B. Sc. (Physics) National College Basavanagudi, Bangalore M. Sc. (Physics) Bangalore University Ph.D from Tata Institute of Fundamental research (TIFR)
Abstract: The dissociation dynamics of multiply charged methanol molecules formed in collision with 1.2 MeV Ar8+projectiles is studied. Using coincidence mapping techniques, we can separate out the different dissociation pathways between carbon, oxygen and hydrogen ionic fragments as well as two- and three-body breakup events. Reactions involving intramolecular bond rearrangements within the CH3 group of the dissociative molecule are discussed in detail. A signature of hydrogen migration in doubly charged methanol is observed. Kinetic energy releases of different breakup channels are reported here and compared with values calculated from a Coulomb explosion model. The shape and orientation of the islands in the coincidence map give further information about the momentum balance in the fragmentation process of two- or many-body dissociation pathways.
Abstract: The projectile charge-state dependence of methane fragmentation is studied at a constant projectile (Arq+, 1q7) velocity of 0.42 a.u. using the technique of time-of-flight mass spectroscopy. The relative cross sections for fragment production are reported as a function of the incident projectile charge state. The fragments appearing as molecular ions show a decrease in cross sections with increasing projectile charge state as opposed to the carbon ions which show an increase with increasing projectile charge state.
Abstract: We report on the results of dissociation dynamics of multiple charged acetylene molecules formed in collision with 1.2 MeV Ar8+ projectiles. Using the coincidence map, we can separate out the different dissociation pathways between carbon and hydrogen ionic fragments as well as complete two-body breakup events. From the measured slopes of the coincidence islands for carbon atomic fragments and theoretical values determined from the charge and momentum distribution of the correlated particles, we observe a diatomlike behavior of the C–C charged complex during dissociation of multiply charged C2H2. We conclude that this behavior in breakup dynamics is a signature of sequentiality in dissociation of this multiply charged molecular species. The shape and orientation of the islands give further information about the momentum balance in the fragmentation process of two- or many-body dissociation pathways. Kinetic energy release of different breakup channels are reported here and compared with values calculated from the pure Coulomb explosion model.
Abstract: We report here the results of dissociation of multiple charged methanol and acetylene molecules in collision with 1.2 MeV Ar8+ projectiles. We observed a wide range of dissociation products from the TOF spectrum starting from undissociated molecular ions, fragments losing an hydrogen atom due to breakage of C-H and/or O-H bonds, to complete rupture of C-C and C-O skeletons for the respective molecules. From the coincidence map of the fragments, we could separate out the different dissociation channels between carbon and oxygen ionic fragments as well as complete two-body dissociation events. The most striking feature in the breakup of CH3OH is the formation of H2+ and H3+ due to intramolecular rearrangement of the C-H bonds within the methyl group. In dissociative ionization studies of C2H2, we observed a diatom-like behaviour of the C-C charged complex as evidenced from the measured slopes of the coincidence islands for carbon atomic charged fragments and theoretical values determined from the charge and momentum distribution of the correlated particles. The shape and orientation of the islands give further information about the momentum balance in the fragmentation process in two-body dissociation.
Abstract: Absolute cross sections for dissociative recombination have been measured for the 4He+2 and 3He4He+ isotopomers as a function of storage time at the ASTRID storage ring. The recombination rate of 3He4He+ with low-energy electrons is found to dramatically decrease with time, giving an upper limit for the thermal rate coefficient for cold ions of the order of 6 $\times$ 10[?]10 cm3 s[?]1, in agreement with flowing afterglow measurements. Two-dimensional imaging of the atomic fragments points to He(1s21S) + He(1s3s 3S) as the lowest dissociation channel to be accessed in the dissociative recombination of rovibrationally hot ions.
Abstract: Microdroplets of 15-ÎŒm diameter are subjected to ultra-short laser pulses of intensities up to 1015Wcm−2 to produce hot dense plasma. The hot electrons produced in the microdroplet plasma result in efficient generation of hard X-rays in the range 50–150keV at an irradiance as low as 8×1014Wcm−2. The X-ray source efficiency is estimated to be about 2 ×10−7%. A prepulse that is about 11ns ahead of the main pulse strongly influences the droplet plasma and the resulting X-ray emission. For a similar laser prepulse and intensity, no measurable hard X-ray emission is observed when the laser is focused on a solid target of similar composition and this indicates that liquid droplet targets are best suited for hard X-ray generation in laser–plasma interactions.
Abstract: Abstract Mass analyzed highly charged ion beams of energy ranging from a few keV to a few MeV plays an important role in various aspects of research in modern physics. In this paper a unique low energy ion beam facility (LEIBF) set up at Nuclear Science Centre (NSC) for providing low and medium energy multiply charged ion beams ranging from a few keV to a few MeV for research in materials sciences, atomic and molecular physics is described. One of the important features of this facility is the availability of relatively large currents of multiply charged positive ions from an electron cyclotron resonance (ECR) source placed entirely on a high voltage platform. All the electronic and vacuum systems related to the ECR source including 10 GHz ultra high frequency (UHF) transmitter, high voltage power supplies for extractor and Einzel lens are placed on a high voltage platform. All the equipments are controlled using a personal computer at ground potential through optical fibers for high voltage isolation. Some of the experimental facilities available are also described.
Abstract: ABSTRACT We present an experimental study of the dissociative recombination (DR) of H3O+ and its isotopomers D3O+ and HD2O+ performed at the ASTRID storage ring. DR cross sections have been measured as a function of energy, while complete branching ratios have been measured at $E=0$ . The H3O+ data yield an accurate determination of the branching ratio for water formation ( $0.25\pm 0.01$ ). The three molecular ions–H3O+, D3O+, and HD2O+–show a marked resemblance concerning cross sections and branching ratios. The only observed isotope effect is in the fragmentation pattern of HD2O+, where the release of a light fragment is favored over release of a heavier fragment. As a consequence, an enhanced production of deuterated molecules takes place as a result of the DR process.
Abstract: We describe an experiment to test a quantum-like theory which predicts quantum-like behavior for an ensemble of electrons in a classical configuration with static magnetic and electric fields. Some of the earlier experiments had supporting evidence for anomalous, quantum-like effects in such a situation showing systematic modulations of electron current when a retarding potential is varied, even though the quantum wavelength of the electrons in such a configuration was less than a billionth of the spatial width of the potential well. Our experiment conclusively rules out any nonclassical, quantum-like behavior in electron transmission through simple electric barriers, when magnetic fields are present. We identify secondary electrons generated at various electrodes as the main source of apparent anomalous behavior. We also present a classical derivation of the quantum-like equation describing the modulations.
Abstract: The interaction of a tetrahedral molecule (carbon tetrachloride) with intense, 532 nm, 35 ps, linearly polarized, laser fields is explored theoretically and experimentally. Distortions caused by the laser field in the molecular electron density distributions are computed in an approximate fashion using quantum-chemical techniques. A comparative experimental study is performed of the dissociative ionization of in such an intense laser field and also by electron impact. Significant differences are observed and attempts are made to explore the possibility that these can be explained using field-distorted electron density distributions. Angular distributions of the fragment ion have also been measured and show a pronounced anisotropy with the ion signal being detected mainly along the direction of the laser's polarization axis.
Abstract: Angular distributions have been measured of the products of dissociative ionization (DI) of the bent triatomic molecules , and induced by intense ( W ), linearly polarized electric fields generated using 35 ps long laser pulses at a wavelength of 532 nm. Both the and fragments produced upon DI of possess isotropic angular distributions. and fragments from DI of are formed with very marked anisotropies, whereas the fragment resulting from DI of exhibits a weak anisotropy. Attempts are made to rationalize these angular distributions in terms of anisotropy of the induced dipole moments of the ground-state precursors and the resulting interaction potentials, in each case. Good accord is reached between the measured data and the results of an approximate, but ab initio, theoretical treatment which indicates that the relative magnitudes of the components of the induced dipole moment, which lie parallel and perpendicular to the molecular symmetry axis, appear to influence the angular distributions.
Abstract: We present high-resolution measurements of the angular distributions of , and fragments from in intense, polarized, picosecond laser fields. Distinctly anisotropic angular distributions are measured in all cases. Some evidence is obtained of lobe-like structure in these distributions; these are indicative of pendular motion of molecules aligned along the polarization axis of the laser field. Results of classical calculations of the dissociation dynamics on an excited state potential energy surface are presented which qualitatively reproduce the global features of the measured angular distributions.
Abstract: The dissociation dynamics of bent molecules in an intense polarized light field produced by 35 ps wide laser pulses at 532 nm has been investigated by measuring the angular distributions of and ions emanating from dissociative ionization of . These angular distributions show a remarkable directional specificity: ions are preferentially formed in a direction which is parallel to the polarization vector of the applied laser field whereas, in contrast, ions are formed in a perpendicular direction.
Abstract: Single and multiple ionization of carbon disulphide by intense picosecond laser fields is the subject of this paper. Mass spectra were measured at five wavelengths from the infrared to the ultraviolet. In terms of the Keldysh adiabaticity parameter, we cover both the multiphoton and the tunnelling regimes. The dynamics of the dissociative ionization process is shown to be dependent upon the regime in which the laser - molecule interaction occurs. Resonances, which may be possible and which could access electronically excited states of the molecule, appear to play little part in the dynamics. Ion abundances have been measured as a function of laser intensity in the tunnelling regime; no correlation is found between measured values of saturation intensity and zero-field molecular properties such as dissociation or ionization thresholds and ionization energies. In addition, the covariance mapping technique is applied to study the dissociation dynamics of multiply charged ions at 1064 nm. The measured values of kinetic energy release accompanying formation of fragment ion-pairs are very much less than those measured in single-photon and electron-impact experiments. It is postulated that this reduction may be a manifestation of the extent to which potential energy surfaces of ions are `flattened' by the action of the intense, linearly polarized laser radiation, akin to the bond-softening process that has been observed in the case of diatomic molecules. Our observations indicate that distortion of molecular potential energy surfaces may be the dominating feature in intense laser - molecule interactions.
Abstract: An attempt is made to relate the dissociation dynamics of highly charged molecular ions (possessing charges in excess of 2+) to molecular structure. High-level, ab initio configuration-interaction calculations are carried out of the potential energy functions of N2q+, for q = 2-12, and predictions are made of the values of the kinetic energies released upon unimolecular dissociation of such ions. The calculated values are found to be significantly lower than those expected from simple Coulombic considerations. Electron density distributions of highly charged molecules are calculated and, on the basis of these, a long-lived state of a diatomic tri-cation, CS3+, is predicted. Mass spectrometric evidence is produced which confirms these predictions.
Abstract: A new application of mass spectrometry to experimental studies of molecular dynamics has been developed and implemented in which ion translational energy spectrometry at very high angular resolutions is utilized to study the unimolecular dissociation of metastable dications. A brief overview is presented of the experimental technique, which utilizes a lsquohome-madersquo, three-sector (equivalent to BEE geometry) instrument. The fragmention kinetic energy spectrum obtained using such apparatus is capable of yielding information on the lifetimes of specific quantal states of dications of a wide variety of stable and radical species; two illustrative examples (CO2+ and SH2+) are presented.
Abstract: Molecular dynamics in intense, pulsed laser fields is an emerging frontier of research in laser-matter interactions. Mass spectrometric techniques are essential to understanding the behaviour of molecules in such fields. We present some salient features of this activity and review some of our recent experimental observations. Results concerning the roles of dissociation and ionization of simple diatomics and triatomics are presented. Further, comparisons are made between the forces experienced by molecules in intense laser fields and in collisions with fast ion projectiles. The dissociative ionization patterns of CS2 and CH4 in the two situations are discussed and the differences in the intense laser field dissociative ionization are attributed to the polarization property of laser light.
Abstract: Lifetimes of doubly charged diatomic and triatomic molecules have been measured by monitoring the decay curves of such ions in a heavy-ion storage ring. CO2+, N22+, CO22+, CS22+ and SH2+ are all found to possess long-lived components which survive for time periods greater than a few seconds. All these dications are found to be essentially stable and their ultimate destruction is due to interactions with residual gases in the ring. CO2+ possesses many more lifetime components in the millisecond range than the isoelectronic N22+ ion. Translational energy spectrometry experiments on the latter species also fail to reveal any short-lived (microsecond) components. Ab initio configuration interaction calculations have been carried out and the potential energy curve for the lowest-energy metastable state of N22+ (1 Sigma g) has been determined, along with Franck-Condon factors for vertical transitions to different vibrational levels from the ground state of neutral N2; tunnelling times of each vibrational level have been computed.
Abstract: The kinetic energies released (KER) upon dissociation of N2q+ (q=2-10) ions are investigated by means of large scale, all-electron, ab initio, configuration interaction molecular orbital calculations. Results indicate that dissociation via non-Coulombic potential energy curves of N2q+ ions yields lower values of KER than would be expected from purely Coulombic considerations due to the significance of electronic charge density distributions in the internuclear region.
Abstract: Ion translational energy spectrometry has been used to determine the lifetime against spontaneous (unimolecular) dissociation of doubly charged molecular ions by studying the kinetic energies of fragment ions. Analysis of the shapes of the fragment ion kinetic energy distributions yields information on the lifetime of the precursor molecular ion. A value of 0.75+or-0.25 mu s is deduced for the lifetime of SH2+ ions. Correlation of this value with the shapes of calculated potential energy curves of low-lying electronic states of SH2+ enables deductions to be made of the importance of spin-orbit induced coupling between the lowest 4 Sigma - and 2 Pi states and provides information on the vibrational level population in the doubly charged molecular ion.
Abstract: A comprehensive comparative study of dissociative ionization of a linear triatomic species, carbon disulphide (S-C-S), has been made using a battery of different interactions involving (i) intense laser fields, (ii) fast, highly-charged, positive ion projectiles (90 MeV Si8+ and F7+), (iii) negative ion projectiles (70 keV O-, Si- and C-) and (iv) fast (700 eV) electrons. The main focus is on the fragmentation channel which gives rise to the formation of S2+ dimer ions from CS2. It is found that while S2+ dimers are formed in all the collisional interactions involving charged projectiles, dissociative ionization of CS2 by intense (1012-1014 W cm-2) 532 nm and 1064 nm laser fields does not proceed along the S2++C dissociation channel. Differences in the interactions which lead to dissociative ionization of CS2 are discussed, particularly with reference the interaction with intense laser fields in which the linear CS2 molecule tends to have its S-C-S axes strongly aligned in the direction of the polarization vector.
Abstract: Ionization and dissociative ionization of N2, CO, CS2, CO2 and NO2 by intense 532 nm radiation has been studied by measuring mass spectra at incident laser intensities over the range 4.1*1012-1.27*1014 W cm-2, covering a region which is intermediate between the multiphoton and tunnelling regimes. For molecules with large zero-field dissociation energies (N2, CO) the mass spectra show the dominance of the molecular ion peak, with relatively small yields of fragment ions. For molecular species which are more loosely bound (CS2, CO2 and NO2), fragment ions dominate the mass spectra. The results appear to indicate that, at 532 nm, molecules with a low zero-field dissociation energy (less than approximately 6 eV) tend to dissociate first, followed by ionization of the fragments. In the case of molecules with high zero-field dissociation energies, molecular ionization occurs first, followed subsequently by fragmentation.
Abstract: Electron impact ionization and dissociative ionization of water in vapour and ice form has been studied using a newly developed apparatus comprising a well-collimated beam of 700 eV electrons, a cryogenic target assembly and a quadrupole mass spectrometer. The target assembly consists of a liquid-nitrogen cooled nickel foil upon which a thin layer of ice is formed. Comparison of the vapour-phase mass spectrum with that obtained in the ice phase reveals significant differences in the dissociative ionization pattern which are of significance in modelling studies of cold astrophysical environments.
Abstract: Ion translational energy spectrometry has been applied to study the kinetic energies of fragment ions resulting from spontaneous (unimolecular) dissociation of doubly charged molecules. Analysis of the shapes of the kinetic energy distributions yields information on the lifetime of the precursor molecular ion. A value of 1.0+or-0.25 mu s is deduced for the lifetime of CO2+ ions.
Abstract: A metastable, triply charged, diatomic molecular ion, CS3+, has been discovered in collisions of O- and Si- with CS2 at an impact energy of 40-70 keV. Concomitant configuration interaction calculations of potential energy curves and electron density distributions have been carried out which indicate that the lowest-energy 2 Sigma g state of this molecular ion is metastable for at least 3 mu s. Predictions are made of the values of the kinetic energies released upon dissociation of the lowest-energy 2,4,6 Sigma g and 2,4 Pi g states of CS3+.
