Valley symmetry point group

Table 8.3. IRs of the site symmetry point groups of the sublevels of np donor states in silicon split by a uniaxial stress, deduced from the IRs of the valley group. The site symmetry group under stress is indicated close to the stress orientation. The magnitudes of the splitting, independent of the value of m, are given in Table 8.2 (after [2])...

The Ch-related donor spectra differ on that point as several parity-forbidden transitions are observed. They start with symmetry-allowed transitions from the Is ground state to the valley-orbit split Is excited states, and are supplemented with 2s (T2) and 3s (T2) lines and Fano resonances within the photoionization spectrum. This is shown in Fig. 6.13 for Se°. Compared to group-V donors, this extends the energy span of the Ch°-related spectra to the ionization energy of the Is (T2) level (35-40 meV in isolated chalcogens) and it can even increase to 40-48 meV when singlet-triplet spin-forbidden transitions are observed. 

Unusual types of temperature dependence of heat capacities can occur in molecular systems. A methyl group in an organic molecule often has a potential for rotating about its symmetry axis that has three peaks and three valleys over its 360° range of positions. It exhibits a vibrational motion called hindered rotation where it twists back and forth within one of the valleys. However, if the molecule is promoted to a higher energy level, it may be above the peaks in this torsional potential. At some point, the torsion of the... 

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