Pulsed laser deposition instrumentation

Figure 6 shows an outline of a PAS instrument designed for fast time-resolved measurements. The excitation light is a laser pulse of some 20 ns duration, at a wavelength which falls within the absorption spectrum of the sample (e.g. 337 nm with a nitrogen laser). Total absorption of this pulse then deposits an energy E in the sample and this will decay in the course of time into heat which will give rise to the pressure sensed by the detector usual microphones have slow response times, so that piezo-electric devices are used to improve the instrument s time resolution [43]. 

Secondary-ion mass spectrometry (SIMS) of a thin layer of nucleic acid bases deposited on a silver foil under bombardment with Ar ions at 3 kV gives intense pseudomolecular ions [M H] but practically no simple bond cleavage fragments. Another new technique is that of (pulsed) laser induced desorption (LD). When applied to nucleotide bases such as cytosine or adenine (266 nm, quadruplet neodymium laser or 347 nm, ruby laser) the technique has good detection limits, particularly for ions with a short lifetime (up to 100 nsec). The technique makes use of a time-of-flight instrument and is utilized in both modes, positive (PI) and negative ions (NI). Both bases exhibit an intense [BH]" ion. These results are similar to those obtained by Cf plasma desorption (PD). 

Infrared Laser Desorption. Following the initial report by Kistemaker et al., Heresch et al. and Cotter combined pulsed IR laser desorption with sector instruments, while Vestal" and Stoll and Rollgen utilized quadrupole mass spectrometers to carry out laser desorption experiments. Further work by Kistemaker and co-workers established a thermal mechanism for laser desorption in which cat-ionized species were produced by gas-phase attachment of alakali ions to codesorbed neutral species. Cationized species were also observed at much shorter wavelengths (256 nm) by Heinen, while M-i-Ag+ ions were reported by Cooks et ah for solid samples deposited onto silver foils with ammonium chloride. The first laser desorption time-of-flight results were reported by Hercules and co-workers using a laser microprobe instrument to observe alkaU- and haUde-ion attachment in positive- and negative-ion mass spectra, respectively, and by VanBreemen et al." ... 

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