Following Resonant Compound States after Electron Attachment - Radiation Damage in Biomolecular Systems - page 80

2 ˙ resonance found for a stretched geometry (CH

1.6 A, Fig. 4.3 Computed features of the D

1.7 A) of linear HCN. Resonance energy E res CN D D 7.714 eV and its width res .R/ D 0:53 eV.

See main text for further details

fragments, along the linear stretching motion of either of them, computed with the
same accuracy of our HCN wavefunction. One clearly sees the greater strength of
the CN bond and its reduced extension compared with CH when supporting an equal
number of vibrational levels (five each in the picture). With this information we can
now analyse more accurately the effects on resonant features upon stretching (and,
later, bending) of these two bonds. One result presented by the data of Fig. 4.3
is given by the appearance of a resonance around 7 eV when both CH and
CN bonds are stretched: the features of the scattering wavefunction are again
similar to those of the 3rd virtual orbital from the neutral calculations, although
the corresponding eigenvalue is 0.53 eV. The additional electron density due to
the trapped particle is extended at this point over the whole molecular space. The
resonance position is found from our calculations to be higher than experiments by
nearly 1 eV, as it also occurred with the resonance.
To summarize, our model calculations find the two resonances seen by experi-
ments, albeit each at an higher energy. We also find that the 2 ˙ resonant state only
appears after stretching both bonds in a linear fashion.

4.3.2 Stretching the C-H and C N bonds

The previous computational studies of the resonances of the present example [ 14 , 16 ]
indicated that the DEA process which produced the CN C H fragmentation orig-

inated from the couplings between 2 ˘ and 2 ˙ states during bending deformations
which undergo avoided crossing between adiabatic states and show decay along
the 2 A 0 ( -like) potential energy curve [ 16 ]. On the other hand, the CN stretching
would lead to HC . 3 ˙ / and N( 4 S ) fragments coming now directly via the 2 ˙
resonance [ 13 ]. It is therefore useful to see how the present modelling could be
employed to follow the resonances' behaviour upon stretching both bonds, thereby
creating a 2-dimensional PES and also examining the spatial modifications of the
associated excess electron wavefunctions upon bond deformation.