Knowledge Bank

The spectroscopist is often confronted with the following problem: a band (or a polyad of bands) is clearly perturbed but the pertubing bands are too weak to be seen. This problem can sometimes be circumvented by resorting to the analysis of hot bands. Several examples can be found in the study of propyne $CH3$$C\equiv CH$. For instance, $v9$ and $2v10$ (at $650cm-1$) are interacting but $2v10$ is invisible. On the other hand, at $330cm-1$, the hot bands $2v10\pm 2-v10$ and $2v100-v10$ are fairly strong and provide an access to the level $v10=2$. In the same way the ‘superhot’ band $3v10\pm 3-2v10\pm 2$ yields information on the $v10=3$ level whereas $3v10$ is by far too weak. In the course of these studies in three spectral regions (9-11$\mu$m, 16 $\mu$m and 30 $\mu$m), we have also analyzed the ($v9\pm 1$+ $v10\pm 1$) ($A1+A2$) and ($v9\pm 1+v10\pm 1$) (E) bands, both directly and indirectly through the associated hot bands $v9+v10-v10$ and $v9+v10-v9$ in the $16\mu$m and $30\mu$m regions respectively. The former band ($v9\pm 1$$v10\pm 1$) belongs to the PAPE type, PAPE meaning Parallel band with a Perpendicular structure. A few details will be given on the methods developed during these studies.