non-equilibrium plasma production by pulsed laser ablation of gold

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Non-equilibrium plasma production bypulsed laser ablation of goldL. Torrisi a b , S. Gammino a & L. Andò aa INFN-Laboratori Nazionali del sud , Catania, Italyb Dipartimento di Fisica , Università di Messina , ItalyPublished online: 29 Oct 2010.

To cite this article: L. Torrisi , S. Gammino & L. Andò (2002) Non-equilibrium plasma production bypulsed laser ablation of gold, Radiation Effects and Defects in Solids: Incorporating Plasma Scienceand Plasma Technology, 157:3, 333-346, DOI: 10.1080/10420150212997

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Radiation Effects and Defects in Solids, 2002, Vol. 157, pp. 333–346

NON-EQUILIBRIUM PLASMA PRODUCTION BYPULSED LASER ABLATION OF GOLD

L. TORRISIa,b,*, S. GAMMINOa and L. ANDOa

aINFN-Laboratori Nazionali del sud, Catania, Italy; bDipartimento di Fisica,Universita di Messina, Italy

(Received 20 February 2002; Revised 18 March 2002; In final form 23 March 2002)

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064 nm wavelength, 9 ns pulse width,900 mJ maximum pulse energy and a maximum power density of the order of 1010 W=cm2. The laser-targetinteraction produces a strong gold etching with a production of a plasma in front of the target. The plasmacontains neutrals and ions having high charge state. Time-of-flight measurements are presented for the analysis ofthe ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits to measure theyield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that theion charge state reaches 6þ and 10þ at a laser fluence of 100 J=cm

2and 160 J=cm

2, respectively. The maximum

ion energy reaches about 2 keV and 8 keV at these low and at high laser fluence, respectively. Experimental ionenergy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields,produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. Theelectrical fields induce ion acceleration away from the target with a final velocity dependent on the ion chargestate. The ion velocity distributions follow a ‘‘shifted Maxwellian distribution’’, which the authors have correctedfor the Coulomb interactions occurring inside the plasma.

Keywords: Plasma by pulsed laser; Ion source; Ion energy distribution; Ion charge states

Pacs Numbers: 79.20.Ds; 79.90.+b; 52.50.Jm

1 INTRODUCTION

In the last years the pulse laser ablation has been employed for several purposes, such as the

thin films deposition, the radiation effects in different materials, and the high temperature

plasma production.

Irradiating a target with a laser pulse having high power density, a strong emission of neu-

trals and ions can be produced in front of the target surface. This effect produces the forma-

tion of a small plasma volume, which expands very fast mainly along the normal to the target

surface, independently on the incidence angle of the laser beam [1].

The expansion generates energetic ions, the kinetic energy of which is of the order of keV

or MeV depending on the pulse energy density [2]. Produced ions can be implanted in

substrates to improve the deposited film adhesion or can be extracted from the plasma and

* Corresponding author. Tel.: þ39 95 542260; Fax: þ39 90 395004; E-mail: [email protected]

ISSN 1042-0150 print; ISSN 1029-4953 online # 2002 Taylor & Francis LtdDOI: 10.1080=1042015021000005207

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injected in ion sources, where further ionization processes take place [3]. A ‘‘Istituto Nazio-

nale di Fisica Nucleare-Laboratorio Nazionale del Sud (INFN-LNS)’’ research project, so

called ‘‘ECLISSE (ECR Coupled to Laser Ion Source for charge State Enhancement)’’,

developed in Catania, has the aim to use a laser ion source for an electron cyclotron resonant

(ECR) [4].

The metal ablation has been induced in many experiments by using IR lasers, with wave-

lengths for which the absorption effect is very high. Interesting results have been obtained

producing plasmas with laser power densities of the order of 1010 W=cm2 or more [5].

Free electrons absorb infrared photons and through the electron–electron, electron–atom

and electron–phonon interactions the absorbed energy is released to the emitted atoms,

ions, electrons, clusters and photons (IR, visible, UV and X-rays). The produced plasma

expands in vacuum at high velocity, of the order of 104 m=s. This is an adiabatic expansion,

which permits to give kinetic energy to neutrals and ions [6]. Generally, at high laser fluence,

a very fast evaporative process is induced and the laser light can be partially absorbed by the

same vapor emission. This interaction, through inverse Bremsstrahlung processes, generates

further ionization and increases the plasma temperature.

Previous measurements, concerning the laser etching rates, laser etching thresholds, ion

and neutral emissions, time-of-flight (TOF) and angular distributions of ejected atoms,

have been presented in recent articles [7, 8]. The actual measurements concern the production

of ions from a gold target, the determination of their energy distribution and of their charge

state distribution as a function of the laser fluence. A new theoretical approach concerning

the ion energy distribution as a function of the ion charge state is discussed.

2 EXPERIMENTAL SECTION

The employed laser is a Q-switched Nd:Yag pulsed laser with 1064 nm wavelength, 9 ns

pulse width, and 1–900 mJ pulse energy. The laser pulse is Gaussian and has 1 cm diameter.

The laser works at 30 Hz repetition rate or in single shot mode. The laser operates in the

TEM00 mode and has an energy stability better than 3%. The pulse energy was measured

with a highly sensible calorimeter. The laser beam is focused, through a convergent lens,

on a gold target placed inside a vacuum chamber at 10�7 torr. The optimum focalization dis-

tance ð50 cmÞ was determined minimizing the spot dimension observed on a gold sheet. The

laser light passes through a thin quartz window and reaches the gold target. The incidence

angle is 45� and the spot size 0:5 mm2.

The target consists of a pure gold sheet, with a polished 2 cm2 surface and 0:5 mm thick-

ness. The target is mounted on a holder the surface normal of which forms 45� with respect

to the incident laser beam. Figure 1 shows a scheme (a) and a photo (b) of the experimental

set-up of the laser laboratory at INFN-LNS of Catania.

Different ion collectors (IC) are placed horizontally, 60 cm distant from the target, at differ-

ent angles with respect to the laser direction: 17�, 30�, 45�, 56�. The IC consist of little Faraday

cups having an input grid connected to ground and of an inner collector biased at �100 V.

A cylindrical electrostatic deflection ion energy analyzer (IEA) is mounted at 45� angle.

The IEA contains a special annular ion collector at the input (60 cm distance from the target)

having a high transmission factor and an electron multiplier placed just behind the deflector

plates, at 1:5 m total distance from the target. Both IC and IEA detectors permit to perform

time-of-flight (TOF) measurements at 60 cm and 150 cm target distance, respectively.

The laser interaction produces a plume emission, which is directed along the normal to the

target surface, independently on the laser incidence angle. The emitted plume is a plasma

334 L. TORRISI et al.

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containing neutrals, ions, clusters and electrons, which are freely expanding in the vacuum.

This plasma has been investigated with IC and IEA combined with the time-of-flight (TOF)

measurements.

Figure 2a shows the electrical scheme of an ion collector. The ion current, Ic, measured at

the time t is given by:

IcðtÞ ¼VcðtÞ

½tRloadð1 þ g=zÞ�; ð1Þ

FIGURE 1 Scheme of the experimental set-up (a) and photo of the apparatus (b).

NON-EQUILIBRIUM PLASMA PRODUCTION BY PULSED LASER ABLATION OF GOLD 335

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where VcðtÞ is the voltage amplitude measured on the oscilloscope, t is the transparence of

the grid (58% in our case), Rload is the load resistance (25O in our case), g represents the

average value of the secondary ion-electron emission coefficient and z is the average charge

state of the ions. The g=z ratio was investigated in several experiments [9]. For the materials

applied for dynodes of the WEM, for the range of the ion energy and the ion charge state

found in our experiment, it is possible to use the approximation g=z 1. More details

about the IC detectors are reported in literature [10].

The output of each ion collector is monitored through a 500 MHz memory oscilloscope,

showing a mV signal as a function of the time-of-flight. A typical IC spectrum is reported

in the bottom of Figure 3. A very fast peak, at left, due to a laser photo-peak, indicates

the TOF start point. At the right, a broader peak is due to the total ions detection; it contains

information about the mean velocity of detected ions but not about their charge state.

The main part of the IEA is the deflection system, which is made of two coaxial metallic

cylinders or radii R1 (inner plate) and R2 (outer plate) maintained at potential V1 and V2,

respectively. A scheme of the IEA is shown in Figure 2b. The particles deflected by the plates

FIGURE 2 Scheme of the ion collector IC (a) and of the ion energy analyzer IEA (b) employed for time-of-flightand energy-to-charge measurements.


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can be detected through a windowless electron multiplier (WEM) connected to a high

frequency digital oscilloscope [10].

Generally the IEA works with a symmetric polarization, at which V1 ¼ �V2 ¼ U=2. In

these conditions, a particle with m mass, v velocity and ze charge is detected only if its

kinetic energy, E, is in agreement with the following relationship:

E=z ¼eU

½2 ln ðR2=R1Þ�¼ k eU ; ð2Þ

where k is a geometrical factor of the IEA which, in our analyzer, is about 10. Thus, the IEA

operates as a filter, which permits the passage of a given energy-to-charge state ratio E=z.

Changing the U voltage it is possible to detect ions with different E=z ratios. Generally, the

plate’s bias ranges between 10 and 120 V.

A typical IEA spectrum is reported in the top of Figure 3. In these cases, for a fixed bias U ,

the spectra show negative peaks each representing a given value of the E=z ratio. The tem-

poral distance between the start and the IEA-WEM peak permits to calculate, through the

known target-collector distance, the ion velocity and the ion energy. By changing, step by

step, the U bias, it is possible to detect different E=z ratios and to give the experimental

ion energy distributions as a function of the ion charge state.

3 RESULTS

Figure 4 shows three IEA spectra relative to the gold irradiation at low (a), average (b) and

high (c) laser fluence by using a U bias of 80 V and an incident angle of 45�.

At a low fluence of 50 J=cm2 only Au1þ, Au2þ and Au3þ are produced; at 100 J=cm2 the

charge state ranges between 1þ and 7þ and at the high fluence of 160 J=cm2 it ranges

FIGURE 3 Typical example of IC (lower section) and IEA (high section) spectrum produced at high laser fluenceirradiating gold.


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FIGURE 4 IEA spectra comparison detected at different laser fluences.


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between 1þ and 10þ. This result is in agreement with literature, which predicts a charge

state of about 50þ by using a laser fluence of about four order magnitude higher [11].

The vertical scale of Figure 4 is different for the three spectra; it is multiplied by a factor

3.0, 1.5 and 1.0 for the 50, 100 and 160 J=cm2 fluence, respectively. Results indicate that the

total ion yield increases about linearly with the laser fluence. Moreover, as previous measure-

ments have demonstrated, the fractional ionization (number of ejected ions with respect to the

total number of ejected atoms) of the plasma increases with the laser fluence and, at the

higher fluence used, the fractional ionization for gold may reaches about 32% [12].

From the time-of-flight measurements performed with IEA, it is possible to calculate the

ion velocity and the ion kinetic energy for each given species. The velocity of the Au1þ ions,

for example, as calculated from Figure 4, ranges between 1:44 � 104 m=s and

1:85 � 104 m=s at 50 J=cm2 and 160 J=cm2 fluence, respectively. At these velocities kinetic

energies correspond to 211 eV and 350 eV, respectively. At high laser fluence high charge

states and high kinetic ion energies are produced. For example, at 160 J=cm2 the Au10þ

ions are emitted with a velocity of 6:52 � 104 m=s, corresponding to a kinetic energy of

4:34 keV.

Changing the U bias of the plates, step by steps, from 10 V up to 120 V, different

E=z values can be selected and an ion energy scan can be performed for a fixed value of

laser fluence. In this way it is possible to measure the experimental energy distribution of

the ions ejected from the laser ablation as a function of their charge state. Repeating many

measurements, at different U biases and different laser fluences, many data concerning the

E=z ratio values can be collected and plotted.

Figure 5 shows two typical example of the ion energy distributions for gold ions obtained

at a laser fluence of 100 J=cm2 (a) and of 160 J=cm2 (b), respectively. Six and ten ion energy

distributions can be plotted for the two cases, in which six and ten charge states are produced,

respectively. For simplicity, Figure 5 reports only the comparison between the first four

charge states. The ion energy distributions as a function of the ion charge state are rich in

information about the generated plasma. They permit to understand the mechanisms of

plasma formation and to evaluate separately the thermal contribution, the plasma center-

of-mass kinetics evolution and the Coulomb interaction contribution to the total ion energy

distribution.

The ion energy distributions are represented by experimental data (points) and by the fit

curves (lines), which show trends similar as Maxwellian–Boltzmann distributions. Increasing

the charge state, the distributions show a shift towards high energies which is, the taller the

higher is the charge state. The distributions are the result of complex phenomena involving

the high plasma temperature, the ‘‘flow velocity’’ of the center-of-mass of the plasma, which

expands very fast from the target surface, and of a ‘‘Coulomb velocity’’ acquired by the ions

because of electrical fields generated inside the plasma. This last contribution is due to non-

uniform spatial charge distribution inside the plasma, as result of the fast escape of the

electrons with respect to the ions.

The fit function employed to plot the curves of Figure 5 is the following:

Fðvx; vk ; vcÞ ¼ Am

2pKBT

� �3=2

v3x exp �

m

2KBT

� �ðvx � vk � vcÞ

2

� �; ð3Þ

where A is a constant of normalization to the experimental data, vx is the gas kinetic velocity

(x denotes the direction normal to the target surface), which follows a Maxwell distribution in

the region where ion collisions occur (Knudsen layer) [13]; m is the mass of the ejected atom,

KB is the Boltzmann constant, T is the plasma temperature, vk is the ‘‘flow velocity’’ along


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the normal to the target surface (referred only to the neutral species) and vc is the ion velocity

in x direction due to Coulomb accelerations. Authors call this Eq. (3) as ‘‘Shifted Maxwell–

Boltzmann–Coulomb distribution’’. Converting Eq. (3) in terms of the energy distribution,

instead of the velocity distribution, the function can be employed to fit the experimental

data of the ion energy distributions, as presented in Figure 5.

The fit parameters are T , the plasma temperature, and vk þ vc, the ‘‘shift velocity’’ term,

containing the shift velocity for neutrals and for ions. The best fits indicate T values of

7 � 105 K and 4 � 106 K for the two laser fluence 100 J=cm2 (low) and 160 J=cm2 (high),

corresponding to 60 eV and 345 eV, respectively.

The thermal average quadratic velocity associated to these temperatures, for mono-atomic

species, is given by:

vx ¼

ffiffiffiffiffiffiffiffiffiffiffiffi3KBT

m

r: ð4Þ

FIGURE 5 Experimental data of ion energy distribution (points) and fit of the data (lines) by using Eq. (3) at lowand high laser fluence.


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It is 9:4 � 103 m=s and 2:3 � 104 m=s for low and high laser fluence, respectively.

According to the theory of the adiabatic plasma expansion of Kelly and Dreyfus [14], the

flow velocity due to neutrals can be calculated through the plasma temperature by the follow-

ing relationship:

vk ¼

ffiffiffiffiffiffiffiffiffiffiffiffigKBT

m

r; ð5Þ

where g is the adiabatic coefficient (g ¼ 5=3 for mono-atomic species). This velocity, direc-

ted along the normal to the target surface, is 7 � 103 m=s and 1:7 � 104 m=s for low and high

laser fluence, respectively.

The fit parameter ‘‘shift velocity’’, vk þ vc, depends on the charge state. For neutrals

vc ¼ 0, and the shift velocity, corresponding only to vk , can be evaluated by Eq. (5). For

ions having single charge state the value vk þ vc is 0:92 � 104 m=s and 2:38 � 104 m=s at

low and high laser fluence, respectively. For ions having charge state 6þ the shift velocity

is 3:80 � 104 m=s and 5:69 � 104 m=s at low and high fluence, respectively. At high fluence

ions having charge state 10þ have a vk þ vc fit value of 6:1 � 104 m=s. Thus, by subtracting

the vk value (constant at a given temperature) from the shift velocity, determined as a fit para-

meter, it is possible to know the vc value. Table I shows the fit parameters employed at low

fluence ð100 J=cm2Þ (a) and high fluence ð160 J=cm2Þ (b) and the data about the Coulomb

velocity vc. This last velocity depends on the ion charge state and, for multiple ionized

atoms, it is of the order of 104 m=s.

There is another way to calculate vc. By observing the experimental ion energy distribu-

tions we can derive a correlation of the average ion kinetic energy as a function of the charge

state. At low laser fluence the maximum value EM of the ion energy distribution increases, in

average, by about 350 eV per charge state, while at high laser fluence it increases in average,

by about 570 eV per charge state. This result lets us, suppose that ions can be submitted to an

electrical field inside the plasma, which produces significant ion accelerations depending on

their charge state. The ‘‘equivalent voltage’’ of acceleration should be 350 V and 570 V for

low and high laser fluence, respectively. Thus, from these observations it is possible to

calculate the Coulomb velocity vc as follows:

vc ¼

ffiffiffiffiffiffiffiffiffiffiffi2DEz

m

r; ð6Þ

here DE is the average Coulomb energy per charge state acquired by the ion inside the

plasma and z is the charge state.

In order to have a confirmation of the vc values, Eq. (6) has been employed and the

obtained results are in good agreement with experimental data reported in Table I.

These calculations indicate that TOF measurements of ions, performed along the normal to

the target surface, furnish an experimental ion velocity ve which corresponds to the following

value:

ve ¼ vx þ vk þ vc: ð7Þ

It represents an average velocity which has the three described components. The ve values at

low and high laser fluence are reported in Table I.

The distribution of the ion energy is characterized by a EM energy value at the maximum

of the distribution, to which corresponds a velocity vM, the most probable velocity of the

distribution. From EM it is possible to evaluate the mean kinetic ion energy hEi of the


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TA

BL

EI

Pa

ram

eter

sA

u0

Au

1þ

Au

2þ

Au

3þ

Au

4þ

Au

5þ

Au

6þ

Au

7þ

Au

8þ

Au

9þ

Au

10þ

Low

lase

rfl

uen

ce(1

00

J/cm

2)

Low

ener

gy

Fit

par

amet

ers

T(�

10

6K

)0

.70

.70

.70

.70

.70

.70

.7v k

þv c

(�

10

4m

/s)

0.7

0.9

21

.80

2.5

03

.00

3.4

03

.80

Vel

oci

ty(�

10

4m

/s),

v x0

.94

0.9

40

.94

0.9

40

.94

0.9

40

.94

Ex

per

imen

tal

dat

av k

0.7

0.7

0.7

0.7

0.7

0.7

0.7

v c0

0.2

21

.11

.82

.32

.73

.1v e

1.6

41

.86

2.7

43

.44

3.9

44

.34

4.7

4

Dis

trib

uti

on

dat

aE

M(e

V)

22

12

92

69

71

07

51

44

01

78

92

13

9v M

(�

10

4m

/s)

1.4

71

.69

2.6

13

.24

3.7

54

.18

4.5

8hEi¼

1.1

5E

M(e

V)

25

43

36

80

21

23

61

65

62

05

72

46

0hvi(�

10

4m

/s)

1.5

81

.81

2.8

03

.48

4.0

34

.49

4.9

1

Co

up

ling

par

amet

erE

c/K

T0

0.0

82

.05

5.4

88

.95

12

.34

16

.26

Hig

hla

ser

flu

ence

(16

0J/

cm2)

Hig

hen

erg

yF

itp

aram

eter

sT

(�

10

6K

)4

.04

.04

.04

.04

.04

.04

.04

.04

.04

.04

.0v k

þv c

(�

10

4m

/s)

1.6

72

.38

3.2

74

.00

4.6

35

.18

5.6

96

.16

6.6

07

.01

7.3

9

Vel

oci

ty(�

10

4m

/s),

v x2

.25

2.2

52

.25

2.2

52

.25

2.2

52

.25

2.2

52

.25

2.2

52

.25

Ex

per

imen

tal

dat

av k

1.6

71

.67

1.6

71

.67

1.6

71

.67

1.6

71

.67

1.6

71

.67

1.6

7v c

00

.71

1.6

02

.33

2.9

63

.51

4.0

24

.49

4.9

35

.34

5.7

2v e

3.9

24

.63

5.5

26

.25

6.8

87

.43

7.9

48

.41

8.8

59

.26

9.6

4

Dis

trib

uti

on

dat

aE

M(e

V)

10

73

14

30

20

00

25

75

31

50

37

20

43

00

48

80

54

50

60

30

66

00

v M(�

10

4m

/s)

3.2

43

.74

4.4

25

.02

5.5

56

.03

6.4

96

.91

7.3

07

.68

8.0

4hEi¼

1.4

5E

M(e

V)

15

56

20

74

29

00

37

34

45

68

53

94

62

35

70

76

79

03

87

44

95

70

hvi(�

10

4m

/s)

3.9

04

.51

5.3

36

.04

6.6

97

.27

7.8

18

.32

8.7

99

.25

9.6

7

Co

up

ling

par

amet

erE

c/K

T0

0.1

50

.76

1.6

12

.60

3.6

54

.79

5.9

77

.20

8.4

59

.69


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distribution. The experimental ion energy distributions indicate that hEi corresponds to about

1:15 � EM and 1:45 � EM for the low fluence of 100 J=cm2 and for the high fluence of

160 J=cm2, respectively. This different proportionality is due to a small difference in the

experimental ion energy distribution shape with the laser fluence. From hEi it is possible

to calculate the average ion velocity of each charge distribution, hvi.

The EM , vM , hEi and hvi values, for low and high fluences, are reported in Table I.

For instance, the mean energy hEi of Au1þ is about 340 eV and 2070 eV, corresponding to

an average velocity hvi of 1:8 � 104 m=s and 4:5 � 104 m=s, for low and high laser fluence,

respectively. The maximum mean energy is reached at high laser fluence for Au10þ ions. It is

about 9570 eV, corresponding to ion velocities of 9:67 � 104 m=s.

The ve values are in good agreement with these hvi values, as evident by Table I.

The Coulomb velocity increases with the charge state and may reach a value which is

about a factor three higher both with respect to the flow velocity and the thermal velocity.

Because vc increases in average by 0:58 � 104 m=s per charge state both at low and at high

energy, the average kinetic energy gained by the Coulomb interactions is about 34 eV per

charge state both at low and at high laser energy. Thus the ion Au6þ, for example, acquires

an energy, due to Coulomb interactions inside the plasma, of about 204 eV while Au10þ an

energy of about 340 eV.

The contribution given by the Coulomb interactions to the total ion energy increases with

the charge state of the ion. At low charge state the ion energy is due mainly to the plasma

temperature, the Coulomb contribution for Au1þ being about 3.5% (from v2c=ðv

2x þ v2

kÞ) at

low laser energy and about 6.4% at high laser energy. At high charge state the Coulomb con-

tribution becomes prevalent; in fact, the Coulomb contribution is about 7 times higher than

the thermal one for Au6þ at low laser fluence and about 4 times higher than for thermal one

for Au10þ at high laser fluence.

An interesting aspect is represented by the ratio between the Coulomb energy of the ions,

Ec, and the plasma temperature expressed in eV, KBT . This ratio, known as coupling para-

meter, depends strongly on the charge state and on the laser fluence, as reported in the

lower sections of Table I. The coupling parameter is about 1 only for low charge state,

while it becomes about 10 at high charge states, at which the Coulomb accelerations are

predominant for the energy acquisition.

Figure 6 shows the Coulomb velocity vc as a function of the ion charge state for two dif-

ferent laser fluences. For comparison, the thermal vx and vk values are also reported. The

FIGURE 6 Comparison of thermal velocity ðvxÞ, flow velocity ðvkÞ and Coulomb velocity ðvcÞ as a function of theion charge state.


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Coulomb velocity increases with the charge state and with the laser fluence while the thermal

and the flow velocity are constant, depending only on the plasma temperature.

The area subtended by each ion energy distribution is proportional to the number of pro-

duced ions. The areas generally decrease with the ion charge state. The ion yield production

depends mainly on two processes: the multiple ionization process and the ion recombination

process, as described by Shirkov [15].

Neutrals do not have any Coulomb velocity contribution but only thermal and flow velo-

city contributions. Using the data reported in Table I, the temperature evaluated for neutrals is

0:7 � 106 K and 4 � 106 K for low and high laser fluence, respectively. The velocity of

neutrals along the normal to the target surface is:

vn ¼ vx þ vk; ð8Þ

corresponding to 1:64 � 104 m=s and 3:92 � 104 m=s at low and high laser fluence, respec-

tively. This velocity decreases to the only vx value in other directions at which vk ¼ 0.

Assuming that neutrals can be described by a ‘‘shifted Maxwellian distribution’’ [13], their

energy distribution can be calculated and plotted such as in Figure 7 for low (a) and high (b)

fluence. The neutrals’ yield has been deduced from previous experimental data about the

fractional ionization of the Au plasma generated by laser ablation [12]. This fraction is

about 10% at 100 J=cm2 fluence and reaches about 32% at 160 J=cm2 fluence.

4 DISCUSSION AND CONCLUSIONS

The present work gives important data concerning the different parameter characteristics

of the unstable plasma produced by the pulsed laser interaction with a gold target. Due

to the very fast process, non-equilibrium phenomena are involved and complex aspects

characterize the plasma kinetics. It is very difficult, for example, to measure the tempera-

ture, the density and the ionization fraction of the plasma. Generally indirect measure-

ments, such as those optical ones, are employed to obtain information about the plasma

properties [16].

Moreover, this work presents an interesting experimental–theoretical approach to evaluate

both the plasma mean temperature, occurring at the beginning of its production, and the ion

energy assumed for an anisotropic charge distribution inside the plasma. Many observations

FIGURE 7 Neutral energy distribution, obtained by the shifted Maxwellian distribution, for low and high laserfluence.


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come out from the accurate analysis of the ion energy distribution performed by an IEA

detector.

The fits of the experimental data of the ion energy distributions, obtained by using Eq. (3)

give an improved mean temperature for a given laser fluence. This dependence shows that

the plasma temperature depends exponentially on the laser fluence, as reported in the

semi-logarithmic plot of Figure 8.

The mean flow velocity, corresponding to the temperature values according to Eq. (5), are

in good agreement both with data fit and with the literature [1]. This means that the plasma

expands with the velocity of sound, as described by Kelly and Dreyfus [14], and not with the

velocity of a pure adiabatic expansion described by Zel’dovich and Raizer [17].

The obtained results demonstrate that, although the plasma temperature is high, the

Coulomb component of the ion energy cannot be negligible. The Coulomb contribution to

the ion energy is predominant especially at high charge state and at high laser fluence, as

evident in Figure 6.

The Coulomb interactions produce ion acceleration inside the plasma. Because the angular

distribution of the ions is peaked along the normal to the target surface, as already reported in

our previous contributions [18], this means that an electrical field inside the plasma exists.

Thus the charge distribution is not uniform. On the other side, it is intuitive to suppose

that the higher electron velocity with respect to the ions, causes their rapid escape that pre-

ceedes the one of the ions. Thus an unstable charge distribution may produce ion acceleration

away from the zone immediately near to the target surface. At high laser fluence the equiva-

lent voltage of ion acceleration is of the order of 600 V, this means that the equivalent elec-

trical fields, developed in small plasma zones of the order of 0:1 mm size should be of the

order of 6 kV=mm.

Acknowledgements

The authors thank Dr. L. Laska and Dr. J. Krasa of the Institut of Physics of the Academy of

Science of the Czech Republic in Praha for the scientific cooperation with this experiment.

Many thanks also to Mr. S. Marletta, Mr. C. Marchetta and Mr. C. Percolla of INFN-LNS of

Catania for the useful technical support given to the presented measurements.

FIGURE 8 Calculated plasma temperature as a function of the laser fluence.


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References

[1] Willmott, P. R. and Huber, J. R. (2000). Rev. Modern Physics, 72(1), 315.[2] Wolowski, J., Parys, P., Woryna, E., Krasa, J., Laska, L., Rohlena, K., Gammino, S., Torrisi, L., Boody, F. P.,

Hopfl, R., Hora, H. and Haseroth, H. (2000). Optica Appl., XXX(1), 69.[3] Woryna, E., Wolowski, J., Kralikova, B., Krasa, J., Laska, L., Pfeifer, M., Rohlena, K., Skala, J., Perina, V.,

Boody, F. P., Hopfl, R. and Hora, H. (2000). Rev. Sci. Instr., 71(2), 949.[4] Gammino, S., Ciavola, G., Torrisi, L., Celona, L., Wolowski, J., Woryna, E., Parys, P., Laska, L., Krasa, J. and

Shirkov, G. D. (2000). Rev. Sci. Instr., 71(2), 1119.[5] Jungwirth, K., Cejnarova, A., Luha, L., Kralikova, B., Krasa, J., Krousky, E., Krupickova, P., Laska, L.,

Masek, K., Mocek, T., Pfeifer, M., Prag, A., Renner, O., Rohlena, K., Rus, B., Shala, J., Straka, P. andUllschmied, J. (2001). Physic Plasmas, 8(5), 2495.

[6] Kelly, R. (1992). Phys. Rev. A, 46(2), 860.[7] Torrisi, L., Ciavola, G., Gammino, S., Ando, L., Barna, A., Laska, L. and Krasa, J. (2000). Rev. Sci. Instr., 71(11),

4330.[8] Torrisi, L., Ando, L., Gammino, S., Ciavola, G., Celona, L., Genovese, S., Laska, L. and Krasa, J. (2001).

In: Gammino, S. and Ciavola, G. (Eds.), Int. Phys. Soc., Conf. Proc. PJBHI 2000, SIF, Vol. 72. Bologna, p. 115.[9] Charatis, G. (1975). Plasma Physics and Controlled Nuclear Fusion Research, Vol. 2. IAEA, Vienna.

[10] Woryna, E., Parys, P., Wolowski, J. and Mroz, W. (1996). Laser and Particle Beams, 14(3), 293.[11] Laska, L., Krasa, J., Masek, K., Pfeifer, M., Trenda, P., Kralikova, B., Skala, J., Woryna, E., Farny, J., Paris, P.,

Wolowski, J., Mroz, W., Shumshurov, A., Sharkov, B., Collier, J., Langbein, K. and Haseroth, H. (1966). Rev. Sci.Instr., 67(3), 950.

[12] Torrisi, L. (2001). Nucl. Instr. Methods Phys. Res. B, 183, 271.[13] Kelly, R. and Miotello, A. (1992). In: Chrisey, D. B. and Hubler, G. K. (Eds.), Pulsed Laser Deposition of Thin

Films. J. Wiley & S., New York, p. 55.[14] Kelly, R. and Dreyfus, R. W. (1988). Nucl. Instr. Meth. Phys. Res., B32, 341.[15] Shirkov, G. D. and Zschrnack, G. (1996). Electron Impact Ion Source for Charged Heavy Ions. Vieweg Press.,

Fed. Rep. Germany.[16] Szymanski, Z., Kurzyna, and Kalita, W. (1977). J. Phys. D: Appl. Phys., 30, 3153.[17] Zel’dovich, Y. B. and Raizer, Y. P. (1966). Physics of Shock Waves and High Temperature Hydrodynamic

Phenomena, Vol. 1. Academic Press, New York.[18] Torrisi, L., Ando, L., Ciavola, G., Gammino, S. and Barna, A. (2001). Rev. Sci. Instr., 72(1), 68.


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non-equilibrium plasma production by pulsed laser ablation of gold

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