Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (4): 563-567

The article information

Min Chen, Hao Liang, Chao He, Dong-feng Zhao, Yang Chen
陈旻, 梁昊, 何超, 赵东锋, 陈旸
Ultraviolet Photodissociation Dynamics of m-Bromofluorobenzene at around 240 nm
间氟溴苯分子240 nm紫外光解动力学
Chinese Journal of Chemical Physics, 2018, 31(4): 563-567
化学物理学报, 2018, 31(4): 563-567
http://dx.doi.org/10.1063/1674-0068/31/cjcp1806136

Article history

Received on: June 8, 2018
Accepted on: July 26, 2018
Ultraviolet Photodissociation Dynamics of m-Bromofluorobenzene at around 240 nm
Min Chen, Hao Liang, Chao He, Dong-feng Zhao, Yang Chen     
Dated: Received on June 8, 2018; Accepted on July 26, 2018
CAS Center for Excellence in Quantum Information and Quantum Physics, 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: Dong-feng Zhao studied physical chemistry at the Department of Chemical Physics, University of Science and Technology of China (USTC) from 2000 to 2009. He received his B.S. degree in Chemistry in 2004 and his Ph.D. degree in Physical Chemistry in 2009 for laser spectroscopic experiments of small radicals, under the supervision of Y. Chen. Shortly thereafter, he joined W. Ubachs group at the VU University Amsterdam (2009-2012) and H. Linnartz group at the University of Leiden (2012-2015) in the Netherlands, where he has worked on the combined laboratory and observational spectroscopic study of interstellar molecules. After that, he was appointed as a senior researcher at the Hefei National Laboratory for Physical Sciences at the Microscale, USTC. His main research interest currently lies in the high resolution laser spectroscopy and reaction dynamics of transient gas-phase radicals and ions with relevance to cold interstellar clouds.
*Author to whom correspondence should be addressed. Dong-feng Zhao, E-mail:dzhao@ustc.edu.cn; Yang Chen, E-mail:yangchen@ustc.edu.cn
Part of the special issue for celebration of "the 60th Anniversary of University of Science and Technology of China and the 30th Anniversary of Chinese Journal of Chemical Physics"
Abstract: The photodissociation dynamics of m-bromofluorobenzene has been experimentally investigated at around 240 nm using the DC-slice velocity map imaging technique. The kinetic energy release spectra and the recoiling angular distributions of fragmented Br(2P3/2) and Br(2P1/2) atoms from photodissociation of m-bromofluorobenzene have been measured at different photolysis wavelengths around 240 nm. The experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying 1ππ* excited states dominate the production process of the ground state Br(2P3/2) atoms. Because of the weak spin-orbit coupling effect among the low-lying triplet and singlet states, the spin-orbit excited Br(2P1/2) atoms are mainly produced via singlet-triplet state coupling in the dissociation step. The similarity between the present results and that recently reported for o-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.
Key words: Photodissociation dynamics    Kinetic energy release spectra    Spin-orbit coupling effect    
Ⅰ. INTRODUCTION

The ultraviolet (UV) photodissociation dynamics of organobromides has long been topic in gas-phase photochemistry because of the significance of such chemical processes in atmospheric chemistry and in fundamental studies of intramolecular processes [1-6]. In recent years, with the fast development of the experimental techniques in detecting and characterizing photofragmentation products, many studies on the UV photodissociation dynamics of bromobenzenes have been performed [7-12]. Using the femtosecond laser spectroscopy, Borg et al. investigated the photodissociation processes of a series of fluorinated bromobenzenes at $\sim$266 nm, from which the substitution effect of fluorine in the photodissociation dynamics was suggested [7-9]. Later, Tang et al. revisited the 266 nm photodissociation dynamics of the three mono-fluorinated bromobenzenes ($o$-, $p$-, and $m$-bromofluorobenzene) using the velocity map imaging (VMI) technique, and summarized two dissociation pathways to the photofragmented bromine atoms at the photolysis wavelength of 266 nm [10, 11, 12]. On the theoretical side, several ab initio studies on the potential energy surfaces (PESs) of bromofluorobenzenes have also been reported [7, 13, 14]. These calculated PESs provide a theoretical support in understanding earlier experimental results.

Very recently, we have studied the photodissociation dynamics of $o$-bromofluorobenzene (2-BrFPh) in the whole wavelength range of its first UV absorption system ($^1$$\rm{L}_b$) using the DC-slice VMI technique [15]. In this study, we find that the production of Br atoms from UV photodissociation of 2-BrFPh is dominated by (pre-)dissociation of one or more singlet $^1$$\pi$$\pi$$^*$ excited states, and it involves weak spin-orbit couplings among the low-lying electronic states. The latter leads a small quantum yield ($\sim$0.02) of spin-orbit excited Br($^2$$\rm{P}_{1/2}$). Meanwhile, a new dissociation channel, which was not observed in the 266 nm photodissociation [11], is also found at wavelengths shorter than 247 nm. This channel is concluded to a nearly barrierless dissociation pathway that is formed by an avoided crossing between the 2$^1$$\pi$$\pi$$^*$ and 1$^1$n$\sigma$$^*$ states. These new findings have provided a new understanding on the UV photochemistry of aryl bromides, specifically the way they differ from aryl iodides. In this contribution, we extend our experimental study to the $m$-bromofluorobenzene (3-BrFPh) molecule, aiming at a more in-depth understanding in how the position of the fluorine substituent affects the photodissociation dynamics of bromofluorobenzenes upon photoexcitation of its $^1$$\rm{L}_b$ absorption system.

Ⅱ. EXPERIMENT DETAILS

Experiments are performed using our home-made DC-slice velocity map imaging setup, details of which can be found in our recent publications [15-17]. Briefly, a gas mixture of 1% 3-BrFPh vapor seeded in argon at a stagnation pressure of $\sim$3 bar is supersonically expanded into the first vacuum chamber through a pulsed nozzle (General Valve, Series 9, 0.25 mm orifice) and then skimmed by a 1 mm skimmer before entering the second differentially pumped chamber. During the experiment, the two vacuum chambers, namely, the source and detection chambers, are maintained at $\sim$$10^{-6}$ and $10^{-7}$ mbar, respectively, by using two turbomolecular pump stations. The skimmed molecular beam is intersectedr perpendicularly by two linearly polarized laser beams in the detection chamber. The frequency-doubled outputs of two sets of Nd:YAG laser pumped dye lasers (PrecisionScan, Sirah) are used as pulsed UV laser sources in this work. The jet-cooled 3-BrFPh molecules in the molecular beam are photolyzed by the first UV laser, and the photofragmented Br atoms are immediately ionized by the second UV laser beam or by the same UV laser beam via a (2$+$1) resonance-enhanced multiphoton ionization (REMPI) scheme. In two laser measurements, the photolysis laser and probe laser are set at $\sim$1.0 and $\sim$0.1 mJ/pulse, respectively, and are focused onto the molecular beam by a plano-convex lens ($f$$=$250 mm). Using a set of ion optics designed for VMI measurements, the produced $\rm{Br}^+$ ions are accelerated and get projected onto a Chevron-type dual multichannel plate (MCP, 40 mm) detector that is coupled to a phosphor screen (P47). To realize the DC slicing of the ion packet in VMI experiments, a fast high-voltage pulse (DEI, PVM-4210, pulse amplitude of 850 V and pulse duration of 80 ns) is applied to the front MCP to gate the gain of detection system. The raw images on the phosphor screen are captured by a charge-coupled device (CCD) camera (LaVision, Imager Intense, 1276$\times$1024 pixels), and are transferred to a computer on every shot for accumulation. The whole experiment runs at a repetition rate of 10 Hz, and two multichannel digital delay pulse generators (SRS, DG 535) are used to control the time sequence of the pulsed nozzle, the two pulsed laser, and the pulsed gating.

Ⅲ. RESULTS AND ANALYSIS

We have measured DC sliced images of both ground-state Br($^2$$\rm{P}_{3/2}$) and spin-orbit excited Br($^2$$\rm{P}_{1/2}$) atoms (hereafter referred as Br and $\rm{Br}^*$, respectively) at different selected wavelengths in the 240 nm region. FIG. 1 shows the experimental images recorded in the present study. Each image displays a two-dimensional recoiling speed and angular distributions of photofragmented Br/$\rm{Br}^*$ atoms, the fine structure in each illustrates different production pathways of Br/$\rm{Br}^*$ atoms. The results for 3-BrFPh presented in FIG. 1 are measured for the first time.

FIG. 1 Experimental images of Br($^2$$\rm{P}_{3/2}$) (upper panel) and Br($^2$$\rm{P}_{1/2}$) (lower panel) atoms following photodissociation of 3-BrFPh at different photolysis wavelengths.

As shown in FIG. 1, experimental images for Br and $\rm{Br}^*$ are very similar. All the recorded images show a component ($\rm{C}_1$$^*$) near the center of the image and two well recognized ring components ($\rm{C}_2$$^*$ and $\rm{C}_3$$^*$). It is noticed that the $\rm{C}_1$$^*$ and $\rm{C}_2$$^*$ components have also been observed in the 266 nm photodissociation [11] of 3-BrFPh, while the $\rm{C}_3$$^*$ components are newly observed at photolysis wavelengths around 240 nm. This indicates the presence of a new dissociation pathway at shorter wavelengths.

Total kinetic energy release (TKER, $E_\rm{T}$) spectra have been obtained for both Br and $\rm{Br}^*$ by integrating the experimental images over the whole angular range, and are given in upper and lower panels of FIG. 2, respectively. Least squares fits of individual TKER spectra in terms of three overlapping Gaussian functions are performed to determine the $E_\rm{T}$ distributions. Since the three Gaussian components used in the fits are correlated with the fine structures of the experimental images, they are also denoted by $\rm{C}_{1/2/3}$$^*$, i.e., the same as in FIG. 1. From the fits, we have determined the $E_\rm{T}$ peak positions and relative intensities of the three components for both Br and $\rm{Br}^*$ production channels. The results are summarized in Table Ⅰ and Table Ⅱ for Br and $\rm{Br}^*$, respectively.

FIG. 2 TKER spectra of Br($^2$$\rm{P}_{3/2}$) (upper) and Br($^2$$\rm{P}_{1/2}$) (lower) in the photodissociation of 3-BrFPh at different photolysis wavelengths. The sum spectrum and individual component from the fits are also shown by solid and dashed lines, respectively.
Table Ⅰ Experimentally determined parameters for the photofragmented Br($^2$$\rm{P}_{3/2}$) atoms. (RI: relative intensity.)
Table Ⅱ Experimentally determined parameters for the photofragmented Br($^2$$\rm{P}_{1/2}$) atoms. (RI: relative intensity.)

Angular distributions of the fragmented Br and $\rm{Br}^*$ atoms, $P$($\theta$), where $\theta$ is the angle between the laser polarization and the recoil velocity of the photofragments, have been extracted from the experimental images by integrating a velocity distribution over an appropriate range of speed at each angle for the $\rm{C}_2$$^*$ and $\rm{C}_3$$^*$ features. The raw data for $\rm{C}_1$ and $\rm{C}_1$$^*$ components are not processed here, as they may contain contributions from multiphoton processes. Using the angular distributions shown in FIG. 3 and FIG. 4, the corresponding values of the anisotropy parameter $\beta$ are determined from a least square fit by:

$ \begin{eqnarray*} P(\theta)\propto1+\beta{{P}_2}(\cos\theta) \end{eqnarray*} $
FIG. 3 Angular distributions of Br($^2$$\rm{P}_{3/2}$) in the photodissociation of 3-BrFPh at three photolysis wavelengths. The left and right panels are for the $\rm{C}_2$ and $\rm{C}_3$ components, respectively. The solid lines indicate the least squares fits to determine the anisotropic parameter $\beta$.
FIG. 4 Angular distributions of Br($^2$$\rm{P}_{1/2}$) in the photodissociation of 3-BrFPh at three photolysis wavelengths. The left and right panels are for the $\rm{C}_2$$^*$ and $\rm{C}_3$$^*$ components, respectively. The solid lines indicate the least squares fits to determine the anisotropic parameter $\beta$.

where the term $P_2$(cos$\theta$) is the second-order Legendre polynomial [18]. For a fast dissociation process, the parameter $\beta$ has a physical range between $-$1 and $+$2, with the minimum limiting value of $-$1 corresponding to excitation via a pendidencular transition, and the maximum limiting value of $+$2 corresponding to excitation via a pure parallel transition. The resulting $\beta$ values have also been summarized in Table Ⅰ and Table Ⅱ for Br and $\rm{Br}^*$, respectively.

Ⅳ. DISCUSSION

Our recent work [15] on the photodissociation dynamics of 2-BrFPh shows that the quantum yield of the spin-orbit excited $\rm{Br}^*$ is only $\sim$0.02. Due to the UV absorption spectrum of 2-BrFPh [7, 12, 19] and the general picture proposed for iodobenzenes by Ashfold and co-workers [20, 21], this has led to the conclusion of a very weak spin-orbit coupling effect among the low-lying singlet and triplet states. Consequently, the UV photodissociation dynamics of 2-BrFPh mainly undergoes within the singlet electronic state manifold. In the present case of 3-BrFPh, the 266 nm results by Tang et al. have shown that the $\rm{Br}^*$ yield ($\sim$0.02) is also very small [11]. Furthermore, it can be found from Refs.[7, 12, 19] that the UV absorption spectra of 2-BrFPh and 3-BrFPh are very similar to each other. Specifically, the UV absorption spectrum shows that 3-BrFPh in the 280$-$300 nm region has nearly no absorption. Because the photoexcitation energy in the 280$-$300 nm range is significantly lower than the lowest-lying singlet excited state (1$^1$$\pi$$\pi$$^*$), any measurable absorption should arise from excitations to lower lying triplet states with appreciable transition strength given by spin-orbit coupling. We estimate from the UV absorption spectrum that the probability of photoexcitation to low-lying triplet states of 3-BrFPh is likely three orders of magnitude weaker than that of singlet low-lying states. Therefore, although the $\rm{Br}^*$ yield is not measured in the present experiments, the spin-orbit coupling effect in the low-lying excited states of 3-BrFPh is considered to be very weak, and the production mechanisms of ground-state Br atoms from UV photodissociation of 3-BrFPh can be discussed mainly within the singlet electronic state manifold. In FIG. 5, a simplified PES diagram with inclusion of the three lowest-lying singlet excited states, i.e., 2$^1$A$'$, 3$^1$A$'$, and 1$^1$A$''$, is reconstructed to understand the photodissociation mechanisms of 3-BrFPh at around 240 nm. Here, the PES curves are drawn based on the previously reported ab initio calculations [7, 13, 14]. In this diagram, the three states are the only singlet excited states that may be accessible by one UV photon in the studied wavelength range, where 2$^1$A$'$ (1$^1$$\pi$$\pi$$^*$) and 3$^1$A$'$ (2$^1$$\pi$$\pi$$^*$) correspond to the first and second $\pi$-$\pi$$^*$ excitation statess, respectively, and the repulsive 1$^1$A$''$ (1$^1$$\pi$$\sigma$$^*$) state corresponds to a $\pi$-$\sigma$$^*$ excitation.

FIG. 5 A schematic diagram of the relevant potential energy surfaces of 3-BrFPh (adapted from Refs.[7, 14]). See the main text for details.

For the photofragmented Br atoms, the $\rm{C}_2$ component is also observed in the 266 nm photodissociation. The anisotropy parameter $\beta$($\rm{C}_2$) is found to be $+$0.6 in the present experiment, very similar to the value of $\sim$$+$0.65 at 266 nm [11]. No significant wavelength dependence of the parameter $\beta$($\rm{C}_2$) can be found in the 234$-$267 nm range. The much smaller value than the limiting value ($\sim$$+$2 for a parallel transition) indicates that the corresponding pathway is a relatively slow predissociation process. Therefore, the $\rm{C}_2$ component corresponds to a predissociation channel from the bound 2$^1$A$'$ (1$^1$$\pi$$\pi$$^*$) state to a dissociative 1$^1$A$''$ (1$^1$$\pi$$\sigma$$^*$) state. This conclusion is also in good agreement with the previously reported predissociation time constant of 5.6 ps [7, 9].

The $\rm{C}_3$ component of the photofragmented Br is a new production channel observed in the 240 nm region. The determined values of the anisotropy parameter $\beta$($\rm{C}_3$) show a clear wavelength dependence (increasing from $+$0.9 at 243 nm to $+$1.4 at 234 nm). Meanwhile, the relative intensity of the $\rm{C}_3$ component in the TKER spectra also significantly increases with increasing the photoexcitation energy. These results are indicative for a relatively fast dissociation process originating from a parallel type photoexcitation. Based on the $\beta$($\rm{C}_3$) values, this dissociation channel may be fully open with a photolysis wavelength at $\sim$237 nm (5.20 eV). We attribute the $\rm{C}_3$ component to a dissociation pathway on the singlet 3$^1$A$'$ state surface (FIG. 5). To confirm this point, we performed additional time-dependent density functional theory (TD-DFT) calculations at the level of B3LYP/6-311G$^{**}$ to test relevant transition dipole moment. The calculations show that the 3$^1$A$'$ (2$^1$$\pi$$\pi$$^*$) state transition dipole moment is approximately parallel to the C$-$Br bond in the molecular plane, well consistent with our observed $\beta$($\rm{C}_3$) values. In Refs.[7, 14], the ab initio PES of the 3$^1$A$'$ state is found to be formed by an avoided crossing between the 2$^1$$\pi$$\pi$$^*$ and 1$^1$n$\sigma$$^*$ states, having a barrierless dissociation pathway. Such a PES well explains the experimentally observed wavelength dependence, namely, a higher photoexcitation energy leads to a faster dissociation process. Although the lowest photoexcitation energy ($\sim$5.1 eV) studied in this work is slightly lower than the vertical excitation energy ($\sim$5.3 eV) of the 3$^1$A$'$ state, on one side, the small energy discrepancy is reasonably within the accuracy of the theoretically calculated PES; on the other side, the possible involvement of additional vibrational mode [9] may further lower down the vertical excitation energy. All these arguments support our conclusion of the $\rm{C}_3$ component arising from dissociation on the singlet 3$^1$A$'$ state surface.

It is noticed from the present results that the wavelength dependent behavior of the photofragmented $\rm{Br}^*$, for both TKER spectra and the anisotropy parameters $\beta$($\rm{C}_2$$^*$, $\rm{C}_3$$^*$), is very similar to that of the photofragmented Br. As discussed above, the spin-orbit coupling effect in the photoexcitation step is not sufficient to explain the $\rm{Br}^*$ yield ($\sim$0.019) at 266 nm. Thus, the production channels of $\rm{Br}^*$ are likely due to weak spin-orbit couplings in the dissociation step, as schematically illustrated by a dashed line in FIG. 5. Such a mechanism also explains the similar wavelength dependent behaviors between Br and $\rm{Br}^*$ that have been observed in our experiment.

Based on the discussions above, it is found that the production mechanisms of both Br and $\rm{Br}^*$ from UV photochemistry of 3-BrFPh are very similar to those we recently reported for 2-BrFPh [15]. This indicates that the UV photodissociation dynamics of mono-fluorinated bromobenzenes is nearly independent of the (ortho or meta) position of the fluorine substituent on the benzene ring. The only difference we can find is in the results of $\beta$($\rm{C}_2$) and $\beta$($\rm{C}_2$$^*$) at $\sim$234 nm, which values are $\sim$$+$0.66 for 3-BrFPh and $\sim$$+$0.88 for 2-BrFPh. The smaller $\beta$ values for 3-BrFPh indicate that the 1$^1$$\pi$$\pi$$^*$ state predissociation process is still relatively slow even for a photoexcitation energy of $\sim$5.30 eV. In other words, the coupling between the bound 2$^1$A$'$ (1$^1$$\pi$$\pi$$^*$) and the dissociative 1$^1$A$''$ (1$\pi$$\sigma$$^*$) states in 3-BrFPh is slightly weaker than in 2-BrFPh. However, this minor difference does not change our understanding in the overall UV photochemistry of bromofluorobenzenes, i.e., by a simplified model of the spin-conserving (pre-) dissociation with one or more singlet $^1$$\pi$$\pi$$^*$ excited states.

Ⅴ. CONCLUSION

The kinetic energy release spectra and the recoiling angular distributions of fragmented Br($^2$$\rm{P}_{3/2}$) and Br($^2$$\rm{P}_{1/2}$) atoms from photodissociation of 3-BrFPh at around 240 nm have been experimentally measured using the DC-slice velocity map imaging technique. The present experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying $^1$$\pi$$\pi$$^*$ excited states dominate the production process of the ground state Br($^2$$\rm{P}_{3/2}$) atoms. For the spin-orbit excited Br($^2$$\rm{P}_{1/2}$) products, because of the weak spin-orbit couplings among the low-lying triplet and singlet states, the singlet-triplet state couplings in the dissociation step play an important role. The similarity between the present results and that recently reported for $o$-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.

Ⅵ. ACKNOWLEDGMENTS

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

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间氟溴苯分子240 nm紫外光解动力学
陈旻, 梁昊, 何超, 赵东锋, 陈旸     
中国科学技术大学化学物理系, 中国科学院量子信息与量子科技创新研究院, 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研究了间氟溴苯分子在240 nm附近的紫外波段的光解动力学.实验上在三个不同光解波长处测量了光解产生的Br(2P3/2)和Br(2P1/2)原子的平动能释放谱和反冲速度的角分布.实验结果表明,通过两个最低1ππ*激发态的预解离或直接解离生成Br(2P3/2)原子的机理主导着间氟溴苯分子的紫外光化学;由于分子中较弱的自旋轨道耦合作用,激发态的Br(2P1/2)原子主要通过单重激发态在分子解离过程中与三重激发态的耦合产生.将当前的实验结果与我们最近报导的邻氟溴苯分子的结果对比可以发现,氟原子取代位的变化不会显著影响氟溴苯分子的紫外光解动力学机理.
关键词: 光解动力学    平动能释放谱    自旋轨道耦合