Using time-dependent density functional theory coupled nonadiabatically with molecular dynamics (TDDFT-MD), we systematically investigate the real-time nonadiabatic electron-nuclear dynamics of cytosine irradiated by 800 nm (1.55 eV) laser pulses. The laser intensity ranges from 1 \(\times10^{13}~\text{W/cm}^2\) to 8 \(\times10^{15}~\text{W/cm}^2\) . The calculated ground-state electronic structure, highest occupied molecular orbital (HOMO) distribution, and optical absorption spectrum are in excellent agreement with experimental data, validating the theoretical model. Two distinct ionization regimes (multiphoton, \(\gamma > 1\) ; tunneling, \(\gamma < 1\) ) are identified, with distinct orbital contribution mechanisms and structural responses. In the multiphoton regime, ionization exhibits a near-resonance energy alignment tendency. HOMO‑3 contributes the most (20.8%) owing to its favorable \(\sigma\) -type spatial symmetry without nodal planes perpendicular to the laser polarization, together with near-resonant energy matching with six-photon energy. In the tunneling regime, the intense laser field depresses the Coulomb barrier, activating deep orbitals and reducing the total contribution of the four outermost orbitals to 22.5%. Laser intensity modulates structural stability: low intensities induce small-amplitude vibrations without dissociation, while high intensities trigger NH/CH bond cleavage via intramolecular Coulomb repulsion, with the pyrimidine ring remaining intact. The ionization yield follows a power-law relationship with laser intensity in both regimes, with logarithmic slopes of 3 (multiphoton) and 0.8 (tunneling), reflecting intrinsic differences in ionization mechanisms. This work provides atomic-level insights into cytosine’s strong-field dynamics, laying a foundation for understanding laser-DNA interactions.
