H. Zacharias

Spin Selectivity in Electron Transmission Through Self-Assembled Monolayers of Double-Stranded DNA

Photoelectrons emitted from a DNA-covered gold surface can have an unbalanced spin population of up to 60%. In electron-transfer processes, spin effects normally are seen either in magnetic materials or in systems containing heavy atoms that facilitate spin-orbit coupling. We report spin-selective transmission of electrons through self-assembled monolayers of double-stranded DNA on gold. By directly measuring the spin of the transmitted electrons with a Mott polarimeter, we found spin polarizations exceeding 60% at room temperature. The spin-polarized photoelectrons were observed even when the photoelectrons were generated with unpolarized light. The observed spin selectivity at room temperature was extremely high as compared with other known spin filters. The spin filtration efficiency depended on the length of the DNA in the monolayer and its organization.

Rotational line strengths in two- and three-photon transitions in diatomic molecules

Abstract Rotational line intensity factors are calculated for two- and three-photon excitation of diatomic molecules belonging to Hunds case (a) and intermediate (a)-(b) coupling schemes. The intensity factors compare well with experimentally measured normalized line intensities for two-photon transitions in the Lyman-Birge-Hopfield system of N2, the γ-bands of NO and the fourth-positive system of CO and for three-photon excitation of the fourth-positive system of CO.

Laser photofragment spectroscopy of the NO2 dissociation at 337 nm. A nonstatistical decay process

The dynamics of the photodissociation of the NO2 molecule into NO(X 2Π1/2,3/2; v, J)+O(3P0,1,2) at the N2 laser wavelength 337 nm (=29 965 cm−1) has been investigated by laser photofragment spectroscopy at room temperature in bulk. The internal energy distribution of the NO(X 2Π) fragment has been analyzed state selectively with respect to the electronic energy (Ω=1/2, 3/2) and rotation–vibration energy (v, J) using laser induced fluorescence excitation for the NO observation by the A–X transition in the ultraviolet. The rotational and vibrational populations strongly deviate from equilibrium distribution indicating a nonstatistical decay mechanism. The partitioning of the maximum available excess energy of 4842 cm−1 is about 70% into internal and 30% into recoil fragment energy. The distribution of the internal energy is about 51% into vibration and 16% into rotation of NO, with an inversion in the vibrational population of the accessible levels v=0,1,2.

Two-photon excitation of NO(A2Σ+; ν′ = 0,1,2) and radiation lifetime and quenching measurements

Abstract The NO molecule has been excited with tunable pulsed dye laser by two-photon absorption into ν′ = 0,1,2 levels of the A 2 Σ + state at bandwidths down to Doppler width. The method has been used to measure the radiation lifetime and the collisional deexcitation of rotationally resolved levels in the A 2 Σ + state.

Spatio-temporal coherence of free electron laser pulses in the soft x-ray regime

The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two time-delayed partial beams directly on a CCD camera. The partial beams are obtained by wave front beam splitting in an autocorrelator operating at photon energies from h nu = 30 to 200 eV. At zero delay a visibility of (0.63+/- 0.04) is measured. The delay of one partial beam reveals a coherence time of 6 fs at 23.9 nm. The visibility further displays a non-monotonic decay, which can be rationalized by the presence of multiple pulse structure.

Multiphoton ionization of methyl iodide and selective stepwise ionization of methyl radicals

The collisionless multiphoton ionization of methyliodide in the isotopic forms CH3I and CD3I has been investigated with pulsed laser light of two wavelengths: fixed‐frequency 266 nm of high intensity (∼1×107 W/cm2) and tunable around 215 nm of relatively lower intensity (∼2×106 W/cm2). The 215 nm was chosen because the methyl radicals CH3 and CD3 absorb there resonantly through the (0,0) (B 2A′1←X 2A′′2) band transition with the B state being very heavily predissociated. Objectives have been: (1) the ionization mechanism of CD3I to form ion fragments D+, CD+, CD2+, CD3I+, and I+, obtained with 266 nm alone, and 266 nm together with tunable 215 nm, (2) the possibility of influencing the MPI by selective excitation of an intermediate (CD3, CH3), and (3) detection and spectroscopy of the methyl radical by two‐photon ionization through the decaying B 2A′1 state as intermediate step. Mass spectra have been taken and measurements have been made with respect to the abundance of fragments, power dependence, se...

Rotational and electronic relaxation in pulsed supersonic beams of NO seeded in He and Ar

Resonantly enhanced two‐photon ionization has been used to obtain detailed information about the rotational and electronic relaxation in pulsed supersonic expansions of nitric oxide seeded in helium and argon. Due to the sensitivity of the method we were able to measure rotational distributions up to high quantum numbers J″>24.5. Measurements were made for stagnation pressures ( p0) and nozzle diameters (d) in the range 0.44≤p0d≤22 [Torr cm] for NO/Ar beams, and 0.88≤p0d≤36 [Torr cm] for NO/He beams. In general non‐Boltzmann rotational population distributions were observed. Furthermore, we found that for NO/He beams, the two electronic substates 2Π1/2 and 2Π3/2 of NO were not in local equilibrium. These observations can be understood in terms of a simple model using state‐to‐state collision cross sections combined with the empirical hydrodynamic equations to describe the isentropic expansions.

Laser spectroscopy of desorbing molecules

In this article the application of tunable dye lasers to desorption phenomena is illuminated. These lasers provide radiation continuously tunable from 105 nm in the vacuum ultraviolet to about 10 μm in the mid-IR. By employing either laser induced fluorescence (LIF) or resonance enhanced multiphoton ionization (REMPI) spectroscopy almost all diatomic and many polyatomic molecules can be probed with the sensitivity required to detect desorbing molecules under UHV conditions. The spectral resolution of the lasers is sufficiently high that rotational state selectivity is achieved. Recent developments permit in addition the velocity distributions of molecules to be determined with internal quantum state resolution. Therefore very detailed information about the molecular dynamics has been obtained. In most experiments so far reactive recombinations off surfaces have been investigated. In this paper special emphasis will be given to the recombination of hydrogen on copper and palladium surfaces. For these systems very detailed data about the internal state populations at various surface temperatures have been obtained. The rotational cooling previously observed in molecular beam scattering has also been established for desorption. Strong vibrational excitation has been observed, which in the case of desorption from copper may be associated with the recombination dynamics, whereas for desorption of D2 from Pd(100) a molecular precursor state might be responsible. By measuring the velocity distribution in each quantum state, the complete energetics of the desorbing molecules has been determined. Some first experiments on laser induced desorption with state selective detection of the desorbing molecules will also be discussed. Finally, making use of the polarization analysis of the signal, alignment effects in the desorption can be observed, permitting observation of molecular dynamics with a “magnifying glass”.

Resonant two photon ionization of H and D atoms

Abstract H and D atoms have been ionized with tunable VUV + UV laser light by two-photon absorption, resonantly through the 2 2 P 1 2 , 2 2 P 3 2 fine structure doublet, with total energies very close to the ionization potential. Experiments have been carried out in the ionization chamber of a quadrupole massfilter, in a crossed atomic-laser beam configuration with sub- Doppler resolution of ∼ 10 GHz. The sensitivity is ⪆ 2.5 x 10 5 atoms per cm 3 .

State selective step-wise photoionization of NO with mass spectroscopic ion detection

State and isotope selective two-step photoionization of NO with mass spectroscopic ion detection has been demonstrated and investigated. Using saturation condition the photoionization cross section for a single rotational level of the intermediate state, No(A2Σ+, ν′=0), has been measured: σi=(7.0±0.9) X 10−19 cm2. The charge transfer15NO++14NO→15NO+14NO+ has been observed and investigated, yielding a cross section of the order of 13×10−16 cm2, consistent with recent measurements at about 1 eV.