Hari R. Varma

Pronounced effects of interchannel coupling in high-energy photoionization

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 2p thresholds, i.e., at photon energies about an order of magnitude above the 3d thresholds. Calculations based on the relativistic-random-phase approximation show that this effect is due to interchannel coupling of the 3d photoionization channels with the 2p channels affecting the cross section over a wide range of energies. It is argued that this is a general phenomenon in high-energy photoionization throughout the periodic table.

Photoionization of atomic krypton confined in the fullerene C60

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 distribution of photoelectrons from the 4p, 4s, 3d and 3p subshells are reported within the framework of the relativistic-random-phase approximation.

Theoretical and experimental demonstrations of the existence of quadrupole Cooper minima

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.

X-ray absorption in neon modulated by a strong laser pulse

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 to laser light that was temporally overlapped with the x-rays.

Confinement, correlation and relativistic effects on the photoionization of 6s subshell of Hg

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 photoelectron angular distribution.

Resonance-induced deviations of β from 2.0 for rare gas s-subshell photoionization

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 the case of Kr the largest deviation from 2.0 is a mere −0.030(6), β4s = 1.970(6), while in Xe it is definitively larger at −0.080(6), β5s = 1.920(6). To understand these results in more detail, we have carried out relativistic random phase and relativistic multi-channel quantum defect calculations. Our theoretical results clearly indicate deviations of β from 2.0 for these photolines in these resonance regions, although the degree of variation is predicted to be larger than what is observed.

Effect of coulomb confinement resonances on time delay in Ne@C60-5

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 the time delay in 2s and 2p ionization channels through interchannel coupling at energies far above their thresholds.

Inner-shell autoionization resonances along Mg isoelectronic sequence

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.

Effect of confinement and interchannel coupling on high-Z atoms

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 neighboring subshells. In the present work, we report the combined effect of confinement, correlation and relativistic interactions on the structure of these multiple minima in high-Z atoms by using the RRPA.

Photoionization of Ca 4s in a spherical attractive well potential: dipole, quadrupole and relativistic effects

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 in the photoelectron angular distribution. The existence of Cooper minima in the 4s dipole channels give rise to very significant effects of quadrupole interactions, even at extremely low energies, in the vicinity of the E1 Cooper minimum. It is shown that the entrapment of Ca in a spherical attractive well (Ca@SAW) further enhances the importance of quadrupole interactions determining the photoionization parameters. The complicated behavior of both dipole and quadrupole Cooper minima, as functions of well depth, is delineated, along with the importance of relativistic effects.