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Product-state specific observation of Li(32D) quenching collisions with inert-gas atoms
CH~hlICAL I’IIYSICS LCTTCRS
Volume 88, number 5
PRODUCT4TATE
SPECIFIC OBSERVATION
WlTH INERTGAS
ATOMS
OF Li(3 2D) QUENCHING COLLISIONS
G. ENNEN* and Ch. OTT1NGER
Rcceivcd 3 March 1982
Lithium atoms m the 3 ‘D state wcrc prcpwd by laser irradnllon of lithmm vapor. Cmissronfrom scvcnother stews was observed, resulting from coIlmona electronic energy transfer. From depcndenccon argon and tenon gas pressure, &k-toslate rate constants acre delcrmmcd Energy transfer by colbs~onswith ground-&k Llhrum atoms ws also obscrvcd.
resonance level, but not in the cxtrcmc Rydberg rcgimc, appear to have been little studied. Buvbcn et al. [2f j measured coll.isionatNa(4 *D--t 4 *F) transfer (5 mcV eodotherrmc)and found cffcctwecrossncltons of 36 and 50 AZ with Ar and Xc, rcspcctlvcly. in the present work we report on detadcd studies of the quenching of the Li(3 *D) state by colhsions with inert-gasand ground-st~tc lithium atoms. It was possible to identify the final lithium atomic states by means of theu optical emission. For rcfcrencc fig. 1 taken from ref. [22] shows Ihe relevant energy levels.
1. Introduction
Electronic energy transfer and reacttvc collisions of excited alkali atoms have recently
been studied very BC-
lively [ 11. From an experunental pomt of view, alkali systems offer attractive possibilitiesbecause of the convenient state-selective reactant preparatton by means of laser excitation. Also alkali vapors have practical mlportance as laser media 1231. Fwlly, alkali systems are of fundamental theoretical interest since interaction potentials can be calculated, for the lighter atoms ab mitlo [4-61 and for heavier atoms usmg the pseudopotenttal method [7-91. In most experimental studies, the first resonance level of alkali atoms was exated. Collision partners were other alkali atoms, for example in experiments on laser-assistedionlzatlon [lo], or simple molecules as in quenching experiments [I 1,121. Inert-gas atoms, although much used in studes on line broadenmg [ 13,141, do not quench the first alkali resonance levels;upper hmits of 10-S to lo-* A?-have been measured for the correspondmg cross sectlons [11,15--371 ** _High-lymg alkali Rydberg levels, on the other hand, have huge quenching cross sections (=I03-to4 A’!) wzth inert-gas atoms [ 19,203. Qucnching of intermediate alkali levels, i.e. higher than the first
* Present address: Fakultjt fiir Phystk, UnwcrsltHt Bielefeld, Biefefeld, West Germmy. **The as-e of boron is one example where quenching atomic resonance line does occur, see ref. [ 18 1.
of an
2. Experimental The apparatus, which has been described [23,241, cell (at *650°C) rnciudmg a gas inlet system, a cw dye laser (Coherent 490) operated with rhodamrne 6G or, in a few cases, a VlS/ UV argon ion laser, and a 314 m Spcx spectrometer fitted with a 9658 EMI photomultIplier. Lithium metal of natural isotopic composition was used. The L1(3 2D) atoms were prepared by irradtatmg the hthmm vapor consists basically of P fluorescence
wrth 6103 ,% laser It&i.
This wavelength corresponds
to the Li(2 2P -+ 3 2D) transition. Li(2 *P) atoms are always present under these conditions, partly due to thermalexcttation, partly resulting from collision-induced dissociation of Li2(A * 2:) molecules which arc simultaneously generated by the 6 103 A radiation [25]. The Li(22P) concentration is quite high, as is evidenced
0 009.2614~82/0000-OOOO/~02.75 Q 1982 North-Ho~i~d
Voluinc 88.
number 5
CHEAI KAL
21 May 1982
PHYSICS LITERS
by the extremely bright 2 2P-3 *D fluorescence ap pearmg as soon as the laser is tuned onto this tranntion. The intensity of this fluorescence Light,IDP, W”JS
measured. It is proportional to the Con~~nt~tio~~D of Li(3 2D) atoms. I,,
(0
= lV& Dplrc&pllDp’
where ADS = 7.16 X 10’ S-I is the spontaneous 3 2D + 2 ZP transition probability [76], and i&, r&p are the corresponding wavenumber and spectral sensitivity, respectively. Eq. (I) holds irrespective of simultaneous stimulated 3 21) + 3,2P emission. With the laser fwed on the 6103 a line, the emission spectrum of the lithium vapor was scanned. A surprising number of other well-known atormc lithium lines 11-71was observed. Their intensities Zx were measured, wluch are related to the corresponding steady-state concentntionsNX Ix
=
of emitting
NXA Xkc?xqX
zo-
IS -
0 Fii. 1. CnerEy level diipm
atoms via
v
(2)
The ratio I,#,,
was then detested as a function of inert-gas pressure. fDP was always much larger than f,, so that neutral density filters with an attenuation factor of 300 had to be used for the I,, measurement. Laser light scattered from the cell walls was not a problem, because the detector was arranged to view only the center of the cell. Tests with a cold cell confiied that all of the iDP signal was due to fluorescence in the vapor.
of Ihe lrthium atom. In Ihis work
collisionalenergy transferhas been observedfrom the laserexcited 3 ‘D level to the levelswith pnncipal quantum numbers n = 3.4 and 5 as shown. 3. Results Table 1 lists the seven observed lithium lines. In fig f the upper states of the observed transitions are seen to lie from ZQOOOcm-* below to =8LlOOcm-* above the initial 3 21) level. The pressure dependence was measured for four of these lines, using argon and xenon as buffer go. Under steady-state conditions
‘fable 1 Ekitation transfer Li(3 2D) Ar,xe. L$X) Fmal state LGQ
3% 32P 4% 42P 42D 5% 5% ”
Observedemission
32S122P ~*P-c~~S 42S-+22P 42P-+22S 42D-r22P S*s-c*aP
8126.38 3232.63 4971.72 2741.19 4602.87 4213.11
5*D-2*P
4132 60
Emissionpartly set-absorbed, see text.
488
Wavelength (A)
Excitation transfer rate constant kx (IfIr” cm3 S’)
Excitation transfer cross sectIon ueff (A2)
ilIg0l.l
xenon
argon
xenon
11 .8.5x lg3 a) 0.52
24 -
5.5 >4x lo-3a) 0.26
13
5.3
I2
2.4
-
6.2
CHEhllCAL
Volume 88. number 5
+ kxNDn = 0.
dtVx/dt = -Nx/rx where
l/rx
(3)
is the sum over the radiative
= Z, A,
tion probabilities
PHYSICS
1261. From (3), usmg (1) and (2), we fmd the intensity ratio ~X~~DP = kX4TXAXIA
&
Cxr;;,,>
(4)
rlxlqDP.
Here kx is the rate constant for colliwon-induced Li(3 2D + X) transitions, and tz is the buffer gas density. Ix/fDP is plotted UI fig,2 versus the buffer gas pressure. From the slopes of these lines, k, was found and is given in table 1, Of the four collision-induced transitions measured m this way, two are endothermic (namely those to 4 2S and 4 2D, by AE = 3729 cm-l and 5340 cm-l, respectively), so that only buffer gas atoms in the highenergy portIon of the maxwelhan velocity distribution wn contribute. The two corresponding rate constants as given in table 1 have been corrected for this by multiplication with exp(AE/kT)/(l t AE/ kT), as outlined by us in ref. [24, appendix]. The correction factors amount here to 50 and 450 for the 4 2S and 4 2D levels, respectively. Thus the uncorrected /
+
rate constants, as directly measured, are smaller by these factors*.
trami-
from level X to all accessible levels i
LITTCRS
Among
the transitions
observed, 3232.63
A is tbc
emisslon IS therefore self-absorbed in the hthium vapor to an unknown extent. Whde it is true that all Li(32P) atoms formed ~111ultunately emit 3233 A hght (ncglecting back-transfer) irrespective of the - probably considerable - lengthening of their effective radiative Metime [24], this emission will occur from a dlffusc area, and will partly not be detected. In ref. [24) we have pointed out that this geometrical effect cannot be described by the effective hfetune. For this reason only a lower ltmit can be given in table I for the 3 2D + 3 2P transfer rate constant and cross section. The last column of table 1 gives effective cross sections, only one terminating
uC-f= lqii,
on the Li ground state. This
(5)
where E is the mean relatrve collision velocity [24, ap-
pendix] . *
Note that these results supersedethe prehmmxy d313 communicatcd in the footnote of ref. 1241. p. 136.
+
l
Argon
0
Xenon
0
intensity. as a function ot Fig. 2. Dependence of the intensity of four LJ em&on lines, normalued to the 3 3D + 2 *P emkon buffer g;ls (Ar, Xe) pressure. The (partly self-absorbed) 3233 A cmmon IS shown on a ten fold expanded scale.
489
In addition to the seven lmes hsted in table 1, the 3 1D --L2 % transition at 3 196 A was observed. Its micnsily. nonnahzed to IgP, is independent of the buffer 6”s pressure.
4. Discussion tt IStmportdnt to ascertam that the observed populatton of cmittmg statesNX does indeed result from colilsio~mduccd ~l~enchlng transitions between L1(320) and buffer gas atoms. Since the 6103 I$ laser light also populates LiZ(A ‘2:) molecular levels, the reaction mtght tnvolve these cxcrred molecules and not the L$3 2D) atoms at all. By tunmg the laser shghtly (within its ~~nd\vidth) m the vicinityof 6103 A, one can diffcrcntiste between the two possible reaction routes, because the molecular absorption lmes do not comcide exactly wnh the atomic 2 ZP + 3 *D absorption. Detunmg the Inset m this way, it was found that the emission Intensity ratio IX/l,, remained constant, which identGes the Li(3 2D) atoms umquely as the reactant species. Conside~n8 the potential energy curves for Li*inert gas interaction as given in ref. [8l , the statespecific quenching processes measured here can easily be interpreted as non-adiabatic transitions at near curve crossings in the repulsive part of the potent&, where aU cuwcs crowd t~geti~ertn a spaceof ~0.1 A. It is interestmg to note, however, that the magnitude of the rale constants and cross sections is correlated with the well depth of the attraetrve L1(32D)-mert gas potential. The 7-Aand 7-il potent& have, respecttvely, well depths of -*J3000and 1000 cm-l for Xe, whde for Ac they are only *I200 and -300 cm-t deep. The curves shown UI fig. 2 exblblt a very large ordmate mtereept. We ascnbe this to quenching collisrons of Lt(3 2D) with ground-state lithium atoms, wtuch have, at the cell temperature used, a partial pressure of 0.12 Torr [28] _A confirmation of our tentative interpretation would require a systematic variation of the Li atom density, which was not done in this work. Pendmg tlus, the measured ordinate intercept indicates a cross section for quenchmg of Li(3 2D) by Li atoms which is about two orders of magnitude larger than with inert-gas atoms. An exceptionally large efficiency of lithfum atoms in electronic energy transfer collisions \vm tecentty also found by us in the use of electronk-
ally excited Liz molecules[29]. 490
Related to these Li*-Li energy transfer colhsions is the process of colkon-induced dissocration of electronically exclfed Li molecules, to give Li* t Li. The ener,y transfer Li(3 I D) + Li + Li(X) + Li must proceed via a tempomrllyformed L.1;molecule.The observed drstnbution over various states Li(X) indicates that several LIP potential energy curves and transitions between them are involved. in the case of colhsion-mduced dissociation of some hi-Iy~g state Li;: I_$ f M * Li(X) + Li + M, one would therefore also expect a variety of products Li(X). In conclusion we would like to report on some experiments which coniirm this. Using the bIue.green lines of an argon ton laser instead
of the 6103 Wdye laserlight, we observedemis-
sion of the 6 103 and 3 I26 A atomic Iin~s. Their IIItensity increased linearly with the pressure of added argon. Energy conslderations show that two laser photons are necessary to populate the L1(32D) and Li(3*S) states from ground-state species. A measurement showed that the 6103 and 8 126 Wemission intensities mcreased ahnost quadratically with the laser power, the deviation from a strict quadratic dependence being ascribed to saturation effects. The L$ state, here termed “Y” state, excited by this two-photon transition, cannot be identified uniquely, except that ;t has to be of “g” symmetry and is expected to lie well above the C tfl, state. Very hkely, the Li,(B ~I&} state IS excited first, and from it the unknown “Y” state. The
6103 and 8126 A emission intensities were measured using five different laser lines between 4579 and 5145 PI. Dividing these intensities by the integral Liz@ + X)
emission intensity (as a measure of the U,(B) poputation) and by the laser power (to which the Liz(B + u) transitIon probablbty is proportional), we found no smooth correlation with the exciting laser wavelength. We attribute the scatter in the data after this norrnahzation to the Franck-Condon re~u~ements and to the more or less perfect coincidences between laser lines and molecular lines of the B -+ Y excitation step. Moreover this “scatter” was different for the 6103 and 8126 8, lines. This must be due to the details of the non. adiabatic processes leading from the “Y” state to the Li(3 2D) and Li(3 2S) a~ptotes. Similarly we found emission from these same Li atomic states during irradiation of the vapor with the W lines of the argon ion laser (see fig. 5 of ref. [29]). In this case it is known that the Li2(C In,) state is excited in a single photon tm~tion [SO]. Pre~ma~
CIILIIICAL
Valumc 88. number 5
measurements showed
agsin that the 6 103 and 8 I16
PIIYSICS
8,
emission intensities were dependent on the buffer gas
pressure. Since, by the Wigner-Witmer rules, Li,(C Ill,) cannot dissociate into Li(3 2S) t Li(2 2S), at lcast the appearance of the 8 126 A line must be due to a nonadiabatic step in the collision-induced break-up of Liz(C). Li(3 2D) + Li(2 2S) is one of three adiabatically possible dissociation asymptotes of Liz(C), see fig. 1 in ref. [24] . From the asymptote Li(3 ‘P) t Li(2 2S) *, also allowed, one might expect emission at 3233 A. Table 1 shows that this line, as produced by energy transfer from Li(3 ‘D), IS very weak compared to the others, probably as a result of self-absorption. In the present case of Liz W excitation, this line would then bc expected to be unobservably weak.
.?I Bl.ly 1982
LETTCRS
121 C. York and A. CaUaghcr, JlLA Report No. 11-S. Boulder (1974). 131 B. Wcllcgchauscn, K H. Stcphan. D. rricdc and II Wllmg, Opt. Commun. 23 (1977) 157. 141 D.D. Konowalow and M.L. Olson, J Chcm. Phys 71 (1979) 450. [S] D D. Konowalow and P.S. Juhcnnc. I. Chcm. Phys. 72 (1980) 5815. [6] R.P. Saxon, R.E. Olson and B. Liu. J. Chcm. Phys. 67 (1977)
2692.
171 W.C. Bayhs. J Chem. Phys. 51 (1969) 2665. m Progcss III atonuc spectroscopy, Part A. cds. W. Hank and IL Klcmpoppen (Plenum Press, New York, 1978) p_ 207. IS] J Pascalcand J. Vandephnquc, J. Chcm Phys. 60 (1974) 2278, Molecular Terms of the Alkah-Rdrc Cd5 Mom POE.- Numcrlcal rcsuks and potcnt~al encrg) CUNCF. I Lithium. Scrvicc de Physique Alomiquc, Ccnlrc d’Ctudcs Nuclcarrcs dc Saclay (March 1974). 191 R. Duren. Advan. At Mol. Phys. 16 (1980) 55. P. Polak-Dmgcls, J. Keller, J. Wcincr. J.C. Gaulhler and N. Bras, Phys. Rev. A24 (198 1) I 107.
[ 101 5. Conclusion
[I I] S.-M. Lin and R.E. Weston. J. Chcm. Phys 65 (1976) Redand, J. Chcm. Phys. 74 (1981)
1443.
[ 121I.V.Hertcl and W. Energy transfer from laser-excited Li(3 2D) atoms (“quenching”) was observed. The product atoms were detected in a state-specific way by their emission. While the fist resonance level, Li(2 2P), ISknown lo be not measurably quenched by inert-gas collisions, we find for Li(3 2D) cross sections of a few A2, for xenon twice as large as for argon. Ground-state lithium atoms appear to have much larger quenching cross sections. The observed product states are clustered around the Initial state and are coupled IO tt by non-adiabatic transirlons between the correspondmg Li*-inert gas or Li*-LI
potential energy curves. Evidence for this coupling in the latter case is also obtamed from results on collisioninduced dissociation of laser-excited Li;.
6757. and rcfcrcnccstherem. [ 131 A. Callaghcr, Procccdmgs of the 4th lnlcrnatlonal
Con-
feercnce on Atonnc Phyws. 1974 (Plenum Press,New York. 1975) p. 559. 1141 W. Bchmcnburg, m ProgressIII atomic spcclroscopy.PM B, eds. IV. Hanlc and II. Klcmpoppcn (Plenum Press, New
York, 1979) 1187.
[ 151 [ 161
L. Krause, Advan. Chcm. Phys. 28 (1975) 267. G. Coplcy. B.P. Rlbblc and L Krause, Phyr Rev. 163
(1967) 34. I171 J N Dodd, C. Cncmruk and A. Gallagher. J. Chcm. Phys. 50 (1969)4838.
[ 181 P. Hannaf’oord and R.M. Lowe. whys. RCV. LC~!CIS 38 (1977) 650. [ 191 T.l-. CaLhghcr, S A. Edclstem and R.M. HdI, Ph)s. Rev. Lcttcrs 35 (1975) 644. [20] F. Gounand, P.R. Fourmer and J. Llcrlandc,Phys. Rev.
Al5 (1977) 2212. [2l] r. Busbcn, K. Bcroff, C. Giacobino and G. Crynbcrg, J. Phys. (Paris) 39 (1978)
Acknowledgement
LlO8.
1221S. Bashkin and J.O. Stoner, Atomic cncrgy levelsand Thanks are due from one of us (GE) for a grant from the Deutsche Forschungsgemeinschaft. * In
fii. 1 of ref. 1231 thrs was crroncously labeled as 3 2P +
22P.
grolrhn diagrams (North-Holland,
Amsterdam. 1975). C Enncn and Ch. Ottinger, Chem. Phys. 3 (1974) 404. C Enncn and Ch. Ottmgcr. Chcm. Phys. 40 (1979) 127. G. Enncn andCh.Ottingcr.Chcm. Phys.41(1979)415. W.L. WICKS,h1.W. Smith and B.M. Clcnnon, Atomic Transltlon ProbabUrcs, Vol. 1, NSRDS-NBS 4 (1966). [27] A.R. Striganov and N.S. Sventitskir, Tables of spectral bnes of neutral and ionized atoms (Plenum Press, New
[23] 1241 [ZSj 126 ]
York, 1968).
References [l]
1281 A.W. Nesmkyanov, Vapour prcssurcs of the clcmcnts (Van Nostrand, Pnnceton. 1950).
B. Sayer, M. Ferray. J. Lozingot and J. Berkmde, J. Chem. Phys. 75 (1981)
3894, and references thcrem.
1291 C. Enncn and ch. Ottinge;. J. &em. Phys. (May 1982). C. Ennen. Ch. Ottinger, K.K. Verma and W.C Stwalley. J. Mol. Spectry. 89 (1981) 413.
[ 301
491
Volume 88, number 5
PRODUCT4TATE
SPECIFIC OBSERVATION
WlTH INERTGAS
ATOMS
OF Li(3 2D) QUENCHING COLLISIONS
G. ENNEN* and Ch. OTT1NGER
Rcceivcd 3 March 1982
Lithium atoms m the 3 ‘D state wcrc prcpwd by laser irradnllon of lithmm vapor. Cmissronfrom scvcnother stews was observed, resulting from coIlmona electronic energy transfer. From depcndenccon argon and tenon gas pressure, &k-toslate rate constants acre delcrmmcd Energy transfer by colbs~onswith ground-&k Llhrum atoms ws also obscrvcd.
resonance level, but not in the cxtrcmc Rydberg rcgimc, appear to have been little studied. Buvbcn et al. [2f j measured coll.isionatNa(4 *D--t 4 *F) transfer (5 mcV eodotherrmc)and found cffcctwecrossncltons of 36 and 50 AZ with Ar and Xc, rcspcctlvcly. in the present work we report on detadcd studies of the quenching of the Li(3 *D) state by colhsions with inert-gasand ground-st~tc lithium atoms. It was possible to identify the final lithium atomic states by means of theu optical emission. For rcfcrencc fig. 1 taken from ref. [22] shows Ihe relevant energy levels.
1. Introduction
Electronic energy transfer and reacttvc collisions of excited alkali atoms have recently
been studied very BC-
lively [ 11. From an experunental pomt of view, alkali systems offer attractive possibilitiesbecause of the convenient state-selective reactant preparatton by means of laser excitation. Also alkali vapors have practical mlportance as laser media 1231. Fwlly, alkali systems are of fundamental theoretical interest since interaction potentials can be calculated, for the lighter atoms ab mitlo [4-61 and for heavier atoms usmg the pseudopotenttal method [7-91. In most experimental studies, the first resonance level of alkali atoms was exated. Collision partners were other alkali atoms, for example in experiments on laser-assistedionlzatlon [lo], or simple molecules as in quenching experiments [I 1,121. Inert-gas atoms, although much used in studes on line broadenmg [ 13,141, do not quench the first alkali resonance levels;upper hmits of 10-S to lo-* A?-have been measured for the correspondmg cross sectlons [11,15--371 ** _High-lymg alkali Rydberg levels, on the other hand, have huge quenching cross sections (=I03-to4 A’!) wzth inert-gas atoms [ 19,203. Qucnching of intermediate alkali levels, i.e. higher than the first
* Present address: Fakultjt fiir Phystk, UnwcrsltHt Bielefeld, Biefefeld, West Germmy. **The as-e of boron is one example where quenching atomic resonance line does occur, see ref. [ 18 1.
of an
2. Experimental The apparatus, which has been described [23,241, cell (at *650°C) rnciudmg a gas inlet system, a cw dye laser (Coherent 490) operated with rhodamrne 6G or, in a few cases, a VlS/ UV argon ion laser, and a 314 m Spcx spectrometer fitted with a 9658 EMI photomultIplier. Lithium metal of natural isotopic composition was used. The L1(3 2D) atoms were prepared by irradtatmg the hthmm vapor consists basically of P fluorescence
wrth 6103 ,% laser It&i.
This wavelength corresponds
to the Li(2 2P -+ 3 2D) transition. Li(2 *P) atoms are always present under these conditions, partly due to thermalexcttation, partly resulting from collision-induced dissociation of Li2(A * 2:) molecules which arc simultaneously generated by the 6 103 A radiation [25]. The Li(22P) concentration is quite high, as is evidenced
0 009.2614~82/0000-OOOO/~02.75 Q 1982 North-Ho~i~d
Voluinc 88.
number 5
CHEAI KAL
21 May 1982
PHYSICS LITERS
by the extremely bright 2 2P-3 *D fluorescence ap pearmg as soon as the laser is tuned onto this tranntion. The intensity of this fluorescence Light,IDP, W”JS
measured. It is proportional to the Con~~nt~tio~~D of Li(3 2D) atoms. I,,
(0
= lV& Dplrc&pllDp’
where ADS = 7.16 X 10’ S-I is the spontaneous 3 2D + 2 ZP transition probability [76], and i&, r&p are the corresponding wavenumber and spectral sensitivity, respectively. Eq. (I) holds irrespective of simultaneous stimulated 3 21) + 3,2P emission. With the laser fwed on the 6103 a line, the emission spectrum of the lithium vapor was scanned. A surprising number of other well-known atormc lithium lines 11-71was observed. Their intensities Zx were measured, wluch are related to the corresponding steady-state concentntionsNX Ix
=
of emitting
NXA Xkc?xqX
zo-
IS -
0 Fii. 1. CnerEy level diipm
atoms via
v
(2)
The ratio I,#,,
was then detested as a function of inert-gas pressure. fDP was always much larger than f,, so that neutral density filters with an attenuation factor of 300 had to be used for the I,, measurement. Laser light scattered from the cell walls was not a problem, because the detector was arranged to view only the center of the cell. Tests with a cold cell confiied that all of the iDP signal was due to fluorescence in the vapor.
of Ihe lrthium atom. In Ihis work
collisionalenergy transferhas been observedfrom the laserexcited 3 ‘D level to the levelswith pnncipal quantum numbers n = 3.4 and 5 as shown. 3. Results Table 1 lists the seven observed lithium lines. In fig f the upper states of the observed transitions are seen to lie from ZQOOOcm-* below to =8LlOOcm-* above the initial 3 21) level. The pressure dependence was measured for four of these lines, using argon and xenon as buffer go. Under steady-state conditions
‘fable 1 Ekitation transfer Li(3 2D) Ar,xe. L$X) Fmal state LGQ
3% 32P 4% 42P 42D 5% 5% ”
Observedemission
32S122P ~*P-c~~S 42S-+22P 42P-+22S 42D-r22P S*s-c*aP
8126.38 3232.63 4971.72 2741.19 4602.87 4213.11
5*D-2*P
4132 60
Emissionpartly set-absorbed, see text.
488
Wavelength (A)
Excitation transfer rate constant kx (IfIr” cm3 S’)
Excitation transfer cross sectIon ueff (A2)
ilIg0l.l
xenon
argon
xenon
11 .8.5x lg3 a) 0.52
24 -
5.5 >4x lo-3a) 0.26
13
5.3
I2
2.4
-
6.2
CHEhllCAL
Volume 88. number 5
+ kxNDn = 0.
dtVx/dt = -Nx/rx where
l/rx
(3)
is the sum over the radiative
= Z, A,
tion probabilities
PHYSICS
1261. From (3), usmg (1) and (2), we fmd the intensity ratio ~X~~DP = kX4TXAXIA
&
Cxr;;,,>
(4)
rlxlqDP.
Here kx is the rate constant for colliwon-induced Li(3 2D + X) transitions, and tz is the buffer gas density. Ix/fDP is plotted UI fig,2 versus the buffer gas pressure. From the slopes of these lines, k, was found and is given in table 1, Of the four collision-induced transitions measured m this way, two are endothermic (namely those to 4 2S and 4 2D, by AE = 3729 cm-l and 5340 cm-l, respectively), so that only buffer gas atoms in the highenergy portIon of the maxwelhan velocity distribution wn contribute. The two corresponding rate constants as given in table 1 have been corrected for this by multiplication with exp(AE/kT)/(l t AE/ kT), as outlined by us in ref. [24, appendix]. The correction factors amount here to 50 and 450 for the 4 2S and 4 2D levels, respectively. Thus the uncorrected /
+
rate constants, as directly measured, are smaller by these factors*.
trami-
from level X to all accessible levels i
LITTCRS
Among
the transitions
observed, 3232.63
A is tbc
emisslon IS therefore self-absorbed in the hthium vapor to an unknown extent. Whde it is true that all Li(32P) atoms formed ~111ultunately emit 3233 A hght (ncglecting back-transfer) irrespective of the - probably considerable - lengthening of their effective radiative Metime [24], this emission will occur from a dlffusc area, and will partly not be detected. In ref. [24) we have pointed out that this geometrical effect cannot be described by the effective hfetune. For this reason only a lower ltmit can be given in table I for the 3 2D + 3 2P transfer rate constant and cross section. The last column of table 1 gives effective cross sections, only one terminating
uC-f= lqii,
on the Li ground state. This
(5)
where E is the mean relatrve collision velocity [24, ap-
pendix] . *
Note that these results supersedethe prehmmxy d313 communicatcd in the footnote of ref. 1241. p. 136.
+
l
Argon
0
Xenon
0
intensity. as a function ot Fig. 2. Dependence of the intensity of four LJ em&on lines, normalued to the 3 3D + 2 *P emkon buffer g;ls (Ar, Xe) pressure. The (partly self-absorbed) 3233 A cmmon IS shown on a ten fold expanded scale.
489
In addition to the seven lmes hsted in table 1, the 3 1D --L2 % transition at 3 196 A was observed. Its micnsily. nonnahzed to IgP, is independent of the buffer 6”s pressure.
4. Discussion tt IStmportdnt to ascertam that the observed populatton of cmittmg statesNX does indeed result from colilsio~mduccd ~l~enchlng transitions between L1(320) and buffer gas atoms. Since the 6103 I$ laser light also populates LiZ(A ‘2:) molecular levels, the reaction mtght tnvolve these cxcrred molecules and not the L$3 2D) atoms at all. By tunmg the laser shghtly (within its ~~nd\vidth) m the vicinityof 6103 A, one can diffcrcntiste between the two possible reaction routes, because the molecular absorption lmes do not comcide exactly wnh the atomic 2 ZP + 3 *D absorption. Detunmg the Inset m this way, it was found that the emission Intensity ratio IX/l,, remained constant, which identGes the Li(3 2D) atoms umquely as the reactant species. Conside~n8 the potential energy curves for Li*inert gas interaction as given in ref. [8l , the statespecific quenching processes measured here can easily be interpreted as non-adiabatic transitions at near curve crossings in the repulsive part of the potent&, where aU cuwcs crowd t~geti~ertn a spaceof ~0.1 A. It is interestmg to note, however, that the magnitude of the rale constants and cross sections is correlated with the well depth of the attraetrve L1(32D)-mert gas potential. The 7-Aand 7-il potent& have, respecttvely, well depths of -*J3000and 1000 cm-l for Xe, whde for Ac they are only *I200 and -300 cm-t deep. The curves shown UI fig. 2 exblblt a very large ordmate mtereept. We ascnbe this to quenching collisrons of Lt(3 2D) with ground-state lithium atoms, wtuch have, at the cell temperature used, a partial pressure of 0.12 Torr [28] _A confirmation of our tentative interpretation would require a systematic variation of the Li atom density, which was not done in this work. Pendmg tlus, the measured ordinate intercept indicates a cross section for quenchmg of Li(3 2D) by Li atoms which is about two orders of magnitude larger than with inert-gas atoms. An exceptionally large efficiency of lithfum atoms in electronic energy transfer collisions \vm tecentty also found by us in the use of electronk-
ally excited Liz molecules[29]. 490
Related to these Li*-Li energy transfer colhsions is the process of colkon-induced dissocration of electronically exclfed Li molecules, to give Li* t Li. The ener,y transfer Li(3 I D) + Li + Li(X) + Li must proceed via a tempomrllyformed L.1;molecule.The observed drstnbution over various states Li(X) indicates that several LIP potential energy curves and transitions between them are involved. in the case of colhsion-mduced dissociation of some hi-Iy~g state Li;: I_$ f M * Li(X) + Li + M, one would therefore also expect a variety of products Li(X). In conclusion we would like to report on some experiments which coniirm this. Using the bIue.green lines of an argon ton laser instead
of the 6103 Wdye laserlight, we observedemis-
sion of the 6 103 and 3 I26 A atomic Iin~s. Their IIItensity increased linearly with the pressure of added argon. Energy conslderations show that two laser photons are necessary to populate the L1(32D) and Li(3*S) states from ground-state species. A measurement showed that the 6103 and 8 126 Wemission intensities mcreased ahnost quadratically with the laser power, the deviation from a strict quadratic dependence being ascribed to saturation effects. The L$ state, here termed “Y” state, excited by this two-photon transition, cannot be identified uniquely, except that ;t has to be of “g” symmetry and is expected to lie well above the C tfl, state. Very hkely, the Li,(B ~I&} state IS excited first, and from it the unknown “Y” state. The
6103 and 8126 A emission intensities were measured using five different laser lines between 4579 and 5145 PI. Dividing these intensities by the integral Liz@ + X)
emission intensity (as a measure of the U,(B) poputation) and by the laser power (to which the Liz(B + u) transitIon probablbty is proportional), we found no smooth correlation with the exciting laser wavelength. We attribute the scatter in the data after this norrnahzation to the Franck-Condon re~u~ements and to the more or less perfect coincidences between laser lines and molecular lines of the B -+ Y excitation step. Moreover this “scatter” was different for the 6103 and 8126 8, lines. This must be due to the details of the non. adiabatic processes leading from the “Y” state to the Li(3 2D) and Li(3 2S) a~ptotes. Similarly we found emission from these same Li atomic states during irradiation of the vapor with the W lines of the argon ion laser (see fig. 5 of ref. [29]). In this case it is known that the Li2(C In,) state is excited in a single photon tm~tion [SO]. Pre~ma~
CIILIIICAL
Valumc 88. number 5
measurements showed
agsin that the 6 103 and 8 I16
PIIYSICS
8,
emission intensities were dependent on the buffer gas
pressure. Since, by the Wigner-Witmer rules, Li,(C Ill,) cannot dissociate into Li(3 2S) t Li(2 2S), at lcast the appearance of the 8 126 A line must be due to a nonadiabatic step in the collision-induced break-up of Liz(C). Li(3 2D) + Li(2 2S) is one of three adiabatically possible dissociation asymptotes of Liz(C), see fig. 1 in ref. [24] . From the asymptote Li(3 ‘P) t Li(2 2S) *, also allowed, one might expect emission at 3233 A. Table 1 shows that this line, as produced by energy transfer from Li(3 ‘D), IS very weak compared to the others, probably as a result of self-absorption. In the present case of Liz W excitation, this line would then bc expected to be unobservably weak.
.?I Bl.ly 1982
LETTCRS
121 C. York and A. CaUaghcr, JlLA Report No. 11-S. Boulder (1974). 131 B. Wcllcgchauscn, K H. Stcphan. D. rricdc and II Wllmg, Opt. Commun. 23 (1977) 157. 141 D.D. Konowalow and M.L. Olson, J Chcm. Phys 71 (1979) 450. [S] D D. Konowalow and P.S. Juhcnnc. I. Chcm. Phys. 72 (1980) 5815. [6] R.P. Saxon, R.E. Olson and B. Liu. J. Chcm. Phys. 67 (1977)
2692.
171 W.C. Bayhs. J Chem. Phys. 51 (1969) 2665. m Progcss III atonuc spectroscopy, Part A. cds. W. Hank and IL Klcmpoppen (Plenum Press, New York, 1978) p_ 207. IS] J Pascalcand J. Vandephnquc, J. Chcm Phys. 60 (1974) 2278, Molecular Terms of the Alkah-Rdrc Cd5 Mom POE.- Numcrlcal rcsuks and potcnt~al encrg) CUNCF. I Lithium. Scrvicc de Physique Alomiquc, Ccnlrc d’Ctudcs Nuclcarrcs dc Saclay (March 1974). 191 R. Duren. Advan. At Mol. Phys. 16 (1980) 55. P. Polak-Dmgcls, J. Keller, J. Wcincr. J.C. Gaulhler and N. Bras, Phys. Rev. A24 (198 1) I 107.
[ 101 5. Conclusion
[I I] S.-M. Lin and R.E. Weston. J. Chcm. Phys 65 (1976) Redand, J. Chcm. Phys. 74 (1981)
1443.
[ 121I.V.Hertcl and W. Energy transfer from laser-excited Li(3 2D) atoms (“quenching”) was observed. The product atoms were detected in a state-specific way by their emission. While the fist resonance level, Li(2 2P), ISknown lo be not measurably quenched by inert-gas collisions, we find for Li(3 2D) cross sections of a few A2, for xenon twice as large as for argon. Ground-state lithium atoms appear to have much larger quenching cross sections. The observed product states are clustered around the Initial state and are coupled IO tt by non-adiabatic transirlons between the correspondmg Li*-inert gas or Li*-LI
potential energy curves. Evidence for this coupling in the latter case is also obtamed from results on collisioninduced dissociation of laser-excited Li;.
6757. and rcfcrcnccstherem. [ 131 A. Callaghcr, Procccdmgs of the 4th lnlcrnatlonal
Con-
feercnce on Atonnc Phyws. 1974 (Plenum Press,New York. 1975) p. 559. 1141 W. Bchmcnburg, m ProgressIII atomic spcclroscopy.PM B, eds. IV. Hanlc and II. Klcmpoppcn (Plenum Press, New
York, 1979) 1187.
[ 151 [ 161
L. Krause, Advan. Chcm. Phys. 28 (1975) 267. G. Coplcy. B.P. Rlbblc and L Krause, Phyr Rev. 163
(1967) 34. I171 J N Dodd, C. Cncmruk and A. Gallagher. J. Chcm. Phys. 50 (1969)4838.
[ 181 P. Hannaf’oord and R.M. Lowe. whys. RCV. LC~!CIS 38 (1977) 650. [ 191 T.l-. CaLhghcr, S A. Edclstem and R.M. HdI, Ph)s. Rev. Lcttcrs 35 (1975) 644. [20] F. Gounand, P.R. Fourmer and J. Llcrlandc,Phys. Rev.
Al5 (1977) 2212. [2l] r. Busbcn, K. Bcroff, C. Giacobino and G. Crynbcrg, J. Phys. (Paris) 39 (1978)
Acknowledgement
LlO8.
1221S. Bashkin and J.O. Stoner, Atomic cncrgy levelsand Thanks are due from one of us (GE) for a grant from the Deutsche Forschungsgemeinschaft. * In
fii. 1 of ref. 1231 thrs was crroncously labeled as 3 2P +
22P.
grolrhn diagrams (North-Holland,
Amsterdam. 1975). C Enncn and Ch. Ottinger, Chem. Phys. 3 (1974) 404. C Enncn and Ch. Ottmgcr. Chcm. Phys. 40 (1979) 127. G. Enncn andCh.Ottingcr.Chcm. Phys.41(1979)415. W.L. WICKS,h1.W. Smith and B.M. Clcnnon, Atomic Transltlon ProbabUrcs, Vol. 1, NSRDS-NBS 4 (1966). [27] A.R. Striganov and N.S. Sventitskir, Tables of spectral bnes of neutral and ionized atoms (Plenum Press, New
[23] 1241 [ZSj 126 ]
York, 1968).
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1281 A.W. Nesmkyanov, Vapour prcssurcs of the clcmcnts (Van Nostrand, Pnnceton. 1950).
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1291 C. Enncn and ch. Ottinge;. J. &em. Phys. (May 1982). C. Ennen. Ch. Ottinger, K.K. Verma and W.C Stwalley. J. Mol. Spectry. 89 (1981) 413.
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491