Knowledge Bank

Low resolution $NO2$ fluorescence spectra were recorded as a function of pressure, using second harmonic lines (1 $cm-1$ \ FWHM) of a pulsed Nd-YAG laser (100 nsec pulse width) as an excitation source. The 72 $\ell$ fluorescence cell is large enough to allow intensity measurements to be obtained, free from geometric discrimination. At 532 nm excitation, collision free (0.01 mTorr) spectra consist of banded structure superimposed on an apparent continuum extending from $\sim$500 $cm-1$ above the excitation energy to beyond the long wavelength cutoff of the detector (890 nm). The bands can be assigned to ground state vibrational progressions. The continuum-to-banded structure intensity ratio is strongly pressure dependent, increasing by about a factor of five between 0.01 and 10 mTorr. These results do not support conclusions drawn from magnetic quenching $experiments1$ which find no pressure dependence, and uphold the previously proposed step ladder mechanism for vibrational $deactivation.2,3$ The states excited at 532 nm have an average lifetime of about $70\mu sec$. The quenching rate constants of the banded features and continuum features are $6\times 10-10$ and $1.0\times 10-10cm3sec-1$, respectively.