Charge transport in a polypeptide chain

Charge transport is one important example of signal transduction in a protein which is responsible for action at a distance, and is a fundamental process in biochemical action. A model is presented in which electronic effects interact with motional processes to combine into a bifunctional model. This model is investigated with new detailed molecular dynamics calculations and successfully explains such action at a distance. Received 1st February 2002 / Received in final form 26 May 2002 Published online 13 September 2002     

Two-color resonance photoionization spectrum of nickelocene in a supersonic jet

Two-color photoionization of nickelocene molecules cooled in a supersonic jet is performed using a tunable nanosecond pulsed laser. The first stage of the multiphoton excitation is the transition from the highest occupied molecular orbital of nickelocene to the lowest Rydberg level. Conditions are found under which molecular ions (η ⁵-C₅H₅)₂Ni⁺ are the only product of the multiphoton ionization in the one-color experiment. Irradiation of an excited molecule by an intense pulse of another laser increases significantly the yield of molecular ions. The dependence of the yield of (η⁵-C₅H₅)₂Ni⁺ ions on the frequency of the second laser makes it possible to determine the adiabatic ionization potential of nickelocene as 6.138±0.012eV.     

Two-color photoionization spectroscopy of jet-cooled nickelocene

Two-photon ionization of the nickelocene molecules cooled in a supersonic jet was performed for the first time by simultaneous excitation of (⁵–C₅H₅)2Ni with two tunable nano- second dye lasers. The one-photon transition from the HOMO to the Rydberg R4p level was used as the initial step of the multiphoton excitation. In a one-color experiment, the conditions were found for generation of the intact molecular ion, (⁵–C₅H₅)2Ni⁺, as the only ionic product of the multiphoton ionization. The use of an intense pulse of the second dye laser lead to an increase in the yield of the molecular ion.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1604–1606, August, 2004.     

Single rotational lifetimes of formaldehyde in a hypersonic jet

Radiationless transitions have recently been shown to also depend on their rotational energy content. Careful documentation of this effect demand lifetime measurements of single rotational states. These must, however, be documentable by a complete computer simulation. The molecule cannot be too large to afford complete spectroscopic separability of lines and accurate spectroscopy for use in the computer simulation. Furthermore measurements should be carried out in a beam to avoid the almost impossible extrapolation to the rotationally “collisionless” regime. Formaldehyde is apprarently a good candidate due to its highly documented spectroscopy including magnetic perturbations. The latter is particularly prominent for the 2²4¹ vibronic absorption, but the rotational transitions are still grossly congested (1902 transitions with a peak intensity above 1% are calculated to contribute to the band contour). Experiments in a hypersonic jet cool the molecules to 17 K without leading to aggregates. The rotational states are well separated and very pure, as shown by detailed computer simulation for the asymmetric rotor including contrifugal distortion. Lifetimes of some 18 states are reported as a function of rotational quantum numbers and shown to vary substantially. The symmetric and antisymmetric nuclear spin forms of formaldehyde here show up as separate isomers during the expansion.     

Quick assessment methodology for reliability of solder joints in ball grid array (BGA) assembly—Part I: Creep constitutive relation and fatigue model

In this study, a new unified creep constitutive relation and a modified energy-based fatigue model have been established respectively to describe the creep flow and predict the fatigue life of Sn−Pb solders. It is found that the relation successfully elucidates the creep mechanism related to current constitutive relations. The model can be used to describe the temperature and frequency dependent low cycle fatigue behavior of the solder. The relation and the model are further employed in part II to develop the numerical simulation approach for the long-term reliability assessment of the plastic ball grid array (BGA) assembly. The project supported by the National Natural Science Foundation of China (59705008)     

Effect of collision and magnetic field on quantum beat in biacetyl

Molecular quantum beats are shown for the case of biacetyl, to vary strongly under the influence of weak magnetic fields. Values for dephasing constants for a gas-phase molecular system were determined. Information on sub-Doppler level splittings in the intersystem-crossing process is obtained.     

Intensity and lifetimes of ZEKE spectra

The questions addressed in this Letter are whether the lifetime or intensity of long-lived molecular ZEKE/Rydberg states are influenced by the intensity of background ions and hence whether these background ions are mechanistically involved in the formation of ZEKE states. We performed an experiment changing the ion density which had no appreciable effect on the intensity of the ZEKE signal. ZEKE states with a lifetime of many tens of microseconds are here not changed by background ions, either in lifetime or intensity.