Two-Color Laser Induced Evaporation Dynamics of Liquid Aerosols Probed by Time-of-Flight Mass Spectrometry

The first results are reported from an investigation into the evapora tion dynamics of liquid aerosols using a newly constructed two color laser time-off light mass spectrometer, which utilizes a CO₂ laser (IR) for vaporization and an excimer laser (UV) for ionization. The results indicate that the CO ₂ laser with an intensity greater than 1.2 = 10⁸ W cm2 completely vaporizes the aerosol particle and that this preevaporation of the liquid aniline particle increases the ion signal by as much as two orders of magnitude compared to direct ionization of the particle with the excimer laser only. The present focus is on exploring the vaporization process by varying both the timing between the two lasers and the CO ₂ laser fluence. A three-dimensional (3D) model of the laser-induced gas plume is used to extract the associated velocity distribution and its temperature of 1300 and 2500 K for CO₂ laser intensities of 1.2 = 10⁸ W cm2 and 2.8 = 10⁸ W cm2, respectively. At the higher CO₂ laser energies, the data could only be fitted by the addition of a 350 m/s stream, or entrainment, velocity to the three-dimensional Maxwell-Boltzmann distribution, consistent with hydrodynamic expansion.