A New kHz Velocity M ap Ion/Electron Im aging Spectrom eter for Fem tosecond Tim e-Resolved M olecular Reaction Dynam ics Studies

Zhi-gng HeZhi-cho ChenDong-yun YngDong-xu DiGuo-rong WuXue-m ing Yng

a.State Key Laboratory ofMolecular Reaction Dynam ics,Dalian Institute ofChem ical Physics,Dalian 116023,China

b.University of Chinese Academy of Sciences,Beijing 100049,China

A New kHz Velocity M ap Ion/Electron Im aging Spectrom eter for Fem tosecond Tim e-Resolved M olecular Reaction Dynam ics Studies

Zhi-gang Hea,b?,Zhi-chao Chena,b?,Dong-yuan Yanga,b,Dong-xu Daia,Guo-rong Wua?Xue-m ing Yanga

a.State Key Laboratory ofMolecular Reaction Dynam ics,Dalian Institute ofChem ical Physics,Dalian 116023,China

b.University of Chinese Academy of Sciences,Beijing 100049,China

A new velocity m ap im aging spectrometer is constructed for m olecular reaction dynam ics studies using time-resolved photoelectron/ion spectroscopy method.By combining a kHz pulsed valve and an ICCD camera,this velocity map imaging spectrometer can be run at a repetition rate of 1 kHz,totally com patiblew ith the fs Ti:Sapphire laser system,facilitating time-resolved studies in gas phasewhich are usually time-consum ing.Tim e-resolved velocity map imaging study ofNH3photodissociation at 200 nm wasperformed and the time-resolved total kinetic energy release spectrum of H+NH2products provides rich information about the dissociation dynam ics of NH3.These results show that this new apparatus is a powerful tool for investigating themolecular reaction dynam ics using tim e-resolved methods.

Photoelectron spectrum,Pum p/probe,Fem tosecond time-resolved

I.INTRODUCTION

Various experimentalmethods have been developed in the field of m olecular reaction dynam ics.They can generally be classified into two categories:energyresolved measurements and time-resolved measurements.In the former method,the quantum state distribution,translationalenergy distribution,angular distribution,etc.of the final reaction products are measured.The reaction mechanism and dynam ics are often indirectly inferred from the information of the final products,usually in concert w ith high-level theoretical calculations.Tim e-resolved m ethods,founded upon sem inal work of Zewail and coworkers[1?3]directly follow the energy and/or charge flow w ithin molecular system s during chem ical reactions w ith ultrafast laser pulses,X-ray pulses,electron pulses,etc.Unified pictures involving both energy-resolved and time-resolved measurements are often required in order to provide them ost detailed information on the dynam ics of a reaction.

In time-resolved studies of molecular reaction dynam ics,experim ental methods are m ainly diff ered in the way of probing a reaction triggered by the pum p pulse,such as nonlinear optical spectroscopy and transient absorption spectroscopy in the condensed phase,laser-induced fluorescence and resonant multiphoton ionization in the gas phase,and time-resolved X-ray or electron diff raction in condensed or gas phases,tim eresolved photoelectron spectroscopy(TRPES)in gasor condensed phases.Among them,the TRPESmethod has become a very powerfulm ethod in the molecular reaction dynam ics studies,especially in the excitedstate nonadiabatic dynam ics studies,due to a number of practical and conceptual advantages brought by the photoionization probingm ethod[4?19].

There aremany ways tomeasure photoelectron spectra,includingmagnetic bottlespectrometer,hem ispherical analyzer,cold target recoil ion momentum spectrom eter,velocity m ap im aging(VM I)spectrom eteretc.By combining w ith a fs laser system,time-resolved velocitymap imaging(TRVM I)ofphotoelectron hasbecom e a popular way tom easure the TRPES spectrum, w ith the capability of m easuring the translational energy and angular distributions of photoelectron simultaneously[10,17,20,21].A TRVM Iexperiment needs taking im ages atmany diff erent delays,including those w ith very small signal.It typically takes m any hours or even days to get enough statistics for data analysis. Therefore,it is very critical to increase the repetition rate of the TRVM Ispectrom eter in order to shorten the tim e needed or even m ake som e of the experim ents feasible.Commercial Ti:Sapphire fs laser systems can be run w ith a kHz repetition rate.However,most of the VM I spectrometers are lim ited to tens of hertz,due to the low fram e rate of CCD cameras and som etim es also the low repetition rate of pulse valves.It is practically im portant to increase the repetition rateof VM Ispectr-ometers up to kHz and tomatch w ith the fs laser systems.Suzuki and coworkers have developed a very elegant m ethod to run VM I at 1000 fram es/s(w ith a 512×512 pixel array)[22].Com p lem entary m etal oxide sem iconductor(CMOS)image sensor w ith multistage image intensifier was used to increase to frame rate of the camera,and programmable gate array circuit(FPGA)was used to perform real-time centers of gravity(COG)calculation[23]of the light spots on the phosphor screen and data transfer between the camera and a personal com puter.Suzuki’sm ethod is very advanced,but also relatively com plicated.In the present work,we report a new ly constructed VM Ispectrometer in which a combination of an Even-Lavie pulsed valve and an ICCD camera is used.W ith this sim p lem odification,charged particlescan be recorded at a frame rate of 1 kHz w ith a 1024×1024 pixel array.A lthough the readout rate is only lim ited to the order of 10 Hz and the real-tim e COG calculation is infeasiblew ith current configuration,the1 kHz imaging on theMCP/phosphor screen is collected w ith a high effi ciency,w ithout noise problem inherent to long exposuremethod which is often im p lemented when a low frame rate cam era is used. The abandonment of COG calculation makes the speed resolution lim ited to 2%?3%.But this is not a real sacrifice since the inherent broad bandw idth of fs laser pulsesw ill eventually lim it the speed resolution achievable in TRPES experiments.

II.NEW APPARATUS

A.Velocity map im aging spectrom eter

Thenew ly constructed VM Ispectrometer isvery similar to the setup asdescribed by Eppink and Parker[24], w ith necessary modifications suitable to 1 kHz repetition and fs tim e-resolved experim ents.The apparatus consists of a source chamber and an interaction chamber,both diff erentially pum ped,as shown in FIG.1. The source chamber and the interaction chamber are evacuated w ith a 3200 and a 1000 L/s magnetically levitated turbo-molecular pum p(Edwards,3306c and 1003c),respectively,sharing a backing pum p station (Pfeiff er,HiCube 80 Eco).The vacuum for both chamber w ith themolecular beam off are as good as 1×10?9Torr.The source and interaction chambers are separated by a skimmer w ith a 1-mm diameter aperture (Beam Dynam ics Inc.),which is mounted on a translatable gate-valve assembly[25].W ith this design,the source chamber could bevented to atmospheric pressure while the interaction chambermaintains base vacuum. This design is practically im portant for experiments w ith liquid or solid sam pleswhich need to be loaded into the cartridge of the Even-Lavie pulsed valve.By this design,this can be donew ithout theneed for a time consum ing pum p-up of the interaction chamber before the experiments are restarted.A typical turnaround time for sam p le rep lacem ent and subsequent recomm encement of data acquisition is about 1.5 h.

FIG.1 Schem atic view of the velocity m ap im aging spectrom eter.

The m olecular beam is produced by expanding gas samp le into vacuum through a pulsed valve[26](Even-Lavie valve,200μm diameter conical nozzle)operating at 1 kHz.For gas sam p les or liquid sam p les w ith reasonable vapor pressure at room tem perature,a prepared m ixture of samp le and carrier gas is used.W hile for liquid or solid samp lesw ith low vapor pressure,the sam p lesare preloaded into the cartridgewhich isw ithin the body of valve and then carrier gas flows through the cartridge before expanding into vacuum through the nozzle.The valve can be heated up to 250?C to ad just the concentration of the sam p le in the m ixture.The distance between the nozzle and the skimmer is ad justable from 20 mm to 120 mm.A fter passing through the skimmer,them olecular beam is intersected by laser beam s at the center of ion optics which consistsof three electrodes.The electrodesare 1-mm-thick stainless steel p lates of 56 mm in diameter mounted w ith alum inum oxide spacers of about 5 mm in length (K imball Physics Inc).The repeller electrode isa p late, while both the extractor and ground electrode have a 5-mm-diameter hole at the center.The ion optics could be used for ion or electron im aging by sim p ly changing the polarity of the power supp lies for them.The tim eof-fl ight axis is perpendicular to the p lane containing molecular and laser beam s.The ion opticsand the timeof-fl ight(TOF)path to the detector are shielded from the stray electric and m agnetic fields by a stainless steel and aμ-metal shielding tubes.A 75-mm dualm icro channel p late(MCP)detector backed by a P31 phosphor screen(Photek Lim ited,V ID275.The decay tim e of P31 to 10%brightness quoted by Photek is 40μs.) ismounted onto a flange at the end of the TOF tube. There isa 10-mm gap between the end of these shielding tubes and MCP detector to ensure adequate gas pum ping in the region close to the detector.The distancefrom the interaction point to the front surface of the MCP detector is about 558mm.The MCP p lates and the phosphor screen are powered by a 3-channel high voltage power supp ly(Photek Lim ited,DPS3),w ith the back plate of MCP assembly gated w ith a high voltage gate unit(Photek Lim ited,GM-MCP).Charged particles im pinging on the MCP/phosphor screen are recorded by an ICCD cam era w ith a 1024×1024 pixel array(Princeton Instruments,Pi-max3 1024i).The intensifier of the ICCD camera is operated at a repetition rate of 1 kHz,fully com patiblew ith the fs laser system. The intensifier is timed w ith the laser system and gated. The intensifier is only open when the ion or electron cloud im pinges on the MCP/phosphor screen,greatly reducing the background.The accumulated signal on the CCD chip w illbe read out before getting saturated, typically at a rate of 10 Hz or even less,depending on the signal level.By thism ethod,the 1 kHz imaging on the MCP/phosphor screen is collected w ith a high efficiency,w ithout noise problem inherent to long exposure method which is often used when a low repetition rate CCD camera is used in a 1 kHz experiment.A photomultip lier tube(Hamam atsu,H10722-20)m ounted beside the ICCD camera is used to record TOF spectrum of charged particles arriving at MCP/phosphor screen detector,by sw itching off the gate unit and running the detector at the DC m ode.

B.Fem tosecond laser system

The laser system,shown schematically in FIG.2, consists of a fully integrated Ti:Sapphire oscillator/am p lifier system(Coherent,Libra-HE),two commercial optical parametric am plifiers(OPAs,Coherent,OPerA-Solo)and a fourth harmonic generation (FHG)box.The regenerative am p lifier delivers about 3.8m J/pulse at a repetition rate of 1 kHz,w ith a pulse duration slightly less than 50 fs and a central wavelength of 800 nm.The output of the am plifier is sp lit into 4 beam s,two of them w ith about 1.3 m J/pulse, the third 0.9 m J/pulse and the last 0.3 m J/pulse.The fi rst two are used to pum p two OPAs.The outputs of OPAsarewavelength tunable from 240 nm to 2600 nm, w ith m icrojoule pulse energy level even at wavelengths of the lowest effi ciency.The output of one of the OPAs is furtherm ixed w ith the 0.3m J/pulse beam in a BBO crystal to push the wavelength down to 200 nm.The 0.9 m J/pulse beam is used to pum p the FHG box in which the fundamental is frequency doubled,trip led, and then quadrupled by a series of BBO crystals.Depending on the specific experiment,either the third harm onic or the fourth harm onic isused,w ith typicalpulse energiesof12 and 1μJ,respectively.Thereare twomotorized high-performance travelling linear stage(Newport,M-ILS250HA),one between the am p lifier and one of the OPAs and the other between the am p lifier and the FHG box,which provide the capability to change the tem poral delay between any two of the three arm s w ith a m inimum step size of 2 fs and a maximum delay of 1.5 ns.These laser beam sare combined w ith one or two thin dichroic m irrors and focused into the VM I spectrom eterwhere they intersected w ith them olecular beam at 90?.

FIG.2 Schem atic overview of the fem tosecond laser system.

C.Data acquisition program

A tim e-resolved photoelectron or photoion imaging experiment needs taking images at m any diff erent delays,including thosew ith very small signal.Therefore, it usually takesmany hours or even days to get enough statistics for data analysis.The data acquisition needs to be done by systematically changing the pump/probe delay back and forth many times in order to m inim ize the uncertainties caused by the variations in pulse energies of laser pulses,molecular beam intensity,pointing of laser beam s,etc.A reliable and automated data acquisition program is therefore necessary.

We have developed such a program in LabVIEW environm ent.Them ain featuresare listed below:(i)Data acquisition parameters setting including the number of delays scanning,the number of laser shotsat each delay in a delay scanning for pum p/p robe signal,the number of laser shots at each delay in a delay scanning for pum p or probe only signal(single color signal is delay independent and subtracted in data analysis as background.),the delays(equally or non-equally spaced) where the im ages are taken.(ii)Autom atically change the pum p/probe delay bymoving themotorized travelling linear stageand read thestatusofstageas feedback. (iii)Autom atically block/unblock pum p or probe laser beam using optical shutters.This provides the capability of taking signal and background alternatively at each delay.(iv)Real-time data analysis,including the reconstruction of the 3D distribution of charged particles from the 2D images[27],and the further derivation of the velocity and angular distributions.Two methods of reconstruction of the 3D distribution are im p lemented:one is the basis-set expansion m ethod[28?30] and the other is sim ilar to that proposed by Cho andNa[31]and Townsend and coworkers[21],but developed independently.(v)COG calculations[23]are also integrated into this program although it is disabled in the fs tim e-resolved experim ents due to the lim ited resolution caused by the bandw idth of fs laser pulses.

III.EXPERIMENTAL RESULTS

A.Velocity calibration of VM I spectrom eter

The velocity calibration of the apparatus was done by measuring the photoelectron im age from multiphoton ionization of Xenon atom at 400 nm(second harmonic of the fundamental).The Xebeam wasgenerated by expanding neat Xe of 5 bar using the Even-Lavie pulsed value operating at 1 kHz repetition rate.The raw photoelectron image accumulated over 0.2 m illion laser shots is shown in FIG.3(a)after the subtraction of background which is taken w ith the laser and molecular beam s tem porally separated.The photoelectron 3D distribution is reconstructed using theβ-basis expansion method[29]and the cut through the center of the 3D distribution is shown in FIG.3(b).Integrating the 3D distribution over recoil angles gives the distribution of photoelectron along the radial direction,as shown in FIG.3(c).The peaks in this distribution are readily assigned to photoelectron of Xe ionized w ith 4,5 and 6 photons of 400 nm.There are two peaks correspond to 5 photons ionization process,resulting into the ground(Xe+(2P3/2))and spin-orbit excited (Xe+(2P1/2))cation,respectively.The kinetic energy of the photoelectron at each peak in this distribution is readily calculated.In FIG.3(d),the square root of the kinetic energy of photoelectron at each peak is p lotted against the radius(in pixel)of the corresponding ring in the im age.This p lot is very well fi tted w ith a straight line crossing the ordinate at 0 which proves the VM I spectrometer is properly working[24].Measurem ents at diff erent voltages,w ith a fixed ratio between repeller and extractor electrodes were also perform ed (not shown)which proves that the VM I spectrometer works properly over a w ide range of voltages.

B.Fem tosecond tim e-resolved experim ent

In a test experiment,the TRVM Istudy of NH3photodissociation at 200 nm is performed[20,32].The time-resolved totalkinetic energy release(TKER)spectrum obtained by im aging the H atom products provides rich information about the H-atom elim ination and the coupling between the initially excited S1state and the ground state S0of NH3.

FIG.3(a)The raw photoelectron image from multiphoton ionization of Xe at 400 nm.(b)The cut through the center of the 3D distribution derived from(a)using p-Basex transform ationm ethod.(c)The speed distribution(in pixelunit) obtained from(b).(d)The square root of kinetic energy of electronvs.the radius of the rings in(b).The double arrow indicates the polarization direction of the laser pulse.

The NH3molecular beam was generated by expanding a m ixture of 1%NH3in He carrier gas of 3 bar using a pulsed valve.The pum p laser was at 200 nm (～0.5μJ/pulse)which excites NH3from the ground state toν2′=4 in the fi rst electronically excited state (A?(1A′2′))[33,34].The nascent H-atom products were detected using 2+1 resonance-enhanced multiphoton ionization(REMPI)at 243.1 nm(～6μJ/pulse).By systematically changing the delay between the pum p and probe lasers,the time-resolved TKER spectrum is derived.The H-atom signal w ith pum p or probe laser only was also recorded and subtracted as background. TheT0and the cross-correlation between pump and probe laser pu lses were measured using the delay dependence of the total NH3cation yield which gave a cross-correlation of200 fs.It has to be em phasized that while a resolution of 200 fs is suffi cient for the current study amuch short cross-correlation is expected if both laser pulses have been compressed w ith prism pairs.

FIG.4 Tim e-resolved TKER spectrum of the H+NH2fragm ents resu lting from the photodissociation of NH3at 200 nm. The energetic lim its for generating the ground state NH2eX(2B1)and the fi rst excited state NH2?A(2A1)are indicated w ith dash arrow s.

In FIG.4,the time-resolved TKER spectrum is shown.At negative delays where the probe pulse proceeds the pum p pulse,there isno appreciable H+signal, suggesting the H+signalat positive delays is generated by the pum p pulse(asneutralH atom)and then probed by the probe pulse.Att>0,the H+signal rises quickly and then approachesa p lateau at 1 psand remains constant beyond,indicativeofa fast H atom elim ination.A cut of this time-and energy-resolved distribution along the delay coordinate at 1 ps is also p lotted beneath the 2D data,which represents the kinetic energy distribution of H atom productswhen the reaction is com p leted entirely.At 200 nm,there are two channels open:the ground state NH2eX(2B1)and the fi rst excited state NH2eA(2A1).The energetic lim its for generating these two electronic states of NH2are indicated w ith dash arrows in FIG.4.This distribution is very sim ilar to that derived in previous photodissociation study[32], except a lower energy resolution in current study due to amuch broader bandw idth of the fs laser pulses used here.M ost interestingly,it is very clear from FIG.4 that the appearance time of the H+signal is dependent on the kinetic energy of the H+ion:the H+ionsw ith higher kinetic energiesappearmore prom ptly and those w ith lower kinetic energies relatively slow ly,indicating that the H atom s produced at different delays have a different kinetic energy distributions.The H+signal transients integrated over a low and high kinetic energy ranges are p lotted at the left figurewhich clearly shows the kinetic energy dependence of the time scales for H atom elim ination.Using an error function,convoluted w ith the Gaussian cross-correlation between pum p and probe pulsem easured independently,these two curves arewell fi tted,giving appearance time of 322 and 91 fs for“slow”and“fast”H atom products,respectively. The im p lications of these observation to the photodissociation dynam ics of NH3and the nonadiabatic coup ling between the fi rst excited and ground states are exp lained in detail in previous study[20]by Stavros and coworkers.We are not going to reiterate here.

IV.CONCLUSION

A new VM I spectrometer has been constructed for molecular reaction dynam ics studies using a tim eresolved photoelectron/ion spectroscopy m ethod.By combining a kHz pulsed valve and an ICCD camera, thisVM Ispectrometer can be run at a repetition rateof 1 kHz,totally com patible w ith the fs Ti:Sapphire laser system,facilitating time-resolved studies in gas phase which are usually time-consum ing.A fully automated data acquisition program was also developed for this TRVM I spectrometer.In a test experiment,TRVM I study of NH3photodissociation at 200 nm was performed.The time-resolved TKER spectrum of H+NH2products shows that the elim ination time of the H products is clearly dependent on their kinetic energy, suggesting a rich dynam ics in the photodissociation of NH3.These results show that this new apparatus is a powerful tool for investigating the m olecular reaction dynam ics using time-resolved m ethods.

V.ACKNOW LEDGM ENTS

This work was supported by the National Basic Research Program of China(No.2013CB922200), the M inistry of Science and Technology of China (No.2012YQ12004704),and the National Natural Science Foundation of China(No.21573228).

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ceived on February 19,2017;Accepted on March 14,2017)

?These authors contributed equally to this work.

?Author to whom correspondence shou ld be addressed.E-m ail: wugr@dicp.ac.cn