A crossed-beam scattering study of CH4+ and CH3+ formation in charge transfer collisions of Kr+ with CH4 at about 1 eV

Article abstract of DOI:10.1063/1.469095

The dynamics of CH4⁺ and CH3⁺ ion formation in collisions of Kr⁺(²P_3/2'1/2) with thermal CH4 has been investigated in a crossed beam experiment at a hyperthermal collision energy of 1.18 eV.The scattering data show that the CH4⁺ product is formed in a near-resonant exoergic process in which the most probable energy transferred to the target is practically equal to the recombination energy of the Kr⁺ projectile (resonant energy transfer); in addition a wide band of internal states of CH4⁺ up to +/-0.6 eV is populated in inelastic and superelastic collisions.In contrast, the CH3⁺ product is formed in dissociative charge transfer, with about one-half of the yield due to nonresonant, endoergic collisions of Kr⁺ (²P_3/2).The other half of the CH3⁺ product is found to originate in near-resonant exoergic collisions of Kr⁺ (²P_1/2).An estimate is given of the distribution of the total energy deposited in methane by the above processes.

Full text of DOI:10.1063/1.469095

A crossedbeam scattering study of CH+ 4 and CH+ 3 formation in charge transfer
collisions of Kr+ with CH4 at about 1 eV
Zdenek Herman and Břetislav Friedrich
Citation: The Journal of Chemical Physics 102, 7017 (1995); doi: 10.1063/1.469095
View online: http://dx.doi.org/10.1063/1.469095
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/102/18?ver=pdfcov
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A crossed-beam scattering study of CH؉4 and CH3؉ formation in charge
transfer collisions of Kr؉ with CH4 at about 1 eV
a)
ˇ
Zdenek Herman and Bretislav Friedrich
´
ˇ
J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3,
CZ-182 23 Prague 8, Czech Republic
͑Received 18 November 1994; accepted 30 January 1995͒
The dynamics of CHϩ4 and CH3ϩ ion formation in collisions of Krϩ (2P3/2 1/2) with thermal CH4 has
Ј
been investigated in a crossed beam experiment at a hyperthermal collision energy of 1.18 eV. The
scattering data show that the CHϩ4 product is formed in a near-resonant exoergic process in which
the most probable energy transferred to the target is practically equal to the recombination energy
of the Krϩ projectile ͑resonant energy transfer͒; in addition a wide band of internal states of CH4ϩ
up to Ϯ0.6 eV is populated in inelastic and superelastic collisions. In contrast, the CHϩ3 product is
formed in dissociative charge transfer, with about one-half of the yield due to nonresonant,
endoergic collisions of Krϩ ͑2P3/2͒. The other half of the CHϩ3 product is found to originate in
near-resonant exoergic collisions of Krϩ ͑2P1/2͒. An estimate is given of the distribution of the total
energy deposited in methane by the above processes. © 1995 American Institute of Physics.
I. INTRODUCTION
ciative and dissociative charge transfer processes between
Krϩ and CH4,
Crossed beam scattering studies of charge transfer be-
tween atoms and molecules have provided a more detailed
insight into the mechanism and energy exchange in these
important elementary collision processes. Both diatomic and
simple polyatomic targets have been studied.1–5 Mostly non-
dissociative processes have been investigated; studies of dis-
sociative charge transfer have been rather rare and limited to
diatomic molecules.6 In an important series of high-
resolution studies of inelastic and charge transfer collisions
of protons with molecules the energy loss of the projectile
was measured;7,8 these experiments, however, could not dis-
tinguish between nondissociative and dissociative encoun-
ters. The studies showed that in most cases exoergic charge
transfer processes between ions and molecules populate pre-
dominantly those internal states of the molecular product ion
that lie close to the recombination energy of the projectile.
However, the range of states which were populated with de-
creasing probability on both sides of this peak value could be
quite considerable and depended on the scattering angle and
on the collision energy.
In our earlier paper we dealt with nondissociative charge
transfer between Krϩ and CH4 at collision energies below 1
eV.5 The results showed that the charge transfer process oc-
curred with highest probability in encounters which did not
require energy exchange between translational and internal
degrees of freedom of the system, i.e., that the most probable
energy transferred was equal to the recombination energy of
the Krϩ projectile ͑resonant charge transfer͒. In addition,
however, internal states of CHϩ4 were populated in inelastic
and superelastic collisions which extended to about Ϯ0.5 eV
on both sides of the peak value.
KrϩϩCH4→KrϩCHϩ4 ,
͑1͒
͑2͒
KrϩϩCH4→KrϩCHϩ3 ϩH.
Processes ͑1͒ and ͑2͒ were investigated in crossed beam scat-
tering experiments at the collision energy of 1.18 eV ͑c.m.͒.
II. EXPERIMENT
The experiments were carried out on the crossed beam
apparatus EVA II using the same arrangement as described in
our previous study of the charge transfer process ͑1͒.5 The
reactant ions Krϩ were produced in a low-pressure ͑about
10Ϫ2 Pa of krypton gas͒ ion source by impact of 120 eV
electrons and the two spin–orbit states of Krϩ, P3/2, and
2
2P1/2, were presumably formed in the statistical ratio, 2:1.
The ions were extracted from the ion source, mass analyzed,
and decelerated to the energy of the experiment. The ion
reactant beam was crossed at right angles by a modulated,
thermal beam of methane molecules effusing from a colli-
mated multichannel jet. The laboratory energy and angular
spread of the reactant ion beam at the collision center was
0.25 eV and 1° ͓full-width at half-maximum ͑FWHM͔͒, re-
spectively; the neutral beam angular width was 6° ͑FWHM͒.
Angular distributions were measured by rotating the two
beams about the scattering center. The energy distributions of
the reactant and product ions were analyzed by a stopping-
energy analyzer placed behind the detection slit. The ions
were then accelerated to 1 keV into the detection mass spec-
trometer, and after the mass analysis accelerated by a poten-
tial of 2.8 kV onto the first dynode of an open electron mul-
tiplier. Modulation of the neutral beam and phase-sensitive
detection of the ion product was employed to deal with the
effects of scattering in the background gas. Accumulation of
the translational energy spectra and signal averaging was
used to improve the signal to noise ratio.
In this paper we extend our study to both the nondisso-
a͒
Present address: Department of Chemistry, Harvard University, 12 Oxford
St., Cambridge, Massachusetts 02138.
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Z. Herman and B. Friedrich: Collisions of Krϩ with CH4
FIG. 1. Contour scattering diagram of CHϩ4 from reaction ͑1͒. The cross marks the position of the tip of the c.m. velocity vector, the RCT circle shows the
velocity of the product ion, corresponding to zero translational exoergicity, ⌬Tϭ0 ͑its radius is equal to the initial c.m. velocity of the reactant CH4 which
moves from right to left along the 180°–0° relative velocity axis͒. An inset at the top shows the respective Newton diagram ͑note scale 0.5͒.
Laboratory angular distributions of CHϩ4 and CH3ϩ prod-
uct ions were recorded over the laboratory angular range of
Ϫ6° to 95°, and energy profiles of the ions were measured
several times at discrete scattering angles over this angular
range in intervals of 5°–10°. The construction of scattering
diagrams from these data and obtaining the c.m. angular dis-
tributions, P͑␽͒ vs ␽, and the relative product translational
energy distributions, P(TЈ) vs TЈ, from the scattering dia-
grams followed the usual procedure.5,9 It should be noted,
respond to the product formed in superelastic collisions, with
a net conversion of internal energy into translational energy
of the products.
The product ion CHϩ4 from reaction ͑1͒ peaks at the c.m.
angle 0° ͑corresponding to a straight trajectory of the mo-
lecular reactant without angular deflection͒ and practically
on the RCT circle. The distribution decreases sharply, but its
ridge follows closely the RCT circle, even for large-angle
scattering, where it assumes a small value of about 2.3% of
the peak intensity, constant for scattering angles larger than
90°. The respective c.m. angular distribution ͑Fig. 3͒ reflects
this behavior. Figure 4͑a͒ summarizes data on the relative
translational energy distribution of products of reaction ͑1͒.
The lower three curves show P(TЈ) along a certain ͑small͒
scattering angle, the two upper curves sum up the transla-
tional energy distributions for small-angle ͑0°–80°͒ and
large-angle ͑80°–180°͒ scattering. The uppermost curve is
the relative translational energy distribution resulting from
integration over the entire scattering diagram. The relative
translational energy is presented as ⌬TϭTЈϪT, the net con-
version of translational energy into internal energy of the
system ͑⌬TϾ0 indicates superelastic collisions͒. The
hatched curve in the lowest section of the figure shows, for
orientation, the initial relative translational energy distribu-
tion of the reactants. All P(TЈ͒ curves peak practically at
⌬Tϭ0; the deviations are smaller than 0.05 eV, which is
within the limits of accuracy of the experiment. All in all, the
results for the nondissociative reaction ͑1͒ at Tϭ1.18 eV
however, that for reaction ͑2͒ the product of the charge trans-
ϩ
*
fer process was assumed to be first an excited ͑CH4 ͒ which
would only subsequently dissociate in an unimolecular pro-
cess to CHϩ3 ϩH. The relative translational energy TЈ was
ϩ
*
therefore calculated for the product pair Krϩ͑CH4 ͒ ͑see
also Results and Discussion͒.
III. RESULTS
The contour scattering diagrams of CHϩ4 and CH3ϩ from
reaction ͑1͒ and ͑2͒ at the collision energy Tϭ1.18 eV are
shown in Figs. 1 and 2, respectively. The cross marks the
position of the tip of the c.m. velocity vector. The resonant
charge transfer ͑RCT͒ circle indicates the loci of the c.m.
velocities of the molecular ion product formed by charge
transfer in which there is no exchange of energy between
internal and translational degrees of freedom of the system.
The product ion velocities inside this circle correspond to the
product formed in inelastic collisions, with a net conversion
of translational energy into internal energy, those outside cor-
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Z. Herman and B. Friedrich: Collisions of Krϩ with CH4
7019
FIG. 2. Contour scattering diagram of CHϩ3 from reaction ͑2͒. The RCT circle refers to the initial velocity of CH4 .
confirm the findings reported earlier for this reaction at col-
lision energies below 1 eV.5
mum peak, corresponding to energy losses of about 0.5 and
1.1 eV. The considerable portion of the inelastic large angle
scattering can be well seen from the c.m. angular distribution
of CHϩ3 in Fig. 3, where the intensities are normalized to the
same area under the curve.
Figure 4͑b͒ shows the product relative translational dis-
tributions derived from the scattering diagram of CHϩ3 ,
P(TЈ͒. For small scattering angles the P(TЈ͒ curves exhibit
two maxima; one of them lies very close ͑within the experi-
mental error on it͒ to the RCT circle indicating CHϩ3 formed
in a near resonant charge exchange process. Its intensity de-
creases quickly with the scattering angle in a way which
resembles the fall-off of the angular distribution for the prod-
uct CHϩ4 .
The scattering diagram of the CHϩ3 product differs mark-
edly from that of CHϩ4 . It shows a peak at the c.m. scattering
angle 0° which lies close to the RCT circle, but in addition
there is a strong inelastic portion of the scattering at all scat-
tering angles, with ridges of about 20%–30% of the maxi-
The second maximum in the P(TЈ͒ curves suggests CH3ϩ
originating from CHϩ4 formed in a charge transfer process
with Krϩ which is inelastic by about 0.4 eV; the relative
weight of this process decreases more slowly with the scat-
tering angle than that of the quasiresonant process mentioned
above. The P(TЈ͒ distributions integrated over the small
͑0°–80°͒ and large ͑80°–180°͒ c.m. scattering angles, as
well as over the entire scattering diagram ͑uppermost curve͒
peak at TЈ which correspond to a charge transfer process
inelastic by about 0.45 eV and extend from ⌬TϭϪ1.18 eV
͑inelastic͒ to about ⌬Tϭϩ0.6 eV ͑superelastic͒.
IV. DISCUSSION
In order to interpret the scattering results, we assumed
that both the nondissociative and dissociative processes ͑1͒
and ͑2͒ involve an impulsive, single-charge transfer between
Krϩ and CH4 that populates vibronic states of the methane
cation predominantly below and above the dissociation limit,
respectively. The excited methane cation is assumed to un-
dergo a subsequent unimolecular decay to yield CHϩ3 :
FIG. 3. C.m. angular distributions, P͑␽͒ vs ␽, of CHϩ4 and CH3ϩ from
reactions ͑1͒ and ͑2͒, respectively.
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7020
Z. Herman and B. Friedrich: Collisions of Krϩ with CH4
FIG. 4. Relative translational energy distributions, P(T ͒, of CHϩ ͑a͒ and CHϩ3 ͑b͒; the curves are plotted against the translational exoergicity ⌬TϭT ϪT,
Ј
Ј
4
for specific angles of small-angle scattering, integrated over small-angle ͑0°–80°͒ and larger-angle ͑80°–180°͒ scattering region, and finally over the entire
ϩ
scattering region 0°–180° ͑uppermost curve͒. The dashed curve at 0° is P(T ͒ calculated by convoluting the recoil velocity of CHϩ from CH4ϩ →CH4 ϩH
*
Ј
3
into the velocity profile of CHϩ4 at 0° ͑see text͒.
ϩ
ϩ
*
͑CH4 ͒ →CH3 ϩH. The unimolecular decomposition theory
was shown to be applicable to the methane cation as exem-
plified by the unimolecular breakdown pattern obtained ex-
perimentally by charge transfer,10,11 and threshold electron–
photoion coincidence,12 and by the agreement of these data
with the quasiequilibrium theory calculations.13
the light H leads to a non-negligible, but acceptable blurring
effect in the scattering diagram of CHϩ3 . In the following we
will discuss the CHϩ3 scattering as reflecting in angle and
ϩ
*
velocity the scattering of ͑CH4 ͒ before dissociation having
in mind that the data include this blurring effect ͑e.g., in
comparison with the scattering data of CHϩ4 in Fig. 1͒. The
effect of the recoil velocity on the broadening of the CH3ϩ
distribution was quantitatively estimated for the translational
energy distributions at ␽ϭ0°, where CHϩ3 is formed mostly
by near-resonance charge transfer and the inelastic scattering
contributions perturb least the CHϩ3 distribution ͑see further
on͒. This was done by convoluting the recoil velocity of
CHϩ3 , 3ϫ104 cm/s, into the velocity distribution of CHϩ4 at
␽ϭ0° ͓Fig. 4͑a͔͒. The resulting calculated P(TЈ͒ of CHϩ3 is
shown as a dashed curve in Fig. 4͑b͒, P(TЈ͒ at ␽ϭ0°, bot-
tom. It can be seen that the broadening corresponds well to
that experimentally observed P(TЈ͒ in the region over which
the two curves can be compared.
The observed distribution of the scattered CHϩ3 traces
ϩ
*
the ͑CH4 ͒ distribution provided the lifetime of the excited
ϩ
*
͑CH4 ͒ is much longer than the collision time. This condi-
tion is fulfilled at the collision energy of 1.18 eV; for an
interaction radius of 6 Å the collision time is 1.5ϫ10Ϫ13 s;
on the other hand, the mean unimolecular dissociation time
ϩ
*
of ͑CH4 ͒ after charge transfer excitation by 14.7 eV ͓re-
combination energy of Krϩ ͑2P1/2͔͒ as calculated by the RRK
model is longer than about 10Ϫ9 s, orders of magnitude
longer than the collision time.
ϩ
*
The tracing of the original ͑CH4 ͒ distribution is some-
what blurred by the translational energy released in the dis-
sociation process. The average energy released in the disso-
ciation ͑CH4 ͒ →CH3 ϩH has been determined in
photoionization experiments to be 140 meV ͑Ref. 12͒ ͓which
agrees well with earlier value of 112 meV ͑Ref. 14͔͒. This
value leads to an average recoil velocity of CHϩ3 of 3ϫ104
cm/s. Thus the average recoil velocity of the heavy CHϩ3 on
ϩ
ϩ
*
The results on the nondissociative charge transfer pro-
cess ͑1͒ ͑formation of CH4ϩ͒ confirm our earlier results5 ob-
tained at energies below 1 eV.
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Z. Herman and B. Friedrich: Collisions of Krϩ with CH4
7021
͑a͒ The c.m. angular distribution peaks at 0° and exhibits a
small ridge of almost isotropic large-angle scattering
͑Fig. 3͒;
͑b͒ The relative translational energy distribution P(TЈ͒
peaks within about 50 meV at the ‘‘resonant charge
exchange’’ value ͑i.e., no exchange between transla-
tional and internal energy͒ and stretches to about Ϯ0.5
eV on both sides of the peak value thus indicating the
occurrence of both inelastic and superelastic charge
transfer events ͓Fig. 4͑a͔͒;
͑c͒ These findings are consistent with the conclusion that
the reactant state is the ground state Krϩ ͑2P3/2͒, re-
combination energy 14.00 eV; the excess energy of the
exoergic charge transfer is deposited with highest prob-
ability as the internal energy of the ion product CHϩ4 .
The results on the formation of CHϩ3 come somewhat as
a surprise in this connection. They show that a sizable frac-
tion of CHϩ3 is formed in inelastic processes of a most prob-
able endothermicity of about 0.4–0.5 eV, though the scatter-
ing diagram exhibits at small scattering angles a region
which indicates that CHϩ3 is also formed by a near-resonance
charge transfer ͓see Figs. 2 and 4͑b͔͒, similarly as in the case
of CHϩ4 .
These findings on the CH3ϩ formation can be explained
by two different processes connected with the two spin–orbit
states of Krϩ. The near-resonant charge transfer is due to the
reaction of the upper state, Krϩ ͑2P1/2͒ ͓IPϭ14.66 eV ͑Ref.
ϩ
*
15͔͒. The energy is deposited into ͑CH4 ͒ in a quasiresonant
FIG. 5. Energy deposited in the product molecular ion in reactions ͑1͒ and
͑2͒. ͑a͒ Energy deposited in reactions of Krϩ ͑2P3/2͒; curve CH4ϩ , in reaction
͑1͒; CHϩ3 , portion from reaction ͑2͒; ͚CH4ϩϩCHϩ3 , sum of both contribu-
tions. ͑b͒ Energy deposited in reactions of Krϩ ͑2P1/2͒. ͑c͒ Summary of data:
estimated distribution of energy deposited by Krϩ ͑P3/2͒ and Krϩ ͑P1/2͒;
HϩϩCH4 , energy deposited by 30 eV protons ͑Ref. 8͒; PES, photoelectron
spectrum of CH4 ͑Ref. 17͒; solid lines CHϩ4 , CH3ϩ , CH2ϩ , breakdown pattern
of the methane cation ͑Ref. 13͒.
way and the excess energy ͓about 0.4 eV above the
AP͑CHϩ3 ͒ϭ14.25 eV ͑Ref. 15͔͒ leads to its dissociation to
CHϩ3 ϩH. In the P(TЈ͒ curves in Fig. 4͑b͒ this shows as the
peaks which lie close to ⌬Tϭ0 and which decrease quickly
with the increasing scattering angle. In the c.m. angular dis-
tribution ͑Fig. 3͒ most of the sharp peak at 0° presumably
originates from this process. The angular and energy distri-
bution connected with this reaction channel thus resembles
that of the slightly exoergic nondissociative process ͑1͒
where, however, Krϩ ͑2P3/2͒ is the reactive state.
tion of all products, i.e., the formation of both CHϩ4 and CH3ϩ
in reactions ͑1͒ and ͑2͒, and their respective energy disposal.
To do so, one needs information on the relative integral cross
sections of both processes at Tϭ1.2 eV which, unfortunately,
cannot be derived with sufficient accuracy from the present
differential data. Tosi16 found in his beam experiments that
the ratio of the integral cross sections ␴͑CHϩ3 ͒/␴͑CH4ϩ͒ was
0.4 at 1.2 eV. This value will be used in further estimation;
the conclusions can be, however, easily adopted to a different
value of the integral cross section ratio.
The estimation starts from the P(TЈ͒ curves ͚͑0°–180°͒
of CHϩ4 and CHϩ3 ͓uppermost curves in Figs. 4͑a͒ and 4͑b͒,
respectively͔. The area under the P(TЈ͒ curves ͑total scat-
tered intensity͒ is proportional to the integral cross section;
normalization of this area and multiplication by the integral
cross section ratio of 0.4 makes the two curves comparable
with one another ͑the final scaling factor CHϩ3 /CH4ϩ was
0.22͒. In the second step, the P(TЈ͒ of CHϩ3 was divided into
contributions due to reactions of Krϩ ͑2P3/2͒ and Krϩ ͑2P1/2͒:
it was assumed that the reaction of Krϩ ͑2P1/2͒ determines
the shape of P(TЈ͒ at ⌬TϾ0, that this distribution peaks
close to ⌬Tϭ0, and that it has in general the same shape as
A large fraction of CHϩ3 is formed in an inelastic process
where some of the relative translational energy is converted
into the internal energy. This strongly suggests that the reac-
tant ion state is the ground state Krϩ ͑2P3/2͒; the reaction of
this state requires, at the threshold, a conversion of 0.25 eV
into the internal energy to open the dissociative charge trans-
fer channel ͑2͒. The observed most probable inelasticity of
0.4 eV ͓Fig. 4͑b͔͒ can be linked to the breakdown pattern of
CHϩ4 ͑Fig. 5͒ which shows a maximum of CHϩ3 at about 14.4
eV.
The scattering results thus lead to conclusions which dif-
fer from the traditionally accepted interpretation of the
charge transfer between Krϩ and methane.10 It was assumed
earlier that CH4ϩ was formed in collisions of Krϩ ͑2P3/2͒ and
CH3ϩ in collision of Krϩ ͑2P1/2͒; the lower abundance of
CHϩ3 was assumed to reflect the lower statistical population
of Krϩ ͑2P1/2͒ in the reactant beam.
In order to obtain an estimate of the overall distribution
of the energy transferred in the charge transfer collision be-
tween Krϩ and CH4, one has to take into account the forma-
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7022
Z. Herman and B. Friedrich: Collisions of Krϩ with CH4
P(TЈ͒ curve of CHϩ4 . This made it possible to break up the
P(TЈ͒ curve of CHϩ3 into contributions which could be as-
cribed to reactions of Krϩ ͑2P1/2͒ and Krϩ ͑2P3/2͒, respec-
tively ͑see dashed lines in the uppermost curve in Fig. 4͒, the
former peaking close to ⌬Tϭ0, the latter clearly inelastic.
The estimates are given in Fig. 5. Its upper panel shows
the energy deposited in methane by charge exchange with
Krϩ ͑2P3/2͒ as it results from the above-mentioned analysis.
Note that the abscissa in Fig. 5 is inverted with respect to
that of Figs. 4͑a͒ and 4͑b͒, in order to comply with the usual
form of presenting energy deposition distributions ͑e.g., in-
elastic processes mean less energy than ⌬Tϭ0 in product
translation, but more energy than the recombination energy
left in the molecular ion͒. The dashed curve labeled CH4ϩ
corresponds to the CHϩ4 formation ͓uppermost P(TЈ͒ in Fig.
4͑a͒, abscissa inverted͔, the dotted curve labeled CHϩ3 is a
contribution from CHϩ3 formation plotted to scale ͓left part
of the uppermost P(TЈ͒ curve in Fig. 4͑b͒, inverted͔. The
solid line is the sum of both and it represents an estimation
of the overall energy deposited in methane by Krϩ ͑2P3/2͒.
The middle part of Fig. 5 shows the estimate of energy dis-
any, should be less than 10% ͓see overlap of the ͑2P1/2
͒
curve with the CHϩ4 curve of the breakdown pattern in
Fig. 5, upper part͔;
͑4͒ The integral charge transfer cross sections of the two
Krϩ spin–orbit states appears to be different, assuming
the statistical 2:1 ratio of ͑2P3/2͒/͑2P1/2͒ in the beam,
␴͑2P3/2͒/␴͑2P1/2͒Ӎ3.
V. CONCLUSIONS
͑1͒ The dynamics of the formation of CHϩ4 and CH3ϩ in
charge transfer collisions between Krϩ and CH4 was in-
vestigated at the hyperthermal energy of 1.18 eV ͑c.m.͒
in a crossed beam scattering experiment; scattering dia-
grams of the ion products, c.m. angular distributions,
P͑␽͒, and distributions of product relative translational
energies, P(TЈ͒, were obtained from the scattering data.
͑2͒ The scattering of CHϩ4 shows that the nondissociative
charge transfer process is most likely, if little or no en-
ergy interchange takes place between internal and trans-
lational degrees of freedom of the system ͑⌬Tϭ0͒, i.e.,
when the recombination energy of the Krϩ projectile is
deposited in the internal energy of the product ion ͑reso-
nant charge transfer͒; however, internal states of CHϩ4 up
to Ϯ0.6 eV on both sides of the resonant charge transfer
value are populated with decreasing probability in in-
elastic and superelastic collisions; the angular distribu-
tion peaks sharply at 0° ͑direction of the CH4 initial ve-
locity͒.
tribution deposited in the methane target by the Krϩ ͑2P1/2
͒
projectile. The curve is drawn to scale with the upper portion
of the figure.
The resulting overall energy deposition curves are shown
again in the lower part of Fig. 5. The curves are plotted with
respect to the recombination energy of Krϩ ͑2P3/2͒, 14.00 eV,
and of Krϩ ͑2P1/2͒, 14.66 eV, which corresponds to ⌬Tϭ0 in
the upper part of the figure. The dashed curve labeled ͑2P3/2
͒
͑3͒ The CHϩ3 product is formed in two processes in about
equal amount at this collision energy; in an exoergic dis-
sociative charge transfer with Krϩ ͑2P1/2͒ which occurs
in the resonant way, analogous to the case of CHϩ4 for-
mation; in an endoergic reaction with the ground state
Krϩ ͑2P3/2͒ in which a part of the collision energy ͑peak
value about 0.5 eV͒ is used to overcome the 0.25 endo-
ergicity barrier; the latter process leads to an almost iso-
tropic scattering of CHϩ3 .
represents an estimate of the energy deposited in the methane
target by charge transfer with Krϩ ͑2P3/2͒, the dash–dotted
curve ͑2P1/2͒ an estimate of energy deposited by Krϩ ͑2P1/2͒.
The shape and the width of this energy deposition curve can
be compared with the data obtained for charge transfer col-
lisions between protons and methane at the collision energy
of 30 eV ͑Ref. 8͒ ͑dotted curve͒. The shapes of the energy
deposition curves from Krϩ and Hϩ compare well, though
the Hϩ curve is somewhat broader, presumably due to a
much higher collision energy. For a general orientation, the
photoelectron spectrum of CH4 ͑Ref. 17͒ and the breakdown
pattern of the CH4ϩ molecular ion10,13 as a function of total
energy of the system is also plotted in the figure.
ACKNOWLEDGMENTS
This work was partly supported by the Grant No. 203/
93/0246 of the Grant Agency of the Czech Republic. One of
the authors ͑Z.H.͒ gratefully acknowledges the Alexander
von Humboldt Award Fellowship during which most of this
manuscript was prepared.
Though the estimate is only approximate, several inter-
esting conclusions can be drawn from it which provide more
insight into the charge transfer process between Krϩ and CH4
at this hyperthermal collision energy of 1.18 eV.
͑1͒ The CHϩ3 product ions formed by dissociative charge
transfer process ͑2͒ come in about equal amounts from
the exoergic reaction with Krϩ ͑2P1/2͒ ͑50%͒ and the
endoergic ͑inelastic processes on the expenses of relative
kinetic energy͒ reaction with Krϩ ͑2P3/2͒ ͑50%͒; evi-
dently, the small endoergicity of the latter process helps
in making it probable;
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J. Chem. Phys., Vol. 102, No. 18, 8 May 1995
128.240.225.44 On: Sat, 20 Dec 2014 06:34:42