Femtosecond stimulated Raman spectroscopy of ultrafast biophysical reaction dynamics

Access restricted to the OSU CommunityI have developed the technique of femtosecond stimulated Raman spectroscopy (FSRS), which enables the rapid acquisition of vibrational spectra with < 100-fs time-resolution and < 15-cm⁻¹ frequency-resolution. FSRS uses three laser pulses: (1) a femtosecond visible actinic pump that initiates the photochemistry, (2) a narrow bandwidth picosecond Raman pump that provides the energy for amplification of the probe, and (3) a femtosecond continuum probe that is amplified at Raman resonances shifted from the Raman pump. FSRS has the ability to collect Raman spectra and depolarization ratios with only seconds of data averaging and negligible fluorescence interference. The capabilities of FSRS are explored through studies of the polyene [beta]-carotene. My initial experiments used picosecond time-resolved Stokes and anti-Stokes spontaneous resonance Raman spectroscopy to determine that vibrational relaxation in the S₁ (2A[subscript g]⁻) electronic state is nearly complete within 2 ps and to quantify the intramolecular vibrational energy redistribution (IVR) processes in S₀. FSRS studies on [beta]-carotene revealed that following optical excitation to S₂ (1B[subscript u]⁺) the molecule relaxes to S₁ in 160 fs where it undergoes rapid two-step IVR with 200- and 450-fs time constants. In later work, the FSRS spectrum of S₂ [beta]-carotene was observed, which consists of three intense and broad bands at ~1100, 1300 and 1650 cm⁻¹ that exhibit kinetics matching the decay of the S₂ near-infrared absorption. These data show that there is no additional intermediate 1B[subscript u]⁻ electronic state involved in the relaxation pathway of [beta]-carotene. FSRS was also used to study the photoisomerization dynamics in bacteriorhodopsin (bR). Spectra obtained during bR's excited state lifetime exhibit dispersive lineshapes at the ground-state frequencies that decay in 250 fs and are attributed to a nonlinear emission process. This relaxation is significantly faster than the decay of the stimulated emission (~500 fs), indicating that the excited population moves away from the ground-state geometry in 250 fs. Spectral changes between 1.5 to 100 ps reveal that a significant fraction of the isomerization occurs on the ground state photoproduct surface. The many benefits FSRS will make it a valuable tool for vibrational spectroscopy of reaction dynamics in ultrafast photochemical and photophysical processes