Volume 25A, number 4 PHYSICS L E T T E R S 28 August 1967

P H O T O I O N I Z A T I O N OF ATOMIC NITROGEN

F. J. COMES and A. ELZER Institut f l i t Physikalische Chemie der Universititt Bonn, Bonn, Germany

Received 28 July 1967

The photoionization of atomic nitrogen has been measured using a cross beam technique.

Recent ly the photoionization of a tomic hy- drogen and a tomic oxygen has been m e a s u r e d [1,2]. The expe r imen t s were pe r fo rmed using a uv -monochroma to r , an a tomic beam, and a m a s s spec t romete r . A pulsed capi l la ry d i s - charge was chosen as uv- l ight source on in t en - s i ty r ea sons . The n i t rogen a toms were produced in a condensed d ischarge through ni t rogen with t r a c e s of oxygen added ( < 0.1%).

In fig. 1 par t of the ionizat ion curve is shown. The photoionization c ros s section exhibits con- s iderab le s t ruc tu re in the given energy region due to exci tat ion to one of the 2s -e l ec t rons . The configurat ion of the ground state of the n i t rogen atom is l s 2 2s 2 2p 3. Three equivalent p - e l e c - t rons give 4S, 2p, 2D. Of these 4S is the lowest and therefore the ground state of the n i t rogen atom. The 2D and 2p t e r m s do not l ie far above the ground s tate , s ince they belong to the same e lec t ron configurat ion. By absorpt ion of a pho- ton with a m i n i m u m energy of 14.54 eV one of the p - e l ec t rons can be excited into a continuous state (1).

o

N(4S) + hv ~ < 851A N+(3p) +e- . (1)

Other states of the ion belonging to the same electron configuration are ID and IS. Transi- tions to these states from the ground state are forbidden because of the change is multiplicity. 5S is the lowest excited state to which a transi- tion can be expected. Its excitation energy cor- responds to 46 784.6 cm -1 as determined by Eriksson [3]. By absorption measurements in ac- tive nitrogen Carrall et al. [4] found a Rydberg series which they interpreted as arising from absorption by neutral atomic nitrogen. The con- vergence limit as determined from the measure- ments agrees within the experimental uncer- tainties with the position of the 5S state of the atomic nitrogen ion. They concluded that the

334

Rydberg series can be represented by the tran- sition 4S°-2s2p3(5S°)np 4p. From the selection rules for autoionization one would expect these levels to interact with the continua associated with the 4p terms of the series 2p2(3p)ns and 2p2(3p)nd. Then the absorption lines should show the typical asymmetric structure observed for autoionization. The occurrence of asymmetric peaks is well to be seen in the photoionization curve and also from the absorption experiments. From fig. 1 the line shape parameter may be estimated to have a value of approximately one. This means that both the maxima and the minima of the autoionizing levels have nearly the same absolute height. As the light source emits a line spectrum the measured values represent the ioni-

ut

~15 J~

10 ̧

photon energy (eV) 17 18 19 20 21 22

i I , I i i

o!3 I I 700 ~50 60O 550 w a v e l e n g t h ( A }

Fig. 1. Cross section for photoionization of atomic ni- trogen in arbitrary units. The positions of the auto- ionizing levels as indicated are taken from the mea-

surements of Carra11 et al. [4].

Volume 25A, number 4 P H Y S I C S L E T T E R S 28 August 1967

zation curve only f o r the cha rac t e r i s t i c wave- lengths as given by the emi s s ion spec t rum. The ionizat ion curve drawn in fig. 1 is therefore only a l ine which connects these points . If the half - width of the autoionizing levels is la rge com- pared to the l ine spacings of the spect rum of the source this curve r e p r e s e n t s the rea l shape of the ionizat ion curve, in other cases not. As can be seen f rom the f igure five m e m b e r s of the Hydberg s e r i e s a r e r e so lved and have the pos i - t ions taken f rom the absorp t ion m e a s u r e m e n t of C a r r a l l et al . [4]. F r o m the se lec t ion ru les this Rydberg s e r i e s should be the only one to occur in absorpt ion . However, the photoionization c u r - ve shows some s t r u c t u r e in the energy region above 700 ~ which is outside the exper imenta l e r r o r . A fu r the r i n t e r e s t i ng r e s u l t i s that the photoionization c ros s sect ion has near ly the same value in the region before the f i r s t t e rm of the Rydberg s e r i e s as well as behind the conver - gence l imi t of the s e r i e s , indicat ing a smal l osc i l l a tor s t rength f o r the t e r m s of the s e r i e s .

The ionizat ion s tudies were ca r r i ed out to de te rmine the photoionization c ros s section of a tomic n i t rogen [5]. The ionized par t i c les which a re fo rmed by the photoionization of ni t rogen

a toms and molecu les in the beam which c r o s s e s the ionizat ion r e g i o n a r e both found to be in the i r e lec t ron ic ground s ta tes . Contr ibut ions to the ion c u r r e n t due to excited species was --< 0.3% for a tomic ions and --< 1.5% for mo lecu l a r ions, r e s - pect ively. This could eas i ly be es tab l i shed by m e a s u r i n g the ionizat ion at photon energ ies lower than the respec t ive ionizat ion potent ia ls of the pa r t i c l e s in quest ion. The atom concent ra t ion in the beam was found to be 25-30%. Somet imes va lues up to 50% have been measu red . When us ing a glow discharge ins tead of a condensed d ischarge the contr ibut ion of metas tab le a toms as well as molecu les to the m e a s u r e d ion c u r r e n t s was a l - ways s m a l l e r than 1-2%.

Ref e~'ences 1. F.J . Comes, A. Elzer and F. Speier, Z. Naturforsch.,

to be published. 2. F.J. Comes, F. Speier and A. Elzer, Z. Naturforsch.,

to be published. 3. K.B.S. Eriksson, Ark. Fys. 13 (1958)303. 4. P.K. Carrall, R. E. Huffman, J.C. Larrabee and Y.

Tanaka, Astrophys. J. 146 (1966)553. 5. F.J . Comes and ~,.Elzer, Z. Naturforsch., in pre-

paration.

A N I S O T R O P I C P L A S M O N B E H A V I O U R IN G R A P H I T E

K. Z E P P E N F E L D Institut fll~, Angewmuite Physik der Universit~t Hamburg, Germany

Received 27 July 1967

Energy loss spectra of 60-keV-electrons were measured in transmission of thin graphite single crystals at non normal incidence and at different scattering angles. The plasmon energy and the loss intensity were found to be dependent on the angle between the direction of the plasmon pulse and the c-axis of the crystal.

The dependence of the energy of the volume p l a sma osc i l la t ion on the wave vector k (d i sper - s ion re la t ion) has been m e a s u r e d in recent y e a r s by means of cha rac t e r i s t i c energy loss exper i - ments . The re has been found no influence of the angle of ~k agains t the c ry s t a l axis .

In graphi te one has obse rved two p la sma lo s - ses , for ~r-electrons (1 pe r atom) at about 7 eV and for # - plus a - e l e c t r o n s (4 per atom) n e a r 25 eV [1,2] in ag r eemen t with optical ref lec tance

data [3]. However one expects in a high a n i s o t r o - pic c rys t a l as graphite a dependence of the p l a s - mon energy on the d i rec t ion of i t s pulse com- pa red to the c r y s t a l or ienta t ion .

In this l e t t e r we r epor t the observa t ion of such a k-dependence in the energy loss spec t ra of 60 keV e lec t rons having passed thin l aye r s (about 500 - I000 ~), c leaved perpend icu la r to the optical axis c f rom Ceylon graphi te .

F r o m energy and pulse conserva t ion one ob-

335