Energy Transfer between Excited Atomic, Molecular and
Ionic Species in a Laser Produced Spark in Ar–O2 and Ne–O 2
Gas Mixtures
M. A. Khan1, Faisal Saeed1 and M. A. Baig2
1
Physics Department, COMSATS Institute of Information Technology, Park Road, Islamabad 44000,
Pakistan
2
Physics Department, Quaid-i-Azam University, Islamabad 44000, Pakistan
Abstract
We have investigated laser induced breakdown in Ar - O2 and Ne - O2 gas mixtures with
different O2 compositions to study possible energy transfer between excited atomic and ionic
species (Ne, O, Ar, Ne+, Ar+, O+) in these gas mixtures. Laser pulses of 5 ns duration with a
repetition rate of 10 Hz from a frequency doubled Nd: YAG (λ = 532nm) delivering up to 75
mJ energy per pulse were used to create sparks. Energy transfer was inferred from the
changes occurring in the intensities of specific atomic and ionic transitions in the respective
emission spectra of Ar - O2 and Ne - O2 gas mixtures compared with the corresponding
intensities in the spectra of pure Ar, Ne and O2 gases. Strong evidence of energy transfer from
Ar and Ne to O was recorded. High lying metastable levels of Ar and Ne are believed to play
important roles in this collisional energy transfer.
Keywords: Laser Sparks, Gas Mixtures, Collisional Energy Transfer, Optical Emission Spectroscopy
1. Introduction
When a high power laser pulse is focused in a
gas, a spark is instantly created [1-4].
Simultaneous absorption of many laser photons
by atoms or molecules of the gas results in some
ionization (multi-photon ionization, MPI) and
release of electrons. The free electrons gain
energy from the laser field through inverse
Bremsstrahlung and some nonlinear processes
[5-6]. The energetic free electrons spearhead
further collisional ionization of gas atoms or
molecules creating additional free electrons.
Such collisions occur again and agarepeatedly
and form a cascade ionization [2, 7]. The
temperature of the gas in the focal volume
increases and expansion starts immediately.
Initially, the energetic free electrons diffuse into
the surrounding volume where they create
excitations and/or ionization of atoms and
molecules. The excited atoms, molecules and
ions also expand into the surrounding gas where
energy exchange continues. During the laser
pulse and afterwards, rapid recombination of
free electrons and ions in the plasma takes place.
This is responsible for the emitted radiation as
the electrons cascade down through excited
states.
Energy transfer through collisions between
atomic and ionic species is important in laser
sparks in Ar – O2 and Ne – O2 gas mixtures
because this leads to a significant enhancement
in the density of atomic oxygen that has many
applications in material processing. Although
energy transfer between noble gas atoms and
oxygen has been investigated in some lowpressure glow discharges [8-11], it has not been
studied in laser-produced sparks in gas mixtures
in any significant detail.
2. Experimental Set Up
Our experimental set up consisted of a stainless
steel chamber evacuated down to a pressure of
10-6 mbar before being filled with different
compositions of Ar-O2 and Ne-O2 gas mixtures
with varying percentage (5% to 40%) of O2 at a
total pressure of 1 bar. Laser pulses of 5 ns
duration with a repetition rate of 10 Hz from a
frequency doubled Nd: YAG (λ = 532nm)
delivering up to 75 mJ per pulse were focused
with a 5 cm focal length lens. The light emitted
from the spark was transferred by an optical
fiber to HR2000 high-resolution spectrometer
(Oceans Optics) with a resolution of ~ 0.05 nm
(FWHM). Spectra in the 200–725 nm region
were recorded.
3. Results and Discussions
The recorded spectra contained emissions from
various atomic, molecular and ionic species.
However, our primary interest was in collisional
energy transfer from highly excited atomic and
ionic states of Ar and Ne to O2. In this context,
excited Ar and Ne atoms in metastable levels
play a very important role in energy transfer
because these levels have long radiative
lifetimes and can contribute significantly
through collisions. We compared the line
intensities in the spectra of gas mixtures with the
corresponding line intensities in the spectra of
pure gases taken as a reference.
When analyzing the spectra from laser sparks in
gas mixtures with varying compositions of
constituent gases in the mixture, the line
intensities are expected to increase or decrease
in a systematic way as the percentages of
particular excited atoms or ions increase.
However, our interest was focussed on cases
where drastic changes occured. As an example,
figure 1a shows the changes in line intensities of
two transitions, 4p ® 4s of Ar-II at 480 nm and
4p ® 4s of Ar-I at 706 nm with increasing O2
fraction in Ar-O2 gas mixture. The 480 nm line
simply shows a linear decrease in the intensity
when O2 is added in the mixture and the
percentage of Ar is reduced proportionately. The
upper level of this transition is not significantly
disturbed by the addition of O2. On the other
hand, the 706 nm transition shows an
exponential decrease in intensity with increasing
percentage of O2 in the mixture suggesting a
strong influence of O2 possibly through direct
collisional transfer of energy from the upper
level of this transition or some other collisional
or radiative channel feeding this level. Fig. 1b
shows the intensity behavior of two transitions
in O atoms, the 3p ® 3s of O-I at 715 nm and
the 3p ® 3s of O-II at 375 nm in the Ar-O2 gas
mixtures. The 375 nm line shows a much higher
intensity when a small amount of O2 is added to
the Ar gas. Particularly noteworthy is the
comparison between the intensity of this
transition when 100% O2 is there and when only
5% O2 is added to Ar. Interestingly, the intensity
of this line goes on increasing as O2 fraction is
reduced from 40% down to 5%.
Figure 2a shows the exponential decrease in
intensities of several transition including the 5s4p and 4p-4s transitions of Ar II, and 4d-4p and
4p-4s transitions of Ar I. A corresponding
exponential increase in the intensities of several
transitions of O-II is shown in figure 2b. This
would suggest that energy transfer from Ar II
takes place from 5s level of Ar II. However, it is
equally probable that energy transfer takes place
at the metastable levels because of which higher
excitation to Ar II is reduced.
The situation in the case of laser spark in Ne-O2
gas mixture is quite similar. Figure 3 shows the
exponential decrease in intensity of several 3s 3p transitions of Ne I at 597, 616, 630 and 703
nm when O2 is added to Ne in different
proportions. There are similar increases in the
intensities of several O II and O I transitions.
O-II at 375 nm in Ar-O2 gas mixture with
different O2 fraction.
These provide strong evidence of collisional
energy transfer from Ar or Ne to O atoms.
(a)
(a)
(b)
(b)
Figure 1: Intensity behavior of different
transitions a) 4p ® 4s and 4p ® 4s transitions
of Ar-II at 480 nm and Ar-I at 706 nm Ar, and
b) 3p ® 3s of O-I at 715 nm and the 3p ® 3s of
Figure 2: Intensity behavior of different
transitions of a) Ar, and b) O in Ar-O2 gas
mixture with different O2 fraction.
As is well known, changes in electron density
and electron temperature due to addition of O2
strongly influence the excitation and ionization
rates. We have estimated changes occuring in
the electron temperature when O2 proportion is
increased in the mixtures.
Acknowledgment
The support of Higher Education Commission,
Pakistan and COMSATS Institute of
Information
Technology,
Islamabad,
is
gratefully acknowledged.
References
1. A. J. Alcock, M. C. Richardson, Phys.
Rev. Lett. 21, 667 - 670 (1968)
2. T. X. Phouc, Optics and Lasers in
Engineering, 44, 351 - 397 (2006)
3. A. Gold, H. B. Bebb, Phys. Rev. Lett.,
14, 60 - 63 (1965)
4. T. Keldysh, Sov. Phys. JETP, 20, 1307 1314 (1965)
Figure 3: Intensity behavior of different
transitions of Ne in Ne-O2 gas mixture with
different O2 fractions.
3. Conclusion
Analysis of the intensity behavior of various
atomic and ionic transitions recorded in the
emission spectra from laser created sparks in ArO2 and Ne-O2 gas mixtures with varying
fractions of O2 has indicated strong collisional
energy exchanges between atoms and ions of Ar
or Ne and O2 leading to enhanced O atom
production. Some of the energy levels may be
directly involved in energy transfer while some
others may be located on the path of decay
cascades during the recombining phase. Efforts
are underway to identify the exact energy
transfer channels involving resonant or nearresonant energy transfers.
5. H. Brysk, J. Phys. A: Math. Gen., 8,
1260 - 1264 (1975)
6. C. Grey - Morgan, Rep. Prog. Phys., 38,
621 - 665 (1975)
7. P de Montgolfier, J. Phys. D: Appl.
Phys., 5, 1438 - 1447 (1972).
8. M. A. Khan, A. M. Al - Jalal, J. Appl.
Phys., 104, 123302 (2008)
9. A. M. Al-Jalal, M. A. Khan, Plasma
Chem. and Plasma Processing, 30, 173 182 (2010)
10. M. A. Khan, A. M. Al-Jalal, J. Appl.
Phys., 99, 0333302 (2006)
11. M. A. Khan, A. M. Al-Jalal, Appl. Phys.
Lett.., 89, 171501 (2006