States, atomic normal

If the temperature were raised, more molecules would attain the excited state, but even at 50,000°C there would be only one excited-state atom for every two ground-state atoms, and stimulated emission would not produce a large cascade effect. To reach the excess of stimulated emissions needed to build a large cascade (lasing), the population of excited-state molecules must exceed that of the ground state, preferably at normal ambient temperatures. This situation of an excess of excited-state over ground-state molecules is called a population inversion in order to contrast it with normal ground-state conditions. 

A special mention is in order of high-resolution electron microscopy (HREM), a variant that permits columns of atoms normal to the specimen surface to be imaged the resolution is better than an atomic diameter, but the nature of the image is not safely interpretable without the use of computer simulation of images to check whether the assumed interpretation matches what is actually seen. Solid-state chemists studying complex, non-stoichiometric oxides found this image simulation approach essential for their work. The technique has proved immensely powerful, especially with respect to the many types of defect that are found in microstructures. 

For all other elements the electronic configuration of a single atom does not correspond to a thermodynamically stable state at normal conditions. Only at very high temperatures do single atoms occur in the vapor phase. At more ordinary temperatures atoms have to be linked to produce stable structures. 

Consider continuous radiation with specific intensity I incident normally on a uniform slab with a source function 5 = Bv(Tex) unit volume per unit solid angle to the volume absorption coefficient Kp and is equal to the Planck function Bv of an excitation temperature Tcx obtained by force-fitting the ratio of upper to lower state atomic level populations to the Boltzmann formula, Eq. (3.4). For the interstellar medium at optical and UV wavelengths, effectively S = 0. 

The compounds in question are classified by oxidation state of the central metal atom. Assignment of oxidation state is normally straightforward when the formula is known, but there are cases where the choice is somewhat arbitrary. Thus bis(cyclopentadienyl)nickel, (CcH Ni, may be considered as the nickel (II) cation complexed by two cyclopentadienyl anions, or as a combination of an uncharged nickel atom with two cyclopentadienyl... 

Almost all atomic emission is carried out with an inductively coupled plasma, whose temperature is more stable than that of a flame. The plasma is normally used for emission, not absorption, because it is so hot that there is a substantial population of excited-state atoms and ions. Table 21-3 compares excited-state populations for a flame at 2 500 K and a plasma at 6 000 K. Although the fraction of excited atoms is small, each atom emits many photons per second because it is rapidly promoted back to the excited state by collisions. 

ATOMIC ENERGY LEVELS. 1. The values of the energy corresponding to the stationary states of an isolated atom. 2. The set of stationary states in which an atom of a particular species may be found, including the ground state, or normal state, and the excited states,... 

TABLE 5. ELECTRON STRUCTURE OF ATOMS (NORMAL STATE)... 

Fig. 9.9. Contour plot of the potential energy surface of H2O in the AlB state the bending angle is fixed at 104°. Superimposed are the total stationary wavefunctions I tot( ) defined in (2.70). The total energies are —2.6 eV and -2.0 eV corresponding to wavelengths of A = 180 nm and 165 nm, respectively. Energy normalization is such that E = 0 corresponds to three ground-state atoms.

Fig. 13.3. (cont.) and (13.7). The wavefunctions are calculated using the empirical fit PES of Sorbie and Murrell (1975). The arrows indicate qualitatively the route of the evolving time-dependent wavepacket in the excited state. The dashed contours in (a) and (b) represent the A-state PES. Right-hand side The corresponding absorption spectra as functions of the energy in the excited electronic state. E = 0 corresponds to three ground-state atoms, H + O + H. Within this normalization the lowest eigenvalue of H20(A) is -9.500 eV. 

The g = 2 Puzzle. When an atom vapor passes through a magnetic field, the "normal" Zeeman effect splits an optical absorption or emission line into an odd number of lines for example, in a P state the normal Zeeman effect splits the optical spectrum of an atom into three lines, which argues for Mi = —l, 0, +1 states, whence L = 1, and gL = 1. 

The coordination number of the halogens in the -1 state is normally only 1, but exceptions are found in HXH+ cations, in polyhalide ions such as FC1F and I3, and when X ions occur as bridging ligands where the coordination number is 2. There are also triply bridging X ions in some metal atom cluster compounds. 

In the preceding sections we have outlined the requirements a cluster has to fulfill in order to dissociatively chemisorb H in summary, the cluster first has to contain at least one atom with a d occupation including at least one open d-orbital. Second, there has to be at least one open shell valence (s-character) orbital in the cluster wave-function. If there is only one open shell orbital, a dihydride or possibly a molecularly chemisorbed state will be formed. If there are at least two open shell orbitals, atomically chemisorbed hydrogen atoms of the type found on surfaces will be formed. The formation of the latter state is normally more exothermic. Finally, if these requirements are not fulfilled by the ground state wave-function of the cluster, excitation to a low lying state which satisfies the requirements and which has an excitation energy less than the exothermic ty 20 kcal/mol) will lead to... 

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