Hari Varma

Hari Varma
Indian Institute of Technology Mandi | भारतीय प्रौद्योगिकी संस्थान मंडी · School of Basic Sciences

Doctor of Philosophy

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53
Publications
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309
Citations

Publications

Publications (53)
Article
Full-text available
The present work reports the photoionisation studies of 2p subshell of Na atom, just above the threshold region. The calculations are performed in the framework of multiconfiguration Dirac-Hartree-Fock method (MCDHF). It unravels the effect of 3d orbital on the final state leading to the appearance of Cooper minimum (CM) in the region close to the thr...
Article
Full-text available
The photoionization dynamics of the inner subshell of an endohedral anion, $Ar@{C}_{60}^{q=-4},$ using relativistic random phase approximation (RRPA), is investigated. The charged nature of the fullerene induces Coulomb confinement resonances (CCRs) not only on the 2s cross-section but also on other dynamical attributes such as angular distribution...
Research
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A non-technical abstract of our recent paper "collective" quasi-particles, known as the plasmons. "Collective effects in photoionization of sodium clusters: plasmon resonance spill, induced attractive force and correlation minimum"
Article
Full-text available
The photoionization cross-section of an atom trapped inside a fullerene anion in the near-threshold region is dominated by the Coulomb confinement resonances. These prominent structures facilitate the possibility of an enhanced spin–orbit interaction activated interchannel coupling (SOIAIC) leading to significant modifications of the photoionizatio...
Article
Full-text available
Photoionization studies of Na20 and Na92 clusters are carried out in a framework of linear response density functional theory. Cross sections show substantial spillover of plasmon resonances to the near-threshold ionization energies which are in reasonable agreements with photoabsorption measurements. The analysis of the oscillator strength, consum...
Article
Confined atomic systems are of great importance owing a multitude of possible applications in various areas of science and technology. Of particular interest are atoms encaged in the \(\hbox {C}_{{60}}\) molecule, \(\hbox {A}@\hbox {C}_{{60}}\), since the near-spherical symmetry of \(\hbox {C}_{{60}}\) simplifies theoretical studies, and the stabil...
Article
An earlier work of Dolmatov et al. [Phys. Rev. A 73, 013201 (2006)] on Ne inside a charged fullerene (Ne@C60^q=−3,−5) has revealed the presence of two unusually large confinement resonances termed Coulomb confinement resonances (CCRs); the first CCR is narrow and the second one is broad and their origins are attributed to the presence of charge on...
Preprint
Full-text available
Photoionization studies of Na20 and Na92 clusters are carried out in a framework of linear response density functional theory. Cross sections show substantial spillover of plasmon resonances to the near-threshold ionization energies which are in reasonable agreements with measurements. The analysis of the oscillator strength, consumed by the cross...
Article
In contrast to the conventional finite difference methods, two transient phase methods have been effectively used in the present work to directly compute the photoionization phase shift and Wigner time delay of confined atoms (A@C 60) in the single-active electron (SAE) approximation. The different phase methods: (A) employing logarithmic derivativ...
Article
Full-text available
Synopsis We study the photoionization properties of the Na n (n=40 and 92) clusters in a spherical jellium frame using the Kohn-Sham density functional method. Two well known form of exchange-correlation (xc) functional in the framework of local density approximation (LDA) with Gunnarsson-Lundqvist parametrization[1] are employed: i) the electron s...
Article
Full-text available
Synopsis The present work we study the photoresponse of isoelectronic systems (IES) such as Ar and K ⁺ when they are confined inside neutral and charged fullerenes. The nature of the external cage (charged or neutral) play a significant role in determining the photoionization parameters along these isoelectronic members.
Article
Full-text available
Photoionization from H@C60 and Ar@C60 is studied by employing two different model potentials to simulate the endohedral environment. The contrast between the diffusive Gaussian annular square well (GASW) and the compact annular square well (ASW) model potential on the photoionization dynamics is explored by studying the cross-section, phase shift,...
Article
Full-text available
An empirical ansatz for the complex photoionization amplitude and Wigner–Eisenbud–Smith time delay in the vicinity of a Fano autoionization resonance are proposed to evaluate and interpret the time delay in the resonant region. The utility of this expression is evaluated in comparison with accurate numerical calculations employing the ab initio rel...
Article
Characteristic features of Wigner photoemission time delay from endohedral anions A@C60q along with their dependence on the anion charge q are unraveled. Specifically, significant enhancement of the time delay in the innermost dipole photoionization channels near threshold is found, owing to the presence of the Coulomb confined resonances (CRs). Mo...
Article
Studies of the 2s → np autoionization resonances using a combination of Relativistic Random Phase Approximation (RRPA) and Relativistic Multichannel Quantum Defect Theory (RMQDT) of Mg isoelectronic sequence are reported.
Article
Full-text available
Wigner time delay for inner and outer shells of Ne@C60^{-5} have been studied using relativistic random phase approximation (RRPA) [1]. Coulomb confinement resonances (CCR) due to the charged nature of fullerene cause prominent effects on the photoionization time delay of 1s subshell. These CCRs in 1s also results in significant modifications of th...
Conference Paper
Full-text available
Synopsis Wigner time delay for inner and outer shells of Ne@C60-5 have been studied using relativistic random phase approximation (RRPA) [1]. Coulomb confinement resonances (CCR) due to the charged nature of fullerene cause prominent effects on the photoionization time delay of 1s subshell. These CCRs in 1s also results in significant modifications...
Article
Full-text available
Photoionization cross sections and dipole angular distribution asymmetry parameters, β, of 5s and 4d shells of the closed-shell ions (La3+,La9+, and La11+) in the La isonuclear sequence have been studied using the relativistic random phase approximation. The positions of the 5s Cooper minima in La3+ and La9+ ions are found to be extremely sensitive...
Article
Full-text available
scrutinized within the framework of the dipole relativistic random phase approximation. The focus is on the variation of time delay in the vicinity of the Cooper minima in photoionization of the outer subshells of neon, argon, krypton, and xenon, where the corresponding dipolematrix element changes its sign while passing through a node. It is revea...
Article
Full-text available
A study of the outer 4s subshell of the Ca atom in a spherical attractive potential well (SAW) of variable depth is performed to determine the effect of an external potential on dipole (E1) and quadrupole (E2) photoionization processes. As the depth of the potential well increases, dramatic changes are observed in the 4s cross-section, as well as i...
Article
Full-text available
Photoionization studies of outer shells of high-Z atoms like Radon (Z=86) and Radium (Z=88) have been studied using relativistic random phase approximation (RRPA) [1,2]. Multiple minima found in the pho-toionization cross section were attributed to many electron correlations resulting from interchannel coupling with photoionization channels from ne...
Data
Full-text available
Synopsis Photoionization studies of outer shells of high-Z atoms like Radon (Z=86) and Radium (Z=88) have been studied using relativistic random phase approximation (RRPA) [1,2]. Multiple minima found in the pho-toionization cross section were attributed to many electron correlations resulting from interchannel coupling with photoionization channel...
Article
Full-text available
The 3d core level photoemission of metallic Ag and In was measured over a wide energy range including the 2p ionization thresholds. The intensities (photoionization cross sections) of the 3d5/2 and 3d3/2 lines were observed to modulate significantly with photon energy, both absolute and relative, and this is most pronounced in the vicinity of the 2...
Conference Paper
Full-text available
Photoionization studies of Cd@C 60 ASHISH KUMAR, SINDHU KALYADAN, HARI R. VARMA, IIT-Mandi, PRANAWA C. DESHMUKH, IIT-Madras, STEVEN T. MANSON, Georgia State Uni-versity — Atoms trapped in fullerene (A@C 60) cages have attracted con-siderable attention in the recent past owing to their importance in many areas of physics [1]. The additional potentia...
Conference Paper
Full-text available
Time-delay associated with the ejection of photoelectrons with respect to the instant at which light pulse is incident on the absorbing atom can now be measured using recent developments in atto-second streaking. A number of experimental and theoretical studies have already been reported. We study the time-delay in the ionization of Xe which is tra...
Article
Full-text available
The effect of interchannel coupling and confinement on the photoionization of the Kr atom is examined. It is found that the position of the Cooper minimum is sensitive to interchannel coupling in both free and confined Kr. The strengths of the confinement oscillations are also sensitive to interchannel coupling.
Article
Full-text available
Dipole and quadrupole photoionization parameters for intermediate subshells of mercury have been studied using the relativistic-random-phase-approximation (RRPA). Multiple Cooper minima are observed in 5d quadrupole cross sections, even at the level of intrashell coupling. It has been found that the effect of interchannel coupling is much more sign...
Article
Full-text available
The combined effects of interchannel coupling, relativistic interactions and endohedral confinement on the photoionization of atomic Kr are studied. The confinement of the Kr atom placed at the centre of the C60 cage is modelled by placing the atom inside an annular spherical potential. Cross sections for the photoionization and angular distributio...
Article
The importance of first-order nondipole effects in low-energy photoionization is well known [1], and, the significance of second-order [O(k^2r^2), where k is the photon wave number] nondipole terms has been stressed even at photon energies as low as ˜11 eV [2]. In the present work, valence dipole and quadrupole photoionization of free atomic Ca and...
Conference Paper
Full-text available
Submitted for the DAMOP12 Meeting of The American Physical Society Sorting Category: 2.1 (C) Time delay in photoionization near Cooper min-ima — The connection be-tween the energy dependence of the scattering phase shift and time delay is known [1]. With the developments of techniques in attosecond physics, it has become possible to measure the tim...
Article
Using the example of photoionization of 3s and 4s atomic subshells, we study the energies of quadrupole Cooper minima (QCM) over the entire range of Z for which they appear in the continuum. The results show that s->d continuum QCM show up over a much larger range of Z than their s->p dipole (DCM) counterparts. For 3s photoionization QCM exist for...
Article
Full-text available
Ultrafast X-ray science is an exciting frontier that promises the visualization of electronic, atomic and molecular dynamics on atomic time and length scales. A largely unexplored area of ultrafast X-ray science is the use of light to control how X-rays interact with matter. To extend control concepts established for long-wavelength probes to the X...
Article
Full-text available
Two-photon above-threshold ionization processes in the x-ray regime are studied using atomic hydrogen as a model system. Within the minimal-coupling formalism of nonrelativistic quantum electrodynamics, two distinct interactions—Â⋅p̂ in second order and Â2 in first order—contribute to the two-photon absorption amplitude. The relative importance of...
Article
We have measured the absorption of x-rays in neon gas in the presence of a strong laser pulse. The femtosecond x-rays were tuned to energies near the neon 1s-3p resonance, and the laser intensity of 1013 W/cm2 was below the intensity required to alone ionize neon. We observed strong modification of the x-ray absorption when the neon was subjected t...
Conference Paper
We report, to our knowledge, the first dressed absorption spectrum at an X-ray probe wavelength. An ultrafast optical pulse induces transparency for X-rays, demonstrating a promising route to femtosecond X-ray pulse shaping and measurement.
Article
Using an improved value for the energy splitting between the 1s −1 3s 1 S0 and 1s −1 3p 1 P1 states, electromagnetically induced transparency for x rays at the 1s → 3p resonance in Ne is reassessed. The energy splitting between the 1s −1 3s 1 S0 and 1s −1 3p 1 P1 states is calculated with two indepen-dent approaches. In the first approach, first-or...
Article
Full-text available
Calculations and measurements of the Xe 5s and 5p nondipole photoelectron asymmetry parameters are obtained, which present clear evidence of the existence of quadrupole Cooper minima, i.e., minima in quadrupole matrix elements as a function of energy, in the photoionization process. This verifies earlier predictions of quadrupole Cooper minima.
Article
Studies of confined atoms have attracted considerable attention in recent years [1, 2]. We have studied the sensitivity of atomic wavefunctions and probability densities to confinement parameters using the Dirac-Fock self-consistent field method. The confinement potential is modelled as an annular attractive potential of a certain radius, width and...
Article
Selective interchannel coupling has been used in the present work as an effective computational strategy to idintify specific autoionization resonances and other correlation effects in the photoionization angular distribution asymmetry parameter for the case of valence ns subshell photoionization of atomic krypton and mercury. It is found that in t...
Article
Full-text available
A theoretical study of photoionization parameters of Hg atom confined in a spherical shell potential has been carried out using the relativistic random phase approximation. It is found that confinement, correlation and relativistic effects all bring significant and separate changes in the 6s photoionization cross-section, and also in the photoelect...
Article
Full-text available
A study of the combined effects of correlation, confinement and relativistic effects on the photoionization of the outer ns subshells of the alkaline-earth-metal atoms, Mg, Ca, Sr and Ba has been performed with particular emphasis on the Cooper minima using the relativistic-random-phase approximation (RRPA) methodology. It is found that the respons...
Article
Interchannel coupling effects involving non-dipole matrix elements on photoelectron angular distribution have been recently found to be of significant importance even at energies not so high above ionization thresholds. Studies have been reported on the angular distribution of photoelectrons from some inert gas atoms: Ne, Ar, Kr and Xe. In the pres...
Article
Full-text available
We investigate the case of resonance-induced deviations of the angular distribution anisotropy parameter, β, from its nominal value of 2.0 for the Kr 4s-subshell at the 3d → mp(m ≥ 5) excitations and the Xe 5s-subshell in the region of the 4d → mp(m ≥ 6) excitations. We observe small but unmistakable variations of β from 2.0 at these resonances. In...
Article
Studies of atoms confined in an endohedral environment have aroused significant recent interest [1]. In this work, the photoionization @Ca and @Zn have been studied using the Relativistic-Random-Phase Approximation, modified to include the confinement potential. Photoionization of the 4s and 3p subshells of free and confined atomic calcium, along w...
Article
Full-text available
We present the absolute photoionization cross section of doubly ionized silicon as a function of photon energy. These were obtained by merging a Si2+ ion beam generated in an electron cyclotron resonance source with monochromatized synchrotron radiation from an undulator. The photoion yield measurements were carried out in the photon energy range b...
Article
It has recently been found that interchannel coupling between the photoionization channels arising from 3d_5/2 and 3d_3/2 in Xe produces structure in the 3d_5/2 in the vicinity of the 3d_3/2 threshold [1], and this structure has been observed experimentally [2]. In this work, the generality of this effect is explored. The photoionization of a numbe...
Conference Paper
Full-text available
Synopsis We study the photoionization dynamics of Na40 confined in a giant fullerene C540 using a linear response density functional theory. The ionization spectra reveal the plasmonic contamination and the presence of inter-cluster Coloumbic decay (ICD) resonances.

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