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 14thinsp;600ndash;18thinsp;400 cmminus;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 pathlengthtimes;pressure of about 34 kmthinsp;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 12thinsp;600ndash;13thinsp;800 cmminus;1were measured using a multipass cell. Optical pathlengthstimes;pressure from 10 to 15 kmthinsp;Torr were used for these three bands. The band origins in cmminus;1, the band labels (CN, bend, CH), and the intensities in cm/mol are:12thinsp;636, (0,0,4), 154(3); 12thinsp;658, (3,0,2), 9.7(3); 13thinsp;702, (2,0,3), 3.1(2); 14thinsp;654, (4,0,2), 8.0(4); 14thinsp;670, (1,0,4), 7.2(4); 15thinsp;552, (0,0,5), 17.5(4); 15thinsp;711, (3,0,3), 0.05(1); 16thinsp;164, (0,11,5), 0.047(8); 16thinsp;640, (5,0,2), 0.40(4); 16thinsp;674, (2,0,4), 1.7(1); 17thinsp;550, (1,0,5), 13.9(4); 18thinsp;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)ndash;(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 withabinitioresults.
展开▼
