diagnostics of laser ablated plasma plumes

Thin Solid Films 453–454(2004) 562–572

0040-6090/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2003.11.137

Diagnostics of laser ablated plasma plumes

S. Amoruso *, B. Toftmann , J. Schou , R. Velotta , X. Wanga,b, c c a,b a

Coherentia INFM and Istituto Nazionale per la Fisica della Materia, Complesso Universitario di Monte S. Angelo, Via Cintia,a

I-80126 Napoli, ItalyDipartimento di Scienze Fisiche, Universita degli Studi di Napoli Federico II, Complesso Universitario di Monte S. Angelo, Via Cintia,b `

I-80126 Napoli, ItalyOptics and Fluid Dynamics Department, Risø National Laboratory, DK-4000 Roskilde, Denmarkc

Abstract

The effect of an ambient gas on the expansion dynamics of laser ablated plasmas has been studied for two systems byexploiting different diagnostic techniques. First, the dynamics of a MgB laser produced plasma plume in an Ar atmosphere has2

been investigated by space-and time-resolved optical emission spectroscopy. Second, deposition rate and fast ion probemeasurements have been used to study the plume propagation dynamics during laser ablation of a silver target, over a large rangeof Ar background gas pressures(from high vacuum tof100 Pa). A comparative analysis of the experimental results allows usto identify different regimes of the plume expansion, going from a free plume at low pressure, through collisional and shock-wave like hydrodynamic regimes at intermediate pressure, finally reaching a confined plume with subsequent thermalization ofthe plume particles at the largest pressure of the background gas. The experimental findings also show that a combination ofcomplementary techniques, like optical emission spectroscopy, close to the target, and fast ion probe and deposition ratemeasurements at larger distances, can lead to a more detailed understanding of the laser ablated plasma plume propagation in abackground gas.� 2003 Elsevier B.V. All rights reserved.

PACS: 34.50 Ion-molecule collisions; 52.50.J Laser produced plasma; 52.75.R Plasma application-film deposition; 79.20.D Laser ablation

Keywords: Laser ablation; Spectroscopy; Electrical probe; Plume dynamics

1. Introduction

During the past decade laser ablation has been utilizedfor a number of applications, emerging as a versatiletechnique for pulsed laser deposition(PLD), nanoparti-cles and clusters production, material sampling, etc.w1–3x. Further progress in these applications requires adeeper understanding of the mechanisms of plasmaplume formation and evolution. Laser ablation plasmais transient in nature, quickly evolving in space andtime and depends strongly on processing parameters likelaser fluence and pulse duration, ambient gas pressureand composition, etc.w1,2x. In spite of the extensiveliterature on the subject, the different mechanismsinvolved during a laser ablation process are rathercomplex and the expansion dynamics of the plasma

*Corresponding author. Fax:q39-081-676376.E-mail address: [email protected](S. Amoruso).

plume, in vacuum or in a background gas, are not stillfully understood.

In a number of papers we reported on the complexdynamics of multicomponent materials, namelyYNi B C borocarbide targets, in high vacuum and on2 2

the effect on thin film depositionw4–6x. The mainoutcome of these investigations was the dependence ofthe spatial distribution of the emitted plume ions withthe heavier species in its center and the lighter ones atits edges and the identification of target-to-substratedistance as a critical parameter for the optimal conditionsfor deposition of this material by PLD.

Here we present an overview of our more recentexperiments on plume dynamics in a background gas.In particular, we report results for two different systemsby exploiting different investigation techniques. The firstexperiment, which was carried out at the plume diag-nostic setup of the Coherentia-INFM centerw7x, dealswith the analysis of the expansion dynamics of the new

563S. Amoruso et al. / Thin Solid Films 453 –454 (2004) 562–572

Fig. 1.(a) Schematic of the experimental setup. M is a diverter mirrorwhich switches between the intensified charge coupled device(ICCD)camera and the photomultiplier tube(PMT). (b) Typical spectrum ofthe MgB laser ablated plasma plume acquired at an Ar pressure of2

7 Pa and at a distanceds2 mm from the target surface. An UVabsorbing filter was used to avoid multiple orders from the emissionlines from BI and MgI at wavelengths shorter than 300 nm.

MgB superconductor in Ar ambient gas by applying2

optical emission spectroscopy. In the second experimentperformed at the existing setup in Risø National Lab-oratory w8,9x, combined diagnostic measurements of iontime-of-flight signals and deposition rate have beenemployed to investigate the propagation of a laserablation plume from an elemental target, silver, into abackground gas. In both cases, the expansion behaviorin the background gas was studied at different pressures,showing interesting effects like plume-splitting andsharpening, excitationyionization revival, shock wavesand plume thermalization.

This paper is organized in five sections. In Sections2 and 3 the experimental procedure and findings of thetwo experiments are discussed, respectively, while Sec-tion 4 contains a description of the plasma plumepropagation into a background gas and a comparativeinterpretation of the experimental results. Finally, thesummary and conclusions are given in Section 5.

2. MgB plasma dynamics investigated by optical2

emission spectroscopy

The plasma dynamics of the plume formed duringexcimer laser(XeF, 351 nm, 20 ns) irradiation of astoichiometric MgB target at a laser fluence of 3 J2

cm has been investigated by space- and time-resolvedy2

optical emission spectroscopy. The target was mountedon a rotating holder and placed in a vacuum chamberwith a residual pressure off10 Pa. During they5

experiment, the chamber was filled with Ar gas and thepressure was varied in the range 10 –50 Pa. An opticaly2

system was used to image the plume onto the entranceslit of a monochromatoryspectrograph system(JobinYvon HR250), to have a 3:1 correspondence betweenthe sampled area of the plume and the image, with aspatial resolution off300 mm wsee Fig. 1ax. Themonochromator was equipped with a turret of twointerchangeable gratings(100 and 1200 grovesymm).One of the exit ports of the spectrograph was coupledto an ICCD camera(Andor Technology) operated in thevertical binning mode of the array to obtain spectralemission intensity vs. wavelength. In order to record thetemporal profile of a selected emission line, the otherexit port was coupled to a photomultiplier tube(PMT),whose output signal was registered by a 500 MHzdigital oscilloscope(Tektronix TDS5054) triggered bya fast photodiode collecting the light scattered by thelaser beam focusing lens. The collected light was guidedto the selected output by means of a diverter mirror(M). This setup provides time- and space-resolvedanalysis of the emission from the constituent specieswithin the plume, in particular in normal direction.

In order to acquire the plume emission in the largestwavelength range the spectra were acquired with the100 grovesymm grating, and an UV subtractive filter

was used to discern the emission lines from high orderpeaks of B and Mg lines with wavelengths shorter than300 nm. Moreover, the 1200 grovesymm grating wasused to obtain a more reliable and fine identification ofthe most significant lines. In Fig. 1b, a characteristicspectrum of the MgB plume in the spectral range 300–2

800 nm is shown. The observed emission lines wereidentified according to standard dataw10x and correspondto Mg and B neutrals and ions.

Fig. 2 shows the emission intensity temporal profilesof the MgI 383 nm line observed at two differentdistances from the target surfacew(a) ds3 mm,(b) ds7 mmx, for three different Ar background gas pressures.These profiles represent a convolution of the signal fromthree very close MgI spectral emission linesw382.935,383.229 and 383.23 nmx involving transitions between3s3d D and 3s3p P levels, which cannot be resolved3 3 0

by our experimental setup. All the transitions are char-acterized by a lifetime off10 ns. To facilitate the

564 S. Amoruso et al. / Thin Solid Films 453 –454 (2004) 562–572

Fig. 2. Emission intensity temporal profiles of the MgI atoms at 383 nm at two distances from the target surface,(a) ds3 mm and(b) ds7mm, and for three different pressuresP.

comparison, the emission profile forPs0.5 Pa has beenmultiplied by a factor 0.4 atds3 mm and 0.05 atds7 mm, respectively. At a distanceds3 mm wsee Fig.2ax, we observe a very intense emission at a pressure of0.5 Pa, with a peak maximum at a timet f150 ns andM

a tail extending to not more thanf1 ms. By increasingthe pressure toPs7 Pa, the peak signal intensity reducesby factor f3, while t is still f150 ns. Then, atPsM

23 Pa the appearance of a long tail, which extends upto f 5 ms, is observed. Atds7 mm, Fig. 2b, the peaksignal intensity reduces by factorf20 by increasing thepressure fromPs0.5 to 7 Pa, while the time at whichthe peak maximum is observed,t f300 ns, remainsM

almost unchanged. Then, atPs23 Pa, a second delayedcomponent appears in the emission profile with a max-imum atf4 ms and a very long tail, which extends upto f70 ms. Moreover, a fast peak characterized by asignal level and timing quite similar to the one observedat lower pressure is still present. Similar features havebeen observed for the other spectral lines investigated.The analysis of the temporal emission profiles revealsthat this peak splitting effect is observed only above adistance of 3 mm at a pressure of 23 Pa. This showsthat plume splitting appeared during the later stage ofthe plasma expansion as a consequence of plume-background gas interaction.

The plume splitting effect has been observed forplume ions expanding into an ambient gas, and has beenthoroughly discussed in the literaturew11–14x. Morerecently, by studying the expansion of an Al plume intoa background airw15x, Hariral et al. have reported thiseffect also for neutrals, in agreement with our present

observation. Nevertheless, it remains just one stage ofthe complex plume dynamics in an ambient atmosphere.

In Fig. 3a the integrated yield of excited speciesemission as a function of the distanced from the targetis reported. The integrated yield has been obtained byintegrating the whole area under the emission profile,and gives an indication of the amount of plume excitedspecies reaching a given distanced. The data show thatgoing from 0.5 to 7 Pa, the emission intensity decreasesat almost all distances, whereas atPs23 Pa it growsup again, becoming even larger than that observed atPs0.5 Pa, for distancesd larger 3 mm. Similar resultshave been observed for Mg ionsw7,16x.

Coincidentally, at an Ar pressure of 7 Pa, the meanfree pathl of an Mg atom in Ar becomes of the orderof f3 mm, as estimated by using the equations givenby Westwoodw17x. This means that the plume speciesstart to experience collisions with the background gasatoms in this pressure range on a distance of fewmillimeters. In this respect, it is worth observing that aMg atom in a collision with an Ar atom, changes, onaverage, its direction byf 668 and losesf60% of itskinetic energyw17x. Thus, the strong reduction of plumeluminescence can be ascribed to the escape of excitedplume species from the luminescence region.

To support this interpretation, in Fig. 3b we show theratio between the integrated emission yield at a pressureof 7 and 0.5 Pa,rw7 Pax. The ratio between 23 and 0.5Pa,rw23 Pax is also reported for comparison. We observethat at 7 Pa,rw7 Pax is fairly well described by anexponential decrease,rw7 Pax A exp(ydyl) with ls(3.7"0.5) mm, while a completely different behavior


Fig. 4. Distanced from the target surface as a function of the timet at which the maximum in the emission intensity profile is registeredM

for the 383 nm MgI line at different Ar background pressures. At 23Pa only thed–t data for the delayed component is shown, while forM

the fast peak almost the same behavior as at lower pressures isobserved. The solid line and dashed lines are fits todAt andM

dAt with as0.3, respectively. The inset shows an enlarged view ofaM

the first microsecond.

Fig. 3. (a) Integrated emission intensity as a function of the distanced from the target surface for the MgI 383 nm line. The lines are a guide tothe eye.(b) Ratio between the integrated emission yield at a pressure of 7 and 0.5 Pa,rw7 Pax, and 23 and 0.5 Pa,rw23 Pax, respectively. Thesolid line represent a fit to an exponential decrease behaviorrw7xA exp(ydyl), with ls(3.7"0.5) mm. The dashed line is a guide to the eye.

is observed forrw23 Pax. This suggests that atPs23 Paa different plume dynamics occurs, which influences theexcitationyionization kinetics of the expanding plume.

To elucidate the underlying mechanisms leading tothe delayed component observed atPs23 Pa, we haveshown the dependence of the timet at which theM

maximum in the emission intensity profile is observedas a function of the distanced from the target. We havemade d–t plots for the three different pressures asM

shown in Fig. 4. At 23 Pa only thed–t plot for theM

delayed component is shown, while for the fast peakalmost the same behavior as at lower pressure wasobserved. We note that the plume expansion in the earlystage(-0.5 ms) is linear irrespective of the backgroundpressure, then at longer distances and times a differentbehavior is observed at the larger pressures investigated.In particular, at 0.5 Pa a linear dependence is observedover the whole range of distances for which emissioncould be detected, resembling a behavior similar to afree-plume expansion in high vacuum. The velocityestimated from thed–t plots at this pressure results ofM

(2.4"0.1)=10 mys for the Mg neutral atoms and4

(2.9"0.3)=10 mys for the Mg ions. At larger pres-4

sures, the dependence is no longer linear at distancesdlarger than 7 mm atPs7 Pa and 3 mm atPs23 Pa,respectively. There is a clear distance-related pressurebraking effect on the emitting species. Thus, we observe,both for ions and neutrals, a fast component whichmoves with almost the free expansion velocity, and adelayed component which appears at larger pressures,above a specific distance, and whose dynamics is strong-ly influenced by the pressure of the background gas.

Zel’dovich and Raizer have shown that the expansionof a spherical blast wave is nearly self-similar with a

velocity gradient such that the velocity of the frontR asa function of the timet can be described byRAt witha

as0.4 w18x. In the present experiment, we are notfollowing specifically the temporal evolution of theplume front R, but rather the dependence of the timet at which the maximum in the intensity emissionM

profile occurs at the distanced. Nevertheless, we haveshown that such a parameter is normally representative


Fig. 5. Axial distribution of Mg excited atoms obtained by the emission intensity of the MgI 383 nm line for different time delayst after theD

laser pulse and at three different Ar background gas pressure:(a)–(d) Ps0.5 Pa;(b)–(e) Ps7 Pa;(c)–(f) Ps23 Pa. The lines are a guide tothe eye.

of the plume front dynamicsw7x. In fact, the temporalevolution of the distanced as a function of the maximumemission timet for the delayed component at 23 PaM

follows fairly well the dependencedAt , with anaM

expansion coefficientaf0.3, thus showing a shock-wave-like propagation of the plume at this pressure.

The background gas pressure also influences thespatial properties of the expanding plume. To investigatethis effect we have analyzed the spatial distribution ofthe emission intensity along the normal to the targetsurface as a function of the time delayt with respectD

to the laser pulse. Such plots give a snapshot of theaxial profiles, and they are shown in Fig. 5a–f for thethree different pressures investigated and at different

values oft . The experimental data points have beenD

obtained by integrating over a 100 ns interval centeredat t . The intensity distributions show that the back-D

ground gas largely affect the plume spatial evolution,leading to two major effects:(i) by increasing thepressure the plume emission becomes more confined;(ii) a strong increase in the emission is observed atPs23 Pa compared withPs7 Pa.

In particular, at low pressurewPs0.5 Pa, see Fig. 5a–dx, the plume emission intensity drops down rapidly inthe first microsecond, while the position of the intensitymaximum d moves away. Then, at later timeswtG1M

ms, see Fig. 5dx, the plume emission extends up to morethan 10 mm from the target showing two spatial com-


Fig. 6. Relative emission intensity spatial profiles for different timedelayst after the laser pulse atPs23 Pa.D

ponents. The fast one, corresponding to the trailing edgeof the highly forward-directed expanding plume, movesaway from the target while its emission drops downrapidly. On the contrary, the second one shows an almoststationary behavior as a function of the distanced, whileits luminescence decreases on a timescale of few micro-seconds. The latter one can be ascribed to very slowplume atoms, which are thermally emitted in the latestage of target evaporation, as already observed duringlaser ablation of different materials by ICCD photogra-phy w19x and time-of-flight mass spectrometryw20x.

By increasing the pressure toPs7 Pa wsee Fig. 5bx,we observe a decrease in the overall emission, and theluminescent plume appears more confined, while thefast component observed at the lower pressure and latertimes is no longer presentwsee Fig. 5ex, as a consequenceof collisions between plume species and background gasatoms.

Finally, when the pressure is increased toPs23 Pawsee Fig. 5c–fx, a much more intense plume backgroundinteraction occurs, leading to a stronger confinement atearly times(up to f1 ms) accompanied by a muchlarger emission from the plume core, with respect to thelower pressures. Moreover, a different temporal dynam-ics is observed with the maximum of emission increas-ing, going from 0.1 to 0.5ms, and then slowlydecreasing as a function of time. Then, at longer timeswsee Fig. 5fx the tail of the spatial distribution extendsto larger distances and shows an increase of the numberof excited species at the plume front with respect toPs7 Pa due to the interaction dynamics with the back-ground gas. This is in good agreement with the obser-vation of a shock wave-like dynamics in thed–t plotM

at this pressure. In this regime, the expanding plumecompresses the surrounding gas like a piston due to alarge number of collisions between the ablated andbackground gas atoms. The adjoint mass of the ambientgas at the plume edge breaks the plume expansionresulting in a large deviation from the free expansionwhen the mass of the gas surrounding the leading edgeof the plume becomes comparable with the plume massw18x. The generation of a shock wave produces aredistribution of kinetic and thermal energies betweenthe plume and the ambient gas, leading to a transfer ofpart of the particle flux velocity into plume thermalenergy, then resulting in plume heatingw21x. Thisprocess mainly affects the plume frontw21x and can leadto the observed increase of excited species in the plumetail, at larger distances and longer timeswsee Fig. 4fx,as well as to the different behavior of the integratedemission yieldwsee Fig. 3x, at Ps23 Pa with respect toPs7 Pa. To show more clearly this effect we report inFig. 6 the variation of the relative emission intensity asa function of the distanced from the target for differenttime delays t at Ps23 Pa. The relative intensityD

distributions have been obtained by normalizing the

intensity profiles of Fig. 5 to the integral of the emissionyield. We note that the fraction of luminescence fromthe plume front gradually increases at larger time delayst . This shows that an even larger number of excitedD

species is present at the plume front during the propa-gation of the plume species into the background gas asa consequence of plume heating.

The results analyzed in this section clearly show thatthe interaction with the background gas, above certaindistance and pressures, strongly affect the temporal andspatial dynamics of the plume, but also influences theplume excitationyionization kinetics.

3. Silver plasma dynamics investigated by ion probeand quartz crystal techniques

The effect of the pressure of the background gas onthe dynamics of a laser ablation plasma plume producedby a simple one component target, silver, in Ar back-ground gas in a large pressure interval(from f10 toy4

f10 Pa) has been investigated by using combined2

measurements of ion signals and deposition rates. Theexperiments were carried out at the existing setup atRisø National Laboratoryw8,9x. A frequency-tripledNd:YAG laser beam(ls355 nm, pulse widthf6 nsFWHM, laser fluencef2.5 J cm ) was used in they2

present study to ablate a silver target in a vacuumchamber with a residual pressure of 10 Pa. The lasery5

beam was focused to a circular beam spot on the targetat normal incidence.

The ion flux was measured by using a planar Lang-muir probe, oriented to face the target spot, located at adistance of 75 mm from the target and at an angle off208 with respect to the normal to the target surface.The probe collecting area was a 2=2 mm square2


Fig. 7. Time-of-flight profiles of the silver ions as a function of the Ar background gas pressure.

copper plate insulated on the rear side, similarly to thosein Ref. w9x. During the ion collection, the probe wasbiased aty10 V. The collected ion current was meas-ured by acquiring the voltage signal developed across aload resistor by a 500 MHz digital oscilloscope. Theamplitude of the probe signals on a new target spot wasfound to reach a constant regime after approximately 50laser shots. All the measurements were carried out aftersuch a target conditioning with the ion flux signalsaveraged over five consecutive shots.

The deposition rate was measured by a quartz crystalmicrobalance(QCM), with a 6-mm diameter activearea, located at a distance of 80 mm from the target andat an angle off238 with respect to the normal to thetarget surface. Each run was taken on a fresh targetspot, typically with 200–1000 pulses with a repetitionfrequency of 0.1 Hz and with a subsequent relaxationtime for the crystals of 4–6 h.

Fig. 7 shows the influence of the Ar background onthe ion time-of-flight(TOF) signals for different valuesof the ambient gas pressure,P. Each signal has beennormalized to its own maximum value to facilitate thecomparison. The fast narrow peak at almost zero timeis due to photoelectrons from the metal of the probe tipinduced by scattered laser photons and UV photons fromthe plume. The additional peak with a short arrival timeobserved at larger pressures is presumably due to back-ground gas atoms ionized in the immediate vicinity ofthe collector by UV light or fast electrons from theplasmaw22,23x and collected on the biased probe. Then,the peak with a maximum atf5 ms and extending upto f20 ms, at the lower pressure, is due to the collectionof plasma plume ions reaching the probe. This single

peak structure was observed from high vacuum up to apressure off4 Pa, where the signal temporal profile ismainly characterized by the appearance of a tail whichextends to longer times as the pressure increases. Byfurther increasing the pressure, the long tail transforms,at first, into a shoulder, and then, atPf5 Pa, into asecond delayed peak of slow ions. This means that ionplume splitting at such an Ar background pressureoccurs at distances of several centimeters from the targetsurface w11,12,14x. At even higher gas pressure thearrival time of the delayed ion peak increases, while thenumber of fast ions(first peak) becomes progressivelysmaller than that in the second delayed peak.

We also analyzed the collected ion yieldY and thei

deposition rate as a function of pressureP. The data arereported in Fig. 8a,b. We clearly observe that the numberof ions reaching the Langmuir probe is essentiallyconstant and equal to the vacuum yield up to a pressureof f0.5 Pawsee Fig. 8a, insetx. This indicates that theslight broadening of the signal is due to fast ionsexperiencing elastic collisions with the background gasatoms with do not significantly deflect them out of thecollecting solid angle in this pressure interval. In thisrespect, it has to be noticed that in a collision betweenAg and Ar atoms the Ar atoms are efficiently pushedaway, because of the large difference between the massesof projectile and target species. In particular, the direc-tion of the Ag atoms changes, on average, only off178while the loss of kinetic energy isf40% w17x.

A progressive decrease ofY is observed for pressuresi

larger than 1 Pa such that the collected ion yield isreduced at one-half of the value observed in highvacuum for an Ar pressurePf5 Pa, while the long tail


Fig. 8. (a) Collected ion yield and(b) atoms deposition rate as afunction of background gas pressure. In the insets the data are shownon a linear-logarithmic scales plot to show the behavior at very lowpressures. The lines are guide to the eye.

in the ion signal becomes more and more significantwsee Fig. 7x. At larger pressure, plume splitting occursin the ion signal and the delayed component start tobecome more and more significantwsee Fig. 7x, whereasthe collected yield continues to slow down. Finally, atthe largest pressure(PG20 Pa) a quasi-stationary regimeis reached, in which the collected ion yield slowlydecreases as a function of the pressure. A similarbehavior is observed for the deposited atoms on theQCM shown in Fig. 8b. Eventually from the observedvalues in high vacuum an ionization degree of the orderof f30% can be estimated at the detection distance.

To further elaborate on the effect of ambient gas onthe plasma dynamics, in Fig. 9 we report the time ofarrival (obtained by considering the time at which themaximum in the ion TOF-signals is observed) of thefast and slow peak of the collected ion flux as a functionof pressure. The scattering of the arrival time of thedelayed peak at high pressure is due to signal noisewhich does not allow a very precise identification ofthe peak maximum. It is interesting to note that the fastions propagate through the background gas almost with-out deceleration up to a pressure off10 Pa. At higher

pressure, the peak merges into the fast backgroundionization peak. The delayed ion component shows aclear shock-wave-like dependence,tAP w18x, in the0.5

pressure rangef 5–10 Pa and is then somewhat delayedat higher pressure. This indicates a further decrease ofthe kinetic energy towards a complete plume thermali-zation with background gas atoms.

4. Physical description of laser produced plasmaplume expansion into an ambient gas

In nanosecond laser ablation, target evaporationbegins just after the impact of the leading edge of thelaser pulse on the sample surface. The interaction of thefollowing part of the laser beam with the vapor in thevicinity of the target surface leads to strong heating andionization of the vapor and to plasma formation. Initially,the ablated atoms and ions undergo collisions in a highdensity region near the sample surface, leading to ahighly directional expansion along the normal to thetarget surface.

In high vacuum condition, and for background gaspressures so low that the mean free path of the ablatedparticles is larger than the characteristic length of theexperiment, an adiabatic expansion occurs with theplasma plume rapidly reaching an inertial stage. In thisstage almost all the initial thermal energy deposited inthe laser produced plasma has been converted intodirected plume kinetic energy(free-plume). A physicaldescription of plume expansion in this regime has beenmodeled by using self-similar profiles of the plumethermodynamic variables based on gas-dynamics andenergy conservation equationsw24x.

In an ambient gas, the expansion of the laser producedplasma plume can be quite complex and stronglydependent on a number of properties like atomic massof the plume and background atoms, initial plume energyand density, etc.w7,21,25,29x. Obviously, if the initialmass and energy of the laser produced plasma is nothigh enough, the plume can be so faint that no hydro-dynamic effects can take place and ablated particles justdilute into the ambient gas. However, when an energeticplasma is formed, the plume expansion into the back-ground gas can pass through different stages, which caninvolve plume confinement, splitting and snowplowingeffects, plume-gas mixing, shock wave formation, etc.The expansion of a laser produced plasma plume intoan ambient gas has been treated by different authorsaddressing different aspects of the process. For example,the evolution of spherical laser plume has been analyti-cally modeled in terms of internal and external shockwave formation to describe the different stages of plumeexpansion by Arnold et al.w21x. Gas-dynamics equationswith collisional plume-gas interactions have been con-sidered by Wood et al. to explain plume splitting in Ref.w26x. In addition, other numerical models based on the


Fig. 9. Peak arrival time vs. pressure for the two observed ion peaks. The solid line corresponds to a shock-wave-like behavior,tAP , while0.5

the dashed line is a guide to the eye.

dynamics of shock wave formation and propagation hasbeen also proposed by Bulgakov and Bulgakovaw27x.More recently, a hybrid approach based on the combi-nation of continuous gas-dynamical modeling and directMonte Carlo simulation has been used to describe plumeexpansion of Al neutral ground-state atoms in oxygenenvironment by Itina et al.w28x.

The experimental results discussed in the previoussections allow us to give a physical description of thelaser produced plume expansion into an ambient atmos-phere. In particular, the combination of complementarytechniques, like optical emission spectroscopy, fast ionprobe and deposition rate measurements, enables us toinvestigate the effects of the background gas both atshort and long distances from the target surface follow-ing the behavior of neutrals, ions and excited species.

From the experimental findings reported in the pre-vious sections, we can identify three different regimesof the plume expansion as a function of the backgroundgas pressure:(a) free-plume, where the plume propa-gation is nearly vacuum-like;(b) collisional regime,where more and more plume particles from the plumefront start to collide with background gas atoms. Abovea certain pressure a strong plume-background interactionoccurs leading to shock wave generation and plumesplitting; (c) plume thermalization, where the plumebecomes strongly confined and, above a certain distance,the ablated species tend to diffuse out the plume coreinto the ambient gas.

In the following, the basic features of the differentregimes will be analyzed and discussed.(a) Free-plume: At low pressures, the plume moves

away from the target with a constant average velocity,

while adiabatically expanding, resembling the plumeexpansion in high vacuum conditions. During expansion,plume density and temperature gradually drop down,leading to the evolution of the MgB plume emission at2

a pressure of 0.5 Pa observed in Fig. 5a. However, thearrival time of the first ion peakwsee Fig. 9x as well asthe number of collected silver atoms and ionswsee Fig.8x at a distance of several cm from the target is almostindependent of the pressure up toPf1 Pa.(b) Collisional regime: At intermediate pressures,

when the mean free path of the ablated species becomescomparable with the observation distances, collisionswith the background gas atoms start to play a role.Initially, the collisions mainly involve ablated species atthe plume edge, and influence the overall plume propa-gation dynamics only slightly. This effect can beobserved in the evolution of the temporal maximum ofplume emission,t , at Ps7 Pa wsee Fig. 4x, where weM

observe a small delay only above a distancedf7 cmfrom the target surface. The effect on plume emissioncan be clearly observed by comparing the axial distri-bution of the plume emission at 0.5 and 7 Pa. Weclearly observe that while the emission close to thetarget is almost comparable at the two pressures, theluminescence at 7 Pa is progressively reduced at largerdistances from the target as a result of collisions sufferedby emitting species. The reduction of the emission signalcan be ascribed to scattering processes as confirmed bythe dependence of the integrated emission yield on thedistanced shown in Fig. 3. A behavior in agreementwith the analysis reported above can also be draw outfrom the dependence of the Langmuir probe and QCMdata observed in Figs. 7 and 8. In fact, by increasing


the pressure of the background gas, we observe nosignificant variation in the arrival time of the Ag ionspeak, up to a pressure off4 Pa, while the collectedyield is gradually attenuated.

When the ambient gas pressure is further increased, astronger plume-background gas interaction regimeoccursw7,21,25,27–29,29x. Through an increasing num-ber of collisions the ablated materials begins to effec-tively push away the ambient gas atoms. As a result,both the plume and the adjacent background gas start tocompress. Then, the transition to a hydrodynamic regimeof the plume propagation takes place, with the plumeacting as a piston on the surrounding atmosphere, whosecounteraction breaks the ablated atoms expansion. Thisleads to plume confinement, which also influences theplume core. This is shown by the appearance of adelayed component in the temporal profiles of theemission intensity atPs23 Pawsee Fig. 2x and of theions time-of-flight distribution above a pressure off4Pa wsee Fig. 7x. The delayed component becomes moreand more important as the ambient gas pressure increas-es, as clearly observed in Fig. 7. Nevertheless, a smallnumber of fast species is yet observable in both casesand even at particularly high pressure. This indicatesthat a small fraction of the energetic species present atthe leading edge of the plume still penetrate into theambient gas and reaches the detection region withnegligible delay.

The analysis of the delayed peak temporal dynamicsshows that the plume-gas hydrodynamics couplingresults in the generation of a shock-wave-like motion,as observed by the distance–time plot at 23 Pa, in Fig.4, and by the arrival time vs. pressure, in the interval5–10 Pa, in Fig. 9. The formation of a shock-wave-likeexpansion leads to plume confinement and plume heat-ing effect compared to free-plume, and an increase ofthe overall plume emission is observed atPs23 Pa asshown in Fig. 5c–e. In particular, plume confinementresults in a larger amount of emitted photons from theinner part of the plume due to the formation of a highdensity peak in this areaw28,29x. It also tends to increaseexcitationyionization at the plume front as a result ofplume heatingw7,21x, due to transfer of plume streamvelocity into thermal energy, as observed in Figs. 3 and6.

However, we observe that the amount of atoms andions reaching a collector located at several centimetersfrom the target surface decreases with the ambient gaspressure. During the hydrodynamic regime, the plumeis braked mainly along the normal to the target surface,because the larger the expansion velocity, the larger theadjoint mass due to background gas. This leads to asignificant modification of the plume angular distribu-tion in this regimew29x, which results in the reductionof the number of atoms reaching the QCM and theLangmuir probe, as observed in Fig. 8. Moreover, the

decrease of the collected ions yield observed in Fig. 8acan be partly ascribed to an enhancement of three-bodyrecombination of the delayed ions as a consequence ofplume confinement.(c) Thermalization: At still higher pressures, namely

above 10 Pa for silver in Ar ambient gas, a thermaliza-tion stage is gradually approached. This is noticeablyseen in Figs. 8 and 9. Fig. 8 shows a rapid decrease ofthe slope of the collected yield as a function of thepressure abovef10 Pa. This is well correlated to therise of the delayed peak arrival time observed at thesame pressures in Fig. 9, indicating the transition froma shock-wave-like propagation of the plume to a diffu-sion-like motion of the plume ions into the backgroundgas followed by a thermalizationw7,25,29x. At thisbackground gas pressures also, the remaining high-energy ions starts to break, finally confusing with thebackground ionization peak, in Fig. 7.

5. Conclusions

In summary, two experiments exploiting differentdiagnostic techniques have been carried out in order toinvestigate laser produced plasma plume dynamics in abackground gas. In the first one, reported in Section 2,space- and time-resolved optical emission spectroscopyhas been used to investigate the effect on an Aratmosphere on the dynamics of an MgB plasma plume.2

In the second experiment, described in Section 3, fastion probe and deposition rate measurements have beenused to study the propagation dynamics of ablated ionsand atoms from a silver target in Ar background gas,over a large range of pressures. From a comparativeanalysis of the experimental findings we have demon-strated the evolution of the plume expansion throughdifferent regimes as a function of the ambient gaspressure. In particular, by increasing the pressure, theplume propagation dynamics goes from a vacuum-likefree-plume expansion to a diffusion-like regime, wherethe ablated species become gradually thermalized withbackground gas atoms, passing through collisional andshock-wave-like hydrodynamic regimes at intermediatepressures. Additionally, the experimental data also showthat the combination of complementary techniques, likeoptical emission spectroscopy, close to the target, andfast ion probe and deposition rate measurement at largerdistances leads to a more detailed understanding of thelaser ablated plasma dynamics. However, due to thecomplexity of the physical processes leading to thedifferent features observed in the present work, webelieve that further detailed experimental and theoreticalstudies will help to completely clarify the propagationdynamics of a laser ablation plasma plume in gasenvironment.


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diagnostics of laser ablated plasma plumes

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