Large variations in the propensity of aqueous oxychlorine anions for the solution/vapor interface

E. M.

 

Knipping

,

M. J.

 

Lakin

,

K. L.

 

Foster

,

P.

 

Jungwirth

,

D. J.

 

Tobias

,

R. B.

 

Gerber

,

D.

 

Dabdub

, and

B. J.

 

Finlayson-Pitts

,  , ().

P. K.

 

Dasgupta

,

P. K.

 

Martinelango

,

W. A.

 

Jackson

,

T. A.

 

Anderson

,

K.

 

Tian

,

R. W.

 

Tock

, and

S.

 

Rajagopalan

,  , ().

N.

 

Kang

,

T. A.

 

Anderson

, and

W. A.

 

Jackson

,  , ().

L.

 

Onsager

and

N. N. T.

 

Samaras

,  , ().

P.

 

Jungwirth

and

D. J.

 

Tobias

,  , ().

L. M.

 

Pegram

and

M. T.

 

Record

,  , ().

L. X.

 

Dang

and

T. M.

 

Chang

,  , ().

S.

 

Hüfner

, (, , ).

S.

 

Ghosal

,

J. C.

 

Hemminger

,

H.

 

Bluhm

,

B. S.

 

Mun

,

E. L. D.

 

Hebenstreit

,

G.

 

Ketteler

,

D. F.

 

Ogletree

,

F. G.

 

Requejo

, and

M.

 

Salmeron

,  , ().

N.

 

Ottosson

,

M.

 

Faubel

,

S. E.

 

Bradforth

,

P.

 

Jungwirth

, and

B.

 

Winter

, “,” (in press).

B.

 

Winter

,

R.

 

Weber

,

I. V.

 

Hertel

,

M.

 

Faubel

,

P.

 

Jungwirth

,

E. C.

 

Brown

, and

S. E.

 

Bradforth

,  , ().

B.

 

Winter

,

E. F.

 

Aziz

,

N.

 

Ottosson

,

M.

 

Faubel

,

N.

 

Kosugi

, and

I. V.

 

Hertel

,  , ().

B.

 

Winter

,

R.

 

Weber

,

W.

 

Widdra

,

M.

 

Dittmar

,

M.

 

Faubel

, and

I. V.

 

Hertel

,  , ().

B.

 

Winter

,  , ().

P.

 

van der Straten

,

R.

 

Morgenstern

, and

A.

 

Niehaus

,  , ().

R. F.

 

Fink

,

M.

 

Kivilompolo

,

H.

 

Aksela

, and

S.

 

Aksela

,  , ().

H. J. C.

 

Berendsen

,

J. P. M.

 

Postma

,

W. F.

 

Vangunsteren

,

A.

 

Dinola

, and

J. R.

 

Haak

,  , ().

U.

 

Essmann

,

L.

 

Perera

,

M. L.

 

Berkowitz

,

T.

 

Darden

,

H.

 

Lee

, and

L. G.

 

Pedersen

,  , ().

J. W.

 

Caldwell

and

P. A.

 

Kollman

,  , ().

E.

 

Lindahl

,

B.

 

Hess

, and

D.

 

van der Spoel

,  , ().

L. C.

 

Adam

,

I.

 

Fabian

,

K.

 

Suzuki

, and

G.

 

Gordon

,  , ().

P.

 

Pianetta

, see: http://xdb.lbl.gov/section3/sec_3-2.html.

At a high enough KE, we would expect to obtain 1:1 signal ratios of $ClOn−$ and $Cl−$ $(n=2–4)$⁠, corresponding to the macroscopic stoichiometric proportions, given our assumption that the Cl $2p$ photoionization cross section is the same for all ionic species. While this assumption might not be fully accurate, e.g., due to small variations in the intensity of satellites which redistribute signal from the main PE line (Ref. 8) different cross sections for the ions are highly unlikely to explain the $ClOn−/Cl−$ ratio of 1.2 at 900 eV KE. Rather, one must keep in mind that the photoemission signals are integrals of the total density distributions from the surface into the bulk, exponentially attenuated by the electron mean free path. In a recent photoemission study of aqueous NaI we have shown that the surface enhancement of iodide relative to sodium was observable only for electron KEs smaller than 600 eV; higher KEs primarily probe the bulk solution (Ref. 10). Probably, for the systems investigated here the preferential surface segregation is significantly stronger than for NaI solutions, and hence surface contributions to the total signal remain detectable at the highest KE achieved here. It would require considerably higher KEs for the surface contributions to become negligible.

While the rest of the $ClO−/Cl−$ data set is consistent with a lower surface segregation of hypochlorite relative to chloride, the sharp jump in Fig. 2 from 0.47 to 0.70 when going from 100 to 50 eV hardly seems to be consistent with the bulk stoichiometry of $ClO−$ to $Cl−$—if the bulk ratio is 1:2 the PE ratio should not exceed 0.5. Neither can any single IMFP function explain this large variation with KE for hypochlorite, and at the same time account for the much smaller changes $ClOn−/Cl−$ in the same KE range for the larger oxychlorine anions. One possibility would be to assume a dramatic dependence of the IMFP values on the ionic species, but this assumption could hardly be physically justified. Rather, at 50 eV KE we are apparently too close to the ionization threshold for making the approximation that the effective Cl $2p$ cross section is ion independent. In the condensed phase, low KE electrons can give rise to complex scattering phenomena and even in the gas phase, severe dependence of the effective core-electron cross sections has been observed on the local molecular structure at the ionized site. Above 100 eV KE these effects should however be very small;

L.

 

Saethre

(private communication).

R.

 

Moberg

,

F.

 

Bökman

,

O.

 

Bohman

, and

H. O. G.

 

Siegbahn

,  , ().

L.

 

Vrbka

,

M.

 

Mucha

,

B.

 

Minofar

,

P.

 

Jungwirth

, and

E. C.

 

Brown

,  , ().

P.

 

Jungwirth

and

D. J.

 

Tobias

,  , ().

M. D.

 

Baer

,

I. -F. W.

 

Kuo

,

H.

 

Bluhm

, and

S.

 

Ghosal

, “,” (submitted).