- Email: [email protected]
Electronic spectroscopy of large van der waals molecules by resonant two-photon ionization
Volume 86, number 5.6
ELECTRONIC BY RESONANT
CHlXICAL
SPECTROSCOPY
Tunable WE applied transihon
16 November
1981,
laser two-photon
ihrersqv.
m final form
Tel AIW, Israel
15 Dcccmbcr
1981
ol the clcctiomc
orlgn
wrth tune-of-fl@t cwtatlons
mass spectroscopy.
of the Su -
S, clcctromc
fluorcnc AI, and lluorcnc Krr produrcd In supcrsomc c\p.mslons
41 ,pentacene 131 and ovalene [S], were undertaken to explore excited-state energetics and dynamics of such aromatic molecules embedded in a well-characterized “solvent” con!?guration of the vdW complex. A meaningful description of microscopic solvent effects on escrted-state energetics and dynamics of large molecules embedded m a vdW complex requires the charactcrizatron of the composition and the structure of the complex. For the mediumazed vdW molecule tetrazineHe,, the rotational contour analysis of Levy et al. [S-IO] provided a complete charactenzation of the complex. Unfortunately, such an approach IS not readily apphcable to very large vdW molecules, where thermal inhomogeneous broadening due to sequence and hot bands of the low-frequency mtermolecular vlbrations may prevail even at the low temperatures attamed in supersonic expannons. The laser-Induced fluorescence (LIF) method revealed spectral features on the PhysiktdtschChembches Basel, CH4056 Basel, Swtzcrland.
0 009-2614/82/0000-0000/S
combmcd
and of some low wbnllonal
The structure, encrgetrcs and intramolecular dynamIC of van dcr Waals (vdW) molecules are of considerable interest for the elucidation of the features of weakly bound molecular systems. Recent experimental [I-6] and theoretxal [7] studies of very large vdW molecules, consisting of rare-gas atoms(R) bound to large aromatIC molecules(M), such as anthracene [6], tetracene [2,
Universitat
MOLECULES
IONIZATION
1. Introduction
* Permanent address:
5 hlsrch 1987
VAN DER WAALS
1on1za11on oTkugc van dcr Wdsls molcculcs,
to the idcntfxatlon ol fluorcwhr,,
LCI-KRS
*, UZI EVEN and Joshua JORTNER
Dcportrncnt of Clronrstr_v, Tel-Aw Rcccwcd
OF LARGE
TWO-PHOTON
Samuel LEUTWYLER
PHYSICS
Institut
der
02.75 0 1982 North-Holland
low-energy side of the clcctromc origin of the SB -r S, transitIon of the bare molecules clpanded m NC, Ar, Kr and Xe, WIIIC~ were assigned to vrbratronless clectromc transitrons of the MR,, complexes [Z-G]. wlule some spectral features on the red side of the vibrational excltatrons of hl could be attributed to intramolccular vrbrational excitations of MR,, [2,3]. Qudnutatwc dragnostic spectroscopic methods were advanced for the ldentlficatlon of these mdrvrdual spectral fcaturcs, which could be asslgned to drstmct RIR,, vdW molccules [2,3]. This diagnostic LIF tcchmquc [2,3] IS inherently hmrted to hlR,, complexes with a low coordlnation number II, which reveal a distinct spectral structure, and is apphcable only to the promment absorption bands of fluorescent vdW complexes The apphcation of the two-photon Ionization (2PI) method [I I- 161 to vdW molecules is expcctcd to provide a powerful dlagnostrc tool for the rdentrfication of their electronic ekCitatlons and for the unambiguous charactenzation of the composition of these MR,, complexes. WC utrhzcd the 2PI method [ 1 I-16], together with time-of-flight (TOF) mass-sclectne detcction, to record the S, + St spectra of large vdW molccules of a well-characterzed composrtlon. Thus approach also provides mformation on secondary multrplet structure m these 2PI spectra, whoseweak features are ma&cd m the LIF spectra by an overlap with e\crtations of other complexes and promises to overcome other inherent IimItations of the LIF method. In thrs paper, we report the wavelength- and mass-resolved 439
Volume 86, number 5,6
CHEMICAL PHYSICS LiTlXRS
spectra of several vdW complexes of the lluorene (FL) molecule (C,,II,,) wuli mcrt gases. The O-O excitattoo and some low-Iymg vtbrattonal excitations of the Sn + St transitron of FL*Art , FL-Ar2 and FLKrl were momtored by recording the ton signals of the corresponding tonic species, i.e. FL-Ari, FL*Ari and FL.Kr;. 2PI studtes of other vdW complexes were conducted by Smalley et al. [ 161 on benzene clusters and by Schlag et al. [If] on benzene-Art. The present exploration of the electronic ongm and of some low intramolecular escitattons of FL.R,, (II = I ,2) molecules results III an unanlbiguousidenttficatton of thedetarled spectral features of these complexes and provtdes nov-
el InfornIatlon on the dlssociatIon energtes of these interesting, weakly bound, vdW molecules.
2. Experimental The apparatus for 7-PI induced by a tunable iaser combined with TOF mass spectrometry has been described [IS]. A molecular beam was formed by skimming a pulsed supersonic expanston of fluorene seeded mto Ar or Kt, as well as He-Ar and He-Kr matures. The fluorene sample was heated m the nozzle chamber to 100°C (vapor pressure 1.4 Torr) and mixed with dtluent gas at the stagnation pressure p = 300-2000 Torr. The shape of the gas pulse, which was measured by delayed LIF and 2PI of the seeded fluorene, exhtbits a rise ttme of 20~s and a width of 200 p (fwhm). The nozzle dtameter was D= 300 m. The supersomc jet was skimmed twice and then passed through the ion source of a TOF mass spectrometer. The frequencydoubled laser output of a ~olectron DL2 dye laser in the spectral region 2880-3000 A was characterized by a spectral wtdth of 0.3 cm-t, a pulse duration of 4 ns and a pulse energy of I-2 J. The laser beam was focused partialfy m the ion source of the TOF mass spectrometer with an f= 80 mm quartz lens. The laser beam has the intenstty of 1-2 MW cm-* in the source region. The laser crossed the skimmed seeded supersome beam at a distance of 8 cm from the nozzle, where the fluorene density was = 1011 molecules cm-s, The positive ions produced by 2PI were accelerated at right angles to both the molecular beam and to the laser beam into the TOF mass spectrometer, using a double-field focusmg arrangement_ Laser fluorescence excitation spectra m the seeded
supersonic Jet were measured by crossmg the unskimmed pulsed supersonic expansion by the unfocused laser beam at a distance of IO mm from the nozzle. The fluorescence in the range 3300-4000 A was detected by a photomultlplter and recorded as previously descrtbed [IS].
3. Results Fig. I shows the LIF excttation spectra of the fluor. ene molecule m the range 2880-2990 13,in pulsed supersomc cspansions of Ar and of Kr. Effective internal coohng of the vtbration~ degrees of freedom was accomplished at Ar and Kr pressures exceedmg p > 230 Torr, i.e. at pD values exceeding 7 Torr cm, where the relative intensittes of sequence bands were lower than 1%. The spectral features, whose energies are mdependent of the sta~ation pressure, are due to the O-O and to the vtbrational excitations of the So -+ St (I At + lBz) [ 19,201 electronic transition of the bare molecule and are in complete accord with a recent spectroscopic study of the bare fluorene molecule in a pulsed planar supersonic Jet [ 191. The weak spectral features appearing on the low energy side of the electronic origin, whose positions depend on the dduent and whose Intensities depend on the stagnation pressure, are attnbuted to FL-R, vdW complexes. The weak features appea~ng to the red side of the totally symmetnc (a,) 208,408 and 728 cm-l fundamental vtbrations were assrgned to corresponding vrbrationaf excitations of the vdW complexes. On the basis of the order of appearance of these spectral features, we were able to identify the O-O excitattons and some intramolecular vibrational excitations of the chemically distinct FL*Ar,, FL-ArZ and FLKrt complexes, which are marked in fig. 1 and summarized in table 1. The following four notable results are of interest. First, red spectral shifts are exhibited originating from dispersive interactions [4] and posstbly also from rhpoleinduced-dipole interactions [ 191. Second, the spectral shifts for FL-Art and for the prominent peak of FL-Krt are proportional to the pola~ab~ties of these rare gases, as expected for both dispersive [4] and for dipole-induced-dipole interactions [19]. Third, the spectral shifts for FL-Art and FL.Ar2 are proportional to the coordination number n. Fourth, for FL-Ar, and for FL-Arz only ? single spectral fea-
CflChffCAL DIIYSICS LCTTCRS
Volume 86, number 5,6
PHOTON
ENERGY
f crri’~
4 33&C I
34100 ,
IO
3-a
l\r p- 210 TORR a,(2081
Kt p=380
I
TORR
1 I
0
I
29m
,
I
2930
WAVELENGTH
I_
1
1
2950
2970
1
(ii)
ture could be identified for each complex, wtie for FL.Kr, two spectral features are exhiblted. Novel info~ation concerning the identi~cation and characte~~ion of the electro~c-vibrations excitations of these large vdW molecules was obtained from wavelength.resoIved and maswesolved resonance TPI spectra. Figs. 2 and 3 show the mass-resolved ton signal spectra of FL*, FL-Arf, FL-Ar; and FL*Kri. These
5 &rch 1982
f% 1 IIuorcsrcnce e~tstron spectra m the rungc 78902980 A of Ruorenem pulsed su~crs0111c of Ar .md Er I‘luarcnc at IOO”C (vapor pressure I 4 Tow) wx sccdcd tnto the rare gas.at the prcs~~rcsmdrcad on the cwcs and expanded tflrou~fl the 300 pm noztfe Tfwzfr~queney~oubfed dye lasercrossedthe supcrsontcrxpanston at 7 mm downstrcant. The clcctromc ongm of the bare tt~ol~culcis labclfcd by O-O. The al vlbntional fcaturcs of tftc hrc molcculc arc labelled by 3, ( ), where the number m parcnthcscsdenotes frequency tn cm-t. Features due to fhrorcncAr,) co~~~pic~cs x(: denoted TL Art for n = 1 and fI-AI* for rr = 3. The \pcctrai features due to the wbr,ttronlcssewttation 01 lluorcnc%rt arc dcnotcd TL Mrl, whtle the weak feature tn the lluorcnc spectrum tn Kr at 22 cm- bcfow the O-O of the bare molecule is due to a sequenceband
rxpmsvinc
spectra correspond to the spectral features of the mtermediate resonant ‘6, state, which mvolvcs the vlbrdtionfess So + St exc&ations of the vdW molccuies From the excellent agreement between the LIF spectra and the ton current spectra (table 1) two COIIC~USIO~IS emerge. Firstly, conclusive assqmlents of the spectral features of the individual FL.R,, vdW molecules wcrc obtamed. Secondly, the resonant 2PI. VU the clectronICOrion of these vdW nloleculcs, results m the pfiotoselective producrton of large vdW ions. The photofmgmcntation probabrhty of these FL-R,, vdW Ions 1squite low The FL’ 3PI spectrunt for FL seeded tn Ar (fig 2) exlublts, in addition to the O-O e~cttatton of the bare n~ole~uie, Z.Iweak feature at 7,963 8 tf, whtch comcidcs with the vIbratIonless cxeitatlon of FL*Ar rn the LIF spectrum and m the FL.Ari ton spectrum. This weak feature orrgmates from the fragitlent~tton of the FkAri ion produced via a remnant 2PI process, where the excess total energy above the lontzatton threshold is &, z 3800 cm-l *. Fragmentation of a large M-R+ ion at such excess total energy can occur by two mechanisms (I) Direct dtssoctatron m the romzation step VIP cscitatton of the Mf-R stretching mode. Model culcul~tlons [?I] for benzene-Art dnd benzene-Art mdrcdtc that this process is precluded by small Fran&-Condon factors for the intermolecular slretchmg vibration. (Ii) Vlbrationnl predissociation of the M+-R bond. From * Thhc~YCCSS energy mvolvcd In the 2Pi processof the bare fluorcnc molecule, whose ronkalmn potcntd IS IP = 7 94 eV 1221, excited at the electronicoryin (33784 cm-t) is E, = 3840 cm-t _Nqfccttng small encrgch shifts of the IP of the vd\V complcws, the excessenergy involved m 7PI of the vdW complcvcs ts close lo thts value off&.
441
Volume 86. number 5,6
CHEhflCAL
PHYSICS
5 March 1982
LIXTERS
Table 1 Spcctni shtf&sof the eIfctromc or&m of the St (’ Bz) state of fhiorenc~K,rfomptexcs SpCCICS
m/e
hfass.rcsoIvcd2PI a~~)
LIT a1b)
lluorcne
166 206
tluorene Arz ftuorcnc Kr
246 248-252
2960.2 2960.3 2963 8 2966.9 2962.9 29655
2960.0 (0)
Iluorcnc~Ar
(0) (-1) (-41) (-76) f-33) (-63)
1963 6 2966.6 2962 9 2965.4
(-41) (-75) (-33) (-63)
a) ~Vaveleo~thsm d; numbers m parentheses represent sh&s in cm- 1 from the efectromc or& of the bare molecufc. b) Absolute accuracy of ~vavelcn~thscale is 20.5 A The relatrvc accuracy of thewavelength scale IS rO.1 A, while the accuracy of the spcclial shifts is r
I 5 ed.
ENERGY ( cm-’I 33800
33750
r
the Fran&-Condon factors deduced from the first band in the He1 photoelectron spectrum of tluorene [X?j configurational changes accompanymg the ionization of the molecule are modest. Accordingly, one expects that follo~ving lonizatjon at .!?, = 3800 CIII-~ 3 substantral fraction of the MRt ions will be produced m the vibrattonless state and at low vibrational levels of Al+, which are stable wrth respect to the vtbratronal predissocration. Thus, the fragmentatron probability, I,, of FL.R; ions at Ee = 3800 cm-t ISexpected to be 10w.f~ of FL.Arl was estrmated from the ratio of the relative intensity of the FL.Arl excitatron in the ion current spectrum of FL*, and the relative intensity of the FL-Art band m the LIF spectrum. We fmd f, m 0.2 for FL-Arf at Ec = 3800 cm-‘. For FL.-Krl no evidence for fragmentation was observed in the ion current spectrum of FL’ (fig. 3) and we estimate thatf, Q 0.05 for the FL-Kri ton at this total excess energy. The interrogation of FL.R,, molecules by 2PI unveils some further details of the spectra of these complexes, providing unambiguous identification of some weak features which are obscured in the LIF spectra. The LIF spectra of FL in Kr (fig. 1) reveal, in addition
33700
FLUORENE* Ar* mle : 206
m/e
=246
Fig. 2. Ion current
Y~ISUS
the laser wavelength For Ruorene+
(uppercurve),tluorene Art (middel curve) and ftuorene AI:
3
2962 WAVELENGTH
442
2966 (;I
2’
(lower curve). Fluorene at 100°C (vapor pressure 1.4 Torr) was seeded into Ar at p = 1200 Ton. The seeded gas was WCpanded through the 300 am nozzle. The focused Laserbeam crossed the skimmed supersoac expensron at 80 mm from the nozzle. Each spectrum is displayed on a different relative intenaty scale. The relalivc intensity tie for FL-AI+ IShigher by a numerical factor of four than that for FL&.
CHChllCAL
Volume 86, number 5,6
ENERGY ( cti’ 300 I /
33750 I I
I
1
33700 I I II
I
PHYSICS LIXTTCRS
I
33’ I
0
FLUORENE * de
FLUORENE . Kr*
= 166
,
mle - 246 - 252
I 8
2962
,
I
2966
I
2970
WAVELENGTH 6) rig. 3. Ion current versus laser wsvclenglh for fluorene+ (upper curve) and for fluorcnc-Kr+ (lower curve). lluorcnc was seeded into Kr at p = 1000 Torr. All other experimental condltlons as in fig. 2. Each spectrum is dlsplayed on a dlffcrcnt relstlvc in-
tcnslty scale.
to the prominent feature corresponding to the vlbrationless transItIon of FL.Kr, , an addItional weak feature at a higher energy, which was also attributed to the FL-Krl complex. The same feature appears III the FL.Kri ion current spectrum (fig. 3), providmg conclusive evidence for a doublet structure of the electronic origin of FL-Krl (table 1). The FL-Art ion current spectrum (fig. 2) also reveals an additional feature on the high-energy side of the mam feature of FL-Ar,, which overlaps the O-O transition of the bare molecule. We have excluded the possibility that this weak feature in the FL-Ari ion spectrum or&mates from the three-body recombination process FL+ + Ar + Ar
5 hlrrch 1982
+ FL-Ar; t Ar in the ion source region, as the relative intensity of this feature was found to be independent of the Ar pressure III the beam. Accordmgly, the vibntionless excltation of both FL-Kr, and FLAr, near the electromc ongin show a characteristic doublet, the separation of the second weak peak from the main feature being 6v = 40 cm-l for the Ar compleh and 6~ = 30 cm-1 for the Kr complex. This doublet structure of FL-R, (R = Ar, Kr) can origmate either from the existence of two dlstmct chemical Isomers of FL-R,, e g the R Jtom sitting above the sti-membered rmg and above the five-membered ring, or from the cxc~tation of a vibrational mode of the R atom with respect to the aromatic plane m a single energetically favored FL-R, structure. We are reluctant to accept the first alternative as one of the two transItions of FL-Arl practically coincides with the O-O transitron of the bare molecule, and it is unthinkable that the binding of an Ar atom to FL will exert the same interaction with the ground as with the excited aromatlc molecule. The second alternattve is favored by the 6~ values, which are of reasonable magmtude for the stretching vibrational frequency of a rare-gas atom with respect to an aromatic ring [7]. 2PI studies can provide pertment mformatlon on the bmdmg energies of large vdW molecules. Resonant 2PI of M-RI ions, proceeding via an intcrmcdlate excitatlon of an intramolecular vibration in St, WIII result essentially In the M+ ion when the vIbratIonal predlssociation (VP) channel IS open in the intermediate state, while when the VP channel is closed estenslve
production ofhI_Ri ion is ekpectcd.The LIF spectrum (fig. I) reveals vibrational excltatlons of the FL-Art complex wltb the prominent mtramolecular fundamental vibrations of 208,408 and 728 cm-l, which are marked m fig. 1. We have searched for the production of FL-Arl ions m the energy- and mass-resolved TP spectrum resultmg from these vrbrationally excited states of FL.Art . Excltatlons of the 208 and 408 cm-1 intermolecular fundamentals of the FL-Art resulted In prominent features m the FL-Art Ion spectrum at the same energies as evhiblted in the LIF spectrum, whde excitation of the 728 cm-l intramolecular vibration of FL-Aq did not result in any observable Ion current of FL-Ari. Accordmgly, lower and upper limits are set for the dissociation energy D of the FL-Ar, complex m the St state, 408 G D Q 723 cm-t . This estimate of a relatively high value of D concurs with the results 443
Volume 86, nun&r
Cll~~llCAL
$6
of model calculations to 13rgc aromaw
j7]
for the bindmg
of rare gases
molcculcs
4. Conclusions
We hvc sllowll that ( I) Resonant “smglc-color”
1PI is apphcable for the ~~r~~~~tio~~ of large vdW eons III cases where no marked strucliiral changes lake place during the ionization proccss (2) The clcctromc by the LIF
orlgrns assigned
technque
to vdW complehes
were ~ln~rnbi~ons!y
identified
by the 2PI. (3) Dctatlcd turcs,
whrch
were clearly
mformation overlap
dlstmgulshed
(4) Pcrtment
on secondary
in the LIF
spectral
Tea.
spectrum,
in the 1PI method
Inforill3tion
zy of the fluorene Arl
excitation
on the dawciation
ener-
complcs was inferred
Acknowledgement We arc grateful sions. Tlus research Commlttec Academy Bln~tion~
141 A. Ammv, U. Even and J Jortncr, J. Chcm. Phys 75 (1981) 2489. 15 I A. Amuav, U Cvcn and J. Jortncr, J. Chem. Phys 74 (1981) 3745 [6] T R. Hayes. W. He&c, H L Selzlc and C.W. Schtag. Chcnl Phys. Lcttcrs 77 (1980) 19 171 XI J. Ondrcchcn, 2. IJerkovatch-Yclhng and J. Jortncr, J Am. Chcm Sot , to bc published [8 1 D.fl. Levy, III Advances m chemrcal physics scrrcs. Vol. 47 Photo~lective chcn~~try, cds J. Jortner, R.D. Lcvinc and S A. Rice (Wdey-intencicncc, New York, 1981) [P] R E Smalley, L. Wharton, D H Levy and D.W Chandler, J. hlol Spcctry. 66 (1977) 375. [IO] R C: Smalley, L. Wharton, D.H. Levy and D W Chnndlcr. J Chem Phys. 68 (1978) 2476. [ t 11A. Hcrrman, S Lcutwyler. E. Schumacher and L Waste, tlelv. Chml Act? 61 (1978) 453. [ 121U Boesl. H J Ncusscr and E W Schlag, Z Naturforsch A33 (1978) 1546. 1131 T G. Dtetz, KA. Duncan, Bf C Livcrman and R.E Smalley. Chem. Phys Letters 70 (1980) 246. [ I4 J T G Dxetz, M A Dunnn. KC. Llvcnnan and R E. Smalley, J. Chcm. Phys 73 (1980) 4816, 81 A. Duncan, T G. Dictz and R.E. Smatlcy, J Chem Phys 75 (1981) 2118.
1IS I
to AVIVAmirav was supported
for Basic Research of Sciences, Science
for helpful dacusin part by the
of the Israel National
and by the United
Foundation
(No
164!),
States-Israel Jerusalem,
iSKll?l.
References
S Leutwylcr and U. Even, Chcm. Phys Letters 81 (1981) 578. [ t6j J B. Hopkins, D C. Powers and R C SmaUcy. J. Phys.
Chem , to bc pubbshed [ I71 C.W Sch1.g and H L. Selzlc. prlvatc communicalon; IO bc published
[ 1st A. Amuav. U Even and J Jortncr, 3. Chcm. Phys 7.5 (1981) 3770.
( 191 A Ammv, U. Even and J. Jormncr,Spectroscopy of the Fluorenc MolcLule in Plrnar Supersomc E\pansrons, Chcm. Phys., to bc published /201 A Bree and R. Zwutch, J. Chem. Phys. 51 (1969) 903 [Zl]
111II Ii. Levy, Ann. Rev. Phys Chew. 31 (1980) 197 121 A Anww, U. Even and J Jortner. Chcm. Phys Lcttcrs 61(1979) 9. 131 A AniuW U. Even .md J. Jortncr, J Phys. Chem. 85 (19811309.
444
5 March 1982
PIIYSICS LE-MXRS
2 Berkowtch-Y&n,
J Jortner, S. Leutwylerand
U.
Even, to be published 1221 J P. hlarer and D.W. Tumcr, faraday Discussion Chcm sot. 54 (1972)
149
ELECTRONIC BY RESONANT
CHlXICAL
SPECTROSCOPY
Tunable WE applied transihon
16 November
1981,
laser two-photon
ihrersqv.
m final form
Tel AIW, Israel
15 Dcccmbcr
1981
ol the clcctiomc
orlgn
wrth tune-of-fl@t cwtatlons
mass spectroscopy.
of the Su -
S, clcctromc
fluorcnc AI, and lluorcnc Krr produrcd In supcrsomc c\p.mslons
41 ,pentacene 131 and ovalene [S], were undertaken to explore excited-state energetics and dynamics of such aromatic molecules embedded in a well-characterized “solvent” con!?guration of the vdW complex. A meaningful description of microscopic solvent effects on escrted-state energetics and dynamics of large molecules embedded m a vdW complex requires the charactcrizatron of the composition and the structure of the complex. For the mediumazed vdW molecule tetrazineHe,, the rotational contour analysis of Levy et al. [S-IO] provided a complete charactenzation of the complex. Unfortunately, such an approach IS not readily apphcable to very large vdW molecules, where thermal inhomogeneous broadening due to sequence and hot bands of the low-frequency mtermolecular vlbrations may prevail even at the low temperatures attamed in supersonic expannons. The laser-Induced fluorescence (LIF) method revealed spectral features on the PhysiktdtschChembches Basel, CH4056 Basel, Swtzcrland.
0 009-2614/82/0000-0000/S
combmcd
and of some low wbnllonal
The structure, encrgetrcs and intramolecular dynamIC of van dcr Waals (vdW) molecules are of considerable interest for the elucidation of the features of weakly bound molecular systems. Recent experimental [I-6] and theoretxal [7] studies of very large vdW molecules, consisting of rare-gas atoms(R) bound to large aromatIC molecules(M), such as anthracene [6], tetracene [2,
Universitat
MOLECULES
IONIZATION
1. Introduction
* Permanent address:
5 hlsrch 1987
VAN DER WAALS
1on1za11on oTkugc van dcr Wdsls molcculcs,
to the idcntfxatlon ol fluorcwhr,,
LCI-KRS
*, UZI EVEN and Joshua JORTNER
Dcportrncnt of Clronrstr_v, Tel-Aw Rcccwcd
OF LARGE
TWO-PHOTON
Samuel LEUTWYLER
PHYSICS
Institut
der
02.75 0 1982 North-Holland
low-energy side of the clcctromc origin of the SB -r S, transitIon of the bare molecules clpanded m NC, Ar, Kr and Xe, WIIIC~ were assigned to vrbratronless clectromc transitrons of the MR,, complexes [Z-G]. wlule some spectral features on the red side of the vibrational excltatrons of hl could be attributed to intramolccular vrbrational excitations of MR,, [2,3]. Qudnutatwc dragnostic spectroscopic methods were advanced for the ldentlficatlon of these mdrvrdual spectral fcaturcs, which could be asslgned to drstmct RIR,, vdW molccules [2,3]. This diagnostic LIF tcchmquc [2,3] IS inherently hmrted to hlR,, complexes with a low coordlnation number II, which reveal a distinct spectral structure, and is apphcable only to the promment absorption bands of fluorescent vdW complexes The apphcation of the two-photon Ionization (2PI) method [I I- 161 to vdW molecules is expcctcd to provide a powerful dlagnostrc tool for the rdentrfication of their electronic ekCitatlons and for the unambiguous charactenzation of the composition of these MR,, complexes. WC utrhzcd the 2PI method [ 1 I-16], together with time-of-flight (TOF) mass-sclectne detcction, to record the S, + St spectra of large vdW molccules of a well-characterzed composrtlon. Thus approach also provides mformation on secondary multrplet structure m these 2PI spectra, whoseweak features are ma&cd m the LIF spectra by an overlap with e\crtations of other complexes and promises to overcome other inherent IimItations of the LIF method. In thrs paper, we report the wavelength- and mass-resolved 439
Volume 86, number 5,6
CHEMICAL PHYSICS LiTlXRS
spectra of several vdW complexes of the lluorene (FL) molecule (C,,II,,) wuli mcrt gases. The O-O excitattoo and some low-Iymg vtbrattonal excitations of the Sn + St transitron of FL*Art , FL-Ar2 and FLKrl were momtored by recording the ton signals of the corresponding tonic species, i.e. FL-Ari, FL*Ari and FL.Kr;. 2PI studtes of other vdW complexes were conducted by Smalley et al. [ 161 on benzene clusters and by Schlag et al. [If] on benzene-Art. The present exploration of the electronic ongm and of some low intramolecular escitattons of FL.R,, (II = I ,2) molecules results III an unanlbiguousidenttficatton of thedetarled spectral features of these complexes and provtdes nov-
el InfornIatlon on the dlssociatIon energtes of these interesting, weakly bound, vdW molecules.
2. Experimental The apparatus for 7-PI induced by a tunable iaser combined with TOF mass spectrometry has been described [IS]. A molecular beam was formed by skimming a pulsed supersonic expanston of fluorene seeded mto Ar or Kt, as well as He-Ar and He-Kr matures. The fluorene sample was heated m the nozzle chamber to 100°C (vapor pressure 1.4 Torr) and mixed with dtluent gas at the stagnation pressure p = 300-2000 Torr. The shape of the gas pulse, which was measured by delayed LIF and 2PI of the seeded fluorene, exhtbits a rise ttme of 20~s and a width of 200 p (fwhm). The nozzle dtameter was D= 300 m. The supersomc jet was skimmed twice and then passed through the ion source of a TOF mass spectrometer. The frequencydoubled laser output of a ~olectron DL2 dye laser in the spectral region 2880-3000 A was characterized by a spectral wtdth of 0.3 cm-t, a pulse duration of 4 ns and a pulse energy of I-2 J. The laser beam was focused partialfy m the ion source of the TOF mass spectrometer with an f= 80 mm quartz lens. The laser beam has the intenstty of 1-2 MW cm-* in the source region. The laser crossed the skimmed seeded supersome beam at a distance of 8 cm from the nozzle, where the fluorene density was = 1011 molecules cm-s, The positive ions produced by 2PI were accelerated at right angles to both the molecular beam and to the laser beam into the TOF mass spectrometer, using a double-field focusmg arrangement_ Laser fluorescence excitation spectra m the seeded
supersonic Jet were measured by crossmg the unskimmed pulsed supersonic expansion by the unfocused laser beam at a distance of IO mm from the nozzle. The fluorescence in the range 3300-4000 A was detected by a photomultlplter and recorded as previously descrtbed [IS].
3. Results Fig. I shows the LIF excttation spectra of the fluor. ene molecule m the range 2880-2990 13,in pulsed supersomc cspansions of Ar and of Kr. Effective internal coohng of the vtbration~ degrees of freedom was accomplished at Ar and Kr pressures exceedmg p > 230 Torr, i.e. at pD values exceeding 7 Torr cm, where the relative intensittes of sequence bands were lower than 1%. The spectral features, whose energies are mdependent of the sta~ation pressure, are due to the O-O and to the vtbrational excitations of the So -+ St (I At + lBz) [ 19,201 electronic transition of the bare molecule and are in complete accord with a recent spectroscopic study of the bare fluorene molecule in a pulsed planar supersonic Jet [ 191. The weak spectral features appearing on the low energy side of the electronic origin, whose positions depend on the dduent and whose Intensities depend on the stagnation pressure, are attnbuted to FL-R, vdW complexes. The weak features appea~ng to the red side of the totally symmetnc (a,) 208,408 and 728 cm-l fundamental vtbrations were assrgned to corresponding vrbrationaf excitations of the vdW complexes. On the basis of the order of appearance of these spectral features, we were able to identify the O-O excitattons and some intramolecular vibrational excitations of the chemically distinct FL*Ar,, FL-ArZ and FLKrt complexes, which are marked in fig. 1 and summarized in table 1. The following four notable results are of interest. First, red spectral shifts are exhibited originating from dispersive interactions [4] and posstbly also from rhpoleinduced-dipole interactions [ 191. Second, the spectral shifts for FL-Art and for the prominent peak of FL-Krt are proportional to the pola~ab~ties of these rare gases, as expected for both dispersive [4] and for dipole-induced-dipole interactions [19]. Third, the spectral shifts for FL-Art and FL.Ar2 are proportional to the coordination number n. Fourth, for FL-Ar, and for FL-Arz only ? single spectral fea-
CflChffCAL DIIYSICS LCTTCRS
Volume 86, number 5,6
PHOTON
ENERGY
f crri’~
4 33&C I
34100 ,
IO
3-a
l\r p- 210 TORR a,(2081
Kt p=380
I
TORR
1 I
0
I
29m
,
I
2930
WAVELENGTH
I_
1
1
2950
2970
1
(ii)
ture could be identified for each complex, wtie for FL.Kr, two spectral features are exhiblted. Novel info~ation concerning the identi~cation and characte~~ion of the electro~c-vibrations excitations of these large vdW molecules was obtained from wavelength.resoIved and maswesolved resonance TPI spectra. Figs. 2 and 3 show the mass-resolved ton signal spectra of FL*, FL-Arf, FL-Ar; and FL*Kri. These
5 &rch 1982
f% 1 IIuorcsrcnce e~tstron spectra m the rungc 78902980 A of Ruorenem pulsed su~crs0111c of Ar .md Er I‘luarcnc at IOO”C (vapor pressure I 4 Tow) wx sccdcd tnto the rare gas.at the prcs~~rcsmdrcad on the cwcs and expanded tflrou~fl the 300 pm noztfe Tfwzfr~queney~oubfed dye lasercrossedthe supcrsontcrxpanston at 7 mm downstrcant. The clcctromc ongm of the bare tt~ol~culcis labclfcd by O-O. The al vlbntional fcaturcs of tftc hrc molcculc arc labelled by 3, ( ), where the number m parcnthcscsdenotes frequency tn cm-t. Features due to fhrorcncAr,) co~~~pic~cs x(: denoted TL Art for n = 1 and fI-AI* for rr = 3. The \pcctrai features due to the wbr,ttronlcssewttation 01 lluorcnc%rt arc dcnotcd TL Mrl, whtle the weak feature tn the lluorcnc spectrum tn Kr at 22 cm- bcfow the O-O of the bare molecule is due to a sequenceband
rxpmsvinc
spectra correspond to the spectral features of the mtermediate resonant ‘6, state, which mvolvcs the vlbrdtionfess So + St exc&ations of the vdW molccuies From the excellent agreement between the LIF spectra and the ton current spectra (table 1) two COIIC~USIO~IS emerge. Firstly, conclusive assqmlents of the spectral features of the individual FL.R,, vdW molecules wcrc obtamed. Secondly, the resonant 2PI. VU the clectronICOrion of these vdW nloleculcs, results m the pfiotoselective producrton of large vdW ions. The photofmgmcntation probabrhty of these FL-R,, vdW Ions 1squite low The FL’ 3PI spectrunt for FL seeded tn Ar (fig 2) exlublts, in addition to the O-O e~cttatton of the bare n~ole~uie, Z.Iweak feature at 7,963 8 tf, whtch comcidcs with the vIbratIonless cxeitatlon of FL*Ar rn the LIF spectrum and m the FL.Ari ton spectrum. This weak feature orrgmates from the fragitlent~tton of the FkAri ion produced via a remnant 2PI process, where the excess total energy above the lontzatton threshold is &, z 3800 cm-l *. Fragmentation of a large M-R+ ion at such excess total energy can occur by two mechanisms (I) Direct dtssoctatron m the romzation step VIP cscitatton of the Mf-R stretching mode. Model culcul~tlons [?I] for benzene-Art dnd benzene-Art mdrcdtc that this process is precluded by small Fran&-Condon factors for the intermolecular slretchmg vibration. (Ii) Vlbrationnl predissociation of the M+-R bond. From * Thhc~YCCSS energy mvolvcd In the 2Pi processof the bare fluorcnc molecule, whose ronkalmn potcntd IS IP = 7 94 eV 1221, excited at the electronicoryin (33784 cm-t) is E, = 3840 cm-t _Nqfccttng small encrgch shifts of the IP of the vd\V complcws, the excessenergy involved m 7PI of the vdW complcvcs ts close lo thts value off&.
441
Volume 86. number 5,6
CHEhflCAL
PHYSICS
5 March 1982
LIXTERS
Table 1 Spcctni shtf&sof the eIfctromc or&m of the St (’ Bz) state of fhiorenc~K,rfomptexcs SpCCICS
m/e
hfass.rcsoIvcd2PI a~~)
LIT a1b)
lluorcne
166 206
tluorene Arz ftuorcnc Kr
246 248-252
2960.2 2960.3 2963 8 2966.9 2962.9 29655
2960.0 (0)
Iluorcnc~Ar
(0) (-1) (-41) (-76) f-33) (-63)
1963 6 2966.6 2962 9 2965.4
(-41) (-75) (-33) (-63)
a) ~Vaveleo~thsm d; numbers m parentheses represent sh&s in cm- 1 from the efectromc or& of the bare molecufc. b) Absolute accuracy of ~vavelcn~thscale is 20.5 A The relatrvc accuracy of thewavelength scale IS rO.1 A, while the accuracy of the spcclial shifts is r
I 5 ed.
ENERGY ( cm-’I 33800
33750
r
the Fran&-Condon factors deduced from the first band in the He1 photoelectron spectrum of tluorene [X?j configurational changes accompanymg the ionization of the molecule are modest. Accordingly, one expects that follo~ving lonizatjon at .!?, = 3800 CIII-~ 3 substantral fraction of the MRt ions will be produced m the vibrattonless state and at low vibrational levels of Al+, which are stable wrth respect to the vtbratronal predissocration. Thus, the fragmentatron probability, I,, of FL.R; ions at Ee = 3800 cm-t ISexpected to be 10w.f~ of FL.Arl was estrmated from the ratio of the relative intensity of the FL.Arl excitatron in the ion current spectrum of FL*, and the relative intensity of the FL-Art band m the LIF spectrum. We fmd f, m 0.2 for FL-Arf at Ec = 3800 cm-‘. For FL.-Krl no evidence for fragmentation was observed in the ion current spectrum of FL’ (fig. 3) and we estimate thatf, Q 0.05 for the FL-Kri ton at this total excess energy. The interrogation of FL.R,, molecules by 2PI unveils some further details of the spectra of these complexes, providing unambiguous identification of some weak features which are obscured in the LIF spectra. The LIF spectra of FL in Kr (fig. 1) reveal, in addition
33700
FLUORENE* Ar* mle : 206
m/e
=246
Fig. 2. Ion current
Y~ISUS
the laser wavelength For Ruorene+
(uppercurve),tluorene Art (middel curve) and ftuorene AI:
3
2962 WAVELENGTH
442
2966 (;I
2’
(lower curve). Fluorene at 100°C (vapor pressure 1.4 Torr) was seeded into Ar at p = 1200 Ton. The seeded gas was WCpanded through the 300 am nozzle. The focused Laserbeam crossed the skimmed supersoac expensron at 80 mm from the nozzle. Each spectrum is displayed on a different relative intenaty scale. The relalivc intensity tie for FL-AI+ IShigher by a numerical factor of four than that for FL&.
CHChllCAL
Volume 86, number 5,6
ENERGY ( cti’ 300 I /
33750 I I
I
1
33700 I I II
I
PHYSICS LIXTTCRS
I
33’ I
0
FLUORENE * de
FLUORENE . Kr*
= 166
,
mle - 246 - 252
I 8
2962
,
I
2966
I
2970
WAVELENGTH 6) rig. 3. Ion current versus laser wsvclenglh for fluorene+ (upper curve) and for fluorcnc-Kr+ (lower curve). lluorcnc was seeded into Kr at p = 1000 Torr. All other experimental condltlons as in fig. 2. Each spectrum is dlsplayed on a dlffcrcnt relstlvc in-
tcnslty scale.
to the prominent feature corresponding to the vlbrationless transItIon of FL.Kr, , an addItional weak feature at a higher energy, which was also attributed to the FL-Krl complex. The same feature appears III the FL.Kri ion current spectrum (fig. 3), providmg conclusive evidence for a doublet structure of the electronic origin of FL-Krl (table 1). The FL-Art ion current spectrum (fig. 2) also reveals an additional feature on the high-energy side of the mam feature of FL-Ar,, which overlaps the O-O transition of the bare molecule. We have excluded the possibility that this weak feature in the FL-Ari ion spectrum or&mates from the three-body recombination process FL+ + Ar + Ar
5 hlrrch 1982
+ FL-Ar; t Ar in the ion source region, as the relative intensity of this feature was found to be independent of the Ar pressure III the beam. Accordmgly, the vibntionless excltation of both FL-Kr, and FLAr, near the electromc ongin show a characteristic doublet, the separation of the second weak peak from the main feature being 6v = 40 cm-l for the Ar compleh and 6~ = 30 cm-1 for the Kr complex. This doublet structure of FL-R, (R = Ar, Kr) can origmate either from the existence of two dlstmct chemical Isomers of FL-R,, e g the R Jtom sitting above the sti-membered rmg and above the five-membered ring, or from the cxc~tation of a vibrational mode of the R atom with respect to the aromatic plane m a single energetically favored FL-R, structure. We are reluctant to accept the first alternative as one of the two transItions of FL-Arl practically coincides with the O-O transitron of the bare molecule, and it is unthinkable that the binding of an Ar atom to FL will exert the same interaction with the ground as with the excited aromatlc molecule. The second alternattve is favored by the 6~ values, which are of reasonable magmtude for the stretching vibrational frequency of a rare-gas atom with respect to an aromatic ring [7]. 2PI studies can provide pertment mformatlon on the bmdmg energies of large vdW molecules. Resonant 2PI of M-RI ions, proceeding via an intcrmcdlate excitatlon of an intramolecular vibration in St, WIII result essentially In the M+ ion when the vIbratIonal predlssociation (VP) channel IS open in the intermediate state, while when the VP channel is closed estenslve
production ofhI_Ri ion is ekpectcd.The LIF spectrum (fig. I) reveals vibrational excltatlons of the FL-Art complex wltb the prominent mtramolecular fundamental vibrations of 208,408 and 728 cm-l, which are marked m fig. 1. We have searched for the production of FL-Arl ions m the energy- and mass-resolved TP spectrum resultmg from these vrbrationally excited states of FL.Art . Excltatlons of the 208 and 408 cm-1 intermolecular fundamentals of the FL-Art resulted In prominent features m the FL-Art Ion spectrum at the same energies as evhiblted in the LIF spectrum, whde excitation of the 728 cm-l intramolecular vibration of FL-Aq did not result in any observable Ion current of FL-Ari. Accordmgly, lower and upper limits are set for the dissociation energy D of the FL-Ar, complex m the St state, 408 G D Q 723 cm-t . This estimate of a relatively high value of D concurs with the results 443
Volume 86, nun&r
Cll~~llCAL
$6
of model calculations to 13rgc aromaw
j7]
for the bindmg
of rare gases
molcculcs
4. Conclusions
We hvc sllowll that ( I) Resonant “smglc-color”
1PI is apphcable for the ~~r~~~~tio~~ of large vdW eons III cases where no marked strucliiral changes lake place during the ionization proccss (2) The clcctromc by the LIF
orlgrns assigned
technque
to vdW complehes
were ~ln~rnbi~ons!y
identified
by the 2PI. (3) Dctatlcd turcs,
whrch
were clearly
mformation overlap
dlstmgulshed
(4) Pcrtment
on secondary
in the LIF
spectral
Tea.
spectrum,
in the 1PI method
Inforill3tion
zy of the fluorene Arl
excitation
on the dawciation
ener-
complcs was inferred
Acknowledgement We arc grateful sions. Tlus research Commlttec Academy Bln~tion~
141 A. Ammv, U. Even and J Jortncr, J. Chcm. Phys 75 (1981) 2489. 15 I A. Amuav, U Cvcn and J. Jortncr, J. Chem. Phys 74 (1981) 3745 [6] T R. Hayes. W. He&c, H L Selzlc and C.W. Schtag. Chcnl Phys. Lcttcrs 77 (1980) 19 171 XI J. Ondrcchcn, 2. IJerkovatch-Yclhng and J. Jortncr, J Am. Chcm Sot , to bc published [8 1 D.fl. Levy, III Advances m chemrcal physics scrrcs. Vol. 47 Photo~lective chcn~~try, cds J. Jortner, R.D. Lcvinc and S A. Rice (Wdey-intencicncc, New York, 1981) [P] R E Smalley, L. Wharton, D H Levy and D.W Chandler, J. hlol Spcctry. 66 (1977) 375. [IO] R C: Smalley, L. Wharton, D.H. Levy and D W Chnndlcr. J Chem Phys. 68 (1978) 2476. [ t 11A. Hcrrman, S Lcutwyler. E. Schumacher and L Waste, tlelv. Chml Act? 61 (1978) 453. [ 121U Boesl. H J Ncusscr and E W Schlag, Z Naturforsch A33 (1978) 1546. 1131 T G. Dtetz, KA. Duncan, Bf C Livcrman and R.E Smalley. Chem. Phys Letters 70 (1980) 246. [ I4 J T G Dxetz, M A Dunnn. KC. Llvcnnan and R E. Smalley, J. Chcm. Phys 73 (1980) 4816, 81 A. Duncan, T G. Dictz and R.E. Smatlcy, J Chem Phys 75 (1981) 2118.
1IS I
to AVIVAmirav was supported
for Basic Research of Sciences, Science
for helpful dacusin part by the
of the Israel National
and by the United
Foundation
(No
164!),
States-Israel Jerusalem,
iSKll?l.
References
S Leutwylcr and U. Even, Chcm. Phys Letters 81 (1981) 578. [ t6j J B. Hopkins, D C. Powers and R C SmaUcy. J. Phys.
Chem , to bc pubbshed [ I71 C.W Sch1.g and H L. Selzlc. prlvatc communicalon; IO bc published
[ 1st A. Amuav. U Even and J Jortncr, 3. Chcm. Phys 7.5 (1981) 3770.
( 191 A Ammv, U. Even and J. Jormncr,Spectroscopy of the Fluorenc MolcLule in Plrnar Supersomc E\pansrons, Chcm. Phys., to bc published /201 A Bree and R. Zwutch, J. Chem. Phys. 51 (1969) 903 [Zl]
111II Ii. Levy, Ann. Rev. Phys Chew. 31 (1980) 197 121 A Anww, U. Even and J Jortner. Chcm. Phys Lcttcrs 61(1979) 9. 131 A AniuW U. Even .md J. Jortncr, J Phys. Chem. 85 (19811309.
444
5 March 1982
PIIYSICS LE-MXRS
2 Berkowtch-Y&n,
J Jortner, S. Leutwylerand
U.
Even, to be published 1221 J P. hlarer and D.W. Tumcr, faraday Discussion Chcm sot. 54 (1972)
149