Field: Author: Kar AND Ho)AND(Journal: Journal of Physics B: Atomic, Molecular and Optical PhysicsDispersion coefficients for interactions between hydrogen and helium atoms with Coulomb and screened Coulomb potentialsSabyasachi Kar,Y K HoandZishi Jiang2011J. Phys. B: At. Mol. Opt. Phys.44165004doi:10.1088/0953-4075/44/16/165004Hide abstractWe have carried out calculations to investigate Van der Waals two-body dispersion coefficientsC8,C10andC12for the H(1s2S)He(1s21S) system with screened Coulomb interactions in the framework of a powerful computational technique. The dispersion coefficients for different screening parameters ranging from 0 to 1a10are reported. In the unscreened case, theC12coefficient is reported for the first time in the literature.

Resonances in electronhydrogen scattering in Debye plasmasSabyasachi KarandY K Ho2011J. Phys. B: At. Mol. Opt. Phys.44015001doi:10.1088/0953-4075/44/1/015001Hide abstractWe have investigated the influence of Debye plasmas on resonance states in electronhydrogen scattering using highly correlated exponential wavefunctions in the framework of the stabilization method. Resonance parameters (resonance positions and widths) below theN= 2 H threshold are reported for selected screening parameters. Critical values of the screening parameters are reported from which S, P, D Feshbach resonances are transformed to shape resonances. Behaviour of the resonance widths obtained from this work as a function of screening parameters is in agreement with the recently reported work for weaker screening, but not in close agreement with those for stronger screening.

Strong dc electric-field effects on the lowest doubly excited singlet states of helium using highly correlated exponential wavefunctionsSabyasachi KarandY K Ho2010J. Phys. B: At. Mol. Opt. Phys.43135003doi:10.1088/0953-4075/43/13/135003Hide abstractWe have investigated the dc electric-field effect on the doubly excited 2s21Se, 2s2p1P and 2p21Destates of a helium atom using highly correlated exponential wavefunctions in the framework of the complex-coordinate rotation method. Strong electric-field strengths up toF= 0.01 au are used in our present study. Block matrices up toLmax= 6 (I states) are used to investigate the convergence behaviour for the resonance parameters (resonance energy and width). When the external electric field is turned on, the 'classic' Stark effect is observed for theM= 0 components of those two neighbouring 2p21Deand 2s2p1P states. Our results are in good accord with the results in the literature.

Doubly excited P, D and F unnatural parity states of hydrogen negative ion using correlated wavefunctionsSabyasachi KarandY K Ho2009J. Phys. B: At. Mol. Opt. Phys.42185005doi:10.1088/0953-4075/42/18/185005Hide abstractWe have investigated the doubly excited1,3Pe,1,3D and1,3Festates of the hydrogen negative ion using highly accurate correlated wavefunctions. The doubly excited 2p23Pemetastable bound state (so-called second bound state) energy of His obtained using the Ritz variational principle. The upper bound of the energy of the 2p23Pebound states is 0.125 355 451 242 au, which is comparable with the available best results 0.125 355 451 24 (Bylicki M and Bednarz E 2003Phys. Rev.A67022503). We employ the complex-coordinate rotation method to extract resonance parameters. The resonance parameters (both resonance position and width) of Hfor1,3Pe,1,3D below then= 3, 4, 5, and 6 thresholds and1,3Festates below then= 4, 5, and 6 thresholds are repo...

Isotope shift for the1Deautodetaching resonance in Hand DSabyasachi KarandY K Ho2009J. Phys. B: At. Mol. Opt. Phys.42055001doi:10.1088/0953-4075/42/5/055001Hide abstractThe doubly excited1Deresonance state of Hbelow then= 2 threshold has been investigated using highly correlated exponential basis functions in the framework of the complex- coordinate rotation method. The resonance energies and widths for this system having infinite, finite proton and deuteron, nuclear masses are reported. The resonance energies and widths are in excellent agreement with the experimental results. The isotope shift of the1Deresonance of Hand D, measured from the respective ground state, is also compared with experimental determination.Effect of screened Coulomb potentials on the resonance states of two-electron highly stripped atoms using the stabilization methodSabyasachi KarandY K Ho2009J. Phys. B: At. Mol. Opt. Phys.42044007doi:10.1088/0953-4075/42/4/044007Hide abstractResonance states of C4+, O6+, Ne8+ions interacting with screened Coulomb potentials have been investigated using the stabilization method. We employ highly correlated exponential basis functions for S-, P-state calculations and employ CI-type basis functions for D-state calculations. We report a total of seven S-wave resonances of which four belong to 2sns (n= 2, 3, 4, 5) series and three belong to the 2pnp (n= 2, 3, 4) series. In addition, a total of eight P-wave resonances of which three belong to 2snp+(n= 2, 3, 4), three belong to the 2snp(n= 3, 4, 5) series, two belong to the 2pnd (n= 3, 4) and the lowest D-wave resonance for each systems and each screening parameters below then= 2 thresholds of the respective subsystems are reported. In the unscreened case, we have found some resonance states that have not been reported before. In the...

Resonance states of three self-gravitating bosons and fermions below theN= 3 threshold of the two-body subsystemSabyasachi KarandY K Ho2008J. Phys. B: At. Mol. Opt. Phys.41065302doi:10.1088/0953-4075/41/6/065302Hide abstractThe resonance states of systems consisting of three identical bosons and three fermions interacting with an attractive 1/rpotential are investigated using highly correlated exponential basis functions. The method of complex-coordinate rotation is used to extract energies and widths for the resonances lying below theN= 3 excited state threshold of the respective two-body subsystem. We have obtained a total of 24 S-wave resonances for the three-boson system and a total of 19 S-wave resonances in total spin-3/2 (the4S state) and a total of 15 S-wave resonances in total spin-1/2 (the2S state) below theN =3 threshold. For the resonances associated with theN =3 threshold, we have obtained two, three and four series of Rydberg states for the three-boson system, three-fermion system for2S states and three-fermion system for4S states, respectively. The resonance energies and widths of these states for the t...

Doubly excited1,3Pemeta-stable bound states and resonance states of helium in weakly coupled plasmasSabyasachi KarandY K Ho2007J. Phys. B: At. Mol. Opt. Phys.401403doi:10.1088/0953-4075/40/7/009Hide abstractThe doubly excited1,3Pemeta-stable bound states and resonance states of helium embedded in weakly coupled plasmas are calculated using Slater orbital basis functions. The plasma effects are taken care of by using the screened Coulomb potential obtained from the Debye model. We have calculated the 2pnp1Pe(n= 3, 4) and 2pnp3Pe(n= 2, 3, 4) meta-stable bound state energies of He in plasmas for various Debye lengths. In addition, a total of ten resonances (five each for1Peand3Pestates) below theN= 3 He+thresholds has also been obtained by calculating the density of states using a stabilization method. For the1Pestates, this includes three members from the 3pnp (n= 4, 5, 6) series, and two members from the 3dnd (n= 4, 5) series. For the3Pest...

Doubly excited inter-shell P-wave resonances of helium in weakly coupled plasmasSabyasachi KarandY K Ho2006J. Phys. B: At. Mol. Opt. Phys.392445doi:10.1088/0953-4075/39/11/010Hide abstractWe have investigated the doubly excited1,3Poresonance states of helium embedded in weakly coupled plasmas. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charge particles. A correlated wavefunction consisting of a generalized exponential expansion has been used to represent the correlation effect. A total of 18 resonances (9 each for1Poand3Postates) below then= 2 He+thresholds has been estimated by calculating the density of states using a stabilization method. For each spin state, this includes four members in the 2snp+(2 n 5) series, three members in the 2snp(3 n 5) series, and two members from 2pnd(n= 3, 4) series. The resonance energies and widths for various Debye parameters ranging from infinity to a small value for these1,3Po...

S-wave resonances in the positronhydrogen system with screened Coulomb potentialsSabyasachi KarandY K Ho2005J. Phys. B: At. Mol. Opt. Phys.383299doi:10.1088/0953-4075/38/18/002Hide abstractWe have made a first calculation for S-wave resonances in positronhydrogen scattering under the influence of Debye plasma environments. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the nucleus, the electron and the positron. Hylleraas-type wavefunctions are used to represent the correlation effect between the charge particles. The stabilization method is used to calculate the density of resonance states, from which resonance energies and widths are deduced. We have found two resonances of which one is associated with the hydrogenn= 2 threshold, and the other one is found lying below the Ps(2S) threshold. The resonance parameters (energies and widths) calculated for various Debye lengths are reported.ONE REMAINS

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PRA2 citationsBorromean windows for H+2with screened Coulomb potentialsSabyasachi Kar and Y. K. HoPhys. Rev. A86, 014502 Published 5 July 2012We have carried out calculations to search Borromean windows for the boundFandGstates of the H+2system, where the nuclei and the electrons interact through a screened Coulomb (Yukawa-type) potential. The boundFand G state energies for different values of the screening parameters are obtained using correlated exponential wave functions in the framework of the Ritz variational principle. The critical values of the screening parameters for the boundFandGstates are reported for which the H+2system is stable, while all the possible fragments are unbound; that is, it shows windows for Borromean binding.

PRA4 citationsShape resonance in the PssystemSabyasachi Kar and Y. K. HoPhys. Rev. A86, 014501 Published 3 July 2012We have searched forS-wave shape resonances in the Pssystem. The calculations are carried out in the framework of the complex-coordinate rotation method using both Hylleraas-type wave functions involving powers of interparticle coordinates, and the exponential correlated wave functions. We have located anS-wave shape resonance lying above the Ps (N=2) threshold. By changing the mass of the positively charged particle from one unit of the electron mass to infinitely heavy, we have traced this resonance pole from Psto H. Results for the shape resonance in Hare comparable with the available results in the literature.

PRA6 citationsPolarizabilities of two-electron positive ions with screened Coulomb potentialsZishi Jiang, Sabyasachi Kar, and Y. K. HoPhys. Rev. A84, 012504 Published 11 July 2011We have carried out calculations of the polarizabilities of the two-electron positive ions Li+, Be2+,B3+,C4+,N5+, andO6+interacting with screened Coulomb potentials. Highly accurate correlated exponential wave functions are used to represent correlation effects on the charged particles. The dipole, quadrupole, and octupole polarizabilities for the screening parameters in the range 01a10are reported. Reported results for the unscreened case are comparable with the available results and for the screened case show some interesting behavior with increasing nuclear charge.

PRA6 citationsDoubly excited1,3Peresonance states of helium and the hydrogen negative ion interacting with Coulomb and screened Coulomb potentialsSabyasachi Kar and Y. K. HoPhys. Rev. A83, 042506 Published 13 April 2011We have investigated the doubly excited1,3Peresonance states of helium and the hydrogen negative ion interacting with Coulomb and screened Coulomb potentials using exponential correlated wave functions. In the pure Coulomb case, calculations have been carried out by using the complex-coordinate rotation and the stabilization method. The1Peresonance states of He below theN=3, 4, and 5 thresholds of He+, and the3Peresonance states of He below theN=3 thresholds of He+, are reported. The5p23Pestate, which has attracted recent interest, is also reported and discussed. In the screened Coulomb case, we have used the stabilization method to obtain two different series (3pnpand 3dnd) of resonance states below theN=3 He+threshold as a function of the screening parameters. Resonance widths for the 3dndseries show some interesting behaviors. The resonance parameters (position and width) for helium and the hydrogen negation ion as functions of the screening parameters are reported.

PRA2 citationsComment on Doubly excited bound and resonance (3Pe) states of heliumSabyasachi Kar and Y. K. HoPhys. Rev. A82, 036501 Published 22 September 2010Recently a study [J. K. Saha and T. K. Mukherjee, Phys. Rev. A80, 022513 (2009)] on the doubly excited3Pestates of helium reports some resonance states that are not in conformity with previously published works [Y. K. Ho and A. K. Bhatia, Phys. Rev. A47, 2628 (1993)]. Owing to discrepancies between the works, we investigate the resonance parameters (positions and widths) of the doubly excited Feshbach resonances of the3Pesymmetries associated withN=3,4, and5He+thresholds using a different choice of correlated wave functions in the framework of the complex-coordinate rotation method and the stabilization method. Resonance parameters below theN=4andN=5He+thresholds obtained from the present calculations are not consistent with the results and discussions made in the recent work. Here we point out an error in assessment made by Saha and Mukherjee in an earlier work as well as the lack of numerical accuracy of the results for some high-lying doubly excited states reported by Saha and Mukherjee. The bound2p23Peand2p3p3Pestate energies obtained from this calculation are consistent with the best reported results but not with the reported values of Saha and Mukherjee.

PRA11 citationsDispersion coefficients for interactions between helium atoms in Debye plasmasSabyasachi Kar and Y. K. HoPhys. Rev. A81, 062506 Published 11 June 2010Effect of Debye plasmas on the dispersion coefficientsC8,C10, andC12for interactions between atoms has been investigated using highly accurate correlated exponential basis functions. In the free-atom case, the dispersionC12coefficient and the hexadecapole polarizability for helium are reported. The dispersion coefficients, hexadecapole polarizability, and1s5g1Gestate energy of helium for different screening parameters are also reported.

PRA8 citationsMultipole polarizabilities of helium and the hydrogen negative ion with Coulomb and screened Coulomb potentialsSabyasachi Kar and Y. K. HoPhys. Rev. A80, 062511 Published 10 December 2009We have carried out calculations of multipole polarizabilities of helium and the hydrogen negative ion interacting with pure Coulomb and screened Coulomb potentials using highly accurate correlated exponential wave functions with exponent generated by a quasirandom process. The dipole, quadrupole, and octupole polarizabilities for the ground1s2S1estate for different screening parameters starting from infinity (pure Coulomb case) to small values of the screening parameters, are reported. The octupole polarizability of the hydrogen negative ion is reported for the first time in the literature. The bound4F1state energy of helium for different screening parameters are also reported.

PRA11 citationsDoubly excited nonautoionizingP,D, andFstates of helium with Coulomb and screened Coulomb potentialsSabyasachi Kar and Y. K. HoPhys. Rev. A79, 062508 Published 17 June 2009We have investigated the doubly excitedP1,3e,D1,3o, andF1,3enonautoionizing states of helium atom with Coulomb and screened Coulomb (Yukawa) potentials in the framework of Ritz variational principle. Highly accurate correlated exponential wave functions with exponents generated by a quasirandom process are used to represent the correlation effect. For the Coulomb case, the metastable bound-excited2pnpP1e(3n5),2pnpP3e(2n5),2pndD1,3o(3n9), and2pnfF1,3e(4n9)states energies are reported. Our upper-bound results for the2pndD1o(3n7),2pnfF1e(4n8),D3oandF3estates are the lowest values up to this date. For the screened Coulomb case, the lower-lying metastable bound states energies for different screening parameters and for each spin states are also reported.

PRA9 citationsRatio of double-to-single photoionization cross sections of plasma-embedded helium atoms at x-ray energiesSabyasachi Kar and Y. K. HoPhys. Rev. A77, 022713 Published 28 February 2008The asymptotic ratios of double-to-single photoionization cross sections of plasma-embedded helium atoms at very high but nonrelativistic photon energies are calculated for different shielding parameters that are based on the Debye model for weakly coupled plasmas. The ratio of double-to-single photoionization for free atom is determined as 1.644%, in good agreement with the synchrotron measurements. The asymptotic ratio of double-to-single photoionization cross sections for different screening parameters are reported.

PRA7 citationsResonance states of three self-gravitating bosons and fermionsSabyasachi Kar and Y. K. HoPhys. Rev. A76, 032711 Published 18 September 2007The resonance states of the systems consisting of three identical bosons and of three fermions interacting with an attractive1/rpotential are investigated using highly correlated exponential basis functions. The method of complex-coordinate rotation is used to extract energies and widths for the resonances lying below then=2excited state threshold of the respective two-body subsystem. For the system consisting of three identical bosons, we have obtained a total of 12S-wave resonances. It is concluded that nine of them belong to one series of Rydberg states. For the system consisting of three fermions, we have obtained a total of 23S-wave resonances in total spin three-halfs (theS4state) and 17S-wave resonances total spin one-half (theS2states). The resonances for theS4states belong to three different series, while those for theS2states belong to two Rydberg series. The resonance energies and widths of these states for the three-body self-gravitating bosons and fermions are reported.

PRA26 citationsBound states and resonance states of the plasma-embeddedppmolecular ionSabyasachi Kar and Y. K. HoPhys. Rev. A75, 062509 Published 19 June 2007We have investigated the effect of weakly coupled hot plasmas on the bound states and resonance states of theppmolecular ion. The plasma effect is taken care of by employing a screened Coulomb (Yukawa-type) potential, and highly correlated wave functions are used. TheSe1andPo3resonances of theppmolecular ion in plasmas for different screening parameters below thep(2S)threshold are calculated using the stabilization method. The boundSe1andPo3state energies of theppmolecular ion are determined using a variational method. Thep(1S)andp(2S)threshold energies for various screening parameters are also reported here.

PRA25 citationsDoubly excitedPo1,3resonance states ofPsin weakly coupled plasmasSabyasachi Kar and Y. K. HoPhys. Rev. A73, 032502 Published 2 March 2006ThePo1,3resonance states of positronium negative ions embedded in dense plasma environments are determined by calculating the density of resonance states using the stabilization method. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charge particles, together with employing highly correlated wave functions to represent the correlation effect between them. We have calculated onePo1and twoPo3resonances associated with and below then=2threshold of the positronium atom. The resonance energies and widths for various Debye lengths ranging from infinity to a small value are reported.

PRARapid Communication34 citationsDoubly excited2s2pPo1,3resonance states of helium in dense plasmasSabyasachi Kar and Y. K. HoPhys. Rev. A72, 010703(R) Published 29 July 2005We have made an investigation on the2s2pPo1,3resonance states of helium embedded in dense plasma environments. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charge particles. A correlated wave function consisting of a generalized exponential expansion has been used to represent the correlation effect. Resonance energies and widths for the doubly excited He embedded in plasmas with various Debye lengths are determined using the stabilization method by calculating the density of resonance states. The resonance energies and widths for various Debye parameters ranging from infinity to a small value for the lowestPo1,3resonance states are reported.

PRA39 citationsGround state and resonance state ofPsin plasmas with various Debye lengthsSabyasachi Kar and Y. K. HoPhys. Rev. A71, 052503 Published 11 May 2005We have made an investigation on the resonances for positronium negative ionPsin various model plasma environments. The2s2Se1autoionization resonance state inPsion is determined by calculating the density of resonance states using the stabilization method. We have also performed accurate variational calculations to obtain ground-state energy eigenvalues ofPsfor various Debye lengths. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charged particles. A correlated wave function has been used to represent the correlation effect between the three charge particles. The calculated resonance energies and widths for various Debye parameters ranging from infinity to a small value along with the ground-state energies are reported.

PRE47 citationsAutoionizingSe1resonance ofHin Debye plasma environmentsSabyasachi Kar and Y. K. HoPhys. Rev. E70, 066411 Published 30 December 2004We have made an attempt to investigate atomic resonances in various Debye plasma environments. The2s2Se1autoionization resonance for a two-electron systemHis determined by calculating the density of resonance states using the stabilization method. Highly correlated Hylleraas-type wave functions are used to represent the correlation effects between the three charged particles. The calculated resonance energies and widths for the various Debye parameters ranging from infinity to a small value are reported.

.(Content contains Research Publications)AND(Authors contains Ho Y K)AND(About contains aip/journal/popOne- and two-photon ionization of hydrogen atom embedded in Debye plasmasT. N. Chang,T. K. FangandY. K. HoPhys. Plasmas20, 092110(2013);http://dx.doi.org/10.1063/1.4821123

We present a detailed analysis of the plasma-induced resonance-like atomic structures near theionizationthreshold in one- andtwo-photonionizationof hydrogen atom. Such resonance-like structures result from the migration of the upper bound excited states of bound-bound atomic transitions into the continuum due to the less attractive screened Coulomb potential which simulates the external environmental effect for an atom embedded in Debyeplasma.The change from the resonance-like narrow structures into broad continuousspectraas theplasmaeffect increases could be accounted for by the overlap between the respectivewavefunctionsof the atomic electron in the initial state and its corresponding outgoing ionized state in the continuum.

Doubly excited P-wave resonance states of Hin Debye plasmasL. G. JiaoandY. K. HoPhys. Plasmas20, 083303(2013);http://dx.doi.org/10.1063/1.4818986

We investigate the doubly excitedP-wave resonance states of Hsystem in Debyeplasmasmodeled by static screened Coulomb potentials. The screening effects of theplasmaenvironment on resonance parameters (energy and width) are investigated by employing the complex-scaling method with Hylleraas-typewave functionsfor both the shape and Feshbach resonances associated with the H(N=2 to 6) thresholds. Under the screening conditions the H(N) threshold states are no longerldegenerate and all the Hresonance energy levels are shifted away from their unscreened values toward the continuum. The influence of Debyeplasmason resonance widths has also been investigated. The shape resonance widths are broadened with increasingplasmascreening strength whereas the Feshbach resonance widths would generally decrease. Our results associated with the H(N=2) and H(N=3) thresholds are compared with others in the literature.

Investigation of charge transfer and ionization in He-like systems (Li+, Be2+, B3+, C4+, N5+, O6+)-hydrogen atom collisions in Debye plasmasM. K. Pandey,Y.-C. LinandY. K. HoPhys. Plasmas20, 022104(2013);http://dx.doi.org/10.1063/1.4790663

Thecharge transferandionizationcross sections have been calculated for He-like system (Li+Be2+B3+C4+N5+O6+) and hydrogenatom collisionsin Debyeplasmasfor energies ranging from 1 to 500keV using the classical trajectory Monte Carlo method. Interactions of the activeelectronswith the projectile ions have been described by model potentials. Cross sections are determined in both screening and unscreening environments and a comparative study between both environments has been carried out. In particular an interesting feature of sudden increase in theionizationcross sections at lower velocities is also observed in all the systems like pandeyet al.[M. K. Pandeyet al.Phys.Plasmas19062104 (2012)] calculations for O8++ Hcollision.The feature of sudden increase inionizationcross sections at lower velocities and the effect ofplasmascondition on it are explained in terms of the classical trajectory framework. We have found the cross sections for both capture andionizationare dependence on Debye screening lengths throughout thecollisionenergies range but are particularly pronounced at low projectilecollisionsenergies. The calculated cross sections for the unscreened case are found in reasonable agreement with available experimental and theoretical results.

Cross sections of charge exchange and ionization in O8++H collision in Debye plasmasM. K. Pandey,Y.-C. LinandY. K. HoPhys. Plasmas19, 062104(2012);http://dx.doi.org/10.1063/1.4729680

Charge exchange and ionization processes in O8++H collision system in a Debye plasma are studied using the classical trajectory Monte Carlo (CTMC) method in the collision energy ranging from 1keV/amu to 500keV/amu. Total charge exchange and ionization cross sections have been determined in both screening and unscreening environments. In the unscreened case partial cross sections for transfer into individualnshells of the projectile have also been determined. An interesting and remarkable feature of sudden increase in the ionization cross sections at lower velocities is discussed in terms of the CTMC framework. Results are analyzed in light of available theoretical and experimental results. The cross sections dependencies on Debye screening lengths have been investigated and plasma screening effect on charge exchange and ionization cross sections has been found throughout the collision energies range but is particularly pronounced at low projectile collision energies. The sudden rise in the ionization cross sections towards lower energies is explained qualitatively in terms of the multiple encounter model.

Energies and transition wavelengths for Li II, Be III, B IV, C V embedded in Debye plasmasZishi Jiang,Sabyasachi KarandY. K. HoPhys. Plasmas19, 033301(2012);http://dx.doi.org/10.1063/1.3692085

We have carried out non-relativistic calculations to investigate the effect of Debye plasmas screening on the doubly excited nonautoionizing1 3Peand1 3Dostates of Li II Be III B IV and C V using highly correlated exponential wave functions within the framework of Ritz variational principle. The 2p23Pe2p3p1Peand 2p3d1 3Dostates energies and wavelengths for the 2p3p1Pe 2p3d1Do2p23Pe 2p3d3Do2p3p3Pe 2p3d3Doand 2p3p3Pe 2p3d3Dotransitions for different Debye lengths are reported. Comparisons are made with the existing results. Results for B IV and C V for different screening parameters are reported for the first time in the literature. Transition wavelengths show interesting behavior with increasing screening parameters and nuclear charge.

Quadrupole photoionization of hydrogen atoms in Debye plasmasC. Y. LinandY. K. HoPhys. Plasmas17, 093302(2010);http://dx.doi.org/10.1063/1.3480641

Although a great deal of effort has been devoted to investigating dipole photoionization of plasma-embedded atoms far less is known about the corresponding quadrupole transitions. In the present work quadrupolephotoionization processes for the ground and excited states of hydrogen atoms in Debye plasma are explored using the method of complex coordinate rotation. The plasma shielding effects on the quadrupolephotoionization cross sections are reported for a variety of Debye screening lengths and compared to the dipole results accordingly. Under the perturbation of plasma screening shape resonances and Cooper-type minima occurring in both dipole and quadrupolephotoionization cross sections are presented and discussed. Comparisons are made to other theoretical calculations for the dipole photoionization with good agreement. The present quadrupole results are the first predictions for hydrogen photoionization in Debye plasmas.

Combined effect of Debye plasma environment and external electric field on hydrogen atomS. PaulandY. K. HoPhys. Plasmas17, 082704(2010);http://dx.doi.org/10.1063/1.3463708

We consider weakly coupled plasmas characterized by DebyeHuckel model potential and an external electric field alongz-axis. Due to plasma environment the energy levels of atom are shifted up bound states are merged to continuum. For external electric field the excited energy levels also split up; degenerate energyeigenvalues become nondegenerate. In the presence of external electric field energy levels are shifted up and down except ground state. The ground stateenergy value is shifted only down. Therefore it is very interesting to study the combined effect of plasmas and external electric field on a simple atom (hydrogen). To calculate the energy levels and the corresponding states we expand the wave function in terms of linear combination of the basis functions. The basis is generated by hydrogenic wave functions. Here we estimate various plasma surroundings and electric field strengths. We observe converged results for the basis size 45 with angular momentum states up to eight.

Application of variational method for three-color three-photon transitions in hydrogen atom implanted in Debye plasmasS. PaulandY. K. HoPhys. Plasmas16, 113301(2009);http://dx.doi.org/10.1063/1.3258665

Multiphoton processes where transparency appears have long fascinated physicists. Plasma screening effects are investigated on three-color three-photon bound-bound transitions in hydrogen atom embedded in Debye plasmas; where photons are linearly and circularly polarized two left circular and one right circular. All possible combinations of frequency and polarization are considered. Analytical wave functions are used for initial and final states along with the pseudostate wave functions for intermediate states. The analytical wave functions are obtained from the modified wave functions for screening Coulomb potential (Debye model) using Ritz variational method. Here we have found three-photon transparency for lower values of Debye length. This type of phenomenon occurs due to energy level shifting in the presence of Debye plasma environments. The description of resonance enhancements and three-photon transparency is reported in the present context along with the region of resonance enhancements and three-photon transparency.

Ratios of double-to-single photoionization cross sections of helium atom embedded in dense quantum plasmas at high nonrelativistic photon energiesArijit GhoshalandY. K. HoPhys. Plasmas16, 073302(2009);http://dx.doi.org/10.1063/1.3191723

An investigation on the asymptotic ratios of double-to-single photoionization cross sections of the helium atom (He) embedded in dense quantum plasmas at very high nonrelativistic photon energies has been made. Exponential cosine-screened Coulomb potential has been used to represent the effective potential of a test charge in a dense quantum plasma. Making use of highly correlated wave functions for the ground states of He results of the asymptotic ratios(R)of double-to-single photoionization cross sections for the screening parameter in the range [0 0.5] a.u. are reported. The reported value ofRfor the unscreened case confirms its established value. A comparative study of the behavior ofRin weakly coupled plasmas and in dense quantum plasmas has also been made.

Hydrogen atoms in Debye plasma environmentsS. PaulandY. K. HoPhys. Plasmas16, 063302(2009);http://dx.doi.org/10.1063/1.3152602

Plasma-screening effects are investigated on hydrogen atoms embedded in weakly coupled plasmas. In the present context bound statewave functions are introduced related to the screening Coulomb potential (Debye model) using the Ritz variation method. The bound energies are derived from an energy equation which contains one unknown variational parameter. To calculate the parameter numerically fixed-point iteration scheme is used. The calculated energyeigenvalues for various Debye lengths agree well with the other available theoretical results. The radial wave functions and radial probability distribution functions are presented for different Debye lengths. The outcomes show that the plasma affects the embedded hydrogen atom.

Two-photon transitions in hydrogen atoms embedded in weakly coupled plasmasS. PaulandY. K. HoPhys. Plasmas15, 073301(2008);http://dx.doi.org/10.1063/1.2953802

The pseudostate method has been applied to calculate energyeigenvalues and corresponding eigenfunctions of the hydrogen atom in Debye plasma environments. Resonant two-photon transition rates from the ground state of atomic hydrogen to2sand3sexcited states have been computed as a function of photon frequency in the length and velocity gauges for different Debye lengths. A two-photon transparency is found in correspondence to each resonance for1s3s. The transparency frequency and resonance enhancement frequency vary significantly with the Debye length.

Photodetachment of the hydrogen negative ion in weakly coupled plasmasSabyasachi KarandY. K. HoPhys. Plasmas15, 013301(2008);http://dx.doi.org/10.1063/1.2829070

The effect of weakly coupled plasmas on photodetachment cross sections of the hydrogen negative ion is investigated by using the asymptotic from of the bound statewave function and a plane wave form for the final-state wave function. The Debye model is used to represent the plasma effects. The required normalization constant is determined from highly accurate completely nonadiabatic wave functions for the three-particle systems. Photodetachment cross sections for the plasma-embeddedHion are calculated for different Debye lengths (D) ranging from infinite (pure Coulomb) toD=1.0.

Photoionization of Li and Na in Debye plasma environmentsSatyabrata SahooandY. K. HoPhys. Plasmas13, 063301(2006);http://dx.doi.org/10.1063/1.2200290

A calculation of the photoionization cross sections is presented for alkali-metal atoms such as Li and Na in plasma environments. The computational scheme is based on the complex coordinate rotation method. A model potential formalism has been used to simplify the computational complexity of the problems of making quantitative predictions of properties and interactions of many electron systems in Debye plasmas. The plasma environment is found to appreciably influence the photoionization cross sections. In this regard the photoionization cross sections of isolated atoms are also discussed that is found to be in good agreement with the previous theoretical results. It is observed that the strong plasma screening effect remarkably alters the photoionization cross sections near the ionization threshold. The Cooper minimum in the photoionization cross sections of Na shifts toward the higher energy as the plasma screening effect increases. For Li the Cooper minimum is uncovered in strong plasma environments. This is the first time such structures have been determined.

Stark shifts and widths of a hydrogen atom in Debye plasmasA. C. H. YuandY. K. HoPhys. Plasmas12, 043302(2005);http://dx.doi.org/10.1063/1.1867493

A computational scheme has been developed and used to investigate the influence of the plasma environments on modified atomic autoionization for isolated atoms/ions by using the complex coordinate rotation method which is proved to be a very simple and powerful tool to analyze the position and the width of a resonance. The Debye screening potential is employed to describe the effects of the plasma environments. Stark shifts and widths on the ground state of hydrogen are reported for field strength up toF=0.12a.u.Slater-type basis wave functions are used to describe the system and angular-momentum states up toL=11are included when the external electric field is turned on. Converged results are obtained by using different maximum angular-momentum states. The modified autoionization for various Debye lengths ranging from infinite to a small value of 0.86 are reported. It has been observed that for a given temperature and under the influence of a given external electric field the resonance energy and the autoionization width increase for increasing electron density in the plasma. A discussion on the physical implication of our results is made.

Influence of plasma environments on photoionization of atomsL. B. ZhaoandY. K. HoPhys. Plasmas11, 1695(2004);http://dx.doi.org/10.1063/1.1650351

Within the framework of the method of the complex-coordinate rotation a computational scheme has been developed to investigate influence of plasma environments on photoionization for atoms and ionized atoms. The Debye screening potential is used to describe effects of the plasma environments. Photoionization cross sections are reported for hydrogen and hydrogen-like helium in the ground states and compared with the available data. Investigation shows that near the ionization thresholds photoionization is sensitive to the plasma screening. The strong screening remarkably alters the photoionization cross sections near the ionization thresholds. It is concluded that a great error may arise if photoionization cross sections for isolated atoms or ions are employed for numerical simulations of astrophysical and laboratory plasmas.

Electron bunch acceleration and trapping by the ponderomotive force of an intense short-pulse laserQ. Kong,S. Miyazaki,S. Kawata,K. Miyauchi,K. Nakajima,S. Masuda,N. MiyanagaandY. K. HoPhys. Plasmas10, 4605(2003);http://dx.doi.org/10.1063/1.1622952

By utilizing a pulsed laserbeam ofTEM(1 0)+TEM(0 1)mode it was found numerically for the first time that an electron bunch can be effectively trapped by the transverse ponderomotive force in the transverse direction and at the same time accelerated by the longitudinal ponderomotive force to about 378 MeV at the laser peak intensity ofI5.481018W/cm2.In addition the electron bunch size is preferably small: at this laser intensity the electron bunch thickness is10in the longitudinal direction and the bunch radius is about 625 in the transverse direction.

.Content contains Research Publications)AND(Authors contains Kar S)AND(About contains aip/journal/popThe positive ion temperature effect in magnetized electronegative plasma sheath with two species of positive ionsA. K. Shaw,S. KarandK. S. GoswamiPhys. Plasmas19, 102108(2012);http://dx.doi.org/10.1063/1.4759460

The properties of a magnetized multi-component (two species of positive ions negative ions and electrons) plasma sheath with finite positive ion temperature are studied. By using three fluid hydrodynamic model and some dimensionless variables the ion (both lighter and heavier positive ions and negative ions) densities the ion (only for positive ions) velocities and electric potential inside the sheath are investigated. In addition the absence and presence of magnetic field and the orientation of magnetic field are considered. It is noticed that with increase of positive ion temperature the lighter positive ion density peaks increase only at the sheath edge and shift towards the sheath edge for both absence and presence of magnetic field. For heavier positive ions in the absence of magnetic field the density peaks increase at the sheath edge. But in the presence of magnetic field the density fluctuations increase at the sheath edge. For both the cases the density peaks shift towards the sheath edge.Top of FormBottom of Form

Numerical investigation of the ion temperature effect in magnetized plasma sheath with two species of positive ionsA. K. Shaw,S. Kar,K. S. GoswamiandB. J. SaikiaPhys. Plasmas19, 012120(2012);http://dx.doi.org/10.1063/1.3678199

The effect of ion temperature magnitude of magnetic field and its orientation on a magnetized plasmasheath consisting of electrons and two species of positive ions are investigated. Using three-fluid hydrodynamic model and some dimensionless variables the dimensionless equations are obtained and solved numerically. It is found that with the increase of the ion temperature and magnetic field strength there is a significant change in ion densities and energies in the sheath. It is also noticed that increase of magnetic field angle enhances the ion density near the sheath edge for a constant ion temperature. With increase in ion temperature and magnetic field angle the lighter ion density near the sheath edge enhances and reverses for the heavier ion species.Top of FormBottom of Form

Excitation of electrostatic plasma waves using a dielectric covered metallic electrodeS. KarandS. MukherjeePhys. Plasmas18, 112303(2011);http://dx.doi.org/10.1063/1.3658035

Plasma response to high positive and negative voltage pulses is studied using pulsed capacitive excitation in a uniform and unmagnetized plasma. The positive or negative voltage pulse is applied to a metallic electrode covered by a dielectric (Kapton) film immersed in a low pressure argon plasma. The pulse magnitude is much greater than the electron temperature (U0kTe/e). Experiments are carried out for different plasma parameters to find how the plasma perturbations propagate for various applied pulse widths in comparison to ion plasma period (f1i). Plasma perturbations are studied by varying the thickness of the dielectric. For positive pulse bias depending on the dielectric thickness excitation of solitary electron holes or solitary ion holes are observed. For negative pulse bias varying the dielectric thicknesses only ion rarefaction waves are excited.Top of FormBottom of Form

Observation of plasma density dependence of electromagnetic soliton excitation by an intense laser pulseG. Sarri,S. Kar,L. Romagnani,S. V. Bulanov,C. A. Cecchetti,M. Galimberti,L. A. Gizzi,R. Heathcote,R. Jung,I. Kourakis,J. Osterholz,A. Schiavi,O. WilliandM. BorghesiPhys. Plasmas18, 080704(2011);http://dx.doi.org/10.1063/1.3625261

The experimental evidence of the correlation between the initial electron density of the plasma and electromagneticsoliton excitation at the wake of an intense (1019W/cm2) and short (1 ps) laser pulse is presented. The spatial distribution of the solitons together with their late time evolution into post-solitons is found to be dependent upon the background plasma parameters in agreement with published analytical and numerical findings. The measured temporal evolution and electrostatic field distribution of the structures are consistent with their late time evolution and the occurrence of multiple merging of neighboring post-solitons.Top of Form

Excitation of ion rarefaction waves in a low pressure plasma by applying a short high negative voltage pulseS. Kar,S. MukherjeeandY. C. SaxenaPhys. Plasmas18, 053506(2011);http://dx.doi.org/10.1063/1.3591360

The ion rarefaction response to a high negative voltage pulse (U0>>kTe/e) applied to a metal plate immersed in a low pressure argon plasma for time duration lower than ion plasma period is experimentally examined. In the present experiment the pulse duration is kept intermediate between the ion and electron plasma response times. Such a pulse duration is chosen so that ions are collectively undisturbed and according to general understanding no force is given to ions. Hence no ion rarefaction wave should be excited. But contrary to the general understanding excitation of a rarefaction wave is observed. The results indicate that the speed of the rarefaction waves for various conditions (like plasma density applied pulse magnitude and pulse duration) is supersonic. After a distance from the exciter (biased plate) typically three-fourth of the exciter diameter the rarefaction waves are turned into ion acoustic waves. The experimental results indicate that even though the bias durations are shorter than the ion plasma period if the bias magnitude is large enough some collective plasma behavior can still be excited.Top of Form

Conditions for efficient and stable ion acceleration by moderate circularly polarized laser pulses at intensities of 1020W/cm2B. Qiao,M. Zepf,P. Gibbon,M. Borghesi,B. Dromey,S. Kar,J. SchreiberandM. GeisslerPhys. Plasmas18, 043102(2011);http://dx.doi.org/10.1063/1.3577573

Conditions for efficient and stable ion radiation pressure acceleration (RPA) from thin foils by circularly polarized laser pulses at moderate intensities are theoretically and numerically investigated. It is found that the unavoidable decompression of the co-moving electron layer in Light-Sail RPA leads to a change of the local electrostatic field from a bunching to a debunching profile ultimately resulting in premature termination of ion acceleration. One way to overcome this instability is the use of a multispecies foil where the high-Z ions act as a sacrificial species to supply excess co-moving electrons for preserving stable acceleration of the lower-Z ion species. It is shown by 2D particle-in-cell simulations that 100 MeV/u monoenergetic C6+ion beams are produced by irradiation of a CuC-mixed foil with laser pulses at intensities 51020W/cm2which can be easily achieved by current day lasers.Top of Form

Possible excitation of solitary electron holes in a laboratory plasmaS. Kar,S. Mukherjee,G. RaviandY. C. SaxenaPhys. Plasmas17, 102113(2010);http://dx.doi.org/10.1063/1.3501994

Plasma response to a fast rising high positive voltage pulse is experimentally studied in a uniform and unmagnetized plasma. The pulse is applied to a metallic disk electrode immersed in a low pressure argon plasma (np109cm3andTe0.52eV) with the pulse magnitudeU0kTe/ewhereTeis the electron temperature. Experiments have been carried out for various applied pulse widthspranging from less than3f1ito greater than3f1iwherefiis the ion plasma frequency. For pulse widths less than3f1ipotential disturbances are observed to propagate in two opposite directions from a location different from the actual exciter (metal disk electrode) indicating the presence of a virtual source. For pulse widths equal or greater than3f1ithere is no indication of such virtual source. These disturbances propagate with two phase speeds i.e.vp/ve=1.360.11and0.40.15whereveis the electron thermal speed. It is also observed that by increasing plasma density the speed of these disturbances increases whereas the speed is independent of pulse magnitude. Analysis of these disturbances indicates the excitation of solitary electron holes.Top of Form

Measurements of fast electron scaling generated by petawatt laser systemsTsuyoshi Tanimoto,H. Habara,R. Kodama,M. Nakatsutsumi,Kazuo A. Tanaka,K. L. Lancaster,J. S. Green,R. H. H. Scott,M. Sherlock,Peter A. Norreys,R. G. Evans,M. G. Haines,S. Kar,M. Zepf,J. King,T. Ma,M. S. Wei,T. Yabuuchi,F. N. Beg,M. H. Key,P. Nilson,R. B. Stephens,H. Azechi,K. Nagai,T. Norimatsu,K. Takeda,J. ValenteandJ. R. DaviesPhys. Plasmas16, 062703(2009);http://dx.doi.org/10.1063/1.3155086

Fast electron energy spectra have been measured for a range of intensities between1018and1021Wcm2and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University where the pulse duration was varied from0.5to5psrelevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40 inp-polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions.Top of Form

Magnetic field measurements in laser-produced plasmas via proton deflectometryC. A. Cecchetti,M. Borghesi,J. Fuchs,G. Schurtz,S. Kar,A. Macchi,L. Romagnani,P. A. Wilson,P. Antici,R. Jung,J. Osterholtz,C. A. Pipahl,O. Willi,A. Schiavi,M. NotleyandD. NeelyPhys. Plasmas16, 043102(2009);http://dx.doi.org/10.1063/1.3097899

Large magnetic fields generated during laser-matter interaction at irradiances of51014Wcm2have been measured using a deflectometry technique employing MeV laser-accelerated protons. Azimuthal magnetic fields were identified unambiguously via a characteristic proton deflection pattern and found to have an amplitude of45Tin the outer coronal region. Comparison with magnetohydrodynamic simulations confirms that in this regime theTenesource is the main field generation mechanism while additional terms are negligible.Top of Form

Study of electron behavior in a pulsed ion sheathS. KarandS. MukherjeePhys. Plasmas15, 063504(2008);http://dx.doi.org/10.1063/1.2934640

In many plasma processing applications like plasma immersion ion implantation(PIII) the substrate is immersed in low pressureplasma and is biased with negative voltage pulses. In typical PIII the pulse duration is much larger than the ion response time and hence the ion matrix sheath expands and ion implantation happens on the biased substrate. It is assumed that for pulse duration shorter than ion response times the ions remain stationary and electrons are repelled by the negative bias. In the present investigation the negative pulse duration is varied between ion and electron plasma response times; so as to study the electron behavior assuming ions are stationary. The results indicate that the electrons that are lost to the walls come from the ion matrix sheath and probably from the bulk plasma as well. The pulse duration when it is less than the ion response time plays a crucial role in determining the number of electrons lost to the walls.

Ho and collaborators in their research have had extensive practice of Debye screening model in weakly coupled plasmas.