  We have performed first principles electron-correlated calculations employing
large basis sets to optimize the geometries, and to compute linear optical
absorption spectra of various low-lying conformers of silicon hydrides:
Si$_{2}$H$_{2n}$, $n=1,2,3$. The geometry optimization for various isomers was
carried out at the coupled-cluster singles-doubles-perturbative-triples
{[}CCSD(T){]} level of theory, while their excited states and absorption
spectra were computed using a large-scale multi-reference singles-doubles
configuration-interaction (MRSDCI) approach, which includes
electron-correlation effects at a sophisticated level. Our calculated spectra
are the first ones for Si$_{2}$H$_{2}$ and Si$_{2}$H$_{4}$ conformers, while
for Si$_{2}$H$_{6}$ we obtain excellent agreement with the experimental
measurements, suggesting that our computational approach is reliable. Our
calculated absorption spectra exhibit a strong structure-property relationship,
suggesting the possibility of identifying various conformers based on their
optical absorption fingerprints. Furthermore, we have also performed geometry
optimization for the selected optically excited states, providing us insights
into their character.
