Radiative depletion

Bound electronic states exhibit a discrete spectrum of rovibrational eigenstates below the dissociation energy. The interaction between discrete levels of two bound electronic states may lead to perturbations in their rovibrational spectra and to nonradiative transitions between the two potentials. In the case of an intersystem crossing, this process is often followed by a radiative depletion. Above the dissociation energy and for unbound states, the energy is not quantized, that is, the spectrum is continuous. The coupling of a bound state to the vibrational continuum of another electronic state leads to predissociation. 

Several conditions must be fulfilled for an anionic electronic state to exist (i) it should possess positive electron affinity with respect to its parent neutral state (ii) it should exhibit slow depletion by spin-forbidden autodetachment for at least one fine-structure component and by radiative depletion and (iii) its wave-function should undergo weak interaction with the electron continuum wave. Such stable and metastable states have been identified for several negatively charged atoms and molecules, in both ground and electronically excited states. Long-lived electronically excited molecular systems, where the anionic ground state is not bound, do exist and have been observed experimentally. For a detailed presentation of the examples already known is referred in Refs. [1-3]. 

Fig. 3 The outline of fast dynamic processes that proceed at rates comparable with that of the radiative depletion of the excited state and can be studied by time-resolved fluorescence techniques (TRF) the rate of the polymer chain dynamics (vibrational motion and relaxation) strongly overlaps that of electronic relaxation and can be studied by TRF. Adapted from Springer, Self Organized Nanostructures of Amphiphilic Block Copolymers I, 241, 2011, 187-249, figure 3, [2], Copyright 2011. With kind permission from Springer Science and Business Media...

The physical meaning of the individual symbols is as follows k t) is the time-dependent rate constant of the radiative depletion of the excited state, p v,t) is the normalized time-dependent emission spectmm, i.e., the denominator k f)p v, t))) describes the total fluorescence, S t), P2 is the Legendre polynomial of the second order which correlates with the mutual angular orientations of /energy states. He also employed the master equation, which describes the time change of the conditional probabilityp(/, i2 tj(j, J2o t = 0) that the fluorophore is in energy state i and... 

The age at which Li depletion occurs increases with decreasing mass (and Li-burning temperatures are never reached for M < 0.06 M0). As luminosity, L oc M2 for PMS stars, the luminosity at which complete Li depletion takes place is therefore a sensitive function of age between about 10 and 200 Myr [6]. This relationship depends little on ingredients of the PMS models such as the treatments of convection and interior radiative opacities because the stars are... 

Fig. 1. Evolutionary tracks (labelled in Mq) and isochrones (in Myr) for low-mass stars taken from two models [8,31]. The epochs of photospheric Li depletion (and hence Li-burning in the core of a fully convective star or at the convection zone base otherwise) and the development of a radiative core are indicated. The numbers to the right of the tracks indicate the fraction of photospheric Li remaining at the point where the radiative core develops and at the end of Li burning.

Opacity effects are also important. This can refer to differences in the treatment of interior opacities or to the effects of uncertain stellar compositions on the opacities. An increase in opacity makes temperature gradients larger, keeps the star convective for longer, raises Tf,cz once the radiative core develops and so leads to enhanced Li depletion. Opacity is increased by an increase in overall metallicity or a decrease in the Helium abundance. Changes of only 0.1 dex in metallicity can lead to an order of magnitude change in Li depletion (e.g. see Fig. 2 of [37]). 

Structural Effects of Rotation Rapid rotation in a fully convective star decreases the core temperature, but actually increases Tt,cz once a radiative core has developed. The net effect on Li depletion seems to be rather small and cannot explain the dispersion of Li abundances seen among the slow rotating ZAMS stars [24]. 

The lifetime of the excited state will be influenced by the relative magnitudes of these non-radiative processes and thus time-resolved spectroscopy can provide information on the dynamics of excited state depletion... 

Nitrous oxide (N2O) is an important greenhonse gas with a radiative forcing effect 310 times that of CO2 and a lifetime in the troposphere of approximately 120 years. Part of the N2O is converted to NO in the stratosphere, and so contributes to depletion of ozone. Nitric oxide (NO) is very reactive in the atmosphere and has a lifetime of only 1-10 days. It contribntes to acidification and to reactions leading to the formation of ozone in the troposphere, and so also to global warming. 

NO, Peak 50-100% increase Ozone depletion, change in radiative forcing... 

Human activity has also caused ozone changes, due to emissions of substances that deplete ozone in the stratosphere and precursors that generate ozone in the troposphere. The ozone changes, in particular in the troposphere, vary on regional scales. As discussed in WMO (1995 1999) and IPCC (1996) the radiative forcing due to ozone has a longwave as well as a shortwave component and there is a critical dependence on the vertical distribution of ozone changes. 

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