Wavelength selective modulation in femtosecond pump-probe spectroscopy andits application to heme proteins

We demonstrate novel lock-in detection techniques, using wavelength selecti ve modulation of ultrafast pump and probe laser pulses, to discriminate bet ween vibrational coherence and electronic population decay signals. The tec hnique is particularly useful in extracting low frequency oscillations from the monotonically decaying background, which often dominates the signal in resonant samples. The central idea behind the technique involves modulatin g the red and/or blue wings of the laser light spectrum at different freque ncies, Omega (R) and Omega (B), followed by a lock-in detection at the sum or difference frequency, Omega (R)+/- Omega (B). The wavelength selective m odulation and detection discriminates against contributions to the pump-pro be signal that arise from degenerate electric field interventions (i.e., on ly field interactions involving different optical frequencies are detected) . This technique can be applied to either the pump or the probe pulse to en hance the off-diagonal terms of the pump induced density matrix, or to sele ct the coherent components of the two-frequency polarizability. We apply th is technique to a variety of heme-protein samples to reveal the presence of very low-frequency modes (similar to 20 cm(-1)). Such low-frequency modes are not observed in standard pump-probe experiments due to the dominant sig nals from electronic population decay associated with resonant conditions. Studies of the diatomic dissociation reaction of myoglobin (MbNO --> Mb+NO) , using wavelength selective modulation of the pump pulse, reveal the prese nce of an oscillatory signal corresponding to the 220 cm(-1) Fe-His mode. T his observation suggests that the spin selection rules involving the ferrou s iron atom of the heme group may be relaxed in the NO complex. Mixed iron spin states associated with adiabatic coupling in the MbNO sample could exp lain the fast time scales and large amplitude that characterize the NO gemi nate recombination. (C) 2001 American Institute of Physics.