Gauge invariance in the theoretical description of time-resolved angle-resolved pump/probe photoemission spectroscopy

Nonequilibrium calculations in the presence of an electric field are usually performed in a gauge, and need to be transformed to reveal the gauge-invariant observables. In this work, we discuss the issue of gauge invariance in the context of time-resolved angle-resolved pump/probe photoemission. If the probe is applied while the pump is still on, one must ensure that the calculations of the observed photocurrent are gauge invariant. We also discuss the requirement of the photoemission signal to be positive and the relationship of this constraint to gauge invariance. We end by discussing some technical details related to the perturbative derivation of the photoemission spectra, which involve processes where the pump pulse photoemits electrons due to nonequilibrium effects.

Electric field and temperature dependence of the local structural disorder in the lead-free ferroelectric Na0.5Bi0.5TiO3: An EXAFS study

The complex nature of the structural disorder in the lead-free ferroelectric Na1/2Bi1/2TiO3 has a profound impact on the perceived global structure and polar properties. In this paper, we have investigated the effect of electric field and temperature on the local structure around theBi and Ti atoms using extended x-ray absorption fine structure. Detailed analysis revealed that poling brings about a noticeable change in the bond distances associated with the Bi-coordination sphere, whereas the Ti coordination remains unaffected. We also observed discontinuity in the Bi-O bond lengths across the depolarization temperature of the poled specimen. These results establish that the disappearance of the monoclinic-like (Cc) global distortion, along with the drastic suppression of the short-ranged in-phase octahedral tilt after poling B. N. Rao et al., Phys. Rev. B 88, 224103 (2013)] is a result of the readjustment of theA-O bonds by the electric field, so as to be in conformity with the rhombohedral R3c structure.

Study of band offsets at the Cu2SnS3/In2O3: Sn interface using x-ray photoelectron spectroscopy

Cu2SnS3 thins films were deposited onto In2O3: Sn coated soda lime glass substrates by spin coating technique. The films have been structurally characterized using x-ray Diffraction (XRD) and Atomic Force Microscopy (AFM). The morphology of the films was studied using Field Emission Scanning Electron Microscopy (FESEM). The optical properties of the films were determined using UV-vis-NIR spectrophotometer. The electrical properties were measured using Hall effect measurements. The energy band offsets at the Cu2SnS3/In2O3: Sn interface were calculated using x-ray photoelectron spectroscopy (XPS). The valence band offset was found to be -3.4 +/- 0.24 eV. From the valence band offset value, the conduction band offset is calculated to be -1.95 +/- 0.34 eV. The energy band alignment indicates a type-II misaligned heterostructure formation.

Study of band offsets at the Cu2SnS3/In2O3: Sn interface using x-ray photoelectron spectroscopy

Cu2SnS3 thins films were deposited onto In2O3: Sn coated soda lime glass substrates by spin coating technique. The films have been structurally characterized using x-ray Diffraction (XRD) and Atomic Force Microscopy (AFM). The morphology of the films was studied using Field Emission Scanning Electron Microscopy (FESEM). The optical properties of the films were determined using UV-vis-NIR spectrophotometer. The electrical properties were measured using Hall effect measurements. The energy band offsets at the Cu2SnS3/In2O3: Sn interface were calculated using x-ray photoelectron spectroscopy (XPS). The valence band offset was found to be -3.4 +/- 0.24 eV. From the valence band offset value, the conduction band offset is calculated to be -1.95 +/- 0.34 eV. The energy band alignment indicates a type-II misaligned heterostructure formation.

Triplet excited electronic state switching induced by hydrogen bonding: A transient absorption spectroscopy and time-dependent DFT study

The solvent plays a decisive role in the photochemistry and photophysics of aromatic ketones. Xanthone (XT) is one such aromatic ketone and its triplet-triplet (T-T) absorption spectra show intriguing solvatochromic behavior. Also, the reactivity of XT towards H-atom abstraction shows an unprecedented decrease in protic solvents relative to aprotic solvents. Therefore, a comprehensive solvatochromic analysis of the triplet-triplet absorption spectra of XT was carried out in conjunction with time dependent density functional theory using the ad hoc explicit solvent model approach. A detailed solvatochromic analysis of the T-T absorption bands of XT suggests that the hydrogen bonding interactions are different in the corresponding triplet excited states. Furthermore, the contributions of non-specific and hydrogen bonding interactions towards differential solvation of the triplet states in protic solvents were found to be of equal magnitude. The frontier molecular orbital and electron density difference analysis of the T-1 and T-2 states of XT indicates that the charge redistribution in these states leads to intermolecular hydrogen bond strengthening and weakening, respectively, relative to the S-0 state. This is further supported by the vertical excitation energy calculations of the XT-methanol supra-molecular complex. The intermolecular hydrogen bonding potential energy curves obtained for this complex in the S-0, T-1, and T-2 states support the model. In summary, we propose that the different hydrogen bonding mechanisms exhibited by the two lowest triplet excited states of XT result in a decreasing role of the n pi* triplet state, and are thus responsible for its reduced reactivity towards H-atom abstraction in protic solvents. (C) 2016 AIP Publishing LLC.