## 1 Introduction  
Under irradiation with intense femtosecond laser pulses, solids may undergo rapid structural phase transitions. In particular, ultrafast nonthermal melting in semiconductors attracts attention. At a pulse intensity that is sufficiently high, such nonthermal melting induces a loss of atomic periodicity in a semiconductor crystal on a femtosecond timescale without any significant increase in the observed ion temperature [1, 2]. Understanding the dependence of the nonthermal melting on light pulse parameters and beam geometry could enable control of this ultrafast process. This would open a prospect for high-precision processing of silicon and other semiconducting materials with wide-ranging scientific and technological applications. Nonthermal melting occurs after the absorption of a large amount of photons (typically leading to an absorbed dose of \(\geq\)1 eV/atom), which triggers a massive excitation of electrons into the conduction band and their subsequent thermalization through collisional processes. As a result, anti-bonding states become populated, and bonding states become depleted. This induces atomic dynamics, which result in a transformation of the material structure. Experimentally, such transitions were first observed for materials irradiated with high-fluence lasers at near-infrared or visible wavelengths [1; 3; 4; 5; 6; 7; 8; 9; 10]. Currently, free-electron lasers (FELs) [11], emitting photons in the ultraviolet and X-ray range, can also generate pulses intense enough to trigger nonthermal melting in semiconductors after a single shot. Such X-ray-induced transformations have been experimentally observed, e.g., in silicon [12], and in diamond [13]. In parallel to the measurements, a dedicated theory apparatus has been developed to study light-induced nonthermal processes. In the infrared regime, radiation-triggered nonthermal melting of silicon has been studied in detail, e.g., in [2; 14; 15; 16; 17]. For studying the response of materials to X-ray irradiation, a dedicated hybrid approach has been developed [18]. The resulting code, XTANT (see next section for details) has been so far successfully applied to model ultrafast X-ray-induced structural transitions in carbon and in silicon materials [18; 19; 20]. The code applicability is currently restricted to femtosecond timescales, which correspond to the timescales of the pump and probe pulse, and the delays between them. Extension of the code to longer timescales would require the consideration of long-timescale processes, such as, e.g., nucleation and solidification [21; 22]. Such computationally expensive extension is not required for the present work, which focuses on X-ray diffraction studies. Diffraction imaging experiments, performed at various XFEL facilities, exploit the ultrabright, ultrashort X-ray pulses to determine the structure of matter, in particular, of nanoparticles and biomolecules [23; 24]. This powerful technique is being improved in its spatial resolution [25; 26; 27] toward atomic resolution. It can also provide an access to ultrafast dynamics within X-ray-irradiated samples with femtosecond temporal resolution [28], using the X-ray pump-X-ray probe experimental scheme. In particular, this should enable the study of ultrafast structural transitions in solid materials, triggered by an X-ray pump pulse. However, the results of such experiments have to be interpreted with care [24; 28], due to the non-uniformity of the X-ray beam [29]. As the imaged object, e.g., a single crystal, is large in comparison with the beam focal size, the imaging signal includes contributions from the crystal regions illuminated weaker or stronger, depending on their position in respect to the beam focus. The total diffraction signal recorded will then be a superposition of the contributions from the different crystal regions [29] and has to be interpreted and modeled as such.