Triplet excitation of water and methanol by low-energy electron-impact spectroscopy

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Cti@ICAL.

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iFebruw.1972 -:

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EJiCITATION OF WATER ,AND METHANOL BY LOW-ENERGY ELECTRON-IMPACT $J?C;rROSCOPY

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F.W.i. KNOtiP, H.H. BRONGEkSMA$ . FOh~Jnstitute fat A torn@ and Molecule Physics, Amsterdam, The Netherlands

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and ,;,

.. r;.J. OOSTERHOFF Depvtmetlt

of Theoretical Organic Chemistry.

University of Leideir, The Netherlands

Received 10 December 1971

; -. Electronic excitation by lowenergy electron impact has been studied For water and for methanoL In the case of W.&r a, triplet excitation process was observed at 7.2 eV energy loss while for methanol probabIy a similar process : .&asftiund at 6.5 eV. The weak and broad absorption peaking at 4.4-4.6 eV in the water spectra seems to have an analogue ii methanol observed at an energy loss of 4.4 eV_

The interaction of low-energy electrons and single mo!ecules is & important source of information for the excited states of these molecuies. While for highenergy electron impact the selection rules are very similar to those for.optical excitation, practically all electronic transitions, including spin change; are allowed when low-eneru electrons are used. By means of the Trapped Electron (T.E.) method [l, 21 thre.shold excitation spectra of atoms or molecules can be ob.tairied. With this technique a potential well depth (w) is’ereated which enables one, to coliect only those scattered electrons with a residual energy ‘M of eWor less. For reasons of resolution this method is ojy applicable for small well depths. Recently B new technique has been developed, the Dou@ Retarding Potentia! Difference (DrR.P.D.) methdd:[3] ; which allows one to study excitation ‘proce&frbm,zero:up ‘to more &an 10 eV above the excitaiion ,$ehold v& reasonable energy resolution. lYh~~infop+tion is’im+ortant for the classification of

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CHEMICAL PHYSICS LETTERS ‘,

of helium .+d the gas under investigation using as .. .. deference the 2% excitation value of helium (19.82 ev). In the case of kat$r vapour an unaFbiguous.de-

H,O ctE.ZeV

, h,

2

‘tkniination &the triplet states is especially important as a large discrepancy ,between the theoretically. tire- : -. dieted and expkimentdly de&mined excitation energieS for the&z stat& has been found. Moreover this information is iinportant. for the radiation che_mistry of aqueous solutions. With water (figs:. i, 2), the dominant transition is : observed at 7.2 eV. Th% agrees with the 7.3:eV value -.

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6

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8

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found by Schulz [I] ,using the T.E. method. However,‘ it is known from higher-energy electron-impact studies [4, S] and photC@mpact work [6] that the maxiqum of the lowest singlet state (‘El) lies in the range 7.44-7.49 eV (table 1). it is unlikely that the difference between these values and otir value of 7.2 eV is due to an error in energy calibration. The identification of the 7.2 eV transition as a triplet state (3A2 or 3B,) seems more reasonable. Up to now it has been’ assumed that the ‘B, state (7.5 eV) will be the lowest excited singlet level. This view is supported by experimental data 14-61 and by theoretical’studies [7,‘8]. For these reasons any excited state lying below this level should be a triplet state. Additional support for the assignment was obtained from the energy dependence of the total cross section studied with the D.R.P.D. method. A plot of this de-

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12

10

Eiectron energy loss E CM L I 400 300

I 200

i 100

I 150

Fig. 2. Excitation spectrum of Hz0 measured with the

D.R.P.D. method at an excess energy pf the electrons of 2 eV. triplet, dipole or quadrupole transition. in the D.R.D.P. spectra the excess energy AE of the electrons after scattering is directly related with the applied well depth ir/ (AE = ew). Caiibration js effected by introduction of a mixture

Table 1 Water vapour (energies in eV) Electron

impact

Photon@ Other work

This reyaxha)

Assignment

impact

see text

4.5 b)

4.4-4.6

~6.2 C)

‘AZ(?)

H-

6.5

6.sd)

7.2

7.3 d)

peak (see ref. [ 11) triplet state 3BI 0r’~A2

7.44 c)

7.5 b)

7.49

8.8

‘AZ(?) or triplet. Hate

9.2d)

9.1

9.7oc)

9.7,

9.67b).

.lO.O

10.1 d)

1o.00 b,c)

lo.17’d

10.14~)

‘..,.

9.81b)

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*B1 or triplet state’ .,

-10.15

.. b) ref. [4] ,; c) yef.. [5] ;‘&) ref_ [I]

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.,

‘Al or triplei state

9.75 --9.99

.-

aj ColIected dab from TiE. and.D.RP.D..me~:ements;

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‘B*

H-peak (ref. [ 11)

8.8 d)

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CHEMICAL’PHYSICS

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iETTERS

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Electron energy .,I

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6 Electron energy

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spectrum of methanol measured with the T.E. method at A&= 0.05 eV.

‘pendence‘&ows a maximum at 2 eV above threshold, ‘.foIlowed by a continuous decre& with increasing ‘en&y ‘Thisbehavfour is characteristic for a triplet excitation p@z+ (See for instance Aarts et al. [9] and Smit .et al. [IO] .) The rather rapid decrease

could

.exp!&why Trajmar et al. [4] did not observe this process ai an incident eriergy of 20 eV. Due to the large n&t.+ width, Iok resolution of our apparatus an< apparent1y.a relatively we* intensity oi‘the “,l@vest singlet transition, no peak 0; shpulder could . be observed at 7.5 eV.‘This_is not surprising, because ‘it js,known @iat optically allowed singlet-singlet transitions somciimes appear very weakly in thre&,old ‘. .excitatipn splct+.(see ref. [I I J); ;,A wek’and broa_d peak at_ 4.4-4.6 eV,is clearly ob$ed in ou< spectra (fig; 2). This absorption-cannot ,.b$ dtie to &e fo,rmation of negative ions; because such :. contributidr& aie elimiSted in the D.R.P.D. method.

I 600

I 333

I 200

I 150

Fig:& Excitation spectrum of methanol meaked with the D.R.P.D. method at AE =.2 eV. The dot+ line presents a part of the D.R.P.D. spectrum measured at an excess energy of 4 s-1. A maximum can be observed peaking at -4.4 eV.

The methanol spectra (figs. 3,4 and table 2) disgreat similarity to thoSe df water. The first op tica! singlet state is known to peak at 6.75 ey [ 12, 131. In our spectra, an inten&transition is observed at 6.5 eV. Following the analogy with water, a’triplet assignment is probable_ In contrast to water no H- ions’could be.detected using the TX. techniqu& This could easily be established since no positive signal w& found applying a negative well de+. : For excess energies _up to 3 eV a weak and broad absorption is observed in the 3.0-6.0 eV region .’ showing a si&ar behaviour as found in .the water spectra. At AE= 4 eV the miximum of the peak can be found at ti.4 et, possibly b&g the analogue of the play a

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a) Collected da& fiomT.E. b) Ref. [13];c)ref. 1121. absorptidn

and D.R.P.D. .’

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J. Kistemakei The discussions with Dr; J. Hag&ge, Mr. M-C. van Hemert and Mrs. J.A. v.d. of Professor

ure to

Koek

were

highly

appreciated.

mention the effective assistance of Mr. Tj. v.d.

+ From our exlCmenk the absorption at a4.5 eV is not tikety to be due to contamjn~tion effects, though the= cannot be excluded totally. In the spectra of the deuterakd compounds of water and methanol identicat absorptions xe observed..

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[9] J.F.M. Aarts and.F.j.de Heer, Chem. Whys. Letters 4 (1969j116. -[IO) C&nit, H.G,M.Heideman and J.A.Smit, Phy&a 29 (1963) 245. [ I l] H.H. Brongersma and L..J.‘O&er&off? Chem. ,Phyr : Lette’ri l(L967) 169: [ 121 A.J.Harriion, ‘d.J.Ce@rholti and M.A.Te%liiger, j. : Chem. Phys. 30 (1959) 355. [13] J.HagGge,P.C.i&berge and C.Vermeif. Ber. Bunseng& Physik. C&em. 72 11968) 138. ..

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3. Chem. Phys. 33 (1960) 1661.

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1141 J.R:EIendersan and M.hb.uam‘ot?. 1. Chem. Phys. 43 ._ ,,’ ,’: (1,965) 1215. ‘r ‘.

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[l] G.J.Schulz,

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5461971) 3799.

sponsored by F.O.M. with financial support by Z-W-0. is

Ref&&s

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753. [73 W.J.Hqnt and W.A.Godd& III, Chcm. Phys; Letters 3, : (1969)414. .’ _[8] C.R.Claydori, G,A.Sega! Ud H.S.Taylor, J. Chem:Phys;

It &‘a pleas-

Hauw (eelectronics). This work

‘, .. ..

Phys. 49 (1968) 5042.. [61 K.Watanabe and M.Zelikoff, J. Opt. Soc..Am. 43 (19%)

is gratefully acknowledged; f&t-v.d.

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,, CIteA. Phys. Letters S (1970) 450. (41 S.Trajmar, ~V.WUiams and A.Kupperm$nn, .I. Chem. ,‘.. :-,Phys. Sb (l97.1) 2274. [S] ASiierbele; M_.A.DiUon and EXLassettre. J. Chemi ‘,

of the same symmetry (A,>. interest

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.[2] H.~&ongersma, Thesis, tciden (1968): .: {3j F.W.E.Knoo& H.H.‘Brongersr&and ~_.J.H.Boerboom,

water might consist of a triplet and a singlet transition

The cotiti&ed

,,

trip&t excitations

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eV$ in

at 4.5

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9.0 - 9.6bj

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8.3

Therefore

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