THE INTENSITY AND SELF-BROADENING OF OVERTONE TRANSITIONS IN HCN

Abstract

$^{1}$ G. Kortum and H. Verleger, Proc. Phys. Soc. 63, 462 (1949). $^{2}$ H. A. Rabitz and R. G. Gordon, J. Chem. Phys. 53, 1831 (1970). Address of Smith and Klemperer: Deparment of Chemistry, Harvard University, Cambridge, Massachusetts, 02138. Address of Lehmann: Department of Chemistry, Princeton University, Princeton, New Jersey, 08544.Author Institution:The intensity of the (0,0,5)-(0,0,0) transition at $15, 552 cm^{-1}$ in HCN has been measured using a cw dye laser and an optical pathlengths of 0.5 Km. Also from a photoaconstic spectrum of HCN, the relative intensity of the (0,1,5)-(0,1,0) hot band transition was determined. In the (0,0,5) (0,0,0) band, the self-broadening coefficients were measured for both the P and R branches for J between O and 19. For the P branch, they range from about 15 to 45 Mhz/torr. For both branches, the variation of the self-broadening coefficient with J resembles the J thermal population factor vs. J, 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 rolaxation. Our self-broadening coefficients agree quite well with those of Kortum and $Verleger^{1}$ for the (0,0,3)-(0,0,5) band at $9,634 cm^{-1}$ of HCN. They also agree reasonably well with the rotational only linewidth calculations of Rabitz and Gordon.$^{2}$ These results show that a high degree of vibrational excitation has little or no effect on the line-broadening