Sanjeev Joshi | Kumaon University (original) (raw)

Papers by Sanjeev Joshi

Journal of Electron Spectroscopy and Related Phenomena, 2011

Photoelectron-Auger electron coincidence spectroscopy probes the dicationic states produced by Au... more Photoelectron-Auger electron coincidence spectroscopy probes the dicationic states produced by Auger decay following the photoionization of core or inner valence levels in atoms, molecules or clusters. Moreover, the technique provides valuable insight into the dynamics of core hole decay. This paper serves the dual purpose of demonstrating the additional information obtained by this technique compared to Auger spectroscopy alone as well as of describing the new IPP/FHI apparatus at the BESSY II synchrotron radiation source. The distinguishing feature of the latter is the capability to record both the photoelectron and Auger electron with good energy and angle resolution, for which purpose a large hemispherical electrostatic analyser is combined with several linear time-of-flight spectrometers. Results are reported for the K-shell photoionization of oxygen (O 2 ) and the subsequent K-VV Auger decay. Calculations in the literature for non-coincident O 2 Auger spectra are found to be in moderately good agreement with the new data.

Physical Review B, 2009

The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacu... more The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacuum ultraviolet photoionization with synchrotron radiation. We show the dependence of the spectrum on cluster size, and the change of its shape with photon energy. Inelastic losses due to intracluster photoelectron scattering are most important for changes in the photoelectron main line, and explain the appearance of additional peaks. A comparison of our results to earlier work on bulk condensed Ar and Ar thin films is given. Evidence for a deviation of the photoionization cross sections for clusters from the atomic ones has not been found.

Chemical Physics, 2005

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvon... more The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C=O and CH2= carbon 1s orbitals were determined to be 292.8+/-0.2 and 289.8+/-0.2 eV, respectively. The remaining C-H C 1s levels substantially overlap under an intense central peak centered at 290.5+/-0.2 eV. The angle-resolved photoemission from the carbonyl carbon C=O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of approximately 6% at the magic angle of 54.7 degrees to the light beam propagation direction. This corresponds to an expected 0 degrees -180 degrees forward-backward electron emission asymmetry of approximately 10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation.

Physical Chemistry Chemical Physics, 2006

We use an autoionization process that involves ultrafast energy transfer to neighbouring sites to... more We use an autoionization process that involves ultrafast energy transfer to neighbouring sites to characterize the formation of NeAr van der Waals bonds in clusters formed by a coexpansion of both gases. This autoionization process, the so-called Interatomic or Intermolecular Coulombic Decay (ICD), is ubiquitous in weakly bonded systems. The energy of the electron being emitted in the ICD process is shown to be characteristic for the two neighbouring entities and is therefore suggested as a new means for structural investigation, such as interface identification, of weakly bonded complexes.

Physical Review B, 2009

The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacu... more The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacuum ultraviolet photoionization with synchrotron radiation. We show the dependence of the spectrum on cluster size, and the change of its shape with photon energy. Inelastic losses due to intracluster photoelectron scattering are most important for changes in the photoelectron main line, and explain the appearance of additional peaks. A comparison of our results to earlier work on bulk condensed Ar and Ar thin films is given. Evidence for a deviation of the photoionization cross sections for clusters from the atomic ones has not been found.

Journal of Physical Chemistry A, 2008

Measurements made with a dilute, non-oriented, gas-phase sample of a selected fenchone enantiomer... more Measurements made with a dilute, non-oriented, gas-phase sample of a selected fenchone enantiomer using circularly polarized synchrotron radiation demonstrate huge chiral asymmetries, approaching 20%, in the angular distribution of photoelectrons ejected from carbonyl C 1s core orbitals. This asymmetry in the forwardbackward scattering of electrons along the direction of the incident soft X-ray radiation reverses when either the enantiomer or the left-right handedness of the light polarization is exchanged. Calculations are provided that model and explain the resulting photoelectron circular dichroism with quantitative accuracy up to ∼7 eV above threshold. A discrepancy at higher energies is discussed in the light of a comparison with the closely related terpene, camphor. The photoelectron dichroism spectrum can be used to identify the absolute chiral configuration, and it is more effective at distinguishing the similar camphor and fenchone molecules than the corresponding core photoelectron spectrum.

Physical Review A, 2008

We present the first measurements of a photoelectron circular dichroism in photoionization from O... more We present the first measurements of a photoelectron circular dichroism in photoionization from O and 1s core levels of the R-enantiomer of glycidol (C 3 H 6 O 2 ) in the gas phase. This dichroism emerges from a forward-backward asymmetry in the angular distribution of electrons created on ionization with circularly polarised synchrotron radiation and is already fully present in the pure electric dipole approximation. Asymmetry factors obtained for the core levels in this study range up to a few percent, but it is likely that these values are limited by a failure to resolve photoemission from individual atomic sites. Theoretical modelling is provided to examine potential differences between these alternative atomic photoemission sites, and between different conformational structures of glycidol. The calculated chiral angular distribution parameters that support the circular dichroism display a much enhanced sensitivity to the molecular conformation compared to the conventional photoionization cross-section and the β-parameter. Likely conformer structures can be suggested after comparison with the experiment.

Journal of Electron Spectroscopy and Related Phenomena, 2011

Photoelectron-Auger electron coincidence spectroscopy probes the dicationic states produced by Au... more Photoelectron-Auger electron coincidence spectroscopy probes the dicationic states produced by Auger decay following the photoionization of core or inner valence levels in atoms, molecules or clusters. Moreover, the technique provides valuable insight into the dynamics of core hole decay. This paper serves the dual purpose of demonstrating the additional information obtained by this technique compared to Auger spectroscopy alone as well as of describing the new IPP/FHI apparatus at the BESSY II synchrotron radiation source. The distinguishing feature of the latter is the capability to record both the photoelectron and Auger electron with good energy and angle resolution, for which purpose a large hemispherical electrostatic analyser is combined with several linear time-of-flight spectrometers. Results are reported for the K-shell photoionization of oxygen (O 2 ) and the subsequent K-VV Auger decay. Calculations in the literature for non-coincident O 2 Auger spectra are found to be in moderately good agreement with the new data.

Physical Review B, 2009

The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacu... more The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacuum ultraviolet photoionization with synchrotron radiation. We show the dependence of the spectrum on cluster size, and the change of its shape with photon energy. Inelastic losses due to intracluster photoelectron scattering are most important for changes in the photoelectron main line, and explain the appearance of additional peaks. A comparison of our results to earlier work on bulk condensed Ar and Ar thin films is given. Evidence for a deviation of the photoionization cross sections for clusters from the atomic ones has not been found.

Chemical Physics, 2005

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvon... more The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C=O and CH2= carbon 1s orbitals were determined to be 292.8+/-0.2 and 289.8+/-0.2 eV, respectively. The remaining C-H C 1s levels substantially overlap under an intense central peak centered at 290.5+/-0.2 eV. The angle-resolved photoemission from the carbonyl carbon C=O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of approximately 6% at the magic angle of 54.7 degrees to the light beam propagation direction. This corresponds to an expected 0 degrees -180 degrees forward-backward electron emission asymmetry of approximately 10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation.

Physical Chemistry Chemical Physics, 2006

We use an autoionization process that involves ultrafast energy transfer to neighbouring sites to... more We use an autoionization process that involves ultrafast energy transfer to neighbouring sites to characterize the formation of NeAr van der Waals bonds in clusters formed by a coexpansion of both gases. This autoionization process, the so-called Interatomic or Intermolecular Coulombic Decay (ICD), is ubiquitous in weakly bonded systems. The energy of the electron being emitted in the ICD process is shown to be characteristic for the two neighbouring entities and is therefore suggested as a new means for structural investigation, such as interface identification, of weakly bonded complexes.

Physical Review B, 2009

The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacu... more The shape of the outer valence (3p) photoelectron spectrum of Ar clusters is investigated by vacuum ultraviolet photoionization with synchrotron radiation. We show the dependence of the spectrum on cluster size, and the change of its shape with photon energy. Inelastic losses due to intracluster photoelectron scattering are most important for changes in the photoelectron main line, and explain the appearance of additional peaks. A comparison of our results to earlier work on bulk condensed Ar and Ar thin films is given. Evidence for a deviation of the photoionization cross sections for clusters from the atomic ones has not been found.

Journal of Physical Chemistry A, 2008

Measurements made with a dilute, non-oriented, gas-phase sample of a selected fenchone enantiomer... more Measurements made with a dilute, non-oriented, gas-phase sample of a selected fenchone enantiomer using circularly polarized synchrotron radiation demonstrate huge chiral asymmetries, approaching 20%, in the angular distribution of photoelectrons ejected from carbonyl C 1s core orbitals. This asymmetry in the forwardbackward scattering of electrons along the direction of the incident soft X-ray radiation reverses when either the enantiomer or the left-right handedness of the light polarization is exchanged. Calculations are provided that model and explain the resulting photoelectron circular dichroism with quantitative accuracy up to ∼7 eV above threshold. A discrepancy at higher energies is discussed in the light of a comparison with the closely related terpene, camphor. The photoelectron dichroism spectrum can be used to identify the absolute chiral configuration, and it is more effective at distinguishing the similar camphor and fenchone molecules than the corresponding core photoelectron spectrum.

Physical Review A, 2008

We present the first measurements of a photoelectron circular dichroism in photoionization from O... more We present the first measurements of a photoelectron circular dichroism in photoionization from O and 1s core levels of the R-enantiomer of glycidol (C 3 H 6 O 2 ) in the gas phase. This dichroism emerges from a forward-backward asymmetry in the angular distribution of electrons created on ionization with circularly polarised synchrotron radiation and is already fully present in the pure electric dipole approximation. Asymmetry factors obtained for the core levels in this study range up to a few percent, but it is likely that these values are limited by a failure to resolve photoemission from individual atomic sites. Theoretical modelling is provided to examine potential differences between these alternative atomic photoemission sites, and between different conformational structures of glycidol. The calculated chiral angular distribution parameters that support the circular dichroism display a much enhanced sensitivity to the molecular conformation compared to the conventional photoionization cross-section and the β-parameter. Likely conformer structures can be suggested after comparison with the experiment.