vuv absorption spectra of mercury atoms trapped in solid inert gasesSalvador A. Malo Citation: The Journal of Chemical Physics 61, 2408 (1974); doi: 10.1063/1.1682342 View online: http://dx.doi.org/10.1063/1.1682342 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/61/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Exciplex absorptions and emissions in solid rare gases involving atomic hydrogen J. Chem. Phys. 113, 7446 (2000); 10.1063/1.1312829 Absorption and emission spectra of Na atoms trapped in raregas matrices J. Chem. Phys. 69, 1670 (1978); 10.1063/1.436743 Photoelectron spectra of condensed gases on an inert substrate J. Vac. Sci. Technol. 12, 286 (1975); 10.1116/1.568765 Recombination of Oxygen Atoms in the Presence of Inert Gases J. Chem. Phys. 38, 1495 (1963); 10.1063/1.1776908 Ultraviolet Absorption Spectra of Mercury in Solution J. Chem. Phys. 18, 1679 (1950); 10.1063/1.1747562

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vuv absorption spectra of mercury atoms trapped in solid' inert gases*

Salvador A. Malot

I. B. P. Instituto Mexicano del Petroleo. Apdo. 14-805, Mexico 14, D.F., Mexico (Received 29 April 1974)

The photoelectrically obtained vuv absorption spectra of Hg-doped rare-gas matrices are shown. The main features observed are correlated with those of the I PI_IS 0 transition of Hg in its gaseous phase: These are shifted and show fine structure. Matrices of Ar, Kr, Xe, and Ne were used. Spectra were also obtained under different experimental conditions, and the results are compared with previous work.

I. INTRODUCTION

Since the first paper of McCarty and Robinson on Hg trapped in solid gases,l many papers have been published on the subject. Due to its high vapor pressure and its vapor consisting mainly of monoatomic species, it is very amenable to experimental work.

McCarty and Robinson studied the 3pl-1S0 transition, occurring in the gas phase at 253.6 nm, finding a shift of the transition to a higher energy and a splitting of the transition into several components. Later work by Roncin and Damany-Astoin2 using argon matrices, con­firmed the shift although their reported value was small­er. Roncin and Damany-Astoin also found other absorp­tion features at shorter wavelengths including one at­tributedtothe l p1_l.so, 184.9 nm, resonance line. This last result was also reported by Dressler3 for xenon matrices.

A few years later Brewer, Meyer, and Brabson4 un­dertook an extensive investigation of the 3P1_ 1S 0 transi­tion in argon, krypton, xenon, and neon matrices, their results falling in good agreement with those of McCarty and Robinson. Simultaneously Duley, working at Im­perial College, found not only the lPl-1S0 and 3pl-1S0

transitions but also several other features. 5,6 Finally, Merrithew, Marusak, and Blount7 studied the 253.6 nm line using solid xenon matrices. All these studies were performed photographically. We report here an investi­gation of the spectra of mercury atoms trapped within solid argon, krypton, xenon, and neon matrices using photoelectric detection.

II. EXPERIMENTAL

The experimental set-up was similar to those used by other investigators.a A hydrogen discharge was placed at the entrance slit of a one-meter normal incidence monochromator. The hydrogen leaked from the dis­charge container to the monochromator, where it was pumped out to a pressure better than 10-4 mm Hg while the lamp was running.

The monochromator had a 1200 lines/mm grating driven by a stepping motor and the useful range was from - 50 to 300 nm, the resolution being 0.08 nm.

At the exit slit of the monochromator and isolated from it by a LiF window, a cryostat (described pre­viously9), was placed. At the bottom of the cryostat a

LiF window, used as substrate, was held in good ther­mal contact by copper clamps and an indium seal. The pressure in the sample chamber was better than 10-6

mm Hg and the temperature of the substrate was moni­tored with a copper-constantan thermocouple.

The samples were prepared by previously mixing rare gases of high purity with mercury vapor at room tem­perature. By varying the pressure of the gases, differ­ent matrix to metal-atom (M/ A) ratios could be obtained. The mercury-doped gases were then fed to the sample chamber through a 3 mm bore tube, and allowed to strike the cold LiF surface. On touching the surface the gas solidified, leaving the metal atoms trapped within.

The depositions lasted at the most a few minutes and were stopped when a sufficiently thick layer had been ob­tained. This was detected by observing the decay of in­tensity of the monochromatic light passing through the sample. The light was detected using a 13-stage EMI photomultiplier with a p-terphenyl coated quartz window. The phosphor converted the ultraviolet light into the spectral region of the photomultiplier response. A mi­croammeter amplified the detector signals which were then displayed on a strip chart recorder.

Before the deposition was made, a plot of the intensi­ty Io vs wavelength was recorded in the interval of in­terest, usually between 165 to 300 nm. Once this was completed the deposition was made and the intensity I was again recorded for the same wavelength region. By comparing I with Io the absorption regions were detected and, if desired, slower scans could be carried out around these regions.

III. RESULTS

Figure 1 shows a typical recording, the sharp lines belonging to the onset of the many-line region of the hydrogen discharge spectra are particularly noticeable in the upper curve. The lower curve shows a gross ab­sorption feature with fine structure, indicated here by three vertical lines.

Figure 2 shows the absorption spectrum of a xenon matrix containing well isolated mercury atoms (M/ A ~ 2000/1). The dots represent the natural logarithm of Io/Itaken at 0.5 nm intervals, except in the vicinity of the two main features where the interval was reduced to 0.2 nm. Two absorption features are clearly marked,

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Salvador A. Malo: Spectra of mercury atoms 2409

>­f-

U)

Z W f­Z

170 180

A..nm

Kr: Hg

190 200

FIG. 1. Typical recording of intensity vs wavelength obtained from mercury-doped solid samples.

a very strong line at around 189.5 nm. and a weaker line at 253.5 nm. These two features which appear to show fine structure are attributed to the lpl-1SO and 3Pl-1SO

transitions, respectively. A very broad weak band cen­tered at - 215 nm can also be seen.

Spectra similar to that shown for Hg: Xe were obtained for Hg: Ar, Hg: Kr, and Hg: Ne. Also, spectra were ob­tained under different experimental conditions. Figure 3, for example, shows the spectra of several layers of Hg: Xe. This was done by repeatedly depositing on the

0.6 Xe:Hg

,'; 189.5

0.(; ".

0.4

::::: 0.3

.= 0.2

0.1

180 200 220 240

A.nm

FIG. 2. Absorption spectrum of a xenon matrix containing mercury as an impurity. Dots at 0.5 and 0.2 nm intervals (see text).

"­o

185

A.nm

190

00 • o •

• 0 •

195

Xe:Hg

I. 0 1---------i--~-___!----___1

. o _ •• _.. 0

0000°00° • - •• : 0°000

. . . . ..... . 3

, ••••• 2 0.51-----___!----___!..::....!.---"---___1

. .

6.8 6.6 6.4 6.2

h~. eV

FIG. 3. Absorption spectra of a xenon matrix containing mer­cury as an impurity. Three subsequent depositions on same substrate.

same substrate an amount of Hg-doped gas, and' taking a spectrum of the region of interest between each deposi­tion. Figures 4 and 5 show on the other hand the spec­tra of various samples while warming. This was achieved by rapidly scanning back and forth in the vicini­ty of the feature, while the sample temperature was ris­ing. The numbers and arrows accompanying the curves shown in Fig. 4 indicate the beginning and order of each particular scan and therefore the direction of tempera-

"­.

3.

2

1.0 _ 4

-+ 3

---I

185

A. nm 190 195

Xe:Hg

-2

O~---~----~----~

6.8 6.6 6.4 6.2 hi. eV

FIG. 4. Absorption spectra of gross features obtained while the sample was warming.

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2410 Salvador A. Malo: Spectra of mercury atoms

>­I-

(J)

Z w I­z

TEMPERATURE,OK

4 10 20 3040 50 60

XeHg 189.3 nm

TIME

70 74

FIG. 5. Intensity obtained at the fixed wavelength of 189.3 run lying in an absorption feature of aXe : Hg sample while it was warming.

ture rise. It can be seen from the figure that as the temperature rises a general increase in absorption is evident which is not limited to the feature but extends to the background on both the blue and red sides. The fea­ture broadens to the red with a gradual decrease in structure while the blue maximum of the triplet remains in the same position; finally the whole structure disap­pears. Frequently, the disappearance of structure was preceded by a sudden drop in the absorption followed im­mediately by an increase. This drop-rise cycle was sometimes repeated 2 or 3 times before the final level of absorption was reached.

Varying the ratios of M/ A did show a decrease in structure with lower ratios, as well as broadening of the feature. Sometimes a change in the position of the fea­ture center was noted.

The absorption features observed are as follows: In xenon, the most intense feature appears at around 189.5 nm and other much weaker at 253.5 nm. These two show fine structure. A broad weak feature, extending

over several tens of angstroms and centered at 215.0 nm can also be distinguished. A more common feature is a peak on the red side of the 189.5 nm absorption and sometimes merging into it. Finally, on at least two oc­casions a very sharp feature was observed on the blue side of the 189.5 nm transition.

In krypton an intense absorption feature centered at 183.5 nm and possessing fine structure is the most ob­vious one. Also present are a sharp absorption at 170.8 nm, a broader band at - 194.0 nm, and two bands at 239.0 and 244.0 nm merging into each other occasionally.

Finally, while the argon spectrum closely resembles that found for xenon and krypton in having a strong fea­ture on the short wavelength of the spectra, that of neon only shows a weak band in this region. Furthermore, in the argon feature fine structure can still be discerned, although less clearly defined than in the krypton or xenon matrices, but none can be distinguished in the neon spec­trum. Besides these features, both argon and neon show a broad weak absorption at 247.0 and 248.5 nm, re­spectively. Argon and neon matrices also show with de­creasing M/ A ratio a tendency to develop a shoulder on the red side of the main absorption band and, in one oc­casion, another band appeared at 212.4 nm.

The main absorption features observed are given in Table I.

IV. DISCUSSION

In xenon the main absorption features at 189.68 nm and 253.60 nm have been correlated with the (6S21 S 0-

6s6p 1P 1) and (6s2 lSo-6s6p 3P1) transitions of mercury in the gas phase. Comparison of the 253.6 nm line shift with previous work shows a very good general agree­ment, all reported values lying within 0.3 nm. The agreement extends to the fine structure, although some differences do appear. McCarty and Robinson, 1 Brewer et al., 4 Merrithew et al., 7 and the present author ob­served a triplet in contrast to Duley,6 who observed only a doublet. McCarty and Robinson reported a splitting separation of 120 cm-t, Duley, Brewer, and Merrithew's work reported this separation as 200 cm-t, and the re­sult reported here is 140 cm -1. While Brewer's and Merrithew's result can be attributed to their higher sam­ple temperature, no similar arguments can be put for­ward to explain the difference in Duley's case. Never-

TABLE 1. Position, width, and maxima of main absorption features found in matrices doped with mer­euryatoms.

Type of matrix

Correlated transition

Feature center

Shift from gas transition

Half width

Absorption maxima

(run) (em-I)

(em-I)

(em-I)

(em-I)

Xenon Krypton Argon Neon

ISo_3p~ ISo-lp~ IS0_lp~ ISo-lp~ ISo-lpr

253.60 189.68 183.46 178.5 181. 0 39420 ± 15 52710 ± 15 54507 ± 15 56023 ± 50 55249 ±50

8 ± 15 -1359 ± 15 438 ± 15 1954 ± 50 1180 ± 50

279 840 826 ± 15 1260 ± 50 1800

39478 ± 15 52995 ± 15 54794 ± 15 56449 ±50 39409 ± 15 52710 ± 15 54620 ± 15 56142 ±50 39339 ± 15 52458 ± 15 54306 ± 15 55841 ± 50

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Salvador A. Malo: Spectra of mercury atoms 2411

theless, it is so small that it can be accounted for by experimental uncertainties.

In regard to the 187.68 nm line, the only results avail­able are those of Dressler3 and Duley.5,6 The former re­ported a complex feature appearing at -1400 cm-I of the free state energy. Duley reported the shift to be -1239 cm- l and the feature to consist of a triplet whose maxi­ma were 550 cm-I apart. We found the separation as being 537 cm- l and the shift to be -1359 cm-t, therefore lying in close agreement.

The correlation of the other features with gas-phase transitions still remains uncertain. There is no general agreement between the various published results indi­cating that they might be due to impurities.

In krypton, argon, and neon the 1 SO_3PI transition was just barely seen as a broad weak rise in absorption with no resolved components. The determination of the fea­ture's center wavelength was rather difficult, and the author has them at 243.8, 247.0, and 248.5 nm for kryp­ton, argon, and neon, respectively. The corresponding shifts given in the same order are 605, 1073, and 828 cm-I. The agreement with values previously reported is good for krypton, but poor for argon especially when compared to the data of Brewer et al. In neon the only value previously reported for this transition is that of Brewer et al., 4 who give the shift as being 940 cm-l

compared to ours of 828 cm -1.

The ISO-lpl transition was clearly seen in krypton as centered at 438 cm-l and having a triplet structure with outer maxima separation of 488 cm-I. This result does not compare favorably with Duley's5 776 cm-t, or with his later value. 6 For argon the situation is better, our reported shift, 1954 cm-t, agreeing with all previous work except that of Roncin and Damany-Astoin. 2 No data were available for neon.

The values reported above for xenon, krypton, and argon seem to conform to the explanation put forward by McCarty and Robinsonl who explained the shift by assum­ing it could be expressed as the difference of the total interaction potential with the lattice of the atom in its ground and excited state. This approach was shown to fail in the case of a neon lattice where the values pre­dicted were quite different than those reported, this is still true for the values here reported. Recently McCartyIO has modified the McCarty and Robinson treat­ment by allowing the 12 nearest neighbors around the metal atom to expand or contract radially, therefore giving a different internuclear distance between neigh­bors. He has calculated this new internuclear distance by minimizing the potential of a cluster of 54 atoms in a fcc arrangement. With this provision he has been able to fit the neon data for the ISO-

3P l shift. Preliminary calculations by the author repeating McCarty's approach seem however to give different values.

An interesting aspect of the sample warming studies

common to all matrices was a decrease in transmiSSion followed by an increase and then by a decrease. This is shown in Figs. 4 and 5. Figure 4 shows scans being made back and forth while the sample was warming, in Fig. 5 the wavelength was fixed. While a possible ex­planation of this effect is to consider it as due to a gen­eral relaxation of the lattice causing interference effects, nevertheless since it was observed in xenon at tempera­tures well below its melting point, other explanations might be nearer to the truth. One of these is related to diffusion properties in solids. Pimentelll has given am­ple evidence through calorimetric, infrared, and ESR studies to indicate that atoms in solids can diffuse at temperatures 50% below the melting temperature of the matrix accompanied by heat release and light emission.

The evidence from the present work shows that the splitting of levels is highly reproducible although the in­tensities and separation of the individual components vary in a unexplained fashion. Furthermore, mounting evidence seems to indicate that variations might be due to the presence of impurity gases. In view of this and the controversial interpretation of the fine structure, further work under more controlled experimental con­ditions is necessary.

ACKNOWLEDGMENTS

The author would like to express his gratitude to Professor W. R. S. Garton and members of his staff at Imperial College for their help; in particular H. Gold­stein, J. Jenkins, Dr. R. J. M. Learner, and J .. E. G. Wheaton. Thanks are also due to Dr. A. Blake, Dr. W. Duley, and Dr. W. Hunter for their contributions. Fi­nally, the author wishes to acknowledge the financial support of the Instituto Nacional de Energfa Nuclear and the Instituto Mexicano del Petr6leo, Mexico.

*Work performed in the Spectroscopy Group, Imperial College, London, England. .

t Also at Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, Mexico.

1M. McCarty, Jr. and G. W. Robinson, Mol. Phys. 2, 415 (1959).

2J. Y. Roncin and N. Damany-Astoin, Compo Rend. 253, 835 (1957).

3K. Dressler, J. Quant. Spectrosc. Radiat. Transfer. 2, 683 (1962).

4L. Brewer, B. Meyer, and G. D. Brabson, J. Chern. Phys. 43, 3973 (1965).

5W. W. Duley, Phys. Lett. 19, 361 (1965). BW. W. Duley, Proc. Phys. Soc. 90, 263 (1967). 7R. B. Merrithew, G. V. Marusak, and C. E. Blount, J. Mol.

Spectrosc. 29, 54 (1969). 8W. Weyhmann and F. M. Pipkin, Phys. Rev. 137, A490 (1965). 9W. R. Hunter and S. A. Malo, J. Phys. Chern. Solids 30,

2739 (1969). 10M. McCarty, Jr., J. Chern. Phys. 52, 4973 (1970). I1G. C. Pimentel, in Formation and Trapping of Free Radicals,

edited by A. M. Bass and H. P. Broida (AcademiC, New York, 1960).

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