Femtosecond measurements of geminate recombination in heme proteins

This chapter discusses the femtosecond measurements of geminate recombination in heme proteins. The disappearance of the unliganded (Hb) species, nitrogen (N) (t), is monitored by probing the amplitude of the induced transient absorption in the 438- nm region. Because the maximum achievable yield of dissociation with femtosecond light pulses is only 15 to 30% in order to avoid nonlinear processes (such as stimulated Raman or continuum generation within the solvent), the population of the deoxy species is evaluated by measuring the transient difference spectra ΔA(t) at different time delays. The underlying assumption is that the observed spectral changes in the investigated spectral and time windows are because of only ligand rebinding. Any optoelectronic device is too slow to record directly spectroscopic changes occurring on the femtosecond or picosecond time scales; the required time resolution therefore is obtained by utilizing two separate femtosecond pulses.