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

Abstract

The pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the origin band of the X ˜+ 2 A′′ r X 1A′ transition of propene (C 3 H 6 ) and perdeuterated propene (C 3 D 6 ) have been recorded at high resolution, allowing for the partial resolution of the rotational structure. The analysis of the spectra in the realm of the orbital ionization model for rigid-rotor asymmetric-top molecules enabled the determination of the adiabatic ionization energy of propene and the rotational constants of C 3 H 6 + and C 3 D 6 +. The tunneling splittings resulting from the hindered rotation of the methyl group could not be resolved, and the analysis was therefore carried out in the C s molecular symmetry group. Angular momentum contributions of p π , d π , and d δ character were included in the single-center expansion describing the molecular orbital out of which ionization occurs, leading to the selection rules |∆N| ) |N + -N′′| e 2 and ∆K a ) K a + -K a ′′ ) (1, (2 and to photoelectron partial waves with angular momentum quantum number up to l ) 3. The observation of a strong spectral feature associated with ∆K a ) 0 indicates the importance of vibronic interactions. † Part of the "Klaus Mu ¨ller-Dethlefs Festschrift".

Torsional vibrational structure of the propene radical cation studied by high-resolution photoelectron spectroscopy

“…Consequently, no rotational structure could be observed for these bands that would have allowed the unambiguous determination of the vibronic symmetry of the cationic levels using rovibronic photoionization selection rules, as was possible for the origin band. 30 To nevertheless assign the vibrational structure, the photoelectron spectra of several deuterated isotopomers of propene, CH 3 CDCH 2 , CH 3 CHCD 2 , CD 3 CHCH 2 , and CD 3 CDCD 2 , were also recorded and the identity of each vibrational band was deduced from the magnitude of the isotope shifts. Indeed, the Franck-Condon active modes are expected to be unambiguously identifiable by the isotopic shifts caused by deuteration at specific carbon sites.…”

“…In our previous study of the rotational structure of CH 3 CHCH 2 and CD 3 CDCD 2 , the adiabatic ionization energies of these two isotopomers were determined to be 78 602.0(4) cm −1 and 78 759.9(5) cm −1 , respectively. 30 To determine the adiabatic ionization energies of CH 3 CDCH 2 , CH 3 CHCD 2 and CD 3 CHCH 2 , the PFI-ZEKE photoelectron spectra of their origin bands were measured at higher resolution, as explained in Sec. II, and the partially resolved rotational structures were analyzed with the procedure described in Ref.…”

“…II, and the partially resolved rotational structures were analyzed with the procedure described in Ref. 30. The high-resolution spectra of CH 3 CDCH 2 , CH 3 CHCD 2 and CD 3 CHCH 2 are displayed in Fig.…”

“…This orbital has a nodal plane containing the three carbon atoms, and its single-center expansion consists primarily of ( = 1, λ = ±1), ( = 2, λ = ±1) and ( = 2, λ = ±2) contributions, λ = ±0 being forbidden by symmetry (see Ref. 30 for a complete discussion). In the orbital ionization approximation, 44 the rotational selection rules can be expressed as ( N, K a ) = ( , λ ).…”

“…14 The general aim of the present work was to characterize and model the vibrational energy level structure of the propene cation at low energies and so obtain a better understanding of the large-amplitude torsional motions. More specific goals were (i) to resolve ambiguities in previous assignments of the low-lying vibrational levels of the propene radical cation by recording photoelectron spectra of several deuterated isotopomers and systematically analyzing the isotopic shifts, (ii) to derive quantitative information on the potential energy surface along the torsional modes, primarily in the form of barrier heights and widths and verify theoretical predictions concerning the relative energies of the conformers and the torsional barrier along the CH 3 internal rotation coordinate, and (iii) to gain information on vibronic interactions in the cation that could complement that obtained in our previous study of the rotational structure of the vibrational ground state, 30 and in studies of vibronic interactions in the ground state of C 2 H …”