Molecule coordinate frame anisotropy in dissociative photoionization of CF3I

Chemical Physics 1fJOi1985) 401413 No&-Holland. Amsterdam

.

.~ ..

: .._+-

:...

_ : .,. -_:

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COOtiINAti FRAME ANISOTROPY IN DISSOCIATIVE PHOTOIONIZATION OF CF,I tiC&ECULJZ

KhahGek

LOW,

Paul D. HAMPTON

and Ivan POWIS t

Physicd Chemisny LaboraroT, Sou!h Parks Road, OxJorrd.OXI 3QZ. UK Received 25 February 1985; in foal form 29 July 1985

Electron-ion coincidence studies of dissociative photoionization can establish correlations between the directions of photoelectronand fragknt ion ejection_Where rapid dii ation of the initi.zl parent ion occurs under axial rumit wnditions the fragment ion direction indicates molecular orientation at the moment of photoikization. Molecule coordinate frame anisotropies in the photoelectron distribution may then be inferred. These conditions apply to the C-I bond ckavage dissociationsof A’At CF,I+ and the data indicate highIy directionalphotoelectron ejection. In the experimentsdescribed here it is shown how. as a result. an oriented sample of mo1ecnIarions can be achieved. Data for the dissociaiionsof other electronic states of CF,I* up to a limit of 212 eV are reported_

1. Introduction The utility of measurements of the angular distribution of particles in photoejection processes is well established and these measurements have been widely used for the investigation of molecular photoionization and photodissociation dynamics_ Experimentally, such work has expanded with the ready availability of laser and synchrotron sources of tuneable, polarised light and there has been a concomitant growth in the theoretical literature on these topics. In the case of photoionization systems; the simplest and most common experimental measurement is of the photoelectron distribution relative to the photon (lab) coordinate frame from a randomly oriented (gas phase) -sample [l-3]. This gives rise to distributions of the well-known form da/da=

(u/4n)[l

+fiP2(cos

6)1,

(1)

Legendre polynomial and 8 the angle from the electric vector of the radiation_ However, the general theoretical treatment of molecular photoionization cross sections is most naturally formulated initially in terms of molecule coordinate frame wavefunctions [4]_ Here -specific angular dependences and terms arise as coefficients in the partial wave expansion of the final state continuum wavefunction [4,5]; the contribution of each partial wave to the observed cross section nevertheless depends on the electric dipole interaction which is in turn dependent upon molecular orientation in the photon frame. Cross-section expressions may then be separated into system specific dynamical terms (the transition amplitudes) and general geometric terms, the latter which may be transformed end/or averaged to achieves expressions appropriate to various types of experimental situations [4]. In general these will have a greater harmonic content than (1) of the form

which are conveniently characteriied by the single parameter fi_ Here u is the total cross section, Pz a

da/da

’ Current addressand ad-

A,,

for all wmrmnications: Department of Chemistq, Nottingham University.Nottingham NG7 ZRD*UK:;

=c

~&L,(@, KM

+I,

:

(2):

where -Y,,,,( 0, +) &e spherical harmonics and appropriate expansion coefficients A number of specific experimental arrangements from which cross-section data might be

0301-0104/85/$03_30 0 Elsevier Science Publishers B-V_ (NorthiHolland Physics Publishing Division)

obtained have been considered with a view to identifying those which yield the maximum dynamical information [6,7]. Common to many of these alternative schemes is the use of oriented molecules. although clearly this will be much less readily achieved in practice than the usual unoriented gas-phase sample. Consequently only a very limited number of oriented systems have actually received investigation. One example is that of CO weakly physisorbed on Ni surfaces, where calculations 181 of the molecule frame photoelectron distributions from non-bonding orbitals were used to confirm the orientation of CO on the surface [9,10]. A more general method for preparing oriented molecules, where molecule-surface interactions are not a potential complication, seems desirable_ A related formalism applies to the study of angular distributions [11.12] photodissociation where again differential cross sections of the form of eq. (1) are encountered for randomly oriented targets, and the more richly structured eq. (2) for aligned systems [13]. Analysis of the P parameter for molecular photofragments yields information not only on the symmetries of the states involved but also relating to the dissociation lifetime as molecular rotations act to reduce, but not eliminate, any anisotropy [11,14-171. In comparison to the above, the anisotropies inherent in dissociative photoionization have received much less attention. This process is in principle complicated by the production of three. not two, fragments AB+hv+A”+B+e-, but is treated conceptually as ionization with a subsequent ion dissociation. It is apparent that the photoelectron and ion fragment directions in each event will be correlated in some way because of the common molecular coordinate frame. From this comes the opportunity to use the molecular dissociation to elucidate further details of the photoionization step and vice versa. For example, in. the dissociative photoionization of Hz the direction of the H+ axial recoil dissociation product has been used to indicate the molecular orientation at the moment of photoionization and so yields directly the photoionization differential cross section

as a function of lab frame orientation [18]_ -Moreover, the dynamical information is enhanced cornpared to the more usua! photoelectron lab frame differential cross section because of the absence of interferences between alternative symmetry photo. electron channels [7,18]. A natural and general way to determine the interdependent electron and fragment ion distributions arises in photoelectron-photoion coincidence experiments (PEPICO) where the electron and ion from a single event, and thus sharing a common molecular orientation, are identified by time correlation_ It will be shown that when the dissociation process is dynamically characterised in such experiments, information on the molecule frame photoelectron distributions may be obtained without an initially oriented sample being required 18% It has been widely assumed in the analysis of BEPICO dissociation data that the relevant processes were effectively isotropic in the lab coordinate frame. Although this assumption has generally been recognised as such, it has only once been found necessary to revoke it to explain otherwise anomalous results [19]_ The present experimental data now constitute a second such, and perhaps by their nature less ambiguous. case. They arose in the course of a complete study of the electronic state-selected dissociations of CF,I+, brief results of which are included in this paper_

2. Experimental The experiments were carried out in a PEPICO apparatus which has previously been described in detail [20]. The salient-features of the instrument geometry are summa&d here and can be seen by reference to the schematic diagram, fig. l_ Unpolarised VUV radiation from a rare-gas resonance lamp enters along the z axis and intersects a sample beam (taken to define the x axis). A dc electric field is applied, perpendicular to these two directions, to extract the positive ions and electrons from the resulting ionization region. Electrons are thus accepted by the electron energy selector from a small solid angle about the instrumental (y) axis. Ions pass in the opposite direc-

,-. .. -:

0-:.:. ‘:

-_

.:

Resonance lamp

II Sample inlet jet

-.

Source ELectron detector C

r-l I’egionl ! ]Iy Electron

energy +

-

1. Sch.~~tic

diagram

or the

PEPICO

II

ion drift

selector Fig

Ion

detector

apparatus(lop). and

tion through a time-of-flight analyser. Other than for the light and sample beams the instrument is fully symmetric around its y axis. The electron-ion-coincidence detection scheme is fully explained elsewhere 1201;its consequence is that effectively only those ionic species which originate in the same ionization event as one of the energy-selected electrons are detected. Detection of the electron thercfore indicates both the time of formation and initial energy (state) of the ion system. The time-of-flight analyser permits both mass and kinetic energy analysis of the ion fragments. A commercial sample of CF,I was used under similar experimental conditions to those employed in earlier studies of CFsX systems [21]; in particular source fields of 6 and 8 V cm- ’ were employed- This results in photoelectron bandwidths of= 150 and = 200 meV due, largely, to the finite width of the ionization volume. The intrinsic elec-

region

a plan vim

of the =me

(bottom).

trostatic analyser performance is better than this so that nominal (band centre) ionization energies. may be quoted to higher precision. The precision with which kinetic energy releases are determined from the ion time-of-flight peak shapes is, of course, related not to the electron kinetic energy resolution (i_e_ the degree of initial state/ener,T selection in coincidence mode), but to the mode of operation of the time-of-flight analyser, to the translational temperature of the effusive beam inlet (10 K), and to the signal-to-noise ratio of the time-of-flight peak [20]_ The signal-to-noise ratio of the coincidenk ,mode time-of-flight (CTOF) peaks is. Mth the exception of the X band data, worse than has previously been reported by our group, despite the use of veq long counting periods (> 48 h per .CTOF spectrum)_ A number of factors contribute to this, including low electron count rates for the excited state bands and the very large cm. energy

releases

and consequent high geometric diScrimination which are inferred for the excited-state fragmentations. In all CTOF data presented in this paper we have assumed an uncertainty or error given by Poisson counting statistics; however after such long counting periods with io-.v count rates other sources of noise and e,rror can become. significant. The CTOF data were in fact obtaixied in two sessions of experimental work, between &rich the apparatus was completely stripped and re-assembled. The earlier results were then found to be fully reproducible.

Table 1 Appearance thresholds for CF,I fragmentations

0 K =ppearan- -F’w
Re3ctiod hr

CF,I+c-+CF;

+I(‘Pxp_)

11.36 =’

+CFa+ +I(zP1/z)

1230 ab’

I-I+(~P~)~CF, +I+(3P,)i-CF ‘I+(3P,)+CF;

12.70 *’ 1350 ab’ 13.58 =b’

+CF?I++F

14.58 =’

+CF;

17.62 -A’

+F+I

.~

values from ref. [25]. b’ I and I+ &a-orbit separationsfrom ret 1231. =’ Tbermoebemistryof CF,I + is tmeertein.Value quoted is en appearance energy from ref_ 1241. d, AH,_ CF2’ = 939 kl mol- ’ from ref_ 1241.

a1 Thermochemical 3. Results 3.1. General The He1 photoelectron spectra of CF,I, recorded under the conditions used for coincidence

electron bands and indicates thermodynamic

dis-

mode work, were in broad detail identical to earlier

sociation

0 K

high-resolution spectra [22] although under these conditions the vibrational features previously noted in the % and I? bands were not observed. Similarly the % and A spectra recorded with a Ne resonance lamp showed no significant differences. Fig. 2 summ arises the assignment of the photo-

heats of formation (table 1). Actual observations of the dissociation channels from each initial ion state are tabulated in table 2 while a breakdown diagram is presented in fig_ 3 to indicate the trends_ The fractional abundances have not been corrected for the effects of ion discrimination be-

thresholds

calculated

from

literature

He limit

EaF --_ Be -.

EaF ---__

___

F

.xLit F

-_

-.

--‘--__C

ZtF ---

_

_

6eF BeF ---____ a 0-F EaCF-F ---__-__ --Eat= aaC_cl~~-------__- BaF

‘_-_-

--,ZaC

___,-

g a F EeF

CF2++F+

I

_-EaCF-p GaF

5r_

Chl++

F

I’(3Po.3P,l + CF3 I+(“@1 + CFJ CG + WP*,2)

CG CFsCl

CF3Br

l

I(3P3/21

CF31

Pig. 2. Photoelectronband assignmentsfor the series CF,X and thermodynamic limits for tbe dissociativephotoionization of CF,I_

Ionisationenergy(eV) productsas a function

Fig. 3. Breakdo*= diagram showing fractional abundances of CF,I+ and elecrronic state (r) CF,I+; (0) CF$; (0) I+: (A) CF,I+_

and its dissociation

of ioniiarion

cause of uncertainty about the exact features of the ion trajectories, particularly for the A band data (see below)_ Other than undissociatcd CF,I’ only CF:* is observed from the % state. The CTOF peaks have been analysed as described for the other CF3X+ systems [21,27] and an example of the kinetic energy release distribution is given in fig. 4. All other states were fully dissociated_ The CF3+ and I+ peaks recorded in coincidence with A band photoelectrons were anomalous and are wnsidered further in the next section. From the B and C bands the principal CTOF peak is identified as

CF-J++. The peak shapes here are broad and rather square and, as with all the excited-state data, are afflicted with poorer coincidence counting statistics than might have been desired. We. note that there seems to be some unusual structure to these peaks which although less than tbel sta*tical fluctuation, does appear to be consistently present in all cases. With this caveat the CF,l+ peak shapes were analysed and the mean energy releases are plotted in fig_ 5. A representative KER33 is included in fig. 6. The CF,i peaks observed in this ionization energy range were of low intensity and so were not fully analysed; however, from the

Table 2 Summary of ~observexldissociation products a) Initial phoroelectron band (ioa state)

CF3Br ‘)

x A

CF,+

CF,Cl

b’

CF,I

=’

D-tion ink-red

mahanisms d)

CF; CF3+

CF; CF;.

s

CF$.?l+. CF,+

CF,I+(CF=.I+)

dir.xL repul&e

C

CFzI+(CF,+.I+)

direct, repukive

b

CF&l+ CF,Cl+

CF-$+.

direct. repulsive

E

CF> . CF;

I+

CF;

products in parentheses.b’ Refs. 121,275 cPThis work. d’ Classification. based on aoalysis of cm. energy releases.

a’ Minor

I+

I+

sIatitical direct, repulsive

direct, re.pulsive

energy

KG.

406

Low et a.! ,I Anisotropic pho;oionizasion of

CF,I

O-6

1

-t Q02

0-w

006

008

o-10

o-5

Et IeV) Fig 4. Kinetic energy release distribution (showing the probability oi a transIationaI energy cI) for CF31+ (2 ‘Et,=) CF3+ + I at an energy 0.07 eV above the 0 K threshold for I(‘P3r_) formation The solid curve is obtained from a phase-space modei calculation-

CTOF peak widths we estimate an energy release of = 0.5 eV. The b band I’ and CF,I’ CTOF peaks are

I-

l-0 l-5 Et Ievl

-l 2-O

2-5

Fig. 6. An experimental kinetic energy release distribution. P(r,). for CF,I+ + CF21’i-F at an ionization energy of 16.1 CV.

generally unremarkable. The I + energy release is t: 0.4 eV, that of the CF,I‘ is considerably higher (fig. 5). Finally the i? band I’ fragment peak was found to be broadened by an energy release of = 0.6 eV. 3.2. A- band CTOF peaks

17-o

17-g

Ionization energy (eVj Fig. 5. Mean kinetic energy releases (Z,) for CF,I+ - CFzIi -+ F_ The dashed line indicates an adiabatic modeI in which aII the energy in -excess of the threshold is released into translation; the solid line is the prediction dissociation model (see text).

of a late release impulsive

Concurrent dissociations to CF;f and I+ products are observed from the A band and the CIOF peak shapes are very remarkable for both these fragmentation channels. At the onset of this band, around an ionization energy of 13 eV, the CF;t .fragment appears in the CTOF spectrum as a single peak, with a shoulder to long flight time; by 133 eV it has clearly developed into a.“split” peak shape having two distinct components about the expected mid-point of the CFs. CTOF peak. This remains true throughout the remainder of this band (to an ionization energy of = 13.7 ev). The I+ peak simultaneously has a split appearance across the entire band. Such split peak shapes are not unwmmon (see e.g. ref. 1211 and fig. 7) and are generally indicative of a l&e cm_ energy release with a relatively

_.

26CKKl-

24ooo -

Fig. 7. Coincidence time-of-fligbr peak for CFT + CF,’ +F showing the characteristicsplit appearance wbicb results from a large. narrowly distributedcentreof-mass energy release.The smooth curve rhrough the dara is a computer simulation of the

peak.assuming an iostropiefragmentdistribution.

narrow distribution_ The split form is readily understood_ The cm. energy release imparts an additional lab frame velocity to the fragment ion. If the net initial lab frame velocity is directed toward the ion detector then the fragment arrives earlier than the nominal flight time; if away from the detector then the ion’s motion is eventually reversed by the source field, but the overall flight time to the detector is lengthened. Hence the peak is broadened. When the energy release is large and narrowly distributed those ions with only a small component of velocity along the lab y axis and a consequent near nominal flight time to the detector must have a correspondingly large component of transverse velocity. This can cause such ions to be lost at an aperture before reaching the detector_ C~nseq~ently.there.is a depletion of the recorded peak intensity at the nominal flight time, giving rise to the split app earance- One additional point to bring out is that those ions which have an initial backward component of velocity alorig the .y +, but retain some transverse velocity. ‘componen.t, wilI tend to_ experience .more severe losses at the apertures than those ions with a similar f&-ward y

_-_:.. -- :--:_: -:. - .-i __

axis velocity component because of i&r &&&r’ flight time and. consequently gr&ater trans&rse excursions. In other words the&:& t+ possibiit~ of greater geometric discrimination’ &a&i’ thelonger flying ions so that the ielative intensity $f the long flight-time component of a spIit peak..wiK be reduced. An example of such a p&, recorded with the present apparatus for the. &so&&on CF,+ -, CF,I + F, is shown in fig_ 7. Also included in t@s drawing is a smooth curve from a computer Simulation of the ion trajectories which is seen to reproduce exactly the peak shape. When such simulations were attempted for the -two dissociations under consideration here we were unable to reproduce the observed peak shapes. The backward flying, long flight time, component of the CFJ+ peaks is apparently of too low an’ intemity (figs. 8a and 8b)_ The I+ peak shape is even more striking_ since here the forward flying component of the CTOF peak is of lower intensity, in corn-. plete contrast to the expectations outlined above, and it proves impossible to reproduce this peak with the stadard simulations (figs. 8d and Se). Both CF5+ and I+ peaks are deemed anomalous_ The peaks discussed so far were recorded using He1 radiation These io_nization energies are also accessible with a Ne resonance lamp and the CTOF measurements were thus repeated with the following differences noted. The CF;’ peak shape does not become clearly split until an ionisation energy > 13.4 eV. Even then, and throughout the rest of the band. the depth of the cleft in the peak at the nominal flight time is less than for the He1 results at a similar ionization energy_ Below 132 eV the Ii peaks, though broad, are barely split; at higher ionization energies the anomalous forward/backward flying component peak ratios are readily visible. Again, however, the extent of the splitting is less marked than for the corresponding He1 result (figs. Sb, SC, Se, Sf)_ At the high&t ionization energies in the band it still pro.vcd difficult to obtain a satisfactory reproduction of rhe experimental data for both CFs+ and I’ from the standard simulations, although the discrepancies were less pronounced than with He1 radiation. The interpretation of the above data is taken up in the discussion. To comp!ete this section we

KG.

408

Low et aL / Animtropicphotr%mization

of CF,Z

(al 13-27eV

2

z3

0

-2-o

O-0

2-o

-2-o

O-O

Relative time-of-flight

2-o

-2-o

O-O

2-o

(JIS)

Fig. 8. Examples of the coincidence time-of-night peaksfor fragment ions from CF,I’(A’A,). (a). (b): CFT with He1 radiation: (c) CF,i with Ne radiation; (d). (e) I+ with He1 radiation; (f) I+ with Ne radiation. In alI cases the curves through tie data are best-fit computer simulationswhich assume isotropic distributions.Nominal ionization energies(IE) of the experimentaldata are as shown in C.V.

consider the magnitude of the c-m_ energy releases. An exact determination of this information is dependent upon the ability to reproduce the CTOF peak shapes by computer simulation [20]. This, as

10. Mean kinetic energy releases(best estimates), Z,. for A CF,I* + CFZ +I. Predictionsof two phase-space cakulations are shown_ assumingeither I(‘P& (7) or I(‘Pt& ( -) Also included are the predictions of an impulsive dissckttion model which wumes 1(2P,,) product in the extremes 0f so-called “late’! (- - -) or “early” (-- -) energy releaseFig

Fig 9. Mean kinetic energy rekases (best estimates). 1,. for A CF,I+ + CF, i-1’ The solid line would be the expected release if aI1the energy in excess of the 0 K thresholdappearedin translation.

noted above, cannot. be achieved here.. Nevertheless it is possible :to obtain -an estimate of- the momentum along the y axis .fiom’ the-peak~widths and so estimate a_lower bound on the cim. energy release. We have used the less anomalous Ne radiation data for this purpose_ Mean energy releases are presentedin figs. 9 and 10.

4. Discussion 4. I. Unimolecrdar 4.1.1. k CF,I

dissociation

chantiek

-+

The unimolecular dissociation of ground-state CF,I+ has been previously inve&gated by both single [28] and multi-photon [29] IR photodissociation and PEPICO [26]. A slow dissociation CF,I* (X *Elfi) + CF;i(%‘A,)

+ I(‘P,,&

is indicated [26] following, in the ease of IR photodissociation at least, vibrational energy redistribution in the parent species [28]_ The mean kinetic energy releases were found to be small, of the order of a few tens of meV near threshold [26,28]. Energy release data obtained in the present work corroborate the conclusion drawn from the above, that this is a statistical dissociation mechanism. Although generally a few meV higher than those of Bombach et al. [26], the mean energy releases determined from our experiments are in accord with a phase-space calculation we performed. A KERD predicted by this calculation is included in fig. 4 and can be seen to be in close agreement with the corresponding experimentally determined distribution; (Parameters used for this calculation are as given in ref_ [271; additionally CF,I rotational constants of O-051-and 0191 cm-’ [30] and an I atom polarizability of 3.9 X 10-2a A3 [31] were assumed_) 4.1.2. & 2, -5 band dissociations to CF,I i The magnitude of the translational energy (compared to the ionization energy in excess of threshold) atimpanying CF,I+ formation (fig 5) and the sharp RERDs (fig 6) are both characteristic of rapid dissociation occuning on a. repulsive

potential energy surface [ill. .A cornpa&& .with-. model calculations for the~kineticenergy *eIeaseis impeded because of the large~uncertainty-3 the appearance energy of the CF21* fragment [23,24].. However, -using the tabulated value (table 1). the predictions of (a) an adiabatic (i.e.: 13% energy release) and (b) an ea?ly release impulsive energy partitioning are included in fig. ,5 [l&21,32]: It is seen that these are compatible wirh the experimental-data in the B and C band. The b band releases are still substantial, and in no way indicative of a statistical dissociation mechanism 4.1-S. A- CFsI + As pointed out in section 3.2 one.has to be cautious in interpreting the experimental data for this state while the peak shape anomalies remain unexplained Nevertheless it is clear from fig 9 that nearly 100% of the ionization energy in excess of the thermodynamic threshold for I +( 3Pz) formatioa appears as translational energy in the CFS + Ii product system, at least across the first half of the A band Even at the higher ionization -. energies> 50% appears as translational energyFurthermore, the CIOF peak shapes do at least strongly suggest a sharply peaked distribution of energy. This behaviour is highly reminiscent~of that observed for several other dissociations in the series CF,X’ [21] and elsewhere,and-it attributed to a repulsive state direct dissociation without redistribution of the excitation energy- One presumes the reaction to be much more rapid than the statistical 8 state dissociation, and this may be a factor in explaining the lack of vibrational structure in the A band PES whilst it is observed in the X band 1221. Interpretation of the CFS+ dissociation data is less clear cut. As fig 10 shows there is a large scatter in the experimentaI data, but these values are bounded on either side by the predictions of a phase-space calculation which assumes either I(‘P& or I(*Pt& as the neutral dissociation product The impulsive energy release model fOi C-I cleavage indicates = 25% of the “available” energy appearing as trLnslation_With the specific assumption of excited I(‘Ptfl) formation the “early” and “late” energy release variants of this model [21,32] again bound the experimental data

410 Overall we- tend to the interpretation of a direct type dissociation, probably producing I( 2P,r-), as being the more likely- First, the splitting of the CTOF peaks is indicative of a direct repulsive. rather than a statistical, dissociation_ Secondly, the correlation diagram deduced by Bombach et al. [26] shows a strongly repulsive A state surface leading asymptotically to I( *P& product. It may also be noted that the A state of CF,I+ is produced by ionization of a C-I bonding electron which could specifically favour direct C-I bond cleavage reactions [22]. Thirdly, the CFs+ dissociation channel is here competing with the Ii dissociation channel and both must therefore deplete the A CF,I* population at a comparable rate. Having decided that the latter channel is more rapid than a statistical mechanism implies that CF;’ formation, either by direct dissociation or some internal conversion, is comparably rapid. Finally, but importantly, we draw upon the comparisons with other members of the CF,Xi series. These have been indicated in fig. 2 and table 2 and can be found in the above discussions and refs. [21,27]. On a state by state basis there are very marked similarities in the dissociation behaviour of these systelms. Such a comparison reinforces the contention that the A state dissociation to CF;i is direct and impulsive, since that is the case in the analogous CF3Br+ and CF;Cl* data. In conclusion, both the I+ and CF;’ channels are thought to proceed by rapid direct dissociation on a repulsive surface. This will be an important element in the interpretation of the peak shape anomalies which follows. 4.2. The A- band CTOFpeak

symmetrical about the y axis.other than for the sample inlet and the photon beam_ It is recalled that the remarkable feature of the A band CTOF peaks is the inability of an ion trajectoryealculation to simulate the-peak shapes. The calct+ation used does, however, allow for the asymmetry of the sample inlet [20]. Moreover, an attempt is made to allow for other factors (e.g. stray fields and asymmetric broadening induced by misalignment), effectively by deconvoluting the instrument function from a parent ion peak (unaffected by cm. energy releases) and subsequently folding this function into the computed peak shapes. The major assumption made in the simulation is in fact that of an isotropic energy release on fragmentation_ (2) Collisional scattering. Collisions in the apparatus could cause the observed peak shapes to be modified- Such effects would principally affect the backward flying ions, since they have a longer flight path pass through zero velocity as they are turned round and recross the relatively higher pressure sample inlet before travelling toward the ion detector_ Simple scattering losses would thus tend to reduce the relative intensity of the backward flying, long flight time component of a split peak_ This is qualitatively the trend seen for the CFJ+ CTOF peak, but is exactly the opposite of that found for the I’ peak, and so is clearly not an adequate explanation. If reactive collisions are considered the following ion-molecule reactions are pertinent 1251: I++CF,I--,CF;c+IIz, k = 3-S X lo-‘*

shapes

To explain the anomalous A band CTOF peak shapes (section 3.2) we will postulate that the A’A, CF,I’ ions whose dissociation is recorded are preferentially oriented in the lab frame_ How &is circumstance might arise is discussed in subsequent sections. The implications of having oriented CF,I’ are discussed here, but first it is shown why alternative exp!anations have been rejected: (1) Apparatus anisotropy. As discussed in section 2 (see also fig. 1) the instrument is essentially

-

cm3 molecule-’

CF,I++

I,

k = 3.9 X 10-r* cm3 molecule-’ CF;’

s-‘;

+ CF,I + CF,I++

s-r;

CF,,

k =4_8 x 1O-'o cry? molecule-’

s-l_

The rate constants are, however, several orders of magnitude below the lo-‘cn? molecule-’ s-’ we estimate would be required to account for the loss of 25% of the ions under these experirirental conditions; and again backward flying ions would be more strongly depleted by such processes, in opposition to what is observed for the II per&-

Additional .to these &guments it is noted that these anomalies are very specific to this ion, and indeed to its A state,- out of many other systems studied under similar experimental conditions (see e-g. fig. 7). This suggests that the explanation is to be found by considering the particulars of this system rather than general instrumental effects. If then it is supposed that for CF,I+(A’A,) .the C-I bond tends to be oriented along the y instrumental axis with the I toward the electron detector and assuming, as seems likely, that axial i&oil conditions apply to C-I bond cleavage dissociations (see section 4-l-3) it is seen that the I+ (or I) fragments would tend to be initially backward flying in the sense previously used and the CF;t (or CF,) fragments would tend to be forward flying. Thus a single supposition accounts for both CF;C and I+ peak shape anomalies. Eland [19] has previously attributed anomalous PEPICO peak shapes to anisotropic behaviour, the most striking example being a reduced intensity of backward flying N+ from c311 NO’. In the present case the observation of the opposite distortion for the I” peak shapes is unique and the simultaneous observation of oppositely distorted peak shapes for CF,+ and I* is very compelling evidence in favour of the postulated orientations4.3. Anisotropic

photoionization

The lab frame orientations invoked in the previous section can be explained in tern of molecule frame photoelectron angular distributions_ Specifically, if the photoelectron distribution is sharply peaked along the C-I bond from the I atom (fig_ to electron ; - - _ __ detector I--: _ -&.-c/l c//-I_\\ \ ‘-0 __I -t

photoelectron

FF F

I =

to ion detector -_)y

11) .then, since the apparatus accepts .&ct&o* emitted in a particular lab fra.me.dire&on;~~&er~ will be a preferred lab frame orientation for.those ions which are detected in coincidence with’their photoelectron_ It -is stressed that the- observed orientation of those ions is therefore. a. wnsequence of the coincidence detection arrangement and does not make any reference to lab fra& anisotropies in the.overall photoionization process. In general of course, since a directional beam of light is used these overall anisotropies will be present; however the symmetry of the ion and electron detection axes will not lead to the forward/backward asymmetries which are se$t. This suggestion can be checked_ The effective solid angle of electron acceptance depends on instrument geometry and also, as a sour& field is employed, on the magnitudes of the field-and the photoelectron initial velocity. Experimental constraints preclude making significant variations of the source field but an attempt was made to reduce the photoelectron energy by using Ne ionizing radiation_ It is a simple matter to calculate that in the current apparatus this increases the effective solid angle from which electrons are accepted by = 25%_ Consequently one expects the degree of orientation of the coincident ions to be reduced. This is entirely consistent with the Ne lamp experimental data which, as remarked in section 32 are somewhat more readily simulated_ Moreover the other feature of the Ne peak shapes is that they are less deeply split than the corresponding He peak and this too would be as expected for a more isotropic (lab frame) dissociation_ Another factor in this comparison may be wavelength-dependent resonances in the photoionization pro& which lead to an enhanced directional character in photoelectron emission at the He1 wavelength_ The photoionization process in question may be writt& as

I

distriblrtbn

Fig. 11. Figurative representation showing how oriented CF,I+ is obtained by co-incidence detection as a result of a marked molecule frame photoelectron anisotropy. Lab frame distt-ibution of fragments from an impulsive rupmre of the C-I bond would be expected to reflect the initial orientation. (See discussion in tat)

IA, CF,I 2 ‘A, CF,I++

(a,, e)e-.

where the alternative symmetry photoelectron channels arise from electric dipole selection rules which permit A, and E total final state s-etries. -Naively, one can image that promotion of the a, C-I bonding.electron to the a, continuum

412

KG. LowexaL

/Anirorropicphofojonj~afjonroionirolionfCF~I

orbital, a parallel type transition, will lead to electron ejection along the direction of the electric vector i.e. along the molecule symmetry axis. However, as the electron leaves the vicinity of the ion core it is scattered by interaction with the molecular ion field which will be different at the CF3 and I ends of the molecule, giving rise. to the forward-backward asymmetry we have suggested_ More rigorously one can, following the work of Dill [4], derive an expression for the differential cross section for molecule frame photoelectron ejection from randomly oriented molecules, which has the general form of eq. (2) [33]. Forwardbackward asymmetry of the photoelectron distribution requires the presence of odd harmonics in eq. (2) which can arise from interference terms between odd and even I angular momentum components of the continuum function partial wave expansion_ The maximum harmonic content can likewise be shown to be 2Z_. where I_ is the highest partial wave required in the expansion. Calculated photoionization amplitudes are not available for CF,I; however, detailed calculations of the molecule frame photoelectron distribution for two fixed molecular orientations of the CO molecule do indeed show the highly directional distributions that are postulated here [S]. Since the first draft of this paper was prepared, Kaesdorf et al. [34] have published pre’hminary rest&& in which the photoelectron current from CH,I, oriented by a hexapole field, was measured along the C-I bond axis. These authors report a greater current along the direction C --, I, much as we have postulated here.

5_ Conclusions Data have been presented for the dissociation of electronic state-selected CF,I+. Lab frame anisotropies are inferred for the A state fragment ion, based on the following qualitative features of the A band CTOF peak shapes: (i) The intensity ratios of the “forward”- and “ backward-flying” components of the split CTOF peaks are anomalous with respect to simulations in which isotropic fragment distributions are assumed.

(ii) While, -at a -given ionization. energy, the forward/ha&ward ratio is enhance&; for CF;’ fragment peaks, remarkably for I+ it falls below unity_ We believe this obse_ovation is unique. (iii) When recorded with a Ne resonance lamp the CTOF peak shapes are visibly changed; they become more square in shape and are less split in appearance_ Various explanations are discussed but are rejected in favour of a postulated photoionization anisotropy referred to the molecule coordinate frame. This is the only model adequately to explain the above features, particularly (ii)_ As has been pointed out before [19] anisotropic dissociations have various implications for the interpretation of fixed wavelength PEPICO data. The most general issue raised is to know how reliable are energy releases and branching ratios determined from analysis of CTOF peak shapes. Two conditions are assumed to apply to the anisotropic system studied here. The first is that the molecule frame photoelectron distribution is strongly peaked. How generally true this is is not known and requires further investigation by theory and experiment_ The second assumption was of axial recoil in the dissociation process. CF,I may be a particularly favourable case for this condition to be observed as its large moment of inertia leads to a low rotational speed around a perpendicular axis. In any event it has to date been the case that the CTOF peak shape data most closely analysed have been those pertaining to slow dissociations, where initial orientation effects can be assumed to be small. Data for direct dissociations where axial recoil might apply have received less detailed analysis. It is probable that most conclusions reached in the past would not need significant modification were anisotropies to have been mistakenIy neglected_ An exception may be the dissociation A CF,Cl*-+ CFJ* + Cl where a peak shape anomaly present with He but not Ne ionizing radiation was attributed to autoionization [21]. Because of the close similarities with CF,I which were emphasised in section 4.13 one is led to wonder whether this could instead be indicative of anisotropy. The experiments reported in this paper were not initially undertaken to investigate anisotropic distributions_ They have, however, motivated fur-

ther experiments, which are now. being prepared, specifically to investigate photoelectron and photofragment distributions and their interdependence in lab and molecule coordinate frames. The potential for such studies based on exploitation of the features of dissociative photoionization discussed here can be summarised as follows: (1) The use of axial recoil dissociations to determine or select initial ion orientations and to allow molecule frame photoelectron distributions to be determined. As a probe of photoionization dynamics this represents an attractive alternative to some of the experiments considered by Dill and co-workers [7,18]. Molecule frame distributions have a richer harmonic content and, in the case of linear molecules, fewer interference terms than lab frame photoelectron distributions_ The availability of photoelectron distributions for gas-phase oriented molecules may have value for determining the orientation of surface adsorbed molecules [8-lo]. (2) The converse situation would be to take advantage of highly directional photoelectron distributions to obtain oriented ions. In the field of reaction dynamics this could find application in studies of bimolecular collisions, or the investigation of uuimolecular processes by measurement of fragment angular distributions.

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