IONIZATION AND DISSOCIATION OF BENZALDEHYDE USING SHORT INTENSE LASER-PULSES

In a recent series of experiments, femtosecond laser mass spectrometry (FLMS) was applied to benzaldehyde utilizing laser pulse widths in th e range 90 fs to 2.7 ps. Beam intensities up to 2 x 10(14) W cm(-2) we re used with wavelengths of 750 and 375 nm. Different ionization-disso ciation channels were found compared with previous studies by other au thors based in the nanosecond regime. The general theme emerging is on e of predominant above-threshold ionization-dissociation (ID) in which predissociative states are largely bypassed via rapid optical up-pump ing to the molecular ionization continuum. Above the parent ionization threshold, ladder-switching is seen to be a function of laser pulse w idth, intensity, and wavelength with exclusive parent ion formation be ing achievable in the lower intensity regions at all pulse widths. Inc reasing fragmentation occurs as the laser intensity increases, althoug h parent supremacy remains. At 750 nm however, the increase of fragmen tation with intensity is greatly reduced compared to 375 nm, leading t o the conclusion that FLMS at longer wavelengths is preferred for chem ical analysis. Moreover at 750 nm and at laser intensities close to 10 (14) W cm(-2) C7H6O2+ becomes evident. It is also shown that the C7H6O +/C7H5O+ ratio is strongly dependent on the pulse width, suggesting th at a hydrogen loss pathway has a dissociation time of about a picoseco nd.