Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (6): 735-740

#### The article information

Zhi-guo Zhang, Min Xin, Yan-ning Wu, Shu-tao Zhao, Yi-jia Tang, Yang Chen

Imaging HNCO Photodissociation at 201 nm: State-to-State Correlations between CO ($X^1{\Sigma}^+$) and NH (${a}^1{\Delta}$)
HNCO在201 nm的离子成像光解动力学:CO($X^1$$\Sigma^+)与NH(a^1$$\Delta$)态相关性
Chinese Journal of Chemical Physics, 2018, 31(6): 735-740

http://dx.doi.org/10.1063/1674-0068/31/cjcp1808192

### Article history

Accepted on: September 21, 2018
Imaging HNCO Photodissociation at 201 nm: State-to-State Correlations between CO ($X^1{\Sigma}^+$) and NH (${a}^1{\Delta}$)
Zhi-guo Zhanga, Min Xina, Yan-ning Wua, Shu-tao Zhaoa, Yi-jia Tanga, Yang Chenb
Dated: Received on August 26, 2018; Accepted on September 21, 2018
a. School of Physics and Electronic Engineering, Fuyang Normal University, Fuyang 236041, China;
b. Department of Chemical Physics and Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Zhi-guo Zhang, E-mail: zhgzhang@mail.ustc.edu.cn; Yi-jia Tang, E-mail: yangchen@ustc.edu.cn; Yang Chen, E-mail: tyjfync@163.com
Abstract: The NH($a^1$$\Delta)+CO(X^1$$\Sigma^+$) product channel for the photodissociation of isocyanic acid (HNCO) on the first excited singlet state S$_1$ has been investigated by means of time-sliced ion velocity map imaging technique at photolysis wavelengths around 201 nm. The CO product was detected through (2+1) resonance enhanced multiphoton ionization (REMPI). Images were obtained for CO products formed in the ground and vibrational excited state ($v$=0 and $v$=1). The energy distributions and product angular distributions were obtained from the CO velocity imaging. The correlated NH($a^1\Delta$) rovibrational state distributions were determined. The vibrational branching ratio of $^1$NH ($v$=1/$v$=0) increases as the rotational state of CO($v$=0) increases initially and decreases afterwards, which indicates a special state-to-state correlation between the $.1$NH and CO products. About half of the available energy was partitioned into the translational degree of freedom. The negative anisotropy parameter $\beta$ indicates that it is a vertical direct dissociation process.
Key words: HNCO    Ion velocity map imaging    Energy distributions
Ⅰ. INTRODUCTION

As a simple four atom molecule, isocyanic acid (HNCO) has received considerable attention due to its significance for life and major constituents of chemical species in biology and chemistry. HNCO and its aqueous anion isocyanate have been shown to be toxic and linked to human health such as rheumatoid arthritis, cataracts and atherosclerosis through carbamylation reactions [1]. Isocyanic acid is a product of various forms of combustion such as cigarette smoking, automobile exhaust, biomass burning, and cooking [1, 2]. Dixon et al. and Rabalais et al. have measured the ultraviolet absorption spectrum (180-280 nm) of HNCO [3-5], which has been assigned to an S$_1$($^1$A$''$)$\leftarrow$S$_0$($^1$A$'$) transition. The spectrum is diffuse at wavelengths shorter than 265 nm and is unstructured below 244 nm. Photodissociation dynamics of HNCO has been the subject of wide investigations, both theoretically and experimentally, in the past few decades [6-33]. The photodissociation dynamics of HNCO is quite complex. There are three low-lying electronic states relevant to the dissociation process. The main photodissociation channels are summarized below:

 $\textrm{HNCO}+ h\nu \rightarrow\textrm{NH} (X^3\Sigma^-) + \textrm{CO} (X^1\Sigma^+) \nonumber\\ \;\;\; D_0=(30060\pm30) \; \textrm{cm}^{-1}$ (1)
 $\textrm{HNCO} + h\nu \rightarrow \textrm{H} (^2\textrm{S}) + \textrm{NCO} (X^2\Pi) \nonumber\\ \hspace{0.3cm}D_0=(38370\pm30)\; \textrm{cm}^{-1}$ (2)
 $\textrm{HNCO} + h\nu \rightarrow \textrm{NH} (a^1\Delta) + \textrm{CO} (X^1\Sigma^+) \nonumber \\ \hspace{0.3cm} D_0=(42750\pm30) \hspace{0.15cm}\textrm{cm}^{-1}$ (3)

For convenience, NH($a^1\Delta$) and NH($X^3\Sigma^-$) are denoted by $^1$NH and $^3$NH in the following sections.

The spin-forbidden channel (1) has been studied by Reisler and co-workers [18, 24]. The channel is observed at a wide range of excitation wavelengths (280-217 nm). By ion imaging technique, Reisler et al. probed CO product around 230 nm. Their results showed that the CO originating from channel (1) has isotropic angular distribution. The isotropic angular distributions of CO imply that the lifetime of the intermediate state exceeds 5 ps from S$_1$ to T$_1$. They proposed that the most likely dissociation pathway to channel (1) is S$_1$$\rightarrowS_0$$\rightarrow$T$_1$. The quantum yield is relatively small at a wide range of excitation wavelengths, but the channel is still the main dissociation channel in the range just above the opening of channel (2).

Dissociation to spin-allowed channel (2) has attracted much attention. Spiglanin and co-workers [7] probed NCO product by laser-induced fluorescence (LIF) technique. Zhang et al. [14] researched this channel at 193 nm photolysis energy via the high-$n$ Rydberg H atom time-of-flight (TOF) spectroscopy. Their results showed that the translation energy release peaked near the maximum available energy and accounted for about 70% of the total energy. The NCO product was extensive bending excited and an anisotropic angular distribution of $\beta$=-0.85 was observed. It suggested that the products of channel (2) accrued via direct dissociation on a repulsive potential energy surface. Crim et al. [11-13, 31, 32] studied this channel by LIF technique. Their study demonstrated that approximately 65% of the total available energy appeared in relative translation of the photoproducts near the channel (2) threshold, while about 30% went into vibration (dominated by the bending excitation of NCO) and 5% into rotation of NCO photoproducts. Furthermore, Crim's group performed a series of experiments to investigate mode-selective dissociation dynamics to channel (2). Reisler and co-workers [15, 17, 24] studied this channel at excitation wavelengths in the range of 217-260 nm. They deduced that channel (2) dissociation proceeded not directly on S$_1$, but rather via IC (internal conversion) to S$_0$ followed by decomposition on S$_0$ without a barrier near its threshold. The barrier on S$_1$ state to channel (2) direct dissociation was found to be at least 8140 cm$^{-1}$. Using the hydrogen atom Rydberg tagging TOF technique, Yu and co-workers [28] reinvestigated this channel at excitation wavelengths in the region of 200-240 nm. They observed two competitive dissociation pathways. One was the indirect dissociation on the S$_0$ surface, following IC from S$_1$ to S$_0$, which is consistent with Reisler's results. The other was the direct dissociation on the S$_1$ surface. As the photon energy increased, the direct dissociation pathway became much more important.

Photodissociation to spin-allowed channel (3) has been researched in the recent years. Fujimoto and co-workers [6] probed CO product with an average vibrational energy about 4.6 kcal/mol at 193 nm. Spiglanin and co-workers [7-10] examined the internal state distributions of $^1$NH and CO following photodissociation of HNCO at several wavelengths between 193 nm and 230 nm. Their results showed that the rotational state distribution of the CO was hot, but the $^1$NH rotational state distribution was cold. Reisler et al. [15, 17, 18, 23-25] performed a series of experiments to investigate channel (3) at excitation wavelengths in the range of 217-230 nm. Their results indicated that channel (3) dissociation proceeded via predissociation on S$_0$ surface following IC from S$_1$ to S$_0$, but after exceeding a small barrier of (470$\pm$60) cm$^{-1}$, direct dissociation on S$_1$ surface commenced and quickly dominated. Recently, we studied channel (3) by sliced velocity map imaging technique. The $^1$NH photoproduct was state-selectively probed via resonance enhanced multiphoton ionization (REMPI). We found that the rotational state distribution of CO($v$) was bimodal. By full-dimensional theoretical calculation, Bonnet and co-workers reproduced the bimodality of CO [33].

In this work, we further investigate channel (3) of HNCO photodissociation dynamics at 201 nm by the sliced velocity map ion imaging method probing the CO photoproduct. From the image of CO photoproduct, the $^1$NH internal state populations, photoproduct total kinetic energy distributions, and the angular distributions are obtained.

Ⅱ. EXPERIMENTS

The sliced velocity map ion imaging arrangement has been described in detail elsewhere [34, 35], so a brief description of the experimental process will be given here. The repetition rate of the whole experiment is 10 Hz. A 2% mixture of HNCO and He with a stagnation pressure of 1 bar is expanded into vacuum through a pulsed valve (General valve series 9) with a 0.5 mm diameter nozzle orifice. About 22 mm downstream from the nozzle, the pulsed free jet expansion is collimated by a 1 mm diameter aperture skimmer and reaches the main chamber, where the HNCO/He beam is crossed at right angles by the focused photolysis and probe lasers pulses.

The focused photolysis laser (0.5 mJ per pulse) is produced by the tripled output of a tunable dye laser, which is pumped by the second harmonic of a Nd:YAG laser. The CO photofragment is detected about 20 ns later by a focused probe laser beam (0.3 mJ per pulse) generated by doubling the output of a tunable dye laser, which is pumped by the third harmonic of a second Nd:YAG laser. The CO products are interrogated via the B$^1$$\Sigma^+$$\leftarrow\leftarrow$$X^1$$\Sigma^+$ (2+1) REMPI process around 230 nm. The linearly polarized direction of the photolysis light is parallel to the detector plane, while that of the probe light is set to be perpendicular to the detector plane.

The resulting CO ions are accelerated by the focusing electric fields of ion optics and pass through a 500 mm long time-of-flight tube before hitting a dual 40 mm diameter Chevron-type microchannel plates (MCP) coupled to a phosphor screen (P-47). A fast high-voltage switch is used to gate the central slice of the ion products at a specific mass. The typical pulse width is about 50 ns. The resulting electron avalanche strikes a P-47 phosphor screen, thereby creating the ion image, which is captured by a charge-coupled device (CCD) camera (ImagerPro2 M 640$\times$480 pixels, LaVision) and transferred to a computer on an every shot basis for event counting [36] and data analysis. The final images are accumulated over 2$\times$10$^4$ laser shots or more, depending on the signal-to-noise ratio. The timing of the pulsed valve, the photolysis and probe lasers, and the gate pulse applied to the MCP detector are controlled by using two multichannel digital delay generators (DG645, SRS).

Ⅲ. RESULTS AND DISCUSSION

The CO product ion images of HNCO photodissociation were measured at 201 nm. The images were obtained by accumulating the CO$^+$ signals with probe laser tuned to the Q branch of $B^1$$\Sigma^+$$\leftarrow\leftarrow$$X^1$$\Sigma^+$ transition of the CO product around 230 nm. All measured signals appearing in the images are pump-probe dependent. The background was taken with the photolysis light and molecular beam on. FIG. 1 displays typical CO($v$=0) sliced images after photodissociation of HNCO at 201 nm. The vertical red arrow indicates the polarization direction of the photolysis light. The polarization direction of the probe light is perpendicular to the image plane and the polarization direction of the photolysis light. No significant effects are observed in the ion images when the polarization direction of the probe light is changed. As seen in FIG. 1, two anisotropic rings are clearly displayed in the CO($v$=0) sliced images. The outer ring corresponds to vibrational ground state $^1$NH($v$=0$|j$) partner product, while the inner ring corresponds to the vibrationally excited $^1$NH($v$=1$|j$) partner product.

 FIG. 1 Raw sliced images of CO($v$=0$|$$j) products after photodissociation of HNCO at 201 nm. The double arrow indicates the polarization direction of the photodissociation light. From the raw sliced images, the CO speed distributions were extracted and converted to the total translational energy distributions of ^1NH+CO. The energy information of the whole system is shown as the following:  \begin{eqnarray} E_{h\nu} - D_0 = E_\textrm{T} + E_{\textrm{int}}(^1\textrm{NH}) + E_{\textrm{int}}(\textrm{CO}) \end{eqnarray} (4) where E_{h\nu} denotes the energy of photolysis light, D_0 represents the dissociation threshold energy of channel (3), E_\textrm{T} is the photoproduct total kinetic energy, E_{\textrm{int}}(^1NH) and E_{\textrm{int}}(CO) are the internal energy of ^1NH and CO products. The corresponding center-of-mass total kinetic energy distributions for the individual rovibrational states of CO(v=0|j) are shown in FIG. 2. It is obvious that FIG. 2 reflects the rovibrational state distributions of the correlated ^1NH product. Two vibrational peaks (v=0 and 1) of ^1NH are clearly resolved. It is interesting that the proportion of vibrationally excited ^1NH(v=1) does not decrease monotonously with the increase of CO rotational energy. We have done a more in-depth analysis for this.  FIG. 2 Product total kinetic energy distributions for CO(v=0|$$j$) after the photodissociation of HNCO at 201 nm.

To extract the vibrational branching ratio ($v$=1/$v$=0) of $^1$NH products, a qualitative fitting of the total kinetic energy distributions was carried out. FIG. 3 displays the correlation between vibrational branching ratio ($v$=1/$v$=0) of $^1$NH products and rotational states of CO($v$=0). As can be seen in the graph, the vibrational branching ratio ($v$=1/$v$=0) of $^1$NH products increases first and then decreases as the rotational excitation of CO($v$=0) increases. At $j$=23 of CO($v$=0), it reaches the maximum value. The vibrational excitation of the $^1$NH products is not simply anti-correlated to the rotational excitation of CO($v$=0), which indicates a special state-to-state correlation between the $^1$NH and CO products.

 FIG. 3 Vibrational branching ratio ($v$=1/$v$=0) of $^1$NH products for different rotational states of CO($v$=0).

FIG. 4 shows vibrationally excited CO($v$=1$|j$) sliced images of HNCO photodissociation at 201 nm. From the images, the ring sizes of vibrationally excited CO($v$=1$|j$) become smaller compared with that of CO($v$=0$|j$). For $j$=21 of CO($v$=1), the intensity of the inner ring is very weak. FIG. 5 displays the corresponding total kinetic energy distributions for the individual rovibrational states of CO($v$=1$|j$). It can be seen that the correlated $^1$NH products are mainly distributed in the vibrational ground state. The rotational excitation of $^1$NH decreases as the CO rotational excitation increases. This result shows that the rotational excitation of the $^1$NH products is anti-correlated to the rotational excitation of CO.

 FIG. 4 Raw sliced images of CO($v$=1$|j$) products after photodissociation of HNCO at 201 nm. The double arrow indicates the polarization direction of the photodissociation light.
 FIG. 5 Product total kinetic energy distributions for CO($v$=1$|j$) after the photodissociation of HNCO at 201 nm.

Our previous studies [29, 30] found that the rotational state distributions of CO are bimodal, which is further confirmed by Bonnet and co-workers [33] via theoretical calculation. In this study, however, we did not find the bimodal rotational distribution of $^1$NH from the image of CO product. It may be that the rotational energy of $^1$NH is relatively low and it is not easy to observe the bimodal phenomenon. As seen in FIG. 2 and FIG. 5, the $^1$NH products are mainly distributed in the low rotational states ($j$$\approx2-12), which is in consistence with the results of Spiglanin et al. [9] and Reisler et al. [15]. Based on total kinetic energy distributions, excitation energy and bond dissociation energy, we obtained the ratio of the average total kinetic energy to the available energy, [E_\textrm{T}]/E_{\textrm{avl}}, as shown in FIG. 6(a) and (b). For the CO(v=0|j) product, about 50% of the available energy is partitioned into the translational degree of freedom. For the CO(v=1|j) product, about 40% of the available energy is partitioned into the translational degree of freedom.  FIG. 6 Ratio of the average total kinetic energy to the available energy for (a) the CO(v=0|j) product and (b) the CO(v=1|j) product. Anisotropy parameter for individual ^1NH vibrational state correlated to (c) CO(v=0|j) and (d) CO(v=1|j). The angular distributions were obtained by integrating the imaging signals over the relevant radius region. FIG. 6(c) and (d) display the corresponding anisotropy parameters. As seen in FIG. 6(c), the anisotropy parameters are near -0.8 and -0.6 for ^1NH(v=0 and v=1)+CO(v=0). The ^1NH+CO(v=1) products display a similar anisotropic angular distribution as shown in FIG. 6(d). However, the ^1NH(v=1)+CO(v=1) anisotropy parameters are as low as -0.4, the low anisotropy should be due to the weak signal-to-noise ratio. The negative anisotropic angular distribution usually indicates that the dissociation process is fast. Previous studies have shown that there are two different dissociation pathways leading to ^1NH+CO channel [24]. Near the threshold of channel (3), excited HNCO dissociates on S_0 surface by IC from S_1 to S_0. At higher excitation energy, the barrier on S_1 surface to channel (3) will be exceeded and direct dissociation on S_1 prevails. It is obvious that the barrier on S_1 surface to channel (3) is exceeded at 201 nm. Under this condition, S_1 surface has a strong repulsive gradient along the C-N bond. So the photodissociation of HNCO should be a fast direct dissociation process at 201 nm. The negative anisotropic angular distribution further confirms this conclusion. Ⅳ. CONCLUSION The photodissociation dynamics of isocyanic acid for ^1NH+CO channel has been investigated by means of time-sliced velocity map imaging technique at photolysis wavelengths around 201 nm. The CO product has been detected by (2+1) REMPI. From analysis of the product total kinetic energy distributions, the correlated ^1NH rovibrational state distributions for CO(v=0 and v=1) products are obtained. The vibrational branching ratio of ^1NH (v=1/v=0) increases first and then goes down as the rotational state of CO(v=0) increases, which indicates a special state-to-state correlation between the binary ^1NH and CO products. Because of the low rotational energy of ^1NH, we do not observe the bimodal rotational distribution of ^1NH from the image of CO product. The negative anisotropy parameter \beta indicates that it is a vertical direct dissociation process at 201 nm. These results shown here provide a sensitive testing basis for the study of HNCO photodissociation dynamics. Ⅴ. ACKNOWLEDGMENTS This work was supported by the National Science Foundation of Anhui Province (No.1608085QA19), the National Science Foundation of China (No.11604052), the Natural Science Research Project of Education Department of Anhui Province (No.2014KJ020), the PhD Research Startup Foundation of Fuyang Normal University (No.FSB201501005), the Quality Engineering Project of Anhui Province (No.2017jyxm0277) and the Open Foundation of State Key Laboratory (Nos.SKLMRD-K201611, SKLMRD-K201711 and SKLMRD-K201810). Reference  [1] J. J. B. Wentzell, J. Liggio, S. M. Li, A. Vlasenko, R. Staebler, G. Lu, M. J. Poitras, T. Chan, and J. R. Brook, Environ. Sci. Technol. 47 , 7663 (2013). DOI:10.1021/es401127j [2] J. M. Roberts, P. R. Veres, A. K. Cochran, C. Warneke, I. R. Burling, R. J. Yokelson, B. Lerner, J. B. Gilman, W. C. Kuster, and R. Fall, J. De Gouw, Proc. Natl. Acad. Sci. USA 108 , 17234 (2011). [3] R. N. Dixon, and G. H. Kirby, Trans. Faraday Soc. 64 , 2002 (1968). 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Sci. Instrum. 73 , 4206 (2002). DOI:10.1063/1.1520732 HNCO在201 nm的离子成像光解动力学:CO(X^1$$\Sigma^+$)与NH(a$^1$$\Delta)态相关性 张志国a, 辛敏a, 吴言宁a, 赵书涛a, 唐义甲a, 陈旸b a. 阜阳师范学院物理与电子工程学院，阜阳 236041; b. 中国科学技术大学化学物理系，合肥微尺度物质科学国家研究中心，合肥 230026 摘要: 本文通过时间切片离子速度成像技术在201 nm附近研究了HNCO分子在S_1电子激发态的光解动力学. CO产物通过共振增强多光子电离的方法进行了选态探测，获得了CO产物的振动基态和激发态切片影像.从CO的影像得到了解离产物的能量分布和空间角分布，确定了NH(a$$^1$$\Delta$)产物的振转态分布信息.研究发现$^1$NH的振动分支比($v$=1/$v$=0)随CO($v$=0)转动能的增大先增大后下降，展现了$^1$NH与CO之间特殊的态态相关性.大约一半的可资用能分配给解离产物的平动自由度.负的各向异性参数表明HNCO的光解是个快速的直接解离过程.