Coulomb explosion of CD3I induced by single photon deep inner-shell ionisation
M. Wallner, J.H.D. Eland, R.J. Squibb, J. Andersson, A. Hult Roos, R. Singh, O. Talaee, D. Koulentianos, M.N. Piancastelli, M. Simon, R. Feifel
CCoulomb explosion of CD I induced by single photon deep inner-shell ionisation
M. Wallner, J.H.D. Eland,
2, 1
R.J. Squibb, J. Andersson, A. Hult Roos, R. Singh, O. Talaee,
1, 3
D. Koulentianos,
1, 4
M.N. Piancastelli,
4, 5
M. Simon, and R. Feifel ∗ Department of Physics, University of Gothenburg, Origovägen 6B, 412 58 Gothenburg, Sweden Department of Chemistry, Physical and Theoretical Chemistry Laboratory,Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom Nano and Molecular Systems Research Unit, University of Oulu,P.O. Box 3000, FI-90014 University of Oulu, Finland Sorbonne Université, CNRS, Laboratoire de Chimie Physique-Matière et Rayonnement, F-75005 Paris Cedex 05, France Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
L-shell ionisation and subsequent Coulomb explosion of fully deuterated methyl iodide, CD I,irradiated with hard x-rays has been examined by a time-of-flight multi-ion coincidence technique.The core vacancies relax efficiently by Auger cascades, leading to charge states up to 16+. Thedynamics of the Coulomb explosion process are investigated by calculating the ions’ flight timesnumerically based on a geometric model of the experimental apparatus, for comparison with theexperimental data. A parametric model of the explosion, previously introduced for multi-photoninduced Coulomb explosion, is applied in numerical simulations, giving good agreement with theexperimental results for medium charge states. Deviations for higher charges suggest the need toinclude nuclear motion in a putatively more complete model. Detection efficiency corrections fromthe simulations are used to determine the true distributions of molecular charge state produced byinitial L1, L2 and L3 ionisation.
Molecules exposed to sufficiently energetic photonsmay be totally destroyed in a process where all bondsare broken and all or most of the atoms become posi-tively charged and repel each other by the Coulomb force.This rapid repulsion and subsequent fragmentation of amolecule was termed Coulomb explosion in 1966 by T.A.Carlson and R.M. White [1]. Since then, Coulomb ex-plosions have been studied in many different ways, forinstance, by photoion-photoion coincidence spectroscopyin the vacuum ultraviolet (VUV) and soft X-ray region[2–4], by coincidence imaging [5], by time-resolved pump-probe techniques involving ultraviolet (UV) [6], and byX-ray free electron laser (XFEL) ionisation utilising few-photon absorption processes [7–12]. The dynamics ofthe charge rearrangement essential for the Coulomb ex-plosion have previously been studied using time-resolvedpump-probe techniques on molecular iodine [13, 14] andin methyl iodide [15, 16] where the results match wellwith a classical over-the-barrier model.In this work we have used a multi-ion coincidence tech-nique in combination with hard X-rays to investigate sin-gle photon-induced Coulomb explosion of fully deuter-ated methyl iodide, CD I, upon creation of a core va-cancy in the n = 2 shell of iodine, building on the originalwork of Carlson and White [1], where Coulomb explo-sion was first explored experimentally by a coincidencemethod. For the data interpretation, we explore a two-parameter model recently introduced by Motomura et al.[9] , who studied Coulomb explosion of CH I from ionisa-tion induced by the absorption of several X-ray photonswithin a pulse duration of ∼
10 fs.In the present experiment, photoionisation occurs pri-marily in selected L-shells of iodine induced by a single photon followed by subsequent Auger cascades, leadingto high charge states of CD I. We measure the time offlight of the ions created in a multi-ion coincidence appa-ratus, shown schematically in figure 1, which is describedin more detail in the section on experimental methods.After leaving the source volume, a sideways-flying ionof sufficiently high energy will hit the extractor platerather than pass through its aperture, so the number ofions being detected decreases with increasing kinetic en-ergy. This means that for higher molecular charge states,whose fragment ions gain more kinetic energy, a largerproportion of ions are lost. To quantify the results fromour spectrometer in the presence of these effects a nu-merical investigation was performed. In particular, nu-merical calculations are used to determine the collectionefficiency for each fragmentation channel D + + C n + +I m + . The collection efficiencies are subsequently used tocorrect the coincidence intensities so that the total num-ber of created events can be determined. Low-order (two-fold and threefold) coincidences are used in most of theanalysis to take advantage of their relatively favourablestatistics. More details on the implementation of ournumerical investigation can be found in the section onnumerical methods.To simulate our experimental data numerically,molecules are placed within the source volume with a ran-dom orientation and are then dissociated and ionised. Todetermine the mutually dependent angles and kinetic en-ergies with which the ions leave the source, the equationof motions are integrated under mutual Coulomb repul-sion. Initially, the atoms start in the normal equilibriumconfiguration of the molecule, with exact C v symmetry,making all deuterium atoms equivalent. Time depen- a r X i v : . [ phy s i c s . a t m - c l u s ] A ug h ν * Target gas Lens
MCPelectrondetector Position sensitiveMCP ion detectorRepeller AcceleratorExtractor 0.5mFlying ions
FIG. 1: Schematic diagram ofthe multi-ion coincidencespectrometer used. Theelectrodes around the sourceregion are generating an electricfield that accelerates the ionsthrough the aperture of theextractor plate. The ions travelthrough the drift tube and hit aMCP detector where thetime-of-flight is registered.dence of the ionic charges can, in principle, be calculatedby solving a large set of rate equations involving all Augertransition probabilities [17] where a Monte Carlo typeapproach is imperative [18]. However, a comparativelysimple model introduced recently by Motomura et al. [9]is computationally more attractive, because it condensesthe large set of parameters to two generalised parame-ters that describe the charge build up and the chargereconfiguration processes simultaneously. The charge isassumed to be sequentially built up, here predominantlyby Auger cascade, at the site of the iodine atom, becauseof its dominant cross section for photon absorption. Inthis model, the total charge Q tot is supposed to build upaccording to Q tot ( t ) = ( m + n + 3) (cid:16) − e − t/τ (cid:17) , (1)where τ is a parameter for the charge build up time,and m and n are the final charges for iodine and carbon,respectively. When a high charge is created on the iodineatom the molecule becomes unstable due to the chargeimbalance and electrons from the deuterium atoms andthe carbon atom are transferred to the iodine. The rateof transfer is described by ddt Q CD ( t ) = R · Q I ( t ) , (2)where R is a rate constant for the charge transfer. Q I and Q CD are the charges at the iodine site and the methylgroup, respectively, and they obey Q tot ( t ) = Q I ( t ) + Q CD ( t ) . (3)The ions are allowed to have fractional charges during thecharge build up, which is assumed by considering effectssuch as delocalisation and screening, but are all requiredto have integer final charges. Charges of at least 4+on the methyl group are apportioned as three units tothe three equivalent deuterium atoms and the residue tothe carbon atom. For comparison with the charge buildup model, we also calculate the result of instantaneouscreation of the final charges on the atoms in their originalpositions. Using the model described above in comparison withour experimental results, starting from the parameters in[9], we find the best overall agreement with τ = 7 fs and R = 0 . fs − . However, varying R has only to a mi-nor effect on the kinetic energy release and therefore thesimulated flight times. The parameters are determinedby comparing the experimental and numerical time-of-flight distributions from triple coincidence detectionsfrom different fragmentation channels D + + C n + + I m + .The lighter ionic species are the most sensitive to changein the model parameters; in particular the deuterium ionis the most suitable for comparison between the model’spredictions and the experimental data. To ensure that aspecific decay channel is involved in each comparison itis required that iodine and carbon ions are detected inaddition to one deuterium ion. Triple coincidences areused as the numbers of fourfold and fivefold coincidencesare very low. The simulations suggest that only a verysmall set of initial molecular orientations lead to trajec-tories allowing detection of more than one D + ion. Fig. 2shows a comparison between the experiment and the sim-ulations in form of triple detections showing the ion paircontours of deuterium-carbon ions of charges 1+ and n+,respectively, together with different iodine species. Thefigure contains experimental data in red contour lines,blue contours show the numerical data utilising Moto-mura’s model [9] with the previously mentioned param-eters, and the green contour lines show numerical datausing the instantaneous model. The ion pairs form twoislets, which is a consequence of the high momenta gainedwhich result in sideways flying ions missing the detector.As evidenced by the figure, the charge build up modelfor these intermediate decay channels, n ∈ [1 , and m ∈ [3 , , matches well with the experiment whereas theinstantaneous model overestimates the kinetic release en-ergies, especially for the deuterium fragment. For a com-prehensive reflection of all the resolvable decay channels,an islet separation plot is shown in Fig. 3, showing theseparation of the deuterium 1+ ion correlated with car-bon and iodine ions with positive charges of 1-4 and 1-9,respectively. The islet separation is selected as the timeat half the full peak height on the outside of each peak(forward and backward), and the figure shows the exper-FIG. 2: Contours of deuterium-carbon ion pairs of charges 1+ and n+, respectively, correlated with different chargesm+ of iodine. The vertical axis shows the carbon separation and the horizontal axis shows the deuteriumseparation. Dashed lines represent the flight times for zero kinetic energy ions. The red contours are theexperimental data, the blue are the numerical data based on the charge build up model with charge transfer and thegreen are the numerical data for an instantaneous model. The contour lines are for 6% of maximum intensity.FIG. 3: Comparison of the islet peak separation of thedeuterium ion involved in triple coincidence events withC n + + I m + . Markers: experimental data; dashed lines:instantaneous model; solid lines: charge build-up model.imental data in symbols, the data based on the numericalcharge build up model in solid lines and the data basedon the instantaneous model in dashed lines. The figureshows that the model recreates Coulomb explosion forthe intermediate decay channels in good agreement withthe experiment, whereas decay channels involving a highcarbon or iodine charge deviate and the fragments gain less momenta from Coulomb explosion. The deviationsin low iodine charges for carbon 4+ may be explained bythe lack of charge imbalance in favour of the iodine ion,as assumed by the charge build up model, when in factthe carbon ion has a higher charge during the majorityof the charge build up.In order to determine the true charge distributions pro-duced by each specific initial hole creation, we use the rel-ative intensities of threefold coincidences for each chan-nel. Although only one D + ion is usually detected allthree are assumed to become charged in the explosionsand this assumption underlies all the derived distribu-tions. Several corrections must be applied to the rawintensities to account for differences in ion and electrondetection probabilities.Because of the geometry of the apparatus shown inFig. 1, fragmentation releasing higher initial kinetic en-ergy implies a greater loss of sideways flying ions. Thecollection efficiency is non-linear as a function of kineticenergy and is different for each ionic species and con-sequently the fragmentation channels are affected dif-ferently. In order to determine the true distribution ofmolecular charge states this non-linear dependence in theapparatus is determined numerically through simulation.Another factor is the risk of not detecting any electronsto initialise a flight time measurement. The probabilityof detecting a single electron is f i ≈ and thus theprobability of not detecting any electrons for n ejectedelectrons is (1 − f i ) n . In the present case, the lowestresolvable molecular charge state involves 5 emitted elec-trons, yielding a probability of not detecting any startelectron of ∼ .Experimental charge distributions are extracted at fourdifferent photon energies near the L-subshell thresholdsin iodine such that each set of data contains a differentblend of ionisation events from the different subshells.A subtraction method is implemented to determine thedistributions of charge states from the individual L1, L2and L3 ionisations, on the basis of the theoretical pre-diction that the total photoelectric cross-section [19] foreach ionisation declines as a function of photon energy, E , in line with a fitted polynomial, with E − / as thedominating term [20]. The four data sets are denoted as D − D , where D corresponds a to photon energy of5290 eV which is above L1, L2 and L3 thresholds, D to4950 eV which is above the L2 and L3 thresholds, D to4660 eV which is only above the L3 threshold, and D at 4300 eV which is below all the L threshold and showsionisation only from shells at lower binding energies.Using the theoretical relative partial cross-sections de-rived as above, we obtain a set of equations allowingextraction of the charge distributions produced by eachpure subshell ionisation:L3 = D − . · D (4)L2 = D − . · D . · D (5)L1 = D − . · D − . · D . · D . (6)where the data set D has considerably poorer statis-tics than the other three. However, the uncertainty of D is a major factor only in the extraction of the pure Charge state A bundan c e i n % FIG. 4: Relative charge state abundances estimatedfrom the experimental data, and corrected for thesimulated collection efficiency. The bars reflect theabundance of the charge states present in initialionisation from the iodine 2s, 2p / and 2p / shells,represented in black, white and grey, respectively. L3 distribution. The distribution for pure L1 has a rel-atively large uncertainty as the subtracted terms consti-tute a large fraction of the D1 data set. The distributionsof total molecular charge produced by ionisation fromthe individual shells are shown in Fig. 4, which showsthat the most probable charge number for L1 ionisationis roughly two units higher than those from L2 and L3ionisation. This can be related to a fast Coster-Kronigtransition where the L1 hole is filled by an L3 electron,providing enough energy to eject an electron from theM shell after which the relaxation proceeds similarly tothe case where the initial hole is created in L3, with anadditional M hole. A similar transition for an initial L2hole is expected to be much less probable as the energyavailable from an L3 to L2 transition is only sufficientto eject an N electron and the increased radial distancelimits the transition rate.The present study shows that the charge creationmodel introduced by Motomura et al. [9] using only afew empirical parameters gives a satisfactory descriptionof single photon induced Coulomb explosion for the decaychannels involving low and intermediate charges. But thediscrepancy for decay channels involving higher chargestates of carbon and iodine indicates that a better modelmay need to incorporate specific effects of nuclear motionon charge development and/or charge transfer when ioni-sation is by deep hole creation followed by Auger cascade.A fuller analysis may also need to allow for the possibleinvolvement of neutral fragments in the Coulomb explo-sion.A related effect ignored in the present model is thepossible role played by molecular vibrational modes.The energetic contribution of vibrational modes is mostlikely negligible, but some degenerate modes deform themolecule in such a way that the most probable struc-ture at any single instant is different from the time-averaged (C v ) structure. Such a deformation may affectthe Coulomb explosion of CD I by making the deuteriumatoms inequivalent at the instant of ionisation. The nor-mal mode expected to be dominant in the zero-point mo-tion of such a study is the ν ( e ) mode. The present ex-periments were unfortunately not sensitive enough to in-vestigate such effects, but they should be detectable infuture experiments with fully operational ion detectionposition sensitivity.In conclusion, we have investigated single photon in-duced Coulomb explosion of fully deuterated methyl io-dide, CD I, using X-ray pulses in the 4660-5290 eV pho-ton energy region. Our experimental ion time-of-flightdata were compared to the results of a numerical modelwhich took into account the dimensions of our appara-tus and the electrical fields applied. The few-parametermodel used here has previously been shown to work wellin a Coulomb explosion study of CH I induced by multi-photon ionisation involving an XFEL. The present studyshows that this model is also applicable to the single pho-ton case for intermediate charge state decay channels, asevidenced by the good agreement between our experi-mental and numerical results. Decay channels involv-ing high carbon or iodine charge show a systematicallylower kinetic energy release than predicted by the model,which we attribute to a stronger effect of competitivenuclear motion. If our interpretation is correct, for thelight hydrogen isotopologue, CH I, stronger deviationsin kinetic energy release from the model predictions maybe expected. The success of the simple model is per-haps somewhat surprising in view of the very differentcharge generation pathways in deep hole creation andAuger cascade as opposed to x-ray multi-photon ionisa-tion. The model may serve as a useful tool for descriptionof Coulomb explosions more generally, and as a start-ing point for more sophisticated modeling. Experimentalabundances together with detection efficiencies derivedfrom numerical modeling of the apparatus were used todetermine the true relative abundances of initial chargestates. The charge states from L2 and L3 ionisation havesimilar distributions, whereas L1 ionisation gives a distri-bution displaced roughly two units higher in charge. Thisdifference is explained by a rapid Coster-Kronig transi-tion turning an L1 electron hole into an L2 or L3 electronhole with an additional electron hole in a higher shell.
EXPERIMENTAL METHOD
The experiments were carried out using synchrotronradiation provided by the LUCIA beam line of the stor-age ring SOLEIL in Paris. The ring was operated insingle bunch mode at a frequency of . MHz. The syn-chrotron radiation pulses interacted in a crossed-beamconfiguration with an effusive jet of the target gas froma hollow needle located in the source region of the appa-ratus shown in Fig. 1. The electrodes provide an electricfield that accelerates the negative electrons to a nearbydetector, serving as a start for the ion coincidence mea-surements, while the positive ions fly in opposite direc-tion to a more distant, position sensitive microchannelplate detector [21]. The position information on the datahits has not been used in the analysis because of a stronglensing effect which essentially guided almost all ions toa small area in the centre. Some angular effects are pre-served, but they are too small to be interpreted reliably.Since the ionic fragments created by the Coulomb ex-plosion may gain a fairly high kinetic energy release, notall of the ions reach the detector. In practice, only about21% of the deuterium atoms, 50% of the carbon atomsand 78% of the iodine atoms reach the detector. Thisis due to the geometrical configuration of the apparatuswhere sideways flying ions of sufficient energy will hit theinside of the extractor plate rather than passing throughthe aperture. The loss of sideways flying ions create ahollowness in the overall time-of-flight peak shapes, yield- ing a forward- and a backward component separated by √ mU qE .Ions that do reach the detector may still not be reg-istered because of the non-unit efficiency of the detectorwhich is predominantly determined by the open area ra-tio of the channel plate detector. NUMERICAL METHODS
In the model of the Coulomb explosion process used,the molecule is assumed to start at its nominal equilib-rium geometry with exact C v symmetry, representing alldeuterium atoms equivalently. A time dependent chargeis given to the atoms as they are allowed to evolve, nu-merically calculated using ode45 in Matlab, under theirmutual Coulomb repulsion until they have moved suffi-ciently far apart that their energies and relative anglesare fixed. At this point the energies and angles are trans-formed into LAB coordinates with a random orientationand the further motion of the ions is determined by theapplied electric fields.A geometric model of the apparatus was constructedin SIMION [22], to mimic the experimental conditionsas realistically as possible. The initial positions, kineticenergies and angles of all ions from each explosion deter-mined by the simulation are fed into this model of theapparatus, where they travel towards the detector underthe influence of the applied electric fields. This allows thefragments of a molecule to be modelled as coincidenceevents, incorporating the loss of sideways-flying ions toallow comparison with the actual experiment. The mod-elling takes into account the limited efficiency of the de-tector by random deletion of half the ions that arrive atthe detector surface. Once the simulated data resemblethe experimental data set sufficiently closely the initialcharge state abundances can be extracted. AUTHOR INFORMATIONAuthor contribution
J.H.D.E., M.N.P., M.S. and R.F. devised the re-search, R.J.S., O.T., R.S., J.A., A.H.R., D.K., andR.F. participated in the conduction of the experimen-tal research, M.W. constructed the numerical model,M.W. and J.H.D.E. performed the data analysis, M.W.,J.H.D.E. and R.F. wrote the paper and all authors dis-cussed the results and commented on the manuscript atseveral instances.
Notes
The authors declare no competing interests.
DATA AVAILABILITY
The datasets generated during and/or analysed duringthe current study are available from the correspondingauthor on reasonable request.
ACKNOWLEDGEMENT
This work has been financially supported by theSwedish Research Council (VR) and the Knut and Al-ice Wallenberg Foundation, Sweden. We thank SOLEILfor the allocation of synchrotron radiation beam time andwe want to warmly acknowledge the staff and colleaguesof this facility for their technical assistance and adminis-trative support. ∗ [email protected][1] Carlson, T. A.; White, R. M. Measurement of the Rela-tive Abundances and Recoil-Energy Spectra of FragmentIons Produced as the Initial Consequences of X-Ray In-teraction with CH3I, HI, and DI. J. Chem. Phys. ,44, 4510-4510[2] Eland, J. H. D. Dynamics of three-body reactions inICN2+ and related molecules.
Chem. Phys. Lett. ,203, 353-362[3] Eland, J. H. D.; Singh, R.; Pickering, J. D.; Slater,C. S.; Hult Roos, A.; Andersson, J.; Zagorodskikh, S.;Squibb, R. J.; Brouard, M.; Feifel, R. Dissociation ofmultiply charged ICN by Coulomb explosion.
J. Chem.Phys , 145, 074303[4] Ueda, K.; Eland, J. H. D. Molecular photodissociationstudied by VUV and soft x-ray radiation.
J. Phys. B:at., Mol. Opt. Phys. , 38, S839-S859[5] Luzon, I.; Livshits, E.; Gope, K.; Baer, R.; Strasser, D.Making Sense of Coulomb Explosion Imaging.
J. Phys.Chem. Lett. , 6, 1361-1367[6] Amini, K.; Savelyev, E.; Braue, F.; Berrah, N.; Bomme,C.; Brouard, M.; Burt, M.; Christensen, L.; Dsterer, S.;Erk, B.; et al. Photodissociation of aligned CH3I andC6H3F2I molecules probed with time-resolved Coulombexplosion imaging by site-selective extreme ultravioletionization.
Structural Dynamics , 5, 014301[7] Rudenko, A.; Inhester, L.; Hanasaki, K.; Li, X.; Ro-batjazi, S. J.; Erk, B.; Boll, R.; Toyota, K.; Hao, Y.;Vendrell, O.; et al. Femtosecond response of polyatomicmolecules to ultra-intense hard X-rays.
Nature ,546, 129 EP -[8] Takanashi, T.; Nakamura, K.; Kukk, E.; Motomura, K.;Fukuzawa, H.; Nagaya, K.; Wada, S.-I.; Kumagai, Y.;Iablonskyi, D.; Ito, Y;. Sakakibara, U.; et al. UltrafastCoulomb explosion of a diiodomethane molecule inducedby an X-ray free-electron laser pulse.
Phys. Chem. Chem.Phys. , 19, 19707-19721 [9] Motomura, K.; Kukk, E.; Fukuzawa, H.; Wada, S.-I.;Nagaya, K.; Ohmura, S.; Mondal, S.; Tachibana, T.;Ito, Y.; Koga, R.; et al. Charge and Nuclear Dynam-ics Induced by Deep Inner-Shell Multiphoton Ionizationof CH3I Molecules by Intense X-ray Free-Electron LaserPulses.
J. Phys. Chem. Lett. , 6, 2944-2949[10] Nagaya, K.; Motomura, K.; Kukk, E.; Takahashi, Y.; Ya-mazaki, K.; Ohmura, S.; Fukuzawa, H.; Wada, S.; Mon-dal, S.; Tachibana, T.; et al. Femtosecond charge andmolecular dynamics of I-containing organic molecules in-duced by intense X-ray free-electron laser pulses.
FaradayDiscuss , 194, 537-562[11] Nagaya, K.; Motomura, K.; Kukk, E.; Fukuzawa, H.;Wada, S.; Tachibana, T.; Ito, Y.; Mondal, S.; Sakai,T.; Matsunami, K.; et al. Ultrafast Dynamics of a Nu-cleobase Analogue Illuminated by a Short Intense X-rayFree Electron Laser Pulse.
Phys. Rev. X , 6, 021035[12] Erk, B.; Rolles, D.; Foucar, L.; Rudek, B.; Epp, S. W.;Cryle, M.; Bostedt, C.; Schorb, S.; Bozek, J.; Rouzee,A.; et al. Ultrafast Charge Rearrangement and NuclearDynamics upon Inner-Shell Multiple Ionization of SmallPolyatomic Molecules.
Phys. Rev. Lett. , 110, 5[13] Schnorr, K.; Senftleben, A.; Kurka, M.; Rudenko, A.;Schmid, G.; Pfeifer, T.; Meyer, K.; Kübel, M.; Kling, M.F.; Jiang, Y. H.; et al. Electron Rearrangement Dynam-ics in Dissociating I n +2 Molecules Accessed by ExtremeUltraviolet Pump-Probe Experiments.
Phys. Rev. Lett. , 113, 7[14] Schnorr, K.; Senftleben, A.; Schmid, G.; Rudenko, A.;Kurka, M.; Meyer, K.; Foucar, L.; Kübel, M.; Kling, M.F.; Jiang, Y. H.; et al. Multiple ionization and fragmen-tation dynamics of molecular iodine studied in IR–XUVpump–probe experiments.
Faraday Discuss , 171,41-56[15] Erk, B.; Boll, R.; Trippel, S.; Anielski, D.; Foucar, L.;Rudek, B.; Epp, Sascha W.; Coffee, R.; Carron, S.;Schorb, S.; el al. Imaging charge transfer in iodomethaneupon x-ray photoabsorption.
Science , 345, 288-291[16] Boll, R.; Erk, B.; Coffee, R.; Trippel, S.; Kierspel,T.; Bomme, C.; Bozek, J. D.; Burkett, M.; Carron,S.; Ferguson, K. R.; et al. Charge transfer in dissoci-ating iodomethane and fluoromethane molecules ionizedby intense femtosecond X-ray pulses.
Structural Dynam-ics , 3, 043207[17] Inhester, L.; Hanasaki, K.; Hao, Y.; Son, S-.K.; Santra,R. X-ray multiphoton ionization dynamics of a watermolecule irradiated by an x-ray free-electron laser pulse.
Phys. Rev. A , 94, 023422[18] Fukuzawa, H.; Son, S.-K.; Motomura, K.; Mondal, S.;Nagaya, K.; Wada, S.; Liu, X.-J.; Feifel, R.; Tachibana,T.; et al. Deep Inner-Shell Multiphoton Ionization by In-tense X-Ray Free-Electron Laser Pulses.
Phys. Rev. Lett.
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