Spectroscopy and Predissociation of Acetylene in the np Gerade Rydberg States

Multiphoton breakdown of acetylene; formation of organic building block fragments

Mass resolved REMPI spectra and electron and ion velocity map images were recorded for REMPI of acetylene in the case of two-photon resonant excitations to low lying 3p and 4p Rydberg states. Combined data analysis of ion signal intensities and electron and ion kinetic energy release distribution revealed multiphoton-fragmentation processes in terms of photodissociation and photoionization channels to form the molecular ion, C₂H₂⁺ and the fragment ions H⁺, C⁺, CH⁺, CH₂⁺, C₂⁺ and C₂H⁺. The ratio of fragment ion formation over the parent ion formation increases with excitation energy. To a large extent, multiphoton-fragmentation involves the initial breakdown of the molecule into ground and excited state neutral fragments by two-, three- and four-photon dissociation processes prior to multiphoton ionization. The three-photon dissociation processes via superexcited molecular state(s) are found to be the most important and electronically excited fragment species playing a significant role in the overall multiphoton-fragmentation. Furthermore, the data are indicative of the involvement of secondary photodissociation processes and provide information on fragment energetics as well as state interactions. The question, whether acetylene could be an important source of building block fragments for the formation of organic molecules in interstellar space, is addressed.

Probing the predissociated levels of the S1 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 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 à - X ̃ 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.

Analysis of low-lying gerade Rydberg states of acetylene using two-photon resonance fluorescence excitation spectroscopy

The np gerade Rydberg states of acetylene were analyzed using two-photon resonance fluorescence excitation spectroscopy in the 72,000-93,000 cm(-1) energy region. The npπ(1)Σ(g)(+) and npπ(1)Δ(g) Rydberg series (n = 3-5) were identified in the fluorescence excitation spectrum measured by monitoring the C(2) d(3)Π(g)-a(3)Π(u) Swan system. Some vibronic bands were assigned to the npπ(1)Δ(g)-X̃(1)Σ(g)(+) transition on the basis of rotational analysis. The 5pσ(1)Π(g) state was observed, which is the first such observation in an npσ(1)Π(g) series. Rotational analysis of the 5pσ(1)Π(g)-X̃(1)Σ(g)(+) transition showed e/f-symmetry dependent predissociation of acetylene in the 5pσ(1)Π(g) state. The 0(0)(0) band of the deuterated acetylene (C(2)D(2)) 4pπ(1)Σ(g)(+)-X̃(1)Σ(g)(+) transition exhibits an atypical structure, which was satisfactorily reproduced by a simple model of quantum interference between the discrete and quasi-continuum states. The predissociative lifetimes of the npπgerade Rydberg states were estimated from the spectral profiles. The predissociation mechanism of acetylene in the Rydberg states is discussed.