Download - Time-resolved fluorescence as a direct experimental approach to the study of excitation and ionization processes in different atom reservoirs

Specrrochrmrca Am, Vol. 498. Nos 12-14. pp 1519-1535. 1994 Copyright 0 1994 Elsevrer Snence Ltd

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Time-resolved fluorescence as a direct experimental approach to the study of excitation and ionization processes in different atom reservoirs

N. OMENET~O and 0. I. MATVEEV~

European Commission, Joint Research Centre, Environment Insitute, 21020 Ispra, Italy

(Received 19 May 1994; accepted 26 July 1994)

Abstract-Several examples of laser-excited, time-resolved fluoresence waveforms are discussed to show how the essential parameters describing the interaction between the radiation and the atomic system can be directly evaluated. The results reported here have been experimentally obtained, with relatively fast detection electronics, in different atom reservoirs operated at atmospheric pressure, that is, an air-acetylene flame, an inductively coupled argon plasma and a graphite furnace. It is shown that the study of the complete temporal evolution (i.e., during and after excitation) of the population density of selected atomic levels, directly pumped by the laser or collisionally coupled to the laser-excited level, can provide important information about the dyamics of the interaction and the saturation behaviour of the transition as a function of the different atomization environments. In simple cases, collisional mixing and ionization rate coefficients can also be evaluated. The measurements discussed here have been obtained with the following elements: Au, Hg, Mg, Na, Pb, Sr and Tl. It is shown that the analytical relevance of the information gained from the waveforms is particularly significant when two or more lasers (i.e., excitation steps) are used in the techniques of Laser Induced Fluoresence (LIF) and Laser Enhanced Ionization (LEI) in flames and electrothermal atomizers.

1. INTR~DUC~~N

THE MOST common meaning of time resolution, when applied to the LIF technique, refers to the measurement of the decay time of the signal after the excitation pulse has subsided [l, 21. In other words, one should ideally use an excitation pulse much shorter than the fluorescence decay time, so that no deconvolution is needed to extract the true decay. The parameter obtained in this way represents the actual fifefime or decay time of the excited state reached by the laser radiation which contains both radiative rate coefficients (Einstein spontaneous emission probabilities) and collisional quenching (intermultiplet) or mixing (intramultiplet) coefficients. The actual lifetime is of considerable theoretical and practical importance in LIF experiments, since it is related to the quantum efficiency of the process, which is given by the ratio between the measured lifetime and the purely radiative lifetime [ 1, 31.

With the advent of lasers, the concept of saturation has also become important from the analytical and diagnostic point of view. The laser radiation is in fact capable, in a short time (sometimes referred to as “pumping time”), of equalizing the weighted populations of the connected levels and this holds until the irradiance of the pulse no longer exceeds the saturation irradiance, a fundamental parameter characterizing the selected transition. In view of the high population of the excited level and its efficient coupling, by collisions, with nearby levels, many non-resonance fluorescence transitions can be measured with the resulting blessing that spurious scattering signals, occurring at the laser excitation frequency, are minimized. In addition, this efficient coupling provides the basis for the analytical use of disconnected, multi-step excitation schemes, in which the final level reached by the first excitation step does not coincide with the

* This paper is dedicated to Jim Winefordner, as a memento of the countless hours of meditation and excitement spent together on these topics and was published in his Special Honor Issue of Spectrochimica Acta Part B.

t On leave from “Mozaika II”, Moscow, Russia.

1519

1520 N. OMENETTO and 0. I. MATVEEV

initial level of the second excitation step. A deeper insight into the relaxation processes between the excited levels is also essential in the technique of LEI [4], which relies upon the collisional (thermal) ionization of the excited atoms. In collision-dominated environments, it is indeed an “effective” ionization coefficient, involving many excited levels, which is responsible for the efficient creation of free charges, even during the short interaction time of an exciting laser pulse [l, 5, 61. It should then be interesting to follow directly the evolution of the population of selected excited levels, lying l-2 eV away from the laser-pumped level. This information, together with the experimental measurement of the rise time of the ionization pulse, could be the key to distinguishing between the processes of direct photoionization versus collisionally-assisted ionization in flames [7].

If one measures the actual lifetime of a level, for example, by monitoring the decay of the fluorescence following the excitation by a short laser pulse, it is irrelevant whether the transition is optically saturated or not (apart from a welcome gain in the signal-to-noise ratio, if scattering is not the limiting noise). However, in recent years, the importance of time resolution within the excitation pulse, recognized a long time ago [8], has been repeatedly stressed [9-111. In this case, saturation of the transition can offer some new information. In fact, any decay of the signal occurring while the pulse is still on constitutes direct evidence of a depletion process, which can be either ionization or accumulation of atoms into a metastable state: in both cases, the atoms will not be recycled during the fast excitation pulse. The fluorescence waveform should then be characterized by a fast rise time indicating that the levels are locked by the laser, followed by a decay dictated by the depletion rate of the laser coupled two- level system, and by the attainment of a steady state plateau when the depopulation process is counterbalanced by the corresponding replenishment. Whether steady state conditions are reached or not will depend essentially upon the duration of the interaction, the spectroscopic characteristics of the levels involved and the atomization environment, that is the type and number density of the major colliding species.

The purpose of the present paper is to present and discuss several time-resolved fluorescence waveforms, at the nanosecond level, for the elements. Au, Hg, Mg, Na, Pb, Sr and Tl. Most measurements were performed in an inductively coupled argon plasma (ICP), except for Au, which was atomized in a graphite furnace, and for Pb, which was also atomized in a flame. Many of the theoretical predictions related to the saturation behaviour and to the depletion of the excited state population are clearly borne out in these experiments. As an example, from the evaluation of the resonance fluorescence waveform of magnesium atoms, it is shown that atom losses due to ionization occurring during the excitation pulse are almost complete. Moreover, many important hints concerning the timing of the laser pulses, in the case of analytical LIF and LEI experiments with multi-step excitation, are given. On the other hand, no specific theoretical modeling was undertaken for any of the experimental data presented, since this was considered out of the scope of the present paper. The data illustrated here in fact refer to several independent experiments whose goal was either strictly analytical (e.g., Au, Tl and Pb) or, more generally, the study of the excitation and ionization processes.

2. EXPERIMENTAL

All the data discussed in this paper were obtained with two pulsed, tunable dye lasers (Model FL3002E, Lambda Physik, Gbttingen, Germany and a special eight-cuvette system made by Jobin-Yvon, Longjumeau, France) pumped by an excimer laser (Model LPX-200, Lambda Physik, Gottingen, Germany) operated with XeCl. The beams were directed into the various atomizers by fused silica prisms and plane mirrors. Typical characteristics of the lasers and of the experimental set-up are given below.

2.1. Lasers Both laser beams have a pulse duration of 15-20 ns and an energy per pulse which varies

from a few microjoules (in the case of frequency doubled operation around 250 nm) to several

Time-resolved fluorescence in atom reservoirs 1521

millijoules in the fundamental. The spectral bandwidth in the case of the Jobin-Yvon laser varies from 1.5 cm-’ at 400 nm (0.024 nm) to 3.5 cm-i at 600 nm (0.126 nm), while the Lambda Physik (without the cavity etalon) has a nominal bandwidth of about 0.2 cm-i and therefore cannot be considered as a pseudo-continuum excitation source. However, the conclusions derived from the time-resolved information presented in this paper should hold for both continuum and line excitation sources. The spatial structure of the beam has been characterized in a few cases with a Laser Beam Analyzer (Model LBA-lOOA, Spiricon, Inc., Logan, UT, U.S.A.). The selection of the dye for the different elements was made according to the efficiency curves provided by the manufacturers. With the Jobin-Yvon laser, three different dyes (Rhodamine 590 Chloride, 2-7 Dichlorofluoresceine and Coumarin 480, dissolved in methanol or in a mixture of methanol, water and Ammonix LO) were used. The dyes were supplied by Exciton, Dayton, Ohio, U.S.A. With the Lambda Physik laser, the dyes used (all dissolved in methanol and supplied by Lambda Physik) were Rhodamine 6G. Furan 2 and Coumarin 102, 120, 307 and 153. The UV excitation wavelengths were obtained by frequency doubling the fundamental laser output with KDP, KPB and BBO crystals. It was found essential to avoid the use of focusing optics to increase the laser irradiance at the atomizer since, especially in the ICP, which is characterized by a high fluoresence quantum efficiency, severe scattering effects occurring outside the laser volume [12] can hinder the unequivocal interpretation of the resulting waveforms. One or more apertures and a beam expander were then placed into the optical path of the beam to improve its shape and approach a uniform irradiance distribution. A subtle effect, on the other hand, was observed on some fluorescence waveforms. It was found that, at different locations within the same laser spot, different time profiles exist as well as a slight shift in the laser frequency. This effect, which was thoroughly investigated in the case of gallium fluorescence, is fully described in another paper [13]. Finally, the characterization of the laser pulse, which often shows several oscillations repeating periodically and whose modulation depth can be significant, will also be reported elsewhere [14].

2.2. Flame, ICP and graphite furnace set-ups An air-acetylene flame supported on a conventional, home-made circular capillary burner, a

pre-mix chamber and nebulizer assembly were used. The fluorescence was collected at right angles with two plano-convex lenses and dispersed with an f-4 grating monochromator (Model 1670 Minimate, Spex, Metuchen, NJ, U.S.A.). The ICP (Model 2500, Plasma Therm, Kresson, NJ, U.S.A.) was operated at a forward power of 0.6-1.5 kW, with an outer flow of 0.4 I/min, an intermediate flow of 0.45 l/min and a nebulizer flow of 0.4 l/min. Fluorescence, excited at 20 mm above the load coil, was collected at right angles and imaged onto the entrance slit of a 1.29 m focal length, grating monochromator (Model 1269, Spex, Metuchen, NJ, U.S.A.). Finally, the fluorescence from the graphite furnace (Model HGA-400, Perkin-Elmer, Uberlingen, Germany), was collected longitudinally with a plane mirror pierced in its center to allow exiting of the laser beams while at the same time collecting a fraction of the isotropically emitted fluorescence photons and directing them into an f-3 double monochromator (Model DH-10, Jobin-Yvon, Longjumeau, France). This particular arrangement has been described previously in more detail [15]. Pure acidified solutions for the different elements were prepared from analytical grade chemicals and diluted according to the desired concentration.

2.3. Detection In all cases described above, two types of detectors were used: a Proximity Microchannel

Plate Photomultiplier (Model R 1564-U-07, Hamamatsu Photonics, Japan) and a High Speed Photomultiplier (Model H3376, Hamamatsu Photonics, Japan). The Microchannel Plate Photomultiplier (MCP-PMT) has a rise time of 128 ps and is operated at a bias voltage of -3000 V, corresponding to a current gain of 6 X lo5 and a dark current of 0.3 nA. Its quantum efficiency ranges from 0.04 to 0.2 throughout the entire UV-visible region (200-700 nm). The maximum continuous anode current rating is 100 nA while the pulsed peak current can reach 700 mA. Due to the exceedingly low dc anode current rating needed to preserve the linearity of operation, many plasma experiments are not possible with this tube. The High Speed Photomultiplier (HS-PMT) on the other hand, operated at -2500 V with a current gain of 106, has a slower rise time (400 ps) but a much higher maximum dc current rating. For every experiment, the dc current was monitored with a picoammeter (Model 414S, Keithley, Cleveland, OH, U.S.A.) to assure linearity of operation. The output of both detectors was passed directly into the 50-ohm input of a Digitizing Signal Analyzer (Model DSA 602A, Tektronix, Beaverton, OR, U.S.A.). The scope has a 1 GHz bandwidth and a maximum sample rate for real time acquisition of 2 Gsample/s (1 Gsamplels for concurrent two channel acquisition). The trigger


was provided by a fast photodiode, biased at 2000 V, which sensed a small fraction of the excimer beam: in this way, the excimer temporal profile could also be monitored. Since the main purpose of the work was to monitor the time behaviour of the fluorescence, most of the waveforms discussed in the next section have been obtained under exactly the same settings of the oscilloscope time base. For example, in this way, by comparing the leading edges of the same signal obtained both with the MCP-PMT and the HS-PMT, the electron transit time of the last detector (7 ns) could easily be measured. On the contrary, the amplitude of the signals were adjusted in each case to emphasize their different temporal behaviour.

3. DISCUSSION

As discussed in the Introduction, obtaining a complete time-resolved fluorescence waveform together with that of the corresponding excitation pulse allows one to follow the history of the population of several excited states, including that reached by the laser, in addition to achieving a better understanding of the saturation behaviour of the transition. In the following discussion, the experimental results have been grouped in three sections to show:

(i) some peculiar characteristics of the saturation process; (ii) several collisional relaxation effects and the influence of the quenching

environment; and (iii) the depletion of neutral atoms due to ionization.

The interpretation of most of the data presented here can only be considered on a qualitative basis, due to the complexity of the system investigated. However, many theoretical insights as well as practical hints will be clearly outlined.

3.1. Saturation effects Sodium appears to be the obvious choice to show the effect of saturation since its

ground state (2S1,2) and the doublet excited state (2P3,2,1,2) constitute what can be considered, to a very good approximation, as a two-level system. If quenching collisions (P+S) are negligible compared to mixing collisions between the P levels (as expected in an argon atmosphere) the saturation irradiance is very low. In fact, it can be calculated [l] that, in an argon plasma, a spectral irradiance of 0.53 kW cme2 nm-’ is sufficient to equalize the weighted population of the ground state and the excited doublet. Figures 1 and 2 show a typical resonance fluorescence waveform obtained in the ICP for a low concentration of sodium atoms. These figures illustrate two effects. First, the fluorescence signal is practically unaffected by a decrease of the laser irradiance by one order of magnitude [Fig. l(B)] and decreases about 2.5 times in 30 ns for a 200-fold decrease of the laser irradiance. In fact, it is interesting to note that the second hump in the fluorescence shape is due to a small laser after-pulse, barely observable in Figs l(A) and 2(A), which has been made clearly visible in Fig. 2(B). For a 100 PJ, 15 ns pulse, with a bandwidth of 0.05 nm and a cross section of 0.8 cm2, the excitation spectral irradiance is 166 kW cmp2 nm-r, that is, approximately 300 times greater than the saturation irradiance. Therefore, the laser is able to redistribute the sodium atoms between the ground and excited levels in a pumping time much shorter than one nanosecond. Second, because of saturation, the temporal width of the fluorescence waveform is much broader than the laser pulse [Fig. l(A)]. This explains why a saturation curve (i.e., a plot of the fluorescence signal versus laser irradiance) will hardly reach a plateau if the time-integrated fluorescence signal is measured [16]. On the other hand, such strong saturation allows the evaluation of the lifetime of the level when the laser irradiance is practically reduced to zero, that is, about 50 ns after the laser pulse has reached its maximum [Fig. 2(b)]. To the authors’ knowledge, this peculiar outcome, the possibility of measuring even a short decay time of a level with a much longer, but strongly saturating, laser pulse, has not yet been pointed out in the literature. The lifetime which can be deduced in this way is close to 16 ns, thus confirming the high quantum efficiency of the sodium fluorescence in an argon atmosphere.


Time, 25 nsldiv

z - \ I 2

I \\

.s co-

_ I - laser

1 \Lk!L

resonance /fluorescence

(A)

09

I> b I! I I co I I Time, 2.5 ns/div

Fig. 1. Saturation behaviour of sodium atoms in the argon ICP. The laser excitation was set at 589.995 nm and fluorescence measured at the same wavelength with the HS-PMT tube, Energy/pulse = 1OOu.I; Na concentration = 3ug ml-‘. In (B), the acquisition time was faster and therefore the resolution was decreased as compared to (A). All waveforms are negative and the signal levels have been purposely adjusted. The rise time of the fluorescence is limited

by the time response of our apparatus.

The high fluorescence quantum efficiency of sodium atoms in the ICP can also be deduced from the fact that the ratio between the time-integrated signals of the Di and D2 lines is close to 2, irrespective of which line is chosen for excitation [17, 181. The time-resolved waveforms shown in Fig. 3 demonstrate that the above argument is valid at any time during the excitation pulse, indicating that the mixing rate between the P levels is much higher than the quenching rate into the ground state, and that the mixing process is indeed effective at the nanosecond time scale.

A second example chosen to validate the above discussion is illustrated in Fig. 4 and refers to the resonance fluorescence waveform of mercury atoms in the ICP. This element was selected because the transition probability of the 253.652 nm line (intercombination singlet-triplet, forbidden) is relatively low, that is, 8.5 x 10e6 ss1[19] with a resulting radiative lifetime of 118 ns. The difference in the rise and decay times should then reflect the occurrence of saturation of the transition. An inspection of Fig. 4 clearly shows that the resonance fluorescence quantum efficiency of the transition is 0.9, since the observed decay time [Fig. 4 (A)] is 106 ns, and that the rise time of the fluorescence (10 ns), being shorter than the duration of the laser pulse, is essentially dictated by the pumping time of the laser excitation process. This is in agreement

1524 N. OMENE~TO and 0. I. MATWTEV

Time, IO ns/dlv

Time, 10 nsldw

Fig. 2. Saturation behaviour of sodium atoms in the argon ICP. The experimental conditions are the same as those of Fig. 1. Labelling letters in (A) have the following meaning: (a) laser profile; (b) resonance fluorescence; (c) same as (b), but with the laser irradiance decreased 200 times. In part (B) of the figure, the laser signal (a) has been expanded 100 times as

compared to the fluorescence signal (b). All waveforms are negative.

with expectations, since the calculated saturation spectral irradiance of mercury (40 kW crnw2 nm-l) is 75 times higher than that of sodium.

3.2. Collisional relaxation between excited levels The possibility of observing many fluorescence transitions originating from levels

close to that directly pumped in the excitation process was recognized as one of the important advantages of lasers over conventional excitation sources, since the problems due to scattered light could then be minimized. Moreover, because of the efficient coupling between the levels, when more than one excitation step is used, as in the case of double-resonance LIF or LEI in atmospheric pressure atomizers, the two transitions do not necessarily need to be connected via a common level. Finally, in ionization experiments, it would in principle, be possible to obtain a time-resolved waveform in which the effect (if any) of photoionization could be discriminated from the more common collisionally-assisted process [7]. It would then be interesting to have at least a partial knowledge of the time scale in which collisions redistribute the excited state population among the various energy levels until a final steady state equilibrium is reached.

This problem is understandably rather complicated, since one can anticipate the existence of different relaxation times depending upon the type of levels involved and


their

Time, 10 ns/div

Fig. 3. (a) Resonance fluorescence and (b) stepwise line fluorescence waveforms of sodium atoms in the argon ICP. Laser excitation: 588.995 nm. Fluorescence waveforms: (a) 588.995 nm and (b) 589.592 nm. Na concentration = 3hg ml-‘. The laser irradiance was decreased 200 times. The waveforms are negative and the signal levels are directly comparable. Both

waveforms are characterized by the same rise time (5 ns).

energy separation from the pumped level. The most direct experimental approach to the study of the above collisional effects is to experimentally observe several spectrally separated, time-resolved, non-resonance fluorescence transitions and to compare them with the resonance fluorescence waveform. In this way, for the particular system investigated, the time required to populate and depopulate a level and to reach or only approach a steady state condition can be directly evaluated.

In the following, several examples have been selected and are derived from analytical and diagnostic work performed in an air-acetylene flame, an argon ICP and a graphite furnace. As pointed out earlier, the experimental data are not compared in a quantitative manner with a theoretical model since in most cases such a model would be far from simple. On the other hand, several important outcomes will be stressed.

Figure 5, which refers to the excitation of lead atoms in the flame [Fig. 5 (A)] and in the ICP [Fig. 5 (B)], is reported here with the aim of illustrating the practical time- resolving capability of our apparatus. The laser excitation is set at 283.306 nm (6p* 3P,-7~3P”,). The waveforms shown correspond to a transition originating from the laser pumped level (Direct Line Fluorescence at 363.958 nm) and from a level (7s3P,“) lying only 0.041 eV below (Stepwise Line Fluorescence at 368.348 nm). Both fluorescence transitions terminate into a metastable level. The time difference, -0.5 ns in the flame and -2 ns in the ICP, between the peaks of the waveforms is clearly resolved.

Figure 6 shows the energy level diagram pertinent to six transitions of the strontium atom which were excited in the ICP. In Figs 7 and 8, five transitions originating from levels whose energy difference with the laser pumped level ranges from 0.099 eV to 2.134 eV, are compared with the direct line fluorescence waveform, that is, with a transition originating from the laser-excited level. As seen in these figures, from a comparison of the direct line fluorescence (curve a in Fig. 7) with the collisionally assisted fluorescence from the 5d-3D3 level (curve e in Fig. 7), it takes -10 ns, during the laser excitation, to reach the maximum population of this level, which lies 0.48 eV below the 7p level, and -15 ns for the 5p’P,“ level, which lies 2.134 eV below the 7p level (curve c in Fig. 7). This is more clearly visualized in Fig. 8, where the three waveforms are displayed on a normalized scale.

As another example, Fig. 9 shows a comparison between a resonance fluorescence and a stepwise line fluorescence waveform obtained for thallium atoms in the ICP. In this case, excitation was set at the 2P1,2+2D 3/2 transition (276.787 nm). In the non- resonance case, fluorescence was observed at the *S1,2+*P3,~ transition (535.046 nm).


Time, 50 ns/dw

resonance fluorescence

Time, 5 ns/div

Fig. 4. Saturation behaviour of mercury atoms in the argon ICP. The laser excitation was set at 253.652 nm and the fluorescence measured at the same wavelength. Energy/pulse = 1OuJ; Hg concentration = 1OOpg ml-‘. In (B), the time acquisition was faster and therefore the time resolution was decreased as compared to (A). The waveforms are negative and the signal

levels have been purposely adjusted. The data were taken with the HS-PMT tube.

The energy difference between the levels 2D,,2 and *Si,* is 1.21 eV. The non-resonance fluorescence waveform reaches its peak intensity in about 20 ns from the onset of the laser excitation. It is interesting to note that the 6D-2D3,2 level reached by the laser can be efficiently coupled with the 7P levels which are situated 0.11 and 0.24 eV below. These last levels are then radiatively coupled with the 7S-2S1,2 level from which fluorescence at 535.046 nm is observed. The radiative coupling occurs via two infrared lines at 1301.32 nm and 1151.28 nm, whose spontaneous transition probabilities are 1.71 x 10’s_l and 2.37 x 107sW1, respectively [19]. In the presence of collisions, it should then take less than 50 ns to populate the fluorescing level from the laser pumped level.

As a final example, the fluorescence of gold atoms present in a graphite furnace with an argon buffer gas is shown in Fig. 10. The analytical determination of gold by LIF and electrothermal atomization has been undertaken in our laboratory with the aim of developing a method capable of subfemtogram detection sensitivity. The scheme used involves the measurements of fluorescence photons in the low UV region after excitation with two lasers counterpropagating along the axis of the tube. The first laser (A, in the figure) excites the atoms from the ground state (2S1,2) to the ~P-P”,~ level (267.595 nm). From this level, the atoms are further excited into the 6d-*D3,*


(4

t I I I ! Time, 1 ns/dw

Time, 1 ns/div

Fig. 5. Direct line fluorescence and stepwise line fluorescence waveforms of lead atoms obtained with the MCP-PMT detector. Laser excitation: 283:306 nm; Pb concentration = 5Oug/ ml. Part (A) air-acetylene flame; Part (B) ICP. In both figures, (a) refers to the fluorescence line at 363.598 nm and (b) to the fluorescence line at 368.348 nm. The waveforms are negative. In (A), the acquisition time is faster and therefore the resolution is decreased as compared

to (B). The signal levels have been purposely adjusted.

level with a second laser (A, in the figure) tuned at 406.508 nm. Collisional relaxation brings the atoms down by 0.39 eV to the ~s-P“~,~ level from which fluorescence is measured, at 201.200 nm, down to the 6~~-~D5/2 level. The advantages of this scheme would be that scattering is avoided, thermal emission from the furnace is minimal and a solar blind photomultiplier can be used. It was felt interesting to investigate the efficiency of the coupling of the levels 6d-2D 3,2 and T’s-~PO~,~ (AE = 0.39 eV) in a pure argon atmosphere. In Fig. 10(A), a thermally-assisted fluorescence waveform at 312.278 mu, resulting from the collisional mixing between the 6p-Pm and the ~P-~P”,, levels (AE = 0.47 eV) is shown in comparison with the laser scatter waveform, obtained by diffusing the laser beam from the center of the furnace into the monochromator. The results of a similar procedure followed for the analytical fluorescence waveform at 201.200 nm are shown in Fig. 10(B). From these data, despite the high noise in the fluorescence waveforms, one can see that, for similar energy gaps, high-lying excited states mix much faster than the lower ones. The fluorescence scheme chosen would then effectively work. On the other hand, one should be aware of the possibility of losing excited atoms because of collisonal ionization since the ionization continuum lies 1.54 eV above the 6d-2D3n level.

1528 N. OMENEITO and 0. I. MAWEEV

36560 36045

21b98

I 20150

I,$,9

6d JD, 6d 3D, 7p ‘P;

5p’- 3D; 5d - 3D,

5p. ‘P;

4d-‘D,

4d JO,

5p 3P;

Fig. 6. Partial energy level diagram of the strontium atom. The energy (cm-‘), the spectroscopic notation of the various levels and the corresponding wavelengths (nm) are also indicated. I.P.

refers to the Ionization Potential. The laser excitation is set at 256.947 nm.

I I

Time, 5 nsldiv

Fig. 7. Experimental fluorescence waveforms (negative) for the strontium atoms in the argon ICP pertinent to the transitions indicated in Fig. 6. The laser excitation profile at 256.947 nm is also indicated. The signals were detected with the HS-PMT tube. Sr concentration = 100 pg/ ml-‘. (a) 532.982 nm; (b) 403.038 nm; (c) 460.733 nm; (d) 554.336 nm (e) 496.226 nm; (f) 548.084 nm. Except in (a) and (c), the signals have been recorded with the same sensitivity settings and are therefore directly comparable. In (a) and (c), the signal levels have been

reduced 10 times and 4 times, respectively.

Several interesting considerations can now be summarized from the results presented in Figs 5-10:

(i) in the examples presented, the major emphasis was reserved for the temporal characteristics of the various waveforms rather than for their absolute signal levels, which are needed to evaluate the fraction of excited atoms which are not contributing to the analytical fluorescence signal, that is, to the signal measured from the laser- pumped level. This information is essential when one attempts to place the LIF technique on an absolute basis. Among the various data, those pertinent to the strontium transitions shown in Figs 7 and 8 can be interpreted in a quantitative


Time, IO nsldiv

Fig. 8. Experimental fluorescence waveforms (negative) for three selected transitions shown in Fig. 7, i.e., (a), (c) and (e). The signals have been normalized to the same level and the normalization factor is indicated in parentheses. The energy level schemes of each waveform are also indicated. The waveforms refer to one direct line fluorescence process at 532.982 nm and two stepwise line fluorescence transitions at 496.226 nm (0.48 eV) and at 460.733 nm

(2.134 eV).

, I,, , #- Time, 5 n&Iii

Fig. 9. Resonance fluorescence and stepwise line fluorescence waveforms (negative) for thallium in the ICP (20 mm above the load coil; 0.8 kW). h,,.=276.787 nm; A,=276.787 nm and 535.046 nm. Tl concentration = 1oOug ml-‘. The signal levels cannot be compared directly.

The waveforms were taken with the HS-PMT tube.

manner. In fact, in Fig. 8, when the absolute signal levels relative to the waveforms shown are taken into account, the integration of the direct line fluorescence signal with a nurrow gate (-3 ns) right at the beginning of the excitation pulse, assures that over 85% of the excited atoms are measured;

(ii) for disconnected excitation with more lasers, the optimum time delay between the first and the second excitation step can be directly determined from the time- resolved waveforms. It is important to note that such delay can, in some cases, be longer than the duration of the laser pulse: if this is the case, it is obvious that no conclusion can be given about the real time scale of the process: it can only be said that it is longer than the laser duration;

(iii) when a metastable state is involved as the final level of strongly allowed transitions, the population of the excited level accumulates in the metastable state in a time scale which can be much shorter than the duration of the excitation pulse [see, for example, the flame results for lead in Fig. 5(A) and for strontium in Fig. 81. This outcome has already been pointed out in the literature 17 years ago [20]. On the

1530 N. OMENET~O and 0. I. MAWEEV

TIME (10 ns/div)

TIME (5 n&v)

Fig. 10. Experimental fluorescence and excitation waveforms (negative) for the gold atom in the graphite furnace with argon as buffer gas. (A) A, = 267.595 nm; hF = 312.278 nm. (B) A, = 267.595 nm; A, = 406.508 nm; A, = 201.200 nm. The measurements were taken with the MCP-PMT detector. The absolute amount of gold in the furnace was 100 pg. The signal levels have been purposely adjusted. The noisy fluorescence waveforms were obtained by

manually freezing the acquisition on the display during the atomization cycle.

other hand, in more recent double-resonance fluorescence and ionization experiments, it is clear that the rising edge of the second excitation pulse cannot lag behind the first one by more than one or two nanoseconds, since in this case a very modest ionization enhancement, if any, or a very weak fluorescence signal will be observed, despite the fact that the two pulses overlap for about 15-20 ns. This point might have been overlooked in some multi-step excitation experiments;

(iv) the lack or establishment of steady state conditions during the excitation pulse can be directly deduced from the non-resonance fluorescence waveforms. This is essential, for example, in the correct application of the Thermally-Assisted Fluorescence Method for determining flame and plasma temperature [21].

All the waveforms discussed until now refer to a particular set of atomization conditions (ICP power, flame composition, etc.), which were kept constant during the measurement. It was then felt interesting to investigate the responsivity of the relaxation time parameter to a change of the operating conditions. Figures 11 and 12 show the results of such experiments for thallium and strontium atoms in the ICP. In the former case, the rise time of the resonance fluorescence waveform at 377.572 nm (curve a) is compared with that of a thermally-assisted fluorescence line (351.924 nm) originating from a level lying 1.21 eV above the pumped level (curve b) under identical


Time, 1 ns/ch

Fig. 11. Effect of the ICP forward power on the rise time of a thermally assisted fluorescence signal (negative) for thallium atoms in the argon ICP. Laser excitation: 377.572 nm; Tl concentration = 100 p,g ml-‘. (a) resonance fluorescence; (b) thermally assisted fluorescence at 351.924 nm, ICP: 1.5 kW; (c) thermally assisted fluorescence at 351.924 nm, ICP: 0.8 kW. The signal levels cannot be compared directly. The data were taken with the HS-PMT tube.

Time, IO ns/div

Fig. 12. Effect of the ICP forward power on the decay time of the stepwise line fluorescence signal for strontium atoms in the ICP. Sr concentration = 100 pg ml-i. A,,, = 256.947 nm; A, = 460.733 nm. The signal levels (negative) are directly comparable. The data were obtained

with the HS-PMT tube.

operating conditions of the plasma. In addition, there is a striking five-fold prolongation of the rise time of the thermally assisted fluorescence waveform when the forward power to the plasma varies from 1.5 kW (curve b) to 0.8 kW (curve c). A remarkable difference, shown in Fig. 12, is also observed in the decay time of a strontium transition when the power is varied from 0.6 kW to 1 kW. These preliminary data, which refer to one particular location in the plasma, are well worth pursuing on a more systematic and quantitative basis, since the time variations observed could bear a direct relationship with the electron number density in the plasma. Finally, the well- known different quenching characteristics of an air-acetylene flame compared to an argon plasma are highlighted in Fig. 13, which shows the direct line fluorescence waveform of lead atoms (283.306 nm excitation, 405.783 nm fluorescence) as observed in both atom reservoirs. Here, the same considerations previously given in item (iii), concerning Figs 5(A) and 8, apply.

1532 N. OMENE~TO and 0. I. MATVEEV

h hF -_--I ILZ em m

Time, 5 nsldiv

(B)

Time, 5 ns/div

Fig. 13. Experimental evidence of the depletion of lead atoms due to their accumulation into a metastable trap. Both waveforms (negative) have been measured with the MCP-PMT detector. Laser excitation: 283.306 nm; direct line fluorescence: 405.783 nm. Pb concentration = 50 pg ml-‘. (A) Flame; (B) ICP. The overshoot (ringing) observed in (A) is an artifact of the

detector. The signal levels in (A) and (B) are not directly comparable.

3.3. Depletion effects due to ionization It has been stressed many times, both theoretically and experimentally, that the

ionization of atoms in different atom reservoirs can be studied by measuring the modulation in the fluorescence signal resulting from the onset of ionization [9, lo]. A particularly well studied case is that of magnesium. The concept here is very simple: if the atoms are ionized during a saturating laser pulse, the entire atom population will decay with a time constant reflecting the ionization rate coefficient. This depletion occurs since ion-electron recombination cannot take place on such a fast time scale. This concept is more general, since it applies to any process which acts as a trap for the excited atoms, for example, accumulation into a metastable state (see Pb and Sr cases before) or a charge transfer mechanism [ll].

The ionization experiment described for magnesium atoms in the air-acetylene flame [9] was repeated in the ICP. The transition l&-lP”, (285.213 nm) was barely saturated, to avoid photoionization effects, with one laser. The population of the ‘PO1 level, monitored by the resonance fluorescence signal, was then further excited with a second, strongly saturating laser tuned at 470.299 nm into the ‘Dz level, which lies 0.66 eV below the ionization continuum. Figures 14-16 report the fluorescence as well as the laser waveforms of the experiment. The degree of depletion of the lp0, level


Time, 5 nsldw

Fig. 14. Experimental evidence of the depletion of neutral magnesium atoms in the ICP (0.8 kW, 20 mm above the load coil) due to ionization within the laser pulse. Both resonance fluorescence and laser excitation waveforms (negative) are shown. The data were taken with the HS-PMT tube. Mg concentraltion = 5 kg ml-‘. (a) Resonance fluorescence with both lasers: A, = 285.213 nm, 1OpJpulse; A, = 470.299 nm, 7 ml/pulse; (b) resonance fluorescence in absence of AZ; (c) laser temporal profile at 470.299 nm (A,); (d) laser temporal profile at

285.213 nm (A,).

Time, 5 nsldiv

Fig. 15. Experimental evidence of the depletion of netural magnesium atoms in the ICP, due to ionization, as a function of the irradiance of the second excitation step (A*). (a) AZ at full irradiance; from (b) to (e) the power of A, was decreased 4,20, 40 and 200 times, respectively; in (f), only A, was present. All the signals (negative) have been obtained with the same

sensitivity settings and are directly comparable.

follows clearly the irradiance of the second excitation step (Fig. 15). A more informative way of presenting the results is that of taking the ratio of the waveforms obtained with and without the second laser: when their ratio no longer increases with time, the ionization process has halted. This can be seen to occur about 25 ns after the onset of the second excitation step (Fig. 16).

As anticipated in Section 2, a very peculiar effect, thoroughly discussed in another paper [13], was also observed here with regard to the frequency doubled laser shape at 285.213 nm (see Fig. 14). From a comparison of the resonance fluorescence shape (curve b) and the laser shape (curve d) it can be seen that the first oscillation peak in the laser output does not contribute to the fluorescence as much as the rest of the pulse. This is even more evident in waveform (a), the depleted waveform. The effect is attributed to a slight detuning in the frequency of the oscillation with respect to that of the rest of the pulse. By readjusting the tuning of the grating in the oscillation

1534 N. OMENET~O and 0. I. MAWEEV

Time, 5 ns/div

Fig. 16. Experimental evidence of the depletion of neutral magnesium atoms in the ICP; due to ionization. Mg concentration = 5 kg ml-‘. (a) resonance fluorescence without A,; (b) resonance fluorescence in the presence of A, at full irradiance; (c) ratio of waveforms (a) over (b). The signal levels in (a) and (b) are directly comparable. Compared to Figs 14 and 15, the first laser (X,) was spectrally readjusted to optimize its tuning into resonance with the

absorption line of magnesium.

cavity and the frequency doubling crystal, the effect disappeared, as clearly seen by comparing waveform (a) in Fig. 14 with waveform (b) in Fig. 16. It must be emphasized that the occurrence of such artifacts is totally random and can therefore be easily overlooked.

4. CONCLUSIONS

Several examples of time-resolved fluorescence waveforms for different elements have been presented in order to prove the usefulness of such measurements in the application of the fluorescence and ionization techniques in atmospheric pressure atomizers. It has been conclusively demonstrated, in the authors’ opinion, that many important parameters describing the interaction between the laser radiation and the atomic system can be directly evaluated, even if, in most cases, no quantitative agreement is claimed with any proposed theoretical model.

From an analytical point of view, time resolution would certainly help to establish the correct timing in a multi-step excitation experiment and, depending upon the type of atomizer used, to choose the most suitable integration time for the analytical signal. In many cases, an accurate comparison between the temporal profile of the laser and the fluorescence pulses can reveal some misadjustments in the tuning of the oscillator cavity which would have been otherwise overlooked. Although these effects may be unimportant for analytical work (provided that they remain constant during the analysis), their recognition is certainly essential in diagnostic studies.

The systematic use of time resolution in flame and plasma work with laser excitation is still lacking. Other processes, such as Amplified Spontaneous Emission (ASE) in the laser output and mode beating, can be investigated by studying the time-resolved profile of the laser oscillation. It is hoped that the experiments and the results described in this paper will also stimulate further interest and research in other laboratories.

PI

PI

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