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.

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. (C) 2013 AIP Publishing LLC.

Laser-induced condensed phase reactions are often interpreted as nonequilibrium phenomena that go beyond conventional thermodynamics. Here, we show by Langevin dynamics and for the example of femtosecond-laser desorption of hydrogen from a ruthenium surface that light adsorbates thermalize rapidly due to ultrafast energy redistribution after laser excitation. Despite the complex reaction mechanism involving hot electrons in the surface region, all desorption product properties are characterized by equilibrium distributions associated with a single, unique temperature. This represents an example of ultrahot chemistry on the subpicosecond time scale.

Submitted for the MAR06 Meeting of The American Physical Society Ultrafast dynamics of the mid-infrared response of carbon nanotubes CHRISTIAN FRISCHKORN, Physics Department, Freie Universitat Berlin, Germany, TOBIAS KAMPFRATH, LUCA PERFETTI, MARTIN WOLF — We report on time-resolved measurements of low-energy excitations in carbon nanotubes and compare these with results obtained for graphite. The systems mid-infrared response has been obtained from time-resolved THz spectroscopy data in the 10 30 THz spectral range. We find essentially two processes governing an electronic current dynamics in carbon nanotubes. First, strongly bound excitons are the main photoproduct in large-band gap tubes and thus prevent a typical free-carrier response, while in small-gap and metallic tubes carrier localization due to defects is observed as manifested in a substantial dichroism. In these measurements, the reduced polarizability perpendicular to the tube axis is exploited. In the case of graphite, our results show that strongly coupled optical phonons in the graphite layer dominate the ultrafast energy and transport relaxation dynamics after optical excitation [1]. These phonon modes heat up on a femtosecond time scale and cool down with a time constant of several picoseconds. Moreover, the observed pronounced increase in the Drude relaxation rate significantly originates from these few active lattice vibrations. [1] PRL 95, 187403 (2005). Christian Frischkorn Physics Department, Freie Universitat Berlin, Germany Date submitted: 30 Nov 2005 Electronic form version 1.4

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.

The charge carrier dynamics of photoexcited bismuth generates a Drude response that evolves over time. Our data show that the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed ab initio calculations on bulk bismuth within the density functional theory and show that this peculiar behavior is due to local extrema in the valence and conduction bands. It follows that most of the carriers first accumulate in these extrema and reach the Fermi level only 0.6 ps after the photoexcitation.

The photon-driven substrate-mediated dissociation of N2O on thin MgO films grown on an Ag(100) crystal has been investigated with postirradiation thermal desorption spectroscopy (TDS). After excitation with 248 nm photons, we observe the simultaneous formation of N-2 and a high-temperature oxygen species, accompanied by a decrease in the parent N2O signal. On the basis of the generation and depletion of N-2 and N2O as a function of the photon dose, we determine cross sections of about 10(-18) and 10(-19) cm(2), respectively, whereas for the concurrent desorption of the N-2 photoproduct a cross section of 10(-20) cm(2) is found. If the desorption of molecular oxygen is completed at 650 K, then the MgO film is virtually restored to its initial reactivity condition. However, only partial removal of the high-temperature oxygen results in the diminished formation of N-2 in subsequent reduction cycles, which we explain by blocking the reactive sites through oxygen at annealing temperatures that are not high enough. Our findings can be rationalized in terms of the initial photogeneration of electron hole pairs that then upon electron trapping lead to reactive sites that cause the dissociation of the parent N2O molecules.

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 5 K. These results suggest that the energy relaxation in graphite at room temperature and above is dominated by the anharmonic decay of hot A1′ phonons at the K point into acoustic phonons with energies of about 10 meV.

Electron injection into thin, crystalline D2O layers on a Ru(001) surface after UV excitation has been investigated using broadband sum-frequency generation spectroscopy, whereby a signal enhancement by several orders of magnitude is observed.

The optical properties of single-wall carbon nanotube sheets in the far-infrared (FIR) spectral range from few THz to several tens of THz have been investigated with terahertz spectroscopy both with static measurements elucidating the absorption mechanism in the FIR and with time-resolved experiments yielding information on the charge carrier dynamics after optical excitation of the nanotubes. We observe an overall depletion of the dominating broad absorption peak at around 4THz when the nanotubes are excited by a short visible laser pulse. This finding excludes particle-plasmon resonances as absorption mechanism and instead shows that interband transitions in tubes with an energy gap of ~10meV govern the far-infrared conductivity. A simple model based on an ensemble of two-level systems naturally explains the weak temperature dependence of the far-infrared conductivity by the tube-to-tube variation of the chemical potential. Furthermore, the time-resolved measurements do not show any evidence of a distinct free-carrier response which is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed.

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.

Ultrashort broadband THz pulses are applied to probe the electron dynamics of various gases following ionization by an intense femtosecond laser pulse. The dielectric function of the plasma is found to be Drude-like and yields the temporal evolution of the density and collision rate of the free electrons. The electron decay in a plasma with molecular ions such as O2 + is much faster than in monatomic plasmas like Ar+/e- due to dissociative recombination which is only possible in molecular plasmas. However, adding a small amount of the electron scavenger SF6 to Ar substantially accelerates the electron decay and enables one to reliably determine the electronic temperature. Furthermore, anomalously high, metal-like electron collision rates are found. Kinetic plasma theory dramatically under-estimates these rates pointing towards additional velocity-randomizing processes like collective excitations.

For a microscopic understanding of chemical reactions at surfaces, it is essential to obtain detailed knowledge on the underlying elementary processes. The reaction mechanism, the pathways and timescales of energy flow and the energy partitioning between different degrees of freedom of the reaction products are of key interest. Reactions of species adsorbed on a metal surface are generally mediated through electron and/or phonon excitations of the substrate. Since thermal equilibration between these excitations occurs on a femto- to picosecond timescale, chemical reactions initiated by ultrashort laser pulses provide the base to investigate processes beyond equilibrium conditions. The recombination of two hydrogen atoms forming an H2 molecule, which leaves the surface, represents one of the most basic surface reactions one could think of and thus may serve as a prototype system for femtosecond laser-induced surface chemistry. In particular, the Hads+Hads H2,gas associative desorption from a Ru(001) surface has been studied in great detail. Ultrafast energy transfer times of less than 200 fs in conjunction with a pronounced isotope effect between H2 and D2 unambiguously indicate a hot-substrate electron-driven reaction mechanism. Measurements of the energy partitioning between external (translational) and internal (vibrational, rotational) degrees of freedom of the product molecule reveal predominantly translational excitation of the desorbing hydrogen. Theoretical modelling based on a multidimensional frictional description of energy transfer between the ruthenium substrate and the hydrogen layer excellently reproduces the experimental findings. Furthermore, peculiar characteristics like a threshold-like coverage dependence of the desorption yield and promotion effects in isotopically substituted adlayers have been observed in the experiment which demonstrate the importance of strong adsorbate–adsorbate interactions in the H2/D2 association, yet still awaiting a quantitative theoretical treatment.

The far-infrared conductivity of single-wall carbon-nanotube ensembles is dominated by a broad absorption peak around 4 THz whose origin is still debated. We observe an overall depletion of this peak when the nanotubes are excited by a short visible laser pulse. This finding excludes optical absorption due to a particle-plasmon resonance and instead shows that interband transitions in tubes with an energy gap of approximately 10 meV dominate the far-infrared conductivity. A simple model based on an ensemble of two-level systems naturally explains the weak temperature dependence of the far-infrared conductivity by the tube-to-tube variation of the chemical potential.

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.

The femtosecond (fs)-laser-induced associative desorption of CO from a C/O coadsorbate on Ru(001) has been investigated. The recombination of the atomic reactants is found to originate predominantly from oxidation of isolated 'reactive' carbon atoms, whereas oxidation of surface carbon with carbon-carbon bonds is not observed. Due to the excess of oxygen atoms (C coverage in the few- percent range) the C-ads + O-ads -> COgas formation exhibits first-order kinetics. For both excitation wavelengths 400 and 800 nm, a strongly nonlinear fluence (F) dependence of the CO desorption yield Y is observed with exponents n approximate to 4 in a power law parametrization Y alpha < F >(n). Furthermore, excitation with 400 nm pulses leads to a significantly higher desorption yield as compared to 800 nm laser light with cross sections and desorption probabilities for 400 and 800 nm excitation of sigma(eff) = 4(9 x 10(-18) cm(2), P-des = 0.17 and sigma(eff) = 4.9 x 10(-18) cm(2), P-des = 0.17, respectively, at an absorbed fluence of < F > = 170 Jm(-2). This wavelength dependence is attributed to the shorter optical penetration of 400 nm light in the Ru substrate leading to higher surface temperatures at the same absorbed energy rather than to nonthermalized hot electrons. In addition, two-pulse-correlation measurements show a fullwidth at half-maximum of similar to 20 ps excluding a purely electron-driven reaction mechanism, which should exhibit a subpicosecond response time. However, careful qualitative and quantitative analyses based on frictional modelling of the adsorbate-substrate coupling reveals that the C-O association reaction is mediated by both substrate phonons and electrons. The electronic, i.e. nonadiabatic contribution with a coupling constant of eta(e1) = 1/500 fs(-1) is responsible for the ultrafast activation of the reaction found in the frictional modelling to occur within similar to 1 ps after excitation. Similarities to the associative desorption of N-2 (isoelectronic with CO) from N/Ru(001), a system for which density-functional calculations exist, can be drawn. Finally, the energy transfer to nuclear degrees of freedom during the C-O association process on the Ru(001) surface has been studied with time-of-flight measurements. The obtained translational energies expressed by T-trans = < E-trans >/2k(B) approximate to 700 K exhibit only a weak dependence on the absorbed laser fluence and are by a factor of similar to 3 lower than the calculated surface temperatures present after fs-laser excitation. Possible origins of this discrepancy, such as unequal energy partitioning between the molecular degrees of freedom or nonadiabatic damping, are discussed.

The optical properties of single-wall carbon nanotube sheets in the far-infrared have been investigated with THz time-domain spectroscopy. Over a wide frequency range from 1 THz to 40 THz, the complex dielectric function of the nanotube sample has been derived. Our data can be excellently reproduced by a Drude-Lorentz model function. The extracted fit parameters such as Lorentz resonance frequency and plasma frequency are consistent with values obtained by scanning tunneling techniques. We discuss the origin of both the Lorentz and Drude contribution in terms of direct and indirect optical transitions. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.