Raman spectra of negative molecular ions doped in alkali halide crystals

JOURNAL

OF MOLECULAR

Raman

SPECTROSCOPY

Spectra

32, 13-23 (1969)

of Negative Molecular Alkali Halide Crystals

Ions

Doped

in

HOLZER,~ W. F. MURPHY, AND H. *J. BERNSTEIN Division

of Pure Chemistry, National Research Council of Canada, Ottawa, Canada

Raman bands arising from S2- and 83 have been observed in alkali halide single crystals which had been heated in t’he presence of sulphur vapor. The S3 ion could also be detected in the mineral ultramarine. Since the excitation frequency was sometimes close to the absorption band of t,hese species, a resonance Raman effect was observed. This u-as strongest for the Sz- species in KC1 where the fifth overtone of the symmetrical stretching mode vibrational could be seen. In similarly treat.ed crystals, the vibrational Raman bands of Se, and SeSwere observed. Raman data for Ni_ in potassium halide single crystals are also reported. INTRODUCTION

In a previous paper (1) we reported on Raman spectra of 02- ions doped at low concentration in alkali halide single crystals. This study has now been extended to other negative ions. There are no data reported in the literature, on the Raman spectra of the species studied here. They have been observed, however, in alkali halide crystals by means of electron spin resonance and fluorescence spectroscopy. The fluorescent emission (2) spectra of Sz-, SSe-, and Sez-, were observed at 4.YK; there is also an earlier report on S, fluorescence (3) at 77% The Sz- ion has been studied by EPR (d), Sa- and Ses- and mixed triatomic species by EPR (5) and ENDOR (6). EPR signals of nitrogen centers (7) in KCl, BBr, ICI and KaCl below 30°K have been observed and assigned to N2-. This has been recently confirmed by low-temperature (3-3001<) EPR experiments (8) . EXPERIMENTAL

The Raman spectra were excited with an argon ion laser (Spectra Physics Model 140) operating at 4880 A with 50%300 mW power, and observed using the Spex 1400 double monochromator, an EM1 6256s photomultiplier and photon counting detection. 1 Issued as NRCC No. 10890. 2 National

Research

Council

of Canada

Postdoctoral 13

Fellow

196749.

HOLZEII,

14

MURPHY,

AN11 BEWSTEIX

The laser beam was focussed into the crystals with a j’ = 3S-mm lens. 911 spectra were obtained at room temperature with crystals from 5 X 5 X 8 mm to 2 X 2 X 4 mm in size. The argon plasma lines and the plasma lines of a helium-neon laser were used as wavelength standards, so that the frequencies could be determined to fl cm-’ accuracy. The depolarization ratios ps , of the Raman bands of the doped species were measured by analyzing the light scattered perpendicular to the plane of polnrization of the incident beam. Infrared spectra (obtained with a Perkin-Elmer 521 Spectrometer) and visible absorption spectra (obtained with a Cnry Spectrometer 11 M 1 were also recorded for some of the crystals. The sulfur and selenium crystals were prepared by heating the crystals in the presence of sulfur and selenium vapor (4) and supplied by Dr. J. Morton. In the case of N2- (also prepared by J. Morton) the crystals were doped with nitrate, electrons were then injected, and finally the crystals were photolyzed with uv light, ( 7, 8‘). RESULTS

AND

DISCUSSION

S,- and S3-. Raman spectra observed with the sulfur doped crystals support previous identification of a diatomic species Ss- (Refs. 2, 4) and a bent triatomic species SI- (Refs. ,5, 6). The Raman spectrum observed in sulfur doped RI is shown in Fig. 1. The intensities of these bands are quite high due to a Resonance

sz-

s3-

543cm-' 594cm-I

I

KI:

TIME 45 II

1

Se-

, S,-

SLITS

k

5 cm-’ CONST.

2 SW

%/ml” 4880

%

FREWENC; FIG.

1. The Raman

bands of Hz- and Sp- in a single crpstal

of RI

RAMAN

SPECTRA

OF MOLECULAR

IONS

15

ABSClRPlJON (arbitrary

I

3000

units)

I

I

4000

5000

Wavelength

/

6000

1

7000

[I]

FIG. 2. The visible tbsorption spectrum of KI doped with sulfur ions. The position the excitation at 4880 A and 5145 A is indicated.

of

Raman effect. The visible absorption spectrvm of this crystal in Fig. 2 shows that the exciting line with wavelength of 4880 A lies in the wings of the absorption bands of both species (see below). With 5145 8 excitation, which is closer to the Sa- absorption band, the Sa- Raman band intensity increases, while that for the S2- band decreases considerably. The existence of two species has been confirmed by irradiating the RI crystal with ultraviolet radiation. After irradiation, only the Raman band at 594 cm-’ can be observed, along with the absorption band having a maximum at 4000 8. For this species, no infrared absorption band can be found indicating that this species is symmetrical and diatomic. EPR examination of this crystal confirms that the species which remains is that previously identified (4) as Sz-. The measured Raman frequencies for Ss- in various crystals are included in Table I. The species which disappears on irradiation has a Raman band at 546 cm-‘, an infrared absorption band at 585 cm-‘, and the broad visible absorption band centered at 6100 B. The noncoincident infrared and Raman frequencies indicate that this species has more than two atoms. The difference in the vibrational frequencies of the isotopic species of Szcan be predicted. The relative shift of the (32S34S)- band from the 32S2-band should be -1.56%. Figure 3 shows a spectrum of the 546 cm-’ band in NaCl obtained at higher resolution. The shoulder due to the isotopic component is f 0.1) %, which is further evidence that this band shifted 5.6 cm-l, i.e. (-1.0 is not due to a diatomic species. This also suggests t,hat the band is not due to a linear triatomic species, since the same isotopic shift as in the diatomic case

HOLZER,

16

MURPHY,

AND

TABLE

NaCl NaBr

BERNSTEIX

I

1144 1131

NaI KC1 KBr KI

1145 1135 1123

RbCl RbBr RbI

1141 1132

B All frequencies are from Ref. 1. b SeT7 isotope.

1836 f 1821 f

3 3

612 *

1870 f

2

594

2 325

4G4

611 598

are accurate

to &l cm-l

unless otherwise

NaCI-

indicat,ed.

The data for Ot-

S;

SLITS

I9cm-’

TIME

CONST.

10%~

FREOUENCY

FIG. 3. The S3 Raman

band in NaCl

should then be observed. Because of the large half width of the S2- bands (S cm-’ m ’ RI, RbI, and NaI) , the isotopic shift cannot be resolved in the diatomic case. A careful search in HI and KaCl over the region 20&600 cm-’ for a Raman band due to the Ss- bending vibration was unsuccessful. The frequency and widths at half height of the totally symmetric stretching vibration of S3- in

RAMAN

SPECTRA

OF MOLECULAR

TABLE RAMAN

AND I.R.

FREQUESCIW

(cm-‘)

Ramana N&l XaBr KC1 KBr KI RpCl RpBr RbI x Error b Error

II of Ss- IN ALIMLI

Halfwidth

cm-1

H.LLIDL I.R.”

3.5 3.5 3.8

531 523 527 523 543 528 555 544

17

IONS

540 58.5 585

8.5

CRYST.\LS Halfwidth

cm-’

-10

-5 -8 8.5

fl cm-l =t2 cm-‘. 3’d overtone

4 x 528cm-’ u IOcm-’

2”d overtone

A

1” overtone

2 x 530cm-’ ++ IOcm-t

m

NaCL: S;

fundamental

531 cm-’

IOcm-

SLITS

7.5 cm-’

TIME CONST. 4 set

ahIn x 4880% 4.5

_;

J \_.FIG. 4. R.esonance

Raman

effect

of 63

in NaCl

the various crystals is given in Table II, along with infrared data for the antisymmetric stretching vibration. The strong overtone spectrum of Sa- due to a Resonance Raman effect is shown for NaCl:Sg- in Fig. 4. All bands are recorded using the same instrumental

IS

IIOLZER,

MURPIIY,

AND BEltNSTEIN

TABLE

III

KMAN SCATTERING CROSS SECTIOXS (BRSC) FOR PLANE POLARIZED INCIDENT LIGHT AT 4880 d OF THE FUNDAMENT.J.L AND OVERTOXES OF &- IX ITaCl

I~IS.~TIVIS

Band [cm-‘]

RRSC

531 2 x 530 3 x 529 4 x 528

1.00 O.lli 0.23 -0.12

546cm-’

Ultramarine

S;

SLITS

5cm-’

TIME

CONST

I set

11.5 ii/min

Frequency

FIG. 5. The Rsman

band due to Y, in Ultramarine

conditions, except for the chart speed, as indicated. The integrated areas of t,he bands, corrected for the spectral sensitivity of the spectrometer, give the relative Raman scattering cross sections listed in Table III. In these measurements, the focussed laser beam was kept as close as possible to the crystal surface in order to avoid absorption of the scattered radiat’ion. ENDOR studies (6) indicate that the Ss- ion in KC1 and K&l is bent and occupies a trivacancy (one cation, two anions) in the crystal lattice. In this position the ion has a Lvell-defined geometry with respect to the host lattice. Indeed, as expected, the Raman band width in KC1 and NaCl is quite narrow\-! This is also true when IiBr, NnBr, and RbCl is 3.S a,nd 3.5 cm-‘, respectively. the host lattice, as shown in Table II. The Sa- band in t.he crystals having ;t larger lattice constant (e.g. ICI, RbBr, and Rbl) is quite broad, about 8 cm-‘, and the frequency is shifted by about 20 cm-‘, indicating that the vibration is more strongly affected by the crystal. This suggests that in the 1:ltter cases the S,- ion occupies only one halogen vacancy. The S,- ions also occupy a single halogen vacancy, as shown using ESR (4‘1. The S2- band could not be found in chloride crystals due to the small size of the anion vacancy, although a weak R.aman band observed at 6’22 cm-’ in Kc’1 mn>’

RAMAN

SPECTRA

OF MOLECULAR

IOSS

20

HOLZER,

MURPHY,

AND

BERNSTEIN

182lmr KBr: SLITS TIME

N,7.5 cm-’ CONST.

IO set

2ii/min 5 cd

FIG. 6. The Raman

x

488Og

band of N?

in KBr

be due to S2-. Also, there is structure in the SC band observed in ICI and RbI, for which we have no explanation. These results for St- and S8- in alkali halide crystals may be used to identify the sulfur species in the mineral ultramarine. EPR results (9) indicate that ultramarine contains Sa-. This is confirmed by our Raman spectrum of a pellet of polycrystalline ultramarine, which is shown in Fig. 5. This band could not be found in sodalite, which has a similar structure, but contains no sulfur. A Resonance Ramnn effect is observed for ultramarine, with the fundamental at ,546 + 1 cm-l, and overtones at 2 X 547, 3 X 54s and 4 X 549 cm-‘. This is similar to the spectrum of Sa- in ICI and RbI. The origin of the small shoulder at about .?S;? cm -’ (see Fig. 5) has not been determined. SK and Sez-. In the ICI crystals heat’ed in the presence of sulfur and selenium vapor, bands at 464 and 325 cm-’ are obtained in addition to the Sp- band at 594 cm-l (Table I). The regular spacing suggests an assignment of these bands The 464 cm-’ band was also found in infrared to SSe- and Se--, respectively. absorption as expected for a heteroatomic molecule. In ICI heated in the presence of selenium vapor, a Raman band at about 280 cm-’ was also observed, but it vanished rapidly when the crystal was irradiuted n-ibh the laser. This band may be due to Sen-. The ions Se?- and SeS- also exhibit a Resonance Rnman effect. The frequencies of their overtones, along with those for Ss- and Sa- are collected in Table IV. N%-. The frequencies of the Raman bands found in nitrogen doped crystals (Icig. 6) are shown in Table I. In the same crystals, the EPR spectrum W:IB observed at low- temperature (8) and was assigned to x2-. From t,he approximately constant ratio of the frequencies vo,-,iuo2 , vsi- vs, , :md ~~<.~-i..‘v~e~ , one may estimate :L vibrational frequency for Ns-. Here the frcquencp of the negative ion is that observed in KI. The frequencies of the neutral molecules have been taken from the literature ( IO). The predicted frequency for S,- in Ii1 of lSl0 f 140 cm-’ is consistent with bhc observed value of 1807 cm-’ (Table I). Frequencies of the Isolated Iom. A detailed theoretical study (11‘) of the dependence of the infrared vibration frequency of CN- on the repulsion potent,ial

RAMAN

SPECTRA

OF MOLECULAR

21

IONS

cm-’ t’s,-

540 L2030

I

I

2050

2070 cm-’

‘CNFIG. 7. The Raman crystals.

frequency

of SZ- vs. the infrared

TABLE DEPOLARIZATION RATIOS

pa ,

OF THE

IN ALULI

02-

Ny

I 2090

frequency

of CN-

V

RAMAN LINES OF NEGATIVE 10~6 DOPED

HALIDE

CRYSTALS

S-

Sez-

NaCl NaI KC1 KBr KI RbCl RbBr RbI

in the same

&0.60 0.65 0.55

4.04


4.40 0.45

0.30

0.60

4.04 4.35 0.45

-0.5 0.70

in alkali halide crystals has been carried out which predicted a frequency of 2038

cm-’ for the isolated CN- ion. Using these data, we could derive by a graphical method the frequency of the 02- isolated ion to be about 1090 cm-’ (1). Under the assumption that the Sz- ion experiences similar repulsion in the crystal host,

HOLZER,

22

AND

BERNSTEIK

VI

COMPARISON OF FLUORESCENCE AND Frequency

RAM)\N

[cm-l]

I)ATA

OF IONS

Anharmonicit>

DOPED

IN

Ii1

[cm-‘]

-siSes-

595.7% 327.8%

594 f 1” 325 ztz lh

SSe-

462.3”

464 dz lb

2.58 0.75% 1 .I?

* Fluorescence. See Ref. 2; temperature 4.2OK b Raman. This work; room temperature. (The anharmonicities cryst,aI data).

3.4 It 0.1) 0.75 zk 0.“5’,

2.0 * 0.25’,

are averaged

over all

the same method can be applied to S,. In l?ig. 7, vs; is plotted versus vcN- for the same alkali halide crystals. The intersection point gives the frequency of the isolated SZ ion to be about 550 cm-l. In a similar way the frequency of the isolated N,- ion was derived to be about 1760 cm-‘. This result can be independentlyaverified using Clark’s rule (f2). If one uses an interatomic distance of 1.18 A (derived from the isoelectronic sequence Nz-, NO (1.15), and O?+ (l.la)), a vibrational frequency of 1730 f 50 cm-’ is obtained. Since Sez- and SeS- were observed only in the iodide crystals, the graphical method cannot be applied. One would expect, however, that the relative difference between the frequency vKI observed in the KI crystal and that of the isolated ion v0 would be even larger for Sep- and SeS, since these large ions experience a greater lattice repulsion. If one correlates the differences Ye, - y. with yliI for the ions CN-, iY2-, O,- and S-, an estimate of about 275 and a.bout 415 cm-’ for the frequencies of the isolated Se2- and SeS molecules is obtained. SUMM,4RY

A summary of the observed Raman frequency data, discussed in this paper and our previous one (1)) is given in Table I. The observed depolarization ratios are presented in Table V. The depolarization ratios of the first overtones of Stin KI and S3- in KI and KC1 were also measured to be close to that listed for the corresponding fundamentals. In Table VI our results for Sz-, Se2-, and SSe-, in 13 a.re compared with the fluorescence data (2). Deviations in the frequencies and anharmonicities could be due to the difference in sample temperature for the two experiments. ACKNOWLEDGMENTS The authors the ultramarine RECEIVED:

thank Dr. J. R. Morton sample.

January

23, 1969

for providing

the doped alkali halidr crystals

and

RAMAN

SPECTRA

OF MOLECULAR

23

IONS

1:EFERENCES 1 W. HOLZIX, W. F.

P. 3.

4. 5. 0. 7. 8. 9. 10.

MURFHY,

Ii.

J. BI,:RNST~;IN.\xD J.

ROLFIS,

J. Mol.

Spectry,

26, 534

(1968). J. RoL~J$, J. ChewI. ~‘l~!y.s.49, 4193 (1968). J. H. SCHULMAN AND R. D. BI~I;, “Solid State Communications,” 2, 105 (1964) Pergaman, New York. I,. II:. V.\N~TTI AND J. I:. MOILTOS, Phys. IZev. 161, 282 (1967). L. SCIINEIDIGR, B. DISHLISR, ASI) A. J:.LuBP:R, Phya. Slat. Sol. 13, 141 (1966). J. Sun-a~ssr .2ND H. SEIDEL, Phjgs. Sfat. 801. 13, 159 (1966). A. H~USM.\KX, R. HIIXH, AND W. SAXDLR, Z. Ph~silc 179,4Gl (196-L). J. R. BRIILSFORD, J. It. MORTON, AKD L. E. V1k~o~r~, J. Chew Phgs. 60, 1051 (1969). J. IL. MORTON, 15th Colloque A.M.P.E.R.E. Grenoble, September 1968. Princeton, of niatonlic Molecules, ” 2nd ed. Van Nostrand, G. HERZBERG, “Spectra

N. J., 1950. 11. CT. R. FIELD AND W. F. SHERUN, J. Chem. 1%. C. H. D. CIARK, Phgs. Rev. 47, 238 (1935).

Phys.

47, 2378 (1967).