Dissociative electron attachment to the H2O molecule II: nucleardynamics on coupled electronic surfaces within the local complexpotential model Page: 2 of 25
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2
E (eV)
14
12
10
8
6 -2B
B1r1=r
(units of ao)0 60 120 180 240 300 360
0 (degrees)
FIG. 1: (Color online) Real parts of resonance energies ER
as constructed in (I) within C2 geometry (r1 - r2), plotted
with respect to bending angle and symmetric stretch distance.
solid lines. The surfaces extend forward in Fig. 1 along
the symmetric stretch direction to geometries at which
r1 r2 2.7a0. The conical intersection comprises the
set of points along which the 2A1 and 2B2 surfaces in-
tersect. The Renner-Teller degeneracy between the 2B1
and 2A1 states occurs at 0 180g.
Although Fig. 1 shows only one cut of the potential
energy surfaces, and only their real part, it is useful for
introducing certain features of these surfaces and the dy-
namics that will result. Dissociative attachment via the
lower 2B1 and 2A1 states leads primarily to the product
H- + OH (X 2H). The two OH bond lengths for such
an arrangement are unequal, and therefore this product
arrangement cannot be seen in Figure 1. However, we
can see that at the equilibrium geometry of the neutral,
the 2B1 surface is relatively flat with bend, while the 2A1
surface slopes steeply downward towards linear geometry
(0 180 ). As a result, the dynamics beginning on the
2A1 surface will lead towards linear geometry, and we ex-
pect that the Renner-Teller coupling between these two
states will be more important for DEA via the 2A1 than
via the 2B1 state.
The channel H2 + O- is the minor channel for DEA
via the 2B1 and 2A1 states, but the major channel for the
2B2 state. We can see why this is the case from Fig. 1;
the gradient of the 2B2 (2 2A') surface leads downward
from the ground state equilibrium geometry towards the
conical intersection, where the system may make a nona-
diabatic transition to the lower surface and access the
clearly visible H2 + O- well on the 1 2A' (lower cone)
surface. The 2 2A' surface does not have a low energy
asymptote in this geometry, instead correlating to 0- +
H2 (uoj)-
The outline of this paper is as follows. In Section
II we summarize previous experimental and theoretical
work on this problem. In Section III we present the local
complex potential model, which forms the foundation ofour theoretical implementation. The Hamiltonian for the
rovibrational nuclear motion of a triatomic molecule, and
the additional terms which arise when the Renner-Teller
effect is included, are described in Section IV. In Section
V we describe the Multi-Configuration Time-Dependent
Hartree (MCTDH) method, which we use to calculate the
nuclear dynamics, and the formalism for calculating the
DEA cross sections. In Section VI we present the final
results of this study: cross sections, as a function of inci-
dent electron energy, resolved into the final rovibrational
product states.
II. PREVIOUS EXPERIMENTAL AND
THEORETICAL RESULTS
Dissociative electron attachment to water molecules
has been the subject of previous experimental investi-
gation, starting as early as 1930 [2], and as recently as
the present year (2006) [20]. Early experiments on dis-
sociative electron attachment to H20 focused mainly on
the identification of the negative ion species formed, the
measurement of the total cross sections, and the energy
locations of the structures in the resonance process [6].
Buchel'nikova [3] and Schultz [4] established that the
main products of dissociative electron attachment to wa-
ter are H- and O- , with the production of O- being
almost ten times smaller than that of H- at lower ener-
gies, but with O- dominating at higher electron-impact
energies.
Both Compton and Christophorou [5] and Melton [6]
carried out comprehensive studies of negative ion forma-
tion in water and measured absolute cross sections for
DEA. Three resonance peaks were observed. H- produc-
tion was observed at approximately 6.5 eV and 8.6 eV,
with the second peak much less intense than the first.
The species O- was observed in increasing intensities in
three peaks at 7.0 eV, 9.0 eV, and 11.8 eV [5].
The species OH- is also observed in the dissociative
electron attachment experiments, though at an intensity
one order of magnitude below the minor O- + H2 chan-
nel, which is itself observed at an intensity approximately
one order of magnitude lower than the dominant H- +
OH channel. Melton [6] argued that OH- + H was a
true channel of dissociative electron attachment to H2O
molecules, while in subseqent studies (e.g. Ref. [26]) it
was argued that OH- is produced by DEA to water clus-
ters [H20]n. The question of OH- production has been
reexamined in the recent experimental study of Fedor et
al.[20]. These authors have concluded that, indeed, it is a
direct product of dissociative electron attachment to wa-
ter. This minor channel is not examined in the present
treatment, and no mechanism has, as yet, been advanced.
The effects of isotopic substitution have also been an
issue of some debate. The replacement of H2O by D20 as
the molecular target has the effect of nearly doubling the
reduced masses corresponding to OH (OD) bond motion.
One would expect, at least in a simple one-dimensional
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Haxton, Daniel J.; Rescigno, Thomas N. & McCurdy, C. William. Dissociative electron attachment to the H2O molecule II: nucleardynamics on coupled electronic surfaces within the local complexpotential model, article, December 21, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc900698/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.