attosecond transient absorption*

Advances in attosecond technologies have opened the way to the experimental time-resolved study of correlated electronic excitations in atoms and molecules [1,2]. These experiments require a time-dependent wave-function approach to represent the coherent superposition of multiple ion-photoelectron pairs. To tackle this challenge, we have developed ASTRA (AttoSecond TRAnsitions), a molecular-ionization code based on a new transition-density-matrix formalism that allows for efficient algorithms to build the necessary observables in a close-coupling space [3], with the support of the LUCIA general configuration-interaction code [4] and the GBTOlib hybrid-integral library [5]. We apply ASTRA to study the ultrafast electron dynamics triggered in molecular oxygen by sequences of attosecond laser pulses. We focus in particular on the effect that electronic correlation has on the attosecond transient absorption spectrum of the laser-dressed molecule, across many ionization thresholds up to the the c ⁴Σ_u^- ionic channel, within the Franck-Condon region. To compute the optical response of the dressed target, we propagate the wave function in a basis of essential Siegert states, which greatly accelerates the calculation without appreciably compromising its accuracy. Our computed photoionization and transient absorption spectra are compared with other theoretical and experimental results reported in the literature [6,7]. This work is supported by the DOE CAREER grant No. DE-SC0020311. The calculations used NERSC resources under the contract No. DE-AC02-05CH11231 and the award BES-ERCAP0024720.

[1] J Duris et al, Nat. Phot. 14, 30 (2020).

[2] N Saito et al, Optica 6, 1542 (2019).

[3] J M Randazzo et al, arXiv:2211.10316 [physics.atom-ph] (2022).

[4] J Olsen et al, J. Chem. Phys. 104, 8007 (1996).

[5] Z Mavšín et al, Comp. Phys. Comm. 249, 107092 (2020).

[6] C T Liao et al, Phys. Rev. A 95, 043427 (2017).

[7] S Marggi  et al, Phys.Chem.Chem.Phys. 21, 16497 (2019).