VIBRONIC STRUCTURE IN THE SPECTRUM OF FLUORESCENCE LIFE TIMES AND QUANTUM YIELDS OF ANILINE VAPOR

Author Institution: Department of Chemistry, Illionis Institute of TechnologyAniline vapor has a well resolved vibronic absorption spectrum in the region from 3000 to 2750 \AA. By means of high resolution phase fluorimetry we have determined the spectrum of fluorescence life times and quantum yields over this region with a band width of 2.5 \AA. The emission spectrum was not resolved. Relative values of quantum yields for different wavelengths were obtained directly, while absolute values were estimated from literature data. Quantum yields fluctuate between 0.25 and 0.5 with a downward tendency as the energy increases. The reciprocal life time, $1/\tau=k_{isc}+k_{j}$ (the sum of intersystems crossing and radiative transition probabilities) varies in the range (1.3 to 2.3) $\times 10^{3} sec^{-1}$ with an upward trend toward higher energies. Moreover, the reciprocal life times show sharply pointed minima whereas the quantum yields have maxima at the wavelengths of all major absorption peaks in the aniline spectrum. The larger portion of this effect can be attributed to changes in $k_{isr}$ which are interpreted as vibronic fine structure, superimposed on spin-orbit coupling. The latter mixes the $^{1} B_{2}$ and $^{2} A_{1}$ states involved here. Vibronic coupling between these states is symmetry-forbidden but can be induced by certain vibrational distortions. In aniline, the major absorption peaks are due to totally symmetric ($a_{1}$) or to ($b_{1}$) modes. For molecules selectively excited to these modes, the selection rule is not likely to be lifted and hence, vibronic coupling and the value of $k_{isr}$ will be at a minimum

STUDY OF NON-ADIABATIC TRANSITIONS IN TRIATOMIC MOLECULES

$^{1}$A. E. Douglas, J. Chem. Physics 45, 1007 (1966).""Author Institution: Department of Chemistry, Saint Louis University; Department of Chemistry, Illinois Institute of TechnologyNon-adiabatic coupling between electronic states in triatomic molecules has been studied with special consideration of the possibility of internal conversion. Approximate calculations of electronic and vibrational coupling elements and density of states, which determine the transition probability, have been performed for $H_{2} O$ and some other molecules. The results suggest that the model mechanism for large molecules, which is of statistical type and implies an irreversible process, should not be applied to triatomics. Under favorable conditions, e.g. sufficiently different shape or position of the two potential curves involved, strong coupling might occur which is large enough to play a role in radiationless transition processes. This mechanism may explain anomalously long fluorescence lifetimes in small molecules. However, observations of lifetime $effects^{1}$ in $NO_{2}$, $SO_{2}$, and $CS_{2}$ cannot be explained with this mechanism, because internal conversion is electronically forbidden

REMPI - TOF mass spectra of CO2-laser desorbed thermally unstable molecules in a supersonic jet Example: Tryptophane

TIB Hannover: D.Dt.F. AC 1000(35,27) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman