The intensity and self-broadening of overtone transitions in HCN

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Abstract
The intensity of the (0,0,5)–(0,0,0) transition at 15 552 cm−1 in HCN has been measured using a cw laser dye laser, and a White cell with an optical path length of about 0.5 km. From a photoacoustic spectrum of HCN, the relative band intensity of the (0,0,5)–(0,0,0) transition to the (0,1,5)–(0,1,0) hot band transition has been determined. The integrated band intensity of the (0,0,5)–(0,0,0) band is 2.91(7)×10−23 cm−1/(molecule cm−2) or 17.5(4) cm/mol, which implies a dipole transition moment of 6.70(8)×10−5 D. The (0,1,5)–(0,1,0) band is equal in intensity to the (0,0,5)–(0,0,0) band within experimental error. The self-broadening and pressure shifts of these bands have also been examined. In the (0,0,5)–(0,0,0) band, the self-broadening coefficients for both the P and R branch for J between 0 and 19 were measured. For the P branch, the self-broadening coefficient is 23.7 MHz/Torr for J=1, increases monotonically to a maximum of 48.9 MHz/Torr at J=9, and then decreases monotonically to 21.7 MHz/Torr at J=19. For the R branch, the coefficient is 24.1 MHz/Torr for J=0, rises to a maximum of 45.4 MHz/Torr at J=7 and then falls to 14.7 MHz/Torr at J=19. For both branches, the variation of the self-broadening coefficient with J follows the J thermal population factor, which suggests that the collisional broadening is dominated by rotational resonances. Even though the vibrational density of states in the upper state is much larger than the density of rotational states, the line broadening appears to be fully accounted for by rotational relaxation.

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