Spectroscopic study of novel silicon and carbon clusters formed by a laser ablation technique

Abstract

Fourier transform infrared studies of small pure silicon and cyclic carbon clusters have been carried out by trapping the products of the pulsed laser ablation of silicon and $\sp{13}$C/$\sp{12}$C rods in solid Ar matrix at 10 K. Isotopic data obtained from each of the species studied enabled the unambiguous assignments. Comparison of these assignments with theoretical calculations yielded very good agreement. The two stretching modes of Si$\sb3$ with $C\sb{2\nu}$ symmetry in its ground state have been identified. Bands at 550.8 and 525.4 cm$\sp{-1}$ are assigned to the $\nu\sb1\ (a\sb1)$ and $\nu\sb3\ (b\sb2)$ modes of this cluster. The most intense IR active mode, $\nu\sb3\ (b\sb{1u}),$ of Si$\sb4$ with $D\sb{2h}$ symmetry in its ground state has been identified at 501.1 cm$\sp{-1}.$ Cyclic C$\sb6$ clusters has been made by the laser ablation technique and detected in an Ar matrix by FTIR isotopic studies. The most intense infrared active mode of cyclic C$\sb6$ with $D\sb{3h}$ symmetry, $\nu\sb4(e\sp\prime)=1694.9$ cm$\sp{-1}$ which is doubly degenerate, has been measured and assigned for the first time based on the excellent agreement of the frequency, isotopic shifts, and their relative intensities with the predictions from DFT calculations. Cyclic C$\sb6$ is the first cyclic carbon cluster so far discovered spectroscopically. The cyclic C$\sb8$ clusters with $C\sb{4h}$ symmetry has been detected in Ar matrices produced by laser ablation of $\rm\sp{12}C/\sp{13}C$ rods. Its most intense infrared active mode, $\nu\sb{12}(e\sb{u}),$ is doubly degenerate and mixes with two originally infrared inactive modes upon $\sp{13}$C substitution. A isotopic perturbation model (IPM) which combines DFT ab initio calculation with isotopic perturbation theory is used to solve this complex situation. Band observed at 1844.2 cm$\sp{-1}$ has been assigned to the $\nu\sb{12}(e\sb{u})$ mode. Cyclic C$\sb8$ is the second cyclic carbon cluster so far discovered in the TCU Molecular Physics Laboratory using the FTIR isotopic technique in concert with the results of ab initio calculations.