Absorption and Excited-State Coherences of Cryogenically Cold Retinal Protonated Schiff Base in Vacuo.

Retinal protonated Schiff base (RPSB), found in its all-trans conformer in Bacteriorhodopsin, undergoes barrier-controlled isomerization upon photoabsorption through polyene- chain torsion. The effects of the protein environment on the active vibrations during photoabsorption and their redistribution are still not understood. This paper reports on femtosecond time-resolved action-absorption measurements of cryogenically cooled gas-phase all-trans RPSB, which exhibit two coherent vibrational oscillations, 167(14) cm-1 and 117(1) cm-1, of the first excited state with dephasing times of ∼ 1 ps. The absence of the high-frequency vibration in solution and the low-frequency vibration in the protein indicates that these vibrations are sensitive to environments. An action-absorption spectrum of cryogenically cold all-trans RPSB, reveals a ∼ 310 cm-1 active vibration when using a hole-burning technique and 1500 cm-1 C=C stretching modes.