Resonance-enhanced laser-induced plasma spectroscopy for sensitive
elemental analysis: Elucidation of enhancement mechanisms

S. L. Lui and N. H. Cheunga)Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China ͑Received 4 September 2002; accepted 30 October 2002͒ When performing laser-induced plasma spectroscopy for elemental analysis, the analytesignal-to-noise ratio increased from four to over fifty if the plume was reheated by a dye laser pulsetuned to resonant absorption. Time-resolved studies showed that the enhancement was not due toresonance photoionization. Rather, efficient and controlled rekindling of a larger plume volume wasthe key mechanism. The signal-to-noise ratio further increased to over a hundred if the atmospherewas replaced by a low-pressure heavy inert gas. The ambient gas helped confine and thermallyinsulate the expanding vapor. 2002 American Institute of Physics. ͓DOI: 10.1063/1.1532774͔ Laser-induced plasma spectroscopy ͑also known as tions were studied, the fluence of the 532 nm laser pulse was laser-induced breakdown spectroscopy͒ is a versatile tech- only about 800 mJ cmϪ2 ͑Ref. 9͒. The fluence of the dye nique for elemental analysis. It can be performed remotely laser pulse was about 460 mJ cmϪ2, which was marginally on practically any sample, and applications ranging from Unless stated otherwise, ablations were done under 1 demonstrated.1–3 Unfortunately, laser ignition of plasmas is atm of air although the sample chamber could be filled with chaotic and violent. Reproducibility and sensitivity are, different gases to any pressure. Light emissions from the therefore, compromised. We recently showed that the sensi- plasma were imaged axially onto the entrance slit of a spec- tivity could be improved if the plume was photoexcited trograph equipped with an intensified charge-coupled device resonantly.4 For example, we laser ablated a potassium-rich ͑ICCD͒. Analyte spectra were captured using a slit width of target doped with sodium as a test analyte. The expanding 300 ␮m, giving a 0.2 nm resolution. Lithium linewidths were plume was then intercepted by a dye laser pulse ͑404 nm͒ to measured with a 100 ␮m slit at 0.08 nm resolution. Time- resonantly excite the K atoms (4 2S integrated spectra were taken with an intensifier gate delay t 589 nm emissions were enhanced. Undoubtedly, the analyti- of 40 ns ͑relative to the onset of the 532 nm pulse͒ and a 5 ␮s gate width tb . For time-resolved studies, tb was 50 ns plasma spectroscopy ͑RELIPS͒ can be improved if the un- while td was scanned. The 50 ns tb was the electronic pulse derlying processes are better understood. In this letter, we width. The actual optical gate width was narrower.10 It was report a study of the RELIPS mechanism and illustrate how characterized by scanning the gate across the 532 nm light the sensitivity could be further enhanced.
pulse. The resultant pulse width was about 37 ns ͓full width Cylindrical pellets of potassium iodate (KIO at half maximum ͑FWHM͔͒, and the occurrence of the pulse ing 35 ppm of sodium were used as targets ͑for details, see maximum was located to within Ϯ 5 ns. For that reason, td Ref. 4͒. Lithium ͑55 ppm͒ was added when the plasma tem- was stepped at 5 ns for time-resolved studies. In all cases, perature T and electron density ne were to be measured:5 Twas determined from the intensity ratio of the Li 610.3 and670.8 nm lines, which was shown to be consistent with thatdeduced from the more conventional iron lines.6 ne was de-termined from the Stark width of the Li 610.3 nm line, whichwas shown to agree with estimates based on the linewidthsof the H␣ and H␤ transitions.6,7 However, ne deduced fromthe width of the Li 670.8 nm line, was found to be overesti-mated, and corrections were necessary. In separate experi-ments, pellets of sodium bicarbonate (NaHCO3) doped with50 ppm lithium were used as targets.
The experimental setup, shown schematically in Fig. 1, was similar to the one used previously.4,8 Briefly, a laserpulse ͑532 nm, 10 ns, and 10 Hz͒ from a Nd:YAG laser was FIG. 1. Schematic diagram of RELIPS setup. A rotating cylindrical pellet of apertured and imaged onto the side of the rotating target. 30 KIO3 housed in a sample cell was ablated by the second-harmonic ͑532 nm͒ ns later, a dye laser pulse ͑9 ns and 10 Hz͒ was focused output of a Nd:YAG laser pulse of 10 ns width. 30 ns later, the expanding normally onto the same spot. Because near-threshold abla- plume was intercepted by a dye laser pulse of 9 ns duration and 0.3 nmlinewidth centering on 404.4 nm. The plume emissions were directed ontothe entrance slit of a spectrograph equipped with an intensified array detec- a͒Author to whom correspondence should be addressed; electronic mail: tor. The sample cell could be evacuated or filled with various ambient gases Downloaded 27 Dec 2002 to Redistribution subject to AIP license or copyright, see
Appl. Phys. Lett., Vol. 81, No. 27, 30 December 2002 FIG. 2. Plume emission spectra generated by the RELIPS scheme. KIO3pellets containing 35 ppm of Na were ablated in air by a 532 nm laser pulseat a fluence of about 800 mJ cmϪ2. 30 ns later, the expanding plume wasintercepted by a minimally ablative dye laser pulse of about 460 mJ cmϪ2.
The plume emissions were time integrated for 5 ␮s starting 10 ns after thefiring of the dye laser. Emission spectra produced by ͑a͒ the dye laser alone, ͑b͒ the 532 nm laser alone, ͑c͒ the 532 nm laser followed by an off-resonance 407 nm dye laser pulse, ͑d͒ similar to ͑c͒ except with an on-resonance 404 nm dye laser pulse, and ͑e͒ similar to ͑d͒ except the ambientgas was 13 mbar xenon instead of open air. Since the edge pixels of thecharge coupled device ͑CCD͒ were not intensified, the spectral trace at theedges served conveniently as baselines in all cases. All spectra were offsetvertically for clarity. The SNR for the various traces were about ͑a͒ 1, ͑b͒ 4, 300 events were accumulated before the spectrum was stored FIG. 3. Time-resolved plume emissions generated under conditions identical to that of traces ͑c͒ and ͑d͒ in Fig. 2, except the ambient gas was 350 mbar The effectiveness of RELIPS is shown in Fig. 2. Trace air and the ICCD gate width was 50 ns. The effective optical gate width was ͑a͒ was generated with the 404 nm dye laser alone. The edge about 37 ns. The time axis t was the ICCD gate delay td plus half of the gatewidth t pixels were not intensified so the two edges served as base- b . Time t was, therefore, the center of the time window. Both on- resonance ͑circles͒ and off-resonance ͑crosses͒ behaviors were shown for lines. Trace ͑b͒ was generated with the 532 nm pulse alone.
easy comparison. The four panels are: ͑a͒ sodium signal, ͑b͒ continuum Trace ͑c͒ was generated with the 532 nm pulse followed by background, ͑c͒ electron density determined from Stark widths of the Li an off-resonance ͑407 nm͒ dye laser pulse. Trace ͑d͒ was 610.3 and 670.8 nm lines, and ͑d͒ plasma temperature determined from theintensity ratio of the same pair of Li lines, together with the Li 610.3 nm line similar to ͑c͒ except with the dye laser tuned to resonance ͑404 nm͒. Enhancement is clearly demonstrated. The en-hancement may be quantified in terms of the signal-to-noise parent. The cyclic variation in T for the 404 nm case was ratio ͑SNR͒. If we define the Na signal as the average inten- probably due to shock waves.11 The off-resonance T was sity under the doublet minus the average background inten- only briefly measurable between 60 and 65 ns, when it was sity, and noise as the standard deviation of the background, found to be about 0.42 eV. At earlier times, the background then the SNR of traces ͑a͒ through ͑d͒ are, respectively, 1, 4, masked the weak Li 610 nm line. At later times, T dropped 5, and 53. Trace ͑e͒ will be discussed in a later section.
below the measurable threshold of 0.35 eV.
The enhancement mechanism was elucidated by captur- In addition to the obvious, three subtle observations may ing time-resolved spectra. They were taken under conditions be drawn from Fig. 3 that are no less important. First, n identical to that of Fig. 2 ͑traces c and d͒ except with a 50 ns remained high (1016 and 1017 cmϪ3) during much of the gate width and an air pressure of 350 mbar. A lower pressure signal lifetime, be it on or off resonance. It guaranteed local was selected to ensure a reasonable signal even under non- resonant conditions ͑see next͒. The signal and background temperatures.12 An unnecessary resonant boost would only intensities, as defined earlier, are plotted against time in Figs.
contribute to an unwelcome background.
3͑a͒ and 3͑b͒, respectively. The dye laser was tuned to either 404 nm for the on-resonance traces ͑open circles͒ or 407 nm ϭ50 ns ͓Fig. 3͑c͔͒, which was 20 ns after the 404 nm pulse for the off-resonance traces ͑crosses͒. The time axis t is de- was fired. This noticeable delay suggests that 1ϩ1 tb/2. It marks the center of the time window.
Events earlier than 25 ns were not captured because of over-whelming continuum backgrounds. As can be seen, relative h␯͑404 nm͒K͑52P3/2 , to the off-resonance case, 404 nm light induced a small in- K͑52P ͒ϩh␯͑404 nm͒KϩϩeϪ, crease in background but a marked enhancement in signal could not be the dominant electron production process.
The evolution of the electron density ne(t) and plasma Quenching of the excited K (5 p) atoms via electron–atom temperature T(t) is shown in Figs. 3͑c͒ and 3͑d͒ ͑darker superelastic collisions would compete with photoionization symbols͒, respectively. Again, resonant effects are very ap- ͑process 2͒.14 Given the high ne , collision deexcitation was Downloaded 27 Dec 2002 to Redistribution subject to AIP license or copyright, see
Appl. Phys. Lett., Vol. 81, No. 27, 30 December 2002 more likely. The suprathermal electrons so produced eventu- resonant absorption, as opposed to inverse Bremsstrahlung ally impact ionized the ambient atoms to raise ne . At the heating, offers efficient and controllable energy delivery to present dye laser irradiance (ϳ107 W cmϪ2) and electron targeted atoms. Self-capping prevents local overheating. The density (1016– 1017 cmϪ3), the impact ionization time would more extensive heating means a larger heat reservoir at the be of the order of 10 ns.16 That is consistent with the 15–20 optimal temperature, leading to prolonged signals. In other ns delay ͑relative to the 404 nm pulse͒ of the ne maximum.
words, maintaining an optimal temperature is the key to ana- Third, even though the on- and off-resonance tempera- tures were comparable at tϭ65 ns, the 404 nm induced Na Of course, another effective way to maintain the tem- 589 nm and Li 610 nm signals were much stronger than the perature is to reduce the heat lost from the plasma. A rarefied 407 nm case ͓Figs. 3͑a͒ and 3͑d͒, lighter symbols͔. Assuming atmosphere would better insulate the plume and prolong the LTE, the difference in excited state population could not be signal.19 To test the idea, we reduced the air pressure from attributed to the difference in ne .15 The only plausible reason one atmosphere to below 0.1 mbar.20 The analyte signal in- was a difference in the mass heated. Since the vapor plume creased until around 350 mbar and then decreased. At lower density should be similar in both cases, a much larger vol- pressure, the freer expansion and accelerated thinning of the ume of the plume was probably heated in the resonance case.
plume caused a drop in signal and ne . A heavy inert gas, The second observation, concerning the role of 1ϩ1 such as xenon, would confine the plume even at low pressure photoionization ͑processes 1 and 2͒, was further investigated while hardly conducting heat away. Our study showed that by ablating pellets of NaHCO3 doped with lithium as the 13 mbar of xenon gave the best SNR. The corresponding analyte. The Na-rich plume was intercepted by a dye laser RELIPS spectrum is shown in Fig. 2 ͑trace e͒. The SNR, as beam tuned to 589 ͑594͒ nm for on ͑off͒-resonance pumping.
In this case, ionization was energetically impossible even In summary, RELIPS delivered a superior analytical per- with the absorption of a second 589 nm photon. Yet, a simi- formance because it kept a larger plasma volume at the pre- lar boost in the analyte ͑Li 670.8 nm͒ emissions was ob- ferred temperature for a longer time. Interestingly, one may served. This unambiguously showed that resonant photoion- stretch the dye laser pulse to tens or even hundreds of ␮s, ization is not essential for signal enhancement. The spatial such as with a flash lamp pumped device. The sustained extent ͑FWHM͒ of the Li 670.8 nm emissions was also esti- plasma and the extended signal lifetime should enhance the mated by scanning the plume image vertically across a 50 SNR tremendously. RELIPS studies with a long-pulse dye ␮m horizontal slit mounted in front of the spectrograph slit.
The resonant image was about 400 ␮m tall while the off-resonance one was about 130 ␮m. This supports the third This work was supported by the Research Grants Coun- cil of the University Grants Committee of Hong Kong and The difference in heated volume may be explained as Faculty Research Grants from Hong Kong Baptist Univer- follows. An off-resonance laser pulse deposits energy in a plasma via inverse Bremsstrahlung absorption. For visible 1 For a review of LIPS, see D. A. Rusak, B. C. Castle, B. W. Smith, and J.
light, that absorption cross section ␴ is about 10Ϫ21 cm2 D. Winefordner, Crit. Rev. Anal. Chem. 27, 257 ͑1997͒.
e of 1017 cmϪ3, the absorption coefficient A. K. Knight, N. L. Scherbarth, D. A. Cremers, and M. J. Ferris, Appl.
Spectrosc. 54, 331 ͑2000͒.
e) is only 10Ϫ4 cmϪ1. Hotter regions near the beam 3 D. Anglos, K. Melesanaki, V. Zafiropulos, M. J. Gresalfi, and J. C. Miller, focus may absorb more light because of higher ne . So, hot Appl. Spectrosc. 56, 423 ͑2002͒.
spots become hotter in a positive feedback fashion leading to 4 S. Y. Chan and N. H. Cheung, Anal. Chem. 72, 2087 ͑2000͒.
localized and unpredictable heating.16 In sharp contrast, a 5 At lithium concentrations higher than 100 ppm, self-absorption of the resonant laser pulse deposits energy in the host atoms ex- 6 K. M. Lo and N. H. Cheung, Appl. Spectrosc. 56, 682 ͑2002͒.
tremely efficiently. The absorption cross section of the K 404 7 C. W. Ng, W. F. Ho, and N. H. Cheung, Appl. Spectrosc. 51, 976 ͑1997͒.
nm transition is about 2ϫ10Ϫ16 cm2. The number density of 8 J. D. Wu and N. H. Cheung, Appl. Spectrosc. 55, 366 ͑2001͒.
host atoms is about 1018 cmϪ3 ͑Refs. 16 and 17͒. The ab- 9 All laser fluence refers to the peak fluence of the central hottest ͑ϳ90% sorption coefficient is therefore ϳ102 cmϪ1. Uniform depo- 10 According to the ICCD manufacturer, an electronic pulse width of 50 ns sition of light energy in an extended volume is possible be- cause local absorption is automatically capped whenever the 11 H. P. Gu, Q. H. Lou, N. H. Cheung, S. C. Chen, Z. Y. Wang, and P. K.
excited population is saturated.15 Subsequent superelastic Lim, Appl. Phys. B: Lasers Opt. B58, 143 ͑1994͒.
collisions distribute that electronic energy evenly throughout H. R. Griem, Plasma Spectroscopy ͑McGraw–Hill, New York, 1964͒.
13 G. S. Hurst and M. G. Payne, Principles and Applications of Resonance the plasma plume in the form of heat. This sustains an LTE Ionization Spectroscopy ͑Hilger, Bristol, 1988͒.
plasma at a temperature favorable for the spectrochemical 14 R. M. Measures and P. G. Cardinal, Phys. Rev. A 23, 804 ͑1981͒.
15 L. St-Onge, M. Sabsabi, and P. Cielo, Spectrochim. Acta, Part B 53, 407
Our observations have important practical implications.
16 C. R. Phipps and R. W. Dreyfus, in Laser Ionization Mass Analysis, edited First, unlike photoionization, superelastic collisions thermal- by A. Vertes, R. Gijbels, and F. Adams ͑Wiley, New York, 1993͒.
ize the absorbed energy without directly generating more un- 17 The ionization fraction of potassium at Tϳ0.45 eV and n ϳ wanted free electrons. A resonant excitation scheme is also can be estimated from the Saha equation and is about 10%. That gives simpler to devise than resonant photoionization, as borne out ͓K͔ϳ1018 cmϪ3. Typical LIPS plume density is also known to be about1018 to 1019 cmϪ3.
by our 589 nm excitation of Na. An ultraviolet 330 nm 18 S. F. Wong, Honors thesis, Hong Kong Baptist University, 1998.
1ϩ1 scheme or less probable 660 nm 2ϩ1 transitions 19 Y. Iida, Spectrochim. Acta, Part B 45, 1353 ͑1990͒.
would be required to resonantly photoionize Na.13 Second, 20 Effects of ambient gas on RELIPS will be reported elsewhere.
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