Science - USA (2020-06-05)

INSIGHTS | PERSPECTIVES

sciencemag.org SCIENCE

GRAPHIC: C. BICKEL/

SCIENCE

By Christine M. Isborn

S

olutions of alkali metals in liquid am-

monia have fascinated scientists for

more than 200 years. The “fine blue

colour” of a dilute solution indicative

of solvated electrons was first noted

by Sir Humphry Davy in 1808 [( 1 ), p.

63] and independently published by W. Weyl

in 1864 ( 2 ). A bronze sheen as the solution

becomes highly concentrated (see the figure)

develops as these solvated electrons coalesce

into a metallic continuum. Charles Kraus, an

early pioneer of the study of these solutions

( 3 ), noted that “these solutions...constitute a

link between electrolytes, on the one hand,

and metals, on the other” [( 4 ), p. 83]. On page

1086 of this issue, Buttersack et al. ( 5 ) com-

bine low-temperature x-ray photoelectron

spectroscopy with high-level simulations to

reveal the energetics of metal-ammonia so-

lutions across a large concentration range.

These studies provide the missing energetic

link to characterize the journey of solvated

electrons from electrolyte to metal.

Electrons are usually either relatively lo-

calized in atomic or molecular orbitals or

can be delocalized in the energy bands of

extended solids. For the much less common

case of solvated electrons—which exist in a

number of solvents, including water, but are

most stable in ammonia—questions remain

about the extent of localization and the de-

gree of association with the parent ions and

surrounding solvent. The solvent environ-

ment may change to support these species,

and as the concentration increases, the elec-

trons may interact to become spin-paired

dielectrons and ultimately coalesce into a

metallic state.

These questions have motivated many

studies of hydrated and ammoniated elec-

trons over the years ( 6 , 7 ). One of the most

effective techniques for studying electronic

energy levels is photoelectron spectroscopy,

which measures the kinetic energy of elec-

trons emitted after a substance is irradiated

with light (based on the principles of the pho-

toelectric effect), revealing electronic binding

energies. This technique has been used to

characterize hydrated electrons and ammo-

niated electrons, albeit at low concentrations,

well below the electrolyte-to-metal transition.

The development of a new apparatus en-

abled the application of x-ray photoelectron

spectroscopy to a liquid ammonia microjet

( 8 ), allowing Buttersack et al. to collect the

photoelectrons from the volatile, polar re-

frigerated metal-ammonia solution across

the wide concentration range of 0.012 to 9.7

mol % metal. Over this range, the solution

color changes from a lighter to a deeper blue

and finally develops a bronze metallic sheen.

The photoelectron spectra reveal the onset

and growth of a peak beginning at a concen-

tration of 0.08 mol % metal (see the figure).

The energy of this peak is independent of

the identity of the alkali metal in the solu-

tion, which suggests that the peak arises only

from the ammoniated electron and that the

metal parent ion does not play a direct role

in this transition.

As the concentration of the alkali metal

increases, this peak gradually grows into a

metallic conduction band with a sharp Fermi

edge, and an additional plasmon peak ap-

pears. This plasmon peak is responsible for

the characteristic bronze color of the metallic

solution. This gradual transition is in contrast

to the sharp transition proposed in recent

work on metal-ammonia nanodroplets ( 9 ),

which shows that the bulk liquid and small

solvent clusters support different solvated

electron structures at higher concentrations

and therefore different metallic onsets.

Modeling by Buttersack et al. comple-

ments the photoelectron spectroscopy data.

They apply a metallic free-electron gas

model to fit the growth of the conduction

band and the sharp Fermi edge in the pho-

toelectron spectra as the solution undergoes

the electrolyte-to-metal transition. These

Photoelectron spectroscopy maps a gradual transition for solvated electrons in ammonia

Chemistry and Chemical Biology, University of

California Merced, Merced, CA 95343, USA.

Email: [email protected]

Electrolyte

solution

Solvated electrons

A localized peak arises from the

solvated electron. A similar

feature appears for the dielectron.

Metallic electrons

At higher concentrations, metallic features

appear in the spectrum at concentrations

well below the visual color change to bronze.

Coalescence of

solvated electrons

Increase in

metal concentration

Metallic

solution

Plasmon peaks Conduction

band

Fermi edge

Solvated electron

CHEMICAL PHYSICS

The link between electrolytes and metals

A fascinating color change

Alkali metal-ammonia solutions change from blue to bronze as the concentration of the metal is increased.

This color change marks the shift of the solution from electrolyte to metallic. The photoelectron

spectroscopy experiments by Buttersack et al. across the concentration range show how the electron

gradually changes from localized to metallic.

1056 5 JUNE 2020 • VOL 368 ISSUE 6495

Published by AAAS