Vibrational analysis of amorphous carbon-nitrogen alloys by15Nand D isotopic substitution

The origin of the vibrational modes of $a-{\mathrm{CN}}_x$ alloys is investigated by infrared (ir) and Raman spectroscopies. Isotopic substitution of ${}^{15}\mathrm{N}$ and D is used in the identification of vibration modes. In nonhydrogenated materials containing low nitrogen concentration (⩽20 at. %), the ir and Raman spectra are not equivalent. Above this concentration (up to ∼30 at. %), the spectra became very similar to each other. On the contrary, for a suitable hydrogen content the spectra are quite similar, independently of the range of nitrogen concentrations studied. Quantum chemical Parametric Method 3 calculations are performed to obtain the frequencies and intensities of the ir active vibrations of organic molecules and large carbon-nitrogen clusters. These results are used to help in the identification of the features observed in the ir spectra. The importance of both the promotion of bond dipoles and symmetry breaking by nitrogen in the ir spectra is established.