Raman spectrum modification in the presence of solvated electrons. Observation by polarization CARS

Volume

CIIL\lICAL

66. nunrbsr 3

R.&MAN SPECTXUM OBSERVATION

MODlFICAl-lON

BY POLARIZATION

PIIYSICS

LI?I-Il:RS

IN THE PRESENCE

OF SOLVXTED

IS October

1979

ELECTRONS_

CARS

UGns the method of poLw~fmn G\I
in this paper we present d summary of the experiof the soIvated electron (es) on the vibr.itional frequency, intensity ;IIHI depolarization r.itio of Raman active modes of the solvent was observed by ncnlinear poIarization Raman spectroscopy (method of coherent R,mlan ellipsometry or polarization CARS [I] )_ The Raman line, corresponding to the bending vibration of the C-H bond with a frequency v. = 1486 cm-l in Ilex~lnetIlyIphospflortri~tiiide (HB1P.A) was found to shift to higher frequency by 10 cm-l m the presence of the solvated electron. The evaluation of spectroscopic paranleters by numerical fitting of the modified Raman line in the presence of es shows the growth of es intensity, decrease of depoIarization ratio and b.mdwidth of the modified line_ The possibility of utilizing the resuIts for the definition of the microstructure of es is discussed_ Within the past twenty years intensive investigations of es have been made_ There are many experimental results, concerning chemical and physicaIchemical properties of es in different liquids [z] _ Nevertheiess, up to now, direct experimental results on es microstructure are absent_ In principle, Iocalization of es cti be determined by optical methods; the

meets in which for the first tinle the influence

es shouId influence the characteristics of modes of solvent molecules. Just because of this circumstance Raman scattering is of particular interest_ Unique information may be obtained from measurements of the frequencies, intensities, polarization, and Iinewidth of the vibrational and rotational modes influenced by es_ As far as we know, all attempts to register the influence tif e, on spontaneous Raman spectra were unsuccessful. Authors [3] failed to find the influence of the ammoniated electron on spontaneous Raman spectra: the effect was absent in dilute and concentrated solutions with different halogens. i\fegati;e resuits [3] could be attributed to low sensitivity of the methods used. The application of tunable lasers and development of noniinear laser spectroscopy extended the possibiiities of Raman spectroscopy. These new methods are very effective especially in rhe problem of resolving inhomogeneously broadened Raman lines; this problem is very closely connected with the problems of optical detection of es [4] _ The Raman lines which are modified in the presence of es are observed on the background of relatively strong lines of the solvent (in typical conditions the es concentration in HMPA did not exceed 1W4 mol/Q). 479

r-i_ 1. (a>The &per&m curves of the major ellipse axis direction 0 and cltipticity K nmr the Raman Iinc ~0 = 1486 cm” (bending vibration ofC-tt bond) in pure IMPA (1) and in tftc prcrcnco of es (2) as a frequency dispkemcnt au = YL - PZ_ (bl The dispersion ewes * and K near the Ran resonance of fig. In:
cording dud not exceed 30’;_ Tbc concenfr3tion oft, was 5 X 10w5 ,\I. This choice ofconcenrmtion W&S limited b_t absorption ztt fkcqrrcncics wt _cu’? in our espcrimcnrs_ .\leasurernents of1 IMPA R.unsn lines df the frequencies L’*= 1-W cm-* (C-II bond), 1067 cm-* (C-S bond) .Ind 1207 cm -I (P==O bond) in the prescncc sad rrbscncc ofcs wxc pcrformcd. Fig. I dcmonstrata the dispersion trace oft and $ near the C-It bcwd rtbrxtitm resonance. One cm see, the solvated electron leads to the appcxmce of an 3ddition.d 11:.1xiniim al K in ihe Iiig!t-frequcrxy region md an aymmctriwl \hapc of the curve ti = ti(wt - w2)_ It is nccessrtry to remember that the mssimum of the IC spectrurrl shifts with respect to the mdsinttm~ of the spontzmeous firnan Iinc [I ] _ The observed spectrum modifkxtion is LompIeteiy reverslbk. Spectra with salts I and II arc identiwl. Figs. Ia aid lb reproduce spectr.t for s3lt I only_ At the sdme Ime the analogous me.wrrements in the vicinity of the Rsnrsn lines 1207 cmt aid 1067 cm-* do not demonstr.ttc the intluencc at-e, on the line sltapc. The specirti check esperimcnts demonstrated that in the presence of salts I. II and in pure I INPA, wtthin the experimental ;ICcurrtcy, the line shapes of investigated vibrations of I NP.4 remained the same. Thus. one cm conclude that in the ciusters with e, tbc C-AI bending sibration is modified. Paameters of the modified Iine were defined by sokin% the inscrse spectroscopic problem. We utifired A simple fitting method by computer_ CalcuLtted spectroscopic parxneters obt.tined by fitting the theoreticd1 IL and $ dispersion curves to the experimental ones we si~own in table 1 (see tliso kig:. 1bl_

X spccisl analysis ~‘3s carried 0111 to cIarify the resson for &scnce of inform&ion &bout the modified line in the spontaneous spectra (fi_e_2ztf. Fig. 2b shows polarired

and depokized

we c;llculated

spontaneous

for the ptraneters

listed

spec:rrr_

which

in table

I_ One

cm see that changes in the lineshape nex the modified resonance arc sm311. The absence of the tIMPA CREM modifications in the presence of-salts I and II indicates that the observed frequency shift cannot be connected 1%ith the presence of anions in the solvent. although the concentration of anions exceeded e, concentration by more th.m two orders of magnitude m our eupcri~nenr.d condirions. Coincidence of experimental results in soIutions with salts I snd II demonstrates that the effkt is caused bk es_ and cznnot be attributed to (N$. 1,‘) complex, beausc in solution with salt I this com$lex ctlnnot be crcJted [S] . The frequency shift of the C-II vibration in the presence of e, can-

(b)

PO

(cm-‘)

I-

(cm-

P

)

em 3)NR x $ 111

nonperturbed line C-H tnodikd

line

149s

4

0.2

0.35 rt 0.05

Fis_ 2. Pokrued and dcpolsrized spontaneous Ramrtn spectra near vo = 1486 cm -I in the presence of es_ (ri) Experimental daa are obtained by “Ramalog 6”_ The resolution is equal ro 1 Cin-1 _ (5) Calculated curves, which irre piotted wirh parameters extracted from CRE% specta

481.

Refereuces