U. Happek

Observation of coherent synchrotron radiation at the Cornell linac

Abstract The behavior of the far infrared radiation produced by the passage of a mm long bunch of electrons through a vertical 0.44 T magnetic field is found to

Frequency and temperature dependence of the vibrational relaxation of the SH stretch mode in vitreous As2S3

Abstract The non-radiative relaxation of the SH vibrational stretch mode in hydrogen doped As2S3 glass has been measured using infrared picosecond laser saturation spectroscopy. A lifetime on the order of 10−10 s is found at low temperature that varies by a factor of 6 across the broad SH absorption line, demonstrating a site-dependent relaxation rate. A temperature dependence study indicates that the stretch mode relaxes through a multi-vibrational decay into modes of the glassy host.

Ultrafast relaxation of the fundamental vibrations of SH- and OH- in potassium halides

Abstract The vibrational relaxation times of SH- and OH- in potassium halides, measured by incoherent laser saturation, vary from 0.8 to 7 ns at 1.7 K. A librational side band at +300 cm-1 provides a possible decay channel. The strength of this mode is strongly temperature dependent, possibly owing to barrier hopping.

Observation of site-dependent relaxation of the OH vibrational stretch mode in fused silica

Abstract We have investigated the vibrational relaxation of the OH impurity stretch mode in amorphous SiO 2 . At low temperatures, a short relaxation time of ≈10 −10 s is found that varies strongly across the inhomogeneously broadened absorption line. The inhomogeneous broadenining is caused by a site-dependent frequency shift proportional to the strength of the hydrogen bond of the OH to neighboring atoms. Thus the observed lifetime decrease across the absorption line can be attributed to increasing hydrogen bond strength. A simple relation between the site-dependent vibrational lifetime and the frequency shift is presented.

Ultrafast vibrational relaxation of diatomic chalcogen hydrides in alkali halides

The energy relaxation times of the fundamental stretching modes in the electronic ground state of OH−, OD−, SH−, and TeH− in a variety of alkali halides are measured by incoherent laser saturation and found to vary from 0.3 to 3 ns at 1.7 K. These vibrational lifetimes are between 4 and 8 orders of magnitude smaller than those of other heteronuclear diatomics diluted in crystals, including the ionic systems of CN− in salts and the neutral deuterides, DCl and ND, and oxides, CO and NO, in rare-gas matrices. Unlike these other systems, the chalcogen-hydride-doped alkali halides have a librational mode at frequencies well above the top of the host phonon band. This makes the librational decay channel a lower order process than relaxation into phonons. An energy gap law can be fit to the data, in which the relaxation times vary exponentially with the number of accepting reorientational modes. This model can explain the fact that OH− and OD− in KCl have nearly the same lifetimes, since the vibrational and libr...

Relaxation of the SH vibrational stretch mode in As2S3 glass

The frequency dependent relaxation properties of the SH vibrational stretch mode in hydrogen doped As 2 S 3 glass have been measured using IR picosecond laser saturation spectroscopy. A low temperature lifetime in the order of 10 -10 s is found that varies by a factor 6 across the broad SH absorption line

Relaxation of terahertz phonons in mixed crystals exhibiting glass-like properties

Abstract High-frequency phonons in mixed crystals exhibiting glass-like properties show a frequency dependence of the decay rate that differs from the v5 dependence of the anharmonic phonon decay in crystalline insulators. We present a scaling law that describes the transient frequency distribution of non-equilibrium phonons in the glass-like system.

Energy transfer between UV generated color centers and the CN- ion in alkali halides

Abstract In alkali halide crystals containing CN - ions, pulsed UV radiation below the band gap is observed to produce F centers and then IR vibrational fluorescence, in that order. The stretch mode excitations are driven by inelastic collisions from conduction band electrons produced by the UV ionized F centers.

Terahertz Phonons in Highly Disordered Crystals

The relaxation of non-equilibrium THz phonons in the mixed-crystal systems Cal−xSrxF2 and Cal−2LaxF2+x has been studied at low temperature. In the Ca1−zSrxF2 system we find only a moderate (up to a factor of two) increase of the relaxation rate, even at high Sr concentrations, and a v5 frequency dependence of the relaxation rate, as is characteristic for spontaneous phonon decay due to lattice anharmonicity, similar to that found in pure CaF2 and SrF2. In Cal−xLaxF2+x however, La concentrations in the order of even 1% reduce the lifetime by up to two orders of magnitude. Furthermore, the relaxation rate increases with the cube of the frequency, indicating the existence of a new mechanism dominating the relaxation of the phonons.