447
J. Chem. Phys., Vol. 111, No. 2, 8 July 1999
Role of Al–O chemistry
2
A possible mechanism by which the emissions can be
prolonged invokes a chemistry-dependent model involving
The resonantly enhanced intensities exemplified in Fig. 2
provide evidence that additional energy is deposited on the
AlO produced. It partly explains the behavior of the en-
16
laser vaporization of solids in air, and combustion of Al
1
7
droplets. The long time scales of Al emission observed can
be attributed to the presence of air. It has been shown4,16,18
that an increase to mbar pressures or introduction of oxygen
drastically affects the vaporization process. An increase in
emission lifetimes18 and rapid decrease in velocities resulting
in a more localized spatial extent of reaction products has
been reported. Furthermore, evidence for the formation of
fragments ejected from the surface that would eventually de-
hanced intensities for ϭ514 nm in Fig. 3. When L
L
ϭ514 nm, part of the intensity plotted at tϭ20 s arises
from the resonantly enhanced intensities described in Fig. 2.
The AlO intensity is therefore enhanced compared with the
AlO intensity when ϭ506 nm. When the laser is off at t
L
у8 s, the resonance no longer exists. However, it is seen
in Fig. 3 that the AlO intensities are still higher. More strik-
ingly, the Al intensities are drastically affected by the laser
wavelength, even if the laser is resonant with the reaction
product. Equation ͑1͒ is therefore insufficient to describe the
19,20
compose have also been reported.
If Al fragments are
indeed formed, and these vaporize as droplets, then long
chemical lifetimes are expected. The combustion of Al drop-
lets is a slow process.17 It is hypothesized to occur through
the peeling off of layers on the surface resulting in a decrease
in diameter ͑d͒ through the equation
enhanced intensities with resonant at extended times.
L
The interactions between the laser, the film, and the oxy-
gen atmosphere are complicated. Increased collisions16 due
to the added energy, and energy also added from the resonant
laser are expected to contribute in the emission increase in
the following manner:
2
2
d ϭd Ϫt,
͑2͒
0
where d0 is the initial diameter and  is the burning rate
coefficient. In this process, vaporization is limited by the size
of the droplets, and the thermal conductivity to transfer heat
from the interior to the outside surface where radiative tran-
sitions can take place. The polynomial function involving the
combustion of Al droplets in time is consistent with the slow
decay curves of Al in Fig. 3. The Al curves are amenable to
polynomial fits that arise upon proper consideration of the
dependence of intensities on the droplet diameters. The func-
tional form IϰexpϪt/ , where is a chemical lifetime,
conventional for gaseous reactions, falls too quickly in time
to fit well the Al decay curves.
The temporal evolution of AlO emission intensity de-
pends on several factors, such as the rate constant for the
equilibrium reaction ͑1͒ and the subsequent reaction of AlO,
the radiative lifetime of excited AlO, the population of Al
AlO*ϩAl→AlOϩAl*.
͑3͒
Reaction 3, which is facilitated by AlO* molecules in the
vЈϭ5 state, can explain the early enhancement of the Al.
This process, however, cannot thermodynamically persist for
long times, up to 100 s in Fig. 3. It is not likely to last
because the radiative transitions for cooling are readily avail-
able for both Al and AlO vapors.
Chemical reactions peculiar to the extreme temperature
attained may occur in laser vaporization measurements. The
dissociation of AlO is not predicted to occur at ambient con-
ditions. Its dissociation energy in the ground state is reported
to be 5.24 eV.22 At room temperature and assuming a Bolt-
c
c
zmann ͓exp(ϪE/k T)͔ distribution of molecules, virtually no
b
88
molecule ͑1 in 10 ) can dissociate. The reactions may how-
ever proceed in the conditions attained in these measure-
ments. At 4500 K, the temperature inferred from reported
described above, the population of O in air, and the relative
2
11
6
AlO spectra, one in 10 molecules has the energy to disso-
ciate. The probability of dissociation further increases rap-
idly if it occurs in electronically excited surfaces with lower
dissociation energy. Further studies on these excited states
are necessary to make firmer conclusions.
closeness of Al and O for reaction ͑1͒ to occur. The rate
2
6
constant for reaction ͑1͒ is measured to be greater than the
rate constant for the subsequent reaction21 of AlO from 300
to 1400 K. Assuming these rate constants in the conditions of
these experiments would result in a different dynamic behav-
ior for the reactant Al compared with the intermediate AlO.
The rate constants, together with the lifetime of excited AlO
are, however, in the submicro and nanosecond time scales,
much faster then the times measured in these experiments.
What is observed therefore in emission reflects the nascent
population of excited AlO being produced in time. Given the
above model of Al emanating from droplets and the homog-
V. CONCLUDING REMARKS
The importance of chemical reactions in laser ablation
has been alluded to in various reports. With improved under-
standing of the ablation process and its importance in new
applications, especially in those involving chemical bond
formation/dissociation, its inclusion in models is necessary.
On a fundamental basis, these dynamic measurements of the
ablation process provide a window to investigate fast mate-
rial response at extreme temperature conditions difficult to
enous O population in air, the plateau or maximum of the
2
AlO intensities in time in Fig. 3 can now be explained in
terms of diffusion and transport properties for reaction ͑1͒ to
occur. AlO can be produced from Al vapor that emanates
from the Al droplets reacting with the surrounding oxygen.
Initially, the Al is concentrated near the original film position
so that AlO formation is limited by the availability of O2
around Al. Al then diffuses through the air, resulting in in-
creased AlO production. As the Al or oxygen is consumed,
and with subsequent cooling, the AlO intensity decreases in
time.
11,23
obtain otherwise.
ACKNOWLEDGMENTS
The Naval Research Laboratory and Office of Naval Re-
search support this work. Dr. D. Ladouceur, Dr. N. Garland,
and Dr. J. Owrutsky are acknowledged for discussions re-
garding Al combustion and AlO gas-phase spectra.
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