Picosecond-Resonance Coherent Anti-Stokes Raman Scattering in Biophysics: Power Dependence in the Bacteriorhodopsin Photocycle

Abstract

Picosecond resonance coherent anti-Stokes Raman (PR/CARS) spectra in the C=C stretching region (1400–1640 cm⁻¹) of the BR photocycle are recorded with 8-ps pulses and measured as a function of w _l laser power. These PR/CARS data are quantitatively fit to third-order susceptibility (χ⁽³⁾) relationships to obtain the band origins, bandwidths, relative amplitudes, and electronic resonant phase factors of the observed features. Variations in the w _l power alters the relative populations of the primary BR photocycle intermediates from which PR/CARS is generated. For w _l pulse energies of 0.7 nJ or lower, PR/CARS scattering occurs from BR-570 only. An analysis of the BR photocycle species which may contribute to the PR/CARS spectra recorded at higher w _l energies is also presented. These results provide insight into how the w _l power controls these relative populations, an understanding of importance in the design of picosecond time-resolved CARS spectroscopy from the BR photocycle.