Laser-induced plasmas and resonance ionisation spectroscopies

C. Grey Morgan

lndexing terms: Plasmas, Ionisation

Abstract: The mechanisms of laser-beam-induced ionisation of matter are briefly reviewed and dis- cussed in relation to applications in spectroscopic and analytical techniques. Non-selective laser- induced ionisation is described as an example of a rapid, real-time technique applicable in harsh environments. Highly selective resonance ionis- ation, which is much more sensitive, is described with accounts of recent investigations employing resonance ionisation mass spectroscopy.

I

1 introduction

Resonance ionisation spectroscopy is an exceptionally powerful, sensitive and versatile analytical technique which has its roots firmly embedded in studies of electri- cal discharges in gases. It is able to address a wide variety of fundamental and technical studies of atoms and mol- ecules and is especially attractive for elemental character- isation of materials at very low contamination content. It arose out of studies of electronic, atomic and photonic collisional processes and ionisation phenomena in gases. Its modus operandi and applications depend largely on gas discharge phenomena.

An extension of RIS, namely resonance ionisation mass spectroscopy (RIMS), equally depends on ionisation processes and the behaviour of charged particles in elec- tric or magnetic fields; phenomena which have exercised ionisation and plasma physicists for decades. RIS and its wide applications represent one of the most rapidly advancing analytical techniques now available, and, in the limit, are capable of detecting a single atom of selected element in the presence of an overwhelmingly larger number of other atoms of different species. This limit has already been achieved.

The discoverers of this technique, to whom due acknowledgment must be paid, are G.S. Hurst [l] of Oak Ridge National Laboratory, and V. Letokhov [Z, 31 of the Institute of Spectroscopy of the Russian Academy of Sciences at Troitsk, Moscow. They, at roughly the same time in the early to mid 1970s, realised the enormous potential of using tunable laser beams of sufficient intens- ity selectively to ionise a pre-determined atomic species and leave any other species present unchanged. The electron-ion pair created in this way may be separated, detected, counted and used as a marker of the species. This forms the basis of both laser isotope separation and ultra-trace materials analysis.

&? IEE, 1994 Paper 1018A (S3), received 6th January 1994 The author is with the Department of Physics, University College of Swansea, Singleton Park, Swansea, SA2 8PP, United Kingdom

I E E Proc.-Sci. Meas. Technol., Vol. 141, No. 2, March 1994

As the names RIS and RIMS suggest, ionisation is an essential part of the technique. In this case it is photoion- isation. Photoionisation of a target caused by the incid- ence of carefully tuned narrow-bandwidth laser radiation involving the absorption of two or more photons to create an electron-ion pair. At least one of the photons must have energy equal to that between two allowed atomic energy levels, i.e. there must be a resonant inter- action to ensure species selectivity.

Either the electron or ion (or both) may be detected. In the former case electron amplification by Townsend ava- lanches in a gas and the resulting current may be used to detect the selective ionisation as in proportional or G-M counters. In the latter case a mass spectrometer may be used to further enhance selectivity and identification, in which case the ion detection is usually by a channeltron. Because it is generally easier to collect charged particles than photons, RIS and RIMS are superior to methods of analysis such as laser-induced fluorescence, based on light detection.

RIS and RIMS are, in principle, delightfully simple but in practice considerable experimental and theoretical dif- ficulties need solutions for successful application. The apparatus needed is an ionisation chamber containing the sample to be analysed which must be converted by some means into the gas phase, a finely tunable laser of sufficient power and wavelength stability to produce saturated ionisation of all the atoms of the pre-selected element irradiated by the laser beam, and a method of detecting the resulting ionised components.

The coupling of a time-of-flight or magnetic sector mass spectrometer to the basic RIS apparatus is a logical step which provides the very powerful combination, RIMS, with the added stage of species selectivity needed for analytical purposes. Such a system is illustrated dia- grammatically in Fig. l [4].

A variety of techniques have been used to provide the gas phase needed for the RIS process. It should be noted here that although the gas phase is essential, this does not mean that RIS is limited to use only with gas samples. Any vaporised material may be studied and advantage taken of the various methods commonly used for atomisation in mass spectrometer ion sources.

It is worth recalling that atomic and molecular ionis- ation by laser photon irradiation alone is well known and used extensively for analytical purposes. Studies of laser-induced ionisation mechanisms and laser-produced gas discharges have been made since the advent of Q- switched lasers in the sixties; the process of multiphoton ionisation is well documented [ S , 61. In effect a laser radi- ation field of sufficient intensity, usually - l O I 3 w cm’, can very rapidly ionise any atom even though the ionis- ation potential Ei can be very much larger than the laser photon energy hv and when there are no resonances

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between hv and energies of allowed excited atomic states. Photon absorption takes place through a series of virtual states. A very high photon flux F - m-2 s-* is needed to give successive excitations through the virtual

I acquisition data It

time-of-flight mass spectrometer

laser s y s t e m s hv, + hv2

Fig. 1 Target sample to be analysed is atomised toform vapour plume The plume is irradiated by one or two laser beams and the preselected atomic species resonantly ionised by the absorption of two or more resonantly tuned laser photons. The ions are extracted from the electron-ion cloud and enter the mass spectrometer for detection and display

neutrals 0 background ions

states and so to the continuum and ionisation. In this way, using multi-megawatt pulsed lasers, ionisation and plasmas in gases are readily formed with electron and ion concentration -1016cm-3 in times of a few nano- seconds. The plasma so formed emits radiation character- istic of the gas constituents and this may be used for spectral analysis.

However, if a free electron is initially present in the illuminated volume then ionisation can proceed without multiphoton absorption at much lower intensities (- lo9 - 10" w cm-2) by the process of inverse brems- strahlung absorption, i.e. collision ionisation in which the free electron absorbs sufficient energy from the radiation field of the laser beam in making three-body interactions with photons and gas molecules to cause excitation and ionisation. To a first order of approximation the analyti- cal treatment of this case in which a free initiatory elec- tron is present in the focused laser beam is an extrapolation of microwave breakdown theory to optical frequencies.

As in the case of a plasma created by non-resonant multiphoton absorption the plasma formed by inverse bremsstrahlung absorption also emits radiation which may be used for analytical purposes, for characterising the plasma temperature and the occupancy of the various excited states taking into account the usual constraints of the need for local thermodynamic equilibrium. Fig. 2 shows the variety of information that can be gleaned from a laser-produced plasma. In practical analytical situations the laser plasma emission is often enhanced by a spark, RF or microwave discharge. These remarks apply equally well to liquids and solids and the emitted radiation or charged particles produced form the basis of commercially available laser-based analytic techniques known as LIBS (laser-induced breakdown spectroscopy), LMSA (laser microspectral analysis), LIMA (laser- induced ion mass analysis) and LAMMA (laser micro- probe mass analysis) [7-121.

At Swansea LIBS has been used for the analysis of nuclear reactor steels in which we examined quantitat-

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resonantly generated ions

ively the composition of standard specimens as a basis for assaying steel tie rods in a gas-cooled reactor. The work was specifically directed to producing working curves for the determination of chromium and molyb- denum content.

Qneutral particles A , A z . . . . . A n

charaed DarticIes A , A " ...., A , e

continuum radiation Bremsstrahlung

line emiss ion

target

Fig. 2 Emissionsfrom laser-produced plasma

The equipment is shown in Fig. 3 and typical results are illustrated in the case of Cr present at 0.05% in Fig. 4. These data were obtained in real time using 1200 flashes from the Nd : YAG laser at 1.06 nm in 60 s.

A limitation of LIBS, and indeed of all emission spec- troscopic techniques, stems from the possible coincidence of strong emission lines from two or more species of interest, and one is therefore forced to use much weaker non-coincident emission with loss of sensitivity.

Nevertheless, despite this inherent limitation LIBS is being used to advantage, in 'real world' situations, remotely, on-line in hostile environments to provide information of economic importance. As an example, Krupps [13] have successfully developed a LIBS system for the rapid assessment of carbon content of molten steel in an 80 ton argon oxygen decarbonisation convertor. LIBS spectra, obtained in less than 10 s, yields data which enables optimum melt control and tap-to-tap time reduction to optimise energy, gas, flux and refractory consumption. The system is clearly applicable to a wide variety of metallurgical situations.

The mechanisms of plasma formation by non-resonant multi-photon ionisation and ionisation by inverse bremsstrahlung absorption are rather brute force forms of photo-ionisation insofar as all the species of atoms in the intense laser beam are ionised. Although this is advantageous as it provides a rapid, in real time, some- times using only a single laser flash, complete composi- tional analysis of a micron-sized area of sample, nevertheless there is no sense of elemental selectivity. This limits the ultimate sensitivity attainable, in contrast to resonance ionisation. Such a species selective ionisation mechanism overcomes this deficiency.

The idea that tuning the laser into resonance with allowed optical atomic transitions could assist efficient ionisations and create plasmas at very much lower laser intensities is due to R.M. Measures [14], who showed, in detailed calculations for a potassium-seeded argon plasma, that it should be possible to sustain a steady state plasma with an electron and ion concentration of

at 3600 K by using only 15 W C ~ - ~ of resonant radiation. But no tunable laser of sufficient power was available then to test this experimentally. Fur- thermore if one did not start from the ground state of an alkali metal in vapour form then it could be possible to

IEE Proc.-Sci. Meas. Technol., Vol. 141, No. 2, March 1994

sustain a relatively cold, low density plasma (10l2 cm-') by a combination of associative ionisation and resonant illumination of only a few milliwatts focused to an inten- sity as low as lo4 Wcm-* in sharp contrast with the lo9 to lo'' Wcm-' needed to produce plasmas with non- resonant laser radiation.

These essentially resonantly-assisted methods for producing low temperature plasmas are somewhat select- ive, but of little use for material analysis purposes. Never- theless they demonstrated the importance of resonance absorption and the possibility of laser assisted collisional ionisation and plasma formation by single photon absorption.

Hurst and Letokhov and their colleagues however showed independently the unassisted laser ionisation of pre-selected atomic species using two or more photons (which are not necessarily of the same wavelength). Multiple photon absorption led to ionisation with relatively low laser beam energies and intensities when the photon energies were deliberately made to resonate with allowed atomic energy levels. The principle of direct multiphoton resonance ionisation was established.

The first experiment by Hurst et al. [15] employing the RIS principle was the photoionisation of metastable He (2's) by two-photon absorption via the intermediate He (3'P) state to produce He' and an electron which

was detected in an ionisation chamber. The population of singlet helium atoms was created in the helium gas in the chamber by protons from a 2 MeV Van de Graaff gener- ator. Only if the laser radiation was tuned to 501.5 nm, i.e. into resonance with the energy gap between 2's and 3'P levels were these singlet metastables ionised. At other wavelengths no such ionisation could take place due to the lack of resonant energy transfer from the laser beam to the singlet metastable atoms. This experiment estab- lished the RIS concept in practical form.

Hurst and his colleagues have perfected techniques, which, for the first time, can detect and identify a single free atom of any element with the current exception of ground state helium.

Although it has been possible to detect single electrons and unstable, i.e. radioactive, atoms since the invention of the proportional counter there was no assured and reli- able technique for single stable thermal-energy atoms until 1977 when Hurst and his colleagues [16] unam- biguously demonstrated the detection of single atoms of caesium. They achieved this notable advance by using a finely-tuned dye laser to photo-ionise a caesium atom by two-photon absorption and then used amplification of the electron so liberated by Townsend avalanches in a proportional counter to indicate its presence, and, by inference, that of the Cs ion and hence atom. In this

detector control ler

4

Fig. 3 Schematic diagram of LIES apparatus developed in Physics Department, University College ofswansea, for remote analysis

crucial experiment to demonstrate the extraordinary selectivity and sensitivity of the RIS technique they showed that it was possible to detect a single Cs atom even when there were l O I 9 atoms of argon and 10" mol- ecules of methane in the same laser beam. These gases

formed A further the counter spectacular gas. demonstration of the power of RIS for single atom detection which again involved Cs

-3.5 - 3 -2.5 - 2 -1.5 - 1 -0.5 0 was the application to the spontaneous fission decay of 252Cf in which a Cs atom is emitted. This experiment

LIBS generated working curve for chromium in steel proved that a single neutral daughter atom could be

- r;bL U 2 1 0

- 1-5

log [c(Cr)/c(Fe) ]

Fig. 4

I E E ProcSc i . Mens. Technol., Vol. 141, No. 2, March 1994 85

counted in temporal coincidence with the nuclear decay of its parent atom [17].

Fig. 5 illustrates the RIS principle in its simplest form for the case of two-photon absorption leading to ionisa- tion and simultaneously occurring competitive processes of spontaneous decay and collisions resulting in depopu- lating the intermediate state. The selected atom in its ground state (0) is excited to level (1) lying more than half-way up to the continuum by single resonant photon absorption from a flux of photons having energy hv equal to the energy difference between these states. If hv has some other value the absorption cross-section is negligi- ble and no excitation takes place and the atom remains in its ground state. If a second resonance photon is absorbed by the excited atom before it is de-excited by spontaneous decay at a rate A , , or by collision with gas atoms and decay to another lower level, say at a com- bined rate D, it may be ionised and hence detected. With an appropriately large photon flux the ground and excited states quasi-equilibrate because the rates of absorption and stimulated emission greatly exceed the combined photoionisation, spontaneous, and collision decay rates. Detailed consideration of the rate equations describing the populations in the levels as a function of the laser flux F , irradiation time T and photoionisation cross-section a of level (1) show that the number of ionised atoms n,(T) at time T is given by

aFn, nAT) = ~ (1 -exp[-(oF+D)g'T]}

(UF + D) where no is the number of ground state atoms illuminated and g1 = gl/(go + gl). where 9, and g1 are level degener- acies. So, to achieve saturated ionisation, i.e. to ionise all the atoms of the selected species in the irradiated volume, and make nAT) = n o , two conditions must be satisfied, namely

aF p D and

aFT 9 1

With typical values for a the corresponding laser energy requirement is FT > 100 mJ cm-' at the resonance wavelength. This is readily achievable with commercially available dye lasers.

For some other atoms it is necessary to use more com- plicated schemes which involve two or more synchronous tunable beams to excite the ground state atom to a first intermediate selected quantum state using a resonant photon hv, and then to a second higher allowed level by a resonant photon hv, from a second laser, and finally into the continuum by a photon from either. Excitation from a higher level upper resonant state can also be effected by other means such as a local applied electric field, auto-ionisation, radiation from a CW IR laser, etc. Fig. 5 illustrates some of these.

As an example of RIMS work undertaken at Swansea, conducted in collaboration with the Earth Science Department of Cambridge University, we have deter- mined the isotopic ratio of rhenium. Rhenium and osmium isotope ratios are of interest from a geo- chronological point of view. The decay of Re187 with a half-life of 4.35 x 10" years into its isobar Osis7 in rock samples may provide a geological 'clock' based on meas- urement of the isotopic content.

Rhenium has a relatively high ionisation potential of 7.9 eV so that its ionisation in conventional evaporation sources is very ineficient rendering it difficult to analyse

86

in conventional mass spectrometers. However, since RIMS requires ideally neutral atoms advantage may be taken of the fact that for every thermally produced ion roughly 5 x IO6 neutrals are formed giving a relatively abundant supply even from small samples.

a b C

Fig. 5 The energy hv, of first photon resonates with energy diKerence(E, - E,) between ground state and excited atomic state in atom to be detected - whose ionisation energy is E, U photoionisation by second absorption of photon hr, by resonantly excited atom will ionise it provided ( h v , + hv,) > E, h autoionisation occurs in which two electrons ace simultaneously excited such that their total energy exceeds E!. the atom spontaneously decays emitting one electron and forming the atomic ion c field ionisation in presence of moderately strong electric field in neighbourhood of highly excited (Rydberg state) atom is sufficient I O ionise it

Principles of resonance ionisation

Using a two-photon one colour resonance ionisation scheme (Fig. 5a) based on UV at just below 300 nm with a linewidth - 20 GHz from a frequency doubled tunable dye laser pumped by a frequency doubled flash lamp pumped Nd3+ YAG laser, efficient saturated ionisation of rhenium was readily observed. The strong RIMS signal free of background counts and the optogalvanic wavelength calibration are shown in Fig. 6.

194 steps

18 I

596- l6nm 596 55nm 330 steps 380 steps

8 2 0 0 -2

596- l6nm 596 55nm 330 steps 380 steps

8 2 0 0 -2 -6L- , Ne ,&

60 140 220 300 380 grating steps

Fig. 6 calibration using neon-buffered hollow cathode glow discharge

R I M S spectrum of rhenium and an optogalvanic wavelength

The 20GHz bandwidth of the laser photons meant that the ionisation probabilities of the two isotopes RlS5 and Re187 were equal and Fig. 7 shows the isotope separation and their relative abundance achieved by RIMS. The signals were obtained with only 4 x lo7 atoms/cm3 of the laser beam.

I E E Pmc.-Sci. Meas. Technol., Vol. 141. No. 2, March 1994

Isotope ratio evaluation using RIMS is not quite as simple and straightforward as this example suggests. This is especially the case when the isotopes have complex hyperfine structures and isotope shifts which can vary

61.6%

7.3 7.5 m a s s select volloge,V

Fig. 7

from one isotope to another. The spectral characteristics of the laser, its bandwidth, mode structure and stability are other factors which influence the evaluation because the measured quantities (the relative amounts of ionisation) are governed by the convolution of the laser spectral profile with that of the atoms. Even if there is no hyperfine structure in the resonantly excited levels and when the laser bandwidth i s much bigger than the isotope shift, significant variation in measured isotope ratios with laser wavelength i s to be expected, and has been demonstrated for L u " ~ / L u " ~ in work at Los Alamos [18].

These effects have also been observed for the Pbzo6, PbZo7 and Pbzo8 isotopes in a collaborative research pro- gramme [19] involving the Swansea group, the Atomic Weapons Establishment, Aldermaston, and the Moscow Institute of Physics and Technology. The results are shown in Fig. 8.

2

Conventional high power Nd" YAG lasers used to pump tunable dye lasers operate typically at repetition rates not much in excess of 20 -30 Hz. The duty cycle and data-gathering rates are therefore rather poor and ineffi- cient, especially when used in conjunction with thermal atomisation. Gas discharge pumped lasers and atom- isation by ion bombardment can greatly improve the situation. For example, excimer lasers can operate at up to 500 Hz while copper vapour lasers are capable of operating at several kilohertz [20] a fast start-up, high efficiency, 100 W copper-bromide laser capable of 20 KHz repetition rate has recently been reported [21], which if used successfully to pump tunable liquid or solid state lasers will greatly enhance efficiency and reduce observation time.

We have taken advantage of the high repetition rates achievable with gas discharge lasers in a programme of strategic research and development of a user-friendly turn-key RIMS instrument in collaboration with Fisons Instrumentation plc, the Clarendon Laboratory and Oxford Lasers. This has been supported by the UK Science and Engineering Research Council and the Department of Trade and Industry, and we have assembled an instrument for quasi-routine industrial materials analysis at ultra-trace level. Initially it is used primarily for examination and assaying of semiconductor materials and of assaying biological samples.

Atomisation of the sample in an ionisation chamber is achieved by pulsed 10 KeV, 100 nA Ar+ ion bombard- ment from a duo-plasmatron source driven in synchron- ism (with an appropriate advance in time scale -p),

I E E Proc.-Sri. Mens. Technol., Vol. I J I , No. 2, March 1994

Isotope ratio of Re"'""*' determined using RIMS

Gas discharge aspects and applications in RIMS

with a tunable dye laser using frequency doubling and tripling crystals, pumped at 6 KHz by a 40 W copper vapour discharge laser. The resonantly produced ions are accelerated from the ionisation zone between pulses into

0.1 450.312 450.314 450.316 L50.318

o

" Y

450.312 150.311 450.316 450.318 wavelength ,nm

C

Fig. 8 wavelength a Pb206i208 h Pb2071208 c Pb 2061207

Variation of measured isotope ratios of lead as function of laser

experimental points theoretical isotope variation error range

~~~~

. . . . . . . solid horizontal lines are natural abundance

a reflectron type time-of-flight mass spectrometer for further identification, isotope separation and detection.

The ion beam atomisation scheme also creates second- ary ions characteristic of the analyte so the instrument also has a SIMS (secondary ionisation mass spectrometry) capability enabling direct comparison of RIMS and SIMS spectra to be made. The analyte can be viewed and displayed using a vidicon-TV system. The whole instrument is computer controlled for ease of use (Fig. 9).

3 Future developments

RIS and RlMS are not confined to atoms though gener- ally application to molecules is rather more difficult on account of their broad band rotational and vibrational energy levels. Nevertheless significant advances in studies of molecular complexes have been brought about by studies of resonance enhanced multiphoton ionisation, nor are they confined to high power pulsed lasers; the use of CW tunable lasers offers significant advantages in improved duty cycles, and narrower linewidth than can be obtained with pulsed lasers. Again the advances in solid state diode arrays as pump lasers for solid state tunable lasers such as titanium sapphire offer the possi-

87

bility of compact holosteric systems and miniaturisation which could not have been foreseen even a few years ago.

There is no doubt that RIS and RIMS, growing out of studies of gas discharge phenomena, will continue to make major and exciting advances in many fields.

Fig. 9 meter driven by copper-vapour-pumped dye laser at Swansea

Resonance ionisation and secondary ionisation mass spectro-

4 References

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resonance ionization spectroscopy’ (IoP Publishing, U.K., 1988)

U.K., 1986)

(Academic Press, USA, 1987)

4 GREY MORGAN, C., and TELLE, H.H.: ‘Resonance ionisation spectroscopy’, Physics World, 1992,s. (IZ), p. 28

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7 DINGLE, T., GRIFFITHS, B.W., and RUCKMAN, I C . : ‘A novel laser induced ion mass analyser’, Vacuum. 1981,31, p. 571

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12 GREY MORGAN, C.: ‘Laser-based analysis of steels’, Steel World, 1993, 9, p. 73

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14 MEASURES, R.M.: ‘Electron density and temperature elevation of a potassium seeded plasma by laser resonance pumping’, J. Quant. Spec. Rad. Trans., 1970, 10, p. 107

15 HURST, G.S., JUDISH, J.P., NAYFEH, M.H., PARKS, J.E., PAYNE, M.G., and WGNER. E.B.: ’He metastable atom concen- tration’. Proc. 3rd Conf. Applications of Small Accelerators, CONF. 741040, Vol. I, pp. 97-119, Springfeld, Virginia. Nat. Tech. Inf, Service.

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17 KRAMER, S.D., BEMIS, C.E. Jr., YOUNG, J.P., and HURST, G.S.: ‘One atom detection in coincidence with nuclear decay’, Opt. Lett., 3, p. 16

18 MILLER, C.M., FEAREY, B.L., PALMER, B.A., and NOGAR, N.S.: ‘High-fidelity in isotope ratio measurements by resonance ion- isation mass spectrometry’. IoP Conf. Series No. 94, 297, Adam Hilger, Bristol, 1989

19 TELLE, H.H., TELYATNIKOV, A.L., MCNAGHTEN, E., BROWN, R.A., and McCORMICK, A.: ‘Isotope ratio measure- ments in lead using three-photon one-colour resonance ionisation mass spectrometry’, Rapid Comm. Mass Spect., 1993, 7, p. 524

20 WEBB, C.E.: ‘Copper vapour laser pumping of dye lasers in atomic and nuclear research’. l o p Conf. Series No. 114, 365, Adam Hilger, Bristol, 1990

21 JONES, D.R., MAITLAND, A., and LITTLE, C.: ‘A copper hybrid laser producing 149 watts at 2.4% efliciency’. Proc. CLEO Conf., 1992

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