Distinguishing between relaxation pathways by combining dissociative ionization pump probe spectroscopy and ab initio calculations: A case study of cytosine

Abstract

We present a general method for tracking molecular relaxation along different pathways from an excited state down to the ground state. We follow the excited state dynamics of cytosine pumped near the S(0)-S(1) resonance using ultrafast laser pulses in the deep ultraviolet and probed with strong field near infrared pulses which ionize and dissociate the molecules. The fragment ions are detected via time of flight mass spectroscopy as a function of pump probe delay and probe pulse intensity. Our measurements reveal that different molecular fragments show different timescales, indicating that there are multiple relaxation pathways down to the ground state. We interpret our measurements with the help of ab initio electronic structure calculations of both the neutral molecule and the molecular cation for different conformations en route to relaxation back down to the ground state. Our measurements and calculations show passage through two seams of conical intersections between ground and excited states and demonstrate the ability of dissociative ionization pump probe measurements in conjunction with ab initio electronic structure calculations to track molecular relaxation through multiple pathways.

Femtosecond Intersystem Crossing in the DNA Nucleobase Cytosine

“…Figure 3 of archetypal geometries where two or three states are found to be (near) degenerate (energy difference smaller than 0.1 eV). Among the geometries where the S 0 and S 1 states are close in energy, two geometries similar to the twist and sofa S 0 /S 1 conical intersections reported by Kotur et al 48 could be identified. Figure 3a shows our twist structure, with the carbon atoms twisted by about 70°around the CC double bond.…”

The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine

“…36 and to the analogues optimized for cytosine at different levels of theory. [21][22][23][24][25][26][30][31][32] Its relative energy is 31 375 cm −1 . It is characterized by a twisted C 5 -C 6 bond, with a bond length of 1.468 Å and a H-C 5 -C 6 -H dihedral angle of 122…”

“…9 The interest in the photophysics of Cyt has motivated many theoretical studies, whose main aims were to explain the ultrashort excited-state lifetimes and the formation of the long-lived triplet state. [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Most studies have considered the amino-keto tautomer. Among the points of debate is whether the structure of the S 1 minimum of (ππ * ) character, 1 (ππ)* Min , has a quasi-planar or ring-puckered conformation, for a recent discussion see Ref.…”

“…Three different CI structures are accessible, depending on the experimental excitation energies: The lowest-energy one, called (Eth) X , is characterized by an out-of-plane twist of the C 5 -C 6 double bond, similar to the CI structure of ethylene. [21][22][23][24][25][26][30][31][32] The second CI, called (OP) X , has a puckered N 3 atom and the amino group bent out of the plane. 18,[24][25][26]30,32 The third structure, (n O π * ) X , has a semi-planar ring and a stretched C 2 -O 7 bond.…”

Strong-Field Molecular Ionization from Multiple Orbitals

“…The peak intensities of the pulses were about 0:3 TW=cm 2 for the UV (low enough to avoid saturating the S 0 -S 2 transition or producing any ion signal) and about 10 TW=cm 2 for the IR. The experimental setup is described in more detail in [22].…”