Knowledge Bank

Rotationless band intensities and Herman-Wallis parameters are listed in HITRAN tabulations (Rothman, et al. J.Q.S.R.T. 48, 537, 1992) for several hundred $CO2$ overtone-combination bands. These parameters are based on laboratory measurements when available, and on DND calculations for the unmeasured bands. The DND calculations for the Fermi interacting $n\nu 1+\nu 3$ polyads show the $a2$ Herman Wallis parameter varying smoothy from a negative valu for the first member of the polyad to a positive value for the final member. Johns' (J. Mol. Spec. 134, 433, 1989) measurements of the $\nu 1+\nu 3$ dyad are consistent with the DND calculations for the $a2$ parameter, as are our recent measurements (submitted to J. Mol. Spec.) of the $4\nu 1+\nu 3$ pentad. However, the measurement-based values in the HITRAN tables for the $2\nu 1+\nu 3$ triad and the $3\nu 1+\nu 3$ tetrad do not support the DND calculated values for the $a2$ parameters. We therefore decided to make new measurements to improve some of these intensity parameters. With the McMath FTS at Kitt Peak National Observatory/National Solar Observatory we recorded several spectra of the 4000 to $8000cm-1$ region of pure $CO2$ at $0.011cm-1$ resolution using the 6 meter White absorption cell. The signal/noise and absorbance of the first and fourth bands of the $3\nu 1+\nu 3$ tetrad of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O2$ were ideal on these spectra for measuring line intensities and broadening widths. Our selfbroadening results agree with the HITRAN paramaterization, while our measurements of the rotationless band intensities are about 15% less than the HITRAN values. We find a negative value of $a2$ for the $30011\leftarrow 00001$ band and a positive values for the $30014\leftarrow 00001$ band, whereas the HITRAN values of $a2$ are positive for all four tetrad bands. Our $a1$ and $a2$ Herman-Wallis parameters are closer to DND calculated values than the 1992 HITRAN values for both the $30011\leftarrow 00001$ and the $30014\leftarrow 00001$ band.