Again, the intensity of the probe pulse is so weak
that the excited-state population is not affected appreciably by the excited-state absorption process. (4) A fourth possible contribution to the ΔA spectrum is given by product absorption. After TPCA-1 in vitro excitation of the photosynthetic, or more generally photobiological or photochemical system, reactions may occur that result in a transient or a long-lived molecular state, such as triplet states, charge-separated states, and isomerized states. The absorption of such Temozolomide a (transient) product will appear as a positive signal in the ΔA spectrum. A ground-state bleach will be observed at the wavelengths where the chromophore on which the product state resides has a ground-state absorption. A well-known example of such a transient product state is the accessory bacteriochlorophyll (BChl) anion in the bacterial reaction
center (RC), which acts as a transient intermediate in the electron transfer process from buy Vadimezan the primary donor P to the bacteriopheophytin (BPheo). The rise and decay of this species can be monitored through its specific product absorption at 1,020 nm (Arlt et al. 1993; Kennis et al. 1997a). Pulse duration, time resolution, and spectral selectivity Laser pulses as short as 5 fs are now available for transient absorption spectroscopy (see, e.g., Cerullo et al. (2002); and Nishimura et al. (2004)). A short pulse duration Δt implies a large spectral bandwidth Δv according to relation ΔtΔv = 0.44 for Gaussian-shaped pulses. This relation is known as the time–bandwidth product. For instance, a 10-fs pulse with a center wavelength of 800 nm has a spectral bandwidth of 4.4 × 1013 Hz at full-width at half maximum (FWHM), which corresponds to about 100 nm in this wavelength region. Thus, one has to make a trade-off between time resolution and spectral selectivity. Consider the example of the bacterial RC, which has the primary donor absorbing at 860 nm, the accessory BChls at 800 nm, and the BPheos at 760 nm. With a 10-fs pulse at 800 nm, one would simultaneously
excite all the cofactors. In order to selectively excite one of the cofactor pairs to study its excited-state PJ34 HCl dynamics, spectral narrowing to ~30 nm is required, which implies a longer excitation pulse of ~30 fs (Streltsov et al. 1998; Vos et al. 1997). For the photosystem II (PSII) RC, where the energy gaps between the pigments are significantly smaller, the excitation bandwidth has to be narrowed even more to <10 nm for selective excitation, with corresponding pulse durations of ~100 fs (Durrant et al. 1992; Groot et al. 1997). On very fast timescales, transient absorption signals have contributions from processes additional to those described in the previous section. These non-resonant contributions are often lumped together under the terms “coherent artifact” and “cross-phase modulation.