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electron-elecron-scattering:Te-h >> TL
electron-phononscattering:Te-h↓, TL↑
phonon-phononscattering
Te-h ≈ TL
time [s]
10-15
10-14
10-13
10-12
10-11
10-10
10-9
non-thermal
thermalheat conduction
(thermal) melting
ablation
sample
fs-laser pluse
VB
CB
Ultrafast laser excitation of solids
semiconductor
Characteristic time scales:
• Electronic excitation: τL• Thermalization: τR• Energy transport: τE• Mass transport: τM
τL << τR << τE < τM
• Spatial and temporal localization of energy deposition:
• Non-equilibrium• Impulsive heating at constant volume• High temperature and high pressure phases of matter:
„warm dense matter“
Ultrafast laser excitation of solids II
Ultrafast x-ray spectroscopy
Femtosecond time scale:
Changes of nuclear configuration• chemical reactions• vibrations• structural changes
x-ray radiation:
Spatial structure ofatomic configuration
• structure of matter
Ultrafast x-ray spectroscopy: atomic resolution - temporal and spatial
Δx
Δt
Sources for ultrafast x-ray pulses(how to realize ultrafast x-ray diffraction…)1. accelerator based sources:
K. Sokolowski-Tinten et al., PRB 51, 14186 (1995), ibid. 58, R11805 (1998),
optical spectroscopyelectronic excitation
non-thermal melting
ablation
0.47 J/cm2
R(λ) of solid Si
liquid
Ultrafast non-thermal melting:
> 10 % of all valence electrons!
K. Sokolowski-Tinten et al., PRB 61, 2643 (2001)
Intense electronic excitation
P. Stampfli et al., PRB 49, 7299 (1994)
lattice instability
Use ultrafast melting as test case for time-resolved x-ray diffraction
Non-thermal und thermal melting and subsequent re-crystallization
≈ 300 fs
K.Sokolowski-Tinten et al., PRL 87, 225701 (2001)
170 nm Ge on Si; (111)-diffraction spot
800 m/s
X-ray diffraction: Ultrafast melting of Ge
Non-thermal meltingNon-thermal und thermal melting
Si (111)
170 nmGe, Bi (111)
Analysis of x-ray pulse duration
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction
Delay Time [ps]
Ge 0.2 J/cm2
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction"melting"
Delay Time [ps]
Ge 0.2 J/cm2
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction"melting"250 fs
Delay Time [ps]
Ge 0.2 J/cm2
0.8
0.9
1.0
-0.4 0.0 0.4 0.8
X-ray diffraction"melting"250 fs350 fs
Delay Time [ps]
Ge 0.2 J/cm2
τX = (300 ± 50) fs
for analysis phase transition is assumed as “instantaneous”
• Bi is a semimetal
• rhombohedral structure:- small displacement from fcc lattice- two atom basis
Lattice dynamics in Bismut
a
Bi-Bi distance a (0.468 × diagonal c)stabilized by Peierls-Jones mechanism
A1g-Phonon (V = const.)
• Bi is a semimetal
• rhombohedral structure:- small displacement from fcc lattice- two atom basis
• Excitation of coherent optical phonons(A1g-mode) with fs-laser pulses- Zeiger et al., PRB 45, 768 (1992)- DeCamp et al., PRB 64, 92301 (2001)- Hase et al., PRL 88, 67401 (2002)