Table Of ContentFragmentation dynamics of CO3+ investigated by multiple electron capture in
2
collisions with slow highly charged ions
N. Neumann,1 D. Hant,1 L.Ph.H. Schmidt,1 J. Titze,1 T. Jahnke,1 A. Czasch,1
M.S. Scho¨ffler,1,2 K. Kreidi,1 O. Jagutzki,1 H. Schmidt-B¨ocking,1 and R. D¨orner1,∗
1Institut fu¨r Kernphysik, Goethe-Universita¨t Frankfurt am Main, Max-von-Laue-Str.1, 60438 Frankfurt, Germany
2Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA-94720, USA
(Dated: January 28, 2010)
Fragmentationofhighlychargedmolecularionsorclustersconsistingofmorethantwoatomscan
0
proceedinanonestepsynchronousmannerwhereallbondsbreaksimultaneouslyorsequentiallyby
1
emittingoneionaftertheother. Weseparatedthesedecaychannelsforthefragmentation ofCO3+
0 2
2 ions by measuring the momenta of the ionic fragments. We show that the total energy deposited
in the molecular ion is a control parameter which switches between three distinct fragmentation
n pathways: the sequential fragmentation in which the emission of an O+ ion leaves a rotating
a CO2+ ion behind that fragments after a time delay, the Coulomb explosion and an in-between
J
fragmentation - the asynchronous dissociation. These mechanisms are directly distinguishable in
1 Dalitz plots and Newton diagrams of the fragment momenta. The CO3+ ions are produced by
2
2 multipleelectron capturein collisions with 3.2 keV/uAr8+ ions.
]
h
PACSnumbers: 34.50.Gb,34.50.Fa,34.70.+e
p
-
m
As one or more electrons are removed from a neutral ies,includingthedissociationofCOq+ withchargestates
o 2
diatomic or polyatomic molecule or cluster the Coulomb up to q=4, using time-of-flight mass spectrometers were
t
a repulsion between the ionic cores will eventually lead done by Tianet al. [14] in 1998. Up to presentthere are
.
s to fragmentation. The dynamics of this dissociation of only a few more experimental reports on the three-body
c molecular and cluster ions, however, is highly complex. dissociation of CO3+ ions [11]. In last decade, sophisti-
i 2
s A key question is, what parameters control which of the cated experimental techniques to measure not only the
y
various decay mechanisms becomes active. In an ideal time-of-flight of each ionic fragment but also the com-
h
p case,ifsuchparametersareunveiled,theycanbeusedto plete vector momentum gave a better understanding of
[ switch between the breakup channels. The most simple dissociation processes and molecular geometries, see e.g.
of such mechanisms is single step Coulombexplosion[1– [15].
1
v 3] of the multiply charged molecular ion. Here all bonds
6 break simultaneously and the remaining atomic ions are In the present work we have chosen impact of slow
4 driven apart purely by their Coulomb repulsion. This highly charged ions (3.2 keV/u Ar8+ projectiles) to
8 scenario is in many cases successfully used to describe produce CO32+ ions. The multiple capture of electrons
3 fragmentation of molecules in strong laser fields [4–6]. by highly charged ions is a rather gentle and very
.
1 The other extreme are sequential or stepwise processes rapid process which leaves the molecular ion preferen-
0 in which in the first step the molecular ion separates tially in its ground or low lying electronically excited
0 into two fragments. After that - at distances between state. Compared to ionization in a femtosecond laser
1
: the primary fragments where these hardly interact any- field [16], the collision is very fast and does not leave
v more-anotherdissociationtakesplace. Inbetweenthese time for geometrical rearrangement. It leads to a
i
X two scenarios a third one is possible - the asynchronous vertical transition between the linear neutral ground
r breakup: thebondsofthemolecularionbreakinasingle state and the CO32+ potential energy surfaces. Unlike
a step,butatatimewherethegeometryofthemoleculeis earlier experiments [7, 8, 15, 17, 18] we are able to
asymmetric, as vibration and rotation precede the frag- observe the three-body dissociation of CO32+ leading
mentation. to C++O++O+ ions in a kinematically complete way
by applying multicoincidence momentum imaging tech-
Inthe presentstudyweshowfortheprototypesystem niques, explained elsewhere [19–21]. The detection of all
of CO3+ ions that these different dissociation pathways fragments with 4π solid angle allows us to distinguish
2
can be identified and distinguished experimentally and, thevariousfragmentationmechanismsofthetricationin
furthermore,thatthesystemcanbesteeredononeorthe unprecedented detail and completeness. The ion beam
otherpathwaybyvaryingthetotalamountofenergyde- of Ar8+ projectiles at 3.2 keV/u has been generated at
positedintothemolecularion. Thedecayofthedication the Electron Cyclotron Resonance (ECR) Ion Source at
of CO molecules created by electron impact, photoion- Goethe-University, Frankfurt. All three fragment ions
2
izationorheavyionimpacthasbeenstudiedtheoretically are measured in coincidence with the projectile charge
and experimentally by several groups [7–13]. First stud- state using the COLTRIMS (COLd Target Recoil Ion
2
a) b)
1 ǫ ǫ
ǫ vs. O+ − O+ (1)
C+ − 3 √3
13
-+C 0 where ǫ = ~k /(2m W), m is the mass
e C+,O+ C+,O+ C,O
and W is the total energy of the three atoms. A key ad-
vantageoftheDalitzplotis,thatthephasespacedensity
is constant, i.e. all structure in such a plot results from
0 0
(e O + - e O + ) / 3 (e O + - e O + ) / 3 the dynamics of the process, not from the trivial final
state phase space density. Figure 1(b) shows a Dalitz
c)
plot of our measured data. Each region of that diagram
refers to a certain geometry of the momentum vectors
C+ at the instance of breakup as shown in fig. 1(a). Our
data clearly shows that the most likely configuration of
O+ the dissociatingCO32+ ionis linear(markedby the small
violet oval at the bottom). For events in this region the
energyoftheC+ionisverysmallandthetwoO+ionsare
emitted back-to-back, reflecting the linear ground state
O+
geometry of the CO molecule. This island corresponds
2
to the direct,synchronousprocesswhere allbonds break
simultaneously.
In recent experiments on slow and swift heavy ion
collisions with polyatomic molecules like CO only this
2
simultaneous breakup and some contribution from the
FIG. 1: (a) Characteristic momentum vector geometries for
asynchronous reaction mechanism have been observed
specificpointsintheDalitzplotand(b)Daltizplotwithmea-
sured data; (a) + (b) events located in the different marked [17, 18]. Figure 1(b) clearly provesthat in slowion colli-
areas correspond to various reaction mechanisms: magenta sionstheasynchronousdissociationmechanism,preceded
colored oval - direct ionization process, dash-dotted X-shape by molecular bending and asymmetric stretching of the
- sequential breakup, black ovalson left and right side- asyn- molecular ion, take place, as well; events resulting from
chronous stretching and green dotted area - molecular bend-
molecular bending are located within the green dashed
ing. (c) Newton diagram: momentum vector of one O+ ion
oval,whileeventsallocatedtoasymmetricstretchingcan
in the CM frame defines the x-axis, while the relative mo-
mentum vectors of the C+ ion and the second O+ ion are be found inside the black solid-line ovals (left and right
mappedintheupperandlowerhalf, respectively;thedashed at the bottom). Additionally, the Dalitz plot (fig. 1(b))
circle marks the sequential breakup, the symmetric islands showsafourth,X-shapedstructuremarkedbytheyellow
in the upper and lower half identify the direct and concerted dash-dottedlineswhichcontainsabout20%ofallevents.
breakup mechanism, see text. This structure results from the sequential breakup. To
see this more directly, we display the same data in a
Newton diagram in fig. 1(c). Here the momentum vec-
Momentum Spectroscopy) technique [19–21]. The ionic tors are shown with respect to the center of mass (CM)
fragments produced in the interaction region are guided of the fragments. The direction of the momentum vec-
by an electrical field of 39 V/cm onto a microchannel tor of one of the O+ ions is represented by an arrow
plate detector with delay-line anode [22]. By measuring fixed at 1 a.u.. The momentum vectors of the C+ ion
the positions of impact and the time-of-flight of each and the second O+ ion are normalized to the length of
particle one can determine the three-dimensional initial the first O+ ion momentum vector and mapped in the
momentum vector and the mass to charge ratio of each upper and lower half of the plot, respectively. In the
ionic fragment in an offline analysis. Downstream of the Newton diagram the C+ and the second O+ fragment
reaction zone the projectile charge states are separated momenta are located on a circle shifted to the left by
by an electrostatic deflector. These projectile ions are about half of the first O+ ion momentum. This shifted
detected by a time- and position sensitive microchannel circle (marked by the dashed line in fig. 1(c)) is a clear
plate detector, as well. proof of a two step dissociation mechanism. In the first
step the CO3+ molecule dissociates into an O+ ion and
2
We now show how we experimentally identify the var- an CO2+ ion. Then after a delay, when the force of the
ious breakup mechanisms. A very useful tool for visu- departing O+ ion onto the CO2+ fragment is negligible,
alization of three body processes is the Dalitz plot [23]. the second step occurs: the CO2+ ion, which is moving
This probability-density plot displays the vector correla- totheleft,dissociatesintoaC+ ionandasecondO+ion.
tion in terms of the reduced energies of the three atoms, The intermediate CO2+ fragment receives some angular
3
4000 (a)
3000
2000
1000
(c) (d) (e)
15 20 25 30 35 40
(b) seq. mechanism s] (f) (g) (h)
mol. bending nit
direct process b. u C+ C+ C+
r
m [a O+ O+ O+
u O+
nt O+ O+
e
m
o
m
momentum [arb. units]
FIG. 2: left: (a) Total, experimental KER (black solid line) distribution. (b) KER distributions for the various reaction
mechanisms normalized to the maximum, respectively; The different colors represent events selected from different regions in
fig. 1(a) corresponding to the different mechanisms; right: Spectra (c) - (e) show Dalitz plots and spectra (f) - (h) Newton
diagramms for different regions of KER: (c) and (f): 16,5 eV < KER < 17,5 eV, (d) and (g): 30,5 eV < KER < 31,5 eV and
(f) and (h): 37,5 eV < KER < 38,5 eV.
momentum as the O+ ion is expelled. From the mean metric stretching mode (area inside the green dashed
angle of 170deg for the ground state of CO and a mea- and black solid ovals in figure 1(b)). Unlike in this
2
sured momentum of about 150 a.u. of the primary O+ asynchronous reaction mechanism the C+ ion gains
ionthe angularmomentumtransferto the CO2+ ionleft almost no momentum while breaking up via pure direct
behind can be estimated to be about 60 ¯h correspond- processes. Here the O+ ions dissociate back-to-back
ing to 89 fsec for half a turn. The secondary breakup leaving the C+ ion almost at rest. This simultaneous
of this rotating CO2+ wavepacket leads to the observed breakup leads to islands in the upper and lower half of
circle in figure 1(c). Previous studies of CO2+ molecules the Newton diagram, respectively. The slight offset of
createdbyK-shellphotoionizationfollowedbyAugerde- these main spots is an artefact of the Newton diagram.
cay have shown that for a kinetic energy release (KER) By definition all carbon ions are displayed in the upper
below 10.95eV, the CO2+ iondecayswithin 30-100fsec, hemisphere and all oxygenions appear in the lower half.
whichissufficientforrotationtooccurbeforefragmenta- Thus, any spread of a linear configuration looks like an
tion(seefigure4in[24]). Oneindicationforthis lifetime apparent bend. This effect is further enhanced by the
is also the clearly observed vibrational structure in this fact that unlike the Dalitz plot the Newton diagram
KER regime (see figure 3 in [24]). The most important does not have a constant phase space: The solid angle
intermediate states of the excited CO2+ molecule in this and hence the phase space along the horizontal sep-
regime are the 1Π, 3Σ+ and 21Σ+ [24–26]. The poten- aratingthecarbonionfromtheoxygenionregioniszero.
tial energy curves of these states have local minima in
the 1.9-3.8a.u. range which decay by coupling to purely
repulsive states [26, 27]. After unambigously identifying the fragmentation
pathways we now show, that the amount of energy de-
Besides the illustrated sequential dissociation mecha- positedintothe systemby the ionimpact, decideswhich
nism concerted breakups, like the asynchronous decay, pathway is dominant. This energy is converted to ki-
can be identified in the Newton diagram, as well. netic energy of the fragments, which we measure,and to
The geometrical rearrangement of the CO3+ ion after a much smaller extent in possible electronic excitation
2
ionization via electron capture results in a momentum energy, which eventually is emitted as photons. In our
gain of the C+ fragment. This momentum gain leads setup we measure the KER with a resolution of about
to the apparent slight bend angle of the main spots 100 meV, we do not detect emitted photons. Since our
in figure 1(c) which correspond to the data in the multiple electroncapturereactiontypicallydoesnotcre-
Daltiz plot, associated with the bending and asym- ateveryhighlyexcitedstatesofCO3+,mainly,fragments
2
4
in the ionic ground state contribute in our case. Figure directly from the data without the need of knowledge
2(a) shows the total KER distribution and figure 2(b) of the potential energy surface. As we have shown, the
shows the measured KER distributions for the different total energy put into the system is the key parameter
regions in the Dalitz plot indicated in fig. 1(a) and (b) which can be used to control the fragmentation.
by the coloured ovals. They correspond to the different
breakup mechanisms, as well. The KER distribution for
thesequentialbreakupshowsthesmallestonsetenergyof
all three mechanisms. Figures 2(c)-(e) show the Dalitz
plots and fig. 2(f)-(h) show the Newton diagramsgated
∗ Electronic address: [email protected]
on different regions of KER, respectively, i.e. they cor-
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study shows a way out by identifying clear mechanisms
