Rui Mao,Chao He,Min Chen,Dan-na Zhou,Qun Zhang,Yang Chen

a.Scho ol of Mathematics and Physics and Chemical Engineering,Changzhou Institute of Technology, Changzhou 213032,China

b.Hefei National Lab oratory for Physical Sciences at the Microscale and Department of Chemical Physics,University of Science and Technology of China,Hefei 230026,China

Photodissociation Dynamics of Carbon Dioxide Cation via the Vibrationally MediatedState:A Time-Sliced Velocity-Mapped Ion Imaging Study

Rui Maoa?,Chao Heb,Min Chenb,Dan-na Zhoub,Qun Zhangb,Yang Chenb

a.Scho ol of Mathematics and Physics and Chemical Engineering,Changzhou Institute of Technology, Changzhou 213032,China

b.Hefei National Lab oratory for Physical Sciences at the Microscale and Department of Chemical Physics,University of Science and Technology of China,Hefei 230026,China

(Dated:Received on November 11,2016;Accepted on January 25,2017)

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].

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 sourceand detection chambers were maintained at～10-6and～10-7Torr,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 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]resonanceenhanced 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 chargecoupled 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 5×104shots or more.The backgrounds were removed by subtracting the of f-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

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.

B.Ion images and assignments of CO+vibronic distributions

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

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:

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 VOrepresent the mass and velocity of the CO+and O fragments,respectively.

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 formstate at 351.24 nm.(c)The anisotropy parameter β v s.TER profile is plotted.

C.Dissociation dynamics via

TABLE I Energy partitioning on the photodissociation ofchannel.Eavaildenotes 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.

TABLE I Energy partitioning on the photodissociation ofchannel.Eavaildenotes 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.

aCalculated by using the relationbCalculated by using the relation

Mediated state Excitation wavelength/nm Eavaila/eV〈ET〉b/eV〈ET〉/Eavail(5,0,0)293.64 2.773 0.349 0.126 (4,2,0)κ294.64 2.744 0.326 0.119 (4,2,0)μ296.09 2.703 0.387 0.143 (4,0,0)303.53 2.497 0.387 0.155 (3,2,0)κ304.73 2.465 0.323 0.131 (3,2,0)μ306.24 2.425 0.363 0.150 (3,0,0)314.13 2.222 0.486 0.219 (2,2,0)κ315.65 2.184 0.482 0.221 (2,2,0)μ317.21 2.145 0.482 0.225 (2,0,0)325.55 1.945 0.536 0.275 (1,2,0)κ327.36 1.903 0.574 0.302 (1,2,0)μ329.01 1.865 0.508 0.272 (1,0,0)337.93 1.666 0.504 0.303 (0,2,0)κ340.08 1.619 0.490 0.303 (0,2,0)μ341.79 1.583 0.461 0.291 (0,0,0)351.24 1.387 0.676 0.487

IV.CONCLUSION

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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?Author to whom correspondence should be addressed.E-mail: maorui@ustc.edu.cn