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H− formation from collision-induced dissociation of H3+ in He at keV energies
Nuclear Instruments and Methods forth-Holland_ Amsterdam
in Physics
Research
B40/41
H - FORMATION FROM COLLISION-I~UC~~ OF H: IN He AT keV ENERGIES I. ALVAREZ,
H. MARTiNEZ,
C. CISNEROS,
245
(1989) 245-247
DISSOCIATIUN
A. MORALES
and J. DE URQUIJO
r~~~~i~~~ de Fisica, LGVAM, P.O. Box 139-B, Cuernnuaca, Mor., 621Pf, Mexico
We have used a small accelerator with a colutron-type ion source and a computer-controlled rotating chamber housing a parallel plate energy analyzer to measure the angular and energy distributions of the negative ions produced by the collision-induced dissociation of Hz in He. Due to the correlation between center-of-mass energy dist~but~o~sof the dissoclatiag fragments and their angular spread in the laboratory frame we have used the results from both measurements to estimate the internal energy increment of the system (E) and the energy at which the fragments are ejected as a result of the dissociation (W). The total energy E = 22 eV is consistent with the calculated energy from the Hz ground state to the dissociation limit leading H-.
1. Introduction The PI; ion has been the subject of several experimental and theoretical studies [I]. Due to the inherent complexity of a collision-induced dissociation processes, most investigations have been designed so as to simplify the problem to some extent. Focusing attention on the present experiment, the following characteristics are apparent : (a) Only one dissociation channel predominates; in other words, when measuring the I-I- fragment, the other two fragments are restricted to be H+, since the charge transfer process is very unlikely with He as a target. (b) On bombarding the He target with HT at keV energies, the molecule can be considered as “frozen”, and consequently the resulting fragments carry the molecular state at the time of the collision [2]. It is assumed that a two-step model of dissociation applies [2]. The excitation of the molecule above the dissociation limit may be caused by the adiabatic distortion of the eiectron clouds of the two colliding particles [a]. For the reaction Hz + He + I-f+ t H- + Ht a polar dissociation mechanism is suggested, based on the ejection of the two protons in almost opposite directions due to induced polar forces during the collision. The two-step model is strictly applicable only to binary dissociation processes. However, the angular and energy distributions are due to the transverse component of the velocities acquired by the fragments sharing the dissociation energy, so that by using the conservation equations one arrives at analogous expressions corresponding to a binary dissociation when account is taken of the symmetry of this three-body Coulomb breakup. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
2. Experimental method Molecular ions of mass m + M (t?l is the mass of two protons and M is the I$- detected mass), formed in an arc discharge source, were electrostatically accelerated to energies of 1.25 , 2.75, 3.5 and 4.83 keV, and selected by a Wien filter. The selected ionic species was then allowed to pass through a series of collimators before entering the interaction cell, containing He at a pressure of about 0.1 mTorr. The cell was a cylinder of 2.5 cm in length and diameter, with a 1 mm diameter collimator at its entrance, and a rectangular exit aperture of 2 x 6 mm. This cell was fixed at the center of a computer-controlled, rotatable vacuum chamber. Upon rotation, the chamber moves the detection system, located 47 mm away from the interaction cell. The detection system consists of a retarding field, parabolic flight electrostatic analyzer [3] with a channel electron multiplier attached to its exit end. Both energy and angular distributions were obtained with the same analyzer, simply by replacing the collimators 131. The experimental apparatus is shown in fig. 1. The data were analyzed by using the following relations [2,3]: (M+m)U=M(V-W-E) +mW + 2[ mMW(P--
8max = [ mW,,,/M(V-
w-
w-
E)]“‘,
E)]?
(1) (2)
(1) refers to energy distributions, whereas eq. (2) refers to angular distributions [3]; ?_Iis the energy of the fragment of mass M, detected at zero degree in the laboratory frame (LF), I/ is the initial energy of the molecular ion of mass (m + M), E is the internal energy increment of the system and W is the energy at which the fragments are ejected as a result of dissociation. Since the dissociation process is isotropic, this Eq.
I. ATOMIC
PHYSICS
I. Alvarez et al. / H - formation from collision-induced dissociation
246
COLLIMATORS
COLU TRON ION SOURCE I
::“,SE
SCATTERING
CHAMBER RETRACTABLE SECONDARY
EMISSION
L
WIEN I VELOCITY FILTER
STEERING PLATES
CHANNEL ELECTRON MULTIPLIER (NOT
(&So PARALLEL PLATE ANALIZER)
TO SCALE)
Fig. 1. Schematic diagram of the apparatus.
energy must be added or subtracted to the initial energy of the beam, depending on whether the fragment of mass M is ejected in the forward (+ ) of backward ( - ) direction. Fig. 2 shows an energy spectrum of H- at 4.5 keV primary beam energy. Eq. (2) represents the dependence between the maximum kinetic energy, W,,,, at which fragments at an is the angle between the angle em, are released. e,, direction of the observed fragment (M) and the incident beam direction at maximum angular spread. Fig. 3 shows the angular distributions and the position of e,,. The procedure by which E and W are derived from the analysis of both measured angular and energy distributions has been described in detail in ref. [3]. Thus,
from the energy distributions, values of E = 22 & 6 eV and W = 4.5 f 0.4 eV are obtained. The estimated error is one standard deviation. A more careful inspection of the energy distributions indicates that their highest peaks are slightly shifted towards the left of the position that corresponds to zero energy loss, namely, one third of the incident energy. This shift from the peak (W = 0) originates from the loss of energy during the collision which in turn, is a measure of the increment in internal energy of the molecule, E [4]. E can be calculated from eq. (1) by setting W = 0, taking the U value at the maximum on the energy distribution and solving for E, its value being the minimum excitation energy for the molecule to reach the potential energy level at which Hl dissoci-
H?+He-H-
2.75
-1
‘i
1
0-i e (degrees) II 6.59 4.5
I 1x3
I a02
I
1.7
II
4.5
6.0
(eV) ti+eV)
Fig. 2. LF energy spectrum of the H- fragment at
4.5
keV.
Fig. 3. Angular distributions of the H- fragment, showing 6’,,,,,. Each curve is the average of several runs after a careful subtraction of the background. The angular resolution of the apparatus is 0.1 o
I. Aluarez et al. / H _ formation from collision-induced
ation into the fragments H+ + H- + H+ takes place. By doing this, the same value for E is obtained as stated above. Thus we conclude that the H- formation process is caused by the adiabatic distortion of the electron clouds, resulting in the polar dissociation of the molecule, when we compare the present results with those obtained previously when Cs was used as a target [5].
dissociatron
247
References [l] SC. Goh and J.B. Swan, Phys. Rev. A24 (1981) 1624. [2] J. Loss and T.R. Govers, in: Collision Spectroscopy, ed. R.G. Cook (Plenum, New York, 1978) p. 289. [3] H. Martinez, I. Alvarez, J. de Urquijo, C. Cisneros and A. Amaya-Tapia, Phys. Rev. A36 (1987) 5425. [4] G. Comtet and P.G. Foumier, Chem. Phys. 81 (1983) 221. [5] C. Cisneros, I. Alvarez, C.F. Barnett, J.A. Ray and A. Russek, Phys. Rev. Al9 (1979) 631.
I. ATOMIC PHYSICS
in Physics
Research
B40/41
H - FORMATION FROM COLLISION-I~UC~~ OF H: IN He AT keV ENERGIES I. ALVAREZ,
H. MARTiNEZ,
C. CISNEROS,
245
(1989) 245-247
DISSOCIATIUN
A. MORALES
and J. DE URQUIJO
r~~~~i~~~ de Fisica, LGVAM, P.O. Box 139-B, Cuernnuaca, Mor., 621Pf, Mexico
We have used a small accelerator with a colutron-type ion source and a computer-controlled rotating chamber housing a parallel plate energy analyzer to measure the angular and energy distributions of the negative ions produced by the collision-induced dissociation of Hz in He. Due to the correlation between center-of-mass energy dist~but~o~sof the dissoclatiag fragments and their angular spread in the laboratory frame we have used the results from both measurements to estimate the internal energy increment of the system (E) and the energy at which the fragments are ejected as a result of the dissociation (W). The total energy E = 22 eV is consistent with the calculated energy from the Hz ground state to the dissociation limit leading H-.
1. Introduction The PI; ion has been the subject of several experimental and theoretical studies [I]. Due to the inherent complexity of a collision-induced dissociation processes, most investigations have been designed so as to simplify the problem to some extent. Focusing attention on the present experiment, the following characteristics are apparent : (a) Only one dissociation channel predominates; in other words, when measuring the I-I- fragment, the other two fragments are restricted to be H+, since the charge transfer process is very unlikely with He as a target. (b) On bombarding the He target with HT at keV energies, the molecule can be considered as “frozen”, and consequently the resulting fragments carry the molecular state at the time of the collision [2]. It is assumed that a two-step model of dissociation applies [2]. The excitation of the molecule above the dissociation limit may be caused by the adiabatic distortion of the eiectron clouds of the two colliding particles [a]. For the reaction Hz + He + I-f+ t H- + Ht a polar dissociation mechanism is suggested, based on the ejection of the two protons in almost opposite directions due to induced polar forces during the collision. The two-step model is strictly applicable only to binary dissociation processes. However, the angular and energy distributions are due to the transverse component of the velocities acquired by the fragments sharing the dissociation energy, so that by using the conservation equations one arrives at analogous expressions corresponding to a binary dissociation when account is taken of the symmetry of this three-body Coulomb breakup. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
2. Experimental method Molecular ions of mass m + M (t?l is the mass of two protons and M is the I$- detected mass), formed in an arc discharge source, were electrostatically accelerated to energies of 1.25 , 2.75, 3.5 and 4.83 keV, and selected by a Wien filter. The selected ionic species was then allowed to pass through a series of collimators before entering the interaction cell, containing He at a pressure of about 0.1 mTorr. The cell was a cylinder of 2.5 cm in length and diameter, with a 1 mm diameter collimator at its entrance, and a rectangular exit aperture of 2 x 6 mm. This cell was fixed at the center of a computer-controlled, rotatable vacuum chamber. Upon rotation, the chamber moves the detection system, located 47 mm away from the interaction cell. The detection system consists of a retarding field, parabolic flight electrostatic analyzer [3] with a channel electron multiplier attached to its exit end. Both energy and angular distributions were obtained with the same analyzer, simply by replacing the collimators 131. The experimental apparatus is shown in fig. 1. The data were analyzed by using the following relations [2,3]: (M+m)U=M(V-W-E) +mW + 2[ mMW(P--
8max = [ mW,,,/M(V-
w-
w-
E)]“‘,
E)]?
(1) (2)
(1) refers to energy distributions, whereas eq. (2) refers to angular distributions [3]; ?_Iis the energy of the fragment of mass M, detected at zero degree in the laboratory frame (LF), I/ is the initial energy of the molecular ion of mass (m + M), E is the internal energy increment of the system and W is the energy at which the fragments are ejected as a result of dissociation. Since the dissociation process is isotropic, this Eq.
I. ATOMIC
PHYSICS
I. Alvarez et al. / H - formation from collision-induced dissociation
246
COLLIMATORS
COLU TRON ION SOURCE I
::“,SE
SCATTERING
CHAMBER RETRACTABLE SECONDARY
EMISSION
L
WIEN I VELOCITY FILTER
STEERING PLATES
CHANNEL ELECTRON MULTIPLIER (NOT
(&So PARALLEL PLATE ANALIZER)
TO SCALE)
Fig. 1. Schematic diagram of the apparatus.
energy must be added or subtracted to the initial energy of the beam, depending on whether the fragment of mass M is ejected in the forward (+ ) of backward ( - ) direction. Fig. 2 shows an energy spectrum of H- at 4.5 keV primary beam energy. Eq. (2) represents the dependence between the maximum kinetic energy, W,,,, at which fragments at an is the angle between the angle em, are released. e,, direction of the observed fragment (M) and the incident beam direction at maximum angular spread. Fig. 3 shows the angular distributions and the position of e,,. The procedure by which E and W are derived from the analysis of both measured angular and energy distributions has been described in detail in ref. [3]. Thus,
from the energy distributions, values of E = 22 & 6 eV and W = 4.5 f 0.4 eV are obtained. The estimated error is one standard deviation. A more careful inspection of the energy distributions indicates that their highest peaks are slightly shifted towards the left of the position that corresponds to zero energy loss, namely, one third of the incident energy. This shift from the peak (W = 0) originates from the loss of energy during the collision which in turn, is a measure of the increment in internal energy of the molecule, E [4]. E can be calculated from eq. (1) by setting W = 0, taking the U value at the maximum on the energy distribution and solving for E, its value being the minimum excitation energy for the molecule to reach the potential energy level at which Hl dissoci-
H?+He-H-
2.75
-1
‘i
1
0-i e (degrees) II 6.59 4.5
I 1x3
I a02
I
1.7
II
4.5
6.0
(eV) ti+eV)
Fig. 2. LF energy spectrum of the H- fragment at
4.5
keV.
Fig. 3. Angular distributions of the H- fragment, showing 6’,,,,,. Each curve is the average of several runs after a careful subtraction of the background. The angular resolution of the apparatus is 0.1 o
I. Aluarez et al. / H _ formation from collision-induced
ation into the fragments H+ + H- + H+ takes place. By doing this, the same value for E is obtained as stated above. Thus we conclude that the H- formation process is caused by the adiabatic distortion of the electron clouds, resulting in the polar dissociation of the molecule, when we compare the present results with those obtained previously when Cs was used as a target [5].
dissociatron
247
References [l] SC. Goh and J.B. Swan, Phys. Rev. A24 (1981) 1624. [2] J. Loss and T.R. Govers, in: Collision Spectroscopy, ed. R.G. Cook (Plenum, New York, 1978) p. 289. [3] H. Martinez, I. Alvarez, J. de Urquijo, C. Cisneros and A. Amaya-Tapia, Phys. Rev. A36 (1987) 5425. [4] G. Comtet and P.G. Foumier, Chem. Phys. 81 (1983) 221. [5] C. Cisneros, I. Alvarez, C.F. Barnett, J.A. Ray and A. Russek, Phys. Rev. Al9 (1979) 631.
I. ATOMIC PHYSICS