Franck-Condon Shift Research Articles

Photoconductivity of Gallium Selenide Crystals

Single crystals of GaSe have been prepared by reaction of the elements, followed by gradient freeze crystallization. Crystals as grown were $p$-type, probably because of Cu acceptors, with hole mobility of 15 ${\mathrm{cm}}^{2}$/volt sec, hole concentration of ${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and acceptor ionization energy of 0.12 ev. In many ways the properties of GaSe crystals are very similar to those of ZnTe crystals. The absorption edge of GaSe is at 6310 A, corresponding to a band gap of 1.97 ev; the temperature coefficient of band gap is about -4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ev/deg. Insulating and photosensitive GaSe crystals can be prepared by compensating the acceptor impurities by incorporated donors from Groups VII or IV. Rectification tests indicate $p$-type photoconductivity. Thermal quenching of photoconductivity corresponds to an ionization energy of about 0.5 ev, whereas optical quenching corresponds to an ionization energy of about 1.0 ev; thus a large Franck-Condon shift is indicated. Below the temperature at which thermal quenching of photoconductivity occurs, the sensitivity is within the range of sensitive CdS or CdSe crystals.

Absorption Spectra at 4°K of Single Crystals of Normal and Isotopically Substituted Sodium and Potassium Nitrates

Sharp line structure is observed in the low energy shoulder of the 3000-A absorption band of crystalline nitrates that have been cooled to 4°K. Single crystals of normal and of nitrogen-15 isotopically substituted sodium and potassium nitrates were examined spectrographically. Correlation between the spacings, polarization and orientation properties, isotope effect, and crystal field splitting of the spectra permitted their analysis. In terms of the D3h point symmetry of a free nitrate ion the excited electronic state is E′. This electronic configuration is one of three considered likely upon the basis of a LCAO molecular orbital treatment of the ion. The two asymmetric E′ vibrations of the ion predominantly contribute to the band structure. A large Franck-Condon shift, an extensive weakening of force constants, and the character of the excited vibrations indicate that the excited ion is planar but expanded with perhaps some, but not a great amount of asymmetric distortion. Predissociation appears at a level corresponding to a dissociation energy of 3.76 ev for the nitrate ion.

Infrared phosphor-semiconductors

The development of sensitive photoconducting detectors for infrared radiation has made possible the study of luminescence emission from a new variety of solids including those of considerable interest as semiconductors. A review of progress over the past few years is given with special reference to the emission of zinc and cadmium sulphide phosphors, lead sulphide, lead telluride and mercury sulphide. The infrared emission found in zinc sulphide is ascribed to electron transitions from the valence band to the ground states of visible emission centers. In mercury sulphide the characteristics of the excitation and emission spectra suggest a different mechanism. This also applies to lead sulphide and lead telluride. New results on cadmium telluride made p- or n-type under controlled conditions show a number of emission bands extending to 2.6 μ. Excitation and photovoltage excitation curves coincide with emission bands, there being no Franck-Condon shift. The results are given a tentative theoretical explanation.