VUV photoelectron emission spectroscopy of liquid systems

Volume 32, nimber:

CHEMICAiPHYSICSLETTERS

1

:’

1 April 1975

,’

..

~. .,’ V-W

’

PHO&EiXRON’EMISSION

J&ille.CHI~,

:

SPECTROSCOPi

dF EI$JID

SYSTEMS

Ladislav:NEMEC and Paul DEWY

Ciremisrry’Depar,tment,

New

York

University,

New

York,

.’

: New

York

IOOO3, USA

Fekibility and potential usefulness of ‘studyingphotoelectron emissicn by p&e.liquids or soIutions in the vacuum ultraviolet range (5’ 10 eV) tie demonstrated by_,means of relatjvely- simple equipment. Experimental resu!ts include: quantum yield spectral response curves for ferracyanide, iodide and bromide ions; energy diskibution curves for ferrocyanide. The ph6toionization spectrum of ferrocytide iksolution is determined and interpreted

in terms of bound-continuum

transitions and autoionizatibn

of excited bound states.

‘,’

=A method was recently de’veloped [l] for, determining the vari’ations of photoionizalion (PI) cross

high-energy strii light was estimated and found quite small both in the calibration of photon flux

section with photon energy for a species in.2 liquid W, sol_u_tioti. The determination of the PI spectrum, in this.m*thod, intiolves the measurement and analy~i3 of enerw- distibkon curves (EDC) for photo-

and the measurements with liquid systems [3]. The enkance beam was modulated at 2.4 Hz in quantum. yield measurements:Thk exit beam. was inonitored

el&r~n

w&:-elucidation of PI mechanisms in liquids (direct ‘,’bound-continuum transitions, autuionization of bound excited states); testing of theoretical models (e.g., solva.ted elections); Study .of solvation by com. ..p@ison of PI spectra in the Iiquid and gas phases., Methodology-developed this far [I,21 covers the

in front of the liquid target in the celI de&bed below. ., The-exit beam of the monochromator impinged directly on’the, liquid target which was contained in an evacuated glass cell. Thl latter was attached via a metallic flange. to the monochromator.‘The cell was also fitted,with a lower compartment for cryogenic pumping [I!]. The target was compbsed.of a piece

.I400 ?d 190 nin rang& and extension, to-the vacuum .i’itraviolet .(Vw) range seemed desirable to considerably,.increase ‘the. nuniber of potent_ial applications,

of fiberglass cloth soaked with the liquid being studied and ittached tp a platinum foil. The target assembly was mounted on a shaft which could be

einission.by spectia should prove

the liquid of interest

or solution_ k

a.ngmbei

Such PI of

,Ini’tial results obtained with relatively, simple equip; .,’men! are reported here for photon energies up to-10 eV.‘:

-.

by meaqs

salicylate

of a movable.photocliode

converter)

(with

sodium

that could be inserted

rotated from the outside’of the cell, and the fiberglass cloth, thus could be immersed periodical!y in a trough

containing

the liquid or

s01ptioh.

Photo-

electrtin emissionoriginatedsolely from the liquid as -&.Gme~j~l rHethdd$ are descrit;ed in detail ti . long 3s the,cloth was properly wet. The inner wall : a.report. [?] available upon FeqJest to the authors, of the cell was.coated with tin dioxide [5] which and only essential points,will be meniioned. ‘The served as c.Jllector. electrode. Electronics [3] we!e “1VUv-source was a McPherson titinochromator, modgenerally a:;. inprevious work [I ,5]. .!1.218,:.equipped with 2 filament;type [4] HinteregSpeqtraj response curves for ‘emission quantum g& hy_drogen !arnp. ‘fhe _Fonpchromat& was separwere obtained’ for ferrocyanide iti glyceibl [S] : vieid _ :- ated fIo16 the, lamp and .the emission ‘cell by vacu&+ (fig. 1) and iodide and bromide (fig. 2) ~JItetrqlyme ‘tight lithium fluoride wiridows. Stray Eght w&mini,ether] . This ether, : .:tiized tiy fi!ters .wh&ver feasible;. W-grads synthe- ‘, ‘,. [bjs (Z-(2’-rnethoxy-ethoxy)-ethyl) also used in ESR,s+dies [6], seems. a promising. ,, .:tic sapphire .an’d Supmsil II. The contribution of ., .,’ ,: .,_..’ ” : ‘, 1. . ‘. :

Volume 32, ntimbcr

1 April 1975

CHEMICAL PHYSICS LETTERS

1

4-1

5-

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P-

Fis 1. Quantum yield against photon energy for glycerol (0) and 0.16 M lithium fenocyanide in glycerol (T) at = 25°C. Insert shows results for full sctie of photon gies. Pressure at collector was = 10m3 torr.

l-

ener-

--IO 5

PHOTON

tion of the lithium

cation

by means

(eV)

Fig 2. Photoionization cross section against photon cncrg (upper curve) in flyccroi at 25°C for 0.16 hf Lithium ferre cyanide (o) and as deduced from EDC’sin ref. [S] (0). Absorption spectrum of O.OS XI lithium ferrocyanide in glycerol at E 25”C, as measured with 0.01 mm-thick cell (Iowcr curve).

solvent for this type of work because of its low ~apor pressure at room temperature and its rather high PI potential. Adequate solubility (0.1 M) of, lithium iodide or bromide was achieved by complexacrown-6

ENERGY

[7,8] of I&-

(1,4;7,10,13,16-hexaoxacyclooctadecaue).

Figs. 1 and 2 show that interference by the SOI-

Such &own compounds should prove extremely useful, in this respect, in photoelectron emission work

with solutions

of inorganic

vent and/or 1g-crown-6 is minor O@ approtimateIy 2 eV of photon enerw. Tetraglyme is more advantageous than glycerol in this respect. Only f’errocyanide, among inorganic anions, could be investigated with previously developed methods [5,9], whereas now a number of inorganic anions and some cations should be amenable to this type of study. The scope in organic chemistry in the VW range should, of course, be very wide indeed. EDC’s for ferrocyanide in glycerol exhibited the usual features characteristic.of scattered quasifree electrons ‘in liquids: widening high-energy tail upon

anions.

increase

of photon

energy,

kinetic

enerp

at maxi-

mutt\ nearly independent’bf photon eneru [ 131. The ‘PI spectrum 6 10-E .a

I

L

I

7

B ENERGY

9

PHOTON

7

(eV)

Fig. 2. Quantum yield-against photon energy for 0.1 M 18. crown-6 in tetrnglyme (oj, 0.1 M lithium bromide + 0.1 hl l&crown-6 in tdtraglyme.(e), and 0.1 M iitbium.iodide + 0.1 M l&crown-6 in tetraglyme (A) at’= 25°C. Pressure at

collector was B 4 X 10m3 torr;

of ferrocyanide

(fig. 3) was deter-

mined from these EDC’s by the method of EDC sum

D.

‘1

10

perposition of ref. [I]. The following PI mechanisms are suggested on the basis of the PI and absorption spectra (fig. 3) and relevant observations.: (i) PI, of ferrocyanide in solution occurs by direct bound-continuum transitions at ofeven .below 6 eV. It is known [lOI that hy,drated electrons are 91 .‘.

‘..

Voluq32, : ., produced

number 1 .: ‘:

‘,

,,

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CHEMICALPHYSICS

by irradjation

of 'aqueous solutiotis of.fer-

[5,9].‘Moreover, the absorption spxtra in these sol&its ore tieariy the Same (fi$. 3). i onizatiqn of excited bound states (ii) PI via ,. aut o’ : ‘becomcs’predominant above .6 eV. The segment of. ihe PI cross section curve below 6.5 eY in fig. 3 carresponds to a.preddminant autoionization PI mecha-

..

&id or soltition, perhaps to temperatures as low as

._

-lOO’C,-to lower’th” vapor pressure and consequentIy broaden the selection’of liquids; (iii)‘design of a more versatile system than the glass cell used in this

:

in this laboratory, 2nd it is expfxted that PI spectra in the VUV range will becomk available for a variety of inorganic and organic speciei in liquid systems.

work. Work on impioved methodology is in progress

This work was slpported.by the National Science Foundation and the Office of Naval Research.

(iii) The rise in PI cross section above..6.5 eV is .attributed.io’ autdionization of excited bound states.’ lJ-@ rise-in absorbance (fig: 3) above 6.5 eY indeed, svgests the appearance of a new absorption band at higher photon energies. The correspon_ding FI band is well developed in fig. 3. with-a maximum tear 7.2 eV. Assignment tu a significant contribution

I2eferences [l]

H. Auiich, P. Delahay and L. Ncmcc, J. Chem. Phyr ..59 (1973) 2354. [Z] H. Aulich, L. Nemec arid P. Delahay; J. Chem. Phys., fo be pubLished. [3] L. C&. I_.Nemec’gnd P. Del&ay_ Vacuum Ultiavi@ let Photoelectron Emission. Spectroscopy of Liquid

6.5

:eV G-k&d out by the appearance of.EDC’s (no .~hump~ir_thesecurves) and the method of obtaining PI cross section (set discu$ision iriref. [ 1J). However, .thc- 7.2 eV band in fig. 3 may be,somewhat .distorted,.especially above 7.2 eV, for several reasons:

System:;: Experimental Methods, Technical Report No. 28 to thk’Office of Naval Researc$ Task No.

NR 0.51-258 (1974): [4] D.E. Extmon and J.J:Donelon, Rev. Sci’Instr. 41 ‘, ,’ (1970) 1648. [5] L. Ner;.ec. B. Bsron and P. Dclz&ay; Cnem.. Phys. Let-

strqng ittenuation of the i,ncident photon flux by absb$ion

by the solvent-at

spectral characteris&s’than yI_qe) ;eetis in order.

a depth in the liquid

glycerol

‘E!Xs for iodide and .b&ide,

(e.g.-, tetragly-.

could nbt-be deter- ’

studies in the..VUY -range. Improvements in methodoloa are suggest&d by this and past work [1,2] :

:

sd’me~.>iloton ene& of = 6.5 ec. This coincidence ; strongly suggests a PI mechanism via autoionization, ’[l] . Theie is’indeed an absorption band havir.g Its : maximum at 5.59 eV in dycerol.

corresponding to the range of quasifrke electrons; error iesulting from inadequate superpositipn bf ‘, EJX’s over a wide range of photon energies. Fur.ther work with ti -solv&t ha\rir.g Fore favorable..

1 April 1975

(i) continuous renewal of the liquid-gas, interface because’of rapid evaporation; (ii) cooling of the li-

Thus, botkthe PI cross section’curve and the absorption .spectrum exhibit-minima a: nearly jhe

transition_gbove

:,

--some potential useFuln&s of ph&oelebtron. e&ion

...’r$ti.

from a new bound-ccntinuum

‘.

technique (contintibus renewal of solution). : The present work demonstrates t@ feasibility and

rocyanide at photon energies higher than approxirnately. 4.0 cVi Comparisori of ksults for water and -. .glycerol seems reasonable since the spectral,r&ponses -fa,i quantum yields appear to be.nearly the same for these solvents’ in photoelectron emission experiments

LETTERS :

,’ hb~ye-&r,’sh&id he entirely feasibie with improved

‘.

.

teri 16 (1972) 278. [k] J.H. Sharp and M.C._K’Symons,~in;~Ions and ion pairs in organic reactions;ed. &I;Szwarc (Wiley-interscience, New York, 1972)‘pp. 177-262. ii1.C.J. Pederson, J. Am. &em; Sot. 89 (1967) 70’1. [ 81’CJ. Pederson and H.K. Frensdorff, Angcw. C’hkm. ‘Intern; Ed 11 (1972) 16. -[9] L; Ner&c UI~ P..Dclahay, J. Chemi whys. 57 (1972) 2135. : IlO] ,hl. S@& a@ G. Stein, J. Chem.Phys. 55 (1971) .) 3372. .; -,: :. . . .’ ,.

,+kd, by the present technique becausq.the solvent . ..$vaporated tod rapidly from the fiberglass’cloth’& ‘,der,efficient cryoge&,c pumping. ,Suc& det@ninations, ’ .. : : :. ..: : ., .‘_ ,,’ : ,.,. .’ .:’ ., . ,, ;: .. .--. _: .f _,, :.. .,. ,. : ., . : ‘ . ‘ . .I-, .’ ,_ ::. -, : .. _.-/ .:: -, ,,, 1 -. ‘. ..I . :. ,~. .,. ‘. ,’ .‘_. ; ; _ .. .,. ‘,. ‘, ._. .:: .-. I ;, ‘> :, ‘ ,‘ ..,, :. .-’ ;. _, . . : ;; .: gz_... -, ‘.,_:.:. ‘. 1 ,,,-:.,, 2 ‘.I -i ._:, ‘.. ._ ./. . . . . . ,. : .r ,:.,,‘. :. ,, : .’ ,’ e,..._. ., ,.., ., .’ : ,,, :’ _,“ . : ,::. ,‘ . ,:. _,~_. : ..’_:_.:_ .‘. ‘.‘.‘, : : : . ..’ ,_ ._’ >. ,. : ,’ .: y ., .. ‘, _.?‘) .;,:‘ ,,. .. : ^’ ._: :,;. ./ ‘., ‘.:. : ,. ..‘.. ,’ ,‘__ :,,I .,:_ I”, : ‘,