Study of S-T conversion induced by an external magnetic field in gaseous oxalylfluoride excited to the 0(0)-level of the (A)over-tilde(1)A(u) state

The fluorescence intensity and decay of gaseous oxalylfluoride ((COF)(2)) e xcited to the (I)A(u)(0(0)) level by the (A) over tilde <-- (X) over tilde transition were measured as a function of an external magnetic field. On ex citation to this level, the dynamics in zero field may be described in the small-molecule limit, with the fluorescence exhibiting an almost exponentia l decay. However, at increasing field strength the initial fluorescence dec ay becomes faster, the decay profile becoming biexponential at higher field s. Thus, a magnetic field-induced change of dynamics occurs in the (A) over tilde(1)A(u) state from that of a small molecule to that of the intermedia te case. The fast-component decay rate constant K-f = (2.36 +/- 0.19) x 10( 7) s(-1) is independent of the (COF)(2) gas pressure and magnetic field str ength, while the slow-component lifetime depends on both. We find that the magnetic field effect on the slow component grows at lower gas pressures. A n increase of the integrated (COF)(2) phosphorescence was observed at highe r magnetic fields; consequently an external field accelerates singlet-tripl et transitions in the excited (COF)(2). Time-resolved measurements of the e ffect of a microwave field on the fluorescence demonstrated that the slow-c omponent amplitude and lifetime are additionally reduced by an external mic rowave field, at nu(MW) = 9400 MHz, B = 0.3295 T, and P = 30 mTorr; and the fast-component amplitude increases at constant lifetime. We also find an a dditional phosphorescence intensity increase with subsequent saturation at higher microwave intensities. Experimental data are interpreted using the i ndirect mechanism theory in the low level density limit. (C) 1999 Elsevier Science B.V. All rights reserved.