Fluorescence spectra of fluorobenzene cations excited in a supersonic molecular beam

Chemical Physia 58 (1981) 151-162

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CATIONS FLUORESCENCE SPECTRA OF FLUOROBEMZENE EXCITED PN A §UPER§ONKC MOLECULAR BEAM R.P. TUCKET-I? Depamnenrd Chemistry, The Uniuerriiy, SourhampIon SO9 5NH, UK Received15 December 1981 Fluorescence spectra of several fluorobenzene cations in the gas phase have been observed foliowing electron impact on a supersonic beam of the neutraf molecule. The very low rotational temperature of the beam is not disturbed by the ionisation process, so the different vibronic states of the cation are produced rotationally cold: the fiuoresceoce spectra are therefore very simple as every vibronic band has been condensed into a very few rotational components. Vibrationai frequencies obtained from the analyses agree excellently with values from other techniques. The effects of Jahn-Teller distortion are clearly seen in the spectra of CsFg_and 1.3,5-C&H;.

1. Bntroduction Emission spectra in the visible of several fluorobenzene cations in the gas phase were first observed by Maier et al. [I, 21 in 1975 using electron impact excitation of the parent neutral molecule. More recently, Cossart-Magos et al. [3] have photographed emission spectra of these ions from a dc discharge source at higher resolution, and made substantial vibrational analyses. Both techniques, however, suffer the disadvantage that the difierent vibrational bands are superimposed on a large, continuous rotational envelope as the ions are rotationally hot; this has often hindered analysis, and sometimes led to ambiguities in assignment. Complementary absorption experiments have been carried out over the last three years by Miller et al. [4,5] who have observed spectra of these ions both in the gas phase, but more especial!y in solid matrices_ The ions are produced by Penning ionisation of the neutral, and, for example, trapped in a Ne matrix at 5 K. Rotational structure is thus frozen out, and with a tunable dye laser as an absorption source, laser induced fluorescence can probe * Present address: Department of Physical Chemistry, Lensfield Road, Cambridge CB2 IEP, UK.

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vibrational levels in the upper electronic state of the transition. Alternatively, the laser sits on a particular vibronic band (often O-O), and the fluorescence is dispersed, thus prcviding ground state information. These experiments do suffer the disadvantage that the band origin is shifted substantially by the matrix (up to 100 cm-‘), and Miiler et al. are perhaps fortunate that for these Auorobenzene cations vibrational frequencies in the matrix are effectively unchanged from their gas-phase values. The beauty of these experiments, however, is the relative simplicity of the spectra, primarily caused by the freezing of rotational structure in the matrix. We have recently described a novel technique for observing electronic fluorescence spectra of positive ions in the gas phase at very low rotational temperatures 161. The ions are formed by electron impact on a supersonic molecular beam, and the resulting fluorescence is dispersed. It is well known that extensive rotational and vibrational cooling accompanies adiabatic expansion of molecules from a high pressure source to form a collision-free neutral molecular beam, and we showed that the process of electron impact does not substantiaily alter the rotational distribution in the ion, i.e. Publishing Company

there is little transfer of rotational angular momentum. We obtained N: 6 ‘Ei + 2 ‘1; characterised by a rotational temperature of 23 K, NzOf A ‘CT+ 2 ?I at 25 K, and CO;A ?IU-, j(: ‘I& at a similar low temperature. The last spectrum is particularly revealing, as the many different vibronic bands of CO: (including those due to Renner-Teller splitting and Fermi resonances) stand out very cIearly in isolation to each other, as rotational structure has been frozen out of each band. This paper describes the results of this technique applied to much larger fluorobenzene cations (C6FnH& n = 3-6). The greater density of rotational and vibrational states in these species means that the cooling of the neutral in the supersonic expansion is more pronounced (probably to a few Kelvin only), SO the fluorescence spectra of the parent cations become very simple. As will be shown, these experiments perform matrix-like studies in the gas phase, and yet, of course, without any of the disadvantages inherent to matrix isolation spectroscopy. _

3. Theory The Iiuorobenzene cations studied are shown in fig. 1, together with the symmetries of the electronic states invoived in the observed transitions,The ground state (R) and first excited state (A) of the ion arise through loss of an electron from the two highest occupied r molecular orbitals of the neutral. These orbitals are degenerate for the symmetrical species CGF6 and 1,3,5-CbFsHs, hence their ions have doubly-degenerate ground states (*Et, and ‘E” respectively). This n- degeneracy is, however, removed for the remaining less symmetrical fluorobenzenes, hence the ground states of these ions are singly degenerate. The second excited state (6) of the ion arises through ionisation from the next highest occupied $r molecular orbital of the neutral; this orbita is nondegenerate for a11seven lluorobenzenes (e.g. aZu

F 2. Experimental The fluorobenzenes (Flue: ochem. Ltd., 99% purity) are maintained in z constant temperature bath at 23°C and diluted with helium before expansion through a 50 pm stainlesssteel nozzle hole; source pressures of =0.5 atm are used. The beam passes through a skimmer (diameter 0.75 mm), and is crossed by an electron beam. The electron gun has been described elsewhere [6]; electron energies of 100 eV, and currents of S-10 mA are used. The fluorescence is dispersed by a 1.26 m f/9 scanning monochromator (SPEX 1269) and detected by an RCA C31034 PM Tube (cooled to -20°C). The grating has 1800 g mm-’ giving a first order dispersion of 4.3 8, mm-‘. Optical resolutions of 1-k are used for all the huorobenzene spectra. Single photon counting electronics are used, the output being displayed on a y-r chart recorder.

‘2h

Fig. 1. The group symmetries, struchxes, and electronic state symmetries of the fluorobenzene cations studied.

R.P.Tucken / Fluorobenzene cation fiuorescence spectra

in CSF~), so the B states of the ions are all singly degenerate. The B-3 transitions are electric-dipole allowed, occur in the 400450 nm region of the spectrum, and have a structured spectrum. The 8-A transitions are also electric-dipole ahowed, occur to longer wavelengths around 600 nm, and yet have an effectively continuous, non-structured spectrum. This is probably due to c_opious mixing of low vibrational levels of the A state with high vibrational levels of the 2 ground state (the separation of the two states being at most 0.5 eV). This p?per will only be concerned with the structured B-k spectra. If the regular carbon hexagon is preserved in both electronic states of the ion, the two states belong to the same point group (e.g. D2,, for 1,2,4,5-CsF4Hz), and the B-% spectrum is predicted to consist of an origin (voo) and progressions involving vibrational modes which are totally symmetric in that particular point group: progressions involving non-totally symmetric modes will have zero Franck-Condon factors. If, however, the carbon skeleton in either electronic state becomes distorted (by, for example, Jahn-Teller effects) thereby lowering its symmetry, more vibrational modes are totally symmetric in the lower symmetry point group, so further vibronic bands are observed in the B2 spectrum (see later). The spectra of Maier et al. [1] and CossartMagos et al. [3] contain vibrational information on both ground (A) and excited (6) electronic states of the ion. Our spectra, unfortunately, only contain ground state information as nearly all the fluorescence is to low frequency of voo in other words, emission is only observed from the vibrationless level of the B state. At first sight, this would seem to be a consequence of extensive vibrational cooling of the neutral moiecules in the supersonic expansion, only low vibrational levels (mostly the zero level) of the fi state of the ion being populated by electron impact as presumably only these transitions have appreciable Franck-Condon factors. The disadvantage of this argument is that the photoe!ectron (PE) spectra of the thtorobenzenes show that the B state vibrational levels of the

1%

ion can be populated up to several thousand wavenumbers [7]. Even if the vibrational temperature of neutral fluorobcnzcnes in the beam is as low as a few Kelvin (only the zero level then being populated), the PE spectra imply that many vibrational levels of the B state should still be populated. (This point could presumably be confirmed by recording the PE spectra in a molecular beam; unfortunately, this experiment has not yet been performed.) The intensity of a vibronic band in the ion depends, of course, on a different Franck-Condon factor between the 8 and % vibronic levels, and there could in principle be arguments to explain why the (0,O) bands and those to low frequency have appreciable intensity whiist those to high frequency have little intensity. Yet in Allan and Maier’s original experiment [l], which is similar to this one except their source was an effusive rather than a supersonic beam, their spectra show definite structure to high frequency of the band origins. Another possible explanation is that rapid intramolecular vibrational relaxation (IVR) occursin the beam prior to fluorescence. This phenomenon has been observed in a beam in the ‘Bz. (‘~Tz*) state of several aIkylbenzenes by Smalley et al. [S]. They found that the larger the alkyl substituent, the faster the IVR, the main relaxation pathway being via low frequency torsionai modes of the alkyl chain; in n-hexylbenzene, for example, the rate of IVR is almost 100 times faster than the fluorescence rate, so emission is only observed from the vibrationless level of the B state and is independent of the vibrational mode excited by their (tunabIe) laser. The B state Iifetimes of the thtorobenzene cations (~50 ns [2]) are of the same magnitude as the alkylbenzene lifetimes, but there are of course no low frequency torsional modes to assist the vibrational relaxation. In conclusion it does seem likely that this lack of emission from B state vibrational levels is due to some property of the molecular beam, as one keeps returning to the observation that emission is observed from these levels under the (non-molecular beam) conditions of both

154

R.P. Tuckett / Fluorobenzene cation fluorescence spectra

Maier et al. [l] and Cossart-Magos et al. 131. Miller and co-workers [9] have very recently observed the same effect in the 6-j;: fluorescence spectrum of CsFz formed by electron impact on an (unskimmed) molecular beam of C6F6 in helium (see also section 4.6): they observe Little fluorescence to high frequency of YO0and conclude that this is a consequence of vibrational cooling of the neutral in the beam. Whilst this may well be the ,property of the beam that is causing the effect, the physical processes by which it arises are as yet unclear.

4. Results A great simp!itication in the fluorescence spectra compared to those of Maier et al. [l] and Cossart-Magos et al. [3] is observed for all

seven fluorobenzene cations. As mentioned above, this is due to extensive rotational (and probably vibrational) cooiing of the neutral in the supersonic expansion, so the different vibronic bands of the parent cation are rotationally very cold; hence they stand out very clearly on a true baseline without any-underlying rotational structure. The individcal spectra will now be described in detail. 4.1. 1,2,3,5-C&& This molecuIe has Czv symmetry (fig. 1) and has 11 totally symmetric (al) vibrational modes. The fluorescence spectrum is shown in fig. 2, and the assignments are given in table 1. The hydrogen and helium atomic emission lines serve as a useful internal calibration. The simplicity of the spectrum and the narrowness

Fig. 2. 1,2,3,5-C,F,H; fiuorescence spectrum, ii *Sz+jC2Bz.

R.P. Tucken J Fluoroben:ene cation fluorescence specrra Table I 1,2,3,5-C6FaH:

vibronicbands(cm-‘)

Intens@ S

m w 5 m m

Ins m VW m

m VW mw m mw w VW VW

Assignment” 23325 23020 22939 22898 22741 22596 22471 22315 22166 22043 22007 21894 21867 21675 216.57 21585 214.43 21370

0 30s 386 427 584 729 854 1010 1159 1282 1318 1431 1458 1650 1668 1740 1883 I955

voO ~11 VI0 y9

~lDfYt1 2wl

155

excellently with the Ne matrix results of Miller et al. [4], thus demonstrating the absence of shift in vibra?ionaI frequency due to the solid matrix. The matrix band origin, however, is shifted 93 cm-’ to low frequency from the gasphase value. The remarkable similarity in appearance of this cooled gas-phase spectrum with the matrix spectrum (fig. la of ref. [4]) expiains why our experiment is, in effect, performing matrix-like studies in the gas phase.

~9+-PIO ~~lO+-~ll

3% vs 2”1o* %a VP c? vs+ Y,rJ q t “10 Y2-c YE0

a) Approximate intensities. s - strong, ms - medium strong, m - medium, w -weak, VW- very weak. ‘) Assignments refer to ground state vibrations (see text). of the vibronic bands (both due to rotational

cooling) should be noted. The experimental linewidth is determined by the slitwidth of the monochromator (250 @m, optical resolution 1 A), and not by the width of the vibronic bands; hence with increased availabie signal narrower slits and smaller linewidths could be obtained. In assigning the spectrum it is assumed that the ionic vibrational frequencies do not diRer substantially from those of the parent neutral molecule, and all the bands can be assigned to the fundamentals and combinations of totally symmetric vibrations ~~1, ~10, vg, v5, vj and ~2. (The mode numbering scheme adopted by Miller et al. [4, lo] is used, whereby all vibrations of a particular symmetry are grouped together in order; thus vll is the lowest al vibrational frequency, y1 the highest.) The very small (efTectively zero) anharmonicity in symmetric vibrations (e.g. ~~0) and the absence of any non-totally symmetric vibration in the spectrum strongly suggests that planarity and CZVsymmetry are preserved in both electronic states of the ion. The assignments agree

4.2.

1,2,3,4-C&H;

This molecule also has C2” symmetry, and has the same 11 totaIly symmetric vibrations. Part of the spectrum is shown in fig. 3, and the assignments are given in table 2. Again, only totally symmetric vibrations are observed. The vg vibration forms a strong progression, and each band has a,n associated hotband 14 cm-’ to low frequency; the most likely assignment is the low frequency v10 vibration (Y& = 340, V{O= 324 cm-’ [4]). Miller et al. do not see these hotbands as their laser only populates a single selected upper state vibrational level, whereas our method of excitation by electron impact is non-selective. The gas-phase frequencies agree very well with the matrix values (to a few cm-‘, i.e. well within the linewidth of these gas-phase spectra). 4.3.

c&H+

This molecule also has CZ.. symmetry. The spectrum is shown in fig. 4 and the assignments are given in table 3_ The totally symmetric vibrational frequencies agree well with Miller et al.‘s matrix values except for V& (Y;~ (gasphase) = 423, & (matrix) = 438 cm-‘). Miller et al. were unsure of their assignment as it is substantially different from Steele and WhifFen’s value of 327 cm-’ Ill] in the Raman spectrum of the parent. Like Miller, we observe no band near 327 cm-’ in the gas-phase spectrum, and therefore agree with him that their assignment of 327 cm-’ as the Q al symmetric vibration is probably incorrect. However, it is surprising that the gas-phase and matrix values for this

R.P. Tuckeft / Fluorobenzene cation fiuorescence spectra

156

v”nL(cm-’)

Fig. 3.

1.2,3,4-C,F,H;

fluorescence spectrum,6 ‘Bz-+jtzBt.

vibraiion of the ion differ by as much as 15 cm-‘, whereas all the other vibrations agree to within experimental accuracy. 4.4.

1,2,4,5Cfl.;

This molecule has slightly higher symmetry !ZZh) than the other two isomers of C6FsHg, .‘nd has only six symmetric (a& fundamentals. ihe spectrum is shown in fig. 5 and the assignments are given in table 4. Four vibration

Fig. 4. C&H+

S

are readily assigned at 287 (ZJG),482 (YS), 1477 (vs) and 1558 (Q) cm -’ to low frequency of Y”. The value for v6 is in exact agreement with Miller et al.‘s matrix value [4]. but disagrees with Cossart-Magos et al.‘s value of 317 cm-’ [3]_ The reason for this discrepancy is almost certainly due to the problems of assigning emission spectra of these large ions when produced “hot” in a discharge source. As in the matrix spectrum, our gas-phase spectrum shows satellites to high frequency of vg and vz

fluorescence spectnm,

B’Bz+%‘At.

Table 2 1.2.3.4~CsFJH;

vibronic bands (cm-‘) Assignment

Intensity

“==

s

23291 23277 23019 22951 22939 22850 22836 22612 22579 22409 22398 22207 22172 21971 21864 21525 21636 21500 21491

m m w W

s m W m nu

m W w w mw w mw mw

0 14 271 339 352 441 454 678 711 881 892 1083 1118 1319 1426 1665 1755 1790 1800

with

W

us and

vl_ The

Assignment

hv 0 18 42

“O”

ms

22818 22672 22638 22541 22513 22360 22178 22053 21902 21549 21497 21230

277 423 457 554 582 735 917 1042 1193 1546 1598 1565

yt1 +l

m vw W VW

vw vw

“9

2Ull v8

%I+ Ql 2% Qf%. 2v9+w

% ~2

w m w m w w w m w w w

Assignment

a) Miller’s notation [4]. He reasons that Lagand V=are combination bands in Fermi :esonance with us and vz respectively.

violet

2309s 23077 23053

W

+45 22 0 -91 -287 300 317 464 482 573 591 770 967 1250 1477 1503 1539 1558 1644 1844 1958 2041 2130 2329 2521

mw w mw m

s w w

m

24487 24464 24442 24351 24155 24142 24125 23978 23960 23869 23851 23672 23475 23192 22965 22939 22903 22884 22798 22598 22484 22401 22312 22113 21921

S

Y’-

5

Au

w m w mw mw

Intensity

W

9-c

S

Table 3 c6F,H* vibronic bands (cm-‘)

W

Intensity

w

(labelled v3 and vC in table 4 - Miller et al.% notation [4]). They assign these bands to overtones of non-totally symmetric vibrations in resonance

vibronic bands fen-‘)

w

=) Hotbands -14 cm-’ to low frequency of each band. Possible candidates are vs. Y,,, !Y&, = 340, vi,_, = 324, Vi =680, PC = 668 cm-’ [4]); the former is chosen due to its lower frequency.

Fermi

Table 4 1,2,4,5-C6F,H;

degraded band at 23371 cm-’ is present in all the fiuorobenzene cation spectra: and is the head of N;fi’C: (u=O)+%‘& (v=l)_ Nl is readily produced as an impurity in electric discharge sources, and is favoured by the presence of helium [12].

This molecule has very low symmetry (CJ and has 21 symmetric (a’) vibrations. The values of the parent neutral are listed elsewhere [1315], and are labelled ~1 (highest frequency) to vzl (lowest)_ Assuming as before little change in ionic frequencies, the observed bands (fig. 6, table 5) can be assigned to fundamentals or combinations of y19 (404cm-*), vIs (485), v17 (674) and us (1615).

158

R.P. Tuckett / Fluorobenzene cation .flrtorescencespectra

Y *-

(cm*)

-_- ..--_. ._.~~~~~~~~~~~Y~~ .._ i.__._ ..~ ._ i -.-_ _._. ..-- -_. ,____t.___

23,500 9”‘y (cm-‘)

R.P. Tucken / Fluorobenzene cation fluorescence spectra

Table 5 1,2,S-C6F3H$vibronic bands (cm-‘) Intensity

u-=

AV

Assignment

s w m s m m

24275 24238 23870 23790 23600 23405 23308 23187 22660

0 37 405 485 675 870 967 1088 1615

Y*

in w

m

4.6.

VI9 h6 y17 Q8 f

VI9

h8

U17+-%9 us

C&

This is the most symmetrical and perhaps the most interesting of the cations studied. The neutral molecule belongs to Dsh, and as mentioned earlier removal of the highest elg bonding electron from the ‘in molecular orbitals produces a doubly-degenerate ground electronic state of the cation %‘E1,. Being doublydegenerate, such a state of a non-linear molecule is unstable, and by coupling the electronic motion with particular non-totally symmetric vibrational modes, the electronic degeneracy is removed and the molecule assumes a geometry

159

of lower than DBh symmetry: this is the JahnTeller effect. The second excited state of C& 6 *AZ” (arising from removal of an azU bonding r electron) is, of course, nondegenerate, and therefore suffers no Jahn-Teller distortion. In strict Dsh symmetry, C6F6+ has only two alg totally symmetric vibrations, and only these fundamentals and_ combinations of ., them should appear in the B + X fluorescence spectrum. In practice, as the ground state lowers its symmetry, other vibrational modes-become vibronically allowed, and irregular sequences are observed in these modes. The spectrum of C6Fz is shown in fig. 7, and the assignments are given in table 6. The band origin and the symmetric V; vibration (a13 are clearly seen at 21617 and 21061 cm-’ respectively; each has an associated hotband ~25 cm-’ to lower frequency (see below). There are, however, three bands between these two allowed transitions (284, 406 and 497 cm-’ to low frequency of Y”, which cannot be assigned to alg fundamentals; they have approximately equal intensities, and are due to vibrations which become allowed as the molecule lowers its symmetry through Jahn-Teller distortions. Herzberg [16] has shown that the symmetry of the vibration which canses the distortion must

Table 6 CsFl vibronic bands (cm-‘) Intensity ms m m u’ w m m mw ITlS m w w w w w

v’==

AL\v

21617 21590 21.574 21333 21298 21231 21211 21120 2iO92 21061 21038 20926 20828 20776 20707 20656

27 43 284 319 ?86 406 4?7 525 556 579 691 789 841 bl0 961

Assignment”

YOO (“; + “;

)b’

a'Assignments refer to ground state frequencies, unless otherwise stated. ” Hotbands ==23cm-’ to low frequency of Y”, ~1s. 2~1s and IQ. The most likely assignment & ~2 (vi = 534 cm-‘, LJ;= 559 cm-‘) [20]. ” Possibly bIended with He line (,X’“=47-13.21A).

be contained in the symmetrical product of the symmetry of the degenerate el&ronic state

with itself. In Deb, the symmetrical products of E1, x E1, are AL, and EZg, and vibrations of eZs symmetry can thus cause distortion. Neutral C6Fs has four modes z+~-zQ~ of this symmetry at 1659, 1159, 445 and 272 cm-’ [17]. Miller and co-workers have recently completed a global fit to all the available information on the vibronic levels of 2 C& from their gas-phase and matrix experiments [lg]. They found that if the Jahn-Teller effect is considered individually for these four modes, little progress was made in spectral analysis. However, if all four modes are treated simultaneously, their observed spectra can be predicted (both line positions and intensities) exceptionally well. They thus assign these three bands to I& (284cm-‘), v’;, (406) and 2& (497), although such labels are slightly meaningless due to the strong mode mixing that is occurring. It is worth stressing the following additional poiilts.

(a) The simplification in the spectrum upon rotational (and vibrational) cooling is most pronounced for CbFz; there is little visible similarity between Cossart-Magos et al.‘s “hot” spectrum [3] and our “cold” one. (b) As mentioned earlier, Miller and coworkers have very recently reported a similar spectrum for GFz, excited by electron impact on a free (unskiimed) jet expansion of CsF6 in helium [9]_ Our spectra are almost identical, and in particular we both obtain band origins for the s-2 transition of 21617 (*2) cm-‘, to be contrasted with Cossart-Magos et al.% value of 21600.6 cm-’ [3]. The CsFz spectrum of the French group was somewhat special in that the bands were very wide (IO-20 cm-‘) and did not resolve further on the high resolution plates. This does, however, show the error that can arise in assigning the maximum in a fluorescence spectrum of a rotationaily hot large molecule necessarily to the band origin. For the other fluorobenzene ions, their origin bands have much narrower heads, and the differences in YO”values compared to the cold jet spectra are much less (on average l-2 cm-‘, see also section 4.7). (c) The irregularity in spacing between v18. 2Y18, 3~~~ and 4~3 (table 6) is a direct consequence of the Jahn-Teller effect; the distortion parameter Di for ~1s is quite large (0.38)

C=31. (d) Transitions involving v17 and ZQ~(table 6) refer to the j = l/2 Jahn-Teller components, as emission from the vibrationless level of 6 *AZ, can only terminate on these components 1191. (e) The band origin, Yla, 2~18 and ~2 all have a hotband 25-30 cm-’ to low frequency of the main band. The most likely assignment is ~2, since the highest resolution gas-phase experiments give z& = 534 and V: = 559 cm-’ [20]. (The disadvantage of this assignment is, of course, the absence of the V$ = 1 + #I= 0 band to high frequency of vO”.) Miller et al. observe these same bands in their free jet spectrum [9], and suggest they may be due to one.or more helium atoms complexed to the ion. Whilst an extremely attractive proposition, it does, however, seem unlikely as the spectrum is

R.P.Tucken

I Fluombenzene

apparently unchanged by differing helium pressures (0.5 atm in our experiments, 5-25 atm in his). 4.7.

carion fluorescence specira Table 7 1,3,5-C&H:

vibronic bands (cm-‘j

v-

Intensity

1,3,5-c&H:

21877 21869 21861 21854 21847 21828

8 a -8 -1.5 22 41

ms w w w mw w VW w

21319 21296 21277 21258 20891 20838 20741 20312

550 573 592 611 978 1031 1128 1557

.-.-1 ‘,3sjF3H; -

i_ Z’E”

:

.- _-__

._.-_ V,,(J.d

P

e3 u= Pd

l/2)

2Y13 (;= l/2) P3 LJ+Y~~ (j=1/2)

_. __

:

_. _----.--:6:----.-. A --

___

u “‘(cm”) Wg. 8. 1.3.5~C&H;

Asign_ment

350 cm-’ from voo [23], is not seen 2s this mode is almost inactive. The band origin has 4 or 5 associated hot bands, many of which have been identified by Cossart-Magos et al. [3] in their higher resolution work. The band origin of 21869 cm-’ agrees excellently with Cossart-Magos et al’s value of 21868.9 cm-’ [3], but both values difIer from Bondybey et al.% value of 21855 cm-’ reported for gas-phase laser induced fluorescence studied at liquid N2 temperatures 1261. This difference is very surprising as the temperature of their

__. $A’;

AY

s w w mw mw

w

This cation fluoresces with near unity quantum efficiency, and has been the most studied of the fluorobenzene cations. In particular, Cossart-Magos and Leach [21-241 have made a comprehensive analysis of very high resolution (0.1 cm-‘) photographic plates of their discharge source, and obtained many Jahn-Teller parameters for the g *E” ground state. The Bell Labs group have also made a full analysis of their gas-phase and solid matrix results [ZS]. Our very cold free jet spectrum (fig. 8, table 7) has, disappointingly, little new information to add, and will only be described briefly. In strict I&, symmetry, 1,3,5-CeF& has only four a; totally symmetric vibrations, and only ~3 and vd are observed (both very weakly) in @e B *A; + % %” fluorescence spectrum. Seven modes (v~ - vr4) have the correct symmetry (e’) to cause Jahn-Teller distortion of the *E” ground state, but unlike CeFl one mode (vrs) is much more active than the others. The vys (i = 0.5) band is thus clearly seen 550 cm-’ from voo (fig. S), whereas vx (i=OS), for example, which should occur approximately

161

fluorescence spectrum, BZA;+X’E”.

V

00

~. ---~--

.I 62

R.P. Tuckeft j Fiuombenzene

source (77 K) lies between that of my supersonic beam (a few Kelvin) and the Orsay discharge source (>300 K), so it is unlikely that the different source temperatures (and hence different widths of the bands) can account for this discrepancy. The French photographic plates are calibrated by lines from an.Fe/Ne hohow cathode lamp, the beam spectra by H and He atomic lines to low frequency of the spectrum shown in fig. 8, whereas the method of dye laser calibration is not mentioned in Bondybey et al.% paper. A small error in their calibration seems the most Iikely explanation.

5. Conclusions This paper has shown the great simplification that can arise in fluorescence spectra of large potyatomic cations by rotationally cooling the neutral molecules in a supersonic expansion before ionisation. The resolution obtainable is limited either by the signal available (i.e. the number of fluorescing ions per unit area per second), as is the case in this paper, or by the optimum resolution of the spectrometer if enough signal is availabie. The study of rotationally cold ions at very much greater resolution (using a single frequency dye laser as an absorption source for laser induced fluorescence) is a real possibility for the future.

Acknowledgement I thank A. Carrington, S. Leach and T.J. Sears for their help, encouragement and interest in this work, T-A. Miller for sending me preprints of his papers on C,Fz, and the referees for constructive comments. Finally I acknowledge SRC for the award of a Post-Doctoral Research Fellowship.

cation fluorescence specrm

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