title: The Primary Photochemistry of Vision Occurs at the Molecular Speed Limit.
abstract: Ultrafast photochemical reactions are initiated by vibronic transitions from the reactant ground state to the excited potential energy surface, directly populating excited-state vibrational modes. The primary photochemical reaction of vision, the isomerization of retinal in the protein rhodopsin, is known to be a vibrationally coherent reaction, but the Franck-Condon factors responsible for initiating the process have been difficult to resolve with conventional time-resolved spectroscopies. Here we employ experimental and theoretical 2D photon echo spectroscopy to directly resolve for the first time the Franck-Condon factors that initiate isomerization on the excited potential energy surface and track the reaction dynamics. The spectral dynamics reveal vibrationally coherent isomerization occurring on the fastest possible time scale, that of a single period of the local torsional reaction coordinate. We successfully model this process as coherent wavepacket motion through a conical intersection on a ∼30 fs time scale, confirming the reaction coordinate as a local torsional coordinate with a frequency of ∼570 cm-1. As a result of spectral features being spread out along two frequency coordinates, we unambiguously assign reactant and product states following passage through the conical intersection, which reveal the key vibronic transitions that initiate the vibrationally coherent photochemistry of vision.
authors.affiliation: Departments of Chemistry & Physics, University of Toronto , 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.
authors.forename: Philip J M
authors.initials: PJM
authors.lastname: Johnson
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands.|Max Planck Institute for the Structure and Dynamics of Matter , Atomically Resolved Dynamics Division, Luruper Chaussee 149, Hamburg 22761, Germany.
authors.forename: Marwa H
authors.initials: MH
authors.lastname: Farag
authors.affiliation: Departments of Chemistry & Physics, University of Toronto , 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.
authors.forename: Alexei
authors.initials: A
authors.lastname: Halpin
authors.affiliation: Department of Biochemistry, University of Toronto , 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Takefumi
authors.initials: T
authors.lastname: Morizumi
authors.affiliation: Max Planck Institute for the Structure and Dynamics of Matter , Atomically Resolved Dynamics Division, Luruper Chaussee 149, Hamburg 22761, Germany.
authors.forename: Valentyn I
authors.initials: VI
authors.lastname: Prokhorenko
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors.forename: Jasper
authors.initials: J
authors.lastname: Knoester
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors.forename: Thomas L C
authors.initials: TLC
authors.lastname: Jansen
authors.affiliation: Department of Biochemistry, University of Toronto , 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada.|Department of Molecular Genetics, University of Toronto , 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Oliver P
authors.initials: OP
authors.lastname: Ernst
authors.affiliation: Departments of Chemistry & Physics, University of Toronto , 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.|Max Planck Institute for the Structure and Dynamics of Matter , Atomically Resolved Dynamics Division, Luruper Chaussee 149, Hamburg 22761, Germany.
authors.forename: R J Dwayne
authors.identifier: 0000-0003-0884-0541
authors.initials: RJD
authors.lastname: Miller