The Influence of Weak Magnetic Fields on Radiationless Processes

Magnetic effects on the fluorescence from singlet excited states represent an interesting new method of studying the behaviour of excited states. Here the intersystem crossing in a weak magnetic field in both protonated and deuterated glyoxal was studied. Fluorescence quenching was observed at various pressures with different collision partners as a function of the magnetic field. Time resolved emission and excitation spectra were observed and quenching rate constants were determined with and without magnetic field. The collision free fluorescence lifetime of glyoxal was found to be unaffected by the magnetic field. The magnetic fluorescence quenching effect is only observed in the presence of colliders and is seen as an enhancement of intersystem crossing. In protonated glyoxal all phosphorescence is collision dependent, whereas in the deuterated species there exists zero pressure phosphorescence. The magnetic effect is here interpreted as being due to the interaction of the singlet with the triplet state, which is magnetically sensitive. In particular additional intersystem crossing mechanisms are postulated to be operative in the presence of a magnetic field. A grouptheoretical method predicts which new frequencies must be involved in this new process, and this activity is experimentally confirmed in the emission spectrum.