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Fei Xu, Yu-xin Tan, Dao-fu Yuan, Wen-tao Chen, Sheng-rui Yu, Ting Xie, Tao Wang, Xue-ming Yang, Xing-an Wang. Wavelength Dependent Photodissociation of OCS via $ F$ ${3^1\Pi}$ Rydberg State: CO(${X^1\Sigma^+}$)+S($^{1}$D$_{2}$) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 691-696. doi: 10.1063/1674-0068/cjcp2008147
Citation: Fei Xu, Yu-xin Tan, Dao-fu Yuan, Wen-tao Chen, Sheng-rui Yu, Ting Xie, Tao Wang, Xue-ming Yang, Xing-an Wang. Wavelength Dependent Photodissociation of OCS via $ F$ ${3^1\Pi}$ Rydberg State: CO(${X^1\Sigma^+}$)+S($^{1}$D$_{2}$) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 691-696. doi: 10.1063/1674-0068/cjcp2008147

Wavelength Dependent Photodissociation of OCS via $ F$ ${3^1\Pi}$ Rydberg State: CO(${X^1\Sigma^+}$)+S($^{1}$D$_{2}$) Product Channel

doi: 10.1063/1674-0068/cjcp2008147
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  • Corresponding author: Dao-fu Yuan, E-mail: ydfu@ustc.edu.cn; Xue-ming Yang, E-mail: xmyang@dicp.ac.cn; Xing-an Wang, E-mail:xawang@ustc.edu.cn
  • Authors contributed equally to this work.
  • Received Date: 2020-08-20
  • Accepted Date: 2020-09-28
  • Publish Date: 2020-12-27
  • The vacuum ultraviolet photodissociation of OCS via the $F$ $3^1\Pi$ Rydberg states was investigated in the range of 134$-$140 nm by means of the time-sliced velocity map ion imaging technique. The images of S($^1$D$_2$) products from the CO($X^1\Sigma^+$)+S($^1$D$_2$) dissociation channel were acquired at five photolysis wavelengths, corresponding to a series of symmetric stretching vibrational excitations in OCS($F$ $3^1\Pi$, $v_1$=0$-$4). The total translational energy distributions, vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) coproducts were derived. The analysis of experimental results suggests that the excited OCS molecules dissociate to CO($X^1\Sigma^+$) and S($^1$D$_2$) products via non-adiabatic couplings between the upper $F$ $3^1\Pi$ states and the lower-lying states both in the C$_{\infty \textrm{v}}$ and C$_{\rm{s}}$ symmetry. Furthermore, strong wavelength dependent behavior has been observed: the greatly distinct vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) products from the lower ($v_1$=0$-$2) and higher ($v_1$=3, 4) vibrational states of the excited OCS($F$ $3^1\Pi$, $v_1$) demonstrate that very different mechanisms are involved in the dissociation processes. This study provides evidence for the possible contribution of vibronic coupling and the crucial role of vibronic coupling on the vacuum ultraviolet photodissociation dynamics.
  • Authors contributed equally to this work.
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Wavelength Dependent Photodissociation of OCS via $ F$ ${3^1\Pi}$ Rydberg State: CO(${X^1\Sigma^+}$)+S($^{1}$D$_{2}$) Product Channel

doi: 10.1063/1674-0068/cjcp2008147

Abstract: The vacuum ultraviolet photodissociation of OCS via the $F$ $3^1\Pi$ Rydberg states was investigated in the range of 134$-$140 nm by means of the time-sliced velocity map ion imaging technique. The images of S($^1$D$_2$) products from the CO($X^1\Sigma^+$)+S($^1$D$_2$) dissociation channel were acquired at five photolysis wavelengths, corresponding to a series of symmetric stretching vibrational excitations in OCS($F$ $3^1\Pi$, $v_1$=0$-$4). The total translational energy distributions, vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) coproducts were derived. The analysis of experimental results suggests that the excited OCS molecules dissociate to CO($X^1\Sigma^+$) and S($^1$D$_2$) products via non-adiabatic couplings between the upper $F$ $3^1\Pi$ states and the lower-lying states both in the C$_{\infty \textrm{v}}$ and C$_{\rm{s}}$ symmetry. Furthermore, strong wavelength dependent behavior has been observed: the greatly distinct vibrational populations and angular distributions of CO($X^1\Sigma^+$, $v$) products from the lower ($v_1$=0$-$2) and higher ($v_1$=3, 4) vibrational states of the excited OCS($F$ $3^1\Pi$, $v_1$) demonstrate that very different mechanisms are involved in the dissociation processes. This study provides evidence for the possible contribution of vibronic coupling and the crucial role of vibronic coupling on the vacuum ultraviolet photodissociation dynamics.

Authors contributed equally to this work.
Fei Xu, Yu-xin Tan, Dao-fu Yuan, Wen-tao Chen, Sheng-rui Yu, Ting Xie, Tao Wang, Xue-ming Yang, Xing-an Wang. Wavelength Dependent Photodissociation of OCS via $ F$ ${3^1\Pi}$ Rydberg State: CO(${X^1\Sigma^+}$)+S($^{1}$D$_{2}$) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 691-696. doi: 10.1063/1674-0068/cjcp2008147
Citation: Fei Xu, Yu-xin Tan, Dao-fu Yuan, Wen-tao Chen, Sheng-rui Yu, Ting Xie, Tao Wang, Xue-ming Yang, Xing-an Wang. Wavelength Dependent Photodissociation of OCS via $ F$ ${3^1\Pi}$ Rydberg State: CO(${X^1\Sigma^+}$)+S($^{1}$D$_{2}$) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 691-696. doi: 10.1063/1674-0068/cjcp2008147
  • Carbonyl sulfide (OCS), which serves as the primary source of sulfate, is an important component in the sulfur cycle in the atmosphere [1]. Although it is chemical inertness in the troposphere, OCS can be transported to the stratosphere where it either reacts with OH or O radicals or is photodissociated. Moreover, the photodissociation process is the dominant to OCS sink where amounts of reactive products such as S and O radicals, which serve as reactants in various atmospheric chemical reactions, are generated. So it is helpful for atmospheric simulation work to study the photolysis mechanism of OCS and obtain relevant kinetic parameters [2-5]. Hence, photodissociation dynamics of OCS have attracted much attention over the past decades in both theoretical and experimental studies.

    The absorption spectrum of OCS in the ultraviolet (UV) region starts near 250 nm and has a broad Gaussian-like peak centered around 222 nm. The primary product channel in the UV band is:

    The photodissociation dynamics of OCS in this region has been studied extensively [6-10]. Studies in the 222-249 nm region show that the photodissociation of OCS produces low vibrationally populated but high rotationally excited CO products [7-9]. Moreover, it is found that the excited state lifetime of the OCS at 222 nm is short compared to its rotation period and the transition moment is predominantly parallel to the OCS axis.

    Compared with the UV region, due to the high density of electronic states at a high energy level, in general, the photodissociation in the VUV region is also very informative and could provide detailed dynamical information on the intermolecular electronic non-adiabatic couplings and even the vibronic couplings. For OCS photolysis process, as the photon energy increases to the vacuum ultraviolet (VUV) region, the following channels start to occur:

    CO+S($ ^1 $S$ _0 $) is one of the main photodissociation channels in the 110-170 nm region, and predominates in the quantum yield from 142 nm to 160 nm [11-15]. Compared with the CO+S($ ^1 $S$ _0 $) channel, the CO+S($ ^1 $D$ _2 $) product channel has been rarely studied in the VUV region before. There are several factors: the relatively small branching ratios of the S($ ^1 $D$ _2 $) product channel in the VUV region [11]; the difficulty to obtain a tunable VUV laser beam in the laboratory; the quite strong background from the UV detection laser in the (2+1) resonance-enhanced multiphoton ionization (REMPI) scheme of S($ ^1 $D$ _2 $) ionization. As a result, more detailed dynamics of this product channel in the VUV region remains unclear.

    To obtain a more quantitative picture of photodissociation of OCS into the S($ ^1 $D$ _2 $) channel, high-resolution measurements in a wider photolysis wavelength range are beneficial. Here we report an experimental study of the OCS photodissociation dynamics around 140 nm. A tunable VUV laser beam, generated by the nonlinear optic process in rare gas, was employed in the laboratory. The photodissociation experiment was performed at a set of photolysis wavelengths from 139.96 nm to 134.57 nm. The S($ ^1 $D$ _2 $) products were selectively detected with the (2+1) REMPI scheme. The product total kinetic energy release distributions (TKERs), product vibrational distributions and the anisotropy parameter were extracted. The results are helpful in depicting a quantitative picture of OCS photodissociation dynamics in the VUV region.

  • The photodissociation experiment was carried out by using a molecular beam apparatus with a time-sliced velocity map imaging (VMI) detector. The main setup of the apparatus has been described previously [16-19]. In brief, the OCS molecular beam was generated through the supersonic expansion of the gas mixture (5% OCS seeded in Ar) using a general valve (Parker Series 9) with a 1 mm orifice. The repetition rate of the pulsed valve was 20 Hz. The base pressure and the typical operating pressure in the source chamber were around 1$ \times $10$ ^{-8} $ and 1$ \times $10$ ^{-5} $ mbar, respectively. The pulsed valve was mounted perpendicular to the plane of the imaging detector. About 18 mm downstream from the nozzle, a skimmer with a 1.5 mm diameter aperture was used for both collimating the molecular beam and realizing the differential pumping between the source and detection chambers. The base pressure and operating pressure in the detection chamber were around 1$ \times $10$ ^{-8} $ and 5$ \times $10$ ^{-8} $ mbar, respectively. After being collimated, the OCS beam entered the detection chamber and then passed through a 2 mm hole on the first electrode plate before entering the ion optics. The OCS molecules intersected with the VUV photolysis laser and were dissociated in the ion optics. The detection laser selectively ionized the S($ ^1 $D$ _2 $) product of the dissociation. The ions were accelerated and velocity focused in the electric fields of the 23-plate ion optics and then propagated farther along a field-free time-of-fiight tube before reaching the position-sensitive ion detector. The detector consists of a 70 mm diameter dual microchannel plate (MCP) coupled to a phosphor screen (P43) and a CCD camera (Imager pro plus 2M, LaVision). Finally, the raw ion images were acquired using this time-sliced velocity map imaging (VMI) method with the event counting scheme. The timings of the pulsed valve, dissociation and ionization lasers, and the gate pulse applied to the MCP were all controlled by the digital delay generators (DG535 and DG645, Stanford Research Systems).

    The two-photon resonance-enhanced four-wave mixing scheme in Krypton was employed to generate the tunable VUV photolysis laser [20]. A 212.55 nm ($ \omega_1 $) laser beam was generated by the doubled output of a tunable dye laser (Corbra-Stretch, Sirah) pumped by a pulsed Nd:YAG laser (Powerlite 9020, Continuum), and a second tunable laser ($ \omega_2 $) was generated by the fundamental output of the second tunable dye laser (Corbra-Stretch, Sirah) pumped by the same Nd:YAG laser. The two laser beams were spatially and temporally overlapped and focused into a stainless-steel Krypton gas cell with a quartz focusing lens ($ f $ = 30 cm). The generated VUV (2$ \omega_1 $-$ \omega_2 $) radiation and the residual incident laser light were collimated by an MgF$ _2 $ lens and entered the detection chamber. Tuning the wavelength of the VUV laser beam was achieved by fixing the wavelength of $ \omega_1 $ and changing the wavelength of $ \omega_2 $. A 288.19 nm UV laser beam was generated by the doubled output of the third tunable dye laser (Corbra-Stretch, Sirah) pumped by the second Nd:YAG laser (Powerlite 9020, Continuum), and used for the S($ ^1 $D$ _2 $) product channel detection by the (2+1) REMPI scheme. The Doppler wavelength scanning of $ \omega_2 $ was performed to realize the equal detection efficiency of all the S($ ^1 $D$ _2 $) fragments at different velocities. The background was taken and subtracted carefully. No significant polarization effect for the S($ ^1 $D$ _2 $) product channel was detected at the studied wavelengths. So the polarizations of the VUV and UV lasers were set to be parallel and perpendicular to the imaging plane, respectively.

  • The images of S($ ^1 $D$ _2 $) products were acquired at five photolysis wavelengths, namely, 139.96, 138.53, 137.15, 135.84, and 134.57 nm. These wavelengths correspond to the resonant transitions from the vibronic ground state of OCS to the $ F $ $ 3^1\Pi $($ v_1 $, $ v_2 $, $ v_3 $) Rydberg states. Symbol $ v_i $ ($ i $ = 1-3) represent the three vibrational modes of OCS, namely, the symmetric stretching mode $ v_1 $, the bending mode $ v_2 $, and the anti-symmetric stretching mode $ v_3 $. Based on the related spectrum data [21, 22], the five vibrational states of OCS($ F $ $ 3^1\Pi $) are assigned and summarized in Table Ⅰ.

    Table 1.  Corresponding OCS excited states at five photolysis wavelengths.

    The background subtracted images of S($ ^1 $D$ _2 $) products from the photodissociation of OCS at five wavelengths are shown in FIG. 1. The red horizontal arrow illustrates the polarization direction of the photolysis laser. In this experiment, the noises mainly originate from the UV photodissociation of OCS, which can be subtracted by shutting off the VUV and keeping on the UV laser. Due to the huge photon energy difference (more than 4.5 eV) between the VUV photolysis and UV detection lasers, the noises from the UV detection laser only distribute in the inner ring region corresponding to lower velocities, and can be well separated from the VUV signals which mainly distribute in the area at faster product velocity. The images show some distinct wavelength dependent features. At shorter photolysis wavelengths (135.84 and 134.57 nm), the speed distributions of the products are more localized, presented as a distinguished circle in each image, and the angular distributions are also more anisotropic compared with products at longer photolysis wavelengths (139.19, 138.53, and 137.15nm). This is consistent with the wavelength dependent TKERs and anisotropic parameter distributions, vide infra.

    Figure 1.  Experimental images of S($ ^1 $D$ _2 $) products from the photodissociation of OCS at five wavelengths, respectively. (a)139.96 nm, (b) 138.53 nm, (c) 137.15 nm, (d) 135.84 nm, and (e) 134.57 nm. The red double arrow presents the polarization direction of the photolysis laser.

  • From the observed ion images, the speed distributions of the S($ ^1 $D$ _2 $) products were extracted by integrating the signal intensities over the total angular range and then converted to the product TKERs according to the law of conservation of momentum. The energy of the whole system in the photodissociation process can be described as the following equation:

    Among them, $ E_{h\nu} $ is the energy of the photolysis laser, $ D_0 $(OC-S) is the bond energy of the C-S bond, $ E_{\rm{int}} $ (CO) is the internal electronic and rovibrational energy of the CO products, and $ E_{\rm{int}} $(S) is the energy difference between S($ ^1 $D$ _2 $) products and ground state S($ ^3 $P$ _2 $).

    The TKERs of the S($ ^1 $D$ _2 $) products at five photolysis wavelengths in the center of mass frame are shown in FIG. 2. The red solid line in each graph depicts the result of a multipeak fitting of the total kinetic energy and the energy combs correspond to the vibrational quantum number $ v $ of CO($ X^1\Sigma^+ $, $ v $) co-products. By fitting the TKERs, the branching ratios of different vibrational states at each wavelength were obtained and are shown in FIG. 3. The results show a very "hot" vibrational population of the CO co-products and an energy dependency of vibrational distributions. At 139.96 nm, the product distribution is very wide and delocalized. As the wavelengths decrease to 138.53 and 137.15 nm, the product distributions turn to be localized and more dominant at $ v $ = 11-15. When the wavelength further decreases to 135.84 and 134.57 nm, the main peaks of the product distributions located between $ v $ = 11 to $ v $ = 15 become quite prominent. The sum of the branching ratios of these five vibrational states from $ v $ = 11 to $ v $ = 15 occupies more than 80% of the total products.

    Figure 2.  Total kinetic energy release distributions for S($ ^1 $D$ _2 $)+CO($ X^1\Sigma^+ $) channel from the photodissociation of OCS at five photolysis wavelengths. (a) 139.96 nm, (b) 138.53 nm, (c) 137.15 nm, (d) 135.84 nm, and (e) 134.57 nm.

    Figure 3.  Product vibrational branching ratios of CO($ X^1\Sigma^+ $, $ v $) products at five photolysis wavelengths. The sum of the vibrational branching ratios at each photolysis wavelength is set to be unity.

  • The product angular distribution for a single photon dissociation process could be described by the following equation:

    where $ \theta $ is the angle between the polarization direction of the photolysis laser and the velocity vector of the recoil product, and $ P_2 $(cos$ \theta $) is the second-order Legendre polynomial. $ \beta $ is the anisotropy parameter which characterizes the angular distribution of the products in the photodissociation process. The $ \beta $ value ranges from -1 to 2 with $ \beta $ = 2 corresponding to a pure parallel transition ($ \Delta\Omega $ = 0) and $ \beta $ = -1 corresponding to a pure perpendicular transition ($ \Delta\Omega $ = $ \pm $1). The $ \beta $ values at various photolysis wavelengths are plotted as a function of the vibrational quantum number of CO products and displayed in FIG. 4 for the CO($ X^1\Sigma^+ $)+S($ ^1 $D$ _2 $) channel. Since the very weak signals of some vibrational states ($ v $ = 0, 1 and $ v $$ > $15) at 135.84 and 134.57 nm, and the signals for $ v $$ > $16 are overlaid with the quite strong background from the UV detection laser, the $ \beta $ values for these states are difficult to be extracted accurately, and not presented in FIG. 4.

    Figure 4.  Anisotropy parameters for individual CO($ X^1\Sigma^+ $, $ v $) vibrational states at five photolysis wavelengths.

    FIG. 4 shows a distinct feature in the $ \beta $ value distributions. The $ \beta $ values at these five photolysis wavelengths mainly distribute in two different regions: at 139.96, 138.53 and 137.15 nm, which correspond to the $ F $ $ 3^1\Pi $($ v_1 $ = 0, 1, 2) upper states of the excitation process, the $ \beta $ values are very close to zero, which indicates that the nearly isotropic dissociation plays the major role in the photodissociation process at these three wavelengths. While, at 135.84 and 134.57 nm, which correspond to the $ F $ $ 3^1\Pi $($ v_1 $ = 3, 4) upper states of the excitation process, the $ \beta $ values are very close to 1.0. This result indicates that a parallel photodissociation is responsible for the dissociation process at these two wavelengths. This distinguishing feature also can be easily identified from the experimental images at 135.84 and 134.57 nm with the products mainly distributed in the direction parallel to the polarization vector of the photolysis laser.

  • To obtain the detailed photodissociation dynamics, we should know the fine structures on the potential energy surface. Previous theoretical studies mainly focused on the CO+S product channel at the UV region [6, 23, 24]. Recently, Jiang et al. drew an adiabatic correlation diagram with the energy up to 10 eV of both SO+C and CO+S product channels in C$ _{\infty \rm{v}} $ symmetry [25]. The adiabatic correlation diagram is actually quite complicated, and we just focus on the CO($ X^1\Sigma^+ $)+S($ ^1 $D$ _2 $) product channel that are related to our experiment here. In principle, the $ F $ $ 3^1\Pi $ Rydberg state prepared by the VUV excitation laser only adiabatically associates with the CO($ ^3\Pi $)+S($ ^3 $P$ _{J} $) product channel. It is noticed that the CO($ X^1\Sigma^+ $)+S($ ^1 $D$ _2 $) are adiabatically with the low-lying states $ 1^1\Pi $ and $ 1^1\Delta $ and the ground state $ 1^1\Sigma^+ $. Hence, it suggests that the high-lying states undergo non-adiabatic processes to dissociate into the CO($ X^1\Sigma^+ $)+S($ ^1 $D$ _2 $) product channel. The ground electronic state of OCS has a linear structure and was labeled as $ 1^1\Sigma^+ $ in the C$ _{\infty \rm{v}} $ symmetry and 1$ ^1 $A$ ' $ in the C$ _{\rm{s}} $ symmetry [26]. $ 1^1\Sigma^+ $, $ 1^1\Pi $ and $ 1^1\Delta $ electronic states in the C$ _{\infty \rm{v}} $ symmetry and 1$ ^1 $A$ ' $, 1$ ^1 $A$ '' $, 2$ ^1 $A$ ' $, 2$ ^1 $A$ '' $, and 3$ ^1 $A$ ' $ states in the C$ _{\rm{s}} $ symmetry all correlate with the CO($ X^1\Sigma^+ $)+S($ ^1 $D$ _2 $) asymptote [6, 23, 25]. It indicates that the observed CO($ X^1\Sigma^+ $)+S($ ^1 $D$ _2 $) channel is attributed to the non-adiabatically coupling from the $ F $ $ 3^1\Pi $ upper states of excited OCS molecules to the low-lying states.

    As shown in Table Ⅰ, different excitation wavelengths correspond to different symmetric stretching vibrational quantum number in the $ F $ $ 3^1\Pi $ state. The results of branching ratios show that the vibrational populations of the CO co-products are very "hot" and the vibrational state distributions are much more localized at 135.84 and 134.57 nm than the other three longer wavelengths. The $ \beta $ value results show that the angular distributions are more anisotropic at 135.84 and 134.57 nm than the other three longer wavelengths. These phenomena indicate that the products at shorter wavelengths come from a faster dissociation process. However, the products at longer wavelengths are more likely to come from relatively slower process. Furthermore, it is interestingly noticed that there is a considerable potential well locating on the exit channel both of the $ 1^1\Delta $ and $ 1^1\Sigma^+ $ states while the $ 1^1\Pi $ state is purely repulsive [25]. In general, the dissociation through $ 1^1\Pi $ state should be faster than the $ 1^1\Delta $ and $ 1^1\Sigma^+ $ states. The results indicate that the vibronic coupling effect may play crucial roles in the adiabatic coupling process, which can be accelerated by the higher vibrational excitation. The detailed photodissocaition dynamics in this VUV region should be more complicated than that we discussed above. To seek a clearer picture of the OCS photodissociation dynamics in the VUV region, more theoretical and experimental work needs to be performed.

  • In this experiment, state-selective excitation of OCS molecules was performed with the tunable vacuum ultraviolet laser. The CO($ X^1\Sigma^+ $)+S($ ^1 $D$ _2 $) photodissociation channel was measured by using the high-resolution time-sliced VMI method combined with the REMPI technique. The results show that at the photolysis wavelength of around 140 nm, the CO product vibrational populations are very "hot". The results also indicate energy dependent dissociation dynamics from the $ F $ $ 3^1\Pi $ Rydberg states, with quite different faster and slower dissociation pathways involved in the dissociation process. The excitation of the symmetric stretching mode in $ F $ $ 3^1\Pi $ state will speed up the non-adiabatic coupling between the $ F $ $ 3^1\Pi $ and the low-lying states, which provides important evidences that the vibronic coupling effect play critical roles in the photodissociation dynamics for the OCS molecule in the 130- 140 nm VUV region.

  • This work was supported by the National Key R & D Program of China (No.2017YFF0104500), the National Natural Science Foundation of China (No.21473173, No.21590802, No.21673216, and No.21773213), the Strategic Priority Research Program of Chinese Academy of Sciences (No.XDB17000000). We thank Bin Jiang and Xiao-guo Zhou at University of Science and Technology of China for helpful discussions.

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