Knowledge Bank

Nominally forbidden rotational transitions ($|\Delta K|\equiv |K\prime -\ell ""|\le 3$) have been observed in the stimulated Emission Pumping (SEP) spectra of acetylene in the $7000cm-1$ region of the $X¯$ state via rotational-$\ell$- resonance in the $X¯$ state and $axis-switching.1$ It is significant to note that axis-switching, because the acetylene $A¯\to X¯$ transition is bent$\to$linear, allows access to new vibrational levels which would normally be dark solely due to rotational selection rules. From the rotational intensities observed in the $7000cm-1$ region of the $X¯$ state a conservative estimate of the detectable range of SEP transitions in our setup, using 50$\mu$J of DUMP and a maximum observed fluorescence dip of 14%, is 500 fold. Since fluorescence dips of 30% are not uncommon, the practical range from the strongest to the weakest detectable SEP transitions is probably greater than 1000. The dynamic range of IR and Raman spectroscopy is typically smaller in congested regions accessible to SEP, IR and Raman. This unexpected dynamic range may be important for the reinterpretation of previous SEP spectra. For instance, in the case of acetylene, this unexpectedly large sensitivity range coupled with the newly discovered strong vibrational resonances may now reconcile the difference between the larger observed density of states at $15,000cm\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}1$ and the density of states calculated from a symmetry sorted direct anharmonic count.