Photoelectron spectroscopy and inferred femtosecond intramolecular dynamics of C2H+2 and C2D+2

Abstract

The 584 Å photoelectron spectra of rotationally cold C2H2 and C2D2 were obtained with improved resolution, permitting the first three electronic states of the ions to be characterized in greater detail. Temperature-dependent studies led to a definitive assignment of the low intensity features in the X̃ 2∏u state, yielding ν4=837±12 cm−1 for C2H+2 and ν4=702±12 cm−1 for C2D+2. The ν5 origin of the Renner–Teller multiplet was identified. In the case of C2D+2, a Fermi resonance with this multiplet contributed intensity to the ν1 mode, facilitating its evaluation at 2572±16 cm−1. The C2h geometry of the à 2Ag state was determined from the two previously unobserved bending progressions, assigned to ν4 and ν5B, and evaluated at 492±12 and 605±12 cm−1 for C2H+2 and 339±12 and 516±12 cm−1 for C2D+2, respectively. A more extensive vibrational progression than previously evident, comprised of irregular spectral features indicative of nonadiabatic effects, was observed for the B 2∑+u state. Autocorrelation functions were derived from the spectra for all three electronic states, and the two electronically excited states exhibit an ultrafast decay on a 10−14 s time scale. The à 2Ag state decays within one period of bending vibration, while the B̃ 2∑+u state survives only 14 fs, corresponding to a single period of symmetric stretching motion.

Mass-Analyzed Threshold Ionization (MATI) Spectroscopy of Atoms and Molecules Using VUV Synchrotron Radiation

“…The ν 4 band is also weakly observed at 11.533 eV, and this is consistent with a previously reported photoelectron spectrum. 32 …”

How Complex can be the Unimolecular Decomposition of a Simple Molecule? The Case of Acetylene. An Electron Impact and PIPECO Investigation

“…Though the discrepancy between this experiment and theoretical predictions, it could be suggested that at 20.1 eV the reaction takes place, where the C 2 H radical would carry a certain amount of internal energy. The H + (D + ) production very likely runs over a predissociation mechanism ascribed to the coupling of the B 2 A 1 , A 2 A 1 and X 2 A 1 states of C 2 H 2 + in the C 2h configuration [7]. This would also be the origin of the reversed isotope effect observed on the slopes of the straight lines.…”

“…For the easiness and clarity in the following discussion, all the data used to calculate threshold energies for dissociative ionization processes giving rise to the ions to be considered in this work are gathered in Table 1 [7]. The He(II)-photoelectron spectrum shows two more bands with a vertical ionization energy at 23.6 eV and at 27.6 eV successively [10].…”

“…CD + or CH 2 + and N + the ionization energy of Ne at 21.564 eV [6] is used. In the PIPECO-experiment the photoelectron energy scale is calibrated by using the adiabatic ionization energy of C 2 H 2 in the X 2 II u state at 11.403 eV [7].…”

The predissociation of highly excited states in acetylene by time-resolved photoelectron spectroscopy

“…The most obvious reason for this is that the total energy (9.4 ϩ3.14 eV) is not sufficient for accessing excited states of the acetylene ion and that the resolution is not sufficient to resolve the Renner-Teller structure of the ionic ground state. 28 The first rules out observation of any ionization dynamics arising from a change of the electronic interaction in the neutral molecule, 29 while the second prevents observation of nuclear wave-packet dynamics. Figure 7 shows the C 2 H 2 PES recorded at the excitation wavelength 2 at a range of pump-probe delays.…”