Photoion anisotropy in dissociative photoionization of CF3IIvan Powis, Odile Dutuit, Martine RichardViard, and Paul Marie Guyon Citation: The Journal of Chemical Physics 92, 1643 (1990); doi: 10.1063/1.458098 View online: http://dx.doi.org/10.1063/1.458098 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/92/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dissociation limit and dissociation dynamic of CF4 +: Application of threshold photoelectron-photoioncoincidence velocity imaging J. Chem. Phys. 138, 094306 (2013); 10.1063/1.4792368 Infrared multiphoton excitation dynamics of CF3I. I. Populations and dissociation rates of highly excitedrovibrational states J. Chem. Phys. 96, 8863 (1992); 10.1063/1.462243 Dissociative photoionization of tbutyllithium J. Chem. Phys. 69, 2715 (1978); 10.1063/1.436866 Carbon isotope separation by multiphoton dissociation of CF3I J. Chem. Phys. 67, 4819 (1977); 10.1063/1.434685 Photoionization of the CF3 Free Radical J. Chem. Phys. 47, 3439 (1967); 10.1063/1.1712409

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Photo ion anisotropy In dissociative photolonlzatlon of CF31 Ivan powis,a) Odile Dutuit,b) Martine Richard-Viard,C) and Paul Marie GuyonC) LURE-Laboratoire mate CNRS, CEA et MEN-Bat 209D Universite Paris-Sud, 914050rsay-Cedex. France

(Received 29 June 1989; accepted 29 September 1989)

Dissociative photoionization of CF 31 is studied in the region of the CF 31 + A state (13-15 e V). We observe a pronounced anisotropy of the 1+ fragment ions in the time of flight coincidence spectra obtained at 14.24 and 15.0 eV photon energy, but having the same 13.3 eV parent ion internal energy. It is the result of a strong alignment in the photoionization process at these energies, probably related to a specific autoionization process. A new and lower value of 13.40 ± 0.05 eV is measured for the appearance potential of the CF21+ fragment.

I. INTRODUCTION

In a recent paperl the dissociative photoionization of CF31 was investigated in a Photoelectron Photoion Coinci­dence (PEPICO) experiment at two different photon ener­gies (both derived from unpolarized rare-gas (VUV) reso­nance lamps). Pronounced asymmetry of both 1+ and CF3 + fragment ion time-of-ftight peak shapes from the first elec­tronically excited state of CF3 + was observed and was de­duced to result from a strong correlation between the photo­electron and fragment photoion directions. More specifically it was suggested that photoelectrons were prefer­entially ejected in a direction which lies parallel to the molec­ular axis and away from the CF3 group. Because of strong angular discrimination for electron detection only those molecules which were oriented in the LAB frame so as to release electrons toward the electron detector could be sub­sequently detected as coincident ions. For this selective sam­pling of an oriented subset of photoionized molecules to be apparent in the experiment as conducted it was, moreover, necessary to postulate that the dissociation dynamics were highly anisotropic in a molecule fixed frame. Together these conditions would generate the clear forwardlbackward asymmetry in the fragment ion LAB-frame distribution which is apparent from the time-of-ftight peaks (see e.g. Fig. 8 of Ref. 1).

In the present paper we report further experiments de­signed to elucidate the photoionization mechanism responsi­ble for the observed asymmetry. For this we have used tuna­ble, partially polarized synchrotron radiation in an experiment designed to minimize angular discrimination for fast fragment ions. Photoion coincidences with both thresh­old (i.e., zero kinetic energy) photoelectrons (T-PEPICO), and energy selected fast electrons (PEPICO) were exam­ined at a number of photon energies in the range 13-19 eV.

II. EXPERIMENTAL

A. The light source

Synchrotron radiation from ACO, Orsay's electron storage ring, was dispersed by aIm McPherson 225 normal

aJ Address correspondence to this author. Department of Chemistry, Uni­versity of Nottingham, University Park, NottinghamNG7 2RD, UK.

bJ L. P. C. R. Bat 350, Universite Paris Sud 91405, Orsay-Ce<lex, France. cJ L. C. A. M. Bat 351, Universite Paris Sud 91405, Orsay-Cedex, France.

incidence monochromator equipped with a 2400 l/mm hol­ographic grating bIased at 100 nm. A photon bandpass of 0.1 nm was used in the present experiment. The polarization of the light was estimated to be only 65% due to two reftections in the horizontal plane containing the main polarization axis of the synchrotron radiation. The principal polarization axis lies parallel to the electron and ion ftight axes.

B. The double tlme-of-fllght spectrometer

The present experiment was performed with a new ver­sion of a double time-of-flight spectrometer,2a.b which has been designed to maximize time dispersion of the ions due to their initial kinetic energy while at the same time maintain­ing good mass resolution. Figure 1 shows a schematic of the apparatus.

The electronics have also been modified to record simul­taneously time-of-ftight PEPICO spectra in coincidence with both threshold and energetic electrons, hence providing a two dimensional analysis of the dissociative photoioniza­tion dynamics. The corresponding coincidence circuitry is shown in Fig. 2.

The experimental method has been previously de­scribed.2•3 Briefty, parent CF31+ ions are prepared by pho­toionization of neutral molecules effusing from a hypoder­mic needle positioned at the center of a 10 mm long first acceleration region. Photoelectrons are accelerated towards the electron detector by a constant weak electrostatic field (typically 200 V 1m). Threshold and energetic electrons are selected using both angular and temporal discrimination and detected by a 20 mm diam dual channelplate detector. The voltages applied to various electrodes of the electron time-of­flight analyzer are such that the slowest electrons arrive at the detector in a time shorter than the interval between light pulses, i.e., 73.5 ns. The electron signal triggers a positive 60 V repeller pulse to extract the ions from the interaction region. The time delay between the photoionization event and the origin of the ion extraction pulse is the sum of two terms: the electron time-of-ftight « 73.5 ns), and a fixed electronic delay of 150 ns. This is too short to measurably affect the ion time-of-flight. The coincident ions are post accelerated in a second 10 mm long acceleration region to about 175 eV and then fty to the detector in a 140 mm long drift tube. Finally they are detected with a dual channelplate detector. The diameter of the ion apertures and detector are

J. Chern. Phys. 92 (3),1 February 1990 0021-9606/90/031643-10$03.00 @) 1990 American Institute of Physics 1643

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1644 Powis et at: Dissociative photoionization of CF31

hv

1 -

5V -IV Iem

10, 15, and 42 mm respectively. We calculate that under these conditons all ions having an initial kinetic energy less than 4 eV should pass through the TOF instrument without suffering any angular discrimination, and hence expect to be able to detect any such ions with uniform efficiency.

The amplified electron and ion detector signals, togeth­er with the synchrotron r8.diation time-reference signal are analyzed using LeCroy NIM and CAMAC modules (Fig. 2 ). Both the electron and ion delays, referenced to the synchrotron light pulse, are encoded and each delayed coin­cidence between an electron-ion pair contributes a count to a two-dimensional histogram which is accumulated for a pe-

CAMAC I GPIB Interf..:e Bus

~~------------------------------~~

FIG. 2. Coincidence detection logic and timing circuits. Electron and ion flight times (referenced to the ACO photon pulse) are recorded by separate TDCs (Time to Digital Convertors) and encoded as, respectively, S- and 9-bit values. The TDC outputs access a 14 bit histogramming memory to store individual coincidences in a two dimensional array: electron vs ion time-of­flight. A group of scalers simultaneously record the accumulation time, the total number of electron start events, the total number of ions, and the pho­ton flux. Delay I is used to shift the electron TOF spectrum, whereas delays 2 and 3 are used to inhibit the electron and ion discriminators preventing them from retriggering because of the noise signal generated during the rise and fall time of the ion extraction pulse.

Ion detection

FIG. 1. Schematic of double (elec­tron-ion) time-of-fiigbt apparatus. See text for description of operation.

riod of time. One dimension of this histogram corresponds to electron time-of-flight (kinetic energy), the other to ion time-of-ftight (masslkinetic energy).

C. ':xperlmental procedure

The method was similar to that employed in Ref. 2 ex­cept that, as just described, the raw data from each experi­ment consisted of a set of32 PEPICO ion time-of-flight spec­tra correlated with 32 discrete values of the electron flight-time. Conversely, summing the 512 channels of the ion time-of-flight spectra at each discrete electron time-of­flight would reproduce the photoeleptron spectrum. In the photoelectron time-of-ftight spectrum we selected several groups of channels associated with various internal energy states of the parent molecular ion. Within each group the PEPICO spectra have been combined to yield the final re­sults: a PEPICO spectrum as a function of two parameters, photon energy and nominal internal energy of the parent ion.

These spectra were then corrected for the false coinci­dence contribution which was ascertained by accumulating data for the same number of electron start signals but using randomly generated start pulses. The counting rate was maintained below 10 000 counts per second to minimize the false coincidence rate.

Pure research grade CF3I supplied by Fluorochem Ltd. was introduced into the apparatus without further purifica­tion.

III. ION TIME-oF-FLIGHT PEAK SHAPE ANALYSIS

The ion coincidence time-of-flight peak shapes reflect the ion nascent velocity and angular distribution with re­spect to the apparatus axis. The design and operation of the time-of-flight analyzer is such that all fragment ions carry­ing any reasonable kinetic energy perpendicular or parallel to the time-of-flight axis will reach the ion detector without discrimination.

Following previous practicel•4

•s the time-of-flight peak

shapes can be quantitatively analyzed by computing a set of simulated peaks for a selection of discrete CM energy re­leases and fitting to the experimental data to find coefficients for each component. This result can then be expressed ;is a kinetic energy release distribution (KERD). The ion trajec­tory calculations which are performed for the simulation take into account the anisotropic distribution of parent mol-

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Powis et 81.: Dissociative photoionization of CF31 1645

ecule thermal velocity induced by the inletjet.4 Further, in the present case the individual trajectories are weighted by a flux distribution6 of the form7

•8

I(t1)a:[I+~ (3cos2 t1-1)] (1)

where t1is the angle from the polarization vector, E, and Pis the anisotropy parametee·8

( - I<P(2), and which is in tum modified to reflect the partial (65%) polarization of the synchrotron radiation along the time-of-flight axis.

In a idealized case of a single value for the kinetic energy release, and an isotropic distribution, this would give rise to a square, flat topped time-of-flight peak. This can be seen in a simulated peak shape, Fig. 3(b). The width of the peak is determined by the magnitude of the kinetic energy. Anyani­sotropy will, however, modify the ion peak shape. In particu­lar, alignment of the molecular and time-of-flight axes by photoabsorption followed by fast dissociation would pro­duce a more peaked shape for a perpendicular dissociation i.e., for fragments which are ejected predominantly at t1 = 90" with respect to the time-of-flight axis [Fig. 3 (a) ] . This situation corresponds to a negativepparameter. Alter-

a

10.10

b {!>=O

10.10 10.20

c {!> = 2

10.10 10.20

TIME-OF- FLIGHT (,uS)

FIG. 3. Simulated fragment ion time-of-flight peak shapes for a single value kinetic energy release. Different peak shapes illustrate the behavior expect­ed for different fragment ion angular distributions. represented by the indi­cated p values [seeEq. (1)]: (a) p= -1; (b) P=O (isotropic); (c) P=2.

natively a split, but still symmetric, peak shape [Fig. 3 ( c ) ] results for a parallel dissociation where P is positive and fragments are ejected primarily in a direction along the time­of-flight axis.

In a realistic situation where there is a distribution of kinetic energy the observed time-of-flight peak shape will be given by some kind of superposition of such idealized shapes. Thus a pointed time-of-flight mass peak may represent a superposition of square, isotropic peaks or alternatively a different superposition of anisotropic peak shapes. Without further angle resolved data the information concerning the kinetic energy and angular distribution which has been mapped into the time-of-flight peak shape cannot be unam­biguously extracted. However, should the observed peak shape be a split one, this cannot conceivably arise from su­perposition of nonsplit components such as in Fig. 3 (a,b) (P,O); only a superposition of components such as Fig. 3 (c) ({3 > 0) could be responsible. Therefore any observa­tion of a split peak shape in the present apparatus unambigu­ously indicates some parallel anisotropy in the fragment ion distribution.

IV. RESULTS AND DISCUSSION

A. The threshold photoelectron spectrum and photolonlzatlon efficiency spectrum

Figure 4(a) presents the TPES recorded with a photon resolution of 0.2 nm in steps of 0.5 nm and with an integrat­ing time of 3 s per point. It is recorded by counting the elec­trons in a time window corresponding to zero kinetic energy while scanning the monochromator. The electron time-of­flight is measured with respect to the synchrotron radiation photon pulse. The 200 V 1m continuous extraction field in the source region provided an energy resolution of 55 meV (FWHM). The TPES shows seven main bands identified as theX to Fstates, and possibly a weaker Gband around 24 e V.

Figure 4(b) presents the total ion yield curve. The con­tribution from the second order light was checked by record­ing a similar spectrum with Ar (IP = 15.76 eV): it is signifi­cant at 10 e V but decreases monotonically to zero at ::::: 14 eV. The ion yield curve shows shoulders associated with the opening of successive ionization channels. A dip is observed around 20 eV which unfortunately corresponds to a pro­nounced minimum in the grating transmission function. Its depth can be strongly influenced by the local fraction of the scattered light, negligible everywhere else but here.

The structure labelled S 1 can either be attributed to autoionization of Rydberg states converging to the A state or to a sharp decline of the X state cross section with energy as inferred from inspection of the Hel and Hell PES.9

•10 The

structures labelled S 2 and S 3 can possibly be assigned to autoionization of Rydberg states converging to the unre­solved B and C states. The S 3 peak at 14 e V is of particular interest for this work since it falls in the region where our coincidence spectra showed the largest anisotropy. It is re­markable to observe that this peak which occurs in the A, B Franck Condon gap has no corresponding feature in the TPES. If this peak is due to autoionization, it is surprising that it does not produce any resonant autoionization. This is

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1646 Powis et s/.: Dissociative photoionization of CFal

-fd 1.4 .£!!. X ~ 12 z a a ~ 10

B,C 0 CI) z 0 8 a: t; w ..J 6 w A c .4 ..J 0 J: CI)

2 w a: J: I-

0 8 12 28 32

2000

0'1600 w ~

CI)

~ 1200 ::J 0 ~

800 CI) z Q

.400

PHOTON ENERGY (eV)

FIG. 4. (a> Threshold photoelectron spectrum for CF31; (b> Totalion yield curve for CF31. Corrections for second order light from the monochromator grating are indicated as broken curves. Both (a) and (b> are nonnalized to photon flux measured by a photomultiplier mounted behind a sodium sali­cylate coated window.

at variance with what is often observed for polyatomics (see Refs. 11 and 12).

B. Coincidence spectra

As already mentioned in the experimental procedure of Sec. IIC, we recorded simultaneously at each photon energy two types of coincidence time-of-flight spectra: a photoelec­tron TOF spectrum and several PEPICO spectra associated with various final energy states.

1. Electron tlme-of-fllght

Figure 5 shows a typical time-of-flight photoelectron spectrum. It was obtained at 14.24 eV photon energy. The calibration of the electron energy scale was achieved by ex­amining the electron time-of-flight distribution for argon at various energies above its ionization potential. The time-of­flight PES was divided into several regions corresponding to relevant final ionization energies of CF31.

The energy scale shows that one could easily separate the X and A state. The internal energy for the latter is defined within 0.2 eV, but the resolution degrades rapidly at higher electron energies as illustrated by the partially resolved X state spin orbit components (0.7 e V) .

PHOTOELECTRON ENERGY (av) ~.r-____________ ~'~ ____________ ~OT·5~ ____ -; ________ ~Z~3~ ________ -, rg th reshold electrons z hY .14.24 ev ::J ai a: <

2E X state \'.l~ -

o 10 20 30 40 50 60

CHANNEL NUMBER

FIG. 5. Typical CF31 photoelectron time-of-flight spectrum. Shading indi­cates the range of electron flight times utilized in the threshold (TPEPICO) and A state (PEPICO) coincidence measurements which are subsequently reported at this particular photon energy (14.24 eV).

2. Ion tlme-of-fllght

Observations of ion time-of-flight spectra were concen­trated in the region around and just ahove the A 2AI state ionization energy, viz. 13-15 e V. Parent CF 31 + and its three principal dissociation products, CF3 +,1+, CF21+ were ob­served in this study and literature values for the threshold energies of the various fragmentation channels are given in Table I. It should be noticed that the thresholds given for CF 3+ formation 13 have been revised downward compared to values we have quoted previously. 1

A typical ion time-of-flight spectrum, obtained in coin­cidence with threshold photoelectrons, is shown in Fig. 6. At this energy of 14.24 eV, CF31+ is fully dissociated into CF3+ , 1+ and CF21+ fragments. The sharp peak observed at the mass of the parent CF 3+ ion results from a nonperfect cor­rection for false coincidences. To analyze the kinetic energy of the ions, we recorded expanded time-of-flight spectra with a better time resolution (2.5 ns/channel). In these condi-

TABLE I. Appearance thresholds for CF 31 fragmentations.

Reaction

hv

CF31 - e- + CF/ + lep3/2)

+ CF/ + lep'/2)

+ l+ep2 ) + CF3

+ I+epo) + CF3

+ pep,) + CF3

+CF2I+ + F

+CF,+ +F+ I

o K appearance energy (e V)

11.858•b

12.70b

13.50b

13.58b

113.40" 1 17.62b

•d

"Obtained using 6Hr" (CF)+ = 357.7 kl mol-' deduced from Ref. 13. bObtained using thennochemical values from Ref. 14. c300 K appearance energy detennined in the present investigation-see text.

d6Hr"CF,+ = 939 kl mol-' from Ref. 15.

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Powis et al.: Dissociative photoionization of CF31 1647

o 600 CF3+ E hY "14.24 eV

w en "-~ ?i 400 0 0

Z CF3 I+ Q W 200 0

1+ Z CF2 I+ W C {) Z Of-----(5 -0

7 8 9 10 11 12 13 14 15 16

ION TIME OF FLIGHT (Il SEC)

FIG. 6. Typical TPEPICO ion time-of-ftight spectrum for CF3I recorded at a photon energy of 14.24 eV. This spectrum has been corrected for false coincidences. See section lIe.

tions, however, only one mass could be recorded at a time due to memory restrictions. In the specific case ofthe parent ion, where no kinetic energy is released in the center of mass frame, the time-of-flight peak shape reflects the thermal ve­locity distribution of the molecules, from which we can de­duce an effective translational temperature transverse to the direction of flow from the inlet jet of - 20 K. Such a pro­nounced cooling is expected and is a consequence ofthe or­dered flow achieved with the effusive inlet.4

This thermal distribution was then used to deduce the KERD of the fragment ions by obtaining a fit of the experi­mental peak with simulated ones. As discussed in Sec. III different KERDS can be obtained according to the chosen fJ parameter which is not directly measured here. Results for the different fragment ions are considered separately below and are also summarized in Table II.

TABLE II. Fragment ion time-of-ftight peak shape analysis.

Ionization hv/eV region Method

1+

13.30 A band T-PEPICO

14.24 A bandc PEPICO

15.0 A bandc PEPICO

14.24 A/Bbandgap T-PEPICO

15.0 A/Bbandgap T-PEPICO

19.0 Eband T-PEPICO

CF3 + 13.30 A band T-PEPICO

14.24 A bandc PEPICO

14;24 A/Bbandgap T-PEPICO

CF2I+ 14.24 A/Bbandgap T-PEPICO

3./+ ft'll{Jment

We recorded T-PEPICO spectra at four different pho­ton energies as shown in Table II. The peak shape at 14.24 eV and IS.00eVisroughlytriangular [Figs. 7(b), 7(c)] where­as it is more trapezoidal at 13.30 eV [Fig. 7(a)], reflecting different kinetic energy distributions (KERDs). The KERDs shown in Fig. 7 have been obtained by a fit with the explicit assumption of an isotropic distribution (fJ = 0).

(a) At 13.30 eV one produces the CF31+ A state. The mean CM energy released in fragmentation is 0.40 ± O.OS eV, the same value as previously obtained from a PEPICO result for the A state at Ne I wavelengths. 1 This kinetic ener­gy (which represents 66% of the total available energy in the dissociation) has already been interpreted as a result of a fast and direct dissociation into CF3 + I+CP2 ) at 12.70eV.

(b) At 14.24 eV the KERD has approximately the same total width and the same mean value. However, it peaks near zero eV and thus looks more characteristic of a statistical redistribution of internal energy. At this energy two other dissociation limits are energetically accessible, CF3 + 1+ CPo) at 13.S0eV and CF3 + I+CP1) at 13.S8eV. If the above mentioned mechanism of a direct dissociation into CF3 + 1+ C P2 ) was still active one would expect a shift of the distribution and a mean kinetic energy of about 1 e V. This is not the case. We may then consider two alternative possibilities. First, a fast and direct dissociation to the higher limits, since the mean kinetic energy of 0.39 eV would then represent about SO% of the total excess energy. Second, a dissociation towards the ground state limit with randomization of the excess energy. This second mechanism is the one preferred because the shape of the KERD, peaked near 0 e V, is as expected for a statistical distribution, and the mean kinetic energy now would represent only 20% of the total excess energy. The next question concerns the identity

Excess Figure {J (E,)a/eV energyb (eV)

7(a) 0 0.40 0.02 0.60

8(a) 1.25 0.42 0.03

8(b) 0.75 0.41 0.04

7(b) 0 0.39 0.03 LS4(0.74,0.66)

7(c) 0 0.55 0.04 2.30( LSO, 1.42)

0 0.69 0.04

9(a) 0 0.25 0.01 2.39( 1.45)

lOCal 0 0.26 0.02

lO(b) 1.25 0.19 0.01

9(b) 0 0.39 0.02 3.33(2.39)

l1(a) LS 0.37 0.04 0.84

• Mean translational energy derived from analysis of time-of-ftight peak shape as discussed in text, with an assumed {J parameter as indicated in preceding column.

bBased on dilference between selected ionization energy and thermochemical dissociation thresholds (Table I). Values in parentheses apply to higher fine structure dissociation limits.

C Nominal ionization energy for these PEPICO results is 13.3 eV.

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1648 Powis 6t af.: Dissociative photoionization of CFsl

(a.) (b) (c)

., 1 hV=13.3OeV ., BO ., 120 hV=lS.OOeV

c: I=13.30ev c: ; 100 I=lS.OOeV ::s ::s 81 1+ 0 0

t) t) BO 0 04-0 0 60 c: c: c: - "0 '0 4-0 0 t) t) 20 t)

20

-0.2 0.0 0.2 -0.2 0.0 0.2 -0.2 0.0 0.2 Relative TOF xl0(-6)& Relative TOF xl0(-6)& Relative TOF xl0(-6)&

(d) (e) (I) 2.0 (p 2.0

t ~ cp 1.6

> 1.5 ~=O >1.5 ~O -

'f ~=O

l>

" " ~ ~ 1.2 if? - -"- "- "-_1.0 IiQ 1.0 ~ IiQ 0.8 ~ !iii

P - - - ~ g.. g.. c.. 0.5 0.5 0.4-

III Q) I ®,

0.4- 0.8 0.4- O.B 0.4- 0.8 1.2 E (eV) E (eV) E (eV)

FIG. 7. 1+ TPEPlCO time-of-ftight peaks recorded at photon energies of (a) 13.3 eV; (b) 14.24 eV; (c) 15.0 eV. Computed best fit simulations (for an assumed isotropic fragment distribution) are drawn as smooth curves through the eltperimental data. The kinetic energy release distributions which are inferred from the simulations are displayed below each time-of-ftight peak. Error bars on the eltperimental points in the time-of-ftight peaks (a)-(c) represent the standard deviation of each count.

of the precursor parent ion state. The i()nization energy of 14.24 e V lies in the Franck-Condon gap between theA and B states, so we cannot a priori assign the prepared ion state to either the A or the X state. It is, however, possible that reso­nant autoionization produces high vibrational levels of the X ground electronic state of CF31+ which then undergoes a RRKM type dissociation, as previously established for the ground ion state. 14.16-18

(c) At 15 eV, the kinetic energy release gives a wider distribution, showing possibly two maxima, one at low ener­gy (near zero) and another around 0.6 e V. At this energy the higher B electronic state can also be formed and several dis­sociation mechanisms are probably involved.

Among the different recorded PEPICO (energetic elec­tron) spectra, we selected those which correspond to a pro­nounced peak in the electron TOF spectrum, in order to minimize the contribution of coincidences due to back­ground photoelectrons of any energy. Figure 8 shows two PEPICO spectra recorded at 14.24 eV and 15.0 eV photon energy, both corresponding to a nominal parent ion internal energy of 13.3 eV, i.e., in the the center of the A band. The dip in the middle of both peaks clearly and unambiguously shows a strong parallel anisotropy. Since we only measure the time-of-flight distribution and have no direct informa­tion on the ejection angle of the fragment ion, we had to assume some value for {3 in our treatment of the data. This choice of course affects the final KERD which is derived in the fitting procedure. Although no unique result for both anisotropy and KERD can be achieved, limits can be placed on the likely range of f3 parameter which would result in a

peak shape such as is experimentally observed. A minimum value is required to allow for sufficient dip in the peak center, while too great a value suggests a highly structured physical­ly and a priori unrealistic KERD. For both photon energies, we deduce 0.5,{3, 1.5, with a preferred value of 1.25 for the 14.24 eV photon peak and 0.75 for the 15 eV photon result. Such expressed preferences are based only on the preceding considerations of the form of the KERDs, and these are shown in Fig. 8. With these {3 values, a mean CM kinetic energy of 0.4 eV (see Table II) is obtained in both cases, which is in perfect agreement with the A state ionization results obtained from the T-PEPICO spectrum at 13.3 eV photon energy, as indeed is the form ofthe KERD [compare Fig. 7(d) and 8(c)]. One can conclude that CF31+ ions formed in the A state at such photon energies display consid­erable alignment along the polarization axis of the light, which coincides with the TOFaxis, and dissociate rapidly with respect to the molecular rotation period into 1+ frag­ments.

In seeking reasons why such a highly anisotropic frag­ment ion distribution should appear in the current circum­stances, it can be recalled that at 14 e V there is a peak in the total ion yield which is most probably due to autoionization. A bound-bound excitation from the IA I ground neutral state to an A I Rydberg state (that is a parallel transition-as op­posed to an AI-E perpendicular excitation), followed by autoionization rapid compared to the estimated 4.5 ps CF 31 + rotational period would then be capable of generating parallel aligned CF31+. If it is followed by similarly rapid dissociation of the ion, it would explain the observed anisot-

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Powis 9tB/.: DIasociative photoionizatlon of CFal 1649

(<<) (b)

180

JJ ; 120 o u o 80 c:: -o u 40

hv=14.24eV

I .. 13.3eV JJ 80 c:: g 80 u

g4Q a

20

hv=lS.OOeV

I-13.3eV

-0.3 -0.2 -0.1 0.0 0.1 Relative TOF xl0(-6)s

-0.2 0.0 0.2

FIG. 8. 1+ PEPICO time-of-ftight peaks corresponding to a nominal 13.3 eV ioni­zation energy recorded with (a) 14.24eV, and (b) 15.0 eV photon energies. Other details as for Fig. 7. However, the best fit simulations here represent anisotropic fragment distributions. (a) /3 = 1.25 (b) /3= 0.75. CorrespondingKERDsare (c) and (d) respectively.

Relative TOF xl0( -8)s

~ (0)

2.00

1.75 f 3.0

'> 1.50 ~1.2S _2.5

" ~ ,1.25 ,2.0

(d.)

P=O.7S

_1.00 iii 1.5 \iii 'it" 0.75

T -"" 1.0

0.50

0.25 III 0.5

0.2 0.4 0.6 E (eV)

0.8 1.0 0.2 0.4 0.6 0.8 E (eV)

1.0

ropy. In contrast a direct A state ionization will proceed via dipole allowed continuum channels of both A 1 and E sym­metry, which will have different spatial properties. Unless one ofthese dominates, it may be expected that the intrinsic photoionization anisotropy will be masked by an admixture of the two continuum channels. Indeed no significant anisot­ropy was observed in the T-PEPlCO result at 13.30 eV which is very likely dominated by a direct ionization process. It should be noticed that the anisotropy is probably greater at 14.24 eV than at 15.00 eV photon excitation energy. This is consistent with the above mentioned autoionization mech­anism for which a maximum is observed at 14 eV in the total ion yield curve (i.e., the S3 structure).

Let us note that the anisotropy which has been com­mented on here is quite distinct from that observed in the Hel A state ionization;! There the anisotropy was observed as an asymmetric (forward/backward) distortion of the time-of-ftight 1+ peak shape, reflecting an orientation of the parent ion with respect to the apparatus axis. It was inter­preted as an anisotropic photoelectron ejection relative to the molecular axis and fast dissociation of the parent ion: because of the strong angular discrimination in electron de­tection for that experiment, only those molecules which were oriented in the LAB frame so as to release electrons towards the electron detector could be subsequently detect­ed as coincident ions. In the experiment described in this work such electron-ion correlation would not be evident in T -PEPlCO results because most threshold electrons are de­tected, but it might, in principle, be observable in the

PEPICO data because of a similar discrimination for ener­getic electrons. In the previous section we nevertheless pre­ferred to invoke alignment to the molecular axis due to an­isotropic photon absorption. In fact in our PEPICO experiment one cannot dismiss outright the possibility of a correlation between the direction of the ejected electron and the molecular axis, but in this case our PEPICO spectra do not show any evidence for preferential orientation (no back­ward/forward disymmetry).

Our present results are not in contradiction with those of Ref. 1 since dift'erent photon excitation energies were em­ployed. One in fact knows that for the same final ion state (in our case CF31+ A state) the partial dift'erential photoelec­tron cross sections depend on the photon excitation energy as, therefore, should any eventual orientation process. Un­fortunately the restricted resolution in our electron time-of­flight at higher photoelectron energies precllided our mak­ing comparable measurements at 21.21 e V photon energy in the present work.

4. CF~ + frsgment

Figure 9 shows the T-PEPICO spectra recorded at 13.3 eVand 14.24 eV photon energies.

The T -PEPlCO spectrum at 14.24 eV [Fig. 9(b)] gives a KERD which peaks at low kinetic energies and whose form suggests that a statistical dissociation mechanism might here be operative. This inference is supported by the relatively small proportion of the excess energy at this ioni-

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1650 Powis et al: Dissociative photoionization of CF31

(0,) (b) 250 200

hv=13.30eV !J 175 hv=14.24eV !J 200 I-13.3OcV I-14.24eV .:: .:: 150 :3 CF3+

:3 CF3+ 8 150 8 125

c.i c.i 100 .s 100 .:: 75 -0 0 t,) t,)

50 50 25

-0.2 0.0 0.2 -0.2 0.0 0.2 Relative TOF xl0(-6)_ Relative TOF xl0(-6)_

FIG. 9. CFl+ TPEPICO time-of-ftight peaks at (a) 13.3 eVand (b) 14.24eV pho­ton energies. Other details as described in caption to Fig. 7. (c)

3.0 ~ ~ 2.5 p:o 2.0 - -~ ~ >

~ _2.0 ~ 1.5

" Gi 1.5 -- !!!. 1.0 CI.o 1.0 CI.o

0.5 0.5

IZI

0.2 0.4- 0.6 0.8 1.0 0.2 E (eV)

zation energy which appears in product translation (i.e. -12%). Such a deduction is also consistent with our obser­vations for the 1+ fragment under these conditions. As for I + we can invoke a two step mechanism: a resonant autoion­ization of a Rydberg state to the vibrationally excited X CF 31 + ground state, followed by a statistical dissociation with energy randomization amongst the molecular degrees of freedom.

This behavior seems to be different from that observed at the lower 13.3 eV excitation energy. Both the 13.3 eV photon energy T-PEPICO spectrum [I"ig. 9(a)] and the 14.24 eV PEPICO spectrum (Fig. to) are identical and con­sequently similar KERDs are deduced from them. The mean energy release remains a small proportion of the avail­able excess, though the KERDs, which have narrowed com­pared to the 14.24 eV excitation energy result, now possess a maximum shifted to a higher kinetic energy. From this it appears likely that a different route is followed to fragmenta­tion at this 13.3 eV excitation energy. The 13.3 eV threshold ionization experiment should, as previously noted, access theCF31+ A AI state. The observation in our experiment ofa significantly greater abundance ofCF3+ ion as compared to I + ion (4: 1 ) indicates that the CF 3+ channel competes effec­tively with the 1+ channel. Also we recall the earlier Hel result that the CF3+ channel for theA state displays orienta­tion effects. It thus seems that the A state can fragment to both I + and CF 3+ channels with comparable rapidity. Alter­natively the interchannel competition evident in these ex­periments may result from an internal conversion to the X

(d)

rlI (I)

0.4-

= 0.6 0.8 1.0

E (eV)

CF31+ ground state in the CF3+ channel, followed by a slower fragmentation to CF3+ involving some energy ran­domization.

A major feature of interest lies. in a comparison of the 13.3 eV excitation energy CF3+ time-of-ftight peak (Fig. to) with the corresponding 1+ TOFpeak [Fig. 8(a)] where both are recorded with a photon energy of 14.2 eV. In con­trast to the I + peak the CF 3+ peak shows n~ evidence of splitting and hence displays no clear, evidence for alignment of the parent ion. As discussed previously the data can be fitted with different values of the /3 parameter (i.e. /3 = 0 or /3 = 1.25 as shown in Fig. 10) so that parent ion alignment is not precluded by the failure to detect clear splitting. Never­theless this may lend some support to the two step model (fast interconversion, slow dissociation) just proposed since this would be expected to wash out observable alignment.

Clearly more data are required to resolve the mecha­nisms at work here. It is salutary to note the potential com­plexity of the fragmentation mechanisms and their critical dependence upon photoionization mechanism, even at the same nominal excitation energy.

5. CF:z/+ fral/ment

Although the literature value for the appearance energy ofCF21+ is given as 14.58 eV,14 the thennochemistry of this fragment ion is known to be subject to uncertainty. In the course of this work it became clear that .traces of CF21+ could be observed with photon energies below this quoted

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Powis et at: Dissociative photoionization of CFsl 1651

200 hv=14.24eV

l! I=13.3eV s:: g 150 CF3+

t.)

U 100 s:: .... o t.)

50

(a)

200 l! s:: g 150

t.)

U 100 s:: '0 t.)

50

hv=14.24eV

I==13.3eV

CF3+

(b)

-0.2 0.0 0.2 -0.2 0.0 0.2

FIG. 10. CF3 + PEPlCO time-of-tlight peak for a nominal 13.3 eV ionization energy and a 14.24 eV photon energy. The best fit simu­lations drawn as smooth curves in (a) and (b) differ in the assumed anisotropy: (a) p = 0; (b) P = 1.25; (c) KERD inferred from simulation (a); (d) KERD inferred from simulation (b).

Relative TOF xl0( -6)8 Relative TOF xl0( -6)8

3.0

>2.5 ., -'2.0 -~1.5 g.

1.0

0.5

(c) (d)

5

13==0 p==l.25

1 (I)

0.2 0.4 0.6 0.6 1.0 E (eV)

0.2 0.4 0.6 0.8 1.0 E (eV)

threshold. Consequently a series ofT -PEPICO mass spectra were recorded to establish a revised appearance energy of 13.40 ± 0.05 eV. This should be regarded as an upper limit to the thermodynamic threshold. No traces of CF2I+ frag­ments were found in theA state ionization region in previous HeI PEPICO investigations, I but when we re-examined the NeI PEPICO mass spectra, indications ofCF2I+ (amount­ing to a few percent of the total ion yield) have been found at nominal ionization energies down to 13.24 eV. One can only speculate as to the apparent ionizing wavelength dependence of these observations.

The threshold ionization CF2I+ peak becomes some­what more intense at 14.24 eV permitting some estimate of the CM energy release to be made from the T -PEPICO spec­trum (see Fig. 11). Although the statistics are still poor, the observed peak shape is flat topped, if not actually dished, and the inclusion of some parallel anisoiropy ({3 > 1) is required to achieve a satisfactory simulation. The KERD obtained with {3 = 1.5 is presented in Fig. 11.

Any assessment of the significance of the anisotropy pa­rameter here needs to recognize that in this instance the dis­sociation axis no longer coincides with the C3 symmetry axis. In the simplest analysis {3 is given by the expression8

{3 = [3 cos2X - 1] (2 )

where X is the angle between dissociation axis and the transi­tion moment. A nonzero value for X, as here, should lead to a reduction in the observed {3 value. However, we do not feel a more detailed analysis of this result is warranted here.

60

~ 50 ::s 840

~ 30

820

10

(a)

hv=14.24eV I=14.24eV

..,.0.2 0.0 0.2 0.4

2.00 1.75

;;- 1.50 GI

:::::::: 1.25

IiQ 1.00 Ii:' 0.75

0.50 0.25

Relative TOF x10(-6)8

(b)

0.2 0.4 0.6 0.8 1.0 E (eV)

FIG. 11. CF2I+ TPEPICOTOFpeak at photon energy at l4.24eV. Best fit simulation (smooth curve) in (a) is for assumed P = 1.5. Corresponding KERD is shown in (b).

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1652 Powis 8t BI.: Dissociative photoionization of CFsl

The recorded mean kinetic energy of 0.37 eV represents around 45% ofthe estimated excess energy which together with the strong anisotropy points to a fast dissociation mech­anism. This must, therefore, be in competition with the reso­nant autoionization to vibrationally excited X state which was previously invoked for the 14.24 eV T-PEPICO CF3+ and I + fragmentation results. On the other hand the clearly evident aligment is reminiscent of the 14.24 eV photon ener­gy PEPICO result for 1+. Possibly we are observing here a near-threshold autoionization of the same aligned precursor state. At the higher ion excitation implied by the near zero electron energy the branching from this state favors CF21+; at reduced ion excitation energy implied by the more ener­getic electrons detected in the PEPICO data it reverts to the major 1+ channel.

The maximum kinetic energy determined from the width of the peak base is of the order of 1 e V, suggesting that the true threshold is perhaps around 13.2 eV, slightly less than the 13.4 eV onset which is directly observed.

V. CONCLUSIONS

We have observed a variety of fragmentation processes of the CF31+ ion. Often very different behavior is evident even at the same nominal ionization energy; there is a clear dependence not only upon the parent ion excitation energy but also upon its precise mode of formation, via direct ioni­zation, autoionization or resonant autoionization. In the case of CF2I+ and 1+ fragment ions strong anisotropy is evident which can be attributed to alignment resulting from autoionization by the polarized synchrotron radiation source. The subsequent ion fragmentation is then also re­quired to be very rapid for the alignment to be observed as a fragment anisotropy. Threshold ionization conversely ap­pears to produce vibrationally excited ground electronic state CF31+ parent ions which decay statistically, whereas various direct and predissociative channels from the A state produced by direct ionization have been considered. In the course of this work a new, lower threshold for the appear­ance ofCF21+ has been determined at 13.40 ± 0.05 eV. The primary motivation for undertaking these investigations was to examine the anisotropy previously observed in the HeI and NeI photoionization investigations. The alignment ef­fects which are found at the modest photon energies em­ployed here differ in detail from the orientation effects pre-

viously observed. Both observations, however, are manifestations of an extremely rapid dissociation of the A state ion and hence share a common feature. Moreover, they indicate different means whereby one may exert experimen­tal control over the spatial disposition of prepared pho­toions.

ACKNOWLEDGMENTS

We thank the staff of LURE for the financial and techni­cal support of this experiment and for operating the ACO storage ring, as well as NATO for a grant (04-0784-86). We thank M. Lavollee who developed the acquisition software and M. Ait-Kaci for her help in the data acquisition.

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