刘宁亮 沈环,*

(1华中农业大学理学院，武汉430070；2华中农业大学应用物理研究所，武汉430070)

飞秒脉冲作用下氯丙烯的多光子解离和电离动力学

刘宁亮1,2沈环1,2,*

(1华中农业大学理学院，武汉430070；2华中农业大学应用物理研究所，武汉430070)

利用飞秒激光脉冲，在200、400和800 nm下对氯丙烯(C3H5Cl)的光解离和电离动力学进行了研究。实验测量了氯丙烯在飞秒脉冲作用下的飞行时间质谱、光强指数以及光电子影像。结果发现在短波(200 nm)时，母体分子的电离为主要通道，而其他离子均来源于C3H5Cl+的解离。当波长移动到长波(800 nm)时，碎片离子就占据了主导。这些碎片离子来源于中性碎片的多光子电离过程，而这些中性碎片又是由C3H5Cl的中间态直接解离而产生的。这意味着，光解离过程起到非常重要的作用。这是因为800 nm可以激发分子达到能够产生中性碎片的中间态。在400 nm时，虽然中间态的解离过程仍然至关重要，但并没有在800 nm时明显。综上所述，本文的研究证实了氯丙烯的光解离/电离行为具有波长依赖性，并揭示了氯丙烯在200、400和800 nm飞秒激光作用下的复杂动力学过程。

飞秒激光脉冲；飞行时间质谱；光电子影像；氯丙烯；多光子解离和电离

1 Introduc tion

Asa prototype of aliphatic halides,ally l chloride hasattracted agreatdealof interestdue to its complex photodissociation dynamics1－13.The radiation of a UV photon possibly leads to C―Cl bond breaking and/or HClelimination.Myers etal.7measured the photofragment velocity and angular distribution of allyl chloride at 193 nm w ith a crossed laser-molecular beam apparatus and identified three competing channels occurring uponππ*excitation.Later,Morton etal.8found that[allC―Cl]:[fastC―Cl]: [slow C―Cl]:[fast HCl]:[slow HCl]:[all HCl]was 1.00: 0.971:0.029:0.291:0.167:0.458,where fast refers to thehigh recoil kinetic energy channels.Park9and Liu10et al.also investigated the C―Clbond fission dynam ics of ally chlorideat235 nm with velocitymap imaging technique.Both groups found that at least two channelsare involved in the dissociation.As for the ionization dynam ics,important constants,such as the accurate ionization potential,w ere alsomeasured11,12.

Since all studieswere carried outwith nanosecond pulses,Shen etal.13studied the photodissociation dynamicsof allyl chloride in a time-resolved way using fem tosecond lasers recently.The lifetimes of the C―Cl bond fission channelsaswellas the HCl elimination channelwereobtained.Itwas found that C―Clbond fission channelshave two differentpathways:one takes(48±1) fs and the other takes(85±40)ps.They are attributed to the predissociation process on the repulsive nσ*/πσ*state and dissociation after internal conversion to the vibrationally excited ground state.Two different HCl elim ination channels,namely (600±135)and(14±2)ps,were alsomeasured.The(14±2) ps componentwas due to internal conversion from theππ*/πσ* state to the ground state and dissociation on the ground state.The (600±135)fs componentwasproposed to be dissociated on the excited state surface.

Aspreviously demonstrated by Shen etal.13,time-resolvedmass spectra coupledwith photoelectron imagesarea powerful tool to study the time-resolved dynam icsof complexmolecules.Lifetime constants can beextracted from time-resolvedmass spectrawhile information from the intermediate state can be obtained from photoelectron images.Since two differentcolorsareused in timeresolved experiment,and both of them may generate photoelectrons w ith different kinetic energies,it is im portant to clearly identify the photoelectron spectraaswellas the intermediate state when each single-color laser is used.In the presentwork,the multiphoton dissociation and ionization dynam ics of allyl chloride were investigated using femtosecond laser pulsesat200,400 and 800 nm.M ass spectra aswell as photoelectron imageswere recorded ateach wavelength.The photodissociation and ionization dynam icswere found to bewavelength-dependent.Finally,the im plications to the photodissociation dynam ics were also discussed.

2 Experim en talsetup

The experimentalsetup used herehas been described in detail elsewhere14.In brief,～5%allyl chloridewasseeded in He carrier gas.Them ixturewasexpanded through a pulsed valveoperating at 10 Hz.A fter passing a skimmer,which separated the source chamber from the ionization chamber,themolecular beam was interacted with the femtosecond laser pulses at them idway between the repeller and the extractor platesof the electrostatic lens. The generated photoions and photoelectronswereaccelerated in the electric field and detected by a two-dimensional position sensitive imaging detector.In order tominimize theeffectof the earth′smagnetic field on the photoelectron trajectory,a doublelayerμ-metal shield w as installed along the axisof the time-offlight chamber.A photomultiplier tube and a 100 MHz digital oscilloscope(TDS 2012,Tektronix)wereused to acquire the timeof-flightmass spectra of the photoions,while a charge coupled device(CCD)camerawas used to collect the photoelectron images.

The laser sourceemployed herewas a regenerative amplified Ti:sapphire femtosecond laser system(Coherent,Legend).The Ti: sapphireoscillatorwaspumped by the second harmonic of a CW Nd:YVO4 laser.A seed beam was generated and then amplified by a Nd:YLF laser pum ped regenerative am plifier to generate a～50 fs,1m Jpulse centered at800 nm with a repetition rateof 1 kHz.400 nm pulse was produced by the second harmonic generation of the fundamental pulse,while 200 nm pulse was generated by the sum frequency of the fundamental(800 nm)and its third harmonic(267 nm).These laser beam swere focused using a 25 cm focal-length quartz lens.The polarization of the laser beamswasset to be vertical to theoptical tableand parallel to the face of the imaging detector.The typical intensity used was1.0× 1012W·cm－2for200 nm,1.9×1012W·cm－2for400 nm and 4.5× 1012W·cm－2for 800 nm at the focusing region for each laserpulse, respectively.

The photoions and photoelectronsproduced wereanalyzed by an ion time-of-flightmass spectrometer and by a velocity-mapimaging(VM I)device,respectively.The VMIdevice was also used formapping the kinetic energy and theangular distributions of the photoelectrons resulting from the photoionization process. Each image was accumulated over 20000 laser shots.The photoelectron kinetic energy was calibrated using(2+1)resonanceenhancedmultiphoton ionization of iodineatom15.

3 Resu lts and discussion

3.1 Ion spec tra

Fig.1 depicts the ion spectra of allyl chloridew ith the radiation of 800,400 or200 nm femtosecond pulse.Asshown in Fig.1,five peaks can beobserved,and based on their respective flight time, they can be assigned to the parent ion C3H5Cl+and its fragment ions,i.e.,(as labeled in the Fig.1).Five kinds of ionsare observed atall the threewavelengths.However, their relative intensities are different(Table 1).At800 nm,the branch ratio of C3H5Cl+is 17.06%,which is almost three times lower than thatofAt400 nm,thebranch ratio of C3H5Cl+increases to 30.31%,which is close to that of C3H+5(37.69%).However,at200 nm,the branch ratio of C3H5Cl+in-creases to 55.26%,which isabout three times higher than thatofMeanwhile,the branch ratios of other ion fragments,includingshow very slight changesat differentwavelengths asshown in Table 1.The parent ion shows differentbranch ratiosat these threewavelengths,indicating that different formation mechanisms are dom inant at different wavelengths.

Fig.1 Time-of-flightmass spectra of allylchloridew ith the radiation of(a)800 nm,(b)400 nm and(c)200 nm fem tosecond pu lses

Tab le 1 Branch ratio of each ion after the photoion ization of allyl ch lorideat threewavelengths

In the present study,the intensities of the fem tosecond laser pulseswere set to be1.0×1012W·cm－2at200 nm,1.9×1012W· cm－2at400 nm and 4.5×1012W·cm－2at800 nm.At low intensity lim it(～1012W·cm－2),photoionization productsare produced by themultiphoton ionization process normally16.The ionization potential for allyl chloride is10.20 eV17,suggesting thatat least two 200 nm photons(2×6.20=12.40 eV),four400 nm photons (4×3.10=12.40 eV)or seven 800 nm photons(7×1.55=10.85 eV)are required for the ionization of ally l chloride.In order tofurther confirm the number of photons absorbed at the three wavelengths,a power dependencemeasurementof the ion yield was performed(Fig.2).A slope of 1.94±0.03 was found for C3H5Cl+at 200 nm(Fig.2(a)),which is consistent with the assumption of a two-photon process.The pow er indexes for the fragment ions,such aswere found to be 2.13± 0.15 and 2.32±0.19,respectively,which are very close to thatofare produced from the dissociation of C3H5Cl+,since the dissociation of C3H5Cl+can be easily saturated due to its lim ited population.Fig.2(b)show s the sim ilarmeasurementat400 nm.The laser power indexes ofwere recorded to be 4.41±0.14,3.71± 0.16and 4.03±0.44,respectively,indicating a four-photon process. However,the laser pow er indexes forare 5.63± 0.12 and 4.92±0.15,respectively,which are higher than thatof C3H5Cl+.These results suggest thatother channelsmay contribute to these twofragment ions besides the dissociation of C3H5Cl+, which willbe discussed in Section 3.3.Fig.2(c)shows the results at 800 nm.The laser pow er indexes forwere recorded to be 1.44±0.04,2.14±0.25 and 2.10±0.03, while they were5.69±0.19 and 5.13±0.31 forespectively.It seems thatnone of them shows a seven-photon process at 800 nm.This obvious deviation indicates that other channelsmay play a significant role in the formation process of these fragment ionsbesides the dissociation of C3H5Cl+,aswillbe discussed in Section 3.3.Forsimilar resultswere obtained at400 and 800 nm,and they show a laser power indexof～6 and～5,respectively.On the other hand,the laser power index of C3H5Cl+drops to 1.44.Of course,two photonsat800 nm cannot ionize C3H5Cl;instead,at least seven photonsare needed. It is likely thatat800 nm,C3H5Cl+dissociatesand other ions are formed as the laser power increases,resulting in a lower laser power index.

Fig.2 Laser power indexmeasurementof thegenerated ionsat threewavelengths

3.2 Photoelectron spectra and ionization dynam ics

The photoelectron images resulted from photoionization at200, 400 and 800 nm femtosecond laser fieldsare shown in Fig.3.The arrow indicates the polarization of the laser.The left partof each image is the two-dimensional raw image.These raw imagesare the two-dimensional projections of the three-dimensional speed and angular distributions of the photoelectrons.Given that the distributions of the photoelectrons have a cylindricalsymmetry around the polarization axis of the photolysis laser,a full threedimensional photoelectron image can be reconstructed using the basis-setexpansionmethod(BASEX)18,as shown in the righthalf of each image in Fig.3.Thus,the kinetic energy distributions of the photoelectronswere obtained from the angle integration of the reconstructed imageasa function of the radialdistance from the center(Fig.4).Two peaksareobserved at200 nm,and three peaks areobserved at400 nm,while six peaksareobserved at800nm.

The observed peaks can be assigned using the conservation of energy,

where n stands for the number of photons involved in the photoionization process,hv is theenergy for a200 nm,a400 nm or an 800 nm photon,E0is the internalenergy of the parentmolecule, and it can be neglected under the cold supersonic beam conditions19,Eiis the internalenergy of ionic parentat the i th level,and Eeis the kinetic energy of theoutgoing electrons.

At200 nm,themoleculewould obtain an energy of 12.40 eV after absorbing two photons.The first and second ionization potentials(IP)of allyl chloride are 10.20 and 11.17 eV,respectively17.Thus,the kinetic energy obtained by the photoelectron should be located at 2.20 and 1.23 eV,respectively.This is in accordancewith theobserved peaksa2and a1,which centerat2.08 and 1.15 eV(Fig.4(a)).The discrepancy on the orderof～0.1 eV isprobably due to theerror in the identification of the peak center since the peaksare pretty broad.

In our photoelectron spectra,two peaks both result from the ionization of the parent ion at200 nm.Thisobservation is in line with themassspectraobtained when 200 nm wasused.In Fig.1 (a),thebranch ratio of the parent ion is55.26%,which isat least three times higher than those of other ions.Nevertheless,the fragment ions,such as C3H5+,C3H4+,C2H3+and CH2+,are also observed.Normally,the fragment ions are generated from two possible processes20－23.One is the dissociative ionization of the neutral parentmolecule,while the other is the ionization of the neutral fragment dissociated from the neutral parentmolecule. Due to the different ionization potentialsof the parentmolecule and its fragments,electrons from each specieswould show distinctpeaks.In our case,we did notobserveany peak for the ionization of theneutral fragments,implying thattheobserved fragment ions, suchasat200nm,areattributable to the dissociative ionization of theneutralparentmolecule.

Fig.3 Photoelectron imagesof the resu lted electronsin the fem tosecond laser fieldThe leftpartofeach image is the two-dimensionalraw image,while the rightpart is the reconstructed three-dimensinalimage. Thedoubledirection arrow is thepolarization of the laser.

Fig.4 K inetic energy distribu tions of the photoelectrons ob tained from the photoelectron im agesThe observed peaks are labeled as an,bnand cn(n=1,2,3,…,6)at200,400 and 800 nm,respectively.

At400 nm,three peaks are observed,which are located atb1(0.88 eV),b2(1.32 eV)and b3(1.70 eV)(Fig.3(b)).At least an energy of 12.4 eV(four 400 nm photons)isneeded to ionize the parentmolecule since its IPis10.20 eV,and two 200 nm photons also result in an energy of 12.40 eV.Thus,we assign the b2peak to the ionization of the C3H5Clmolecule.However,two other peaks,b1(0.88 eV)and b3(1.70 eV)were observed,implying that these peaksmightcome from other processes,such as ionization of neutral fragments.We assign the b3peak to the ionization of H2C＝CHC:H or the HC·＝CHC·H2radicalafter theabsorption of three 400 nm photons as Shen etal.13,w ho also reported the observation ofa1.67eV peak.As to theb1peak,itprobably comes from ionizationof theC3H5radical,whichhasan IPof8.18eV(6× 1.55－8.18=1.12 eV)24,25.Theseassignments are in linewith the massspectraobserved in Fig.1(b),whereanddominate themassspectraat400 nm.Themeasured photoelectron spectraalso indicate that theobservedith 400 nm fem tosecond pulse are not derived only from the dissociative ionization of C3H5Cl;andmultiphoton ionization of neutral radical also playsa significant role,which isprobably the reasonwhy its power index ishigher than 4.

At800 nm,six peaks,c1(0.65 eV),c2(0.99 eV),c3(1.45 eV), c4(2.23 eV),c5(3.01 eV)and c6(4.58eV),areobserved(Fig.3(c)). With 800 nm pulse,seven photons could achieve an energy level of 10.85 eV,which is 0.65 eV higher than the IPof the parent molecule.Thus,it is possible that the c1peak(0.65 eV)is due to the ionization of the C3H5Clmolecule(7×1.55－10.20=0.65 eV).The c2peak can be assigned to the ionization of C3H5radical, while the c3peak can beassigned to the ionization of H2C＝CHC: H or the HC·＝CHC·H2radical(6×1.55－7.63=1.67 eV)13.As tootherpeaks,wenotice thatc4－c1=1.58 eV,c5－c3=1.56eV,and c6－c5=1.57 eV.Thus,they are from the samemolecule/radical, butabsorb onemore photon.Assuggested previously,ionization of theC3H5Clmolecule isone of the possible pathways to generate c1.Thus,the c4peak implies thatC3H5Cl+ispopulated to ahighly excited state by absorbing one more photon.Hence,the fragmentation of C3H5Cl+would lead to a decrease of itsbranch ratio. On themassspectra in Fig.1(c),thebranch ratio of C3H5Cl+is the lowestamong these threewavelengths,which is in linew ith the observations in the photoelectron kinetic energymeasurements.

3.3 Im p lication to the pho todissociation dynam ics

A lthough five different ions are observed at all the three wavelengths,photoelectron spectrum measurements show that they are generated by differentschemes.At200 nm,C3H5Cl+is predominant and other ions are generated by the dissociative ionization of C3H5Cl.At400 nm,multiphoton ionization of the neutral fragmentsplaysa significant role.At800 nm,thebranch ratio of C3H5Cl+is further decreased by dissociation with the absorption of an additional photon.Meanwhile,multiphoton ionization of the neutral fragments is as importantasat400 nm. This wavelength-dependent ionization behavior implies that photodissociation playsa significant roleat longwavelength,since neutral fragments are supposed to be generated on the potential energy surface of the intermediatestates reached by 400 nm or 800 nm photons.

The photodissociation dynam ics of ally l chloride on theππ* state and the nσ*/πσ*state have been investigated by time-resolvedmass spectroscopy coupled w ith photoelectron spectroscopy13.Afterabsorbing two photonsat400 nm(6.20 eV),the fast predissociation ofC―Clbond on the repulsive nσ*/πσ*state takes (48±1)fs,while the dissociation of C―Clon the vibrationally excited ground state resulting from the internal conversion from the initially preparedππ*state takes(85±40)ps.The HCl elimination on the excited state takes(600±135)fs and on the ground state resulting from the internal conversion from theππ* state takes(14±2)ps.With 266 nm light,themolecule is populated to the nσ*stateand directC―Cl dissociation takes～48 fs.

In the presentstudy,three intermediatestates,ππ*,nσ*andπσ*, are reachablew ith theabsorption of a 200 nm photon.Asmentioned in the previous section,only the fastC―Clbond breaking channelon the nσ*/πσ*potentialsurface is comparable to the pulse duration(50 fs).Thus,ionization or dissociation to C3H5+Cl of C3H5Cl is thedominantchannel.Hence,moleculeson theππ*state and the nσ*/πσ*state corresponding to the slow dissociation channelw illbe ionized.Therefore,it isnotsurprising tofind that C3H5Cl+has abranch ratio as high as 55.26%.As forother ions, they are likely from the dissociation of the parent ion.This is in agreementwith ourobserved photoelectron spectra,whereelectrons from neutral radicalsarenotobserved.At400 nm,sinceno intermediate states can be reached with one 400 nm photon,two 400 nm photons can populate themolecule to the same intermediate statesasone 200 nm photon.However,C3H5+is dom inant in themassspectra,which is different from the resultsobserved at 200 nm.This difference is due to the fact that three 400 nm photons(9.30 eV)populate themolecule to a region which cannot beachieved by 200 nm photons.With thisabundant internalenergy,more dissociative states can be reached and fragmentation ismore likely to occur.A lthough the pathways for ionization are stilldominantafter theabsorptionof two400 nm photons,they are disturbed by the dissociative states before ionization.At long wavelength(800 nm),more dissociative states are reachable.For example,three800 nm photons populate themolecule to the nσ* state,where C―Clbond breaking takes～50 fs.In order to reach theππ*state populated by two 400 nm photons,onemore800 nm photon isneeded.Hence,it can be speculated that lessmolecules are populated on theππ*statesat800 nm than at400 nm.On the mass spectra,the C3H5Cl+signal isweakerat800 nm compared with at400 nm,which is in agreementw ith theaboveanalysis.

4 Conc lusions

In the presentwork,femtosecond laser pulseswere used to study the photodissociation and photoionization dynam ics ofC3H5Cl.The time-of-flightmass spectraaswellasphotoelectron spectra were obtained.We found that the ionization of C3H5Cl induced by two photons is the primary pathway at 200 nm. Fragment ions are mostly generated through dissociative ionization of the parentmolecule.At 400 nm,dissociation of the C3H5Clmolecule playsan important role.The fragment ionsshow an increasing branch ratio,and they are generated from the dissociation of the C3H5Clmolecule produced bymultiphoton ionization.At 800 nm,dissociation on the intermediate states becomesmore significant sincemore intermediate states are involved.The present study provides important information for the ionization and dissociation dynamicsof C3H5Cl togetherw ith our previous two-color time-resolved study.

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Multiphoton Dissociation and Ionization Dynamics of Allyl Chloride Using Femtosecond Laser Pulses

LIU Ning-Liang1,2SHEN Huan1,2,*
(1Co llege ofScience,Huazhong AgriculturalUniversity,Wuhan 430070,P.R.China;
2Institute ofApplied Physics,Huazhong AgriculturalUniversity,Wuhan 430070,P.R.China)

The dissocia tion and photoionization dynam ics of C3H5Clw ere studied at 200,400,and 800 nm with fem tosecond laser pulses.The time-of-flightmass spectra,laser power index and photoelectron images were recorded.At shortwavelength(200 nm),ionization of the parentmolecule was found to be the dom inant channel,while other ions were generated by the dissociation of C3H5Cl+.With the shift to long wavelength (e.g.,800 nm),fragment ions became dom inant,and were generated through themultiphoton ionization of neutral fragments after the photodissociation of C3H5C l.These resu lts im p ly that photodissociation p lays a significant role at long wave length,because neutral fragments are supposed to be generated from the intermediate states reached by 800 nm photons.At400 nm,the dissociation on the intermediate states is also critical,but is no t as high as that a t 800 nm.Taken toge the r,our resu lts dem onstrate tha t the dissocia tion/ ionization behaviors of a llyl ch loride are wavelength-dependent,and reveal the comp lex dynam ics of allyl ch loride a t200,400 and 800 nm.

Fem tosecond laser pulse;Time-of-flightmass spectrometry;Photoe lectron imaging;Allyl ch loride;Mu ltiphoton d issocia tion and ioniza tion

O643；O433；O561

Elshakre,M.Radiat.Phys.Chem.2015,112,49.

10.1016/j. radphyschem.2015.03.016

doi:10.3866/PKU.WHXB201611111

www.whxb.pku.edu.cn

Received:October 12,2016;Revised:November 11,2016;Published online:November11,2016

*Corresponding author.Email:shenhuan@mail.hzau.edu.cn;Tel:+86-27-87282197.

Theprojectwas supported by theNationalNatural Science Foundation of China(21403080,61308028)and Specialized Research Fund for the Doctoral Program of Higher Education,China(20130146120015).

国家自然科学基金(21403080,61308028)及高等学校博士学科点专项科研基金(20130146120015)资助项目©Editorialofficeof Acta Physico-Chim ica Sinica