Ionization photophysics and spectroscopy of dicyanoacetylene

Abstract

Photoionization of dicyanoacetylene was studied using synchrotron radiation over the excitation range 8-25 eV, with photoelectron-photoion coincidence techniques. The absolute ionization cross-section and detailed spectroscopic aspects of the parent ion were recorded. The adiabatic ionization energy of dicyanoacetylene was measured as 11.80 ± 0.01 eV. A detailed analysis of the cation spectroscopy involves new aspects and new assignments of the vibrational components to excitation of the quasi-degenerate A(2)Πg, B(2)Σg(+) states as well as the C(2)Σu(+) and D(2)Πu states of the cation. Some of the structured autoionization features observed in the 12.4-15 eV region of the total ion yield spectrum were assigned to vibrational components of valence shell transitions and to two previously unknown Rydberg series converging to the D(2)Πu state of C4N2(+). The appearance energies of the fragment ions C4N(+), C3N(+), C4(+), C2N(+), and C2(+) were measured and their heats of formation were determined and compared with existing literature values. Thermochemical calculations of the appearance potentials of these and other weaker ions were used to infer aspects of dissociative ionization pathways.

Vibronic structure of the2Πuground electronic state of dicyanoacetylene cation revisited by PFI-ZEKE photoelectron spectroscopy andab initiocalculations

“…The corresponding measured vibrational wavenumbers are reported in Table 5 together with other literature values. The overall agreement with the previous works is good, except for the ν 3 mode which differs from the values found by Leach et al [32] and Maier et al [26]. We suspect that this difference comes from a misassignment due to the lower resolution of their spectra and to the presence of the RT components of the ν 8 vibrational level in the vicinity of the ν 3 mode.…”

“…Table 4 compares our experimental and calculated adiabatic ionisation potential and SO coupling constant values with those reported in the literature. In the previous PES spectra [26,[30][31][32], the SO components of the ground electronic states of C 4 N + 2 were not resolved. Thus, the reported ionisation potentials correspond to the energy of the two SO component centroid with respect to the neutral ground electronic state.…”

“…Whereas the neutral C 4 N 2 species starts to absorb in the UV domain, its cation absorbs in the visible range of the diffuse interstellar bands (DIBs) (400-900 nm). Small hydrocarbon chains have been proposed as good candidates for DIBS [24] and several studies have been carried out to study the vibronic structure of C 4 N + 2 by fluorescence [25][26][27], IR and visible absorption spectroscopy [28,29], as well as by photoelectron spectroscopy (PES) [27,[30][31][32] and mass spectrometry [33]. Up to now, only few calculations are available about the electronic and vibrational structure of the C 4 N + 2 cation [34][35][36][37][38][39].…”

Ionization photophysics and spectroscopy of cyanoacetylene

“…The excitation of a bending vibration on ionization has also been observed as weak features in the SPES of dicyanoacetylene. 4 The geometrical changes on ionization of cyanoacetylene discussed above indicate that on ionization to the X 2 ion ground state one might also expect to excite the ν 3 (C≡C) stretching vibration by virtue of the lengthening of r(C≡C) on ionization. A corresponding SPES feature is not observed as a clearly defined peak but can be suspected to exist as the very weak broad signal around 11.870 eV ( Fig.…”

Photoionization Spectroscopy of Nucleobases and Analogues in the Gas Phase Using Synchrotron Radiation as Excitation Light Source

“…In addition, a fragment AE can be subject to a substantial kinetic shift, ΔE kin since at threshold, the reaction might be too slow to be observed. These effects were discussed in more detail by Schwell and co-workers in [49,50]. Such kinetic shifts can be estimated by statistical models based on the possible existence of a transition state along the reactive pathways (see for example [51]).…”