- Email: [email protected]
Electron impact excitation functions of radiating and metastable states of carbon monoxide
Volume 11, number 2 .. .‘. ‘. : .: ;. ‘. .‘.‘. “. : :
~~MI~~~P~Si&S ::. : . :
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I octob& 1971
.‘.
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: ~MX”EXCiTA’I”ItjN
FiJNCiXOtiS
~IATIN~AND~A~TABLESTA~S'OFC~ON -: .’
.,,
,.
“. ._
ELJ$~Rt.lN
.‘~
I;ETTERs .
..-,.
i;&t?nce Radahm
--AS. NEWTO&UI~G.E~TH~B&S~~ hwratory,
OF
.-
“-$:
MONGER
-,
‘.
iJn&wsity of Cla&rrn~, Berkefey, California 94720, i/sA’~
.’
Received 19 Juiy 1971 ..
-
Excitation functions nek threshold h&e been me&nreh for tie production of photons and for tie produ&on of l&g-Ikd meiastable molecules from c&on monoxide. An h&resting peaked fkatmi existsat ffie threshold for photon production.
hai’been described 6y ~o~~.~d.~~gelsbarg [4]. It is possible to operate the electron gun in this atoms and mole&es is the eiossed electron and neti& instrument as a high resolution electron source using -. the RPD tecbsiique. However, the photon signal from _ ,beamteclmique. The me&stables produced.by electron impact are detected via their ab-Wyto eject an elecCO vvasvery weak, and it was not possible to obtain an Adaequates&w.+-nob ratio in the RFD mode. tron-from a metal surface placed downstream from the collision region in the path of the neutral beam [I]. Therefore, in thiis wo& th;hetwo-cycle BPD operation of the gun was eliminated and only the f&t cyle of Photons hi&er in energy the the work function of the pulsing scheme prev&usIy described [4] was used, the metal detector can also be measured. By pulsing the electron beam and by using time-of-flight (TOF) The electron beam pulse width was IOpsec, and the techniques, the two signals can be measure@ indepen-:. electmn’ctixrent during the pulse ‘was= 1W6 A. To record the photon curve, the data count gate was ’ dently [2] . A TOF technique usbig a diffuse, rather than a directed gas source has recently. been used by openedlonly during the electron pulse. For the metaBorst. and Zipf )P measure excitation.effIciency curves stable‘curve; the count gate was delayed for 60 psec and was then opened to imttercept the majority of the for metastabie production and to deduce lifetimes of metastabfe states+ CO andN2 [3]. T9.F distribution of the CO metastables. The time T’be molecular,beam al@rratus &rd the electron gun between electron pulses was 1 msec. The CO gas usedused in this study. have been’described previously by w& Cp grade: obtained from the Matbeson Company. Qunpitt and Newton [21. The only signif&nt change :. Its purity was,> 99.3%. Impurities 0,tber than 8% in, t.$e appatatus [email protected] an EMI 9603 Be-&u.ele&~~ ‘wereC 0.1% each. .. t!on mi#iplier was used as the detector:This.multi-, The electron’energy scale was calibrated by running. plikr-had.an effe&vephotoele&c work ftinction of,‘. ‘. +he appmpriate CO, &tie and then,, while still scanning,5.1, eV, measured using a ~~e~t~~u~.~~t squrce..an~ 1. .’ +&lirig~~~&nallhnpurity pf argon to the berim source. -a ~~n~~~~rn~t~~~ T@edi~t~~e.be~ee~ the eleled~tron I$5 data -Coat&gate w@ ~~t~eou~y-opener to _ : ~-.countaigon me_tastables.T& signal strength from the .gun-and., the detector w&,19.7 .&n. The data&&l&i& -. : ‘... ‘_ : : : .”:i .,( : _‘. ,’ .‘,,., ( : -1 . ,’ ;iddecj ‘tigo$vaS $Ihi&tiotipared to that from‘the .. ., ____.._. __-, :,. -. -. ,: . ., ~-_~~~~~r~~~~~:~~~~~ ule auaices ~~~~~~~~~~~~~~’ : ~1.- C? ,$haQs&tr@$f threshold~cduld be-superimposed ‘: E&ergqi@m&$~e&~~; ,-:_-’ :._:-.i,,. .,\._‘l’,,, .,,, 1.. ::‘:l. . :: 09 thi=CO~~nre.withinl3C.sec~of,adiling the ar&n to ’ ’ -- th&,&iainA~,tiai &&aq&3 asgi&:,that potential .;_ .t.~~~~1’~d~6:Philipi~e~~h Laborat+ies;E&d&oii,’ ‘. -., One ~c~que for obtaining electron impact excitation functions for long-lived metastable states of
.
system
2.1Fe,Neth;er+n+, -. .iy.;I_.-. .I.:.., ,:-:::, :i ;. -:,Q,,‘.::y:-.. _:.:.’ .- .-dri&‘&t+iidt’o&i$,ki f&j &oti,the;!J&g&* .: : f , : . : .. .’ .. :‘_ ._’ .‘: ..:, ,,_,.‘/’ :*.-,:‘: ..., :,:;_ __ -..y.: .;...-.._ ,.; :.__’ : ,._~ _.:.:.: __ _:. :.I...’1: -:_.: .. .., ,, ;_.:. ._,,-’ .,. --. ._;,_ ” .:..:. ‘.:.. ,:. .I ._: _ I ‘: ., I _: :‘ ., - .:-. ., -, .;;y,,:..,., ‘_: : ._ :,- __ :. .,;,-. ‘J7l . . . .:::i: ... _..-_.,,. ,,.:, .;_ :_*:I’..., .-. _. _..._-:._: -.- ;:. -...‘..__:_-.:,. ,. - , . _ ‘..-. . ._...I ..- : ..‘, 1.’. .’ -..__;‘:.,; .’ _, ;.:. ., ~,.. -3 I :.- .,._ _.. ... ,‘._::F., ._: ..,. .‘,, .,,-,.:y_, .:*:‘_. .,..I ;,,., -,‘:_, .,;-.. ‘;._.. .,I ;.... -.... ‘.... :’ : -._ .-,. <. ‘<...*,’:; ., ,: :.y . .. _ ?_.‘_. .. ., -. , ::~_ . ..::.,.__. .. :. ,:_.:,,._.. ,...I -,.“-‘: :,, ..‘._ , _ I ,:I.’ ,. ,_ j__ .a: ..;::... -._,, : ‘:.... ,:__; .: :; -./-.. . 2:. _ _- ,_..--I . ,. .: .. -. .:.. .... ..:..r:
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t&ken’to be I I-62 eV l[S] _It is es&&ed that the energy s&e established in this’fashion, :., ,.Gti aticurate’to,50.2 eV: : :.Fig,-I- ahoiws the excitation function for metastable :stat$s produced by election impact on CO. ‘l$e.excifati@t&r+@d was found to he S-9 eV as cgmpared to +e spectroscopic .vaiue bf 6.01 N. A3I curves re-’ cord&d in this study showed the same steady rise to a pl+e%u. it approximately JO eV..A reproducibli fea-: ture,wa$ the sma.lIincrease in signal near 13 eV. The shape of the tiurve agrees quite well with tbeWtial .p$ti& if 9 &et&tabie CO c&-e recently published b$ Bars! arid.iipf f3] _It is, however, quite different :&on ~~,~u~e.rne~~~ed by C&nsted, Newton, tid Siree$ [6I . ,Referring t0 %&penie’s compiiatjon of Spe$rpscopic @ata on CO [?] , the’&ly tioivn state *hi& c&d contribute to the me&stable sig&l in the . region- ~e~at~l~ above 6 eV is the a %I state. m3s
Fig. 2: Excitafion %urictionfor the producti0.nof prompt photons of energy > 5.1 eV from CO. _
Fig, 2 shows the excitation fun&m obtained for the production of pro&pt.radiation of energy@eater than 5.1 eV frcim’C0. Ari unusual feature of this ctifle is the sharp peak qb&ved at threshold. ,me onset of, the curve is at 7.4 eV’and t&peak occurs at 8.3 eV. The width at half height of the peak is 9.9 eV, including the broadening caused by the energy spread of the el&trons f&m the thdriate’d iridium fiament: There is a second strong emission threshold at. * 11 eV. It is_. &o-apparejnt that there is weak emission below 11 eV which is probably not connected with the pe&ked feature, but the threshold of ihis emission is obscured. It is not possible in the present ~st~ment to measure the wavelength dis&jbution of the emitted light. -. T&e stiong emission above 11 ‘eV can have s.&eral ]sources:D&ffend&ck and Fox [lo] observed radiation from the,?3X. system of CO (c 3Cf -+ a 311}.The upper state of this system has ari excitatiqn potential of.I~~:l-eV;~l~]. .At least ti‘portion &fth@ b&its& te&.is ~tifticiently en&=getic to b&.detected here. Oth&i t+pl& leveis. in &is energy range &i&t decay in a similti fashion. A&her possible &&rce of the ra&iation
Volume 1 I, number 2
1 Octobei 1971
CHEMICAL PHYSICS LE’kTERS
the Frar&k&ndon regioc in the appropriate energy range. However,.the width of the electron impact peak compared to the optical envelope ti&&icious. It would be expected that the eIect&*&pact peak would be broadened if direct excitation were involved. The electron ‘impact excitation functions for the individual vibrc;nic levels are not expected to be extremely sharp for direct excitation, even in the case of a triplet level. Secondly, the electron impact data must be tierently broadened by the energy spread of the exciting electrons. An alternative that is consist@ with the present data is that selected vibrational IeveIs of the a’ state are being populated via a resonant state. This could certainly account for the narrow width of the excitation functltin. Sanche and Schulz [ 141 have recently searched,for resonances in CO in an eleccon transmission experiment. A resonant state which may be Fig. 3. Excitation Function For the production of prompt photons from CO (dashed curve) compared to photon abresponsible for populating the b 3Cc state of CO was sorption intensities for the forbidden a’ 3 Z’ + X xZ+ tiansifound near 10.4 eV. it would be necessary to postulate tion (points From rcF. 1131). a similar resonant state (undetected in the experiment of Sanche and Schulz) for populating the a’ state. If case of the B state. The trapped electrcn spectrum of it is assumed that the a’ state is indeed the source of the light, some rough conclusions could be drawn reBrongersma and Oosterhoff [ 121 indicates that at least some of the states in this energy region are exgarding its lifetime and modes of de-excitation. This cited by electron impact at threshold. Definite constate can radiate to the metastable a 311via a dipoleclusions regarding the source(s) of the light cannot be allowed transition (the Asundi System). No radiations clrawri without knowledge of the wavelength distribudirectly to the X 1X+ sound state have been obser&, tion. The present.observations would demand that the The sharp peak at 8.3 eV in the photon excitation branching ratio for depopulation of the a’ state by efficiency curve is somewhat difYicult to explain..The direct radiation to the ground state be an appreciable only known states of CO accessible to Franck-Condon fraction of the total population. The lifetime of the a’ transition below 8 eV are the a %I and the a’ 3Z+ [7]. state must be less than, or of the same order of magniThe a 3il state can be eliminated from consideration tude as, the 10 WC observation time used here. on the basis of its long radiative lifetime [3]. its exciAn improbable source of the !ight is by direct excitation function should resemble that of CO in fig. 2. tation to the A ln state which radiates to the ground Furthermore, this state becomes inaccessible to a state (Fourth Positive bands). The threshold for this Franck-Condon transition from the ground state state is at 8.03 eV, well above the observed thresho!d above 7 eV and no new phenomena pieceded by direct of 7.4 eV. This difference is we!l outside the experiexcitation of this state should occur above this energy. mental error in establishing the energy scale. Further, To interpret the peaked feature as arising frqm a there is no precedent in which the electron impact exdirect tra&ion to the a’ 3Z+ s&e followed by the citation of an.allowed state exhibits resonance behavior. decay of this a’ state to the ground state is also diffiExcitation functions for various members of the Fourth cult. Fig. 3 coGpares the peak observed here with. the Positive.band system in CO were determined by optical absorption enveldpe for the transition a’ 3Z+ h@mina et al. [ 151 LWhile the authors do not show +X lCfobser&d.by HeFberg and Hugo [ i3J :The.. detail&d threshold behavior, their curves indicate a -. gradual r&from threshold to a maximum intensity at ordinates of the twb curves have &en normaliid at the peak values.. It is clear the..d state p&sses.thrqugh :‘. a:iS ev. E&it&i&oft&e A.% state could account :. ..I -, .._ ~_ ,-. :’ ‘. :’
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for-thelow ihtdns$&ht in’the 9-1 i eV re&sri of .‘.: ~. .‘.. : ..fig:2;,. ::,’ ., ‘-. .’ :: !f thk $-?Z’. state populated-via a resonant state is not ‘&e scource of&e .radiation,.it see&necessar;.&the? to:post@ate an u&nown:[9] -and unpredicted [ 16;. state of CO &?he appropriate energy range, or .tci consider.ihe p&@ity’of photon emission pr&’ ceeding via a long-lived
CO-
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state at the observed
.: energy and-the unstabie 2Eground state of CO-. : Tjris_js the dielectronic attachment process [ 171, ,@e ground state.of CO- has been,observed ;rn electron. scattering.experiments [ 181 and its potentiai energy birve has been estimated by Boness; Hasted, and Larkin .[-Io] . (&hy [20] has shown that -there are excited .states cf Cd-- which have dissociation limits of 9&? eV: [C(3P) + 0-(2P)] and 10.88 eV [C(lD) + ??(2P)], and Stamatovic and Schultz [21] have shown still a third state with the c!issociation limit of 9.84 eV [CA(%) + Cl(3P)]. Both the states leading to ci’ (2P) have their maximum yield near the threshold wj* the kinetic-energy of O- equal to zero [20]. One can therefore assnme that these are not repulsive states snd that there are bound levels of. these states below the dissociation limit which he within the Franck-
Condon region. The fate of these levels is not known. In.order to clarify the nature ofthe peaked feature o@erved here, it would be useful to measure the wavelength. distribution of the emitted~light. A second useful e)rperimmp might be to search for a resonancti in Co h the energy range between 7 and 8 eV. This resonance might be manifested in the inelastic cross section.of the a’ state or in the cross section for vibrationai excitation of the ground state if the (lielectronic attachment process is taking place..
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[l] Iborresteh, Physica 9 (1942) 447. [2] .R.Clanipitt and A:SNe&on, I. Chem. Physl50 (1969). .!997; Uriiwxsity of C&forma, Lawrence Radiation La-. boratory Report UCRL-18032. ]3] W.L.Borst aud E.C.Zipf, Phys. Rev. A3 (1971) 979. 141 G.E.Thomas and F.E.Vogeisberg, Rev. Sci. Instr. 42 (197.1) 161. [S] W.L.Wiese,M.W.Smithand B.M.Miies, Atomic transition probabilities, Vol. 2. Sodium through calcium, Natl. Std. Ref. Data Series,Natl. Bur. Std: (US), NSRDS-NBS22 (1969).
[6] J.Olmsted III, A.S.Netiton and K.Street, J. Chem. Phys. 42 (1965) 2321.
[7] P.H.K.rupenie, The band spectrum of carbon monoxide, Natl.
Std. Ref. Data S&es, NatI. Bur. Std. (US), NSRDS-
NBS 5 (1966). [8] G.E.Hansche, Phys. Rev. 57 (1940) 289. [S] R.S.Freund. to be publsibed: [lo] O.S.Duffendack and G_W.Foi, Astrophys. J. 65 (1927) 214. [ 111 J.H.Moore ad D.W.Robinson, J. Chem. Phys. 48 (1968) 4870. [ 121 H.H.Brongersma and L.J.Oosterhoff. Chem. Phys. Letters 1 (1967) 169. [ 131 G.Herzberg and T.J.Hugo, Can. J. Phys. 33.(1955) 757. [ 14) L.Sanche and G.J.SchuIz, Phys. Rev. Letters 26 (1971) 943.
[IS] M.J.Mumma, E.J.Stone and E.C.Zipf, J. Chem. Phys. 54 (1971)‘2627. [16] S.V.O’Nefiand H.F.Schaeffer III, J. Chem. Phys. 53
(1970) 3994. [ 17 ] H.S.W.Maaey,
Negative
ions,
2nd Ed. (Cambridge Univ.
Press,London, 1950) p_ 43. [18] H.Ehrhardt, L.Langhans, F.Linder and H.S.Taylor, Phys. Rev. 173 (1968) 222. (191 M.J.W.Boness, J.B.Hasted and I.W.Larkm, Proc. Roy. Sot. A305 (1968) 493.
~.
[20] P.J.Chautry, Phys. Rev. 172 (1968) 125. [21] A.Stamatovlc and G.J.Schultz, J. Chem. Phys. 53 (1970) 2663.
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~~MI~~~P~Si&S ::. : . :
: ._.
_.
I
._.._
:
‘- _.
I octob& 1971
.‘.
‘.
: i ;r.
: ~MX”EXCiTA’I”ItjN
FiJNCiXOtiS
~IATIN~AND~A~TABLESTA~S'OFC~ON -: .’
.,,
,.
“. ._
ELJ$~Rt.lN
.‘~
I;ETTERs .
..-,.
i;&t?nce Radahm
--AS. NEWTO&UI~G.E~TH~B&S~~ hwratory,
OF
.-
“-$:
MONGER
-,
‘.
iJn&wsity of Cla&rrn~, Berkefey, California 94720, i/sA’~
.’
Received 19 Juiy 1971 ..
-
Excitation functions nek threshold h&e been me&nreh for tie production of photons and for tie produ&on of l&g-Ikd meiastable molecules from c&on monoxide. An h&resting peaked fkatmi existsat ffie threshold for photon production.
hai’been described 6y ~o~~.~d.~~gelsbarg [4]. It is possible to operate the electron gun in this atoms and mole&es is the eiossed electron and neti& instrument as a high resolution electron source using -. the RPD tecbsiique. However, the photon signal from _ ,beamteclmique. The me&stables produced.by electron impact are detected via their ab-Wyto eject an elecCO vvasvery weak, and it was not possible to obtain an Adaequates&w.+-nob ratio in the RFD mode. tron-from a metal surface placed downstream from the collision region in the path of the neutral beam [I]. Therefore, in thiis wo& th;hetwo-cycle BPD operation of the gun was eliminated and only the f&t cyle of Photons hi&er in energy the the work function of the pulsing scheme prev&usIy described [4] was used, the metal detector can also be measured. By pulsing the electron beam and by using time-of-flight (TOF) The electron beam pulse width was IOpsec, and the techniques, the two signals can be measure@ indepen-:. electmn’ctixrent during the pulse ‘was= 1W6 A. To record the photon curve, the data count gate was ’ dently [2] . A TOF technique usbig a diffuse, rather than a directed gas source has recently. been used by openedlonly during the electron pulse. For the metaBorst. and Zipf )P measure excitation.effIciency curves stable‘curve; the count gate was delayed for 60 psec and was then opened to imttercept the majority of the for metastabie production and to deduce lifetimes of metastabfe states+ CO andN2 [3]. T9.F distribution of the CO metastables. The time T’be molecular,beam al@rratus &rd the electron gun between electron pulses was 1 msec. The CO gas usedused in this study. have been’described previously by w& Cp grade: obtained from the Matbeson Company. Qunpitt and Newton [21. The only signif&nt change :. Its purity was,> 99.3%. Impurities 0,tber than 8% in, t.$e appatatus [email protected] an EMI 9603 Be-&u.ele&~~ ‘wereC 0.1% each. .. t!on mi#iplier was used as the detector:This.multi-, The electron’energy scale was calibrated by running. plikr-had.an effe&vephotoele&c work ftinction of,‘. ‘. +he appmpriate CO, &tie and then,, while still scanning,5.1, eV, measured using a ~~e~t~~u~.~~t squrce..an~ 1. .’ +&lirig~~~&nallhnpurity pf argon to the berim source. -a ~~n~~~~rn~t~~~ T@edi~t~~e.be~ee~ the eleled~tron I$5 data -Coat&gate w@ ~~t~eou~y-opener to _ : ~-.countaigon me_tastables.T& signal strength from the .gun-and., the detector w&,19.7 .&n. The data&&l&i& -. : ‘... ‘_ : : : .”:i .,( : _‘. ,’ .‘,,., ( : -1 . ,’ ;iddecj ‘tigo$vaS $Ihi&tiotipared to that from‘the .. ., ____.._. __-, :,. -. -. ,: . ., ~-_~~~~~r~~~~~:~~~~~ ule auaices ~~~~~~~~~~~~~~’ : ~1.- C? ,$haQs&tr@$f threshold~cduld be-superimposed ‘: E&ergqi@m&$~e&~~; ,-:_-’ :._:-.i,,. .,\._‘l’,,, .,,, 1.. ::‘:l. . :: 09 thi=CO~~nre.withinl3C.sec~of,adiling the ar&n to ’ ’ -- th&,&iainA~,tiai &&aq&3 asgi&:,that potential .;_ .t.~~~~1’~d~6:Philipi~e~~h Laborat+ies;E&d&oii,’ ‘. -., One ~c~que for obtaining electron impact excitation functions for long-lived metastable states of
.
system
2.1Fe,Neth;er+n+, -. .iy.;I_.-. .I.:.., ,:-:::, :i ;. -:,Q,,‘.::y:-.. _:.:.’ .- .-dri&‘&t+iidt’o&i$,ki f&j &oti,the;!J&g&* .: : f , : . : .. .’ .. :‘_ ._’ .‘: ..:, ,,_,.‘/’ :*.-,:‘: ..., :,:;_ __ -..y.: .;...-.._ ,.; :.__’ : ,._~ _.:.:.: __ _:. :.I...’1: -:_.: .. .., ,, ;_.:. ._,,-’ .,. --. ._;,_ ” .:..:. ‘.:.. ,:. .I ._: _ I ‘: ., I _: :‘ ., - .:-. ., -, .;;y,,:..,., ‘_: : ._ :,- __ :. .,;,-. ‘J7l . . . .:::i: ... _..-_.,,. ,,.:, .;_ :_*:I’..., .-. _. _..._-:._: -.- ;:. -...‘..__:_-.:,. ,. - , . _ ‘..-. . ._...I ..- : ..‘, 1.’. .’ -..__;‘:.,; .’ _, ;.:. ., ~,.. -3 I :.- .,._ _.. ... ,‘._::F., ._: ..,. .‘,, .,,-,.:y_, .:*:‘_. .,..I ;,,., -,‘:_, .,;-.. ‘;._.. .,I ;.... -.... ‘.... :’ : -._ .-,. <. ‘<...*,’:; ., ,: :.y . .. _ ?_.‘_. .. ., -. , ::~_ . ..::.,.__. .. :. ,:_.:,,._.. ,...I -,.“-‘: :,, ..‘._ , _ I ,:I.’ ,. ,_ j__ .a: ..;::... -._,, : ‘:.... ,:__; .: :; -./-.. . 2:. _ _- ,_..--I . ,. .: .. -. .:.. .... ..:..r:
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_
function for’pzoduciion of ih@ived meta-
t&ken’to be I I-62 eV l[S] _It is es&&ed that the energy s&e established in this’fashion, :., ,.Gti aticurate’to,50.2 eV: : :.Fig,-I- ahoiws the excitation function for metastable :stat$s produced by election impact on CO. ‘l$e.excifati@t&r+@d was found to he S-9 eV as cgmpared to +e spectroscopic .vaiue bf 6.01 N. A3I curves re-’ cord&d in this study showed the same steady rise to a pl+e%u. it approximately JO eV..A reproducibli fea-: ture,wa$ the sma.lIincrease in signal near 13 eV. The shape of the tiurve agrees quite well with tbeWtial .p$ti& if 9 &et&tabie CO c&-e recently published b$ Bars! arid.iipf f3] _It is, however, quite different :&on ~~,~u~e.rne~~~ed by C&nsted, Newton, tid Siree$ [6I . ,Referring t0 %&penie’s compiiatjon of Spe$rpscopic @ata on CO [?] , the’&ly tioivn state *hi& c&d contribute to the me&stable sig&l in the . region- ~e~at~l~ above 6 eV is the a %I state. m3s
Fig. 2: Excitafion %urictionfor the producti0.nof prompt photons of energy > 5.1 eV from CO. _
Fig, 2 shows the excitation fun&m obtained for the production of pro&pt.radiation of energy@eater than 5.1 eV frcim’C0. Ari unusual feature of this ctifle is the sharp peak qb&ved at threshold. ,me onset of, the curve is at 7.4 eV’and t&peak occurs at 8.3 eV. The width at half height of the peak is 9.9 eV, including the broadening caused by the energy spread of the el&trons f&m the thdriate’d iridium fiament: There is a second strong emission threshold at. * 11 eV. It is_. &o-apparejnt that there is weak emission below 11 eV which is probably not connected with the pe&ked feature, but the threshold of ihis emission is obscured. It is not possible in the present ~st~ment to measure the wavelength dis&jbution of the emitted light. -. T&e stiong emission above 11 ‘eV can have s.&eral ]sources:D&ffend&ck and Fox [lo] observed radiation from the,?3X. system of CO (c 3Cf -+ a 311}.The upper state of this system has ari excitatiqn potential of.I~~:l-eV;~l~]. .At least ti‘portion &fth@ b&its& te&.is ~tifticiently en&=getic to b&.detected here. Oth&i t+pl& leveis. in &is energy range &i&t decay in a similti fashion. A&her possible &&rce of the ra&iation
Volume 1 I, number 2
1 Octobei 1971
CHEMICAL PHYSICS LE’kTERS
the Frar&k&ndon regioc in the appropriate energy range. However,.the width of the electron impact peak compared to the optical envelope ti&&icious. It would be expected that the eIect&*&pact peak would be broadened if direct excitation were involved. The electron ‘impact excitation functions for the individual vibrc;nic levels are not expected to be extremely sharp for direct excitation, even in the case of a triplet level. Secondly, the electron impact data must be tierently broadened by the energy spread of the exciting electrons. An alternative that is consist@ with the present data is that selected vibrational IeveIs of the a’ state are being populated via a resonant state. This could certainly account for the narrow width of the excitation functltin. Sanche and Schulz [ 141 have recently searched,for resonances in CO in an eleccon transmission experiment. A resonant state which may be Fig. 3. Excitation Function For the production of prompt photons from CO (dashed curve) compared to photon abresponsible for populating the b 3Cc state of CO was sorption intensities for the forbidden a’ 3 Z’ + X xZ+ tiansifound near 10.4 eV. it would be necessary to postulate tion (points From rcF. 1131). a similar resonant state (undetected in the experiment of Sanche and Schulz) for populating the a’ state. If case of the B state. The trapped electrcn spectrum of it is assumed that the a’ state is indeed the source of the light, some rough conclusions could be drawn reBrongersma and Oosterhoff [ 121 indicates that at least some of the states in this energy region are exgarding its lifetime and modes of de-excitation. This cited by electron impact at threshold. Definite constate can radiate to the metastable a 311via a dipoleclusions regarding the source(s) of the light cannot be allowed transition (the Asundi System). No radiations clrawri without knowledge of the wavelength distribudirectly to the X 1X+ sound state have been obser&, tion. The present.observations would demand that the The sharp peak at 8.3 eV in the photon excitation branching ratio for depopulation of the a’ state by efficiency curve is somewhat difYicult to explain..The direct radiation to the ground state be an appreciable only known states of CO accessible to Franck-Condon fraction of the total population. The lifetime of the a’ transition below 8 eV are the a %I and the a’ 3Z+ [7]. state must be less than, or of the same order of magniThe a 3il state can be eliminated from consideration tude as, the 10 WC observation time used here. on the basis of its long radiative lifetime [3]. its exciAn improbable source of the !ight is by direct excitation function should resemble that of CO in fig. 2. tation to the A ln state which radiates to the ground Furthermore, this state becomes inaccessible to a state (Fourth Positive bands). The threshold for this Franck-Condon transition from the ground state state is at 8.03 eV, well above the observed thresho!d above 7 eV and no new phenomena pieceded by direct of 7.4 eV. This difference is we!l outside the experiexcitation of this state should occur above this energy. mental error in establishing the energy scale. Further, To interpret the peaked feature as arising frqm a there is no precedent in which the electron impact exdirect tra&ion to the a’ 3Z+ s&e followed by the citation of an.allowed state exhibits resonance behavior. decay of this a’ state to the ground state is also diffiExcitation functions for various members of the Fourth cult. Fig. 3 coGpares the peak observed here with. the Positive.band system in CO were determined by optical absorption enveldpe for the transition a’ 3Z+ h@mina et al. [ 151 LWhile the authors do not show +X lCfobser&d.by HeFberg and Hugo [ i3J :The.. detail&d threshold behavior, their curves indicate a -. gradual r&from threshold to a maximum intensity at ordinates of the twb curves have &en normaliid at the peak values.. It is clear the..d state p&sses.thrqugh :‘. a:iS ev. E&it&i&oft&e A.% state could account :. ..I -, .._ ~_ ,-. :’ ‘. :’
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CH33iICAL PHYFICS iEli-El+
for-thelow ihtdns$&ht in’the 9-1 i eV re&sri of .‘.: ~. .‘.. : ..fig:2;,. ::,’ ., ‘-. .’ :: !f thk $-?Z’. state populated-via a resonant state is not ‘&e scource of&e .radiation,.it see&necessar;.&the? to:post@ate an u&nown:[9] -and unpredicted [ 16;. state of CO &?he appropriate energy range, or .tci consider.ihe p&@ity’of photon emission pr&’ ceeding via a long-lived
CO-
‘3
-,
._
state at the observed
.: energy and-the unstabie 2Eground state of CO-. : Tjris_js the dielectronic attachment process [ 171, ,@e ground state.of CO- has been,observed ;rn electron. scattering.experiments [ 181 and its potentiai energy birve has been estimated by Boness; Hasted, and Larkin .[-Io] . (&hy [20] has shown that -there are excited .states cf Cd-- which have dissociation limits of 9&? eV: [C(3P) + 0-(2P)] and 10.88 eV [C(lD) + ??(2P)], and Stamatovic and Schultz [21] have shown still a third state with the c!issociation limit of 9.84 eV [CA(%) + Cl(3P)]. Both the states leading to ci’ (2P) have their maximum yield near the threshold wj* the kinetic-energy of O- equal to zero [20]. One can therefore assnme that these are not repulsive states snd that there are bound levels of. these states below the dissociation limit which he within the Franck-
Condon region. The fate of these levels is not known. In.order to clarify the nature ofthe peaked feature o@erved here, it would be useful to measure the wavelength. distribution of the emitted~light. A second useful e)rperimmp might be to search for a resonancti in Co h the energy range between 7 and 8 eV. This resonance might be manifested in the inelastic cross section.of the a’ state or in the cross section for vibrationai excitation of the ground state if the (lielectronic attachment process is taking place..
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1 October 1971 . ..: L11 : ._.
[l] Iborresteh, Physica 9 (1942) 447. [2] .R.Clanipitt and A:SNe&on, I. Chem. Physl50 (1969). .!997; Uriiwxsity of C&forma, Lawrence Radiation La-. boratory Report UCRL-18032. ]3] W.L.Borst aud E.C.Zipf, Phys. Rev. A3 (1971) 979. 141 G.E.Thomas and F.E.Vogeisberg, Rev. Sci. Instr. 42 (197.1) 161. [S] W.L.Wiese,M.W.Smithand B.M.Miies, Atomic transition probabilities, Vol. 2. Sodium through calcium, Natl. Std. Ref. Data Series,Natl. Bur. Std: (US), NSRDS-NBS22 (1969).
[6] J.Olmsted III, A.S.Netiton and K.Street, J. Chem. Phys. 42 (1965) 2321.
[7] P.H.K.rupenie, The band spectrum of carbon monoxide, Natl.
Std. Ref. Data S&es, NatI. Bur. Std. (US), NSRDS-
NBS 5 (1966). [8] G.E.Hansche, Phys. Rev. 57 (1940) 289. [S] R.S.Freund. to be publsibed: [lo] O.S.Duffendack and G_W.Foi, Astrophys. J. 65 (1927) 214. [ 111 J.H.Moore ad D.W.Robinson, J. Chem. Phys. 48 (1968) 4870. [ 121 H.H.Brongersma and L.J.Oosterhoff. Chem. Phys. Letters 1 (1967) 169. [ 131 G.Herzberg and T.J.Hugo, Can. J. Phys. 33.(1955) 757. [ 14) L.Sanche and G.J.SchuIz, Phys. Rev. Letters 26 (1971) 943.
[IS] M.J.Mumma, E.J.Stone and E.C.Zipf, J. Chem. Phys. 54 (1971)‘2627. [16] S.V.O’Nefiand H.F.Schaeffer III, J. Chem. Phys. 53
(1970) 3994. [ 17 ] H.S.W.Maaey,
Negative
ions,
2nd Ed. (Cambridge Univ.
Press,London, 1950) p_ 43. [18] H.Ehrhardt, L.Langhans, F.Linder and H.S.Taylor, Phys. Rev. 173 (1968) 222. (191 M.J.W.Boness, J.B.Hasted and I.W.Larkm, Proc. Roy. Sot. A305 (1968) 493.
~.
[20] P.J.Chautry, Phys. Rev. 172 (1968) 125. [21] A.Stamatovlc and G.J.Schultz, J. Chem. Phys. 53 (1970) 2663.
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