Molecular Dissociation with Ultrashort Laser Pulses

The CLF is a centre for molecular dissociation studies. User scientists from several universities carry out both fundamental and applied research in this area using ultrashort (50 fs) laser pulses focused to intensities greater than 10¹⁶ Wcm^-2. Researchers take advantage of the intense femtosecond pulses in several ways. The laser pulse duration is shorter than the vibrational period of most molecules, and this simplifies the study of molecular Coulomb explosions.

This new laser technology has enabled scientists to induce and study spatial asymmetry during molecular dissociation by carefully controlling the superposition and relative delay of ionising pulses of different colours. Ultrashort laser pulses have also overcome a problem in analytical science. A disadvantage of using long (nanosecond) laser pulses is that polyatomic molecules often dissociate into many small fragments which render the parent molecule unidentifiable. Researchers have found that, with femtosecond pulses, large mass fragments are detected - often including the parent ion.

The appearance of the CH3NO2 + parent peak in the lower time of flight trace demonstrates the advantage of using ultrashort pulses.

Ian Williams of Queen's University, Belfast aligns a time of flight spectrometer to investigate the photodissociation of molecules (98RC1762)