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Non-statistical excitation of the magnetic substates of the 1P1 level of group II metal atoms in collision with 800 eV helium atoms
Volume 65A, number 3
PHYSICS LETFERS
6 March 1978
NON-STATISTICAL EXCITATION OF THE MAGNETIC SUBSTATES OF THE 1P1 LEVEL OF GROUP II METAL ATOMS IN COLLISION WITH 800 eV HELIUM ATOMS ~ D.W. FAHEY and L.D. SCHEARER’ Physics Department, University of Missouri, Rolla, Missouri, USA Received 7 December 1977
A large polarization has been observed for the emission lines of strontium and calcium excited by a beam of neutral helium atoms with 800 eV lab energy. The measured value of 15% indicates a preferential population of the magnetic substates of the correlated atom states of the target species. Depolarization of the emission in a magnetic field has been observed demonstrating the feasibility of Hanle-type lifetime measurements.
Recently Alber et al. [1] reported the obseryation of a nonstatistical 2P population of the magnetic substates of the 4 312 level of potassium. 2SThe potassium was excited in collisions betweenK(4 112) atoms and the rare gases He, Ne, Ar, Kr, and Xe at lab energies between 50 and 1500 2P eV. They2Sobserved polarized emission from the K(4 312—4 112) transition at 766.7 nm. The magnitude of the polarization ranged from +7% to —7% and showed a sharp energy dependence with the polarization tending to zero as the collision energy reached 1000211 eV and ~ j. They concluded B2~correlating that with the the molecular separated states atomsA(K, 2P 1S 312 and rare 0) 2E gas, state popare populated non-statistically with the B ulated preferentially. In an attempt to ascertain the influence of a spin— orbit coupling or an 1S coupling on the polarization observed in neutral—neutral collisions, we looked at collisions of 800 eV He(1S 0) atoms with certain group 1S II( 0) atoms. The group II atoms have zero nuclear spin and the singlet level, of course, has zero electronic spin. A neutral helium beam source was designed [2] to have a high flux on the order of 1015 atoms/sec-sr in order to optimize the optical observations. The beam * 1 *1
Research supported by the Office of Naval Research. Visiting Scientist, JILA. K—He collision energies only went to 150 eV.
~ ~
~ j
I ____
WAVELENGTH (ANGSTROMS) Fig. 1. Partial emission spectrum for~collisionsof 800 eV 1S 1S He( 0) atoms with target Sr( 0) atoms. energy was 800 eV with an energy spread of approximately 10%. The source is capable of both pulsed and dc operation with both modes yielding identical target excitation. The metastable component of the beam
was estimated to be less than 10~of the ground state flux. The target vapor was produced by resistively heating a small crucible placed near the beam. / The optical emission spectrum obtained from the 1S He—Sr( 0) collision at 800 eV lab energy is shown in fig. 1. No evidence for excitation of1the S strontium triplet spectrum is observed. The ‘P1— 0 emission at 460.9 nm was most intense and was viewed at 90 degrees from the beam axis outside the collision chamber. The observed optical emission was polarized along the beam axis with a value of 15%. Similar excitation and
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Volume 65A, number 3
PHYSICS LETTERS
polarized emission was observed in calcium. Excitation was observed but no polarization measurements made in cadmium and zinc. All polarization measurements were made with the target gas at essentially zero magnetic field, The application of a magnetic field orthogonal to both the beam axis and the observation direction reduced the observed polarization and at sufficiently large fields the polarization was reduced to zero. The depolarization of 1Pthe emission with magnetic field indicates that the 1 magnetic sublevels are coherently excited. The decrease in optical polarization with increased magnetic field follows a iorentzian line shape with a half-width given by the product 1P of the g-factor and the radiative decay time for the 1 level. Thus, the collisional excitation process performs the same role as linearly polarized resonance light in convention-
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6 March 1978
al zero-field level crossing spectroscopy, i.e. the Hanle effect [3]. On the basis of our observations, we conclude that at 800 eV collision energy the molecular state B1 ~ correlating with the separated atom states He(1 S 0) 1P and Sr( 1) is preferentially populated. Thus, states 1P of m1 0 in the Sr( 1)level are preferentially populated giving rise to the observed polarization. Refer2nces Ill
H. Alber, V. Kempter and W. Mecklenbrauck, J. Phys. B8
(1975) 913. [2] D.W. Fahey, L.D. Schearer and W.F. Parks, submitted for publication; D.W. Fahey, MS. thesis, Univ. of Missouri-Rolla (1977), unpublished. [3] W. Hanle, Z. Phys. 30(1924) 93.
PHYSICS LETFERS
6 March 1978
NON-STATISTICAL EXCITATION OF THE MAGNETIC SUBSTATES OF THE 1P1 LEVEL OF GROUP II METAL ATOMS IN COLLISION WITH 800 eV HELIUM ATOMS ~ D.W. FAHEY and L.D. SCHEARER’ Physics Department, University of Missouri, Rolla, Missouri, USA Received 7 December 1977
A large polarization has been observed for the emission lines of strontium and calcium excited by a beam of neutral helium atoms with 800 eV lab energy. The measured value of 15% indicates a preferential population of the magnetic substates of the correlated atom states of the target species. Depolarization of the emission in a magnetic field has been observed demonstrating the feasibility of Hanle-type lifetime measurements.
Recently Alber et al. [1] reported the obseryation of a nonstatistical 2P population of the magnetic substates of the 4 312 level of potassium. 2SThe potassium was excited in collisions betweenK(4 112) atoms and the rare gases He, Ne, Ar, Kr, and Xe at lab energies between 50 and 1500 2P eV. They2Sobserved polarized emission from the K(4 312—4 112) transition at 766.7 nm. The magnitude of the polarization ranged from +7% to —7% and showed a sharp energy dependence with the polarization tending to zero as the collision energy reached 1000211 eV and ~ j. They concluded B2~correlating that with the the molecular separated states atomsA(K, 2P 1S 312 and rare 0) 2E gas, state popare populated non-statistically with the B ulated preferentially. In an attempt to ascertain the influence of a spin— orbit coupling or an 1S coupling on the polarization observed in neutral—neutral collisions, we looked at collisions of 800 eV He(1S 0) atoms with certain group 1S II( 0) atoms. The group II atoms have zero nuclear spin and the singlet level, of course, has zero electronic spin. A neutral helium beam source was designed [2] to have a high flux on the order of 1015 atoms/sec-sr in order to optimize the optical observations. The beam * 1 *1
Research supported by the Office of Naval Research. Visiting Scientist, JILA. K—He collision energies only went to 150 eV.
~ ~
~ j
I ____
WAVELENGTH (ANGSTROMS) Fig. 1. Partial emission spectrum for~collisionsof 800 eV 1S 1S He( 0) atoms with target Sr( 0) atoms. energy was 800 eV with an energy spread of approximately 10%. The source is capable of both pulsed and dc operation with both modes yielding identical target excitation. The metastable component of the beam
was estimated to be less than 10~of the ground state flux. The target vapor was produced by resistively heating a small crucible placed near the beam. / The optical emission spectrum obtained from the 1S He—Sr( 0) collision at 800 eV lab energy is shown in fig. 1. No evidence for excitation of1the S strontium triplet spectrum is observed. The ‘P1— 0 emission at 460.9 nm was most intense and was viewed at 90 degrees from the beam axis outside the collision chamber. The observed optical emission was polarized along the beam axis with a value of 15%. Similar excitation and
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Volume 65A, number 3
PHYSICS LETTERS
polarized emission was observed in calcium. Excitation was observed but no polarization measurements made in cadmium and zinc. All polarization measurements were made with the target gas at essentially zero magnetic field, The application of a magnetic field orthogonal to both the beam axis and the observation direction reduced the observed polarization and at sufficiently large fields the polarization was reduced to zero. The depolarization of 1Pthe emission with magnetic field indicates that the 1 magnetic sublevels are coherently excited. The decrease in optical polarization with increased magnetic field follows a iorentzian line shape with a half-width given by the product 1P of the g-factor and the radiative decay time for the 1 level. Thus, the collisional excitation process performs the same role as linearly polarized resonance light in convention-
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6 March 1978
al zero-field level crossing spectroscopy, i.e. the Hanle effect [3]. On the basis of our observations, we conclude that at 800 eV collision energy the molecular state B1 ~ correlating with the separated atom states He(1 S 0) 1P and Sr( 1) is preferentially populated. Thus, states 1P of m1 0 in the Sr( 1)level are preferentially populated giving rise to the observed polarization. Refer2nces Ill
H. Alber, V. Kempter and W. Mecklenbrauck, J. Phys. B8
(1975) 913. [2] D.W. Fahey, L.D. Schearer and W.F. Parks, submitted for publication; D.W. Fahey, MS. thesis, Univ. of Missouri-Rolla (1977), unpublished. [3] W. Hanle, Z. Phys. 30(1924) 93.