Transitions observational

Lovett R 1995 Can a solid be turned into a gas without passing through a first order phase transition Observation, Prediction and Simuiation of Phase Transitions in Compiex Fiuids vol 460 NATO ASi Series O ed M Baus, L F Rull and J-P Ryckaert (Dordrecht Kluwer) pp 641-54... 

Table 5.2 lists some of the molecules which have been detected. It is interesting to note that some of them, such as the linear triatomics C2H, HCO and N2H, were found in the interstellar medium before they were searched for and found in the laboratory. In all molecules, except OH and NH3, the transitions observed are rotational in nature. 

Each vibrational transition observed in the gas phase gives rise to what is called a band in the spectmm. The word line is reserved for describing a transition between rotational levels associated with the two vibrational levels giving rise to the fine stmcture of a band. However, in the solid or liquid phase, where this fine stmcture is not present, each vibrational transition is sometimes referred to as a line rather than a band. 

Nevertheless, 1,4-difluorobenzene has a rich two-photon fluorescence excitation spectrum, shown in Figure 9.29. The position of the forbidden Og (labelled 0-0) band is shown. All the vibronic transitions observed in the band system are induced by non-totally symmetric vibrations, rather like the one-photon case of benzene discussed in Section 7.3.4.2(b). The two-photon transition moment may become non-zero when certain vibrations are excited. 

Region Wavelength limits Erequency or photon energy Transitions observed or excited... 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

In spin relaxation theory (see, e.g., Zweers and Brom[1977]) this quantity is equal to the correlation time of two-level Zeeman system (r,). The states A and E have total spins of protons f and 2, respectively. The diagram of Zeeman splitting of the lowest tunneling AE octet n = 0 is shown in fig. 51. Since the spin wavefunction belongs to the same symmetry group as that of the hindered rotation, the spin and rotational states are fully correlated, and the transitions observed in the NMR spectra Am = + 1 and Am = 2 include, aside from the Zeeman frequencies, sidebands shifted by A. The special technique of dipole-dipole driven low-field NMR in the time and frequency domain [Weitenkamp et al. 1983 Clough et al. 1985] has allowed one to detect these sidebands directly. 

Fig. 52. (a) The frequency swept dipole-dipole driven NMR spectra of thioanisole recorded at a variety of magnetic fields, (b) The NMR and sideband transitions observed in the thioanisole data presented as a plot of magnetic field versus transition frequency. The transitions are defined in fig. 51. 

In our opinion the spin-flip (SF) phetse transition occurring in AF Cr may be connected with the electron spectrum anisotropy. This transition, observed in... 

QLN = quenehing into liquid nitrogen. No glass transition observed. 

We like to conclude this introduction observing, after Lifshitz, that a free energy exeess is involved in each ETT. TTius, it might happen that, in order to avoid the energy cost involved in opening, say, a neck in the FS, the system might prefer to change its phased. This could be the case of the martensitic transition observed in CuPd. 

Recent NEXAFS (11,2A) have confirmed -the ethylldyne structure proposed by LEED analyses (1A,21) and further determined the structure of adsorbed molecular ethylene. Figure 4 shows the NEXAFS spectra for ethylldyne (a) and ethylene (b) on the Pt(lll) surface taken for two Incidence angles of the X-ray beam. The transitions observed In these NEXAFS spectra have been assigned using SCF-Xo calculations (24). For the ethylldyne spectrum taken at 20 Incidence angle peak A Is caused by a C(ls)+o j, transition peak B Is caused by a C(ls)+o (, (, transition. Peak A In the... 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

Figure 9 Differential scanning calorimetric (DSC) curve showing the phase transitions observed on heating the amorphous form (glass) of polyethylene terephthalate. (Reproduced with permission from Ref. 38.)...

In the paramagnetic regime, the evolution of the EPR line width and g value show the presence of two transitions, observed at 142 and 61 K in the Mo salt, and at 222 and 46 K in the W salt. Based on detailed X-ray diffraction experiments performed on the Mo salt, the high temperature transition has been attributed to a structural second-order phase transition to a triclinic unit cell with apparition of a superstructure with a modulation vector q = (0,1/2, 1/2). Because of a twinning of the crystals at this transition, it has not been possible to determine the microscopic features of the transition, which is probably associated to an ordering of the anions, which are disordered at room temperature, an original feature for such centrosymmetric anions. This superstructure remains present down to the Neel... 

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