Resonant and Nonresonant Hyperpolarizabilities of
Spatially Confined Molecules: A Case Study of Cyanoacetylene
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Abstract
In
this theoretical study we report on resonant and nonresonant electric-dipole
(hyper)polarizabilities of cyanoacetylene molecule confined by repulsive
potentials of cylindrical symmetry mimicking a topology of nanotubelike
carbon cages. The set of investigated electronic properties encompasses
dipole moment, polarizability, first and second hyperpolarizability
as well as the two-photon transition matrix elements. The effect of
external potential on vibrational contributions to electric-dipole
properties is also included in our treatment. The computations are
performed at several levels of theoretical approximation including
state-of-the-art coupled-cluster (CCSD(T)) and multireference configuration
interaction methods (MRCISD(Q)). The results of calculations presented
herein indicate that the decrease in dipole moment observed experimentally
for the HCCCN molecule solvated in helium nanodroplets may be partially
attributed to the confinement effects. The external confining potential
causes a substantial drop of the isotropic average electronic polarizability
and second hyperpolarizability. In contrast, the vector component
of the electronic first hyperpolarizability substantially increases.
Nuclear relaxation contributions to all studied electric-dipole properties
are found to diminish upon confinement. Our calculations also indicate
that the most intense <sup>1</sup>Σ<sup>+</sup> ← <i>X̃</i> one-photon transition is slightly blue-shifted
whereas the corresponding oscillator strength is virtually unaffected
upon confinement. Interestingly, the absolute magnitude of the diagonal
component of the second-order transition moment for the bright state
(<i>S</i><sub><i>zz</i></sub><sup>0→<sup>1</sup>∑<sup>+</sup></sup>) increases
with the strength of external potential. The effect of structural
relaxation on the electric-dipole properties, arising from the presence
of the external potential, is also investigated in the present work