j. jha, d. mathur and m. krishnamurthy- engineering clusters for table-top acceleration of ions

8/3/2019 J. Jha, D. Mathur and M. Krishnamurthy- Engineering clusters for table-top acceleration of ions

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are likely to be larger that 60. We already find ion energyenhancement at this backing pressure. So, we estimate that atleast a 3% doping level of water in the Ar cluster is enoughto bring about discernible enhancement in ion energies. Theion energy spectra shown in Fig. 1 was measured with 1 kVapplied to the retarding potential analyzer in front of the iondetector. This retarding voltage serves as a high-pass filter inthat all those ions of charge state q that possess kinetic en-ergies less than 1q keV are not transmitted to our detector.Our data show that the fraction of high energy ions, for ex-ample of 100 keV energy, is about a factor of 3 larger withdoped clusters.

In earlier experiments it had been shown that chargestates up to 8+ were formed in the cluster explosion ofAr40,000 Refs. 3 and 10 at similar laser intensities. Ourpresent experiments with argon-water clusters clearly showFig. 2 charge states up to 14+, a very substantial enhance-ment in charge state. Data depicted in Fig. 2 are in the formof the ion energy spectrum measured upon application of 3.9kV to our retarding potential analyzer. Ions of charge q, withenergies up to 3900q eV, were electrostatically hinderedfrom reaching the detector, and this manifested itself as aseries of steps in the energy spectrum that corresponded todifferent values of charge state q. The steps are more obviouswhen ion energy spectra are numerically differentiated bro-

ken line in Fig. 2. The labeling of the peaks denotes ioncharge state. Doping not only increases the kinetic energy of

the Coulomb-exploded ions but also enhances formation ofions in charge states higher than 8+.

A priori, one might suspect that such enhancement mightbe associated with change in the cluster size that might occurwhen Ar gas is bubbled through water. If the mean size ofheterogeneous clusters in our experiment is larger, then it isnot a surprise that the high energy fraction in the ion energydistribution would increase. In order to probe this possibility,we made Rayleigh scattering measurements with 400 nmlight to compare the sizes of pure Arn and doped clusters.We note that the polarizabilities of Ar 1.58 3 and water1.47 3 do not differ appreciably; the polarizability-dependent Rayleigh scattering cross section would change byonly 10% if all the Ar atoms were replaced by H2O. Anyappreciable difference in cluster size upon doping wouldreadily manifest itself in the scattering signal, which is ex-pected to increase as the sixth power of the cluster radius.There is no measurable change in the Rayligh scattering sig-nal upon doping.11 Size changes of 1%2% if any, do notaccount for the threefold increase in the high energy ions orthe increased propensity for higher charge states that weobserve.

From earlier measurements12 it is known that a 5%

change in the cluster radius brings about a 10% change in themaximumj ion energy expected from a Coulomb exposion.Using this, the increase in the fraction of high energy ionsdue to the size alone can be estimated to be 15%. Dopedclusters, on the other hand, yield 300% increase in the highenergy fraction. Furthermore, a 5% increase in the clusterradius should actually increase the Rayleigh scattering crosssection by about 30%, which is much less than the change inthe Rayleigh scattering signal that we actually observed forthe doped clusters.

So how do we rationalize our observations? We do so byconsidering the ionization dynamics within the cluster interms of the electron density in pure and doped clusters. The

multiphoton ionization rate W for an atom with ionizationenergy Ip, is given by the ADK theory13 as

FIG. 1. Color online Ion energy spectra measured for Ar9200 solid lineclusters and doped ArH2O clusters broken line exposed to laser intensi-ties of 1016 W cm2, when a 1 kV retarding potential is applied to the re-tarding potential analyzer. The inset shows the ion arrival time spectra forAr40,000 brokenline clusters and doped ArH2O clusters solid line.

FIG. 2. Color online Ion arrival time spectrum measured for Ar-waterclusters solid line exposed to laser intensities of 1016 W cm2, when 3.9kV retarding potential is applied to the retarding potential analyzer. Differ-entiated spectrum showing the different charge states is shown as a brokenline.

041107-2 Jha, Mathur, and Krishnamurthy Appl. Phys. Lett. 88, 041107 2006

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W32Ip9/4

z5/216eIp2

zF2z/

2Ip3/2

exp 22Ip3/23F

, 1where F is the field strength. For H2O, which has lowerionization energy 12.6 eV than Ar 15.7 eV, the ionizationrate can be as large 1200 times that for Ar. Simple estimatesof over-the-barrier ionization thresholds show that the inten-sity threshold for water is three times lower that for Ar.

When an ultrashort laser pulse is incident on the cluster,multiphoton and tunnel ionization dominate in the initialstages as the intensity increases. If a cluster is doped with aneasily ionizable species, like H2O, the ionization becomeslarger in the first few optical cycles: the electron densitygrows faster in doped clusters.

We adopt the cluster ionization formalism of Krainovand Smirnov6 to be more quantitative. Taking atomic ioniza-tion to be due to classical above-barrier ionization, the ion-ized electrons are driven by the laser field to oscillate aboutthe cluster, giving rise to an electrostatic field due to chargeseparation that is expressed as:

EB =

Zn

R23 2 sin

2 sin2

2 , 2

where n is the number of the atoms in the clusters of radiusR ,z is the charge on the atoms, and is the angle indicatedin Fig. 4 of Ref. 1. This ignition field is added to theexternal optical field, and we use the sum to calculate theinner ionization. The most energetic electrons leave the clus-ter, leading to outer ionization, and the total charge of theionized cluster is Q = Zn/42 3 cos +cos3 . Thecharged cluster expands under the Coulomb pressure and fi-nally explodes, giving rise to electrons and ions. For Ar 40,000,90 rad, we find that initiation of inner ionization occursat tpeak-85 fs, for a peak intensity of 10

16 W cm2 and apulse width of 100 fs. If this cluster comprised only watermolecules, initiation of inner ionization would be expected tooccur 10 fs earlier, at tpeak-95 fs. The ignition field attpeak- 95 fs is deduced to be 0.04 a.u., which is about 90% ofthe optical field 0.043 a.u. at this time. Ionization ignitionis a cascade process and, consequently, earlier initiation of itand that too at a rate that is enhanced by the presence ofeasily ionizable H2O leads to formation of a hotter nano-plasma the radius of which is still not too much larger than90 that, upon subsequent explosion, results in an en-hanced yield of higher-energy and higher charge state ions.Furthermore, mechanisms like laser dephase heating14 willalso be strongly influenced by the number of ionized elec-trons that are driven by the large optical field to oscillate

with respect to the clsuters ionic core.We note that in our arrival time measurements there

could be ambiguity as to whether the high energy fraction in

the ion signal is, indeed, due only to highly charged Ar ionssince our doped clusters can also produce O ions and H+. Wemeasured Ar K x rays 3 keV to evaluate the consistencyin the experimental observations and reasoning.11 If the ob-served change in ion signal were only due to protons, thenthe 3 keV emission should not change at all; in fact, it shoulddecrease as the number of Ar atoms per cluster is smallerupon doping. A larger K x-ray yield would indicate a hotter

nanoplasma which correlates with the enhanced ionizationignition scheme that we propose. We measured Kx-ray emis-sion for doped clusters to be 12-fold larger than for pure Arclusters at a backing pressure of 8 atm.

In summary, we have studied intense field ionization ofdoped and undoped clusters. The high-energy ion fractionthat we obtain is threefold larger for the former. The chargestate spectrum for doped clusters clearly shows charges up to14+ while we could clearly decipher only charges up to 8+for pure Arn clusters. Rayleigh scattering measurementsshow that there is no appreciable change in the cluster sizewhen the mixture is used. The change in the ionization pro-pensity is argued to be due to the presence of easily ionizable

water molecules that are doped in the clusters. The electrondensity rises much more steeply in heterogeneous clusters.When there are a larger number of electrons available forrescattering that is driven by the optical field, we expect alarger contribution to the overall ionization and ion accelera-tion, as is, indeed, observed in the experiments that we reporthere.

1T. Ditmire, R. A. Smith, J. W. G. Tisch, and M. H. R. Hutchinson, Phys.Rev. Lett. 78, 3121 1997.

2T. Ditmire, E. Springate, J. W. G. Tisch, Y. L. Shao, M. B. Mason, N. Hay,J. P. Marangos, and M. H. R. Hutchinson, Phys. Rev. A 57, 369 1998.

3V. Kumarappan, M. Krishnamurthy, and D. Mathur, Phys. Rev. Lett. 87,085005 2001.

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041107-3 Jha, Mathur, and Krishnamurthy Appl. Phys. Lett. 88, 041107 2006

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j. jha, d. mathur and m. krishnamurthy- engineering clusters for table-top acceleration of ions

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