Atomic and Molecular Electron Affinities

52 Pages·2001·0.46 MB·English

Chem.Rev.2002,102,231- 282 231
Atomic and Molecular Electron Affinities: Photoelectron Experiments and
Theoretical Computations
Jonathan C. Rienstra-Kiracofe,† Gregory S. Tschumper, and Henry F. Schaefer III*
CenterforComputationalQuantumChemistry,UniversityofGeorgia,Athens,Georgia30602-2525
Sreela Nandi and G. Barney Ellison*
DepartmentofChemistryandBiochemistry,UniversityofColoradoatBoulder,Boulder,Colorado80309-0215
Received June13,2001
Contents 3. Polycyclic Aromatic Hydrocarbons (PAHs) 252
4. CO 253
4
I. Introduction and Scope 231 V. Conclusions 253
A. Definitions of Atomic Electron Affinities 233 VI. Acknowledgments 253
B. Definitions of Molecular Electron Affinities 233 VII. Compilations of Experimental and Theoretical 253
II. Experimental Photoelectron Electron Affinities 235 Electron Affinities
A. Historical Background 235 A. Report of Theoretical DFT Electron Affinities, 253
B. The Photoeffect 236 Tables 8 and 9
C. Experimental Methods 237 B. Review of Experimental Photoelectron 261
Electron Affinities, Table 10
D. Time-of-Flight Negative Ion Photoelectron 239
Spectroscopy VIII. Supporting Information 277
E. Some Thermochemical Uses of Electron 241 IX. References 277
Affinities
F. Layout of Table 10: Experimental 242
I. Introduction and Scope
Photoelectron Electron Affinities
III. Theoretical Determination of Electron Affinities 242 The energy difference between an uncharged spe-
A. Historical Background 242 cies and its negative ion, referred to as an electron
1. Theoretical Predictions of Atomic Electron 242 affinity(EA),isanimportantpropertyofatomsand
Affinities molecules. Negative ions, or anions, result from
2. Theoretical Predictions of Molecular 243 neutral molecules (often radicals) binding an ad-
Electron Affinities ditionalelectron.TheimportanceandutilityofEAs
B. Present Status of Theoretical Electron Affinity 243 extend well beyond the regime of gas-phase ion
Predictions chemistry.1 Indeed, there are many areas of pure
C. Basis Sets and Theoretical Electron Affinities 244 chemistry,2-6 materials science, and environmental
D. Density Functional Theory (DFT) and 245 chemistry7wherethepropertiesofnegativeionsand
Electron Affinities radicalsareimportant.Asurveyofrecentexamples
E. Layout of Tables 8 and 9: Theoretical DFT 247 illustrates the diversity of areas in which electron
Electron Affinities affinities play a role: silicon8 and quantum dot
F. Details of Density Functional Methods 247 (nanocrystal)semiconductorchemistry,9-12Schottky
Employed in Tables 8 and 9 diodes,13 molecular clusters,14,15 fullerene chem-
IV. Discussion and Observations 248 istry,16-18 interstellar chemistry,19,20 polymer photo-
A. Statistical Analysis of DFT Results Through 248 luminescence,21-23 microelectronics,24 flat panel dis-
Comparisons to Experiment and Other plays,25 and even hypotheses regarding the shuttle
Theoretical Methods glow phenomenon.26
B. Theoretical EAs for Species with Unknown 251 Furthermore, the stabilities of free radicals and
Experimental EAs anionsareofgreatimportanceinthedetermination
C. On the Applicability of DFT to Anions and the 251 ofbiochemicalpathwaysforelectrontransfer,photo-
Future of DFT EA Predictions synthesis, oxidative phosphorylation, and oxidative
D. Specific Theoretical Successes 251 stress.27 Recent examples here include the binding
E. Interesting Problems 252 of type 1 human immunodeficiency virus (HIV-1) to
1. C 252 nucleicacids,28toxinchemistry,29photosynthesis,30-32
2
2. SF and SeF 252 electrontransferinbiologicalsystems,33-35andelec-
6 6
tron attachment to nucleic acid bases.36,37
Such examples demonstrate the importance of
*Towhomcorrespondenceshouldbeaddressed.E-mail: (H.F.S.)
electron affinities in chemistry. Clearly, the magni-
[email protected].(G.B.E.)[email protected].
†E-mail: [email protected]. tudes of the electron binding energies are of great
10.1021/cr990044uCCC:$39.75 ©2002AmericanChemicalSociety
PublishedonWeb01/09/2002
232 ChemicalReviews,2002,Vol.102,No.1 Rienstra-Kiracofeetal.
JonathanC.Rienstra-KiracofereceivedhisB.S.degreeinchemistryfrom HenryF.SchaeferIIIearnedadoctorateinchemistryatStanfordUniversity
CalvinCollege,GrandRapids,MI,in1996.In2000hereceivedhisPh.D. underthesupervisionofProfessorFrankE.Harris.Hewasaprofessor
fromtheUniversityofGeorgia,alsoinchemistry.Hehasnowjoinedthe ofchemistryatBerkeleyfor18yearsandnowteachesattheUniversity
Department of Chemistry at Emory University as a lecturer, where he ofGeorgia.Hisresearchinterestscenterontheoreticalstudiesofmolecular
teaches, directs the undergraduate physical chemistry laboratory, and electronicstructure.
conductscomputationalchemistryresearchwithundergraduatestudents.
SreelaNandihasjustfinishedherstudiesforaPh.D.inchemistryand
biochemistryattheUniversityofColorado.
GregoryS.TschumperreceivedaB.S.degree(chemistryandmathemat-
ics)fromWinonaStateUniversity,Winona,MN,in1995andadoctorate
(chemistry) from the University of Georgia in 1999. After spending one
yearasscientificcollaboratorattheETH-Zentrum(SwissFederalInstitute
of Technology) in Zu¨rich, Switzerland, and one year as a postdoctoral
fellow at Emory University, he joined the Department of Chemistry and
BiochemistryattheUniversityofMississippi,whereheteachesphysical
chemistry. His research interests include hydrogen bonding and other
subtle interactions in biochemical systems, excited-state chemistry and
chemistryinvolvingmultipleelectronicsurfaces,andthedevelopmentof
theoreticalmethodsforthepredictionofmolecularproperties.
interest.Theexperimentalmeasurementand/ortheo-
reticaldeterminationoftheseenergeticquantitiesis
an important task. In this review, the experimental G.BarneyEllisonearnedadoctorateinchemistryatYaleUniversityin
1974underthesupervisionofProfessorKennethB.Wiberg.Heteaches
determinationofelectronaffinitiesisexamined.Also
organic chemistry at the University of Colorado. His research interests
examined is the ability of computational chemistry
centerontheopticalanddynamicalpropertiesoforganicradicals.
methodstopredictelectronaffinities.Amajorportion
of this work is a review (through January 2000) of niques are currently the most accurate and reliable
1101 experimentally determined electron affinities experimentalmethodsformeasuringelectronaffini-
viaphotoelectrontechniques,foundinTable10.Also ties, and DFT is perhaps the only widely applicable
included are reports of theoretical values (Tables 8 andeasilyemployedtheoreticalmethodinusetoday
and 9) for electron affinities of 163 atoms or mol- whichachievessatisfactoryaccuracy(within0.2eV)
ecules, as predicted by density functional theory inthepredictionofEAsforlargemolecules(withno
(DFT) from results by the Schaefer group. elements of point group symmetry), by which we
Unfortunately,acompletesurveyofallexperimen- mean molecules with more than 50 first-row atoms
tal techniques and theoretical methods currently C, N, and O. An entire review of all theoretical EA
used for the determination of EAs is not possible, predictionsisbothimpossibleandimpractical.Thus,
even in a review, hence the limitation in scope of our statistical analysis is restricted to a careful
Tables8-10.Ontheotherhand,photoelectrictech- evaluation of the DFT results in Table 8. Nonethe-
AtomicandMolecularElectronAffinities ChemicalReviews,2002,Vol.102,No.1 233
less, discussion is not limited to DFT, and compari- allowforstraightforward,directexperimentalinves-
sons to the coupled-cluster (CC), Gaussian-X (G2, tigations. Anions of atoms with negative electron
G3), complete basis set (CBS-M, CBS-Q), and Weiz- affinities do not exist for any chemically significant
mann-X (W1, W2) theoretical methods are made. time period (typically only a few picoseconds) and
Additionally, DFT EA predictions for 53 atoms or thus are usually of less interest to chemists. Only
moleculeswithunmeasuredoruncertainexperimen- positive atomic electron affinities are discussed in
tal EAs are presented in Table 9. this review.
This assessment of electron affinities presents a The interpretation of an atomic electron affinity
sourceforthoseseekingaccurateexperimentalelec- dependsonachemist’sperspective.Forexample,an
tron affinities of atoms or molecules or for those experimentalistmayviewthequantityastheenergy
desiring an introduction to EA determination via required to remove the excess electron from the
photoelectric techniques. For experimentalists or negative ion,40 while it may be more convenient for
theoreticians desiring to learn the definitions, con- a theoretician to consider the energy gain upon
ventions,concepts,anddifficultiesencounteredinthe additionoftheelectron.41Bothdefinitionsarecorrect
studyofelectronaffinities,thisreviewshouldanswer but imply important differences which must be
mostquestions.Fortheoreticianswishingtocompare delineated in any comparison of experimental and
and contrast computational methods, the summary theoreticalelectronaffinities.Specificallyforatoms,
anddiscussionofvarioustheoreticalmethodsalong- an experimentally determined EA automatically
side the results presented in Table 8 will serve as a includesallelectroniceffects,suchasrelativisticand
valuableresource.Thisworkshouldfacilitatefurther spin-orbiteffects,andislimitedonlybytheaccuracy
collaborations between the complementary fields of andprecisionofaparticulartechnique.Conventional
experimental and theoretical chemistry. quantum theoretical approaches do not typically
include such “difficult” electronic effects; although
Inthisreviewwestatedefinitionsandconventions
theoretical evaluation of all electronic terms is pos-
used in determining atomic and molecular electron
sibleviaconstructionofanappropriateHamiltonian.
affinities from both experimental and theoretical
For practical reasons, many terms are often not
points of view. After a brief historical introduction
includedandconsequentlyignoredoraddedlateras
anddiscussionofthephotoeffect,experimentalmeth-
energycorrections.Thus,thetheoreticaldetermina-
ods which exploit the photoeffect are described. A
tion of an EA is limited by the completeness of the
detailed examination of experimental photoelectron
selected method, that is, the approximations made
electronaffinitiesispresentedinTable10,andsome
withinthemethodtothefullelectronicHamiltonian.
thermochemicalusesofelectronaffinitiesareexam-
ined.Thedevelopmentandpresentstatusofcompu-
B. Definitions of Molecular Electron Affinities
tational chemistry techniques for the prediction of
atomic and molecular EAs is also reviewed in a Theelectronaffinityofaneutralmoleculeisthe
historicalcontext.SpecificdiscussionofDFTfollows binding energy of an electron to the molecule. From
with a particular emphasis on the applicability of anexperimentalviewpoint,itisusefultothinkofthis
DFT to anionic species. Through comparisons to in analogy to a spectroscopic transition, as depicted
experimentalEAsandresultsfromothertheoretical inFigure1.Consideranegativeion,R-,thatisstable
methods, analysis of DFT EAs is given (see Tables with respect to the corresponding neutral, R, and a
6-9).Inthecontextofothertheoreticalmethods,the freeelectron.TheelectronaffinityofRisthetransi-
successes and failures of DFT EA predictions are tion energy:
detailed.Thereviewisconcludedwithabriefexami-
nationofsomespecificEAswhichareofexperimental EA(R))¢E(RrR-) (2)
and theoretical interest.
Familiar examples of R- might be the hydroxide,
A. Definitions of Atomic Electron Affinities methide, and vinyl anions [OH-, CH -, and CH d
3 2
CH-];allareboundwithrespecttothecorresponding
Atoms represent the simplest chemical systems
neutral and a free electron, e-. More specifically,
from which a discussion of electron affinities can
Figure 1 indicates that the electron affinity of R is
begin.Earlyreviewsonatomicelectronaffinitiesby
the transition energy from the ground vibrational/
Hotop and Lineberger38,39 provide a clear definition
rotationalstateoftheaniontothegroundvibrational/
ofatomicelectronaffinities,“Theelectronaffinity,
rotational state of the neutral:
EA,ofanatomAisthedifferencebetweenthetotal
energies (E ) of the ground states of A and its
negative iontoAt -”: EA(R))EjR,v¢ )0,J¢ )0æ rEjR-,v¢¢ )0,J¢¢ )0æ (3)
The electron affinity associated with the (0, 0) band
EA(A))E (A)-E (A-) (1)
tot tot inFigure1isoftenreferredtoasthe“adiabaticEA”.
Itiscommonforspectroscopiststorefertotheground
Notethattheelectronaffinityispositiveforsystems electronicstateofapolyatomicspeciesasX(cid:247) andany
in which the neutral atom lies energetically above excited electronic states of the same multiplicity as
theanion.Timescalesmustbeconsideredwhenthe A(cid:247), B(cid:247), etc. Occasionally, the spectroscopic threshold
stabilityofthenegativeionisdiscussed.Ingeneral, correspondingtothe(0,0)bandcannotbeidentified
anions of atoms with positive EAs exist sufficiently byexperimentalmethods,andthus,theadiabaticEA
longenoughtoplayaroleinchemicalreactionsand is not available. In these cases the transition corre-
234 ChemicalReviews,2002,Vol.102,No.1 Rienstra-Kiracofeetal.
theanion.NotethattheVDEissometimesreferred
to as the first vertical ionization potential of the
anion.41Thetheoretical“adiabatic”electronaffinity,
AEAorsimplyEA,representsthedifferencebetween
the total energies of the neutral and anion at their
respectiveequilibrium(r )nuclearconfigurations.A
e
conceptualmodelofthecorrespondingphysicalpro-
cess is a bit more difficult to imagine. An infinitely
slow removal/addition of an electron which changes
theanion/neutralnuclearconfigurationbyinfinitesi-
mal increments would require/liberate an energy
equaltotheEA.Insummary,theoreticaldefinitions
of VAE, VDE, and EA in terms of optimized, r ,
e
geometries are
VAE)E(optimizedneut.)-
E(anionatoptimizedneut.geometry) (4)
VDE)E(neut.atoptimizedaniongeometry)-
E(optimizedanion) (5)
EA)E(optimizedneut.)-E(optimizedanion)
(6)
Experimental measurements, which do not directly
correspondtor geometries,inprinciple,differfrom
e
all of these theoretical energy separations. That is,
experimental molecular adiabatic electron affinities
Figure 1. A qualitative diagram of potential energy
surfaces for an anionic molecule, R-, and a neutral mol- (eq 3) should be viewed as an EA0 (explicitly shown
ecule,R.Thesurfacesaremeanttorepresentalldiatomic in Figure 1), whereas the theoretical molecular
and polyatomic molecules, though all anions are not electron affinities should be viewed as EA . In
e
necessarilylowerinenergythantheneutrals(cf.thetext). general,however,bothvaluesarereferredtosimply
Transitions shown represent the vertical detachment en-
as “EA”, and the experimental EA is usually well
ergy (VDE), adiabatic electron affinity (EA), and the
approximated by the theoretical adiabatic value.
verticalattachmentenergy(VAE).Inthecaseofanonlin-
earpolyatomicwithnatoms,thereare3n-6modes,and The vertical quantities offer limiting values for
thisfigureshowsacutthroughtheactivemode(s). most molecules. In situations where the nuclear
configurationofthenegativeiondoesnotdrastically
differ from that of the uncharged parent species (as
sponding to the most intense Franck-Condon fea- in Figure 1), the VAE and VDE provide lower and
tures of the detachment spectrum is sometimes upper bounds, respectively, for the EA. These in-
reported as the “vertical detachment energy”, VDE. equalities logically follow from the definition of an
From a theoretical perspective, the progression equilibriumgeometryasthegeometryattheenergy
from atoms to molecules necessarily complicates minimumonthepotentialenergysurface.Changing
matters. A change in nuclear configuration accom- the geometry relative to the equilibrium structure
panies the transition between the neutral molecule must increase the energy. When the change occurs
and its anion. In electronic structure theory this on the potential energy surface of the anion, the
introducestwoBorn-Oppenheimerpotentialenergy neutral-anion energy separation decreases (VAE),
surfacesrequiringthespecificationofthegeometries whiletheoppositeistrueforchangesonthepotential
ofboththeneutralandanion,therebygivingriseto energy surface of the neutral molecule (VDE). This
a myriad of possible energy differences. Only three relationshipcanbreakdowninpathologicalcasesin
separations are particularly useful. Figure 1 repre- whichthenuclearconfigurationdramaticallychanges
sents these cases graphically. The vertical attach- upon the addition or removal of an electron. ClF
7
ment energy (VAE) and VDE are conceptually the provides an example of this behavior.42 For this
simplest. A physical interpretation of a VAE is the molecule, the theoretical EA near 9 eV lies ap-
energyreleasedfromthenear-instantaneousaddition proximately2eVabovetheVDE.Theinstabilitywith
of an electron to a neutral molecule. During such a respect to dissociation of this molecule provides a
processthereisnotimeforgeometryrelaxation;both reasonable explanation of this phenomenon. The
the anion and neutral species are thus at the opti- equilibrium nuclear configuration of the anion cor-
mized equilibrium nuclear configuration (r ) of the responds to some point along the pathway to dis-
e
neutral molecule. The VDE is conceptually equiva- sociationofneutralClF intoClF andF(orevenClF
7 6 5
lent,butisinsteadtheenergyrequiredforthenear- and F ).
2
instantaneousremovalofanelectronfromananion. In addition to the difficult electronic effects noted
Here both anion and neutral species are at the in atoms, another complicating factor introduced at
optimized equilibrium nuclear configuration (r ) of themolecularlevelarisesfromvibrations.Harmonic
e
AtomicandMolecularElectronAffinities ChemicalReviews,2002,Vol.102,No.1 235
zero-point vibrational energies (ZPVEs) electrical discharge generates the N - ion, which
2
drives CO lasers. The scattering resonances are
3(cid:229)n-6 “negative”2electron affinities because the potential
ZPVE)(1/2) h(cid:246) (7)
e curvesoftheionandneutralinFigure1areinverted.
Theanioncurveisabovethatoftheneutralspecies.
wherehisPlank’sconstantand(cid:246) the3n-6set(n
e Useful reviews of the spectroscopy of temporary
)numberofatoms)ofharmonicfrequencies,provide
negative ions have been written.50,51 Jordan and
a reasonable approximation of zero-point energy
Burrow’s review has a complete report of electron
contributions from the fundamental, (cid:238), vibrations.
transmission spectroscopic resonances for all mol-
Addition of ZPVE corrections to electron affinities
eculesthrough1986.Itispossibletoobtaintheoreti-
allows one to approximate a theoretical EA as an
e cal predictions of negative electron affinities. At a
EA , though the r optimized geometry is still used:
0 e first approximation, a negative VAE often corre-
ZPVEEA)[E(optimizedneut.)+ZPVE ]- sponds to the resonant electron scattering energy.
neut.
[E(optimizedanion)+ZPVE ] (8) Although they have no formal, positive electron
anion affinity, water and benzene will bind an electron in
clusters52-54orinsolution.55Thenaphthaleneanion
In this review, the theoretical molecular electron
affinities are presented both with and without the is stabilized by a single water molecule.56 However,
inclusionofZPVEcorrections.Inmanyinstancesthe for simplicity in this review we focus on the EA of
effect is negligible, as the ZPVEs of the neutral and single molecules. Other molecules without an EA
anionic species are usually quite similar (cf. Figure maybestabilizedbyothermeans;forexample,C6H6-
1) and consequently the ZPVE and ZPVE has a beautiful EPR spectrum in cryogenic matri-
neut. anion
nearlycancelineq8.However,incertainmolecules, ces.57 In condensed phases, solvent molecules help
such as polycyclic aromatic hydrocarbons, ZPVE stabilizebindingoftheelectrontothehostmolecule
correctionscanaccountforalargepercentageofthe by a wonderful set of dipole and multipole cou-
total electron affinity43 (see Section IV.3). plings.58 Such stabilizations are also not discussed
Tominimizeconfusion,44,45itisimportanttospecify here.
theelectronicstatesofallspeciesfortheEAresults Finally, recent research has focused on multiply
presented here. Whenever possible, the electronic charged anions (molecules binding more than one
states for both the neutral and anionic species extraelectron).18,59-67Becausethenumberofknown,
examined in this review are specifically stated.
well-characterized, multiply charged anions is few,
It should also be noted that an electron affinity is
only singly charged anions are analyzed in this
typically only a fraction of the size of the ionization
review.
energy(IE)or,equivalently,theionizationpotential
(IP). Photoionization of a neutral species (R) in-
duceschargeseparationandproducesafreeelectron- II. Experimental Photoelectron Electron Affinities
positiveionpair,R+h(cid:238)fR++e-.Asaconsequence
A. Historical Background
of Coulomb’s law, charge separation requires a
considerable amount of energy. In contrast to pho-
Historically, the experimental determination of
toionization, photodetachment of an anion (R-)
electronaffinitieshasprovendifficult.Priorto1970,
produces a free electron and a neutral atom or
molecule, R- + h(cid:238) f R + e-. For example, consider experimental measurements of this chemical prop-
ertyweretypicallyindirectandunreliable.68,69Early
thesimplestatom,hydrogen,andrecall46thatIP(H)
methods of anion photodetachment using conven-
is 13.6 eV while47 EA(H) is only 0.75 eV. For most
tionallightsourceswereimplementedbyBranscomb
molecules, ionization energies are about 10 eV, and
and co-workers for the direct determination of EAs,
the electron affinities of most species are roughly 1
eV; IP(R) (cid:25) 10 eV, but EA(R) (cid:25) 1 eV. but often there were large uncertainties associated
with the results.38,70-76 This experimental situation
While every atom and molecule has an IP, they
quickly improved with the advent of tunable laser
neednothaveanEA.Thereisalargeclassofanions
light sources. Advancements in the production of
which are not bound species. Many common mol-
atomic and molecular negative ions have also led to
eculessuchasN ,H O,andC H donotformstable
2 2 6 6
anions.InthegasphaseH O-decaystoH Oplusa significant improvement in this field.38 A survey of
2 2
freeelectronandC H -isnotstablewithrespectto atomic electron affinities beautifully illustrates the
6 6
benzene and e-. The carbon dioxide anion is meta- profound impact of these recent technological ad-
stable for roughly 100 (cid:237)s before it disintegrates: vancements. In 1970, Lineberger and Woodward
CO - f CO + e-. Generally, radicals, such as OH, reported the first application of dye lasers to the
2 2
CH , or CH dCH, bind an electron into their “half- measurementofanelectronaffinity,specificallythat
3 2
filled molecular orbital” and form stable anions. In of the sulfur atom determined via threshold photo-
contrast,manyclosed-shellspecies,suchasN ,H O, detachmentspectroscopy.77By1975,laserthreshold
2 2
or C H , have filled valences and do not bind an photodetachment techniques had successfully mea-
6 6
electron. sured the electron affinities of 19 atoms.38 A decade
In the gas phase “temporary” negative ions, such later, the number of atomic systems conquered had
asN -,H O-,andC H -,canbestudiedbyresonant morethandoubledto40.39Today,inTable10ofthis
2 2 6 6
electron scattering.48,49 Several of these temporary review, 1101 atomic and molecular EAs determined
negative ions are of great practical importance; an through photoelectron experiments are reviewed.
236 ChemicalReviews,2002,Vol.102,No.1 Rienstra-Kiracofeetal.
Forlinearlypolarizedlightproducingphotoelectrons
ofkineticenergyE,theangulardistributionhasthe
general form
(cid:243)
(cid:243) (E)) D[1+(cid:226)(E)P (cosı)] (11)
D 4(cid:240) 2
where P (cos ı) ) (1/2)(3 cos2 ı - 1), (cid:243) represents
2 D
thetotalphotodetachmentcrosssection,ımeasures
theanglebetweenthedirectionoftheejectedelectron
and the polarization of the incident light, and (cid:226)(E)
isanasymmetryparameter.Commonly,experimen-
talists fix the angle ı to be 54.7°, so P (cos 54.7°) is
2
zero; consequently, eq 11 ensures that the angular
Figure 2. Negative ion photodetachment occurs at the distribution of the detached photoelectrons will be
intersectionofabeamofnegativeionswithalaserbeam. isotropic at this “magic angle”.
Time-dependentopticalperturbationtheory,with
B. The Photoeffect dipoleselectionrules,yieldsasimpleexpression82,83
for the photodetachment cross section, (cid:243) :
Several methods have been employed to measure D
theelectronaffinitiesofisolatedmolecules.Charge- 16(cid:240)3m2(-e)2ø(cid:238)
transferreactionsinamassspectrometer,collisional (cid:243) ((cid:238))) jÆ (cid:190)¢¢ (q)(cid:237)(q)(cid:190)¢(q)æ j2 (12)
ionizationwithfastalkali-metalbeams,plasmaand D 3pc2
optogalvanicspectroscopies,andcollisionalionization
have been used to deduce molecular electron affini- Ineq12,mande-aretheelectronmassandcharge
ties.1 However, the most effective methods to mea- while ø is the asymptotic electron velocity and (cid:238) is
sureelectronaffinities,foreitheratomsormolecules, thephotonfrequency.Ifthedipoletransitionmoment
use the photoelectric effect.78 These measurements operator is written as (cid:237)(q) (where q is the nuclear
routinely produce EA values of “chemical accuracy” coordinate), then the transition moment integral is
with uncertainties of less than (1 kJ mol-1 ((0.24 Æ (cid:190)¢¢ (q) (cid:237)(q) (cid:190)¢(q)æ , with (cid:190)¢¢ (q) being the state of the
kcal mol-1, (0.01 eV). initialanionand(cid:190)¢(q)thestateofthefinalneutral.
The essential experiment is to bombard a target It is common to assume that the rotational/vibra-
ion, R-, with a light beam of frequency (cid:238) and to tionalandelectronicdegreesoffreedomareseparable
monitor either the photodestruction of R- or the in both the anion, (cid:190)¢¢ (q), and the final neutral,
appearance of the scattered electrons, e-: (cid:190)¢(q). If (cid:237)(q) is slowly varying or a constant, then
(cid:237)(q) (cid:25) (cid:237) and the Condon approximation yields
0
R-+h(cid:238)fR+e- (9)
Æ (cid:190)¢¢ (q)(cid:237)(q)(cid:190)¢(q)æ (cid:25)Æ ª¢¢ (cid:237) ª¢ æÆ (cid:230)¢¢ (Q¢¢ )(cid:230)¢(Q¢)æ
elect 0 elect
Howdoesthephotoeffectineq9work?79,80Consider (13)
in Figure 2 a light beam that strikes an ion beam.
Suppose the frequency of the light beam is fixed at The rovibrational wave functions, (cid:230)¢¢ (Q¢¢ ), are func-
(cid:238) , and the photon flux, measured as photons per tions of the normal coordinates, Q¢¢ ; the vibrational
0
second,is(cid:30).Thetargetanions,calledR-ineq9,are overlaps give rise to the familiar Franck-Condon
formed into a beam of velocity ø (cm s-1) and beam factors.
0
width l (cm). By simple conservation of energy, if Cananestimateofhowmanyphotoelectrons,j ,
0 elect
h(cid:238) <EA(R),thennophotodetachmentcanoccurand whichwillbeproducedwhenalasercrossesastream
0
noscatteredelectronswillbeproduced.Ifh(cid:238)0equals of anions, janions (ions s-1), be made? The expression
or exceeds this photodetachment threshold, then for Beers’ law in eq 10 is not very convenient.
photoproductionofelectronsispossible.Typicallyone Typically,experimentalistsmanipulatepropertiesof
uses Beers’ law to describe photodetachment. thenegativeionbeam(thecurrent,thebeamwidth,
InFigure2,theincidentfluxofphotonsthatstrike andvelocity)andtrytogeneratethemostlaserlight
theionbeamis(cid:30)andtheintensityofthetransmitted possible.Bydoingsomeelementarysums,theBeers’
lightis(cid:30)e-F(cid:243)Dl0.Consequentlythecurrentofscattered law expression in eq 10 can be manipulated into a
photoelectrons, j (electrons s-1), in Figure 2 is differentformwhichrelatesthescatteredphotoelec-
elect
related to the incident photon flux (cid:30) through the tronstothecurrentofnegativeions(oranions),j .
anions
photodetachment cross section, (cid:243) (cm2), the ion Ithasbeenshown84thateq10canberecastsothat
D
density,F(cm-3),andtheopticalpathlength,l (cm): the current of scattered photoelectrons, j , is
0 elect
j )(cid:30)[1-e-F(cid:243)Dl0] (10) j )j [1-e-(cid:243)D(cid:30)/ø0l0] (14)
elect elect anions
The photoelectrons in Figure 2 are not scattered Ineq14,thecurrentofscatteredphotoelectrons,j ,
elect
randomly or isotropically. Instead, the angular dis- can never exceed the flux of anions, j . If the
anions
tributionofthescatteredelectronscanbedescribed detachmentcrosssection((cid:243) )becomeshugeorifthe
D
by a simple expression81 that is based on dipole photonflux((cid:30))isgigantic,thene-(cid:243)D(cid:30)/ø0l0f0andjelect
selectionrulesandangularmomentumconservation. (cid:25) j . On the other hand, if the ion beam velocity
anions
AtomicandMolecularElectronAffinities ChemicalReviews,2002,Vol.102,No.1 237
(ø0) is very large, then e-(cid:243)D(cid:30)/v0l0 (cid:25) 1 with the conse- ( 0.0004 eV. The selection rules for atomic and
quence that j (cid:25) 0. molecularthresholdprocesseshavebeenreviewed.93
elect
What are some common experimental values? Asthetargetionsbecomelargerandmorecomplex,
Typically janions is approximately 0.1 nA or 109 ions itbecomesverydifficulttoidentifythetruethreshold
s-1.ArIIIlaserradiationat351nmhasafrequency for electron detachment. So instead of scanning the
((cid:238))of8.5(cid:2)1014Hzorawavenumberof28490cm-1. frequencyoftheradiation(cid:238)ineq9,afixed-frequency,
IfthelaserbeaminFigure2isthoughtofasastream (cid:238) ,lasercanbeusedtodetachtheelectronprovided
0
of photons, then (cid:30) is nh(cid:238), where h is Planck’s that h(cid:238) is greater than h(cid:238) .
0 thresh
constant, 6.6 (cid:2) 10-34 J s. Consequently, a 50 W Ar
III laser generates roughly 9 (cid:2) 1019 photons s-1. R-+h(cid:238) fR+e-(KE) (16)
Suppose the photodetachment82,85 cross section, (cid:243) , 0
D
is 6 (cid:2) 10-18 cm2 and that the ions are focused down
toasmallbeamdiameter,typicallyl (cid:25)0.1mm.The Ineq16aneutral,R,isproducedalongwithdetached
0
ion beam energy is roughly 40 V, so the ions have a electronsthathaveadistributionofkineticenergies
velocity,ø ,of2.2(cid:2)106cms-1.Insertingparameters, (KEs). To study the photoelectron spectroscopy
oneestima0tes(cid:243) (cid:30)/v l tobe0.02.Thissmallnumber (PES)ofR-,thescatteredelectronsmustbecollected
D 0 0
ensures that eq 14 can be linearized and used to and their kinetic energies measured. The electron
compute the flux of scattered photoelectrons: affinity is simply the difference (Figure 1) between
theincidentphoton,h(cid:238) ,andtheKEoftheelectrons
[ ] 0
(cid:243) (cid:30) arisingfromthe(0,0)band: EA(R))h(cid:238)0-KE(0,0).
j (cid:25)j D (15) In Table 10 photoelectron EA values are denoted
elect anions ø l
0 0 “PES”.Inadditiontotheelectronaffinity,thephoto-
electron spectrum of a negative ion provides the
Insertingnumbersintoeq15,itcanbeconjectured experimentalist with a set of different electronic
that the rate of photodetachment will be 2 (cid:2) 107 bands each with their attendant Franck-Condon
electrons s-1. This estimate of the photodetached factors (vide infra).
electrons supposes that they are scattered into all Theclassicalnegativeionphotoelectronexperiment
space (4(cid:240) steradians), but the experimental ap- is carried out with continuous laser irradiation fol-
paratus generally has a hemispheric analyzer to lowed by electron analysis with a hemispherical
count the electrons. This detector only collects the analyzer.94,95
smallfractionofthephotoelectronsthatarescattered
Figure 3 is a sketch of a continuous wave (CW)
into a 5° acceptance angle, so d¿ (cid:25) 4(cid:240)/2000. This
negativeionphotoelectronspectrometer.Theionsin
impliesthattheelectrostaticanalyzerwillexperience
eq 16 are produced in a variety of high-pressure
count rates of (4(cid:240)/2000)(2 (cid:2) 107 electrons s-1) or
sources. An effective method for generating anions
roughly2(cid:2)105electronss-1.Thisistheapproximate is to use ion-molecule chemistry in a flowing after-
electroncountratethatisobservedfrommostatoms.
glow device94 to generate target anions. A flowing
Theratesformoleculesarecommonlymuchsmaller
afterglow ion source is chemically versatile and has
because of molecular Franck-Condon factors and
the virtue of producing collisionally relaxed target
lower ion beam currents.
ions which suffer roughly 104 collisions with the
buffer gas.96-100 For example, one can easily pro-
C. Experimental Methods
duce101 ion beams of the o-benzyne anion from the
oxideanionandbenzene: O-+C H fH O+C H -.
One of the most powerful methods to study the 6 6 2 6 4
In Figure 3, ions are extracted from the ion source
photoelectric effect in eq 9 is to scan the frequency
ofthelaser,(cid:238),andmonitor86-88eithertheappearance at pressures of 10-4 mbar, formed into a beam in a
ofthescatteredelectrons,e-,orthephotodestruction separate chamber (10-5 mbar), and accelerated to
oftheions,R-.Thiscanbeaveryprecisemethodto approximately 700 V. A Wien filter velocity-selects
theanions;sincetheionsaredeliveredataconstant
find the threshold for photodetachment (h(cid:238) )
thresh potential(so(1/2)mv2isfixed),thisistantamountto
because it is straightforward to control the laser
massselection.Themass-selectednegativeionsthen
frequency to a few megahertz; thus, (cid:238) is measured
tobe(cid:24)(3(cid:2)10-4cm-1or(4(cid:2)10-8eV!InTable10 enter a chamber outfitted with a laser/electrostatic
analyzer.
threshold EA values are denoted “ThD”. Thus, the
electron affinity of the H atom has been measured Thenegativeionbeamentersthescatteringcham-
byobservationofthephotodestructionthreshold47for ber at an energy of roughly 700 eV, and beam
H- at 6082.99 ( 0.15 cm-1, corresponding89 to currents are typically about 0.1 nA. The ions are
EA(H) ) 0.754195 ( 0.000019 eV. The deuterium deceleratedtoabeamenergyofabout40eVandare
atom, D, has a slightly different electron binding carefully focused onto a laser/ion interaction region
energy. The photodetachment threshold for D- was (Figure4)wheretheyaresubjectedtoirradiationby
found to be 6086.2 ( 0.6 cm-1, implying that EA(D) a CW laser.
) 0.754593 ( 0.000074 eV. When threshold mea- Generally,thecontinuouslightsourceisanargon
surements can be carried out on a polyatomic mol- ionlaseroperatingononeofthe488.0nm(2.540eV),
ecule, very precise electron affinities can be deter- 364.0 nm (3.406 eV), or 351.1 nm (3.531 eV) lines;
mined. As an example,90-92 the threshold for the laser powers of 50-75 W can be achieved. The
detachment CH2CHO- f CH2CHO is measured to resolutionforthesehemisphericalelectrostaticana-
be (cid:238) ) 14718+2 cm-1 or EA(CH CHO) ) 1.8248 lyzers is approximately (0.005 eV.
thresh -5 2
238 ChemicalReviews,2002,Vol.102,No.1 Rienstra-Kiracofeetal.
Figure 3. An overview of a negative ion photodetachment spectrometer. Negative ions are produced by microwave
bombardmentofanappropriategasmixture,extractedintoanionbeam,selectedbyaWienfilter,anddeliveredtoalaser
interaction/hemisphericalanalyzerchamber.
Figure4. Aschematicviewoftheelectrostatichemisphericalanalyzerusedforanalysisofthescatteredphotoelectrons.
Thelaserbeamisdirectedoutoftheplaneofthefigureandfocusedatthespotmarked“Laser/IonInteraction”.
A typical negative ion photoelectron spectrum,102 voltage is V. The beam energy is W, m is the mass
e
namely, that for the hydroperoxide anion, HO -, is of an electron, and (cid:231) is a dimensionless scale com-
2
shown in Figure 5. This figure shows the origin for pressionfactor(typically1.000(0.006): E)E +
cal
HO - + h(cid:238) f HO + e-(KE) is observed at a (cid:231)(V-V )+m W(1/M -1/M).The“raw”electron
2 351nm 2 cal e cal
center-of-mass(CM)kineticenergyof2.448(0.006 affinity of HO - is extracted from the (0, 0) band at
2
eV. To correct for voltage drifts in the electrostatic 2.448 eV (see Figure 1) and yields an initial value
analyzer due to contact potentials, etc., the photo- for EA(HO ) of 1.083 eV (3.531 - 2.448 eV).
2
electron spectra must be internally calibrated with Onemustincludeafinal,smallcorrection.Photo-
a“known”atomicion.InFigure5thephotoelectron electron spectroscopy may be considered a low-
spectrum is calibrated103 with respect to O- and resolution technique since the resolution of the
transformed to the CM frame by a standard104 electronanalyzerisroughly(0.005eV(or40cm-1).
expressionwhereEistheCMkineticenergy(eV)of This means that a photoelectron spectrometer can
an electron detached from an ion of mass M (Da) resolve most electronic and vibrational bands but is
whichispassedbytheenergyanalyzerwhentheslit inadequate to separate rotational transitions. Con-
AtomicandMolecularElectronAffinities ChemicalReviews,2002,Vol.102,No.1 239
stretchingvibrationoftheneutralradical,HO .The
2
vibrationalfrequenciesforthehydroperoxylradicals
thatareextractedfromFigure5arefoundtobe1098
cm-1fortheHO ground-statestretch((cid:238)¢¢ ),whilethe
2 3
excited HO * state stretch is 929 cm-1 ((cid:238)¢ ).
2 3
As mentioned earlier, the yields of photodetached
electronsareangledependent.Theanisotropyfactor
(cid:226)(E)ineq11canvaryfrom-1to+2(-1e(cid:226)e+2).
The photoelectron spectra shown in Figure 5 are
collectedunderconditionswhereıissettothemagic
angleof54.7°sothatI(ı))(cid:243)/4(cid:240)andisindependent
of (cid:226)(E). If spectra are collected at ı ) 0° (E and
laser
collection direction parallel) and ı ) 90° (E and
laser
collection direction perpendicular), one can extract
a value for the anisotropy factor:
I -I
(cid:226)) 0° 90° (19)
Figure 5. Negative ion photoelectron spectrum of HO2- (1/2)I0°+I90°
asafunctionofthecenter-of-masselectronkineticenergy
(eV). No scattered electrons will have energies exceeding
The value of (cid:226) provides important information as
that of the laser, 351.1 nm or 3.531 eV. The origin of the
spectrum at 2.448 ( 0.006 eV marks the HO (v¢ ) 0) r tothenatureofthephotodetachedelectron.Inatoms,
2
HO -(v¢¢ )0)transitionandislabeled(0,0).Whensuitably detachment of an s-electron leads to an outgoing
2
corrected (see the text), this feature yields the electron p-wave (l ) 1) and (cid:226) ) +2, independent of the
affinityofground-stateX(cid:247) 2A¢¢ ofHO2;EA(HO2))1.078( electron kinetic energy. Detachment of a p-electron
0.006eV.TheoriginfordetachmentofHO2-totheexcited resultsinamixtureofinterferings-andd-wavesand
stateofHO isindicatedby(0,0)at1.576eV.Thesplitting
2 leads to an energy-dependent value for (cid:226)(E). At the
betweenthetwooriginsyieldsthetermvaluefortheHO
2 photodetachment threshold, s-wave (l ) 0) detach-
radical,¢E(A(cid:247) 2A¢-X(cid:247) 2A¢¢ ),of0.872(0.007eV. ment dominates, giving (cid:226) ) 0 and yielding an
isotropicphotoelectronangulardistribution.Atpho-
sequently, the EA (see Figure 1) is rotationally toelectron kinetic energies roughly 1 eV above the
uncertain. Suppose that the target anions in eq 9 threshold, d-wave detachment becomes important
haveameanrotationalenergy,(cid:15)¢¢ ,where(cid:15)¢¢ isthe and (cid:226) f -1. Electron detachment from molecular
rot rot
Maxwell-Boltzmannaverageoftheiondistribution, ionsismorecomplicatedthantheatomiccase,but(cid:226)
(cid:15)¢¢ ) Æ E¢¢ æ . Consequently, the measured, raw EA is isgenerallyfoundtobepositivefordetachmentfrom
rot rot
simply (cid:243) (s-like) orbitals and negative for detachment from
(cid:240) (p-like) orbitals.
rawEA)EA-(cid:15)r¢¢ ot+(cid:15)r¢ ot)EA+¢rot (17) For the HO2- spectrum in Figure 5, (cid:226) ) -0.80 (
0.15fortheX(cid:247) stateand-0.59(0.15fortheA(cid:247) state.
Engelking105hasderiveduseful,approximateexpres- Thesedifferentvaluesof(cid:226)indicatedetachmentfrom
sions for the rotational correction, ¢rot (cid:17) (cid:15)¢rot - (cid:15)¢¢rot. two different orbitals of the negative ion; they are
In his expressions, kB is the Boltzmann constant, T both negative, indicating detachment from (cid:240)-type
is the “effective” temperature of the ion beam, and orbitals, as predicted102 from general valence bond
Ae,Be,andCearetherotationalconstantsoftheion (GVB) diagrams.
(double prime state) and final neutral (single prime
state): D. Time-of-Flight Negative Ion Photoelectron
Spectroscopy
[ A¢ B¢ C¢ 3] (B¢¢ -B¢)
¢ (cid:25)k T + + - + (18)
rot B 2A¢¢ 2B¢¢ 2C¢¢ 2 3 Amajoradvanceinphotoelectronspectroscopywas
the use of pulsed lasers to photodetach negative ion
Use of the rotational correction (using calculated beams.106 These devices offer the use of photons in
and/orknownrotationalconstants)improvestheraw eq 16 that are more energetic than the 488 and 351
EA(HO ) and results in the final adiabatic electron nm lines of a continuous wave argon ion laser. The
2
affinity. The 200 K rotational correction, ¢ in eq pulsed negative ion beams are intersected with the
rot
18, is found to be 0.005 eV, which yields the final output of a pulsed yttrium aluminum garnet (YAG)
value for EA(HO ) as 1.078 ( 0.006 eV. solid-state laser and the resultant detached elec-
2
Atanelectronkineticenergyof1.576(0.010eV, trons detected by time-of-flight (TOF) spectrome-
the (0, 0) band or origin for detachment to the first try.107,108 Use of a pulsed laser (20 Hz) provides the
excited state of HO is observed. After rotational third (354.6 nm/3.496 eV), fourth (266.0 nm/4.661
2
correction (eq 18) to determine the (0, 0) feature for eV), and fifth (212.8 nm/5.826 eV) harmonics of the
theA(cid:247) state,thesedatafixtheseparation¢E(ground YAG.
state-excitedstate)[HO ]as0.872(0.007eV.The Thefirstnegativeionphotoelectronspectrometers
2
vibrational fine structure in the photoelectron spec- werebasedoncontinuous(bothlaserandion)beam
trum of HO - is controlled by the Franck-Condon methods.104Whiletheseinstrumentswerecapableof
2
factors via eqs 12 and 13. Both spectral bands in exquisiteresolution,theywerelimitedtospecieswith
Figure 5 show extensive activity in the HO-O lowelectronaffinities(<3.5eV)duetotheirreliance
240 ChemicalReviews,2002,Vol.102,No.1 Rienstra-Kiracofeetal.
Figure6. Schematicdiagramofatime-of-flightnegativeionphotoelectronspectrometer.EM1andEM2aremicrochannel
plateelectronmultipliers,DP)diffusionpump,TP)turbomolecularpump,and(cid:236)/2)half-waveretardationplate.The
“potential switch” drops from 2.5 kV to ground while the ions are inside to allow both the ion source and the electron
analyzertobemaintainedatgroundpotentialduringoperation.
on argon ion lasers to carry out photodetachment.
Pulsedlasers(e.g.,Nd:YAG),ontheotherhand,have
no such limitation since their energy coverage can
bereadilyextendedusingnonlinearopticalschemes
to generate high harmonics of the powerful funda-
mentalbeam.Theioncommunitywasslowtoadopt
pulsed laser sources, however, due to the dramatic
differenceindutycyclesbetweenthecontinuousion
beam spectrometers and the rather low repetition
rates(10-30Hz)atwhichNd:YAGlaserscanoper-
ate.
Thedutycycleproblemwasovercomein1986when
PoseyandJohnson106developedanewtypeofphoto-
electronspectrometer(shownschematicallyinFigure
6) based entirely on time-of-flight concepts for both
massselectionandelectronenergyanalysis.Intheir
method, the ions were generated using an electron-
Figure 7. Photoelectron spectrum of O - taken using a
beam-ionized,pulsedsupersonicexpansiontocreate 2
TOFapparatus(cf.Figure6)ath(cid:238))2.33eV.Noticethat
an overall electrically neutral plasma containing a
thesinglet/tripletsplitting,¢E(3“-,1¢ ),ofthe“diradical”
high density of thermally equilibrated electrons.109 g g
O canbedirectlyreadfromthespectrum.
Negative ions were formed by attachment of these 2
electronsinthedenseregionnearthenozzle,andthe
plasma was allowed to dissipate until its Debye O - photodetached with 2.33 eV photons (second
2
length increased to the point that molecular ions harmonicfromaNd:YAGlaser).Eachmass-selected
could be extracted into a time-of-flight mass spec- ion packet contains on the order of 105 ions, 90% of
trometer by simply pulsing the voltage across two which can be photodetached with readily achieved
grids.Toachievemassselection,theyusedaso-called outputorfluencesfromcommerciallasers((cid:24)100mJ/
“Wiley-McLaren” two-field acceleration configura- pulse).TheYaleinstrumentusedafield-freeelectron
tion110tospatiallyfocustheionsalongthedriftaxis. velocityanalyzer,shieldedfromstraymagneticfields
In its usual implementation, a particle detector is by two concentric (cid:237)-metal cylinders. Like the CW
placed at this transient focus so that the resulting instruments, this arrangement has a rather poor
TOFmassspectradisplayoptimalresolution.Inthe electron collection efficiency ((cid:24)10-3), resulting in
Posey-Johnson scheme, however, the photodetach- typical count rates of a few photoelectrons per laser
mentlaseristimedtointersectthefocusedionpacket pulse.Dataarethereforeacquiredfor105lasershots
just at it arrives at the sampling orifice of a photo- or so to obtain spectra with signal-to-noise ratios
electrondrifttube.Thisstrategy,inessence,bunches comparable to those of the spectra generated using
theionbeamforefficientoverlapwiththeinfrequent the CW spectrometers. The resolution of the TOF
pulses from the laser.109 instrument depends on the electron kinetic energy,
The performance of the TOF instrument is il- andisabout10meVinthe1eVkineticenergyrange.
lustrated in Figure 7, displaying the spectrum from This is comparable to that obtained using CW