Intramolecular dynamics by photoelectron spectroscopy. I. Application to N2 +, HBr+, and HCN+

peer reviewedThe Fourier transform of an optical electronic spectrum leads to an autocorrelation function C(t) which describes the evolution in time of the wave packet created by the Franck-Condon transition, as it propagates on the potential energy surface of the electronic upper state. This correlation function is equal to the modulus of the overlap integral between the initial position of the wave packet and its instantaneous position at time. The original data resulting from an experimentally determined spectral profile must be corrected for finite energy resolution, rotational, and spin-orbit effects. The behavior of the system can then be followed up to a time of the order of 10 -l3 s, i.e., during the first few vibrations which follow immediately the electronic transition. The method is applied to photoelectron spectra and the results are compared to the available information on potential energy surfaces of ionized molecules, in order to study their unimolecular dissociation dynamics. In the case of the X 2Σg +, A 2Πu, and B 2Σu + states of N2 +, an oscillatory pattern is obtained for the correlation function. This indicates that the nuclear motion is taking place in a bound potential. Effects due to anharmonicity are visible in the case of the A 2Πu state. The study of the X 2Π state of HBr+ demonstrates the overwhelming importance of spin-orbit coupling when heavy atoms are present in the molecule. Finally, the method is applied to a polyatomic molecule. The potential energy surface of the B̃ 2Σ+ state of HCN+ is characterized by two energy minima separated by a saddle point. The corresponding band in the photoelectron spectrum is characterized by an irregular vibrational structure superimposed upon a broad continuum. A study of the correlation function shows that the wave packet undergoes a complicated, two-component motion: while oscillating across the saddle point, it spreads away at the same time along the dissociative degree of freedom. This gives information on the rate of energy redistribution within the molecule. © 1982 American Institute of Physics

Vibrational excitation of methylamine by electron impact in the 4.5-30 eV energy range

Vibrational excitation of gaseous methylamine induced by 4.5-30 eV energy electrons has been investigated by the electron energy loss spectroscopy. The ratios of the differential cross sections for excitation of the vibrational modes and for elastic scattering measured as a function of the electron kinetic energies show that at 15 and 30 eV, the vibrational excitation occurs mainly through a direct mechanism. The absolute vibrationally elastic and inelastic differential cross sections have been measured at these impact energies. The cross sections for the inelastic scattering are strongly dependent on the vibrational mode which is excited. © 1992 American Institute of Physics

Photoelectron spectroscopy of a series of acetate and propionate esters

The electronic state and photoionization spectroscopy of a series of acetate esters: methyl acetate, isopropyl acetate, butyl acetate and pentyl acetate as well as two propionates: methyl propionate and ethyl propionate, have been determined using vacuum-ultraviolet photoelectron spectroscopy. These experimental investigations are complemented by ab initio calculations. The measured first adiabatic and vertical ionization energies were determined as: 10.21 and 10.45 eV for methyl acetate, 9.99 and 10.22 eV for isopropyl acetate, 10.07 and 10.26 eV for butyl acetate, 10.01 and 10.22 eV for pentyl acetate, 10.16 and 10.36 eV for methyl propionate and 9.99 and 10.18 eV for ethyl propionate. For the four smaller esters vibrational transitions were calculated and compared with those identified in the photoelectron spectrum, revealing the most distinctive ones to be a C–O stretch combined with a C[dbnd]O stretch. The ionization energies of methyl and ethyl esters as well as for a series of formates and acetates were compared showing a clear dependence of the value of the ionization energy on the size of the molecule with very little influence of its conformation

Isobutyl acetate: electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, He(I) photoelectron spectroscopy and ab initio calculations

The high-resolution vacuum ultraviolet photoabsorption spectrum of isobutyl acetate, C6H12O2, is presented here and was measured over the energy range 4.3–10.8 eV (290–115 nm). Valence and Rydberg transitions with their associated vibronic series have been observed in the photoabsorption spectrum and are assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. The measured photoabsorption cross sections have been used to calculate the photolysis lifetime of this ester in the Earth’s upper atmosphere (20–50 km). Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of isobutyl acetate and are compared with a photoelectron spectrum (from 9.5 to 16.7 eV), recorded for the first time. Vibrational structure is observed in the first photoelectron band of this molecule

Electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, He(I) photoelectron spectroscopy and ab initio calculations of ethyl acetate

Abstract: The high-resolution vacuum ultraviolet photoabsorption spectrum of ethyl acetate,C4H8O2, is presented over the energy range 4.5−10.7 eV (275.5−116.0 nm). Valence and Rydberg transitionsand their associated vibronic series observed in the photoabsorption spectrum, have beenassigned in accordance with new ab initio calculations of the vertical excitation energiesand oscillator strengths. Also, the photoabsorption cross sections have been used tocalculate the photolysis lifetime of this ester in the upper stratosphere(20−50 km). Calculationshave also been carried out to determine the ionisation energies and fine structure of thelowest ionic state of ethyl acetate and are compared with a newly recorded photoelectronspectrum (from 9.5 to 16.7 eV). Vibrational structure is observed in the firstphotoelectron band of this molecule for the first time

A position-sensitive electron detector for use in electron spectroscopy

peer reviewedA home-made position-sensitive electron detection system has been mounted on an electron energy loss spectrometer. It makes use of advanced technology, is easy to handle with and not too expensive. The K-shell excitation spectrum of benzene is given as an example of the quality of the signal-to-noise ratio while keeping the resolution. (C) 2002 Elsevier Science B.V. All rights reserved

Electronic and vibrational excitation of acrylonitrile by low and intermediate energy electrons

Electronic and vibrational excitation of acrylonitrile induced by 3-50 eV energy electrons has been investigated by the electron energy loss spectroscopy. Electronic excitation spectra have been recorded for 30 and 50 eV impact energies at a 10° scattering angle in the energy loss range from 5.5 to 11.5 eV, corresponding to the excitation of electrons belonging to the outermost-valence-shell molecular orbitals. We have reviewed the assignment of the valence excited states occurring in the 5.5-9 eV energy loss region. The vibrational patterns associated with the two lowest-energy singlet valence excited states have also been re-examined. Moreover, we have proceeded for the first time to the analysis and attribution of several Rydberg series converging to the ionic ground state and to its two lowest-energy electronic excited states. The study of the excitation function of the C-H stretching modes of acrylonitrile in the 3-11 eV electron impact energy range has shown evidence of a broad shape resonance built on the electronic ground state of the molecule and centered at 5.85 eV. This resonance contributes to a preferential excitation of the C-H stretching modes suggesting that the charge distribution of the additional electron is very likely that of a σ* (C-H) valence molecular orbital. A comparison has been made between the resonances observed in C2H4 and CH2CHCN, in order to discuss the symmetry of the resonant state and also to analyze the substitution effect of the cyanogen group. © 1995 American Institute of Physics

Resonant vibrational excitation of methylamine by low energy electron impact

The study of the excitation functions of the NH2 and CH 3 stretching modes of methylamine in the 4-12 eV electron impact energy range shows evidence of a broad shape resonance built on the electronic ground state and centered at 7.5 eV. This resonance is formed by the trapping of the incident electron in the second unoccupied molecular orbital and is of A " symmetry. The elastic and inelastic angular differential cross sections measured in the 10°-90° range suggest a dominant p character. A second shape resonance located at higher energy contributes mainly to the enhancement of the CH3 deformation modes and is very likely of A ' symmetry. A comparison is made between the resonances observed in NH3 and CH 3 NH2. The effect of the methyl substitution is briefly discussed. © 1990 American Institute of Physics

Core shell excitation of furan at the O1s and C1s edges: An experimental and ab initio study

The K-shell spectra of gaseous furan have been measured using the inner-shell electron energy loss spectroscopy (ISEELS) method at the carbon and oxygen thresholds. Large-scale ab initio configuration interaction calculations have been carried out in order to help in the assignments of the observed bands. The spectra are close to previous low resolution ones obtained using ISEELS in the gas phase and photoabsorption with the synchrotron radiation in gaseous and condensed phases. The presence of a new feature located at 287.3 eV in the C1s spectrum and recently detected by photoabsorption with synchrotron radiation is confirmed. At both edges, the calculations confirm the assignments proposed in earlier experimental works, with the exception of several C1s pre-edge features for which a new interpretation is given. (C) 2003 American Institute of Physics

Ab initio and experimental study of the K-shell spectra of s-triazine

The carbon and nitrogen K-shell spectra of gaseous s-triazine have been studied using inner-shell electron energy loss spectroscopy (ISEELS) method. Ab initio Configuration Interaction calculations have been carried out in order to assign the observed bands. As in many similar molecules, both spectra are dominated by an intense pi* peak, followed by lower intensity features. At the C1s edge, the calculations show that some previous assignments made using an underestimated core ionisation energy of about 2.5 eV have to be revisited. At the nitrogen edge, the calculations predict a high intensity pi* doubly excited state lying below the ionisation threshold, which could be responsible for one the most intense observable bands at 405.32 eV