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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msup><mml:mrow></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> <mml:annotation>${^{ - 1} }$</mml:annotation> </mml:semantics> </mml:math> and 117(1) cm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msup><mml:mrow></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> <mml:annotation>${^{ - 1} }$</mml:annotation> </mml:semantics> </mml:math> , of the first excited state with dephasing times of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>∼</mml:mo> <mml:mn>1</mml:mn></mml:mrow> <mml:annotation>${{\rm{ \sim }}1}$</mml:annotation> </mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>∼</mml:mo> <mml:mn>310</mml:mn></mml:mrow> <mml:annotation>${{\rm{ \sim }}310}$</mml:annotation> </mml:semantics> </mml:math> cm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msup><mml:mrow></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> <mml:annotation>${^{ - 1} }$</mml:annotation> </mml:semantics> </mml:math> active vibration when using a hole-burning technique and 1500 cm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msup><mml:mrow></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> <mml:annotation>${^{ - 1} }$</mml:annotation> </mml:semantics> </mml:math> C=C stretching modes.