339
Views
34
CrossRef citations to date
0
Altmetric
Original Articles

Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

, , , , &
Pages 16-45 | Published online: 04 Mar 2011
 

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 (108 to 1010 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.

Notes

In this technique, a nitrogen-pumped dye laser is focused by a cylindrical lens in a thin ribbon parallel to the target surface at a height of several millimeters, and a telephoto lens images the silhouette of the illuminated ablation plume onto a color charge-coupled device.

Self-focusing or defocusing occurs if the real part of the refractive index of the target varies locally as a function of irradiance. Thermal self-focusing is related to the temperature-dependence of the refractive index.

*At saturation vapor pressure.Citation 125 , Citation 126

The threshold is defined as the temperature at which the nucleation rate equals unity.

It is commonly accepted that the general requirement for a negative heat capacity is a small number of particles, i.e., a system far from its thermodynamic limit. Its implications based on classical and quantum mechanics theories are discussed elsewhere.Citation 140 , Citation 142 , Citation 143 , Citation 144

Reprints and Corporate Permissions

Please note: Selecting permissions does not provide access to the full text of the article, please see our help page How do I view content?

To request a reprint or corporate permissions for this article, please click on the relevant link below:

Academic Permissions

Please note: Selecting permissions does not provide access to the full text of the article, please see our help page How do I view content?

Obtain permissions instantly via Rightslink by clicking on the button below:

If you are unable to obtain permissions via Rightslink, please complete and submit this Permissions form. For more information, please visit our Permissions help page.