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Optical Detection of Laser-Induced Ionization: A Study of the Time Decay of Strontium Ions in the Air-Acetylene Flame

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Optical Detection of Laser-Induced Ionization: A Study of the Time Decay of Strontium Ions in the Air-Acetylene Flame
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  0584-8547/87 s3.cxl+o.Dtl Q 1987 Perganon zyxwvutsrqponmlkji ournals td. zyxwvutsrqpo Optical detection of laser-index ionization in the ind~ti~~ly coupled plasma for the study of ion-electron r~umbin~tion and ionization equilibrium G. C. TURK*, 0. AXNER~ nd N. CMENETT~ Joint Research Center, Chemistry Division, 21020 Ispra (Varese) Italy (Received 7 November 1986; in revised orm 7 Februnry 1987) Abstract-The recombination kinetics between the strontium ions and the electrons have been studied in an inductively coupled plasma. TWO xcimer lasers have been used to pump two dye layers, which were made spatially and temporally coincident in the piasma region investigated. With the first zyxwvutsrqponmlkjihgfedcbaZYXWV aser system the strontium atoms were photoion~ and the second Iaser was then used to probe the ions form& by rn~u~ng the resuking onic fluorescence. Since the econd laser was delayed by external triggering with respect to the ionizing laser, the temporal fate of the ions could be continuously monitored. The recombination time constant was found to be 15.5 ps, indicating the absence of fast ion chemistry (in the tens of ns range) which was observed in an air-acetylene flame. Moreover, it was found that: (i) the addition of an easily ionizable element (K, Li) increased the rate of recombination; and (ii) the recombination time constant was direcrly proportional to the ion/atom ratio of strontium. It was concluded that a change in the electron energy distribution is more relevant to the rate of r~ombination than changes in the absolute number density of ekcectrons. 1. INTRWWTION IN A PREVIOUS publication we have described a method for optically detecting the laser- induced ionization of strontium atoms in an air-acetylene flame El]. We now report the results of the application of this method to the inductively coupled argon plasma (ICP) as a diagnostic measurement yielding information relevant to the study of the complex subject of ionization equilibrium and interferences in the ICP [Z-lo]. Briefly summarizing the experiments, Sr atoms in the ICP are specifically ionized by a two step mechanism in which the Sr atoms are first excited by the absorption of laser light tuned to a ground state resonance Sr transition and then photoionized by U.V. adiation from XeCl excimer laser beam in spatial and temporal coincidence with the dye laser beam. The resulting Sr ’ ions are then detected by laser-induced ionic fluorescence using a third laser beam from a dye laser tuned to a Sr+ transition. A variable time delay between pulsing of the ianizing lasers and that of the fluorescence probe laser allows a direct measurement of the rate of decay (recombination) of the laser produced zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA r ’ ions. In flames, such laser-induced ion~tion can be electrically detected directly inside the flame by using biased electrodes to monitor changes in electrical current passed through the flame, *On leave from: Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, MD 20899 U.S.A. ?Gn leave from: Department of Physics, Chafmers University of Technofogy- 412 96, Gotbtmburg, Sweden. *Author to whom co~esponden~ should be addressed. [l] G. C. TURK and N. OMENETTO, ppt. Spectrosc. 40, 1085 (1986). 123 M. W. BLADES nd G. HORLICK, gectrochim. Acra 36B, 881 (1981). f3] G. Grmor+ and HORLICK ~~c~~~~. Acta 4lB 619 1986). [4] S+ R. KOIRTYOHANN, . S. JONES, C. F. JESTER nd D. A. YATES, ~~c~~~rn. Actu 3 B, 49 fl981). [S] G. R. KORNBLUM nd L. DE GALAN, S~c~r~him. Actu 32B, 455 (1977). [6] L. M, FAIRE, C. T. APEL and T. M. NIEMCZYK, Appi. Spectrosc. 37,558 (1983). [7] J. P. RYBARCZ~K, . P. JESTER, . A. YATES nd S. R. KOIRTYOHANN, nd. Chem. 54,2162 (1982). [8] H. KAWAGUCHI, . ITO, K. OTA and A. MIZUIKE, Specrrochim. cta 358, 199 (1980). (93 J. E. ROEDERER, . L. BASTIAANS, . A. FARNANDEZ nd K. J. FREDEEN, ppl. Spectrosc. 36,383 1982). [IO] Y. NOJIRE, K. TANABE H. UCHHM H. ~A~~u~HI~ K. FUWA and J. D. WINEF~~~NER ~~cr~~~~ Acta 3m.4 61 (1983). 873  874 G. C. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM URK d al. as is done in laser-enhanced ionization spectroscopy (LEI) [l 1, 121. This method exhibits extremely low detection limits, and has been applied to over 30 elements and utilized for trace metal anaiysis. Colhsional ionization of laser populated excited states is the normaf m~hanism of LEI sp~tro~opy, A suitable means ofel~tricaI detection of ioni~tion in the ICP has not yet been developer afthough some measurements were performed using electrodes very high in the tail flame of an extended torch ICP C13J. A variety of problems with electrical arcing and radio-frequency interference are encountered, if one attempts ionization measurement with electrodes in the normal analytical zone of the ICP. Although the method of optical detection of ionization presented here is unsuitable for high sensitivity analytical measurements, it does shed some light on the possibilities for laser-induced ionization in the ICP if a suitable means of eiectrical detection could be deveIoped. The ability to observe the recombination of the laser produced ions is a zyxwvutsrqponmlkjihgfedcb nique aspect ofthis ~~~ure~~nt with much diagnostic u e. Changes in the recombination rate imply changes in the electron number density and shifts in the atom-ion equifibrium. Shifts in the ionization equilibrium of Ca in the ICP in response to the presence of easily ionized elements (EIEs) have been studied recently by GILLWN and HORLICK 3]* who measured the effect of a Na matrix on the atomic and ionic fluorescence of Ca, We report here on simitar measurements for Sr. However, such measurements are always somewhat uncertain since the presence of an EIE matrix has a variety of efI ts in the ICP besides the effect on ionization equilibria, as described in reference [2]. In this’work we present the results of measurements we have made of the effect of EIE matrices on the rate of recombination of laser-produ~d Sr * ions, which we fee1 s a more direct measurement of the effect of such elements on ionization equilibria in the ICP. A schematic diagram of the ex~rim~ntal set-up for this work is given in Fig. 1. The two ionizing laser beams are generated from a X&l exckmer (308 nm) (Lambda Physik, Mode1 EMG 102, Gottingen, F.R.G.) pumped dye laser (instruments, S.A., Division Jobin-Yvon, Longjumeau, France) operated at 10 pulses per second. The reflector which normally directs 80 % of the pumping beam into the amplifier section of the dye laser was completely removed, allowing the unused portion of the excimer beam to exit the dye laser for use as the second step photoioni~ng beam. The dye laser, operated without the amplifier, was tuned to the 460.733 nm transition of Sr (O-21698 cm-i). A second XeCI excimer laser (~rn~a-Physik, Modd EMG-50) pumped a second dye laser (~nstru~nts, S.A., Jobin-Yvon) to give the ionic Auorescence probe beam at 421.522 nm (O-23715 cm-l 1. Since optically saturating power densities wereachievable using only the oscillator of thedye laser, the amplifier was removed in order to extend dye life. All three laser beams were aligned through telescopes in order to control the beam diameters. At the ICP torch, the two dye laser beams were approxi~t~ly 1 mm in diameter and carefully aligned to be collinear at the central axial channel of the ICP. The minimum cross sectional area obt~nable for the ex~imer with our two lens telescope was 0.5 cm2 at the torch, and was aligned coIlinearIy with the much smafier dye laser beams. Laser pulse energies at the torch were as follows: 150 ~3 at 460.7 nm; 9 mJ at 308 am; and 300 PJ at 421.5 nm. The ICP was a standard commercial unit (Plasma-Therm, Model 2500, Kresson, NJ, USA) operated at 27 MHz with a cross-flow nebulizer (Labtext, Ratingen, FRG) fed by a peristaltic pump. Argon gas flows were 12 I min- ’ outer gas, 0.5 I min- ’ carrier gas, and 1.6 1 min-* intermediate gas. Plasma power levels between 0.8 and X.5 kW were used, but most of the data presented here was taken at 1 kW. Laser excitation heights between 7 and 19 mm above the load coil (ALC) were studied by moving the torch up or down with most data being collected at 15 mm ALC. The laser induced fluorescence of Sr * was detected at 407.771 nm (O-24517 cm - ‘), thus avoiding any possibIe problems from scattering ofthe excitation beam. Fluorescence was collected at a right angle to the laser beam using a fens to form a 1: 1 image of the laser irradiated volume of the plasma on the entrance sht of a 1.29 m grating monochromator. The spectrometer is described in more detail in Ref. [14]. A slit width of 1 mm was used with a slit height of 2 mm. The slit was aligned with the crossing (1 X] J. C. TRAVIS, . C. TURK, J. R. DEVOE, P. K. fhiENcKand C. A. VAN DIJK, hog. Anal. Atom. Spectrose. 7,199 (1984). [l2] 3. C. TRAVIS, . C. TURK and R. B, GREEN, Anal. Chem. 54, 1006A (1982). [13] G. C. TURK and R. L. WATERS, JR, Awl. Cfwm. 57, 1979 (1985). [14] H, G. C. HUMAN, N. OMENETTO, . CAVALU and G. Row, ~~ec~r~~. Acre 39B, 1345 (t984).  Laser-induced ionization 875 Fig. 1. Schematic of experimental set-up used. PG: pulse generator; PD: photodiode; M: mirrors; L: lenses; ICE inductively coupled plasma; LT: light trap; FM: fluorescence monochromator; PMT: photom~tiplier tube, H.V.: high voltage. point of the dye laser beams and the central axial channel of the ICP, and defines the spatial observation zone. Signal averaging and data storage was done using gated integrators interfaced with a microcomputer. The two excimer lasers were triggered exter&ly with a variable delay to control the time between the laser induced ionization and the ionic fluorescence probe. The timing and &tacoll~tion electronics are described in more detail in Ref. Cl]. 3. RESULTS zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK ND DISCUSSION Figure 2 shows the relative ionic fluorescence signal of Sr+ measured as a function of time delay between the firing of the ionizing lasers and the fluorescence probe laser. The data were collected with the laser beams aligned 15 mm ALC, at a plasma power level of 0.9 kW with 6OO&ml of Sr being nebulized. Included in the plot are a few data points collected at negative time delay, when the pro& laser fires before the ionizing lasers, they measure fluorescence from the normal (not laser-induced) level of zyxwvutsrqponmlkjihgfedcbaZYXWVUT r +. Note that the ordinate does not  876 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA . zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH . TURK et zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP l. i 5 10 15 20 25 30 35 40 45 Time delay Irs) Fig. 2. Experimentally observed time decay of the strontium ionic fluorescence. The zero in the abscissa indicates temporal coincidence between the ionizing and the probe beams. Sr concentration: 600 &ml. The observation is made at the plasma centre. begin at zero ionic fluorescence, in order to show more clearly the laser-induced ionization. When the probe laser reaches time coincidence with the ionizing lasers, a clear enhancement in the level of ionic fluorescence is observed, increasing from the natural level of 5.3 relative units to approximately 7.5 r.u., as a result of the laser-induced ionization of Sr. As a time delay is then added, an exponential decay back to the natural level of ionic fluorescence is observed. As will be shown later, this rate of decay is increased by the addition of EIEs, and is therefore attributed to the recombination of Sr+ ions with electrons. If we then define rRas the recombination time constant, or the time required for the laser produced ion population to decay to l/e of its initial value, a value of zR equal to 15.5 pus s obtained from a non-linear least squares fit of the data to an exponential function. This decay behaviour is much different from that which was observed when the same experiment was performed in the air-acetylene flame. Briefly summarizing the results which are reported in an earlier publication [l], two distinctly different modes of Sr’ decay were observed. The first was a fast decay, occuring with an exponential decay time constant of 58 ns, caused by flame reactions between Sr+ and flame species present under oxygen-rich conditions. This fast decay consumes N 85 % of the laser produced St-” ions before reaching equilib~um. The remaining 15 % of the ions decay at a much slower rate, with an exponential time constant of 57 ,US, aused by ion-electron recombination and/or gas flow and diffusion. The most sibilant difference between the Sr ’ decay in the flame and the ICP is the absence of the fast decay caused by flame chemistry. This is certainly a striking example of the absence in argon ICP of complex chemical interactions, which are often the srcin of interferences in game and other excitation sources in molecular gases. The observed r~ombina~on rate is somewhat faster in the ICP than in the flame. This seems reasonable since the electron number density is much higher in the ICP, but comparison simply on the basis of electron number density is not justified since the detailed mechanisms of recombination are different. The absence of Nz in the ICP as a third body collisional partner for r~ombination is particularly relevant. 3.2. Effect of easily ionisab~e elements As alluded to earlier, the primary reason for attributing the Sr + decay observed in the ICP to ion-electron r~ombination is the response of the decay rate to the presence of EIEs. These effects are summarized in Tables 1-3. Table 1 shows the effect which equimolar concen- trations of the elements Ca and K have on the Sr + decay rate observed 8 mm ALC at a plasma power level of 0.9 kW. The matrix elements were present at a 23: 1 molar concentration ratio to the 100 pg./ml Sr concentration. Both elements increase the rate of recombination, with the  Laser-induced ionization 877 Table 1. Effect of easily ionized element on the decay rate of laser produced Sr+ Ions Matrix Ionization potential W) Decay time constant (Its) Dist. water Ca K - 15 6.1 7.6 4.3 3.6 Table 2. Concentration dependence of Sr+ decay rate Concentration Molar ratio Decay time constant &/ml) (Li/Sr) (I1s) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON 0 0 15 100 12.6 8.1 10 126 3.3 Table 3. ES&t of Li matrix at various observation heights Height (mm ALC) Sr+ decay time constant (gs) pure water 1000 fig/ml Li 7 13.1 2.1 11 13.0 2.2 1.5 14.7 3.3 .I9 - 6.8 more easily ionized element K having the greater effect. In Table 2 the concentration dependence of the matrix element lithium on the recombination is shown, this time measured 15 mm ALC at I kW. As expected, the highest concent~tion caused the greatest increase in rate. Similar effects were observed at a variety of observation heights and plasma power levels. In Table 3 the effect of a 1000 pg/ml Li matrix on the recombination rate is given for four different observation heights with the plasma power at 1 kW. The Sr concentration is again 100 pg/ml. At the 19 mm ALC position no significant enhanced ionization could be observed without the Li matrix present, this being the result of the already very high natural ion fraction for Sr at this position. Measurements made at 1.5 kW of plasma power and 13 mm ALC also showed an increased rate of recombination in the Li matrix. The most important point in all of these EIE matrix effect measurements is that changes in the observed rate of recombination must be associated with shifts in the ionization equilibrium between Sr atoms and ions. This is a more direct measurement of an equilibrium effect than the more common approach of measuring changes in signals from atomic and ionic lines. As has been pointed out by BLADEsand zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM ORLICK 2], the addition of an EIE has at least five effects on atomic and ionic emission thus making it impossible to study ionization equilibria simply on the basis of changes in emission intensity. In particular, increased collisional excitation rates induced by EIEs obscure any effect on ionization equilibria. The situation is somewhat simpler in the case of atomic and ionic fluorescence measurements, and GILLSON and HORLICK [3] have reported shiffs in the ionization equilibrium of Ca under certain conditions on the basis of such measurements. 3.3. Discussion and possible interpretation of the eflect To further characterize the Sr ionization equilibrium we have also made measurements of the normal (non laser-induced) ionic fluorescence of Sri and atomic fluorescence of Sr, at
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