Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material
The results of study of the emission spectra of metals excited by laser pulses bursts under atmospheric conditions are presented. It is demonstrated that the area responsible for the atom irradiation of evaporated substance is essentially distant from that of the continual spectrum irradiation. As a...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
1999
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| Zitieren: | Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material / E. Zabello, V. Syaber, A. Khizhnyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 142-146. — Бібліогр.: 8 назв. — англ. |
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irk-123456789-1179552017-05-28T03:05:19Z Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material Zabello, E. Syaber, V. Khizhnyak, A. The results of study of the emission spectra of metals excited by laser pulses bursts under atmospheric conditions are presented. It is demonstrated that the area responsible for the atom irradiation of evaporated substance is essentially distant from that of the continual spectrum irradiation. As a result, the sensitivity of the qualitative analysis increases. For quantitative analysis it is necessary to select the analytical lines differing from those used for spark or arc emission analysis. 1999 Article Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material / E. Zabello, V. Syaber, A. Khizhnyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 142-146. — Бібліогр.: 8 назв. — англ. 1560-8034 PACS 52.50.J, 42.62 http://dspace.nbuv.gov.ua/handle/123456789/117955 621.37: 543.42 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The results of study of the emission spectra of metals excited by laser pulses bursts under atmospheric conditions are presented. It is demonstrated that the area responsible for the atom irradiation of evaporated substance is essentially distant from that of the continual spectrum irradiation. As a result, the sensitivity of the qualitative analysis increases. For quantitative analysis it is necessary to select the analytical lines differing from those used for spark or arc emission analysis. |
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Article |
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Zabello, E. Syaber, V. Khizhnyak, A. |
| spellingShingle |
Zabello, E. Syaber, V. Khizhnyak, A. Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Zabello, E. Syaber, V. Khizhnyak, A. |
| author_sort |
Zabello, E. |
| title |
Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material |
| title_short |
Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material |
| title_full |
Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material |
| title_fullStr |
Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material |
| title_full_unstemmed |
Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material |
| title_sort |
influence of temporal parameters of laser irradiation on emission spectra of the evaporated material |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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1999 |
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http://dspace.nbuv.gov.ua/handle/123456789/117955 |
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Influence of temporal parameters of laser irradiation on emission spectra of the evaporated material / E. Zabello, V. Syaber, A. Khizhnyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 1. — С. 142-146. — Бібліогр.: 8 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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AT zabelloe influenceoftemporalparametersoflaserirradiationonemissionspectraoftheevaporatedmaterial AT syaberv influenceoftemporalparametersoflaserirradiationonemissionspectraoftheevaporatedmaterial AT khizhnyaka influenceoftemporalparametersoflaserirradiationonemissionspectraoftheevaporatedmaterial |
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2025-07-08T13:04:09Z |
| last_indexed |
2025-07-08T13:04:09Z |
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1837084023732568064 |
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142 © 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 1. P. 142-146.
PACS 52.50.J, 42.62; UDK 621.37: 543.42
Influence of temporal parameters of laser irradiation on
emission spectra of the evaporated material
E. Zabello, V. Syaber, A. Khizhnyak
International Center «Institute of Applied Optics» of National Academy of Sciences of Ukraine, 254053, Kyiv, Ukraine,
phone: (38044) 212-21-58; fax: (38044) 212-48-12 khizh@writeme.com
Adstract. The results of study of the emission spectra of metals excited by laser pulses bursts under
atmospheric conditions are presented. It is demonstrated that the area responsible for the atom
irradiation of evaporated substance is essentially distant from that of the continual spectrum irra-
diation. As a result, the sensitivity of the qualitative analysis increases. For quantitative analysis it
is necessary to select the analytical lines differing from those used for spark or arc emission analy-
sis.
Keywords: laser, emission spectrum, erosive torch.
Paper received 16.02.99; revised manuscript received 12.05.99; accepted for publication 24.05.99.
I. Introduction
Despite evident advantages of the laser emission spec-
troscopy (LES) [1] the method did not find yet wide prac-
tical application . The primary problem here is low de-
tection ability, i.e., high threshold level in detecting the
low concentrated elements in a sample. This low detec-
tion sensitivity of the LES technique is basically caused
by high intensity continuos spectra that comes along
with discrete spectra whose components are associated
with material under the study and therefore of primary
interest.
An improvement of the laser-induced emission spec-
tra characteristics through complementary electrical dis-
charge of the evaporated media [2] substantially
complicates the whole system and adds limitations
immanent to this type of excitation. Emission spectra of
much better quality can be also achieved by reducing
atmospheric pressure around a sample under test [3],
however this will require vacuum chamber, which
complicates the entire system.
The process of the emitted media torch formation is
entirely non-stationary and can be divided onto several
consequent stages. First, it starts with absorption of the
laser radiation by a target under investigation with its
local overheating above melting temperature. This leads
to an explosive outbreak of the object�s substance, flash
of plasma, breakdown and overheating of the surround-
ing air layer, and finally to emission of continuos spec-
tra. The formed cloud of vapor enlarges and produces a
shock wave. This moving shock wave leaves behind it-
self an area with reduced pressure. Then as the tempera-
ture of the expanding cloud of vapor decreases, the at-
oms of the evaporated substance are getting down to
lower levels, the ions recombine, and quasi-steady ther-
modynamic equilibrium is established. However, the
vapor of the substance formed after a single pulse action
can�t move away from the target�s surface, being resist-
ed by the pressure of the surrounding atmosphere. So
the area of luminous vapor responsible for linear spec-
trum is overlapped with the area of continuous spectrum,
which decreases the detection limit of laser emission
method.
It was demonstrated in [4-6] that a significant trans-
formation of the laser induced torch radiation is observed
when two laser pulses (with ten microsecond interval)
143SQO, 2(1), 1999
E. Zabello et al.: Influence of temporal parameters of ...
expose the surface, as being compared with single pulse
action. Basing on those results and taking into account
the complicated multi-step mechanism of the formation
of excited atoms, it is interesting to learn the influence of
temporal characteristics of laser radiation on a charac-
ter of erosive torch radiation, when the initiating laser is
composed by a train of the pulses. The results of such
investigations are presented below.
II. Experimental set-up
A Q-switched YAG:Nd3+ laser with two different LiF
crystals (initial transparency T
01
= 70 % and T
02
= 90 %,
respectively) as passive Q-modulator was used in these
experiments. Placing the crystal into laser cavity result-
ed in formation of the train of pulses with an interval
10 µs or 15 µs respectively. The 40 µs time delay was
observed when both crystals were inserted simultaneous-
ly. The output energy 10, 15 and 20 mJ was obtained at
pumping energy 75 J and flash lamp time 200 µs. The
pumping energy variation affected both the generated
pulses number and the interval in their sequence, leaving
the output energy of the every pulse unchanged. The la-
ser radiation was focused into the target�s surface by lens
with F = 75 mm in the spot 100 µm diameter. The stud-
ied samples of copper, iron and tin, as well as their al-
loys were used for analysis. Such selection was made
because of difference in thermo-physical properties,
which allows to expand the obtained results onto some
wider class of materials.
Detection of the luminescent spectra was made with
spectrograph DFS-452 (on photographic film) or linear
CCD-array. By using the film, we could receive addi-
tional information about change of torch radiation along
of its axis. The CCD-array in its turn made it possible to
achieve direct PC data processing. However, because of
relatively small dimension of the array only limited nar-
row spectral band (about 10 nm) was taken.
III. Experimental results
Performed measurements revealed an essential depen-
dence of the emission spectrum behavior upon temporal
characteristics of the laser pulses. The most striking
changes have been observed with first type of the Q-
modulator (T
01
). In this case, the maximum range of re-
producability for a quantity of pulses and interpulse sep-
arations could be achieved. By varying the pumping en-
ergy from threshold level 15 J up to 75 J, the number of
the pulses in train was changed from 1 to 18-20, while
interpulse separation smoothly decreased from 20 µs (two
pulses) down to 10 µs.
Fig. 1 illustrates the character of the emission spec-
tra for the steel sample DM-2 which is composed by 98 %
of iron. Let�s admit that all these spectra were obtained
at equal exposure level of the sample. This means that
the total amount of energy affecting the sample was
managed to remain the same, i.e., the variation of the
number of pulses was compensated by corresponding
variation of their energy.
For single 10 mJ pulse of 10 ns (Fig. 1a) an intensive
continuous background was observed within all spec-
tral range, being completely overlapped with entire range
of the discrete spectra.
In double-pulse mode (pulse separation 20 µs, 10 mJ
in each pulse) certain increase of the discrete spectrum
radiation intensity took place at the torch�s top, with
slightly narrowed range of continuous radiation (Fig.
1b). As the pumping level increases (which is accompa-
nied by concurrent increase in the number of pulses in a
train and decrease in their time interval) a certain in-
crease of the discrete spectrum range takes place with
simultaneous narrowing of a continuous radiation com-
ponent. A lasting regime with 18-10 pulses in train and
10 µs separations was found to be optimal (Fig. 1c). The
results of measurements of the torch�s height as a func-
tion of the number of pulses in a train are shown in the
Table 1.
As follows from the above results, the action of the
train of pulses on the target leads to a significantly in-
crease of the torch�s area that emits the discrete spec-
trum. An area of radiation of continuous spectrum de-
creases. Therefore the area of the torch available for car-
ing out spectral analysis decreases. Moreover, in this case
we observed strong increase in the number of the spec-
tral lines of the discrete spectrum as their luminous zone
is beyond that of the continuous spectrum. Thus, the an-
alytical spectrum quality and detecting limit increase.
When crystal number 2 or combination of both crys-
tals was used, qualitatively similar changes of spectrum
were observed. However, in this case both the intensity
of the discrete spectrum and the ratio between the width
of «discrete zone» and the corresponding width of «con-
tinual zone» became worse.
Fig. 1. The emission spectrum photo of steel sample for differ-
ent number of laser pulses: a � monopulses, b � double pulses,
c � 18-20 pulses in packet. Vertical coordinate is distance along
the input aperture of spectrograph.
λ, nm
360 344
5
m
m c
b
a
E. Zabello et al.: Influence of temporal parameters of ...
144 SQO, 2(1), 1999
Measurements of the spectral line intensities (I) were
made with a CCD-array. The spectrograms for the sam-
ple of steel for three above described cases are presented
in Fig. 2. The measurements were performed for zone of
the torch lying above the area of continual radiation.
With increasing the number of pulses in the train and
decreasing their interval, the growth of the intensity of
the weak lines is observed. Such an enrichment of the
detected spectra at the expense of the weak lines increas-
es sensitivity for both qualitative and quantitative anal-
ysis.
The spectrograms corresponding to different zones
of the torch are presented in Fig. 3. Here, the plot a (Fig.
3a) matches to the lower zone of the torch (but higher
than zone with continual spectrum). Figs 3b and 3c be-
long to the central and upper torch�s zones respectively.
Apparently, the ion�s associated lines as well as slan-
der background have been observed in the area of the
torch that is close to the target�s surface. With increasing
distance from the surface, because of the ion recombina-
tion, the whole spectra acquire the character typical for
atoms which simplifies its interpretation. Furthermore,
with transition to distant regions of the torch the line
width in the emission decreases (Fig. 4), which indicates
the temperature decrease in this area.
It follows from the performed measurements of the
integral torch luminescence that its duration is about 1 µs,
which is considerably shorter than an interval between
the laser pulses themselves. When this interval was se-
lected to be longer than 20 µs, the ratio between the am-
plitude of the torchluminescence and the amplitude of
corresponding laser pulses remains unchanged within the
limit of all train. By reducing the time interval in the train
to about 10 µs this ratio became dependent upon the num-
ber of pulse, reaching its maximal value for the every third
pulse.
Table 1. Dependence of torch height on number of pulses in train for copper and steel target.
Fig. 2. Emission spectrum of steel sample: a � monopulses, b �
double pulses, c � 18-20 pulses in packet.
Fig. 3. Emission spectrum of tin sample for different parts of
torch: a � lower zone, b � middle zone, c � upper zone.
Conditions Material Torch height, mm Background height, mm
1 pulse Cu 1,15 0,85
Fe 1,10 0,80
2 pulses Cu 1,30 0,85
Fe 1,80 0,80
7 pulses Cu 3,20 0,45
Fe 4,50 0,35
12 pulses Cu 4,30 0,35
Fe 6,40 0,25
145SQO, 2(1), 1999
E. Zabello et al.: Influence of temporal parameters of ...
IV. Discussion
According to [4, 5], where the double-pulse action was
studied, the observed improvement in the signal-to-noise
ratio can be explained by formation of zone with low
pressure around the interaction area. Therefore the sec-
ond laser pulse takes place under lower pressure condi-
tions.
However, the effects detected in our experiments can
not be explained by only this factor [7]. Indeed, the com-
ponent with continuous emission spectra can be reduced
with the atmospheric pressure decrease. Moreover, the
vapors ejected by subsequent pulse will propagate in the
area with lower pressure, whicht makes it possible for
this vapor to keep its kinetic energy and to shift the shell
of the atmosphere away on a greater distance. This can
explain an enlargement of the torch (see Tab. 1).
Moreover, every consequent pulse ejects the vapor
into the area that contains the evaporated substance gen-
erated by previous pulses. As a result, the propagating
front of the new portion of the evaporated substance
affects and interacts not only with pure air, like the first
pulse does, but with the vapor already containing the
target�s substance. All that makes the excitation process
to be more effective and in this way enhances the intensi-
ty of the signal.
Finally, as the ejected vapors propagate in a space
with decreased pressure, they can move on larger dis-
tance and move away from the target�s surface, in other
words, leave the area where the continuous spectrum is
formed. Due to larger propagation length the ions have
enough time for their recombination, while the atoms
have not time for deactivation, so they drop to quasi-
steadystate condition. This last process is accompanied
by discrete spectra emission as it is seen from the above-
presented picture.
Apparently, when the interval between the pulses in
train coincide with time of the formation of maximum
concentration of vapor in the torch area [8], the condi-
tions of excitation becomes nearly optimal. In our ex-
periments such regime corresponds to 10 µs interval be-
tween the pulses.
Thus, in going to multiple pulse laser regime a signif-
icant enlargement of the radiating zone of the torch takes
place, and, as a result, the number of registered lines in-
creases. By using such form of multiple pulse action the
achieved detection limit was about 10 ppm at standard
atmospheric conditions without any additional means
of excitation.
The potentials of quantitative analysis were investi-
gated on the samples of steel standards 461-464, one of
which being used for calibration. The paires of the lines
analogous to those for quantitative analysis with spark
discharge method were selected in these experiments. For
a concentration of the impurities not less than 10 %, the
error of measurements was about 2 %, and at impurity
levels less than 1 % the error reached 50 %. The dispro-
portional changes in correlation between the intensities
of the spectral lines for the material with strongly differ-
ent concentration of components are responsible for this
result. The comparison of spectra obtained at laser exci-
tation and those at spark discharge shows their radical
difference, for both the number of the registered lines
and their relative intensities. That is why, performing the
analysis with LAS technique, there is a strong necessity
in selection of the additional analytical lines, as well as
the most optimal spectral range for such measurements.
Acknowledgement
Authors wish to express their thanks to Dr. S. Anokhov
of the International Center �Institute of Applied Optics�
for fruitful discussions contributed much to the obtain-
ing results.
References
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num Press, New York and London (1983).
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torch, 2 � lower zone of torch.
E. Zabello et al.: Influence of temporal parameters of ...
146 SQO, 2(1), 1999
6. 6. R. Satman, V. Sturm, R. Noll, Laser-induced break down spec-
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