Line shape study of Ba ions in laser produced plasmas

___ i!B

c2@3

applied

surface science

ELSEVIER

Applied Surface Science 99

(I 996)

345-35

I

Line shape study of Ba ions in laser produced plasmas R.A. Al-Wazzan, J.M. Hendron, T. Morrow Received 25 August 199.5: accepted

IO December

I995

Abstract Detailed measurements of spatio-temporal variations in the line strengths and line shapes for Ba ion absorption lines, observed in laser-ablated YBCO plasma plumes, show that the presence of ambient oxygen leads to enhanced local number densities and temperatures at the expanding front of the plasma plume. Enhanced local temperatures (and local excitation rates) are attributed to the increased collisional redistribution of high initial :-directed plume velocities in the presence of ambient oxygen.

1. Introduction Pulsed laser deposition has proved [I] to be a convenient technique for deposition of various materials in thin film form; including the high-T’ superconductor YBa,Cu,O, (i.e. YBCO). For 248 nm laser ablation of YBCO optimum quality in situ thin films are obtained for deposition at laser fluences of 2-4 J/cm’ in the presence of 100-200 mTorr ambient oxygen [2,3]. Several techniques including mass spectroscopy [4], ion probes [5,6], optical emission spectroscopy [7], absorption spectroscopy [8,9], laser induced fluorescence [IO, I I] and direct imaging [9,12] have been used to study the plasma parameters following laser ablation. Both emission and absorption spectroscopy measurements on plasma plumes yield spectra1 line widths [13,14] which are markedly wider than the corresponding natural line widths. Strongly distorted line shapes, with apparent line widths > 5 A, may be observed experimentally during the early stages of plume expansion close to target. In these early stages high density gradients within the expanding 0169.4332/96/$lS.O0

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SSLll 0169.4332(95)00604-4

plume lead to strong wavelength dependent refraction of both plasma self-emission and/or any visible probe beam used for absorption studies. In addition to their dependence on time and position within the plume such refraction effects vary markedly, both in magnitude and direction, with wavelength in the vicinity of a strong plume absorption resonance i.e. exhibit both normal and anomalous dispersion effects. The magnitude of such refraction effects depends on the laser fluence used; typically for a laser fluence of * 3 J/cm’ incident on target, plume refraction effects limit valid absorption measurements to > 1 cm from target and delays > I p_s. Probe beam refraction and other line broadening effects, such as Stark broadening, which influence line shapes close to target at short delays, are discussed in detail elsewhere [14]. The present paper deals with plume absorption observed at distances > 1 cm from target for delays > I p,s where absorp tion line widths are dominated by thermal Doppler broadening and/or the motional Doppler shifts hX, due to directional expansion of plasma species with velocity components u, in the direction of the probe

1996 Published by Elsevier Science B.V. All rights reserved.

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rt 01. /Applied

dye laser beam (i.e. AX, = &u,/c, where A, is the unshifted line centre wavelength). In the present work absorption line strengths and line shapes are used to determine velocity components, Boltzmann temperatures and number densities of BaII following laser ablation of YBCO in vacuum and ambient oxygen. Absorption data is also correlated with fast time-resolved ICCD images of the plasma self-emission.

2. Experimental The experimental arrangement used for the present study was similar to that reported previously [9]. The 248 nm output from a Lambda Physik LPX 210i laser, fitted with unstable resonator optics, was incident in the x,z-plane at 45” to the rotating YBa,Cu,O, target (see Fig. 1). All measurements were made on plasmas produced in either vacuum (N IO-’ Torr) or 180 mTorr oxygen at an incident laser fluence of 2.6 J/cm2 and a laser spot area on target of 4 mm*. Images of the spectrally-integrated plume emissions were recorded by imaging the sideon view of the luminescent plume onto a fast gated intensified CCD system (ORIEL Instruments INSTASPEC V) which provided full software facilities for image processing. Images of the spatial distribution of the BaII (62Pp,2 -+ 6’S,,2) 493.41 nm emission within the plume at 2 ps delay were recorded by imaging the plume onto the ICCD through a narrow-band optical filter with a transmission bandwidth (FWHM) of 8.34 nm. Plume spectra, recorded under the latter conditions, showed that the filter transmitted only the strong 493.41 nm BaII line and

Surface Science 99 (19%) 345-351

a much weaker BaII 490.00 nm line (intensity < 10% of BaII 493.41 nm intensity). For absorption studies, the plasma was backlighted at pre-set delays after the excimer laser pulse by the expanded beam from a short pulse (5 ns) narrow bandwidth (full half-width, Au _ 0.15 cm- ’, AA 5 0.03 A) Nd:YAG-pumped dye laser (Lambda Physik SCANMATE 2~). The spatial profile (in the z-direction) of the dye laser radiation transmitted by a central slice of the plume, centred on the z-axis and selected by a 0.5 mm wide slit located in the x, z-plane, was recorded by a 1024 pixel photodiode array system (ORIEL Instruments INSTASPEC III). At the demagnification used (i.e. 2.96) the spatial resolution within the plume in the y- and :-directions was 1.5 mm and 0.5 mm respectively. A broadband optical filter (AA - 50 nm, centred at 450 nm) was placed in front of the diode array in order to reduce the signals arising from ambient light and from plasma self-emission. The system software provided full facilities for signal averaging over a preselected number of laser events and for subtraction of contributions to the signal arising from dark current, ambient light and plasma self-emission. The line-integrated plume absorbance values D, (i.e. D, = log ,“( I,,/(), ) were recorded in the direction of the probe dye laser beam (i.e. x-direction). The incident dye laser intensity Z,) was recorded by firing the dye laser before the excimer laser pulse. All I,, f, and plasma self-emission signals were recorded as average values over 100 consecutive laser events accumulated at a repetition rate of 10 Hz, on the normal to the target surface at the laser ablation spot (i.e. on the z-axis).

3. Results and discussion Dye Laser

er Laser

Broadband

Filter

Fig. 1. Schematic of the experimental absorption measurements.

configuration

used for

Spatial profiles of the BaII ground state absorption in the z-direction were recorded at various delays and for various probe dye laser wavelengths within the line width of the BaII 455.4 nm resonance. The absorbance values recorded as a function of wavelength at various distances z from target and at delays of 1 ks and 2 I.LSafter ablation are summarised in the form of 2D-plots in Fig. 2. The grey scales in Fig. 2a-d specify the local absorbance values whilst a slice through the data at fixed z on

R.A. Al-Wa;:an

lcm

et al./Applied

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Surface Science Y9 CIYY6J 345-351

455.42

I

-

Absorbance

1cmI

4

3

2

1

Position

0 z (cm)

Fig. 2. Absorption line shapes recorded for the 355.4 nm transition of ground state BaII at 180 mTorr oxygen (b. d). The grey scales used specify local abxorbance values.

these figures corresponds to the absorption line shape at that distance. It should be noted that for the 2 p_s data recorded in vacuum (i.e. Fig. 2c) the plasma length in the z-direction exceeds the aperture of the observation port (i.e. 4 cm). At small distances from the target (< 1 cm) and short delay timts ( < 1 ps), the large apparent linewidths of * 0.5 A in Fig. 2a and b arise from a combination of Stark broadening, due mainly to collisions with plasma electrons, and wavelength dependent refraction effects which influ-

I ks (a, b) and 2 ks (c. d) in vacuum (a.

C)

and

ence both the lineshapes and absolute absorbance values recorded in this spatio-temporal regime of the plume. Stark widths can not therefore be deduced directly from absorption measurements made in this way. At distances greater than w 1.5 cm from target the line-integrated lineshapes recorded in vacuum (i.e. Fig. 2a and c) are dominated mainly by the motional Doppler shifts AX, (where AX, = h,u,/c) arising from the different velocity components II, of the ions in different spatial regions of the directional

Fig. 3. ICCD images of the spectrally-integrated visible plasma self-emission mTorr oxygen (b), in 80 mTorr oxygen (c) and in 180 mTorr oxygen (d).

as recorded

at

I ps delay after ablation in vacuum (a), in 40

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Surface Science 94, (19961335-351

plume. The position of the front of the expanding plume in Fig. 2a indicates that the maximum ion velocity in the z-direction, i.e. u:(max), at I p,s is = 4 X IO6 cm/s. Whilst the largest motional Doppler shift AX, of N 0.5 A, observed for z > 1.5 cm in Fig. 2a and c, indicates that the maximum value of u,, the component of the ion velocity Since in in the x-direction, is _ 7 X 10’ cm/s. vacuum the plasma plume undergoes essentially free expansion with a constant velocity u,(t) = x(t)/t then the wavelengths within the line shapes in Fig. 2a and c (which were recorded in vacuum) can be directly correlated with spatial position I of absorbing species within the plume (i.e. x(t) = AA,.c.t/X,). The appropriate distance scales in the x-direction are therefore provided in Fig. 2a and c. The spatial resolution in the x-direction in Fig. 2a and c is limited by the dye laser bandwidth (full half-width) of 0.03 A, which is equivalent to a velocity resolution of 2 X 10’ cm/s and a corresponding spatial resolution of 0.2 cm/ps of delay. The relatively small motional Doppler shifts in Fig. 2a and c indicate that in the vacuum case the fast moving BaII ground state ions are strongly forwarddirected along the z-axis. From Fig. 2b and d it can be seen that the fast z-directed ion component has been spatially confined in the presence of 180 mTorr

Absorbance

?? O.18-0.21 ?? 0.15-0.18 E 0.12-0.15

oxygen and a significant increase in the absorption line width is observed at the front of the plume (i.e. i.- = 1S-2.5 cm). Since the latter increase is due, at least in part, to normal thermal Doppler broadening, resulting from the increased local collision rates within the plume in ambient oxygen, the different wavelengths within the absorption line shapes in Fig. 2b and c can not be correlated directly with spatial positions x(t) of species within the plume. As we reported previously [9], the absorbance values in Fig. 2 indicate that the local BaII ground state number densities at the expanding front of the plume are significantly increased in the presence of ambient oxygen. ICCD images of the spectrally-integrated visible plasma self-emission, as recorded at 1 ps in the presence of various oxygen pressures, are shown in Fig. 3. It can be seen that as the oxygen pressures increases a zone of enhanced visible emission develops at the front of the expanding plume and the intensity (and size) of this zone increases as it moves closer to target due to progressive confinement of the fast z-directed ion component and correspondingly increased local collisional excitation rates. The direct correlation between absorption line shape measurements and plasma self-emission images is illustrated in Fig. 4. Absorption line shapes

452.484 452.488 452.492 452496 452:500

% ?? O.O9-0.12 0

% 0.06-0.09 t% 452.476 k= 452.484 00.03-0.06 452.492 no-o.03 452.500 452.508

Position

(cm)

Fig. 4. (a) and (b) show line shapes of the 452.49 nm absorption transition from the excited 6’ Pf,2 state of BaII, as recorded at 2 ks after laser ablation in vacuum and 180 mTorr oxygen respectively. (c) and (d) are ICCD images of 493.41 nm BaII (6’P$, --t 6’S,,z) plume emission recorded using a 20 ns detection gate width at 2 us delay in vacuum and 180 mTorr oxygen respectively.

R.A. Al-Wazan

et al./Applied

for the BaII (7*S,,2 +- 62PF,,) transition at 452.49 nm, as recorded at 2 ps delay in vacuum and in 180 mTorr oxygen, are plotted as a function of distance from target in Fig. 4a and b respectively. Fig. 4c and d show corresponding ICCD images of BaII (6’Pp,, +6’S,,,) 493.41 nm plume emission, from the same excited 62Pp/, state at 2.51 eV, recorded through a narrow band filter (transmission details provided in experimental section) using a 20 ns detection gate width at 2 ps after laser ablation. For plume conditions which approximate to thermodynamic equilibrium, the local Boltzmann temperature of a particular plasma species may be determined 1161 from the Boltzmann relation, viz.

4

-

P)

n(q)

g,

AE( ~3 4)

= -exp

kT

[

gy

I

(1)

where n(p)/n(q) is the ratio of the local populations in states p and q, with degeneracies g, and gy, and energy difference AH p, q). In the present work local population ratios and corresponding local Boltzmann temperatures were estimated from the line-integrated ground and excited state absorbance values for BaII shown in Figs. 2 and 4. i.e. n( PI -z-d

N

Ph,

n(q)

DC dh,

4q)x, 4

Ph,

(2)

where D, represents the absorbance value recorded at wavelength A and crx is the corresponding absorption cross section. Since for expansion into vacuum the motional Doppler shift, A A,, from the line centre wavelength, A,, correlates directly with spatial position x(t) within the plume, this spatial information was retained by computing population ratios, and hence local temperatures, using wavelengths A, and A, (in expression (2)) which corresponded to the same motional Doppler shift from both line centres (i.e. 452.49 nm and 455.40 nm respectively). The absorption cross sections (TV used were estimated from the experimentally measured absorption lineshapes and the integrated absorption cross section calculated from:

P

,dA=

A:, g2 --A,, 8rrc g,

(3)

where A,, is the spontaneous emission probability, A,, the line centre wavelength and g, and g? are the

349

Surface Science 99 (IYY6) 345-351 Temperature

[eVJ

i-l W 2.1-2.4

0 3 -0.1 2

0.1

4

3

2

1

0

??1 8-2 1 ??1.5-1.8

[UU-u 5

)

Position z (cm) Fig. 5. Boltzmann temperatures (in eV) of BaII at 2 ks delay following ablation in vacuum (a) and in 180 mTorr oxygen (b) as determined from the absorption data in Fig. 2c. d and Fig. 4a, b.

degeneracies of the lower and upper states respectively. The A?, values used were taken from Ref.

[‘Y. Boltzmann temperatures of BaII at 2 us after laser ablation in vacuum and in 180 mTorr oxygen are shown in Fig. 5, where the grey scales used correspond to Boltzmann temperatures in eV. These temperatures were calculated from the ratio of populations in the BaII ground state (estimated from the absorption data in Fig. 2c and d) and in the excited 6’P:/, state at 2.5 1 eV (estimated from the absorption data in Fig. 4a and b). For ablation in vacuum, illustrated in Fig. 5a, the strongly z-directed plume exhibits a monotonic decrease in Boltzmann temperature with increase in both delay time and distance from target, as would be expected for essentially collision-free expansion into a vacuum. Our line shape measurements in Fig. 2b and d show that the presence of ambient oxygen leads to a significant increase in absorption line widths, corresponding to increased n-direction velocities, at the expanding front of the plume. In the case of Fig. 2d, recorded at 2 ps delay, this increase in linewidth is observed in the region ; = 2.0-2.5 cm. As can be seen from Fig. 4b and d the latter corresponds to a region of the plume where local excited state number densities (and emission intensities) are high and which therefore exhibit high local (electronic) temperatures, as illustrated in Fig. 5. At 2 I.LS delay the maximum expansion velocity of the plume in the z-direction was reduced from - 2.5 X IOh cm/s (in vacuum) to * I .4 X lo6 cm/s in the presence of 180 mTorr oxygen, which would correspond to a reduction in

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the BaII kinetic energy from w 450 eV to * 140 eV. Under corresponding conditions the electronic Boltzmann temperature at the front of the plume (see Fig. 5) is increased from N 0.5 eV (in vacuum) to N 2.5 eV (in 180 mTorr oxygen). The high Boltzmann temperatures observed at the expanding front of the plume in oxygen presumably arises from enhanced recombination and progressive collisional redistribution of high initially z-directed ion velocities into the x- and y-directions [17]. The latter, along with increased local number densities, leads to enhanced collisional excitation rates, and markedly enhanced plume emission intensities at the expanding front of the plume in oxygen. Since at a local temperature of 2.4 eV (i.e. 2.78 X 10” K) the full half-widths, due to normal thermal Doppler broadening, of both transitions
Surface Science 99 (19%)

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behaviour is closer to that of a blast wave [ 18,191 with the front of the expanding plume, as observed in our absorption data, following a relationship of the form z(t) = at”. In agreement with previous work ]121 n = 0.6 was found to give the best fit to our absorption data. We would also stress that the effects reported in the present paper (i.e. increased local temperatures, increased local number densities and marked modification of particle dynamics at the expanding front of the plume in the presence of ambient oxygen) are consistent with the behaviour expected during the build up and propagation of a shock front [ 181 at the interface between the plume and the ambient oxygen.

4. Conclusion The absorption line shapes of BaII observed following laser ablation of YBCO show that the ambient oxygen leads to enhanced local number densities and enhanced local Boltzmann temperatures at the expanding front of the plasma plume. These enhanced local temperatures result from increased collisional redistribution of high initial z-directed plume velocities into the X- and y-directions.

Acknowledgements We wish to acknowledge the EPSRC for financial support, the personnel of Oriel Instruments Ltd for loan of the gated ICCD system, Dr. M. Catney and Mr. M. Pringle for assistance with image processing and Mr. W.A. Montgomery for essential laser support.

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