A comparison of femtosecond and nanosecond multiphoton ionization and dissociation for some nitro-molecules

Resonance‐enhanced multiphoton ionization spectrometry (REMPI), using time‐of‐flight mass spectrometers and tuned lasers, has proved an important ultra‐sensitive analytical technique. Nevertheless, conventional nanosecond REMPI suffers from a number of shortcomings: most importantly, REMPI often fails through rapidly (pre)dissociating states. In the case of thermally labile molecules, which include the nitro‐molecules, either no or very small parent or high/mass fragment ion peaks exist, making the interpretation of the mass spectra ambiguous at best and often impossible. Femtosecond laser mass spectrometry (FLMS) can often ‘defeat’ these dissociative states, resulting in large parent or high‐mass fragment ion peaks which make the interpretation less ambiguous. In the present paper, nanosecond and femtosecond multiphoton ionization and fragmentation are compared using time‐of‐flight mass spectrometry for NO2 gas and a number of different nitro‐molecules: nitromethane, nitrobenzene, m‐nitrotoluene, dinitrotoluene and trinitrotoluene.