Laser-induced phase change

In addition to pulse duration, laser-induced phase changes must be considered carefully in the studies and/or models of particle heating dynamics. First, the particle melting and the subsequent shape transformation may be observed at high temperatures. Second, a phase change in the surrounding medium of the particle may be observed. Either of these phase transformation will cause drastic changes in the heat transfer properties and lead to a requirement for more complicated models (i.e. a solution for the fuU set of compressible equations) to represent the effects of pressure and bubble formation around the particle. 

Lindenberg AM, Kang I, Johnson SL, Missalla T, Heimann PA, Chang Z, Larsson J, Bucksbaum PH, Kapteyn HC, Padmore HA, Lee RW, Wark JS, Ealcone RW (2000) Time-resolved X-ray diffraction from coherent phonons during a laser-induced phase transition. Phys Rev Lett 84 111-114... 

If the pump beam was polarized in the plane, even a relatively weak pump intensity could induce a molecular reorientation in the M-z plane, in addition to the laser heating effect. The probe beam polarized perpendicular to 6. again monitored only the thermally induced refractive-index change, but when polarized parallel to H, it should feel both the thermal effect and the molecular reorientation effect. The refractive-index changes due to both effects were actually fairly small we can therefore write the laser-induced phase shift experienced by the probe beam in traversing the sample cell of thickness das... 

In the vicinity of the phase transition temperature Tc, molecular correlations in liquid crystals give rise to interesting so-called pretransitional phenomena. This is manifested in the critical dependences of the laser-induced index change and the response time on the temperatiue. These critical dependences are described by... 

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

Hg. 6. Laser flash-induced absorbance changes (AA) observed in a core complex at 819 nm (A) and 380 nm (B) at 298 K aA at 819 nm presented on three time scales with parameters derived by computer cun/e fitting AA at 380 nm without and with ferricyanide (C) absorbance difference spectra in the UV/vis region constructed from the 10-//s- and 100-ps decay phases. Figure source Brettel and Golbeck (1995) Spectral and kinetic characterization of electron acceptor A, in a photosystem I core devoid of iron-sulfur centers Fx, Fb and Fa- Photosynthesis Res 45 185,187. 

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