Simulation of Impulsive Vibrational Spectroscopy Federico J. Herna ́ ndez,* ,†,‡ Franco P. Bonafe ́ , †,‡ Ba ́ lint Aradi, ¶ Thomas Frauenheim, ¶ and Cristia ́ n G. Sa ́ nchez †,‡ † Universidad Nacional de Có rdoba. Facultad de Ciencias Quı ́ micas, Departamento de Quı ́ mica Teó rica y Computacional, Có rdoba Argentina ‡ Instituto de Investigaciones en Fisicoquímica de Có rdoba, INFIQC (CONICET-Universidad Nacional de Có rdoba), Có rdoba 5000, Argentina ¶ Bremen Center for Computational Materials Science, Universitä t Bremen, Bremen 28359, Germany * S Supporting Information ABSTRACT: In the present work we applied a fully atomistic electron-nuclear real-time propagation protocol to compute the impulsive vibrational spectroscopy of the five DNA/RNA nucleobases in order to study the very first steps (subpico- second) of their energy distribution after UV excitation. We observed that after the pump pulse absorption the system is prepared in a coherent superposition of the ground and the pumped electronic excited states in the equilibrium geometry of the ground state. Furthermore, for relatively low fluency values of the pump pulse, the dominant contribution to the electronic wave function of the coherent state is of the ground state and the mean potential energy surface within the Ehrenfest approximation is similar to that of the ground state. As a consequence, the molecular displacements are better correlated with ground-state normal modes. On the other hand, when the pump fluency is increased the excited-state contribution to the electronic wave function becomes more important and the mean potential energy surface resembles more that of the excited state, producing a better correlation between the molecular displacements and the excited-state normal modes. Finally, it has been observed that the impulsive activation of several vibrational modes upon electronic excitation is triggered by the development of excited-state forces which accelerate the nuclei from their equilibrium positions causing a distribution of the absorbed electronic energy on the nuclear degrees of freedom and could be closely related to the driving force of the ultrafast nonradiative deactivation observed in these systems. 1. INTRODUCTION Since the advent of picosecond and then femtosecond light sources, a vast new research field has emerged both for photophysics and for photochemistry allowing the study of the dynamics of molecular systems on ultrashort time scales. In this sense, the development of time-resolved pump-probe spectroscopies has increased tremendously during the last three decades, and the study of the nonlinear response of complex systems upon interaction with coherent laser light pulses has become a frontier research topic in molecular quantum physics. Particular interest has been focused in the investigation of coherence in molecular processes occurring in the condensed phase, leading to a new kind of spectroscopy, namely, vibrational coherence spectroscopy (VCS). 1-7 The principle of VCS is based on the interaction of the molecular system with an ultrashort coherent laser pulse (pump pulse) which is shorter than the period of the molecular nuclear motions and has a spectral width larger than the corresponding vibrational levels spacing. Such laser pulse may produce a vibrational wave packet (i.e., preparation of the molecular system in a coherent superposition of vibrational levels) in almost any molecule in a impulsive manner. Moreover, if this ultrashort pulse is resonant with any ground-state electronic absorption band, for example, the S 0 -S 1 , it will produce a coherent superposition of electronic and vibrational quantum states. Hence, a nonstationary population will be produced upon light absorption, and the generated wave packet will be in a coherent superposition between the S 0 and the S 1 states until decoherence occurs. Due to the fact that the quantum superposition of vibrational states results in the classical oscillation of the vibrational degrees of freedom and this oscillation changes the molecular structure, the absorbance signals of the pumped molecule oscillates as well. Then a second laser pulse (probe pulse) may interact with the system at different time delays recording the spectrum of the transient (pump-driven) species. Finally, the Fourier transform of the oscillatory absorption signals reveals the Raman activity of the system, providing similar information to that available in the frequency domain. Thus, this sequential pump-probe transient absorption spectroscopy using ultra- Received: January 10, 2019 Revised: February 13, 2019 Published: February 15, 2019 Article pubs.acs.org/JPCA Cite This: J. Phys. Chem. A 2019, 123, 2065-2072 © 2019 American Chemical Society 2065 DOI: 10.1021/acs.jpca.9b00307 J. Phys. Chem. A 2019, 123, 2065-2072 Downloaded via DEUTSCHES ELEKTRONEN-SYNCHROTRON on March 21, 2019 at 19:37:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.