Dual IR laser shattering of a water microdroplet

Ion desorption from the infrared (IR) laser shattering of water microdroplets (ø90 μm in diameter) was experimentally examined by ion current measurements coupled with time-resolved imaging by a charge-coupled-device camera. When a microdroplet was shattered by simultaneous illumination by two IR lasers (λ=2.9 μm) from both the left- and right-hand sides, the time-resolved imaging shows that a lot of small fragments of splash spread around the droplet. The spatial distributions of the small fragments were symmetrically compressed. The resulting fragment swarm was effectively introduced into a vacuum chamber through an inlet skimmer ø0.3-0.4 mm in diameter. The ion current measured from a 10 ^-6 mol/m ³ NaCl water solution microdroplet using two lasers was considerably enhanced compared to that by single IR laser shattering. When one of the two IR lasers was delayed by 0-1000 μs, the ion current gradually decreased with the delay time, and dropped substantially at delays longer than 100 ns. The results are ascribed to dynamical processes following the multi-photon excitation. The dual IR laser ablation of a liquid droplet can enhance the efficiency of ion formation with a lower dispersion velocity, which can be conveniently combined with time-of-flight mass spectrometry.