Term internal conversion

Should the isomerization route be shown to be important in some molecules, the question arises as to whether the term internal conversion should be used to describe this kind of relaxation. It may be worthwhile to distinguish between physical and chemical processes by confining the term internal conversion purely to the physical process, and coining a new term for the isomerization route. However, until the isomerization route has been definitely substantiated by careful quantitative work, this is a moot point. 

According to Figure 5.1 the term internal conversion (IC) is used for a transition or T,I T between two isoenergetic vibrational levels denoted... 

The term internal conversion describes interinolecu-lar processes by which a molecule passes to a lower-energy electronic state without emission of radiation. These processes are neither well defined nor well understood. but they are often highly ellicienl. 

Polymer photophysics is determined by a series of alternating odd (B ) and even (Ag) parity excited states that correspond to one-photon and two-photon allowed transitions, respectively [23]. Optical excitation into either of these states is followed by subpicosecond nonradiative relaxation to the lowest excited state [90]. This relaxation is due to either vibrational cooling within vibronic sidebands of the same electronic state, or phonon-assisted transitions between two different electronic states. In molecular spectroscopy [146], the latter process is termed internal conversion. Internal conversion is usually the fastest relaxation channel that provides efficient nonradiative transfer from a higher excited state into the lowest excited state of the same spin multiplicity. As a result, the vast majority of molecular systems follow Vavilov-Kasha s rule, stating that FT typically occurs from the lowest excited electronic state and its quantum yield is independent of the excitation wavelength [91]. 

The excited state Si can also return to So without the emission of a photon. The excess energy is usually given off as heat to the medium. This energy wasting process is termed internal conversion (IC). Conceptually, it is similar to a productive photochemical reaction, and so we delay further discussion of IC until we are ready to discuss reactions. 

The radiationless transition between two states of same spin is called internal conversion, the one occuring with inversion of spin being termed intersystem crossing. In both processes the excess energy is liberated as heat. All these transitions between different electronic states are customarily preceded by vibrational relaxation, i.e. the deactivation from a higher vibronie level to the v0-level of the same electronic state (Fig. 5). 

A term in photochemistry and photophysics describing an isoenergetic radiationless transition between two electronic states having different multiphcities. Such a process often results in the formation of a vibrationally excited molecular entity, at the lower electronic state, which then usually deactivates to its lowest vibrational energy level. See also Internal Conversion Fluorescence... 

A subsequent picosecond electronic absorption spectroscopic study of TPE excited with 266- or 355-nm, 30-ps laser pulses in cyclohexane found what was reported previously. However, in addition to the nonpolar solvent cyclohexane, more polar solvents such as THF, methylene chloride, acetonitrile, and methanol were employed. Importantly, the lifetime of S lp becomes shorter as the polarity is increased this was taken to be evidence of the zwitterionic, polar nature of TPE S lp and the stabilization of S lp relative to what is considered to be a nonpolar Sop, namely, the transition state structure for the thermal cis-trans isomerization. Although perhaps counterinmitive to the role of a solvent in the stabilization of a polar species, the decrease in the S lp lifetime with an increase in solvent polarity is understood in terms of internal conversion from to So, which should increase in rate as the S -So energy gap decreases with increasing solvent polarity. Along with the solvent-dependent hfetime of S lp, it was noted that the TPE 5ip absorption band near 425 nm is located where the two subchromophores— the diphenylmethyl cation and the diphenylmethyl anion—of a zwitterionic 5ip should be expected to absorb hght. A picosecond transient absorption study on TPE in supercritical fluids with cosolvents provided additional evidence for charge separation in 5ip. 

A time-dependent process, such as radiative absorption, internal conversion, intersystem crossing, unimolecular isomerization, or collision, may be treated in terms of a zero-order Hamiltonian H0 and a perturbation T. An unperturbed eigenstate of H0 evolves in time, since it is not an eigenstate to the perturbed Hamiltonian... 

Radiationless transitions among electronic states of molecules represent a class of relaxation processes that are electronic in nature. The general term electronic relaxation appears to be appropriate for these processes,23 but it is convenient to divide those transitions involving a change in the bound electronic states of a molecule into two categories Transitions between states of the same multiplicity, referred to as internal conversion, and transitions between states of different multiplicity, referred to as intersystem crossing. Although there are several early experimental... 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

The contribution of fluorescence to the deactivation of the excited singlet state of the phytochromobilin chromophores in Pr and Pfr (see Section II.D) is negligible in quantitative terms. The total fluorescence quantum yield of the photochromic P and P3 components amounts only to from Tables 1 and 4). This means that deactivation proceeds predominantly through nonradiative channels, i.e., via internal conversion back to the electronic ground state of Pr and via primary photoreaction(s). Nevertheless, the fluorescence efficiency suffices to serve as a sensitive tool to monitor certain aspects of the competing primary reaction(s) of P (see Sections III.A and III.C). 

Define or describe the following terms or phenomena radiochemistry, isotone, internal conversion, gluon, lepton. 

External Conversion.—Transfer of excitation energy to the environment. The term is now little used but was originally coined to contrast with internal conversion (vide supra). In the usual context the term has about the same meaning as energy transfer. 

Energy Transfer—(Excitation transfer).— As used in photochemistry the term is nonspecific and refers to any transfer of energy from an excited molecule to other species. The energy acceptor may itself be promoted to an excited electronic state or the electronic energy may be donated to a host system as vibrational, rotational or translational energy. The term is used to describe variously overall processes which may involve two or more steps (e.g., internal conversion followed by transfer of vibrational energy) and the individual steps in which the energy passes from one molecule to another (or others). 

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