photoelectron emission spectroscopy of inorganic anions in aqueous solution

Volume 78, number 2 CHEMICAL PHYSICS LETTERS 1 March 1981

PHOTOELECTRON EMISSION SPECTROSCOPY OF INORGANIC ANIONS IN AQUEOUS SOLUTION

Kathrin VON BURG and Paul DELAHAY Department of Chemistry, New York University. Nero York, New York 10003, USA

Received 31 October 1980

Threshold energies Et are determmed for photoelectron emrssion by 20 inorgamc anions m aqueous solution (7.1 <Et C 9.1 ev). Calculated values of Et for Cl-, Br-, I- agree with experiment. The Et are correlated with charge-transfer-to- solvent absorption spectra.

1. Introduction

Experimental methods were recently described [l] for the photoelectron emission spectroscopy of water and aqueous solutions up to photon energies of 10.5 eV_ These methods will be applied in the present pa- per to solutions of inorganic anions and threshold energies thus obtained will be compared with calculated energies and charge-transfer-to-solvent absorption spectra.

2. Determination of threshold energies

The yield for emission of photoelectrons by the solution into water vapor was measured as a function of photon energy. Experimental methods were the same as in ref. [l] except for minor details. Special attention was paid as before to the absence of contam- ination by organic impurities and spurious photoelectron emission by a surface fihn. The proportionality between yield and anion concentration (O-2,0.5, 1 M) was verified for all anions over the full range of photon ene@es to ascertain the absence of emission by an im- purity surface film.

Typical emission spectra are displayed in fig. 1 (450 data points in the 120 to 180 nm interval). The thiosulfate and bicarbonate exhibited, respectively, the highest and lowest emission yields among all the anions studied here. Emission by water was entirely negligible except for a few of the anions with the high-

‘? I

-Y CNS- i 0 2- - n .X--_ 5

D, “F opr,,-__

8 9 10

PHOTON ENERGY (eV)

Fig. I _ Photoelectron emission spectra of water and 1 M aruon aqueous soiutions

est threshold energies_ A minor correction of the yield for emission by water was made for these anions.

Threshold energies were obtained from emission spectra by application of the Brodsky-Tsarevsky theory [2]. This theory was developed for emission into vacuum whereas electrons were emitted into water vapor in the present experiments. Yields are lowered by backscattering of electrons in the gas phase, but the shape of the yield against photon energy cur- ves is not affected significantly [ 11. The occurrence of backscattering does not invalidate the present application of the Brodsky-Tsarevsky theory because

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Volume 78, number 2 CHEMICAL PHYSICS LETTERS 1 hfarch 1981

the scattering length of electrons in the gas phase at the prevailing pressure 1s much greater than the char- acterrstrc Iengt!l for the image force.

Plots of the quantum yield Y to the power 0.4 or 0.5 against photon energy are linear according to the Brodsky-Tsarevsky theory, and extrapolation to zero yield gives the threshold energy Et (table 1). These plots correspond to two hmitmg cases of a more gen- eral relationship, also given in ref. [2] _ between yreld and photon energy. Condrtions for linearity of the Y*-4 and Y” 5 plots are dtscussed briefly below. Ex- amples of Y” 4 and Y”e5 plots are shown in fig. 2 for the anions of tig. 1. The plots are mdeed hnear, except for the initial curved segment and minor devia- tions resulting mostly from scattered light and the ensuing error in the normalization of yields to a constant photon flux. Stmilar plots were obtamed for all ions studied in this work. The initial curved segment in the Y” 4 and Y”a5 plots (fig. 2), which was observed in ail casts, is attributed to the neglect of configura- tion distnbution of the emitter in the liquid and/or complications [2] very near the threshold.

The best linear fit was determined by least squares, the curved initial segment being omitted, and correlation coefficients were computed for the Y”-4 and Y”*5 plots. The best fit was obtamed with Y”-5 plots for the amens (OH-, CI-, CIO,, SO:-, HPOg-) with the higher threshoId energies whereas the Y" 4 plots fitted best the data for anions with lower threshold energies. These observations agree with theoretical conditions stated in ref. [2] and they enabled us to assign the proper exponent in the three cases [Clot (OS), NT (O-4), (I-ICO~ (O-5):

Table 1 Threshold enegles (eV) a)

___I OH- (8.45)

Cl- (S-77), Br- (7.95), I- (7.21)

CIO; (8.21), BrO; (7.85), IO; (7.44)

CIO,- (8.45)

so;- (7.17), szo:- (7 27), so;- (8.65), s>o;- (7.33)

NO; (7.57), NO; (7.46). & (7.35)

HPO;- (S-79), PO,3- (7.44)

HCO; (g-07), CO,‘- (7.40), CNS- (7.20)

a) Standard deviation of 0.01-0.02 eV, but the accuracy 1s lower for reasons stated in the takt.

288

PtiOTON ENERGY(W)

Fig 2. Yreid Y to the pon er 0.4 (top) or 0.5 against photon energy for the anions of fig. 1 Arrows Indicate extrapolated threshold energies-

in which correlation coefficients were nearly the same for YO-4 and Yo 5 plots.

Conditions for linearity of the Y”-4 and Y”-5 plots are stated in ref. [2]. The less stringent condition corresponds to E 4 U (with E +fio), where E is the energy of the electron upon emission into the vapor phase and U the energy of delocalized electrons in the liquid (U= 1.3 eV for water [S]). Actually, Y”-4 and Yo-5 plots were linear over a broader (~1-2 eV) range than expected from ref. [2]. Linearity over such a broad range probably results from the fortuitous compensation of energy-dependent factors, namely the photoionization cross section and/or the damping length of the electron wave (in the liquid). Confidence m the threshold energies obtained by extrapolation is strengthened by the agreement obtained in sections 4 and 5.

3. interpretation of threshoId energies

Photoelectron emission In the gas phase corresponds to the process

F-(aq) = X(=-lI-(aq) +e-(g) , (1)

where (aq) and (g) denote species in the liquid and gas

Volume 78, number 2 CHEMICAL PHYSUS LETTERS I March 1981

phases, respectively. The species XF-l)-(aq) is the product of the “oxidation” of Xr-(as> for a vertical transition. Oxidation under adiabatic conditions (in the spectroscopic sense) yields the species Xc=- l)-(aq). The vertical and adiabatic processes will be correlated, and threshold energies wdl be related to thermodynamic data_

Consider the cycle,

Xz-(as) + K+(aq) = X(z-l)-(aq) + iH2(g) ,

~I-$&) = Waq) + e-(q),

e-(aq) = e-(g) ;

on summing, this yields

(2)

(3)

(4

Xr-(as) = Xc=- i I-(aq) i e-(g),. (5)

Analogous cycles have been used [4--61 in calculations of energies for photoelectron emission by solutions. The free energy (in eV) for (5) is,

AF, = FIX(Z-l)-(aq)] - F[Xz-(as)] + 4.39 , (6)

where 4.39 eV is the sum of the free energies for (3)

and (4), namely 2.77 eV [7] and 1.62 eV [S] _ The process,

X(2-ii- = X(=-I>-(aq) , (7)

must be added to (5) to obtain (1). Process (7) repre- sents the reorganization of the solvent about the ion and possibly a contribution from structural charges, e.g., a variation of interatomic distances in polyatomic ions. The free energy for (l), that is, for photoe!ectron emission in the gas phase is

AFe=AF1 +AF2, (8)

where AF2 pertains to (7). The quantity AF, should be equal or nearly equal to the threshold energy, except for a minor contribution from the surface potential at the solution-water vapor interface.

The free energy AF, for (7) is by no means negligible (al-3 eV), but one expects this quantity to vary monotonically for a series of anions with the same ionic charge and similar chemical properties, e.g., the halide ions. Hence, the threshold energies for such a series are expected to vary monotonically with the standard free energy for (5) A linear plot with unit slope [6] for such a relationship should be obtained only if AF2 is constant for that series. This is only approximately the case for the halide ions (section 4).

4. Calculation of the threshold energies for the halide

ions

In the case of halide ions, AF2 pertains solely to reorganization of the medium since the atom X,(aq) is produced by photoionization. The solvent organi- zation about X,(aq) corresponds to the solvation of X-(aq) except for the loss of electronic polarization. The standard free energy of solvation of the halides in aqueous solution can be expressed quite accurately as a function of the ionic radius r- [S] _ The Born term (e2/2r)(l - e,-l) - m this expression is now replaced by (e’/2r)($ - ETA, where eoP and es are the optical and static dielectric constants, respectively_ Thus, one has

AF2 = - 0.0685r2 + 0.0246 + 3.976/r - 0.858/r2 .

(9)

The r-l term in eq. (9) corresponds to the Born equa- tion, and the sum of the other terms 1s nearly indepen- dent of r (-0.46 to -0.57 eV for the halide ions).

Values of AF, [eq. (6)] for the halide ions, calculated from data from ref. [9], are listed in table 2 to- gether with the AF, (eq. (9); radii from ref. [lo] )- The free energy AF2 is by no means constant but does vary monotonically from F- to I-. The calculated AF, [eq. (8)] agree with the experimental threshold energies within 0.2 eV. This discrepancy is compa- rable to the energy for the surface potential whrch has indeed been estimated to be = 0.1-0.2 eV [l 11. Vari- ation of the surface potential from Cl- to I- is negligible (a.03 V) f or our purpose- The agreement is good, especially in view of the approximate nature of the calculation of AF2_

One calculates from the data of table 2, Et - AF, = 1.93, 1.64,1.56 eV for Cl-, Br-, I-, respectively. A plot of Et against AF, therefore is only approximately

Table 2 Calculated free energy aFe versus experimental threshold energy Et

I- AFI AF2 AFe Et (iu <eV) (ev) WCI (ev)

F- 1.36 7.87 2.35 10.22 - cl- 1.81 6.84 1.74 8.58 8.77 Br- i-9.5 6.31 1.57 7.88 7.95 I- 2.16 5.65 1.36 7.01 7.21

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Volume 78, number 2 CHEMICAL PHYSICS LETTERS 1 March I981

linear with a slope different from unity (z-1 -1-l -2 here) as noted in section 3.

The results of table 2 may be compared to the electron affinities for the halide ions in the gas phase [ 121: 3.45,5.61,3.36,3.07 eV from F- to I-. These values are not very different, and it is the effect of the solvent (variation of v) that causes the threshold energes for solutions to vary significantly in the halide ion series.

5. Comparison with charge-transfer-to-solvent absorption spectra

Threshold energies Et are plotted in fig. 3 against the energy Em, at the nzumum of the charge-transfer-to-solvent (CTTS) absorption band. Data on CTTS spectra are from ref. [13]. One has Et = Em, + 1.7 (in eV) in agreement with a prediction 1141 based on the interpretation [ 133 of CTTS spectra of morganic anions. Agreement for NT would have been better If the value, Et = 7.66 eV, from a Y”-5 plot had been used (see sectlon 2).

The shift of al.7 eV is the sum of the binding energy B of the electron in the CTTS excited state and the energy corresponding to the surface potential of the solution (0.1-0.2 eV [l I])_ One has B = 1.58 and 1.55 eV, respectively, for ionic charges of -1 and -2. The agreement in fig. 3 therefore IS as good as one can expect from a simple model for CTTS spectra.

5 6 7 ABSORPTION MAXIMUM (eV)

FG. 3. Threshold energy Et against encr,9 E- at the maei- mum of CTTS absorption band for different anions. Lme cor- esponds to Et = Emax + 1.7.

6. Conclusion

The experimental and theoretical methods applied in this work yield reliable (= 50.1 eV) energies for photoelectron emission by aqueous solutions in agreement with calculated energies and results from CTTS spec- tra_

Acknowledgement

This work was supported by the Office of Naval Research and the National Science Foundation. The authors are indebted to Dr.1. Watanabe for introduc- ing one of them (K-v-B.) to the experimental methods used in this work.

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[ 11) J-E-B. Randles, in. Advances in eiectrochemistry and electrochemical engineering, Vol. 3, eds P. Delahay and C-W. Tobias (Wiley-interscience, New York, 1963) pp. l-30.

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photoelectron emission spectroscopy of inorganic anions in aqueous solution

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