Lifetime of the bound-states

A further term, which has no analogue in hydrogen, arises in the fine structure of positronium. This comes from the possibility of virtual annihilation and re-creation of the electron-positron pair. A virtual process is one in which energy is not conserved. Real annihilation limits the lifetimes of the bound states and broadens the energy levels (section 12.6). Virtual annihilation and re-creation shift the levels. It is essentially a quantum-electrodynamic interaction. The energy operator for the double process of annihilation and re-creation is different from zero only if the particles coincide, and have their spins parallel. There exists, therefore, in the triplet states, a term proportional to y 2(0). It is important only in 3S1 states, and is of the same order of magnitude as the Fermi spin-spin interaction. Humbach [65] has given an interpretation of this annihi-... 

It is the relationship between the bound potential energy surface of an adsorbate and the vibrational states of the molecule that detemiine whether an adsorbate remains on the surface, or whether it desorbs after a period of time. The lifetime of the adsorbed state, r, depends on the size of the well relative to the vibrational energy inlierent in the system, and can be written as... 

Signal transduction between Ras protein and Raf kinase is based on a specific interaction of the two proteins, which can only be performed by the activated, GTP-bound form of Ras protein. The inactive, GDP-form of Ras protein shows significantly weaker binding to Raf kinase (Herrmann et al., 1995). Complex formation is not linked to stimulation of GTPase activity of Ras protein, and thus it is assumed that termination of signal transduction only occins on dissociation of Raf kinase or hydrolysis of the boimd GTP. The lifetime of the GTP state is determined by the GTPase activity of the Ras protein, which itself is subject to regulation by GAP proteins. 

We can exclude a predissociation process [39] responsible for the decrease of the lifetime for three reasons, (i) Dispersed emission spectra did not show any indication of emission from the fragment monomer [40]. Thus no dissociation occurs on the time scale of the fluorescence emission, (ii) The additional excitation of the van der Waals stretching vibration in benzene-Ar does not lead to a further decrease of the lifetime, (iii) The stronger decrease of the lifetime of the 61 state in benzene-Kr would not be expeced for a predissociation process since the benzene-Kr complex is more strongly bound and has only a slightly higher density of states since the frequencies of the three van der Waals modes do not differ very much from that of benzene-Ar [41]. 

V. Engel Let me come back to the distribution of lifetimes of the ZEKE Rydberg states. I wonder if there is a simple picture behind. Consider a much simpler molecule, namely the Nal molecule Prof. Zewail told us about. There you have a bound state coupled to a continuum. It can be shown that in such a system the lifetimes of the quasibound states oscillate as a function of energy. In fact, Prof. Child showed with the help of semiclassical methods that there are lifetimes ranging from almost infinity to zero [1]. That can be understood by the two series (neglecting rotation) of vibrational levels obtained from the adiabatic and diabatic picture. If two energy levels of different series are degen-... 

A medium with bound electrons can be regarded as an ensemble of oscillators each of which is characterized by three parameters its natural frequency oscillator strength/, and the damping constant y(, which equals the inversed lifetime of the corresponding state ("y, = 1 /r,). The well-known expression for the permittivity of the medium91,95 can be presented in the form89... 

Many other fluorophores are temperature-sensitive only when they are bound to macromolecules. Figure 10.16 shows the effect of temperature on the fluorescence intensity of native and guanidine unfolded AEDANS-RNase. Increasing the temperature from 10 to 30°C induces a decrease in fluorescence intensity for both protein states. The intensity decrease in native protein is more affected by temperature than the guanidine-unfolded protein. This thermal quenching is the consequence of rapid movements of the protein structure around the fluorescent probe. These movements occur during the lifetime of the excited state, and their rate is temperature-dependent. 

Abstract. The Coulomb interaction which occurs in the final state between two particles with opposite charges allows for creation of the bound state of these particles. In the case when particles are generated with large momentum in lab frame, the Lorentz factors of the bound state will also be much larger than one. The relativistic velocity of the atoms provides the oppotrunity to observe bound states of (-n+fx ), (7r+7r ) and (7x+K ) with a lifetime as short as 10-16 s, and to measure their parameters. The ultrarelativistic positronium atoms (.4oe) allow us to observe the effect of superpenetration in matter, to study the effects caused by the formation time of A e. from virtual e+e pairs and to investigate the process of transformation of two virtual particles into the bound state. 

Based on the Ps bubble model, [10, ll] experimental data on o-Ps lifetimes, and the widths of the narrow component of the ACAR spectrum, it was found that in practically all investigated liquids of various chemical nature, values of V0Ps = U are close to +3 eV. Thus, from Eq. (6) we obtain Eb —(0.5 to 1) eV. This means that qf-Ps is a loosely bound structure although the long-range Coulombic attraction between e+ and e always provides for the existence of the bound state. Because e+-e separation, rep, in qf-Ps is expected to be large, we may rewrite E, in the following form ... 

Fig. 10 Smith lifetimes for Ai and A2 rovibronic wavefunctions. The vertical arrows indicate the energies of the bound states on the upper electronic potential energy surface, with adiabatic corrections...

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