Time-resolved spectroscopy ground electronic states

Time-Resolved Spectroscopy of Electronic Ground States... 

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Any reduction initiated by electron transfer might be completed by either H - or H+/e-transfer. In one case at least, it has been shown that the H+/e sequence would be followed. Photochemical reaction of fluorenone with NMAH (Peters et al., 1982) involves initial electron transfer from NMAH to singlet fluorenone yielding a contact radical ion pair, presumably identical to that expected from a hypothetical ground state reaction with initial electron transfer. Fast time-resolved spectroscopy shows that this decays to the ketyl-NMA radical pair by proton transfer. 

While there are many papers in the literature on the solvent relaxation in solutions of proteins, surfactant micelles, and phospholipid vesicles studied by time-resolved emission spectroscopy [25-27], similar studies focused on block copolymer micelles are scarce. In most cases, amphiphilic fluorescent probes localized in the inner part of the shells of amphiphilic block copolymer micelles in aqueous solutions were used for the studies. The studies reveal the heterogeneity of the binding sites of the probe that manifest itself by multiple-exponential fluorescence decays. In the case of block copolymer micelles, interpretation of the relaxation behavior can be complicated by redistribution of the probe molecules in the micelles during its excited-state lifetime of the probe [28]. The redistribution occurs as a result of the increased polarity of the excited probe as compared with its ground electronic state. 

In this paper we report the use of spectrally resolved two-colour three-pulse photon echoes to expand the information that can be obtained from time-resolved vibrational spectroscopy. The experiments allow the study of intramolecular dynamics and vibrational structure in both the ground and excited electronic states and demonstrate the potential of the technique for studying structural dynamics. 

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

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