The resonance Raman spectrum of ultramarine blue

Volume 34, number

CHEMICAL PHYSICS LmERS

1

THE RESONANCE RAMAN SPECTRUM OF ULTRAMARINE R.J.H. CLARK

1 Jllly 1975

BLUE

and M.L. FRWKS

Christopher hgohi

I.aboratorier,

Wniversity

College

London,

London

WCIH

OAf,

UK

Received 24 March 1975

JThe resonance

Raman spectrum

647.1 run excitation. from sulphurdoped

of ultramarine

blue has been observed by the use of 457.9,488.0,514.5,568.2 -ad are observed; comparison of the results with mrlier data that ultrsmarine blue contains the ST .zs well as the S; anion.

With 457.9 nm excitation three progressions alkali halide crystals indiutes

I. Intruduction Under conditions in which a molecule is excited with a laser line whose frequency corresponds with the maximum of an allowed electronic transition, a rigorous resonance Raman (RR) spectrum may be obtained. Such spectra are characterised by an enhancement in the intensity of a totally symmetric fundamental of the scattering molecule and by high-intensity overtone progressions in this fundamental [ I,2 j _ A previous report [3] on the Raman spectra of negative molecular ions doped in alkali halide crystals demonstrated thjs effect, RR spectra being observed for the SF, ST, Se? and SeS- species. Further, a pellet of polycrystalline ultramarine is reported to exhibit an RR spectrum closely similar to that of the ST ion, this observation having ied to the conclusion that the sulphur-containing species in the mineral is indeed the ST ion. However, ESR results 15-71 indicate that ultramarine contains both the SF as well as the ST ions. This different conclusion, together with the sur-

prisingly large sepaiaiion of the laser excitation wavelength used (488.0 nm) from that of the lowest allowed electronic transition of the mineral (6 IO run; 2A1 * 2B,) [6,7] has led us to reinvestigate the Raman spectrum of a sample of ultramarine blue using a variety of laser excitation wavelengths. In this way, RR progressions based on both the ST and ST ions have now been observed.

2. Experimental The Raman spectra Nere recorded by use of 2 Spex 1401 spectrometer in conjunction with Coherent

Radiation Model 52 Arf and Krf lasers, 90” col!ection optics, and two Bausch and Lomb

12CKl linesjmm

was by means of photon counting in conjunction with a cooled RCA C 31034 phototube (Grade 1: linear display). The

gratings blazed at 500 run. Stection

power available For 457.9, 488.0, 514.5, 568.2 and 647.1 nm excitation was ca. 300 m’N, 1.5 W, 1.8 W, 100 m\V and 500 m\V respectively. Spectra were calibrated by reference to the emission lines of neon, which were superimposed directly on the recording. Band areas were determined by the cut-and-weigh procedure and then corrected for the relative spectral response of the instrument. Samples were rotated at ca. 1600 r.p.m. in order to minimize thermal decomposition of the sample at the beam focus [8,9]. The spectra of both artificial and mineral ultramarine have been studied but the differences between these are insignificant.

3. Results and discussion The Raman spectra obtained from ultramarine blue are in a!1 cases characteristic of the RR effect. The

69

Vplume 34, number Table

1’

1

CHEMICAL

PHYSICS LEmERS

1 July 1975

’

Band inlensitirs slsndard

observed

in the resonance

Rnman spectrum

Relative intensity

of ultramarine

blue relative to the V, (a,) band of SOi- as internal

a) --

Band

IOll

457.9 run

488.0

0.41 0.055 0.02 0.015 0.01 0.015 0.01 0.11 0.02 5 0.015 0.01

nm

514.5 nm

568.2 run

647.1 run

0.81 0.24 0.10 0.04 0.015 0.01 0.04 0.02 0.015

0.93 0.32 0.16 0.085 0.02 0.01 0.G5 0.03 0.01

0.95 0.53 0.30 0.22 0.16

1.00 0.56 0.34 0.26 b)

0.11

b)

0.14 0.08 0.05s

0.20 0.13 0.08

0.03

0.025

0.01

0.01

3) Corrected far the spectral response of ihe instrument. Band intensities are given at each exciting wavelength (nm) relative to that of u1 (Sj) at 647.1 nm. NXe that whereas the intensities of VI and its overtones fall with decrease in exciting wavelength, that of U; increases (see text). b) Higher overtones obscured by high fluorescent background.

All of the exciting

lines used, namely

457.9,483.0,

514.5,568.2 and 647.1 nm, lie within the envelope of this electronic band and an overtone progression in p1 (ai) of ST (the totally symmetric stretching vibration occurring at 548.9 + 0.5 cm-l) is observed for some lines as far as 6v, (previously observed, although with lower accuracy, as far as 4vl) [3 ] _The relative

ktensities of this fundamental to its overtones for the different excitation wavelengths are shown in table 1. Taking the 1(2~~)/l(~~) ratio as an example, it is seen that this is highest when the excitation frequency (vo)

-

cm-’

Fig. 1. Resonance Roman spectrum of ultramarine obtained with Ar+457.9 and 488.0 nm excitation; slit widths 250!375/250 pm (5 cm-‘), sunGIg speed 100 cm-l/min.

lowest eleqtronic band of ultramarine is very broad and has a maximum at co. 610 run, as determined by diffuse

reflectance

spectral

messurements

[6]; it is as-

signed [4] as the “A, + ZB, transi;ion of the ST ion. 70

is nearest to the electronic absorption frequency (v,) and lowest when v. is furthest from o,. The overtones (IW~) show a continuous increase in half-band widths and a continuous decrease in both band areas and peak heights as the vibrational quantum number increases, and both these features are characteristic of the RR effect [ 11. There

is also apparent

in the spectrum

(fig.

I) a

second progression in v 1, namely y2 + nvl (where Y,(al) is the bending vibration of SF occurring at 259.0 & 0.5 cm-l) extending as far as n = 3. The observation cf second progressions involving a totally

symmetric fundamental

have been noted in the cases

Volume 34, number

!

Table 2

within the envelope of this electronic band and the spectrum obtained with this exciting line (fig. 1) shows a further progression, namely in nv; up to n = 4, where Vi is thz stretching vibration of the ST’ ion (586.5 f 0.5 cm-l) [14,15]. This progression shows all the characteristic features of the RR effect as described above. In tabie 2 are listed the frequencies and half-band widths observed for all three progressions. When an overtone progression is observed for a particular fundamental it is possible to determine both the harmonic frequency (wl) and anharmonicity constant (-ril). In an anharmonic oscillator the observed wavenumber, v(n), is given by the expression [I 61

Wavenumbers and half-band widths (can-’ ) of the nyL, Ye f “~1 (both Sj), and nui (ST> progressions observed in the resonance Runan spectrum of ultramarine blue IOII

%

Band assignment

a)

0.5

AV112

(cm-’ )

IS+

1

VI + 2U, vz -I-3Vl

1096 1641 2187 2730 3270 807 1355 1899

* f k f ? 2 + +

1 1 3 5 8 1 2 3

30+ 1 5oc 2 64k 3 EO? 5 100 + 20 27+ 1 492 2 76* 4

u; 2Vi 3v; 4LJi

586.5 1168 1747 2320

+ ? 2 f

0.5 2 5 10

2oc 1 40* 2 ask 5 140 f 10

2Vl 3Vl 4Vl 5Vl 6~1 V2 +Vl

S,

Band maximum km-‘)

548.9 I

“I

a) 2LJ;, 3v;,4v;; (see text).

only observed

v(n) = no1

- (&‘2)X11

+ higher order terms .

A plot of u(n)/n versus n should therefore be a straight line, slope xll, intercept w1 - xll; hence both w1 and xl1 may ba determined. In the case of the nvl progression of the ST ion, this analysis leads to w1 and xi1 values of 550.3 c 0.3 and 0.75 -C0.08 cm-1 respectively. Similarly values for o1 and xl1 can be found from the frequencies of the members of the u2 + nvl progression by plotting [(v2fnvl) - v2]/” versus n. The plot in this case leads to o1 andxlz Mlues of 549.9 2 0.3 and 0.75 i 0.25 cm-l respectively (table 3). The xl1 values determined from the two progressions are the same as each orher. However, the o1 value from the nvI progression is slightly (0,4 cm-l) bigger than that from the v2 + lzvl progres-

with 457.9 nm excitation

ion [l 11, the [I23 and the Au&, - ion [13]. As in previous cases the relative intensities of the bands in the ~2 + rrrl progression are less than those in the nvI progression, but the half-band widths of the former * are larger than those of the latter for 2 given value of n. The v2 band also shows an increase in iiltensity as v. approaches v,, as seen by comparison of its intensity with that of the broad band at 225 cm-’ due to the sodium aluminosilicate framework (the idea!ised formula of ultramarine is (NagA16Si6024S4jn). The diffuse reflectance spectrum of ultramarine [6] and the visible absorption spectrum of potassjum iodide doped with sulphur ions [3] both display a broad band at ca. 400 m-n, albeit very weak in the case of ultramarine; it is identified [14] 2s being due to the ST ion. The 457.9 nm exciting line lies just

of the $InO, ion [lo], the Mo$

Mo2C18-

I July 1974

CHEMICAL PHYSICS LETTERS

ion

sion, the difference representing 2 direct, tho&h highly imprecise, measure of the cross term xlz

in the

potential energy expression. It is, however, closely similar in magnitude to similar cross terms deduced from RR spectra of the AuBrl and ,Mo2Cli- ions [12,13]. In the case of the nv; progression

of the ST ion,

the o, and x, w, values obtained are 590.4 k 0.6 and 2.1 2 0.1 cm--l respectively. These values are similar to those found for this ion by resonance fluorescence

Table 3

Spectroscopic constants for the S; and ST ions ir! ultramarine (cm-‘) Ion S;

s;

WI (cm-‘)

x,1 (cm-‘)

550.3 r 0.3

0.75 _L0.08

549.9 * 0.3

0.75 f 0.25

we (cm-‘;

Xewe (cm-‘)

590.4 t 0.6

2.1 = 0.1

71

Volume 34, number

I

CHEMICAL

at 4.2 K (595.7 and 2.5 crt~-~ respectively) Thus as in the case of pl,evious results on fundamentals displaying the RR effect, both v1 and 1~; modes are c!ose to behaving as harmonic oscillators. studies

P3YSICS

LETTERS

1 July 1975

Reference5

[ !5].

4. Conclusion The technique of RR spectroscopy indicates that, in agreement with the ESR results, ultramarine does no! contain only one sulphur-containing anion, but two, the ST as well as (in greater proportion) the ST ions. Accurate values for the spectroscopic constants for these ions are presented. The resonance is most effective, as measured by the ratio I(%, j/I(ul) when the exciting frequency is closest to the maximum of an allowed electronic transition of the scattering species. Thus, of the exciting frequencies used, resonance is closest with the 568.2 and 647.1 nm lines in the case of the ST ion and with the 457.9 nm line in the case of the ST ion.

[I] W. Holzer. W.F. Murphy and H.J. Bernstein, J. Chem. Phys. 52 (1970) 399. [2] L.A. Nafie, P. Stein and W.L. Peticolas, Chem. Phys. Letters 12 (1971) 131. [31 W. Holzer, W.F. Murphy and H.J. Bernstein, J. Mol. Spectry. 32 (1969) 13. [$I J.K. hforton, Colloque Ampere I5 (1968) 299. 151 K.-H. Schwarz nnd U. !jofmann, 2. Anorg. Allg. Chem. 378 (197Oj 152. [61 U. Hofmann, E. Herzcnstiel, E. Schiinemann and K.-H. Schwarz, 2. Anorg. Ally. Chem. 367 (1969) 119. [7I S.D. hicLaughlan and D.J. hfarsball, J. Phys. Chem. 74 (1970) 1359. [81 W. Riefer and ti.J. Bernstein, Appi. Spectry. 2.5 (1971)

501.

PI R.J.H. Clark, Spex Speaker 18 (1973) 1. 1101W. Kiefer and H.J. Bernstein, hlol. Phys. 23 (1972) 835. 1111 A. Rnnade and M. Stockburger, 191 [I31 I141 r151 [16]

Chem. Phys. Letters 22 (1973) 257. R.J.H. Chrk and h1.L. Franks, Chem. Commun. (1974) 316;J. Am. Chem. Sot., to be published. Y.hi. Bosworth and R.J.H. CIark, Chem. Phys. Letters 28 (1974)611. E. Vanolti and J.R. hforton, Phys. Rev. 162 (1967) 282. J. Rulfe, J. Chem. Phys. 49 (1968) 4193. G. Herzberg, Infrared and Raman spectra of polyatomic molecules (Van Nostrand, Ptinceton, 1966) p_ 205.