VUV Photodissociation Dynamics of Nitrous Oxide: The O(1SJ=0) and

More than 10 vibrational states were identified for the O(1S0) + N2(X1Σg+) channel. Five energy accessible vibrational states were identified for the...
1 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCA

VUV Photodissociation Dynamics of Nitrous Oxide: The O(1SJ=0) and O(3PJ=2,1,0) Product Channels Shengrui Yu,†,∥ Daofu Yuan,†,∥ Wentao Chen,† Xueming Yang,*,†,‡ and Xingan Wang*,†,§ †

Center for Advanced Chemical Physics and Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, Anhui Province, P. R. China ‡ State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, Liaoning Province, P. R. China § iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), University of Science and Techonology of China, Jinzhai Road 96, Hefei 230026, Anhui Province, P. R. China ABSTRACT: Vacuum ultraviolet photodissociation dynamics of nitrous oxide was investigated using the time-sliced velocity ion imaging technique. Images of the O(1SJ=0) and the O(3PJ=2,1,0) products were measured at nine photolysis wavelengths from 124.44 to 133.20 nm, respectively. Three main dissociation channels: O(1S0) + N2(X1Σg+), O(3PJ=2,1,0) + N2(A3Σu+), and O(3PJ=2,1,0) + N2(B3Πg) were observed and identified in the product images where vibrational states of N2 were fully resolved. Product total kinetic energy releases and angular distributions were acquired. In all product channels, the branching ratios of vibrational states of N2 products were determined. In addition, the O(3PJ=2,1,0) + N2(A3Σu+)/O(3PJ=2,1,0) + N2(B3Πg) branching ratios were determined. We found that in the O(3PJ=2,1,0) channels the O(3PJ=2,1,0) + N2(B3Πg) channel becomes dominant at long photolysis wavelength, indicating a strong coupling between the singlet D(1Σg+) state and the triplet 3Π state. For both O(1S0) and O(3PJ=2,1,0) products, the derived angular anisotropy parameters (β values) are very close to 2 at lower vibrational states of the correlated N2 electronic states and gradually decrease with the increasing vibrational quantum number. These behaviors suggest that the photodissociation processes are primarily governed by a fast dissociation in a linear geometry, while the N2 products at excited vibrational states are very likely produced via a more bent transition state.

1. INTRODUCTION Nitrous oxide (N2O) plays a very important role in atmospheric chemistry.1 Although N2O is only a trace gas, it efficiently affects the ozone depletion process in the earth’s atmosphere,2,3 and on a per molecule basis, N2O is a potent greenhouse gas that has an even higher global warming potential than CO2. The photodissociation process in the stratosphere is a main sink of N2O, and the radicals such as NO and O produced in N2O photodissociation are actively involved in various atmospheric chemical reactions. Therefore, photodissociation dynamics of N2O has attracted much attention over the past decades in both theoretical4,5 and experimental6−10 studies. In the VUV region, a very intense and smooth peak between 124 and 134 nm is displayed in the absorption spectrum of N2O.11,12 The central wavelength of this peak is ∼128 nm, corresponding to an excitation to the D(1Σg+) electronic state.13 By measuring the relative quantum yield of the product channels, Black et al.14 have shown that N2O photodissociation is mainly via the following channels to produce O+N2 N2O → O(1S0) + N2(X1Σg +) © XXXX American Chemical Society

N2O → O(3P J = 2,1,0) + N2(A3Σ u+)

(2)

N2O → O(3P J = 2,1,0) + N2(B3Πg)

(3)

Using a conventional flash lamp as the light source, Gilpin et al.15 and Stone et al.16 measured the time-of-flight spectra for photodissociation products in VUV region, respectively. Several metastable reaction products were identified in their experiments. Results from Stone et al. also provided some information about the vibrational distributions of the products; however, the limited resolution hindered further detailed analysis. In the development of laser-based detection techniques, in 2005, Witinski et al.17 studied O(3PJ=2,1,0) + N2(A3Σu+) and O(3PJ=2,1,0) + N2(B3Πg) product channels using pulsed lasers, in combination with the oxygen Rydberg time-of-flight method. A VUV laser (near 130 nm) was employed in both the photodissociation and product detection processes. They determined the overall branching ratio between N2(A3Σu+) Received: May 8, 2015 Revised: June 23, 2015

(1) A

DOI: 10.1021/acs.jpca.5b04438 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 1. Schematic view of the experimental apparatus.

and N2(B3Πg) formation for three J components. At similar photolysis wavelength, Lambert et al.18 investigated the photodissociation dynamics of N2O using the ion imaging detection technique. They obtained the branching ratios of five product channels, among which the O(1S0) + N2(X1Σg+) channel is found to be the dominant one. Previous studies have shed some light on the mechanism of the photodissociation process in the VUV region. A more quantitative picture of photodissociation of this molecule in the D(1Σg+) state requires high-resolution results in a wider photolysis wavelength range. Here we report an experimental study of the photodissociation dynamic of N2O. A tunable VUV light source, generated by the nonlinear optic process in rare gas, was employed in this experiment. The studies of photodissociation dynamics were carried out at various wavelengths. The resolution of the measured images was sufficiently high to resolve the vibrational structures in the O(1S0) and O(3PJ) product channels. This study provides a quantitative picture of the O + N2 product channels in the VUV photodissociation of N2O.

chambers is rotatable. As shown in Figure 1, the apparatus equips with three molecular beam sources: two beams are designed for crossed beam experiments and a third one in the rotatable source chamber is used for the current photodissociation experiment. The ion imaging detector is mounted in the detection chamber. In this study, the source chamber was pumped by a 2000 l/s turbo molecular pump. The detection chamber was pumped by a 2000 l/s turbo molecular pump and a 1600 l/s turbo molecular pump. The background pressures in the source chamber and the detection chamber were 9 × 10−9 and 1.5 × 10−8 mbar, respectively. The N2O molecular beam was generated in the source chamber by a supersonic expansion of the gas mixture (5% N2O in Ar for the O(1S0) channel and 30% N2O in Ar for the O(3PJ) channel) using a general valve (Parker Series 9) with a 1 mm orifice. The repetition rate of the pulsed valve was 20 Hz. The stagnation pressure behind the nozzle was 1 bar. The typical operating pressure in the source chamber was 1 × 10−5 mbar. The general valve was mounted 18 mm away from a skimmer with 1.5 mm diameter aperture, with its axis perpendicular to the plane of the imaging detector. The N2O beam passed through the skimmer and entered the detection chamber with a typical operating pressure of 5 × 10−8 mbar. 75 mm downstream from the nozzle, the collimated beam passed through a 2 mm hole in the first electrode plate and propagated

2. EXPERIMENT The photodissociation experiment was carried out using a new crossed molecular beam apparatus with a time-sliced ion imaging detector.19−21 The new apparatus consists of two source chambers and one detection chamber. One of the source B

DOI: 10.1021/acs.jpca.5b04438 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A further along the axis of the 650 mm long time-of-flight tube of the ion imaging detector. The VUV photodissociation laser in this study was generated using the nonlinear four-wave mixing method. A 212.55 nm (ω1) laser beam was generated by the doubled output of a tunable dye laser (Corbra-Stretch, Sirah), and a second tunable laser (ω2) was generated by the fundamental output of another tunable dye laser. The two dye lasers were pumped by an Nd:YAG laser. The two laser beams were spatially and temporally overlapped and focused collimated through a stainless-steel krypton gas cell with an MgF2 collimating lens. The generated VUV (2ω1−ω2) radiation and the residual incident laser light entered the detection chamber. The polarization of the resulting VUV laser is determined by the second laser (ω2). A third pulsed dye laser (labeled as ω3) pumped by a second Nd:YAG laser was employed for the detection, enabling the study of photodissociation dynamics at various wavelengths. The detection laser was generated in two different ways for the O(3PJ) and O(1S0) channels, respectively. For the O(3PJ) product channel, the detection was carried out by the ultraviolet (UV) resonance-enhanced multiphoton ionization (REMPI) method using 226.23, 226.06, and 225.65 nm laser for the three J components, respectively. The detection for the O(1S0) product channel was done by a (1 + 1′) VUV+UV ionization scheme. A second VUV radiation (2ω1−ω3, ω3 = 835.48 nm) was produced in the same gas cell as the photolysis VUV laser. During the experiment, the polarization of the photodissociation laser is parallel with the plane of the imaging detector, and the polarization of the detection laser is perpendicular to the plane of the imaging detector. The N2O molecular beam intersected with the photodissociation laser inside the 23-plate time-sliced ion optics.22 The atomic oxygen photofragments of the dissociation were then detected by the detection laser. The oxygen ions images were collected by a 70 mm microchannel plate (MCP) coupled to a phosphor screen (P43). The transient images on the phosphor screen were captured by a charge-coupled device (CCD) camera (Imager pro plus 2M, LaVision). Every image was evaluated by the event counting scheme. The timing of the pulsed valve, dissociation, and ionization lasers and the gate pulse applied to the MCP were controlled by two delay generators (DG 645 Stanford Research System). The speed of the product was calibrated using the O+ signal from the photodissociation of O2 at ∼225 nm.

Figure 2. Raw ion images of O(1SJ=0) and O(3PJ=2,1,0) products from the photodissociation of N2O at 125.55 nm, respectively. The rings in the images correspond to the vibrational state of the coincident N2 products.

were observed in the O(1S0) raw images. Each ring corresponds to a single vibrational state of the correlated N2(X1Σg+) products. For the O(3PJ) product channels, two main features, the inner rings and the outer rings, are clearly displayed on the raw images. Furthermore, vibrational states of the coincident N2 products were fully resolved as well. The inner rings with low kinetic energies correspond to the O(3PJ) + N2(B3Πg) channel, and the outer rings correspond to the O(3PJ) + N2(A3Σu+) channel. Product Total Kinetic Energy Releases. From the observed raw ion images, the speed distributions of the O(1S0) and O(3PJ) products were extracted. The product speed distributions were then converted to the product total kinetic energy release distributions (TKERs). The energy of the whole system in photodissociation can be described by the eq 4 TKER = Ehv − D0(N2−O) − Evibronic(N2) − E int(O) (4)

where Ehv is the energy of the photolysis laser, D0(N2−O) is the bond energy of N−O bond, which was determined by previous study,23 Evibronic (N2) is the internal electronic and vibrational energy of N2 products, and Eint(O) is the energy difference between the oxygen atom products and the ground state O(3P2). Because a separate detection laser was employed, the detection laser was independent of the photolysis laser, all values in eq 4 are independent of others. The TKERs of O(1S0) products in the center of mass frame at 125.55 nm are displayed in Figure 3a. It is obvious that vibrational states of the correlated N2(X1Σg+) products are very well resolved in the TKER. The v = 1 state is the most populated state at 125.55 nm. The very well separated vibrational peaks indicate that the rotational state distributions of the correlated N2(X1Σg+) products are very “cold”. The TKERs of the O(3PJ) products for all the oxygen spin−orbit J levels in the center of mass frame at 125.55 nm are also shown (Figure 3b−d). Two main manifolds are observed, corresponding to the O(3PJ) + N2(B3Πg) and the O(3PJ) + N2(A3Σu+) channels, respectively. Very weak structures shown in the shoulder of peaks in the TKERs are believed to be from the vibrationally excited state of

3. RESULTS AND DISCUSSION The O(1SJ=0) and O(3PJ=2,1,0) product ion images of N2O photodissociation were measured at nine photolysis wavelengths, namely, 124.44, 125.55, 126.51, 127.65, 129.17, 130.22, 131.19, 132.19, and 133.20 nm. At each wavelength, the images were taken by accumulating the O+ signals with detection laser wavelengths tuned to REMPI transitions to O(1SJ=0) and O(3PJ=2,1,0) states. All observed signals were checked to be dependent on the photolysis laser, the detection laser, and the molecular beam. The background was taken with the probe lasers and molecular beam on but without the photolysis laser. Figure 2 displays the raw images of O(1S0), O(3P2), O(3P1), and O(3P0) products obtained at 125.55 nm. The vertical arrow shows the polarization direction of the photolysis laser. The polarization direction of the probe laser was perpendicular to the photolysis laser polarization and images’ plane. No significant effects were observed on the images when the polarization of the probe laser was rotated. A series of rings C

DOI: 10.1021/acs.jpca.5b04438 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

where θ is the angle between the polarization direction of the photolysis laser and the velocity vector of the recoil product, P2(cos θ) is the second Legendre polynomial, and β is the anisotropy parameter that could be used to characterize the photodissociation process. Because the N2 vibrational states are well resolved in the raw ion images, the anisotropy parameter for each vibrational state could be derived by fitting the angular distributions in these images with eq 5. The β values at various photolysis wavelengths are plotted as a function of the vibrational quantum number and displayed in Figures 4 and

Figure 4. Anisotropy parameters for individual vibrational states in O(1S0) + N2(X1Σg+) channel at nine photolysis wavelengths.

5 for the O(1S0) and O(3PJ) channels, respectively. We found that the O(1S0) and O(3PJ) channels share certain common features: the β values for all ground vibrational states (v = 0) in N2(X1Σg+), N2(A3Σu+), and N2(B3Πg) are very close to 2, and as the vibrational quantum number increases, the β value decreases and eventually reaches a lowest valve of 0.46 for the O(3P0) + N2(A3Σu+) channel, 0.05 for the O(3P0) + N2(B3Πg) channel, and 0.88 for the O(1S0) + N2(X1Σg+) channel. This behavior indicates that the photodissociation process is a parallel photodissociation process from a linear configuration at lower vibrational states and becomes less anisotropic for the N2 product in vibrationally excited states. Several differences are also found. In the O(3P0) + N2(A3Σu+) and O(3P0) + N2(B3Πg) product channels, the β values are very sensitive to the photolysis wavelength, while in the O(1S0) + N2(X1Σg+) channel, the β value for individual vibrational state is quite similar at different photolysis wavelengths. Product Branching Ratios. Using the multipeak fitting for the TKERs previously mentioned, we obtained the branching ratios of different vibrational states in N2(X1Σg+), N2(B3Πg), and N2(A3Σu+) (Figures 6 and 7). More than 10 vibrational states were identified for the O(1S0) + N2(X1Σg+) channel. Five energy accessible vibrational states were identified for the O(3PJ) + N2(A3Σu+) channels. The number of observed vibrational states for the O(3PJ) + N2(B3Πg) varies with the photolysis wavelengths. The population of vibrational states in the O(1S0) + N2(X1Σg+) channel is inverted: the most populated state changes from v = 1 to 2 as the photolysis laser wavelength increases. In the O(3PJ) + N2(A3Σu+) and O(3PJ) + N2(B3Πg) channels, v = 0 are almost always the major

Figure 3. Total kinetic energy release (TKER) distribution for O(1S0) + N2(X1Σg+) and O(3PJ) + N2(A3Σu+, B3Πg) channels from the photodissociation of N2O at 125.55 nm. Results for all J levels of the oxygen atom products are shown. In each plot, peaks are assigned to the vibrational states of the coincident N2 product.

N2O. Almost no signal from the N2O vibrational states was observed in the O(1S0) channel because a lower concentration N2O mixture was used. This resulted in a better cooling of the molecular beam compared with the beam employed in the O(3PJ) channels. In Figure 3, the red solid line in each graph is the result of a multipeak fitting of the total kinetic energy. The branching ratios of vibrational states in each product channel were determined through the fitting. The N2(B3Πg)/N2(A3Σu+) electronic states ratios were also evaluated. As shown in Figure 3b−d, significant changes of the relative population of the N2(B3Πg) and N2(A3Σu+) channels are noticed between different J (J = 2,1,0) levels at 125.55 nm. This behavior is different from the case in previous studies near 130 nm, where the branching ratio between the two electronic states is very close to 1, suggesting that the dissociation dynamics of N2O varies considerably across the electronic absorption band. Product Angular Distributions. The product angular distribution for a dissociation process could be described by the following equation I(θ ) = (1/4π )(1 + β P2(cos θ ))

(5) D

DOI: 10.1021/acs.jpca.5b04438 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 5. Anisotropy parameters for individual vibrational states in O(3PJ) + N2(A3Σu+, B3Πg) channels at nine different photolysis wavelengths between 124 and 133 nm.

increases. As for the O(3PJ=2) product, the O(3PJ=2) + N2(B3Πg) channel is always predominant except for the 128− 130 nm range, at which the branching ratio of the O(3PJ=2) + N2(B3Πg) channel is only a little more than that of the O(3PJ=2) + N2(A3Σu+) channel. Because of the different detection efficiencies, branching ratios of the three J levels in the O(3PJ) channels and the O(1S0)/O(3PJ) ratios were not derived in this experiment. Photodissociation Dynamics. Detailed photodissociation dynamics clearly relies on the knowledge of the fine structures on the potential energy surface. Theoreticians have expended much effort to investigate the photodissociation of N2O in UV region. For example, Schinke et al. performed a series of studies on various topics of the N2O photodissociation in UV region, including the potential energy surface and absorption spectrum,24,25 contribution of triplet states,26 and isotope dependence.27 In 1980s, Hopper performed a study on the potential energy surfaces of N2O in a wide energy range of 0− 13 eV. The ground electronic state of N2O has a linear NNO structure and was labeled as X1Σg+ in the C∞v symmetry and 11A′ in the Cs symmetry. Theoretical calculation in ref 4 performed by Hopper suggests that the O(1S0) + N2(X1Σg+) channel is correlated with the D(1Σg+) state of N2O. While the D(1Σg+) ← X(1Σg+) transition is optically allowed. The upper D(1Σg+) state can be effectively populated by the VUV light source. The observed high β value (2.0) for the N2(X1Σg+) product in its lower vibrational state suggests that these products (v = 0, 1, 2) are formed via a parallel dissociation in a linear geometry. The N2(X1Σg+) product in higher excited vibrational states has smaller β values. This may indicate that the highly excited vibrational states are formed via a more bent

Figure 6. Product branching ratios of vibrational states in the N2(X1Σg+) electronic states for O(1S0) + N2(X1Σg+) channel at nine photolysis wavelengths. The sum of the vibrational branching ratios at each photolysis wavelength is set to be unity.

dissociation channel, whereas at 124.44 nm, v = 1 state becomes more important in the O(3PJ) + N2(A3Σu+) channel. Meanwhile, the total branching ratios between the O(3PJ) + N2(B3Πg) and O(3PJ) + N2(A3Σu+) channels were determined at different photolysis wavelengths and displayed in Figure 8. For O(3PJ=0,1) products, it is found that the branching ratios N2(B3Πg): N2(A3Σu+) are about 1:1 at 124.44 nm; as the photolysis laser wavelength increases, the contribution from O(3PJ=0,1) + N2(B3Πg) channel goes down first and then E

DOI: 10.1021/acs.jpca.5b04438 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

Figure 7. Product branching ratios of vibrational states in the N2 (A3Σu+) and N2(B3Πg) electronic states for O(3PJ) + N2 channels at nine photolysis wavelengths. The sum of the vibrational branching ratios at each photolysis wavelength is set to be unity.

The O(3PJ) + N2(A3Σu+) channel can be formed via several possible electronic states: (1) the 11A′ state (the D state); (2) the 3Π state in a linear configuration; and (3) the upper 3A″ state (Renner−Teller component of the 3Π state in a bent configuration). The high β value (∼1.9) observed in the lower vibrational states of N2 products is clearly consistent with a dissociation in the linear configuration. The higher vibrational states may be formed through a more bent configuration. It might be qualitatively explained that the potential energy in bent configuration (C s ) is lower than that in linear configuration (C∞v); therefore, the available energy in the Cs symmetry is greater and might be likely to form products with vibrational excitations. We also notice that the branching ratio of vibrational states of O(3PJ) + N2(A3Σu+) channel varies with the photolysis wavelengths. At 124.44 nm, the population of the v = 1 state reaches the maximum; this may also indicate that there are different dissociation pathways leading to different vibrational distributions. As reported by Schinke in ref 25, the energy difference between the O(3PJ) + N2(B3Πg) channel and the O(1S0) + N2(X1Σg+) channel is very small in a slightly bent configuration. Therefore, strong coupling between the D(1Σg+) state and the 3 Π state (correlated with the O(3PJ) + N2(B3Πg) channel) is expected. Because of the slightly bent configuration, its β value for this dissociation channel is slightly lower than the other two channels. The O(3PJ) + N2(B3Πg) channel is more preferred at longer photolysis wavelength. This might be due to the fact that the Franck−Condon region at longer photolysis wavelengths is closer to the coupling region such that the coupling is stronger, leading to more O(3PJ) + N2(B3Πg) products. To this point, the details about the coupling as well as the observed

Figure 8. Products branching ratios of N2(B3Πg) and N2(A3Σu+) electronic states for O(3PJ) + N2 channels at nine photolysis wavelength. The sum of the electronic branching ratios at each photolysis wavelength is set to be unity.

transition state. The average β value for all vibration states is ∼1.5. This is consistent with the result by Lambert et al. F

DOI: 10.1021/acs.jpca.5b04438 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A differences between the spin-orbit J level in the O(3PJ) channels have not been fully understood yet. New theoretical studies with higher accuracy in the VUV range are desirable to account for the detailed experimental observations presented in this work.

(6) Nishide, T.; Suzuki, T. Photodissociation of Nitrous Oxide Revisited by High-Resolution Photofragment Imaging: Energy Partitioning. J. Phys. Chem. A 2004, 108, 7863−7870. (7) Harding, D. J.; Neugebohren, J.; Grutter, M.; Schmidt-May, A. F.; Auerbach, D. J.; Kitsopoulos, T. N.; Wodtke, A. M. Single-field SliceImaging with a Movable Repeller: Photodissociation of N2O from a Hot Nozzle. J. Chem. Phys. 2014, 141, 054201. (8) Hanisco, T. F.; Kummel, A. C. State-resolved Photodissociation of Nitrous Oxide. J. Phys. Chem. 1993, 97, 7242−7246. (9) Johnson, M. S.; Billing, B. D.; Gruodis, A.; Janssen, M. H. M. Photolysis of Nitrous Oxide Isotopomers Studied by Time-Dependent Hermite Propagation. J. Phys. Chem. A 2001, 105, 8672−8680. (10) Neyer, D. W.; Heck, A. J. R.; Chandler, D. W. Photodissociation of N2O: J-Dependent Anisotropy Revealed in N2 Photofragment Images. J. Chem. Phys. 1999, 110, 3411−3417. (11) Zelikoff, M.; Watanabe, K.; Inn, E. C. Y. Absorption Coefficients of Gases in the Vacuum Ultraviolet. Part II. Nitrous Oxide. J. Chem. Phys. 1953, 21, 1643−1647. (12) Lee, L. C.; Suto, M. Production and Quenching of Excited Photofragments of N2O. J. Chem. Phys. 1984, 80, 4718−4726. (13) Nee, J. B.; Yang, J. C.; Lee, P. C.; Wang, X. Y.; Kuo, C. T. Detection of O(1S) Produced in the Photodissociation of N2O. J. Phys. B 1998, 31, 5175. (14) Black, G.; Sharpless, R. L.; Slanger, T. G.; Lorents, D. C. Quantum Yields for the Production of O(1S), N(2D), and N2(A3Σ+u) from the Vacuum UV Photolysis of N2O. J. Chem. Phys. 1975, 62, 4266−4273. (15) Gilpin, R.; Welge, K. H. Time-of-Flight Spectroscopy of Metastable Photodissociation Fragments. N2O Dissociation in the Vacuum UV. J. Chem. Phys. 1971, 55, 975−978. (16) Stone, E. J.; Lawrence, G. M.; Fairchild, C. E. Kinetic Energies and Angular Distributions of Oxygen Atom Photofragments produced by Photodissociation of O2 and N2O in the Vacuum Ultraviolet. J. Chem. Phys. 1976, 65, 5083−5092. (17) Witinski, M. F.; Ortiz-Suárez, M.; Davis, H. F. Photodissociation Dynamics of N2O at 130 nm: The N2(A3Σu+, B3Πg) + O(3PJ=2,1,0) Channels. J. Chem. Phys. 2005, 122, 174303. (18) Lambert, H. M.; Davis, E. W.; Tokel, O.; Dixit, A. A.; Houston, P. L. Photodissociation Channels for N2O near 130 nm Studied by Product Imaging. J. Chem. Phys. 2005, 122, 174304. (19) Chandler, D. W.; Houston, P. L. Velocity and Internal State Distributions by Two-Dimensional Imaging of Products Detected by Multiphoton Ionization. J. Chem. Phys. 1987, 87, 1445−1447. (20) Eppink, A. T. J. B.; Park, D. H. Velocity Map Imaging of Ions and Electrons using Electrostatic Lenses: Application in Photoelectron and Photofragment Ion Imaging of Molecular Oxygen. Rev. Sci. Instrum. 1997, 68, 3477−3484. (21) Lin, J. J.; Shiu, W.; Liu, K. Application of Time-Resolved Ion Velocity Imaging to Crossed Molecular Beam Experiments. Rev. Sci. Instrum. 2003, 74, 2495−2500. (22) Lin, J. J.; Zhou, J. G.; Shiu, W.; Liu, K. Application of TimeSliced Ion Velocity Imaging to Crossed Molecular Beam Experiments. Rev. Sci. Instrum. 2003, 74, 2495. (23) Dibeler, V. H. N2O Bond Dissociation Energy by Photon Impact. J. Chem. Phys. 1967, 47, 2191. (24) Schinke, R.; Suarez, J.; Farantos, S. C. Communication: Photodissociation of N2O-Frustrated NN Bond Breaking Causes Diffuse Vibrational Structures. J. Chem. Phys. 2010, 133, 091103. (25) Schinke, R. Photodissociation of N2O: Potential Energy Surfaces and Absorption Spectrum. J. Chem. Phys. 2011, 134, 064313. (26) Schinke, R.; Schmidt, J. A.; Johnson, M. S. Photodissociation of N2O: Triplet States and Triplet Channel. J. Chem. Phys. 2011, 135, 194303. (27) Schmidt, J. A.; Johnson, M. S.; Schinke, R. Isotope Effects in N2O Photolysis from First Principles. Atmos. Chem. Phys. 2011, 11, 8965.

4. CONCLUSIONS Photodissociation dynamics of N2O have been studied by the time sliced velocity ion imaging technique with the tunable vacuum ultraviolet laser. The total kinetic release distributions and the angular distributions of the O(1S0) and O(3PJ=2,1,0) product channels were acquired at nine photodissociation wavelengths from 124 to 134 nm. Photolysis wavelengthdependent product anisotropy parameters as well as product branching ratios have been determined. With the vibrational state resolved images, it is clear that the anisotropy parameters change quite dramatically with the vibrational quantum number. The analysis for the O(3PJ=2,1,0) + N2(B3Πg) channel reveals that there could be a strong coupling between the singlet D(1Σg+) state and a triplet 3Π state. For the O(1S0) + N2(X1Σg+) and O(3PJ) + N2(A3Σu+) channels, the results show that the photodissociation processes are primarily governed by a parallel dissociation in a linear geometry, while the N2 products in excited vibrational states are very likely formed via a more bent transition state. The results shown here provide a clear picture of vibrational state specific photodissociation dynamics for the N2O molecule in the VUV region.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 008641184695174 (X.Y.). *E-mail: [email protected]. Phone: 008655163600726 (X.W.). Author Contributions ∥

S.Y. and D.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (nos. 21403207 and 21473173), China Postdoctoral Science Foundation (grant no. 2014M551810), and University of Science and Technology of China. We thank Xiaoguo Zhou, Qing Guo, Fuyi Liu, and Rongjun Chen for their help in optimizing the molecular beam apparatus.

■

REFERENCES

(1) Wayne, R. P. Chemistry of Atmospheres, 2nd ed.; Oxford University Press: New York, 1991. (2) Crutzen, P. J. The Influence of Nitrogen Oxides on the Atmospheric Ozone Content. Q. J. R. Meteorol. Soc. 1970, 96, 320− 327. (3) Crutzen, P. J. Ozone Production Rates in an Oxygen-HydrogenNitrogen Oxide Atmosphere. J. Geophys. Res. 1971, 76, 7311−7327. (4) Hopper, D. G. Ab Initio Multiple Root Optimization MCSCF Study of the C∞v/Cs Excitation Spectra and Potential Energy Surfaces of N2O. J. Chem. Phys. 1984, 80, 4290−4316. (5) Chutjian, A.; Segal, G. A. Theoretical Interpretation of the Optical and Electron Scattering Spectra of Polyatomic Molecules. III. N2O and the Discovery of Resonant Phenomena in the B Region at 6.8 eV. J. Chem. Phys. 1972, 57, 3069. G

DOI: 10.1021/acs.jpca.5b04438 J. Phys. Chem. A XXXX, XXX, XXX−XXX