Knowledge Bank

Vib-rotational spectra of $CO2-HBr$ and its deuterated counterpart have been recorded utilizing the $CO2$ asymmetric stretch $(2349cm-1)$ chromophore. These weakly bonded dimers are created by supersonic expansion of a seeded carrier gas $(1\%CO2,1\%HBrinArNe)$ through a pulsed slit nozzle $(.15\times 38mm2)$. Direct absorption spectra are obtained by passing the output of a tunable diode laser through the expanding effluent and monitoring laser attenuation as a function of laser frequency. Rapid tuning of the diode laser allows a complete spectrum $(\sim .75cm-1)$ to be obtained during each pulse of the nozzle. As in our previous work on $CO2-HBr.1CO2-DBr$ also exhibits spectral features associated with an asymmetric rotor. In all, 68 transitions were assigned $(Ka=0,1and2)$ and fit to a Watson type Hamiltonian. Due to efficient rotational cooling, characteristic in supersonic expansions, only transitions up to $J=12$ and $K3=2$ was available; hence only the rotational constants A, B, C. and DJK were determined for both the vibrational ground and excited states. A combination-difference method was used to obtain vibrational ground-state constants, which were then fixed for subsequent fitting of the upper-state constants as well as the band origin. $v0$. Results from the two isotopically different experiments allows us to determine the distance of the H(D) atom from the center of mass of $CO2-H(D)Br$. In addition, the angle between the H(D)Br molecular axis and the center-of-mass line is estimated as $\sim$. Substantial differences of the hydrogen location in bent $CO2-H(D)Br$ vs. linear $CO2-H(D)Cl$ and linear $CO2-H(D)F$ have important ramifications for the interpretation of precusor-geometry-limited photoinitiated reactions such as $H+CO2\to CO+OH$, which will also be discussed in the following talk