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He2 molecule in a molecular beam
Volume 3. number 7
CHEMICAL PHYSICS LETTERS
He2
MOLECULE
EN
A
JuIy 1969
MOLECULAR
BEAM
J. GRAY * and R. H. TOMLINSON hfchfaster University, Hamilton. Ontario.
Canada
_
Received 4 June 1969
The
He2 molecule
is
produced by
neutralization
The He2 molecule has been identified spectroscopically and details of its various states summarized by Herzberg [l], and more recently by Ginter [2]. Calculations have shown [3] that some of the excited states may be expected to be bkding. The present letter describes the production of beams which are interpreted as showing the existence of He2 molecules with lifetimes in excess of 1O-8 seconds. The apparatus is shown schematically in fig. 1. It is essentially in a tandem mass spectrometer employing two 4” radius of curvature magnetic mass analysers and a 60 Mcps Radio Frequency source which is capable of producing intense beams in excess of 10-6 A. It has been employed to produce beams from hydrogen, helium and declterium and mixtures of these gases. The positive ions produced in this source are accelerated through 5000 volts and focussed using a gap lens followed by an electrostatic quadrupole lens, at the entrance to the first mass analyser. A movable viewer is located at this point to facilitate this operation. After mass selection the beam enters the intermediate region between the two mass analysers. Here the beam is aligned and further focussed at the entrance to the charge transfer cell. This consists of a narrow canal into which may be passed, lithium vapour, or a variety of charge transfer gases, hydrogen, helium etc. Immediately following this, are located sweep plates which may be charged to 5000 volts to remove all ions from the beam and a normal electron impact source of the Nier type, employing a tungsten filament, capable of producing up to one milliampere of electrons for ionizing neutral * Submitted in partial fulfillment of the requirements
for a Ph. D. degree at McMaster University, Hamilton, Ontario, Canada.
of
zx Het,
beam
in ZImolecular
beam
apparatus_
species in the beam. Since there is no accekrating potential associated with the second mass
analyser only those ions which have come from neutral atoms or molecures, originating from the primary ion beam, can be focussed by the second mass analyser. The energy of such ions wiU not necessarily be the same as weld be the case in
the primary beam. Thus a 5000 volts beam of HeH+ could give, on neutralization and dissociation a He atom of 4000 volts (ignoring contributions from the binding energies) and a H atom of 1000 volts energy. With this arrangement then, only those moLecules or atoms which are neutral may pass through the charged plates and become avaiLabLe for analysis by the second mass analyser. Thus, since the velocity of the beam is of the order 106 cm s-l, only those species with lifetimes in excess of 10-6 s may pass through the plates as neutrals and subsequently be analysed as ions. In order
to calibrate
the machine and estab-
ce
Fig.
1.
Moleculnr benm apparatus. nat to scale. Totat beam path is approximately 2.5 m.
523
Volume 3. number 7
CHEMICAL PHYSICS LETTERS
July 1969
a)
aeld
Strength
Fig. 3. The magnetic scan of (a) the primary He5 beam and (b) the reionized He2 beam, plotted on the .same axes. Fig. 2. The magnetic scan of (a) the primary HD+ beam and @) the reionized HD beam plotted on the sa2ne axes.
lish thd feasibility of the experiment a number of different atomic yd m+olecu+lar stpeciez we+re studied, including H , D , He , H HD , D2, 9 Typicai HII;, 4, D$, HeH+, HeDt and He2. results are shown in fig. 2 for the HD+ system. Fig. 2a shows the spectrum obtained after final mass analysis of the mass 3 primary beam having undergone no neutralization or ion removal in the intermediate region. With charges up to 5000
volts on the deflector plates and introduction of gas.into the neutralizer, the beam intensity is reduced by five orders of magnitude at the second mass analyser. With a gas pressure of the order
10-4 Torr in the neutralization canal either from heating the lithium oven, ot by introduction of H2 gas the spectrum in fig. 25 is obtained after removal of the primary beam. Peaks corresponding to &, D+ and I-& may be observed. Empioying the same technique for the mass 8 beam in a helium discharge figs. 3a and b are obtained for the He3 system. We interpret these observations as follows: HD system
Neutralization HD+ + e-(bound) - HD charge-transfer with high cross section and HD+ + e-(bound) - H + D dissociative recombination. 5i4’ -..
Afterremoval of the ion beam the neutrals undergo subsequent reionization *. HD+e-
HD++2e
HorD+e+HforD++2e with low cross section [4]. He2 system He;&=)
+ e(tw.md)
-
He2
(singlet
states) --t
-
He2 (triplet
2He
states).
The He; in the primary beam would be large1 in the 2CG ground state since approximately lo- 5 seconds elapsed from production of the ion in the
rf source to subsequent neutralization. The He2 singlet states are unstable with respect to the ground state 1X; of the molecule and would be expected to lead to production of two He atoms. Some of the He2 triplet states are, however, bonding end would rapidly decay to the 3C: state which is metastable with spin forbidden transition to the non bonded singlet ground state. There would be essentially no decay of the 3Z;h state of the He2 molecule in the time between formation of the molecule and subsequent reionization. The observations are therefore consistent * These reactions may dso take place with bound elktrons due to ionizing collisions with residual gas in the beam path.
Volume 3, number 7
CHEMICAL
with the assumed existence of a mktastable beam of neutral He2 molecules in the 3Zi state having a long lifetime in relation to the 10-6 seconds which would be the minimum for their detection in this apparatus. Work is being done on other systems of a similar nature including H3, D3, HeH and HeD. The latter two molecules in particular have bonding excited states [5,6] which, although not metast-
able to a non bonded ground state, may permit existence of the molecule for a time appropriate to detection in this apparatus. Data obtained on these systems has not yet been fully resolved but appear to indicate their existence.
JuIy 1969
PHYSICS LETTERS
REFERENCES 111G. Herzberg. Spectra of diatomfc molecules (Van Nostrand, Princeton, NJ.. 1957) p_ 535 I21 M. L. Ginter. J. Chem. Phys. 45 c1966) 248. [3] F. D. Peat, Can. J. Chem. 45 0967) 847. 141 - _ H. S. W. Maesev and E. H. S. Hurhoc. Electronic and ionic impact phenomena (Clarendou Press, Oxford, 1952). [5] H. H. Michels and F. E. Harris,
063) 1464. [S] H. H. Michels. 0.966) 3334.
J. Chem. Phys. 39
HeH+ calculatiou, J. Chem. Phys. 4C
525
CHEMICAL PHYSICS LETTERS
He2
MOLECULE
EN
A
JuIy 1969
MOLECULAR
BEAM
J. GRAY * and R. H. TOMLINSON hfchfaster University, Hamilton. Ontario.
Canada
_
Received 4 June 1969
The
He2 molecule
is
produced by
neutralization
The He2 molecule has been identified spectroscopically and details of its various states summarized by Herzberg [l], and more recently by Ginter [2]. Calculations have shown [3] that some of the excited states may be expected to be bkding. The present letter describes the production of beams which are interpreted as showing the existence of He2 molecules with lifetimes in excess of 1O-8 seconds. The apparatus is shown schematically in fig. 1. It is essentially in a tandem mass spectrometer employing two 4” radius of curvature magnetic mass analysers and a 60 Mcps Radio Frequency source which is capable of producing intense beams in excess of 10-6 A. It has been employed to produce beams from hydrogen, helium and declterium and mixtures of these gases. The positive ions produced in this source are accelerated through 5000 volts and focussed using a gap lens followed by an electrostatic quadrupole lens, at the entrance to the first mass analyser. A movable viewer is located at this point to facilitate this operation. After mass selection the beam enters the intermediate region between the two mass analysers. Here the beam is aligned and further focussed at the entrance to the charge transfer cell. This consists of a narrow canal into which may be passed, lithium vapour, or a variety of charge transfer gases, hydrogen, helium etc. Immediately following this, are located sweep plates which may be charged to 5000 volts to remove all ions from the beam and a normal electron impact source of the Nier type, employing a tungsten filament, capable of producing up to one milliampere of electrons for ionizing neutral * Submitted in partial fulfillment of the requirements
for a Ph. D. degree at McMaster University, Hamilton, Ontario, Canada.
of
zx Het,
beam
in ZImolecular
beam
apparatus_
species in the beam. Since there is no accekrating potential associated with the second mass
analyser only those ions which have come from neutral atoms or molecures, originating from the primary ion beam, can be focussed by the second mass analyser. The energy of such ions wiU not necessarily be the same as weld be the case in
the primary beam. Thus a 5000 volts beam of HeH+ could give, on neutralization and dissociation a He atom of 4000 volts (ignoring contributions from the binding energies) and a H atom of 1000 volts energy. With this arrangement then, only those moLecules or atoms which are neutral may pass through the charged plates and become avaiLabLe for analysis by the second mass analyser. Thus, since the velocity of the beam is of the order 106 cm s-l, only those species with lifetimes in excess of 10-6 s may pass through the plates as neutrals and subsequently be analysed as ions. In order
to calibrate
the machine and estab-
ce
Fig.
1.
Moleculnr benm apparatus. nat to scale. Totat beam path is approximately 2.5 m.
523
Volume 3. number 7
CHEMICAL PHYSICS LETTERS
July 1969
a)
aeld
Strength
Fig. 3. The magnetic scan of (a) the primary He5 beam and (b) the reionized He2 beam, plotted on the .same axes. Fig. 2. The magnetic scan of (a) the primary HD+ beam and @) the reionized HD beam plotted on the sa2ne axes.
lish thd feasibility of the experiment a number of different atomic yd m+olecu+lar stpeciez we+re studied, including H , D , He , H HD , D2, 9 Typicai HII;, 4, D$, HeH+, HeDt and He2. results are shown in fig. 2 for the HD+ system. Fig. 2a shows the spectrum obtained after final mass analysis of the mass 3 primary beam having undergone no neutralization or ion removal in the intermediate region. With charges up to 5000
volts on the deflector plates and introduction of gas.into the neutralizer, the beam intensity is reduced by five orders of magnitude at the second mass analyser. With a gas pressure of the order
10-4 Torr in the neutralization canal either from heating the lithium oven, ot by introduction of H2 gas the spectrum in fig. 25 is obtained after removal of the primary beam. Peaks corresponding to &, D+ and I-& may be observed. Empioying the same technique for the mass 8 beam in a helium discharge figs. 3a and b are obtained for the He3 system. We interpret these observations as follows: HD system
Neutralization HD+ + e-(bound) - HD charge-transfer with high cross section and HD+ + e-(bound) - H + D dissociative recombination. 5i4’ -..
Afterremoval of the ion beam the neutrals undergo subsequent reionization *. HD+e-
HD++2e
HorD+e+HforD++2e with low cross section [4]. He2 system He;&=)
+ e(tw.md)
-
He2
(singlet
states) --t
-
He2 (triplet
2He
states).
The He; in the primary beam would be large1 in the 2CG ground state since approximately lo- 5 seconds elapsed from production of the ion in the
rf source to subsequent neutralization. The He2 singlet states are unstable with respect to the ground state 1X; of the molecule and would be expected to lead to production of two He atoms. Some of the He2 triplet states are, however, bonding end would rapidly decay to the 3C: state which is metastable with spin forbidden transition to the non bonded singlet ground state. There would be essentially no decay of the 3Z;h state of the He2 molecule in the time between formation of the molecule and subsequent reionization. The observations are therefore consistent * These reactions may dso take place with bound elktrons due to ionizing collisions with residual gas in the beam path.
Volume 3, number 7
CHEMICAL
with the assumed existence of a mktastable beam of neutral He2 molecules in the 3Zi state having a long lifetime in relation to the 10-6 seconds which would be the minimum for their detection in this apparatus. Work is being done on other systems of a similar nature including H3, D3, HeH and HeD. The latter two molecules in particular have bonding excited states [5,6] which, although not metast-
able to a non bonded ground state, may permit existence of the molecule for a time appropriate to detection in this apparatus. Data obtained on these systems has not yet been fully resolved but appear to indicate their existence.
JuIy 1969
PHYSICS LETTERS
REFERENCES 111G. Herzberg. Spectra of diatomfc molecules (Van Nostrand, Princeton, NJ.. 1957) p_ 535 I21 M. L. Ginter. J. Chem. Phys. 45 c1966) 248. [3] F. D. Peat, Can. J. Chem. 45 0967) 847. 141 - _ H. S. W. Maesev and E. H. S. Hurhoc. Electronic and ionic impact phenomena (Clarendou Press, Oxford, 1952). [5] H. H. Michels and F. E. Harris,
063) 1464. [S] H. H. Michels. 0.966) 3334.
J. Chem. Phys. 39
HeH+ calculatiou, J. Chem. Phys. 4C
525