Himadri Chakraborty

Single attosecond pulses from high harmonics driven by self-compressed filaments

We show that isolated subfemtosecond, extreme ultraviolet (XUV) pulses can be generated via harmonic generation in argon by few-cycle infrared pulses formed through filamentation-induced self-compression in neon. Our calculations show that the time structure of the XUV pulses depends sensitively on both the amplitude and the phase modulation that are induced in the driving pulse during the self-compression process.

Beyond the dipole approximation: angular-distribution effects in valence photoemission

Angular distributions of valence photoelectrons showing effects due to higher-multipole photon interactions have been measured for the first time. Neon 2s and 2p photoemission exhibits effects beyond the dipole approximation throughout the 250 - 1200 eV photon-energy range studied. The results suggest that any photoemission experiment, on any sample, can be affected at relatively low photon energies, pointing to a general need for caution in interpreting angle-resolved-photoemission measurements.

Photoionization of C60: a model study

Time-dependent density functional theory is used to calculate the total and subshell photoionization cross sections of C60. The core of 60 C4+ ions is smeared into a classical jellium shell before treating the correlated motion of the 240 valence electrons quantum mechanically. The calculation reveals two collective plasmon resonances in the total cross section in agreement with the experiment. It is found that a phase-coherent superposition of amplitudes leading to enhancements in the ionization from various C60 subshells in two distinct energy regions essentially builds the plasmons. While the result shows good qualitative agreement with the experiments, the limitation of the model to describe the data in quantitative detail is discussed.

Time delay in the recoiling valence photoemission of ar endohedrally confined in C60.

The effects of confinement and electron correlations on the relative time delay between the 3s and 3p photoemissions of Ar confined endohedrally in C60 are investigated using the time-dependent local density approximation--a method that is also found to mostly agree with recent time delay measurements between the 3s and 3p subshells in atomic Ar. At energies in the neighborhood of 3p Cooper minimum, correlations with C60 electrons are found to induce opposite temporal effects in the emission of Ar 3p hybridized symmetrically versus that of Ar 3p hybridized antisymmetrically with C60. A recoil-type interaction model mediated by the confinement is found to best describe the phenomenon.

Plasmon?plasmon coupling in nested fullerenes: photoexcitation of interlayer plasmonic cross modes

Considering the photoionization of a two-layer fullerene?onion system, C60@C240, strong plasmonic couplings between the nested fullerenes are demonstrated. The resulting hybridization produces four cross-over plasmons generated from the bonding and antibonding mixing of excited charge clouds of individual fullerenes. This suggests the possibility of designing buckyonions exhibiting plasmon resonances with specified properties and may motivate future research to modify the resonances with encaged atoms, molecules or clusters.

RELATIVISTIC EFFECTS IN THE PHOTOIONIZATION OF ATOMIC BERYLLIUM

Calculations of the cross section, angular distribution asymmetry parameter and photoelectron spin-polarization parameters for valence photoionization of neutral beryllium using the relativistic-random-phase approximation plus relativistic-multichannel quantum defect theory are performed in the region of Rydberg resonances just below the 1s ionization threshold. A new autoionizing series, 1s2s2np 3P, which occurs solely due to relativistic effects, is found. A quantitative study of the character of resonances has been carried out and the implications of these relativistic resonances for angular distributions and spin polarization are investigated in detail.

Spurious oscillations from local self-interaction correction in high-energy photoionization calculations for metal clusters

We find that for simple metal clusters a single-electron description of the ground state employing self-interaction correction (SIC) in the framework of local-density approximation strongly contaminates the high-energy photoionization cross sections with spurious oscillations for a subshell containing node(s). This effect is shown connected to the unphysical structure that SIC generates in ensuing state-dependent radial potentials around a position where the respective orbital density attains nodal zero. Non-local Hartree-Fock that exactly eliminates the electron self-interaction is found entirely free from this effect. It is inferred that while SIC is largely unimportant in high photon energies, any implementation of it within the local frame can induce unphysical oscillations in the high-energy photospectra of metal clusters pointing to a general need for caution in choosing appropriate theoretical tools.

Coherence of Auger and inter-Coulombic decay processes in the photoionization of Ar@C60 versus Kr@C60

Abstract For the asymmetric spherical dimer of an endohedrally confined atom and a host fullerene, an innershell vacancy of either system can decay through the continuum of an outer electron hybridized between the systems. Such decays, viewed as coherent superpositions of the single-center Auger and two-center inter-Coulombic (ICD) amplitudes, are found to govern leading decay mechanisms in noble-gas endofullerenes, and are likely omnipresent in this class of nanomolecules. A comparison between resulting autoionizing resonances calculated in the photoionization of Ar@C60 and Kr@C60 exhibits details of the underlying processes. Graphical abstract

Resonant Auger-intersite-Coulombic hybridized decay in the photoionization of endohedral fullerenes

Considering the photoionization of Ar@C60, we predict resonant femtosecond decays of both Ar and C60 vacanciesthroughthecontinuaofatom-fullerenehybridfinalstates.ForAr3s → np excitations,theseresonances are far stronger than the Ar-to-C60 resonant intersite-Coulombic decays (ICD), while for C60 excitations they are strikingly larger than the corresponding Auger features. The results indicate the power of hybridization to enhance decay rates and modify lifetimes and line profiles, offering a unique probe, more powerful than regular ICDs, for multicenter decay processes.

Unique role of orbital angular momentum in subshell-resolved photoionization of C60

We predict that the oscillations in the subshell photoionization of C60 evolve with the orbital angular momentum of the bound electrons such that the structures of the highest and the lowest angular momentum subshell cross sections differ dramatically. The effect results from a decrease in the photoelectron production at the molecular inner edge due to the angular momentum generated repulsion on the electron. Fourier analysis of the cross sections at energies below the carbon K-shell continuum indicates that the effect can be observed by photoelectron spectroscopy. The phenomenon should be generic in the photoionization of nanoparticles containing delocalized electrons.