S. M. Kazakov

Electron energy loss under scattering by complex organic compound vapors

Abstract Low-energy electron scattering by anthracene and 1, 4-di 2-(5-phenyloxazolyl) -benzene (POPOP) vapors at low pressures is studied. The primary electron beam energy EO was varied between 10 and 60 eV. The scattering angle was θ = 90°. It has been found that in the electron-energy-loss spectra, bands with maxima of 3·7 and 5·41 eV for anthracene, 4.02 and 7 eV for POPOP correspond to the SO → S1 and So → S2 transitions. Low intensity electron-energy loss due to T2 -states excitation has been observed in the region of 2·15 eV for anthracene and 2·54 eV for POPOP. From comparison with optical absorption spectra and analysis of spectra structure changes at different EO the nature of other bands has been determined. From the obtained results it was concluded that the probability of T1 -state excitation is low. It is shown that the main mechanism of populating the T1 -state by electric methods of vapor pumping is the process of intersystem crossing. The development of free complex molecule spectroscopy ...

The Function of Electron Impact Excitation of Fluorescence of Complex Organic Molecules in the Gas Phase

Abstract Excitation functions of fluorescence of complex organic compound vapors by a monokinetic energy-tuned electron beam are studied. It is shown for the first time that the maximum of these functions is in the region of - 4ET where ET is threshold excitation energy of a lower singlet state. Emission spectra have been recorded for POPOP molecules at collision with high-energy electrons. Analysis of the spectra, as well as of the character of excitation functions changes enabled us to draw a conclusion about weak destruction of complex organic molecules as a result of direct electron impact.

Determination of first ionization potentials from spectra of electronic energy loss in the vapor of polyatomic organic compounds

It is shown that an analysis of energy loss spectra of electrons scattered at θ=90° in the vapor of polyatomic organic molecules allows determination of the first ionization potentials. Thus, for pyrene, perylene, and POPOP molecules they are 7.7, 7.2, and 8.7 eV, respectively. It is found that the first ionization potentials measured depend considerably on the thermal distribution of the free molecules.

Spectral‐Luminescent Properties of Oxazoles and Oxadiazoles in a Gas Phase on Excitation by Electrons

The energy‐loss spectra of electrons, fluorescence excitation functions, and the fluorescence spectra on excitation of the vapors of a number of oxazoles and oxadiazoles by monokinetic beams of electrons of various energies are determined. In contrast to optical absorption spectra, in the energy‐loss spectra of a number of studied substances a band associated with the S0–T1 singlet‐triplet transition is observed. The π–π*‐type transitions are fixed up to S0–S5 on excitation of molecules by high‐energy electrons, including the region of vacuum ultraviolet. The cross sections of elastic and inelastic collisions of electrons of different energies with POPOP molecules have been measured. The dependences obtained differ substantially from those calculated in the Born approximation. The cross section of elastic scattering is in a rather good correspondence with the geometric section of the molecule.

Spectral‐Luminescence Characteristics of Polyphenyls and Polyacenes in the Gas Phase in Electron‐Beam Excitation

The spectra of electron‐energy loss, excitation functions, and fluorescence spectra in excitation of the vapor of polyphenyls and polyacenes by electron beams of different energies are determined. The influence of successive complication of the molecules under study on these spectral‐luminescence characteristics is tracked. Unlike the optical absorption spectra, in the spectra of electron‐energy loss of all the substances studied one observes a band which is related to the singlet‐triplet transition S0–T1. The transitions up to S0–S5 are recorded in excitation of the molecules by high‐energy electrons, including the region of vacuum ultraviolet. From the functions of fluorescence excitation the authors have determined the excitation thresholds that correlate with the energies of the S1 levels, except for pyrene in which the S0–S1 transition is forbidden and does not show up not only in photon excitation but also in electron‐beam excitation, although the intercombination forbiddenness in the latter case is removed and the S0–T1 band is observed.

Losses in the Energy of Low-Energy Electrons in Fluorene, Fluorenone, and Diiodofluorenone Vapors

Electron energy loss spectra (EELS) of fluorene, fluorenone, and diiodofluorenone vapors excited by monokinetic electrons of energies 15–50 eV have been obtained. The singlet and triplet absorption bands of these molecules have been calculated. Comparison of these bands with the experimental EELSs and optical absorption spectra has shown that the forbiddenness of singlet-triplet transitions is not completely removed in the process of interaction of molecules with electrons. The presence of heavy iodine atoms in the diiodofluorenone molecule enhances singlet-triplet transitions. Bands of overtones of stretching vibrations of the CH groups of the benzene rings have been detected near the peak of elastic scattering of electrons of the molecules studied.

Electron Energy Loss Spectra of the Organic Complexes of Europium

In the electron energy loss spectra (EELS) of the organic europium complexes Eu3+ (BTFA)3TPPO and Eu3+(Br‐BTFA)3TPPO in a gas phase obtained on excitation by monokinetic beams of electrons of different energies in the range 12–50 eV, we have identified the bands associated with the electron transitions S0–S1, S0–S2, and S0–S3. The connection of these transitions with the structural groups of the complexes is established. The addition of the bromine atom to the phenyl ring of β‐diketonate leads to the rise in the relative intensity of the S0–S2 band. The singlet‐triplet transitions manifest themselves in the region 2.5–3.2 eV and contribute to the S0–S2 band of the electron energy loss spectra.

Spectral-Luminescence Characteristics of Carbazole, Dibenzofuran, and Dinaphthofuran in the Gas Phase in Excitation by Electrons and Photons

The spectra of electron-energy loss, the excitation functions, and the fluorescence spectra in excitation of carbazole, dibenzofuran, and dinaphthofuran by monoenergetic beams of electrons of different energies are determined. The singlet-triplet transitions S0–T1 and the singlet-singlet transitions up to S0–S7 are recorded, which covers the region 2–11 eV. In the spectra of electron-energy loss, bands that refer to the nπast and ππ* transitions are identified. The replacement of the heteroatom of nitrogen by the atom of oxygen in the five-membered ring has no substantial effect on the spectra of electron-energy loss.

Electron Energy Loss Under Scattering by Naphthalimide Derivatives Vapours

Abstract Electron energy loss spectra under scattering of electrons on 4-amino-N-(n-butylphenylene)-1, 8-naphthalimide and 4-methylamino-N-(o-tolyl)-1, 8-naphthalimide in a vapour phase is studied. The primary electron energy was varied from 6 to 60 eV and the scattering angle was 90°. It is found that the differential cross section of excitation of complex organic molecules into some triplet states can reach the same value as singlet ones.

Formation of fluorescence bands of polyatomic organic molecules in the gaseous phase upon excitation by electrons

Information on paths in absorption and deactivation of energy gained by molecules in their excitation by electron impact to low-lying singlet states has been obtained from an analysis of changes in the fluorescence spectra of these molecules. It is shown that there is a significant difference in the formation of fluorescence spectra when free molecules are excited by optical radiation and by electrons. It contrast to optical excitation, the interaction of an electron with a molecule is nonselective in character. All electronic states have a chance to be excited, which results in ensembles of emitting molecules with a different store of vibrational energy, and these ensembles each contribute to the fluorescence spectrum.