Early time dynamics

Tamai and Masuhara [26] also worked on NOSH, but in 1-butanol. They could examine femtosecond dynamics for the C—O bond breaking and formation of a primary photo-product X, which formed within 1 psec and had a broad absorption with peaks at 450 and 700 nm. The spectrum of X then evolved, forming a broad merocyanine-type spectrum, which itself evolved with time to form the usual merocyanine spectrum in that solvent after less than 400 psec. The spectral broadening was said to be either due to the formation of a vibrationally hot ground state or to an equilibration between isomeric forms because the spectrum that formed at early times was similar to the spectrum usually obtained in cyclohexane. Tamai s spectra are shown in Fig. 3. 

Other crossover behavior can arise when one moves to a regime where the continuum picture is not valid. For examples, Giesen-Seibert et al. (1995) show that for PD, at very early times w behaves like t rather than t " because the dynamics are dominated by random walks of kinks. In their simulations the effective exponent decreases smoothly with increasing temperature, with no evident crossover in any of the fixed-Tlog-logplots of w vs. t. They also show how to take into account fast events, viz. rapid, inconsequential... 

We present a preliminary study on the structural dynamics of photo-excited iodine in methanol. At early time delays after dissociation, 1 - 10 ns, the change in the diffracted intensity AS(q, t) is oscillatory and the high-q part 4 -8 A 1 is assigned to free iodine atoms. At later times, 10-100 ns, expansive motion is seen in the bulk liquid. The expansion is driven by energy released from the recombination of iodine atoms. The AS(q, t) curves between 0.1 and 5 (is coincide with the temperature differential dS/dT for static methanol with a temperature rise of 2.5 K. However, this temperature is five times greater than the temperature deduced from the energy of dissociated atoms at 1 ns. The discrepancy is ascribed to a short-lived state that recombines on the sub-nanosecond time scale. 

Fig. 5. (a) Kinetics recorded for peridinin in methanol at the ICT state emission maximum at 950 nm after excitation at 425 nm (squares), 500 nm (circles), 525 nm (triangles) and 550 nm (stars). All kinetics are normalized and corresponding fits of the kinetics are represented by solid lines. Enlargement of the early time dynamics is shown in inset, (b) Dependence of peridinin lifetime on excitation wavelength in different solvents. The lifetimes are normalized to the longest observed lifetime in the corresponding solvents. 

Figure 2a compares the time-resolved Stokes shift of the normal sequence and the abasic sequence. For ease of comparison, the data is shifted to overlap the sequences at early times. In the first nanosecond, the Stokes shifts from both sequences overlap almost perfectly. This results suggests that there is not a large scale collapse of the normal DNA structure at the abasic site. However after 1 ns, the abasic sequence has additional dynamics beyond those of the normal sequence. The fit of the abasic sequence has the same logarithmic component of the normal sequence fit, but with an additional exponential term for the fast rise in the Stokes shift after 1 ns S(t) = S0 + A0 logl0(t/t0) + 4,(l-exp(-f/r)), with an exponential time constant r of 25 ns. 

To sum up, this chapter has endeavored to show that chemical processes in solution often proceed in a deterministic fashion over chemically significant distances and time scales. Ultrafast spectroscopy allows real-time observation of relative motions even when spectra are devoid of structure and has stimulated moleculear level descriptions of the early time dynamics in liquids. The implication of these findings for theories of solution phase chemical reactions are under active investigation. 

Since the factorization of H into H[j res effective Hamiltonian blocks is approximate, each polyad res will describe the low-resolution spectrum of each polyad and the early-time intrapolyad dynamics. Since the acetylene DF spectrum has the special property that each g,l = 0+, 2 polyad is illuminated by exactly one known ZOBS, for example, (0, n, 0,2m0>2,00)0,2, the polyad is specified by the ZOBS quantum numbers... 

What we see in a low-resolution DF spectrum is the early-time dynamics of the ZOBS. In other words, what region of V(Q) does the ZOBS explore... 

Figure 27 shows p(z, t) calculated for BA with the appropriate static and dynamic parameters for BA in the polar aprotic solvent, propylene carbonate. The results show how the charge transfer in Sj occurs. At early times p(z, t) is highly peaked near zero z, where the LE probability is high. As time progresses, the charge transfer occurs and the system approaches the equilibrium configuration (see Section III.A). 

In Fig. 25 the dynamics of the characteristic length scales lx and ly is presented for the case a = 0.05 > ac. Typical snapshots are shown in Fig. 26. The peculiar nonmonotonic behavior of lx at early times can be understood as follows in the linear range the noise-initiated fastest mode grows exponentially as i//oexp(f/4) and... 

A case of solvent-driven electronic relaxation has been observed [76] for [Re(Etpy)(CO)3(bpy)]+ in ionic liquids TRIR spectra have shown at early times a weak signal due to the II. state, in addition to much stronger bands of the 3MLCT state. Although no accurate kinetic data are available, the II. state converts to MI.CT with a rate that is commensurate with the solvent relaxation time. Fluorescence up-conversion provided an evidence [10] for population of an upper II. state in MeCN, which converts to CT with a much faster lifetime of 870 fs (Table 1). The solvent dynamic effect on the 3IL—>3CT internal conversion can be rationalized by different polarities of the II. and JCT states, Fig. 11. The solvent relaxation stabilizes the 3CT state relative to II., driving the conversion. 

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