Tunable, Coherent Vacuum Ultraviolet Radiation for Photoionization Mass Spectrometry

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Abstract
Coherent vacuum ultraviolet radiation between 118 and 129 nm (10.5 and 9.6 eV) is generated by third-harmonic conversion of radiation between 355 and 390 nm. The conversion efficiency of a single negatively dispersive rare gas (xenon or krypton) is compared to the efficiency of a mixture of a negatively dispersive gas with a positively dispersive gas (argon). The rare-gas mixtures are found to give significantly higher third-harmonic conversion efficiencies. They also have much narrower wavelength tuning ranges than the single gases. Optimum gas pressures, mixing ratios, and conversion efficiencies are tabulated at selected wavelengths. The photoionization characteristics of compounds that exhibit little or no parent ion abundance with conventional 70-eV electron impact ionization are evaluated with tunable vacuum ultraviolet radiation. n-Alkanes, alkenes, aldehydes, amines, carboxylic acids, ethers, and ketones are ionized without significant fragmentation by using wavelengths close to the ionization thresholds. Alcohols and esters, however, fragment extensively, even at the ionization threshold.