130 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 27, NO. 1, FEBRUARY 1999

Two-Dimensional Imaging of Low Temperature Laser Produced PlasmasG. W. Martin, T. P. Williamson, A. Al-Khateeb, L. A. Doyle,

I. Weaver, D. Riley, M. Lamb, T. Morrow, and C. L. S. Lewis

Abstract—Laser induced fluorescence images of a low tempera-ture laser-produced plasma expanding into vacuum are presentedand compared to a computer simulation. The complex nature of aplume expanding into background gas is highlighted, along witha potential means of simplifying the study of such systems.

Index Terms—Ablation, modeling, optical spectroscopy, plasmaproperties.

T HE combination of absorption spectroscopy (AS) withsimultaneous two-dimensional (2-D) imaging of the cor-

responding laser induced fluorescence (LIF) can be used toinvestigate the spatio-temporal variations of species numberdensity [1] within low temperature laser produced plasmas(LPP). In this paper, we present LIF images obtained forexpansion of a titanium LPP into vacuum (10 torr) andcompare the experimental data to images generated usinga simple analytical model. The more complicated situationof plume expansion into ambient gas is also illustrated anda method of simplifying the investigation of such systemsproposed.

A KrF excimer laser [248 nm, ns full width at halfmaximum (FWHM)], incident at 45 to a rotating titaniumtarget, is focused using cylindrical optics and a random phaseplate [2] to provide a circular spot on target mm.A 1 mm thick slice of the resulting laser produced plasmais probed using a tunable narrowband ( 0.15 cm ) dyelaser ( ns FWHM). In-line absorption measurementsare recorded on a photodiode array whilst the LIF emittedfrom the slice surface is simultaneously imaged onto a fast(5 ns gate) 2-D intensified charge-coupled device (ICCD).Absorbance data yields the line-integrated number density ofabsorbers while the LIF images show the spatial location ofthe absorbing species. A comprehensive description of theexperimental detail is given in [1].

Typical data acquired using this technique is illustrated inFig. 1, which shows the LIF images recorded at detunings of

, 60 mA from the centre of the Ti II338.3768 nm transition at 3 s after the ablation

pulse. In this case, the plume is expanding into vacuum andthe absorption lineshape is strongly dominated by motionalDoppler effects. The narrow bandwidth dye laser thereforeonly interacts with species having appropriate velocities in thedirection of the dye laser probe beam, described by the simple

Manuscript received June 29, 1998. This work was supported by theEngineering and Physical Sciences Research Council and the Department ofEducation for Northern Ireland.

The authors are with the School of Mathematics and Physics, The Queen’sUniversity of Belfast, Belfast, BT7 1NN U.K. (e-mail: g.martin@qub.ac.uk).

Publisher Item Identifier S 0093-3813(99)02563-1.

Fig. 1. LIF images of Ti II(4G�

5=2 4F3=2) 338.3768 nm transition for

detunings of�� � 0, �60 mA at 3 �s delay.

expression

where is the direction vector of the dye laser,the positionvector of the species relative to the centre of the laser spoton target, and is the detuning from line centre at whichabsorption will occur. This dependence of absorber locationwith probe wavelength is evident from the LIF images shownin Fig. 1.

A simple analytical model has been developed usingMath-ematica to predict the expected spatial distribution of theLIF signal, Fig. 2. The model uses a self-similar expansiondescription of the density distribution in the expanding plasma.Absorption of the dye laser pump radiation is calculated, takinginto account the motional Doppler effects in addition to thedye laser bandwidth and detuning from line centre. The LIFsignal, integrated along the line of sight of the detector, isevaluated for an elliptical Gaussian density distribution andcompared with the experimental data. Although the ellipticalGaussian distribution of material density normal to the target isnot entirely accurate, this relatively simple model can providea good description of the plume expansion into vacuum.

Typical pulsed laser deposition (PLD) conditions, whichinvolves expansion into ambient gas (illustrated in Fig. 3), aremuch more complex than in the vacuum case. Here elastic,inelastic, and reactive collisional processes play importantroles and investigation of these processes using spectroscopictechniques presents a greater challenge, since the relative

0093–3813/99$10.00 1999 IEEE

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 27, NO. 1, FEBRUARY 1999 131

Fig. 2. LIF intensity for Ti II 338.3768 nm transition, as predicted by theanalytical model, for comparable experimental conditions.

Fig. 3. Spectrally integrated emission image of a YBa2Cu3O7 plume ex-panding into 80 mtorr O2 at 1 �s delay.

contributions of motional Doppler and thermal Doppler tomeasured lineshapes will vary with time and position withinthe plume. A technique which may prove valuable in such

Fig. 4. LIF image of Ti II 338.3768 nm transition at 2�s delay showingnarrow jet of plasma selected by a� = 0:5 mm aperture placed 2 cm fromthe target. (�� � 0 mA, plume expanding into vacuum.)

studies is illustrated in Fig. 4, which shows the LIF imageof a jet of plasma, selected by an aperture placed in frontof the expanding plasma. Such jets, directed along the targetwill normallly exhibit minimum motional Doppler broadeningof their absorption lineshapes hence facilitating spectroscopicinvestigation of any collisional processes resulting from inter-action of the jet with ambient gases.

In conclusion, a simple analytical model is shown to becapable of reproducing the salient features of experimentalLIF images of plume expansion into vacuum. The practicalcase of expansion into ambient gas is more complex and apotential means of simplifying investigation of such systems,by selecting and injecting a narrow jet of the plume, isproposed.

ACKNOWLEDGMENT

The authors wish to thank W. A. Montgomery for specialistlaser support.

REFERENCES

[1] G. W. Martin, L. A. Doyle, A. Al-Khateeb, I. Weaver, D. Riley,M. J. Lamb, T. Morrow, and C. L. S. Lewis, “Three dimensionalnumber density mapping in the plume of a low temperature laserablated magnesium plasma,”Appl. Surf. Sci., vol. 127–129, no. 1–4,pp. 710–715, 1998.

[2] L. A. Doyle, G. W. Martin, A. Al-Khateeb, I. Weaver, D. Riley, M.J. Lamb, T. Morrow, and C. L. S. Lewis, “Electron number densitymeasurements in magnesium laser produced plumes,”Appl. Surf. Sci.,vol. 127–129, no. 1–4, pp. 716–720, 1998.