Darmstadt 2008 – scientific programme
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MO: Fachverband Molekülphysik
MO 18: Femtosekundenspektroskopie
MO 18.6: Poster
Tuesday, March 11, 2008, 14:00–19:00, Poster C1
Long living, coherent phonons in quantum-crystals: cryogenic normal- and para-hydrogen — •Falk Königsmann1, David T. Anderson2, and Nikolaus Schwentner1 — 1Freie Universität Berlin, Institut für Experimentalphysik, Arnimallee 14, D-14195 Berlin — 2Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA
Coherent optical phonons are a hot topic in metals and insolators, but did not show up in previous ultroshort pulsed excitations of quantum crystals. We grow 3 cm long, transparent para- and normal-hydrogen crystals in an enclosed cell at 8 K, which are than cooled down to 4 K. The crystals are pumped with 150 fs pulses at 780 nm of a Ti:Sa amplified laser system and the induced birefringence is detected with the second harmonic in a colinear way. We observe long lasting (> 20 ps) sinusodial birefringence modulation with a period of 900 fs in para-hydrogen. While the phonon density of states would cover a broad range from 0-180 wavenumbers, the observed coherent phonon has a sharp frequency centered at 36,8 wavenumbers. It coincides with the transverse, optical phonon at the center of the Brillouin-zone, which is observed exclusively in Raman scattering. The phonon wave packet is anisotropic and the amplitude lies perpendicular to the c axis. In normal-hydrogen the same class of phonon, slightly shifted to higher frequencies and more strongly dampened, is observed.