Knowledge Bank

We have generated NO$3$ in supersonic free jet expansions and observed laser induced fluorescence (~LIF~) of the $B~$ $2E\prime$ -- $X~$ $2A2\prime$ transition. We have measured LIF excitation spectra and dispersed fluorescence (~DF~) spectra from the single vibronic levels (~SVL's~) of the $B~$ $2E\prime$ state of $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$NO$3$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}$NO$3$. The vibrational structure of the $X~$ $2A2\prime$ state has been analyzed by comparing the vibrational structures of the DF spectra of the two isotopomers. The 1,053 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ band of $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$NO$3$ is observed as two bands at 1,039 and 1,053 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ with an intensity ratio of 4 : 5, respectively, for $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}$NO$3$, which are observed in the DF spectra with our standard resolution (~$\sim$ 7 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ in FWHM~). Higher resolution measurements (~$\sim$ 2 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ in FWHM~) of the DF spectra show that the 1,053 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ band of $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$NO$3$ is also observed as two bands at 1,051 and 1,056 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ with an intensity ratio of 5 : 3, respectively. The 1,051 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ band is attributed to be the $\nu 1$ (~$a1$'~) fundamental, because of its little isotope shift. There are two possibilities for another band, the band at 1,056 and 1,038 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ for $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$NO$3$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}$NO$3$, respectively; (1) the $\nu 3$ (~$e\prime$~) fundamental band, and (2) the $\nu 2+\nu 4$ (~$a2″$ and $e\prime$, respectively~) combination band. If this is the case (1), the $\nu 3$ band should be observed in IR spectrum, but it has yet to be observed. If (2), the intensity must be stolen from the $B~$ $2E\prime$ -- $A~$ $2E″$ transition through the $\nu 2$ mode, the considerable transition moment of which has been predicted. A simple consideration for the vibronic coupling between the $A~$ $2E″$ and $X~$ $2A2\prime$ states through the $\nu 2$ mode can understand about 20 % of the combination band intensity to that of the $\nu 1$ fundamental. The higher resolution measurements of the DF spectra also show that the 1,499 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ band of $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}$NO$3$ is much stronger than the 1,492 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ band in the electronic spectrum, while the latter is the strongest band in the IR absorption spectrum.