arXiv subjects

Siddhartha Chattopadhyay

Publications and source records attributed to Siddhartha Chattopadhyay.

8 recordsLinked to original sources

arXiv2025

Behavioral Expectations in New Keynesian DSGE Models: Evidence from India's COVID-19 Recovery and Vaccination Program

This paper extends the New Keynesian Dynamic Stochastic General Equilibrium (DSGE) framework by incorporating behavioral expectations to analyze the moments of India's output gap and inflation rate, with a particular focus on the impacts of COVID-19 and vaccination programs. While DSGE models traditionally rely on rational expectations, we demonstrate that behavioral expectations more accurately capture the distributional characteristics of India's output gap and inflation rates data. Utilizing both Hodrick-Prescott and Kalman filters, we estimate the output gap and establish congruence with the moments of the simulated output gap. Concurrently, employing the initial negative demand shock values of the output gap, we calibrate the persistence parameters of negative aggregate demand (AD) and positive aggregate supply (AS) shocks, alongside the initial magnitude of the positive supply shock to achieve correspondence with post-COVID actual average values of the output gap and inflation rate. To ensure model precision, we implement Mahalanobis distance minimization for model calibration. Our findings elucidate that vaccination programs generated significant positive supply shocks, counterbalancing the prolonged negative demand shock precipitated by the COVID-19 pandemic. Moreover, the analysis reveals that the positive supply shocks' persistence parameter exceeds that of the negative demand shocks, indicating the exceptional efficacy of India's vaccination strategy. Furthermore, this research advances the scholarly contributions of Dasgupta and Rajeev (2023) by furnishing a quantitative DSGE framework that complements their static simple Keynesian analysis.

econ.GN↗

arXiv2023

Two-electron interference in two-photon attosecond double ionization of neon

The pump-probe experiments enabled by X-ray free-electron lasers (XFEL) will allow us to directly observe correlated electronic motion with attosecond time resolution by detecting photoelectron pairs in coincidence. In helium, the transition between the non-sequential and sequential regime in two-photon double ionization (TPDI) is well explained by a virtual-sequential model. Much less is known, however, about the TPDI process in more complex atoms. Recently, we extended the virtual-sequential model to arbitrary light pulses [Chattopadhyay {\it et al.,} Phys. Rev. A~{\bf 108}, 013114 (2023)]. This extension employs multi-channel scattering states for the single ionization of both the neutral and the ionized target, which we initally applied to helium. In the present study, we show that our extended virtual-sequential model reproduces the qualitative features of the angularly integrated observables with available experimental results for neon, a considerably more complex target. We observe an intriguing feature of inverted two-particle interference in the joint energy distribution of $\mathrm{Ne}$ compared to $\mathrm{He}$. This phenomenon, attributable to the presence of a final doubly ionized state with triplet symmetry coupled to the two photoelectrons, should be observable with current experimental technologies.

physics.atom-ph↗

arXiv2023

ASTRA: a Transition-Density-Matrix Approach to Molecular Ionization

We describe \ASTRA{} (AttoSecond TRAnsitions), a new close-coupling approach to molecular ionization that uses many-body transition density matrices between ionic states with arbitrary spin and symmetry, in combination with hybrid integrals between Gaussian and numerical orbitals, to efficiently evaluate photoionization observables. Within the transition-density-matrix approach, the evaluation of inter-channel coupling is exact and does not depend on the size of the configuration-interaction space of the ions. Thanks to these two crucial features, \ASTRA{} opens the way to studying highly correlated and comparatively large targets at a manageable computational cost. Here, \ASTRA{} is used to predict the parameters of bound and autoionizing states of the boron atom and of the N$_2$ molecule, as well as the total photoionization cross section of boron, N$_2$, and formaldehyde, H$_2$CO. Our results are in excellent agreement with theoretical and experimental values from the literature.

physics.atom-ph↗

arXiv2023

Two-photon double ionization with finite pulses: Application of the virtual sequential model to helium

As a step toward the full \emph{ab-initio} description of two-photon double ionization processes, we present a finite-pulse version of the virtual-sequential model for polyelectronic atoms. The model relies on the \emph{ab initio} description of the single ionization scattering states of both the neutral and ionized target system. As a proof of principle and a benchmark, the model is applied to the helium atom using the {\tt NewStock} atomic photoionization code. The results of angularly integrated observables, which are in excellent agreement with existing TDSE (time-dependent Schrödinger equation) simulations, show how the model is able to capture the role of electron correlation in the non-sequential regime, and the influence of autoionizing states in the sequential regime, at a comparatively modest computational cost. The model also reproduces the two-particle interference with ultrashort pulses, which is within reach of current experimental technologies. Furthermore, the model shows the modulation of the joint energy distribution in the vicinity of autoionizing states, which can be probed with extreme-ultraviolet pulses of duration much longer than the characteristic lifetime of the resonance. The formalism discussed here applies also to polyelectronic atoms and molecules, thus opening a window on non-sequential and sequential double ionization in these more complex systems.

physics.atom-ph↗

arXiv2023

Measuring autoionization decay lifetimes of optically forbidden inner valence excited states in neon atoms with attosecond noncollinear four wave mixing spectroscopy

Attosecond noncollinear four wave mixing spectroscopy with one attosecond extreme ultraviolet (XUV) pulse and two few-cycle near-infrared (NIR) pulses was used to measure the autoionization decay lifetimes of inner valence electronic excitations in neon atoms. After a 43-48 eV XUV photon excites a 2s electron into the 2s2p6[np] Rydberg series, broadband NIR pulses couple the 2s2p6[3p] XUV-bright state to neighboring 2s2p6[3s] and 2s2p6[3d] XUV-dark states. Controllable delays of one or both NIR pulses with respect to the attosecond XUV pulse reveal the temporal evolution of either the dark or bright states, respectively. Experimental lifetimes for the 3s, 3p, and 3d states are measured to be 7 +/- 2 fs, 48 +/- 8 fs, and 427 +/- 40 fs, respectively, with 95% confidence. Accompanying calculations with two independent ab initio theoretical methods, NewStock and ASTRA, verify the findings. The results support the expected trend that the autoionization lifetime should be longer for states that have a smaller penetration in the radial region of the 2s core hole, which in this case is for the higher angular momentum Rydberg orbitals. The underlying theory thus links the lifetime results to electron correlation and provides an assessment of the direct and exchange terms in the autoionization process.

physics.atom-ph↗

arXiv2022

ASTRA, A Transition Density Matrix Approach to the Interaction of Attosecond Radiation with Atoms and Molecules

A new formalism and computer code, ASTRA (AttoSecond TRAnsitions), has been developed to treat the interactions of short, intense radiation with molecules. The formalism makes extensive use of transition density matrices, computed using a state-of-the-art quantum chemistry code (LUCIA), to efficiently calculate the many-body inter-channel-coupling interactions required to simulate the highly correlated electron dynamics due to atoms and molecules exposed to attosecond laser radiation.

physics.atom-ph↗

arXiv2018

Electron correlation effects in enhanced-ionization of diatomic molecules in near-infrared fields

We investigate electron correlation effects in internuclear-distance-dependent enhanced ionization of $\mathrm{H}_2$, $\mathrm{LiH}$, and $\mathrm{HF}$ molecules by intense near-infrared laser pulses using a 3D description of the systems with the time-dependent generalized-active-space configuration-interaction method. This method systematically incorporates electron-electron correlation of the quantum many-electron system under consideration. Our correlated description of diatomic molecules shows that enhanced ionization occurs at certain critical internuclear separations and electron correlation systematically improves the ionization probability in this process until convergence is reached. We demonstrate the failure of the single-active-electron and the configuration-interaction singles approximations to produce the correct internuclear position and probability of the strong-field enhanced-ionization process. We elucidate the role of low-lying electronic excited states in the enhanced ionization process of diatomic molecules. There is clear evidence that an accurate description of low-lying electronically excited states is important to describe the non-perturbative enhanced ionization phenomenon in the ultrashort intense near infrared laser pulses.

physics.atom-ph↗

arXiv2016

RCCPAC: A parallel relativistic coupled-cluster program for closed-shell and one-valence atoms and ions in FORTRAN

We report the development of a parallel FORTRAN code, RCCPAC, to solve the relativistic coupled-cluster equations for closed-shell and one-valence atoms and ions. The parallelization is implemented through the use of message passing interface, which is suitable for distributed memory computers. The coupled-cluster equations are defined in terms of the reduced matrix elements, and solved iteratively using Jacobi method. The ground and excited states coupled-cluster wave functions obtained from the code could be used to compute different properties of closed-shell and one-valence atom or ion. As an example we compute the ground state correlation energy, attachment energies, $E$1 reduced matrix elements and hyperfine structure constants.

physics.atom-ph↗