Rotationally resolved photoelectron spectroscopic study of the Jahn–Teller effect in allene

Abstract

The pulsed-field-ionization zero-kinetic-energy photoelectron spectra of allene (C3H4) and perdeuterated allene have been recorded from the first adiabatic ionization energy up to 2200 cm−1 of internal energy in the cations at a resolution sufficient to observe the full rotational structure. The intensity distributions in the spectra are dominated by vibrational progressions in the torsional mode, which were analyzed in the realm of a two-dimensional model of the E⊗(b1⊕b2) Jahn–Teller effect in the allene cation [C. Woywod and W. Domcke, Chem. Phys. 162, 349 (1992)]. Whereas the rotational structure of the transitions to the lowest torsional levels (00 and 41) are regular and can be qualitatively analyzed in terms of a simple orbital ionization model, the rotational structure of the spectra of the 42 and 43 levels are strongly perturbed. The photoelectron spectrum of C3H4 also reveals several weak vibrational bands in the immediate vicinity of these levels that are indicative of (ro)vibronic perturbations. A slight broadening of the transitions to the 41 levels compared to that of the vibronic ground state and the increase of the number of sharp features in the rotational structure of the spectrum of the 42 level point at the importance of large-amplitude motions not considered in previous treatments of the Jahn–Teller effect in the allene cation.

The Rotational Structure of the Origin Band of the Pulsed-Field-Ionization, Zero-Kinetic-Energy Photoelectron Spectra of Propene-h6 and Propene-d6

“…Thanks to the continual improvement of its resolution resulting from the exploitation of electric field ionization sequences, 15,24,25 PFI-ZEKE photoelectron spectroscopy, initially applied to resolve the rotational structure in the photoelectron spectra of diatomic molecules, has rapidly been applied to obtain information on the rovibrational photoionization of more com- ethene, 31,32 and allene. 33 The present results on propene illustrate the possibility of partially resolving the rotational structure in single-photon photoelectron spectra of structurally more complex molecules. This possibility results in the following advantages: The rotational structure of a photoelectron spectrum enables the determination of the vibronic symmetry of the cationic states by means of rovibronic photoionization selection rules; it permits the distinction between different conformers of the neutral or ionized molecule, it gives access to the rovibronic photoionization dynamics and provides 9 information on the electronic structure.…”

Internal rotation in Jahn–Teller coupled systems: The ethene and allene cations

“…The barrier at D 2h geometry on the potential energy surface of C 2 H + 4 is low so that the tunneling splitting of the lowest vibrational levels are easily resolvable in the photoelectron spectrum [29]. This barrier is higher in C 3 H + 4 , and the tunneling splittings of the low vibrational levels are much more difficult to resolve [30]. As a result, different nomenclatures are used to label the torsional levels of these two ions: Whereas the two E + and the two E − levels correspond to the 0 0 and 4 1 levels of C 2 H …”

“…As explained previously [22,30], the intensities I i ( X + ← X) of the transitions to the final states v i from the X(0 0 ) initial vibronic state ( X + (ν …”

“…In the calculations of the vibronic intensity distributions in the photoelectron spectra of C 2 H 4 and C 3 H 4 presented in the next section, we only considered transitions from the X(0 0 ) ground vibronic state because of the low temperature in the supersonic expansion [29,30]. As explained previously [22,30], the intensities I i ( X + ← X) of the transitions to the final states v i from the X(0 0 ) initial vibronic state ( X + (ν …”

“…Successive refinements of vibronic coupling models by including (i) an increasing number of coupled electronic states, (ii) and increasing number of nuclear displacement coordinates, and (iii) an increasing number of terms in the Taylor series used to approximate the potential energy functions of the coupled electronic states have led to a detailed understanding of the He I photoelectron spectra of ethene [18,19,20], allene [21,22,23,24,25,26,27], and butatriene [28], and to a global and coherent description of the associated vibronic structure and dynamics. In our recent studies of the pulsed-field-ionization zero-kinetic-energyPFI-ZEKE photoelectron spectra of C 2 H 4 [29] and C 3 H 4 [30], we have observed spectral structures resulting from the tunneling along the torsional motion through the potential barrier located at the planar (D 2h ) structure.…”