Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

Abstract

For the last decade, a variant of pulsed laser ablation, Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), has been studied as a deposition technique for organic and polymeric materials. RIR-MAPLE minimizes photochemical damage from direct interaction with the intense laser beam by encapsulating the polymer in a high infrared-absorption solvent matrix. This review critically examines the thermally-induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices.Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. Normal vaporization is a highly inefficient polymer deposition mechanism as it relies on collective collisions with evaporating solvent molecules. Diffusion length calculations for heterogeneously nucleated vapor bubbles show that normal boiling is kinetically limited. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIR-MAPLE. Phase explosion occurs when the target is rapidly heated (10 8 to 10 10 K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited process, while surface evaporation can occur even at very low fluences.

Nanosecond laser-induced shock propagation in and above organic liquid and solid targets

“…In addition, unlike liquid targets, solid targets can support a shear stress. Simple calculations show that under typical MAPLE conditions the target surface can fracture with a typical crack length of ∼20 nm [3]. This process is considered to serve as a retarding mechanism for shock propagation.…”

“…Both can solubilize a wide range of polymers and other materials, and have relatively low reactivity and health concerns. Toluene has been frequently used as a MAPLE matrix [8,9,3,10], whereas acetone is a common choice for both MAPLE [11] and LAL experiments [12][13][14]. In this work, we utilize the fourth harmonic of a Nd:YAG laser (266 nm) in order to ablate liquid and solid surfaces of acetone and toluene under ambient conditions.…”

“…Toluene and acetone are examples of organic solvents that are frequently used in laser processing experiments such as matrixassisted pulsed laser evaporation (MAPLE) [1][2][3], and laser ablation in liquids (LAL) [4][5][6][7] because of their versatility. Both can solubilize a wide range of polymers and other materials, and have relatively low reactivity and health concerns.…”

Material properties and applications of blended organic thin films with nanoscale domains deposited by RIR-MAPLE

“…Therefore, a different strategy was reported by Bubb, et al [15], and Toftmann, et al, in 2004[16], which used infrared lasers that are resonant with vibrational bond stretches in the solvent only, thereby separating the absorption spectrum of the solvent from that of the polymer. Thus, a distinction was made between different excitation wavelengths used in MAPLE: ultraviolet excitation (UV-MAPLE) and infrared excitation that is resonant with particular chemical bond vibrational modes within the liquid solvent (resonant-infrared MAPLE (RIR-MAPLE)) [17][18][19].…”

Finite element simulation of infrared laser ablation for mass spectrometry

“…A large number of studies use ultraviolet (UV) lasers because the high energy per photon results in both direct ionization and photon emission . Infrared (IR) lasers have also been used in cases where efficient removal of material is important . Mid‐IR laser systems such as the optical parametric oscillators (OPO) and Er:YAG lasers with wavelengths near the 3 µm OH stretch absorption are particularly useful for laser ablation.…”