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Angular distribution of ions produced by laser ablation of magnesium with special reference to sublimation energy
Vacuum 85 (2010) 170e175
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Angular distribution of ions produced by laser ablation of magnesium with special reference to sublimation energy M. Khaleeq-ur-Rahman, Dilawar Ali, M.Z. Butt* Department of Physics, University of Engineering and Technology, Lahore 54890, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2010 Received in revised form 9 April 2010 Accepted 16 May 2010
A 99.99% pure Mg target was irradiated with 100 shots of a Q-switched pulsed Nd:YAG laser (1.064 mm, 9 ns, 1.1 MW) in a vacuum w103 mbar to generate ions from laser-produced plasma (LPP). CR-39 detectors were positioned at 0 , 30 , 60 and 90 with respect to the normal of the target surface. The LPP Mg ions made tracks on the detectors, which were then etched in 6 N NaOH solution for 8 h at 70 1 C. The etched detectors were then analyzed using computer controlled Motic DMB series optical microscope. It is found that the Mg ions with maximum energy and maximum flux were obtained at an angle 0 with respect to the normal of the target surface, whereas both energy and flux of the ions decrease with the increase in angle. The angular distribution of Mg ions is encompassed by a cosine power-law in which the exponent n of the cosn q distribution is found to be 0.24. This together with the wealth of data (n ¼ 3e24) obtained by Konomi et al. (2009) for 11 different metals has been shown to follow a linear relationship between the exponent n and the sublimation energy of target metals. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Nd:YAG laser Mg ions Angular distribution Sublimation energy
1. Introduction The phenomenon of laserematter interaction has been extensively studied during the last three decades. When photons of laser light having intensity greater than the threshold breakdown intensity of the material target impinge on it, ejection of a mixture of energetic species, namely electrons, ions, atoms, molecules, micronsized particles and molten globules etc takes place. This forms plasma in the vicinity of the target surface, and is termed as laserproduced plasma (LPP) [1]. The LPP is a big source of ions which has very important applications such as ionography [2], surface modification of metals, ceramics, and plastics [3], and for the production of nanocrystal semiconductors [4]. The LPP ions can also be used in materials synthesis techniques for the fabrication of multicomponent magnetic films [5] and metastable metallic alloys [6]. The ions emitted from the LPP plume are found to be distributed angularly in the form of a cone [7e22]. The angular distribution of LPP ions with respect to energy and flux can be studied with the help of different techniques, e.g. CR-39 detectors [13e15,23e25], time of flight method [26e29], and film deposition [11,17]. Recently Konomi et al. [17] performed systematic experiments of great significance on the angular distribution of laser-ablated
particles for thirteen different metals (atomic weight: 47.90e207.21 amu) using fourth harmonics (wavelength ¼ 266 nm) of Nd:YAG laser. The laser fluence was close to threshold value (2.3 J/cm2). They found the angular distribution of atoms ejected by laser ablation of these metals to follow the cosine power-law, in which the value of exponent n of cosn q distribution varied over a very broad range 3e24. Konomi et al. [17] explicitly demonstrated that the sublimation energy of metals has a great influence on the angular distribution of ejected atoms. In general, the exponent n (3e24) was found to increase with the increase in the sublimation energy (195e850 kJ/mol) of the target metals. There was, however, no direct or indirect systematic correlation of n values with the atomic weight of the target metals, which is in accord with the observations of Antoni [20], and in contrast of Buttini et al. [8]. In the present work, we have extended the investigations of Konomi et al. [17] to 99.99% pure Mg metal, which has atomic weight 24.31 amu and sublimation energy 146.5 kJ/mol. The LPP Mg ions have been characterized with special reference to the angular distribution of energy and flux using CR-39 detectors. The use of CR-39 was preferred over other techniques because it needs no electronics, and is sensitive to the ions only while insensitive to the electromagnetic radiation pulses and electron flux up to very high dose [30]. 2. Experimental techniques
* Corresponding author. Tel.: þ92 42 99029204 (office), þ92 3234306032 (mobile). E-mail address: [email protected] (M.Z. Butt). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.05.007
The schematic of experimental setup has been depicted in Fig. 1. A passive Q-switched pulsed Nd:YAG laser of 1.064 mm wavelength,
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175
171
Fig. 1. Experimental setup.
9 ns pulse width, 0.5 Hz repetition rate, 10 mJ energy, and 1.1 MW power was used to irradiate 99.99% pure Mg circular target of 4 cm diameter and 2 mm thickness. The target, fixed on a holder, was placed at the centre of a stainless steel spherical chamber evacuated to the order of w103 mbar. Laser was focused on the target at an angle 45 with respect to the normal of the target surface through an infrared (IR) window and using an IR plano-convex lens of focal length 100 mm. The experimentally measured spot diameter was about 400 mm [31], whereas laser fluence and the laser intensity at the focus were 7.9 J/cm2 and 8.7 108 W/cm2, respectively. To avoid local heating and crater formation, the target was kept on rotating at 6 rpm. Four CR-39 detectors were placed inside the chamber at 0 , 30 , 60 , 90 with respect to the target normal at a distance of 50 mm from the target in each case. The
target was irradiated with 100 laser shots, which produced a plasma plume consisting of electrons, ions, and neutrals etc. The ions projected out of the plasma plume made tracks on CR-39 detectors. To make the tracks visible, the exposed detectors were etched in 6 N NaOH solution at 70 1 C for 8 h. The etched detectors were then examined using Motic DMB Series computer controlled optical microscope.
3. Results and discussion The ions emitted from the LPP plume made tracks on the CR-39 detectors, which can be seen in Fig. 2. These tracks which are in the form of cones, are distributed uniformly on the surface of the
Fig. 2. Optical micrographs of CR-39 detectors irradiated with LPP Mg ions positioned at angle (a) 0 , (b) 30 , (c) 60 and (d) 90 with respect to the normal of the target surface. 100.
172
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175
detectors. The diameter of the cones depends upon the energy of ions impinging on the detector [15]. A wide variation in the diameter of these cones indicates that ions of different energies are emitted from the LPP plume. The cone tracks having large diameters correspond to the ions having maximum energy. However, it is speculated that a few cone tracks having very large diameters, e.g. Fig. 2(d), may be caused by the spurious particles like a-particles, nitrogen etc. One can readily note from Fig. 2 that the planar concentration of tracks is maximum on the detector pasted at angle q ¼ 0 with the normal to the target surface, and is minimum for q ¼ 90 . According the Waheed et al. [32], an empirical relation between the energy E of the incident ions and the diameter D of the cone tracks is given by
E ¼
6:16 D 1:8409
(1)
where D is in mm and E is in keV. It should be pointed out that Eq. (1) was obtained by Waheed et al. [31] from the EeD data for ion energy in the range 60e727 keV. However, it is found to be reasonably valid for ion energies far below 60 keV as well [15]. In order to study the anisotropic behaviour of the Mg ions with reference to energy and flux, the diameters of the cones over an
area 228 169 mm2 of the detectors pasted at an angle q ¼ 0 , 30 , 60 , 90 with the normal to the target surface, were measured after etching the exposed detectors with the help of computer controlled optical microscope (MOTIC). The energy of Mg ions was determined from the measured values of the diameter of the cones using Eq. (1). The data obtained in this manner have been depicted by points in Fig. 3, and are encompassed by relation:
E ¼ E0 exp
(2)
The values of the pre-exponential factor Eo and the constant Do, along with the correlation factor r for q ¼ 0 , 30 , 60 and 90 are given in Table 1. One can readily note that the energy of the Mg ions increases exponentially as the cone diameter increases for each value of angle q in the range 0 e90 . However, the ions making tracks on the detector pasted at q ¼ 0 include the most energetic ones, and as the angle q increases, the maximum energy of the ions decreases showing an angular distribution of Mg ions. This is further supported by Fig. 4, which illustrates that the average energy, Eav, of Mg ions (Table 1) for a given value of q in the range 0 e90 (denoted by squares) decreases as q increases, i.e. when one moves away from the normal of the target surface. The leastsquares linear fit to the data points is given by the relation:
800
800
a
b 600
E ( keV )
600
E ( keV )
D D0
400
400
200
200
0
0
0
1
2
3
4
5
6
0
1
2
800
4
5
6
0.8
c
d
600
0.6
E ( keV )
E ( kev )
3
D ( μm )
D ( μm )
400
0.4
200
0.2
0
0.0
0
1
2
3
D ( μm )
4
5
6
0.0
0.5
1.0
1.5
2.0
D ( μm )
Fig. 3. Relation between the energy E and the diameter D of cone-shaped tracks made by LPP Mg ions on CR-39 detectors positioned at angle (a) 0 , (b) 30 , (c) 60 and (d) 90 with respect to the normal of the target surface.
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175 0.8
Table 1 Values of constants Eo and Do in Eq. (2), flux F and average energy Eav of LPP Mg ions for various angles in the range 0 e90 .
q (degree)
Eo (keV)
Do (micron)
r
F (109 ions m2)
Eav (keV)
0 30 60 90
0.431 0.298 0.131 1.804 105
0.718 0.674 0.591 0.164
0.998 0.998 0.998 0.999
1.47 1.40 1.30 0.18
53.3 32.7 17.1 0.634
Eav ¼ 51:97 0:579q
173
Do ( μm )
0.6
0.4
(3)
with the correlation factor r ¼ 0.998. We shall now examine the dependence of the values of constants Eo and Do in Eq. (2) on the angle q (Table 1). Referring to Fig. 5, the points (squares) denote the values of Do taken from Table 1, whereas the dotted curve drawn through the data points by leastsquares fitting is represented by
Do ¼ 0:703cos0:23 q
(4)
with the correlation factor r ¼ 0.999. This means that the values of constant Do in Eq. (2) follow the cosine power-law, which is generally expected for the angular distribution of the flux of LPP ions. However the value of pre-exponential factor Eo in Eq. (2) decreases linearly with the angle q, as shown in Fig. 6. The points (squares) denote the values of Eo taken from Table 1, and the line drawn through the data points by least-squares fitting is given by:
Eo ¼ 0:434 4:87 103 q
(5)
with the correlation factor r ¼ 0.999. The angular distribution of LPP Mg ions was also studied with the help of computer controlled optical microscope (MOTIC). The flux was calculated by counting the number of tracks over an area 228 169 mm2 of the etched CR-39 detectors which were pasted at an angle q ¼ 0 , 30 , 60 and 90 with the normal to the target surface prior to their exposure to the LPP ions. The measured values of the flux for a given value of angle q in the range 0 e90 have been listed in Table 1, and depicted by symbols (triangles) in Fig. 7. The dotted curve drawn through the data points by least-squares fitting is encompassed by:
F ¼ 1:48 109 cos0:24 q
(6)
0.2
0.0
0
20
40
60
80
100
θ ( degree ) Fig. 5. Relation between the constant Do and the angle q for LPP Mg ions.
with the correlation factor r ¼ 0.999. One can see that the flux is maximum for the angle q ¼ 0 and is minimum for q ¼ 90 , i.e. it decreases as one moves away from the normal to the target surface. This is in accord with the previous studies of angular distribution of LPP ions for a number of other metal targets [8,11,17,19e22]. Moreover, the angular distribution may also be described in terms of the “half angle “ of the distribution cone, q1=2 , which is defined as the value of q at which the flux is equal to the half of its maximum value [33]. Given a distribution FðqÞ ¼ Fo cosn q, the “half angle” q1=2 ¼ cos1 ½ð1=2Þ1=n . The value of q1=2 for Mg ions in the present work is found to be 86.8 . It is rather interesting to note that the value of exponent n (0.24) of cosn q term for LPP Mg ions distribution (Eq. (6)) is the same as that for the empirical constant Do (Eq. (4)). Thus, the value of constant Do in the ion energyetrack relation (Eq. (2)) depends on the angle q (Fig. 5) in a manner identical with that for the ion flux (Fig. 7). This aspect has not been reported earlier in the literature. Furthermore, it is illustrated graphically in Fig. 8 that greater the value of flux F, greater is the value of average energy Eav of the Mg ions. Thus the average energy of plasma ions along the normal to the target surface (q ¼ 0 ) is maximum whereas it is minimum for 0.5
60
0.4 45
Eo ( keV )
Eav ( keV )
0.3 30
15
0.2
0.1
0.0
0 0
20
40
60
80
100
θ ( degree ) Fig. 4. Relation between the average energy Eav and the angle q for LPP Mg ions.
0
20
40
60
80
θ ( degree ) Fig. 6. Relation between the constant Eo and the angle q for LPP Mg ions.
100
174
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175 30
1.6
Ta 20
Zr
Exponent n
2
F ( 10 ions / m )
1.2
9
0.8
Ti Sn Au
Ni
Pb
0.4
Mg
0
0.0
0
20
40
60
80
100
Pt
W
10
0
θ ( degree )
Mo
In
200
400
600
800
1000
Sublimation Energy ( kJ / mol )
Fig. 7. Dependence of the LPP Mg ions flux F on the angle q.
Fig. 9. Dependence of the exponent n of cosn q distribution on the sublimation energy of 12 targets metals.
q ¼ 90 . The least-squares fit to the data points (squares) taken from
after ward to study the angular distribution of ablated particles. Also, the other laser parameters, e.g. laser wavelength (256 nm), fluence (2.3 J/cm2) etc, and background pressure (4 106 mbar), used by Konomi et al. [17], are rather different from those used in the present work. In spite of all these, the value of n for Mg found in the present work lends further support to the general trend observed by Konomi et al. [17] that metals which have lower sublimation energy show broader angular distribution, i.e. the value of exponent n of cosn q distribution will be smaller. One can readily foresee that if the distance of the detectors from the target, which is 50 mm in this work, is increased, the ion flux or the number of tracks per unit area on the detectors will decrease but the angular distribution of LPP ions will not change. In other words, the value of exponent n of cosn q distribution is independent of distance between the target and the detector. This has been investigated and confirmed by Rafique et al. [13]. However, the angular distribution can broaden with the increase in the background pressure due to enhanced scattering of LPP ions by gas atoms [14]. The rather low value of n (¼0.24) for Mg found in this work can therefore be attributed to the relatively high background pressure (w103 mbar) compared to that of 4 106 mbar used by Konomi et al. [17]. Nevertheless, even an increase in the n-value of Mg by a factor of 10 on reducing the background pressure to the level employed in [17] will not have a marked effect on the relationship between n and sublimation energy depicted by Eq. (8).
Table 1 is encompassed by the relation:
Eav ¼ 2:71 103 exp
F 1:488 108
(7)
with the correlation factor r ¼ 0.999. Finally, reference to Fig. 9 shows a correlation between the sublimation energy (SE) and the exponent n of cosn q distribution for 12 different metallic targets with face-centred cubic, bodycentred cubic, hexagonal close-packed and tetragonal crystal structures. The data points for all metals other than Mg belong to Konomi et al. [17] whereas the one for Mg denotes the result of present investigations. A linear fit to all the data points is given by
n ¼ 2:04 þ 2:21 102 SE
(8)
with the correlation factor r ¼ 0.702. It should be noted that in their experimental work, Konomi et al. [17] employed the film-based technique, in which ablated particles are deposited on a substrate, and the film thickness is measured
60
4. Conclusions
Eav ( keV )
45
From the foregoing evidence, we conclude that: 30
15
0 0.0
0.4
0.8 9
1.2
1.6
2
F ( 10 ions / m ) Fig. 8. Relation between the average energy Eav and the LPP Mg ions flux F.
1. The relationship between the energy of LPP Mg ions and the diameter of cone-shaped tracks made by these ions on CR-39 detectors follows an exponential growth for each value of angle q with respect to the normal of the target surface in the range 0 e90 . 2. The average energy of LPP Mg ions for a given value of q decreases linearly as q increases from 0 to 90 . 3. The angular distribution of LPP Mg ions follows the cosine power-law in which the value of exponent n of cosn q distribution is 0.24. 4. The experimental value of n ¼ 0.24 for Mg lends further support to the general trend observed by Konomi et al. [17] that metals which have lower sublimation energy show broader
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175
angular distribution, i.e. the value of exponent n of cosn q distribution will be smaller. Acknowledgement Special thanks are due to Dr. Hameed Ahmad Khan, Scientist Emeritus, Pakistan Institute of Nuclear Science & Technology, Islamabad, for providing CR-39 detectors. We are also indebted to one of the reviewers for his constructive criticism and valuable suggestions to improve the manuscript. References [1] Bauerle D. Laser processing and chemistry. 3rd ed. Berlin: Springer; 2000. [2] Faenov AY, Pikuz TA, Fukuda Y, Kando M, Kotaki H, Homma T, et al. Appl Phys Lett 2009;95:101107. [3] Conrad JR, Radtke JL, Dodd RA, Worzala FJ, Tran NC. J Appl Phys 1987;62:4591. [4] Wolowski J, Badziak J, Czarnecka A, Parys P, Pisarek M, Rosinski M, et al. Laser Part Beams 2007;25:65. [5] Yang CJ, Kim AW, Kang JS. J Appl Phys 1998;83:6620. [6] Krebs HU. Int J Non-Equilib Process 1997;10:3. [7] Hansen TN, Schou J, Lunney JG. Europhys Lett 1997;40:441. [8] Buttini E, Thum-Jager A, Rohr K. J Phys D Appl Phys 1998;31:2165. [9] Woryna E, Parys P, Wolowski J, Krasa J, Laska L, Kralikova B, et al. Laser Part Beams 1999;17:307. [10] Thum-Jager A, Sinha BK, Rohr KP. Phys Rev E 2000;61:3063. [11] Torrisi L, Ando L, Ciavola G, Gammino S, Barna A. Rev Sci Instrum 2001;72:68. [12] Laska L, Krasa J, Pfeifer V, Rohlena K, Gammino S, Torrisi L, et al. Rev Sci Instrum 2002;73:654.
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[13] Rafique MS, Khaleeq-ur-Rahman M, Siraj K, Shafiq R, Afzal S, Mahboobur-Rahman, Shahzad F. Proceedings of 30th EPS Conference on Control Fusion and Plasma Phys.. St. Petersburg Russia; 7e11 July, 2003. ECA Vol 27A: P-3.84. [14] Rafique MS, Khaleeq-ur-Rahman M, Anwar MS, Mahmood F, Ashfaq A, Siraj K. Laser Part Beams 2005;23:131. [15] Rafique MS, Khaleeq-ur-Rahman M, Munazza S, Bhatti KA. Plasma Sci Tech 2008;10:450. [16] Laska L, Jungwirth K, Krasa J, Krousky E, Pfeifer M, Rohlena K, et al. Laser Part Beams 2008;26:555. [17] Konomi I, Motohiro T, Asaoka T. J Appl Phys 2009;106:013107. [18] Thum-Jager A, Rohr K. J Phys D Appl Phys 1999;32:2827. [19] Toftmann B, Schou J, Lunney JG. Phys Rev B 2003;67:104101. [20] Antoni F, Fuchs C, Fograssy E. Appl Surf Sci 1996;96e98:50. [21] Akhsakhalyan AD, Gapanov V, Luchin VI, Chirimanov AP. Sov Phys Tech Phys 1988;33:1146. [22] Lorusso A, Velardi L, Nassisi V. Radiat Eff Defects Solids 2008;163:429. [23] Cavallaro S, Margarone D, Torrisi L, Laska L, Krasa J, Ullschmied J. Radiat Eff Defects Solids 2008;163:371. [24] Szydlowski A, Badziak J, Fuchs J, Kubkowska M, Parys P, Rosinski M, et al. Radiat Meas 2009;44:881. [25] Szydlowski A. Radiat Meas 2003;36:35. [26] Franghiadakis Y, Fotakis C, Tzanetakis P. J Appl Phys 1998;84:1090. [27] Franghiadakis Y, Fotakis C, Tzanetakis P. Appl Phys A 1999;68:391. [28] Harilal SS. J Appl Phys 2007;102:123306. [29] Torrisi L. Rad Eff Defects Solids 2003;158:689. [30] Durrani SA, Bull RK. Solid state nuclear track detection: principles, method and application. Oxford: Pergamon Press; 1987. [31] Khaleeq-ur-Rahman M, Siraj K, Rafique MS, Bhatti KA, Latif A, Jamil H, et al. Nucl Instrum Method B 2009;267:1085. [32] Waheed A, Moschini G, Khan HA, Cherubini R, Lembo L, Haque AMI. Nucl Tracks Radiant Meas 1988;15:129. [33] Eason R, editor. Pulsed laser deposition of thin films. New York: Wiley; 2006.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Angular distribution of ions produced by laser ablation of magnesium with special reference to sublimation energy M. Khaleeq-ur-Rahman, Dilawar Ali, M.Z. Butt* Department of Physics, University of Engineering and Technology, Lahore 54890, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2010 Received in revised form 9 April 2010 Accepted 16 May 2010
A 99.99% pure Mg target was irradiated with 100 shots of a Q-switched pulsed Nd:YAG laser (1.064 mm, 9 ns, 1.1 MW) in a vacuum w103 mbar to generate ions from laser-produced plasma (LPP). CR-39 detectors were positioned at 0 , 30 , 60 and 90 with respect to the normal of the target surface. The LPP Mg ions made tracks on the detectors, which were then etched in 6 N NaOH solution for 8 h at 70 1 C. The etched detectors were then analyzed using computer controlled Motic DMB series optical microscope. It is found that the Mg ions with maximum energy and maximum flux were obtained at an angle 0 with respect to the normal of the target surface, whereas both energy and flux of the ions decrease with the increase in angle. The angular distribution of Mg ions is encompassed by a cosine power-law in which the exponent n of the cosn q distribution is found to be 0.24. This together with the wealth of data (n ¼ 3e24) obtained by Konomi et al. (2009) for 11 different metals has been shown to follow a linear relationship between the exponent n and the sublimation energy of target metals. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Nd:YAG laser Mg ions Angular distribution Sublimation energy
1. Introduction The phenomenon of laserematter interaction has been extensively studied during the last three decades. When photons of laser light having intensity greater than the threshold breakdown intensity of the material target impinge on it, ejection of a mixture of energetic species, namely electrons, ions, atoms, molecules, micronsized particles and molten globules etc takes place. This forms plasma in the vicinity of the target surface, and is termed as laserproduced plasma (LPP) [1]. The LPP is a big source of ions which has very important applications such as ionography [2], surface modification of metals, ceramics, and plastics [3], and for the production of nanocrystal semiconductors [4]. The LPP ions can also be used in materials synthesis techniques for the fabrication of multicomponent magnetic films [5] and metastable metallic alloys [6]. The ions emitted from the LPP plume are found to be distributed angularly in the form of a cone [7e22]. The angular distribution of LPP ions with respect to energy and flux can be studied with the help of different techniques, e.g. CR-39 detectors [13e15,23e25], time of flight method [26e29], and film deposition [11,17]. Recently Konomi et al. [17] performed systematic experiments of great significance on the angular distribution of laser-ablated
particles for thirteen different metals (atomic weight: 47.90e207.21 amu) using fourth harmonics (wavelength ¼ 266 nm) of Nd:YAG laser. The laser fluence was close to threshold value (2.3 J/cm2). They found the angular distribution of atoms ejected by laser ablation of these metals to follow the cosine power-law, in which the value of exponent n of cosn q distribution varied over a very broad range 3e24. Konomi et al. [17] explicitly demonstrated that the sublimation energy of metals has a great influence on the angular distribution of ejected atoms. In general, the exponent n (3e24) was found to increase with the increase in the sublimation energy (195e850 kJ/mol) of the target metals. There was, however, no direct or indirect systematic correlation of n values with the atomic weight of the target metals, which is in accord with the observations of Antoni [20], and in contrast of Buttini et al. [8]. In the present work, we have extended the investigations of Konomi et al. [17] to 99.99% pure Mg metal, which has atomic weight 24.31 amu and sublimation energy 146.5 kJ/mol. The LPP Mg ions have been characterized with special reference to the angular distribution of energy and flux using CR-39 detectors. The use of CR-39 was preferred over other techniques because it needs no electronics, and is sensitive to the ions only while insensitive to the electromagnetic radiation pulses and electron flux up to very high dose [30]. 2. Experimental techniques
* Corresponding author. Tel.: þ92 42 99029204 (office), þ92 3234306032 (mobile). E-mail address: [email protected] (M.Z. Butt). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.05.007
The schematic of experimental setup has been depicted in Fig. 1. A passive Q-switched pulsed Nd:YAG laser of 1.064 mm wavelength,
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175
171
Fig. 1. Experimental setup.
9 ns pulse width, 0.5 Hz repetition rate, 10 mJ energy, and 1.1 MW power was used to irradiate 99.99% pure Mg circular target of 4 cm diameter and 2 mm thickness. The target, fixed on a holder, was placed at the centre of a stainless steel spherical chamber evacuated to the order of w103 mbar. Laser was focused on the target at an angle 45 with respect to the normal of the target surface through an infrared (IR) window and using an IR plano-convex lens of focal length 100 mm. The experimentally measured spot diameter was about 400 mm [31], whereas laser fluence and the laser intensity at the focus were 7.9 J/cm2 and 8.7 108 W/cm2, respectively. To avoid local heating and crater formation, the target was kept on rotating at 6 rpm. Four CR-39 detectors were placed inside the chamber at 0 , 30 , 60 , 90 with respect to the target normal at a distance of 50 mm from the target in each case. The
target was irradiated with 100 laser shots, which produced a plasma plume consisting of electrons, ions, and neutrals etc. The ions projected out of the plasma plume made tracks on CR-39 detectors. To make the tracks visible, the exposed detectors were etched in 6 N NaOH solution at 70 1 C for 8 h. The etched detectors were then examined using Motic DMB Series computer controlled optical microscope.
3. Results and discussion The ions emitted from the LPP plume made tracks on the CR-39 detectors, which can be seen in Fig. 2. These tracks which are in the form of cones, are distributed uniformly on the surface of the
Fig. 2. Optical micrographs of CR-39 detectors irradiated with LPP Mg ions positioned at angle (a) 0 , (b) 30 , (c) 60 and (d) 90 with respect to the normal of the target surface. 100.
172
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175
detectors. The diameter of the cones depends upon the energy of ions impinging on the detector [15]. A wide variation in the diameter of these cones indicates that ions of different energies are emitted from the LPP plume. The cone tracks having large diameters correspond to the ions having maximum energy. However, it is speculated that a few cone tracks having very large diameters, e.g. Fig. 2(d), may be caused by the spurious particles like a-particles, nitrogen etc. One can readily note from Fig. 2 that the planar concentration of tracks is maximum on the detector pasted at angle q ¼ 0 with the normal to the target surface, and is minimum for q ¼ 90 . According the Waheed et al. [32], an empirical relation between the energy E of the incident ions and the diameter D of the cone tracks is given by
E ¼
6:16 D 1:8409
(1)
where D is in mm and E is in keV. It should be pointed out that Eq. (1) was obtained by Waheed et al. [31] from the EeD data for ion energy in the range 60e727 keV. However, it is found to be reasonably valid for ion energies far below 60 keV as well [15]. In order to study the anisotropic behaviour of the Mg ions with reference to energy and flux, the diameters of the cones over an
area 228 169 mm2 of the detectors pasted at an angle q ¼ 0 , 30 , 60 , 90 with the normal to the target surface, were measured after etching the exposed detectors with the help of computer controlled optical microscope (MOTIC). The energy of Mg ions was determined from the measured values of the diameter of the cones using Eq. (1). The data obtained in this manner have been depicted by points in Fig. 3, and are encompassed by relation:
E ¼ E0 exp
(2)
The values of the pre-exponential factor Eo and the constant Do, along with the correlation factor r for q ¼ 0 , 30 , 60 and 90 are given in Table 1. One can readily note that the energy of the Mg ions increases exponentially as the cone diameter increases for each value of angle q in the range 0 e90 . However, the ions making tracks on the detector pasted at q ¼ 0 include the most energetic ones, and as the angle q increases, the maximum energy of the ions decreases showing an angular distribution of Mg ions. This is further supported by Fig. 4, which illustrates that the average energy, Eav, of Mg ions (Table 1) for a given value of q in the range 0 e90 (denoted by squares) decreases as q increases, i.e. when one moves away from the normal of the target surface. The leastsquares linear fit to the data points is given by the relation:
800
800
a
b 600
E ( keV )
600
E ( keV )
D D0
400
400
200
200
0
0
0
1
2
3
4
5
6
0
1
2
800
4
5
6
0.8
c
d
600
0.6
E ( keV )
E ( kev )
3
D ( μm )
D ( μm )
400
0.4
200
0.2
0
0.0
0
1
2
3
D ( μm )
4
5
6
0.0
0.5
1.0
1.5
2.0
D ( μm )
Fig. 3. Relation between the energy E and the diameter D of cone-shaped tracks made by LPP Mg ions on CR-39 detectors positioned at angle (a) 0 , (b) 30 , (c) 60 and (d) 90 with respect to the normal of the target surface.
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175 0.8
Table 1 Values of constants Eo and Do in Eq. (2), flux F and average energy Eav of LPP Mg ions for various angles in the range 0 e90 .
q (degree)
Eo (keV)
Do (micron)
r
F (109 ions m2)
Eav (keV)
0 30 60 90
0.431 0.298 0.131 1.804 105
0.718 0.674 0.591 0.164
0.998 0.998 0.998 0.999
1.47 1.40 1.30 0.18
53.3 32.7 17.1 0.634
Eav ¼ 51:97 0:579q
173
Do ( μm )
0.6
0.4
(3)
with the correlation factor r ¼ 0.998. We shall now examine the dependence of the values of constants Eo and Do in Eq. (2) on the angle q (Table 1). Referring to Fig. 5, the points (squares) denote the values of Do taken from Table 1, whereas the dotted curve drawn through the data points by leastsquares fitting is represented by
Do ¼ 0:703cos0:23 q
(4)
with the correlation factor r ¼ 0.999. This means that the values of constant Do in Eq. (2) follow the cosine power-law, which is generally expected for the angular distribution of the flux of LPP ions. However the value of pre-exponential factor Eo in Eq. (2) decreases linearly with the angle q, as shown in Fig. 6. The points (squares) denote the values of Eo taken from Table 1, and the line drawn through the data points by least-squares fitting is given by:
Eo ¼ 0:434 4:87 103 q
(5)
with the correlation factor r ¼ 0.999. The angular distribution of LPP Mg ions was also studied with the help of computer controlled optical microscope (MOTIC). The flux was calculated by counting the number of tracks over an area 228 169 mm2 of the etched CR-39 detectors which were pasted at an angle q ¼ 0 , 30 , 60 and 90 with the normal to the target surface prior to their exposure to the LPP ions. The measured values of the flux for a given value of angle q in the range 0 e90 have been listed in Table 1, and depicted by symbols (triangles) in Fig. 7. The dotted curve drawn through the data points by least-squares fitting is encompassed by:
F ¼ 1:48 109 cos0:24 q
(6)
0.2
0.0
0
20
40
60
80
100
θ ( degree ) Fig. 5. Relation between the constant Do and the angle q for LPP Mg ions.
with the correlation factor r ¼ 0.999. One can see that the flux is maximum for the angle q ¼ 0 and is minimum for q ¼ 90 , i.e. it decreases as one moves away from the normal to the target surface. This is in accord with the previous studies of angular distribution of LPP ions for a number of other metal targets [8,11,17,19e22]. Moreover, the angular distribution may also be described in terms of the “half angle “ of the distribution cone, q1=2 , which is defined as the value of q at which the flux is equal to the half of its maximum value [33]. Given a distribution FðqÞ ¼ Fo cosn q, the “half angle” q1=2 ¼ cos1 ½ð1=2Þ1=n . The value of q1=2 for Mg ions in the present work is found to be 86.8 . It is rather interesting to note that the value of exponent n (0.24) of cosn q term for LPP Mg ions distribution (Eq. (6)) is the same as that for the empirical constant Do (Eq. (4)). Thus, the value of constant Do in the ion energyetrack relation (Eq. (2)) depends on the angle q (Fig. 5) in a manner identical with that for the ion flux (Fig. 7). This aspect has not been reported earlier in the literature. Furthermore, it is illustrated graphically in Fig. 8 that greater the value of flux F, greater is the value of average energy Eav of the Mg ions. Thus the average energy of plasma ions along the normal to the target surface (q ¼ 0 ) is maximum whereas it is minimum for 0.5
60
0.4 45
Eo ( keV )
Eav ( keV )
0.3 30
15
0.2
0.1
0.0
0 0
20
40
60
80
100
θ ( degree ) Fig. 4. Relation between the average energy Eav and the angle q for LPP Mg ions.
0
20
40
60
80
θ ( degree ) Fig. 6. Relation between the constant Eo and the angle q for LPP Mg ions.
100
174
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175 30
1.6
Ta 20
Zr
Exponent n
2
F ( 10 ions / m )
1.2
9
0.8
Ti Sn Au
Ni
Pb
0.4
Mg
0
0.0
0
20
40
60
80
100
Pt
W
10
0
θ ( degree )
Mo
In
200
400
600
800
1000
Sublimation Energy ( kJ / mol )
Fig. 7. Dependence of the LPP Mg ions flux F on the angle q.
Fig. 9. Dependence of the exponent n of cosn q distribution on the sublimation energy of 12 targets metals.
q ¼ 90 . The least-squares fit to the data points (squares) taken from
after ward to study the angular distribution of ablated particles. Also, the other laser parameters, e.g. laser wavelength (256 nm), fluence (2.3 J/cm2) etc, and background pressure (4 106 mbar), used by Konomi et al. [17], are rather different from those used in the present work. In spite of all these, the value of n for Mg found in the present work lends further support to the general trend observed by Konomi et al. [17] that metals which have lower sublimation energy show broader angular distribution, i.e. the value of exponent n of cosn q distribution will be smaller. One can readily foresee that if the distance of the detectors from the target, which is 50 mm in this work, is increased, the ion flux or the number of tracks per unit area on the detectors will decrease but the angular distribution of LPP ions will not change. In other words, the value of exponent n of cosn q distribution is independent of distance between the target and the detector. This has been investigated and confirmed by Rafique et al. [13]. However, the angular distribution can broaden with the increase in the background pressure due to enhanced scattering of LPP ions by gas atoms [14]. The rather low value of n (¼0.24) for Mg found in this work can therefore be attributed to the relatively high background pressure (w103 mbar) compared to that of 4 106 mbar used by Konomi et al. [17]. Nevertheless, even an increase in the n-value of Mg by a factor of 10 on reducing the background pressure to the level employed in [17] will not have a marked effect on the relationship between n and sublimation energy depicted by Eq. (8).
Table 1 is encompassed by the relation:
Eav ¼ 2:71 103 exp
F 1:488 108
(7)
with the correlation factor r ¼ 0.999. Finally, reference to Fig. 9 shows a correlation between the sublimation energy (SE) and the exponent n of cosn q distribution for 12 different metallic targets with face-centred cubic, bodycentred cubic, hexagonal close-packed and tetragonal crystal structures. The data points for all metals other than Mg belong to Konomi et al. [17] whereas the one for Mg denotes the result of present investigations. A linear fit to all the data points is given by
n ¼ 2:04 þ 2:21 102 SE
(8)
with the correlation factor r ¼ 0.702. It should be noted that in their experimental work, Konomi et al. [17] employed the film-based technique, in which ablated particles are deposited on a substrate, and the film thickness is measured
60
4. Conclusions
Eav ( keV )
45
From the foregoing evidence, we conclude that: 30
15
0 0.0
0.4
0.8 9
1.2
1.6
2
F ( 10 ions / m ) Fig. 8. Relation between the average energy Eav and the LPP Mg ions flux F.
1. The relationship between the energy of LPP Mg ions and the diameter of cone-shaped tracks made by these ions on CR-39 detectors follows an exponential growth for each value of angle q with respect to the normal of the target surface in the range 0 e90 . 2. The average energy of LPP Mg ions for a given value of q decreases linearly as q increases from 0 to 90 . 3. The angular distribution of LPP Mg ions follows the cosine power-law in which the value of exponent n of cosn q distribution is 0.24. 4. The experimental value of n ¼ 0.24 for Mg lends further support to the general trend observed by Konomi et al. [17] that metals which have lower sublimation energy show broader
M. Khaleeq-ur-Rahman et al. / Vacuum 85 (2010) 170e175
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