Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (1): 27-32

The article information

Zhi-guo Zhang, Min Xin, Shu-tao Zhao, Yang Chen
张志国, 辛敏, 赵书涛, 陈旸
Imaging Isocyanic Acid Photodissociation at 193 nm: the NH(a1Δ)+ CO(X1Σ+) Channel
异氰酸分子NH(a1Δ)+CO(X1Σ+)通道在193nm的离子成像光解动力学研究
Chinese Journal of Chemical Physics, 2018, 31(1): 27-32
化学物理学报, 2018, 31(1): 27-32
http://dx.doi.org/10.1063/1674-0068/31/cjcp1706120

Article history

Received on: June 15, 2017
Accepted on: July 26, 2017
Imaging Isocyanic Acid Photodissociation at 193 nm: the NH(a1Δ)+ CO(X1Σ+) Channel
Zhi-guo Zhanga, Min Xina, Shu-tao Zhaoa, Yang Chenb     
Dated: Received on June 15, 2017; Accepted on July 26, 2017
a. School of Physics and Electronic Engineering, Fuyang Normal University, Fuyang 236041 China;
b. Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, 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; Min Xin, E-mail:xinmin_0916@163.com; Yang Chen, E-mail:yangchen@ustc.edu.cn
Abstract: The photodissociation dynamics of isocyanic acid (HNCO) has been studied by the timesliced velocity map ion imaging technique at 193 nm. The NH(aΔ) products were measured via (2+1) resonance enhanced multiphoton ionization. Images have been accumulated for the NH(aΔ) rotational states in the ground and vibrational excited state (v=0 and 1). The center-of-mass translational energy distribution derived from the NH(aΔ) images implies that the CO vibrational distributions are inverted for most of the measured NH(v|j) internal states. The anisotropic product angular distribution observed indicates a rapid dissociation process for the N-C bond cleavage. A bimodal rotational state distribution of CO(v) has been observed, this result implies that isocyanic acid dissociates in the S1 state in two different pathways.
Key words: Isocyanic acid    Photodissociation dynamics    Sliced velocity map ion imaging    
Ⅰ. INTRODUCTION

The photochemistry and thermochemistry of isocyanic acid (HNCO) is fundamentally as well as practically interesting. It plays a significant role in the rapid reduction of nitrogen oxides in the atmosphere [1, 2]. It can serve as a benchmark system for understanding multiple decomposition pathways in a four-atom molecule, such as internal conversion (IC), intersystem crossing (ISC), and direct dissociation.

Its first UV absorption band (180$-$280 nm) is broad and continuous with weak superimposed vibrational bands above 220 nm. It has been analyzed by Dixon and Kirby [3] and by Rabalais et al. [4, 5] as an S$_1$($^1$A$''$)$\leftarrow$S$_0$($^1$A$'$) transition. Photodissociation dynamics of HNCO have been revealed by a great deal of experimental and theoretical studies and are found to be exceptional complicated on account of at least three potential surfaces (S$_0$, S$_1$, and T$_1$) participating in the dissociation [6$-$29]. There are three different dissociation channels:

$ \begin{eqnarray} \textrm{HNCO}(\textrm{S}_1) \rightarrow &&\hspace{-0.25cm}\textrm{NH} (X^3\Sigma^-) + \textrm{CO}(X^1\Sigma^+) \\ && D_0=(30060\pm30) \hspace{0.15cm}\textrm{cm}^{-1} \end{eqnarray} $ (1)
$ \begin{eqnarray} \textrm{HNCO}(\textrm{S}_1) \rightarrow&&\hspace{-0.25cm}\textrm{H}(^2\textrm{S})+\textrm{NCO} (X^2\Pi) \\ &&D_0=(38370\pm30) \hspace{0.15cm}\textrm{cm}^{-1} \end{eqnarray} $ (2)
$ \begin{eqnarray} \textrm{HNCO}(\textrm{S}_1) \rightarrow&&\hspace{-0.25cm}\textrm{NH} (a^1\Delta) + \textrm{CO}(X^1\Sigma^+) \\ && D_0=(42750\pm30) \hspace{0.15cm}\textrm{cm}^{-1} \end{eqnarray} $ (3)

In the following paragraphs, NH(X$^3$$\Sigma^-$) and NH($a^1$$\Delta$) are denoted by $^3$NH and $^1$NH, respectively.

Channel (1) is a spin-forbidden dissociation pathway. It is observed from 280 nm to at least up to 217 nm. Zyrianov et al. [23] detected CO around 230 nm via ion imaging technique. Their results showed that the angular distribution of CO originating from channel (1) is essentially isotropic. The isotropic angular distributions indicate that the lifetime of the intermediate state exceeds 5 ps. They suggested that the dissociation of the channel follows the internal conversion (IC) from S$_1$ to S$_0$ and then ISC from S$_0$ to T$_1$. The exact quantum yield is indefinite, but channel (1) should still be the primary channel in the region just above the opening of channel (2).

Channel (2) is a spin-allowed dissociation pathway and has been investigated in recent years. Spiglanin et al. [10] detected NCO via laser induced fluorescence (LIF) technique. Zhang et al. [11] studied this channel at 193 nm using the high-Rydberg H atom time-of-flight method. They found that the NCO fragment is substantial bending excited and the anisotropy parameter $\beta$=$-$0.85. It indicated that the channel is a direct dissociation process on a repulsive surface. Zyrianov et al. [23] studied this channel in the photolysis wavelength range of 215$-$243 nm. Their results indicated that NCO was rotationally cold and the angular distribution was isotropic. Recently, Yu et al. [28] reinvestigated this channel in 200$-$240 nm photolysis by high resolution HRTOF technique. Their results indicated that at low photon energy excitation HNCO dissociates in the ground state S$_0$, following IC from S$_1$ to S$_0$, which is consistent with an indirect dissociation mechanism. While as the photon energy increases, a new pathway of direct dissociation through S$_1$ appears and is more and more important.

Channel (3) is also a spin-allowed dissociation pathway and has been studied at several different wavelengths. Fujimoto et al. [6] probed CO($v$) at 193.3 nm and levels up to $v$=4 were observed. Spiglanin and co-workers [7, 8] studied the internal state distributions of CO and rotational state distributions of $^1$NH at several wavelengths between 230 and 193 nm. They found that the $^1$NH rotational distribution is cold, but the CO distribution is hot. Wang et al. [27] investigated this channel at 210 nm via ion velocity slice imaging technique for CO product detection. Zyrianov et al. [15, 18, 23, 24] performed a series of experiments to study this channel in the wavelength region of 217$-$230 nm. This dissociation pathway is thought to evolve initially on S$_0$, but after exceeding a small barrier on S$_1$, estimated at 400$-$600 cm$^{-1}$, direct dissociation on this surface quickly dominates. Channel (3) becomes the major channel. Recently, we investigated channel (3) at 201 nm using sliced velocity map ion imaging technique. From the image of $^1$NH product, a bimodal rotational distribution has been observed in the vibrational ground state of CO.

In this work, we further study channel (3) of HNCO photodissociation dynamics at 193 nm by the sliced velocity map ion imaging the $^1$NH fragments. The center-of-mass total translational energy distributions and angular distributions were derived from the images. Experimental results show that both the vibrational ground state and the vibrational excited state of CO($v$) show a bimodal rotational distribution. This result further deepens our understanding of the photodissociation mechanism of HNCO.

Ⅱ. EXPERIMENTS

The photodissociation experiments were carried out on a time-sliced velocity map ion imaging apparatus described elsewhere [29, 30]. In brief, a skimmed molecular beam containing 2% HNCO seeded in He was produced by a pulsed valve (General valve series 9) with a 0.5 mm nozzle. At a distance of 22 mm downstream from the nozzle, the supersonic expanded beam was skimmed to form a well collimated molecular beam by a 1 mm diameter aperture skimmer. After passing through a 2 mm hole in the first electrode plate, the HNCO/He beam aligned along the time-of-flight axis was intersected by two counter-propagating (pump and probe) laser beams in the detection zone.

The pump laser was generated by an ArF excimer laser(EX50/250), the typical power of $\sim$0.3 mJ per pulse was focused by a spherical quartz lens with f=300 mm. The radiation could be polarized by an eight-plate stack of quartz slides placed at the Brewster angle, resulting in approximately 90% polarization. While the probe laser was produced by doubling the output of a tunable dye laser (Cobra-Stretch, Sirah), which was pumped by the third harmonic of a continuum Nd: YAG laser (Continuum PL8000), the typical power of $\sim$0.5 mJ per pulse was focused by a spherical quartz lens with $f$=200 mm.

The $^1$NH fragments were ionized by the $g^1\Delta$(3p$\pi)$ $\leftarrow\leftarrow$$a^1\Delta$(2+1) REMPI scheme $\sim$265 nm for $^1$NH($v$=0$|$$j$) and $\sim$267 nm for $^1$NH($v$=1$|$$j$) [31]. The polarization direction of the photolysis laser was set to be parallel to the detector face, while that of the probe laser was set to be perpendicular to the detector face.

The $^1$NH ions were accelerated by the focusing electric fields and projected onto a 40 mm diameter Chevron multi-channel plates (MCP) coupled to a P-47 phosphor screen (APD 3040FM, Burle Electro-Optics). A fast high-voltage switch (PVM-4210, DEI; typical duration$\approx$50 ns) was used to gate the gain of the MCP's for mass selection as well as the time slicing of the ion packets. The transient images shown on the phosphor screen were captured by a charge-coupled device (CCD) camera (ImagerPro2 M 640$\times$480 pixels, LaVision) and transferred to a computer on an every shot basis for event counting [32] and data analysis. Meanwhile, the total fluorescence from the phosphor screen was monitored by a photomultiplier tube (PMT) to optimize the experimental conditions. Timing of the pulsed valve, pump and probe lasers, the gate pulse applied to the MCP detector was controlled by two multichannel digital delay pulse generators (DG645, SRS).

Ⅲ. RESULTS AND DISCUSSION

A series of $^1$NH images were recorded by setting the probe wavelength at the resolved $^1$NH($v$=0 and 1$|$$j$) rotational states. FIG. 1 shows $^1$NH($v$=0$|$$j$) sliced images of HNCO photodissociation at 193 nm, obtained by accumulating the $^1$NH$^+$ signals over 2$\times$10$^4$laser shots. Well-resolved anisotropic rings were obtained and these structures are assigned to the rovibrational states of the partner CO product in the $^1$NH+CO binary dissociation process, channel (3). These results provide the correlated information of HNCO photodissociation at 193 nm.

FIG. 1 Raw sliced images of $^1$NH($v$=0$|j$) products after photodissociation of HNCO at 193 nm. The double arrow indicates the polarization direction of the photodissociation laser. (A) $j$=2, (B) $j$=5, (C) $j$=8, (D) $j$=10.

As seen in the images of FIG. 1, the relative intensities of the structures in each image change with the $^1$NH rotational states. With the rotational energy increasing, the number of rings gradually decreases, indicating clear pair-correlation between the $^1$NH($v$=0$|$$j$) and CO($v$$|j$) products.

The corresponding total kinetic energy distributions for the different $^1$NH($v$=0$|$$j$) products were derived from the images, results are depicted in FIG. 2. The structure in these distributions represents the internal energy distribution of CO formed in conjunction with the particular probed level of $^1$NH($v$=0$|$$j$). As seen in FIG. 2(a), the CO vibrational excitation distributions extend to $v$=4, this result is consistent with Fujimoto et al.'s [6]. Clearly, the CO vibrational distributions are inverted and peak at $v$=2 for most of the measured $^1$NH($v$=0$|$$j$) products. And the vibrational level of CO($v$) decreases as the $^1$NH($v$=0$|j$) rotational energy increases. Most of the vibrational ground state and the excited state of CO($v$) shown in FIG. 2 can be consistently described by the sum of two broad rotational distributions which peak at low-$j$ and high-$j$ values.

FIG. 2 The product total kinetic energy distributions (black empty circles) from FIG. 1 for $^1$NH($v$=0$|j$). The red lines are the fitting results and the dash lines are the individual CO rotational components. (a) $j$=2, (b) $j$=5, (c) $j$=8, (d) $j$=10.

A qualitative fitting of the energy distributions were carried out for extracting the correlated vibrational distribution of the CO product. The structure was fitted in total kinetic energy space using a Gaussian line shape. The CO vibrational states correlated with $^1$NH were fitted by a bimodal distribution which could account for the obvious two peaks in the distribution. The simulated results are also depicted in FIG. 2.

FIG. 3 shows $^1$NH($v$=1$|j$) sliced images after photodissociation of HNCO at 193 nm. Compared with $^1$NH($v$=0$|j$), the ring sizes of vibrational excited $^1$NH($v$=1$|j$) are smaller. The correlated CO vibrational state distribution shifts to lower vibrational state, as shown in FIG. 4. As can be seen, the vibrational level of CO($v$) decreases as the $^1$NH rotational energy increases. This indicates that the vibrational excitation of the correlated CO product is anti-correlated to the $^1$NH rotational excitation.

FIG. 3 Raw sliced images of $^1$NH($v$=1$|j$) products after photodissociation of HNCO at 193 nm. The double arrow indicates the polarization direction of the photodissociation laser. (A) $j$=2, (B) $j$=5, (C) $j$=7, (D) $j$=9.
FIG. 4 The product total kinetic energy distributions (black empty circles) from FIG. 3 for $^1$NH($v$=1$|j$). The red lines are the fitting results and the dash lines are the individual CO rotational components. (a) $j$=2, (b) $j$=5, (c) $j$=7, (d) $j$=9.

By integrating the images over the relevant radius region, angular distribution of the $^1$NH($v$$|$$j$)+CO($v$) channel was obtained for the $^1$NH($v|j$) rotational states. FIG. 5(a) shows the corresponding product anisotropy parameters determined by fitting the angular distributions. It is easy to see that anisotropy parameters of the $^1$NH($v$=0$|j$)+CO($v$) are negative. The angular distribution becomes slightly less anisotropic as the $^1$NH($v$=0$|j$) rotational energy increases. The $^1$NH($v$=0$|j$)+CO($v$=0 and 4) anisotropy parameters become as low as $-$0.3, however, this low value should be due to the weak signal compared with the roughly isotropic background.

FIG. 5 (a) Anisotropy parameter for individual CO vibrational state correlated to the $^1$NH($v$=0$|j$). (b) Anisotropy parameter for individual CO vibrational state correlated to the $^1$NH($v$=1$|j$).

As seen in FIG. 4 only $v$=0, 1 and 2 of CO can correspond with vibrational excited $^1$NH($v$=1$|j$). The $^1$NH($v$=1$|j$) products also display anisotropic angular distribution (FIG. 5(b)). With $j$ increasing, the angular distribution also becomes slightly less anisotropic.

The anisotropic product angular distribution (FIG. 5) suggests a perpendicular transition and a fast dissociation process which most likely occurs on a repulsive surface, which is consistent with the results of Reisler et al.'s [23]. HNCO has nearly a linear geometry in the S$_0$ state. When HNCO is excited from the ground state to the S$_1$ state, the N$-$C$-$O angle is strongly bent. At 193 nm excitation energy the barriers to all the channels are exceeded. In this condition, since excited HNCO on the S$_1$ surface has a strong repulsive gradient along the CN bond, preference should be given to the rapid dissociation on S$_1$. However, the anisotropy parameter $\beta$ becomes considerably smaller as $^1$NH rotational level increases. This phenomenon has been also seen previously in HNCO photodissociation at 210 and 201 nm [27, 29], which could be explained by a non-axial recoil impact model [33].

As shown in FIG. 2 and 4, both the vibrational ground state and the excited state of CO($v$) show a bimodal rotational distribution. And the CO low-$j$ component and the CO high-$j$ component both change as $^1$NH $j$ increases in both the $^1$NH($v$=0$|j$) and $^1$NH($v$=1$|j$) channels. By integrating the images over the relevant radius region, angular distributions of the corresponding CO low-$j$ and high-$j$ components were obtained for the $^1$NH($v|j$) rotational state. The anisotropy parameters are depicted in Table Ⅰ. Clearly, all the CO products display anisotropic angular distributions, and the values for the low-$j$ and high-$j$ components are only slightly different. This result is similar to our previous results [29]. It implies that both low-$j$ and high-$j$ components are from HNCO rapid dissociation via a repulsive surface on S$_1$. Recently, by full-dimensional theoretical calculation, Bonnet et al. [34] reproduced the bimodal rotational state distribution of CO. Our experimental results are further verified by theoretical calculation. Moreover, Bonnet et al. provided new insight into the dissociation mechanism. The observed bimodal rotational distribution of CO should be the consequence of an impulsive-deflective mechanism due to two repulsive walls of the 6D PES, the first one producing rotationally hot CO products and the second one cooling part of them [34].

Table Ⅰ Anisotropy parameters of the CO high-$j$ and low-$j$ components corresponding $^1$NH($v|j$).
Ⅳ. CONCLUSION

In this report, using the sliced velocity map ion imaging technique, the photodissociation dynamics of HNCO at 193 nm for NH($a^1$$\Delta$)+CO($X^1$$\Sigma^+$) channel has been studied. The NH($a^1$$\Delta$) images have been measured. The state-resolved imaging results show anisotropic product angular distribution and correlation between the NH($a^1$$\Delta$) and CO rovibrational state distributions. The vibrational excitation of the correlated CO product is anti-correlated to the $^1$NH rotational excitation. The anisotropic product angular distribution suggests that the NH($a^1$$\Delta$)+CO($X^1$$\Sigma^+$) dissociation channel is a fast dissociation process. A bimodal rotational distribution of CO has been observed from the $^1$NH images. It should be the consequence of an impulsive-deflective mechanism. This experimental result further deepens our understanding of the HNCO photodissociation dynamics.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.21573227, NO.11604052), the National Science Foundation of Anhui Province of China (No.1608085QA19), the Natural Science Research Project of Education Department of Anhui Province of China (No.2014KJ020), the Open Foundation of State Key Laboratory (No.SKLMRD-K201503, No.SKLMRD-K201611, and No.SKLMRD-K201711), and the Doctoral Foundation of Fuyang Normal University (No.FSB201501005).

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异氰酸分子NH(a1Δ)+CO(X1Σ+)通道在193nm的离子成像光解动力学研究
张志国a, 辛敏a, 赵书涛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: 利用时间切片离子速度成像技术在193 nm研究了异氰酸分子(HNCO)的光解动力学. NH(aΔ)产物通过共振增强多光子电离的方法进行了探测.对NH(aΔ)产物的振动基态和激发态(v=0和1)进行了影像探测.从NH(aΔ)的影像可以发现对于大部分的NH(v|j)产物,CO的振动分布是反转的.各向异性的角分布表明N-C键的断裂是个快速的解离过程. CO(v)的转动分布展现了明显的双峰现象,这一双峰现象应为HNCO经S1电子态的两种解离途径所致.
关键词: 异氰酸    光解动力学    切片离子速度成像