Proceedings of SPIE, v. 8385, "Sensors and Systems for Space Applications V"; Baltimore, MD, 2012. ABSTRACT: A concept of a compact device for analyzing key isotopic composition in surface materials without sample preparation is presented. This design is based on an advanced modification of Laser Induced Breakdown Spectroscopy (LIBS). First, we developed Laser Ablation Molecular Isotopic Spectrometry (LAMIS) that involves measuring isotope-resolved molecular emission, which exhibits significantly larger isotopic spectral shifts than those in atomic transitions. Second, we used laser ablation to vaporize the sample materials into a plume in which absorption spectra can be measured using a tunable diode laser. The intrinsically high spectral resolution of the diode lasers facilitates measurements of isotopic ratios. The absorption sensitivity can be boosted using cavity enhanced spectroscopy. Temporal behavior of species in a laser ablation plasma from solid samples with various isotopic composition was studied. Detection of key isotopes associated with signs of life (carbon, nitrogen, hydrogen) as well as strontium and boron in laser ablation plume was demonstrated; boron isotopes were quantified. Isotope-resolved spectra of many other molecular species were simulated. The experimental results demonstrate sensitivity to 86 Sr, 87 Sr, and 88 Sr with spectrally resolved measurements for each of them. It is possible to measure strontium isotopes in rocks on Mars for radiogenic age determination. Requirements for spectral resolution of the optical measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric analysis of spectra.

1. Laser ablation – optical cavity isotopic spectrometer for Mars rovers
Alexander A. Boľshakov
a
, Xianglei Maob
, Christopher P. McKayc
, Richard E. Russoa,b
a
Applied Spectra, Inc., 46661 Fremont Blvd., Fremont, CA 94538, USA
b
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
c
NASA–Ames Research Center, Moffett Field, CA 94035, USA
ABSTRACT
A concept of a compact device for analyzing key isotopic composition in surface materials without sample preparation is
presented. This design is based on an advanced modification of Laser Induced Breakdown Spectroscopy (LIBS). First,
we developed Laser Ablation Molecular Isotopic Spectrometry (LAMIS) that involves measuring isotope-resolved
molecular emission, which exhibits significantly larger isotopic spectral shifts than those in atomic transitions. Second,
we used laser ablation to vaporize the sample materials into a plume in which absorption spectra can be measured using
a tunable diode laser. The intrinsically high spectral resolution of the diode lasers facilitates measurements of isotopic
ratios. The absorption sensitivity can be boosted using cavity enhanced spectroscopy.
Temporal behavior of species in a laser ablation plasma from solid samples with various isotopic composition was
studied. Detection of key isotopes associated with signs of life (carbon, nitrogen, hydrogen) as well as strontium and
boron in laser ablation plume was demonstrated; boron isotopes were quantified. Isotope-resolved spectra of many other
molecular species were simulated. The experimental results demonstrate sensitivity to 86
Sr, 87
Sr, and 88
Sr with spectrally
resolved measurements for each of them. It is possible to measure strontium isotopes in rocks on Mars for radiogenic age
determination. Requirements for spectral resolution of the optical measurement system can be significantly relaxed when
the isotopic abundance ratio is determined using chemometric analysis of spectra.
Keywords: Laser ablation, optical emission spectrum, isotope, chemical analysis, molecular spectrometry, LIBS, LAMIS.
INTRODUCTION
Isotopic records provide answers to fundamental questions regarding the development and evolution of stars and
planets in the universe, including the solar system. Life processes lead to distinctive isotope patterns, which give clues to
the origin of life and evolution in a galactic context. Radioisotopic age dating is the primary method in which accurate
geochronological ages can be established. Isotopic information in paleoclimatology plays a critical role for
reconstruction of variations in past climate conditions. Consequently, isotopic data hold keys to predicting future climate
changes that may influence global temperature, energy needs, availability of drinking water, and food supplies.
Currently, mass spectrometers that determine mass-to-charge ratio of ionized particles are employed to accomplish
isotopic measurements. Deep vacuum is required to differentiate and analyze the ions. Only a few such instruments or
prototypes have been developed for in situ measurements on surfaces of other planets.1
None of these low-resolution
mass spectrometers can discriminate isobaric masses (e.g., 87
Rb versus 87
Sr or 12
C16
O versus 14
N2) and even in compact
form, these devices are relatively heavy. For high-resolution isotopic measurements, only large, heavy and complex
mass spectrometers are found in research laboratories. Such measurements require prior separation and concentration of
analytes via sophisticated physico-chemical treatment of the samples. Conventional mass spectrometers normally provide
high sensitivity but measurements are generally laboratory-based and time-consuming with complex chemical dissolution
routines that produce acidic waste.
Our concept is based on the integration of two analytical techniques that are both onboard of the Mars Science
Laboratory “Curiosity” but presently unconnected to each other. These techniques are stand-off LIBS sensing for
elemental analysis of solids and tunable semiconductor laser spectrometry for isotope analysis of gases. We intend to
utilize and further develop our recently published technology: Laser Ablation Molecular Isotopic Spectrometry
(LAMIS),2-4
which exploits optical spectra of molecular species produced in ablation plumes. The intrinsically high
spectral resolution of the diode lasers facilitates the measurement of isotopic ratios. The absorption sensitivity can be
*alexandb@appliedspectra.com (AAB); x_mao@lbl.gov (XM); chris.mckay@nasa.gov (CPM); rerusso@lbl.gov (RER).
Invited Paper
Sensors and Systems for Space Applications V, edited by Khanh D. Pham, Joseph L. Cox,
Richard T. Howard, Henry Zmuda, Proc. of SPIE Vol. 8385, 83850C · © 2012 SPIE
CCC code: 0277-786X/12/$18 · doi: 10.1117/12.919905
Proc. of SPIE Vol. 8385 83850C-1

2. boosted using cavity enhanced, cavity ring-down or wavelength modulation spectroscopy. In this fashion, the carbon,
hydrogen, nitrogen, oxygen, chlorine and other stable isotopes can be determined in rocks and minerals. Moreover, the
age at which rocks solidified can be obtained from the isotopic ratios of 87
Rb/86
Sr and 87
Sr/86
Sr following the well-known
isochron method.5
The measurement of strontium and rubidium isotopic ratios can result in determination of the
geological age with uncertainty less than ±500 million years that corresponds to errors within ±2% in the 87
Rb/86
Sr ratio
and ±0.2% in the 87
Sr/86
Sr ratio. At present, there are no means to make direct age dating measurements on Mars but
indirect estimates have uncertainties in billions of years, and validity of these age estimates for Mars geological
components is unknown.6
Thus, our approach adds a major new dimension to the planetary instrumentation, enabling the
high-resolution isotopic determination.
ISOTOPE-RESOLVED EMISSION FROM LASER ABLATION PLASMA
Optical isotope shifts in molecular spectra are significantly larger than those in atomic spectra. This is because the
difference in isotopic masses has only a small effect on electronic transitions (as in atoms) but appreciably affects the
vibrational and rotational energy levels in molecules.2
LAMIS can be implemented in a way similar to the conventional
elemental LIBS analysis, however in LAMIS a few times longer delays after the ablating pulse are applied to allow
formation of molecules in plasmochemical reactions during plasma plume expansion into the air. Benefits of this
technique are rapid and direct isotopic characterization for real-time analysis of condensed samples that does not require
sample preparation or vacuum environment and has the potential for stand-off operation. For practical applications,
LAMIS is poised to speed up, to simplify and to make isotopic analysis more affordable than now, while it will remain
generally less sensitive than the traditional mass spectrometry.
Examples of emission spectra of diatomic radicals SrO measured in laser ablation plasma generated from isotope-
enriched SrCO3 samples are presented in Fig. 1. These emission spectra were collected using a suitable for space
operations, compact echelle spectrograph EMU-65 (Catalina Scientific, Tucson, AZ) fitted with an Electron Multiplying
Charge-Coupled Device (EMCCD). The results plotted in Fig. 1 clearly illustrate that isotope shifts are larger for higher
vibrational quantum numbers. The isotope shift in the head of the (2,0) band is ~0.15 nm, while the shifts in the heads of
the (1,0) and (0,1) bands are both approximately 0.08 nm but of the opposite sign. The shift in the head of the (0,0) band
at 919.6 nm was not resolvable. All three major isotopes of strontium (88
Sr, 87
Sr, 86
Sr) can be resolved in this manner as it
was demonstrated in our previous work.4
Earlier we have also shown that partially resolved spectra can be sufficient for
the quantitative isotopic detection especially if a range of multiple spectral features, such as rotational lines, is measured
at once.3
Ability to measure isotope abundance with a low-resolution spectrometer is a significant attribute of LAMIS.
Fig. 1. Spectral bands of SrO A
1
Σ+
→X
1
Σ+
emission measured in laser ablation of the isotope-enriched 88
SrCO3 and 86
SrCO3
solid samples; (a) vibrational band 2-0, (b) vibrational band 1-0, (c) vibrational band 0-1.
We also measured spectra of diatomic radicals OH, BO, CN and C2 in laser ablation plasma (Fig. 2) to demonstrate
excellent spectral resolution for isotopes of B, C, H and quantitative determination of 11
B. The spectra shown in Fig. 2
were measured using a Czerny-Turner spectrograph fitted with an Intensified Charge-Coupled Device (ICCD).2,3
The
detected molecular radicals are either directly vaporized from a sample or formed by association of atoms or associative
recombination of atomic ions in the plume. Spectra of OH and OD were obtained by ablating the ordinary water ice as
well as vapors of H2O and D2O. The prominent features of the OH A2
Σ+
→X2
Πi (0,0) transition at ~306 nm (R1, R2
branch heads) and ~309 nm (Q2 branch head) with partially resolved individual rotational lines are significantly different
from the corresponding spectrum of OD (Fig. 2a). The experimental shift between the Q2 branch heads of OH and OD is
824 826 828 830 832 834
0
2000
4000
6000
8000
Emissionintensity
Wavelength, nm
88
SrO
86
SrO
868 870 872 874 876 878 880
0
2000
4000
6000
8000
10000
12000
Wavelength, nm
88
SrO
86
SrO
976 978 980 982 984 986 988
0
2000
4000
6000
Wavelength, nm
88
SrO
86
SrO(a) (b) (c)
Proc. of SPIE Vol. 8385 83850C-2

3. ~0.68 nm. Fig. 2b displays a tail fraction of the BO A2
Πi→X2
Σ+
(0,2) band, spectrum of which extends from 503.0 nm
toward the red. Two enriched samples of 10
B2O3 and 11
B2O3 were used; the third sample was boron nitride containing the
natural abundance of boron isotopes. In the latter case, BO molecules were formed in the plasma plume reacting with
atmospheric oxygen. A comparison of the 10
BO and 11
BO spectra in Fig 2b reveals the differences in their rotational
structure, both contributing to the sum spectrum of BO with natural abundance (80.2% 11
B and 19.8% 10
B). The spectra
of C2 and CN (Fig. 2c) were measured in laser ablation of ordinary graphite (99% 12
C) and isotope-enriched urea (99%
13
C). For the C2 d3
Πg→a3
Πu and CN B2
Σ+
→X2
Σ+
transitions, the (0,0) bands in both cases are shown. The isotopic
shifts in the band heads of these radicals are approximately 0.03 nm. Spectrum of the heavier isotope in CN is shifted
toward the violet, but the counterpart in C2 is shifted toward the red.
Fig. 2. Emission spectra of diatomic radicals in laser ablation plasma; (a) OH and OD vibrational band 0-0 of A
2
Σ+
→X2
Πi,
(b) BO vibrational band 0-2 of A2
Πi→X2
Σ
+
, (c) CN vibrational band 0-0 of B
2
Σ
+
→X2
Σ
+
and C2 vibrational band 0-0
of d3
Πg→a3
Πu (Swan system). The data traces are vertically offset for clarity.
We simulated the 16
OH, 18
OH, and 16
OD spectra from 307 to 314 nm as well as the spectra of 12
C14
N, 13
C14
N, and
12
C15
N in the region of the CN A2
Πi→X2
Σ+
(1,0) band from 925 to 940 nm. The simulation demonstrated that spectral
resolution of ~0.03 nm is sufficient to resolve individual rotational lines of all these species. Therefore, modern compact
echelle-based spectrographs can selectively measure multiple isotopomeric molecules at the same time. Multivariate
statistical processing of the spectra can facilitate quantitative analysis even with only partially resolved spectra.
While working with strontium samples, we found that diatomic strontium halides are also readily formed in laser
ablation.4
Logically, not only strontium but other alkaline earth metals will exhibit a similar behavior. Both CaCl and
MgCl are in significant abundance on Mars, and thus they can be utilized for determination of the 35
Cl/37
Cl ratio. The
isotopic shift for chlorine in MgCl was computed as ~0.1 nm, and in CaCl was up to 0.2 nm. These values are larger than
the isotopic shifts presented in Fig. 1 for strontium, and evidently are resolvable. Chlorine isotopes reveal large
fractionation in terrestrial geology and by analogy can shed light on history of past water and volatiles on Mars surfaces
with large isotopic variations to be expected. Perchlorates detected by Phoenix and both Viking missions can be a source
of microbial metabolism, which predictably alters isotopic ratios that can be useful in search for extinct life.
Potentially LAMIS can operate in a stand-off mode collecting emission through a telescope, similar to the LIBS
instrument ChemCam currently onboard the MSL Curiosity. In general, light elements yield the largest isotopic shifts in
LAMIS.2
Consequently, LAMIS is the most advantageous for isotopic analysis of life-forming elements (H, C, N, O).
Carbon isotopes are indicative of primary bio-productivity and energy cycling and are most important for understanding
of biochemistry. The biological enhancement of 12
C over 13
C can be up to 5% (any biological matter on Mars would
probably also reflect an enhancement in 12
C relative to the source material). Hydrogen isotopes are indicative of water
and hydrologic history. Measurement of the D/H isotopic ratio is essential in paleoclimatology, material sciences,
biological and medical research, among many other areas. Oxygen isotopes can characterize the exchange of CO2 and
H2O reservoirs and thus this measurement can help interpret the carbon isotope fractionation.
ABSORPTION MEASUREMENTS IN LASER ABLATION PLASMA
The diode laser absorption spectrometers cannot be outperformed for very high sensitivity afforded with a low-
mass, low-volume optical cavity and electronics package.7
The wavelength of diode laser emission is tunable and
normally has a very narrow linewidth (<1 MHz) enabling high-resolution absorption measurements without the need for
a dispersion spectrograph. The spectral resolution is determined by the diode laser linewidth, while echelle or Czerny-
305 306 307 308 309 310 311 312 313 314
0
5000
10000
15000
20000
Emissionintensity
Wavelength, nm
OH
OD
515.0 515.5 516.0 516.5
Wavelength, nm
13
C
12
C
387.2 387.6 388.0 388.4
C2
CN
512 513 514 515 516
0
200
400
600
Wavelengh, nm
10
BO
11
BO
Natural
(b) (c)(a)
Proc. of SPIE Vol. 8385 83850C-3

4. 0
0.2
0.4
0.6
0.8
894.32 894.33 894.34 894.35 894.36 894.37
Turner spectrographs become unnecessary. A simple photodiode can be a detector. As a result, the tunable laser
absorption spectrometers can be miniaturized to very small devices.
A simplified diagram of our experimental setup is sketched in Fig. 3a. A small vacuum chamber with 4 windows
was used to simulate low pressure environment as on the surface of Mars (~600 Pa). The diode laser beam was steered
through the laser ablation plumes at a distance of 1 to 30 mm above the surface of ablated samples. The intensity of the
diode beam was measured by a fast silicon photodiode. A diaphragm of 3 mm in diameter and a shortpass dielectric
optical filter were installed in front of the photodiode to restrict otherwise intense plasma emission and scattered light
from the ablating laser at 1064 nm. An external cavity diode laser DL-100 (Toptica Photonics, Munich, Germany) with
two interchangeable chips was continuously tunable over spectral intervals of 815–855 and 866–950 nm.
Fig. 3. Schematic diagram of the setup (a) including a vacuum chamber with four windows, a pedestal to support the ablated
samples, and a tunable diode laser, (b) emission of the atomic Cs resonant line in laser ablation plasma and the diode
laser tuned at this line, (c) calculated hyperfine structure of this Cs resonant line with tabulated center at 894.347 nm.
In order to test the sensitivity of the absorption measurements, two spectral lines of Cs with different oscillator
strengths were probed. The stronger line (894.347 nm) was resonant, i.e. originating at the ground state of the Cs atom.
The weaker line (920.85 nm) was a transition between the excited states, which are less populated in plasma than the
ground state. The sample was a tablet of pressed powder of CsNO3 that was initially ablated through open windows at
atmospheric pressure in ambient air. Fig. 3b illustrates an example of the diode laser being tuned at the Cs resonant line
as observed in plasma emission at the delay of 2 µs and the acquisition gate of 20 µs. This emission line exhibits the
spectral width of about 1 nm due to Stark and other kind of broadening. The diode laser line appears to have a finite
width owing to the limit of resolving power of our spectrograph, but in reality is significantly narrower. The resonant
line of Cs has four hyperfine components (Fig. 3c) unresolved in emission. The NIST-tabulated wavelength of 894.347
nm is the value of the center of gravity for these four components.
The absorption measurements presented in Fig. 4 are the oscillograms of the photodiode output voltage. The time
scale originates at the instance of the ablating pulse. The first peak at the moment t=0 is the remainder of scattered
ablation pulse that passes anyway through the optical filter (cut-off wavelength at 1000 nm). Absorption manifests itself
as a negative signal. When the diode laser was tuned at the wavelength 894.386 nm, close to the Cs resonant line, the
absorption was near 100% during initial 20 µs of the plasma plume development. The weaker line at 920.85 nm showed
Fig. 4. Absorption of Cs atoms in laser ablation of CsNO3, (a) at atmospheric pressure ~100 kPa and 1.5 mm from the sample
surface, (b) at reduced pressure ~2.5 kPa and 10 mm from the sample surface.
PD
FFiilltteerr
YAG laser
Diodelaser
898 nm890 nm 894.3 nm
Wavelength, nm Wavelength, nm
0 10 20 30 40 50 60 70 80 90
0.00
0.02
0.04
0.06
0.08
(Cs 894.35 nm)
(Cs 920.85 nm)
off line
Photodiode,V
Time, μs
894.386
920.934
912.212
0 100 200 300 400 500
0.00
0.01
0.02
0.03
Diode laser:
Photodiode,V
Time, μs
894.340 nm
894.334 nm
Cs 894.35nm
Diode laser
(a) (b) (c)
(a) (b)
Diode laser
Proc. of SPIE Vol. 8385 83850C-4

5. respectively weaker absorption when the diode laser was tuned into proximity of it. The excited atoms represented by
absorption at 920.85 nm have shorter life periods in the plasma plume relative to the ground-state atoms represented by
absorption at 894.347 nm (Fig. 4a). If the diode laser is tuned out of any spectral lines, there was no significant absorption
detected. These results indicate that tuning of the diode laser on the spectral features does not have to be precise because
these features are fairly broad in early afterglow of the laser ablation plasma. There were some fluctuations due to
acoustic and thermal shockwaves common for ablation at atmospheric pressure.8
These fluctuations significantly
diminish when pressure is reduced below ~15 kPa and the plasma plume becomes visibly larger. A comparison of the
absorption signal after ablation at atmospheric and reduced pressure is shown in Fig. 4 with legends indicating the
wavelengths at which the diode laser was tuned.
The effect of spectral lines narrowing in time after the event of laser ablation is apparent in Fig. 4b. Detuning of the
diode laser from one of the hyperfine components of the Cs resonant line by 6 pm caused the reduction of the absorption
time down to ~200 µs. When the diode laser was tuned closer to this hyperfine component, the direct absorption lasted
for about 300 µs in the plume. Detuning shortens the absorption time because the diode laser beam can be absorbed only
until the spectral features in the plume are sufficiently broadened. As the plasma cools and spectral lines promptly
narrow down, the detuned diode laser is no longer absorbed, although the absorbing free Cs atoms are still present in the
plume. It is worth to note that hyperfine splittings are not resolvable in emission from laser ablation plasma, and there is
no emission from the plume at delays longer than ~100 µs.
The excited states are not the most favorable for absorption testing but can anyway provide useful diagnostics. The
atomic Cl line at 837.59 nm is usually one of the most intense in the chlorine emission spectrum. This is a transition
between the excited states but involves the most populated (in plasma) metastable chlorine level 3p4
4s 4
P5/2 that should
result in strong absorption at this line. In order to examine pulse-to-pulse variations in absorption, the diode laser
intensity was acquired through a Czerny-Turner spectrograph at the instances synchronized (and duly gated) with the
ablating laser pulses in a sequence of 45 triggering events. The sample was a crystal KCl tablet ablated in open air. The
first 15 acquisitions were recorded without laser ablation, then 15 acquisitions with the laser ablation, and finally another
15 acquisitions without ablation. The results measured at different delays (0.5, 1.0, and 2.0 µs) are illustrated in Fig. 5. If
the diode laser is tuned at the chlorine line, absorption of the beam by chlorine atoms is recorded when the ablating laser
fires and generates the plasma. The intensity of the diode laser emission remained stable and unchanged as evidenced by
measurements before and after the 15 ablating pulses.
Fig. 5. Absorption of Cl atoms at 837.59 nm in laser ablation of KCl at atmospheric pressure and ~1mm from the sample
surface. Measurements were time-gated with the gate width of 1 µs and gate delay of 0.5 µs (a), 1 µs (b) and 2 µs
(c) after the ablating laser pulse. Diode laser was tuned at 837.5945 nm (±0.6 pm).
The data in Fig. 5 indicate that chlorine absorption increased with the delay from 0.5 to 2.0 µs after the ablating
pulses. This means that the number of the absorbing chlorine atoms was growing in a plasma afterglow. A similar effect
of “recombinative population” was generally known in low-pressure plasmas but not in LIBS. Populating of the chlorine
level 3p4
4s 4
P5/2 is attributed to the population mechanism via recombination of electrons with chlorine ions. The level
3p4
4s 4
P5/2 is metastable with a radiative lifetime of ~4 µs, and therefore its population can accumulate in a recombining
plasma. When the diode laser wavelength was detuned about 2 nm from the chlorine line, no absorption (also no
scattering, no beam deflection by shockwaves) was observed at delays longer than 1 µs. In comparison with the
shockwave disturbances after ablation of a tablet of pressed CsNO3 powder shown in Fig. 4a, the KCl crystal lattice
probably generates a fast and repeatable shock pattern that does not interfere with the absorption measurements at long
830 840 850 830 840 850 830 840 850
Wavelength, nm Wavelength, nm Wavelength, nm
(a) (b) (c)
Proc. of SPIE Vol. 8385 83850C-5

6. delays. Nine hyperfine components of this Cl line (837.59 nm) have splittings on the order of 50 to 300 MHz, which are
far below Doppler broadening in the ablation plasma. Precision of diode laser tuning is better than 1 pm and a short-term
wavelength jitter is less than 0.4 pm (<200 MHz).
Preliminary experiments with laser ablation of a RbCl sample yielded no observable emission of any Rb-bearing
molecules in ablation plumes, while the atomic Rb spectrum was intense. It is possible that rubidium molecules do not
form effectively in ablation plasma, or they are very poor emitters. Therefore, the isotopic rubidium determination by
absorption is required to complement the SrO measurements described above (see Fig. 1), if the radiogenic age dating of
rocks is sought using this method. Fortunately, the tunable laser absorption measurements of resolved atomic Rb
isotopes were already demonstrated9
in laser ablation plasma at pressure ranging from 20 to 1300 Pa, which includes
ambient pressure on Mars. A detection limit of 25 µg/g for the individual Rb isotopes in calcium carbonate samples was
obtained illustrating that this method is applicable for measuring 85
Rb and 85
Rb concentrations in geological samples.
Further work is in progress. Our general approach is to augment a ChemCam-like instrument with a miniature
proximal diode laser absorption module enabling isotopic measurements in ablation plumes. This module, Laser
Ablation – Optical Cavity Isotopic Spectrometer (LAOCIS) will provide high-resolution atomic and molecular vapor
density measurements with the use of only a few miniature laser diodes and photodiodes. Tunable single-mode diode
lasers – e.g., vertical-cavity surface-emitting lasers (VCSEL) and distributed feedback (DFB) lasers – are commercially
available. Sensitivity of absorption measurements can be increased by 3 to 4 orders of magnitude using cavity enhanced,
cavity ring-down or wavelength modulation spectroscopy. Neither dispersion optics nor vacuum pumps are required.
Similar packages for exploration of Mars were described to include two lenses, a thermo-stabilized laser, a photodiode, a
cavity and mirrors.7,10
Total mass of individual isotopic diode laser spectrometer for one element was estimated as 230 g
(for H2O)7
or 360 g (for CH4).10
Absorption spectroscopy has some advantages over emission and can be complementary. In an ablation plume, the
absorbing species can dwell significantly longer than the emitting species because the former can be in their ground or
metastable states, while the latter must be excited and radiating, and consequently, short-lived. Molecules usually have
multiple channels to dissipate energy, and their radiative transitions may be less probable than the other channels. As a
result, some species can absorb but will not emit effectively. Absorption can be measured by a simple photodiode with
very high resolution, while emission requires an echelle or other kind of spectrograph with a CCD camera. Contrary,
emission can be measured remotely by a telescope, while absorption requires a close proximity to the sample.
CONCLUDING REMARKS
A schematic picture of our conceptual design is presented in Fig. 6 as one of the possible solutions. A ChemCam-
like instrument is located on a mast and used for the primary purpose of stand-off LIBS measurements (as in MSL
Curiosity). The LAOCIS module is an augmentation installed on a front arm of the rover. An ablating laser pulse hits the
sample. A plume extends upward, perpendicularly to the sample surface and propagates toward the analytical zone.
Light from an array of the diode lasers can be used for simultaneous multi-isotope detection. This laser light pumps a
piezo-modulated cavity (or several crosswise intersecting optical cavities) with an optical axis passing through the
analytical zone. The output from the cavity or cavities is detected by photodiodes. The use of ChemCam laser ablation of
surface materials enables robust sampling that is nearly independent of the sample nature.
For the LAMIS mode (emission detection), if we determine that isotopic molecular spectra can be measured only
from a short distance, then a robotic arm can bring an optical fiber cable close to the location of the ablated sample and
collect the plasma plume emission. The cable will deliver optical emission to the ChemCam's spectrograph which is
fitted inside the rover body (as in MSL). The ChemCam's laser can ablate any location within a radius of ~8 meters. In
the best case scenario, isotopic molecular emission will be measured remotely using ChemCam's telescope.
The main uncertainty of measurements arises from the irreproducibility of the laser ablation. Applied Spectra’s
industrial LIBS instruments routinely achieve elemental concentration precision of 2–4%.11
The reproducibility
demonstrated elsewhere in the LIBS measurements of elemental abundances was significantly better: ±0.03%.12
Therefore, precision and spectral resolution of the proposed device can be sufficient for accurate age determination.
The sensitivity obtained by others in similar experiments on tunable laser absorption in laser ablation plumes is
encouraging. Individual isotopes were determined at levels of 25 µg/g (87
Rb)9
and 50 µg/g (235
U)13
in mineral samples.
However, the previous researchers did not use a cavity enhancement technique. For a comparison, basalts can have up to
Proc. of SPIE Vol. 8385 83850C-6

7. 600 µg/g of Sr, while in magmatic carbonates the strontium content can reach up to ~3000 µg/g. Therefore, strontium can
likely be detected in direct absorption without a cavity but a cavity enhancement greatly increases the dynamic range. .
ACKNOWLEDGMENT
This work was supported by the Defense Threat Reduction Administration (DTRA) of the DoD under Federal
Awards No. LB09005541 and LB09005541A, and Contract No. DE-AC02-05CH11231 awarded by the DOE through
the National Nuclear Security Administration (NNSA) and NASA Contract No. NNX10CA07C awarded to Applied
Spectra, Inc.
Fig. 6. Concept of a rover with the mast-based ChemCam and the LAOCIS installed on a front arm.
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ChemCam
LAOCIS
Laser beam
Proc. of SPIE Vol. 8385 83850C-7