Accurate ionization potential of Li2 from resonant two-photon ionization

CllChllCAL

Volume 88. number 5

PIIYSICS LCTTCRS

II

x13y 1982

ACCURATEIONIZATIONPOTENTIAL OF Li, FROM RESONANTTWO-PHOTONlONlZATlON D. EISEL and W. DEMTRODER Fachbereich Physlk. Utrrwrs~f~fh2iserslaurern. D-6750 Karserslaurern, IVcsr Ccrnlor~~

Rccclved 16 February

1982

Sequcnthl two-photon ionization of Liz m B supersonic beam with Iwo pulsed tunable dye lasers yxlds the adubatlc ionization potentird IP(Li2)=41475 * 8 cm-’ and the dusoaatlon energy of the Ion Do(LI; z~L) = 10353 * 25 cm-’ TIIC lnflucnce of ~kctnc fidds on the measured vnluc of the ionuation po~cntx~l and the app-nce of numerous nuhzknum:mn

1. Introduction The lonizatlon potential of alkah molecules has recently been the subject of intensive expenmental [l-

beam by sequential two-photon lonlzation with two tunable pulsed dye lasers. Mathur et al. [2] pubhshed upper and lower limits for this iomzation potential usmg different lines of argon lasers. Our value confinns

41 and theoretical [5,6] studies. Two-step photoioni-

the results of these authors. The uncertainty, however,

zatlon

could be reduced

with two lasers V” B selected

intermediate

level

by more than one order of magnitude.

represents a very convenient method to obtain reliable and accurate values of the adiabatic ionization potential of molecules [3] . The wavelength of the first laser is tuned to a selected transition from a level (u”,J”) in the electronic ground state to a real mtermediate level (u’,J’) III an excited electronic state. The second laser pumps the selectnely excited molecules either directly

into the ionization continuum or to high-lyingmolecular Rydberg levels which subsequently can autoionize. When the ion current is monitored as a function of the wavelength of the second laser, sharp autoionization lines are found superimposed on a broad iomzatlon continuum.

If the measurements

are performed

under

collision-free conditions in a molecular beam, competing ionization processes, such as collisional ionization from high-lying levels of the neutral molecule below the ionization limit, can be excluded_ Field ionization of the neutral molecule in high-lying Rydberg levels may be caused by the electric field which is used to collect the ions. Since this effect decreases the apparent ionization limit, it has to be considered and experimentally excluded. Thts letter reports on the accurate determination of the ionization

potential

of Li, molecules

0 009-26 14/82/0000-0000/$02.75

in a supersonic

0 1982 North-Holland

2. Experimental The experimental arrangement is shown schcmatlcally in fig. I. The supersonic Li/Li2 beam is formed by adiabatic expansion of lithmm vapor through a nozzle Hnth diameter 0.5 mm and subsequent collimation through a sht wth 8 mm width, ~80 mm downstream from the nozzle. The oven consists of a ceramic hollow cylinder which ISheated by tungsten wire wrapped around the outer envelope and which is shielded by three layers of radla-

tton reflectors and a water-cooled outer jacket. A cylindricA artridge

of stainless

steel (outcr

diameter

4 cm,

165 cm3) with a 20 mm long nozzle was filled with pieces of solid lithium (natural isotope composition with V=

93% ‘Li and 7% 6 Ll) through an open end which wds subsequently welded. A lithium filling lasts for --SO h at oven temperatures of 4 120 K. The cartridge fits into the ceramic cylinder, and changmg cartndgcs is a fast and easy procedure. This oven system has proved

to be superior to all other constructions we have tned SO

far. since no problems

arise with liquid lithium

crccp-

Volunlc

sx. nllmbcr 5

CIICMICAL

ION

rug I. Schcmc of the e\pcnmcnk~l

?I May

PHYSICS LCTTCRS

OEIECIOR

arnngemcnt.

mg through the seahng surface of any flange. The system can be safely operated up to temperatures of 1400 K limited by the softening point of stainless steel The usual operating temperatures were =1120 K III the coldest part of the cartridge and 1200 K at the nozzle. The two dye kxers, which are pumped by n common mtrogen laser, are of the “Hinsch type” [7] and of the “Ltttmnnn type” [S] , respectively. The pulse width IS ~2.5 ns and the spectral hnewrdth generally ~5 GHz. For high-resolution measurements, however, the hnewidth could be reduced to =:I .5 GHz by an extra etalon inside the “Hinsch-type” dye laser resonator. The output from both dye lasers passes single-stage amplifiers. The collinear beams from the two dye lasers mtersect the supcrsomc beam perpendicularly at a distance of I IO mm from the nozzle. The laser-mduced fluoresccnce emitted from the mtermedlate levels of IA, is monitored by a photomultiplier followed by a boxcar integrator. The eons produced in the Interaction regon by two-step photoionizatIon are collected without mass selectlon through an electron-optical lens system onto the cathode of an open electron multiplier. The wavelength Xl of the fist laser is chosen suffi-

1981

the pe& of this transition. The second laser is then turned on and tuned through the spectral region of interest. Both the fluorescence intensity f&2) and the number of ions Nion(“l + ~2) are recorded simultaneously as a function of “2 at a futed wavelength ht. Ion spectra are taken alternately with the first laser on and off. The difference N,,,,,(vI + u3 - N,,,(219) of the two Ion spectra yields those iomzing two-step transitions which arc mduced by one photon 1102from the second laser and which start from the intermediate level B tll,(u’, J’) selectively populated by the fist laser. The fluorescence intensity I&) monitored as a function of the wavelength X2 of the second laser represents the excitation spectrum of the X TV+ j B t ll, system. The lines could be readily ass&e l! to both ISOtopes 7Li7Li and 6L17Li using the molecular constants given by Hessel et al. [9] and Stwalley et al. [IO]. The line posltions served as wavelength marks for the wavelength 1, of the second laser. Due to the internal cooling during the supersomc expansion through the nozzle [ 1 I] most Liz molecules are in the lowest vibrational level u” = 0 and only the lower rotatronal levels arc noticeably populated. This simphfies the excitation spectrum considerably. In order

to examine

the influence

of non-resonant

f q) which do not proceed vta the selected memediate level (IJ’,~‘),

two-photon transitions X

1 Cc’ + k(q

the pulse from the second laser was delayed relative to that of the first by a time interval At larger than the pulsewidth. It turned out that non-resonant two-photon transitions were negligible at intensities of less than 50 kW/cm* used in our experunents.

3. Results and discussion Fig. 2 shows an expanded section of the two ion spectraN,,,(2u2) w1t.hthe first laser off (lower trace) andiVic,&t + 9) with the first laser stabilized onto the transition X t Zi(v” = OJ” = 7)+ B tIIu(u’ = 0,J’ = 7) (upper trace). Both spectra are recorded as a function of the wavelength

X2 of the second

laser. The lower

theresonanttwo*photon iordza. cientlylowto excludephotoronization by twophotons spectrumrepresents tion spectrum Nion(2Y2) and reflects essentially the exciXq. Ions can be produced only by the two-photon combinations IUQ + hv2 or ?izu2. With the second laser off, the fluorescence I&) induced by the first laser ISused to tune the frequency v1 to a selected transition X 1$(u”,J”) + 11~1--*B 1ll,(u’,J’) and to keep it at 482

tation spectrum X IZ+(u” J”) + hu 2 + B ‘Ilu(u’,J’) of the second laser. Thisghas ieen proved by simultaneous recording of the fluorescence intensity In(u2) which is not shown in fig. 2. Although the line intensities are

Volume

CIICXIICALPllYSlCS

88, number 5

21 MJy 198’

LCTTLRS

et 31. [ 121 have shown

Q. 2. Resonant two-photon Ionization spcclrs of LIZ as a function of X2. Tbc lower tract (a) rcprcscnts the ion spectrum fVIOn(Zu2) wlrh tic litst Iascr ofr. whllc the upper Imcc @)

showsN (VI + ~2) Wh li~scr1 stabthzedonto the Q tnnsi“onX1!Y$U ” = 0, J. = 7) - B ‘n,(u = 0, J’ = 7). Both specln were taken under otherwIse ldentiul

conchtions.

generally different for the two spectra N,0n(3~2) I&) the line positions are the same.

and

The upper spectrum Nion(Ut + ~2) in fig. 2 shows a increase of the continuous background for wavc-

marked

lengths X2 < 475

05 MI

and moreover

many

new lmes

which do not appear In the lower spectrum N,,n(?v~), We attrtbute the increase of the continuum to the onset of the duect photoionization process Li,

B

t ll,(u’

= OJ’ = 7) + iru,

+Li~*ZgC(V=O,N=?)te-.

(I)

The hues m the spectrum are caused by excitation of Rydberg levels (rz*. u*,J*) above the Iomzatton limit which can decay through autolonization. The effective principal quantum number II* of these molecular Rydberg levels, their vibrational quantum number u* and their total angular momentum number J’ can be obtained if sufficient spectroscopic information ISavailable. This analysis will be the subject of another paper neutral

1121. The anticipation

in eq.

(1) that

the ion is formed

at

that

LIIC two potcnt~sl

nifii!nifl

of the neutral B 1tl, state and the ‘Yp’ ion state should bc nearly at the same internuclear distances R,. The Fran&-Condon factors are therefore dommant for Au = 0 transitions Starting from u’ = 0 In the B t flu state the transitron to u = 0 rn the “Ci ion state should be therefore most mtcnse. Thus has been conlirmcd in our experiments which have shown that transrtions IJ’ = 0 + u* = I h,tve much less mtcnsity than the u’ = 0 + u* = 0 transition. For an accurate dctermmatron of the adiabatic ION zatron potential one has furthermore to know in which rofariotlal /eve/N the LIP ton is formed at the onset of the continuous background in lig. 7,. Although the photolomzation process of eq. (I) starts from an mtermedrate level (u’,J’) with well deftncd rotattonal quantum numher

J’ and

momentum

is initiated

by one photon A+

?ii, the ejected photoclcctron

with angular can t&c

ZIWI~

p3rt of the angular momentum [J*(J* + I)] ‘1% of the ionizmg complex (J * = J or J’ + I), leavmg the ion core m a rotational lcvcl with quantum number N
maycarryawaya large angular mo-

the ionizationthresholdin Itslowestvibrationallevel IJ= 0 can in prmciple be proved by stabilizing the fast

the

laser onto different X + B bands and thus populating different vrbrational levels u’ in the Intermediate B Ill, state [3]. However, theoretical calculations of the I-1; *Xi potential curve by Konowalov [6] and Botschwina

This can be mterpretcd as follows [ I5 ] : The photoiomzation process (I) can bc though of as procecdmg vta an exerted molecubr state R* to the final ton state plus ejected electron’

483

Li, B tll,(u’.J’)

tuning of the laser wavelength. Fig. 3 demonstrates

+ IIU, + R*(u*,J*)

+ Ll,+ %;(u, N) + e-.

(2)

If the excltatlon enerm of the state R(u*,J*) is above the tct-n~ energy of the zE~(u=O,N) level for IV= I but below that of htgher rotatloml levels the only channel open for photoionizotion is the formation of M = 0 or N = I Ion levels, depending on the symmetry selectlon rules. If all other posslblc decay processes for R(u*,J*).

such as radiative decay, predlssociation or field ionization have sufficiently small probabilities, even ionization transitlons with large values of .&V = IJ* - NI can be detected in spite of their small probability. From the measured linewldth of autolonizatron hnes an upper limit for the total decay rate of autoionizmg

levels can be obtained. For these measurements the bandwdth

of the second

21 htoy 1982

CHEMICAL PHYSICS LETTCRS

Volume 58. number 5

laser was reduced to AU, = 1 .S GHz

by insertmg a second Fabry-Perot etalon inside the laser cavity. Fig. 3 shows three autotonization lines closely above the onset of the continuum measured at two dL ferent field strengths of the electnc field in the ionization region. The steps in the line profiles are due to the discrete wavelength steps during the computer controlled

that the linewidth drastically Increases with increasing electric field. At low fields the linewidth of the autoioniza-

tion lines approaches that of the fluorescence lines in the laser excitation spectrum shown in the lower part of fig. 3. The line profiles are convolutions of the laser lme profile and the fluorescence line profile, respectively. The latter IS agam a convolution

of natural hnewidth and reduced Doppler width in the collimated molecular beam.

With our experimental arrangement this gives a reduced fluorescence linewidth of about Au, = I50 MHz. The comparison of the measured line profiles for fluorescence lines and autoionization lines gives, withm our accuracy, an upper limit of Au < 300 MHz for the inherent linewidth of the autoionuation lines close above threshold. This imphes lifetimes T > 5 X lO’*o s for the corre-

sponding autoionizing levels in low fields, which demonstrates that competing decay processes indeed have small probabilities and explains why ionizing transitions even for large values of AN are observed. onset of the continuum

The llalrened

observed for higher values of

J’ is in accordance with these considerations, because

just above the ionization threshold only processes with large AN are possible while with increasing energy 11v2 lower AN contribute more and more to the autoionizatlon process.

A, = L90 t.35 nm

R ‘r;

--lrt

Iv”;

0

(“‘-0

3”

=6)

J’ ~5)

4. Field ionization The broadening of the autoionizatlon lines with IIIcreasing electric field can have two possible causes: (a) A shortening of the lifetime of the autoionizing levels by field ionization and (b) a field dependent Stark shift of the level energy which will result in a spectral broad-

ening of the B tll,(u’,J’) --f R(u*,J*) transition if the electric field is inhomogeneous over the ionization volume. The effect of field ionization on high-lying Rydberg

L -

levels with large principal quantum number n* is demonstrated in fig. 4. The upper part A shows the two ion

z

spectraN,,,(q + u,)[curve (b)] and A’ion(Zq) [curve (a)] close to the io&tion threshold at an electric field Es

50 V/cm while part B shows the same spectra at a

lower electric field E = 20 V/cm. The lower trace re-

Fig. 3. Autoionmtion linewidthsobserved at two different electric fields in the mtcnctmn region of E = 20 V/cm (a) and E= 100V/cnl (b)

484

presents

the fluorescence

excitation

spectrum

I&J~)_

The lines (1) and (2) which are basely seen at low fields become very intense at higher fields. Within out accu-

CllEhllCAL PHYSICS LlXTLRS

Volume 88. numbur 5

?I Xhy 1982

from the intermediate level (u’ = 0,J’ = 7). The threshold indeed increased with decreasing field. However, the total shift wlthin a field variation between 20 and 50 V/cm was less than 8 cm-l. From an extrapolation of u*(E) at the onset against zero field we obtain the

@I

adiabatx lonuation

potential, defined rls the term dlf-

fcrcncc IPad,ab

= 7-&i;

?q””

= 0.J”

= 0))

-T(Li2’Z~(u=0,J=0))=41475~8cm~‘. The error limits are partly due to the uncertamty of +7_ cm” in the determmation of the continuum onset but mainly to the uncertainty of extrapolation towards zero electric field since a sufficient collectton of the photolons requires a minimum field which might not be homogeneous. Verma et al. [IO] have recently pubhshcd a more accurate value of the dissociation energy D,(* X t xi)

SS16 + 18 cm-t of the Lil ground state, mewred from the potential muumum.This value g~vcswith

=

,

,lb)

T(u” = 0,J” = 0) = 175 cm-’ a value Do(u” = 0.J” 0) = 8341 f 18 cm-l. From D 0 (2C+ Li+)=D g’ 2 - IP(’ z;,

0

=

(lr’ Yf’ Li,) + IP(2 2S,,2, LI)

LI,)

(3)

we obtain with IP(:! 2Sl,Z, Lt) = 43487. I9 + 0.02 cm-l [W LIL 900

47L.700

L7L.000 x2 inm)

Pig. 4. Phototoniza!ion spectraNion(ut

uz) sndN,,,(2u2) m the thresholdrange taken at electric fields of E = 50 V/cm (A) and E = 20 V/cm (B) The traces(b) represcnlN,on(q t 4) wHh laser 1 stabllizcdonto a P tnns~tion ’ I@” = 0. .I” = I)-+ ’ flu(v’ = 0, J’ = 6). Note that in part A trace (h) has been verhcally shifted by rm amount indicated by the arrow. l

racy, however, no field-induced line shifts have been found although these lines correspond to Au = u* - u = 0 transitions from high-lying Rydberg levels wth II* = 50 and u* = 0 This indicates that the lime broadening, shown in fig. 3, is more probably due to field ionization and not so much caused by Stark broadening. For zxcorrect deterr&rt;on of the adiabatic ~OTUZAtion potential a possible shift of the onset of the con-

tinuum with the electric field has to be examined. Thus was done for the ion spectrum Nion(~l + v2) excited

D,(‘Xg’, LI;)=

10353 2 25 cm-‘,

which showsthat the molecular Ion ISmore tightly bound in Its ground state than the neutral molecule. 5. Conclusion The adiabatic ionization potential of L12 and the ion ground-state dissociation energy Do(*Zi) have been determined from the onset of a continuous background III the ion spectra obtamed with resonant two-photon ionization of Liz moleculesIII a supersonicbeam. The effect of field ionization has been taken into account. A much more ;Iccuratc way of determining IP(Li2)

the vibrational and rotational spacings in the ion ground state can be obtained from convergencehmlts and

of the molecular Rydberg levels which manifest themselves through their autoionizaGon hnes. Such an analysis 485

Volume

based on multichannel

21 hlay 1982

CHLhllCAL PIIYSICS LCI-I-ERS

SS. number 5

quantum defect theory has been

by Jungen et al. [ 171. A detailed analysrs of measured autoionizatlon spectra of Liz, which is by no means trivral. is under way, and the results and their comparison with accurate ab initro calculations wrll be published in another paper. performed for H2

121 BP. h!~tbur. E.W. Rolhe, C.P. Reckand A.J Lighlmm, Cbcm. Phys. Letters 56 (1978) 336. [3] S. Leutwyler. A. H~~KIM. L. WV&kand E. Schumacher, Chem. Phys. 48 (1980) 253. [4] S. Leulwyler, h!. Hofmann, H.P. H&i and E Schumacher, Cbem. Phys. Letters 77 (1981) 257. [S] C. Bottcher and A. Da&no, Chem. Phys. Leltcrs 36 (1975)

137.

D.D.Konowalov,lo be pubhshed. 171T.\V.Hinsch, Appl. Opt. 11 (1972) 895. [6]

Acknowledgement

We thank Drs. P. Botschwma and W. Miiller, Fachbereich Chemre, University of Kaiserslautem, for providing us with theu ab initio calculatrons of the Lit ground state. Thanks also to Dr. Ch. Jungen for helpful discussions. We are grateful to B. Bickrnann and L. Meyer for therr skdlful technical assrstance especially during the laborious setting up of the oven assembly. This work was supported by the Deukche Forschungsgememschaft within the Sonderforschungsbereich 9 1: “Energy transfer m atomic and molecular collisions”.

[S] LX. Littman, Opt. Letters 3 (1978) 138. [9] hl.hf. Hessel and C.R. Vi&l, J. Chem. Phys. 70 (1979) 4439. [ 101 K.K. Vcrma. ME. Koch and W.C. Stwalley, J. hiol Spectry. 87 (1981) 548. [ 1 I ] D.H. Levy, L. Wharton, R-E. Smalley. m: Chemical and biochemicalapplicatronsof laser.cd. C-B. hloore (Audemit Press, New York, 1977) [ 121 D. Eisel. W. Demtrijder, P. Botschwina and W. hfuUer, Chem Phys , to be pubhshcd. [ 131 hf. Raoult and Ch. Jungen; J. Chem. Phys. 74 (1981)

3388.

[ 141 J hl. Slchcl. MoL Phys. 18 (1970) 95. [IS] U. Rno, J. Opt. Sot. Am. 65 (1975) 975.

[ 161 C.E. Moore; Atomic Energy Levels, Vol. 1, Nat. Stand. ReL Data Ser. 35 (National Bureauof Stsnduds, \Vashu@ References

[ l] E.H.A. Granneman, hl. Mever, E J. Nygaard nnd h1.J. van der WI& J. Phys B9 (1976) 865.

486

ton, 1971). 1171 Ch. Jungen and D. Dill, .I. Chem. Pbys. 73 (1980) 3338.