Observation of persistent ir hole burning in the vibrational spectrum ofCN−in KBr

Both coherent and incoherent high-resolution ir techniques have been used to explore the vibrational-stretch-mode region (∼2000 ${\mathrm{cm}}^{\mathrm{-}1}$) of ${\mathrm{CN}}^{\mathrm{-}}$ matrix isolated in KBr single crystals. At low temperatures the spectrum consists of a number of sharp lines with inhomogeneous widths ∼0.05 ${\mathrm{cm}}^{\mathrm{-}1}$. We find that most of the complexity is produced by ${\mathrm{CN}}^{\mathrm{-}}$ centers which have an alkali-metal or halogen impurity ion nearby. Fourteen new centers have been identified. Some of these complex centers show persistent spectral hole production within the inhomogeneous line when the transitions are excited with a low-power ir diode laser. One extremely stable defect complex, ${\mathrm{CN}}^{\mathrm{-}}$:${\mathrm{Na}}^{+}$, has been investigated in detail both with vibrational hole burning and with fluorescence techniques. For low laser intensities and small defect densities the hole width is 50 MHz and from fluorescent decay curves the excited-state $T_1$ time is 10 msec, the same order of magnitude found for isolated ${\mathrm{CN}}^{\mathrm{-}}$ in KBr. By combining broadband Fourier transform interferometry with narrow-band diode laser hole burning, we are able to show that the persistent effect occurs when, during vibrational deexcitation, the ${\mathrm{CN}}^{\mathrm{-}}$ molecule flips by 180° between inequivalent energy configurations generated by the presence of the nearby ${\mathrm{Na}}^{+}$ ion.