Electron energy loss spectroscopy of acetone vapor

Abstract

High resolution, inelastic electron scattering data can provide new spectroscopic information on the electronic structure of polyatomic molecules. Features in the acetone energy loss spectrum from 0 to 15 eV obtained for 100 eV incident electrons correspond to vibrational, electronic discrete, and electronic continuum excitations. These data are compared with optical measurements in a wide spectral region extending from the infrared to the vacuum ultraviolet. A comprehensive interpretation of the energy loss spectra is attempted with the use of photochemical and photoelectron data, as well as quantum-chemical calculations in the literature. Three Rydberg series with quantum defects of 1.03, 0.81, and 0.315 join onto bands previously discussed in terms of transitions to valence orbitals. These series converge to an ionization limit of 9.705 eV in good agreement with previous optical determinations. Dissociative continua underlie the Rydberg region and give rise to a variety of neutral products observed in recent photolysis work. Broad features in the ionization continuum appear to correlate generally with higher ionization potentials observed by photoelectron spectroscopy. Apparent oscillator strengths derived from the energy loss data for the bands at 4.4 and 6.35 eV and for a region (9.7–11.78 eV) of the ionization continuum agree very well with the photoabsorption measurements. Integrated oscillator strengths of 0.46 below 9.7 eV and 3.93 below 15 eV were derived from the electron impact data.

Low-energy electron-energy-loss spectroscopy of condensed acetone: Electronic transitions and resonance-enhanced vibrational excitations

“…The broad rise between 5.3 and 7.1 eV with a maximum around 6.2 eV is similar to the one observed between 5 and 6.2 eV in the EEL study of St. John, III et al 41 and where it has been attributed to the transition from the ground state to the 1 3 A 1 valence state involving the CO bonding and the CO * antibonding orbitals ͑i.e., →*͒. This transition has also been observed by threshold electron-impact spectroscopy by van Veen et al 43 However, the maximum could not be reported in that study because of the strong n→3s transition to the 1 1 B 2 Rydberg state near 6.35 eV, 7,10,40,41,43 which is in agreement with various theoretical predictions. 2,4,42 Interestingly in gas-phase photolysis studies, [13][14][15][16][17][18][19][20][21][22] the dissociation of acetone into two methyl radicals and a CO has been attributed to this Rydberg transition.…”

“…In the early electron energy loss ͑EEL͒ spectrum of acetone in the gas phase, which has been reported by Huebner et al, 10 just 3 out of the 24 normal vibrational modes observed by optical means are resolved. 29 The vibrational excitation by electron impact at low energy has been investigated and attributed, by Benoit et al, 27 to the formation of a transient anion state that has been previously located around 1.5 eV by threshold electron-impact spectroscopy by van Veen et al 30 In the solid phase, a detailed assignment of the vibrational modes of acetone has been carried out by Harris and Levin using low-temperature Raman spectroscopy.…”

“…The relatively smaller peaks located below 1 eV result from the excitation of various vibrational modes of acetone and will be discussed in more detail in the following two sections. From a comparison to the gas-phase electronic spectra, 9,10,40,41 one can immediately see that all sharp energy-loss bands due to various Rydberg transitions converging to the first ionization potential ͑9.7 eV, adiabatic͒ 38,40 are absent in the solid phase. Owing to their relatively large size, which make them more susceptible to various perturbations from the condensed phase, these excitations are generally shifted to higher energy or simply obliterated from a spectra, hence confirming their Rydberg character.…”

Ultrafast studies of the photodissociation of the acetone 3s Rydberg state at 195 nm: Formation and unimolecular dissociation of the acetyl radical

“…[19][20][21][22][23] Early indications of a discernable intermediate and a stepwise dissociation were reported by Baba et al 3 based on the power dependence for their multiphoton ionization ͑MPI͒ spectra. In contrast to their results for excitation at 248 nm, North et al 15 did not detect any acetyl radical products for 193 nm excitation.…”

Vibronic coupling in excited states of acetone

“…7 Concentrating on the singlet manifold, we see that excitation from this lone pair orbital n y to the 3b 1 (*) orbital leads to the first excited singlet valence state 1 1 A 2 that has its 0-0 transition at 30 439.9145 cm Ϫ1 . 16 This dipole-forbidden transition has been the subject of many experimental studies [17][18][19][20] concentrating on aspects such as the role of vibronic coupling and interaction with triplet states. 16,21,22 The first Rydberg state derives from excitation of a lone pair electron to the 3s Rydberg orbital, giving rise to the 1 1 B 2 state located at 6.35 eV.…”