The intensities of HCN overtone transitions from 12 600–18 400 cm−1

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Abstract
The absolute intensities of 12 visible overtone bands of HCN have been determined using intracavity photoacoustic detection and a long pathlength absorption cell. The relative intensities of nine of these weak overtone bands (from 14 600–18 400 cm−1) were obtained by using intracavity photoacoustic detection. The absolute intensity of the (0,0,5) band, obtained previously using a multipass cell with an optical pathlength×pressure of about 34 km Torr, was used to calibrate the photoacoustic response, and thus to obtain the absolute intensities for eight more HCN overtone bands. The absolute intensities of three HCN overtone bands between 12 600–13 800 cm−1 were measured using a multipass cell. Optical pathlengths×pressure from 10 to 15 km Torr were used for these three bands. The band origins in cm−1, the band labels (CN, bend, CH), and the intensities in cm/mol are:12 636, (0,0,4), 154(3); 12 658, (3,0,2), 9.7(3); 13 702, (2,0,3), 3.1(2); 14 654, (4,0,2), 8.0(4); 14 670, (1,0,4), 7.2(4); 15 552, (0,0,5), 17.5(4); 15 711, (3,0,3), 0.05(1); 16 164, (0,11,5), 0.047(8); 16 640, (5,0,2), 0.40(4); 16 674, (2,0,4), 1.7(1); 17 550, (1,0,5), 13.9(4); 18 377, (0,0,6), 2.4(2). This work represents the most complete study to date of the overtone intensities of a polyatomic molecule. We have shown that the typical method of intracavity laser power correction is likely to introduce serious errors in relative intensity measurement. We also have found evidence, namely the anomalously large intensity of the (1,0,5)–(0,0,0) transition, for the breakdown of the typically invoked bond dipole model for overtone and combination band intensities. As well as discussing the astromonical interest in predicting overtone intensities of HCN, we present a thorough comparison of the measured absolute overtone intensities with ab initio results.