Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (1): 129-133

#### The article information

Yu-jie Ma, Fang-fang Li, Jia-xing Liu, Feng-yan Wang

Imaging the Dissociation Dynamics of Si2+ via Two-Photon Excitation at 193 nmy

Chinese Journal of Chemical Physics, 2019, 32(1): 129-133

http://dx.doi.org/10.1063/1674-0068/cjcp1901011

### Article history

Accepted on: January 22, 2019
Imaging the Dissociation Dynamics of Si2+ via Two-Photon Excitation at 193 nmy
Yu-jie Ma , Fang-fang Li , Jia-xing Liu , Feng-yan Wang
Dated: Received on January 13, 2019; Accepted on January 22, 2019
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, Shanghai 200433, China
Abstract: In the one-color experiment at 193 nm, we studied the photodissociation of Si2+ ions prepared by two-photon ionization using the time-sliced ion velocity map imaging method. The Si+ imaging study shows that Si2+ dissociation results in two distinct channels: Si(3Pg)+Si+(2Pu) and Si(1D2)+Si+(2Pu). The main channel Si(3Pg)+Si+(2Pu) is produced by the dissociation of the Si2+ ions in more than one energetically available excited electronic state, which are from the ionization of Si2(v=0-5). Particularly, the dissociation from the vibrationally excited Si2(v=1) shows the strongest signal. In contrast, the minor Si(1D2)+Si+(2Pu) channel is due to an avoided crossing between the two 2Πg states in the same symmetry. It has also been observed the one-photon dissociation of Si2+(X4Σg-) into Si(1D2)+Si+(2Pu) products with a large kinetic energy release
Key words: Slice imaging    Photodissociation    Silicon dimmer Si2    193 nm
Ⅰ. INTRODUCTION

Silicon is abundant in interstellar space and silicon materials are important to semiconductor industry. Over the past four decades, the properties of small silicon clusters have received a great deal of interest in order to understand how their structure and properties evolve with size[1].As a first step in understanding the nature of electronic states in large clusters, there have been numerous experimental and theoretical studies reported on the spectroscopic and electronic states of the simple yet important Si2 dimers[218].In these studies, most focused on the potential energy surfaces on the neutral Si2 molecules, and there were only a few studies on the electronic structure of Si2+.

Combined with theoretical and experimental studies, the ground state of neutral Si2 is X3Σg (σu2πu2σg2), and the lowest excited 13Πu (σu2πu3σg) state is located at Te=669 cm−1[7, 19].Considering both X3Σg and 13Πu of Si2, which have nearly equal minimal energies, the ground states of the neutral and ionic species are related by a direct σg or πu ionization in Si2.Specifically, the ground state of the Si2+ is X4Σg (σu2πu2σg) and the first excited 12Πu (σu2πuσg2) state lies at Te=4194 cm−1 (0.52 eV)[10, 15].Dixon and coworkers studied the role of excited states of Si2 in photoionization[15]. Marijnissen and ter Meulen used the N 3Σu state at 46763 cm−1(5.80 eV) as the intermediate state in the two-photon ionization process of Si2 and measured its ionization potential at 63884 cm−1(7.92 eV)[20].Sefyani and Schamps calculated the lowest-lying Rydberg state of Si2 in the range of 35000−55000 cm−1 (4.34−6.82 eV)[15].For the electronic states of Si2+ ions, Bruna and co-workers predicted series of potential curves of the electron configurations which dissociate into Si (3Pg)+Si+(2Pu) through large-scale ab initio CI calculations[10].Liu et al.performed ab initio allelectron relativistic calculations of the low-lying excited states of Si2+with and without consideration of spinorbit interaction on Si2+[21].

The previous experimental and theoretical studies have provided details on the spectroscopy and thermodynamics properties of Si2 dimers.However, to the best of our knowledge, we found no experiments on the photodissociation dynamics of silicon dimers.In this work, the photodissociation of Si2+molecule prepared by twophoton ionization at 193 nm was studied by using timesliced ion velocity map imaging method.Two types of ionic photodissociation channels Si (3Pg)+Si+(2Pu) and Si (1D2)+Si+(2Pu) were observed and more details on the related dissociation mechanisms are given below.

Ⅱ. EXPERIMENTS

The photodissociation experiment of Si2+at 193 nm was performed in self-designed crossed-molecular beam setup and only one molecular beam was used in the present experiment[2224].Briefly, the Si2 molecules were generated by laser ablation on a silicon rod at 532 nm, which was produced by frequency-doubling of the output of Continuum Minilite Ⅱ.Then the ablated Si2 molecules were seeded in a supersonic argon gas beam formed via an Even-Lavie valve and expanded into the main vacuum chamber.The free expansion design in the laser ablation source allows the generation of mainly ablated atoms and small clusters[22]. In the center of ion optics region, the Si2 molecular beam intersected with a linearly polarized laser pulse at 193 nm (~0.3 mJ) generated by an excimer laser (GAM-Laser, Inc.EX5), which was focused by a circular convex lens (f=50 cm).A Brewster window was used to set the polarization of the laser.Under the action of the ion optics system with a total voltage of 1800 V, the produced ions were accelerated to the position sensitive detector, which is composed of two microchannel plates (MCP, 75 mm diameter, Photek) and one Phosphor Screen (P43, Photek).Finally, the center slice of Si+ion cloud was gated by adding a 30 ns pulse width on the back MCP and the emitted light from P43 was captured by the Lavision Elite CCD camera. The image was acquired and processed by an improved software"Davis 8.2".

Ⅲ. RESULTS AND DISCUSSION

The raw slice image of Si+ ions from one color experiment at 193 nm is shown in FIG. 1.The Si+ ions lying near the center and having a speed of almost zero are ionized Si monomers generated by laser ablation of Si rod, and the Si+ ions having a recoil speed are produced from the dissociation process.As the Si+ speed increases in FIG. 1, four distinct dissociation channels appear, marking regions A, B, C and D, respectively. The linear polarization direction of the laser is shown in FIG. 1.We define θ as the angle between the polarization vector of the photolysis laser and the recoil velocity vector of the product.By integrating the signal over the θ=0°−360° angular range, the corresponding speed distribution is present in FIG. 2(a).In order to obtain better speed resolution, the integrated speed distribution in the small angular range of θ=0°−5° is shown in FIG. 2(b), and clear vibrational structures are resolved in regions B and C.The species observed in the molecular beam are mainly Si+ and Si2+, and trace amounts of SiO+.The Si+ ions in the region A are related to the dissociation of SiO, and when the carrier gas of argon is changed to O2, the dissociation channel of SiO in region A becomes stronger.The dissociation dynamics of silicon oxide will be discussed in detail in the next paper and will therefore not be discussed in this work.The sharp rings of Si+with large recoil velocity in regions B, C and D are related with the dissociation dynamics of silicon dimmer, which is the focus of this paper.The similar ring structure in regions B and C reflects the vibrational excitation of the parent molecule, i.e., silicon dimmer.

 FIG. 1 Raw slice image of Si+ ions from the one-color experiment at 193 nm.As the radius of the ring increases, four distinct dissociation pathways are divided into four regions
 FIG. 2 Speed distributions of Si+ ions obtained by integrating the signals over (a) the whole angular range (0°−360°) and (b) a small angular range (0°−5°), where the integration of a small angle gives a better speed resolution

According to the recoil momentum conservation in Si2/Si2+ photodissociation, the speed distribution of Si+is converted to the total kinetic energy release (TKER) distribution of Si+ + Si, as shown in FIG. 3. The TKER interval between adjacent peaks in regions B and C respectively is approximately 500 cm−1, which is consistent with the experimental obtained vibrational frequency (511 cm−1) of the Si2(X3Σg) molecule[25]. The first excited electronic state of Si2(D 3Πu) of the low energy level (Te=669 cm−1) can also be involved in photon excitation, where the vibrational frequency ωe=536 cm−1 and the equilibrium inter-nuclear distance Re=2.12Å (comparatively, 2.25 Å for X3Σg) [7].The first peak in region C has a total kinetic energy release of 10030 cm−1 which is about 6000 cm−1 higher than the first peak at 4030 cm−1 in region B. The energy analysis shows that the signals in regions B (C) come from the dissociation of Si2+ ions after twophoton ionization of Si2 of v=0−4(v=0−5), as shown in FIG. 3.The ionization potential energy of Si2(X3Σg) is 7.92 eV, requiring two photons at 193 nm to ionize the ground Si2(X3Σg) state.The processes involved in the two-photon absorption are

 $\text{S}{{\text{i}}_{2}}({{X}^{3}}\sum\nolimits_{\text{g}}^{-}{, v})\xrightarrow{2h\nu }\text{Si}{{(}^{1}}{{\text{D}}_{2}})+\text{S}{{\text{i}}^{\text{+}}}{{(}^{2}}{{\text{P}}_{\text{u}}})+\text{e}(\text{region}\ \text{B})$ (1)
 $\xrightarrow{2h\nu }\text{Si}{{(}^{3}}{{\text{P}}_{\text{g}}})+\text{S}{{\text{i}}^{\text{+}}}{{(}^{2}}{{\text{P}}_{\text{u}}})+\text{e}(\text{region}\ \text{C})$ (2)
 FIG. 3 The total kinetic energy release distribution of Si+Si+ products converted from FIG. 2(b).The assignments of vibrational structures of Si2 are shown in the two channels, Si (3Pg)+Si+(2Pu) and Si (1D2)+Si+(2Pu), respectively
 FIG. 4 The associated channels for the formation of Si+ products from Si2+ ions

Based on energy conservation, the available energy for the ionic dissociation channel is shown as

 $\begin{array}{l} 2h\nu + {E_{{\mathop{\rm int}} }}({{\mathop{\rm Sin}\nolimits} _2}) - {\rm{IE}}({{\mathop{\rm Si}\nolimits} _2}) - {D_0}({\mathop{\rm Si}\nolimits} _2^ + )\\ = {E_{{\mathop{\rm int}} }}\left( {{\rm{S}}{{\rm{i}}^{\rm{ + }}} + {\rm{Si}}} \right) + {E_{KER}}\left( {{{\rm{e}}^ - } + {\rm{S}}{{\rm{i}}^{\rm{ + }}} + {\rm{Si}}} \right) \end{array}$ (3)

where the two-photon excitation energy 2= 103627 cm−1(12.85 eV), Eint (Si2) is the internal energy of Si2, IE (Si2) is the ionization energy of Si2, i.e., 63884 cm−1(7.92 eV)[20], D0(Si2+) is the ground state dissociation energy of Si2+, i.e., 28827 cm−1(3.57 eV)[21], Eint (Si++Si) is the electronic energy of Si+ and Si products, 6298.85 cm−1 for Si+(2Pu)+Si (1Dg) channel and 0 for Si+(2Pu)+Si (3Pg) channel, and EKER (e+Si++Si) is the translational energy distributed in the Si+, Si and electrons.The internal energy difference between the Si+(2Pu)+Si (1D2) and Si+(2Pu)+Si (3Pg) channels is 6298.85 cm−1 which is consistent with the observation of the TKER difference between the two channels for Si2(X3Σg, v=0).

As shown in FIG. 3, the channel of Si+(2Pu)+Si (3Pg) from the excitation of Si2(X3Σg, v=0) has the TKER of 10030 cm−1, and corresponds to the electron recoil energy about 886 cm−1 according to Eq.(3).Hence the energy analysis indicates that, according to the FranckCondon factors, one or even more than one repulsive state of Si2+with Tv≈38857 cm−1(4.82 eV) is involved in the dissociation at R≈2.246 Å (the equilibrium bond length of the ground Si2).Meanwhile, the repulsive excited state of Si2+ has an avoided crossing with another electronic state in the same symmetry, and then the Si+(2Pu)+Si (1D2) channel is also observed.

The peak in region D has a total kinetic energy release of 16000 cm−1.The energy analysis shows that the Si+ ions in region D are from the one-photon dissociation of Si2+(X4Σg), which is produced by two-photon ionization of Si2(X3Σg).The process is:

 $\text{S}{{\text{i}}_{2}}({{X}^{3}}\sum\nolimits_{\text{g}}^{-}{{}})\xrightarrow{2h\nu }\text{S}{{\text{i}}_{\text{2}}}^{\text{+}}({{X}^{4}}\sum\nolimits_{\text{g}}^{-}{{}})+{{\text{e}}^{-}}$ (4)
 $\text{S}{{\text{i}}_{2}}^{+}({{X}^{4}}\sum\nolimits_{\text{g}}^{-}{{}})\xrightarrow{h\nu }\text{Si}{{(}^{1}}{{\text{D}}_{2}})+\text{S}{{\text{i}}^{\text{+}}}{{(}^{2}}{{\text{P}}_{\text{u}}})$ (5)

Based on energy conservation, the TKER of Si (1D2)+ Si+(2Pu) fragments will be simplified as

 ${E_{\rm{T}}}_{{\rm{KER}}}\left( {{\rm{S}}{{\rm{i}}^{\rm{ + }}} + {\rm{Si}}} \right) = h\nu - {D_0}({\mathop{\rm Si}\nolimits} _2^ + ) - {E_{{\mathop{\rm int}} }}\left( {{\rm{S}}{{\rm{i}}^{\rm{ + }}} + {\rm{Si}}} \right)$ (6)

where ETKER (Si++Si) is the total kinetic energy release in the Si (1D2)+Si+(2Pu) products, is 51813.5 cm−1, D0(Si2+) is the ground state dissociation energy of Si2+, i.e., 28827 cm−1(3.57 eV), and Eint (Si++Si) is the internal energy of Si (1D2)+Si+(2Pu) products, i.e., 6298.85 cm−1(0.78 eV).Then the total kinetic energy ETKER for the Si (1D2)+Si+(2Pu) products is predicted to be approximately 16688 cm−1, which is consistent with the measured TKER of 16440 cm−1.The slight energy difference of 250 cm−1 can be attributed to the excitation of the spin-orbit coupling state of Si+(2P3/2) (287.24 cm−1 above the 2P1/2 state).Considering that the channel in region D requires more photons in the associated dissociation dynamics than the channels in regions B and C, it is reasonable to observe a relatively weak signal in this region.

According to the two theoretical calculations by Bruna et al.[10]and Liu et al.[21], more than one of the excited potential curves of ionic state can be involved in the ionic dissociation into Si (3Pg)+Si+(2Pu). According to the information of the excited electronic state obtained above, that is, Tv≈38857 cm−1(4.82 eV) at the bond length of R≈2.25 Å, compared with the potential energy curves of Liu et al.[21], the repulsive doublet 22Σu state and the quartet 24Σ−u and 14Πu states can be involved in the ionic dissociation.In the work of Bruna et al.[10], in addition to the contributions of quartet 24Σu (Tv≈4.59 eV at R≈2.33 Å), 14Πu (Tv≈4.94 eV at R≈2.33 Å) and doublet 22Σu (Tv≈4.56 eV at R≈2.33 Å) states, the doublet 22Π g state (Tv≈3.94 eV at R≈2.33 Å) can also contribute to the dissociation of Si (3Pg)+Si+(2Pu).Bruna et al.predicted that the energy of 22Πg state decreases suddenly at large distance due to the avoided crossing with the 32Πg state, which is decomposed into Si (1D2)+Si+(2Pu).The role of 22Πg state helps to explain the Si (1D2)+Si+(2Pu) channel.For clarity, FIG. 4 shows the associated channels for the formation of Si+ ions from Si2+.In the present experimental data, we cannot distinguish the exact Ω state of the Si+(2P) state, and therefore the spin-orbit coupling is generally not considered in the present discussion.

For the (1+1) ionization of Si2, the possible intermediate state 25Σu or 53Σu near the one-photon energy of 193 nm has the equilibrium bond length Re≈2.35(6)Å [15].Since the equilibrium bond length of the excited state is significantly longer than that of the ground state, the Franck-Condon transition occurs when the ground state and the intermediate state are in excited vibrational states.Moreover, the Franck-Condon factors occurs in the transition from the intermediate states to the final electronic states.Therefore, according to the Franck-Condon principle, it can be understood that the strongest Si (3Pg)+Si+(2Pu) products are produced from the vibrationally excited Si2+(v=1) and the most intense Si (1D2)+Si+(2Pu) are from the vibrationally excited Si2+(v=3) state.

The angular distributions of Si+ions in regions B and C are shown in FIG. 5 and FIG. 6, respectively.The angular distribution of photofragment produced from the two-photon photolysis of unaligned molecules with a linearly polarized laser can be expressed as[26]

 $I(\theta ) = {I_0}\left[ {1 + \sum\limits_1^n {{\beta _{2n}}{P_{2n}}\left( {\cos \theta } \right)} } \right]$ (7)
 FIG. 5 The angular distributions of the peaks observed in region B of FIG. 2(b).The solid lines represent fits to the measured angular distributions using Eq.(7)
 FIG. 6 The angular distributions of the peaks observed in region C of FIG. 2(b).The solid lines represent fits to the measured angular distributions using Eq.(7)

where n is the number of photons involved in photolysis, and n=2 in regions B and C, and β2n is a coefficient that weights contributions from various order P2n Legendre polynomials.The parameters β2 and β4 are fitted by Eq.(7) and are shown in FIG. 5 and FIG. 6.It can be easily seen from the figures that the experimental data are well fitted.The small anisotropy parameters obtained for the regions B and C can be explained if we consider that more than one potential energy surface is involved in the parent ion dissociation that leads to the production of this channel, but also if the dissociation to the channel is slow compared to the rotational period of the parent ion.

Ⅳ. CONCLUSION

Two types of the ionic dissociation after the twophoton ionization of Si2 molecules at 193 nm were observed by using time-sliced velocity map imaging technique.One leads to Si (3Pg)+Si+(2Pu), and the other is Si (1D2)+Si+(2Pu).More than one excited electronic state of Si2+ obtained by two-photon ionization can directly produce Si (3Pg)+Si+(2Pu) channel. The avoided crossing between two excited electronic states in the same symmetry was contributed to the Si (1D2)+Si+(2Pu) channel.In order to better understand the dissociation dynamics of Si2/Si2+ molecules, more studies on Si2/Si2+ photodissociation in the ultraviolet region will be reported.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.21673047, No.21327901, and No.21322309), the Shanghai Key Laboratory Foundation of Molecular Catalysis and Innovative Materials, and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

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