External cavity tunable diode laser spectrum of the ν1+ ν3 N–H stretching combination band of 15NH 3

Abstract

The infrared spectrum of the 15NH 3 isotopic species of ammonia has been investigated in the 6370–6800 cm −1 region of the ν 1+ ν 3 N–H stretching combination band with an external cavity tunable diode laser (ECTDL) spectrometer. A group of the stronger absorption lines form a pattern that is systematically shifted downwards by 12.2 cm −1 from a closely similar pattern of known transitions in the ν 1+ ν 3 band of normal 14NH 3, allowing immediate line assignments. This isotopic shift is almost exactly compensated by changes in rotational energy such that the r R[( J+1) K+1 ] and p P[( J−1) K−1 ] transitions of 15NH 3 lie very close to the 14NH 3 r R[ J K ] and p P[ J K ] transitions, respectively, giving a convenient comparison of relative intensity. From spectra of a 50:50 mixture of the two species, the absorption strengths appear comparable, with a slightly greater transition moment for 15NH 3.