j) is around 50%. Because of the available energy limit, the CO(v = 1) relative population for all the detected 1NH(v = 1|j) decreases linearly down to zero with increasing j (Figure 3C). The negative anisotropy parameters (Figure 3B,D) at low rotational excitation of 1NH suggest that the 1NH(v|j) + CO(v) dissociation channel is a fast dissociation process, which is in agreement with previous results by Reisler et al.14 With 1NH rotational excitation increasing, however, the anisotropy parameter β becomes considerably smaller. Such phenomenon has been seen previously in HNCO photodissociation at 210 nm26 and can be explained by a nonaxial recoil impact model.41 In the present study, the most interesting observation is the bimodal rotational distribution of CO(v = 0) that correlates to 1 NH(v|j). As can be seen in Figure 2, the CO(v = 0) low-j component and the CO(v = 0) high-j component both grow as 1 NH j increases in both the 1NH(v = 0|j) and 1NH(v = 1|j) channels. Figure 4 shows the measured ratio of the low-j and the high-j components that correlate with 1NH(v = 0 and 1). At low rotational excitation of 1NH, the corresponding CO(v = 0) high-j component is larger. However, the CO(v = 0) low-j component becomes more important as the rotational excitation of 1NH(v = 0|j) increases. When j equals 8, the

Figure 4. Ratio of CO(v = 0) low-j over CO(v = 0) high-j coproducts for different j states of the NH1(v|j) product.

CO low-j component seems to become larger than the CO high-j component even though the two components are not well separated. The CO(v = 0) high-j component correlated to the 1NH(v = 1|j) products, however, is always larger. Interestingly, the ratio between the low-j and the high-j components goes down slightly first and then increases as the rotational excitation of 1NH(v = 1) increases. Angular distributions of the CO(v = 0) high-j and low-j components were obtained for all the 1NH(v|j) rotational states by integrating the images over the corresponding center-ofmass velocity region. The anisotropy parameters are shown in Table 1. All the CO(v = 0) products show negative anisotropy distributions, and the values for the low-j and high-j components are slightly different. This suggests that both high-j and low-j components are from HNCO dissociation on the S1 surface. There are two lowest energy structures for HNCO on the S1 surface: trans- and cis-isomers.33 HNCO has nearly a linear geometry in its ground state. When HNCO is excited from the S0 state to the S1 state, the molecule retains the structure with a very high excited internal energy. It is possible that both trans- and cis-HNCO intermediates are formed in the excited S1 state, then dissociates from two isomers directly. The observed bimodal rotational distribution of CO(v = 0) might result from the two HNCO isomers. The bimodality of the CO 2416

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(5) Rabalais, J. W.; McDonald, J. M.; Scherr, V.; McGlynn, S. P. Electronic Spectoscopy of Isoelectronic Molecules. II. Linear Triatomic Groupings Containing Sixteen Valence Electrons. Chem. Rev. 1971, 71, 73−108. (6) 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 Systems: Application to HNCO(S0, S1). J. Chem. Phys. 2003, 118, 6816−6829. (7) 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. (8) 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 Directed Bond-Selected Photodissociation. J. Chem. Phys. 1996, 105, 10911−10918. (9) 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. (10) 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. (11) Stevens, J. E.; Cui, Q.; Morokuma, K. An Ab Initio Study of the Dissociation of HNCO in the S1 Electronic State. J. Chem. Phys. 1998, 108, 1452−1458. (12) Conroy, D.; Aristov, V.; Feng, L.; Sanov, A.; Reisler, H. Competitive Pathways via Nonadiabatic Transitions in Photodissociation. Acc. Chem. Res. 2001, 34, 625−632. (13) Yi, W. K.; Bersohn, R. The Hydrogen Atom Channel in the Photodissociation of HNCO. Chem. Phys. Lett. 1993, 206, 365−368. (14) Zyrianov, M.; Droz-Georget, T.; Reisler, H. Fragment Recoil Anisotropies in the Photoinitiated Decomposition of HNCO. J. Chem. Phys. 1999, 110, 2059−2068. (15) 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. (16) Droz-Georget, 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) Droz-Georget, T.; Zyrianov, M.; Sanov, A.; Reisler, H. Photodissocation of HNCO: Three Competing Pathways. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 469−477. (18) 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. (19) 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. (20) 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. (21) 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. (22) 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. (23) 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.

Table 1. Anisotropy Parameters of the CO(v = 0) High-j and Low-j Components β ∼CO(v = 0) NH (v = 0)

NH (v = 0)

j

low-j

high-j

low-j

high-j

2 3 4 5 6 7 8 9

−0.63 −0.67 −0.57 −0.51 −0.65 −0.59 −0.67 −0.65

−0.77 −0.79 −0.70 −0.73 −0.72 −0.69 −0.63 −0.59

−0.51 −0.43 −0.38 −0.33 −0.38 −0.41 −0.39

−0.60 −0.55 −0.54 −0.52 −0.50 −0.46 −0.38

rotational distributions might be some sort of rotational rainbow effect as well.42,43

IV. CONCLUSIONS The photodissociation dynamics of HNCO at 201 nm for the NH(a1Δ) + CO(X1Σ+) channel has been investigated via the sliced velocity map ion imaging technique. The 1NH(v = 0 and 1|j) fragments were probed. The energy distributions and fragmentation angular distributions are obtained from the 1NH and pair-correlated CO velocity imaging. The negative anisotropy parameter β measured indicates a direct dissociation process for HNCO at 201 nm. From the image of 1NH fragment, a bimodal rotational distribution of CO(v = 0) has been observed clearly. We speculated that it derived from two different dissociation pathways in the S1 state and gave a possible explanation as being related to the two different stable isomers (trans- and cis-HNCO). These detailed correlated product state distributions of CO(v) and 1NH(v|j) should provide a sensitive testing ground for theoretical studies on the dissociation dynamics of HNCO on the S1 surface.

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AUTHOR INFORMATION

Corresponding Authors

*(X.Y.) E-mail: [email protected]. *(D.H.P.) E-mail: [email protected]. *(Y.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 21203186 and No. 21073187), the National Key Basic Research Program of China (No. 2010CB923302), and the Chinese Academy of Sciences, and partially by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek, via the NWO−Dutch Astrochemistry Network.

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REFERENCES

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dx.doi.org/10.1021/jp500625m | J. Phys. Chem. A 2014, 118, 2413−2418