5314027781

POSTER SESSION, H39

LIFETIME MEASUREMENTS OF THE COLLISION-FREE SLOW FLUORESCENCE FROM S,-T, GATEWAY LEVELS OF GLYOXAL

CH. GÓRLING, CH. OTTINGER, Max-Planck-Institut fur Strómungsjorschung, Bunsenstr. 10, D-37073 Góttingen, Germany;

E. .IALYISTE. Insłitute oj Physics, Tartu Uniuersity, Riia IĄ2, EE2400 Tartu, Estonia; N. OH TA, Department of Molecular Chem-istry, Graduate School oj Engineering, Hokkaido Unwersity, Sapporo 060, Japan.

In the case of Iow triplet State density the coupling hetween accidentally near resonant zero-order singlet and triplet States gives rise to two eigenstates, one of mostly St character and the other of mostly Ti character. The lifetime of the Ti-like eigenstate (called also gateway State) can be much longer than the zero order S| lifetime (2.4 /js for glyoxal). The long-lived e miss i on form the gateway State can thus be called slow fluorescence (SF) or fast phosphorescence.

First experimental observation of the SF for jet-cooled glyoxal was reported in the previous work of this group.° In recording the slow fluorescence excitation spectrum (SFES), the photons arriving in the time interval 30-80 ps after the laser pulse were counted. The SFES were much sparser than the ordinary short-lived FES of the same vibronic bands. This indicates that ordy a few rotational singlet lcvels are coupled with the triplet levels. Herc we report on a reinvestigation of the SFES of the Oj} band of glyoxal with improved resolution as well as on measurements of slow fluorescence lifetiines for selected gateway States. Rotational analysis of tłie 0® band SFES enabled us to identify 23 gateway States by their ./', K'_a, and K[ ąuantum numbers. The measured SF lifetimes were between 20 and 83 ps. The difficulties in lifetime determination arising from the movement ofemitting molecules along with the flow and from the presence of strong ordinary fluorescence will be discussed. The decay of the gateway States with quantuin numbers 17,1.17 and 12,1.12 showed superimposcd quantum beats wliose frequencies are 76 and 64 kHz, respectively. The SF lifetimes were found to inerease with the energy of the excitation pulse. For exainple, raising the laser pulse energy from 2 p.I to 2 mJ the SF lifetime of the 17.1.17 gateway State inereases from 25 to 37 ps. This is explained by power broadening and the coupling to the; ground state vibrational continuum. A stronger laser pulse excites eigenstates in a broader energy rangę near the molecular resonancc. The inclusion of long-lived So-like eigenstates. carrying less oscillator strength, then leads to the lengthening of the resulting lifetime.

°J. Heldt, Ch. Ottinger, A. F. Vilesov, and T. Winkler, ./. Phys. Chan. 101, 740 750 (1997)