Spectra of mercury in inert solids

JOURNAL

OF MOLECCLAR

SPECTROSCOPY

37, 408-413 (1971)

Spectra of Mercury T. HANSEN, Departmenl

of Physics,

W.

in Inert Solids*

TIMMONS, AND C. E. BLOUNT

Texas Christian

Fniversit:y,

Fort Worth, Texas 76159

The absorption spectra of mercury in argon, krypton, xenon, and nitrogen were obtained for deposit temperatures from 4-20°K. The spectra exhibit multiple structure for argon, krypton, and xenon hosts but o111y a narrow band and a broad band for nitrogen as a host. These results can be explained by assuming t,he existence of relat,ively isolated mercury atoms in the host and interacting pairs of mercury atoms trapped at nonneighborillg sites in the host. INTRODUCTION

The multiple structure observed in the spectrum of mercury trapped in inert, solids at, low temperat’ure has been interpreted as resulting from removal of orbit,al degeneracy of t,rapped mercury atoms by the host solid, (I), Jahn-Teller Effect, (2), both relat’ively isolated mercury atoms trapped in the host and interacting pairs of mercury atoms trapped at, nonnearest neighbor sites in the host, (3) (&), or relatively isolated mercury at.oms and mercury at,oms having a nearest neighbor mercury at’om (5). The object of t’his paper is to show that w&h the inclusion of nitrogen as a host solid all observable result,s can best> be explained by assuming the existence of both relatively isolated mercury atoms and interacting pairs of mercury atoms trapped at’ nonnearest neighbor sites in i he host,. DATA AND RESULTS

The absorption spectra of mercury corresponding to the X36.5~ line were obtained in solid argon, krypton, xenon, (4) and nitrogen for deposit temperatures of 4, 16, and 20X, using a molar ratio (host:mercury) of 1000: I. The procedure for making the deposit has been reported previously (4). The spectrum of mercury in solid xenon (4), Fig. 1, consists of a triplet having a widbh at half optical density of %8cm-‘. At low molar ratios ( 65 : 1) a broad band is observed --570cm-’ from the high energy component of the triplet The frequencies of the observed bands and their widths are given in Table I. Andrew (3) has suggested that some of the components of multiple st,ruct.ure can be explained by assuming interact’ions bet,ween pairs of foreign atoms trapped * Work supported

by the Office of Naval Research 408

and the Robert

A. Welch Foundat,ion.

SPECTRA

OF MERCURY

FIG. 1. Absorption spectra of mercury 16°K with static polarixabilities.

IN INERT

SOLIDS

in solid xenon, krypton,

argon, and nitrogen

409

at

at nonnearest neighbor sites in the host solid. This assumption (that the multiple structure results from interactions between pairs of atoms) was extended by Merrithew (4) to explain the triplet structure observed in the spectrum of mercury in xenon. The assignments were based on changes in the relative intensities of the bands for changes in molar ratio. The spectrum of mercury in solid krypton at 16X, Fig. 1, consists of a poorly resolved triplet having a width of 330cm-I. At low molar rat,ios (65: 1) a broad band is observed -624cm-’ from the high energy component of the t,riplet. The frequencies of the observed bands and their widths are given in Table I. The triplet structure observed in the spectrum of mercury in krypton may be explained by extending the assumptions made by Merrit~hew (4). The band at 248lli, marked in Fig. 1 corresponds to transitions of well isolated mercury atoms. The remaining bands correspond to the ?,-l&,+ transition of Hg, for mercury atoms having mercury atoms at sites [3] or [.5] of the krypton lattice.

410

HANSEN,

TIMMONS, TABLE

AND

BLOUNT

I

SPECTRUM OF MERCURY IN SOLID ARGON, KRYPTON, XENON, Host

Xenon

Krypton

Argon

Nitrogen

Wavelength A

0.5 0.3 0.3 0.1 1.0 0.8 0.8 0.8

Band Width (cm-l)

Frequency (cm’) 38 39 39 39 39 40 40 40

967 345 423 540 670 100 187 294

zt zt f zk zk * f f

8 5 5 2 15 12 12 12

430 100 100 70 460 130 120 110

2565.5 2540.9 2535.8 2528.3 2520.0 2492.9 2487.6 2481.0

f zt f f f zk f f

2500.7 2461.5

zk 0.5 f 0.5

39 976 f 8 40 613 zt 8

450 360

2497.0

f

0.6

40 036 f

9

420

2470.1

zt 0.6

40 472 h

9

110

TABLE

Assignment

Uimer 3 5 Isolated Uimer 3 5 Isolated IXmer Nonneighbor & Isolated IXmer & Nonneighbor Isolated

ANLI NITROGEN

Shift Matrix (cm-‘) -______

Shift 31,-G,+ (cm-“)

- 573

-560

-189 - 116 0 -624 - 194 -107 0

- 200 -!)4 0 - 560 - 257 - 12(1 0

-637

- 560 0

0 -436

-560 0

0

II

WIDTH OF TRIPLET Host

Lattice Constant A

Argon Krypton Xenon

3.76 4.00 4.34

Width Exptl cm-’ --._

--__

360 320 ZGO

Hgp Potential Curve cm-’ 300 260 200

There are neighboring mercury atoms at sites [a], [4], [6], and [7] of the krypton lattice. However, the population of sites [2], [4], and [B] relative to the population of sites [Z-3] and [5] are not large enough to produce any resolvable spectral components, and the transit,ion energy at. site [7] is t.oo close to that. of the isolated atom to be resolved in t)he spectrum. The increase in int)ensity of the low energy band, 39670cm -l, at low molar ratios is associated with an increase in the possibility of mercury atoms being trapped in neighboring sit’es in the host and forming dimers. Table II gives a comparison of the experimental results with those obtained for Hgz. For molecular separations less than 4.00& the Hg, results are obtained from Finkelnburg (6). For separations greater t.han 4.00& the results are calculat8ed from the curves in Fig. 2. The calculat’ed energy differences are within 10 % of those obtained experimentally. The difference in the width of the triplet observed in krypt,on and xenon can be associated with the difference in the size of t.he t.wo lat.tices. The width of the triplets may be approximated from the diat’omic pot#ent#ial curve: Fig. 2, by ob-

SPECTRA

OF MERCURY

IN INERT

SOLIDS

411

iloki’) 30; 40

31p

30

I__--------A

Nitrogen

:-:--

Argon

20 I

Krypton

FIG. 2. The Hgz potential curve with location of nonnearest-neighbor sites [3] and [5] in xenon, krypton, and argon and location of nonnearest-neighbor site region in nitrogen. taining the difference in the transition energy of an isolated mercury atom and a pair of mercury atoms having a separation equivalent to the separation of site [3] in krypton or xenon. Since krypton has the smaller lattice, site [3] of krypton will be closer to the potential minimum of the 31, excited state than site [3] of xenon resulting in a larger energy difference. The difference in the width of the individual components of the triplet can be associated with the relative positions of sit’es [3] and [5] on the diat,omic potential curve, Pig. 2. A small change in the separation bet,ween a pair of mercury atoms will result in a change in the transition energy. Due to the relative positions of sites [3] and [5] of krypton and xenon on t,he pot’ential curve, Fig. 2, a small change in t)he separat’ion of a pair of mercury atoms in krypton will result in a larger change in transition energy than the same change in the separation of a pair of mercury atoms in xenon since krypton has the smaller lattice (sites [3] and [5] of krypton closer to potential minimum of 311Lexcited state than sites [3] and [5] of xenon). Since t,he width of the band is a result of all possible transition energies, the components of the triplet observed in the spectrum of mercury in krypton should be wider than hhose observed in the spectrum of mercury in xenon. The spectrum of mercury in solid argon at 4 and 16X, Fig. 1, consists of a broad band at 2461.5& The width of the band is 360cm-‘. At low molar ratios

412

HANSEN, TIMMONS, AND BLOUNT

a band was observed -637cm-’ from the high energy band. The frequencies of the observed bands and their widths are given in Table I. The lack of any resolvable triplet structure in the spectrum of mercury in argon is associated with the region of the potential curves occupied by sites [3] and [5] of argon, Fig. 2. In this region, uncertainty in the positions of the trapped mercury atoms produces so much uncertainty in the transition energies that t,he components of the triplet are too broad to be resolved in the spectrum. As would be expected from the previous discussions, the width of the entire triplet observed in the spect’rum of mercury in argon is broader than triplets observed in the spectra of mercury in krypton and xenon. The band observed at molar rat8ioa below 65: 1 may be attributed to the formation of mercury dimers. The spectrum of mercury in nitrogen, Fig. 1, consists of bands at 247O.lb and 2497.Oi. The width of the 247O.lA band of IOScm-’ is similar to the widths of the components of the triplet observed in krypt)on and xenon. The spectrum of mercury in nitrogen for molar ratios of 1000: 1 is similar to the spectrum of mercury in xenon for a molar ratio of 40: 1. Additional spect’rw of mercury in nitrogen were taken for molar ratios of 6.500: 1, but no triplet was observed. The assumption of nonnearest neighbor interactions was extended to explain the results obt’ained for mercury in nitrogen. Ntrogen crystallizes into two crystal structures, face center cubic P-nitrogen above 35°K and tetrahedra a-nitrogen below 35’K. Assuming structure at 4°K and 16°K is a-nitrogen

I

2440

2460

2480

2500

2520

2540

YAVELENSTH i

FIG.

3. Microdensitometer

tracing

of spectrum

of mercury

in nitrogen

at 16°K.

SPECTRA

OF MERCURY

IN INERT

SOLIDS

413

i”h6( Pa3) (i’) , the population and nonnearest neighbor distances were calculated. For a-nitrogen there are no highly populated sites as found in the fee structure of argon, krypton, and xenon. Thus all possible nonnearest neighbor sites in (Ynitrogen have roughly the same population, 4 to 6. As can be seen in the microdensitometer tracing, Fig. 3, the broad band at 2497.1w is shaded to the blue. Possible cause of this shading is the absorption by mercury atoms having a nonneighboring mercury at,om. Since there are no highly populated nonneighboring sites as found in the fee structure, a diffuse region is observed rather than the triplet observed in krypton and xenon. SUMMARY

The association of the multiple struct>ure observed in the spectra of mercury trapped in solid xenon, krypton, and argon to bot.h relatively isolated mercury atoms and interacting mercury atoms trapped at nonneighboring sites is based on the abiliby of this assumption to explain results obtained for mercury trapped in solid nitrogen. All the ot,her assumpt,ions predict multiple structure for the specbrum of mercury in nitrogen. In addiCon, the assumption t,hat multiple structure results from both relatively isolated and t’rapped mercury atoms having neighboring mercury atoms, places the neighboring atom at a neighboring lattice site. A distance of 3.756 for argon, 3.991 for krypton, and 4.33Fi for xenon which is larger than the interatomic separation of 3.26 for Hgz . This may be possible if both of the atoms are in their ground state. In the excited state the interaction between the mercury atoms dependence due to dipole-dipole resonance force which is of much has an ? van der Wstals ground state interaction (8). The greater range t)han the 1-6 stronger interaction should reduce separation between neighboring mercury atoms. The migration of trapped atoms in inert solids to form diatomic molecules under optical excitation has been observed experimentally for lithium in xenon

(5,Q).

RECEIVED: August

10, 1970 REFERENCES

(1) M. BRITH AND 0. SCHNEPP,J. Chem. Phys. 39, 2714 (1963). (2) G. W. RORINSON,J. Mol. Spectrosc. 6, 58 (1961). (3) L. ANDRE~S AND G. C. PIMENTEL, J. Chem. Phys. 47, 2905 (1967). (4) R. B. MERRITHEW, G. V. MARUSAK, AND C. E. BLOUNT, J. i%foZ.Speclrosc. 29, 54 (1969). (5) M. MCCARTY, JR., J. Chew Phys. 62, 4973 (1970). (6) W. FINKELNBURG,“Kontinuierliche Spektren, ” pp. 193-213, Verlag Van Julius Springer, Berlin, 1938. (7) J.,. H. BOLZ, M. E. BOYD, F. A. MAUER,

AND

H. S. PEISER, Acta Cryslallogr. 12, 247

(1959). (8) G. W. KING ANDJ. H. VAN VLECK, Phys. Rev. 66,1165 (1939). (9) B. MEYER, Science 168, 783 (1970).