Metastable excited states

Neutron capture always is exothermic, because the neutron is attracted to the nucleus by the strong nuclear force. Consequently, neutron capture generates a product nuclide in a metastable, excited state. These excited nuclei typically lose energy by emitting either y rays or protons ... 

X")Ar+] or in multiple collisions [X+(Ar,X)Ar+ X(Ar,X )Ar+]. The translational-energy spectrum of singly charged negative ions formed is recorded. The energy-loss spectrum demonstrates peaks at positions calculated for ground, as well as for metastable excited states, provided these are present in the original beam. 

Particle A is an atom in a metastable excited state, and particle B is an atom. 

Up-conversion relies on sequential absorption and luminescence with intermediate steps to generate shorter wavelengths. Hence, the presence of more than one metastable excited state is required the intermediate metastable states act as excitation reservoirs. One typical example is ground-state absorption followed by inter-mediate-state excitation, excited-state absorption, and final-state excitation to give the up-conversion (the intermediate states and final states are real states) [1, 35], There are many types of up-conversion mechanisms such as excited-state absorption, energy transfer up-conversion and cooperative up-conversion. All these up-conversion processes can be differentiated by studying the energy dependence, lifetime decay curve, power dependence, and concentration dependence by experimental measurements [36-39]. 

Metastable excited states Outlined in gray To.078 MeV above ground state 2+ spin 2, parity even half-life 39.3 hours decav bv 37 (separate channel) IT=internal transition /(separate channel) a emission/(separate channel) EC electron capture /(separate channel) SF=spontaneous fission... 

The minimum prerequisite for generation of upconversion luminescence by any material is the presence of at least two metastable excited states. In order for upconversion to be efficient, these states must have lifetimes sufficiently long for ions to participate in either luminescence or other photophysical processes with reasonably high probabilities, as opposed to relaxing through nonradiative multiphonon pathways. The observed decay of an excited state in the simplest case scenario, as probed for example by monitoring its luminescence intensity I, behaves as an exponential ... 

For certain nuclides, different physical properties (half-lives, mode of decay) are observed. They are due to different energetic states, the ground state and one or more metastable excited states of the same nuclide. These different states are called isomers or nuclear isomers. Because the transition from the metastable excited states to the ground states is forbidden , they have their own half-lives, which vary between some milliseconds and many years. The excited states (isomers) either change to the ground state by emission of a y-ray photon (isomeric transition IT) or transmutation to other nuclides by emission of cc or particles. Metastable excited states (isomers) are characterized by the suffix m behind the mass number A, for instance Co and Co. Sometimes the ground state is indicated by the suffix g. About 400 nuclides are known to exist in metastable states. 

Isomeric transition (IT) E Photons (hv) Delayed emission of excitation energy Amy y Metastable excited states preferably below magic numbers... 

Thus the considerable majority of laboratory work reported in the literature has dealt with production of atomic oxygen in the ground state or in the first two metastable excited states. The conclusions which can be drawn on the basis of this work are summarized below for the different spectral regions identified above. 

Below 85 nm it is energetically possible for photodlssocla-tlon to leave atomic oxygen in the metastable excited state,... 

Lindholm and co-workers [190, 192] obtained some information on the relative abundance of metastable excited states in ion beams during the study of charge-transfer reactions using their perpendicular tandem mass spectrometer. For example, O ion beams produced by electron bombardment of CO, CO2 and N2O react with H2 to produce H2 and H". Since the production of H according to the reaction... 

Finally, HHG can be studied by Fourier transforming the induced time-dependent dipole moment, D(f) = ( F(r,f) D vl/( ,f)), where D is the dipole operator in the polarization of the EMF [33]. For the one-electron hydrogen atom, our choice of the form of the dipole operator has been the acceleration form since, in this case, the computation of the free-free matrix elements is both efficient and accurate [119]. However, in later work on the multiphoton ionization from the two-electron metastable excited state of He, the ls2s S, we stressed that for many-electron systems, "calculations of harmonic spectra based on the use of the acceleration form of the dipole operator will produce, in general, unreliable results even when some correlations are accounted for" [120]. 

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