focal point

BY RICHARDE. Russo
LAWRENCEBERKELEYNATIONALLABORATORY
BERKELEY,CALIFORNIA94720

Laser A b l a t i o n
INTRODUCTION a solid, resulting in vaporization; longer and the irradiance is less then
ejection of atoms, ions, molecular approximately 10 6 W / c m 2, vaporiza-
~ lr aser ablation" conjures species, and f r a g m e n t s ; s h o c k tion is likely a dominant process in-
L up star-wars images of
a h i g h - p o w e r e d laser
waves; plasma initiation and expan-
sion; and a hybrid of these and other
fluencing material removal from a
target. Phonon relaxation rates are on
beam obliterating anything in its processes. Many models have been the order of 0.1 ps, and absorbed op-
path! In reality, this view is accurate. developed to describe these process- tical energy is rapidly converted into
When a short-pulsed, high-peak- es, but each pertains only to a sepa- heat. Heat dissipation and vaporiza-
power laser beam is focused onto rate c o m p o n e n t of the interaction tion are fast in comparison to the la-
any solid target, a portion of the ma- and is applicable only under limited ser pulse duration. The thermal and
terial instantaneously explodes into conditions. There are no models that optical properties of the sample in-
vapor. The drawing in Fig. 1 is a completely describe explosive laser fluence the amount of material re-
conceptual interpretation of laser ab- ablation processes (unless the star- m o v e d during the laser pulse. The
lation. Photographs in Figs. 2 and 3 wars or fusion-energy folks have optical properties (absorption and re-
show target results after laser abla- worked out such m o d e l s - - a n d they flection) determine both the fraction
tion (with the use of multiple pulses are classified). L a s e r i r r a d i a n c e of the incident power that is ab-
and different laser-beam energies). (power density) and the thermo-op- sorbed and the depth of optical ab-
Laser " c r a t e r s " resemble those tical properties of the material are sorption within the sample. Different
caused by meteorites striking a plan- critical p a r a m e t e r s that influence heating and cooling rates are expect-
et or volcanic eruptions. these processes. Two general de- ed if the depth of absorption is great-
This paper briefly describes laser scriptions for the laser-material in- er or less than the thermal diffusion
ablation as it is used with analytical teraction are described on the basis length in the material. Although this
spectroscopy for chemical analysis. [ of irradiance: vaporization and ab- interaction is defined as vaporiza-
apologize for not citing numerous lation.7 ~2 tion, the energy is delivered in a very
excellent papers related to the stud- Vaporization. W h e n the laser short time and it is localized; ther-
ies presented herein; however, this pulse duration is microseconds or m o d y n a m i c models do not complete-
paper is not a review, but rather
serves as an introduction to the field.
The name "laser ablation" is used
generically to describe the explosive Atc~rn~ I~n~ ~ l ~ m ~

laser-material interaction, a more

appropriate definition that does not
imply a mechanism. " L a s e r sam-
pling" will refer to the removal of
material from a solid with the use of
a pulsed laser beam with vapor trans-
port to an a n a l y t i c a l e x c i t a t i o n
source for analysis. Laser sampling
has been coupled with practically ev-
ery analytical source, with the most
prevalent today being the inductively
coupled plasma (ICP). ~-6
L a s e r - m a t e r i a l i n t e r a c t i o n s in-
volve coupling of optical energy into Ft6. I. Conceptual drawing of laser ablation.

14A Volume 49, Number 9, 1995

ly describe the interaction. Also, op-
tical and thermal properties of the
material vary during the laser pulse,
which makes it difficult to accurately
predict the amount of energy cou-
pled to the target and the quantity of
mass removed. Howevel; the inter-
action is p r e d o m i n a n t l y thermnal.
Melting is c o m m o n and fractional
vaporization is possible; elements of
higher vapor pressure will be en-
riched in the vapor relative to their
concentration in the solid. Amazing-
ly, this vaporization laser-material
interaction is considered the easier
case! When the irradiance is highel;
the interaction is more complicated.
A b l a t i o n . At higher irradiance,
beyond 10 9 W / c m 2 with nanosecond
and shorter laser pulses focused onto
any material, an explosion occurs.
The term "laser ablation" has been
adopted to describe this interaction;
it must sound better than "laser ex-
plosion". Phenomenologically, the
surface temperature is instantane-
ous@ heated past its vaporization FtG. 2. Photograph of laser ablation "crater" in copper surface with the use of 30-ps
temperature through linear one-pho- pulses from a Nd:YAG laser at A = 1064 nm.
ton absorption, multi-photon absorp-
tion, dielectric breakdown, and ad-
ditional undefined mechanisms. 8,9.~
The vaporization temperature of the
surface is exceeded within a fraction
of the laser pulse duration; energy
dissipation through vaporization
from the surface is slow relative to
the laser pulse width. Before the sur-
face layer can vaporize, underlying
material will reach its vaporization
temperature. Temperature and pres-
sure of the underlying material are
raised beyond their critical values,
causing the surface to explode. The
pressure over the irradiated surface
from the recoil of vaporized material
can be as high as 10 5 M P a (10 6 at-
mospheres)? 3 This explosive inter-
action has been described as " n o n -
thermal", and melting is often not
observed around the crater. Fraction-
al vaporization should be negligible!
However, during an ablative inter-
action, a plasma is initiated at the
sample. Plasma temperatures are in
excess of 10 4 K, and radiative heat
transport can establish a plasma-ma-
terial interaction. 8,9,~4The plasma du- Ft6. 3. Photograph of laser ablation "crater" in copper surface with the use of 30-ps
ration is microseconds, which is long pulses from a Nd:YAG laser at A = 266 nm.

APPLIED SPECTROSCOPY 15A

focal point
samples for elemental analysis by
the ICP with the use of either atomic
e m i s s i o n s p e c t r o s c o p y ( A E S ) or
mass spectrometry (MS) detection.
In turn, analytical atomic emission
spectroscopy is an excellent technol-
o g y for s t u d y i n g laser a b l a t i o n
mechanisms. AES can provide fun-
d a m e n t a l information on the laser
plasma such as electron density, tem-
perature, and temporal properties. By
transporting the sampled vapor to a
separate analytical atomization/exci-
tation source (the ICP is emphasized
in this work), one can exploit addi-
tional capabilities of AES for study-
ing laser ablation mechanisms. If the
ICP conditions are constant, changes
FIG. 4. Photograph of laser-induced plasma during laser ablation of copper with the in the interaction due to laser param-
use of the nanosecond excimer laser (A = 248 nm) at a power density of 3 × 10~ W~ eters, inert gas environment, and ma-
cm2. Gas environment was 100 mTorr of oxygen. terial properties can be studied by
observing elemental emission inten-
in comparison to the short laser merely approximate. Power densities sity in the ICR temporally and spa-
pulse. Fractional vaporization may in the 106-109 W / c m 2 range can tially. ICP-AES is probably the best
occur during this p l a s m a - m a t e r i a l cause vaporization, ablation, both of technology for studying laser abla-
interaction, and to a greater extent these processes simultaneously, or tion at atmospheric pressure. AES is
than is the case in the direct laser additional mechanisms that have not emphasized over mass spectrometry
vaporization interaction. A photo- yet been identified. One should view (ICP-MS) in order to eliminate is-
graph of a laser-induced plasma these examples as ideal cases; ejec- sues related to vacuum sampling
(LIP) during UV nanosecond laser tion of solid fragments and ions, from the atmospheric-pressure ICR
ablation of copper is shown in Fig. condensation of clusters, and shock However, ICP-MS would provide in-
4. The luminous laser-induced sur- waves occur. Table I lists some of the creased sensitivity for studying trace
face plasma is evident, with cooler mechanisms that have been studied. constituent behavior and fundamen-
spectral emission at the outer parts The existence of shock waves is eas- tal properties such as the ablation
of the expanding plasma. As an ex- ily confirmed by the sonic b o o m threshold. Chemical analysis benefits
ample of how easy it is to achieve heard by anyone who has witnessed from laser ablation sampling and la-
high p o w e r densities for laser abla- a focused short-pulse laser exploding ser ablation benefits from analytical
tion, a 10-ns laser pulse with only 1 target materials at atmospheric pres- a t o m i c e m i s s i o n s p e c t r o s c o p y for
mJ of energy, focused to a 10-txm sure. studying fundamental mechanisms.
spot size' (radius), has fluence equal The remainder of this paper de-
to 300 J/cm 2 and irradiance of ap- scribes some symbiotic relationships.
LASER ABLATION AND Laser Ablation Sampling for
proximately 4 × 10 m W/cm2!
ATOMIC EMISSION A n a l y t i c a l S p e c t r o s c o p y . A gener-
The classification of these mech-
SPECTROSCOPY alized experimental system for laser
anisms is simplified and phenome-
nological; the power densities given Laser ablation is an excellent tech- sampling and analysis by I C P - A E S
for vaporization and ablation are nology for directly vaporizing solid is shown in Fig. 5. The beam from
a pulsed laser is directed into the
TABLE I. Laser ablation mechanisms. " a b l a t i o n " c h a m b e r and f o c u s e d
o n t o the s a m p l e (target) s u r f a c e .
• Absorption (single, multiphoton, defect initiated . . . . )
• Reflection (time-dependent) Samples are usually placed before
• Thermodynamics (melting, latency, phase change . . . . ) the effective focus of the lens to
• Plasma ignition e l i m i n a t e b r e a k d o w n and p l a s m a
• Shock waves (gas) formation directly within the gas.
• Stress waves (solid)
• Laser-plasma interaction (inverse b r e m s s t r a h l u n g .... )
The ablated-sample vapor is carried
• Plasma radiation/heating by the gas flow to the central channel
• Gas-dynamic expansion of the ICP torch. Dual gas-inlet ports
• Hydrodynamic expansion allow mixing of gases in the ICP for
• ????
fundamental studies. Spectral emis-

16A Volume 49, Number 9, 1995

 Monochromator

Ablation
cha~,
Auxiliary
gas
Computer

Chambergas
" /

Mirror

FIG. 5. General experimental diagram for laser oblation, with the use of inductively coupled plasma atomic emission spectroscopy
and laser-induced plasma atomic emission spectroscopy.

sion from the ICP is measured by a easily be predicted or controlled. For sampled material will be a stoichio-
monochromator with several detec- unknown samples, it is impossible to metric representation of the solid.
tors including a photomultiplier tube know, a priori, how much sample The quality of ablated sample and its
(PMT), photodiode array (PDA), and will be ablated, or whether the laser- composition depend on the laser and
charged-coupled device (CCD). If
the ablation chamber is constructed
of quartz, emission from the laser-
induced plasma can be imaged onto
a monochromator with any of the de-
tectors mentioned above.
Laser ablation is the only technol-
ogy that offers direct solid sampling
from any material and w i t h o u t sam-
ple preparation. Sampling is initiated
by optical a b s o r p t i o n p r o c e s s e s ,
thereby eliminating all restrictions
on sample type, size, or geometry.
Elimination of sample preparation is
important, especially for hazardous
materials. Another unique feature is 1
that the sampling area is microme-
o i o l o 1 o 1 o
ters in diameter, so that spatial anal-
ysis is possible. With these advan-
: ,meCm,n
tages, why is laser ablation not used
more w i d e l y - - a r e there no prob-
lems? Of course there are! The ex- FIG. 6. Ni emission time response from the ICP during repetitive pulsed laser ablation
plosive laser material interaction is of steel at five separate locations on a stainless steel surface; 8-ns pulses from a 10-
not fundamentally defined; it cannot Hz Nd:YAG laser.

APPLIED SPECTROSCOPY 17A

focal point

FIG. 7. Cu emission time response from the ICP during repetitive laser ablation of copper at O. I GW/cm 2. Spikes in intensity are
observed on a continuous Cu emission background. Obtained with a 100-ps rise-time photomultiplier tube detector with a 500-Hz
ADC rate.

material characteristics. Fluctuations piing for controlled and accurate solved. Unfortunately, the history of
in the laser's temporal and spatial chemical analysis? Several advance- laser sampling is plagued by poor
profiles and nonlinear power density ments in understanding laser abla- accuracy and precision for single-
dependence contribute to this uncer- tion for chemical analysis are de- pulse sampling (30-70% R S D ) . 4-6
tainty. The sample's physical and scribed below. Single-pulse sampling is like bomb
chemical matrix will affect ablation Single versus Repetitive Laser testing; you have only one shot to
behavior, and variations in the par- Sampling. Most laser sampling stud- accumulate data about an explosion!
ticle-size distribution of the ablated ies e m p l o y h i g h - p o w e r e d , single- For homogeneous samples (and
material will influence transport ef- pulse lasers. Because the laser is inhomogeneous samples when either
ficiency to the excitation source. But pulsed, the emission signal from the an internal standard or external mon-
these problems are not show-stop- ICP will be transient, with a tempo- itor is used---to be discussed later),
pers, o n l y c o n c e r n s . Significant ral profile characteristic of the cham- a repetitively pulsed laser allows the
progress already has been made in ber volume, the transport tube length sampled material from successive
understanding and using laser sam- and diameter, the carrier gas and its ablations to mix, providing c o n t i n u -
pling technology, and commercial flow rate, and the detection electron- o u s and/or c o n s t a n t ICP-AES signal
devices already exist for coupling to ics. For spatially resolved analysis of response. An example of continuous
ICP sources. The issue is, How can inhomogeneous samples, only a sin- and constant response is shown in
we overcome these remaining con- gle laser pulse is preferred, and the Fig. 6 for Ni emission (h = 352 rim)
cerns and better utilize laser sam- temporal emission signal can be re- intensity in the ICP at five separate

18A Volume 49, Number 9, 1995

 h : (5¸¸¸¸¸

i!i!i
' iiili!ii!i
!! ii!iiiiiiiil
i;iiii!II
iijiiiiili/iiiiii!iii!i

:77: : :: :

~ r

w
000 o
0 oo
3 105
o
o%
o if"
0 CDq~ 2 105
°°o o
oo
0

i ¸ t111
•

0
o i 105
: oo
0
i,~,, ' ....... I I I I I I I11 I I I I I I 111 I I , 0
0.1 1 10 100

Power Density ( G W / c m 2)

FIG. 8. ICP-AES from Cu with a picosecond Nd:YAG laser (A = 266 nm, energy = 6 mJ) and a nanosecond excimer laser (A =
248 nm, energy = 30 mJ). Each power density data point was measured during continuous emission.

locations on a steel surface. '5 Each by using lasers with better spatial from the improved precision. Opti-
temporal profile represents the emis- and temporal properties. The nano- mization of the ICP fl)r direct intro-
sion intensity during repetitive laser second N d : Y A G laser exhibits ran- duction of solids is possible during
sampling at a different spot on the d o m mode spikes on its temporal constant sampling. The power level
sample, with the use of the third har- profile. Injection seeding of this laser to the ICE gas flow rates, and detec-
monic (L = 365 nm) of a N d : Y A G may provide an additional improve- tion height can be varied to obtain
laser (pulse width = 8 ns) with a ment in laser sampling precision. the best signal-to-noise (S/N) and
power density of 0.6 G W / c m 2. The C o m p a r e d to a single-pulse inter- s i g n a l - t o - b a c k g r o u n d (S/B) ratios.
repetition rate was 10 Hz. The tem- action, continuous or constant re- A t o m - f o r m a t i o n p r o c e s s e s in the
poral behavior is dependent on the sponse provides better reproducibil- ICP will be different during laser
focal length of the lens, power den- ity for analysis, and a unique "con- sampling than they are in liquid neb-
sity, etch rate of the material, and as- trolled" environment for studying la- ulization, and can be studied. Impor-
pect ratio of the induced crater. The ser ablation mechanisms. '5,'6 In the tantly, the excellent precision allows
initial response is usually irreprodu- remainder of this paper, ICP-AES studies of ablation mechanisms as a
cible as shown, after which time the data are reported only when the tem- function of laser properties, materi-
intensity drops initially but then re- poral intensity behavior is approxi- als, and gas environments.
mains at a level that is essentially mately constant during repetitive la-
constant. The constant signal pro- ser sampling. For every sample we
LASER ABLATION STUDIES
vides excellent reproducibility (RSD have studied, continuous emission
DURING CONTINUOUS
= 4.3%) c o m p a r e d with that of the intensity can be obtained, although it
EMISSION
m a x i m u m peak response (RSD = is not always constant. Several ca-
25.6%) for the five measurements. pabilities beneficial to chemical anal- " S p i k e s " . In addition to atoms,
The 4.3% RSD could be improved ysis are i m m e d i a t e l y r e c o g n i z e d ions, and molecules, laser sampling

APPLIED SPECTROSCOPY 19A

focal point

1 10

FIG. 9. Mass ablation rate behavior measured with the use of ICP-AES during nanosecond laser ablation of brass; ~ = 248 nm.
Data show different rate behavior for Zn and Cu at power densities above and below approximately 0.3 GW/cm 2. This change
suggests a difference in ablation mechanism.

generates particles. The particle-size of copper. The excimer laser (h = the case for the data in Fig. 6. Fun-
distribution depends on the sample 248 nm, pulse width = 20 ns) was damentally, it m a y be possible to
material and p o w e r density of the la- used at a 10-Hz repetition rate. The correlate emission intensity of the
ser beam. Sample entrainment into p o w e r density was 0.1 G W / c m 2. A spikes to particle-size d i s t r i b u t i o n - -
the carrier gas is particle-size depen- continuous signal intensity is ob- important knowledge for understand-
dent, with the majority of the parti- tained, with large excursions caused ing laser ablation mechanisms.
cles larger than 2 ~ m left behind be- by individual larger particles. The Roll-Off. Continuous atomic emis-
cause of low transport efficiency. ~7,~8 emission spikes are due to Cu emis- sion from the ICP provides the nec-
The measured constant signal inten- sion at 324 nm as the particle va- essary precision to accurately mea-
sity is primarily f r o m the ensemble porizes and atomizes, and are not sure changes in the quantity and
of " s m a l l e r " particles. However, due to i n c a n d e s c e n c e . T h e l a r g e composition of the laser-sampled va-
some large particles will be en- number o f spikes measured here is a por. For increased sensitivity, the la-
trained, and they can be observed as w o r s t - c a s e s c e n a r i o ; the low 0.1 ser power density can be changed to
positive " s p i k e s " on the emission G W / c m 2 p o w e r density is k n o w n to increase the quantity of sample de-
signal if the detection electronics and cause substantial melting and ejec- livered to the ICE However, the ef-
data acquisition are fast enough. ~9 tion of molten droplets. The n u m b e r ficiency of laser-energy coupling to
Figure 7 shows the emission from of particles is reduced as the p o w e r the sample changes with p o w e r den-
spikes that results when a monochro- density is increased. The spike-inten- sity, as demonstrated by ICP-AES
mator with a fast P M T detector mea- sity will be " a v e r a g e d " into the measurements for both picosecond
sures Cu emission (h = 324 nm) baseline if a detector with long in- and nanosecond repetitive laser sam-
from the ICP during laser sampling tegration time is employed, as was piing (Fig. 8). 7,9'20 These data are for

20A Volume 49, Number 9, 1995

 i:AI p0.26

Cu

I=A3p0.22

105

~ Z n

o
I0" J . _ _

t 10 1O0

Power Density(GW/cm2)

FIG. 10. Mass ablation rate behavior measured with the use of ICP-AES during picosecond laser ablation of brass; ~ = 266 nm.
Data show similar rate behavior for Zn/Cu as did those for the nanosecond laser (cf. Fig 9).

Cu emission during laser sampling than the higher-energy (30-mJ) nan- samples, including metals, alloys,
of pure copper samples (pure mate- osecond laser (k = 248 nm), point- oxide insulators, and glasses, t9-22 The
rials are used for demonstration). ing out the importance of the energy roll-off occurs for all materials, for
Each data point was measured after deposition per unit time. For both la- b o t h p i c o s e c o n d and n a n o s e c o n d
constant response was obtained dur- sers, the quantity of material increas- sampling. An interesting observation
ing repetitive laser pulsing (cf. Fig. es with power density, but only up is that the roll-off is usually mea-
6). ICP-AES changes reflect changes to a certain power density, at which sured at about 0.2-0.3 G W / c m 2 for
in the laser-material interaction (as- point the quantity goes down. The conducting materials as well as for
suming at this point that sample plateau in intensity and roll-off (de- some insulators, when UV nanosec-
transport is not affected). Several im- crease) are not due to sample loading ond laser pulses (and Ar as the car-
portant observations from these data or optical self-absorption in the ICE rier gas) are used.
are that (1) the picosecond laser is Instead, the roll-off is due to a In these experiments, the spot size
more efficient than the nanosecond change in the efficiency of laser en- of the laser beam was reduced in or-
laser for removing sample, and (2) ergy coupling to the target by in- der to increase the power density.
the sample quantity (ICP-AES inten- creased absorption and/or reflection Mass ablation rate is defined as the
sity) exhibits a strange power density from the laser-induced plasma, a total mass ablated per unit time and
dependence. The lower-energy (6- process known as plasma shielding unit area. Therefore, emission inten-
m J) picosecond laser (k = 266 nm) (to be discussed below). Similar be- sity divided by laser beam area is
provides higher ICP-AES intensity havior was observed for numerous proportional to mass ablation rate.

APPLIEDSPECTROSCOPY 21A
focal point
sample surface and its characteristics
(electron density, temperature, tem-
poral behavior) are governed by the
laser b e a m properties (pulse duration
and wavelength), by the material
composition, and by the gas environ-
ment. The plasma is initiated and
sustained by inverse bremsstrahlung
absorption during collisions a m o n g
5
sampled atoms and ions, electrons,
and the gas species. 25-27 P l a s m a
shielding can be studied by measur-
ing mass ablation rate behavior in
the ICP as a function of laser pulse
duration and wavelength, and gas
environment. The atmosphere in the
ablation chamber (which is also the
ICP sample carrier gas) can be any
of the inert gases. As long as the to-
tal gas composition is constant, the
ICP temperature and excitation char-
acteristics should be constant for
studying effects of gas environment
<~< N 4% . ~ . N 4% on laser ablation sampling.
Ar is easier to ionize than He be-
cause of its higher ionization cross
FIG.1 I. Conceptualmodels for plasma shielding. (Left) Picosecond laser plasma section and lower ionization poten-
shielding based on collisions between fast electrons and gas atoms. (Right) Nanosec- tial; as a result, plasma shielding is
ond laser plasma shielding based an collisions between vaporized atoms/ions. expected to be more severe in Ar
than in He. Plasma shielding can
best be demonstrated by using the
The mass ablation rates for Zn and essentially constant after the roll-off. 1064-nm wavelength from the pico-
Cu (brass sample) vs. power density For the picosecond case, the rates second N d : Y A G laser, since inverse
are shown in Figs. 9 and 10, with and the ratios are essentially the bremsstrahlung is proportional to the
n a n o s e c o n d and p i c o s e c o n d laser same in both regions. What mecha- laser wavelength squared (k2). With
sampling, respectively (the same la- nism is responsible for the fraction- this longer wavelength, Cu emission
ser conditions as above were used ation in the nanosecond case? Is the intensity was 16 times higher with
for these measurements). 23 Brass is thermal c o m p o n e n t of the interaction He then it was with Ar in the abla-
an excellent sample to employ in significant in the lower power den- tion chamber. 2~ In support of the en-
these studies. The brass sample con- sity range? If plasma heating were hanced ICP-AES data, crater vol-
tains 60% copper (bp, 2567 K) and dominant, the rates would be ex- umes were measured to be greater in
40% zinc (bp, 907 K). The rates fol- pected to deviate at higher power the He atmosphere. (Note: a direct
low a power law dependence with density. A p r e l i m i n a r y t h e r m a l - c o r r e l a t i o n o f c r a t e r v o l u m e to
two distinct slopes over this laser based model with inverse brems- changes in ICP-AES measurements
p o w e r density region. The mass ab- strahlung plasma absorption predicts is impossible; see Figs. 2 and 3).
lation rate increases exponentially in the vaporization rate change and Nevertheless, a larger crater pro-
the low power density regime and roll-off behavior. 24 duced with the He atmosphere sup-
drops to near unity, or less, at high ports a r e d u c e d p l a s m a - s h i e l d i n g
power densities. The mechanisms mechanism and enhanced laser en-
LASER-INDUCED PLASMAS
governing this p o w e r law depen- ergy coupling to the target. With the
AND PLASMA SHIELDING
dence are under investigation. For picosecond laser at 266 nm, the en-
the UV nanosecond sampling, Zn The laser-induced plasma itself hancement in ICP-AES intensity was
and Cu exhibit different mass abla- was hypothesized earlier as the cul- only a factor of 3.3 in He in c o m -
tion rates with power density before prit responsible for reducing the ef- parison to that in Ar. This wave-
the roll-off. Unfortunately, this dif- ficiency of laser sampling at elevated length dependence supports the in-
ference means that the ratio of Zn to power density. Several studies have verse bremsstrahlung m e c h a n i s m .
Cu varies as a function of power been performed to test this hypoth- For the excimer laser with 20 ns
density. The rates and ratios b e c o m e esis. The initiation of a plasma at the pulses and k = 248 nm, the gas at-

22A Volume 49, Number 9, 1995

mosphere had a smaller influence on
I C P - A E S signal levels. T h e Cu i i!¸!iIIIi!
e m i s s i o n i n t e n s i t y was o n l y 1.3
times higher in He than in Ar, in
contrast to the behavior with the pi-
cosecond laser (X = 266 nm). This
different influence of gas atmosphere
is due to the laser pulse duration,
since the photon energy is essentially
the same.
The difference is plasma shielding
for picosecond and nanosecond laser
ablation can be explained on the ba-
sis of collisions a m o n g atoms, ions,
and e l e c t r o n s in the a t m o s p h e r e
above the target surface (Fig. 11).
During the picosecond laser pulse,
ejected atoms/ions travel only a few =~U "JU

hundred .~ngstroms from the sur- 10 .6 I 0 "4 10 "2 10 ° 10 2

face, assuming velocities on the or-
der of 106 cm/s. 28 In contrast, high- pressure I1;I
energy ( > 1 0 0 eV) electrons are gen-
erated d u r i n g p i c o s e c o n d interac-
tions, and these electrons can acquire FIG. 12. Experimental change in the crater depth (data points; right axis) vs. pressure
velocities on the order of 109 cm/s. for Ar and He. Solid curves (left axis) calculated from model based on inverse brems-
These fast electrons travel several strahlung for change in number of electrons (ionization) in the plasma.
hundred micrometers during the la-
ser p u l s e duration, and u n d e r g o
many more collisions with the gas
atoms than the ejected atoms or
ions. 29 The fast electrons absorb laser
photons during collisions with the
support gas atoms (Fig. 1 1, left). Ar
and He at pressures ranging from
10 -5 Torr to 1 atmosphere were used
to d e m o n s t r a t e p l a s m a s h i e l d i n g
based on an inverse bremsstrahlung
model involving fast electrons and
the support gas atoms. 2~ The model
p r e d i c t i o n s e x h i b i t e d g o o d agree-
ment with measured changes of the
crater depth with gas pressure (Fig.
12). As pressure increases, the num-
ber of electrons (ionization) increas-
es; the laser energy incident on the
sample and available for removing
material is correspondingly reduced.
D u r i n g the n a n o s e c o n d laser
pulse, the distance that atoms and
ions travel from the sample surface
is several hundred micrometers (Fig.
1 1, right). (Fast photoelectrons have
not been observed for nanosecond
laser ablation.) The sample atoms/
ions undergo collisions with each
other as they expand into the gas,
and absorb photons from the laser FIG. 13. Spatial distribution of the Cu(I) 521.82-nm emission intensity as a function of
beam (inverse bremsstrahlung). For laser beam power density. Nanosecond excimer laser; ,~ -- 248 nm.

APPLIED SPECTROSCOPY 23A

focal point
CORRELATION OF SPECTRAL
EMISSION INTENSITY IN THE
ICP AND LIP
The laser-induced plasma itself
can be used as a monitor to verify
that the ICP-AES intensity behavior
is related to ablation processes, and
not to sample transport. Measuring
and interpreting spatial and time-de-
pendent emission intensity from
these plasmas can be complicated. 3°
Spatial emission intensity is a func-
tion of p o w e r density as shown in
Fig. 13. However, by measuring L I P
emission intensity with a long inte-
gration time and over the plasma's
entire spatial extent at each power
density, one can determine mass ab-
lation rate behavior? ~ The LIP emis-
sion intensity shows a linear rela-
tionship with laser power density,
but with distinct regions, just as did
the mass ablation rate measurements
FIG. 14. Mass ablation rate (Cu emission intensity normalized to laser beam area de-
described earlier (cf. Figs. 9 and 10).
pendence on power density for the ICP and LIP). The inset shows the correlation of the The data in Fig. 14 show the LIP-
two curves. AES and ICP-AES behavior during
laser ablation of copper. The inset in
second laser pulse. Therefore, the the figure shows the correlation be-
laser ablation of copper, the plasma
gas environment influence is not sig- tween the LIP-AES and the ICP-
will be c o m p o s e d mainly of copper
nificant. AES response over this same power
atoms and ions (instead of Ar or He
U n d e r s t a n d i n g p l a s m a initiation density range. Similar data were ob-
ions). The ionization potential of
and reducing plasma shielding is tained for brass, aluminum, iron,
copper (7.724 eV) is significantly
beneficial to the use of laser ablation steel, zinc, aluminum oxide, iron ox-
lower than that of Ar and He. The
for c h e m i c a l a n a l y s i s . I m p r o v e d ide, and glass. The good correlation
photon energy of the excimer laser
sampling efficiency as a function of was obtained for all the samples ex-
is 5.0 eV; only two photons are need-
laser power density can provide in- cept zinc, which m a y be due to the
ed for multiphoton ionization of the
creased sensitivity in chemical anal- low melting point of this metal. The
Cu atoms, c o m p a r e d to four and five
ysis. On the basis of the above ICP- correlation of ICP to LIP emission
for Ar and He, respectively. The Cu
AES data, ultraviolet wavelengths was observed in two cases, when
plasma is established during the nan-
osecond laser pulse on the basis of are better than those in the infrared, only the central channel of the LIP
these atom/ion collisions and multi- and a picosecond pulse duration is or the entire LIP was imaged by the
photon absorption. Collisions of gas better than is a nanosecond. Follow- spectrometer. The correlation with
species with the ejected atoms and ing this trend, X-ray femtosecond LIP demonstrates that the ICP ac-
ions are negligible during the nano- pulses may be even better! curately reflects changes in laser ab-
lation, and that transport does not in-
fluence these data.

TABLE II. Emission intensity ratios to Si and the precision for the LASER ABLATION ICP-AES OF
major elements from an SRTC prototypic glass. G L A S S (AN A N A L Y T I C A L
Avg. STUDY)
spectroscopic Relative
intensity standard UV laser ablation sampling with
Composition ratio Standard deviation ICP-AES was applied to the elemen-
Oxide (%) (5 spots) deviation (%) tal analysis of prototypic vitrified
MnO~ 2.11 0.3279 0.0010 0.31 waste glass samples from the Savan-
Fe203 12.80 0.2676 0.0011 0.42 nah River Technology Center. 22 La-
MgO 1.42 2.4360 0.0181 0.74 ser ablation sampling has been pro-
AlzO3 4.88 0.4176 0.0065 1.56 posed as an excellent alternative to

24A Volume 49, Number 9, 1995

dissolution procedures when vitrified
waste products that may be highly
radioactive are being dealt with. Five
major elements in the prototypic
glass samples were selected for anal-
ysis (Table II). These elements were
chosen to provide good signal-to-
b a c k g r o u n d ratios s i m u l t a n e o u s l y
with the use of the P D A spectrom-
eter, which is not a particularly sen-
sitive instrument.
The data in Figs. 15a (top) and
15b (middle) show the temporal be-
havior of ICP-AES emission mea-
sured for Mn and Si, with the use of,
for ablation, the nanosecond-pulsed
excimer laser (k = 248 nm) with a
power density equal to 1.2 G W / c m 2 Time (s)
at a repetition rate of 10 Hz. At this
power density, fluctuations in laser
power are linearly related to those in
the signal. The curves in Figs. 15a
and 15b show the sample removal r Si
time response from Mn and Si dur-
ing repetitive (continuous) ablation 4000
at five different locations on a shard
of prototypic glass. As expected, the ,3
response was continuous although 3000 2
not constant, because of fast etching = 2500
i
rates for these samples. For each
spot, the signal from the first one 2000
hundred pulses (the initial peak in 1500
the profiles) varies significantly (up 1000
to 50% RSD). The precision for the
five spots after 60 s of pulsing im- 5OO
proves to 5 - 8 % RSD, consistent 0
with the power fluctuations of the la- 50 100 150
ser. The temporal profile of emission Time (s)
for the elements in Table II exhibited
similar behavior. Si was chosen as an
internal standard, and the intensity
ratio to Si for the other four elements 0.8 5
was measured to be constant both in
time and between spots, except for 4
the first 10 s [Fig. 15c (bottom)]. The
0.6
intensity ratios have excellent preci-
sion; for the five craters, the RSD
varied from 0.3 to 1.7% (Table II).
0.4
1

FIG. 15. Time-dependent ICP-AES inten- 0.2

sity during repetitive ( l O-Hz) nanosecond
excimer (,~ = 248 nm) laser ablation of
SRTC prototypic glass. Power density = 0.0
1.2 GW/cm 2. (Top) Mn; (middle) Si; (bot- 50 100 150
tom) Mn/Si ratio. Each time trace is a
Time (s)
separate spot on the glass.

APPLIED SPECTROSCOPY 25A

focal point
The constant ratios demonstrate
that there is no time-dependent frac-
tionation of these elements, at this
power density. Also, these data dem-
onstrate that the glasses are homo-
geneous in elemental composition
throughout the crater depth. The cra-
ter diameters are about 200 ~ m and
the depth is on the order of 1000 Ixm
after 900 laser pulses. The sampling
rate is, therefore, --1 peru per laser
shot. Depth profiling with this reso-
lution or better is therefore possible.
With a fixed laser beam energy
and translation of the focusing lens
to obtain spot sizes from approxi-
mately 1000 to 200 pom, the p o w e r
density was varied from 0.13 to 2.12
G W / c m 2. The ablated quantity of
glass sample increased with the pow-
er density and reached a m a x i m u m
at - 0 . 3 G W / c m 2 (Fig. 16). The pla-
teau in i n t e n s i t y due to p l a s m a
shielding is evident (these data are
not normalized to area). Figures 16a
(top) and 16b (middle) show the data
for Fe and Si as a function of power
density, respectively. Each data point
in these graphs represents the aver-
age intensity from five spots, mea-
sured during c o n t i n u o u s emission.
Relative to that for Si, the intensity
ratio for Fe remains essentially con-
stant over this power density range
[Fig. 16c (bottom)]. Similar behavior
was measured for Mg, AI, and Mn.
Therefore, the irregular surface of a
glass shard and thus slight variation
in lens-to-sample distance did not in-
fluence the mass ablation behavior.
The fact that the ratio remains con-
stant for these elements indicates
that p r e f e r e n t i a l v a p o r i z a t i o n o f
these elements also is not signifi-
cantly influenced by power density,
over this range.
S e v e n S R T C p r o t o t y p i c glass
samples were available, with slightly
varied oxide concentrations. A quan-

FIG. 16. ICP-AESsignal vs. laser power

density with the use of excimer laser ab-
lation. Each data point represents the
average of five spots, measured once
continuous sampling was obtained.

26A Volume 49, Number 9, 1995

 O

~
Z

i i i J ~ i i ~ I I I I i , , , , , , i I r ~ I I I

1,~1.026 0.028 0.030 0.032

CONCENTRATION RATIO CONCENTRATION RATIO

(A) (C)

/:
AI/Si

O
0.4

! o3]
i

0.15 0.20 0.25 0.30 0.050 0.075 0.1 O0 0.125 0.150

CONCENTRATION RATIO CONCENTRATION RATIO

(B) (O)

FIG. 17. Calibration curves for measured elemental: Si ratios vs. the prepared (nominal) composition. Nanosecond excimer laser
sampling of SRTC prototypic glass with ICP-AES was employed.

titative study showed linear calibra- scissa is the prepared oxide concen-
THE FUTURE
tion curves when Si was used as the tration ratios. The correlation coef-
internal standard (Fig. 17). The or- ficients for these data are 0.98 to This intentionally brief discussion
dinate is the measured emission in- 0,99, except for that of Mg, which is turned out to be long, and laser ab-
tensities, ratioed to Si, and the ab- 0.70. lation has barely been addressed!

APPLIED SPECTROSCOPY 27A

focal point
TABLE III. "Ideal" laser sampling characteristics for chemical anal- 16. E Arrowsmith, Anal. Chem. 59, 1437
ysis. (1987).
17. R Arrowsmith and S. K. Hughes, Appl.
• Laser absorbed linearly with energy and power Spectrosc. 42, 1231 (1988).
• No interaction of laser with ambient medium 18. M. Thompson, S. Chenery, and L. Brett,
• Heat-affected zone = optical penetration depth J. Anal. At. Spectrom. 5, 49 (1990).
• No melting or fractionation 19. W. T. Chan and R. E. Russo, Spectrochim.
• No interaction of vapor plume with ambient gases Acta 46B, 1471 (1991).
• Narrow particle size distribution 20. W. T. Chan, X. L. Mao, and R. E. Russo,
• 100% transport efficiency J. Appl. Spectrosc. 46, 1025 (1992).
• Fixed concentration of an element in any matrix gives the same ICP- 21. X. L. Mao, W.-T. Chan, M. A. Shannon,
AES intensity and R. E. Russo, J. Appl. Phys. 74, 4915
(1993).
22. R. E. Russo, W.-T. Chan, M. Bryant, and
E J. Kinard, J. Anal. At. Spectrom. 10,
Where do we go from here? Laser 2. L. Moenke-Blankenburg, Spectrochim. 295 (1995).
a b l a t i o n is h e r e to stay. It h a s f o u n d Acta Rev. 15, 1(1993). 23. W. T. Chan and R. E. Russo, "Character-
3. L. Moenke-Blankenburg, Laser Micro istics of Laser-Material Interactions Mon-
a niche in numerous applications, Analysis, V 105, A Series of Monographs itored by Inductively Coupled Plasma-
extending far beyond the analytical on Analytical Chemistry and its Applica- Atomic Emission Spectroscopy", in La-
realm, into areas of fusion energy, tions, J. D. Wineforner and I. M. Kol- ser Ablation Mechanisms and Applica-
medicine, materials, materials pro- thoff, Eds. (John Wiley and Sons, New tions, J. C. Miller and R. E Haglund, Jr.,
York, 1989). Eds. (Springer-Verlag, New York, 1991),
c e s s i n g , a n d a n e w e x c i t i n g field,
4. D. A. Cremers and L. J. Radziemski, in p. 53-59.
pulsed laser deposition (PLD). For Laser Spectroscopy and its" Applications, 24. X. L. Mao and R. E. Russo, "Thermal
PLD, any material can be "sam- L. J. Radziemski, R. W. Solarz, and J. A. Vaporization and Inverse Bremsstrahlung
p l e d " a n d t h e v a p o r d e p o s i t e d as a Paisner, Eds. (Marcel Dekket; New York, Model to Describe Nanosecond Plasma
thin film. The new high-temperature 1987), pp. 351-415. Shielding", deliver at the Conference on
5. K. Dittrich and R. Wennrich, Prog. Anal. Laser Ablation (COLA), EMRS Meeting,
superconducting (HTSC) materials Atom. Spectrosc. 7, 139 (1984).
h a v e p r o v i d e d a n " e x p l o s i o n " o f la- Strasbourg, France (1995).
6. K. Laqua, in Analytical Laser Spectros- 25. P. L. Kelley, Phys. Rev. Lett. 15, 1005
s e r a b l a t i o n t e c h n o l o g y . 32-36 I n t h e copy, N. Omenetto, Ed. (John Wiley and (I965).
w o r d s o f R i c h a r d L. G o r d o n , " l a s e r Sons, New York, 1979), pp. 47-118. 26. R. Y. Chiao, E. Garmire, and C. H.
7. M. yon Allmen, Laser-Beam Interactions Townes, Phys. Rev. Lett. 13, 479 (1964).
ablation applications are outpacing
with Materials: Physical Principles and 27. R. Fabbro, E. Fabre, E Amiranoff, C.
f u n d a m e n t a l u n d e r s t a n d i n g s " . 37 T h e Applications (Springer-Verlag, New York, Garbau-Labaune, J. Virmont, M. Wein-
same issues influencing laser abla- 1987), p. 16. feld, and C. E. Max, Phys. Rev. Ser. A
tion for chemical analysis hold for 8. N. Bloembergen, in Laser Solid Interac- 26, 2289 (1982).
t h e s e o t h e r a p p l i c a t i o n s . O n e t h i n g is tion and Laser Processing, S. D. Ferris, 28. O. L. Landen and W. E. Alley, Phys. Rev.
H. J. Leamy, and J. M. Poate, Eds. A 46, 5089 (1992).
clear, though: analytical spectrosco- (American Institute of Physics, New 29. G. Farkas and C. Toth, Phys. Rev. A 41,
p y is a n i m p o r t a n t t e c h n o l o g y f o r York, 1979). 4123 (1990).
elucidating laser ablation behavior. 9. J. E Ready, Effect of High-Power Laser 30. X. L. Mao, M. A. Shannon, A. J. Fernan-
Through these and related studies, Radiation (Academic Press, New York,
dez, and R. E. Russo, Appl. Spectrosc. 49,
1971). 1054 (1995).
laser ablation can evolve from an un-
10. Y. V. Afanasyew, O. N. Kroklin, and G. 31. A. J. Fernandez, X. L. Mao, M. A. Shan-
controlled and unpredictable explo- V. Sktizkov, IEEE J. Quant. Elec. JQE- non, and R. E. Russo, Anal. Chem. 67,
s i o n to o n e t h a t is r o u t i n e l y u s e d f o r 2, 483 (1966). 2444 (1995).
analytical chemical analysis. Table 11. J. E Ready, J. Appl. Phy. 36, 462 (1965). 32. T. V. Venkatesan, in Laser Ablation: Prin-
III s u m m a r i z e s i d e a l c o n d i t i o n s w e 12. A. A. Rostami, R. Greif, and R. E. Russo, ciples and Applications, J. C. Miller, Ed.
Int. J. Heat Mass Transfer 35, 2161 Springer Series in Material Science 28
e n d e a v o r to a c h i e v e . (1992). (Springer, New York, 1994), pp. 313-327.
13. T. J. Goldsack, J. D. Kilkenny, B. J. 33. Pulsed Laser Deposition of Thin Films,
ACKNOWLEDGMENTS MacGowan, S. A. Veats, R E Cunning- D. B. Chrisey and G. K. Hubler, Eds.
ham, C. L. S. Lewis, M. H. Key, R 32 (John Wiley and Sons, New York, 1994).
This paper would not be possible without Rumsby, and W. T. Toner, Opt. Commun.
excellent discussions, and experimental and 34. R. E Reade, P. Berdahl, L. W. Schaper,
42, 55 (1982). and R. E. Russo, Appl. Phys. Lett. 66,
theoretical contributions, from Xianglei Mao, 14. W. W. Duley, Laser Processing and Anal-
Mark Shannon, Wing-Tat Chan, Alberto Fer- 200l (1995).
ysis of Materials (Plenum Press, New' 35. R. E. Russo, X. L. Mao, and D. L. Perry,
nandez, Manuel Caetano, and Marvin Kilgo. York, 1983).
This research was supported by the U.S. De- Chemtech, December, 14 (1994).
15. R. E. Russo, "Direct Solid introduction 36. R. P. Reade, P. Berdahl, S. M. Garrison,
partment of Energy, Office of Basic Energy to the ICP Using a Repetitively Pulsed
Sciences, Chemical Sciences Division, Pro- and R. E. Russo, Appl. Phys. Lett. 61,
Nd:YAG Laser", delivered at the Feder- 2231 (1992).
cesses & Techniques Branch, under Contract ation of Analytical Chemistry and Spec-
No. DE-AC03-76SF00098. 37. R. L. Gordon, Pacific Northwest Labora-
troscopy Societies Meeting, Philadelphia tories, personal communications.
(1984).
1. S. A. Darke and J. E Tyson. J. Anal. At.
Spectrom. 8, 145 (1993).

28A Volume 49, Number 9, 1995