External cavity tunable diode laser spectrum of the ν1+ν3 N-H stretching combination band of 15NH3

The infrared spectrum of the 15NH3 isotopic species of ammonia has been investigated in the 6370-6800 cm−1 region of the ν1+ν3 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 ν1+ν3 band of normal 14NH3, allowing immediate line assignments. This isotopic shift is almost exactly compensated by changes in rotational energy such that the rR[(J+1)K+1] and pP[(J−1)K−1] transitions of 15NH3 lie very close to the 14NH3rR[JK] and pP[JK] 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 15NH3.