Ž .Applied Surface Science 127–129 1998 686–691

Absorption spectroscopy of an expanding laser produced lithiumplasma in the extreme ultraviolet using the Dual Laser Plasma

technique

William Whitty, John Costello, Eugene Kennedy, Christopher Moloney,Jean-Paul Mosnier )

School of Physical Sciences, Dublin City UniÕersity, GlasneÕin, Dublin 9, Ireland

Abstract

Ž .We describe the essential features of the Dual Laser Plasma DLP vacuum ultraviolet photoabsorption spectroscopytechnique and the characteristics of our DLP apparatus. We show that the time- and space-resolved capabilities of thistechnique are suited to the monitoring of the dynamics of expanding plasma plumes in the regime used for pulsed laserdeposition of materials. Examples of spectra showing the spatial and temporal evolution of a lithium plasma expanding invacuum are presented. A model based on a self-similar expansion for the plume is developed and used to analyse the shapeof absorption lines. Measurements in the photoionisation continuum of Liq are also presented. q 1998 Elsevier Science B.V.

PACS: 52.70 La; 32.70 Jz

Keywords: Laser produced plasma; Extreme ultraviolet photoabsorption; Lithium

1. Introduction

ŽWhen the output of a Q-switched laser typically,.1 J, 10 ns is focused onto a solid target a hot, dense

plasma is formed. Such plasmas constitute intenseŽ .sources of vacuum ultraviolet VUV and X-ray

w xradiation 1,2 . The laser plasma has thus gainedacceptance as a standard laboratory-based pulsed

w xsource of short wavelength radiation 3 .The inherent time-resolved nature of the laser

plasma light source is suited to the study of thedynamics of transient species, including laser plas-mas themselves. The technique of probing the struc-

) Corresponding author. Tel.: q353-1-704-5303; fax: q353-1-704-5384; e-mail: mosnierjp@dcu.ie.

ture and dynamics of a laser plasma using the lightemitted by another laser plasma is known as the

Ž .Dual Laser Plasma DLP photoabsorption techniquew x4 . DLP absorption involves probing the absorbingplasma in different conditions. Spectra of multiply orsingly charged ions or neutrals are obtained whenprobing the plume in different spatio-temporalregimes, thus introducing selectivity of absorbingspecies. Most DLP experiments have hitherto beenconcerned with the study of fundamental aspects of

w xthe photoionisation process 5 .Photoabsorption is also a powerful analytical tool,

finding application in the study of the processesŽ .governing pulsed laser deposition PLD of oxide

w xsuperconducting thin films 6,7 . Yet, the interpreta-tion of the optical absorption spectra of plasma

0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0169-4332 97 00726-5

( )W. Whitty et al.rApplied Surface Science 127–129 1998 686–691 687

plumes is complex. Severe broadening and distortionw xeffects, e.g., refraction 8 , alter the line shapes ob-

served. A quantitative interpretation of such spectrathus requires a knowledge of the basic physicalmechanisms involved in the plume, together withvalues of the relevant atomic parameters characteris-

Ž .ing the transition s in question.In this paper, we show that under certain condi-

tions the quantitative interpretation of the ExtremeŽ .UV XUV absorption spectra of laser plasmas is

Ž .comparatively simpler. This is due to: 1 the broad-ening or distortion effects resulting from local plumeconditions affect electronic inner-shells less severelythan valence levels and thus the observed spectral

Ž .broadening may even be purely instrumental, 2refractive effects are minimised for probe frequen-cies much larger than the plasma frequency.

The XUV absorption spectra presented here wereobtained using the DLP technique, which has rarelybeen used to study the dynamics of the PLD process.Some recent DLP studies reported on the ablation of

w xsilicon 9,10 .In the following, we describe our DLP apparatus,

present results showing the spatio-temporal evolutionof a lithium plume and develop a model to analyseabsorption line shapes. Finally, measurements in thephotoionisation continuum of Liq are presented.

2. Experimental and results

A diagram of the apparatus is given in Fig. 1. ItŽ .comprises three parts: 1 the lasers and laser plasma

Ž . Ž .sources, 2 the coupling toroidal optics and 3 thespectrometer and data acquisition system. A general

w xdescription of the DLP system is given elsewhere 5and here we only provide new or additional informa-tion relevant to the present paper.

The source of continuum XUV radiation CS isŽ .created by a Nd:YAG laser 0.75 J, 15 ns tightly

focused onto a tungsten rod using a 100 mm uncor-Ž .rected plano-convex lens FL1 . The duration of the

pulse of EUV radiation is about the duration of thew x Ž .laser pulse 11 . The absorbing plasma AP Fig. 1c

Ž .is created by a Nd:YAG laser 0.3 J, 15 ns using acylindrical lens FL2. The irradiance on target can bevaried between 108 and 1011 Wrcm2. The firing

Ž .sequence between the two lasers DT is controlled

Ž . Ž .Fig. 1. a Diagram of apparatus in the horizontal plane: CSŽ . Ž .Continuum Source, AP Absorbing Plasma, TM Toroidal Mir-

Ž . Ž . Ž .ror, ES Entrance Slit, GS Grating Spectrometer MCPrPDAŽ .Microchannel PlaterPhotodiode Array, RC Rowland Circle,

Ž . Ž .OMA Optical Multichannel Analyzer, PC486 486 PersonalŽ . Ž .Computer, DG Digital Delay Generator. b Diagram of appara-

Ž . Ž .tus in the vertical plane: K Knife-edge along O x axis. cxŽ .Detailed view of plasma chamber: D x Distance above plane of

Ž . Ž .sample target, K Knife-edge along O z axis, FL1 FocusingzŽ .Lens 1, FL2 Focusing Lens 2.

by a digital delay generator DG, the system jitterbeing less than 3 ns.

The optical layout conforms with the configura-w xtion prescribed by Rense and Violett 12 to increase

the efficiency of a grating spectrometer by removalof astigmatism with the help of a toroidal mirror TM.The radii of curvature of TM were chosen to producespectral lines of uniform length on the Rowlandcircle RC. Thus, the system is capable of quasi-stigmatic imaging in the available 3–30 nm rangeprovided by a 1200 linesrmm grating GS operated

( )W. Whitty et al.rApplied Surface Science 127–129 1998 686–691688

Ž . 0 Ž .Fig. 2. a Temporal and spatial evolution of Li species. bTemporal and spatial evolution of Liq species.

at an 848 angle of incidence. Details on stigmaticimaging of laser produced plasmas in the EUV can

w xbe found in 13 . In order to determine the spatialresolving capability of our system, we carried out adetailed ray-tracing study using SHADOW 1 and a

Žseries of knife-edge measurements see Fig. 1b and.c . The key idea is to determine the region of the

continuum source and hence of the absorbing mediumthat effectively contributes to the final image on RCw x14 . The dimensions of this region are found to beapproximately 300=500 mm along the O x and O z

1 SHADOW is a software package developed and licensed bythe University of Wisconsin Madison; for further reference seewww.xraylith.wisc.edu.

axes, respectively. The spectral resolving power is ofthe order of 1500 at 150 eV photon energy beforeremoval of instrumental broadening. The determina-tion of the instrument function is of prime impor-tance, as the true absorption profile mirrors the

Žconditions existing in the plume see Section 3 be-.low . The instrument function was estimated by mea-

suring the profile of the Li2q Lyman-b absorptionŽ .line 108.845 eV for which accurate values of the

w xDoppler and Stark widths are computable 15 . ALorentzian function of 0.105 eV FWHM centred at108.845 eV accurately depicts the instrumental func-tion. This value is comparable to typical Doppler and

w xStark widths of EUV lines in laser plasmas 15 .The present experimental results were obtained

with a lithium plasma expanding in vacuum. Thetemporal and spatial evolution of Li0 was mappedby integrating the spectrum of the optically thinabsorption coefficient of the 1s2 2s™1s2s2p transi-

Ž .tion line A , at 58.92 eV, for varying distancesŽ . Ž .above the target surface D x and time delays DT

between the laser pulses. The resulting plot is shownin Fig. 2a. In the case of Liq, the same procedureunder the same conditions was used for the 1s2 ™

Ž . Ž .1s2p transition line B at 62.22 eV Fig. 2b . Thetwo vertical scales are not directly comparable since

q Ž .Fig. 3. The Li photoabsorption spectrum. a Dotted line: mea-sured at D xs0.4 mm, DT s30 ns; solid line: absorption spec-

Ž . Ž .trum resulting from model see text . b Theoretical absorptioncoefficient.

( )W. Whitty et al.rApplied Surface Science 127–129 1998 686–691 689

Fig. 4. The absolute photoionisation cross-section of Liq as aŽfunction of photon energy. Theoretical cross-section from Ref.

w x .16 .

the two transitions have different oscillator strengths.The irradiance of the focused laser beam on targetwas estimated at 9=109 Wrcm2.

The relative absorption cross-section spectrum ofLiq as a function of photon energy in eV is pre-

Žsented between 72 eV and 75 eV in Fig. 3a D xs0.4.mm, DTs30 ns . The observed lines correspond to

the He-like resonance series 1s2 ™1snp with ns4,5, 6 and 7. A synthetic absorption spectrum obtainedfrom the corresponding theoretical absorption coeffi-

Ž . Žcients Fig. 3b is also presented see Section 3 for.explanations . Fig. 4 depicts the measured and theo-

retical cross-section curves for the 1s2 ™1s´ p pho-Ž .toionisation process between threshold 75.64 eV

Ž .and 130 eV D xs0.4 mm, DTs30 ns .

3. Analysis and discussion

The spatio-temporal distribution maps of Fig. 2give insight into the dynamics of the evolution of thelithium plasma. It can be seen that the Liq ionsappear in the earlier stages of the plume expansionand are concentrated close to the target surface alongits normal. Conversely, the population of neutral Liatoms appears to peak at a later stage of the expan-sion and tends to occupy a comparatively larger area.This observation is consistent with earlier detailedstudies on the ion and velocity structure of a laser

w xproduced plasma 17 .The expansion is symmetric about O x with radial

Žsymmetry assumed in planes normal to O x see Fig.

.5 for geometry . A fixed observer in the laboratoryframe looking at the plasma side-on will see Dopplershifted light proportionally to the sign and magnitudeof the radial component of the instantaneous velocityŽif the line-of-sight is along the diameter of the

. Ž . Žplume or the corresponding Õ z projection if the.axis of observation is along a chord as in the figure .

It is assumed that the instantaneous velocity of anabsorbing ion or atom at any point in the plasma canbe represented by the sum of a thermal Dopplercomponent which contributes the Gaussian compo-nent of the total line width and a streaming compo-nent which contributes a frequency shift. In thismodel, a photon of frequency n in the vicinity of anatomic resonance centred at n will only be ab-0

Ž .sorbed by those ions atoms whose thermal andstreaming velocities combine to give a frequency

w xshift Dnsnyn along the path of the photon 18 .0

This frequency shift is equal toÕ

Xn sn 1" 1Ž .0 ž /c

Ž .in the non-relativistic limit. In Eq. 1 , Õ representsŽ . Ž .either Õ r , the radial component, or Õ z the z

Ž . Ž .component with Õ z sÕ r =zrr. The minus signcorresponds to a velocity component away from the

Ž .observer red shift and the plus sign to a velocityŽ .component toward the observer blue shift . Knowl-

Fig. 5. Geometry of the plasma plume in a plane normal to theexpansion direction.

( )W. Whitty et al.rApplied Surface Science 127–129 1998 686–691690

Ž . Ž .edge of the variations of Õ r and Õ z as a functionof r and z respectively is therefore required to

Ž .predict the line profile. It has been shown that Õ rŽ Ž . .skr and thus, Õ z skz where k is a positive

constant which may vary with time and positionŽ .along O x is an acceptable self-similar solution forthe expansion of a laser plasma from a plane targetw x18,19 . The attenuation of radiation at frequency n

incident on the expanding plasma with intensity In0

and emerging with intensity In is given by:L

InqLr2L

sexp y x z d z 2Ž . Ž .H nn ž /I yLr20

Ž .where x z is the absorption coefficient such thatn

Ž . Ž . Ž . Ž .x z ss z N z , s z is the total photoabsorp-n n n

Ž .tion cross-section discrete or in the continuum andŽ .N z is the number density of absorbers, introducing

Ž .a spatial z dependence. In the case of a discretetransition between two levels labeled i and j respec-tively of weighted oscillator strength f the cross-i j

Ž 2 .section in m is written as:

1 p e2i js z s f F z 3Ž . Ž . Ž .n i j nž /4p´ mc0

Ž .in which F z is the normalised Atomic Frequencyn

Ž .Response AFR function. The most versatile func-tion depicting the AFR is a Voigt profile charac-terised by a Doppler width Dn and a LorentzianD

width Dn . The explicit spatial dependence of theLŽ Ž .. Ž Ž ..cross-section in Eq. 3 arises from Eq. 1 and in

Ž .particular, the choice of Õ z skz.We now apply the model just described to the

2 Žcase of the Rydberg series 1s ™1snp ns4, 5, 6,. q Ž .7 in Li Fig. 3 for which accurate values of the

w xoscillator strengths are known 20 . First, we notethat the widths of these lines are comparable to theFWHM of the instrument function previously esti-mated. The broadening is therefore mostly instru-mental. This implies that the experimental conditionsbelong to the low expansion velocity regime as

w xdefined in 18 , i.e., streaming plays a negligiblerole. For each of these transitions an absorptioncoefficient is generated using a Voigt profile for the

qŽ Lr2. Ž .AFR and a trial value of N sH N z d z. AnL yŽ Lr2.initial estimate for the width is obtained using the

w xformulae given in Ref. 15 for the Doppler and Starkcomponents. Natural line widths are negligible for

Ž .these transitions. A value for the transmission 2 isthen computed and the procedure repeated over thefull line profile. The synthetic transmission spectrumobtained is then convolved with the instrumentalfunction and finally compared with the experimentalone. The entire procedure is repeated until a satisfac-tory level of convergence between experiment andmodel is achieved simultaneously for all four transi-tions. At each iteration, the FWHM is allowed tovary differently for each transition, since Stark

Žbroadening depends on n principal quantum num-.ber , whereas the same value of N is used for allL

four transitions. A theoretical absorption coefficientŽ .spectrum results from the fitting procedure Fig. 3b

as well as a value for N . This is equal to ;3=1016L

y2 Žcm in the conditions of Fig. 3a D xs0.4 mm,. q

DTs30 ns . This corresponds to a ground state Lidensity of ;9=1017 cmy3 for a plasma length of0.3 mm and zero density gradient along the line-of-sight. These values are in good agreement with other

w xplume density measurements or estimations 21 .The photoionisation cross-section for the 1s2 qhn

™1sq´ey process in Liq can be accurately com-w xputed using the universal fitting formula of 16 . The

corresponding curve is plotted in Fig. 4. A transmis-sion measurement in this case directly provides avalue for N since the continuum cross-section doesL

not depend on any particular line shape factor. Wehave measured the photoionisation continuum of Liq

between 75.64 eV and 130 eV photon energies inconditions where the neutral population is negligible.The cross-section varies between 2.5 Mb and 0.75Mb in this range which corresponds to a factor of 6on the value of transmission. Thus, in order to carryout the measurements in a satisfactory absorbanceregime over this energy range the length of theabsorbing column was increased with increasingphoton energies. All other experimental conditionswere identical to those of Fig. 3. The entire rangewas recorded using six different positions of thedetector along the Rowland circle with large over-laps between adjacent settings. For each setting, thebest value of N was obtained by a non-linear leastL

square fit of the measured absorbance data to thew xanalytical cross-section 16 . The excellent agree-Ž .ment between the curves Fig. 4 further confirms

the high purity of the Liq content in the plume in theconditions used and also the accuracy of the theoreti-

( )W. Whitty et al.rApplied Surface Science 127–129 1998 686–691 691

cal cross-section. Assuming zero gradient in the Liq

distribution along the line-of-sight, the N values areL

converted to number density values averaging to;1=1018 cmy3. This value is in good agreementwith the value obtained from modeling the discrete

Ž .part of the spectrum see above .

Acknowledgements

The authors wish to thank Forbairt and the EUŽ .HCM Programme for financial support.

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