Picosecond Coherent Raman Scattering (ps CARS) is used to study vibrational dynamics in low temperature hydrogen bonded crystals including various amino acids and peptides. At low temperature this technique can be used to determine the vibrational lifetime T1 when the ps CARS decay is exponential, and a lower limit to T1 when it is not. Vibron (Ω = 150-1600 cm-1) lifetimes are less than 10 ps, whereas librons (Ω = 30-120 cm-1), which are torsional oscillations of the amino acid or peptide chains, have long lifetimes (10 ps < T1 < 5 ns). The theory of anharmonic processes is used to calculate the frequency dependence of T1 for low temperature librons. Most crystals show T1 ∝ Ω-4, which is characteristic of spontaneous decay to two counterpropagating acoustic phonons at Ω/2. The crystal l-alanyl-l-tyrosine · 3H2O shows T1 ∝ Ω-2, which is characteristic of decay to one libron and one acoustic phonon. This mechanism dominates when the unit cell is large and complex. The temperature dependent ps CARS data is discussed and is consistent with a mechanism involving absorption of acoustic phonons. Finally the relationship between the crystal dynamics and protein dynamics, and preliminary results on crystalline lysozyme are discussed.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry