Laser multiphoton mass spectroscopy of organometallic compounds: State selective and mass resolved detection of neutral fragment tellurium atoms from C2H5TeC2H5

Typical features in laser multiphoton ionization of organometallic compounds are well evident in the case of the diethyltelluride C₂H₅TeC₂H₅ molecule. The use of a tunable dye laser coupled with time-of-flight (TOF) mass spectroscopy has allowed to establish that a large amount of tellurium is eliminated from the parent molecule as a neutral atom either in its ground or low excited states. Sharp two- and three-photon atomic Te resonances, which give origin to extraproduction of Te⁺ ions, have been identified in the optical spectra measured by varying the laser wavelength.     

Dead time correction of photon correlation functions

The dynamic range of single photon counting measurements in quasi elastic light scattering is restricted by detector and counter dead time effects. While distortions of single interval statistics have been treated at great length, only lowest order corrections or very special cases of dead time effects on temporal correlation functions were computed in the past.Dead times result in a direct distortion of correlograms on time scales comparable to the dead time. This effect exists even at low count-rates. It is independent of the count rate for paralyzable systems. Nonparalyzable systems show a count rate dependence with increasing correlation times at high count rates.Furthermore, counting saturation produces additional distortions extending to all lag times. These distortions are computed for the rather general case of -distributed intensities with arbitrary shape of the photon correlation function. Such signals are commonly found in multiparticle homodyne experiments with a finite size detector, i.e. arbitrary value of the intercept or contrast of the correlogram. Exact results are provided for the paralyzable system including the effect of fluctuating dead times. The latter case is then used to compute a useful approximation for nonparalyzable systems as well.     

Laser FM spectroscopy with photochemical modulation

We have combined the techniques of frequency-modulation spectroscopy (FMS) and photochemical-modulation spectroscopy to carry out high-resolution, high-sensitivity absorption measurements on the formyl and amino radicals. Using the (0,9⁰, 0)-(0,0¹, 0) band of theÃ,² A- ,² A transition of HCO at 614 nm, we obtained a sensitivity limit for absorption of 1.5×10^–6. Reconstructed spectra of several HCO lines are presented.     

Mechanism and controlling strategy of the production and accumulation of propionic acid for anaerobic wastewater treatment

The production and accumulation of propionic acid affect significantly anaerobic wastewater treatment system, but the reasons are not approached until now. Based on the results of continuous-flow tests and the analysis of biochemistry and ecology, two mechanisms of producing propionic acid have been put forward. It is demonstrated that the reasons of propionic acid production and accumulation are not caused by higher hydrogen partial pressure. The combination of specific pH value and ORP is the ecological factor affecting propionic acid production, and the equilibrium regulation of NADH/NAD+ ratio in cells is the physiological factor. Meanwhile, it is put forward that using the two-phase anaerobic treatment process and the ethanol type fermentation in anaerobic reactor to avoid propionic acid accumulation are efficient methods.     

Time-resolved photo-electron spectroscopy on mass-selected metal clusters using a regenerative femtosecond amplifier up to 100 Hz

A femtosecond photo-electron experiment is described which provides excellent conditions for measuring time-resolved photo-electron spectra of free, monodispersed cluster anions using repetition rates up to 100 Hz. Cluster anions are synthesized in an electric arc and subsequently cooled in a helium carrier gas. A time-of-flight spectrometer is used for mass separation of the negatively charged clusters. The kinetic energy of the photo-electrons is analyzed by a magnetic-bottle time-of-flight spectrometer, which guarantees a maximum collection efficiency. Femtosecond laser pulses are generated by a seeded regenerative Ti:Sa amplifier, which is externally pumped with the second harmonic of a diode-pumped solid-state Nd:YAG laser. A retroreflector mounted on a computer-controlled translation stage serves as a reproducible time delay of the probe pulse. The high energy output of the laser pulses (∼3 mJ) in combination with the variable repetition rate and the high stability of the amplified pulses provide excellent conditions for recording pump-probe photo-electron spectra of mass-separated cluster anions even at the fairly low ion density of pulsed plasma cluster sources. First results on the electron dynamics of the Pt₃ ^- cluster demonstrate the reliability of the whole system. Further experimental investigations will concentrate on electron-relaxation processes in transition- and noble-metal clusters as well as on the nuclear and transition-state dynamics of chemically reacted adsorbate clusters. Received: 7 January 2000 / Published online: 7 August 2000     

Double photoionization of rotating linear molecules

This communication presents a theoretical study of the angular distribution of one or both of the two electrons emitted in one-photon, one-step double ionization of a linear molecule. Experiments which do or do not detect spin of the photoelectrons have been considered. Effects of molecular rotation on double photoionization have been studied in both Hund's coupling schemes (a) and (b) by using parity-adapted states. Selection rules obtained in this paper are very different from those derived earlier for single photoionization and for Auger decay following the absorption of a photon in a rotating linear molecule. It is shown that complete specification of the spin-unresolved and of spin-resolved angular distributions of both photoelectrons require, respectively, three and seven parameters which depend, among other things, on their energies as well as directions of emission. The approach developed in this paper has been used to analyze spin-unresolved double photoionization in the shell of the molecule. The angular distribution is quite different depending on whether or not molecular rotation has been taken into account. Also, it is found to change significantly for different rotational transitions. Effects of electron-electron correlation are clearly manifested even in non-coincident, both rotationally resolved--as well as unresolved--double photoionization. Received: 7 August 1998     

Magnetostructural transition in NdCu2

The magnetic (H , T)-phase diagram of the orthorhombic compound NdCu₂ was investigated for external magnetic fields up to 15 T parallel to the crystallographic c-direction. Magnetization and magnetostriction measurements reveal an anomalous change of the magnetic properties as well as giant magnetostriction (GMS) and large hysteretic effects. This behaviour is similar to that observed in some other RCu₂ compounds where it has been interpreted as a conversion of the magnetic Ising axis. In contrast to these other RCu₂ compounds, however, the easy axis of magnetization in NdCu₂ is the b-axis. The macroscopic measurements are compared with neutron diffraction experiments which reveal GMS along the b-axis and a new magnetic phase with propagation vector in the converted crystal. Received 27 March 2000 and Received in final form 11 September 2000     

Soft X-ray magneto-optics at rare-earth 4d-4f thresholds

We report on X-ray magneto-optics (XMOKE) at the 4d-4f excitation threshold of rare-earth elements and its application to measuring temperature-dependent magnetization as well as element-specific hysteresis curves in heteromagnetic multilayers. We outline some advantages of XMOKE as compared to standard magnetooptics in the visible light regime. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Absolute frequency measurement of a CO2/OsO4 stabilized laser at 28.8 THz

A cw carbon dioxide laser operating on the 10 μm R(0)_I transition (28.832 THz) was frequency stabilized by a servo lock to the saturated absorption dip of the Q(15) transition of ¹⁸⁸OsO₄. The laser frequency was measured with a cesium-clock-based frequency chain. In addition, the absorption line frequencies Q(14) of ¹⁸⁸OsO₄ and sQ(4,3) of ¹⁵NH₃, were measured relative to the frequency of Q(15). Received: 21 August 2000 / Revised version: 15 January 2001 / Published online: 30 March 2001     

O<Subscript>2</Subscript> photodesorption from a Ag<Subscript>8</Subscript>O<Stack><Subscript>2</Subscript><Superscript>-</Superscript></Stack> cluster

The decay path of an Ag₈(O₂)^- cluster photoexcited by a 3.1 eV photon is elucidated using time-resolved photoelectron spectroscopy. Photoabsorption results in the formation of an excited state giving rise to a peak in the photoelectron spectra with well-resolved vibrational finestructure. With a lifetime of about 100 fs this bound state decays into an anti-bonding state which dissociates into O₂ and Ag₈^- on a timescale of 10 ps. In the photoelectron spectra, this corresponds to a broad maximum shifting gradually towards higher binding energy while the O₂ and Ag₈^- separate. Finally, the spectrum of bare Ag₈^- appears. This process is unique to small clusters, because on metal surfaces excited state lifetimes are too short to allow for direct dissociation.