# Mass-Charge and Energy Spectra of Oxygen Ions in a Two-Element Laser-Produced Plasma

Authors: R. T. Khaydarov; U. S. Kunishev
Journal: International Journal of Plasma Science and Engineering
 (2008)

---

## Abstract

Using a static mass spectrometer, we study the characteristics of multicharge plasma ions generated from solid targets under the action of a
15 nanosecond  Nd:YAG laser radiation with maximal intensity1011 W/cm2. We consider two-element solid targets with a mass of the heavy component ranging from 44.9 (Sc) to 174.9 (Lu) with main attention to the properties of oxygen ions. The time-of-flight measurements show that oxygen ions are obtained in the range of the energy
E  =  40–250 eV with maximal charge Zmax=2. The latter is independent on the target composition for the given intensity of the laser radiation. However, the properties of the energy spectra of oxygen ions strongly depend on the second component of the target, which is explained by the interaction between the light and heavy elements of the target.


---

## Body

## 1. Introduction

At present, there  is a number of works
devoted to  the study
of laser-produced plasma  as a source of ions for the
inertial confinement fusion [1–5] together with heavy ion accelerators and the
systems on the base of powerful impulse of electrical charge, that is, Z-pinches. Laser ion source (LIS) has been
recently designed [6] to load the Heidelberg electron beam ion trap with a
pulsed beam of lowly charged ions from solid elements. Due to many
characteristics of laser-produced plasmas, LIS takes advantages over, for example,
a common metal vapor vacuum arc method [7] as a source of ions.Most of the studies of laser-matter
interaction field have been carried out for the laser intensities ranging from
1012  to 1019 W/cm2, where the interests were
directed towards investigations of laser-fusion, X-ray lasers, shock-related
phenomena, and so on. Not much attention was given to the low intensity part of
the laser radiation (109–1012 W/cm2). However, this regime was shown to be very useful for the
material scientists in the fields of material preparation, for example, of thin
films of high-Tc   superconductors [8]. The laser pulsed-deposition
technique shows considerable promise for the fabrication of such films.Many theoretical [9, 10] and experimental
[11, 12] works have been carried out in order to optimize the performance of
the laser ion source and to determine important operating parameters such as
the velocity, mass, and charge-state distribution of the generated ion beam and
plasma temperature. Authors of [9]
considered the case of pulsed laser desorption from planar, binary targets with
the ratio of masses of light and heavy particles being 1 : 5 using the Monte
Carlo simulations. They have shown that the parameters of plasma generated from
two-element targets considerably differ from the ones obtained from single-element
targets: the collisions between light and heavy components of the plasma take
place and, as a result, the particles move predominantly in the direction
normal to the target surface. These particles are more energetic than those
hitting the detector at an oblique angle. They further observed that the energy
is transferred from the lighter to heavier particles. In addition,
lighter species have higher velocity than the heavier particles and focus the heavier particles towards the target normal. However, the mean energy of the
heavier species is always higher than that of the lighter ones. The energy
exchange between different species of the plasma has been recently shown
experimentally by Khaydarov et al. [11]. They investigated time-of-flight (TOF)
spectra of two-element PbMg targets, where the concentration of light element
Mg was fluently changed. It was shown that the energy spectra of both light and
heavy ions were broadened compared to the spectra of one-element plasma due to
the energy exchange between light and heavy ions. For example, the increase of
Mg concentration leads to the broadening of energy spectra of Pb ions for two
times and the impulse duration for 3 to 10 times compared to one-element
targets. Very recently, Srivastava et al. [12] showed that the energy transfers
not only among light and heavy ions of the same species but also among
different ionization states. They have also shown that the results of [9], which were obtained for neutral
particles, are valid for ionized gases as well.In this work, we study parameters of
the plasma ions generated from the surface of two-element targets (i.e., oxides
of elements), where the mass of the main component of the targets changes from
44.9 (Sc) to 174.9 (Lu), under the action of pulsed laser radiation using a
15 nanoseconds Nd:YAG laser,
at the laser intensity 1011 W/cm2. As oxygen atoms are
present in most of the targets used in such experiments, we mainly focus on the
properties of oxygen ions. We show that mass-charge and energy distribution of
O ions strongly depend on the nature of the second component of the target,
which is related to the mutual interaction of the target components.


## 2. Experimental Setup

Experiments were carried out on a
static laser mass spectrometer with mass resolution ofm/Δm∼ 100 and
time-of-flight distance L = 100 cm. The latter helps to separate ions of
different mass
away from the plasma region. The Nd:YAG laser beam was directed normal to the
surface of the target. The duration of the laser impulse was 15 nanoseconds and the
power density of the maximal laser radiation at the target surface is q = 1011 W/cm2. The peak power of laser radiation varied within
5% and each
experimental data is
averaged over five impulses of the laser radiation. The applied
extraction voltage enables us to select ions with given energy. All experiments
were carried out at the same inertial conditions (vacuum (10−6 Torr), focusing condition
of laser radiation, parameters of electrostatic mass spectrometer, etc.). The
following targets (in the form of thick disks of diameter 10 mm) have been used
in the experiments: Sc2O3 (m = 44.955), Y2O3 (m = 88.905), Ce2O3 (m = 140.11), Eu2O3 (m = 151.96), Dy2O3 (m =  162.50), Tm2O3 (m
= 168.93), Yb2O3 (m = 173.05), and  Lu2O3
(m = 174.96). Although the properties of plasma ions show nonlinear dependence on
the intensity of the laser radiation (see Figure 1), we present here
experimental results obtained for the maximal intensity of the laser radiation
q = 1011 W/cm2.Figure 1
Dependence of the maximal chargeZmax of
Sc (solid curve) and O (dashed curve) ions in the plasma on the intensity of
the laser radiation q.


## 3. Experimental Results and Discussion

Experimentally, we obtained
mass-charge spectra, that is TOF spectra of ions, in two-component
laser-produced plasma. Figure2 shows typical TOF spectra
(i.e., the intensity
of ions current as a function of time) of ions in Ce2O3 (Figures
2(a), 2(b)) and Lu2O3
(Figures 2(c), 2(d)) plasmas for two values of
the ions energy: E/Z = 25 eV (Figures 2(a),
2(c)) and E/Z  = 150 eV
(Figures 2(b), 2(d)). It is seen from this figure that the O ions with maximal charge Zmax
=  2
are observed at low energies of the ions and these peaks in the spectra
disappear at higher energies. The ions of the second component of the target
with maximal charge (Zmax = 3) are clearly seen in higher energy part
of the spectra. We note that maximal charge of both light and heavy components
of ions does not depend on the nature of two-element targets for a given
intensity of the laser radiation. The intensity of O ions strongly depends on
the target composition (especially in low energy part of the spectra). For
example, for the energy of ions 25 eV, the intensity decreases with increasing
the mass of the second component of the target (see Figures 2(a) and 2(c)). The
latter may be explained as follows: as both species acquire the same
temperature under the laser irradiation, lighter particles are desorbed with a
higher velocity than the heavy species. Monte Carlo simulations show that in
the latter particles move predominantly in the direction normal to the surface,
where heavy particles are more strongly focused than the lighter ones [9]. Then
the energy is transferred from the light to the heavy species due to the
collisions. To our understanding, as the mass of the heavy species increases
the total number of collisions also increase leading to higher probability of
the light particles to go away from the direction of the detection, as well as
to scatter back towards the target surface. However, this explanation is very
crude keeping in mind very complex physical processes during the plasma
formation and expansion, and more theoretical investigations are needed for
this issue.Typical
mass-charge spectra of ions in two-element plasma, generated by the laser
radiation from (a), (b) Ce2O3 and (c), (d) Lu2O3
at energies (a), (c) E/Z
=   25 eV,  and (b), (d) E/Z  =  150 eV.
(a)(b)(c)(d)From the obtained mass-charge spectra,
we constructed energy distribution of the ions, which allows us to study the
effect of target composition on the parameters of plasma ions. As an example,
we plotted in Figure3 the energy spectra of ions from (a) Sc2O3 and (b) Lu2O3  plasmas. As seen from
this figure, plasma ions have a broad energy spectrum with a single maximum of
the distributions. The spectrum consists of different species of ions located
in different energy ranges. It is also possible to observe that by increasing the charge-state the
energetic distribution is shifted towards higher energy, according to the
Coulomb-Boltzmann-shifted model proposed by Torrisi et al. [13]. But the
decrease of  the ion yields does not follow the exponential low predicted by
Shirko-Lotz theory of the ionization cross-sections [14]. O ions with two ionization states are
located in low energy part of the spectra, while Sc and Lu ions are located in
higher energy part. As expected, the maximal energies of the second component
of the target for all charge multiplicity of ions increase with increasing the
mass of the ions as illustrated in Figure 4 for the cases of Sc, Ce, and Lu
elements. The latter is in agreement with the previous theoretical studies [9].Energy
spectra of ions in two-element (a) Sc2O3 and (b)
Lu2O3 plasmas, obtained at q
= 1011 W/cm2.
(a)(b)Figure 4
Dependence of
the maximal energy of Sc, Ce, and Lu ions on the charge multiplicity of ions.Let us now consider the effect of the
second component of the target to the energy distribution of O ions in more
detail. Figure5(a) shows the energy spectra of O1+ ions in the
two-element Se2O3, Ce2O3, and Lu2O3 plasmas. As we mentioned
above, the character of the energy spectra, for example, the width and the maximal energy, of
O ions depends on the nature of the target. Single charged O1+ ions have a
narrow energy interval (Emax ≤ 100 eV) for
small mass ratio (i.e., Sc ions) (solid curve in Figure 5(a)). With increasing
the mass of the second component (Ce ions) the maximal energy increases more
than two times (dashed curve in Figure 5(a)). Although Emax of O1+ ions slightly decreases with further increasing the mass of the second
component of the target, it is still larger than the case of Sc ions. The
energy spectra of O2+ ions for different mass of
the second component of the
target are shown in Figure 5(b). O2+ ions are mostly located in the
interval of the energy between 40 eV and 120 eV and the energy range of the
ions in this case does not strongly depend on the target composition.Typical
energy spectra of (a) O1+ and (b) O2+ ions in two-element
Sc2O3 (solid curves), Ce2O3 (dashed
curves), and Lu2O3 (dotted curves) plasmas.
(a)(b)It is noticeable that the increase of
the mass of the second component of the target leads to considerable changes in
the spectrum of O ions. For example, the intensity and the maximal energy of O
ions increase with increasing the mass of the second component of the target,
which we relate to energy exchange between heavy and light components of the
plasma. As we mentioned above, this energy transfer was already shown both
theoretically [9] and experimentally [11, 12]. However, in these studies the
mass ratio of the plasma particles was fixed. In what follows, we have
performed our experiment for large number target composition.
Figure 6 shows
the maximal energies of O1+ (solid curves) and O2+ (dashed curves) ions as a function of the mass of the second component of the
target. It is seen from this figure that the maximal energy of single charged
oxygen ions increases with increasing m, which is in good agreement with
the recent experiments [12]. This may indicate that even light species of the
plasma also can gain from the collisions. With further increasing m, the
energy decreases and starts to saturate after m >160 amu. However, we could not explain this
dependence of the maximal energy of the O1+ ions on the mass of the
heavier species. The maximal energy of doubly charged oxygen ions does not
strongly depend on m.Figure 6
Dependence of
maximal energyEmax of O1+ (solid lines) and O2+ (dashed lines) ions on the mass of the second component of the plasma.Figure7 shows the dependence of the
total number of oxygen ions on m for both charge
multiplicities. As expected, the intensity
of single charged O1+ ions is always larger than the one for doubly charged
O2+ ions. The latter considerably decreases with increasing m, whereas
the intensity of O1+ ions increases first for m < 140 amu
and only after that it decreases for larger mass m. This behavior of
light ions is difficult to explain only in the framework of energy transfer
between the ions, but the complex thermal desorption mechanisms should be taken
into account.Figure 7
Total number
of O1+ (circles) and O2+ (squares) ions as a function of
the mass of the second component of the target.Comparative study of O ions and Sc
(Ce, Lu) ions shows that the process of formation and expansion of O ions of
all charge multiplicity is determined by the composition of the target. In
spite of their different thermophysical parameters, these elements differ with
the mass and it is much larger than the mass of oxygen (mass
of Sc, Ce, and Lu atoms are 3, 9,
and 11 times larger than the mass
of oxygen atoms). This difference in mass plays a considerable role in the
formation of two-element plasma, and consequently, to the energy exchange
between the ions. Due to the lack of the experimental results for pure Sc, Ce,
and Lu targets, it is difficult to conclude about the effect of the oxygen ions
to the properties of the ions of the second component of the target.


## 4. Summary

We have reported time-of-flight
measurements of ions produced by laser radiation from solid targets consisting
of light (oxygen) and heavy components of different atomic massm. We
found out that the maximum charge of plasma ions does not depend on the nature
of the target for considered low intensity of the laser radiation. However, the
maximal energy and intensity of light elements of all charge state strongly
depend on the mass of the heavy component of the target, which we related to
the energy-exchange between different kinds of ions.



---

*Source: 180950-2008-02-14.xml*