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Surface analysis with heavy ion induced Auger electrons; basic properties of a new method for materials sciences
Surface analysis with heavy ion induced Auger electrons; basic properties of a new method for materials sciences received 23 July 1974
K 0 Groeneveld,
R Mann,
W Meckbach”
and R Spohr,t
lnstitut ffir Kernphysik Universitat Frankfurt/M,
Germany
Heavy ion induced Auger electron spectroscopy is presented as a new tool for surface analysis. In contrast to conventional Auger electron spectroscopy using electron excitation, the new technique offers the advantage of selective excitation of specific impurity elements present in a solid matrix by choosing a proper value of projectile mass and projectile energy. The technique is characterized by very fast rising cross-sections at threshold. The cross-sections can be very large in comparison to electron excitation. Due to the high energy loss of heavy ions at the energies applied, the projectiles are probing a very thin surface layer of the sample. Another benefit of heavy ion excitation is the possibility of simultaneous detection of back scattered heavy ions as well as the possibility for detection of ultra soft X-rays. In this way three complementary techniques are available simultaneously for mutual comparison.
Introduction SolId
There are two reasons that most likely account for the wide application of Auger electron spectroscopy in surface research : first, the capability of the method to characterize the elements present in the target material by their associated Auger spectrum. Second, the surface sensitivity of the method which is due to the small escape depth of low energy electrons. Over recent years, an enormous amount of qualitative and quantitative information has been accumulated: e.g. in connection with the technique of ion etching, Auger electron spectroscopy using electron excitation of the target has become a standard technique for interface research. If electron excitation of Auger spectra is such a well established technique, why should alternative methods for excitation be sought? Two principal difficulties arise in present application of Auger spectroscopy. On one hand, there is the huge background of inelastically scatterred electrons leaving the target. This background screens weak Auger signals. On the other hand, there exists the problem of interference between Auger lines emitted from different elements present in the same target. Both difficulties may be circumvented using distinctly different methods for excitation. One of these methods is heavy ion excitation. Heavy ion physics recently has aroused much interest in atomic and nuclear physics e.g. for production of super heavy elements. Energetic heavy ion beams of sufficient intensity are becoming available for beam foil spectroscopy and in particular for excitation of Auger spectra in solid targets’.‘. Observation of projectile emitted Auger spectra becomes a new technique in laboratory astrophysics.’ * Centro Atomico, S C Bariloche, Argentine. t Gesellschaft fiir Schwerionenforschung, Darmstadt, Germany. Vacuum/volume
25/number
1.
Pergamon Press LtdlPrinted
target
Figure 1.Schematic representation of heavy ion excitation of a solid
target with ‘delta electrons’ as sidetracks contributing excitation of the target atoms.
to the
Another aspect of heavy ion induced Auger spectroscopy is the possibility for transmission experiments, in which Auger spectra from the back of a foil are observed.’ The geometry of such a transmission experiments is outlined in Figure 1. In transmission experiments, the target-induced Auger lines of the projectile, which appear in the electron spectrum besides the Auger lines form the target itself, can be used as a spectruminternal intensity standard for calibration of the target Auger lines, with respect to the intensity of the exciting beam. Such a spectrum-internal intensity standard does not exist in conventional Auger spectroscopy (Figure 2).’
Advantages of heavy ion excitation
Auger spectra excited by heavy ion impact may in some cases be qualitatively very similar in appearance to Auger spectra excited by electrons, provided that the proper impact energy is chosen for the specific ion. According to a rough estimate (Bohr-Lamb criterion), cross sections for inelastic atomic collision processes, particularly inner shell ionization, are comparable for equal relative velocities between projectile
in Great Britain
9
K 0 Groeneveld
103
ef al: Surface analysis with heavy ion induced Auger electrons;
ProJectlIe energy
basic properties
of a new method for materials sciences
and trace element analysis, a method which offers new capabilities of which the analyst should be aware of. Inherent difficulties of heavy ion excitation should be naturally considered as seriously as in conventional Auger spectroscopy. However, as it seems, sputtering, ion implantation, radiation damage, blistering etc. can be kept small enough to maintain nondestructive conditions for the target to a much higher degree. In the following, three main features of the technique, i.e. background, threshold behaviour, and choice of particle mass, are compared with conventional Auger spectroscopy.
=
Inelastic background
Figure 2. Typical features of the heavy ion excited Auger-spectrum are: smooth background, Auger signal from the target and Auger signal from the projectile. The latter one can be employed as a spectrum internal intensity standard.
and target. As for equal velocity the projectile energy increases linearly with its mass, in the case of heavy projectiles much higher energies are needed. Besides the considerable effort necessary for generation of high energy heavy ions, lack in intensity of the actual ion beams appears to present an important drawback for application since in Auger spectroscopy usually background problems can only be solved by increasing the intensity of the exciting beam. However, heavy ions as projectiles present distinct advantages to justify their use and make the method profitable. One important advantage resides in the fact that-including the method described in this report-three distinct and not interfering methods for analysing the solid surface can be used simultaneously (Figure 3). These methods are: (1) Heavy
ion induced
Auger electron
spectroscopy.g
(2) Heavy ion induced ultra soft X-ray spectroscopy.3 This method is surface sensitive not only because of small penetration depth of heavy ions but also due to the strong absorption of ultra soft X-rays in matter. The advantage, in comparison to excitation by electrons, is that no background of Bremsstrahlung is generated by ion bombardment. (3) Rutherford back scattering spectroscopy,4 which gives reliable absolute concentration values of elements. These methods probe the solid surface with different depths and quality of information and reveal different aspects of one and the same surface. Besides this, the entrance depth range of heavy ions at the energies applied is very small. Together, these methods may represent a step forward towards obtaining a ‘complete’ picture of the surface. Another distinct advantage-which is actually available for the three methods mentioned-results from the possibility of proper choice of mass and energy of the exciting projectile which makes it possible to adapt the experimental situation to a specific problem. Heavy ion induced Auger spectroscopy therefore should find a most serious consideration as a tool for surface analysis 10
The shot noise associated with the continuous background of inelastic electrons screens weak Auger signals. As in conventional Auger spectroscopy primary electron beams of high intensity are obtained with relative ease, such Auger spectra can be recorded in an analog way. In order to avoid overloading of the associated analog amplifier and enable the detection of small Auger signals, the Auger spectrum has to be differentiated via modulation and detected via a lock-inamplifier. In contrast hereto, heavy ion induced Auger spectra, due to the limitation of the beam intensity, usually have to be observed using counting techniques. Spectra must be accumulated via multiple sweep in the memory of a multichannel analyzer. On the other hand, in this way, time variations ot the exciting beam can be eliminated and spectrum information including the background information, can be fully maintained. Therefore Auger line intensities can in the described technique be evaluated in a rather straightforward way. In contrast hereto, in conventional Auger spectroscopy, background information normally is lost by the differentiation technique and, as it seems, is often completely excluded from consideration. Figure 4 shows on a logarithmic scale spectra of electrons emitted at 42.3” from a carbon foil traversed by proton beams of energies between 0.5 and 2.5 MeV. The KLL Auger peak of carbon is observed. The spectra exhibit an almost linear decay of the continuous background up to a certain maximum energy. According to theory this background corresponds, on one hand, to the so-called ‘delta electrons’ (compare Figure 1) of relatively low energy, and on the other hand, to the so-called ‘knock-on electrons’ produced by binary collisions between the ions and electrons of the material. Scattered projectlIe
Faraday
cup
Projectlie 7
x-rays
Electrons
Figure 3. Three simultaneous non-interfering techniques can be employed for surface characterization with heavy ion excitation: Rutherford backscattering ultra soft X-ray spectroscopy, and Auger electron spectroscopy.
K 0 Groeneveld et a/: Surface
analysis
with
heavy
ion induced
Auger
electrons;
basic
properties
of a new
method
for materials
sciences
&Target r9=42 3O
Ed=0 5 MeV E mox
IO
100
EeIec+ron,
eV
1000
Figure 5. Heavy ion induced Auger spectra of a gaseous target, recorded for different angles of observation, indicates possibility of improving signal-to-noise ratio by proper choice of angle.5 E,. eV
Figure 4. Auger spectra of carbon, excited by proton impact at different energies, observed at an angle of 42” with respect to the projectile path. The high energy limit of the background corresponds to the ‘head-on’ collision between projectile and target electron.
The upper limit in energy of the secondary electrons is given by knock-on electrons which are originating close to the surface under the specific angle of observation and which have been able to escape without suffering further collisions. The energy of the knock-on electrons is given by Em,,= 4 (m,/m,) cos 9; 8 2 90”. Except for the effects of scattering of the electrons the contribution from the knock-on electrons disappears if the angle of observations exceeds 90”. This is observed in Figure 5 where secondary electron spectra from a gaseous target induced by 0.5 MeV protons are shown at two different angles of observation. According to expectation at 8 = 90” the contribution from knock on-electrons is practically absent and also the background due to the &electrons is sensibly reduced. Correspondingly, the Auger peak of N is relatively enhanced. We could demonstrate, that this holds also for solids.’ Resuming, we observe that in heavy ion induced Auger spectroscopy the signal-to-noise ratio can be increased by increasing the energy of the ionic projectiles and/or the angle of observation of the electrons.
Projectile energy In conventional Auger spectroscopy the projectile energy is maintained at a fixed value. However, in heavy ion induced Auger spectroscopy the energy parameter plays a very important role. The reason for this is that, due to the formation of a quasi-molecule between projectile and target atom, a spectacular rise of the excitation cross section occurs just above threshold. In this domain of energy the time of inter-
action between projectile and target atom becomes comparable to the orbit frequency for inner electron shells. Figure 6 in a schematic way compares the threshold behaviour due to Coulomb excitation, i.e. the threshold for electron or proton excitation, with the threshold due to Pauli excitation, based on the quasi-molecule concept. The observed spectacular rise above threshold for heavy ion excitation can be used in practice for selective excitation of a specific impurity element present in the target material. Projectile mass
In contrast to the energy parameter, which in conventional Auger spectroscopy, at least in principle, can be used also for discriminative excitation, the projectile mass represents a
log E,
Figure 6. Schematic representation of threshold behaviour for Auger processes excited by different projectiles. Solid line: Coulomb excitation, i.e. electron or proton excitation.6 Dashed line: Pauli excitation for symmetric collisions, i.e. identical projectile and target atom.7*8 Dotted line: Pauli excitation for asymmetric collisions, i.e. different projectile and target atom.‘** For heavy ion excitation, a very pronounced onset at threshold exists, which can be used for discriminatory excitation of a specific impurity element. 11
K 0 Groeneveld
et al: Surface analysis with
heavy ion induced
Auger electrons;
completely new parameter accessible only in heavy ion experiments. In connection with the proper energy selection thus, for each projectile/target combination, a different threshold results. This enables specific excitation of Auger spectra from trace elements in the presence of other elements which may be much more abundant in the matrix of the studied target material. In addition, the cross sections for Auger emission and X-ray emission can reach extremely high values, depending on the target/projectile-combination, which gives this method a powerful potential not only in surface analysis but also in trace element analysis.‘O
Conclusion The special advantages of heavy ion induced Auger electron spectroscopy as a tool in materials surface analysis are: (1) High sensitivity due to the high ionisation cross section. (2) The possibility of highly selective excitation leading to the possibility of element specific trace analysis by (a) the strong dependency of the cross section on the projectile energy,
12
basic properties
of a new method
for materials sciences
(b) the strong dependency of the cross section on the specific target/projectile combination, and (c) the threshold-like behaviour of the excitation function. References
’ E Dietz, K 0 Groeneveld, R Spohr, R Staudte, Nucl Instr Meth, 105, 1972, 467. z K 0 Groeneveld, Proc Conf on Special Problem of Heaoy Iorl Scattering-Atomic Physics, Gesellschaft fiir Schwerionenforschung. Darmstadt, Germany (edited by B Fricke and P Armbruster), GSI 73-11 (1973); K 0 Groeneveld, to be published. 3 J A Cairns, Surfhce Sci, 34, 1973, 638. 4 see e.g. J A Davis, J Denhartog and J L Whitton, Phys Reo, 165, 1968,345; P Staib, B M U Scherzer and R Behrisch, Thin Solid Fi/ms, 19, 1973, 57. ’ N Stolterfoht, Z Physik, 248, 1971, 92. 6 J D Garcia, Phys Rea, A4, 1971, 955. 7 F W Saris, Physica, 52, 1971, 290. ’ W Brandt, Proc Third Int Conf on Atomic Physics, Boulder, Colorado (edited by S J Smith), Plenum Press, New York (1973). g R F Musket and W Bauer, Appl Phys Lett, 20, 1972, 1. lo F Folkmann, C Gaarde, T Huus and K Kemp, N11clImtr Met/z, 116, 1974, 487.
K 0 Groeneveld,
R Mann,
W Meckbach”
and R Spohr,t
lnstitut ffir Kernphysik Universitat Frankfurt/M,
Germany
Heavy ion induced Auger electron spectroscopy is presented as a new tool for surface analysis. In contrast to conventional Auger electron spectroscopy using electron excitation, the new technique offers the advantage of selective excitation of specific impurity elements present in a solid matrix by choosing a proper value of projectile mass and projectile energy. The technique is characterized by very fast rising cross-sections at threshold. The cross-sections can be very large in comparison to electron excitation. Due to the high energy loss of heavy ions at the energies applied, the projectiles are probing a very thin surface layer of the sample. Another benefit of heavy ion excitation is the possibility of simultaneous detection of back scattered heavy ions as well as the possibility for detection of ultra soft X-rays. In this way three complementary techniques are available simultaneously for mutual comparison.
Introduction SolId
There are two reasons that most likely account for the wide application of Auger electron spectroscopy in surface research : first, the capability of the method to characterize the elements present in the target material by their associated Auger spectrum. Second, the surface sensitivity of the method which is due to the small escape depth of low energy electrons. Over recent years, an enormous amount of qualitative and quantitative information has been accumulated: e.g. in connection with the technique of ion etching, Auger electron spectroscopy using electron excitation of the target has become a standard technique for interface research. If electron excitation of Auger spectra is such a well established technique, why should alternative methods for excitation be sought? Two principal difficulties arise in present application of Auger spectroscopy. On one hand, there is the huge background of inelastically scatterred electrons leaving the target. This background screens weak Auger signals. On the other hand, there exists the problem of interference between Auger lines emitted from different elements present in the same target. Both difficulties may be circumvented using distinctly different methods for excitation. One of these methods is heavy ion excitation. Heavy ion physics recently has aroused much interest in atomic and nuclear physics e.g. for production of super heavy elements. Energetic heavy ion beams of sufficient intensity are becoming available for beam foil spectroscopy and in particular for excitation of Auger spectra in solid targets’.‘. Observation of projectile emitted Auger spectra becomes a new technique in laboratory astrophysics.’ * Centro Atomico, S C Bariloche, Argentine. t Gesellschaft fiir Schwerionenforschung, Darmstadt, Germany. Vacuum/volume
25/number
1.
Pergamon Press LtdlPrinted
target
Figure 1.Schematic representation of heavy ion excitation of a solid
target with ‘delta electrons’ as sidetracks contributing excitation of the target atoms.
to the
Another aspect of heavy ion induced Auger spectroscopy is the possibility for transmission experiments, in which Auger spectra from the back of a foil are observed.’ The geometry of such a transmission experiments is outlined in Figure 1. In transmission experiments, the target-induced Auger lines of the projectile, which appear in the electron spectrum besides the Auger lines form the target itself, can be used as a spectruminternal intensity standard for calibration of the target Auger lines, with respect to the intensity of the exciting beam. Such a spectrum-internal intensity standard does not exist in conventional Auger spectroscopy (Figure 2).’
Advantages of heavy ion excitation
Auger spectra excited by heavy ion impact may in some cases be qualitatively very similar in appearance to Auger spectra excited by electrons, provided that the proper impact energy is chosen for the specific ion. According to a rough estimate (Bohr-Lamb criterion), cross sections for inelastic atomic collision processes, particularly inner shell ionization, are comparable for equal relative velocities between projectile
in Great Britain
9
K 0 Groeneveld
103
ef al: Surface analysis with heavy ion induced Auger electrons;
ProJectlIe energy
basic properties
of a new method for materials sciences
and trace element analysis, a method which offers new capabilities of which the analyst should be aware of. Inherent difficulties of heavy ion excitation should be naturally considered as seriously as in conventional Auger spectroscopy. However, as it seems, sputtering, ion implantation, radiation damage, blistering etc. can be kept small enough to maintain nondestructive conditions for the target to a much higher degree. In the following, three main features of the technique, i.e. background, threshold behaviour, and choice of particle mass, are compared with conventional Auger spectroscopy.
=
Inelastic background
Figure 2. Typical features of the heavy ion excited Auger-spectrum are: smooth background, Auger signal from the target and Auger signal from the projectile. The latter one can be employed as a spectrum internal intensity standard.
and target. As for equal velocity the projectile energy increases linearly with its mass, in the case of heavy projectiles much higher energies are needed. Besides the considerable effort necessary for generation of high energy heavy ions, lack in intensity of the actual ion beams appears to present an important drawback for application since in Auger spectroscopy usually background problems can only be solved by increasing the intensity of the exciting beam. However, heavy ions as projectiles present distinct advantages to justify their use and make the method profitable. One important advantage resides in the fact that-including the method described in this report-three distinct and not interfering methods for analysing the solid surface can be used simultaneously (Figure 3). These methods are: (1) Heavy
ion induced
Auger electron
spectroscopy.g
(2) Heavy ion induced ultra soft X-ray spectroscopy.3 This method is surface sensitive not only because of small penetration depth of heavy ions but also due to the strong absorption of ultra soft X-rays in matter. The advantage, in comparison to excitation by electrons, is that no background of Bremsstrahlung is generated by ion bombardment. (3) Rutherford back scattering spectroscopy,4 which gives reliable absolute concentration values of elements. These methods probe the solid surface with different depths and quality of information and reveal different aspects of one and the same surface. Besides this, the entrance depth range of heavy ions at the energies applied is very small. Together, these methods may represent a step forward towards obtaining a ‘complete’ picture of the surface. Another distinct advantage-which is actually available for the three methods mentioned-results from the possibility of proper choice of mass and energy of the exciting projectile which makes it possible to adapt the experimental situation to a specific problem. Heavy ion induced Auger spectroscopy therefore should find a most serious consideration as a tool for surface analysis 10
The shot noise associated with the continuous background of inelastic electrons screens weak Auger signals. As in conventional Auger spectroscopy primary electron beams of high intensity are obtained with relative ease, such Auger spectra can be recorded in an analog way. In order to avoid overloading of the associated analog amplifier and enable the detection of small Auger signals, the Auger spectrum has to be differentiated via modulation and detected via a lock-inamplifier. In contrast hereto, heavy ion induced Auger spectra, due to the limitation of the beam intensity, usually have to be observed using counting techniques. Spectra must be accumulated via multiple sweep in the memory of a multichannel analyzer. On the other hand, in this way, time variations ot the exciting beam can be eliminated and spectrum information including the background information, can be fully maintained. Therefore Auger line intensities can in the described technique be evaluated in a rather straightforward way. In contrast hereto, in conventional Auger spectroscopy, background information normally is lost by the differentiation technique and, as it seems, is often completely excluded from consideration. Figure 4 shows on a logarithmic scale spectra of electrons emitted at 42.3” from a carbon foil traversed by proton beams of energies between 0.5 and 2.5 MeV. The KLL Auger peak of carbon is observed. The spectra exhibit an almost linear decay of the continuous background up to a certain maximum energy. According to theory this background corresponds, on one hand, to the so-called ‘delta electrons’ (compare Figure 1) of relatively low energy, and on the other hand, to the so-called ‘knock-on electrons’ produced by binary collisions between the ions and electrons of the material. Scattered projectlIe
Faraday
cup
Projectlie 7
x-rays
Electrons
Figure 3. Three simultaneous non-interfering techniques can be employed for surface characterization with heavy ion excitation: Rutherford backscattering ultra soft X-ray spectroscopy, and Auger electron spectroscopy.
K 0 Groeneveld et a/: Surface
analysis
with
heavy
ion induced
Auger
electrons;
basic
properties
of a new
method
for materials
sciences
&Target r9=42 3O
Ed=0 5 MeV E mox
IO
100
EeIec+ron,
eV
1000
Figure 5. Heavy ion induced Auger spectra of a gaseous target, recorded for different angles of observation, indicates possibility of improving signal-to-noise ratio by proper choice of angle.5 E,. eV
Figure 4. Auger spectra of carbon, excited by proton impact at different energies, observed at an angle of 42” with respect to the projectile path. The high energy limit of the background corresponds to the ‘head-on’ collision between projectile and target electron.
The upper limit in energy of the secondary electrons is given by knock-on electrons which are originating close to the surface under the specific angle of observation and which have been able to escape without suffering further collisions. The energy of the knock-on electrons is given by Em,,= 4 (m,/m,) cos 9; 8 2 90”. Except for the effects of scattering of the electrons the contribution from the knock-on electrons disappears if the angle of observations exceeds 90”. This is observed in Figure 5 where secondary electron spectra from a gaseous target induced by 0.5 MeV protons are shown at two different angles of observation. According to expectation at 8 = 90” the contribution from knock on-electrons is practically absent and also the background due to the &electrons is sensibly reduced. Correspondingly, the Auger peak of N is relatively enhanced. We could demonstrate, that this holds also for solids.’ Resuming, we observe that in heavy ion induced Auger spectroscopy the signal-to-noise ratio can be increased by increasing the energy of the ionic projectiles and/or the angle of observation of the electrons.
Projectile energy In conventional Auger spectroscopy the projectile energy is maintained at a fixed value. However, in heavy ion induced Auger spectroscopy the energy parameter plays a very important role. The reason for this is that, due to the formation of a quasi-molecule between projectile and target atom, a spectacular rise of the excitation cross section occurs just above threshold. In this domain of energy the time of inter-
action between projectile and target atom becomes comparable to the orbit frequency for inner electron shells. Figure 6 in a schematic way compares the threshold behaviour due to Coulomb excitation, i.e. the threshold for electron or proton excitation, with the threshold due to Pauli excitation, based on the quasi-molecule concept. The observed spectacular rise above threshold for heavy ion excitation can be used in practice for selective excitation of a specific impurity element present in the target material. Projectile mass
In contrast to the energy parameter, which in conventional Auger spectroscopy, at least in principle, can be used also for discriminative excitation, the projectile mass represents a
log E,
Figure 6. Schematic representation of threshold behaviour for Auger processes excited by different projectiles. Solid line: Coulomb excitation, i.e. electron or proton excitation.6 Dashed line: Pauli excitation for symmetric collisions, i.e. identical projectile and target atom.7*8 Dotted line: Pauli excitation for asymmetric collisions, i.e. different projectile and target atom.‘** For heavy ion excitation, a very pronounced onset at threshold exists, which can be used for discriminatory excitation of a specific impurity element. 11
K 0 Groeneveld
et al: Surface analysis with
heavy ion induced
Auger electrons;
completely new parameter accessible only in heavy ion experiments. In connection with the proper energy selection thus, for each projectile/target combination, a different threshold results. This enables specific excitation of Auger spectra from trace elements in the presence of other elements which may be much more abundant in the matrix of the studied target material. In addition, the cross sections for Auger emission and X-ray emission can reach extremely high values, depending on the target/projectile-combination, which gives this method a powerful potential not only in surface analysis but also in trace element analysis.‘O
Conclusion The special advantages of heavy ion induced Auger electron spectroscopy as a tool in materials surface analysis are: (1) High sensitivity due to the high ionisation cross section. (2) The possibility of highly selective excitation leading to the possibility of element specific trace analysis by (a) the strong dependency of the cross section on the projectile energy,
12
basic properties
of a new method
for materials sciences
(b) the strong dependency of the cross section on the specific target/projectile combination, and (c) the threshold-like behaviour of the excitation function. References
’ E Dietz, K 0 Groeneveld, R Spohr, R Staudte, Nucl Instr Meth, 105, 1972, 467. z K 0 Groeneveld, Proc Conf on Special Problem of Heaoy Iorl Scattering-Atomic Physics, Gesellschaft fiir Schwerionenforschung. Darmstadt, Germany (edited by B Fricke and P Armbruster), GSI 73-11 (1973); K 0 Groeneveld, to be published. 3 J A Cairns, Surfhce Sci, 34, 1973, 638. 4 see e.g. J A Davis, J Denhartog and J L Whitton, Phys Reo, 165, 1968,345; P Staib, B M U Scherzer and R Behrisch, Thin Solid Fi/ms, 19, 1973, 57. ’ N Stolterfoht, Z Physik, 248, 1971, 92. 6 J D Garcia, Phys Rea, A4, 1971, 955. 7 F W Saris, Physica, 52, 1971, 290. ’ W Brandt, Proc Third Int Conf on Atomic Physics, Boulder, Colorado (edited by S J Smith), Plenum Press, New York (1973). g R F Musket and W Bauer, Appl Phys Lett, 20, 1972, 1. lo F Folkmann, C Gaarde, T Huus and K Kemp, N11clImtr Met/z, 116, 1974, 487.