N-labeling of di-and tripeptides reveals that electron capture to doubly protonated peptides results almost exclusively in ammonia loss from the N-terminal end, which clearly shows that a significant fraction of electron capture occurs at this end. In accordance with this finding, the competing channel of N-C R bond breakage leads to z +• ions and neutral c fragments after electron capture to small dications. In larger peptides that live long enough for internal proton exchanges to occur, c + ions are also formed and in some cases in dominant yield. Attachment of one or two crown ethers to ammonium groups is likely to reduce the probability of proton transfer, which enhances the formation of z +• relative to c + . The total yield of z +• and c + is, however, more or less unchanged, which indicates that proton transfer or hydrogen transfer from a NH 3 group to the amide group is not required for the N-C R bond breakage.

Direct evidence of the interference effect in the electron emission spectra from ionization of molecular hydrogen in collisions with bare C and F ions at relatively low collision energies is presented. Oscillations due to the interference are deduced by comparing the measured double differential cross sections of the electrons emitted from molecular hydrogen to those emitted from atomic hydrogen, rather than using the calculated cross sections for H as in a previous report. We believe these experimental data provide stronger support for the evidence of the interference effect. We show that it is not only a feature of very high energy collisions, but also a feature to be observed in relatively lower energy collisions.

We use the forward-backward angular asymmetry in the electron emission cross sections in fast ion impact ionization of H 2 as a probe of the inversion symmetric coherence in homonuclear diatomic molecules. The electron energy dependence of the asymmetry parameter for H 2 exhibits oscillatory structure due to Young-type interference in contrast to atomic targets such as He. The asymmetry parameter technique provides a selfnormalized method to reveal the interference oscillation independent of theoretical models and complementary measurements on atomic H target. Angular distribution of various types of radiations ͑particles and photons͒ is known to be quite sensitive to various effects associated with different physical processes in atomic, nuclear, plasma physics and other branches of physics. In fast ion-atom ionization, the long range Coulomb interaction of the final state electrons with the target and the projectile ions influences the evolution of the electron wave function and thereby the angular distribution of electron emission. Such two-center effect is known to cause a large forward-backward asymmetry ͓1-4͔ in the electron emission spectrum. The electron emission spectrum from the simplest diatomic molecule H 2 manifests yet another important aspect of interference ͓5͔ in ion-atom ionization besides the wellknown mechanisms such as soft collision, two-center effect and binary encounter ͓1-4,6-8͔. Since the two indistinguishable H atoms in the H 2 molecule may be considered as the coherent emission sources of phase coupled electrons in a large impact parameter collision, their contributions add coherently and an interference effect should be observed. Therefore, the electron emission from H 2 may be viewed as a natural coherent system which is similar to Young's double slit interference phenomenon ͓5͔. We demonstrate here that the additional mechanism of Young-type interference plays a major role in the angular asymmetry of electron double differential cross section ͑DDCS͒ and asymmetry parameter itself would be a sensitive test to study the interference for a diatomic molecular target.Following the initial theoretical studies on the interference effect in electron scattering ͓9͔ and photoionization ͓5͔, very recently the evidence of Young-type interference was found in the fast-ion collisions with H 2 ͓10-12͔. Ideally one would have expected an oscillation in the DDCS spectrum due to interference. But a steep fall of the DDCS by about four or five orders of magnitude ͑see below͒ does not allow one to observe the oscillation directly. The oscillations, thereby, were observed in the DDCS ratios ͑H 2 -to-2H͒ which was explained due to the interference. However, the experiments using H are rare due to the experimental constraint and oscillations in the DDCS ratios were observed ͓4,12͔ in such experiment with H. Theoretical DDCS for atomic, or effective atomic H have also been employed ͓10,11͔ in the absence of an atomic H target. In such cases, the shapes of the oscillations are sensitive to the atomic parame...

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In this paper, we report the first comprehensive observations of local time asymmetries in densities and scale heights (temperatures) of the Martian upper atmosphere (between 150 and 300 km) measured by the Mars Atmosphere and Volatile Evolution mission/Neutral Gas and Ion Mass Spectrometer. For this purpose, we use the densities and temperatures of Ar. In general, the daytime densities and temperatures are greater than the nighttime values. The maximum and minimum values, however, are observed at the dusk and dawn terminators, respectively. An enhancement at the dusk terminator is persistently observed at all altitudes; however, the time of the peak enhancement shifts toward sunlit hours with increasing altitude. At the dawn terminator, a minimum in density is observed at altitudes of 150–170 km. At higher altitudes, the minimum is observed close to midnight. Accordingly, the dawn‐dusk asymmetry is more prominent at 150–170 km and decreases with increasing altitude. A maximum ratio of six is observed between the dusk and dawn densities at 160 km. In addition, the local time for the maximum ratio at each altitude moves toward sunlit hours with increasing altitude. The observed asymmetry is explained in terms of dynamical heating and cooling due to convergent and divergent winds at the dusk and dawn terminators, respectively. In addition, upward propagating gravity waves generated by the solar terminator wave and O/CO2 radiative cooling are also proposed as important mechanisms contributing to the observed asymmetry.

Circular dichroism (CD) experiments on DNA single strands (dA(n)) at the ASTRID synchrotron radiation facility reveal that eight adenine (A) bases electronically couple upon 190 nm excitation. After n=8, the CD signal increases linearly with n with a slope equal to the sum of the coupling terms. Nearest neighbor interactions account for only 24% of the CD signal whereas electronic communication is limited to nearest neighbors for two other exciton bands observed at 218 and 251 nm (i.e., dimer excited states). Electronic coupling between bases in DNA is important for nonradiative deexcitation of electronically excited states since the hazardous energy is spread over a larger spatial region.

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We report on evaporation studies on positively charged water clusters (H(+)(H(2)O)(N)) and negatively charged mixed clusters (X(-)(H(2)O)(N)) with a small core ion X (X=O(2), CO(3), or NO(3)), in the size range N=5-300. The clusters were produced by corona discharge in ambient air, accelerated to 50 keV and mass selected by an electromagnet. The loss of monomers during the subsequent 3.4 m free flight was recorded. The average losses are proportional to the clusters' heat capacities and this allowed the determination of size-dependent heat capacities. The values are found to increase almost linearly with clusters size for both species, with a rate of 6k(B)-8k(B) per added molecule. For clusters with N<21 the heat capacities per molecule are lower but the incremental increase higher. For N>21 the values are intermediate between the bulk liquid and the solid water 0 degrees C values.