Cross-over transitions

The synthesis of the N03 and PFg salts of the iron(III) complex of the terdentate phenolate anion of the liquid crystal Schiff base (7) provides the first example of the coexistence of spin transition and liquid crystal properties in a single compound. The ligand now needs to be tuned to increase the spin cross-over transition temperature and decrease the transition temperature to the liquid crystal state. ... 

Nitric oxide reacts with an ethanolic solution of [Fe(salen)] [salen = A,7V -ethylenebis(sali-cylideniminato)] at room temperature to give the black complex [Fe(salen)NO]. This compound exhibits185 an S = to S = spin cross-over at 175 K. The corresponding complex of the 5-chloro-salen ligand [Fe(5-Cl-salen)NO], however, shows no sharp spin cross-over transition but a steady decrease in magnetic moment from 3.8 to 0.5 BM over the temperature range 4-100 K, indicative186... 

Recently there has been a lot of interest in the luminescence of BaF2. Its crystals have a large potential as a scintillator material (detection of gamma rays). They show a luminescence at 220 nm with a very short decay time, viz., 600 ps. This short decay time offers the possibility of a good time resolution. This luminescence is of a new t5rpe (cross-over luminescence). Its nature has been unraveled by Russian investigators (75). Excitation with about 10 eV excites anion excitons. Upon annihilation these anion excitons show an emission at about 4.1 eV (300 nm). Excitation with about 18 eV excites cation excitons. These do not annihilate in a simple way, but by a so-called cross-over transition an electron jumps from the F" ion (2p orbital) to the hole in the 5p orbital of Ba (see Fig. 23). This is accompanied by emission at about 5.7 eV... 

The chemical effects on atomic x-ray spectrum has widely been investigated by DV-Xa calculations, where the contribution of interatomic transition, in other word, the cross-over transition to x-ray emission rate can essentially be estimated by eq.(ll). The abnormality in atomic Kp/Ka emission ratio for some 3d transition metal compounds has been observed. 

Transition probabilities for rotational levels. The excitation of a high member of the rotational spectrum in an even-even nucleus is followed by a cascade of 2 radiation. In an even-odd nucleus the cascade is less simple because E2 cross-over transitions are possible and the transition between one level and the next can consist of a mixture oi E2 and Mi radiation. If the transition probabilities were comparable with those to be expected of single particle transitions, E2 radiation at these energies (100 kev or less) would be far weaker than Mi radiation. The existence of strong E2 components and the successful competition of the E 2 components in the cross-over transitions is a clear indication of the collective nature of these processes where presumably the whole nuclear charge contributes to the emission of quadrupole radiation. 

Table 10. Relative probabilities of cascade and cross-over transitions in even-even nuclei.

The transition is fully classical and it proceeds over the barrier which is lower than the static one, Vo = ntoColQl- Below but above the second cross-over temperature T 2 = hcoi/2k, the tunneling transition along Q is modulated by the classical low-frequency q vibration. The apparent activation energy is smaller than V. The rate constant levels off to its low-temperature limit k only at 7 < Tc2, when tunneling starts out from the ground state of the initial parabolic term. The effective barrier in this case is neither V nor Vo,... 

However, for these parameters of the barrier, the cross-over temperature would exceed 500 K, while the observed values are 50 K. If one were to start from the d values calculated from the experimental data, the barrier height would go up to 30-40 kcal/mol, making any reaction impossible. This disparity between Vq and d is illustrated in fig. 34 which shows the PES cuts for the transition via the saddle-point and for the values of d indicated in table 2. 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

The self-similar spectrum is not valid at short times, X < X0, where the details of chemical structure become important (glass transition, entanglements, etc.). The cross-over to the glass transition at short times is typical for all polymeric materials, for both liquids and solids. The critical gel is no exception in that respect. X0 could be used as a characteristic time in the CW spectrum since it somehow characterizes the molecular building block of the critical gel however, it has no direct relation to the LST. At times shorter than X0, the LST has no immediate effect on the rheology. Indirect effects might be seen as a shift in the glass transition, for instance, but these will not be studied here. 

On the day Ter Meer was put on the train for Numberg, Sprech-er released Von Schnitzler from the Numberg jail and put him on the train for Munich, the rail point nearest Dachau. Ter Meer and Von Schnitzler crossed in transit, so to speak. Von Schnitzler was told merely to "rest and think it over."... 

Spin cross-over kinetics for Fe(II) complexes 26,103) have been reviewed. The l.s. h.s. (1A1 5T2) transition in solution is character-... 

Figure 5.12. A revised Periodic Table of the/and d series. According to Smith and Kmetko (1983) this table of the d and/transition elements shows the cross-over between itinerant, fully bonding electrons and fully localized electrons usually forming local magnetic moments.

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