Experimental and simulation study of neon collision dynamics with a 1-decanethiol monolayer

A study of the energy accommodation of neon colliding with a crystalline self-assembled 1-decanethiol monolayer adsorbed on Au(111) is presented. The intensity and velocity dependencies of the scattered neon as a function of incident angle and energy were experimentally measured. Scattering calculations show good agreement with these results, which allows us to examine the detailed dynamics of the energy and momentum exchange at the surface. Simulation results show that interaction times are, at most, a few picoseconds. Even for these short times, energy exchange with the surface, both normal and in-plane, is very rapid. An important factor in determining the efficiency of energy exchange is the location at which the neon collides with the highly corrugated and structurally dynamic unit cell. Moreover, our combined experimental and theoretical results confirm that these are truly surface collisions in that neon penetration into the organic boundary layer does not occur, even for the highest incident energies explored, 560 meV.     

Fano interference with the alternating asymmetry parameter in time-domain experiments

It has been found that the time-domain reflection signal arising after the excitation of a bismuth single crystal at helium temperature by an ultrashort laser pulse exhibits Fano interference in the presence of the interaction between the lattice and charge degrees of freedom. This property is manifested in the characteristic spectral lineshape of the Fourier-transformed signal, whose asymmetry parameter is a periodic function of the delay time τ between the pumping and probing pulses. The most interesting property is the change of the sign of the asymmetry parameter when τ varies, which indicates that time reversal symmetry is broken. Various possible causes of the effect, observed for the first time, are discussed.

     

Dynamics of coherent anharmonic phonons in bismuth using high density photoexcitation

We have investigated the dynamical properties of the coherent anharmonic phonons generated in Bi under high density excitation. The time-resolved reflectivity in the intensely photoexcited Bi film is modulated by the coherent A(1g) phonon oscillation with a time-dependent oscillation period. As the pump power density is increased, the line shape of the A(1g) mode in the Fourier transformed spectra becomes asymmetric, and the redshift of the phonon frequency is observed. Analysis of the transient redshift with a wavelet transform reveals that the frequency of the A(1g) mode depends on the squared amplitude of the oscillation, which is attributed to an anharmonicity of the lattice potential.     

Molekulare konformationen und hyperkonjugation: SiC und GeC hyperkonjugation und die konformation von β-silyl- und β-germyläthyl kation und anion

The conformational consequences of hyperconjugation in β-silyl- and β-germylethyl cations and anions have been investigated using an extended CNDO/2 approach. It is shown that the recently published conformational rules for β-substituted cations and anions fail in the cases considered here. The reason for their failure has been detected.     

In situ spectroscopic measurement of structural change in SiO2 during femtosecond laser irradiation

Spectra of light transmitted through SiO₂ specimens were measured during irradiation of amplified 800 nm ultrashort (400 fs) laser with a repetition rate of 100 kHz. The spectra exhibit a broad dispersion including a peak at around 450 nm, and the peak wavelength and the intensity are found to significantly depend on the irradiation time. Positively charged oxygen vacancies (E center) and oxygen-deficiency centers (ODCs) with neutral charge were observed by electron spin resonance (ESR) and photoluminescence (PL), respectively, after irradiation. The formation of the E center and the subsequent transformation to ODCs significantly depend on the laser power density. The correlation between the intensity of the peak at around 450 nm and the ESR signal intensity of the E center, and also the observation of PL spectrum due to ODCs suggest that self-trapped excitons exhibiting the peak at around 450 nm are created through multi-photon absorption, followed by the formation of the E center, and this finally leads to the formation of ODCs. PACS 78.47.+p; 61.80.Ba; 76.30.Mi     

On the nature of coherent phonons generated by ultrashort laser pulses in single-crystal antimony

Reflectivity oscillations generated by A_1g coherent phonons in an antimony single crystal have been studied by a method involving pumping and probing by femtosecond laser pulses, which was complemented by spectral filtration of the signal. An analysis of the spectrally resolved signal showed that not only the integrated intensity but also the spectrum of the probe pulse are functions of the delay time between the pumping and probing and oscillate between the Stokes and anti-Stokes components at the optical-phonon frequency. A comparison of the integrated lattice excitation relaxation dynamics with the spectrally resolved lattice excitation relaxation dynamics revealed new facets in the nature and generation mechanism of coherent phonons.     

Search for magnetic order in undoped and doped spin‐gap systems by μSR

We introduce our μSR investigations of spin‐gap systems, such as, (1) a 2‐leg spin‐ladder material SrCu₂O₃, (2) a Haldane material (S=1 spin‐chain) Y₂BaNiO₅, (3) a spin‐Peierls material CuGeO₃, (4) a spin‐chain# material Sr₆Ca₈Cu₂₄O₄₁. All of these antiferromagnetic spin systems are characterized by a spin‐gap between the singlet ground‐state and the triplet first excited states. In the above‐mentioned materials, we confirmed the absence of magnetic order down to milli‐Kelvin regime, supporting the non‐magnetic feature of the ground‐state. If a spin‐gap system is doped with charges and/or vacancies at the spin site, unpaired spins are induced out of the singlet ground‐state. In some materials, doping completely destroys the singlet ground‐state and induces a bulk magnetic order. We report μSR investigations of doped materials as well, which clarifies the existence/absence of a magnetic order upon doping. This revised version was published online in July 2006 with corrections to the Cover Date.     

CNDO/2 (complete neglect of differential overlap)-method for third-row molecules

An extension of the CNDO/2 method to compounds containing third-row elements (Germanium, Arsenic, Selenium and Bromine) is presented. Bond lengths, bond angles, dipole moments, and ionization potentials are considered.     

Precolumn derivatization for chromatographic and electrophoretic analyses of carbohydrates

Sensitive detection and high chromatographic resolution are required for analysing the structures of oligosaccharides available in only limited amounts. Precolumn derivatization is one of the most suitable methods for this purpose as it can compensate for the shortcomings of the intrinsic nature of oligosaccharides. Recently published methods of precolumn derivatization are reviewed and advantages and problems are discussed.