Bond-Rearrangement In Water Ionized By Fast Ion Impact
A.M. Sayler, J.W. Maseberg, D. Hathiramani, K.D. Carnes, and I. Ben-Itzhak
James R. Macdonald Laboratory, Department of Physics,
Kansas State University, Manhattan, KS 66506-2604
Abstract. Studies of ionization and fragmentation of water molecules by fast protons or highly charged ions have
revealed an interesting isotopic preference for H-H bond rearrangement. Specifically, the dissociation of H2O+→H2++O
is about twice as likely as D2O+→D2++O, with HDO+→HD++O in between. It is suggested that similar isotopic
preference might exist in the dissociation of multiply charged H2Oq+→H2++O(q-1)+. We present first experimental
observation of such bond-rearrangement dissociation of H2O2+→H2++O+. Furthermore, our data suggests similar relative
rates for this new bond-rearrangement channel and the previously observed H2O+→H2++O.
deuterium labeling and found strong isotopic effects
favoring H2+ + O over D2+ + O production by fast
proton impact ionization [6]. Straub et al. observed a
similar preference for fast electron-impact ionization
[1]. Our work led us to explore the question of whether
such isotopic effects also occur in the dissociation of
transient multiply ionized water molecular ions, for
example H2O2+. Clearly, the first step toward such
studies is determining whether such dissociation
occurs in measurable quantities, as we will show in
this paper. Specifically, we have recently determined
that H2+ fragments are produced not only in single
ionization, H2O+ → H2+ + O, but also in double
ionization, H2O2+ → H2+ + O+, and maybe even in
triple ionization. The relative magnitude of this
reaction channel is very small, but we will show that it
is measurable. The rates of the other isotopes are
expected to be even smaller if they follow an isotopic
dependence similar to the single ionization channel.
INTRODUCTION
Ionization of water molecules by fast protons,
electrons, or energetic photons, results mainly in
singly charged molecular ions or fragmentation of the
molecular ion into ionic and neutral fragments.
Typically, one or both O-H bonds will break, but small
amounts of H2+ fragments have been observed in mass
spectra of water targets (see for example Refs. [1-2]).
This fragmentation channel, namely H2O+ → H2+ + O,
obviously requires bond-rearrangement within the
water molecule, and thus draws more attention than
other more likely dissociation channels. For example,
Rottke et al. have also seen this dissociation channel in
multiphoton ionization of water [3]. Recently,
Piancastelli et al. observed H2+ fragments following a
core excitation of the O(1s) by high-energy photons.
They suggested that the H2+ is formed by excitation of
highly excited bending modes [4]. The suggested
mechanism was further discussed in the theoretical
work of Nobusada and Tanaka [5]; however, the
understanding of this bond-rearrangement mechanism
is far from being satisfactory. One fact that seems
clear is that this bond-rearrangement process is
independent from the ionization mechanism, as H2+
fragments have been observed for different ionization
processes.
EXPERIMENTAL
Ionization by fast proton impact results
predominantly in single ionization with about 1%
double ionization. Thus, all the H2+ were associated
with neutral oxygen and none were found to be in
coincidence with O+. In contrast, ionization by highly
charged ions efficiently ionizes target atoms and
molecules. For this reason we used such beams to
conduct the search for the H2O2+ → H2+ + O+
dissociation channel. The experimental method
(coincidence time of flight), apparatus, and data
The H2+ fragments we observed in our studies had
very low kinetic energy as suggested from the very
narrow width of the time-of-flight (TOF) peak. To
further investigate this mechanism we studied the rate
of H2+ formation for the different water isotopes using
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
48
in Fig. 2. Note that the ratio shown in Fig. 2 is
somewhat smaller than that shown in Fig. 1 due
mainly to the subtraction of the D+ contamination.
analysis used for this study have been described in our
previous publications [7-8] and thus will be only
briefly described here. A 1 MeV/amu bunched beam
of Fq+ (q=4-9) was directed through a target cell filled
with water vapor and collected afterwards in a Faraday
cup. The recoil ions produced in the target cell were
extracted and accelerated by uniform electric fields
onto a microchannel plate detector of a time-of-flight
spectrometer. The times of flight of the different recoil
ions were recorded relative to a signal synchronized
with the roughly 1 ns beam bunch. Recoil ions
produced in the same beam bunch were recorded in
coincidence, event by event, thus separating single,
double, and multiple-ionization events.
R=0.125%
+
[H2 + O ] / [ H2O ] (%)
0.15
RESULTS AND DISCUSSION
+
H
9+
F
e
R
+
First, the yield of H2+ ions relative to H2O+ ions
from single ionization was measured as a function of
the target pressure as shown in Fig. 1. The higher yield
of H2+ at low pressure is caused by ionization of the
residual H2 gas; however, the effect of this
contaminant becomes negligible once the water
pressure is high enough.
0.10
0.05
0.00
1
10
Projectile velocity (a.u.)
9+
F
4+
F
R
0.35
The H2+ + O+ ion-pair from the dissociation of the
transient H2O2+ can be seen in the coincidence
spectrum shown in Fig. 3(a). However, the small
number of events in each of these coincidence
“islands” might raise doubts about the validity of the
claim that H2O2+ → H2+ + O+ has been observed. The
main possible sources for such events, if they are not
what we claim them to be are: (i) random
coincidences, (ii) true coincidences of other water
isotopes (HDO and D2O), and (iii) true coincidences of
contaminant molecules such as O2 and CmHn. The
contribution of the latter was found to be negligible by
measuring without water vapor in the target under the
same conditions and normalizing the two
measurements to the rates of N2+ and O2+. At most 5
counts out of the 1400 in the H2+ + O+ gate marked in
Fig. 3(a) might be due to contributions of the residual
gas.
0.25
+
[H2 + O ] / [ H2O ] (%)
0.30
FIGURE 2. The ratio of H2++O to H2O+ as a function of
impact velocity for: protons and highly charged ions (this
work); electrons (Straub et al. [1]).
0.20
R = 0.17 %
+
0.15
0.10
0.05
0.00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
−6
PChamber [10 Torr]
FIGURE 1. The ratio of H2++O to H2O+ as a function of the
base pressure in the system, for 1 MeV/amu ion impact
ionization. R is the asymptotic measured ratio.
The random coincidence rate, which is not
negligible in this experiment, is mainly due to double
collisions within the same beam bunch leading to a
coincidence between fragments of two different
molecules. Projectiles not within the beam bunch are
The ratio of H2+ + O to H2O+ compares well with
the electron impact data of Straub et al. [1], as shown
49
another source of random events, which contributes an
approximately constant base line to each spectrum
(singles, ion-pairs, etc.). In order to properly subtract
the random events one needs to evaluate the random
coincidence rate as well as the distribution of the
random events within the H2+ + O+ gate. The randomcoincidences rate is found first from the number of
pure random events, i.e. coincidences between ions
from different molecules such as H2O+ + N2+ or O2+
and other similar channels. The shape of each
random- coincidence peak was “simulated” using the
measured singles data as follows. Each single ion
event was grouped into a random pair with the
following single ion. Then the whole random
coincidence spectrum, presented as a 2D matrix in Fig.
3(b) (the vertical scale in Fig. 3(b) was magnified to
show the small channels), was normalized to match the
yield in the H+ + H2O+ random coincidence peak in the
raw data shown in Fig. 3(a). It can be seen that the
random coincidence H2+ + O+ peak is more sharply
peaked than the distribution of true H2+ + O+ events.
Then, we subtracted the simulated random coincidence
spectrum from the measured one and the result is
shown in Fig. 3(c). Note that a significant number of
H2+ + O+ events “survive” this subtraction.
20
+
+
H2 + OH
15
+
H2 + H2O
+
TOF1 (ch.)
10
5
+
H2 + O
2
+
21 32
0
20
+
H + H2O
15
+
10
2 10k 20k
5
0
+
H +O
+
0
+
H + OH
10
20
+
30
40
50
TOF2 (ch.)
20
+
+
H2 + OH
15
+
H2 + H2O
(a)
+
TOF1 (ch.)
10
5
+
H2 + O
2
5
8
2
3k 5k
+
0
20
+
H + H2O
15
+
10
Next, the H2+ + O+ coincidences gate might contain
a significant number of D+ + O+ events from either
HDO or D2O contaminants. In that case one would
expect a measurable number of events of either D+ +
OH+ or D+ + OD+, since the H+ + O+ and H+ + OH+
reaction channels, shown in Fig. 3, are of the same
order of magnitude (a relative yield of about 2.8:1).
5
0
+
+
+
H +O
0
10
+
H + OH
20
30
40
50
TOF2 (ch.)
(b)
20
The fact that only small amounts of D+ + OH+ or
+
D + OD+ can be seen in Fig. 3(c) (labeled as H2++OH+
and H2++H2O+, which have the same mass-to-charge
ratio) allows us to set a limit on the isotopic
contamination, which is a very small fraction of the
H2+ + O+ coincidence events remaining after
subtraction of the random pairs.
+
H2 + OH
15
+
+
H2 + H2O
+
TOF1 (ch.)
10
2 17 32
5
+
H2 + O
+
0
20
+
H + H2O
15
+
10
Finally, the most convincing evidence that the
measured H2+ + O+ coincidence events are true events
is based on the fact that the dissociation channel of
interest is a two-body breakup. Therefore, the times-of
flight of the two fragments should be correlated
forming a slope of –1 on the plot presented in Fig. 3(c)
[7]. The extraction field used in the measurement was
lowered in order to allow more time spread due to the
molecular dissociation resulting in the elongation of
the true coincidence channels in Fig. 3. It can be seen
that the H2+ + O+ events have a distribution similar to
the H+ + OH+ events, thus verifying that they are from
a two-body breakup of a H2O2+ transient molecular
ion.
2 10k 20k
5
0
+
H +O
0
+
10
+
+
H + OH
20
30
TOF2 (ch.)
40
50
(c)
FIGURE 3. Coincidence time-of-flight spectrum of
water molecules ionized by 1MeV/amu F7+, (a) raw
data, (b) simulated random pairs (normalized to
H++H2O+ yield, see text), and (c) data after subtraction
of random pairs, i.e. (a)-(b). Boxed labels mark the
true ion-pair channels, while the random ones are
unboxed. The solid lines mark the expected behavior
of two-body fragmentation, i.e. a slope of –1 (see text).
50
The rate of this small breakup channel was
determined to be about 900±100 events after
subtraction of the base line, i.e. collisions out of the
beam bunch. For comparison we measured
425000±5000 events in the main two-body breakup
channel of H2O2+, H+ + OH+. Thus, the relative rate
was evaluated to be about (2±0.2)×10-3. This ratio is
comparable to the one we measured for the bondrearrangement process following single ionization
suggesting a similar mechanism for both.
dependence of the bond rearrangement is similar for
all levels of ionization.
30
25
(ch.)
10
[ TOF1 + 2 TOF2 ] / 5
15
1/2
20
To determine if an isotopic difference exists in this
bond-rearrangement process similar to the one
discovered for single ionization, further measurements
are needed. It is important to note that even with the
experimental difficulties discussed above the H2O
isotope is the easiest to measure of all the water
isotopes. To determine the bond rearrangement rate in
HDO, that is to measure the rate of HDO2+ → HD+ +
O+, one has to separate that channel from the true
coincidence O27+ → O6+ + O+ and O26+ → O5+ + O+
events, which are barely separated from the channel of
interest in the coincidence TOF spectrum. Thus the
subtraction of the residual gas component needs to be
done more accurately than for the H2O isotope. This
background subtraction is even more problematic for
the D2O isotope because the D2O2+ → D2+ + O+
channel exactly coincides in TOF with the O25+ → O4+
+ O+ dissociation of the residual O2. This competing
channel is larger than those near the HD+ + O+ reaction
channel as only 5 electrons need to be ionized.
Furthermore, D2+ + O+ production is expected to be the
smallest of all these isotopes if the isotopic
1
+
H2 + O
5
9
13
2+
0
30
25
20
15
0
10
+
H +O
5
0
0
1k
3k
2+
5
10
15
20
25
30
TOF2 - TOF1 (ch.)
FIGURE 4. Rotated time-of-flight spectra of the
H++O2+ and H2++O2+ channels (see text).
Finally, if one looks carefully at the rotated
coincidence TOF spectrum shown in Fig. 4, H2+ + O2+
coincidences from H2O3+ dissociation can be seen as a
horizontal distribution similar to the H++O2+ channel
shown for comparison below, though not as spread.
(Both dissociation channels were rotated as discussed
in Ref. 7). The low statistics, however, limit our ability
to verify what this spectrum suggests, i.e. that bondrearrangement might occur in a similar way for all
transient water molecular ions independent of the
number of electrons removed. Further investigation of
this apparent trend is needed.
ACKNOWLEDGMENTS
3. Rottke, H., Trump, C., Sandner, W., J. Phys. B 31, 1083
(1998).
This work was supported by the Chemical
Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science,
U.S. Department of Energy.
4. Piancastelli, M. N., Hempelmann, A., Heiser, F.,
Gessner, O., Rüdel, A., and Becker, U., Phys. Rev. A 59,
300 (1999).
5. Nobusada, K., and Tanaka, K. J. Chem. Phys. 112, 7437
(2000).
6. Sayler, A.M., Hathiramani, D., Carnes, K.D., Esry, B.D.,
and Ben-Itzhak, I., Bull. Am. Phys. Soc. 47, 77 (2002);
Sayler, A.M., ibid 47, 77 (2002); and a paper in
preparation.
REFERENCES
1. Straub, H.C., Lindsay, B.G., Smith, K.A., and Stebbings,
R.F., J. Chem. Phys. 108, 109 (1997).
7. Ben-Itzhak, I., Ginther, S.G., Carnes, K.D., Nucl.
Instrum. and Meth. B 66, 401 (1992).
2. Sayler, A.M., Wells, E., Carnes, K.D., and Ben-Itzhak, I.,
in Application of Accelerators in Research and Industry,
edited by J.L. Duggan and I.L. Morgan, AIP Conference
Proceedings 576, New York: American Institute of
Physics, 2001, pp. 33-35.
8. Ben-Itzhak, I., Carnes, K.D., Ginther, S.G., Johnson,
D.T., Norris, P.J., and Weaver, O.L., Phys. Rev. A 47,
3748 (1993).
51