Laser induced fluorescence spectroscopy of the transition of jet cooled 14NO3 and 15NO3 II: the dispersed fluorescence spectrum from the 3rd E level of the ν4 mode (approximately the 2ν4 (e′), l = 0, level)

Abstract

¹⁴NO₃ and ¹⁵NO₃ isotopomers were generated in a supersonic free jet expansion, and laser induced fluorescence (LIF) of the electronic transition was observed. Dispersed LIF spectra from the vibronic level at ∼770 cm⁻¹ above the vibrationless level have been measured for each isotopomer. One remarkable characteristic of the dispersed fluorescence (DF) spectrum is that the ν₂ fundamental is clearly observed, even though the transition 2⁰₁ is forbidden in the electronic transition. Another is that two sets of vibrational structure are observed in the DF spectrum: one of the two sets is the structure with the origin at the excitation energy, and the other is that with the origin at the ν₂ fundamental. These observations suggest that the fluorescent level has contribution from the ν₂ mode. In part I (M. Fukushima, J. Mol. Spectrosc., 2022, 387, 111646), it has been reported that the ν₄ progressions are one of the typical characteristics of the vibrational structure of the DF spectrum from the vibrationless level; the progressions have intensity gradually decreasing with increasing ν₄ quantum number (named the “regular” distribution), and have members forbidden for the Δl = 0 selection rule. The intensity distribution of the ν₄ progression with the origin at the ν₂ fundamental is similar to that from the vibrationless level, while that with the origin at the excitation energy is drastically different from that from the vibrationless level. The Jahn–Teller (J–T) and Renner–Teller (R–T) interactions in the ²E′ state enable us to interpret the intensity distributions of the ν₄ progressions, in which relatively weak interactions are enough to reproduce the observed distributions. The spectral intensity distribution analyses adopting the two vibronic couplings suggest that the fluorescent level at ∼770 cm⁻¹ above the vibrationless level is the 3rd E eigenstate of the ν₄ mode. The major component of the ν₄ 3rd E state is the |Λ = ±1; v₄ = 2, l₄ = 0〉 level of the ²E′ state, which is a vibrationally and a vibronically E′ level, and it is therefore concluded that the major components of the fluorescent level are both of the ν₂ = 1 level and ν₄ = 2 level .