Probing the predissociated levels of the S1 state of acetylene via H-atom fluorescence and photofragment fluorescence action spectroscopy

Abstract

We report two new experimental schemes to obtain rotationally resolved high-resolution spectra of predissociated S1 acetylene levels in the 47 000-47 300 cm−1 energy region (∼1200 cm−1 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 S1 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 S1 acetylene levels are probed by two-photon laser-induced fluorescence (LIF). The H-atoms are pumped to the 3d level by the two-photon resonance transition at 205.14 nm. The resulting 3d–2p 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 vs. REMPI detection schemes. In the second method, fluorescence from electronically excited C2 and C2H photofragments of S1 acetylene is detected. In contrast to the H-atom detection schemes, the detected C2 and C2H photofragments are produced by the same UV laser as is used for the ÖX̃ 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 S1 acetylene levels with S/N ∼2× better than the HCCH REMPI detection and ∼10× better than H-atom and HCCH LIF detection schemes.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…C-H bond fission is observed following excitation of C2H2( X , v=0) molecules at wavelengths  < 214.5 nm 14 − corresponding to a photon energy ~600 cm -1 above the bond dissociation energy, D0(H-CCH). 15 Energy conservation arguments dictate that the cofragments formed when exciting at such long wavelengths must be ground (X) state C2H radicals, and measurements of the parent excited state lifetimes, 16 their fragmentation probabilities, 17 the product energy disposal, 18 how these quantities vary with excitation wavelength, 19 and companion ab initio theory 20 all suggest that dissociation occurs via coupling to one or more of the nest of triplet states on a relatively long timescale (long when compared to a typical C-H vibrational period). C-H bond fission following excitation of vibrationally excited C2H2 molecules at longer wavelengths has also been reported (at  ~243.1 21,22 and at 248.3 nm 23 ).…”

Diabatic Valence-Hole States in the C2 Molecule: “Putting Humpty Dumpty Together Again”

“…In our implementation of H-atom fluorescence action spectroscopy detection of predissociated S 1 levels of acetylene, we discovered that one-color, resonance-enhanced (S 1 ← S 0 ), multi-photon dissociation of acetylene leads to efficient production of the C 2 C 1 Π g state. − In our analysis of the dispersed fluorescence spectra of the photofragments, we were struck by the highly unusual C -state vibration-rotation structure . In Figure a–c, the vibrational energy spacings (Δ G v + 1/2 ), rotational constants ( B v ), and centrifugal distortion constants ( D v ) are shown, respectively, as a function of the C -state vibrational quantum number, v .…”

“…In our implementation of H-atom fluorescence action spectroscopy detection of predissociated S 1 levels of acetylene, 17 we discovered that one-color, resonance-enhanced (S 1 ←S 0 ), multi-photon dissociation of acetylene leads to efficient production of the C 2 C 1 Π g state. 18−20 In our analysis of the dispersed fluorescence spectra of the photofragments, we were struck by the highly unusual C-state vibration-rotation structure.…”