Research Activities

 

 Double differential distribution of electron emission in the ionization of water molecules by fast bare oxygen ions

The doubly differential distributions of low-energy electron emission in the ionization of water molecules under the impact of fast bare oxygen ions with energy of 48 MeV are measured. The measured data are compared with two quantum-mechanical models, i.e. the post and prior versions of the continuum distorted wave-eikonal initial state (CDW-EIS) approximation, and the first-order Born approximation with initial and final wavefunctions verifying correct boundary conditions (CB1). An overall excellent qualitative agreement is found between the data and the CDW-EIS models whereas the CB1 model showed substantial deviation. However, the detailed angular distributions display some discrepancies with both CDW-EIS models. The single differential and total cross-sections exhibit good agreement with the CDW-EIS models. The present detailed data set could also be used as an input for modeling highly charged ion induced radiation damage in living tissues, whose most abundant component is water. Similar measurements are also carried out for a projectile energy of 60 MeV. However, since the double differential cross-section data show similar results the details are not provided here, except for the total ionization cross-sections results. J. Phys. B: At. Mol. Opt. Phys. 49, 065202 (2016)

Observation of 2p3d(1Po)->1s3d(1De) Radiative Transition in He-like Si, S, and Cl Ions

We present an experimental determination of the 2p3d(1Po) -> 1s3d(1De) x-ray line emitted from He-like Si, S, and Cl projectile ions, excited in collisions with thin carbon foils, using a high-resolution bent-crystal spectrometer. A good agreement between the observation and state-of-the-art relativistic calculations using the multiconfiguration Dirac-Fock formalism including the Breit interaction and QED effects implies the dominance of fluorescent decay over the autoionization process for the 2p3d(1Po) state of He-like heavy ions. This is the first observation of the fluorescence-active doubly excited states in He-like Si, S, and Cl ions.

Ionization of Multi Electron Atoms

Measurement of the energy and angular distributions of the double differential cross section (DDCS) of electron emission from Ne and Xe atoms in collision with 5 MeV/u bare carbon ions is reported. This study aimed to investigate the electron emission processes in the case of multi-electronic systems. In general, several clear differences between the electron emission spectra of Ne and Xe are found, which indicate the influence of the increasing number of electrons. For instance, the sharp peak due to the binary nature of collision is almost absent in the case of Xe, unlike Ne, which could be understood due to the increasing contribution from the strongly bound inner shell (such as 4d) electrons for the Xe atom. The forward-backward angular asymmetry has also been derived from the angular distributions. For Xe, the qualitative behaviour of the asymmetry parameter is seen to be quite different since it reveals structures due to Auger contributions. It is, in general, different and much lower than that for Ne, which shows the smooth behaviour that one finds for other lighter atoms like He. The single differential and total cross sections are also derived. The theoretical calculations based on the prior form of the continuum distorted wave-eikonal initial state (CDW-EIS) approximation have been provided for both the targets. Overall, it gives a very good agreement with the energy and the angular distributions of DDCS for Ne. For Xe, the agreement is not as good as for Ne. We also provide a detailed discussion on the DDCS obtained from different sub-shell ionization, estimated in this framework.

Development of RIMS and CSA

Recoil ion momentum spectroscopy (RIMS) is one of the techniques, which is capable of unfolding molecular fragmentation dynamics by measuring three momentum components of all fragment ions. On the other hand, in case of ion impact fragmentation, the reaction mechanism depends on the energy of the projectile. In the intermediate energy region (~keV/u), electron capture to the projectile and transfer ionization are important reaction channels along with the usual direct ionization channel. To distinguish these channels, post collision projectile charge state analysis is required. Here we report the new development of recoil ion momentum spectrometer coupled with a post collision projectile charge state analyser (CSA), which will be used to study the ion induced fragmentation of different molecules, especially the big molecules of biological relevance or polycyclic aromatic hydrocarbons. The spectrometer provides the momentum information of all recoil ions by measuring their time of flight (ToF) and the hit position on the 2D imaging detector. It is a Wiley-McLaren type spectrometer with three separate parts namely, extraction region, acceleration region and the field free drift region. It ensures excellent time focusing which is required for good longitudinal momentum resolution. For better transverse momentum resolution we have introduce a week nonlinear field just before the drift region. This lensing effect ensures the space focusing at the detector plane as well as larger 4p collection efficiency. We are using the 40 mm active diameter MCP with delay line anode for ion detection. For post collision charge state analysis, an electrostatic trapezoidal plate analyser is employed after the interaction region. Different charged ions, separated from each other, are detected by channeltron detectors mounted at about 1 m away from the interaction region. This whole experimental setup is connected to the TIFR- ECRIA 0o beamline. During the experiment, the base vacuum of all the chambers and the beamline is maintained better than 2 x 10-8 Torr. For data acquisition and analysis we are using the TDC multi-channel card with CoboldPC software.

Electron Impact Ionization of Diatomic Molecules and Interference effect

Double differential cross section (DDCS) of electrons emitted in collision of 7 keV electrons with O2 and N2 were measured for different emission angles. The aim for the present experiments were to check for existence of interference oscillation which was not observed in case of heavy ion impact for the above mentioned molecules. Signature of 1st order oscillation have been observed in the DDCS ratios (molecular/atomic) for both the di-atomic molecules, for all the emission angles, unlike the result seen in heavy ion impact previously. The oscillatory structures were further fitted by the Cohen-Fano model. The frequency distribution as a function of emission angles showed backward angles having higher oscillation frequency as compared to the forward angles. The forward-backward angular asymmetry also showed clear signature of oscillation which was further fitted by the 1st order Cohen-Fano model. The asymmetry parameter showed signatures of higher order effects, which were revealed by dividing the asymmetry parameter by the 1st order model fit. For more details read here

 High-resolution study of x-rays emitted from highly charged S ions using a bent-crystal spectrometer

We have measured the projectile x-rays from 56 MeV sulphur ions in collision with thin carbon foils using a high-resolution bent-crystal spectrometer. The resolution of the spectrometer is good enough to resolve lines arising from transitions in H-, He- and Li-like ions. The line energies are compared with values from the NIST x-ray database. The data have also been used to generate the approximate equilibrium charge state distribution, which is compared with semi-empirical model predictions. Phys. Scr. T156, 014009, 2013

Interaction between bio-molecules and HCI

Ionization of Uracil in Collisions with HCI

Ion-induced collisions on biologically relevant targets like DNA/RNA components (bases, sugar and phosphate backbone) are of prime importance for being able to model the radio-induced cellular death process. In heavy ion therapy, the ions energy lose is continuous and non-uniform along the ion trail. Ions lose maximum energy in the Bragg peak region. Therefore, the study of the interaction of energetic ions with nucleobases over a wide energy range is necessary to model the actual radiation damage. Fast highly charged C and O-ion induced total ionization of a RNA base molecule, Uracil (C4H4N2O2), has been investigated in a wide energy range of keV-MeV. A combined study of the collision products using a ToF mass spectrometer and an electron spectrometer allows one to determine the absolute total ionization cross section (TCS). High energy (20 MeV-90 MeV) ion beams of C, O, F were obtained from the Pelletron accelerator. These studies were then extended, using low energy ion beams of C and O obtained from the ECRIS. Also, the projectile energy range studied here includes the Bragg peak region and therefore is crucial for model calculations for hadron therapy. A comparison of the experimental investigations with the state-of-the-art theoretical models for uracil will help to understand of the collisional mechanisms involving other large molecules or clusters also. The present experimental investigation has been well complemented by the model calculations based on CDW-EIS, CTMC-COB and CB1 approximations. These calculations are performed for the first time by our collaborators for uracil. One interesting observation is the deviation from the well known "q-square" law for ionization and fragmentation by HCIs (submitted to PRA 2011).

We have initiated the study of low energy electron emission from uracil molecule in collisions with fast bare C and O ions. Apart from the total cross section measurement, as was done using recoil ion ToF technique, one needs to study the angular and energy distributions of the DDCS which provide much more stringent clues regarding the collision mechannisms as well as stringent tests to the theoretical models. It is well known that the low energy electrons produced in such collisions are primarily responsible for further damage to the cells in radiation damage as well as in hadron therapy. Therefore the knowledge of the energy distribution of such low energy electrons will be of immense importance for deriving any model for radiation damage. The energy and angular distributions of the electron DDCS are measured. In addition the single differential distributions are also deduced. The initial data are compared with the CDW-EIS and the CB1 model. An unusually large forward backward asymmetry in the electron emission was observed compared to that for ion-atom collision. The details are being published in PRA, soon.


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Influence of Collective excitation processes in clusters

GDPR in Fullerene

In view of the mesoscopic size of clusters, studies on them are believed to bridge the gap between traditional targets namely gases and solids. An important aspect of these studies is to understand the energy loss mechanisms in such large systems. This energy loss is highly influenced by the presence of collective electronic mode of excitation. The C60-fullerene molecule is known to have such an excitation: Giant dipole plasmon resonance (GDPR). Origin of this resonance is due to the delocalized pi electrons of fullerenes which are well described as free electron gas like in solids. In one set of measurements we have studied the influence of collective excitation on the single, double, triple, quadruple ionization of fullerene molecules in collisions with C, O, F and Si projectile ions with varieties of charge states. The experiment was performed using the recoil-ion time-of-flight technique. A linear charge state dependence, in contrast to that for ion-atom collision, was obvious and could be explained qualitatively in terms of a GDPR model.

In addition very recently we have investigated the electron DDCS (double differential cross section) spectrum of C60 in fast collisions. A peak corresponding to GDPR was identified at all angles (to be published).

Influence of Collective Excitation on HCI-induced e- Capture from Fullerene

Earlier, we studied projectile deexcitation Lyman x-ray emission following electron capture and K excitation in collisions of bare ions with fullerenes and different gaseous targets. The intensity ratios of different Lyman x-ray lines in collisions with fullerenes are found to be substantially lower than those for the gas targets, both for capture and excitation. This had been explained in terms of a model based on "solidlike" effect, namely, wakefield induced stark mixing of the excited states populated via electron capture or K excitation: a collective phenomenon of plasmon excitation in the fullerenes under the influence of heavy, highly charged ions.


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Interference effect in Electron Emission from Diatomic Molecules

Young-type Interference effect in e- Emission in Heavy Ion Collisions with H2

Low energy electrons emitted in atomic collisions provide crucial information on the various ionization mechanisms. Especially, the electron spectrum emitted from H2, under heavy ion impact, is very rich since it provides the evidence of the interference effect. Since the two H atoms in molecular hydrogen are indistinguishable, their contributions to the ionization add coherently and an interference effect might be expected. Such electron emission from H2 may be closely related to the well-known Young's two-slit experiment which provided the crucial input to the development of the quantum mechanics. We have seen direct evidence of the interference effect in the electron emission spectra from ionization of molecular hydrogen in collisions with bare ions at relatively low collision energies. 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.

Forward-Backward Asymmetry in e- Emission from H2 in Collisions with Heavy Ions

The forward-backward angular asymmetry in the electron emission cross sections in fast ion impact ionization of H2 can be used as a probe of the inversion symmetric coherence in homonuclear diatomic molecules. The electron energy dependence of the asymmetry parameter for H2 exhibits oscillatory structure due to Young-type interference in contrast to atomic targets such as He. The asymmetry parameter technique provides a self-normalized method to reveal the interference oscillation independent of theoretical models and complementary measurements on atomic H target.

Second-order Interference in Collisions of Bare Ions with H2

We observed the frequency doubling in interference oscillations in fast-ion-induced electron emission spectrum from H2 in collisions with 4MeV/u F9+ ions. Experimentally observed oscillatory structure was well explained by a model calculation based on the rescattering of emitted electron from the second H center. The second-order contribution was found to be as large as 10%. The doubling of oscillation frequency was found out to be independent of angle of observation.

Angular Dependence of Interference Oscillation Frequency

Electrons emitted from H2 in collisions with 5 MeV/u F9+ ions were measured in the energy range from 1 to 300 eV and a wide range of emission angles between 200 and 1600. The DDCS ratios of molecular-to-atomic hydrogen exhibit an oscillatory structure which can be explained in terms of Young-type electron interference. The frequencies of such oscillations were obtained. They showed prominent angular dependence.

Impact Ionization of O2 by Bare C & O Ions : Interference Oscillations ?

The study of interaction of multiply charged heavy ions with diatomic molecules provides important information regarding Coulomb ionization process. It can be implicitly used to detect the Young type interference effect in the electron emission from a diatomic molecule itself, by looking into the oscillatory behavior in the electron double differential cross section (DDCS) spectrum. We have measured the absolute electronDDCS of O2 between 1 to 600 eV, in collision with 42 and 51 MeV bare carbon ions and 72 MeV bare O ions in an angular range of 300 to 1500. The projectile ions were produced in the Pelletron accelerator at TIFR. The experimental set-up consists of a high vacuum chamber equipped with a motorized turntable and a hemispherical electrostatic analyzer. The DDCS ratio of (O2/2O), obtained by using theoretical DDCS for atomic oxygen, does not show prominent oscillatory behavior, unlike in the case of H2. In addition the asymmetry parameter (α) can be defined as the ratio of the difference and sum of DDCS for supplementary forward and backward angles. No oscillation was observed for the α-parameter, which is in contrast to the oscillatory structure observed earlier for H2. However, the KLL-Auger electron single differential cross-section (SDCS) shows an oscillatory structure.

Young-type Interference effect in e- Emission from H2 in Collisions with Fast Electrons

The Young-type interference arising due to the spatial coherence has been investigated in the electron emission spectrum from fast electron impact ionization of the inversion symmetric homonuclear diatomic molecule H2. The evidence of the interference effect in the angular distribution of the double differential spectrum of the secondary electron was found. The signature of constructive interferences had been identified in the soft-collision regions as well as in binary encounters.


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X-ray studies using HCI

High Resolution x-ray spectroscopy and fast electron impact K-ionization

With the rising interest in multiply charged atomic and ionic species, high resolution x-ray spectroscopy has become very important. The applications extend beyond the traditional domain of atomic physics into the fields of astrophysicsand plasma physics. Our group has extensively modified a high resolution bent-crystal x-ray spectrometer for atomic physics experiments. Briefly, this spectrometer consists of an ADP [101] Bragg crystal mounted on a Rowland circle mount, with a gas flow proportional counter as the detector. Using this spectrometer, we have measured the x-ray emission from a thin Al target under 2-10 keV electron impact. The resolution of the spectrometer is ΔE~2.2 eV at Al Kα (E ~ 1.5 keV). The 1st and 2nd Kα satellites are also observed. The entire data acquisition and angle control of the spectrometer is computerized through a LabVIEW user interface, which was developed in-house at TIFR. The yield of the Al Kα line is seen to rise sharply beyond the threshold, reach a maximum and then fall gradually, as predicted by theory.

High Resolution x-ray spectroscopy of H-, He-, Li- like Si, S, Cl Ions

Recently, these experiments are initiated using above HCIs from Pelletron as well as ECRIA. A collaboration with theoretical physicist is in progress to identify the various lines observed.

X-ray measurement in ECR plasma

The TIFR electron cyclotron resonance ion source (ECRIS) emits large flux of bremsstrahlung radiations in the X-ray region. These X-rays are produced due to the collisions of highly energetic electrons within the plasma, and carry information on the state of the plasma, including the electron temperature and the electron and ion density. We have measured the electron bremsstrahlung spectra using GeandNaI (Tl) detectors. The typical end point energy for the x-ray spectrum was found to be ~600 keV. Suitable absorbers and collimators were used to cut-down the huge count rate in the low energy region. In particular, spectra from Ne and Ar plasmas have been studied at different microwave powers between 0 W and 300 W. The gas-pressure and microwave power dependence of the derived plasma electron-temperature (Te) has been investigated. Typical values of Te were found to be ~ 25 - 90 eV. It is proposed to extend this study to other gas plasmas, as well as to study the effect of gas-mixing on these properties. Along the same lines, the high resolution X-ray spectra from the plasma are to be studied using the bent crystal spectrometer to yield more detailed information.


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Study of Inner Shell Processes

L3-subshell Alignment of Au and Bi in Collisions with Heavy Ions

Angular distribution of Au and Bi L and M sub-shell x-rays in collisions with fast C-ions has been measured. The x-ray yields are found to be slightly anisotropic and the anisotropy parameter has been derived over a wide range of beam energy (in collaboration with Dr. Ajay Kumar, presently at BARC).

M-shell X-ray Production Cross Sections of Pb and Bi Induced by HCI

The M-shell x-ray production cross sections were measured for Pb and Bi induced by the highly charged F (only on Pb) and C ions (on both). The measured cross sections were compared with the ECPSSR based on the perturbed-stationary state approximation. The Mγ X-ray cross sections were unusually higher compared to the ECPSSR prediction that was primarily attributed to a dramatic enhancement in the M3-subshell fluorescence yield owing to multiple vacancies in N subshells.

K-shell Ionization and Local Plasma Approximation

K-shell processes in heavy-ion collisions in solids and the local plasma approximation...

Single and Double Electron Transfer

State-selective K-K electron transfer and K ionization cross sections for Ar and Kr in collisions with highly charged C, O, F, S, and Cl ions at intermediate velocities...

Saturation Effect in K-excitation of He-like Ions

Saturation effect in the excitation of helium like Si projectiles in the intermediate velocity range...