Extreme ultraviolet laser excitation of isotopic molecular nitrogen: the dipole-allowed spectrum of ¹⁵N₂ and ¹⁴N¹⁵N

Abstract

Extreme ultraviolet+ultraviolet (XUV+UV) two-photonionizationspectra of the b ¹Πu(v=0–9), c₃¹Πu(v=0,1), o ¹Πu(v=0,1), c′₄¹Σ⁺u(v=1) and b′¹Σ⁺u(v=1,3–6) states of ¹⁵N₂ were recorded with a resolution of 0.3 cm⁻¹ full-width at half-maximum (FWHM). In addition, the b ¹Πu(v=1,5–7) states of ¹⁴N¹⁵N were investigated with the same laser source. Furthermore, using an ultranarrow bandwidth XUV laser [∼250 MHz (∼0.01 cm⁻¹) FWHM], XUV+UV ionizationspectra of the b ¹Πu(v=0–1,5–7), c₃¹Πu(v=0), o ¹Πu(v=0), c′₄¹Σ⁺u(v=0), and b′¹Σ⁺u(v=1) states of ¹⁵N₂ were recorded in order to better resolve the band-head regions. For ¹⁴N¹⁵N, ultrahigh resolution spectra of the b¹Πu(v=0–1,5–6), c₃¹Πu(v=0), and b′¹Σ⁺u(v=1) states were recorded. Rotational analyses were performed for each band, revealing perturbations arising from the effects of Rydberg-valence interactions in the ¹Πu and ¹Σ⁺u states, and rotational coupling between the ¹Πu and ¹Σ⁺umanifolds. Finally, a comprehensive perturbation model, based on the diabatic-potential representation used previously for ¹⁴N₂, and involving diagonalization of the full interaction matrix for all Rydberg and valence states of ¹Σ⁺u and 1Πu symmetry in the energy window 100 000–110 000 cm⁻¹, was constructed. Term values for ¹⁵N₂ and ¹⁴N¹⁵N computed using this model were found to be in good agreement with experiment.