Intruder Peak-Free Transient Inner-Shell Spectra Using Real-Time Simulations

Abstract

Real-time methods are convenient for simulating core-level absorption spectra but suffer from nonphysical intruder peaks when using atom-centered basis sets. In transient absorption spectra, these peaks exhibit highly nonphysical time-dependent modulations in their energies and oscillator strengths. In this paper, we address the origins of these intruder peaks and propose a straightforward and effective solution based on a filtered dipole operator. In combination with real-time time-dependent density functional theory (RT-TDDFT), we demonstrate how to compute intruder-free attosecond transient X-ray absorption spectra for the aminophenol (C6H7NO) oxygen and nitrogen K-edges and the α-quartz (SiO2) silicon L-edge. Without filtering, the computed spectra are qualitatively wrong. This procedure is suitable for both static and transient inner-shell spectroscopy studies and can easily be implemented in a range of real-time methodologies.

Density Functional Theory for Electronic Excited States

“…When computing K-edge x-ray spectra using high-quality basis sets, similar artifacts can manifest as spurious pre-edge features that are not seen in compact basis sets where the density of levels ε pσ is more sparse. In a molecule that contains both nitrogen and oxygen, for example, excitations from N(1s) core orbitals to the highest-energy virtual MOs can manifest as spurious pre-edge features at the oxygen Kedge, 163,469 as shown in Fig. 16.…”

“…( 83), the undesired resonances can be removed from the spectrum. 469 For the oxygen K edge, this means retaining only those rows and columns where j and k refer to the O(1s) orbitals. This is precisely analogous to the frozen-valence truncation of the LR-TDDFT excitation manifold that is used to obtain core-level spectra (Section 2.2.1), and may have similar limitations for L-and M-edge spectra.…”

Accurate Relativistic Real-Time Time-Dependent Density Functional Theory for Valence and Core Attosecond Transient Absorption Spectroscopy

“…DWTA analysis of A′, A, B, and C (shown in Figure 2d and Figures S3 and S4) shares same dominantly contributing transition of π → π* character involving degenerate spinor pair (15,16) → (19, 20). Secondary contributions come from transitions involving degenerate spinor pair (13,14) → (46,45). Interestingly, peak D has completely different character from any of the features obtained in the ground-state absorption spectra, further emphasizing the novelty of this spectroscopic technique to probe states (and therein molecular orbitals) not directly accessible by purely ground-or excitedstate absorption.…”

“…With regard to simulating the response of the laser pulses relevant to pump–probe spectroscopies, the real-time (RT) formalism offers a straightforward approach − over response-theory-based methods as the former is applicable for a large range of field intensities and resembles the experimental setup in a natural way. Addressing core-level spectroscopies has added complexities, as it requires the inclusion of scalar (SC) and spin–orbit (SO) relativistic effects, which are most reliably described by multicomponent relativistic quantum chemical methods.…”

Self-Adaptive Real-Time Time-Dependent Density Functional Theory for Core Excitations

“…Yet, such one-by-one calculations of core excited states may not always be possible due to the non-Aufbau nature, given the availability of many algorithms [6][7][8][9][10][11][12][13][14][15][16] . In contrast, TDDFT can access all core excited states in one shot, by either diagonalization as in linear response (LR) TDDFT [17][18][19][20][21][22][23][24][25][26][27][28][29] or spectral analysis of the time signal generated by real-time (rt) TDDFT [30][31][32][33][34][35][36][37][38] . Yet, both LR-TDDFT and RT-TDDFT lacks orbital relaxation, thereby yielding XANES spectra that usually have large shifts as compared to the experimental ones.…”