A drift tube mass spectrometer apparatus has been used to determine the rate coefficient, energy dependence and product ions of the reaction He(+) +H2. The total rate coefficient at 300 K is 1.1 plus or minus 0.1) 10 to minus 13th power cu cm/sec. The reaction proceeds principally by dissociative charge transfer to produce H(+), with the small remainder going by charge transfer to produce H2(+) and by atom rearrangement to produce HeH(+). The rate coefficient increases slowly with increasing ion mean energy, reaching a value of 2.8 x ten to the minus 13th power cu cm sec at 0.18 eV. The corresponding reaction with deuterium, He(+) + D2, exhibits a value (5 plus or minus 1) x 10 to the minus 14th cu cm/sec at 300K. The reaction rates for conversion of H(+) and HeH(+) to H3(+) on collisions with H2 molecules are found to agree well with results of previous investigations

Biondi, M. A.

Johnsen, R.

English

NASA Technical Reports Server

SPACE RESEARCH COORDINATION CENTER
UNIVRSITY
PITTSBURGH
MEASUREMENTS OF ION-MOLECULE
REACTIONS OF HE , H', AND HEH
WITH H 2 AND D 2
RAINER JOHNSEN AND MANFRED A. BIONDI
DEPARTMENT OF PHYSICS
SRCC REPORT NO. 201
(NASA-CR-138190) HEASUDEMENTS OF N74-22340
ION-MOLECULE REACTIONS OF He PLUS, H
PLUS, HeH PLUS WITh H sub 2 AND D sub
2 (Pittsburgh Univ.) 15 p HC $4.00 Unclas
-.--- CSCL 20 H G3/24 
-37858
UNIVERSITY OF PITTSBURGH
PITTSBURGH, PENNSYLVANIA
APRIL 1974
https://ntrs.nasa.gov/search.jsp?R=19740014227 2020-03-23T09:15:13+00:00Z
APP-45
Measurements of Ion-Molecule Reactions of He+, H+ , and HeH+
with H2 and D2
(Journal of Chemical Physics)
Rainer Johnsen and Manfred A. Biondi
Department of Physics, University of Pittsburgh, Pittsburgh, Pa, 15260
April 1974
Measurements of Ion-Molecule Reactions of He+, H+ , and HeH+
with H2 and D2
Rainer Johnsen and Manfred A. Biondi
Department of Physics, University of Pittsburgh, Pittsburgh, Pa. 15260
Abstract
A drift tube-mass spectrometer apparatus has been used to
determine the rate coefficient, energy dependence and product ions
of the reaction He+ + H2. The total rate coefficient at 300 K is
(1.1 t 0.1) x 10-1 3 cm3 /sec. The reaction proceeds principally
(Z 80%) by dissociative charge transfer to produce H+, with the
small r~mainder going by charge transfer to produce H2 + and by atom
rearrangement to produce HeH+. The rate coefficient increases slowly
with increasing ion mean energy, reaching a value of 2.8 x lO-1 3
cm3 /sec at 0.18 eV. The corresponding reaction with deuterium,
He+ + D2, exhibits a value (5 ± 1) x 10-14 cm3/sec at 300 K. The
reaction rates for conversion of H+ and HeH+ to H3 + on collisions
with H2 molecules are found to agree well with results of previous
investigations.
2I. Introduction
The recent microwave occultation experiment carried out during
the flyby of Jupiter by the Pioneer 10 spacecraft (1) has provided
extensive observational data concerning the Jovian ionosphere which
may be compared with theoretical models proposed by several authors(2-7)o
Models of this kind take into account the physical and chemical
properties of the neutral atmospheric constituents, the fluxes of
ionizing radiation from the sun and the relevant atomic collision
processes between the ions, electrons, and neutral constituents.
The present paper describes drift tube-mass spectrometer
measurements of several ion-molecule reactions believed to be
important in the hydrogen-helium upper atmosphere of Jupiter, in
particular the reaction of He + ions with H2 (and D2 ) molecules to
form H+, HeH+, and H2  ions. Previously, only an upper limit for
the rate of this reaction had been obtained from flowing afterglow
studies(8 ). Measurements of the three-body reaction of H+ and the
two-body reaction of HeH+ with H2 to form H3+ ions are also described.
II. Ecperimental Apparatus and Principle of Measurement
The measurements were carried out in the drift tube-mass
spectrometer apparatus described in a recent publication(9). The
demands on the purity of the reactant gas (hydrogen) were more stringent
here than in studies of faster ion-molecule reactions, since even
very snall amount of the common impurities found in commercially
available hydrogen(10) cause rapid conversion of the highly reactive
helium ion. For this reason some of the measurements were repeated
in hydrogen that had been purified further by diffusion through a
3heated palladium-silver alloy tube. The absence of significant
amounts of background impurities in the apparatus was checked
regularly by measuring the decay rate of He+ ions in pure helium
gas.
As in previous studies, the reaction rate coefficient is deter-
mined from the loss of the parent ion (He+ ) as function of the parent
ions' additional residence time (1) in the drift tube. In order to
obtain the rate coefficient at above-thermal energies, the decay rate
is measured as a function of E/p, the ratio of the drift electric field
to the gas pressure. The ions' mean energy is derived from the
measured drift velocity and Wannier's theoretical relationship(12)
III. Measurements
He+ + H2
The measurements were carried out in two parts; a determination
of the He+ loss rate in H2, yielding the total rate coefficient kl,
and an analysis of the product ions formed by the reaction to establish
the relative contributions of the three exoergic branches,
He+ + H2 - He + H2 (+b)
HeH+ + H. (ic)
The loss rate, v = d(An[He+])/dt, was found to depend linearly
on the H2 pressure over the measured range (0.2 to 1.1 TorrP see
Fig. 1). The two body rate coefficient at 300 K is found to be
k, = (1.1 t 0.1) x 10-13 cm3 /sec in purified H2 (using the palladium-
silver diffusion cell). The slightly higher rate constant observed
in unpurified hydrogen was attributed to a small nitrogen impurity in
the commercial H2 . As a result of its greater molecular weight this
impurity tends to build up in the differentially pumped drift cell by
about a factor [mN2 /H 2] 3.7 over the - 5 ppm contained in the
supply tank.
The variation of k, with the He+ mean energy is shown in Fig. 2.
A steady increase of k1 with increasing ion mean energy is observed up
to the maximum energy attained, - 0.2 eV.
The analysis of the product channels of reaction (1) is
complicated by the fact that all three of the expected product ions
undergo secondary reaction with hydrogen. The reactions are:
H2 + H2 -3 + + H (2)
HeH+ + H2 -He + H3  (3)
H+ + 22 2 -H 3  + H2  (4)
Reaction (2) has been shown to be fast(13 , 14) k2 = 1.4 x 10- 9 cm3/sec
at 300 K. The measured rates of reactions (3) and (4) are discussed
later in this section.
At the relatively high H2 pressures used in the measurements
(0.05 Torr) one would therefore not expect to observe the primary
products of (lb) and (lc) but only their secondary product, H3+, since
the lifetimes of HeH+ and H2 + (< 1 psec at 0.05 Torr) are considerably
shorter than the time of flight of ions through the drift tube (~ 1 msec),
A drastic reduction of the H2 pressure would have been required to make
these lifetimes comparable to the ions' times of flight, which
5unfortunately also would have reduced the number of ions produced in
reaction (1) to below the detection limit.
The measurements were therefore restricted to observations of
the primary product of branch (la), H+ , (which has a lifetime of about
10 msec against three body conversion at 0.05 Torr) and the secondary
product of branches (ib) and (lc), H3+. Both of these ions were observed
in the experiment. Some of the observed "product" ions clearly were
not produced by reaction (1) but arose from ion production in H2 gas
which reached the differentially pumped ion source. This contribution
to the observed signal was separated from the signal of interest by
carrying out the measurements in a mixture of helium and hydrogen. In
this gas mixture the drift velocity of He+ ions is considerably smaller
than that of H3+ and H+ ions, with the result that ions produced by
reaction (1) have a longer transit time than those produced in the ion
source.
It was concluded from these measurements that reaction (1)
predominantly produces H+ ions and that the sum of the reaction channels
(lb) and (ic) accounts for less than 20% of the total rate coefficient.
To arrive at this result, the contribution of reaction (4) to the
observed H3+ signal was calculated from the measured H+ signal, the
rate constant k4 and the H2 density. The ratio of the remaining H3+
signal to the H+ signal is then taken as the ratio (klb + klc)/klao
Since there are experimental indications that our detection sensitivity
for H+ ions may be lower than that for H3+ ions, the actual contribution
of reaction channels (lb) and (lc) to the total rate coefficient may
be considerably lower than 20%o.
6He+ + D2
The total rate coefficient for the He+ + D2 reaction at 300 K
was determined to be t 1) x 10- 1 4 cm3/sec, roughly one half the value
measured for He + + H2 . The reaction exhibited the pressure dependence
expected for a two body process over the pressure range ~ 0.45 to 0.95
Torr (see Fig. 1). Since the deuterium was used without further
purification,it is possible that this rate coefficient includes a N 20%
contribution due to reactions with impurities, so that the actual rate
coefficient may only be - 4 x 10- 1 4 cm3/see.
As in the case of the light hydrogen isotope studies, the
dominant product ion of this reaction appears to be the atomic ion D+.
The dependence of the rate coefficient on the ions' mean energy has
not been studied.
HeH+ and H+ + H2
In the course of these measurements the two-body conversion of
HeH+ to H3+ on collisions with H2 , reaction (3), was observed to occur
with a rate coefficient k3 Z 10-9 cm3 /sec at 300 K, in agreement with
the results of recent measurements (14 15). In the present experiment
the HeHl+ ions were apparently produced from metastable helium atoms
through the reaction HeM + H2 - HeH+ + H.+ e. Our results are given as a
lower bound because the observed loss of HeHll+ was slightly compensated
by a resupply reaction involving the small fraction of He2+ ions
produced in the ion source; viz. He 2 + + H2 - HeH+ + He + H.
The three-body conversion of H+ to H3 +, reaction (4), was
observed to occur with a rate coefficient (3.1 + 0.15)x 10a 29 cm6 /sec
at 300 K, in good agreement with previous determinations(16 ).
7Discussion
The reaction of He+ with H2 has not been previously observed
in laboratory measurements. An upper limit for its rate coefficient,
however, was given by Fehsenfeld et al(8 ), who concluded from measure-
ments in a flowing afterglow system that the rate coefficient at 300 K
was less than or the order of 10-13 cm3 /sec, very close to the value
k = (1.1 t 0.1) x 10- 1 3 cm3 /sec measured in the present experiment.
The present studies also indicate that a majority or all of the reaction
at low energies goes to formation of H+ by dissociative charge transfer,
reaction (la).
Dissociative charge transfer may occur if one of the H2 (i)
electrons is captured into the la state of He and the second H2 (lc)
electron is simultaneously excited into the repulsive lau state of H2 +
or into the attractive lag state above the dissociation limit (see
Fig. 3, wnich shows the relevant potential energy curves for H2 and H2 +)o
Charge transferp in general, is a fast process only if the initial and
final states are nearly resonant in energy; in the He + H2 system this
condition occurs only for two H-H internuclear separations Ri and Ro,
both of which are uitequite different from the euilibri-u separation in
the H2 molecule. Thus, if distortion of the H2 molecule during the
collision with He+ is neglected, one would expect the overlap between
the ground state H2 wave function and that of the H2+ (au) or (ag)
state to be small at energy resonance, leading to a small rate coeffi-
cient for dissociative charge transfer.
However, estimates of the overlap integral based on the usual
approximations to the wave functions (delta functions at Ri and Ro
for the H2 + states and a gaussian function for the H2 ground vibrational
state) lead to prediction of a rate constant many orders of magnitude
8smaller than that observed. In this case one must also consider
possible contributions to the overlap integral from the Franck-Condon
region (cross hatched area in Fig. 3) where the e H2 + (ag) wave function
oscillates rapidly. If the cancellation between the various parts of
this oscillatory wave function as it overlaps the gaussian ground
state wave function is incomplete, the finite contribution to the overlap
integral may be sufficient to account for the observed, slow charge
transfer rate.
Finally, the fact that the He+ + D2 rate coefficient is approxi-
mately one half that of He+ + H2 is in the proper direction for an
isotope effect in which the more compact f%2 ground state wave function
has a smaller overlap with the "delta function" D2+ wave functions at
the energy resonant points and R*. However, the observed difference
between the D2 and H2 rates is far smaller thah would be expected if
the delta function representation of the H2 + and D2+ states were
adequate, again suggesting that contributions to the overlap integral
from the D+ (a ) state in the Franek-Condon region must be considered.
The authors are indebted to J. N. Bardsley for helpful
discussions of the dissociative charge transfer process. This research
has been supported, in part, by the National Aeronautics and Space
Administration under Grant No. NGR 39-011-137.
9References
1. A. Kliori- D. L. Caina G. Fjeldbo B. L. Seidel, and So. I. Rasool
Science 183. 323 (1974).
2. D. M. Hunten., J. Atm os. Sci. 26, 826 (1969).
3. M. shimi~zu, Icarus 149 273 (1971).
4. S. S. Prasad and L. A. Capone, Icarus 15., 45 (1971).
5. T. Tanaka and K. Hirao, Planet. Space Sci. 21, 751 (1973).
6. M. B. McElroy, Space Sci. Rev. 14, 460 (1973).
7. S. K. Atreya, T. M. Donahue and M. B. McElroy, Science 184, 154
(1974).
8. F. C. Fehsenfeld, A. L. Schraeltekopf, P. D. Goldan; H. I. Schiff,
and E. E. Ferguson, J. Chemo Phy. s. 4087 (i966).
9. R. Johnsen and M. A. Biondi, J. Cha.m Phys. %2, 3504 (1973).
10. "Researeh Grade Hydrogen", Air Products and Chemicals., ERMaus
Pennsylvania U.S.A.
11. J. Heimerl, R. Jchnsen, and M. A. Biondi , J. Chem. Phys. 5 1
5041 (1969).
12. G. H. Wannier, Bell Syst. Teeh. J. 2, -170 (1953).
13. M. T. Bowers, D. D. BFl1ran, and J. King Jr., J. Chem.r Phys. 5L,
4787 (1969).
14. L. P. Theard and W. 'T. Huntress, Jr.., J. Cbemo Phys., in press.
15. K. R. -yan and I. G. Graham, J. Chem. Phys. ~g 4260 (1973).
16. E. Graham IV, D. R. James, W. C. Keever, Io R. Gatland, D. L.
Albritton, and E. W MoDaniel., J. Chem. Phys. 5 , 4648 (1973).
10
Figure Captions
Fig. 1 Observew He+ loss rate v as a function of the H2 and D2
pressures. The open circles and squares refer to measure-
ments in "research grade" H2 and D2, the crosses to measure-
ments in H2 which had been further purified by diffusion
through palladiut silver alloy.
Fig. 2 Total rate coefficient k1 for the reaction He+ + H2 as a
function of E/p and of the He + mean energy (laboratory
frame). The solid data point at E/p = 0 represents an
average of 20 points with an rms deviation of 10%.
Fig, 3 Potential energy as a function of internuclear separation R
for the relevant states of H2 and H2 + involved in dissociative
charge transfer. The cross-hatched area indicates the
Franck-Condon region of the H2 molecule in the ground
vibrational state. The horizontal dashed line is at the
ionization energy of He+, indicating the energy resonant
points for the dlssociative charge transfer process.
T 9os 300 K
4
He + H22
x
x He+ + D
I x
0 0.5 1.0
p [H 2 ] (Torr)
Figure 1
-13
3 10 0
-- He + H H/
He + H He + H + + + 2 He+H0
HeH+ H /
o
2-
2 0 /
O~ 9 c '0 TOas 8 300 K
.a- ,-
.I --A
0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18
II I I I I I Oi n (eV )
o 4 6 8 10 12 14 16
E/p (V/cm -Torr )
Figure 2
•H H
4I,
H210
0
SR I R* 2 3 4
internuclear sepration, R ()
Figure 3


Measurements of ion-molecule reactions of He plus, H plus, HeH plus with H sub 2 and D sub 2

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