Femtosecond laser irradiation of metal thin films can induce complex dynamics, such as protrusion, jetting, and nanoparticle formation. These are governed by both energy deposition and film-substrate mechanical coupling. In this study, we use coupled two-temperature molecular dynamics (TTM-MD) simulations to investigate the nonequilibrium thermomechanical response of a gold thin film in the paraxial region under laterally confined laser excitation. We systematically vary the electron diffusion coefficient, absorption depth, rear-side thermal and mechanical boundary conditions to identify how longitudinal energy deposition, electron-lattice coupling, and stress evolution control jet initiation and stability. Our results show that confined electron diffusion significantly enhances near-surface peak temperatures and transient tensile stresses, leading to stress-dominated local material separation. Implementing a dissipative rear-side boundary that mimics a substrate allows stress waves to be transmitted and attenuated, so that jetting is primarily driven by the release of near-surface nonequilibrium pressure and evolves into stable needles or liquid clusters. Spatiotemporal maps of atomic stress and stress rate reveal how localized mechanical stresses nucleate jets and clusters, thereby providing clear mechanical initial conditions for subsequent surface-tension-driven morphology evolution. This atomic-scale stress-based picture refines our understanding of laser-induced nanostructure formation in metal thin films and supplies physically grounded initial conditions for multiscale jetting models.
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