Laser-induced ion formation thresholds of aerosol particles in a vacuum.

Using a time-of-flight mass spectrometer, we have measured the threshold for producing ions from various aerosol particles in avacuum with laser radiation at 248 nm,308 nm, and 10.6µm. In addition, a limited amount of similar data were taken at 193 and 337 nm. At 10.6 µm, two thresholds were observed: one near 3 GW/cm(2), which corresponds to partial ionization, and another at 6 GW/cm(2), which we attribute to plasma formation. At 308 nm, the threshold for ion production is on the order of 200 MW/cm(2). Shorter wavelengths require even less energy, with < 100 MW/cm(2) yielding normal molecular-mass spectra and approximately 500 MW/cm(2) fragmenting the sample to atomic ions.