Vol.6, No 1, 2008 pp. 41 - 55
DOI 10.2298/FUPCT0801041P
UDC 539.17
RESONANT EXCITATION OF MOLECULES BY LOW-ENERGY
ELECTRONS
Goran B. Poparić
Faculty of Physics, University of Belgrade, Studentski trg 12, Belgrade,
P.O. Box 368, 11000 Serbia
Abstract. Low-energy electron impact vibrational and electronic
excitation cross sections of the CO, N2 and CO2 molecules are measured
by use of a high resolution crossed-beams double trochoidal electron spectrometer.
The spectrometer is designed to work in standard and time-of-flight regimes.
The energy dependences of the resonant vibrational excitation of the first
several vibrational levels of the N2, CO, and CO2 molecules, have been
measured. Characteristic substructures in energy excitation spectra in
the cases of N2 and CO have been obtained and discussed for some vibrational
channels for the first time. The ratio of forward-to-backward scattered
electrons from the 2Π resonance in CO is found to be equal to 1, and thus
the angular distribution of scattered electrons to be symmetric relative
to 90°. This conclusion supports the fact that the contribution of the
p partial wave is dominant in the energy region of the 2Π resonance
in CO. The energy dependences of the near threshold resonant excitation
of the valence and Rydberg states of the N2 and CO molecules have been
measured. The cross sections of the near threshold resonant excitation
of the C 3Πu valence state, and the E 3+g and a'' 1+g Rydberg states
of the N2 molecule have been measured. In the case of the CO molecule,
the cross sections of the near threshold resonant excitation of the a 3Π
valence state, and the b 3+ and B 1+ Rydberg states have been measured.
Resonant structures in excitation functions of all measured electronic
states are observed and their locations are compared with resonances obtained
in different decay channels.
Key words: Vibrational excitation; Electronic excitation; The
N2, CO and CO2 molecules.
REZONANTNA EKSITACIJA MOLEKULA ELEKTRONIMA
NISKE ENERGIJE
Mereni su diferencijalni preseci za ekscitaciju molekula CO, N2 i CO2,
pri čemu je korišćen dvostruki trohoidni elektronski spektrometar visoke
rezolucije. Spektrometar je dizajniran za rad u standardnom režimu, i
režimu merenja vremena preleta. Merena je energijska zavisnost rezonantne
vibracione ekscitacije prvih nekoliko vibracionih nivoa molekula N2, CO,
i CO2. Karakteristične podstrukture u spektru energijske zavisnosti u
slučaju molekula N2 i CO su snimljene i diskutovane po prvi put. Određen
je odnos broja elektrona rasejanih unapred i unazad na 2Π rezonanci oblika
u slučaju molekula CO, pri čemu je dobijena vrednost 1, i tako je zaključeno
da je ugaona raspodela rasejanih elektrona simetrična u odnosu na ugao
od 90°. Ovakav rezultat odslikava činjenicu da je doprinos p parcijalnog
talasa dominantan u energijskom regionu 2Π resonance u slučaju molekula
CO. Energijska zavisnost u blizini praga za rezonantnu ekscitaciju valentnih
i Ridbergovih stanja molekula N2 i CO je takođe merena. Mereni su efektivni
preseci za ekscitaciju C 3Πu valentnog stanja, i E 3+g i a'' 1+g
Ridbergovih stanja molekula N2. U slučaju molekula CO, mereni su efektivni
preseci za ekscitaciju a 3Π valentnog stanja, i b 3+ i B 1+ Ridbergovih
stanja. Snimljene su rezonantne strukture u ekscitacionim funkcijama svih
merenih elektronskih stanja i njihova lokacija je poređena sa strukturama
dobijenim u različitim kanalima raspada rezonace.
