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Xi-lin Bai, Dong-feng Zhao, Yang Chen. Photodissociation Dynamics of OCS at 207 nm: S($ ^\textbf{1} $D$ _\textbf{2} $)$ \textbf{+} $CO($ \mathit{\boldsymbol{X}}^\textbf{1} $$ \mathit{\boldsymbol{\Sigma}} $$ ^\textbf{+} $) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(3): 291-295. doi: 10.1063/1674-0068/cjcp1908148
Citation: Xi-lin Bai, Dong-feng Zhao, Yang Chen. Photodissociation Dynamics of OCS at 207 nm: S($ ^\textbf{1} $D$ _\textbf{2} $)$ \textbf{+} $CO($ \mathit{\boldsymbol{X}}^\textbf{1} $$ \mathit{\boldsymbol{\Sigma}} $$ ^\textbf{+} $) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(3): 291-295. doi: 10.1063/1674-0068/cjcp1908148

Photodissociation Dynamics of OCS at 207 nm: S($ ^\textbf{1} $D$ _\textbf{2} $)$ \textbf{+} $CO($ \mathit{\boldsymbol{X}}^\textbf{1} $$ \mathit{\boldsymbol{\Sigma}} $$ ^\textbf{+} $) Product Channel

doi: 10.1063/1674-0068/cjcp1908148
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  • Corresponding author: Xi-lin Bai.bxl5630@mail.ustc.edu.cn
  • Received Date: 2020-08-25
  • Accepted Date: 2020-09-11
  • Publish Date: 2020-06-27
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Photodissociation Dynamics of OCS at 207 nm: S($ ^\textbf{1} $D$ _\textbf{2} $)$ \textbf{+} $CO($ \mathit{\boldsymbol{X}}^\textbf{1} $$ \mathit{\boldsymbol{\Sigma}} $$ ^\textbf{+} $) Product Channel

doi: 10.1063/1674-0068/cjcp1908148
Xi-lin Bai, Dong-feng Zhao, Yang Chen. Photodissociation Dynamics of OCS at 207 nm: S($ ^\textbf{1} $D$ _\textbf{2} $)$ \textbf{+} $CO($ \mathit{\boldsymbol{X}}^\textbf{1} $$ \mathit{\boldsymbol{\Sigma}} $$ ^\textbf{+} $) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(3): 291-295. doi: 10.1063/1674-0068/cjcp1908148
Citation: Xi-lin Bai, Dong-feng Zhao, Yang Chen. Photodissociation Dynamics of OCS at 207 nm: S($ ^\textbf{1} $D$ _\textbf{2} $)$ \textbf{+} $CO($ \mathit{\boldsymbol{X}}^\textbf{1} $$ \mathit{\boldsymbol{\Sigma}} $$ ^\textbf{+} $) Product Channel[J]. Chinese Journal of Chemical Physics , 2020, 33(3): 291-295. doi: 10.1063/1674-0068/cjcp1908148
  • Carbonyl sulfide (OCS) has long been served as a benchmark system of triatomic molecules in understanding the dissociation mechanism, as well as for the multidimensional potential energy surfaces (PESs) [1-3]. The photodissociation of OCS was thought to be a main source of the stratospheric sulfate aerosol, the sudden decrease of OCS concentration in the stratosphere increased the importance of the shorter wavelength photolysis [4]. And the photolysis products of OCS in the stratospheric UV window are also of great significance in the secondary reaction of the atmosphere [5, 6]. Meanwhile, the photolysis products orbital polarization effect of carbonyl sulfide has attracted enormous interest due to its important information of angular momentum [7-11]. In recent years, extensive experimental [12-25] and theoretical [26-29] studies on the photodissociation dynamics of OCS in ultraviolet (UV) region have been reported.

    From the first UV absorption band of OCS [28], we can know that the main source of absorption is caused by excitation of the single repulsive A(21A') state, and the main structure observed in the cross section comes from the excitation of the triple bound c(23A") state. Besides, there is an extremely weak absorption from the excitation of the single repulsive B(11A") state. The spin-allowed channel S(1D2)+CO(X1+) dominates in the UV absorption band of OCS [12]. The total kinetic energy release spectrum (TKER) of the main channel shows a bimodal distribution at λ≥214 nm, and a new channel appears at 210 nm [23]. The highly excited rotational product CO measured in experimental work is caused by strong bending on the excited state surfaces. And the higher-J rotational distribution of CO becomes more obvious with the increasing of the excitation energy [3]. The quantum yield of the spin-forbidden channel, S(3PJ)+CO(X1+), derived from the intersystem crossing is very small.

    Investigation of the angular momentum polarization [30-33] of the atomic photofragments of photolysis reactions can provide an insightful understanding of the photodissociation dynamics. For example, the μ-j correlation parameter β02(02) in the photodissociation of OCS can distinguish the symmetry of the excited state. The determined alignment and anisotropy parameters can distinguish the incoherent excitation and coherent excitation. Recently, these polarization parameters of the atomic S(1D2) photofragments can be determined by using the high-resolution slice imaging technology. Using this method, Sofikitis and co-workers observed the resonant behavior in the photodissociation of OCS at a particular excitation wavelength [11].

    In our previous work, we studied the photodissociation dynamics of OCS at 210 [23] and 217 nm [24] using the slice velocity map imaging (VMI) apparatus. In this work, we report a detailed experimental study on S(1D2) orbital polarization from the photodissociation of OCS at 207 nm, extending to the shorter wavelength. By comparison with the previous experimental data measured from other dissociation wavelengths, a new different dissociation channel is experimentally identified. This work focuses on the detailed discussion on the dissociation mechanism based on the observed anisotropy and polarization parameters, as well as the influence of multidimensional PESs on the photolysis process.

  • The setup of the time-sliced VMI experiment was described in detail in our previous studies [34-37]. In brief, a pulsed supersonic molecular beam of OCS regent with He was expanded into a source chamber and then directed along the central axis of the time-of-flight (TOF). The skimmed molecular beam passed through a 1-mm diameter hole in the repeller plate of the ion optic, then intersected by the photolysis laser beam. Tens of nanoseconds after photolysis laser, the S(1D2) fragments were probed by the (2+1) REMPI scheme via the 1P1 and 1F3 resonant intermediate states. The photolysis-probe laser beams were parallel to the detection plane, and crossed the molecular beam between the repeller (R) and extractor (E) plates. The ionized S+ ions were accelerated by the ion optics and passed through a TOF tube toward the Chevron dual multichannel plate (MCP, 40 mm) detector and provided velocity mapping in the focusing electric fields. Mass selectivity was achieved by applying a 50 ns gate pulse with high voltage to the front of the MCP's detector. The resulting images on the P47 phosphor screen were recorded by a gated intensified charge-coupled device camera (Imager Intense, LaVision) in conjunction with the ion event counting. Timing of the pulsed molecular beam, the lamps of the photolysis and probe lasers, and the pulsed gating switched on the detector was controlled by two delay pulse generators (DG 535, Stanford Research System).

    For two-color experiments, the linearly polarized photolysis laser beam (207 nm) was achieved by the frequency-tripled output of a dye laser (DCM) pumped by a Nd:YAG laser (Spectra Physics, 532 nm), giving only about 0.3 mJ/pulse, for decreasing the background. The polarization of the photolysis laser beam was controlled by a Berek's polarization rotator (Model 5540, New Focus), and monitored by a MgF2 Rochon prism. The sulfur probe beam with a pulse energy of 0.8 mJ (set at 288.18 and 291.48 nm for the transition to 1F3 and 1P1 intermediate states respectively) was generated by the frequency-doubled output of the other tunable dye laser (Rhodamine 610), which was pumped by the second harmonic of Nd:YAG laser (Spectra Physics, 532 nm). A quartz half-wave plate (Thorlabs, 260-400 nm) was used to control the relative orientation of the probe laser to be perpendicular to the pulsed molecular beam (noted as V) or parallel to the pulsed molecular beam (noted as H). The photolysis and probe laser beams were spatially overlapped and focused on the interaction region using two focal length lens (f1 = 250 mm and f2 = 200 mm). The photolysis-probe delay for the whole experiments was optimized at around 20 ns. Narrow scanning of the probe laser was utilized for the S(1D2) fragments in order to avoid the Doppler effect.

  • The time-sliced ion images of the S(1D2) photoproducts in FIG. 1 were recorded with the photolysis of OCS at 207 nm. Two different resonant intermediate states of S(1D2) with four different pump-probe laser polarization geometries are employed to obtain the anisotropy and polarization parameters. VH, VV, HH, and HV were used to describe the polarization geometries of the pump-probe laser. For example, the first letter in ``VH" geometry represents the polarization of the pump laser and the second letter represents the polarization of the probe laser. Here, the label "V" and "H" indicate the polarization of the laser is perpendicular and parallel to the TOF axis, respectively.

    Figure 1.  Raw time-sliced ion images of S(1D2) products from the photodissociation of OCS at 207 nm.

    In FIG. 1, each of the sliced images represents a 2D projection of the recoil velocity distribution of the S(1D2) fragments. The clearly separated velocity rings of the S photofragments in the images correspond to the internal energy distribution of the CO(X1+) co-fragments. According to the law of energy conservation, the total kinetic energy release spectrum in the photodissociation process can be described by the following equation:

    where hν is the photolysis energy of 207 nm, D0(OC-S) is the bond energy of the C-S bond that was determined by previous studies, Eint(CO) and Eint(S) are the internal energy of the products CO(X1+) and S(1D2), respectively. The TKER spectra in the center-of-mass frame were obtained by integrating the time-sliced images overall product velocity angles. FIG. 2 shows the obtained TKER spectra of photodissociation of OCS at 207 nm.

    Figure 2.  (a) Total kinetic energy distributions for the S(1D2)+CO(X1+) product channel from the photodissociation of OCS at 207 nm. (b) Comparison of TKER spectra in different intermediate states with the same polarization geometry. Fitted curves of the TKER spectra in the HH geometry with the 1P1 as resonant intermediate using three Gaussian functions are illustrated with red dot line.

    FIG. 2(b) shows that the obtained TKER spectra can be successfully decomposed into three Gaussian components. It means that there are three different dissociation pathways for S(1D2)+CO(X1+) channel. The features of low ET component and medium ET component are extremely similar to a bimodal structure for dissociation at longer wavelengths [14, 20], although the two components are almost overlapped and have higher rotational distributions. The high ET component also appears as dissociation at 210 nm which is the opening window for the high ET channel [23]. Besides, the obtained CO rotational distributions exhibit a significant dependence on the REMPI transition of S(1D2). FIG. 2(b) shows that the intensity of the high ET component channel via intermediate state 1P1 is larger than that via intermediate state 1F3. This dependence as interpreted in Ref.[32], indicates the polarized characteristic of photofragment S(1D2). The line strength factors for different REMPI transition schemes may be responsible for this difference.

    The angular distributions corresponding to CO(X1+), I(θ), can be extracted from the images by integrating the intensity over the corresponding narrow radial range for each rotational peak and fitted by the following expression:

    where θ is the crossing angle between the recoil velocity of the S(1D2) photofragment and the polarization orientation of the UV photolysis laser, and Pk(2, 4, 6) are the kth-order Legendre polynomials. The Legendre polynomial coefficients βk(2, 4, 6) values were obtained by fitting the angular distributions with Eq.(2) using the least-squares method. The anisotropy parameter and the polarization parameters of S(1D2) from 207 nm photodissociation of OCS can be determined by the Legendre polynomial coefficients βk(2, 4, 6) values using the theoretical model described by Rakitzis and Zare [38].

    The angular distributions of S(1D2) from dissociation at 207 nm have J-dependent characteristics. For the HV pump-probe laser polarization geometry, the angular distribution of S(1D2) changes obviously in different CO(J). For the low ET component, the peak of I(θ) is near 0° and 180°. For the medium ET component, the peak of I(θ) is near 45° and 30° for detection through the 1P1 and 1F3 intermediated states, respectively. For the high ET component, the peak of I(θ) is returned to 0° and 180°. Besides, the angular distributions of S(1D2) fragments from the photodissociation of OCS at 210 nm and 207 nm are also different, especially for the high ET component in VH pump-probe laser polarization geometry and resonance intermediate 1F3 state. In FIG. 3, another difference for the high ET component is that the β value increases to a higher value of 0.78 and the bipolar momentum parameter β02(02) value increases to a positive value with the decreasing of the rotational quantum number of CO. Both of them indicate that there are different dissociation mechanisms in photodissociation at 210 and 207 nm photons.

    Figure 3.  (a) The bipolar momentum parameters β02(02) and (b) the anisotropy parameters β extracted from the time-sliced ion images of S(1D2 products produced by dissociation of OCS at 207 nm.

  • The A(21A') state plays a critical role in the main dissociation pathways, due to the fact that the main excitation comes from the repulsive A(21A') state through the calculated absorption spectrum [27]. From the previous studies, we can get that the low and medium ET component channels arise from the A(21A') state excitation followed by the nonadiabatic crossing to the X(11A') state and the simultaneous two repulsive states (A and B) excitation, respectively. The extremely high CO(J) rotational distributions for these two components indicate that there is greater bending in the dissociation process. This is in agreement with the previous observations and calculations [21, 28].

    The calculated vertical excitation energy of the singlet A(21A') state is only 0.06 eV higher than that of the triplet c(23A") state [27]. However, the energy difference between 207 and 210 nm photons is 0.09 eV. Meanwhile, we notice from Ref.[23] that the experimental photoexcitation energy, 210 nm, can reach the vertical excitation region for c(23A") state. Thus, it indicates that our experimental photoexcitation energy 207 nm can reach the vertical excitation region for A(21A') state. The high β parameters observed in the high ET component just manifest that the symmetry of dissociation excited state is A'. And the low rotational distribution indicates that this channel comes from direct dissociation from the Frank-Condon excitation region. Hence, a direct dissociation from the A(21A') state may lead to the generation of the high ET component channel.

    The translational energy distributions are roughly similar to the dissociation results of OCS at 193 nm [17], although the anisotropy parameters of the high ET component channel become larger. This suggests that the bending degree of molecules by laser excitation is not increased with the increasing of the excitation energy. From the perspective of the PESs, even of the three-atom molecules, the dissociation pathways are also very complex. It needs further theoretical and experimental work accurately to describe molecular dynamics behavior.

    For dissociation of triatomic molecule OCS, the transition to an A" state can obviously result in anisotropic angular distributions with the limiting β value of -1, while the transition to an A' state can generate any value of angular anisotropy, β, between -1 and 2, depending on the intersection angle of the transition dipole moment vector μ with respect to the recoil vector μ. The strong coupling region between the A state and the ground state is off the transition dipole moment, and the β value of the low ET component channel can not reach the limiting value of 2. Unfortunately, we did not find the inversion of the recoil direction (recoil resonance) of the high-J CO products [11]. Maybe the excitation position in our experiment is not in the resonance absorption peak position.

    An anomalously strong J = 81 level presented in the TKER spectra may be induced by a dynamically constrained threshold effect that seems to happen in the photodissociation of HOD [39]. Because there is a centrifugal barrier from the linear ground state to the products via a bending excited state geometry. The inner ring with a negative anisotropy parameter conforms that the photodissociation of hot-band OCS occurs from the repulsive B(11A") state induced by the lonely probe laser.

  • We measured the anisotropy parameter and the polarization parameters of S(1D2) arising from the dissociation of OCS+hν(207 nm)→S(1D2)+CO(X1+) using the time-sliced velocity ion imaging technique. The observed total kinetic energy release spectra show that there are three main dissociation channels: the nonadiabatic process between the A(21A') state and the ground state for the low ET component channel, the two-state excitation for the medium ET component channel, and the direct dissociation from the A(21A') state for the high ET component channel (the new channel). The experiments revealed that the photodissociation mechanism depends on the excitation energy. It is very important to understand the influence of multidimensional PESs from the detailed analysis of the photodissociation mechanism.

  • This work was supported by the National Key R & D Program of China (No.2017YFA0303502), the National Natural Science Foundation of China (No.21773221 and No.21727804), and the Fundamental Research Funds for the Central Universities of China (No.WK2340000078).

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