Metastable states selection

The first excited singlet state, 2 Sq, is also metastable in the sense that a transition to the ground state is forbidden by the Af selection rule but, because the transition is not spin forbidden, this state is not so long-lived as the 2 Si metastable state. 

Gas-discharge lamps are used to optically pump the metastable helium atoms into a higher excited electronic state, which has a dipole-allowed transition to the ground state. Only He (2 S) can be pumped selectively, thereby producing pure He(23S) beams. For the heavier rare gases, both metastable states are equally pumped by gas-discharge lamps. The use of cutoff filters to selectively pump one state is not adequate because of the temperature dependence of the filter transmission and the low / numbers of the pumping transition. Metastable neon can be selectively pumped by a continuous wave (cw) dye laser,60 whereas Ar, Kr, and Xe have so far only been selectively pumped by pulsed dye lasers.61... 

Neimark and Vishnyakov316,317 have carried out an interesting study into the formation of a bubble in a Lennard-Jones fluid confined to a spherical pore at a metastable state. Various simulation techniques are used and compared. The Lennard-Jones parameters are selected to resemble nitrogen and the results are compared with experimental results, with qualitative agreement obtained. 

Abstract A synthetic pure water fluid inclusion showing a wide temperature range of metastability (Th - Tn 50°C temperature of homogenization Th = 144°C and nucleation temperature of Tn = 89°C) was selected to make a kinetic study of the lifetime of an isolated microvolume of superheated water. The occluded liquid was placed in the metastable field by isochoric cooling and the duration of the metastable state was measured repetitively for 7 fixed temperatures above Tn. Statistically, metastability lifetimes for the 7 data sets follow the exponential reliability distribution, i.e., the probability of non nucleation within time t equals. This enabled us to calculate the half-life periods of metastability r for each of the selected temperature, and then to predict i at any temperature T > Tn for the considered inclusion, according to the equation i(s) = 22.1x j Hence we conclude that... 

Wei CC, Gao G and Kispert LD (1997) Selected cis/trans isomers ofcarotenoids formed by bulk electrolysis and iron(III) chloride oxidation. J Chem Soc Perkin Trans 2 pp 783-786 WolffC and Witt HT (1969) On metastable states ofcarotenoids in primary events ofphotosynthesis. Z Naturforsch 24b 1031-1037... 

Is Inconsistent with the dynamic conformational aspect of Ion selectivity developed In the present paper. Furthermore, the conformational options of lonophores are not necessarily a graded function of environmental polarity but may display sudden shifts between metastable states over narrow polarity ranges. Electrostatic interactions between ions and Induced dipoles undoubtedly play a determinative role In cation complexation by lonophores, but the ability of the lonophore to alter Its conformation cannot be ignored as it Is in the assumption of Isosterism (19). 

The fact that an infinity of front velocities occurs for pulled fronts gives rise to the problem of velocity selection. In this section we present two methods to tackle this problem. The first method employs the Hamilton-Jacobi theory to analyze the dynamics of the front position. It is equivalent to the marginal stability analysis (MSA) [448] and applies only to pulled fronts propagating into unstable states. However, in contrast to the MSA method, the Hamilton-Jacobi approach can also deal with pulled fronts propagating in heterogeneous media, see Chap. 6. The second method is a variational principle that works both for pulled and pushed fronts propagating into unstable states as well as for those propagating into metastable states. This principle can deal with the problem of velocity selection, if it is possible to find the proper trial function. Otherwise, it provides only lower and upper bounds for the front velocity. 

A metastable state is defined as an excited state of an atom which does not (according to the selection rules) combine with the ground state of the atom. For example, the (the lowest lying level of the helium triplet system) will not combine with the I Sq (ground) state of helium. Thus, the... 

Figure 2 Excitation scheme for the two laser pump-pump experiment. State-selected benzene cations are produced in a resonantly enhanced two-photon ionization process. A second laser pulse of variable photon energy excites the ions to a well defined energy level above the dissociation threshold and metastable dissociation takes place (taken from ref. /16/).

Figure 3 Scheme of a two laser pump-pump experiment for the production of internal energy-selected molecular ions in a reflectron time-of-flight mass spectrometer (from ref. /15/). Laser 1 produces state-selected molecular ions and 200 ns later laser 2 excites these ions to a well defined internal energy level above dissociation threshold. The dissociation rate constants of the energy-selected ions are measured by the technique of detection and energy analysis of metastable ions. 

Penning ioni2ation electron spectroscopy is also called (i) metastable atom electron spectroscopy (MAES), (ii) metastable deexdtation spectroscopy (MDS), (iii) metastable quenching spectroscopy (MQS), and (iv) metastable impact electron spectroscopy (MIES), where atom at metastable state is used instead of photon to ionize the target material When a slow, long-lived, electronically excited metastable atom hits a solid surface, most of its excitation energy is used to eject electrons from the surface. Unlike photons used for UPS, metastable atoms do not penetrate into the bulk of the solid. PIES, therefore, excites the outermost surface layer selectively (39). 

Construction of the phase diagram followed from DSC DLl and optical microscopy of a number of Individual compositions of the two end members, DSC curves for the end members and for three selected mixed compositions are presented In Fig. 3. The mixed compositions were chosen for presentation since they represent points at low, medium and high concentrations of one component in the other. Heating curves only were used in construction of the phase diagram. Sample handling prior to calorimetry, as described In the Experimental Section, precluded our observation of metastable states. 

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