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Author(s):Fukushima, Masaru
Abstract:We have measured dispersed fluorescence (~DF~) spectra from the single vibronic levels (~SVL's~) of the $tilde{B}$ $^2E'$ state of jet cooled $^{14}$NO$_3$ and $^{15}$NO$_3$, and found a new vibronic band around the $nu_1$ fundamentalfootnote{M.~Fukushima and T.~Ishiwata, paper WJ03, ISMS2013, and paper MI17, ISMS2014.}. This new band has two characteristics; (1) inverse isotope shift, and (2) unexpectedly strong intensity, i.e.~comparable with that of the $nu_1$ fundamental. We concluded on the basis of the isotope effect that the terminated (~lower~) vibrational level of the new vibronic band should have vibrationally $a_1'$ symmetry, and assigned to the third over-tone of the $nu_4$ asymmetric ($e'$) mode, $3 nu_4$ ($a_1'$). We also assigned a weaker band at about 160 cm$^{-1}$ above the new band to one terminated to $3 nu_4$ ($a_2'$). The $3 nu_4$ ($a_1'$) and ($a_2'$) levels are ones with $l = pm3$. Hirota proposed new vibronic coupling mechanismfootnote{E.~Hirota, $J. Mol. Spectrosc.$, in press.} which suggests that degenerate vibrational modes can induce electronic orbital angular momentum (~$L$~) even in non-degenerate electronic states. %It is thus thought the surprisingly wide splitting of $3 nu_4$, $a_1'$ and $a_2'$, is resulted from vibronic coupling, and the explanation we proposed is as follows. We interpret this as a sort of break-down of the Born-Oppenheimer approximation, and think that $pm l$ induces $mpbar{Lambda}$, where $bar{Lambda}$ expresses the pseudo-$L$; for the present system, one of the components of the third over-tone level, $| Lambda = 0; v_4 = 3, l = +3 rangle$, can have contributions of $| bar{Lambda} = -1; v_4 = 3, l = +2 rangle$ and $| -2; 3, +1 rangle$. Under this interpretation, it is expected that there is sixth-order vibronic coupling, $(q_+^3Q_-^3 + q_-^3Q_+^3)$, between $| 0; 3, +3 rangle$ and $| 0; 3, -3 rangle$. The sixth-order coupling is weaker than the Renner-Teller term (~the fourth-order term, $(q_+^2Q_-^2 + q_-^2Q_+^2)$~), but stronger than the eighth-order term, $(q_+^4Q_-^4 + q_-^4Q_+^4)$. It is well known in linear molecules that the former shows huge separation, comparable with vibrational frequency, among the vibronic levels of $Pi$ electronic states, and the latter shows considerable splitting, $sim$10 cm$^{-1}$, at $Delta$ electronic states. Consequently, the $sim$160 cm$^{-1}$ splitting at $v_4$ = 3 is attributed to the sixth-order interaction. The relatively strong intensity for the band to $3 nu_4$ ($a_1'$) can be interpreted as a part of the huge 0-0 band intensity, because the $3 nu_4$ ($a_1'$) level, $| 0; 3, pm3 rangle$, can connect with the vibrationless level, $| 0; 0, 0 rangle$. $3 nu_4$ ($a_1'$) has two-fold intensity because of the vibrational wavefunction, $| 0; 3, +3 rangle + | 0; 3, -3 rangle$, while negligible intensity is expected for $3 nu_4$ ($a_2'$) with $| 0; 3, +3 rangle - | 0; 3, -3 rangle$ due to the cancellation. To confirm these interpretations, experiments on rotationally resolved spectra are underway.
Issue Date:23-Jun-15
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:ACS
Date Available in IDEALS:2016-01-05

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