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Observation of high intensity negative ion pulses by laser impact
Nuclear Instruments and Methods in Physics Research A302 (1991) 379-381 North-Holland
379
Letter to the Editor
Observation of high intensity negative ion pulses by laser impact D. Berkovits a, E. Boaretto and Z. Vager c,e
a,
G. Hollos
b,
W. Kittschera °, R. Naanian
d,
M. Paul
a
Racah Institute of Physics, Hebrew University, Jerusalem, Israel 91904 Accelerator Laboratory, Weizmann Institute of Science, Rehovot, Israel 76100 Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA d Isotope Research Department, Weizmann Institute of Science, Rehovot, Israel 76100 Nuclear Physics Department, Weizmann Institute of Science, Rehovot, Israel 76100
Received 22 October 1990 We report on the observation of intense pulses of sulphur negative ions extracted from a Cs-beam sputter source and produced by the impact of 532 nm photon pulses from a Nd-YAG laser on a FeS cathode. Peak currents of about 3 mA are obtained after extraction and preacceleration of the negative ions, which are subsequently analyzed by accelerator mass spectrometry . The time structure of the 32 S - negative ion current is measured and shows a complex behavior not yet fully understood . The production of positive and negative ions and of cluster ions by the interaction of a laser beam with a solid surface is under thorough investigation [1,2]. Quantitative information on the negative ion production is however scarce (see ref. [3]) and the dominant mode of formation of the negative ions (e.g . plasma formation, thermal ionization) is still unclear. We describe here the observation of high intensity pulses of negative ions produced by the impact of a laser beam
Negative Ion Source
Sample
Power Meter Faraday Cup 1~
Extractor
on a sample surface and analyzed by accelerator mass spectrometry [4,5]. The experimental system, schematically illustrated in fig. 1, is similar to that described in refs . [6,7]. A Cs sputter source (Hiconex 834) is used in a standard reflected-geometry mode [8] to produce a beam of Sions from a 4 mm diameter FeS cathode. Negative ions are extracted at 20 kV and preaccelerated through an acceleration tube to an energy of 100 keV. An electro-
Ion-Laser Interaction Region
Chopper
Injection
532nm 15ns
PreAcc
Master Trigger
100 keV
Horizontal Scale
Fig . 1. Schematic diagram of the experimental system . 0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
Nd :YAG Laser
D. Berkovits et al. / High intensity negative tons
380 200 150
= a) Laser Off
32S
100 50 0r 150
1{IWY1IfYYlh
b) delay=17Ns
100 50 "raWii4ii! 150
c) delay=15ps
LI
100 50 11 .rYhYrVir,oLlLti
0 500
d) delay=13ps
400 300 200 100 0
5
10
15
20
25
Time (us) Fig. 2. Time spectra measured for analyzed 32 S- ions (seethe text for'details on the mode of acquisition). The origin of the time axis corresponds to the opening of the beam chopper: (a) laser off; (b) to (d) laser fired with indicated delay times relative to the start of the ion pulse. The laser is a pulsed frequency-doubled (532 rim) Nd-YAG laser. static triplet quadrupole lens focuses the ions through a system of apertures to a 90 ° magnetic mass analyzer . Analyzed ions are then injected into the Rehovot 14UD Pelletron tandem used as an accelerator mass spec32 trometer [9]. S9+ ions, selected at a final energy of 120 MeV, are transported to a solid-state particle detector and, with adequate attenuation, individually counted. A frequency-doubled (532 rim wavelength) pulsed Nd-YAG laser beam is sent colinearly and opposite to the negative ion beam towards the ion source (fig. 1) and impinges onto the sputter sample. The laser is Q-switched and the pulses, about 15 ns long, are triggered at a frequency of 30 Hz by an external oscillator . The laser energy measured after the aperture system is 3 mJ per pulse and the irradiance on the sample is estimated to be of the order of 107 W/cm2. The oscillator is used with proper delay to chop the negative ion beam
by pulsing the voltage of an electrostatic deflector shortly after the extraction; this produces ion pulses of 18 Ws length . The laser interaction and the time structure of the negative ion beam current are measured by means of a time spectrum generated by a time-to-amplitude converter. A start signal is provided at the time when the beam chopper opens and the stop signal by the particle detector after the accelerator . Figs . 2a to 2d 32Sions measured with and show the time spectra of without laser. The (exponential) decrease seen in fig. 2a is due to the fact that only one particle per beam pulse is time-analyzed, favoring thus earlier particles in the pulse. Figs . 2b to 2d show an intense pulse, about 150 ns long, of 3z S- ions, riding on the continuum of ions produced in the Cs sputter source . The time of appearance of this pulse moves accordingly to the delay 32Sbetween the start pulse and the laser firing. The ion pulse is therefore believed to be produced by the laser impinging onto the cathode surface. The 2 Ws wide dip, also seen in figs. 2b to 2d, results from photodetachment of 3z S - ions by the laser in the region of good overlap near to the focusing apertures, with subsequent loss of the neutralized ions in the bending magnet (see refs. [6,7] for details) . In fig. 3, a more detailed description of the time structure of the 32S- ion beam is shown. The ratio of the peak current to the continuum normally produced by the Cs sputter source is about 220; from a measurement of the continuous (unchopped) beam after the preacceleration tube (Faraday cup 1 in fig. 1), one deduces that the peak current of the negative ion pulse at this position is about 3 mA ; the corresponding values 32Sfor the continuum and peak currents of analyzed ions after the 90 ° magnet (Faraday cup 2) are respectively 0.23 ~LA and 50 1tA, due to the severe collimation at the apertures. From the known velocity of the ions
Fig. 3. Detailed time dependence of analyzed 32S - ions . The laser (delay= 13 Ws, see caption of fig. 2) has an energy of about 3 mJ/pulse ; the irradiance is estimated to be of the order of 107 W/cm2. See the text for an interpretation of the regions 1 to 3 and a to c, indicated in the figure. The right-hand scale is the intensity of the analyzed negative ion beam after the 90* magnet . The peak current in the main pulse 1 after extraction and preacceleration is 3 mA.
D. Berkovits et aL / High intensity negative tons
(78 cm/lis after preacceleration) and the position in the time spectrum, one can deduce the ion location at the time of the laser firing. The edge denoted by a in fig. 3 corresponds to ions being destroyed through photodetachment by the laser just before the 90 ° bend in the injection magnet ; the bend is preceded by a region (dip) where ions and laser photons overlap well near the aperture system . The time difference between the main ion pulse 1 and edge a is 13 .0 Ws and corresponds exactly to the calculated ion flight time between the sample surface and the 90' magnet . Striking in fig. 3 is the appearance of ion pulses 2 and 3 to the right of pulse 1 and corresponding therefore to ions which are formed after the laser was fired. Both pulses 1 and 2 are seen to be preceded in time by dips (b and c) attesting a strong ion suppression in the corresponding regions. The formation of negative ion pulses by laser impact has been studied also by Korschinek and Henkelmann [3] ; they observe a peak current of 1.8 ~tA of Cl - after time-of-flight mass analysis . Further experiments are needed to understand the mechanism responsible for the observed negative ion pulses and for the phenomena described above. It seems, however, that it would be hard to reconcile a hypothesis of formation of negative ions in the main pulse by thermal ionization and direct extraction, with the appearance of delayed pulses, as described in this work . A possible scenario explaining these phenomena might be the formation of a plasma by the laser impact on the surface. Negative ions formed by recombination of electrons and ions, are pulled out by the extraction voltage to form the main negative current peak (1 in fig. 3) while the remaining positive ions get accelerated to the cathode and sputter out a second pulse (2). A similar but attenuated process could be the origin of the weaker peak 3. The strong ion suppression responsible for dip b just before the main pulse 1, may be due to photodetachment interaction of
38 1
the laser in the region very close to the sample surface where the negative ions are continuously formed by Cs sputtering . Dip c could be due, in the scenario described above, to negative ton suppression by charge-changing collisions in the same region and induced by the impinging positive ion pulse. The observation of intense and short pulses of negative ion beams (more than two orders of magnitude above the continuous currents obtained in a standard negative ion source) by laser impact described in this work may lead to new insights in the understanding of negative ion formation and may also lead to interesting applications . References
[2]
[3] [4] [5] [61 [71 [8] [9]
IF Ready, Effects of High-Power Laser Radiation (Academic Press, London, 1971). O. Cheshnovsky, P.J . Brucat, S. Yang, C.L . Pettiette, M.J. Craycraft and R.E. Smalley, Physics and Chemistry of Small Clusters, eds. P. Jena, B.K . Rao and S.N . Khanna, NATO ASI Series B: Physics, vol 158 (Plenum Press, New York, 1987) p. 1 . G. Korschmek and T Henkelmann, submitted to Nucl . Instr. and Meth . A .E. Litherland, Ann. Rev. Nucl . Part Sci. 30 (1980) 437. W. Kutschera and M. Paul, Ann. Rev. Nucl . Part . Sci. 40 (1990) 411. D. Berkovits, E. Boaretto, G. Hollos, W. Kutschera, R. Naaman, M. Paul and Z. Vager, Nucl . Instr. and Meth. A281 (1989) 663. D. Berkovits, E. Boaretto, G. Hollos, W. Kutschera, R. Naaman, M. Paul and Z. Vager, Nucl . Instr. and Meth ., to be published. K. Brand, Nucl. Instr. and Meth . 141 (1977) 519. D. Fink, O. Meirav, M. Paul, H. Ernst, W. Henning, W. Kutschera, R. Kaim, A. Kaufman and M. Magaritz, Nucl . Instr. and Meth. B5 (1984) 123.
379
Letter to the Editor
Observation of high intensity negative ion pulses by laser impact D. Berkovits a, E. Boaretto and Z. Vager c,e
a,
G. Hollos
b,
W. Kittschera °, R. Naanian
d,
M. Paul
a
Racah Institute of Physics, Hebrew University, Jerusalem, Israel 91904 Accelerator Laboratory, Weizmann Institute of Science, Rehovot, Israel 76100 Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA d Isotope Research Department, Weizmann Institute of Science, Rehovot, Israel 76100 Nuclear Physics Department, Weizmann Institute of Science, Rehovot, Israel 76100
Received 22 October 1990 We report on the observation of intense pulses of sulphur negative ions extracted from a Cs-beam sputter source and produced by the impact of 532 nm photon pulses from a Nd-YAG laser on a FeS cathode. Peak currents of about 3 mA are obtained after extraction and preacceleration of the negative ions, which are subsequently analyzed by accelerator mass spectrometry . The time structure of the 32 S - negative ion current is measured and shows a complex behavior not yet fully understood . The production of positive and negative ions and of cluster ions by the interaction of a laser beam with a solid surface is under thorough investigation [1,2]. Quantitative information on the negative ion production is however scarce (see ref. [3]) and the dominant mode of formation of the negative ions (e.g . plasma formation, thermal ionization) is still unclear. We describe here the observation of high intensity pulses of negative ions produced by the impact of a laser beam
Negative Ion Source
Sample
Power Meter Faraday Cup 1~
Extractor
on a sample surface and analyzed by accelerator mass spectrometry [4,5]. The experimental system, schematically illustrated in fig. 1, is similar to that described in refs . [6,7]. A Cs sputter source (Hiconex 834) is used in a standard reflected-geometry mode [8] to produce a beam of Sions from a 4 mm diameter FeS cathode. Negative ions are extracted at 20 kV and preaccelerated through an acceleration tube to an energy of 100 keV. An electro-
Ion-Laser Interaction Region
Chopper
Injection
532nm 15ns
PreAcc
Master Trigger
100 keV
Horizontal Scale
Fig . 1. Schematic diagram of the experimental system . 0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
Nd :YAG Laser
D. Berkovits et al. / High intensity negative tons
380 200 150
= a) Laser Off
32S
100 50 0r 150
1{IWY1IfYYlh
b) delay=17Ns
100 50 "raWii4ii! 150
c) delay=15ps
LI
100 50 11 .rYhYrVir,oLlLti
0 500
d) delay=13ps
400 300 200 100 0
5
10
15
20
25
Time (us) Fig. 2. Time spectra measured for analyzed 32 S- ions (seethe text for'details on the mode of acquisition). The origin of the time axis corresponds to the opening of the beam chopper: (a) laser off; (b) to (d) laser fired with indicated delay times relative to the start of the ion pulse. The laser is a pulsed frequency-doubled (532 rim) Nd-YAG laser. static triplet quadrupole lens focuses the ions through a system of apertures to a 90 ° magnetic mass analyzer . Analyzed ions are then injected into the Rehovot 14UD Pelletron tandem used as an accelerator mass spec32 trometer [9]. S9+ ions, selected at a final energy of 120 MeV, are transported to a solid-state particle detector and, with adequate attenuation, individually counted. A frequency-doubled (532 rim wavelength) pulsed Nd-YAG laser beam is sent colinearly and opposite to the negative ion beam towards the ion source (fig. 1) and impinges onto the sputter sample. The laser is Q-switched and the pulses, about 15 ns long, are triggered at a frequency of 30 Hz by an external oscillator . The laser energy measured after the aperture system is 3 mJ per pulse and the irradiance on the sample is estimated to be of the order of 107 W/cm2. The oscillator is used with proper delay to chop the negative ion beam
by pulsing the voltage of an electrostatic deflector shortly after the extraction; this produces ion pulses of 18 Ws length . The laser interaction and the time structure of the negative ion beam current are measured by means of a time spectrum generated by a time-to-amplitude converter. A start signal is provided at the time when the beam chopper opens and the stop signal by the particle detector after the accelerator . Figs . 2a to 2d 32Sions measured with and show the time spectra of without laser. The (exponential) decrease seen in fig. 2a is due to the fact that only one particle per beam pulse is time-analyzed, favoring thus earlier particles in the pulse. Figs . 2b to 2d show an intense pulse, about 150 ns long, of 3z S- ions, riding on the continuum of ions produced in the Cs sputter source . The time of appearance of this pulse moves accordingly to the delay 32Sbetween the start pulse and the laser firing. The ion pulse is therefore believed to be produced by the laser impinging onto the cathode surface. The 2 Ws wide dip, also seen in figs. 2b to 2d, results from photodetachment of 3z S - ions by the laser in the region of good overlap near to the focusing apertures, with subsequent loss of the neutralized ions in the bending magnet (see refs. [6,7] for details) . In fig. 3, a more detailed description of the time structure of the 32S- ion beam is shown. The ratio of the peak current to the continuum normally produced by the Cs sputter source is about 220; from a measurement of the continuous (unchopped) beam after the preacceleration tube (Faraday cup 1 in fig. 1), one deduces that the peak current of the negative ion pulse at this position is about 3 mA ; the corresponding values 32Sfor the continuum and peak currents of analyzed ions after the 90 ° magnet (Faraday cup 2) are respectively 0.23 ~LA and 50 1tA, due to the severe collimation at the apertures. From the known velocity of the ions
Fig. 3. Detailed time dependence of analyzed 32S - ions . The laser (delay= 13 Ws, see caption of fig. 2) has an energy of about 3 mJ/pulse ; the irradiance is estimated to be of the order of 107 W/cm2. See the text for an interpretation of the regions 1 to 3 and a to c, indicated in the figure. The right-hand scale is the intensity of the analyzed negative ion beam after the 90* magnet . The peak current in the main pulse 1 after extraction and preacceleration is 3 mA.
D. Berkovits et aL / High intensity negative tons
(78 cm/lis after preacceleration) and the position in the time spectrum, one can deduce the ion location at the time of the laser firing. The edge denoted by a in fig. 3 corresponds to ions being destroyed through photodetachment by the laser just before the 90 ° bend in the injection magnet ; the bend is preceded by a region (dip) where ions and laser photons overlap well near the aperture system . The time difference between the main ion pulse 1 and edge a is 13 .0 Ws and corresponds exactly to the calculated ion flight time between the sample surface and the 90' magnet . Striking in fig. 3 is the appearance of ion pulses 2 and 3 to the right of pulse 1 and corresponding therefore to ions which are formed after the laser was fired. Both pulses 1 and 2 are seen to be preceded in time by dips (b and c) attesting a strong ion suppression in the corresponding regions. The formation of negative ion pulses by laser impact has been studied also by Korschinek and Henkelmann [3] ; they observe a peak current of 1.8 ~tA of Cl - after time-of-flight mass analysis . Further experiments are needed to understand the mechanism responsible for the observed negative ion pulses and for the phenomena described above. It seems, however, that it would be hard to reconcile a hypothesis of formation of negative ions in the main pulse by thermal ionization and direct extraction, with the appearance of delayed pulses, as described in this work . A possible scenario explaining these phenomena might be the formation of a plasma by the laser impact on the surface. Negative ions formed by recombination of electrons and ions, are pulled out by the extraction voltage to form the main negative current peak (1 in fig. 3) while the remaining positive ions get accelerated to the cathode and sputter out a second pulse (2). A similar but attenuated process could be the origin of the weaker peak 3. The strong ion suppression responsible for dip b just before the main pulse 1, may be due to photodetachment interaction of
38 1
the laser in the region very close to the sample surface where the negative ions are continuously formed by Cs sputtering . Dip c could be due, in the scenario described above, to negative ton suppression by charge-changing collisions in the same region and induced by the impinging positive ion pulse. The observation of intense and short pulses of negative ion beams (more than two orders of magnitude above the continuous currents obtained in a standard negative ion source) by laser impact described in this work may lead to new insights in the understanding of negative ion formation and may also lead to interesting applications . References
[2]
[3] [4] [5] [61 [71 [8] [9]
IF Ready, Effects of High-Power Laser Radiation (Academic Press, London, 1971). O. Cheshnovsky, P.J . Brucat, S. Yang, C.L . Pettiette, M.J. Craycraft and R.E. Smalley, Physics and Chemistry of Small Clusters, eds. P. Jena, B.K . Rao and S.N . Khanna, NATO ASI Series B: Physics, vol 158 (Plenum Press, New York, 1987) p. 1 . G. Korschmek and T Henkelmann, submitted to Nucl . Instr. and Meth . A .E. Litherland, Ann. Rev. Nucl . Part Sci. 30 (1980) 437. W. Kutschera and M. Paul, Ann. Rev. Nucl . Part . Sci. 40 (1990) 411. D. Berkovits, E. Boaretto, G. Hollos, W. Kutschera, R. Naaman, M. Paul and Z. Vager, Nucl . Instr. and Meth. A281 (1989) 663. D. Berkovits, E. Boaretto, G. Hollos, W. Kutschera, R. Naaman, M. Paul and Z. Vager, Nucl . Instr. and Meth ., to be published. K. Brand, Nucl. Instr. and Meth . 141 (1977) 519. D. Fink, O. Meirav, M. Paul, H. Ernst, W. Henning, W. Kutschera, R. Kaim, A. Kaufman and M. Magaritz, Nucl . Instr. and Meth. B5 (1984) 123.