Angular distributions in the photoelectron spectroscopy of carbon monoxide

doumal of Electron Spectroscopy and Related Phenomena, 16 (1979) 127-145 0 Elsevler Scientrfic Pubhshmg Company, Amsterdam - Pnnted in The Netherlands

ANGULAR DISTRIBUTIONS IN THE PHOTOELECTRON SPECTROSCOPY OF CARBON MONOXIDE*

JEFFREY

A SELL**,

ARON

KUPPERMANN

and DONALD

M MINTZ***

Arthur Amos Noyes Laboratory of Chemrcat Physzcst, Calrfomta Institute of Technology, Pasadena, Coltfomra 91 I25 (US A ) (First received 21 March 1978, m Anal form 1 June 1978)

ABSTRACT The angular Istnbutrons of the photoelectron mtenslty for CO have been rnvestrgated usmg a photoelectron spectrometer that has a rotatable detector and an He1 hght source The asymmetry parameter p was carefully measured for 24 vibrational transrtrons spanrung three Ionization bands Its variation mth final&ate vibrational quantum number was found to have an unusual behavior It 18 concluded that an autolomvng state of CO should exlet at 584 A m order to account for this behavror, and that the value of p 1s very sensitive to the presence of such a state

INTRODUCTION

Angular dlstnbutlons of the photoelectron mtenslty of small molecules are unportant for the understandmg of the mteractlon of ultr~olet radlatlon with molecules They are very helpful m probmg the physics of the loruzation process Itself Lfthere are different zonlzation pathways avEulable’. Angular dlstnbutlons of photoelectrons are especially Important for chem~stry because they can help dlstmgulsh between lonlzation from CJor ?r types of molecular orbltals‘*. Fmally, angular dlstnbutions are needed to correct relative peak mtensltles m fixed-angle photoelectron spectra

* This work was supported m part by a Contract (No EY-76-S-06-767) from the U S Department of Energy Report Code. CALT-767P4-154 ** Work performed m partial fulfilment of the requnements for the Ph D m Chemistry at the Cahfomla Institute of Technology *** Present address TRW Defense and Space Systems Group, Redondo Beach, CA 90278, U S.A ? Contribution No 5747

128

(taken at detector angles other than 54.7’) before makmg the approxnnation that peak intensity 1sproportional to the degeneracy of the molecular orbital from which an electron was removed We have recently been involved m the measurement of the angular distnbulzon of the photoelectron intensity of small molecules m this laboratory. This study was prompted by the theoretical work of Cooper and Zareg and the expenmental angular distributions of photodetached electrons from atoms of Hall and Siegel”. Briefly, the differential cross-section (DCS), do/dS2, for a given transitron and given incident wavelength usmg unpolarized hght has the form da -=da

CJ 477

1 - zp

P2

@OS

@I

(1)

where 8 1s the direction between the incident light beam and the photoelectron, P2(cos 0) IS the second Legendre polynomial, (T is the integral ionization cross-section, dSZ is an element of solid angle around the photoelectron direction, and @ is an asymmetry parameter that ranges from -1 to + 2 (ref. 9). In the case of atoms, it was foundg*lo that the asymmetry parameter depended on the photoelectron energy and also on the angular momentum of the orbital from which the electron was eJected. For molecules it was hoped that there would be a correspondmg difference in the asymmetry parameter for lonlzatlon out of a orbitals compared wrth IJ orbltals Carlson and co-workers 3-* have studied many molecules and found that for unsaturated’** ones the asymmetry parameter 0 for ionization out of ?r orbltals is generally + 0.2-l 0 units higher than for o orbrtals. Early results2*11 from this laboratory confirm this analysis and agreement between the two laboratones LS generally good, especially for strong transitions and photoelectron energies m excess of 1 eV There have been several mves&ations of the angular distributions of photoelectron mtensity m carbon monoxlde3112*13. Carbon monoxide rs an nnportant molecule because of its fundamental role m the chemistry and physics of planetary atmospheres and comet ta&314.Carbon monoxide is also very nnportant m chemistry, the carbonyl group IS an actwe site for the absorption and emission of radiation m many molecules. The results of the angular distributions of CO have not significantly clmfied the physics and chemistry of CO so far, owmg to the disagreement among the various investigators3*12*13. In addition there seems to be no accurate measurement of the variation of the angular drstnbutlon asymmetry parameter 0 with vibrational peak. We have remvestigated angular drstnbulaons m CO using He1 radiation and have paid particularly close attention to that vanation. Only a gradual variation of /3with electron energy for iomzation of electrons from a smgle orbltal m low atomic weight systems is expected and therefore the vanauon of p across the vibrational envelope of an electromc

129

Figure 1 Block dragram of the vanable-angle photoelectron spectrometer He, cylinder of ultrahigh punty hehum, ZT, hquld-mtrogen-Immersed zeohte trap for lamp hehum supply, RB, lamp ballast resistor, LPS, lamp dc power supply, SC, sample chamber, PC, photocathode for light flux measurement, CL, electron lens elements, ANALYZER, hemrspherxal electrostatrc electron energy analyzer, ML, electron lens element, 5, Spiraltron electron multlpher, CPS, Sprraltron cathode power supply, APS, Splraltron anode power supply, R, C, rewstance and capacitance of drfferentratmg network for Sprrsltron pulses

band should be slow and monotonic’5* l6 This conclusion assumes that the mam effect on 0 of varymg the vibrational quantum number m a given electromc band ls due to the corresponding vanatlon m the photoelectron energy and that a slmllar effect could be achieved by varymg the mcident photon wavelength for a fixed vlbronrc transition For &rect photolomzation, the only important effect of the final vlbratzonal quantum number is to change the relative mtens&es of the vibrational peaks of a given band accordmg to the Franck-Condon factors and m a manner mdependent of the direction of electron eJection. This IS the case, for example, for romzatlon of H, by 584-A radiation where the 0 values for the V’ = 0 through v’ = 6 components of the “Xi state are essentially the same wlthm experimental error3 If large and/or non-monotonic variations of p do occur across an electronic band, this may indicate the presence of other ionization channels such as autoionlzation or the breakdown of assumptions leadmg to the Franck-Condon intensrty factors In this paper we present our results and discuss them m terms of several different physical processes that could be occurring during ionlzatlon or shortly thereafter

EXPERIMENTAL

The vanable angle photoelectron

spectrometer used m this work was

130

described m dettlll elsewherel’ and vplll be summarized here only bnefly. A block diagram of the photoelectron spectrometer UJshown m Fig. 1. TJltrahlgh punty (99.999%) hehum flows mto the discharge lamp which 1s located on the mslde of the mam vacuum chamber. The beam of hght (mostly He1 684A) 1s co&mated and passes mto the scattenng chamber which contams sample gas at a pressure of l-10mtorr as measured by an uncalibrated Schulz-Phelps ionlzatron gauge. The photoelectrons produced exit through a slot m the scattenng chamber and are collnnated and decelerated by electrostatic lenses before entermg the electron energy analyzer They are then energyselected m the hemlsphencal electrostatic analyzer which has a mean radius of 6.35 cm and are recolhmated and accelerated by additIonal lenses mto the cone of a Splraltron electron multlpher. The Splraltron sends out a low-voltage pulse for each electron detected, which 1s shaped and amphfied by a preamphfler on the outside of the mstrument. The pulses are directed to a PDP 8/e computer which acts as a multichannel scaler The photoelectron spectrum 1s normally drsplayed on an oscilloscope or plotted by an X-Y recorder. The electrostatic lenses, energy analyzer, and detector are rotated about the center of the scattenng chamber through a vanable angle 8 with respect to the photon beam. In addition, the main vacuum chamber ls lined vvlth mu metal and located at the center of three pairs of Helmholtz colls to reduce the earth’s magnetic field to about 0.3 mG. This 15 necessary smce the photoelectrons have low kmetlc energy and are easily deflected by stray magnetic fields. The data are p&ally reduced while the computer IS on-lme. The spectra are smoothed and a denvatlve algonthm ISused to locate the peak maxlmal*. Spectral mtenslties measured as peak heights at each of nine detector angles UI the range 45-120" are frtted by a weighted least-squares program to eqn. 1, w&h da/d52 a linear function of cos2 6. Correction for background photoelectron counts 1s cntlcal for accurate measurements of 0, especially when the signal-to-background ratio becomes as low as 1.2. At each of those nme angles we accumulated a spectrum w&h no sample gas m the scattenng chamber. As the electron energy decreases from 13 eV to 1 eV, this background mcreases at a rate of about l/6 count eV-L s-l These background data are later used to correct the peak mtensltles at each of the nme angles lqefore 6 18calculated. The performance of the mstrument IS checked by its ab&ty to reproduce the 0 values of 0.88 for the ‘PI2 and 0.86 for the 2P1,2 peaks of argon which are very close to the values reported prevlously3* 13*19. The resolution of the mstrument 1s typically 0 040 eV as measured by the full width at half maximum of the argon 2P 3,2 peak. The energy scale 1s calibrated by mlxmg the sample gas with a small amount of argon.

131

A*l-l

co

54 7*

564It

0WZ

x22* 584A O’h

El

! &*I-I

2x+

56414

0”;?“4

5376

0’

I

I

I

12

1

14

IONIZATION

I

I

I

16

I

I

16

POTENTIAL

20

(eV1

Figure 2 Photoelectron spectrum of CO at a detector angle of 64_7* over the xonrzatron potential range 11 26-20 46 eV The spectrum was obtamed usmg 611 adJaceXJt,13meV wide channels and SO scans Total dwell time per channel wss 50 s Average CO pressure was about 4 mtorr The features labeled 637 A and 622 A are due to the He@ and HeI7 Impurity hes of the predommantiy HeI& lamp The abscassa scale refers to lomzatton by S84-i% hght

r-

1

20 II I

I 2

I

I

x ?z+ t

0

I,,,,,11 0

14

2

f 6

1

537d 4

6

15

IONIZATION

I

-I

584d 1 t 4

A 2R IO -

I

547*

co,

16

POTENTIAL

(e’.‘)

Fqure 3 Spectrum of the regpon between the v’ =: 1 level of the X2 Z+ 684-A hand and ti’ = 7 A2 II S3?-A band The spectrum wss taken over 376 adjacent, 6 meV wade channels at a total dwell &me of 36z per channel Other parameters are the same as for Fxg 2 The abscissa scale refers to mrnzatlon by 684-A l&t

132 RESULTS

Fuced-angle spectrum The photoelectron spectrum of CO was obtamed at the “mwc” detector angle of 54 7’ at which P2 (cos 0) vanishes and the mtensity IS mdependent of 0. As a result, the ratio of the DCS of two transrtlons at this angle is the same as that of the correspondmg mtegral cross-sections, a very useful property The 584-A spectrum (Fig 2) shows three distmct bands labeled X2 EC+,A211, and B2 EC+.In addition, features due to the HeIP (537 A) and He17 (522 A) lmes (which account for 2% and 0.5% of the total light intensity, respectively) are also displayed 20. The most significant result of this fixed-angle spectrum 1s that it shows resolved structure m the abtxssa region between 14.2eV and 16.3 eV. This structure has not previously been reported, probably ovvmg to the low intensity of the peaks. A more detarled spectrum covenng this region IS shown in Fig. 3. This structure does not seem to be due to chemical nnpunties m the sample gas, since the same spectrum 1s obtained usmg 99 9% or 99.99% purity CO Also, rt is unhkely that the structure rs due to chemical Impunties m the hehum lamp gas, smce we used 99.999% purity helium. Furthermore, this structure 26 not present m the backgrouMl spectrum We assign thus structure to high vibrational levels of the X2 C* band as well as 537-R ionxzation of the A211 band. 1 11

Angular dlstnbutlons We have studied the angular distnbutions of the mtensities of 24 vlbronm transitions m the photoelectron spectrum of CO, which 1s twice the number of transitions reported on previously 3. These distnbutions were measured at least three times for all the peaks Each angular distribution 1s obtamed from repeated scans to improve the signal-to-noise ratio, with some runs lastmg up to 12h In spite of the low mtenslty of some of the peaks, the correspondmg anisotropy parameters were usually reproducible to * 0 1 or better. The p values for the most mtense peaks were reproducible to w&m f 0.02 The average values of the asymmetry parameters fi are shown m Table 1 together with the results of Carlson and Jonas3 and Hancock and Samson13 We have not measured the angular distrrbutron of the 584-81, V’ =3andv’= 4 levels of the X2 Z+ state since they are overlapped by the v’=landv’= 2 levels of the A2 Il band formed by 537-A radiation

X2 Z+ band A plot of 0 versus electron energy for the X2 EC+band is shown m Frg. 4. It m&cafes an unusually rapid varration of fl as the electron energy mcreases from 5.9 eV to 7.2 eV as well as a noticeable dip at 6.94 eV. This behavior 1s discussed below.

133

TABLE1 VALUESOFflFORCO

537 A 584 A

537A

x2 z:+ x2 x+

A21-I

v’ = 0

21= , 0 v'= 1 v;= 2 v= 3 , v= 4 I v =5 v’ = 0 ,

Present worka

Carbon and Jontd

Hancock and Samson’

lOSfO12 095fO02 060fO03 0 73+008

08fOl 037+017

095fO05 056fO07

039fO14 027+014 03fOl 03f015 0 23fO16 0 27fO16 0 24fO16 0 29*016

031fO05 0 31+005

lOfO02 lllf03

0 28+005

045fO15 046fO15

v=l v'= 2

8’ = 3 , v= 4 584 A

AaII

I v= I 0 zI= 1 ,

v =2 v’ = 3 v; = 4 v= 6 v; = 6 2, =7 , v= 8 0) = 9 , v =lO 684A

B"IZ:*

v= , I V= , v= ,

0 1 2 v= 3 v'= 4

0 38fO16 0 52fOlO 047fO06 032fO02 0 29fO02 025+005 O10fO04 027fO06 0 24 f:O03 0 22fO09 -007+007 -002fO06 032fO08 041+005 0 54 fOO8 l03fO10 063fO08 122+010

a The valuesreportedareaverage8 of at least threedetermmatlonsThe errorsmdlcated are standarddetnatlons obtamed by the least-squares fit descnbedin the Expemmental tectlon Ref 3 ' Ref 13

A211 band A plot of p versus electron energy for the A”II band IS shown m Frg 5. Agam, we see an unusually rapid and non-monotonic variation of fl ovex the relatively narrow energy range of 2.9-4.1 eV A stra@t he can be drawn through nme of the 14 pomts, as mdlcated m Fig. 5. The 6844 pomts corxespondmg to 0’ = 0, 4, 8, 9, and 10 he sq@hmtly off that Ime. The error bars for t.41~ peaks are fairly small and our value of fi for v’ = 0 agrees well with that of Carlson and Jonas3 as can be seen from

134

ELECTRON

tON UATION

ENERGY (eV1

POTENTIAL

(eV)

Amsotropy parameter fl as a function of electron energy for the X’ Z+ band urcles (0) indicate lonlzatlon by 637A radiation and open duunonds (0) by 584-A radlatlon The upper abscissa 18 common to all points The lower absclruparefers to ionization by 684-A light Wgure

4

Closed

Table 1, but not our value for v’ = 4. Those authors have not reported angular dlstibutions for u’= 8, 9, and 10. It 18 mterestmg to note that when fl values of the AZII band of CO are plotted on the same graph as 0 values of the A211, band of N2 (ref. I), the CO values fall along the general trend hne of the N3 results, except for v’ = 0, 4, 8, 9, and 10 of CO. This may merely be a comcldence, smce plots of 6 versus electron energy for the X2Z+ and B2C+ bands of CO do not comclde at all with plots for the X2 Z; and B2 E; bands of N2. B2 Ic+ band

A plot of 6 versus electron energy for the B” Z+ band ls shown 111Fig. 6 Our value of j3 for v’ = 0 1s outsIde the error bars of the Carlson and Jonas3 value, but It agrees fuly well with that of Hancock and Samson13, as can be seen from Table 1. The plot once agam shows an unusually rapid and non-monotomc dependence of /3 on electron energy A straq@t hne can be drawn through the pomts except for the v’ = 3 one. However, this lme would have a slope of 1 eV_l , which 18 much larger than most other slopes rep~rted~~‘~*l6 for this type of plot

136

ELECTRON 20 I

ENERGY

(eV)

IO ’

’

’

’

I

’

’

’

L

I

d.4 I2

Y’ 1 2

ICI-

ELECTRON ENERGY (eV) 70

60

50

t

40

30

a

I"""~""""'T"r" 06-

08 I-

Y’ 8 3 06

P

I-

-

v’ = t

02t

v'= 4 0 oo-

v'=9 v'=l3 t e

A *l-J BAND -02

,'I 140

0

d-0

04

"

8 '1, 150

IONIZATION

"8 160

'1

POTENTIAL

B

1 L 3 h 1' 8 8 170 180

(eV1

Figure 6 Anlsotropy parameter p as a function of electron energy for the A'T[ band Symbols and abscwa scales have the same meanmgs as m Wg 4

I

90

I’

I

1

IONIZATION

2i=Z+BAND 1

I 200

I

I

POTENTIAL

0

(eV)

parameterfl as Figure 6 Anlsotropy a functmn of electron energy for the B2 Zc+ band formed by 584-A radlatmn

DISCUSSION

In attemptmg to explam the anomalous behavior of p as a function of electron energy (Figs. 4+) it ls nnportant to fust estabhsh whether or not the peaks I.EIthe correspondmg spectra are due to smgle vlbronlc transItions. Overlappmg contnbutlons are possible because of the nature of the hght enntted by our discharge lamp, which includes atomic em=slon lmes at 584A (HeIar), 537 A (HeIP), and 522 ii (HeIT) at Irelativemtenslhes of 100, 2 and 0.5, respectively. We have systematically looked for comcldental electron energy matchups (wlthm 50meV) ansmg from lomzation of dlfferent vlbronlc levels by these three &fferent wavelengths, The near-comcadences obtamed for which we measured p are gwen m Table 2. The expenmental mtenslty at a gxven electron energy for these nearcomcldent

TABLE 2 ELECTRON

LSne number

ENERGY

NEAR

COINCIDENCES

Wavelength (A)

State

637 684 584 S84 584 584 584 584

v”= 4 A=II v’ = 7 A*n v’=SA’Il v’=8A*rl

IN CO FOR

684-A,

637-A.

AND

622-A

PHOTONSa

IP (e V)

Electron energy fe VJ

Wavelength (A)

State

17 278

S 808 3 417 4119 3 266 3 417 3 255

622 537 622 537 622 622

v’=8A3rI

v’=8A211

17 800 17098 17 962 17 800 17 962

u'=lB'Z+ v'=3BaC+ v'=4B'Z+

19862 20334 20450

3224 3407 3291

;;=;;:g:

20334 19 802

0883 1366

537 622

::'=ldII> '=OD'I$

22392 22218

0868 1349

v'=?A*Il

t The energies of these photons Refa 21-23

IP fevj

v'=OB'E+ u’=

OBaZ+

Electron energy

@W

17 a62 6770 19 664 3432 19 6154 4087

are 21 217 eV, 23 086 eV. and 23 741 eV, resuectiveb

should be the 8unple 8um of the mtensltles of the corresponding transitions, mstrumental resolution effect8 mcluded. For the ca8e m which the difference between the electron enewes correspondmg to the center of the contnbutmg peaks 1s smaller than or comparable to the instrumental resolution (which IS the case for all the near-comcldences of Table 2), the observed fl til be a weighted average of the asymmetry parameter8 for the mdlvldual peak8

transitions

P obs

=

~l~lPl+~2~2Pz

_

(2)

The coefficients o1 and u3 are the total cross-sections of the contnbuting 1omzat1ons and I1 and I2 are the mtenslties of the correspondmg incident photon lmes. Usmg this equation we can approximately correct our observed fl for perturbation by overlappmg peaks. For example, consider the pour of transltlons gven m the first lme of Table 2. The observed effective p 18 0 62 We wfl assume that these two transitions are due to direct loruzatlon. This assumption 1s consistent with the subsequent analysis m this section. The p value for the 522-A, u’= 8 A211 ionization can be e&mated a8 0.42, which ~8 obtamed from the /3 value of the solid trend lme m Fig. 6 at an electron kmelzc ehergy of 5.78 eV This 18 the kmet;lc energy of a photoelectron eJected by a 522-A photon leavmg behind an 1on 1n the A211, v* = 8 state. The procedure psumes that ‘this trend describes the variation of p mth electron energy for direct ionization (see &scusslon toward the end of the present sectson). We estimate the ratio of the total ionization cros8-8ections for the 0’ = 8 A2 II state by 522-A photons and the v’ = 4 A2 n state by 537-A photons. Since these wavelengths are farly close to one another, we will as8ume.

137 TABLE

3

COMPARISON OF EXPERIMENTAL FRANCK-CONDON SETS OF THEORETICAL ONES FOR CO

Expenmental FCFa

x2 zI+

A21-I

B2x+

* b c d *

v’=

FACTORS

Present re*ulteb

Theoretical FCFC

Theorekcal FCFd

100 0 3 77 <1o-3

1000

1000

100 0

1 0)> 2

3 46 -

4 49 -0 ge

372
v= , 0 I v= 1 I 21= 2 , v= 3 , v= 4 v; = 5 v= 6 I v =7 I 8 v= I v =9 I v =lO , v= 0 , v= 1 , v= 2 , v= 3 , v= 4 0’ = 5 v’ = 6

39 4 83 4

42 7 75 5

368 818 1000 89 4

, v=

0

WITH TWO

1000

1000

86 7 67 8 50 3 24 2 14 8 8 31 6 08 2 53

85 3 67 1 48 6 23 0 17 1 10 2 5 74 4 27

100 0 35 0 7 75

100 0 43 4 17 2 9 93

36 2 7 68

100 8 81 866

0 22 0 04 0 00

65 5 419 24 2 13 0 6 63 3 24

153 1000

144

36 9 81 9

1000 893 654 417 241 129 6 58 3 21 151 1000 361 7 62 130 0 21 004 000

JL GardnerandJAR Samson.J Chem Phys.60 (1974) 3711 Obtruned~thelectronenergyanalyzerwhlchIsuncorrectedfordetectlonefficlency M Wacks, J Chem Phys.41 (1964)930 Pbys Sot , Ser B, 7 (1968) 1192 R Nlcholls,Proc See Fig 3

that this ratm NJapproximately equal to the correspondmg theoretical ratio of Franck-Condon factors gwen by Table 3 as 6 63/65.5. The 522-81 to 537-A photon mtensity ralao for our lamps 18 0.5/Z. From these numbers and eqn. (2) we obtam for the V’= 4 A211 lonlzalzon by 537-A photons the 6 value of 0.52, which w equal to the value measured for the composrte transltlon bemg considered. The mzun reason that the transltlon to the A’II vt = 8 CO+ state produced by the 522-A lme does not slgmficantiy perturb the 0 value of the A’II v’ = 4 state resultmg from the 537A hne xs the smallness of the correspondmg Franck-Condon factor ratio Just mentioned. By usmg the same kmd of procedure we have exammed the rest of the transltlons m Table 2 and found that the p values for the transltlons

138

indicated m the left half of Table 2 are not ngnlflcantly perturbed by overlap of the translt~ons indicated m the right half. The reason for this IS prnnmly that the 522-A and 537-a lmes are so much weaker m intensity than the 584-A lme Also, for the last two hnes m Table 2, transitions to the D2 II state of CO are believed 2*-23 to be due to two electron processes where one electron rs removed and another is excited [(r2~)~-+ (n2p)’ (n*2p) + e-1 and the correspondmg cross-sections are weaker than those correspondmg to a one-electron process. We conclude that the anomalous energy dependence of p for the X2 ZZ+, A211, and B2 Z+ bands of CO+ depicted m Figs. 4-6, are not due to contnbutlons from overlappmg features resulting from the 537-A and 522-a lines present m low mtenslty m our predommantly 584-A light source. We shall consider next the effect that secondary scattering of the lowenergy photoelectrons by neutral CO molecules would have on the observed angular dlstibutions. In our scattering chamber there IS a steady stream of photoelectrons and a fairly high concentration (-4 mtorr) of neutral CO molecules; colhsions between the two might lead to temporary negative ion states for resonant energies To determme the optimum sample pressure, the mtensity of the most mtense peak m the 584-A spectrum (X2 E+ v’ = 0, photoelectron energy of 7 2 eV) as a function of pressure was determined It was found that the curve representmg this dependence was lmear up to 4 mtorr, bending downwards thereafter. The operating pressure of 4mtor.r was therefore chosen; it maxlmlzes the photoelectron signal intensity wlthout apparently mtroducmg secondary scattermg. Nevertheless, for the 584-A B2 Z+ band, the photoelectrons have an energy around 1.5 eV and are more susceptible to secondary scattering. In addition, m the electron energy range around 1+5 eV there are temporary negative ion resonances m COW. The effect of resonances of that kind on the peak intensities of fixedangle photoelectron spectra of CO, N2 and CO2 at sample pressures between 50 mtorr and 200mtorr has been previously observed by Streets et al.25. Such processes could conceivably be affecting our spectra, although our pressures are significantly lower. To check out this possibility, we determmed the variation of the mtens~ty of the v’ = 1 relatwe to the v’ = 0 B2 E+ 584-A peak with pressure and found an mcrease m this ratio of about 10% m gomg from zero pressure (by extrapolation) to 4mtorr. This small variation should not however have any appreciable effect on the value of p The reason 1s that the small change m the mtenslty of the photoelectron beam attributable to such secondary processes, with the detector at an angular position 6, should be independent of 8 This conclusion has been found to be correct smce we have measured @ for the Q’= 0 and v’ = 1 levels of the B2 I=+ state at pressures of 4 mtorr and 9 mtorr and found them to be equal wlthm experunental error bars Therefore, we conclude that the anomalous trends m the plots of fl versus electron energy shown m Figs. 4-6 are not due to secondary

139

electron scattermg. A possible explanation of these anomalous results might be a breakdown of the Born-Oppenheimer approxlmatron. This has been suggested previously to explain the unusual dependence of /3 on the vibrational quantum number for some transitions m Na and C03*26. Kalman27 has argued agamst such an explanation. The most sigruficant cause of such a breakdown is the strong mteraction between two potential energy curves occumng in the vlcmlty of a crossmg or nearcrossmg For CO+, the potential energy curves of the X2 E+ state and A211 state have been found to cross. Singh and Rai28 have used experimental spectroscopic data to construct the potential energy diagrams of the X, A, and B states of CO+. Their results show that the X2 E+ state at the v’ = 13 level mtersects the A2 II state at the v’ = 5 level. In this regron the relative intensities for the vibrational levels might be expected to disagree with calculated Franck-Condon factors. It would also be possible for the correspondmg asymmetry parameters to be anomalous. Thus, however, would only be the case d the ion1c-rJtat.ecurvecrossmg region fell inside the vertrcal Franck-Condon band determmed by the initial neutral species vibrational wavefunction This is not the case for CO, where this Franck-Condon band extends from 1.06 A to 1.20 &2g whereas the crossing between the X22+ and A211 curves occurs at 1.48 A28. Since this IS the only known instance of curve crossmg m the X, A, and B states of CO+, It is unlikely that a breakdown m the Born-Oppenheimer approxlmatlon is causing the unusual dependence of fl on electron energy. Another possrble explanation of the unusual dependence of j3 on vibrational quantum number IS the vanatlon of the electronic transition dipole matrrx element on the internuclear distance. For the case of N2, Kalma$‘-’ has shown that there is some variation of 0 with vibrational quantum number for the X2 Zz band, but not the large variation observed m the experiments lB3 . Additional data are needed before similar conclusions can be made for CO, but is is unlikely that such arguments would account for the large variation m p for the three bands The most probable cause of the anomalous energy dependence of fi is, in our view, either an electronic autoionization or a shape resonance. In the nomenclature of the present paper, we use the term “electronic autoionization” to denote an mtemal electronic Feshbach resonance. This implies a two-electron, two-step process m which a deeply buned electron is first promoted to an excited bound atomic or molecular orbital, which, however, places the atom or molecule as a whole at an energy above its lowest lomzation potential. After some time, this excltatron energy is transferred to the other electron which 1s m a lower energy orbital, causing its iomzation3’ . (Another form of autoionization 18 vibrational autoionization m whrch ionization of a bound excited electron ls assisted by transfer of vibrational energy to it; this usually mvolves highly excited Rydberg orbitals and results m the geckon of an electron with very low kmetic

140

energy3* I It 1s not relevant for the present case, and from here on we wGl use the word autolonlzat-lon to denote the electronic variety... In a shape resonance, on the other hand, lonlzation takes place from the orbItal ongmally excited, but the nature of the mteractions of the excited electron with the remauung electrons (see below) also results m a delay m the lonlzation32933. Autolomzation has been observed expenmentally34’35 and mvestigated theoretically36 m many atoms and some molecules such as Hz, 02, Nz, and CO. Expenmentally, autoromzatzon 1s observed in photoelectron spectroscopy and m photolomzation experunents, where the ionization crosss&ion 1s measured as a function of photon wavelength. In these experunents3’ autoionlzation IS observed as sharp peaks or dips superunposed on the contmuum spectra attnbutable to direct photoionlzatlon. In photoelectron spectroscopy, autolonlzation US usually suspected when relative vibrational mtensltles m a band de-ate from calculated Franck-Condon factors. Bahr et al.35 have observed relative vibrational mtensltles m the photoelectron spectra of 02, Nz, and CO which they attnbute to autoiomzatlon at wavelengths 6OO-8OOA. They show how hq& vibrational levels of the ion state, which are not populated by direct lonlzatlon through the Franck--Condon prmclple, can be populated by autolomzation These authors dramatically Illustrate how the photoelectron spectra can change appearance when the photon source 16 changed from an off-resonance to a resonance wavelength. The anomalous variation of j3values with vibrational state for the loruzation of the X2 Ez state of Ns by 584-A radiation has also been mterpreted to be due to an autolomzation process’. We shall now examme the posslblllty of autolonlzatlon m CO at the wavelengths used m thy study. Codlmg and Potts21 have obtamed the absorption spectrum of this molecule m the reDon 530-61OA Asbrmk et al.22 made the assignments m theu spectrum and found that there 1s a state at 21.24 eV that corresponds to a 3~ member of a Rydberg senes convergmg to the C2 ZZ+ state of CO+. They argue that this state 1s denved from a two-electron transltlon from the ground state. They represent the ground-state configuration of CO as, KK(02S)2 (a*28)2 (7r2p)4 (02p)”

11=+

(3)

so that the two-election tranwfiop could be wMten as X’E++.

.

(n2~)~ (a2p)l (n*2p)l

(3~0 or 3p7r)l

(4)

or x’z1++.

(n2p)2 (n*2p)* (3~0 or 3p7r)’

(5)

In addlbon, Lee et al.38 have seen the same spectral features as Codlmg and Potts21’m the photoabsorption cross-seclzons of CO from 520 to 730 A. Their absorption cross-section spectrum shows sharp structure m the range

141

550-730& and there appears to be a peak at about 584 ii. They agree with the assignments gven by Codlmg and Potts2’. Samson and Gardner3’ have measured partial photolonlzatlon crosssectlons and branchmg ratios of CO from 750 to 304 A usmg a duoplasmatron and a dc glow discharge m He, Ne, Ar, and N2 as excitation sources coupled to a vacuum ultraviolet monochromator In their plot of partial photolonlzatlon cross-sectlans as a function of wavelength for the produclaon of the CO+ X2 Z+ state, a large dip m the cross-section B seen near 584 ii There 1s a correspondmg increase xn the partial cross-section for the production of the CO+ A211 state at about 584 ii. They tentatively assign the structure m the partial cross-se&on plots to autolonlzatlon resonances and attnbute to such an autolomzatlon state the difference m the p parameters of the v’ = 0 and v’ = 1 levels m the X2 Z+ band observed by Carlson and Jonas3. Plummer et aL40 have measured the partial photolonlzation cross-sections and branchmg ratios of CO usmg synchrotron radiation over the photon energy range 14-50 eV. They observed structure (both dips and peaks) m the pa&al photolomzatlon cross-sections between 19 and 22eV m the A211 and X2 E+ ionic states. They state that some of this structure 1s due to autolomzatlon and some of it may be due to shape resonances. In a photolonlzation shape resonance, the escapmg photoelectron has to overcome a potential energy beer as it moves away from the remammg ion. The barner 1s due to scattermg by the lomc fleldj21 33 which can be the result of dvect coulomb mteractlon or exchange mteractlons of the outgomg electron with the other ionic electrons32. As a result of this bamer, a quasi-bound state can exist which tends to affect the photoelectron crosssection at the excltatlon energy equal to the energy of that state. For the case of molecular systems, this potential can be amsotroplc md can affect the angular dlstnbutions of the photoelectrons. Dehmer and Dti33 have shown, however, that for N2 the vanatlon of 6 from this effect occurs over several eV, which 1s the typical mdth of these shape resonances32w 33*41. Therefore it seems unhkely that a shape resonance at 584 A would cause the anomalous vanation m j3that we observed smce this vanatlon occurs over the energy range of vlbratlonal spacmgs, which are roughly 0 2 eV. In addltlon, the Xa scattenng calculation by Davenport41 , which seems to be the best theoretical description of shape resonances so far, does not mdlcate the presence of any such resonance m the A211 state of CO. Therefore, if the calculation 1s correct, the anomalous varM1on m @ shown m Fig. 5 could not be accounted for by a shape resonance Autolonlzatlon resonances are much narrower (typically < 0.1 eV)j’ than the former because of the relatively weak couplmg between the electrons mvolved. As a result, they are expected to cause a vanabon m p over an equivalently narrow energy range42 43. It seems hkely therefore that an autolonlzation resonance, rather than a shape resonance, IS responsible for the anomalous variation mP

142

The effect of autoionization on vibrational mtensity distributions has Usmg the Fano configuration interaction been investigated by Smith-. theory45, Smith showed that the vibrational intensities I of the resultmg positive ions can be w&ten as the sum of two terms mvolving FranckCondon factors (FCF)

where q 1s a parameter related to the oscillator strengths to the quasiexcited neutral and to the ion state, and the lme width of the neutral state. Fa IS the FCF for the mitral neutral vibrational state to the positive ion state (associated with direct photoionization), F$ 1s the FCF to the excited quasi-bound neutral state, and F& IS the FCF from the latter to the final positive ion state. Expression (6) mdicates that we can observe vlbratronal intensities in photoelectron spectroscopy that do not agree wth the direct FranckCondon factors F’s_ In our spectra of CO we observe gross deviations from calculated Franck-Condon factors for v’ 3 2 of the X2 Z+ and B2 E+ states. As mentioned above, the structure for v’ > 2 of the X2 Z’ state has not been reported previously using 584-A radiation. However, the peaks for v’ = 2-6 of the B2 E+ state can be seen even m the low-pressure spectra of Streets et al.25 These authors attribute these peaks to autolonlzatlon, although they do not discuss this point m deMl We feel that the mtenslty of the v’ > 2 peaks of the X2 C+ and B2 E+ bands must be at least m part due to autoionization. One may attempt to determine the contribution from autolomzat;lon to a mven peak intensity from sn analysis of the correspondmg p value, since autolonrzation peaks m the photoelectron spectrum should m general show a different angular distribution from peaks due to direct lonlzatlon, For the case where the hfelame of the autoronlzmg state is long compared with the rotational penod of the molecule we would expect Pauto= 0, which corresponds to an lsotroplc distnbulaon For peaks which are due to both dnect and autoionization, the observed J3can, to a first approximation, be described as a non-interfering superposition of the two processes, and therefore

Pobs

=

odizPdlx adir

+

%uto&iuto +

Oauto

(7)

where (Jdti and cauto are the total cross-sections for the direct and autoionizatzon processes Smce as stated above the v’ = 2 and V’= 5 peaks of the X22* band are due at least m part to autoionization, let us assume m one extreme case that they are totally due to such a process. This permits us to establish a rough energy dependence of Pauto by memly drawing a straight lme through these two pomts Slrmlarly, we can estimate the energy dependence of &i, for the X2 ZZ+ band d we assume that the v’ = 0 peaks of the 537-A

143

and 584A spectra are populated by dn=ect lonlzation only and take Pdizto be a hnear function of the photoelectron energy. From these assumptions, eqn. (7) and the observed p value for the v’ = 1 peak m the 584-&i spectrum, we can calculate the u,.&J~~~ ratio for this transltlon. The result 1s -0.67, which xs not physically meanmgful. Themfore at least one of these assumptions EJ mcorrect, The assumption of non-mterference between the autolonlzed and duectly loruzed electrons 1s probably not correct42 By studymg j3 as a function of electron energy for xenon through a reaon where autolonlzatlon exists, Dollop and Samson and Gardner4’ have shown that there 1s mterference between these two processes and this leads to complex oscrllations m p as a function of electron energy This has also been shown to occur m neon and argon42s43 To our knowledge thla has not been studied directly for molecules but /3 would be expected to exhibit slmllar behavior for the molecular case Not enough mformatlon 1s therefore available to determme the relative extents of the autoxomxation and direct lonlzatlon contnbutlons to the anomalous peaks of the X2Z+ band. A slmllar concluslon ISvahd for the A2 lI and B2 Z’ bands

SUMMARY

AND CONCLUSIONS

It was found that the relative mtensltles of high (v’2 2) vlbratlonal levels of the X2 I;+ and B2E+ bands resulting from the lonlzation of CO by 584-A hght do not agree filth calculated Franck-Condon factors. The asymmetry parameter /3 was measured for many photoelectron transltlons and was found to have an anomalous dependence on photoelectron energy, varymg non-monotonically and/or very steeply mth this energy. We attnbute this behavior to the contnbution of autolonlzatlon processes to some of the vlbratlonal components of those bands, but not enough lnformatlon 1s avsulable at present to determme quantitatively the extent of such a cont;nbution From this work it ~3 apparent that the asymmetry parameter /3 = very sensltlve to the presence of autoloruzatlon channels. It may turn out that the photon energy dependence of 0 1s a more sensltlve mdlcator of the presence of autolomzatlon than the correspondmg energy dependence of total or partial ionization cross-sections. Photoelectron angular dlstnbution expenments with a contmuously vanable wavelength hght source (such as a synchrotron source) would provide very useful additional mformatlon for the mterpretatlon of the present results It would also be mterestmg to study the photoelectron angular dlstnbutions of Cl30 at 584 A smce the vtbratlonal frequency of C l3 0 1s 2 3% lower than that of Cl2 0 As a result the photoelectrons from high vibrational levels of Cl30 would have sufficiently different kmetlc energy from those of Cl2 0 to provide additional mformation on p at photoelectron energes around 3 eV, where this anlsotropy parameter 1s changmg very rapidly

144

The crossmg known to eglst 28 between the X2 I? and A211 potential energy curves of CO+ occurs at excessively large distances to account for our observations. However, m other systems such crossmgs could occur wlthm the vertscal Franck-Condon envelope. Under these condltlons, a breakdown of the Bom-Oppenhelmer approxunatlon resultmg from such crossmgs could affect the correspondmg /3 values, and it would be mterestmg to use theoretical methods to assess the magnitude of such an effect.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30 31 32

D M Mmtz and A Kuppermann, J Chem Phys ,69 (1978) m press D,C Mason, A Kuppermann and D M Mmtz, in Proceedmga Asr~omor Int Co& Electron Spectroac , 1971, p 269 T A Carlson and A.E Jonas, J Chem Phya, 56 (1971) 4913 T A Carlson, Chem Phya Lett , 9 (1971) 23 T A Carlson and G E McGuxe, J Electron Spectroac Relat Phenom , 1 (1972/73) 209 T A Carlson and R M White, Dlscuaa Faraday Sot ,54 (1972) 285 R M White, T A Carlson and D P Spears, J Electron Spectroac Relat Phenom , 3 (1974) 59 T A Carbon and C P Anderson, Chem Phya Lett , 10 (1971) 661 J Cooper and R N Zare, J Chem Phys ,48 (1968) 942 J L Hall and M W Siegel, J Chem Phya , 48 (1968) 943 D M Mmtz, Ph D Thesis, Cahfomla Institute of Technology, 1975 J Berkowktz, H Ehrherdt and T Tekaat, 2 Phys , 200 (1967) 69 W H Hancock and J A R Samson, J Electron Spectroac Relat Phenom , 9 (1976) 211 P H Krupeme, Nat1 Stand Ref Data Ser. Nat2 Bur Stand , 5 (1966) 1 Rekrt Phenom , 1 (1972/73) 413 S T Manzon, J Electron Spectroac F Hn&a, J Electron Spectroac Rekat Phenom , 9 (1976) 149 D C Mason, D M Mmte and A Kuppermann, Rev Scz In&rum, 48 (1977) 926 A Savltzky and M J E Golay,AnaZ. Chem , 36 (1964) 1627 D J Kennedy and S T Maneon,Phya Rev A, 6 (1972) 227 The mtensltles of these lmes relative to the mam 584-A lme were estnnated from the relative mtensltles of the correspondmg 2p 3,2 peaks m the photoelectron spectrum of argon K Codkng and A W Pot&, J Phye B, 7 (1974) 163 L Asbrmk, C Fmdh, E Lmdholm and K Codlmg, Phya Scr , 10 (1974) 183 A Potts and T Wdhams, J Electron Spectroac ReZut Phenom , 3 (1974) 3 G J Schulz, Nat2 Stand Ref Data Ser, Nat2 Bur Stand, 50 (1973) 85 D Streets, A Potts and W Fnce, Int J Ma88 Spectrom ion Phys, 10 (1972/73) 123 I Thomas, Phys Rev A, 4 (1971) 467 0 Kalman, J Electron Spectroac Relat Phenom , 8 (1976) 335 R Smgh and D Rru, J Mol Spectroac , 19 (1966) 424 G J Schulz, Phys Reu A, 6 (1972) 69 0 Kalman, MoZ Phya ,34 (1977) 397 J Eland, Photoelectron Spectroscopy, Wiley, New York, 1974, p 59 J L Dehmer, J Chem Phys , 56 (1972) 4496

145

36 37 38 39 40

J L Dehmer and D Dlll,Phys Rev Lett , 35 (1975) 213 P Natahs, J Delwlche and J Colhn, Chem Phys Lett , 13 (1972) 491 J Bahr, A Blake, J Carver, J Gardner and V Kumar, J Qucrnt Spectrosc Radzat Tmns, 11 (1971) 1839 R Berry, J Chem Phys , 45 (1966) 1228 J Berkowitz and W A Chupka, J Chem Phys, 51 (1969) 2341 L Lee, R Carlson and D Judge, Mel Phys , 30 (1975) 1941 J Samson and J Gardner, J Electron Spectrosc Relat Phenom , 8 (1976) 35 E Plummer, T Gustafsson, W Gudat and D Eastman, Phys Rev A, 15 (1977)

41 42 43 44 45 46 47

J W Davenport, Phys Rev Lett, 36 (1976) 945 N M Kabachmk and I P Sazhma, J Phys B, 9 (1976) 1681 K T Taylor, J Phys B, 10 (1977) L699, A Smith, J Quant Spectrosc Radutt Trans. 10 (1970) 1129 U Fano, Phys Rev, 124 (1961) 1866 D D111,Phys Rev A, 7 (1973) 1976 J Samson and J Gardner, Phys Rev Lett, 31 (1973) 1327

33 34 35

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