Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (4): 406-410

#### The article information

Rui Mao, Hong Xiao, Yu Hu, Qun Zhang, Yang Chen

Photodissociation Dynamics of Dichlorodifluoromethane (CF2Cl2) around 235 nm using Time-Sliced Velocity Map Imaging Technology

Chinese Journal of Chemical Physics, 2019, 32(4): 406-410

http://dx.doi.org/10.1063/1674-0068/cjcp1812283

### Article history

Accepted on: April 28, 2019
Photodissociation Dynamics of Dichlorodifluoromethane (CF2Cl2) around 235 nm using Time-Sliced Velocity Map Imaging Technology
Rui Maoa , Hong Xiaoa , Yu Hua , Qun Zhangb , Yang Chenb
Dated: Received on December 19, 2018; Accepted on April 28, 2019
a. School of Mathematics and Physics and Chemical Engineering, Changzhou Institute of Technology, Changzhou 213032, 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
Abstract: Photodissociation dynamics of dichlorodifluoromethane (CF$_2$Cl$_2$) around 235 nm has been studied using the time-sliced velocity map imaging technology in combination with the resonance enhanced multi-photon ionization technology. By measuring the raw images of chlorine atoms which are formed via one-photon dissociation of CF$_2$Cl$_2$, the speed and angular distributions can be directly obtained. The speed distribution of excited-state chlorine atoms consists of high translation energy ($E_\rm{T}$) and low $E_\rm{T}$ components, which are related to direct dissociation on $^3$Q$_0$ state and predissociation on the ground state induced by internal conversion, respectively. The speed distribution of ground-state chlorine atoms also consists of high $E_\rm{T}$ and low $E_\rm{T}$ components which are related to predissociation between $^3$Q$_0$ and $^1$Q$_1$ states and predissociation on the ground state induced by internal conversion, respectively. Radical dissociation channel is confirmed, nevertheless, secondary dissociation and three-body dissociation channels are excluded.
Ⅰ. INTRODUCTION

In recent decades, photodissociation dynamics of chlorofluorocarbons has attracted quite a few scientists' interests in experimental and theoretical research because it is related to protection of the ozone layer [1, 2]. The photodissociation dynamics of simple chlorofluorocarbons, CF$_2$Cl$_2$, was studied by several groups during the past years [3-8]. The first report on the photodissociation dynamics of CF$_2$Cl$_2$ at 157 and 193 nm was brought by Bersohn and co-workers [3] and they found that the angular distributions of nascent excited-state chlorine atoms (Cl$^*$) and ground-state chlorine atoms (Cl) are isotropic. Apart from that, White and co-workers [4] suggested that the Cl$^*$ and Cl products were mostly formed via the following dissociation channel on the photodissociation dynamics of CF$_2$Cl$_2$ at 187, 125, and 118 nm:

 $\begin{eqnarray} {\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_{\rm{2}}} + h\nu \to {\rm{C}}{{\rm{F}}_{\rm{2}}}+{\rm{Cl/Cl}^* + \rm{Cl/Cl}^*} \end{eqnarray}$ (1)

However, Baum and Huber [5] investigated the photodissociation dynamics of CF$_2$Cl$_2$ at 193 nm and proposed that the Cl$^*$ and Cl products were formed via the following dissociation channel:

 $\begin{eqnarray} {\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_{\rm{2}}} + h\nu \to {\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{Cl + Cl/Cl^*}} \end{eqnarray}$ (2)

Kawasaki and co-workers [6] used the velocity map imaging (VMI) technology to study the photodissociation dynamics of CF$_2$Cl$_2$ in spectral range of 205–209 nm, and reported that the rupture of C–Cl bond was related to parallel transitions. Takahashi and co-workers [7] also supported dissociation channel (2) by measuring the quantum yields for Cl$^*$ and Cl products at 193 nm. Poterya and co-workers [8] investigated the photodissociation dynamics of CF$_2$Cl$_2$ in argon clusters by means of VMI in order to explore the dissociation process for the slow Cl$^*$ and Cl products.

Above all, various studies have been carried out on the photodissociation dynamics of CF$_2$Cl$_2$, and the suggested dissociation mechanisms are varied, however, the photodissociation dynamics around 235 nm remains unexplored. Here, we present a study on CF$_2$Cl$_2$ by means of time-sliced VMI [9-14], in hope to shed some light on its mechanism. The Cl and Cl$^*$ atoms are probed using a (2$+$1) resonance enhanced multi-photon ionization (REMPI) scheme, directly acquiring the corresponding speed and angular distributions. Three independent dissociation channels are proposed in the Cl and Cl$^*$ formation, respectively.

Ⅱ. EXPERIMENTS

The experimental setup has been described in previous studies [15, 16]. The whole setup consists of source chamber, ionization chamber, and detection chamber. The source chamber and ionization chamber are connected with the skimmer to form vacuum differential pumping system. The CF$_2$Cl$_2$ gases were mixed with helium ($\sim$95%) at backing pressure of $\sim$2 atm and expanded through a pulsed nozzle (General Valve Series 9, 0.5 mm orifice). The molecular beam passed through the skimmer into the ion optics which are installed in the ionization chamber, and was intersected at right angles by the dissociation photons in the ion optics. The vertically polarized dissociation laser radiation around 235 nm was derived from the output of an Nd:YAG (GCR-170, Spectra Physics) pumped by a frequency doubled dye laser (PrecisionScan, Sirah) and was focused by an $f $$= 210 mm lens. Typical ultraviolet pulse energies were in the range of 150 - 200 μJ. The pulsed dissociation laser was intersected by the rising edge of pulsed molecular beam to prevent the formation of clusters. The nascent Cl ^* and Cl atoms were probed at 235.20 and 235.34 nm via the 4{\rm{p}}\left( {{}^2{{\rm{P}}_{{\rm{1}}/{\rm{2}}}}} \right)$$ {\rm{3p}}\left( {{}^2{{\rm{P}}_{1/2}}} \right)$ and $4{\rm{p}}\left( {{}^2{{\rm{D}}_{3/2}}} \right) $$3{\rm{p}}\left( {{}^2{{\rm{P}}_{3/2}}} \right) (2 + 1) REMPI schemes. The ions were accelerated in the ion optics and flew through a field-free region. The high-speed ion packet hitted a 40-mm-diameter Chevron-type dual MCP's coupled to a P-47 phosphor screen (APD 3040FM, Burle Electro-Optics). The slice images which were obtained by using a narrow time gate of 60 ns were captured by a charge-coupled device (CCD) camera (Imager Compact QE 1376 \times 1039 pixels, LaVision) and transferred to a computer. The accumulated images were acquired by using an every shot basis for event counting [17]. The electronic signals from the MCP output were amplified with a preamplifier (SR 245, SRS), and then collected by an oscilloscope (TEK 3052B). Time-of-flight spectrum was transferred to a computer by using a General-Purpose Interface Bus (GPIB) interface card (LPCI-3488A) and a Labview program. The raw images were accumulated over 30000 shots or more. The wavelengths, calibrated by a wavemeter, were scanned over a range of 4 cm ^{-1} to cover all the speed components of the nascent Cl ^* and Cl atoms. Ⅲ. RESULTS The slice images of chlorine atoms are shown in FIG. 1. For Cl ^* atom (FIG. 1(a)), the image shows outer anisotropic distribution and inner isotropic distribution. Whereas, for Cl atom (FIG. 1(b)), the image shows two isotropic distributions. All these features can be coincident with the fact that the speed distributions of Cl ^* and Cl products can both be fitted with two Gaussian components (FIG. 2). The total translational energy, E_\rm{T} , can be calculated with the following equation on the assumption that chlorine atoms are formed via dissociation channel (2):  FIG. 1 Slice images of (a) Cl ^* and (b) Cl atoms for the photodissociation of CF _2 Cl _2 measured at 235.20 and 235.34 nm, respectively  FIG. 2 Speed distributions of (a) Cl ^* and (b) Cl atoms on the photodissociation of CF _2 Cl _2 around 235 nm. Black circles: raw experimental data, blue curves: Gaussian fit to the high and low E_\rm{T} components, red curve: sums of the fitting distributions  \begin{eqnarray} {E_{\rm{T}}} = \frac{{{m_{{\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_2}}}}}{{{m_{{\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{Cl}}}}}} \times \frac{1}{2}{m_{\rm{Cl}}{v_\rm{Cl}}}^2 \end{eqnarray} (3) where m_{{\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_2}} is the mass of parent molecule CF _2 Cl _2 , m_{{\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}}} is the mass of CF _2 Cl radical, m_\rm{Cl} is the mass of chlorine atoms, and v_\rm{Cl} is the speed of chlorine atoms. The internal energy of CF _2 Cl radical, E_\rm{int} , can be calculated with the following equations:  \begin{eqnarray} &&{E_{{\rm{int}}}}\left( {\rm{C}{\rm{F}_2}{\rm{Cl}}} \right) = h\nu - {D_0} - {E_{\rm{T}}} \end{eqnarray} (4)  \begin{eqnarray} &&E_{{\mathop{\rm int}} }^*\left( {\rm{C}{\rm{F}_2}{\rm{Cl}}} \right) = h\nu - {D_0} - {E_{\rm{T}}} - {E_{{\rm{so}}}} \end{eqnarray} (5) where h\nu is the energy of one photolysis photon, D_0 is the dissociation energy of the C–Cl bond (348.9 kJ/mol) [6], E_{\rm{int}} (CF _2 Cl) is the internal energy of CF _2 Cl radical for Cl atom formation channel, {E_{\rm{int}}}^* (CF _2 Cl) is the internal energy of CF _2 Cl radical for Cl ^* atom formation channel, and E_\rm{so} is the spin-orbit excitation energy (10.5 kJ/mol) [18]. All these values are shown in Table Ⅰ. Table Ⅰ Partitions of the available energy and values of \beta on the photodissociation of CF _2 Cl _2 around 235 nm FIG. 3 shows the angular distributions of chlorine atoms, I ( \theta ), which can be fitted with the following equation :  FIG. 3 Angular distributions of chlorine atoms for high E_\rm{T} and low E_\rm{T} components on the photodissociation of CF _2 Cl _2 around 235 nm. Black circles: raw experimental data, blue curves: the fitting according to Eq.(6)  \begin{eqnarray} I\left( \theta \right) \propto 1 + \beta {P_2}\left( {\cos \theta } \right) \end{eqnarray} (6) where \theta is the angle between the photolysis laser polarization and the chlorine atoms recoil velocity, P_2 ( \cos\theta ) is the second-order Legendre polynomial, and \beta is the anisotropy parameter. For Cl ^* atoms, the angular distribution for high E_\rm{T} component was obtained by integrating the speed distribution from 633 m/s to 1868 m/s at each angle, and the angular distribution for low E_\rm{T} component was obtained by integrating the speed distribution from 0 to 1265 m/s at each angle. For Cl atoms, the angular distribution for high E_\rm{T} component was obtained by integrating the speed distribution from 700 m/s to 1856 m/s at each angle, and the angular distribution for low E_\rm{T} component was obtained by integrating the speed distribution from 0 to 1200 m/s at each angle. For Cl ^* atoms, the values of \beta for high E_\rm{T} and low E_\rm{T} components are 1.80 and 0.21, respectively. For Cl atoms, the values of \beta for high E_\rm{T} and low E_\rm{T} components are 0.04 and - 0.01, respectively. Ⅳ. DISCUSSION The first absorption band of CF _2 Cl _2 includes five states ^1 Q _2 , ^3 Q _1 , ^3 Q _0$$ ^+$, $^3$Q$_0 $$^- , and ^1 Q _1 as designated by Mulliken [19], and three of which ( ^3 Q _1 , ^3 Q _0$$ ^+$, and $^1$Q$_1$) are dissociative electronic states. The $^3$Q$_1$ and $^1$Q$_1$ states correlate adiabatically with the formation of Cl atoms via a perpendicular transition, and the $^3$Q$_0$ state does with the formation of Cl$^*$ atoms with a parallel transition character. To clarify the dissociation mechanism more conveniently, a simplified energy diagram of CF$_2$Cl$_2$ is shown in FIG. 4, where the $^3$Q$_1$, $^3$Q$_0$, and $^1$Q$_1$ states are in ascending order of excitation energy. In this VMI study, the reaction mechanisms corresponding to high $E_\rm{T}$ and low $E_\rm{T}$ components of Cl$^*$ and Cl fragments are discussed as follows, respectively.

 FIG. 4 Schematic potential energy surfaces illustrating the photodissociation of CF$_2$Cl$_2$ around 235 nm
A. High $E_\rm{\textbf{T}}$ component

In the VMI experiments, the dissociation laser radiation around 235 nm is also used to probe Cl$^*$ and Cl atoms via (2$+$1) REMPI. One photon energy (h\nu = 508 kJ/mol) is enough to break the C–Cl bond. Furthermore, the high E_\rm{T} component of Cl ^* atoms exhibits a typical parallel transition character. All these features can be ascribed to one-photon photodissociation of CF _2 Cl _2 around 235 nm via the following radical channels:  \begin{eqnarray*} &&{\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_{\rm{2}}} + h{\nu_{{\rm{235}}{\rm{.20 \quad nm}}}}\to {\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{Cl + Cl}}\left( {{}^2{{\rm{P}}_{{\rm{1}}/2}}} \right) \\ &&{\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_{\rm{2}}} + h{\nu_{{\rm{235}}{\rm{.34 \quad nm}}}}\to{\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{Cl + Cl}}\left( {{}^2{{\rm{P}}_{{\rm{3}}/2}}} \right) \end{eqnarray*} It is reasonable that the high E_\rm{T} component of Cl ^* product is related to an initial parallel transition from ground state to the ^3 Q _0 state, followed by direct dissociation on ^3 Q _0 state. Such an interpretation is coincident with the fact that the value of \beta is close to +2. For high E_\rm{T} component of Cl product, the value of \beta is close to 0 (characteristic of predissociation). This can be interpreted as a result of an initial transition from ground state to the ^3 Q _0 state, followed by the nonadiabatic transition from ^3 Q _0 to ^1 Q _1 state. As shown in FIG. 4, the transition from ^3 Q _0 to ^1 Q _1 state has a crossing point. The rate of nonadiabatic transition is probably slowed down as the distance between the excitation position and the crossing point increases in the intersection area. When the rate of the curve crossing is slow, the character of initial parallel transition will be erased. The isotropic high E_\rm{T} component of Cl atoms is produced in this slow predissociation reaction. What is noteworthy is that the image on the photodissociation of CF _2 Cl _2 around 193 nm shows one sharp ring [8], while the images reported here are rater diffuse. As a matter of fact, the photoabsorption cross section around 193 nm is much larger than that around 235 nm [20]. When the parent molecule absorbs the photon energy around 193 nm, the rupture of C–Cl bond is more prompt than that around 235 nm, so vibrational and rotational excitations of CF _2 Cl radical are less sufficient, which is related to the formation of sharp ring. B. Low E_\rm{\textbf{T}} component The low E_\rm{T} components of Cl ^* and Cl atoms exhibit central diffuse distributions as well as isotropic angular distributions. What is the origin of the low E_\rm{T} components? One possible dissociation channel is the secondary dissociation process:  \begin{eqnarray*} {\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{Cl}} + h{\nu_{{\rm{235 \quad nm}}}} \to {\rm{C}}{{\rm{F}}_{\rm{2}}}+{\rm{ Cl}}\left( {{}^2{{\rm{P}}_{3/2}}} \right)/{\rm{Cl}}\left( {{}^2{{\rm{P}}_{1/2}}} \right) \end{eqnarray*} The dissociation energy of this unimolecular decay is about 212 kJ/mol [4-6], so the available energy of 160 kJ/mol (see Table Ⅰ) is not sufficient. Another explanation of the low E_\rm{T} components may be ascribed to the following three-body dissociation channel:  \begin{eqnarray*} {\rm{C}}{{\rm{F}}_{\rm{2}}}{\rm{C}}{{\rm{l}}_{\rm{2}}} + h{\nu_{{\rm{235 \quad nm}}}} \to {\rm{C}}{{\rm{F}}_{\rm{2}}}+{\rm{ 2Cl}} \end{eqnarray*} The dissociation energy of this pathway is 552 kJ/mol [6], so the photolysis photon energy of 508 kJ/mol is also not sufficient. It is to be noted that the slow Cl ^* and Cl atoms may be formed on the photodissociation of clusters. The strong central distribution should be clearly observed on the photodissociation of clusters reported by Poterya and co-workers [8], nevertheless, it has not been observed in the present VMI experiments. In addition, we should also consider the electronic relaxation process, for instance, the internal conversion. Internal conversion is a radiationless de-excitation transition, which is characteristic by isotropic angular distribution and low E_\rm{T} distribution. The low E_\rm{T} components of Cl and Cl ^* atoms in the present VMI experiments are both isotropic. Based on the above discussion, an internal conversion from the excited state followed by an indirect dissociation on the ground potential energy surface (PES) is suggested. It is reasonable that the well on the ground PES and intramolecular relaxation will reduce the initial alignment and increase the vibration and rotation excitations. This is concident with the fact that the value of \beta is smaller than that of direct dissociation. For low E_\rm{T} components of Cl and Cl ^* atoms, the values of \langle E_\rm{int} $$\rangle / E_\rm{avl} ( E_\rm{avl} denotes the available energy) are obviously higher than that of direct dissociation (see Table Ⅰ). The same mechanism was also found in CF _2 BrCl [21] and CH _2 BrCH _2 Cl [22]. To summarize, the relevant energy levels are shown schematically in FIG. 4 to illustrate the reaction mechanism of CF _2 Cl _2 and the (2 + 1) REMPI of Cl and Cl ^* fragments around 235 nm. Radical dissociation channel is confirmed, nevertheless, secondary dissociation and three-body dissociation channels are excluded. Ⅴ. CONCLUSION We have explored the photodissociation dynamics of CF _2 Cl _2 around 235 nm using the time-sliced VMI technology. The nascent Cl and Cl ^* atoms are produced via one-photon dissociation and (2 + 1) REMPI process. The high E_\rm{T} components of Cl ^* fragments are produced via the prompt dissociation on ^3 Q _0 excited PES, nevertheless, the high E_\rm{T} components of Cl fragments are related to predissociation on ^1 Q _1 excited PES by nonadiabatic ^1 Q _1$$ \leftarrow$^3$Q$_0$ transition. The low $E_\rm{T}$ components of Cl and Cl$^*$ fragments are produced via internal conversion.

Ⅵ. ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No.17KJB150005 and No.17KJD510001), the Natural Science Foundation of Changzhou Institute of Technology (No.YN1507 and No.YN1611), Undergraduate Training Program for Innovation of Changzhou Institute of Technology (No.2017276Y), and the National Natural Science Foundation of China (No.21273212).

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a. 常州工学院数理与化工学院，常州 213032;
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