A new broadly tunable (7.4-10.2 eV) laser based VUV light source and its first application to aerosol mass spectrometry

A laser based vacuum ultraviolet (VUV) light source using resonance enhanced four wave difference mixing in xenon gas was developed for near threshold ionization of organics in atmospheric aerosol particles. The source delivers high intensity pulses of VUV light (in the range of 10¹⁰ to 10¹³ photons/pulse depending on wavelength, 5 ns FWHM) with a continuously tunable wavelength from 122 nm (10.2 eV) to 168 nm (7.4 eV). The setup allows for tight (<1 mm²) and precise focusing (μrad pointing angle adjustability), attributes required for single particle detection. The generated VUV is separated from the pump wavelengths by a custom monochromator which ensures high spectral purity and minimizes absorptive losses. The performance of the source was characterized using organic molecules in the gas phase and optimal working conditions are reported. In the gas phase measurements, photoionization efficiency (PIE) curves were collected for seven different organic species with ionization energies spanning the full wavelength range of the VUV source. The measured appearance energies are very close to the literature values of the ionization energies for all seven species. The effectiveness of the source for single particle studies was demonstrated by analysis of individual caffeine aerosols vaporized by a pulsed CO₂ laser in an ion trap mass spectrometer. Mass spectra from single particles down to 300 nm in diameter were collected. Excellent signal to noise characteristics for these small particles give a caffeine detection limit of 8 × 10⁵ molecules which is equivalent to a single 75 nm aerosol, or approximately 1.5% of a 300 nm particle. The appearance energy of caffeine originating from the aerosol was also measured and found to be 7.91 ± 0.05 eV, in good agreement with literature values.