R. Neumann

Fast laser beam position control with submicroradian precision

Abstract Precise control of spatial position and pointing stability of the output beam of a cw free-jet ring dye laser was realized. The fast opto-electronic feedback device, based on four-quadrant photodiode detectors and piezo-driven mirrors, limits beam angle variations to 4 × 10 -7 rad. The importance of the method is discussed and the efficiency is illustrated.

Scanning tunneling microscopy of liquid crystals, perylene‐tetracarboxylic‐dianhydride, and phthalocyanine

Scanning tunneling microscopy measurements conducted on pure liquid crystals and liquid crystals in combination with the dyes perylene‐tetracarboxylic‐dianhydride (PTCDA) and phthalocyanine (Pc), all adsorbed on highly oriented pyrolytic graphite (HOPG), are being reported on. The liquid crystal octylcyanobiphenyl (8CB) was used as a solvent for the dyes, allowing the preparation of ordered dye layers in a very simple way. A combination of vapor deposition of PTCDA and liquid deposition of 8CB provided films, where 8CB and PTCDA are embedded in each other on a small length scale. The samples were prepared in two different ways yielding either 8CB islands within a PTCDA monolayer or PTCDA islands surrounded by 8CB. The Pc films on HOPG exhibit a hexagonal molecular lattice that could be related to the lattice of the graphite substrate and for which a structural model is proposed. For dodekylcyanobiphenyl (12CB) on HOPG the ordering in unit cells containing 10 rather than 8 molecules is reported for the fir...

Intracavity Atomic Beam Laser Spectrometer for Low Intensity Spectral Lines

A laser spectrometer with an intracavity atomic beam having a spectral resolution of about 3 MHz was developed and applied to atomic resonance fluorescence spectroscopy of the very weak Ca intercombination line λ=6573 Å.

Isotope shift of calcium isotopes withA=40, 42, 43, 44, 46 and 48 by laser spectroscopy

Isotope shifts of all the stable Ca isotopes in the intercombination lineλ=6573Å have been studied by laser spectroscopy. The nuclear volume effect for43Ca and46Ca could be evaluated from the measurements and the quadrupole moment of43Ca was obtained.

Contrast mechanisms in photothermal scanning tunneling microscopy

By irradiation of the tunneling junction of a scanning tunneling microscope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal images were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsible for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from either the current variations caused by the non-zero reaction time of the current control loop or from a geometry factor. In both cases the images reflect certain properties of the sample topography. On the other hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the effective barrier height.

Dye laser saturation spectroscopy of the 23S1-23P transition in6, 7Li+ ions

The metastable 23S1 state and the short lived 23P states of the helium-like6, 7Li+ ion spectra have been investigated by dye laser saturation spectroscopy in a low-energy Li+ ion-beam and fluorescence light detection. The hyperfine structure splittings of all the levels, the 23P fine structure intervals and the isotope shift of the 23S1-23P transitions have been measured. These measurements were made by application of a specially constructed tunable dye laser system capable of single-mode laser scans over more than 60 GHz.

Heterodyne laser spectroscopy of lithium ions: 23P fine and hyperfine structure of 7Li+

The fine structure (fs) splittings in the 1s2p3PJ=1,2,0 multiplet of the helium-like7Li+ ion were measured with a laser spectrometer. The results with 3σ errors are: Δv01 (3P0,F=3/2−3P1,F=5/2)=152081.6(2.0) MHz and Δv02 (3P0,F=3/2−3P2,F=7/2)=82704.3(1.9) MHz. Combining the new precise fs measurements with earlier hyperfine structure (hfs) results from laser-microwave spectroscopy provided improved fs constants:D1=−155709.0(2.1) MHz,D2=−93049.2(2.0) MHz, and hfs constants:Ac=3679.0(6) MHz,A0=51.0(4) MHz, andAd=−11.3(1) MHz, thus allowing for a stringent test of available theoretical data. The spectroscopic method used in this work opens up the possibility of determining Li+, 23S−23P absolute transition frequencies with a precision of ∼2·10−9.

Laser-microwave spectroscopy of lithium ions: 23S1 hyperfine structure of7Li+

The hyperfine structure (hfs) splittings of the metastable 1s2s3S1 state of7Li+ have been measured with combined laser optical pumping and microwave resonance. A lowenergy Li+ ion beam, optically excited by an intersecting laser beam, passed a waveguide where radio frequency transitions were induced. The resulting population transfer among the hfs levels of the3S1 was detected via the change in intensity of the fluorescence light from a second crossing region of laser light and ion beam located past the waveguide. The magnetic hfs constantA(7Li+, 1s2s3S1) was measured and compared with theory. A deviation of the two transition frequenciesν(F=3/2−F=5/2) andν(F=1/2−F=3/2) from the interval rule is due to a depression of theF=3/2 hfs sublevel, caused by mixing of the 23S1 and 21S0 states via hyperfine interaction. This shift was never observed so far in a two-electron spectrum, because of absence ofI>1/2 isotopes in He, the only two-electron atom investigated spectroscopically with high precision. The size of the shift is in fair agreement with a theoretical estimate.

An automatic michelson interferometer with frince dropout correction

Abstract An automatic Michelson-type fringe counting interferometer (Lambda-meter) allowing for interpolation to 1/50 of a wavelength is described. The movable part of the interferometer consists of a carriage which slides on two polished steel bars and transports two corner cube retroreflectors. A missing-fringe digital control logic identifies and instantaneously corrects for laser interference fringe dropouts, caused by short-time laser light instabilities. By exact superposition of the beams of a He 22 Ne laser stabilized to a line of I 2 and the laser to be measured in the interferometer, absolute wavelength measurements with a precision of 1 to 2 parts in 10 8 were performed. Real-time control of the measurement, data acquisition and calculation of the wavelength are provided by a microcomputer.

Two-photon intracavity dye laser spectroscopy of the 4S and 3D term in6, 7Li

Doppler-free two-photon spectroscopy of several states of6, 7Li has been achieved by placing an atomic beam inside the dye laser cavity. The hyperfine structure splitting of the 4S state, the fine structure splitting of the 3D level as well as the isotope shifts of the 2S–4S and 2S-3D transition have been measured, by scanning the single-mode laser frequency and detecting the fluorescence light. In addition, signal detection by three-photon ionization via the resonant 3D state was accomplished.