Femtosecond internal conversion

The aforementioned E — D nonadiabatic interactions also lead to efficient, i.e., femtosecond internal conversion processes which we now briefly address. The presence of such processes is indicated indirectly by experimental studies, where Bz+ has been prepared initially in the E state, but fragmentation been found to occur via lower (even the X) electronic states [45,46]. To describe this non-radiative decay... 

The present analysis relies on - and extends - the comprehensive theoretical study of Refs. [23,24] on the multi-state interactions in the manifold of the X — E states of Bz+. Like this recent work, it utilizes an ab initio quantum-dynamical approach. In Refs. [23,24] we have, in addition, identified strong coupling effects between the B — C and B — D electronic states, caused by additional conical intersections between their potential energy surfaces. A whole sequence of stepwise femtosecond internal conversion processes results [24]. Such sequential internal conversion processes are of general importance as is evidenced indirectly by the fluorescence and fragmentation dynamics of organic closed-shell molecules and radical cations [49,50]. It is therefore to be expected that the present approach and results may be of relevance for many other medium-sized molecular systems. 

The red line follows the progress of the reaction path. First, a butadiene compound b excited into its first excited state (either the cis or trans form may be used—we will be considering the cis conformation). What we have illustrated as the lower excited state is a singlet state, resulting from a single excitation from the HOMO to the LUMO of the n system. The second excited state is a Ag state, corresponding to a double excitation from HOMO to LUMO. The ordering of these two excited states is not completely known, but internal conversion from the By state to the Ag state i.s known to occur almost immediately (within femtoseconds). 

Seel, M., and Domcke, W. (1991), Femtosecond Time-resolved Ionization Spectroscopy of Ultrafast Internal-Conversion Dynamics in Polyatomic Molecules Theory and Computational Studies, J. Chem. Phys. 95,7806. 

Sin< recognition that antenna carotenoids have a low lying Sj electronic state it has l en generally assumed that EET to chi proceeds from this state. This is based largely on two considerations. 1. Spectral overlap between the carotenoid Si state and the Qy chi absorption transition is more favourable than for the S2 state. This assumption is however not supported by the recent study of Mimuro et al. [204] in which spectral overlap can be shown to be rather similar for a number of mainly Si and mainly Sj emitters. This point may therefore need further clarification. 2. Extremely fast Sj-Si internal conversion. Recent studies based on fluorescence yield measurements and also on femtosecond absorption measurements [200,205,207] indicate in vitro Sj-Si relaxation times of... 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

The time-scale of molecular vibrations is of the order of 10 13 s, just outside the ps range. Internal conversions and in particular vibrational cascades therefore fall into the femtosecond (10-15s) time-scale. However, the spin-forbidden processes of intersystem crossing take place in times of a few ps to several ns. The case of benzophenone is a good example of the compensation between spin and orbital angular momentum. The rise of the triplet state absorption shows that intersystem crossing is completed within some 20 ps. 

Current photochemical research is strongly linked with the study of photophysical behavior of excited particles. Data on photophysical processes (such as luminescence, internal conversion, intersystem crossing, intramolecular energy dissipation) assist photochemists in the identification and interpretation of chemical deactivation modes. Most of the data related to the elementary steps within deactivation of excited particles have been obtained by fast flash techniques in nano-, pico-, and femtosecond time domains. Photophysics is, in general, as rich a branch of science as photochemistry, and both the parts of excited-state research deserve comparable attention and extent. In the present review, some results on photophysics will be mentioned where suitable and necessary. We will restrict our discussion, however, predominantly to photochemical behavior of metallotetrapyrroles. 

D. R. Cyr and C. C. Hayden, /. Chem. Phys., 104,771 (1996). Femtosecond Time-Resolved Photoionization and Photoelectron-Spectroscopy Studies of Ultrafast Internal Conversion in 1,3,5-Hexatriene. 

Stock, G. and Domcke, W. (1990). Theory of femtosecond pump-probe spectroscopy of ultrafast internal conversion processes in polyatomic molecules, J. Opt. Soc. Am. B 7, 1971. 

Figure 5. A femtosecond pump-probe photoionization scheme for studying excited-state dynamics in DT. The molecule is excited to its S> electronic origin with a pump pulse at 287 nm (4.32 eV). Due to nonadiabatic coupling, DT undergoes rapid internal conversion to the lower lying Si state (3.6eV). The excited-state evolution is monitored via single-photon ionization. As the ionization potential is 7.29 eV, all probe wavelengths <417 nm permit single-photon ionization of the excited state.

Figure 8. Time-resolved photoelectron spectra revealing vibrational and electronic dynamics during internal conversion in DT. (a) Level scheme in DT for one-photon probe ionization. The pump laser prepares the optically bright state S2. Due to ultrafast internal conversion, this state converts to the lower lying state Si with 0.7 eV of vibrational energy. The expected ionization propensity rules are shown S2 —> Do + e (ei) and Si —> D + (b) Femtosecond time-...

These reactions follow the same mechanism [31], whereby the low-energy CT and TL excitations are followed by a femtosecond ISC to the CT state that undergoes an internal conversion to a L-localized triplet state 11.(1.), from which a parallel isomerization and a return to the ground state occur, Fig. 13. This mechanism has several remarkable aspects ... 

Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast Internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example . The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge... 

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