Time-resolved optical emission spectrometry of Q-switched Nd:YAG laser-induced plasmas from copper targets in air at atmospheric pressure

SPECTROCHIMICA ACTA PART B

ELSEVIER

Spectrochimica Acta Part B 50 (1995) 1869-1888

Time-resolved optical emission spectrometry of Q-switched Nd:YAG laser-induced plasmas from copper targets in air at atmospheric pressure B. N6met*, L. Kozma Janus Pannonius University, Department of Physics, H-7624 P~cs, Hungary Received 11 October 1994; accepted 5 June 1995

Abstract

Q-switched Nd:YAG laser induced plasma from a copper target was studied by time-gated optical multichannel analyser. The features of the spectra such as the plasma background, the type of lines of the solid sample (singlet--doublet, doublet-singlet, doublet--doublet, quadruplet-doublet, quadrupletquadruple0, the buffer gas, the intensity-time dependence and the line broadening of the neutral and ionic lines are described. Nitrogen, oxygen ion and metal oxide emission, self-absorption, pressure and Stark broadening is predominant at the onset of plasma formation. A time delay is required to reach the best analytical performance.

Keywords: Time-resolved laser-induced breakdown spectrometry

I. Introduction

The direct spectrochemical measurement (atomic emission spectroscopy, AES) of the laserinduced plasma (laser-induced plasma spectrometry is abbreviated to LIPS or, in other terminology, laser-induced breakdown spectroscopy is abbreviated to LIBS), enables rapid in situ analysis, and is particularly suited to process observations [1--4]. Comprehensive reviews of this analytical spectroscopic techniques and papers on the investigations of the laser/solid interaction process have been published by many authors [3-18,22-24]. The main problem from an analytical point of view is to find what laser conditions are required in order to obtain the best line-shape and a linear relationship between the intensity of the analyte line emission and the bulk composition of the sample. For this reason, the interaction between the laser and the solid target, with particular emphasis on the emission intensities of different analytical lines has been investigated as a function of several experimental parameters. These are (a) the effect of the laser wavelength [11,15], (b) the effect of the laser energy and power density on the amount of material removed [5,7,8,10], (c) the effect of the buffer gases (air, argon) on the plasma ignition and the atom emission intensity [611,18], (d) the effect of the characteristic thermodynamic parameters of the target (such as

* Corresponding author. 0584-8547/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8547(95)01384-9

1870

B. Ndmet, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

I I °°°,,-

1 ler Control I Pulsgenerator e PJ~hotodlode

~ p l e Fig. 1. Block diagram of the experiment. Main parts of the optical detector and time resolution system: diode array (IRY-512), multichannel plate (MCP), controller (FG-100); pulse generator (SI-180). Optical elements: lenses L~, L2; prisms P~, P2. melting temperature of the matrix material, thermal conductivity, heat capacity, and so on) on the amount of the ablated material, and on the plasma temperature, [5,7,9,15], and, (e) the time and spatial evolution of the plasma plume and its dependence on the above-mentioned parameters [7,9-17]. Time-resolved measurements are usually essential for the identification o f the various plasma processes. The t i m e - r e s o l v e d observations o f pulsed ( N d : Y A G , excimer) laser-induced plasmas are k n o w n under the name t i m e - r e s o l v e d laser-induced b r e a k d o w n spectrometry (TRELIBS) [14]. After a short and giant laser pulse ( ~ 1 0 lid, ~-15 ns, G W cm -2) at the early stage of the interaction, the plasma emission consists of an intense radiative continuum which decreases with time. The recombination of electrons and ions plays an important role in the production of excited neutral atoms, and line emission that lasts for several microseconds longer than the continuum is observed. TRELIBS has a significant increase in the line-tobackground (L/B) ratios when compared to LIBS (its time-integrated counterpart). Table 1 Experimental apparatus and characteristics Q-switched Nd:YAG laser Pulsewidth Pulse energy Wavelength Repetition rate Spectrograph Type F-number Grating Reciprocal linear dispersion Stray light Spectral window Optical Detector System Type Number of channels Diode width Diode height Spectral sensibility Controller Time-Resolution System Type Delay time (to) Gate time (q) Voltage of MCP

Russian made 15 ns 15 mJ 1064 nm 1 Hz MDR-6 (LOMO), Russia 0.3 m, double-Czerny-Tumer 1/6 1200 lines mm-~ 1.3 nm ram-J 1:10s 15.5 nm diode array + MCP (Princeton Insmunents, Inc., USA) IRY-512 G/B 512 diodes 25 p.m 2.5 mm 260-800 nm FG-100 Pulse generator (Princeton Instruments, Inc., USA) SI-180 40-1700 ns 50-2500 ns 200 V

B. N~met, I_, Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

1871

The emission lines appear after 10-300 ns and are superimposed on the continuum. These lines are strongly broadened by the Doppler, pressure (collisional) and Stark effects (the latter being due to the high electron density that exists in the plasma during the initial moments). However, the Doppler effect causes a linewidth of less than 0.02 nm, even if one assumes a kinetic temperature up to 50000 K. According to Autin et al. [11], the ions are only present during the laser interaction, so that the Stark effect has been significant only during the laser pulse duration. In contrast with this last statement, it has been proposed that the Stark effect lasts up to several microseconds in argon gas at 140 mbar [8]. At the same time, the plasma could be at a relatively high pressure, lasting of the order of a microsecond, because its expansion was limited by the surrounding air atmosphere. Therefore, pressure effects would also play an important role in the broadening of the lines. The effect of self-absorption, which causes also a significant broadening of the lines, has been reported for laser-produced plasmas in several papers [8,11]. In Ref. Ill], the authors concluded that, on a copper target, the broadening of the 324.75 nm resonance line of Cu(I) is related to self-absorption, which is severe enough to even produce self-reversal. Self-absorption may be reduced by using an analytical line which terminates into an energy level higher than the ground state. In general, after the first microsecond, the continuum emission and the line broadening decrease remarkably, but the line-to-background ratio remains constant [14]. The absorption of the laser radiation by the ambient gas in the plasma plume plays a substantial role in the process of evaporation of the metal target and the spectral characteristics of the emission. In the ambient gas, breakdown plasma emission was observed in air and argon atmospheres at relatively high pressure (>100 Torr) [9]. For example, nitrogen ion (II) emission at 399.5 nm was detected at 760 Torr. The occurrence of an atmospheric plasma prior to the plume of the sample material and its rapid propagation through the gas plasma along the direction of the incident laser beam is possible. Complex high-continuum background can still be observed in air at 760 Torr due to the occurrence of chemical reactions, dissociation of metal species, molecules, and radicals, as well as the formation of stable molecules and ionization. Metal oxide formation with oxygen in air can also result in a broad molecular emission band (only with trace amounts of oxygen). The background continuum has been clearly reduced in an argon atmosphere, and especially under an helium atmosphere, in comparison to an air atmosphere [9,10]. In many analytical studies, the most intense resonance lines (corresponding to transitions between the first excited electron energy level and the ground state) are used, and therefore there is a high probability of self-absorption in the plasma. In most cases these lines fall in the UV region. If the concentrations of the components in a sample are low (low-alloys) one can use these lines for analysis. However, if the sample consists of binary or tertiary alloys, where the concentrations of the components are high (high-alloys), other lines (which are moderately intense) must be chosen and an internal standardization procedure must be applied. In contrast to these investigations of the effects of the laser energy and power, and pressure of the buffer gas on the kinetic behaviour of the line intensity and broadening of plasma emission, only a few studies have been carried out to examine the dependence of the emission kinetics on the type of spectroscopic transitions involved, i.e., on transitions originating between electron energy levels of different multiplicities (singlet--doublet, doublet-singlet, doublet-doublet, quadruplet--doublet, quadruplet-quadruplet, etc.) and inner quantum numbers (S, P, D, F). In this paper, the temporal characteristics of the (Nd:YAG) laser produced plasma formed in air at atmospheric pressure from a copper target are studied by (atomic) time-resolved optical multichannel emission spectrometry. A suite of copper lines of analytical interest have been investigated. Spectral features such as plasma background, the type of lines (transitions), the temporal dependence of the intensity and the broadening of the neutral and ionic copper lines, as well as of the nitrogen and oxygen ion emissions have been investigated and are described over a wide UV and VIS spectral range. The aim of the detailed emission kinetic studies was to enlarge the number of analytically useful copper lines, to draw conclusions pertinent to the emission analysis of other elements and alloys and to develop the TRELIBS technique so as to achieve the best analytical performance.

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1873

B. Ndmet, L Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

mOO" Cu(I)

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Fig. 3. Time evolutionof laser-induced copperplasma emission as observed at different gate and delay times. Cu(I) lines: 324.75 and 327.4urn. 2. Experimental Fig. 1 shows a schematic diagram of the experimental arrangement used in this work. Details of the instrumentation and operating parameters are given in Table 1 [21]. The laser used was a Nd:YAG laser (Russian made) and operated in a passive Q-switched mode. The laser output had a pulse duration of 10 ns and the pulse energy was 10 mJ at surface of the sample, its relative standard deviation measured with a calibrated calorimeter (type Gentec model: ED500) was typically 1-2%. The unfocused output beam of the laser, 2 mm in diameter, was directed on the surface of the investigated solid by a prism at an angle of 25 °, and the beam was focused onto the sample surface by an objective lens ( f = 50 mm). The radiation of the laser-produced plasma was

1874

B. N6raet, L Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888 t

,

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Xg=50 ns

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/4406.0 "98

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Fig. 4. Time evolution of laser-induced copper plasma emission. N(II) lines: (460.15, 460.72, 460.96 nm). Cu(lI) line: 455.59 nm. Cu(I) lines at 450.94 nm, 453.97 nm, 458.70 nm, 448.04 nm and 453.08 nm.

observed perpendicular to the surface. The plasma radiation (at an imaging ratio 2:1) was collimated by a quartz condenser lens ( f = 120 ram) on the entrance slit of a 0.3-m CzernyTurner double-monochromator (from Russia, LOMO-MDR6) equipped with a 1200 grooves per mm grating and characterized by a reciprocal linear dispersion of 1.3 nn mm-L The spectra were detected by a photodiode array (PDA) detector of 512 channels (element size 25 Ixm x 2.5 ram) with a microehannel plate (MCP) image intensifier (Princeton Instruments, Inc., USA). The length of such arrays (typically 12.5 mm) and the number of elements (512) indicates that there must be a compromise between the wavelength range covered and the spectral resolution. The combination of monochromator with this detector system resulted in a practical resolution of typically 0.15 nm when an entrance slit width of 30 Ixm was used with a resulting spectral

B N~met, L Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

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Fig. 5. Time-resolved laser-induced copper plasma emission. Cu(II) lines: 274.54, 276.94, 283.76, 287.67 nm. Cu(I) lines: 276.64, 282.44 nm.

window of ~ 15 nm. The grating-drive mechanism was rotated until the required wavelength window covered the detector elements. Gating the intensified MCP enables the spectra to be temporally resolved. Our detector was time-gated with a gate-width range from 50 to 2500 ns to allow precise control of the integration time (pulse generator type: FG-100, Princeton Instruments, Inc., USA). Spectral data acquisition and processing were carried out with an optical processing system (type: SI-180, Princeton Instruments, Inc., USA). Typically, ten shots were accumulated, with laser pulses delivered to the sample at 1 Hz. The emission signal was corrected by subtraction of the dark current of the detector, which was separately measured for the same exposure time as with the laser firing. Prior to each measurement, the wavelength axis of the monochromator--detector

1876

B. N6met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888 UO0

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Fig. 6. Time-resolved laser-induced copper plasma emission. Nitrogen ionic lines: 566.66, 567.96, 571.07 nm. Cu(I) lines: 570.02, 578.21 nm.

system was calibrated by using a mercury emission lamp and several lines emitted from a laser-induced iron plasma. Seven different spectral regions were chosen for observing the most important transitions in the range 274-580 nm. These seven spectral windows are 274--289, 324-331, 398--410, 426--439, 447--461,510-523 and 565-579 nm and their central wavelengths are 281,327, 404, 433, 454, 516 and 572 nm, respectively. (The emission lines monitored are given in Table 2). The high purity copper was obtained from Vaskut Co. Ltd. Budapest, Hungary, and the aluminium alloys were from MBH Analytical Ltd., Tekmelt Ltd., UK.

B. Ndmet, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

1877

Table 2 Wavelengths, energy levels and spectroscopic term symbols for the emission lines monitored k (nm)

Upper level

Energy (cm-')

Lower level

Energy (cm-~)

324.75 327.39 276.64 282.44 510.55 515.32 521.83 522.00 570.02 578.21 402,23 406.33 406.26 448.04 458.03 427.51 437.82 441.56 450.94 453.94 458.70 465.11 470.47 467.48 469.75

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3. Results and discussions A partial energy level diagram illustrating the main optical transitions o f the neutral copper atom is shown in Fig. 2. The observed transitions (wavelengths, upper and lower levels) of Cu(I) are listed in Table 2. These data have been taken from Refs. [19-21]. The data in Figs. 3 - 1 0 refer to time-resolved plasma-spectra measured at different delay and gate times, while Figs. 12-14 show time-integrated plasma-spectra measured at fixed delay and gate times. These spectra were obtained from the N d : Y A G laser-induced plasmas in air at atmospheric pressure over seven wavelength ranges. The variable delay times were chosen as 40, 80, 160, 320, 500, 1000 and 1700 ns, with corresponding gate times of 50 and 100 ns; the fixed delays were 600 and 1000 ns for a gate time of 2500 ns. 3.1. Time-resolved and time-integrated spectra In general, it can be concluded that the spectra corresponding to delay times shorter than 200 ns indicate that the continuum intensity was very high and also that many Cu ionic lines and a few neutral Cu lines could be observed. Lines or bands from air components (nitrogen, oxgyen) were also observed, under the operating conditions, because the discharge was formed in air. However, for times longer than 200 ns, the continuum intensity as well as the band intensity diminished to a negligible level. The intensity of the Cu(II) lines gradually decreased. In the absence o f the target, no laser-induced breakdown o f air was observed. Although there was a fast expansion o f the plasma in the presence of the target, the air diffused into the discharge. The occurrence of breakdowns was reported for air and argon by Leis et al. [6], for an energy o f 10 mJ o f their N d : Y A G laser at 1064 nm. Some preliminary investigations were carried out with a more powerful XeC1 excimer laser (up to 300 rnJ). No breakdown in air was observed, mainly because o f the use o f a UV wavelength [11]. Therefore, this confirms

B. N~met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

1878 6000

~g=50 ns

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80

160 I

i

.

i

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Fig. 7. Time evolution of laser-induced copper plasma emission. N(II) line: 399.5 nm, O(II) lines: 407.2, 407.59 nm. Cu(II) line: 404.35 nm. Cu(I) lines: 402.23, 406.16, 406.33 nm.

the well known result that a plasma produced by a UV laser differs in nature from that produced by a laser emitting in the IR. The data shown in Fig. 3 indicate a smaller variation in the line width of the Cu(I) 324.75 and 327.40 nm lines than that observed in Ref. [11]. for time delays varying between 0 and 1.7 Ixs (these findings can be better seen in Fig. 10). A wavelength shift of 0.08 nm associated with the line broadening is also shown in the same figure. This broadening may be related to the self-absorption of these resonance lines, which was large enough, but in our experimental conditions did not produce self-reversal (the width of the entrance slit was 25 ~zm). A maximum intensity was observed between 200 and 250 ns because of a longer duration of the selfabsorption effect (see Fig. 11). Many different lines and bands can be seen in Fig. 4. In the first 80-100 ns, no copper line

1879

B. N#met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

2000' ~g=,,,,~nns

517.59

Cu

517.95

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i

=

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0

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emission is observed and only continuum and nitrogen ionic emission (460.15, 460.72, 460.96 nm) can be seen. After allowing a 100 ns delay for the plasma expansion, the continuum decreased significantly. The Cu(II) line at 455.59 nm, observed at 80 ns, disappeared after 1 Ixs. The intensity of three neutral atomic lines (450.94, 453.97, 458.70 rim) did not change from 100 to 1000 ns; these lines result from 3d94sSs-3d94s4p (D-F) transitions (whose upper levels are autoionizing). The 448.04 nm and 453.08 nm lines belong to S-P transitions and their emission could be observed mainly after 1 p~s (Fig. 11). Time-integrated plasma-spectra of these lines are shown in Fig. 12. The intensity ratios of these lines are very different from those given in Refs. [19,20]. Time-resolved spectra of special P-S transitions are shown in Fig. 5 (3d!°5p'-3d94s2, 276.64 nm) and in Fig. 6 (3dl°4pl-3d94s2, 570.02 and 578.21 nm), respectively. In these spec-

1880

B. N@met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

:g=50 ns

A

/ \

O(tl) xd(ns)

Cu

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0 0

431.97

434.94

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i

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,

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=

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i

436

i

o

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xg=lO0ns

nm

Cu

500 .=

Cu(t)

i

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~

434

436

e

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4

o nm

Fig. 9. Time-resolved laser-induced copper plasma emission. O(II) lines: 431.72, 431.97, 434.57, 434.74, 434.94 nm.

Cu(I) lines: 427.51,437.82 nm. tral regions there are also ionic copper lines (274.54, 276.94, 283.76, 287.67 nm), and nitrogen ionic lines (566.66, 567.96, 571.07 nm). There is also another (3d94s4p-3d94s 2 i.e. S-S) transition (282.44 nm). The emission of the nitrogen ion in this region can be detected at longer times (up to 300 ns) than that reported in Fig. 4, while the Cu(II) lines decrease to zero at 1 I~S and the emission at 276.64 nm can be detected only after 1 I~s. The neutral copper atoms radiate at 282.44 nm from the beginning to the last gate interval with little change in intensity and linewidth. The line at 285.47 nm was not observed. The lines at 510.55, 578.21 and 570.02 nm show very similar time and spectral behaviour (see Figs 6, 8 and 11). The observation that line the broadening for these four lines (282.43, 510.55, 570.02 and 578.21 nm) is similar can be related to that fact that they have the same lower energy level and similar transition probabilities. Actually, the discrepancy of about a factor of two can only be explained

B. N6met, L. Kozma/Spectrochimica A c ~ Pan B 50 (1995) 1869-1888

1881 Cu

xd(ns) Xg=50ns 3 2 0 ~ ~

500

.= 0 U

r-

_= 1000

0

T..

¢

,

.

l

.

¢

.

.

386.7'8

!

.

.

.

s

3~7

!

.

.

.

.

387. H

!

.

.

,

327.5

.

!

s

."=".

.

327.75

!

.

.

I

nit

388

600"[ ~d ¢g=50 ns

Cu

320

500 e,.~ 0

C:

1000

L I

-o

o | .

-I 4R7

J

o

=

• 427.

f I

. L:X3

.

•

.

!

427.5

.

,

o

i

I

427.75

.

.

.

.

I 428

.

.

t

s ,laB.

l

, e5

.

J

na

Fig. ]0. Time-resolved laser-induced copper plasma ¢~ssion. Cu(1) lines: 32%4, 427.51 m~.

by other causes of line broadening such as pressure, Doppler, and Stark broadening. The charged particles (as the ionic nitrogen and Cu(II) emissions show) are present in the interval 300-1000 ns, and not only during the laser interaction time, as reported in Ref. [11]. One can therefore assume that the Stark effect would last significantly longer, during several hundred nanoseconds or one microsecond. In addition, the plasma could be at relatively high pressure because its expansion was limited by the surrounding air atmosphere. Therefore, pressure effects would play also a significant role in the line broadening. In conclusion, the selection of the time delay is crucial for obtaining the best operating conditions, in particular to avoid significant line broadening which are a cause of spectral line overlap (Fig. 13 e.g. lines of 276.64 and 276.94 nm). Time-resolved spectra of doublet transition (3dl°5dl-3d~°4p ~ and 3d1°4dl-3dl°4p ~ i.e. D-P)

1882

B. N6met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888 2000

~

I

P-S P-D

0~0

(counts)

0 327.40nm × 510.55nm "1- 578.21 nm 570.02 nm D - P & 515.32 nm ~7 521.82 nm

Cu

\o

1000

I,Tx- m--ta

ill - 0

I

(counts) 100

u-

'-'

S - P o ra D - P & V

I 0.5

448.04 nm 453.08 nm 402.27 nm

0

-

I 1.5

t(~s)

I 1.5

t(~ts)

Cu

4 0 6 . 2 ~

I 0.5

0

-

I 1.0

, ,

v"

I 1.0

I

(counts)

D- F

"

500

/ A----A //~/1~1 ~EJ

0

~

0

0 El D- P A V

Cu ~. _ ~

I

0.5

| 1.0

453.94 458.70 427.51 437.82

nm nm nm nm

I

1.5

t(ps)

Fig. 11. Experimentalbehaviourof the intensity of several copper lines as a function of time.

are shown in Figs. 7 and 8. Their characteristic wavelengths are 402.23, 406.16 and 406.33 nm, and 515.32, 521.83 and 522.00 nm, respectively. In these spectral regions there is also one ionic copper line (404.35 nm), as well as one nitrogen ionic line (399.5 nm) and two oxygen ionic lines (407.2, 407.59 nm). The emission from the nitrogen ion (399.5, 517.59, 517.95 nm) in these regions can be detected only up to 200 ns, whereas the Cu(II) line decreased to zero after 1 p,s. The neutral copper atoms radiate continuously at 515.32, 521.83 and 522.00 nm after 100 ns with little change in the intensity, while their linewidth becomes significantly narrower (Fig. 8), At the same time, the emission at 402 nm and 406 nm can be distinguished the continuum radiation after 500--600 ns and their lineshapes are not homogeneously broadened. The behaviour of two atomic Cu lines (427.51,437.82 nm) and oxygen ionic line (431.72,

1883

B. N#met, L. Kozraa/Spectrochimica Acta Part B 50 (1995) 1869-1888 3000"

xd=lO00 ns

Cu

Xg=2500 ns

450.94 453.94 450.75

CuO) f. o

453.08

C

448.04 441.56

o

•

i

I

a

o

~

442

=

'l

+

o

I

~

444

I

o'

I

o

•

446

'~

4a

J

i

'o

o

I

480

i

l

o

o

I

~

•

l

l

nno

;

I

l

454

2000" ~d=lO00 ns Xg=2500 ns

465.11

Cu

CuO) r. ,.i o u

458.70 c

467.48

4m

~

4154

4118

4811

470.47

470

Fig. 12. Time-integrated spectra of Cu(1) as measured at fixed delay and gate times.

431.97, 434.57, 434.74, 434.94 nm) as a function of the time delay are shown in Fig. 9. The Cu(I) 427.5 and 437.82 nm lines also originate from an autoionizing energy level [11]. The lower energy levels of these two lines are the 3d94s4p metastable levels. For short time delays (up to 500 ns) the line broadening was large, i.e., greater than 0.2 nm, partly owing to selfabsorption, but to Stark and pressure effects, as well (Pig. 10b). For the Cu(I) 427.5 nm line, owing to the short lifetime of the autoionizing level, large natural broadening have been observed. After 500 ns, the experimental linewidth is limited by the spectral bandpass of our spectral detector system (0.12 nm for slit widths of 25 ixm). Time-integrated plasma-spectra of the 427.51 and 437.82 nm and the 515.32, 521.83 and 522.00 nm can be seen in Figs. 13 and 14. The intensity ratios of these lines are also very different from that shown in Refs. [19,20].

1884

B. N~met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888 ~.o00"

Cu

xd=lO00 ns Xg=2500 ns 282.44

CuO) r. o t,t in t,-

276.64

_=

0

o

;~74

o

t

i'

!

i

i

B

;~78

,

i

~

a78

•

i

i

i

~

m

,

o

280

i

,

i

201~

m g

! LNt4

,

m i

o

i

|

ROll

o nm

mO

427.51

Cu

Xd=600 ns Xg=2500 ns

CuO) ¢-t 0 (J

ffl

437.82

e-

450 F i g . 13. T i m e - i n t e g r a t e d

43=!

434

spectra of Cu(1) as measured

436

~

ni

at fixed delay and gate times.

3.2. Analytical application

As an application of the previous time-resolved and time-integrated spectral investigations, the spectra of low-alloyed aluminium (MBH Analytical Ltd.) containing low concentration of copper (in addition to titanium, chromium manganese, zinc and other elements) were taken at the optimum time delay and time gate, and in two spectral regions in order to test the analytical capability of this technique. The representative spectra are shown in Fig. 15. The aluminium background spectra was subtracted from spectra seen in this figure for the calculation of the line intensity. The relative standard deviation of the background was less than 5%. For the construction of the calibration curve, mean values were calculated from 5 repeated measurements, every "measurement" representing an average of 10 shots, made at. different target locations, spaced by 1-2 mm.

B. N~met, L Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888 RO00"

1885

324.75

Td=600ns ~g=2500n s

Cu

327.40

CuO) C :;3 0 o

i-

c

i

.

!

|

!

317.5

I

i

!

!

!

320

I

~

i

i

!

i

.B

I

!

i

|

!

3215

i

.

|

!

!

7"

327.5

!

!

!

330

nI

4000"

xd=lO00ns Xg=2500ns

Cu 521.83

_= e.

CuO)

-¢ o o

c

515.32 510.55 y

,

510

,

512

,

,

,

,

s

514

.

,

.

518

.

,

,~

i

5t8

•

.'

,

'.'=,

580

,

.

522.00

.

.

,

o

.

522

.

.

na

Fig. 14. Time-integrated spectra of Cu(I) as measured at fixed delay and gate times. The calibration curve obtained using the line at 521.83 nm was linear up to 1.15% (this line corresponds to a transition from a higher energy level, with a lower transition probability), while that at 324.75 nm shows some curvature. Since this transition is resonant with the ground state and intense, the self-absorption is not negligible even at copper concentration o f 0.5% [19,20]. These curces are shown in Fig. 16. The detection limit (CL) was calculated according to the conventional definition (which applies when the calibration curves are linear), as the smallest signal that can be distinguished with a given probability from the noise o f the background [4], i.e. CL = zsB/S where z is a constant, s , is the standard deviation o f the background, and S is the slope o f the calibration curve. In our measurements S(324.75 n m ) = 4000 counts per %, and S(521.83 nm) = 350 counts per %, sB = 3. For z = 2, the detection limits calculated are 15 ppm for the 324.75 nm line and 160 ppm for the 521.83 nm. These results are similar to those published in Ref. [11].

B. NEmet, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

1886 ~1000

Xd=600 ns ~g=2500 ns

Ti(ll) c-

323.23 323.66 323.90 324.10

AI - Cu

o u

UI r"

_=

C

322.86 319.09

321.70

\ Zn(I)

d ,~ ~A

I

330.23

320.25 322.28 I ~ , I ,,;./"

'--~*"

''

~.i

' '

'~,'

' ' '~,.i

' ' ' ~,'

'

-

Mg(I) J

3800

zd=lO00 ns Xg=2500 ns

I I

518.36

517.27 P -I

0

cP0)

U

cu(t)

•~

=

520.84

-

521.82

520.60

I s o4s U ~

510.55 __ 0

I

510

i

i

i

*

I

St2

i

l

i

*

I

8t4

i

*

m

i

!

1118

i

i

.

*

I

518

i

i

i

o

!

520

i

.2

i

i

i

I

822

i

i

i

nl

Fig. 15. Time-integrated spectra of low-alloyed aluminium. Copper contents: (a)1.15%, (b) 0.93%, (c) 0.60%, (d) 0.25%.

4. Conclusions

Several Cu(I) and Cu(II) transitions were investigated, covering a large range of excitation energies (2.5-9.0 eV) and including excited electron energy states of several multiplicities and inner quantum numbers (Slr2, P3/2, Pl/2, D5/2, D3/2, F7t2, F5/2, Ps/2,3t2,1t2, D7/2.512.312.112,F9/2.7/2,5/2.3/2). It was possible to establish that the emission starts at P-S, P-D transitions promptly after the laser ablation, while the D-P, S-P and D-F transitions are emitted 200-500 ns later. Many lines and transitions (in addition to the frequently used analyte lines) have been found suitable for quantitative TRELIBS analysis by determining the optimum time delay and time interval, when the measured line intensity depends just on the species composition of the plasma originated from the ablated material.

B. N(met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

1887

/ t

4000

AI - Cu

I

~counts)

x 324.75 521.83

/

400

[

/

(counts)

2000

200

0 0

I 0.5

I

1.0 [Cu] (%)

Fig. 16. Calibration curves for Cu(1) in aluminium alloy. Analyte lines: 324.75, 521.83 nm. For analytical purposes, the most favorable lines belong to 3d~°5pl-3d94s 2 and 3dl°4p l 3d94s 2, i.e. P - S transitions, in addition to 3dl°5dl-3dt°4p 1 and 3dl°4d~-3dl°4p 1, i.e. D - P transitions, and to 3d94s5s-3d94s4p ( D - F ) transitions (whose levels are autoionizing). Depending upon the different spectral and temporal characteristics of the transitions investigated, a long gate time (3000 ns) was used at several different time delays and a short gate (600 ns) at a fixed short time delay (300 ns). In the former case, the time delays were: 300 ns (282.43, 570.02, 578.21,510.55, 427.51,437.82 nm), 1000 ns (450.94, 453.97, 458.7, 515.32, 521.83 nm) and 1500 ns (448.04, 453.08, 276.63, 402.23, 406.26 nm). The last case was applied to the lines: 269.97(11) nm, 283.76(1I) nm, and 287.76(I1) nm. This work is intended as the preliminary step of a systematic wide-ranging investigation on several binary and tertiary high-alloys. A number o f analytical lines of copper, covering a large UV and VIS spectral interval, can be used in the quantitative analysis o f several tertiary and binary high-alloys (Au/Cu/Ag, Cu/Zn, Cu/Sn, Cu/Pb, AL/Cu/Zn), as internal standard to determine the ratio of the components. To this purpose, similar studies have been carried out on high purity silver and gold (because these two elements have similar one-electron energy system) [24]. Moreover, elements characterized by two- and more-electron energy system (Zn, Mg, Pb, Sn, A L and so on) can also be investigated. Work is now in progress along these lines.

References

[1] L.J. Radziemski and D.A. Cremers, Spectrochemical Analysis Using Laser Plasma Excitation, In: Laser-Induced Plasmas and Applications, L.J. Radziemski and D.A. Cremers (Eds.), Marcel Dekker, New York, 1989. [2] L. Moenke-Blankenburg.Laser Microanalysis. J.D. Windefordner and I.M. Kolthoff (Eds.), John Wiley and Sons, New York, 1989. [3] B.L. Sharp, S. Chenery, R. Jowitt, S.T. Sparkes and A. Fisher, J. Anal Atom. Spectrom., 8 (1993) 151R. [4] P. Boumans, J. Anal. Atom. Spectrom., 8 (1993) 767. [5] W. Qiu, J. Watson, D.S. Thomson and W.F. Deans, Opt. Laser Technol., 26 (1994) 157. [6] F. Leis, W. Sdorra, J.B. Ko and K. Niemax, Mikrochim. Acta [Vienna] II, (1989) 185. [7] Z.W. Hwang, Y,Y. Teng, K.P. Li and J. Sneddon, Appl. Spectrosc., 45 (1991) 435. [8] W. Sdorra and K. Niemax, Mikrochim Acta [Vienna], 107 (1992) 318. [9] Y.I. Lee, T.L. Thiem, G.H. Kim, Y.Y. Teng and J. Sneddon, Appl. Spectrosc., 46 (1992) 1597. [10] M. Kuzuya, H. Matsumoto, H. Takechi and O. Mikami, Appl. Spectrosc., 47 (1993) 1659. [11] M. Autin, A, Briand, P. Mauchien and J.M. Mermet, Spectrochim. Acta Part B, 48 (1993) 851. [12] J.A. Aguilera, C. Arag6n and J. Campos, Appl. Spectrosc., 46 (1992) 1382. [13] C.J. Lorenzeh, C. Carlhoff, U. Hahn and M. Jogwich, J. Anal. Atom. Spectrom., 7 (1992) 1029. [14] K.J. Grant, G.L. Paul and J.A. O'Neill, Appl. Spectrosc., 44 (1990) 1711. [15] C. Geertsen, A. Briand, F. Chattier, J.-L. Lacour, P. Mauchien, S. Sj6str6m and J.M. Mermet, J. Anal. At. Spectrom., 9 (1994) 17. [16] J.B. Ko, W. Sdorra and K. Niemax, Fresenius J. Anal. Chem., 335 (1989) 648.

1888 [17] [18] [19] [20] [21] [22] [23] [24]

B. N~met, L. Kozma/Spectrochimica Acta Part B 50 (1995) 1869-1888

A. Quentmeier, W. Sdorra and K. Niemax, Spectrochim. Acta Part B, 45 (1990) 537. M. Owens and V. Majidi, Appl. Spectrosc., 45 (1991) 1463. G.R. Harrison, Wavelength Tables, John Wiley and Sons, Inc., New York (1980). W.L. Wiese and G.A. Martin, NSRDS-NBS 68, Part II. Transition Probabilities. National Bureau of Standards, Washington DC USA, 1980. C.E. Moore (Ed.), Atomic Energy Levels in Circular of NBS 467, vol. II (1952), vol. III. (1958), U.S. Government Printing Office, Washington, DC. G. Lupkovics, B. Ntmet and L. Kozma, Time-Resolved Laser-Induced Breakdown Spectrometry, In: Current Trends and Problems in Optics, J.C. Dainty (Ed.), ch. 5, pp. 69-81, Academic Press Ltd., London, 1994. B. Ntmet, G. Lupkovics and L. Kozma, 'Investigation of Industrial Applications of Time-Resolved Laser-Induced Breakdown Spectrometer', 16th Congress if ICO, Budapest, 9-13 August 1993. SPIE vol. 1985, pp. 977-978. B. Ntmet, and L. Kozma, Special Issue of the 1995 European Winter Conference on Plasma Spectrochemistry, J. Anal. At. Spectrom., 10 (1995).