Research Activities
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.
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.
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.
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.
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...