Dale L. Perry

Zinc telluride films doped with gadolinium (ZnTe:Gd)-made by laser ablation and deposition-have been characterized by X-ray photoelectron spectroscopy (XPS) to determine the molecular species of the elements in the material and their presence as intentionally formed contaminants. The chemical shifts and the shapes of the zinc, gadolinium, and tellurium photoelectron lines have been studied and compared to those of appropriate model compounds that represent potential contaminants occluded in the final product that forms on the subsequent deposition of the laser-ablated plume–derived material.

	Zinc telluride is a semiconductor with well-characterized properties (1–3) and uses such as optoelectronic devices (4), thin film solar materials (5), and other applications. In conjunction with cadmium to form cadmium zinc telluride (CZT), it is valuable for X-ray and gamma–ray detections materials (6) and electronic films and junctions (7). The use of semiconductors in different types of nuclear radiation detection instrumentation necessitates that some area of its surface must be used in conjunction with electrical contacts. Buildup of oxides and oxy-telluro-species on the surfaces of tellurides, for example, can cause insulating effects, modifying the material's performance as a semiconductor for the intended applications, while the bulk materials themselves may possess very small contaminant molecular inclusions that are not intentionally present. These inclusions are introduced into the material matrix during the high-temperature growth of growing bulk crystals.

	The current study is a characterization of one material in this class using X-ray photoelectron spectroscopy (XPS) to profile possible extraneously formed gadolinium, zinc, and tellurium molecules that can be observed both as possible dopants in the material by formation at high temperatures and the formation of surface species because of atmospheric oxidation resulting from handling in the open air. The zinc telluride doped with gadolinium (ZnTe:Gd) studied here was designed and fabricated as a material with proven molecular semiconductor properties that are suitable for model semiconductor radiation detection materials. The samples here additionally incorporate gadolinium, which possesses a large neutron capture cross-section that is useful for neutron-related detection technology.

	XPS (8,9) is ideal for such studies since, being surface-sensitive and relatively nondestructive, the technique can yield information about both the parent materials themselves in a highly pure, bulk state and any products that form on the surface. The photoemission of electrons, using soft X-rays (typically, Al Kα [1486.6 eV] or Mg Kα [1253.6 eV] for most commercial instruments), is conducted under ultrahigh vacuum (UHV) conditions. Figure 1 shows a typical survey XPS spectrum (0–1400 eV) for elemental copper that reflects the impingement of Al Kα (1486.6 eV) X-rays on the surface of the sample, followed by the emission of near-surface electrons of different kinetic energy from the electronic levels. The accompanying photoelectron lines, converted to binding energies (essentially, ionization energies) shown on the 

-axis, correspond to the different levels of copper. By studying the various lines at different binding energies, the spectroscopist can assign them to different elements contained in a sample, along with-under optimum conditions-oxidation states, magnetic states, and electronic spin (diamagnetic versus paramagnetic) states. By using very good standards, one can extract semiquantitative data for elements in a sample in addition to the qualitative existence of each element.


	Figure 1: Basic conditions and processes for experimental X-ray photoelectron spectroscopy. (Adapted from original drawing by B. Vincent Crist.)

	ZnTe:Gd materials (Gd = 10%) were made by pulsed laser deposition (PLD) as previously described (10,11) using appropriate targets and depositing the desired material on a quartz substrate.

	Spectra were obtained using a Physical Electronics Model 5400 photoelectron spectrometer (equipped with an Al Kα [1486.6 eV] anode) with the samples undergoing no pretreatment after fabrication and then being mounted in a holder. All samples were handled openly in air during all experimental operations, with the intentional purpose of introducing atmospheric related surface species. Surface charge compensation and spectral calibration were achieved by assigning a binding energy of 284.8 eV to the adventitious carbon 1

	Zinc telluride and telluride compounds have been extensively characterized by XPS to determine the chemical states and concomitant shifts of the elements present in the materials. Additionally, chemical shifts of gadolinium (12–14), zinc (15–17), and tellurium (18–20) lines in various compounds have previously been reported in the literature. However, a characterization of ZnTe:Gd, fabricated in the manner here-using pulsed laser ablation and deposition (PLD)-by XPS has not been previously reported. The ZnTe:Gd material can be seen as an experimental model to analyze for possible contaminant compounds in radiation detection semiconductors that have been discussed previously (21,22).

	The PLD synthetic approach has several advantages for making molecular defect model materials for semiconductors and related materials useful in nuclear radiation detection materials. It is an energetic technique that vaporizes initial reactant materials to temperatures of several thousand degrees, followed by cooling and condensation of the vapor species, making it a method by which all possible chemical reactions that can occur in a vapor phase will do so. When the process is conducted in air such as here, atmospheric reactants such as CO

O may enter the mechanism of formation of the initial products observed. Secondary reactions such as chemisorption of CO

O with the cooled products may form simple adsorbates or actual molecular surface products that model the handling of the materials in air. Surface products such as these may have effects related to the radiation detection material-electrical contact interface.

X-Ray Pholoelectron Lines of Carbon and Oxygen 

 region contains the adventitious carbon line at 284.8 eV and a line at 289.2 eV (Table I). The 289.2 line is in good agreement with that of CO

 from the atmosphere, either as an adsorbate or as a carbonate (23,24). There is no evidence of any appreciable carbide species, whose carbon 1

 line is considerably lower than the 284.8 eV adventitious carbon line used as the surface charge calibrant. Of course, other products not identified or discussed here might exist at such low levels, they are essentially contained in the baseline. Zinc and gadolinium carbonates are distinct, molecular species that can combine with other atmospheric molecules to form new phases. Gadolinium carbonate hydrates (25) and hydroxy zinc carbonate (mineralogically, hydrozincite) (26) are excellent examples of secondary reaction products. Thus, the carbonate, hydroxy, and water attributed oxygen lines in Table I may also contain slight contributions from other formed carbonate–water–hydroxy complexes that cannot be completely detected with, for example, the carbonate functionality being generically assigned because of photoelectron line characteristics such as the carbon 1

 line discussed above. Multiple hydroxide species also are difficult to differentiate, since even simple, molecular hydroxides fall in a very small range of binding energies (27).

 spectrum consists of a quartet of peaks (Table I). The presence of multiple oxygen species is not surprising in light of both the fabrication technique of the samples and their casual handling in ambient, moist air. The lines at 529.6 and 530.4 eV are representative of ZnO and TeO

, while the peaks at 531.2 and 531.9 are indicative of a water-produced dissociated OH

 core-levels in ZnTe:Gd occur at 1187.6 and 1218.6 eV and 1186.8 and 1217.8 eV, with a corresponding spin-orbit splitting of 31.0 eV (Figure 2). Data from the zinc and tellurium lines strongly suggest that gadolinium is only present in the forms of Gd

 peaks are very broad, with a full width at half maximum (FWHM) of 4.8 eV when fit to two chemical states. While the broadness of the lines may indicate an insulating, charging surface, it is in agreement with previous studies that have observed FWHMs of pure gadolinium compounds of 4.4 and 5.3 eV (28,29). Other formed species such as those discussed above are not conducting and also might contribute to an insulation-based broadness, including multiple, simple adsorbed CO

O to form weak complexes. The presence of a satellite structure near 1196 eV is also observed here and has been previously recorded and explained in detail (29).


	Figure 2: X-ray photoemission lines of gadolinium 3d

 emission lines with a splitting of 23.1 eV as observed in previous studies (15–17). Curve fitting indicates that zinc is present in only two chemical states-ZnTe and ZnO, with two lines with values of 1022.0 and 1021.3 eV, respectively, for the 2

 energy level representing them. Both ZnTe and ZnO are semiconductors (30) rather than insulators and should not appreciably contribute to the observed line broadening of the spectral peaks studied here. They are very stable binary Group 16 compounds of zinc(II), the only stable oxidation state of zinc.


	Figure 3: X-ray photoemission lines for the zinc 2p

 level and their assignments for molecular components in ZnTe:Gd films.

 photoemission lines of Te are illustrated in Figure 4. The 3

 peaks are separated by 10.4 eV. This spin-orbit splitting is in agreement with previously reported numbers for GdTe and other tellurium compounds (18–20). Curve fitting to both of the 3

 level peaks shows that tellurium is present in three chemical states-ZnTe, Gd

 energy levels of 573.0, 572.4, and 575.9 eV, respectively. Again, these energy level values correspond to previous studies on tellurium compounds (18–20). The presence of TeO

, the most stable oxide of tellurium, on the surface might be important for the indicated insulating effects previously discussed in this paper, along with the possibility of oxyanion species such as tellurites and tellurates. The binding energies for these latter two species, however, occur at higher values (20) than any observed in the present studies and would have to exist in minute amounts in the present materials.


	Figure 4: X-ray photoemission lines for the tellurium 3d

	X-ray photoelectron spectroscopy is an effective method for detecting different chemical states and molecular components of solid samples such as films. By use of an analysis of the spectra of the various elements in the sample, one can detect different functional species and groups identified with that element. The films that have condensed from very high temperature vapors exhibit discrete, molecular contaminant species. Further, the technique indicates that the films studied here contain species that are consistent with those observed in much larger bulk samples made by other synthetic routes driven by high temperature effects. The existence of hydroxide and carbonate functional groups, along with water, on the surface are most likely because of the samples interacting with the open atmosphere after initial formation, since all are unstable in the extreme temperatures produced in the synthetic laser plume.

	This work was supported by the U. S. Department of Energy under Contract No. DE-AC0205CH11231 and the Center for Science and Engineering Education (CSEE) at Lawrence Berkeley National Laboratory. The authors wish to thank Beverly Wilgus for her efforts with the artwork related to Figure 1.

	(1) M. Cornet, J. Degert, E. Abraham, and E. Freysz, 

	(2) M. Bazarganipour and M. Salavati-Niasari, 

	(6) T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Brunett, L. Franks, and R.B. James, 

	(7) T.L. Chu, S.S. Chu, C. Ferekides, and J. Britt, 

X-ray Photoelectron Spectroscopy: An Introduction to Principles and Practices 

Applications of Analytical Techniques to the Characterization of Materials 

, D.L. Perry, Ed. (Plenum Press, 1991), pp. 1–23.

	(10) D.L. Perry, A.C. Thompson, R.E. Russo, X.-L. Mao, and K. Chapman, 

	(11) D.L. Perry, X. Mao, and R.E. Russo, 

	(13) Y.-li Li, N.-fu Chen, Jian-ping Zhou, S.-lin Song, L.-feng Liu, Z.-gang, Yin, and C.-lin Cai, 

	(14) B.A. Orlowski, S. Mickievicius, V. Osinniy, A.J. Nadolny, B. Taliashvili, P. Driazwa, T. Story, R. Medicherla, and W. Drube, 

	(15) J.-C. Dupin, D. Gonbeau, P. Vinatier, and A. Levasseur, 

	(19) W.E. Swartz, Jr., K.J. Wynne, and D.M. Hercules,

	(20) M.K. Bahl, R.L. Watson, and K.J. Irgolic, 

	(23) S.L. Wu, Z.D. Cui, F. He, Z.Q. Bai, S.L. Zhu, and X J. Yang, 

	(30) M. Afzaal, M.A. Malik, and P. O'Brien, 

 is a senior scientist in chemistry with the Lawrence Berkeley National Laboratory, University of California, in Berkeley, California. 

 is with PPG Industries in Cheswick, Pennsylvania. 

 is an associate consultant with ZS Associates in San Mateo, California. 

 is an applications engineer with Baker Hughes in Concord, California. Direct correspondence to: 

Articles

Zinc telluride films doped with gadolinium (ZnTe:Gd)-made by laser ablation and deposition-have been characterized by X-ray photoelectron spectroscopy (XPS) to determine the molecular species of the elements in the material and their presence as intentionally formed contaminants.

Molecular Characterization of Gadolinium-Doped Zinc Telluride Films by X-ray Photoelectron Spectroscopy

Laser ablation molecular isotopic spectrometry (LAMIS) involves measuring isotope-resolved molecular emission. Measurements of several key isotopes (hydrogen, boron, carbon, nitrogen, oxygen, and chlorine) in laser ablation plumes were demonstrated. Requirements for spectral resolution of the optical detection system could be significantly relaxed when the isotopic ratio was determined using chemometric regression models. Multiple applications of LAMIS are anticipated in the nuclear power industry, medical diagnostics and therapies, forensics, carbon sequestration, and agronomy studies.

Laser ablation molecular isotopic spectrometry (LAMIS) is a technique that uses optical spectra of transient molecular species produced in ablation plumes in air or buffer gases (Ar, Ne, He, and N

) for rapid isotopic analysis of solid samples (1–8). This technique is similar to laser-induced breakdown spectroscopy (LIBS), but optical emission spectra in LAMIS are measured at longer delays after an ablation pulse than what is used in LIBS. Molecular emissions yield relatively easily detectable isotopic information. Therefore, LAMIS adds a supplementary function of isotopic measurements to the well-established benefits of LIBS (real-time elemental analysis at atmospheric pressure, minimal sample preparation, chemical mapping and depth profiling at high spatial definition, and laboratory and field operation possible at a standoff distance to the sample). The LIBS and LAMIS techniques can be accomplished on the same instrument. 

Molecular radicals are generated effectively when the ablation plume cools down, resulting in an increase of the molecular emission in the plasma afterglow. Several mechanisms contribute to the formation of molecules in the plume such as radiative association of neutral atoms, associative excitation or ionization, recombination of molecular ions, fragmentation of polyatomic clusters or nanoparticles, and vaporization of intact molecules from the ablated surface. Molecular spectra are advantageous for isotopic analysis because the isotopic shifts in molecular emission are considerably larger than in atomic spectra. The difference in isotopic masses has only a small effect on electronic transitions in atoms, but significantly affects the vibrational and rotational energy levels in molecules (1). A compact spectrometer can be sufficient to resolve molecular isotopic spectra; therefore LAMIS measurements can be performed in the field (4,5). The ability to measure isotope abundance using a portable spectrometer with modest spectral resolution is a significant merit of LAMIS, along with no sample preparation and data collection at ambient pressure. 

The qualitative and quantitative studies of isotopes of the light elements are both very important for their use in many fields and for their very extensive scope, with many applications in different fields of their chemistries and materials science. This article describes measurements of isotopes of hydrogen, boron, carbon, nitrogen, oxygen, and chlorine. All of these elements are ubiquitous in nature as well as in practice, with carbon being the basis of many millions of compounds. Variation of the natural isotopic ratio 

C in different materials ranges from 0.96% to 1.15% (9). These variations can be measured using LAMIS. Fractionation in stable isotopes of C, N, and O can be particularly indicative of a range of diverse biotic and abiotic processes relevant to the ecology, biosphere, and geochemistry, both organic (fossil) and inorganic. Fertilizers enriched typically above 40% in 

N are broadly used to track the efficiency of plant uptakes, fertilizer losses, and nitrogen turnover in soil.  

B isotope led to the development of multiple boron-loaded materials for neutron shielding in nuclear reactors and spent fuel storage pools, as well as for screens and curtains in nuclear medicine centers. Enrichment from 50% up to 99% in 

B is often used. Many neutron detection devices are also based on highly borated (

H) scintillator materials. Heavy water nuclear reactors require deuterium enrichment to 99.85%. Water-

O is used as a precursor in the radiopharmaceutical industry. Deuterated drugs and stable isotopic markers are increasingly used in the fields of medicine, pharmacology, nutrition, and physiology for tracing biochemical processes. Medical multinuclear magnetic resonance imaging (MRI) scanners require compounds enriched in nuclear magnetic resonance (NMR)-active isotopes (

O). To assess quality, specifications, or aging of these materials, their isotopic homogeneity or distribution, and  degree of isotopic enrichment can be rapidly tested by LAMIS in open air or inert gas flush without using laborious and expensive mass spectrometry (MS). The feasibility of standoff LAMIS analysis is particularly important for the nuclear industry.  

Natural geochemical heterogeneity in isotopic composition reflects the complex history of our planet. Isotopic ratios of B, C, O, and Cl are particularly variable in nature because of chemical reactivity, high solubility, and volatility associated with the compounds consisting of these elements. Boron and chlorine isotopic fractionation is used to trace the records of evolution and weathering reactions because of interactions of rocks, soil, and sediment materials with water. Biomediated alteration of the 

Cl ratio is distinctive in microbial reduction of anthropogenic perchlorates, biphenyls, halocarbons, and other chlorinated compounds. Simultaneous measurement of multiple stable isotopes is also increasingly used to constrain the carbon cycling in ecosystems better. LAMIS can facilitate the isotopic analysis directly in the field. Numerous applications of LAMIS are anticipated in the nuclear power industry, medical diagnostics and therapies, forensics, carbon sequestration, and agronomy studies.  

A flash-lamp pumped Nd:YAG laser operating at 1064 nm with a pulse energy of 50–150 mJ and a pulse duration of 4 ns was used to ablate the samples. The laser beam was focused onto the sample with a fused-silica lens to a spot diameter of ~100 μm, either in open air or in a cylindrical quartz chamber flushed with inert gas. Inert gas flow removed ambient air when the rare isotopes of nitrogen and oxygen were to be measured. A second lens was used to collect the laser-induced plasma emission in the direction perpendicular to the ablating laser beam. Two spectrometric configurations were interchangeably used. In one of them, collected light was focused onto a fiber-optic cable coupled to a compact echelle spectrograph (EMU-65, Catalina Scientific) fitted with an electron multiplying charge-coupled device (EMCCD). In the other configuration, collected light was focused onto the entrance slit of a Czerny-Turner spectrograph fitted with an intensified charge-coupled device (ICCD). The spectrum acquisition gate widths and delays were optimized to detect molecular emission, while minimizing intensity of atomic and continuum radiation. The collected spectra were averaged over several tens or hundreds of laser shots (see further text and notes in the figure captions). The spectral averaging, background correction, normalization, and final chemometric regression analysis of spectral data were performed using proprietary software designed at Applied Spectra for multivariate calibration and quantification of LIBS spectra. 

Substances with known isotopic content were obtained from commercial sources in the form of powder, then mixed in appropriate proportions and pressed by a 7-ton press into 1-cm-diameter pellets. Isotopically enriched water was frozen into ice tables, which were kept cold by a Peltier thermoelectric plate during ablation for the LAMIS analysis. Samples of borated aluminum materials were provided by Ceradyne, Inc. 

Our previous measurements of boron isotopes 

B were performed using three chemically pure substances, isotope-enriched 

, and natural BN (2,7). The results indicated that LAMIS can be calibrated using multivariate partial least squares regression (PLSR) for quantitative measurements of isotopic ratio with high precision represented by relative standard deviation of only ±0.45% (2σ = 9‰), and this can be further improved (7). Abundances of individual isotopes in material composition can be measured at least down to ~1% (

N can be detected in subnatural concentrations, below 0.3%). High spectral resolution was found unnecessary for isotopic determination (2). However, an ambiguity remained about whether similar results could be obtained on practical samples with possible spectral interferences from other chemical elements.  

The pertinence of LAMIS measurements to industrial applications was tested on neutron absorber materials. Two samples of aluminum alloyed composites with different boron isotope ratios were ablated and analyzed using a 320-mm IsoPlane spectrograph fitted with a PI-MAX4 ICCD detector (Acton/Princeton Instruments). One of these samples was a Boral core (Ceradyne) that consolidated aluminum alloy with 56% boron carbide of natural boron isotope abundance. The other sample was BorAluminum (Ceradyne), a regular unalloyed aluminum (grade 1100) with 4.5% added elemental boron, enriched in the lighter isotope 

B up to 95%. The ablation spectra of these samples recorded in the 591–608 nm spectral interval are presented in Figure 1 (continuum background subtracted). They are different because the isotopic composition is different. These spectra belong to moderately resolved rotational lines of boron monoxide in the vibrational band (0

  emission system. The spectra were acquired using an 1800-grooves/mm (gr/mm) grating with the ICCD delay of 10 μs and a gate width of 100 μs. Spectral intensity was accumulated over 1000 laser pulses (analysis time 100 s at 10 Hz).  

Figure 1: Isotope-specific molecular spectra of BO (AâX; 0-3) emission formed during ablation of two aluminum-based composite materials containing different 11B/10B ratios. The upper spectrum (BorAluminum) is shifted up for clarity. Lower spectrum: Boral.			

  emission was used at both the (0-2) band (4,7) and the (0-3) band (2,7,10). A choice of the spectral interval was driven by quality of the spectra for multivariate quantification with different chemometric procedures. The data in Figure 1 illustrate a significant isotopic shift between spectra of Boral (80% 

B), requiring only modest spectral resolution. Another important factor is that this spectral interval does not have any visible interferences from atomic aluminum or chemical impurities in the samples. When the ablation spectrum was recorded at the usual acquisition delays for LIBS (<1 μs), several spectral lines of Fe, Ti, and Mn atoms coming from impurities in borated aluminum were observed within this range. Provided that the interfering lines are sparse, they do not necessarily preclude the isotopic analyses. Thus, time-gated attenuation of atomic emission is deemed essential for LAMIS measurements in practical analyses.  

A possibility of LAMIS applications for geological analysis was examined using two tourmaline samples of different color (greenish "blue grass" and black minerals). The regular LIBS spectra of these samples indicated that a spectral region from 572 to 585 nm had the least amount of atomic interferences. Aimed at further decreasing atomic emission, the acquisition gate delay was set at 3 μs for LAMIS measurements. However, two spectral lines of atomic iron were persistent in the spectrum of one of the tourmaline samples. The positions of these interfering lines could be masked from the spectrum to determine the isotopic composition using multivariate PLSR described in the literature (2,7). Further quantitative work is needed to determine the limits of isotopic precision and sensitivity in LAMIS of geological samples. However, measuring boron isotopes in real geological samples using LAMIS has already been established as a credible prospect. Our previously attained precision of 9‰ (7) is reasonably adequate because natural variation of the 

B ratio in the terrestrial environment is more than 90‰ (9). 

B content by LAMIS will be useful in other applications. For instance, the 

B isotope is added to semiconductor grade silicon as a doping agent that must be controlled. LAMIS can monitor nuclear facilities remotely, delivering a laser beam through a small window and collecting optical emission either by a telescope or an optical fiber cable.  

Carbon and nitrogen isotopes can be determined simultaneously using spectra of CN molecules in LAMIS measurements. This possibility was briefly mentioned in our earlier publications (1,4,5). The carbon isotope 

C is the most important one for studying biochemistry, but measuring only one isotope is often insufficient. Several key isotopes can follow the biological processes better. To demonstrate simultaneous isotopic measurements, we ablated natural graphite and several solid substances enriched in 

N. The spectra presented in Figure 2 include the B

  vibrational band progression: (0-1), (1-2), (2-3), (3-4) of the CN radical. The rotational structure of these bands is barely resolved in our measurements, but isotopic shifts between isotopologues of 

N are clearly observed. Two pellets of isotope-enriched benzamide with 

N atomic fractions 14% and 99%, respectively, were used as ablation targets. The latter was ablated in a helium flow to preclude nitrogen entrainment from air. Another sample was 99% enriched with 

C amorphous carbon. The spectra were acquired using a 10-μs timing gate delayed after the laser pulse only by 1.5 μs because CN molecules are promptly formed in laser ablation.  

Figure 2: Emission spectra of CN vibrational band progression (BâX; ÎÎ½ = +1) formed during ablation of benzamide pellets enriched in 13C and 15N isotopes. Spectra were averaged over 100 laser pulses.			

Isotopic shifts in the bands with changing vibrational quantum number Δν = +1 shown in Figure 2 are all larger than 0.3 nm and are easily resolvable using a compact optical spectrograph. These shifts are about 10 times greater than the isotopic shift in the (0-0) band head of the same CN B

  transition at ~388 nm (1). The opposite change in a vibrational quantum number Δν = –1 produces another CN band progression around 359 nm (Figure 3). The isotopic differences in these bands are well resolved, but their rotational structure is not. An advantage of using the bands (1-0), (2-1), and (3-2) near 359 nm shown in Figure 3 compared to those shown in Figure 2 is that the spectral peaks with Δν = –1 attributed to the minor isotopes 

N occur on the other side of and apart from the sharp edge of the 

N (1-0) band head. This is important for quantitative calibration. The progression with Δν = +1 requires multivariate calibration because the bands from minor isotopes can be hidden within unresolved rotational lines of the major 

N bands. In contrast, spectral data of the progression with Δν = –1 can be quantified using either univariate or multivariate calibration.  

Figure 3: CN vibrational bands (BâX; ÎÎ½ = â1) formed during ablation of natural graphite and enriched benzamide pellets. Spectra were averaged over 100 laser pulses.			

The results of calibrating LAMIS spectra of a CN molecule using a multivariate PLSR procedure for quantitative determination of 

C content in solid organic samples are presented in Figure 4. Artificial reference samples were prepared by physically blending powders of natural and 

C atomic fraction was assumed as 1.07%, and an additional five blended samples were made nominally at 1.2%, 1.5%, 2.0%, 5.0%, and 9.9%. The CN B→X (Δν = –1) spectra were collected in the interval 353–362 nm, similar to those shown Figure 3. The error bars in Figure 4 correspond to standard deviation of a single measurement within 10 replicates, while each measurement was made by accumulating spectra from 100 laser pulses. The natural 

 C atomic fraction was recalculated from these measurements as 1.09 ± 0.14%. Averaging over 10 measurements (each one made of 100-pulse accumulations) reduces a random error of the average down to ±0.044%. It is obvious from data presented in Figure 4 that LAMIS can measure subnatural abundances of 

C with sufficient precision. However, the decanoic acid powders used to prepare our reference samples consisted of microcrystals and it was difficult to fully homogenize a blended material. Better homogenized standards will improve precision and accuracy of LAMIS measurements.  

Figure 4: Calibration plot with a linear fit and 95% confidence interval for the 13C isotopic content calculated using the PLSR model.			

We tested LAMIS in analyzing the soil samples, in which the total carbon content was 12.26%, and the nitrogen content was 0.27%. For initial experiments, we spiked this soil with 

C-enriched decanoic acid in different proportions. Several spectral lines of iron appear as interferences in the region of CN B→X Δν = –1) bands, but iron lines were well resolved from carbon features. An algorithm that automatically fits a Lorentzian profile to every interfering spectral line and digitally subtracts them from the spectrum can be applied as a remedy. Such a procedure can work relatively well if the interfering lines are no more than three times stronger than the main spectrum.  

A similar multivariate PLSR calibration was performed for measurements of the 

N isotope using reference samples prepared by blending powders of natural and 

N atomic fractions in prepared reference samples were 0.37%, 1.0%, 2.0%, 5.0%, 10%, 20%, and 99%. In these experiments, the CN B→X (Δν = +1) spectra were collected within the range 414–422 nm, similar to those shown Figure 2. The results are displayed in Figure 5 with the error bars corresponding to standard deviation of a single measurement, each made by signal accumulation from 100 laser pulses. Again, quantification precision and accuracy were limited by microscale isotopic inhomogeneity of the reference samples that were prepared. Inaccuracies in preparing these reference mixtures to be equal to the nominal isotopic content also increased the calibration errors.  

Figure 5: Calibration plot with a linear fit and 95% confidence interval for the 15N isotopic content calculated using the PLSR model.			

During laser ablation of carbon-containing samples, both CN and C

 radicals are often generated in comparable quantities. Hence, both of these species can be used for measuring the carbon isotopic ratio. Utilization of diatomic carbon molecules C

 for LAMIS measurements was described previously (1,5,8). Rapid spatial and temporal evolution of the plasma plumes during laser ablation may cause kinetic fractionation effects altering stoichiometric and isotopic equilibrium between the plasma and the sample. Kinetic isotope fractionation was indeed observed at early stages of plume evolution (gate delays <2 μs) in C

 emission (8). At longer delays, the measured isotopic ratio was close to the real ratio in the sample. These results suggest that carbon isotopes 

C can behave differently in laser ablation plumes, and the change in ratio is related to the time and spatial distance above the sample. For the purpose of isotopic analysis, it is important that the isotopic ratio measured at longer delays (>2 μs) equilibrates and approaches the real ratio in the sample roughly at the same time when molecular emission is also reaching its maximum. 

To demonstrate LAMIS for hydrogen and oxygen isotopic measurements, we ablated ordinary water ice (H

O. As in earlier work (1,6), we recorded emission of hydroxyl radicals at the A

 (0-0) transition in the ablation plumes. The major features of OH emission are R

 branch head at ~309 nm. These rotational branch heads are suitable for the D/H ratio determination because the isotopic shifts between OH and OD features are large and can be measured at low resolution. However, isotopic shifts between 

OH are smaller, especially for the lines with low rotational quantum numbers. The isotopic shifts increase with increasing quantum numbers in the band tails.  

The hydroxyl isotopologue spectra recorded in wavelength region 311.8–314.0 nm using an IsoPlane spectrograph (Princeton Instruments) with a 3600-gr/mm grating are presented in Figure 6. The ICCD gate duration was 50 μs and delayed 5 μs after the laser pulse. Argon flow was applied to displace air. The most intense features contributing to the shown spectra belong to OH rotational lines of Q

 branches with quantum numbers from J = 13.5 to J = 18.5. Smaller contributions come from P

, and other minor branches of the (0-0) band, and also from the second vibrational band (1-1). These results demonstrate that hydrogen and oxygen isotopes can be determined simultaneously using spectra of hydroxyl isotopologues 

OD recorded with a compact spectrograph. Quantitative isotopic calibration for ice analysis can be accomplished using multivariate PLSR as previously realized for water vapor (6).  

Figure 6: Spectra of 16OH, 18OH, and 16OD molecules in the AâX (0-0) band formed during laser ablation of ice. Spectra were averaged over a series of 10 replicates, each made of 100-pulse accumulations.			

Isotopic analysis of ice is the main tool in paleoclimatology and glaciology studies, but measuring 

O ratios in rocks and minerals is important for geochemists as well. Aluminum oxide is a common constituent in many minerals. As a result, molecular spectra of AlO are easily detectable in laser ablation of rocks. We observed five band progressions of AlO B

  (Δν = 0,±1, ±2) emission within a broad interval 445–545 nm from several rocks including feldspar, tourmaline, and rock salt. However, this spectral range is generally contaminated by many other spectral lines, and each particular experiment will require choosing a smaller specific segment of the spectrum. We ablated natural and isotope-enriched Al

 pellets, flushed with helium flow to illustrate that Al

O can be resolved in LAMIS. The spectra of AlO B→X (Δν = +1) are presented in Figure 7, indicating that AlO emission can be used to measure the oxygen isotopic ratio 

Figure 7: Emission spectra of AlO vibrational band progression (BâX; ÎÎ½ = +1) formed during ablation of Al2O3 natural and 18O-enriched pellets. Spectra were acquired with a 20-Î¼s delay and averaged over 100 laser pulses.			

Calcium oxide is another common constituent of rocks and minerals. Several emission bands of CaO A

  (2-0, 1-0, 0-0, 0-1) were clearly observed within 765–960 nm during ablation of calcite (CaCO

) in air. Molecular structure and spectrophysical properties of CaO are similar to those of SrO. Emission of the latter was previously used for LAMIS detection of three strontium isotopes (

Sr) (3–5). Our numerical simulation of the Ca

O spectra predicted that CaO emission can be used to infer oxygen isotopic information. Additionally, laser ablation of CaCl

  band progression with Δν = –1. We recorded the spectra of CaCl B→X system using two spectrographs, one of which was 1250-mm Czerny-Turner spectrograph (Horiba JY) and the other one was an echelle spectrograph EMU-65. The results are displayed in Figure 8. The natural isotopic ratio of chlorine 

Cl is 24.2% to 75.8%, respectively. The isotopic shifts because of these two isotopes are apparent in the spectra shown in Figure 8. While the large high-resolution spectrograph recorded a superior spectrum, the compact echelle spectrograph yielded sufficient resolution for LAMIS measurements.  

Figure 8: Spectra of CaCl band progression (BâX; ÎÎ½ = â1) formed during ablation of natural CaCl2 pellets. Spectra were acquired with a 20-Î¼s delay and averaged over 20 laser pulses.			

We demonstrated several possibilities of using LAMIS for rapid optical analysis of rare isotopes of hydrogen, boron, carbon, nitrogen, oxygen, and chlorine. A significant advantage of this technique is that solid samples can be analyzed in their original unaltered condition, without preparation. Precleaning of the sample surface can be completed by laser ablation before or during the analysis, which may be considered as depth profiling. Isotopic two- and three-dimensional mapping is possible. Quantitative measurements can be realized using multivariate regression models that will relate the spectral intensities of the isotope-specific molecular spectra to the original abundances of isotopes in the sample. Quantification is based on measuring spectra of known reference samples. Calibrating the LAMIS response for quantitative determination of C and N isotopes is presented here, while calibration for H and B isotopes was described earlier (2,6,7). Accuracy and precision of determination depends on isotopic homogeneity of the reference standards, but the analyzed samples can be heterogeneous. LAMIS can yield a map of the isotopic distribution.  

 (1) R.E. Russo, A.A. Bol'shakov, X. Mao, C.P. McKay, D.L. Perry, and O. Sorkhabi, 

 (2) X. Mao, A.A. Bol'shakov, D.L. Perry, O. Sorkhabi, and R.E. Russo, 

 (3) X. Mao, A.A. Bol'shakov, I. Choi, C.P. McKay, D.L. Perry, O. Sorkhabi, and R.E. Russo, 

 (4) A.A. Bol'shakov, X. Mao, C.P. McKay, and R.E. Russo, 

 (6) A. Sarkar, X. Mao, G.C.-Y. Chan, V. Zorba, and R.E. Russo, 

 (8) M. Dong, X. Mao, J.J. Gonzalez, J. Lu, and R.E. Russo, 

 (9) T.B. Coplen et al., US Geological Survey, Water-Resources Investigations Report 01-4222 (2002).  

(10) B. Yee, K.C. Hartig, P. Ko, J. McNutt, and I. Jovanovic, 

 is with Applied Spectra, Inc., in Fremont, California. 

 is with Applied Spectra, Inc., and the Lawrence Berkeley National Laboratory at the University of California in Berkeley, California. Xianglei 

 are with the Lawrence Berkeley National Laboratory at the University of California. Direct correspondence to: 

This article describes measurements of isotopes of hydrogen, boron, carbon, nitrogen, oxygen, and chlorine using laser ablation molecular isotopic spectrometry (LAMIS).

Dale L. Perry

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