Transient-hole burning in the infrared spectrum of a polymer with intense picosecond pulses

Transient spectral holes of 70-${\mathrm{cm}}^{\mathrm{-}1}$ width are observed in the OH stretching band (∼3500 ${\mathrm{cm}}^{\mathrm{-}1}$) of hydrogen-bonded poly(vinyl butyral) films at room temperature. The lifetime of the holes is measured to be $T_1$=5±2 ps and interpreted as the population lifetime of the OH stretching vibration. Simultaneously inverse holes with a red shift of 210±10 ${\mathrm{cm}}^{\mathrm{-}1}$ and the same lifetime $T_1$ are observed and explained by excited-state absorption. The inhomogeneous broadening of the OH absorption band presents direct evidence for site-specific frequency shifts of the hydrogen bridges. Breaking of H bonds and establishment of a new thermal equilibrium is indicated with time constant τ=15±5 ps by blue-shifted induced absorption around the monomeric-frequency position.