Dissociative Electron Attachment to H2O via
2B
Resonance
1
Dan Haxton, Zhiyong Zhang, Tom Rescigno, C. William McCurdy (Lawrence Berkeley National Lab and UC-Berkeley)
Hans-Dieter Meyer (The University of Heidelberg, Germany)
THE PROBLEM: Free electrons attach to gas-phase water
molecules, creating transient H2O- species which dissociate to
H2 + O- or H- + OH. We have performed ab initio calculations
on this process in full dimensionality.
H- + OH
C. E. Melton, Journal of
Chemical Physics, 57,
pp.4218-25, (1972).
e-
H2 + O-
+ H2O
DA to H2O proceeds through Feshbach resonances
2
2
2
2
1
2
1a1 2a1 1b2 3a1 1b1 4a1
1a122a121b223a111b124a12
1a122a121b213a121b124a12
2B
D. S. Belic, M. Landau and R.
I. Hall, Journal of Physics B
14, pp.175-90 (1981)
(~ 6.5 eV)
2A (~ 9 eV)
1
2B (~ 12 eV)
2
1
This process has been investigated experimentally
in several studies. H + OH production from the first
two resonance states is observed, and O + H2
production from all three, accompanied by
extensive rotational and vibrational excitation of the
diatomic fragment. The OH- + H channel has been
demonstrated not to be a primary product of DA to
monomeric water.
Local Complex
Potential
Model
 


( E i  k / 2  T  W ( R)) ( R)  (( R) / 2 )  i ( R)
2
1/ 2



1/ 2
 i ( R, t  0)  (( R) / 2 )  i ( R)
V
A+B
-
A+B
Here are several pictures of the real part of the
surface (at left, contours 0.25eV) calculated
from a large-scale CI, and two pictures of the
width of the resonance (above, contours
0.5meV), as calculated by the complex Kohn
variational method. Distance in bohr.
(000) initial state
ISOTOPE EFFECT
(001) initial state
 5.2  1018 cm 2
We do not reproduce the isotope
effect seen in experiment, wherein
the peak cross sections for DA to
D2O are lower than those for H2O.
D
H-
  0
R
We perform the time propagation of the driving term via the Multiconfiguration time-dependent Hartree (MCTDH) method in
collaboration with Prof. H. Dieter-Meyer. The MCTDH wave function
is a time-dependent linear combination of configurations
n1
nf
f
 (Q1 ,..., Q f , t )   ...  A j1 ... j f (t )  (j ) (Q , t )
j1 1
n  N
 1
j f 1
(200) initial state
FINAL ROTATIONAL STATE ANALYSIS
OH v = 0
OH v = 4
D- from D2O
0
-
Solution, z(R), is Fourier
transform:


 i ( E  k 2 / 2i ) t
i
 ( R)  lim i  e
 i ( R, t )dt
(100) initial state
COMPARISON WITH EXPERIMENT:
VIBRATIONALLY RESOLVED CROSS SECTION
Propagate i on complex
resonance potential surface
 6.9  1018 cm 2
1
2
3
-
H from H2O
4
5
6
7
Compton and Christophorou, Phys Rev 113 (1967)
We fit the S-matrix to a Breit-Wigner form….
iA
S  S  (1 
)
E  E R  i / 2
A '      '
And thereby find the partial widths ||2 into each
plane-wave channel . Assuming rotational
suddenness, this gives us the angular distribution of
products.
e-
The analysis of the final rotational state product distribution for the
H- + OH channel is complicated by the fact that this channel is
comprised of an ion-dipole pair. We therefore perform the rotational
state analysis in terms of hindered-rotor or “pendular” states. The
results of this analysis, presented here for DA to the ground
rovibrational state of water, indicate a large degree of rotational
excitation, perhaps larger than that seen in experiment.
Further Directions
ANGULAR DISTRIBUTION OF PRODUCTS
bg
We have achieved nearly
quantitative agreement with
experiment with regard to
total cross section and degree
of vibrational excitation.
Here is plotted experimental
data from Belic et al., shifted
+0.34eV, along with our
results. The agreement in
magnitude and general shape
of these cross sections is
excellent.

Data from Belic, Landau, and Hall, J Phys
B 14, 175 (1981), squares, and Trajmar and
Hall, J Phys B 7, L458 (1974), circles.
The calculation of angular distributions presented in this figure is dependent upon certain
approximations which apply only to the production of H- + OH from the 2B1 resonant state.
H-
We are currently pursuing a full treatment of DA to water, including contributions from each of the
three resonance states. This treatment will take into account not only the existence of the three
resonance states but also the coupling among them: the Renner-Teller coupling between the 2B1
and 2A1 states which occurs at linear
geometry, as well as the conical intersection
between the 2A1 and 2B2 resonance states.
2B 2A
The figure to the right shows preliminary
2/
1
results of a Multiconfiguration SCF (MCSCF)
calculation on the three states, showing the
real part of the resonant PES’s at r1=r2=1.81
2A /2B
1
2
bohr as a function of bending angle. Here you
2B
1
2
2
see both the A1/ B1 degeneracy at linear
geometry and the conical intersection between
the 2A1 and 2B2 states at approximately 75
degrees.