Knowledge Bank

We have recorded excitation spectra of the acetylene $A~Au\leftarrow X~1\Sigma u+$ transition by measuring simultaneously the total emission in two spectral regions: UV (200-400 nm) and NIR (0.9-1.5 $\mu$m). UV emission is due to well characterized $A~\to X~$ transitions. The source of the NIR signal is suggested by the emission patterns within the subbands studied, $Vo2K01$ and $V00K00$. ($V$ denotes the trans-bending mode.) The NIR:UV intensity ratio is approximately constant throughout the $V02K01$ subband. This is consistent with excited $C2H$ or $C2$ as the carrier of the NIR signal because these species, capable of emitting in the $NIR2$,3 could be produced via multiphoton $processes2$ resonantly enhanced by the $A~$ state as intermediate. The $V03K01$ subband shows significantly more fluctuation in the NIR:UV intensity ratio than does the $Va2Ka1$ subband. The one-quantum incvrease in trans-bending excitation enables accidental resonances with backround states to modulate the NIR emission pattern. Triplet perturbers could participate in this process by modifying the resonance enhancement of $C2H$ or $C2$ produciton. Mixed $S1$-triplet states could additionally contribute to the NIR signal through $Tu\to Tl(\pi >l)$ transitions.