Probing the predissociated levels of the S state of acetylene via H-atom fluorescence and photofragment fluorescence action spectroscopy.

We report two new experimental schemes to obtain rotationally resolved high-resolution spectra of predissociated S acetylene levels in the 47 000-47 300 cm energy region (∼1200 cm above the predissociation threshold). The two new detection schemes are compared to several other detection schemes (employed at similar laser power, molecular beam temperature, and number of signal averages) that have been used in our laboratory to study predissociated S acetylene levels, both in terms of the signal-to-noise ratio (S/N) of the resultant spectra and experimental simplicity. In the first method, H-atoms from the predissociated S acetylene levels are probed by two-photon laser-induced fluorescence (LIF). The H-atoms are pumped to the 3 level by the two-photon resonance transition at 205.14 nm. The resulting 3-2 fluorescence (654.5 nm) is collected by a photomultiplier. The S/N of the H-atom fluorescence action spectrum is consistently better by ∼3× than that of the more widely used H-atom resonance-enhanced multiphoton ionization (REMPI) detection. Laser alignment is also considerably easier in H-atom fluorescence detection than H-atom REMPI detection due to the larger number-density of molecules that can be used in fluorescence . REMPI detection schemes. In the second method, fluorescence from electronically excited C and CH photofragments of S acetylene is detected. In contrast to the H-atom detection schemes, the detected C and CH photofragments are produced by the UV laser as is used for the acetylene excitation. As a result, laser alignment is greatly simplified for the photofragment fluorescence detection scheme, compared to both H-atom detection schemes. Using the photofragment fluorescence detection method, we are able to obtain action spectra of predissociated S acetylene levels with S/N ∼2× better than the HCCH REMPI detection and ∼10× better than H-atom and HCCH LIF detection schemes.