Christian Frischkorn

Photodissociation of HBr molecules and clusters: Anisotropy parameters, branching ratios, and kinetic energy distributions

The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around n=9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (β=−0.96±0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (β*=1.96±0.05) with a branching ratio of R=0.20±0.03. At the wavelength of 193 nm, about the same value of R=0.18±0.03 is found, but both channels show a mainly perpendicular character with β=−0.90±0.10 for Br and β*=0.00±0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their ...

UV photolysis of (HBr)n clusters with known size distribution

Abstract The photodissociation of (HBr)n clusters is reported using polarized laser light at 243.1 nm. The cluster size is measured in a deflection experiment with He and Ne atoms. A time-of-flight mass spectrometer with low extraction fields is used to obtain the kinetic energy distributions of the hydrogen atoms. At averaged cluster sizes of n = 8.1 two distributions are observed with fast peaks of uncaged fragments and with a sharp peak at zero velocity which shows an isotropic angular distribution and which is attributed to complete caging.

Ultrafast electron solvation dynamics in D2O/Cu(1 1 1): influence of coverage and structure

Using femtosecond time-resolved two-photon photoelectron spectroscopy we have studied the dynamics of photoexcited electrons injected from a Cu(1 1 1) substrate into the conduction band of ultrathin ice layers. Ultrafast localization of the injected electrons within <50 fs is followed by a stabilization on a time scale of 0.1–1 ps due to local rearrangements of water dipoles. The dynamics of this electron solvation process are very similar for amorphous and crystalline ice, but exhibit a pronounced coverage dependence with two different regimes, which are attributed to solvation sites in the interior and at the surface of the adlayer. Additional information on the adsorbate structure is obtained from low-temperature scanning tunneling microscopy.

Temperature dependence of ultrafast phonon dynamics in graphite

Nonequilibrium optical phonons are generated in graphite following the excitation of electron-hole pairs with a femtosecond laser pulse. Their energy relaxation is probed by means of terahertz pulses. We find that the hot-phonon lifetime increases by a factor of 2 when the sample temperature decreases from 300 to 5K. These results suggest that the energy relaxation in graphite at room temperature and above is dominated by the anharmonic decay of hot A 0 phonons at the K point into acoustic phonons with energies of about 10meV. V C 2011 American Institute of Physics.

Maximizing the amplitude of coherent phonons with shaped laser pulses

We perform model calculations of coherent lattice vibrations in solids driven by ultrashort laser pulses. In order to maximize the amplitude of the coherent phonon in the time domain, an evolutionary algorithm optimizes the driving laser field. We find that only a Fourier-limited single pulse yields the maximum phonon amplitude, irrespective of the actual physical excitation mechanism (impulsive or displacive). This result is in clear contrast to the widespread intuition that excitation by a pulse train in phase with the oscillation leads to the largest amplitude of an oscillator. We rationalize this result by an intuitive model and discuss implications for other nonlinear processes such as optical rectification.

Indication of Te segregation in laser-irradiated ZnTe observed by in situ coherent-phonon spectroscopy

We irradiate a ZnTe single crystal with 10-fs laser pulses at a repetition rate of 80 MHz and investigate its resulting gradual modification by means of coherent-phonon spectroscopy. We observe the emergence of a phonon mode at about 3.6 THz whose amplitude and lifetime grow monotonously with irradiation time. The speed of this process depends sensitively on the pump-pulse duration. Our observations strongly indicate that the emerging phonon mode arises from a Te phase induced by multiphoton absorption of incident laser pulses. A potential application of our findings is laser-machining of microstructures in the bulk of a ZnTe crystal, a highly relevant electrooptic material.

Coupling of spin and vibrational degrees of freedom of adsorbates at metal surfaces probed by vibrational sum-frequency generation.

Vibrational spectroscopy using sum-frequency generation has been used to investigate the coupling between a ferromagnetic thin film and adsorbed molecules, here CO on Ni/Cu(100). The CO stretching vibration exhibits a strong magnetic contrast with a pronounced temperature dependence, underlining the high sensitivity of this adsorbate-specific spectroscopy method. Our results indicate that the strong temperature dependence is caused by dynamical changes in the surface chemical bond when the CO stretch vibration is coupled to thermally excited external vibrational modes.

Ultrafast dynamics of coherent optical phonons in α-quartz

Femtosecond laser excitation of α-quartz launches coherent optical phonons modulating the refractive index of the sample. The observed oscillations in the transmission and ellipticity of probe light decays due to phonon-phonon scattering. With decreasing temperature, the vibrations shift towards higher energies and are accompanied by a rise of the phonon lifetime caused by lattice stiffening and freezing of phonon modes, respectively.

Ultrafast changes in the far-infrared conductivity of carbon nanotubes

The ultrafast charge-carrier dynamics in single-wall carbon nanotubes (NTs) have been investigated by time-resolved THz spectroscopy. Both the equilibrium and non-equilibrium conductivity data of the NTs in the far-infrared (FIR) spectral range from 1 to 40 THz are dominated by optical transitions across the band gap of tubes with gap energies of ~ 10 meV. A simple model based on an ensemble of two-level systems excellently explains all experimental findings. In particular, the surprisingly weak temperature dependence of the FIR conductivity has been shown to arise from tube-to-tube variation of the chemical potential which is ~ 100 meV in our sample. The results strongly suggest to use the temperature dependence of the FIR conductivity as a very sensitive and contact-free probe of the NT sample purity. Finally, the relaxation of the photo-excited NT sheet on a picosecond time scale mainly reflects the cooling of hot phonons which is about five times faster than in graphite. This points to much stronger lattice anharmonicities in NTs.

Surface femtochemistry: Ultrafast reaction dynamics driven by hot electron mediated reaction pathways

Fundamental insights into the ultrafast dynamics of energy transfer processes at surfaces are of central importance for a microscopic understanding of chemical reactions at solid surfaces, e.g. in heterogeneous catalysis. The detailed investigation of the rates and pathways of energy flow in the adsorbate–substrate system as well as the chemical dynamics has become possible utilizing intense femtosecond laser pulses. Surprisingly, the strong non–equilibrium situation that arises upon irradiation with these pulses results in a new reaction channel caused by hot electron excitation for a number of systems investigated. This electron–mediated reaction mechanism is exemplified here for the model reaction Had+Had → H2,g on Ru(001) and discussed in the broader context of other simple reactions such as the formation of H2O, CO2 and the CO desorption on the same surface.