Optical excitation functions

Integral cross sections for selected electron-impact excitation and ionisation processes have been largely obtained by measuring optical excitation functions. These need to be corrected to a varying degree of accuracy for effects such as cascade contributions and photon polarisation. The details of the experimental procedures, sources of errors and data evaluation have been discussed by Heddle and Keesing (1968). 

The most useful measurements of optical excitation functions are made for the first dipole excitation of an atom, which has an allowed photon transition to the ground state. This excitation normally has a large cross section ai. Integral cross sections for other states may be determined relative to oi. 

Die optischen Anregungsfunktionen der Quecksilberlinien (Optical excitation functions of spectral lines of mercury), Z Phys. 62, 106-142 (1930) (by W. Schaflemicht). 

Figure 1. Potential energy plot of the reactants (precursor complex) and products (successor complex) as a function of nuclear configuration Eth is the barrier for the thermal electron transfer, Eop is the energy for the light-induced electron transfer, and 2HAB is equal to the splitting at the intersection of the surfaces, where HAB is the electronic coupling matrix element. Note that HAB << Eth in the classical model. The circles indicate the relative nuclear configurations of the two reactants of charges +2 and +5 in the precursor complex, optically excited precursor complex, activated complex, and successor complex.

INTRODUCTION. A standard and universal description of various nonlinear spectroscopic techniques can be given in terms of the optical response functions (RFs) [1], These functions allow one to perturbatively calculate the nonlinear response of a material system to external time-dependent fields. Normally, one assumes that the Born-Oppenheimer approximation is adequate and it is sufficient to consider the ground and a certain excited electronic state of the system, which are coupled via the laser fields. One then can model the ground and excited state Hamiltonians via a collection of vibrational modes, which are usually assumed to be harmonic. The conventional damped oscillator is thus the standard model in this case [1]. 

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). 

FIGURE 3 Room temperature PL and EL intensity as a function of pumping power comparing above and below bandgap optical excitation wavelengths. The monitor wavelength was 1.54 pm. 

Ideal semiconductor quantum dot structures should exhibit a delta-function-like (atomic-like) density-of-states for both electrons mid holes [4], Optical excitations in such structures are excitonic in nature, since an electron mid a hole confined in a quantum dot necessarily interact via their Coulomb interaction mid, therefore, form mi exciton. Consequently, a single electron-hole pair in a quantum dot corresponds to mi exciton, whereas doubly occupied electron and hole states (both spin states) correspond to a biexciton. Since the Coulomb interaction of the particles is inevitable, it makes no sense to distinguish between excitons mid free electrons mid holes within a quantum dot. Optical gain... 

A widely used type of multiscale simulation combines quantum mechanical and molecular mechanical (QM/MM) simulations. In this approach, the functional core of the molecular system, for example, the catalytic sites of an enzyme, is described at the electronic level (QM region), whereas the surrounding macromolecular system is treated using a classical description (MM region). Some of the biological applications for which QM/MM calculations have been widely utilized are chemical reactions in enzymes, proton transfer in proteins and optical excitations. In QM/MM... 

Fig. 7 Phosphorescence intensity as a function of time for a PSBF thin film after pulsed optical excitation and for two temperatures as indicated. The solid lines are exponential fits to the late part of the data sets yielding an estimate for the phosphorescence lifetime...

Fig. 24 Delayed fluorescence decays after pulsed optical excitation of PF2/6 as a function of temperature in several media as indicated. Details can be found in [24]...

Figure 9 displays the integrated ionization probability of H after pulsed optical excitation as a function of the maximum cycle-averaged laser-power density. Calculations have been performed for wavelengths between 80 and 590 nm and for pulse lengths between 10 and 30 fs (Fig. 8). All results (symbols and fitted thin curves) show a monotonous increase as a function of the power density I and nearly 100% ionization is reached for I = 5 X lO " W/cm. At low power densities the curves are proportional to...

The SIM technique uses the intensity infomiation in two dimensions (2D), which can enhance the sensor detection sensitivity. The intensity distribution in 2D is the function of the optical excitation and the boundary conditions of the optical fiber, whereby changing the boundary conditions results in intensity modulation in 2D [36]. It is known that in SIM technique applications, higher-order modes are excited by off-axis illumination of the optical fiber [16, 38]. Those modes have more interactions with the core/cladding interface therefore, they are more sensitive to changes in the refractive index of the cladding material. 

Proton transfer dynamics of photoacids to the solvent have thus, being reversible in nature, been modelled using the Debye-von Smoluchowski equation for diffusion-assisted reaction dynamics in a large body of experimental work on HPTS [84—87] and naphthols [88-92], with additional studies on the temperature dependence [93-98], and the pressure dependence [99-101], as well as the effects of special media such as reverse micelles [102] or chiral environments [103]. Moreover, results modelled with the Debye-von Smoluchowski approach have also been reported for proton acceptors triggered by optical excitation (photobases) [104, 105], and for molecular compounds with both photoacid and photobase functionalities, such as lO-hydroxycamptothecin [106] and coumarin 4 [107]. It can be expected that proton diffusion also plays a role in hydroxyquinoline compounds [108-112]. Finally, proton diffusion has been suggested in the long time dynamics of green fluorescent protein [113], where the chromophore functions as a photoacid [23,114], with an initial proton release on a 3-20 ps time scale [115,116]. 

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