Chinese Journal of Chemical Physics  2017, Vol. 30 Issue (2): 123-127

The article information

Rui Mao, Chao He, Min Chen, Dan-na Zhou, Qun Zhang, Yang Chen
茆锐, 何超, 陈旻, 周丹娜, 张群, 陈旸
Photodissociation Dynamics of Carbon Dioxide Cation via the Vibrationally Mediated Ã2Πu, 1/2 State: A Time-Sliced Velocity-Mapped Ion Imaging Study
二氧化碳分子离子经由Ã2Πu1/2电子态振动能级共振的光解动力学:一项时间切片离子速度成像技术研究
Chinese Journal of Chemical Physics, 2017, 30(2): 123-127
化学物理学报, 2017, 30(2): 123-127
http://dx.doi.org/10.1063/1674-0068/30/cjcp1611208

Article history

Received on: November 11, 2016
Accepted on: January 25, 2017
Photodissociation Dynamics of Carbon Dioxide Cation via the Vibrationally Mediated Ã2Πu, 1/2 State: A Time-Sliced Velocity-Mapped Ion Imaging Study
Rui Maoa, Chao Heb, Min Chenb, Dan-na Zhoub, Qun Zhangb, Yang Chenb     
Dated: Received on November 11, 2016; Accepted on January 25, 2017
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
*Author to whom correspondence should be addressed. Rui Mao, E-mail:maorui@ustc.edu.cn
Abstract: We report on the photodissociation dynamics of CO2+ via its Ã2Πu, 1/2 state using the scheme of [1+1] photon excitation that is intermediated by the mode-selected Ã2Πu, 1/2(u1, u2, 0) vibronic states. Photodissociation fragment exciation spectrum and images of photofragment CO+ have been measured to obtain reaction dynamics parameters such as the available energy and the average translational energy. Combining with the potential energy functions of CO2+, the dissociation mechanism of CO2+ is discussed. The conformational variation of CO2+ from linear to bent on the photodissociation dynamics of CO2+ is verified.
Key words: Photodissociation dynamics    Velocity map imaging    Carbon dioxide cation    
Ⅰ. INTRODUCTION

Photodissociation dynamics of molecular cations is a significant subject in photochemistry [1-4]. Since the prominent improvement of resolution by Eppink and Parker in 1997 [5], ion imaging method has been playing an important role in studying photodissociation dynamics of many molecular systems, like the recent reported freon [6], bromocyclopropane [7], Criegee intermediates [8], xylene [9], OCS [10], and furan [11], owning to its high detection efficiency and high velocity resolution. Many approaches come out, including time slicing in which subsets of the conventional crushed image are recorded by the gating the detector selecting a small region of △t and no need of the inverse-Abel for further analysis [12].

As an important molecular cation in astrophysics and atmospheric physics, CO2+ has been studied extensively during the past several decades [13-18]. Previously numerous experimental and theoretical studies on the spectrum and photodissociation dynamics of the CO2+ ion were reported [13-18], which supplied abundant information. C. Y. Ng et al. measured the vacuum ultraviolet pulsed field ionization-photoelectron spectra for CO2 in the energy range of 13.6-20.0 eV, revealing vibronic structures for the states of CO2+, they also used theoretical calculations to provide a rationalization that the predissociation for to form and most likely proceeds via the repulsive a4g+ and b4u (or 4B1 in a bent geometry) states [13-15]. Zhang et al. studied the mass-resolved [1+1] two-photon dissocaition spectra of CO2+ via transitions [16-18]. The spectroscopic constants were deduced and the dissociation dynamics was discussed in their research work.

We have reported the subtle and complex photodissociation behavior in a narrow energy region (~280 cm-1) on the photodissociation of CO2+ mediated by its different vibronic states using velocity map imaging (VMI) technology [19], nevertheless our previous work focused mainly on reporting the experimental results [19]. So we further provide details and discussion on the physical insights and dissociation mechanism in this work to explore the photodissociation dynamics of carbon dioxide cation via the vibrationally mediated state.

Ⅱ. EXPERIMENTS

The experiments were performed in a home-built VMI apparatus, details of which can be found elsewhere [20, 21]. Briefly, the carbon dioxide sample seeded in Ar (~30%) at a stagnation pressure of ~3 atm was expanded through a pulsed nozzle (Series 9, General Valve) with an orifice diameter of 0.5 mm in a source chamber and skimmed to form a supersonically expanded molecular beam into a differentially pumped detection chamber. The operating pressures in the source and detection chambers were maintained at ~10-6 and ~10-7 Torr, respectively. After passing through a 1.5 mm hole on the repeller plate, the molecular beam directed along the time-of-flight (TOF) axis was intersected at right angles by the laser beam in the detection zone. For all VMI measurements, the electric vector of the linearly polarized laser was set perpendicular to the TOF axis and thus parallel to the front face of the microchannel plates (MCP's) that form part of the ion detection system. The ionization laser around \mbox{333 nm} is the output of a neodymium-doped yttrium aluminum garnet (Nd:YAG) (GCR-170, Spectra Physics) pumped dye laser (PrecisionScan, Sirah) and is focused by an f=150 mm lens, and the dissociation laser between 278 and 354 nm is the output of a neodymium-doped yttrium aluminum garnet (Nd:YAG) (GCR-170, Spectra Physics) pumped dye laser (PrecisionScan, Sirah) and is focused by an f=250 mm lens. The intensities of the ionization and dissociation lasers were simultaneously monitored during the experiment.

CO2+ ions were prepared by a [3+1] resonance-enhanced multiphoton ionization (REMPI) excitation process. Within a set of ion optics designed for the VMI measurements, photofragment CO+ ions were accelerated by the focusing electric fields and projected onto a 40-mm-diameter Chevron-type dual MCP's coupled to a P-47 phosphor screen (APD 3040FM, Burle Electro-Optics). A fast high-voltage switch (PVM-4210, DEI; typical duration ~50 ns) was pulsed to gate the gain of the MCP's for mass selection as well as the time slicing of the ion packet. The transient images from the phosphor screen were captured by a charge-coupled device (CCD) camera (Imager Compact QE 1376 × 1024 pixels, LaVision) and transferred to a computer on an every shot basis for event counting [22] and data analysis. Timing of the pulsed nozzle, the laser, and the gate pulse applied on the MCP's was controlled by two multichannel digital delay pulse generator (DG 535, SRS). The photofragment excitation spectrum (PHOFEX) was acquired using a photomultiplier tube. The images were accumulated over 5 × 104 shots or more. The backgrounds were removed by subtracting the off-resonance images collected under the same conditions. The wavelengths calibrated by a wavemeter were scanned to cover all the speed components of the nascent fragments.

Ⅲ. RESULTS AND DISCUSSION A. PHOFEX spectrum of transition

Pure ions were generated by [3+1] REMPI at 333.06 nm via 3pπu1u Rydberg state depending on the result of L. M. Zhang et al. [16], that is:

(1)

After the preparation of the cations, the photodissociation laser was introduced, and PHOFEX spectrum (Fig. 1) of CO2+ was obtained by scanning the wavelength of photodissociation laser and recording the photofragment CO+ signals. In order to ensure that CO+ was the cooperative action of two lasers, not one laser only, the power of photoionization and photodissociation laser was carefully optimized with temporally and spatially matched at the laser-molecular interaction point.

FIG. 1 PHOFEX spectrum recorded by monitoring the photofragment CO+ signals in the wavelength range of 278−354 nm. The assignments of the vibronic transitions were used for acquiring images of CO+.

Based on previous results, assignments of our PHOFEX spectrum were carried out giving different vibronic of state. Here v1 and v2 denote vibrational quantum numbers for the symmetric stretching and bending modes, respectively. The vibronic coupling induces Renner-Teller splitting, giving rise to the split μ (lower) and κ (upper) components for each state. With this spectrum, we could study the dissociation dynamics via the vibrationally mediated state using time-sliced velocity-mapped ion imaging technique.

One interesting phenomena should be noticed. The intensities of (υ1, 2, 0; υ1=0−3)μ2u; 1/2 are obviously lower than those of (υ1, 2, 0; υ1=0−3)κ2u; 1/2 and (υ1, 0, 0; υ1=1−4)2u; 1/2, nevertheless the intensities of (4, 2, 0) μ2u, 1/2 are higher than those of (4; 2; 0) κ2u; 1/2 and (5; 0; 0) 2u; 1/2. It is known that the potential energy surface of the lower (υ1; 2; 0) μ2u; 1/2 state (corresponding to a bent geometry) is more complex than that of the upper (υ1; 2; 0) κ2u; 1/2 state (corresponding to a linear geometry) [23], respectively. When the linear parent CO2+ cations absorb the first dissociation photon, the bent μ (lower) components for each state are relatively difficult to be excited. Nevertheless, when the linear parent CO2+ cations absorb the first dissociation photon and begin to bend, the bent μ (lower) components for each state are relatively easier to be excited. It is suggested that the comformational variation occurs in the photodissociation process.

B. Ion images and assignments of CO+ vibronic distributions

The time-sliced ion images of photofragment CO+ were obtained at all the vibronic transition peaks of in the wavelength range we explored. A typical example is shown in Fig. 2, in which the photodissociation is mediated via the states. To obtain the anisotropy parameter, β, the angular distribution P(θ) has been fitted according to:

(2)
FIG. 2 (a) Ion image of CO+ and (b) the associated translation energy release (TER) spectrum and assignments resulting from [1+1] photo-excitation of CO2+ to form via the Ã2u (0; 0; 0) state at 351.24 nm. (c) The anisotropy parameter βvs. TER profile is plotted.

where θ is the angle between the polarization vector of the dissociation laser and the recoil velocity vector of the fragments, and P2 (cosθ) is the second-order Legendre polynomial.

On the basis of energy and momentum conservation, the distribution of total translational energy of fragments (CO+, O) can be routinely obtained. The internal energy of CO+ fragments, Eint, could be obtained using the formula below:

(3)

where 2 is the two-photon energy of the photodissociation laser, D0 is the dissociation limit of the channel. D0 value is 5.6724 eV [15], and the corresponding one-photon wavelength is 176.3 nm, which can not be obtained in our laboratory. So [1+1] resonance-enhanced two photons scheme is used to dissociate CO2+. ET is the total released translational energy of O and CO+ fragments, which can be obtained by

(4)

and

(5)

The calibration for the speed of CO+ fragment was achieved by probing CO rotational bands of OCS at 230 nm [24]. mCO+, mO, VCO+ and VO represent the mass and velocity of the CO+ and O fragments, respectively.

Using vibrational frequency ωe=2214.2 cm-1 and the anharmonic constant ωeχe=15.16 cm-1/2 of [25], the vibronic quantum number of was assigned as shown in Fig. 2(b). The profile of β versus total translational energy of fragments is plotted in Fig. 2(c). The values of β are near zero.

C. Dissociation dynamics via Ã2u, 1/2 (υ1; υ2; 0) states

From the analysis of the raw image, the angular distribution of CO+ fragments is isotropic, indicating that the CO2+ cations are predissociative. The bond dissociation occurring in the time scale is equal to or longer than the rotational period of the parent molecule, maybe a few internal vibrational periods. The process is an indirect photodissociation and the coupling between various modes in CO2+ gradually erases the memory of the initial state before the molecule cation breaks.

From initial analysis, the related reaction dynamics parameters are shown in Table Ⅰ. It is obvious that the ratios of the average translational energy to the available energy decrease as the energy of dissociation photon increases. As the excitation wavelength is varied from 351.24 nm to 303.53 nm, the images of CO+ resemble each other, which verify that the dissociation dynamics of carbon dioxide cation in this excitation energy area is similar. This is because the excitation of bending modes does not influence the dissociation process when the parent molecule is linear. However, as the excitation wavelength is varied from 293.64 nm to 296.09 nm (photon energy varied also by ~280 cm-1), the three images of carbon oxide cation look different in small energy region [19]. The dissociation dynamics of carbon dioxide cation exists dramatic variation, which is suggested that there is an important variation in the dissociation process. According to the potential energy functions calculated by C. Y. Ng and coworkers [15], the predissociation for to form most likely proceeds via the repulsive b4u (or b4B1 in a bent geometry) states. P. Rosmus and coworkers also calculated potential energy functions of CO2+, and they found that the potential energy functions of the Ã2A1 state around α OCO=130° exhibited a barrier at ~9000 cm-1 above the energy of its equilibrium linear geometry [26]. When the first dissociation photon excitation energy exceeds the barrier height, the probability of spanning the barrier enhances, and conformational variation from linear CO2+(Ã2u) to bent CO2+(Ã2A1) probably happen. The dissociation process of bent molecule mostly correlates with excitation of bending modes. Excitation of high level symmetric stretching modes is needed, so vibrational profiles are almost vanished for (5, 0, 0) [19]. The potential energy surface of the lower (4, 2, 0)μ2u, 1/2 state (corresponding to a bent geometry) is more complex than that of the upper (4, 2, 0)κ2u; 1/2 state (corresponding to a linear geometry) [23], so more dissociation channels may be opened up when CO2+ ions are excited to the former surface. This causes the continuous distribution of translation energy [19].

Table Ⅰ Energy partitioning on the photodissociation of CO2+ via channel. Eavail denotes the available energy, 〈ET〉: the average translation energy of CO+ and O, 〈ET〉=Eavail: the ratio of the average translational energy to the available energy.

In our research work, the carbon dioxide cation is dissociated vibrationally mediated by Ã2u, 1/2 (υ1; υ2; 0) using [1+1] resonance-enhanced two photons scheme, and the first resonance step is a rate-decided step which plays an important role on dissociation dynamics of carbon dioxide cation. Conformational variation in the dissociation process when carbon dioxide cation is excited by dissociation laser lead to dramatic variation in the intensities of PHPFEX spectrum and vibrational distributions of CO+.

Ⅳ. CONCLUSION

In conclusion, we have investigated the photodissociation dynamics of carbon dioxide cation by means of time-sliced VMI. The cations are predissociative. The abnormal intensities of PHOFEX spectrum and dramatic reaction dynamics variation in a narrow 280 cm-1 region are correlated with the conformational variation in the dissociation process of CO2+.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of Changzhou Institute of Technology (No.YN1507), Undergraduate Training Program for Innovation of Changzhou Institute of Technology (No.J150245), the China Postdoctoral Science Foundation (No.2013M531506), the National Natural Science Foundation of China (No.21273212).

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二氧化碳分子离子经由Ã2Πu1/2电子态振动能级共振的光解动力学:一项时间切片离子速度成像技术研究
茆锐a, 何超b, 陈旻b, 周丹娜b, 张群b, 陈旸b     
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实验室 (筹), 合肥 230026
摘要: 我们报道了二氧化碳分子离子经由Ã2Πu1/2电子态不同振动能级[1+1]共振的光解动力学, 测量了光解离碎片激发谱和解离产物一氧化碳离子的影像, 得到可资用能、平均平动能等相关反应动力学参数, 结合二氧化碳分子离子的势能面讨论其解离机理, 证实了分子离子在光解离过程中构型由线型向弯曲型改变.
关键词: 光解动力学    离子速度成像    二氧化碳分子离子