DISPERSED FLUORESCENCE SPECTROSCOPY OF THE $\tilde{B}$ $^2E'$ -- $\tilde{X}$ $^2A_2'$ TRANSITION OF JET COOLED $^{14}$NO$_3$ and $^{15}$NO$_3$
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Date
2013
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Ohio State University
Abstract
We have generated NO$_3$ in supersonic free jet expansions and observed laser induced fluorescence (~LIF~) of the $\tilde{B}$ $^2E'$ -- $\tilde{X}$ $^2A_2'$ transition. We have measured LIF excitation spectra and dispersed fluorescence (~DF~) spectra from the single vibronic levels (~SVL's~) of the $\tilde{B}$ $^2E'$ state of $^{14}$NO$_3$ and $^{15}$NO$_3$. The vibrational structure of the $\tilde{X}$ $^2A_2'$ state has been analyzed by comparing the vibrational structures of the DF spectra of the two isotopomers. The 1,053 cm$^{-1}$ band of $^{14}$NO$_3$ is observed as two bands at 1,039 and 1,053 cm$^{-1}$ with an intensity ratio of 4 : 5, respectively, for $^{15}$NO$_3$, which are observed in the DF spectra with our standard resolution (~$\sim$ 7 cm$^{-1}$ in FWHM~). Higher resolution measurements (~$\sim$ 2 cm$^{-1}$ in FWHM~) of the DF spectra show that the 1,053 cm$^{-1}$ band of $^{14}$NO$_3$ is also observed as two bands at 1,051 and 1,056 cm$^{-1}$ with an intensity ratio of 5 : 3, respectively. The 1,051 cm$^{-1}$ band is attributed to be the $\nu_1$ (~$a_1$'~) fundamental, because of its little isotope shift. There are two possibilities for another band, the band at 1,056 and 1,038 cm$^{-1}$ for $^{14}$NO$_3$ and $^{15}$NO$_3$, respectively; (1) the $\nu_3$ (~$e'$~) fundamental band, and (2) the $\nu_2 + \nu_4$ (~$a_2''$ and $e'$, 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 $\tilde{B}$ $^2E'$ -- $\tilde{A}$ $^2E''$ transition through the $\nu_2$ mode, the considerable transition moment of which has been predicted. A simple consideration for the vibronic coupling between the $\tilde{A}$ $^2E''$ and $\tilde{X}$ $^2A_2'$ 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$^{-1}$ band of $^{14}$NO$_3$ is much stronger than the 1,492 cm$^{-1}$ band in the electronic spectrum, while the latter is the strongest band in the IR absorption spectrum.
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Author Institution: Faculty of Information Sciences; Hiroshima City University; Asa-Minami, Hiroshima 731-3194, Japan