Vibronic and Rydberg series assignments in the vacuum ultraviolet absorption spectrum of nitrous oxide

- VUV spectroscopy of N2O in 45,000-95,000 cm−1 region using synchrotron radiation.
- TDDFT computations of excited states, oscillator strengths, potential energy curves.
- Features in 45,000-60,000 cm−1 region assigned to hot band progressions of υ2′. QUOTE
- Vibronic structure in 60,000-72,000 cm−1 region explained by Renner-Teller effect.
- First assignment of 3d Rydberg and vibronic transitions in 85,000-95,000 cm−1 region.

We report a comprehensive photoabsorption study of nitrous oxide (N2O) in the vacuum ultraviolet (45,000-95,000 cm−1) region using synchrotron radiation. The observed spectrum comprises of a few valence transitions and low lying Rydberg series converging to the two spin-orbit components (2П1/2,3/2) of the ground state of N2O+. Spectral analysis is aided by extensive quantum chemical calculations of vertical excited states, oscillator strengths and potential energy curves using the time dependent density functional theory. Vibronic bands observed in the first absorption system (45,000-60,000 cm−1) are assigned to hot band progressions in υ2′ originating from v″=1 or 2. New insights into the assignment of the well-formed progression of bands in the X1Σ+→C1П system (60,000-72,000 cm−1) are afforded by consideration of the Renner-Teller interaction. A set of molecular vibrational parameters (ω2=467 cm−1, x22=−2.9, ε=−0.24) for the C1П state are derived from a fitting of the experimental data. The 3pπ1Σ+ state at ~77,600 cm−1 shows a large quantum defect (0.96) which is explained as arising due to mixed valence-Rydberg character. In the 85,000-95,000 cm−1 region, a number of absorption features are observed with greater clarity than in earlier photoabsorption studies and assigned to Rydberg series of type nlλ (n=3,4; l=s,p,d; λ=σ,π,δ) and accompanying vibronic bands. This work has resulted in clarification of several discrepancies in earlier Rydberg series assignments. Additionally, the 3pπ 3Σ− Rydberg state at 85,788 cm−1, the valence transition 7σ→3π (1П) at 87,433 cm−1 and the 3dλ Rydberg series in the 91,700-92,600 cm−1 region are assigned for the first time.