Timing gases for picosecond chemical timing of molecular fluorescence spectra

The generality of the chemical timing or collisional timing technique for imposing picosecond time resolution on fluorescence from molecular gases is demonstrated by comparing S₁ → S₀p-difluorobenzene fluorescence spectra quenched by NO, CS₂, CF₃NO, and CO₂ with spectra quenched by O₂, the timing gas used in all previous studies. Additions of NO, CS₂ and CF₃NO reproduce the spectral changes previously observed with added O₂. Thus the changes in fluorescence spectra at high gas additions truly appear to be the consequence of fluorescence timing. All of these gases can be used to impose time resolution on molecular fluorescence spectra, and the desirability of each as a timing gas is discussed. CO₂ is not effective because collision-induced vibrational relaxation dominates the collisional interaction. The rate constant for quenching of the S₁ state of naphthalene (zero-point level) is reported for each of fourteen gases.