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Laser-induced fluorescence spectroscopy of an N+2-ion beam
Volume
77, number 3
CHEMICAL
LASER-INDUCED A DING,K
FLUORESCENCE
Berlm
SPECTROSCOPY
LETTERS
1 February
OF AN N;-ION
1981
B:AM
RICHTER
Halrrl-dlelrrler-ltlstrlllr D-1000
PHYSlCS
39,
fur
KernJorsdr~rng
Bcrlm
CmbH
Berc~ch
StralAwciremrr,
West German)
and M MENZINGER of Chem!str~,
i3rr11ernr_t of Toronto.
Recelred
in fiial form 10
Departmetlr
18 August
1980,
Toronto.
Omarro.
Carlada
M.5S
I.41
h’ovembcr 1980
The mternal state dwrlbution of an N;-ion beam whuzh has been produced in a phsma eon source has been determmed b) Liser-Induced fluorescence The results show a marked devlatlon from a thermal Boltzmann dlsrrlbutlon Ion-beam denSIU~Sof less than 500 tons/cm 1x1n pxtlcular wbratron-rotation state have been measured
I. Introduction hlolecular eons play an important role m collusion processes m the gas phase, particularly m the upper armosphere or m a plasma The dlfferentlal colhslon cross sections of such processes often depend sensetlvely on the Internal excitation of the partners whch IS usually not known Laboratory experiments show that It IS hardly possible to produce an ion beam consisting of only one Internal state. Non-spectroscopic techruques to measure the Internal dlstnbutlon are not unique and m general rather inaccurate [ 1] A variety of optlcal methods have been devlsed to determme the excitation of molecular ions Conventlonai methods of enusslon spectroscopy m the vlslble [2] , infrared [3] and mlcrowave range [4] are lumted by the mavoidable space charge effects of molecular ions Laser-Induced evcltatlon [S] and fluorescence methods [6-91 have been recently developed, and seem to combme highest resolution with sufficient senutlvity. The euperlments, which are presented here, describe the measurement of the ground-state vlbratlonrotation dlstrlbutlon of an Ni-ion team In spite of the fact that Nq molecular Ions have been observed m bulk by laser-induced fluorescence and one example 0 009-26
14/8 l/0000-0000/$02
50 0 North-Holland
of a true ion-beam measurement (for CO+ Ions) has been pubhshed the combmatron of both has, to our knowledge, not been performed yet Because of the totaLly different method of productIon of molecular ions in a gas discharge, our results differ slgruficantly from those published by Alhson et al [7] and Dagdlglan and Doermg [8] m that we find slgruflcant devlatlons from a thermal Boltzmann dlstrlbutlon
2. Experimental Fig 1 &plays a schematic view of the experiment Ni ions are formed In a plasma ion source [lo] The ions pass a three-element ekctrostatlc emzellens, are deflected around 90” in a magnetic mass analyzer (eqmpped with permanent magnets) and then are lead - coavlally to the laser beam - throtigh another electrostatlc lens mto the observation range where the laser-mduced fluorescence could be detected Finally, they were deflected onto an Ion collector where they were monitored durmg the course of the euperunent. The purpose of the magnet was not only to mass select the Ion beam, but also to let the laser beam,vlhlch runs in a &rectlon opposite to the ions, pass freely through the chamber. For uutlal adjustment a movable ion detector could be shd into the beam Just beF’ubhshmg
Company
523
Volume
17, number
CHEMICAL
3
1 I-ebrurrry 1981
PH\ SLCS LETTERS
DEFLECTION
DEFLECTION MAGNET .
OPTICAL WINDOW /
wobos HORN DYE-LASER
rg
fore
I t\peru~~cnr~l arrangerrcnt
the obwrvatlon
reglon
This enabled
lor rhr ~nvest~atton
us to meas-
ure the total eon beam current and also its geometrlcal propertles from the current dlstrlbutlon over the three lerses Typlcnl currents were 10-6-10-7 A
which could be measured wlthm a dlnmeter ot 20 (current 11 and I-,) and 3 mm (current 1;), respectively Photons enujted from the observation volume (= 50 mm long. multlpher
3 mm diameter) were focused onto a phototube (RCA 1840) by a spherical mirror
(I 50 mm diameter,
300 mm curvature) This proceplaced optlcal stops along the laser and the detector path enabled us to reach an extremely lugh signal-to-background ratlo No additlonal filters have been used Typical ion energies for Nlf were 800 eV (determmed by the magnetic field of the magnetic mass analyser), which corresponds to a hnear Ion density of 5 X 10J ions/cm A tunable dye laser combmatrcn consrstmg ofnn osclllatol and one amplifier stage (Lambda Physk FL 2000) was employed The laser was pumped by a pulsed N, laser (Lambda Physik MlOOO) The dye used dure
was
together
wrth
carefully
BBQ (4’,4”-bls(butylacetyloxy)
quatrophenyl) of the laser was 0 03 nm, typlcal
The bandwidth repetlon rates were 180 pps at pulse widths of 10 ns The laser beam entered the vacuum chamber through a quartz window after passmg a set of adJustable mu-
of the Iacr-Induced
fluorescence
of on eon bram
rors and left It through an optlcal
wmdow posnroned under the Brewster angle The photomultlpher slgnal was preamplfled, pulse shaped and gated by 3 laser sync!lromzed pulse (50 ns delay. 80 ns wdth) and subsequently counted In a binary counter TIE whole euperu-nent was controlled by a microcomputer m connectlon with a CAMAC Interface (Standard Engmeermg Corporation MIK 11) Tlus
enabled us to do long-term ekperlments (up to several hours), where laser and electroruc equipped were driven by the computer An automatic rescan m relatively short time intervals and savmg of all recorded partral spectra greatly reduced the effect of electrlcal dlsturbances
3 Results
and discussion
FIN 2a shows a typical
emlsslon This corresponds
result
for N~(‘Z~--‘~~)
to a yield of 1 multlpher
pulse for 20 laser pulses. Measurmg time was 150 s per point The background produced by metastable (NC)* ions (z 30 pulses/s during the 80 ns gate time) and the stray radlatlon from the laser Itself (maxmlum of 20 pulses/s at 380 nm) has been subtracted from the measured spectrum. The rates in fig 1 are normalized
Volume
77, number
N;
CHEVICAL
3
(B*ZZ;
-
X *Z;
PHYSICS
LETTERS
1 February
1981
)
30 LASER-INDUCED
FLUORESCENCE
t
387.9 388.L 388.9 38?3L 389,9 39Cl.L 3909
391.4
Alnml
2. lnml
(a)
(b)
FIN 2 (a) Excltatlon
spectrum of h’; ground-state 1011sproduced m a plasma Ion source (discharge current Idrs = 100 mA, dlsfor the Doppler effect of the fast charge volrage ‘ids = 50 V, Ion source pressure ps = 0 3 Torr) The spectrum has been corrected NG molecules, which amounts to a stift of 0 94 X at an mn energ, of 800 eV towards longer waveleng.th (b) Comparison between the measured top and nmulated bottom spectrum ar N; The measured spectrum IS corrected for the wavelength-dependent user mrenstb Bars are rocluded to denote the mean statlstlcal error The simulated spectrum HIS calculated assunung a laser hne wldrh of 0 5 4 and a rotational Bolt?nlann dlstrlbutmn of T = 650 t.
to 1 s acceptance tune per point measured. There 1s a srgmficant amount of I\i;(‘XL, u = 1) In the beam (> 10%). which !las been estimated from the (1 ,l) band head using Franch-Condon factors calculated by Brandt [ 1 1 ] Its rotatlonal dlstrlbutlon IS hard to analyze because of overlap with the (0,O) transition A more accurate study would have to use the stronger (0,l) transnlon near 427 8 nm [ 123 Devlatlons from a room-temperature Boltzmann dlstnbutlon appear much more pronounced than In the work of refs [7,8] In order to quantrtatlvely analyze the data a spectrum has been simulated and compared with the experlmental values, varying lmewidth and temperature of the theoretlcal dlstrlbutlon (cf fig.?b) The evperunentai spectrum has been corrected for the wavelength-dependent mtensrty of the laser The best resulrs were obtamed using a temperature of 650 K and a hnewldth of OS w (fwhm, gausslan shape)
Nevertheless, no optmal
fit to the expernnental
data
could be obtained with such a method. Fig 3 shows a logarithmic plot of the rotatronal state population dlvlded by the statistical weights of the rotatronal and nuclear levels versus rotatlonal energy [aJ(J + I)] It is evident that a thermal
NT Rotational
0
0
100
200
300 N”(N’+lI
Temperoture
LOO
500
;‘o
6C
Frg 3 Plot of log [I/(W’ +A”” + 1)K J versus the rotarlonal energy [a hr”(N” + 1) J I IS the mtenslty In arbttrary uruts, IV’ the rotatlonai quantum number of the N; B stare, Iv” the IV btlowl quantum number of the N; ground state K (A “) IS the nuclear sta.tlstxal werght of the ground-state rotatIona Ievets (A’ = 1 I2 for N” even. K = 1 for N” odd) The bars denote the mean statlstrcal error
Boltzmann dlstrlbutlon (whxh would yield a srrarght hne) does not describe the data properly, especrally the differences between even and odd rotational levels, even if one takes mto account statistical errors and noise. 525
Volume
77. number
3
CHEMICAL
PHYSICS
These fmdlngs could be reprodlrced III several reruns of the spectrun~ Possible e\planatlons for rh!s effect are colhslonal processes 111the observatton regton or dewatlons from a Boltzmann dlstrlbutlon of the nuclear spins, due to colhslonal processes m the 10n source
The most sgruhcant
1 February
1981
Acknowledgement One of us (hlhl) would hke to thank the German Academtc Exchange Serwe (DAAD) and the Hahnhleltner-lnstltut for fmanclal support
example of the former ones IS
the symmetric charge ewhange (Ng + N, + N2 + h’s) which yields slow IV; Ions, whose spectrwn IS shlfted bb = 0 94 X against that of the fast Ni molecules because of the Doppler effect Therefore an overlap occurs of the hnes of even rotntlonal levels of the fast h: wth !he 11112s of the odd levels ot the charge e\cknged Ni, wkch results m sn apparent non-BoltzInann dlstrlbutlon [ 121 Wtttl background pressures In the range of lo- 7 Torr IL seem;, however. very unI~hcly to attribute the effect totally to charge exchange colhslons We therefore favour the e\planatlon that these dewtions are caused by a non-8oltzmar-m dlstrrbutlon of the nuclear spms, which could be due to reactive processes (dlssocmtlon and recomblnatlon) In the pkwna ton source Such effects have been reported [7] This conJecture IS supported by the observntlon that there IS d large amount ot long-hved metastable tons m the beam, which appear as a dc bachground signal (npproxnnately on the order of 1 X 1O-8 A) This fact and the iugli rotatlonal temperature also ekplam the ION observed mtensltles of the different rotation-lqbratlon lines From the wdth of the rowtonal dlstrlbutlon and the mtenstty of the metastable lons one cnlcul~tes an ton density of 500 Ions/cm (nccordmg to IOH A) as an upper bound for the detectablllty of the molerular 1011s m one particular state The observation that saturdtlon has not yet occurred suggests that It may be possible to Improve the sensltlvity of this method so that one could use It for anal>71ng products formed In Ion-molecule redctlons
526
LC-I-I’ER.5
References J A Rutherford and D hl J Compton. J Chem Phvs 48 (1968) 1602 121 J H hloore and J P Doermg. Phys Rev 17-I (1968) 178. Ch OttmFer, m Electromc and atomic colhslons, ed C Watch (North-Holland, Amsterdam. 1978) p 639, H H Hnrrls and J J Leventhnl, J Chem Phks 64 (1976) 3185
ItI B R Turner,
VI 9 Dm,e, Faraday Dlscussmns Chem Sot 67 (1979) 353, J Dmg, A Redpath and U Stclrunetzger, to be pubhshed 131 T A DL.on and R C N’oods. Ph>s Reb Letters 34 (1975) 61 R J Saykallp,
T A DLxon. T G Anderson, P-G Szanto and R C Woods. Astrophls J 205 (1976) Ll 151 W H ii tng. G A Ruff, W E Lamb and J J Spezeskb Phls
Reb Letters
36 (1976)
[6 1 P C Engelhmg and A L Smith.
[7]
1488 Chem
Phls Letters 36 (1975) 11, T A hltller and V C Bondybey, C’hem Phys Letters 50 (1977) 275. V E Bondvbey and T A hlffler, J Chem Phys 67 (1977) 1790, F W Grwmann, B H hlnhan and A O’Keefe, J Chem Phys , to be pubhshcd J Ailnon, T. Kondow and R N Zare, Chcm Ph>s Let-
ters 64 ( 19 79) 202 P J Dagdetan and J P Doermg, Chem Phys Letters 64 (1979) 7-00 [9] R D Bro\\n, P D Godfrey, J C Crofts, Z Nmkov and S Vncum, Chem Phqs Letters 62 (1979) 195. [ 101 H P Wc~se, H U hltttmann, A Ding and A Hengletn. Z Naturforsch 26a (1971) t 112, hi hlenzmger and L Wzihhhn. Rev SCI lnar 40 (1969) 102 [ 111 D Brandt, Ph D Thew., Uruvcrslty of Gottmgen (1979) [ 121 A Dmg und K Rxhter, to be publaneZ [S]
77, number 3
CHEMICAL
LASER-INDUCED A DING,K
FLUORESCENCE
Berlm
SPECTROSCOPY
LETTERS
1 February
OF AN N;-ION
1981
B:AM
RICHTER
Halrrl-dlelrrler-ltlstrlllr D-1000
PHYSlCS
39,
fur
KernJorsdr~rng
Bcrlm
CmbH
Berc~ch
StralAwciremrr,
West German)
and M MENZINGER of Chem!str~,
i3rr11ernr_t of Toronto.
Recelred
in fiial form 10
Departmetlr
18 August
1980,
Toronto.
Omarro.
Carlada
M.5S
I.41
h’ovembcr 1980
The mternal state dwrlbution of an N;-ion beam whuzh has been produced in a phsma eon source has been determmed b) Liser-Induced fluorescence The results show a marked devlatlon from a thermal Boltzmann dlsrrlbutlon Ion-beam denSIU~Sof less than 500 tons/cm 1x1n pxtlcular wbratron-rotation state have been measured
I. Introduction hlolecular eons play an important role m collusion processes m the gas phase, particularly m the upper armosphere or m a plasma The dlfferentlal colhslon cross sections of such processes often depend sensetlvely on the Internal excitation of the partners whch IS usually not known Laboratory experiments show that It IS hardly possible to produce an ion beam consisting of only one Internal state. Non-spectroscopic techruques to measure the Internal dlstnbutlon are not unique and m general rather inaccurate [ 1] A variety of optlcal methods have been devlsed to determme the excitation of molecular ions Conventlonai methods of enusslon spectroscopy m the vlslble [2] , infrared [3] and mlcrowave range [4] are lumted by the mavoidable space charge effects of molecular ions Laser-Induced evcltatlon [S] and fluorescence methods [6-91 have been recently developed, and seem to combme highest resolution with sufficient senutlvity. The euperlments, which are presented here, describe the measurement of the ground-state vlbratlonrotation dlstrlbutlon of an Ni-ion team In spite of the fact that Nq molecular Ions have been observed m bulk by laser-induced fluorescence and one example 0 009-26
14/8 l/0000-0000/$02
50 0 North-Holland
of a true ion-beam measurement (for CO+ Ions) has been pubhshed the combmatron of both has, to our knowledge, not been performed yet Because of the totaLly different method of productIon of molecular ions in a gas discharge, our results differ slgruficantly from those published by Alhson et al [7] and Dagdlglan and Doermg [8] m that we find slgruflcant devlatlons from a thermal Boltzmann dlstrlbutlon
2. Experimental Fig 1 &plays a schematic view of the experiment Ni ions are formed In a plasma ion source [lo] The ions pass a three-element ekctrostatlc emzellens, are deflected around 90” in a magnetic mass analyzer (eqmpped with permanent magnets) and then are lead - coavlally to the laser beam - throtigh another electrostatlc lens mto the observation range where the laser-mduced fluorescence could be detected Finally, they were deflected onto an Ion collector where they were monitored durmg the course of the euperunent. The purpose of the magnet was not only to mass select the Ion beam, but also to let the laser beam,vlhlch runs in a &rectlon opposite to the ions, pass freely through the chamber. For uutlal adjustment a movable ion detector could be shd into the beam Just beF’ubhshmg
Company
523
Volume
17, number
CHEMICAL
3
1 I-ebrurrry 1981
PH\ SLCS LETTERS
DEFLECTION
DEFLECTION MAGNET .
OPTICAL WINDOW /
wobos HORN DYE-LASER
rg
fore
I t\peru~~cnr~l arrangerrcnt
the obwrvatlon
reglon
This enabled
lor rhr ~nvest~atton
us to meas-
ure the total eon beam current and also its geometrlcal propertles from the current dlstrlbutlon over the three lerses Typlcnl currents were 10-6-10-7 A
which could be measured wlthm a dlnmeter ot 20 (current 11 and I-,) and 3 mm (current 1;), respectively Photons enujted from the observation volume (= 50 mm long. multlpher
3 mm diameter) were focused onto a phototube (RCA 1840) by a spherical mirror
(I 50 mm diameter,
300 mm curvature) This proceplaced optlcal stops along the laser and the detector path enabled us to reach an extremely lugh signal-to-background ratlo No additlonal filters have been used Typical ion energies for Nlf were 800 eV (determmed by the magnetic field of the magnetic mass analyser), which corresponds to a hnear Ion density of 5 X 10J ions/cm A tunable dye laser combmatrcn consrstmg ofnn osclllatol and one amplifier stage (Lambda Physk FL 2000) was employed The laser was pumped by a pulsed N, laser (Lambda Physik MlOOO) The dye used dure
was
together
wrth
carefully
BBQ (4’,4”-bls(butylacetyloxy)
quatrophenyl) of the laser was 0 03 nm, typlcal
The bandwidth repetlon rates were 180 pps at pulse widths of 10 ns The laser beam entered the vacuum chamber through a quartz window after passmg a set of adJustable mu-
of the Iacr-Induced
fluorescence
of on eon bram
rors and left It through an optlcal
wmdow posnroned under the Brewster angle The photomultlpher slgnal was preamplfled, pulse shaped and gated by 3 laser sync!lromzed pulse (50 ns delay. 80 ns wdth) and subsequently counted In a binary counter TIE whole euperu-nent was controlled by a microcomputer m connectlon with a CAMAC Interface (Standard Engmeermg Corporation MIK 11) Tlus
enabled us to do long-term ekperlments (up to several hours), where laser and electroruc equipped were driven by the computer An automatic rescan m relatively short time intervals and savmg of all recorded partral spectra greatly reduced the effect of electrlcal dlsturbances
3 Results
and discussion
FIN 2a shows a typical
emlsslon This corresponds
result
for N~(‘Z~--‘~~)
to a yield of 1 multlpher
pulse for 20 laser pulses. Measurmg time was 150 s per point The background produced by metastable (NC)* ions (z 30 pulses/s during the 80 ns gate time) and the stray radlatlon from the laser Itself (maxmlum of 20 pulses/s at 380 nm) has been subtracted from the measured spectrum. The rates in fig 1 are normalized
Volume
77, number
N;
CHEVICAL
3
(B*ZZ;
-
X *Z;
PHYSICS
LETTERS
1 February
1981
)
30 LASER-INDUCED
FLUORESCENCE
t
387.9 388.L 388.9 38?3L 389,9 39Cl.L 3909
391.4
Alnml
2. lnml
(a)
(b)
FIN 2 (a) Excltatlon
spectrum of h’; ground-state 1011sproduced m a plasma Ion source (discharge current Idrs = 100 mA, dlsfor the Doppler effect of the fast charge volrage ‘ids = 50 V, Ion source pressure ps = 0 3 Torr) The spectrum has been corrected NG molecules, which amounts to a stift of 0 94 X at an mn energ, of 800 eV towards longer waveleng.th (b) Comparison between the measured top and nmulated bottom spectrum ar N; The measured spectrum IS corrected for the wavelength-dependent user mrenstb Bars are rocluded to denote the mean statlstlcal error The simulated spectrum HIS calculated assunung a laser hne wldrh of 0 5 4 and a rotational Bolt?nlann dlstrlbutmn of T = 650 t.
to 1 s acceptance tune per point measured. There 1s a srgmficant amount of I\i;(‘XL, u = 1) In the beam (> 10%). which !las been estimated from the (1 ,l) band head using Franch-Condon factors calculated by Brandt [ 1 1 ] Its rotatlonal dlstrlbutlon IS hard to analyze because of overlap with the (0,O) transition A more accurate study would have to use the stronger (0,l) transnlon near 427 8 nm [ 123 Devlatlons from a room-temperature Boltzmann dlstnbutlon appear much more pronounced than In the work of refs [7,8] In order to quantrtatlvely analyze the data a spectrum has been simulated and compared with the experlmental values, varying lmewidth and temperature of the theoretlcal dlstrlbutlon (cf fig.?b) The evperunentai spectrum has been corrected for the wavelength-dependent mtensrty of the laser The best resulrs were obtamed using a temperature of 650 K and a hnewldth of OS w (fwhm, gausslan shape)
Nevertheless, no optmal
fit to the expernnental
data
could be obtained with such a method. Fig 3 shows a logarithmic plot of the rotatronal state population dlvlded by the statistical weights of the rotatronal and nuclear levels versus rotatlonal energy [aJ(J + I)] It is evident that a thermal
NT Rotational
0
0
100
200
300 N”(N’+lI
Temperoture
LOO
500
;‘o
6C
Frg 3 Plot of log [I/(W’ +A”” + 1)K J versus the rotarlonal energy [a hr”(N” + 1) J I IS the mtenslty In arbttrary uruts, IV’ the rotatlonai quantum number of the N; B stare, Iv” the IV btlowl quantum number of the N; ground state K (A “) IS the nuclear sta.tlstxal werght of the ground-state rotatIona Ievets (A’ = 1 I2 for N” even. K = 1 for N” odd) The bars denote the mean statlstrcal error
Boltzmann dlstrlbutlon (whxh would yield a srrarght hne) does not describe the data properly, especrally the differences between even and odd rotational levels, even if one takes mto account statistical errors and noise. 525
Volume
77. number
3
CHEMICAL
PHYSICS
These fmdlngs could be reprodlrced III several reruns of the spectrun~ Possible e\planatlons for rh!s effect are colhslonal processes 111the observatton regton or dewatlons from a Boltzmann dlstrlbutlon of the nuclear spins, due to colhslonal processes m the 10n source
The most sgruhcant
1 February
1981
Acknowledgement One of us (hlhl) would hke to thank the German Academtc Exchange Serwe (DAAD) and the Hahnhleltner-lnstltut for fmanclal support
example of the former ones IS
the symmetric charge ewhange (Ng + N, + N2 + h’s) which yields slow IV; Ions, whose spectrwn IS shlfted bb = 0 94 X against that of the fast Ni molecules because of the Doppler effect Therefore an overlap occurs of the hnes of even rotntlonal levels of the fast h: wth !he 11112s of the odd levels ot the charge e\cknged Ni, wkch results m sn apparent non-BoltzInann dlstrlbutlon [ 121 Wtttl background pressures In the range of lo- 7 Torr IL seem;, however. very unI~hcly to attribute the effect totally to charge exchange colhslons We therefore favour the e\planatlon that these dewtions are caused by a non-8oltzmar-m dlstrrbutlon of the nuclear spms, which could be due to reactive processes (dlssocmtlon and recomblnatlon) In the pkwna ton source Such effects have been reported [7] This conJecture IS supported by the observntlon that there IS d large amount ot long-hved metastable tons m the beam, which appear as a dc bachground signal (npproxnnately on the order of 1 X 1O-8 A) This fact and the iugli rotatlonal temperature also ekplam the ION observed mtensltles of the different rotation-lqbratlon lines From the wdth of the rowtonal dlstrlbutlon and the mtenstty of the metastable lons one cnlcul~tes an ton density of 500 Ions/cm (nccordmg to IOH A) as an upper bound for the detectablllty of the molerular 1011s m one particular state The observation that saturdtlon has not yet occurred suggests that It may be possible to Improve the sensltlvity of this method so that one could use It for anal>71ng products formed In Ion-molecule redctlons
526
LC-I-I’ER.5
References J A Rutherford and D hl J Compton. J Chem Phvs 48 (1968) 1602 121 J H hloore and J P Doermg. Phys Rev 17-I (1968) 178. Ch OttmFer, m Electromc and atomic colhslons, ed C Watch (North-Holland, Amsterdam. 1978) p 639, H H Hnrrls and J J Leventhnl, J Chem Phks 64 (1976) 3185
ItI B R Turner,
VI 9 Dm,e, Faraday Dlscussmns Chem Sot 67 (1979) 353, J Dmg, A Redpath and U Stclrunetzger, to be pubhshed 131 T A DL.on and R C N’oods. Ph>s Reb Letters 34 (1975) 61 R J Saykallp,
T A DLxon. T G Anderson, P-G Szanto and R C Woods. Astrophls J 205 (1976) Ll 151 W H ii tng. G A Ruff, W E Lamb and J J Spezeskb Phls
Reb Letters
36 (1976)
[6 1 P C Engelhmg and A L Smith.
[7]
1488 Chem
Phls Letters 36 (1975) 11, T A hltller and V C Bondybey, C’hem Phys Letters 50 (1977) 275. V E Bondvbey and T A hlffler, J Chem Phys 67 (1977) 1790, F W Grwmann, B H hlnhan and A O’Keefe, J Chem Phys , to be pubhshcd J Ailnon, T. Kondow and R N Zare, Chcm Ph>s Let-
ters 64 ( 19 79) 202 P J Dagdetan and J P Doermg, Chem Phys Letters 64 (1979) 7-00 [9] R D Bro\\n, P D Godfrey, J C Crofts, Z Nmkov and S Vncum, Chem Phqs Letters 62 (1979) 195. [ 101 H P Wc~se, H U hltttmann, A Ding and A Hengletn. Z Naturforsch 26a (1971) t 112, hi hlenzmger and L Wzihhhn. Rev SCI lnar 40 (1969) 102 [ 111 D Brandt, Ph D Thew., Uruvcrslty of Gottmgen (1979) [ 121 A Dmg und K Rxhter, to be publaneZ [S]