Time slicing in 3D momentum imaging of the hydrogen molecular ion photo-fragmentationN. Kaya, G. Kaya, F. V. Pham, J. Strohaber, A. A. Kolomenskii, and H. A. Schuessler

Citation: Rev. Sci. Instrum. 88, 023104 (2017); doi: 10.1063/1.4974743View online: http://dx.doi.org/10.1063/1.4974743View Table of Contents: http://aip.scitation.org/toc/rsi/88/2Published by the American Institute of Physics

REVIEW OF SCIENTIFIC INSTRUMENTS 88, 023104 (2017)

Time slicing in 3D momentum imaging of the hydrogen molecular ionphoto-fragmentation

N. Kaya,1,2,3,a) G. Kaya,1,4 F. V. Pham,1 J. Strohaber,1,5 A. A. Kolomenskii,1and H. A. Schuessler1,21Department of Physics, Texas A&M University, College Station, Texas 77843-4242, USA2Science Program, Texas A&M University, Doha 23874, Qatar3Department of Physics, Giresun University, Giresun 28200, Turkey4Technical Sciences of Vocational School, Canakkale Onsekiz Mart University, Canakkale 17020, Turkey5Department of Physics, Florida A&M University, Tallahassee, Florida 32307, USA

(Received 10 August 2016; accepted 10 January 2017; published online 7 February 2017)

Photo-fragmentation of the hydrogen molecular ion was investigated with 800 nm, 50 fs laser pulsesby employing a time slicing 3D imaging technique that enables the simultaneous measurement of allthree momentum components which are linearly related with the pixel position and slicing time. Thisis done for each individual product particle arriving at the detector. This mode of detection allows usto directly measure the three-dimensional fragment momentum vector distribution without having torely on mathematical reconstruction methods, which additionally require the investigated system to becylindrically symmetric. We experimentally reconstruct the laser-induced photo-fragmentation of thehydrogen molecular ion. In previous experiments, neutral molecules were used as a target, but in thiswork, performed with molecular ions, the initial vibrational level populations are well-defined afterelectron bombardment, which facilitates the interpretation. We show that the employed time-slicingtechnique allows us to register the fragment momentum distribution that reflects the initial molecularstates with greater detail, revealing features that were concealed in the full time-integrated distributionon the detector. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4974743]

I. INTRODUCTION

New opportunities and interest in understanding prob-lems in molecular dynamics such as photo-ionization andphoto-dissociation processes have been generated with bothfast timing and 2-dimensional position determination abili-ties in new ion detector systems. High resolution and effi-ciency are achieved by combining micro-channel plate (MCP)detectors with delay-line anodes and make it possible to mea-sure the three components of the momentum vector for eachcharged fragment from the dissociation processes.1,2 Coinci-dence measurements involving two or more charged fragmentsare improved by the performance of the detectors with bet-ter spatial and temporal detection resolution, efficiency, andreliability, as well as a significantly decreased dead time.Simultaneously, having available delay-line anodes with athree-layer hexagonal design, a better precision of the timedigitizers and advanced signal-reconstruction algorithms wasobtained.3 Most notably, unless the difference between theirtimes of impact and spatial separation is very small, it is pos-sible to separate explicitly the signals produced from the twocharged fragments that hit the detector at the same time, butat different positions, or the same position, but at differenttimes.4

Our experiment contains such delay-line detectors andbuilds on Figger’s groundbreaking work ranging from spec-troscopy of helium hydride5 to strong-field dissociation of the

a)necati@physics.tamu.edu.

hydrogen molecular ion.6–12 The apparatus can produce anintense beam of many kinds of molecular ions based on theduo-plasmatron principle, where the electrons from the cath-ode produce a plasma in the gas introduced at low pressure.The positive ions are then extracted. We have measured currentdensities of more than 0.35 µA/mm2 for the hydrogen molecu-lar ions. The ion beam apparatus now features a MCP detector(HEX 80, RoentDek) with <100 µm position resolution and∼1 ns time resolution.

Despite the unique information that can be gained byusing the hydrogen molecular ion targets, these studies arerare due to the additional experimental complexity of using anion beam as a target. Although theoretical studies commonlyuse the hydrogen molecular ion for model calculations due toits simplicity (e.g., Refs. 13 and 14), experimentalists studythe more accessible neutral hydrogen molecule (e.g., Refs. 15and 16). In some experiments with the hydrogen molecule, theexperimental conditions are tailored to enable the study of thetransient hydrogen molecular ions that are formed early in theinteraction with a laser pulse (e.g., Refs. 17 and 18). However,the physics of the hydrogen molecular ion in an intense laserbeam is more clearly understood by directly using the hydro-gen molecular ion as a target. We note that the result of theaction of the same laser pulse on a molecular ion in an ionbeam can be expected to differ from that on a neutral hydro-gen molecule. Indeed, for the former, the dissociation can beaffected by excited vibrational states, while for the latter theground state before ionization has lower energy, and therefore,it is less likely to dissociate.

0034-6748/2017/88(2)/023104/6/$30.00 88, 023104-1 Published by AIP Publishing.

023104-2 Kaya et al. Rev. Sci. Instrum. 88, 023104 (2017)

We obtain results on the photo-fragmentation of the hydro-gen molecular ion with ultrashort laser pulses by employing atime-sliced 3D imaging technique. Time slicing techniqueshave been successfully used in other experiments such astime-of-flight mass spectrometers to avoid the effects dueto intensity averaging.19 We experimentally reconstruct thelaser-induced photo-fragmentation of the hydrogen molecularions prepared by electron impact ionization, while the neutralmolecules are used as a target in most previous experiments.We describe our ion beam apparatus and its potential for futureapplications.

II. EXPERIMENTAL DETAILS

As it is shown in Fig. 1, our ion beam apparatus hasthree vacuum chambers, and each chamber is pumped bya turbo molecular pump. The main components are an ionsource, a magnet for mass selection, different size aperturesfor beam collimation, and Faraday cups for measuring the ioncurrent.

In our experiment, a duo-plasmatron ion source with a dcelectric discharge was used to generate the molecular ions.20,21

Neutral hydrogen gas is sent into the region between an anodeand a hollow cathode. The electrons are emitted from thehollow cathode, and a potential difference of 450–600 V accel-erates them towards the anode. The intermediate electrode isheld at a potential of �100 V with respect to the anode, soit electrostatically concentrates the electron flux. In addition,we have a solenoid magnet with stabilized current (usuallyin the range of 1 to 1.8 A) surrounding the ion source. Theapplied magnetic field compresses the plasma and towardsthe anode, the magnetic field strength increases so that theirradial velocity component increases while the axial componentdecreases when the electrons come close to the anode. Hence,the electrons’ density is getting higher near the anode in orderto increase the ionization efficiency of the neutral hydrogenmolecules. A high potential difference between the anode andan extraction electrode is applied to extract and accelerate thepositive molecular ions from the plasma. In order to extract

the positive ions, the whole ion source was held at a positivepotential of 12 kV with respect to the rest of the apparatus thatwas grounded. The ion current is typically around 100 µA withthe anode having an aperture of 150 µm. This current of theextracted ions could be optimized by adjusting the solenoidmagnet current surrounding the ion source.

We used a set of vertical and horizontal electrostaticdeflection plates (DP1) and an Einzel lens (EL1) in order todirect the extracted positive molecular ions into a sector mag-net (SM) through the entrance slit A1 (width of 5 mm). Thenby changing the magnetic field of the magnet, we deflected thehydrogen molecular ions by 90◦ to pass through the exit slitA2 (width of 5 mm). The second set of electrostatic deflec-tion plates (DP2) is used to steer the selected ion beam. Thenthe beam enters Einzel lens EL2. The combination of Einzellenses EL1 and EL2 is used as an electrostatic telescope tocollimate the ion beam and maximize the ion beam current.

To achieve the alignment of the ion beam, the ion currentis measured at different points along the beam path. There aretwo collimating apertures: A4 (300 µm circular aperture) andA7 (a 300 × 25 µm aperture) and the secondary pre-aperturesA3 and A6 with diameters of 2 mm and 1 mm, respectively,were placed in front of each collimating aperture to make thealignment easier. All insulated apertures and Faraday cups FC1and FC2 were connected to a switchable electrometer (Keith-ley Instruments, model 617). Intermediate FC1 was set on aside and used for adjustments of the ion beam, i.e., H+2 ionbeam could be redirected to FC1 by the deflecting plates.22

The FC2 in front of the MCP detector was positioned in thecenter to block the unfragmented H+2 ions and to minimize theblocking of the detected fragment (H and H+) structures in theouter areas of the MCP.

For the alignment, we first found the best combinationof voltages on the Einzel lenses, the deflection plates, and thesector magnet by optimizing the ion current at aperture A4 andits pre-aperture A3. After that, we followed the same proce-dure for the aperture A7 and its pre-aperture A6. Finally, wemaximized the current on the Faraday cup FC2. We repeatedthe whole procedure several times with similar initial settingsto achieve an optimal and sufficient ion current.

FIG. 1. Layout of the ion beam apparatus. IE: interme-diate electrode; DP: deflection plates (13 mm × 22.5 mm× 0.85 mm); EL: Einzel lenses; SM: sector magnet, R= 150, B = 10G, 8A; A1, A2: two perpendicular slits of5 mm width each; A3: 60 mm circular plate with 2 mmcircular hole, A4: 60 mm × 60 mm square plate with 300µm circular hole, A5: 8 mm circular hole, A6: 60 mm ×60 mm square plate with 3 mm circular hole, A7: 25µm×200 µm slit and 1 mm × 1 mm square hole; IA: ion accel-erator; S: vertical slit with 0.75 mm gap; IG: ion gate, 92mm × 40 mm deflection plates with 5 mm separation; L:focusing lens; FC1, 2: Faraday cups; MCP: microchannelplate detector. Inset (a): Interaction volume defined by thecrossing of a laser beam (red cylinder) and an ion beam (ablue box). Inset (b): Schematic diagram of the geometryfor the beam collimation along the x-axis.

023104-3 Kaya et al. Rev. Sci. Instrum. 88, 023104 (2017)

The experimental resolution depends on the width anddivergence of the ion beam. They are determined by the sizesof the collimating apertures along the laser polarization axis (zaxis) and the positions of the apertures and the detector on theion beam axis. The collimation geometry in the x-z plane isshown in inset (b) of Fig. 1. The pair of outer lines shows themaximum allowed divergence of the beam, and the inner linesillustrate the collimated ion beam. The widths on the detectorare indicated by the letters s and S. The divergence anglesα = 0.25 mrad and β = 0.30 mrad and the widths of the ionbeam s = 701 µm, S = 884 µm can be easily calculated from thesimilarity of the triangles. By using a 300 µm circular apertureA4 and a 300 µm × 25 µm aperture size A7 for collimatingthe ion beam, we could optimize the current of the 12 keVhydrogen molecular ion beam measured on the Faraday cupFC2 up to 3–4 nA.

The collimated, velocity bunched,23 and therefore nearlymonochromatic hydrogen molecular ion beam was crossedwith the focused laser beam at right angle. Laser pulses froma chirped pulse amplification laser system with a repetitionrate of 1 kHz, pulse energies of 1.0 mJ, duration of 50 fs,and centered at the wavelength of 800 nm were focused byan achromatic lens. The focal position of the femtosecondlaser beam was adjusted coarsely before the actual experi-ment by observing the position of the laser-induced opticalbreakdown, when the apparatus was filled with air. The over-lap of the two beams was assured by using a plate with a 50µm pinhole, placed at 45◦ with respect to the ion beam thatpassed through it, and then the laser beam was adjusted topass through it too. Our focus diameters were greater than theheight of the rectangular molecular ion beam (25 µm), and theRayleigh length was large compared to the width of the molec-ular beam (200 µm), which reduced the volumetric effect ofthe intensity variations within the interaction region (Fig. 1,inset (a)). We have a laser beam diameter of 60 µm, a molec-ular ion velocity of 1.5 × 106 m/s, and a typical current ofabout 80 nA in the interaction region, which corresponds to∼40 molecules within the cross section of the focused laserbeam.

Under the action of laser pulses, the hydrogen molecu-lar ion could dissociate into fragments, H+2 → H+ + H, whichcontinued moving toward the MCP detector, and this time-of-flight (TOF) part of the apparatus is in essence a TOFspectrometer. The dissociated fragments were detected by thethree-dimensional imaging system, namely an arrival time sig-nal at the MCP detector, and the position signals from theanode delay line were recorded allowing the evaluation of thefragment momentum. The distance between the interactionregion of the laser radiation with the ions and the detec-tor was 140 cm. All TOF information was measured relativeto the start signal from a photodiode triggered by the laserpulse.

The data analysis involves the reconstruction of themomenta of the ions after calibrating all the necessary exper-imental parameters. The ion detector provides the timing andposition information of the detected particles. Once the dis-tance of the detector from the interaction region (b), the valueof the electric field (E) over the length of the TOF spectrom-eter, and the time-of-flight (ti) and the hit position (xi, yi) are

known precisely, the three components of momentum gainedby an ion in the laser field in three mutually perpendiculardirections (px ,i, py,i, pz,i) can be calculated by using simplerelations,

px,i =mi(xi − xorig)/ti, py,i = mi(yi − yorig)/ti,

pz,i =mib/ti − Eqiti/2,(1)

where mi and qi are masses and charges of the fragments;xorig and yorig are the coordinates of the dissociation point. Inthis equation the timing information of the detected particle isprovided by the difference between the “start” and the “stop”signals from the photodiode and the MCP detectors with thecorrection, which depends on the relative delay of the signalsfrom the photodetector to the MCP extrapolated to zero TOFdistance. The two transverse positions, xi and yi, of the ion areobtained from the position sensitive detector.

In a photon-induced dissociative ionization process withtwo fragments, conservation of linear momentum determinesthat the fragments have equal values of momenta, but withopposite directions. Using momentum conservation, we obtainthe following expressions:

xorig =m1t2x1 + m2t1x2

m1t2 + m2t1, yorig =

m1t2y1 + m2t1y2

m1t2 + m2t1. (2)

In fact, the dissociation point is localized along the x-axis bythe focusing spot size (∼60 µm), which is less than the ionbeam diameter, and along the y-axis by the smallest of thedoubled Rayleigh range (∼0.6 mm) and the ion beam extensionin the y direction (0.2 mm). Then imposing the conditions ofEq. (2) and the additional conditions

m1(x1 − xorig)/t1 =m1(x2 − xorig)/t2,

m1(y1 − yorig)/t1 =m2(y2 − yorig)/t2,(3)

only those pairs of arrival events (t1, x1, y1), (t2, x2, y2) on thedetector are selected, which satisfy momentum conservationwith the temporal uncertainty defined by the accuracy of timemeasurement (∼1 ns). Such filtering considerably reduces thebackground of false counts. The sum-momenta distributionsthen are approaching δ-functions: they are essentially nonzeroonly in the narrow region close to the x and y zeros.

Assuming the absence of the field for the kinetic energyrelease (KER), we obtain similar to Ref. 24,

KER=(p2

rel,x + p2rel,y + p2

rel,z)(m1 + m2)

2m1m2, (4)

prel,x =(px,2 − px,1

)/2=

m1m2(x2 − x1)m1t2 + m2t1

,

where prel,y =(py,2 − py,1

)/2=

m1m2(y2 − y1)m1t2 + m2t1

,

prel,z =(pz,2 − pz,1

)/2=

m1m2(t2 − t1)b(m1 + m2)t1t2

.

III. RESULTS AND DISCUSSION

A schematic view of the TOF spectrometer part of theapparatus is shown in Fig. 2(a). The laser field is linearlypolarized perpendicular to the ion beam. The dissociated ionfragments, H+ and H, obtain momentum components of the

023104-4 Kaya et al. Rev. Sci. Instrum. 88, 023104 (2017)

FIG. 2. (a). Schematic view of the TOF spectrometer: the laser beam interacts with the ion beam that passes through the acceleration plates resulting in H+ andH fragments; the lower graph shows the voltage distribution on the ion acceleration plates. (b): The neutral and charged fragments resulting from the photo-dissociation of the hydrogen molecular ion after applying ion acceleration voltage with magnitude +750 V for ∆t = 60 ns for the separation of ion and neutralfragments: TOF spectra (upper graphs) and related fragment distributions accumulated for TOF detection intervals 220 ns–340 ns, 220–280 ns, and 280–340 ns(lower graphs), 1 pixel = 25 µm.

opposite signs in the plane normal to the initial ion beam prop-agation direction and thus spatially separate. A longitudinalelectric field, typically 20–30 V/mm, was applied to the accel-eration plates in the ion-beam direction to accelerate the ionfragments in order to clearly separate the charged and neu-tral fragments from each other. The original ion beam wasinitially slowed down, and after dissociation the newly cre-ated fragments were accelerated (see the voltage diagram atthe bottom of Fig. 2(b)), thus differentiating the dissociationproducts from the ions in the original beam, since the energyof the latter was not changed in this process. The applica-tion of an accelerating voltage with a magnitude of +750 Vresulted in two distinct peaks in the TOF spectra for the neu-tral and charged fragments from the hydrogen molecular ionphoto-dissociation, which are shown in the upper graphs ofFig. 2(b). The fragment distributions accumulated in TOFintervals between 220 ns and 340 ns, 220 and 280 ns, and280 and 340 ns are presented in the graphs of Fig. 2(b).

Figure 3 displays a series of slices through the momen-tum sphere of the hydrogen molecular ion fragments afterphoto-dissociation with the laser pulses. The numbers at thebottom of the images indicate the TOF time intervals usedto generate the images. The time interval of 230–240 ns

corresponds to the center of the distribution (most intensesymmetric half-moon shaped curves in Fig. 3). This centerslice of the TOF distribution in the inset of Fig. 3 is sharperthan the momentum distribution of the full data set (197–274ns), which allows to resolve the peaks corresponding to thedifferent vibrational states as is indicated. The KER distribu-tion per fragment (H or H+), calculated for the most distinctapex portion (∼10◦) of the center slice 230–240 ns of Fig. 3,is presented in Fig. 4. The labels v = 6. . .10 correspond tothe peaks due to the prominent transitions from the vibra-tional levels of H+2 , which depend on the initial excitationdistribution in the produced H+2 ions and the Franck–Condonfactors, having for v = 9 the highest transition probability dueto near-resonance conditions for the used laser wavelength.6

The energy spread of the ions extracted from a duoplas-matron ion source is less than 10 eV.25 In the propagationdirection, this leads to a half-width of the velocity distribution∼200 m/s with spread of the TOF times of only ∼0.4 ns, whichresults in the fragment KER spread on the MCP of ∼0.2 meV.The divergence of the ion beam just before the fragmenta-tion gives equivalent energy spread of ∼less than 10 meV. Theobserved KER of fragments also causes variation of the TOF,resulting in the energy spread on the MCP less than ∼30 meV.

FIG. 3. Series of time slices (10 ns intervals between197 ns and 274 ns) through the hydrogen molecularion momentum distribution. The momentum distributionof the center slice (230 ns–240 ns) is clearly sharperthan the momentum distribution of the full data set (197ns–274 ns).

023104-5 Kaya et al. Rev. Sci. Instrum. 88, 023104 (2017)

FIG. 4. KER distribution of H and H+ fragments, calculated for the apexportion (∼10◦) of the center slice 230–240 ns of Fig. 3. The labels v = 6. . .10correspond to peaks due to the most prominent transitions from the vibrationallevels of H+2 .

FIG. 5. Time-of-flight spectrum of fragments from the hydrogen molecularion photo-dissociation. No voltage was applied to the ion accelerator in frontof the detector.

Indeed, the observed variations in the KER distribution withintervals of ∼70–90 meV26 for the H and H+ fragmentsfrom vibrational states with n = 6–10 are well resolved(Figs. 3 and 4).

FIG. 6. The time-sliced 3D reconstruction of the fragment momentum distri-bution consisting of separate slices from photo-dissociation of the hydrogenmolecular ions.

In order to reconstruct experimentally the laser-inducedphoto-fragmentation 3D structure of the hydrogen molecularions, we applied no voltage to the ion accelerator, therebythe distributions of the charged and neutral fragments werenot time-shifted on the MCP detector. The TOF spectrum ofphoto-dissociation of the hydrogen molecular ions is shownin Fig. 5 and a 3D reconstruction of the fragment momen-tum distribution consisting of separate slices is presented inFig. 6.

IV. CONCLUSION

Using an ion beam apparatus with a state-of-the-art detec-tor featuring the position and time resolution, we obtainedresults for the fragmentation of the hydrogen molecular ionwith 800 nm, 50 fs laser pulses by employing a time-sliced3D imaging (a measurement of all the three momentum com-ponents). A series of slices through the hydrogen molecularion momentum sphere after photo-dissociation was obtained.The analysis of these slices indicates that the center slice ofthe TOF distribution is clearly sharper than the momentumdistribution of the full data set, which allows resolving thepeaks corresponding to the different vibrational states. Wehave also demonstrated that by using the time-slicing tech-nique together with an appropriate voltage applied to the ionaccelerator, we could selectively study the charged (H+) orneutral (H) fragments. Consequently, the developed detectionsystem is well-suited for 3D imaging of the fragment momen-tum distribution and is a promising tool for imaging studies ofphoto-fragmentation of the atomic and molecular systems ofinterest.

ACKNOWLEDGMENTS

This work was supported by the Robert A. Welch Founda-tion Grant No. A1546 and the Qatar Foundation under GrantNo. NPRP 6-465-1-091.

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