Precursor-successor modelling

The considerable changes in the form of the pump-probe and dump-probe spectra, the existence of isosbestic regions, and total area changes indicate population relaxation to be involved in the spectral relaxation of DCM after photoexcitation. The spectral dynamics in strongly dipolar solvents after 0.4 - 0.6 ps are attributed mainly to solvent reorientation (4.1.3). In consequence, the investigation of a possible photoreaction concentrates on the early time window. The time-resolved spectra were modelled by a global fitting procedure assuming a precursor-sucessor relationship between two spectrally distinct species. This kinetic model is rather simple; it was chosen because of its descriptive character rather than for any physical reason, as its parameter τ reveals information about the relevant timescale of the underlying population changes for each experiment.

Presuming spectral independence of the apparatus function, the bilinear structure of the time-dependent spectra can be used to model them as the product of a matrix K containing

the kinetics and a matrix A given by the wavelength-dependent amplitudes of the species associated with each kinetic trace [Ruth 98]:

M_λt ≅ A K_λs ⋅ _st . (4.3) Here K_st is the convolution of the instrument response function R(t) with the time evolution E(t) of the species s :

K_st = R(t)⊗ E (t)_s , (4.4) and A_λsis the spectrum of that species :

A_λs = A ( )_s λ . (4.5) The matrix of the residuals S_λt depends linearly on the spectral amplitudes A :

S_λt =M_λt - A K_λs ⋅ _st . (4.6) For any given matrix K they can be determined by multiple linear regression. The nonlinear parameters defining the time evolution E (t)_s may be obtained independently of the amplitudes by a Simplex optimization [Press 92], whereas a gradient-based nonlinear fitting algorithm such as the Levenberg-Marquardt algorithm implicitly takes the gradient dependence on the linear parameters into account.

Corresponding to precursor-sucessor kinetics, the functions E (t)_s were set to be an exponential decay and exponential growth with the same time coefficient τ. This can be reduced to a constant and an exponential term, yielding two wavelength-dependent amplitudes which have to be combined to the spectrum of the precursor component:

A ( ) exp (-t / ) + A ( ) (1 - exp (-t / ) ) A ( ) exp (-t / ) + A ( )

Exemplary fits to the dynamic isolated absorption spectra of DCM in acetonitrile are displayed in Figure 4.1-25 along with the data. Figure 4.1-26 contains the spectra for the precursor and successor components. It should be recognized that for the dump-probe as well as for the pump-probe measurements, the precursor spectra are structured and broader than the successor spectra. A comparison with precursor-successor modelling of the directly measured dump-probe spectra yielded a difference of 0.01 ps in the time coefficient and a spectral difference of the two amplitudes very close to those from fits to the isolated absorption spectra.

400 450 500 550 600 650 700 0.00

0.01 0.02 0.03

∆

1 0 0 , 2 5 0 , 5 0 0 fs

λ

Figure 4.1-25 : Fit (solid lines) with precursor-successor modelling to isolated transient ground state absorption spectra (squares) of DCM in acetonitrile.

40 0 45 0 50 0 55 0 60 0 65 0 70 0

-0 .02 0.00 0.02 0.04

S u cce sso r P recu rsor

∆

λ

Figure 4.1-26 : Precursor and successor spectra from the analysis of isolated absorption spectra of DCM in acetonitrile of the previous figure.

400 450 500 550 600 650 700 -0.12

-0.08 -0.04 0.00

∆^{O D}

50, 21 0, 360 fs

λ ^{/ nm}

40 0 45 0 50 0 55 0 60 0 65 0 70 0

-0 .12 -0 .08 -0 .04 0.00

S u cce sso r P recu rso r

∆^{O D}

λ ^{/ nm}

Figure 4.1-27 : a) Fit (solid lines) to isolated stimulated emission spectra of DCM in methanol (squares). b) Pecursor and successor spectra as obtained from analysis in a).

All optimizations were performed limiting the data to delays up to one picosecond, after which the continous spectral shift in strongly dipolar solvents demonstrated in 4.1.2 and 4.1.3 prevents the application of a kinetic model with species characterized by constant spectra. Precursor-successor modelling could also describe the time-dependent isolated emission spectra (Figure 4.1-27).

The time coefficients obtained from the precursor-successor modelling are compared in Table 4.1.1 and Table 4.1.2 for dump-probe and pump-probe measurements to the values

from exponential fits to the time-dependent bandintegral (see 4.1.4). For the pump-probe measurements, the variation of the relaxation times is less than for those from the integrated area. The mean values are in the range of 0.21 - 0.27 ps and close to the population dynamics indicated by the bandintegral evolution, except for tetrachloromethane, where τ≈ 0.16 ps. The relaxation times from modelling the isolated absorption spectra vary by 0.13 ps for different measurements, but are mostly around 0.3 ps for all solvents investigated.

In conclusion of section 4.1., the spectral response of DCM in strongly dipolar solvents after photoexcitation to the electronic excited state or to the ground state can be explained by solvent reorientation from 0.4 - 0.6 ps onwards.

For excited state relaxation, population changes of DCM were observed with time coefficients of approx. 0.2 ps from changes in the integrated intensity. The relative amplitude of the intensity changes grows with excitation energy. For acetonitrile, the characteristic time coefficient also increases with excitation energy. Modelling of the time-dependent spectra in the first picosecond with a precursor-successor kinetic scheme yielded a nearly solvent- and excitation energy-independent time coefficient of around 0.23 (_±0.04) ps (averaged over all solvents). In the unpolar solvents, this relaxation of DCM seems to continue on a longer (picosecond) timescale.

The time-dependent intensity of the dump-probe spectra in strongly dipolar solvents exhibits solvent-dependent, multiple relaxation times, but the largest changes are within the first 0.2 - 0.6 ps. The latter are assigned to ground state population relaxation. The transient ground state absorption spectra within the first picosecond were modelled with a precursor-successor kinetic scheme as for the excited state, yielding also a nearly solvent-independent time coefficient of 0.28 (_± 0.07) ps (again, averaged over all solvents). The smaller intensity changes in the dump-probe spectra on a picosecond timescale are solvent-specific and follow the spectral response function, indicating transition moment or population changes with solvent reorientation.