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Possible evidence for heavy-ion-induced collective electron emission from solids
Volume 76A, number 2
PHYSICS LETTERS
17 March 1980
POSSIBLE EVIDENCE FOR HEAVY-ION-INDUCED COLLECTIVE ELECTRON EMISSION FROM SOLIDS ~ H.J. FRISCHKORN, K.O. GROENEVELD, S. SCHUMANN, R. LATZ, G. REICHHARDT and J. SCHADER Instirut für Kernphysik der J. W. Goethe-Universitdt, Frankfurt/M., Germany and
W. KRONAST and R. MANN Gesellschaft für Schwerionenforschung, Darmstadt, Germany Received 15 November 1979
A peak has been observed for the first time in the angular distributions of low energy electrons (E~<5 eV) emitted under heavy-ion bombardment of solids. The peak energy and angle coincide with the peak energy and angle predicted recently for collective electron emission in heavy-ion—solid collisions.
Heavy ions penetrating solids at velocities v0> 108 cm/s, say, induce electron density fluctuations behind the projectile [1] which can be expressed by the “wake potential” [2,31with the plasma properties of the solid. It has been predicted by Schafer et al. [4,5] that this leads to a directed emission of electrons from the target to be observed at the angle 0 is given by the Mach relation: cos 0~ u~/v0where v5 denotes the velocity of the Mach shock wave and u0 the projectile velocity. Signatures of the “shock electrons” are: (1) The emission direction follows the Mach relation. (2) The angular distribution peak has a half width of about 10 to 20°depending on the details of the collision system. (3) The electron energies are centred at energies between 1 and 10 eV depending mostly on the projectile velocity v 0. Motivated by this interesting concept an experiment was set up to test the feasibility of measuring the predicted phenomenon. H, C, and 0 beams at 300 keV/N from the Frankfurt 2.5 MV Van de Graaff accelerator 2). An electropenetrated carbon (5 toenergy 20 pg/cm static parallel plate foils electron analyser with a channeltron electron detector served as electron spec~•
~‘
Supported by BMFT, Bonn.
trometer. Between the parallel plate analyser and the channeltron the electrons were accelerated to high channeltron-efficiency energies. All surfaces near the collision region were coated with thin graphite layers. Experimental details wifi be published elsewhere. Fig. I gives an example of the angular distributions (instrumentally uncorrected) at 5 different electron energies between 2.6 and 10.7 eV. The measured electrons originate predominantly from ionization processes in the target [6], i.e. as ionization electrons. A pronounced peak develops at 80°with decreasing electron energy below 6.8 eV in the 2). collision system 0 or C (300 pg/cm A keY/N) number -÷ ofC(20 checks have been made to verify or falsify the observed structure: such a peak is not observed with protons of the same velocity. It is also not observed with gaseous targets (like CH4 or C02) bombarded with H or C at the same velocity. In most experiments the foil surface normal was çb = 45°with respect to the beam direction. Within the experimental errors the results given above are results independent of 0 (30°<~< 60°).Also, the same were obtained with Al targets instead of C targets; this result can be expected since most solid target surfaces are covered with carbon at the standard vacuum (10—6 Torr) condition of this experiment; this would obscure a possible 155
Volume 76A, number 2
PHYSICS LETTERS
Table 1 Comparison of theoretical prediction [5] and experimental
11000 +
—
9000 8000 9000 8000
17 March 1980
+
ELECTRON ENERGY [eV]
+
~
result of the C —~ C collision system. Experiment (this work)
Calculation (ref. [5])
__________________________________________________________
electron energy
peak in angular distribution
Ee <5 eV
Ee <5 eV
0
0 = 70° ± 10°
=
80°±15°
=
220 ±
I’)
z ~ z —
-j
peak (FWHM) width
5500
5~
~0 = 20° ± 5°
5000 4500
N. 2500
“N~
~
-
\~ I
30°
I
I
I
50° ANGLE
90°
~m
A peak has been at theC(20 UNILAC thesimilar collision system Pb(lobserved .4 MeY/N) pg/cmin2) at Ee = (6 ±2) eY. However, more detailed experiments are needed; systematic studies of different collision systems at higher bombarding energies with higher-Z projectiles at GSI are under way to verify the predicted shock electrons. -~
nso 2000
tify the experimental anomaly as “shock electrons”.
•
Fig. 1. Angular distributions of electrons emitted from C(3.6 MeV) -° C(10 Mg/cm2) collisions at different electron energies Ee. The ordinate is proportional to (count number X Ee). The solid line is to guide the eye.
Stimulating and helpful discussions with W. Greiner W. Schafer and H. Jex are gratefully acknowledged.
References dependence of the observed phenomena on the different plasma frequencies (hw) in carbon (hw = 25 eV) and in Al (hwAl = 15 eY). The beam currents were typically of the order of 1 (particle) nA; no dependence on the current or current density was found. Table 1 summarizes the experimental findings of this work and gives a comparison with the theoretical predictions for “shock electrons” [5] under the conditions of this experiment. A surprising agreement is found in the two sets of data. It is tempting to iden-
156
[1] N. Bohr, K. Dan. Vidensk Seisk. Mat. Fys. Medd. 18 (1948) no. 8. [2] V.N. Neelavathie, R.H. Ritchie and W. Brandt, Phys. Rev. Lett. 33 (1974) 302. [3] Z. Vager and D.S. Gemmell, Phys. Rev. Lett. 37 (1976) 1352. [4]
W. Schafer, H. Stocker, B. MUller and W. Greiner, Z. Phys. A288 (1978) 349.
[5] W. Schafer, H. StOcker, B. MUller and W. Greiner, ~obe published and private communication. [6] F. Fo&mann, K.O. Groeneveld, R. Mann, G. Nolte, S. Schumann and R. Spohr, Z. Phys. A275 (1975) 229.
PHYSICS LETTERS
17 March 1980
POSSIBLE EVIDENCE FOR HEAVY-ION-INDUCED COLLECTIVE ELECTRON EMISSION FROM SOLIDS ~ H.J. FRISCHKORN, K.O. GROENEVELD, S. SCHUMANN, R. LATZ, G. REICHHARDT and J. SCHADER Instirut für Kernphysik der J. W. Goethe-Universitdt, Frankfurt/M., Germany and
W. KRONAST and R. MANN Gesellschaft für Schwerionenforschung, Darmstadt, Germany Received 15 November 1979
A peak has been observed for the first time in the angular distributions of low energy electrons (E~<5 eV) emitted under heavy-ion bombardment of solids. The peak energy and angle coincide with the peak energy and angle predicted recently for collective electron emission in heavy-ion—solid collisions.
Heavy ions penetrating solids at velocities v0> 108 cm/s, say, induce electron density fluctuations behind the projectile [1] which can be expressed by the “wake potential” [2,31with the plasma properties of the solid. It has been predicted by Schafer et al. [4,5] that this leads to a directed emission of electrons from the target to be observed at the angle 0 is given by the Mach relation: cos 0~ u~/v0where v5 denotes the velocity of the Mach shock wave and u0 the projectile velocity. Signatures of the “shock electrons” are: (1) The emission direction follows the Mach relation. (2) The angular distribution peak has a half width of about 10 to 20°depending on the details of the collision system. (3) The electron energies are centred at energies between 1 and 10 eV depending mostly on the projectile velocity v 0. Motivated by this interesting concept an experiment was set up to test the feasibility of measuring the predicted phenomenon. H, C, and 0 beams at 300 keV/N from the Frankfurt 2.5 MV Van de Graaff accelerator 2). An electropenetrated carbon (5 toenergy 20 pg/cm static parallel plate foils electron analyser with a channeltron electron detector served as electron spec~•
~‘
Supported by BMFT, Bonn.
trometer. Between the parallel plate analyser and the channeltron the electrons were accelerated to high channeltron-efficiency energies. All surfaces near the collision region were coated with thin graphite layers. Experimental details wifi be published elsewhere. Fig. I gives an example of the angular distributions (instrumentally uncorrected) at 5 different electron energies between 2.6 and 10.7 eV. The measured electrons originate predominantly from ionization processes in the target [6], i.e. as ionization electrons. A pronounced peak develops at 80°with decreasing electron energy below 6.8 eV in the 2). collision system 0 or C (300 pg/cm A keY/N) number -÷ ofC(20 checks have been made to verify or falsify the observed structure: such a peak is not observed with protons of the same velocity. It is also not observed with gaseous targets (like CH4 or C02) bombarded with H or C at the same velocity. In most experiments the foil surface normal was çb = 45°with respect to the beam direction. Within the experimental errors the results given above are results independent of 0 (30°<~< 60°).Also, the same were obtained with Al targets instead of C targets; this result can be expected since most solid target surfaces are covered with carbon at the standard vacuum (10—6 Torr) condition of this experiment; this would obscure a possible 155
Volume 76A, number 2
PHYSICS LETTERS
Table 1 Comparison of theoretical prediction [5] and experimental
11000 +
—
9000 8000 9000 8000
17 March 1980
+
ELECTRON ENERGY [eV]
+
~
result of the C —~ C collision system. Experiment (this work)
Calculation (ref. [5])
__________________________________________________________
electron energy
peak in angular distribution
Ee <5 eV
Ee <5 eV
0
0 = 70° ± 10°
=
80°±15°
=
220 ±
I’)
z ~ z —
-j
peak (FWHM) width
5500
5~
~0 = 20° ± 5°
5000 4500
N. 2500
“N~
~
-
\~ I
30°
I
I
I
50° ANGLE
90°
~m
A peak has been at theC(20 UNILAC thesimilar collision system Pb(lobserved .4 MeY/N) pg/cmin2) at Ee = (6 ±2) eY. However, more detailed experiments are needed; systematic studies of different collision systems at higher bombarding energies with higher-Z projectiles at GSI are under way to verify the predicted shock electrons. -~
nso 2000
tify the experimental anomaly as “shock electrons”.
•
Fig. 1. Angular distributions of electrons emitted from C(3.6 MeV) -° C(10 Mg/cm2) collisions at different electron energies Ee. The ordinate is proportional to (count number X Ee). The solid line is to guide the eye.
Stimulating and helpful discussions with W. Greiner W. Schafer and H. Jex are gratefully acknowledged.
References dependence of the observed phenomena on the different plasma frequencies (hw) in carbon (hw = 25 eV) and in Al (hwAl = 15 eY). The beam currents were typically of the order of 1 (particle) nA; no dependence on the current or current density was found. Table 1 summarizes the experimental findings of this work and gives a comparison with the theoretical predictions for “shock electrons” [5] under the conditions of this experiment. A surprising agreement is found in the two sets of data. It is tempting to iden-
156
[1] N. Bohr, K. Dan. Vidensk Seisk. Mat. Fys. Medd. 18 (1948) no. 8. [2] V.N. Neelavathie, R.H. Ritchie and W. Brandt, Phys. Rev. Lett. 33 (1974) 302. [3] Z. Vager and D.S. Gemmell, Phys. Rev. Lett. 37 (1976) 1352. [4]
W. Schafer, H. Stocker, B. MUller and W. Greiner, Z. Phys. A288 (1978) 349.
[5] W. Schafer, H. StOcker, B. MUller and W. Greiner, ~obe published and private communication. [6] F. Fo&mann, K.O. Groeneveld, R. Mann, G. Nolte, S. Schumann and R. Spohr, Z. Phys. A275 (1975) 229.