Photoexcitation, processes following

A technique which combines the high sensitivity of resonant laser ionization methods with the advantages of nonlinear coherent Raman spectroscopy is called IDSRS (ionization detected stimulated Raman spectroscopy). The excitation process, illustrated in Figure 5, can be briefly described as a two-step photoexcitation process followed by ion/electron detection. In the first step two intense narrow-band lasers (ct L, 0) ) are used to vibrationally excite the molecule via the stimulated Raman process. The excited molecules are then selectively ionized in a second step via a two- or multiphoton process. If there are intermediate resonant states involved (as state c in Figure 5), the method is called REMPI (resonance enhanced multi-photon ionization)-detected stimulated Raman spectroscopy. The technique allows an increase in sensitivity of over three orders of magnitude because ions can be detected with much higher sensitivity than photons. 

The reason why the colour of MnC>4- is so intense follows from the unusual way in which the electron changes its position. There are no restrictions (on a quantum-mechanical level) to the photo-excitation of an electron, so the probability of excitation is high. In other words, a high proportion of the MnC>4- ions undergo this photoexcitation process. Conversely, if a photo-excited charge does not move spatially, then there are quantum-mechanic inhibitions, and the probability is lower. 

Reflecting personal preferences, we focus in this review on the modeling of ultrafast bound-state processes following photoexcitation such as electron transfer, internal-conversion via conical intersections, and nonadiabatic... 

The primary process following a photoexcitation of nltrosamldes XIV Is the dissociation of the N-N bond to form a radical pair XV and the ensuing chemical events are the reactions of amldyl and nitric oxide radicals In the paired state or Individually In the bulk of solutions. Naturally, secondary reactions, thermal or photolytic, have to be taken Into consideration under Irradiation conditions (21). First of all, the relatively straightforward chemistry of selective excitation In the n-ir transition band (>400 nm) will be discussed, followed by the chemistry of Irradiation with a Pyrex filter (>280 nm). As nitric oxide Is known to be rather unreactlve (23), primary chemical processes In the Irradiation with >400 nm light under... 

In general, a thorough spectroscopic study, as routinely carried out in the group of Prof. Dr. Dirk M. Guldi by means of steady-state emission/absorption measurements and time-resolved techniques in numerous solvents, sheds light onto the photophysical processes following photoexcitation of these systems. Equally, a detailed description of the employed spectroscopic methods will be given in the next sections. 

Additionally, we speculated that the lowest OH internal excitations (i.e., v = 0, low N) might derive preferentially from higher-than-binary complexes and/or a mechanism that involves a five-atom HO(Br)CO intermediate produced by a multicenter process following photoexcitation (Hoffmann et al. 1990). 

Electrochemical excitation, photochemistry without light, exhibits many phenomena that are unique to ECL as compared to photochemistry. The efficient production of emission from excimers or exciplexes as compared to excited monomers, efficient generation of excited triplet states, and intense delayed fluorescence caused by triplet-triplet annihilation are the most typical examples. On the other hand, the method offers a chance to populate the excited states that are inaccessible by the processes following photoexcitation. 

FIGURE 1.4. Basic processes following photoexcitation of a -conjugated molecule or polymer. 

The goal of this review is to critically compare — from both a concep-tional and a practical point of view — various MQC strategies to describe non-Born-Oppenheimer dynamics. Owing to personal preferences, we will focus on the modeling of ultrafast bound-state processes following photoexcitation such as internal-conversion and nonadiabatic photoisomerization. To this end, Sec. 2 introduces three model problems Model I represents a three-mode description of the Si — S2 conical intersection in pyrazine. Model II accounts for the ultrafast C B X internal-conversion process in the benzene cation, and Model III represents a three-mode description of ultrafast photoisomerization triggered by a conical intersection. Allowing for exact quantum-mechanical reference calculations, all models have been used as benchmark problems to study approximate descriptions. 

We have reviewed a series of investigations of 7T-conjugated molecules using mixed quantum-classical dynamics simulations. AH these molecules have been chosen as models for biologically relevant systems. Dynamics simulations are able to provide information on time-dependent phenomena, which can only be obtained in a very indirect way by conventional static quantum-chemistry simulations. The main pieces of information brought by dynamics in excited states are the time constants for the relaxation processes following the photoexcitation and the relative importance of each available pathway. 

The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

It has been demonstrated that the whole photoexcitation dynamics in m-LPPP can be described considering the role of ASE in the population depletion process [33], Due to the collective stimulated emission associated with the propagation of spontaneous PL through the excited material, the exciton population decays faster than the natural lifetime, while the electronic structure of the photoexcited material remains unchanged. Based on the observation that time-integrated PL indicates the presence of ASE while SE decay corresponds to population dynamics, a numerical simulation was used to obtain a correlation of SE and PL at different excitation densities and to support the ASE model [33]. The excited state population N(R.i) at position R and time / within the photoexcited material is worked out based on the following equation ... 

Figure 7.5 Schematic presentation of photoactivation and relaxation processes in a CdSe quantum dot aggregate (a) surface-passivation of photoexcited quantum dots by solvent molecules or dissolved oxygen, (b) thermal activation followed by the formation ofa stabilized state, (c) the formation of deep-trap states, (d) non-radiative relaxation of deep-...

The major relaxation processes of 2P - 2S photoexcited Cu atoms in rare gas matrices are summarized in the following scheme (34). 

It is proposed that the B-state of Cu2 (bound in the gas phase) (57) is sufficiently strongly destabilized in the matrix to the extent that it is unstable with respect to dissociation to Cu(2D3/2) + Cu(2S1/2) fragments following photoexcitation of CU2 from the ground state, process (1) in above scheme. The extent to which the dissociation actually occurs depends on the local dynamics following photoexcitation and the details of the Cu2 rare gas potentials for the specific trapping site involved. 

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