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Resonance excitation of helium by electrons at energies near 60 eV
Physica 47 (1970) 159-164
RESONANCE
0 North-Holland Publishing Co.
EXCITATION
OF HELIUM
AT ENERGIES H. G. M. HEIDEMAN,
NEAR
BY
ELECTRONS
60 eV
M. L. H. HUIJNEN
and C. SMIT
Fysisch Laboratorium der Rijksuniversiteit te Utrecht, Nederland
Received 16 September 1969
Synopsis The excitation of the 4% and 3sP levels of helium by electron impact has been studied by observing the light emission following the excitation of the levels concerned. The electron energy region considered extends from 55 to 62 eV. The measurements show that in this energy range a number of resonances occur in the optical excitation curves. These resonances, some of which have been observed previously in an electron scattering experiment, are probably caused by an indirect excitation process via intermediate compound states associated with doubly excited states of the neutral helium atom.
1. Introdzwtion. In recent years much attention has been payed to the study of resonances in electron-atom (or molecule) scattering. These resonances resulting from formation and subsequent decay of negative ion compound states often have a very dramatic effect on the excitation cross sections. It has been shown that the excitation of heliumr-3) in the threshold region is largely determined by resonance effects. Resonances have been found in the electron scattering from many other atomsd-6) and molecules7-13) and it is now understood that they are a generally occurring rather than an exceptional phenomenon. It is assumed that a negative ion compound is formed by a binding of the incident electron to an excited state of the neutral atom. In all previous experiments on helium, except onei3), resonances are studied which result from compound states associated with singly excited states of the target atom and thus lying below the first ionization limit. Simpsoni3) et al. were first to observe resonances in the electron-helium scattering at much higher energies, namely near 60 eV. These authors studied in an electron scattering experiment the excitation, near 60 eV, of the n = 2 states as a function of the scattering angle. The resonances they observed are caused by compound states apparently associated with doubly excited states of neutral helium. In this case a compound state is formed by addition of the incident electron 159
160
H. G. M. HEIDEMAN,
to a doubly
excited
state
M. L. H. HUIJNEN
of the helium
atom.
AND C. SMIT
Radiationless
decay
(unde
emission of an electron) of such a state to a singly excited state of heliur may then give rise to a resonance in the excitation curve of that sing1 excited state. In our experiment 14) we have studied the excitation
of the 33P and 431
states of helium near 60 eV by an optical method (i.e. by observing the ligh emission following the excitation of these levels). It would not be possibl to study the excitation of the 33P and 43s states in an electron scatterin; experiment as with the present energy resolution these states can not b separated from nearby states. In our optical experiment this presents ni problem. A disadvantage of the optical method is that total cross section are measured which give of course less detailed information than th differential cross sections which can be measured in an electron scatterin experiment. Fig. 1 shows a schematic drawing o 2. The eqberimental arrangement. the complete setup lsy l6). A sealed excitation tube contains helium that car be excited by means of a beam of electrons of adjustable velocity. The heliun pressure is about 10-3 torr and may be altered by breaking a thin glas
I -
GRATING MONOCHRO. MATOR
VANE
SWITCHED
OSCIL-
LATOR
UNIT
PHOTOMULTIPLIER TUBE IN COOLlNG BOX
ml I
D
ELECTRONIC COUNTERS
Fig. 1. Schematic diagram of the experimental setup.
RESONANCE EXCITATION OF HELIUM BY ELECTRONS
COlliSiOn Chamber
Electron
161
trap
Fig. 2. Schematic drawing of the excitation tube.
partition between a small reservoir and the tube. The pressure is measured with an ionization gauge connected to the tube. The electrode system consists of an oxide-coated cathode K and five electrodes Er . . . Es (see fig. 2). Er and Es are used to regulate the electron current and the parallelism of the beam. The electrode Es is a Faraday cage and serves as excitation chamber. An electron trap, consisting of E4 and Es, is situated behind the excitation chamber to prevent backscattering of the electrons passing through the excitation chamber and also to prevent interfering secundary emission. The electron current through the excitation chamber amounts to about 25 PA. The light originating from a cross section (narrow disk) MM of the electron beam is focused on the entrance slit of a Bausch and Lomb monochromator. The direction of observation is perpendicular to the electron beam. The light of a selected spectral line leaving the monochromator falls on the photocathode of a cooled multiplier, the temperature of which can be adjusted between -20 and -40 degrees centrigrade with a stability better than 1 degree. Each photo-electron gives a current pulse at the anode of the photomultiplier. These pulses are amplified with a fast amplifier, discriminated, and finally recorded with electronic counters. In cases of small light signals it was desirable to correct for the dark current of the photo-multiplier. Therefore a square-wave voltage is supplied to the excitation chamber. This square-wave voltage has the adjustable value Vs (measuring voltage) during 2 period and is zero during the next $ period. The multiplier pulses corresponding to the light signal can be separated from those corresponding to the dark current by means of an electronic channel switch which operates synchronously with the afore-
H. G. M. HEIDEMAN,
162
mentioned
square-wave
the multiplier
M. L. H. HUIJNEN
voltage.
dark current
AND C. SMIT
In this way a simultaneous
indication
o
is obtained.
3. Results and discussion. Fig. 3 shows our results for the 47 1.3 nm line the upper level of which is the 4% state of helium. Between 56 and 61 eV ; clear resonance structure is observed in the excitation curve. Near 57 eV twc small resonances occur. The larger structure near 59 eV is probably also cause< by two closely spaced resonances judging from the asymmetric left side of the peak. The height of the peak at 59.5 eV relative to the “background” o the direct excitation is about 1 : 20 (note that the zero of the ordinate ha! been displaced). In fig. 4 the measurements on the 388.9 nm line (upper level 3sP) are shown. Also in this curve a distinct resonance structure is present I
I
I
I
I
I
I
I
0 0
B
He 00 0
471.3 4%
nm
---) 2=P
co 0 0 000 0
0 OO 0 0 OO
“SD0
OO
00
0~00 0 0
1
I
56
I
I
ELECTRON Fig. 3. Excitation
I
58
I
I
60 ENERGY
I
62 (eV)
of the 471.3 nm line (43s + 23P) of helium near 60 eV.
RESONANCE
EXCITATION
II
OF HELIUM I
I
I
56
I
I
58 ELECTRON
Fig. 4. Excitation
BY ELECTRONS
I
I
I
I
I
I
60 ENERGY
163
I
I
62 (eV)
of the 388.9 nm line (33P + 2%) of helium near 60 eV.
Apart from the two peaks at 56.7 and 57.9 eV, respectively, there is a sharp dip at 58.2 eV. The sharpness of the observed structure is limited by the energy resolution of our electron beam, which amounts to about 0.3 eV. Our energy scale has been established by shifting the onsets of the excitation curves to the spectroscopically known values of the excitation energies. This method is fairly reliable for energies near the threshold, but may give a less accurate calibration in the energy range we are now dealing with, because of a possible distortion of the energy scale due to space-charge effects. We estimate that the error in the absolute energy scale may be about 0.5 eV. The relative positions of the resonances are probably accurate to better than 0.1 eV. Our results may be compared with those obtained by Simpsonrs) et al.,
164
RESONANCE EXCITATION OF HELIUM BY ELECTRONS
who measured the differential excitation curves of the n = 2 states near 60 eV in an electron scattering experiment. These authors observed two distinct resonances at 57.1 and 58.2 eV, respectively. We observe two resonances in the 33P excitation at 56.7 and 57.9 eV on our energy scale; thus at practically the same spacing. It is therefore tempting to assume that the two resonances we observe near 57 eV and the two resonances observed by Simpson et al. are caused by the same two negative ion states. Simpson et al. attribute these resonances to compound states associated with doubly excited states of neutral helium and having the configurations (2ss2p)sP and (2s2ps)sD, respectively. Between 59 and 60 eV we observe additional structure in the 47 1.3 nm curve, which is not reported by Simpson et al. This structure is probably composed of overlapping resonances caused by compound states associated with higher lying doubly excited states of heliumIT) such as (2ps)lD and (2s2p)iP.
REFERENCES 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17)
Chamberlain, G. E. and Heideman, H. G. M., Phys. Rev. Letters 15 (1965) 337. Ehrhardt, H. and Willman, K., 2. Phys. 203 (1967) 1. Ehrhardt, H., Langhans, L. and Linder, F., Z. Phys. 214 (1968) 179. Kuyatt, C. E., Simpson, J. A. and Mielczarek, S. R., Phys. Rev. 138 (1965) A385 Schulz, G. J., Phys. Rev. Letters 13 (1964) 583. Schulz, G. J., Phys. Rev. 136 (1964) A650. Heideman, H. G. M., Kuyatt, C. E. and Chamberlain, G. E., J. them. Phys. 44 (1966) 355. Schulz, G. J., Phys. Rev. Al35 (1964) 988. Andrick, D. and Ehrhardt, H., 2. Phys. 192 (1966) 99. Heideman, H. G. M., Kuyatt, C. E. and Chamberlain, G. E., J. them. Phys. 44 (1966) 440. Kuyatt, C. E., Simpson, J. A. and Mielczarek, S. R., J. them. Phys. 44 (1966) 437. Golden, D. E. and Bandel, H. W., Phys. Rev. Letters 14 (1965) 1010. Simpson, J. A., Menendez, M. G. and Mielczarek, S. R., Phys. Rev. I50 (1966) 76. Heideman, H. G. M., Huijnen, M. L. H. and Smit, C., Proc. VIth Int. Conf. Electr. Atom Coll. (1969), Cambridge, Mass. (The M.I.T. Press, Cambridge, Mass., U.S.A.) p. 727. Heideman, H. G. M., Ph. D. thesis, University of Utrecht, 1968. Heideman, H. G. M., Smit, C. and Smit, J. A., Physica 45 (1969) 305. Simpson, J. A., Mielczarek, S. R. and Cooper, J., J.O.S.A. 54 (1964) 269.
RESONANCE
0 North-Holland Publishing Co.
EXCITATION
OF HELIUM
AT ENERGIES H. G. M. HEIDEMAN,
NEAR
BY
ELECTRONS
60 eV
M. L. H. HUIJNEN
and C. SMIT
Fysisch Laboratorium der Rijksuniversiteit te Utrecht, Nederland
Received 16 September 1969
Synopsis The excitation of the 4% and 3sP levels of helium by electron impact has been studied by observing the light emission following the excitation of the levels concerned. The electron energy region considered extends from 55 to 62 eV. The measurements show that in this energy range a number of resonances occur in the optical excitation curves. These resonances, some of which have been observed previously in an electron scattering experiment, are probably caused by an indirect excitation process via intermediate compound states associated with doubly excited states of the neutral helium atom.
1. Introdzwtion. In recent years much attention has been payed to the study of resonances in electron-atom (or molecule) scattering. These resonances resulting from formation and subsequent decay of negative ion compound states often have a very dramatic effect on the excitation cross sections. It has been shown that the excitation of heliumr-3) in the threshold region is largely determined by resonance effects. Resonances have been found in the electron scattering from many other atomsd-6) and molecules7-13) and it is now understood that they are a generally occurring rather than an exceptional phenomenon. It is assumed that a negative ion compound is formed by a binding of the incident electron to an excited state of the neutral atom. In all previous experiments on helium, except onei3), resonances are studied which result from compound states associated with singly excited states of the target atom and thus lying below the first ionization limit. Simpsoni3) et al. were first to observe resonances in the electron-helium scattering at much higher energies, namely near 60 eV. These authors studied in an electron scattering experiment the excitation, near 60 eV, of the n = 2 states as a function of the scattering angle. The resonances they observed are caused by compound states apparently associated with doubly excited states of neutral helium. In this case a compound state is formed by addition of the incident electron 159
160
H. G. M. HEIDEMAN,
to a doubly
excited
state
M. L. H. HUIJNEN
of the helium
atom.
AND C. SMIT
Radiationless
decay
(unde
emission of an electron) of such a state to a singly excited state of heliur may then give rise to a resonance in the excitation curve of that sing1 excited state. In our experiment 14) we have studied the excitation
of the 33P and 431
states of helium near 60 eV by an optical method (i.e. by observing the ligh emission following the excitation of these levels). It would not be possibl to study the excitation of the 33P and 43s states in an electron scatterin; experiment as with the present energy resolution these states can not b separated from nearby states. In our optical experiment this presents ni problem. A disadvantage of the optical method is that total cross section are measured which give of course less detailed information than th differential cross sections which can be measured in an electron scatterin experiment. Fig. 1 shows a schematic drawing o 2. The eqberimental arrangement. the complete setup lsy l6). A sealed excitation tube contains helium that car be excited by means of a beam of electrons of adjustable velocity. The heliun pressure is about 10-3 torr and may be altered by breaking a thin glas
I -
GRATING MONOCHRO. MATOR
VANE
SWITCHED
OSCIL-
LATOR
UNIT
PHOTOMULTIPLIER TUBE IN COOLlNG BOX
ml I
D
ELECTRONIC COUNTERS
Fig. 1. Schematic diagram of the experimental setup.
RESONANCE EXCITATION OF HELIUM BY ELECTRONS
COlliSiOn Chamber
Electron
161
trap
Fig. 2. Schematic drawing of the excitation tube.
partition between a small reservoir and the tube. The pressure is measured with an ionization gauge connected to the tube. The electrode system consists of an oxide-coated cathode K and five electrodes Er . . . Es (see fig. 2). Er and Es are used to regulate the electron current and the parallelism of the beam. The electrode Es is a Faraday cage and serves as excitation chamber. An electron trap, consisting of E4 and Es, is situated behind the excitation chamber to prevent backscattering of the electrons passing through the excitation chamber and also to prevent interfering secundary emission. The electron current through the excitation chamber amounts to about 25 PA. The light originating from a cross section (narrow disk) MM of the electron beam is focused on the entrance slit of a Bausch and Lomb monochromator. The direction of observation is perpendicular to the electron beam. The light of a selected spectral line leaving the monochromator falls on the photocathode of a cooled multiplier, the temperature of which can be adjusted between -20 and -40 degrees centrigrade with a stability better than 1 degree. Each photo-electron gives a current pulse at the anode of the photomultiplier. These pulses are amplified with a fast amplifier, discriminated, and finally recorded with electronic counters. In cases of small light signals it was desirable to correct for the dark current of the photo-multiplier. Therefore a square-wave voltage is supplied to the excitation chamber. This square-wave voltage has the adjustable value Vs (measuring voltage) during 2 period and is zero during the next $ period. The multiplier pulses corresponding to the light signal can be separated from those corresponding to the dark current by means of an electronic channel switch which operates synchronously with the afore-
H. G. M. HEIDEMAN,
162
mentioned
square-wave
the multiplier
M. L. H. HUIJNEN
voltage.
dark current
AND C. SMIT
In this way a simultaneous
indication
o
is obtained.
3. Results and discussion. Fig. 3 shows our results for the 47 1.3 nm line the upper level of which is the 4% state of helium. Between 56 and 61 eV ; clear resonance structure is observed in the excitation curve. Near 57 eV twc small resonances occur. The larger structure near 59 eV is probably also cause< by two closely spaced resonances judging from the asymmetric left side of the peak. The height of the peak at 59.5 eV relative to the “background” o the direct excitation is about 1 : 20 (note that the zero of the ordinate ha! been displaced). In fig. 4 the measurements on the 388.9 nm line (upper level 3sP) are shown. Also in this curve a distinct resonance structure is present I
I
I
I
I
I
I
I
0 0
B
He 00 0
471.3 4%
nm
---) 2=P
co 0 0 000 0
0 OO 0 0 OO
“SD0
OO
00
0~00 0 0
1
I
56
I
I
ELECTRON Fig. 3. Excitation
I
58
I
I
60 ENERGY
I
62 (eV)
of the 471.3 nm line (43s + 23P) of helium near 60 eV.
RESONANCE
EXCITATION
II
OF HELIUM I
I
I
56
I
I
58 ELECTRON
Fig. 4. Excitation
BY ELECTRONS
I
I
I
I
I
I
60 ENERGY
163
I
I
62 (eV)
of the 388.9 nm line (33P + 2%) of helium near 60 eV.
Apart from the two peaks at 56.7 and 57.9 eV, respectively, there is a sharp dip at 58.2 eV. The sharpness of the observed structure is limited by the energy resolution of our electron beam, which amounts to about 0.3 eV. Our energy scale has been established by shifting the onsets of the excitation curves to the spectroscopically known values of the excitation energies. This method is fairly reliable for energies near the threshold, but may give a less accurate calibration in the energy range we are now dealing with, because of a possible distortion of the energy scale due to space-charge effects. We estimate that the error in the absolute energy scale may be about 0.5 eV. The relative positions of the resonances are probably accurate to better than 0.1 eV. Our results may be compared with those obtained by Simpsonrs) et al.,
164
RESONANCE EXCITATION OF HELIUM BY ELECTRONS
who measured the differential excitation curves of the n = 2 states near 60 eV in an electron scattering experiment. These authors observed two distinct resonances at 57.1 and 58.2 eV, respectively. We observe two resonances in the 33P excitation at 56.7 and 57.9 eV on our energy scale; thus at practically the same spacing. It is therefore tempting to assume that the two resonances we observe near 57 eV and the two resonances observed by Simpson et al. are caused by the same two negative ion states. Simpson et al. attribute these resonances to compound states associated with doubly excited states of neutral helium and having the configurations (2ss2p)sP and (2s2ps)sD, respectively. Between 59 and 60 eV we observe additional structure in the 47 1.3 nm curve, which is not reported by Simpson et al. This structure is probably composed of overlapping resonances caused by compound states associated with higher lying doubly excited states of heliumIT) such as (2ps)lD and (2s2p)iP.
REFERENCES 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17)
Chamberlain, G. E. and Heideman, H. G. M., Phys. Rev. Letters 15 (1965) 337. Ehrhardt, H. and Willman, K., 2. Phys. 203 (1967) 1. Ehrhardt, H., Langhans, L. and Linder, F., Z. Phys. 214 (1968) 179. Kuyatt, C. E., Simpson, J. A. and Mielczarek, S. R., Phys. Rev. 138 (1965) A385 Schulz, G. J., Phys. Rev. Letters 13 (1964) 583. Schulz, G. J., Phys. Rev. 136 (1964) A650. Heideman, H. G. M., Kuyatt, C. E. and Chamberlain, G. E., J. them. Phys. 44 (1966) 355. Schulz, G. J., Phys. Rev. Al35 (1964) 988. Andrick, D. and Ehrhardt, H., 2. Phys. 192 (1966) 99. Heideman, H. G. M., Kuyatt, C. E. and Chamberlain, G. E., J. them. Phys. 44 (1966) 440. Kuyatt, C. E., Simpson, J. A. and Mielczarek, S. R., J. them. Phys. 44 (1966) 437. Golden, D. E. and Bandel, H. W., Phys. Rev. Letters 14 (1965) 1010. Simpson, J. A., Menendez, M. G. and Mielczarek, S. R., Phys. Rev. I50 (1966) 76. Heideman, H. G. M., Huijnen, M. L. H. and Smit, C., Proc. VIth Int. Conf. Electr. Atom Coll. (1969), Cambridge, Mass. (The M.I.T. Press, Cambridge, Mass., U.S.A.) p. 727. Heideman, H. G. M., Ph. D. thesis, University of Utrecht, 1968. Heideman, H. G. M., Smit, C. and Smit, J. A., Physica 45 (1969) 305. Simpson, J. A., Mielczarek, S. R. and Cooper, J., J.O.S.A. 54 (1964) 269.