ESR spectra of matrix isolated CsO and RbO molecules: 2Σ ground states andinner shell bondingD. M. Lindsay, D. R. Herschbach, and Alvin L. Kwiram Citation: The Journal of Chemical Physics 60, 315 (1974); doi: 10.1063/1.1680787 View online: http://dx.doi.org/10.1063/1.1680787 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/60/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Semiempirical valence bond model of hyperfine interactions and bonding in RbO and CsO J. Chem. Phys. 81, 2411 (1984); 10.1063/1.447943 Inner shell excitation of matrixisolated silver J. Chem. Phys. 78, 1758 (1983); 10.1063/1.444970 ESR spectra of matrix isolated potassium atom clusters J. Chem. Phys. 74, 959 (1981); 10.1063/1.441152 NbO molecule: ESR and optical spectra in inert matrices at 4 °K; establishment of its ground electronicstate as 4Σ J. Chem. Phys. 61, 970 (1974); 10.1063/1.1682044 Matrix reactions of Na, K, Rb, and Cs atoms with N2O: Infrared spectra and geometries of K2O, Rb2O, andCs2O J. Chem. Phys. 58, 713 (1973); 10.1063/1.1679258

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Letters to the Editor

T ABLE I. Measured E - V (kl) and quenching constants for Br++HX (ko+kl) and Br*+Br2 (k 2) in cm3 mole-t. sec-l.

(ko + k 1) X 10-12 kl X 10-12 k2 X 10-12

HCI HBr HCI HBr Br2

5.2 ± 0.4 O. 84± 0.11 +0.61l

5.0_ 2. 5 0.43± 0.22& 0.48 ± 0.05

0.66b 11b

8fl 1 cannot exceed (ko + k l ). The experimental uncertainty is ± 25%. 20% of the remaining estimated uncertainty is allowed for the calculated Br* lifetime (Ref. 15). ~eference 16.

rate constant (k 1) for the E - V process is calculated with Eq. (1) from fluorescence intensities of [Br*]t and [HX(v = 1)]t at the t = 0 intercept of the decaying expo­nentials. The intensity data must be multiplied by ratios of emission lifetimes, 13-15 filter and cell transmissions, and detectivities, requiring the somewhat larger esti­mated uncertainty of kl presented in Table I. Self-ab­sorption of fluorescence is considered negligible at the experimental pressures used, as is the weak absorption of Br* emission by water vapor and CO2 in air.

It is important to note that 50% or more of the.overall quenching of Br* by HCl and HBr is E- V transfer. The relative Simplicity of this chemical system, the wide range of XHX studied, and the high signal-to-noise ratio (20-100) allow us to be confident of these results. Fur­ther experiments in progress together with theoretical calculations may allow greater insight into the funda­mental nature of E - V transfer mechanisms.

We are grateful to Professor C. Bradley Moore for his enthusiastic encouragement of this work and to the NSF

315

and US Army Research Office-Durham for support. One of us, S.R.L., wishes to thank NSF for a graduate fel­lowship.

*Present address: Optical Physics Laboratory, Air Force Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Massachusetts 01730.

lJ. C. Polanyi, Appl. Opt. 10,1717 (1971). 2D. H. Maylotte, J. C. Polanyi, and K. B. Woodall, J. Chern.

Phys. 57, 1547 (1972), and succeeding papers. 3N. C. Craig and C. B. Moore, J. Phys. Chern. 75, 1622

(1971). 4B. A. Ridley and I. W. M. Smith, Chern. Phys. Lett. 9, 457

(1971). 5A. B. Callear and J. D. Lambert, Comprehensive Chemical

Kinetics, edited by C. H. Bamford and C. F. H. Tipper (Elsevier, New York, 1969), Vol. 3, Chap. 4.

sD. Husain and R. J. Donovan, Adv. Photochem. 8, 1 (1971). 7H. Heydtmann, J. C. Polanyi, and R. T. Taguchi, App!. opt.

10, 1755 (1971), and references therein. By. Fushiki and S. Tsuchiya (unpublished). 9R . J. Donovall, D. Husain, and C. D. Stevenson, Trans. Fara­

day Soc. 66, 2148 (1970). lOS. E. Harris and R. W. Wallace, Laser Focus 6 (11), 42

(1970). 11K. R. Wilson, Excited State Chemistry, edited by J. N.

Pitts, Jr. (Gordon and Breach, New York, 1970), p. 33. 12S. R. Leone and C. B. Moore, Chern. Phys. Lett. 19, 340

(1973). 13R . A. Toth, R. H. Hunt, and E. K. Plyler, J. Mol. Spectry.

35, 110 (1970). 14L . A. Gribovand V. N. Smirnov, Sov. Phys. Vsp. 4, 910

(1962). 15R. H. Garstang, J. Res. Nat!. Bur. Std. (V. S.) A 68, 61

(1964). ISR. J. Donovan and D. Husain, Trans. Faraday Soc. 62, 2643

(1966).

ESR spectra of matrix isolated CsO and RbO molecules: 2 k ground states and inner shell bonding·

D. M. Lindsay and D. R. Herschbach Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138

Alvin L. Kwiram Department of Chemistry, University of Washington, Seattle, Washington 98195 (Received 2 October 1973)

The electronic ground state of LiO is 2IT and very de­tailed spectral show it can be represented by a crystal field model based on Li+O-, in accord with ab initio cal­cUlations. 2 Reactive scattering experiments indicate NaO also has a zIT ground state but CsO has a z:E ground state. 3 This conflicts with the simple ionic model, in which the electron donated by the alkali atom enters the 2pa oxygen orbital along the internuclear axiS, leaving a hole in the 2prr orbital and hence giving a zIT state ( .. ·azrrs molecular orbital configuration). Here we re­port ESR spectra for CsO and RbO which confirm the z:E ground state and offer evidence that it results from mix­ing of the filled (n - 1) P alkali orbitals with the 2p oxygen orbitals. This is analogous to the "inner-shell bonding"

in the isoelectronic XeF and KrF radicals. 4,5

The alkali monoxides were formed by reaction of alkali atoms with N20 in matrices of Nz, Kr, and Ar at 4 OK, as in a recent study of the infrared spectra. 6

Figure 1(a) shows the ESR spectrum obtained for Cs + NzO in a N2 matrix.7 The pattern of eight parallel and eight perpendicular lines is characteristic of an axially symmetric molecule containing a single Cs atom. To­gether with the absence of any discernible nitrogen hy­perfine structure, this is evidence for NzO having re­acted to yield CsO. In Kr and Ar matrices the spectra show some "superhyperfine" structure which indicates the Nz mOiety remains near the monoxide molecule. The

The Journal of Chemical Physics, Vol. 60, No.1, 1 January 1974 Copyright © 1974 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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316 Letters to the Ed itor

TABLE 1. Magnetic parameters of CsO and RbO in nitrogen matrices.

Til TJ. (n -1)s spin (n -1)p spin gil gJ. a (gauss) population population

133Csl60 2.001 1.982 +139 +28 -14 2%

87Rbl60 2.001 2.081 +166 +34 -17 1%

infrared spectra likewise indicate the matrix material has some role in the reaction. 6 A similar but weaker ESR spectrum was found for Rb + N20. None was ob­servable for K + N20 under conditions which gave KO in­frared absorption. This is consistent with a 2Il ground state for KO, since random local fields in the matrix will lift the orbital degeneracy and smear out the ESR spectrum. Table I gives parameters obtained by a least squares fit of the CsO and RbO spectra to the spin Ham­iltonian, including hyperfine to second-order. B

For a 2L state gil can differ only slightly from the free electron value (ge = 2.0023). This is observed for CsO and RbO. However, gJ. can show a substantial shift arising from spin-orbit coupling of the ground 2L state ( •• '1T4a) with inverted 2Il; ( .. . 1[3a2) and regular 2Ilr

I

I',cr I"" + 1/2 -1/2 - 3/2 -5/2 -7/2

1200 gauss 1 He

I

o

eV

20

30

40

+7/2

--4

+5/2

VALENCE SCHEME

NaO (2.rr)

2s-/-I+-\

" \ \ \ \ \ \ \ \ \

...\

+3/2

\~-}-2p o \+t-/ No

+1/2

I

Jh A

-1/2 -3/2

INNER SHELL SCHEME

- 5/2

. H

,-It-\ #----5s

/ \ 25-( \

\ )-5s '-It-'

o Cs 2s---#-

F Xe

(a)

-712

(b)

FIG. 1. (a) ESR spectrum of CsO in a nitrogen matrix; He = 3312. 2 G, resonance field of a free electron for a cavity fre­quency of 9.2826 GHz. (b) Molecular orbital scheme for alkali monoxides and noble gas monofluorides.

J. Chern. Phys., Vol. 60, No.1, 1 January 1974

6%

3%

( ... 7T47T) excited states. Although the alkali monoxides are largely ionic, the large spin-orbit coupling parame­ter of the (n -1 )p alkali orbitals9 (for cesium As/>

"" 6500 cm-1 whereas for oxygen >"21>"" 150 cm-1) neces­

sitates use of the g-shift expressions for a covalent molecule. 10 Since RbO has gJ. > ge, the predominant coupling must be to 2Il; and the observed shift implies an approximate upper bound of 4500 cm -1 for the excitation energy to this state. For CsO, where g J. <g e' orbital coefficients10 are necessary to obtain such a bound, al­though again the coupling to 2Il; is probably predominant.

The alkali hyperfine parameters for CsO and RbO are very much larger than for LiO and other ionic alkali salts. 1,7,11 If the unpaired electron is assigned to va­lence shell orbitals (n= 6 for Cs, n= 5 for Rb), an un­reasonably large covalent interaction is required to ac­count for the large hyperfine structure. 12 If the inner shell orbitals (n - 1) are used, however, only a small covalent contribution is required. Table I gives the nominal spin populations derived from the observed isotropiC (a) and anisotropic (T) hyperfine parameters by assuming only the (n - 1) alkali inner shell orbitals mix with the oxygen 2s and 2p orbitals. 13

Figure 1(b) contrasts the molecular orbital scheme for NaO, based on alkali valence orbitalS, with those for CsO and XeF, based on inner shell orbitals. 14 The alkali monoxides are highly ionic molecules, M"O-, in both schemes but the latter gives a 2L ground state ( ••• 7T4a). The change from the valence to the inner shell scheme is governed by the location of the (n -1) p or­bitals of M+, which lie far below the 2p orbitals of 0-for Li or Na but become comparable in energy for Rb or Cs. The same trends occur in many other systems. For example, as noted previously, 3 SCF-MO calcula­tions1S for the series LiF··· RbF show a transition be­tween· .. a2

1[4 (valence scheme) and ... 7T4a2 (inner shell scheme) ground-state configurations. An analogous inversion of the 7Ti and 11: orbitals of the O2 ion may oc­cur in alkali superoxides7 and does occur for O2 in alkali halide crystals16 and can be attributed to inner shell bonding.

We are indebted to Professor W. Klemperer for pointing out the analogybetweenCsO and XeF, to Profes­sor E. R. Davidson for instructive discussions, and to Dr. L. L. Edwards for help with the design and con­struction of the liquid helium Dewar. Support of this work by the National Science Foundation is gratefully acknowledged.

IS. M. Freund, E. Herbst, R. p. Mariella, Jr., and W. Klemperer, J. Chern. Phys. 56, 1467 (1972).

2M. Yoshimine, J. Chern. Phys. 57, 1108 (1972).

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Letters to the Editor

3R. R. Henn and D. R. Herschbach, J. Chern. Phys. 52, 5783 (1970).

4J. R. Morton and W. E. Falconer, J. Chern. Phys. 39, 427 (1963). W. E. Falconer, J. R. Morton, and A. G. Streng, ibid. 41, 902 (1964); R. S. Eachus and M. C. R. Symons, J. Chern. Soc. A 1971, 304.

5Recent calculations dispute the existence of a chemically bound XeF species in the gas phase: D. H. Liskow, H. F. Schaef­er, P. S. Bagus and B. Liu (unpublished).

sR. C. Spiker and L. Andrews, J. Chern. Phys. 58, 702, 713 (1973).

7We have also obtained the spectrum assigned to CsO in a nitro­gen-free system, by photolysis of Cs + 02 in an Ar matrix. See D. M. Lindsay, D. R. Herschbach, and A. L. Kwiram, "ESR spectra of matrix isolated alkali superoxides," Chern. Phys. Lett. (to be published).

8See , for example: A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Oxford U. P. , London, 1970), Chap. 3.

9C. E. Moore, "Atomic Energy Levels," Natl. Bur. Std. (U. S.) Circular 467 (1952).

lOSee, for example: W. C. Easley and W. Weltner, J. Chern. Phys. 52, 197 (1970).

317

l1For example NaS02, Na02: F. J. Adrian, E. L. Cochran, and V. A. Bowers, J. Chern, Phys. 59, 56 (1973).

12If the dipolar hyperfine interaction is assumed to come from spin density in an alkali np orbital, a> 90% population is ob­tained using values of (1/r3) from R. G. Barnes and W. V. Smith, Phys. Rev. 93, 95 (1954).

13The alkali (n -l)p spin density was estimated from values of (1/r3) given by J. A. McMillan and T. Halpern, Argonne National Laboratory Report ANL-7784, 1971. The alkali (n -l)s spin density was derived neglecting other contributions to the isotropiC hyperfine such as hybridization of the (n -UP orbitals with the ns orbitals and/or core polarization. If in­stead only the ns hybridization is considered, the correspond­ingisotropic spin density becomes 13% forRbO and 17% forCsO.

14The positions of the atomic energy levels were obtained from Ref. 8 and from J. C. Slater, Phys. Rev. 98, 1039 (1955). The positions of the molecular orbitals are only qualitative.

15 A. D. McLean and M. Yoshimine, Tables of Linear Molecular Wave Functions (IBM Research Laboratory, San Jose, Cali­fornia, 1967).

ISH. R. Zeller and W. Kanzig, Helv. Phys. Acta 40, 845 (1967); R. T. Shuey and H. R. Zeller, Helv. Phys. Acta 40, 873 (1967).

Two-photon excitation spectrum of benzene in the gas phase and the crystal'"

Robin M. Hochstrasser, J. E. Wessel, and H. N. Sung

Department of Chemistry and Laboratory for Research on Structure of Matter, The University of Pennsylvania, Philadelphia, Pennsylvania 19174 (Received 5 September 1973)

By means of the tunable dye laser excitation technique described previously1 we have made observations of the two-photon excitation spectrum of the benzene 1B2u state in the vapor and of CsH6(CsDs) crystals. The method in­volves focussing a tunable dye laser into the sample and observing the variations of benzene or excitation accep­tor luminescence during a continuous scan of the dye laser wavelength.

In the crystal at 2 oK we observe four prominent lines that originate progressions of a 923 cm-1 (881) mode that we assume is the well known2 a,.(l). We observe four other much weaker lines that appear to be fundamentals, bringing the total observed to eight out of a possible ten ungerade modes in the 1B2u state of benzene. In these experiments we scanned the absorption in the region - 37800 to -41300 cm -1 using photons in the range -18900 to - 20 650 cm-1. USing the benzene crystal ori-gin at 37838 cm-1 described by Broude3 and by Colson4

and fixing the C6D6 crystal origin at 38041 em -1, we re­port prominent 1B211 state fundamentals at 177, 740 (605), 922 (757), and 1545 cm-1 (1537). The 177 and 1545 cm-1

origins are doublets (~= - 6 em -1) providing a strong in­dication that they are ell modes, split by the crystal field. The strongest lines in the spectrum are the first three members of the a .. (1) progressions built on 1545 cm-1, which we tentatively ascribe to the mode elu(19), that is, 1482 cm-l (1333) in the ground state. The re­maining modes can be assigned by polarization studies.

The fact that e1u(19) is the strongest vibronic origin in the spectrum is consistent with detailed analyses 5 ,6 that assume the 7T-electrdn transitions dominate the two-pho­ton absorption cross section. The vibronic interaction of 1 E111 with 1 E 2, is responsible for the appearance of elu (19), and such coupling is predicted to be large from calculations7 of the Herzberg-Teller integrals.

For the first time, we have obtained the two-photon spectrum of a gas: at benzene pressures as low as 4 torr we have observed two-photon excitation spectra of a number of bands. The strongest bands are degraded to the red with high frequency peaks at 39647, 40573, 41493 and 42415 cm-l . We interpret these bands as members of a 925 cm-l (lIl) progression associated with a fundamental at 1560 ± 5 em -1: this mode would then correspond to the 1545 cm-l interval observed in the crystal. At 87 torr the highest energy line observed at 0+4329 cm-l (19a 1~) appears about ten times weaker than expected on the basis of a normal (6~ 18) Franck-Condon pattern and we attribute this to a changed quantum yield of fluorescence following excitation to this higher level. Between 87 and 4 torr the ratio of fluorescence observed following excitation of 19~ 1a to that for 19~ 1~ decreases by about five, signalling vibrational relaxation. The strong band at 0 + 3407 cm -1 (19& 1~) is already at higher energy than the well-known fluorescence cut-Off, 8 or channel three, 9 region. This may mean that during the electronic relaxation process the symmetry center is

The Journal of Chemical Physics, Vol. 60, No.1, 1 January 1974 Copyright © 1974 American Institute of PhysiCS This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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