External cavity tunable diode laser spectrum of the v1+v3 N-H stretching combination band of 15NH3

The infrared spectrum of the (NH3)-N-15 isotopic species of ammonia has been investigated in the 6370-6800 cm(-1) region of the nu(1) + nu(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 nu(1)+nu(3) band of normal (NH3)-N-14, allowing immediate line assignments. This isotopic shift is almost exactly compensated by changes in rotational energy such that the R-r[(J + 1)(kappa+1)] and (p)[(J-1)(kappa-1)] transitions of (NH3)-N-15 lie very close to the (NH3)-N-14 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 (NH3)-N-15. (c) 2006 Elsevier B.V. All rights reserved.