Phase transition at low fluences in laser desorption of organic solids: a molecular dynamics study

Computer Simulations of Laser Ablation of Molecular Substrates

“…As the fluence increases, the near surface region melts and evaporation from a liquid surface region occurs. 23,66…”

“…The growing experimental effort has been supported by theoretical studies, ,− numerical analysis of kinetic models, − and molecular dynamics (MD) simulations of laser desorption/ablation. − The diversity and complexity of the intertwined processes involved in laser ablation and occurring at different time and length scales present a challenge for theoretical and computational descriptions of this phenomenon. The processes include primary elementary excitations of optically active states in a molecular solid, thermalization of the deposited laser energy, formation of a highly energetic high-temperature and high-pressure region, explosive disintegration and prompt forward ejection of a volume of material, intensive processes in the ejected plume, recondensation or deposition of the ejected particles in PLD, and propagation of a pressure wave into the bulk of the target away from the ablation region.…”

Molecular dynamics simulations of matrix-assisted laser desorption—connections to experiment

“…At the lowest fluences where we detect ejected molecules, the surface temperature reaches 735 K, slightly below the melting temperature of the model material, ∼750 K. Therefore, the molecular ejection can be attributed to the desorption from a solid. As the fluence increases towards the ablation threshold, the near-surface region melts and desorption occurs from a liquid surface region [10,21]. We find that the dependence of the desorption yield on laser fluence F can be well described by an Arrhenius-type expression [3,8,22,25],…”

“…We have developed a breathing sphere model aimed at overcoming these limitations [16][17][18][19][20][21][22][23][24][25][26][27][28]. The novel feature of the breathing sphere model is an approximate representation of the internal atomic motions, thus we can significantly expand the time and length scales of the simulation yet still reproduce a realistic rate of the vibrational relaxation of excited molecules.…”

Conformation Changes, Complexation, and Phase Transition in Matrix-Assisted Laser Desorption

“…To address the details of desorption in MALDI at the microscopic level, molecular dynamics (MD) simulations have been applied. − The dynamics of desorption and hydrogen bonding was first described by Bencsura and Vertes in a hemispherical volume of nicotinic acid matrix seeded with leucine enkephalin guest species. Using selective excitation of the matrix molecules, the energy transfer between matrix and guest molecules was also investigated for the same system. , The results supported the previously developed homogeneous bottleneck model that had rationalized the absence of fragmentation in MALDI. , To expand the time and size scale in the MD studies of MALDI, Zhigilei and Garrison introduced the breathing sphere model. ,,− Their results indicated the presence of a fluence threshold separating layer-by-layer desorption from ablation, the formation of clusters in the plume, and the peculiarities of the velocity distributions for the two species. Due to the need to keep the calculations manageable, this model employed a single parameter (the radius of the breathing sphere) for the representation of the internal degrees of freedom.…”