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Far ultraviolet absorption spectra of atoms and molecules trapped in rare gas matrices at low temperature
JOI~RN.IL OF MOLECPLAR SPECTROSCOPY 22, 154164
Far Ultraviolet Trapped
(1967)
Absorption Spectra in Rare Gas Matrices
of Atoms and Molecules at Low Temperature
JEAN-YVEH RONCIN, NICOLE DAMANY, I,abomtoire
des Hastes
Pressions,
Centre
National
AND JACQUES
de la Recherche
ROMAND
Scienti$qiqPLe, Rellevue,
France Several absorption spectra are present,ed which were taken in the spectral range 2600-1050 8 with atoms (Hg, Xe), radical (H) and molecules (CO, Nz, NO) in pure solid form or trapped in solid rare gas matrices at temperatures between 20 and 1.5”K. Shifts of resonance lines of Hg and Xe are compared with those obtained under high pressure in gaseous state. The fourth positive group of CO and the Lyman-Birge-Hopfield bands of NP are perturbed in the pure solid but not in matrices. The intensit.y of the zolA,+ X1 Z,+ transition of N? is enhanced in the pnre solid but not in matrix and the progression is observed up t,o v’ = 17. Rydberg states disappear in solid dilute NO and vibrational spacings of the valence states become regular as the perturbating interactions of the two types of states no longer exist. I. INTRODUCTION
The earliest studies of the optical absorption of pure solid films obtained by condensation of a gaseous beam were first made in our laboratory by Romand and Granier-Mayence down to 1600 i and at a temperature of 20°K (1). Later, as part, of t’he program of research undertaken by the National Bureau of Standards on trapped radicals, Schnepp and Dressler extended the investigations to 1200 8 and 4.2% (2). Since then, our apparatus permitted us to work at 1.5% and to go to 1050 d (the limit’ of transparency of LiF) using a triggered vacuum spark, developed at Bellevue, as the source of continuum. The presentation of the results obtained by us will show that the study of the absorption of condensed gases has a t.hreefold interest: (i) The study may be regarded as an extension of t,he observations of pressure effects on spect,ral lines under high pressure of foreign gas, as outlined by Vodar (3). (ii) It allows one to stabilize free radicals [in the sense of unstable chemical species as defined by Broida (d)] in the solid state during the time necessary for their observation. (iii) Finally the work in t,he Schumann region permits a more complete characterization of the electronic tran&ions of molecules. 154
UV ABSORPTION
OF TRAPPED
SOLID
SPECIES
1.G
II. ESPRRIMENTAL
The great,cr part of the spectra presented here were taken with the experiment,al set,up represented in IPig. 1. The cont’inuous spectrum emit’ted by the source (S) is focussed by means of a toroidal mirror (n/r) on t’he entrance slit (E) of a vacuum normal incidence one-meter spectrograph (R) and t,hc spec+rn recorded photographicaally (1’). In the path of the incident light, bct,ween Al and E, is interposed t.he absorbing deposit formed on a knsparrnt mindon- (1:) caooled by a cryostat (C). The source described elsewhere (5) is a modified t.hree-elec*trode va~um spark whosc continuum extends from the visible t.o t.he soft, x-rays (6). The spcc*trograph has a dispersion of X.5 A/mm and a slit width of 70 p. About 50 sparks, at a rate of 5 per second, are necOessary to give a good blackening of the Kodak I’ath6 SC 5 film down t,o 1050 A, because of the presence of three cleaved I,ilf windows in the light path. We successively used two cryostats: t,he first was limited to work at. 4.2’~ while the second permitted us to work successfully at l.:i”Ii by decrckng the pressure above liquid helium down to 3 t.orr. l’or producing H at,oms an Ar-H?O mixture was ndmit’ted in a Wood’s tubtl. The discharge products were condut4ed to the cryos;tat through a t&on-c*oaterl I’yrex tube cooled with liquid S? or possibly with liyuid Hz. III. A.
RESULTS
AXI)
~lSClX3ION
COMPAHISON WITH HIGH I’RESSUHE EFFECTS IS GASES
(1) .l/ewwy
resonawe
lines
At first we st)udied the two resonanc~c lines “P, t lSO (2537 A) and 11’~+ IS,, (IS.50 8) of Hg trapped in an Ar matrix at 20°K (7). The two lines are shifted and broadened as shown in Table I. An extra line a,ppears whkh might be due to the forbidden t,ransition “ZJ2+ ‘So (2270 8). In Fig. 2 we represent the extrap olnt.ion of Robin’s curve for t’hc displacement, of the line 3.537 8 and its s:itcllitc>s versus the densit,y of Ar (8). The point. for Hg-Ar at 20’3 (*orresponding to :I
FIO. 1. I)rawing of the assembly. (S) Three electrode vacuum spark source. (M) toroidal mirror. (E) F,ntranre slit of the spectrograph (Ii) Conc:tve grating. (I’) Photographic film. CC) Liyuid helium cryostat. (I,) LiF window.
156
RONCIN,
DAMANY,
AND
ROMAND
TABLE I POSITION AND SHIFT OF THE 3 BANDS IN SOLID Hg-Ar
Hg
gas
Hg-Ar
20°K
hA
Y cm-
Y’
/-Y
2537 2270 1850
39 410 44 055 54 055
40 320 48 500 55 700
910 4450 1650
Transition
aP1-‘so 3Py’s” ‘PI-‘So
x4, 2540 2530 2520 2510
2490 2460 2470 2460 2450 --.--~~
/
2440
/ 200
400
600
1 ‘~ 1 \ / ‘a.
1 p(amagatsl
000 989 993 1200
FIG. 2. Shift of the satellite of the 3P1 +- ‘SO line of Hg in compressed Ar (Robin). Extrapolation; 0, McCarty’s result (Hg-Ar: 4.2”K) ; +, our result (Hg-Ar: 20°K).
--
density of 989 amagats obtained by us is in good agreement wit,h the extrapolated part of the S1 curve deduced from the high pressure experiment. The discrepancy between our results and those of McCarty and Robinson (9) could be ascribed t,o differences in temperature and concentration in the light of a recent work (10) undertaken to study the systematic influencezf various factors on the format,ion of the layer. Although the shift of the 1850-A line perturbed by compressed Ar is only known up to a density of 400 amagats (ll), the shift in the solid appears to fit’ the extrapolated curve. (2) Xenon
resonance
lines
We observed the resonance doublet of xenon [9$Jlo+- ‘So (1470 A) and [$$Jl” + ‘X0 (1296 A) in the pure solid and in matrices of Kr, Ar, and Ne at 20°K or at,
UV ABSORPTION
OF TRAPPED
SOLID
SPECIES
FIG. 3. Absorption spect.ra of solid Kr (20°K). For all the figures the thickness layer illcreases from the top to the bottom.
1.57
of the
(12). We obtained the displacement of the t’wo lines and verified the xppearunce of extra bands in the solid. ,4s in the c*ase of Hg, the shift could be wnlpared with the curve giving the displacement of the line 1470 d of Xe perturbed by Ar up to a density of 500 amagats (13). Our point is then located on the curve extrapolated t’o 989 amagat’s according to a p3 law. These results, as well as those concerning solid Kr in the pure state and in matrices of Ar and Ne (Fig. 3), are very close to those obtained simultaneously by Rnldini using a line source and a monochromat’or and direct’ly condensing on the fluorescent support which serves as the receptor (14). Baldini att,ributes the new bands of the solid t’o hydrogen-type series due to exciton levels in the crystal. Phillips gave a det’ailed interpretation of the pure solid Xe spect,rum in terms of band struct’ure (16).
I.YI<
K.
ST.IBILIZATION OF H RADICAL IN AN AR MATRIX AT 4.2”Ii
We suwessfully obtained t’he absorption spectrum of H in solid Ar at, a cow wntration of about l:%O (16). The Lyman (Y resonance line (1216 A) of atomit hydrogen is shifted towards short wavelengths by 45 A (Av - 2340 cm-l)
158
HONCIN,
DAMANY,
AND ROMAND
(Fig. 4). Our results are very close to those obtained by Baldini using a similar technique (17). The large half-widt’h (Avl,z - 1300 cm-‘) might be due to the great mobility of H atoms at 4.2%. Keil and Gold have calculated theoretically t.he energy of the Is-2p transit’ion of H atom in a substitutional site of solid Ar using the tight binding approximation (18). It might perhaps be possible to get a sharper band by cooling H in Ne down to 1.5% for which the isotopic shift would not be masked by the bandwidth, as judging from our results with CO-Ne and NO-Ne (see sect’ion B(3)). B. ELECTRONICSTATES OF DIATOWC MOLECULES Dressler gave preliminary results about pure solid CO and Nz at 4.2% down to 1200 B (2) and recently Brith and Schnepp reexamined the problem (19). Extending the spectral region and the range of temperature we studied solid CO, Kg, and NO both in the pure state and in various matrices (%‘O-.Z%). (1) Fourth Positive Group of CO Figures 5-7 exhibit the absorption spectra of solid CO pure and in matrices of Ar and Ne. Table II summarizes the results. The splitting clearly observed for the most’ intense terms of the pure solid is ascribed to be of the Davydov type; it arises from the coupling between molecules occupying translationally nonequivalent sit’es. The splitting is approximately proportional to the Franck--
FIG. 4. Absorption
spectra
of solid H-Ar
(4.2”K)
IJY ABSORPTION
OF TRAPPED
SOLIJ> SPE( XES
159
FIG. 5. Absorption spectra of pure solid CO c4.a°Kj 70 to 4000 A thick. The weak bands in the long wavelength side of the four first bands may be ascribed eit,her tu the PO’” isotope of CO or IO a secondary trapping site in solid Ne. Condon fxtors. In various mat.rices the interaction between CO molecules vanishes so t,hat the bands become sharper and the shift is constant for each vibrational term, which indicates that. there is no deformation of t,he pot,ential energy curve hut only a vert)ical Cranslation. The films absorb in t,he short wavelength region but not compleOely as can be seen on the first spectrum, reprotlucaed in lcig. 7, obtained with longer exposure time. Although the longer wavclengt,h bands clearly prove the presence of CO no trace is found of t,he Rydberg transitions like RIZ+ +- >Y IS+ But, it,s absence is not conclusive since it is quite possible that the system would be either shiftSed by about 100 w towards shorter wavelengths as in t.he case of Rydberg series of atomic: lines, so that t,his system would be situated below the experimental limit, of observation (1050 8), or broadened so much that. it gives rise to a weak cont,inuous absorption diffic*ult 10 cleteclt .
(2) Forbidden Transitions
of i’-C?
The transit#ions of NZ being forbidden in this spectral region their int,ensities are very low so that’ a relatively thick film (of the order of 0.8 cc) is necessary for getting the bands. The results obtained with pure solid iY2 at 4.2% are shown iI1 I’ig. S. Although the bands are sharper, the split,ting is smaller than in the case of CO so that it is not clear. The series w-S is more int,ense; this means that thr w -37 transition is comparatively enhanced by a facator of lo3 as it becomes
160
RONCIN,
FIG. 6. Absorption
DAMANY,
AND
spectra of solid CO-Ar
FIG. 7. Absorption
spectra
ROMAND
(4.2”K) 5000 to 12 000 .& thick
of solid CO-Ne
(15°K)
dipole-allowed in the solid (19). The observation of this transition in the solid brings an additional support to the identification proposed by Tanaka (W). In spite of the great thickness of t’he layers required for observing absorption bands of N, in a matrix (from 10 to 50 p) we recently succeeded in this attempt, by condensing N, in X’e at 1.5% with the time of evaporation ranging from 20 min to 1 hr. The bands are very much sharper than in the pure solid, the two series are shifted very little as in the case of CO in Ke and the intensities are similar to that in the gas since w-X remains dipole forbidden in the matrix. The remark made with regard to the Rydberg transition of CO still remains valid for the C3n, t X’Z,+ transition of N2.
UV ABSOR.PTION
OF TRAPPED TABLIX
,SHJF.TAND
HALF-WIDTH
OF THE
SOLII)
II
\:IBKA.I'IONAL TERMS
POSI'rIVE t:ROUP
OF SOLID
4.2’K
“,--yfl Av1/2
-350 330
FIG. 8. Absorption
(13) Zhited
States
-590 160
spect,ra
OF THE
of pl~re solid
CO-Ne - 65 110
N?(4.2”K)
~TH
co
l.S”K C0m.k
pure co
161
SPECIES
-___
CO-Kr
CO-Se
- 1000 130
- 1twJ 400
3500 to 50 000 i
thick
oj” NO
While no definite conclusion could be drawn regarding the Rydberg transitions of CO and X2 in the solid due to t,heir t,oo high energy levels, the X0 molecule affords, in contrast, a particularly favorable case, sincae its Rydberg states and also it,s valence states lie het’ween 2300 and 1300 ,k. Partly because of it.s odd number of electrons, t-he behavior of NO is cornplet,ely different from that. of CO and X 2; as a matter of fact in the pure solid stat,e only a large absorption band ascribed to X\;?O?has been observed at, 2100 K (1). We did not observe any mw st,ructure at short wavelengths. In contrast, on isolating SO in a rare gas matrix, we find some 40 bands as shown in Figs. 9 and 10. The remarkable feature of t’hese spectra is the sharpness of the NO-Nc hands which is only limited by the resoluGon of the speckograph (0.6 A). Thanks to the recent analysis of the spectrum of X0 gas by Miescher (k$, 25) we have been able to ascribe prakcally all the bands of the solid t,o the three valence st.at.es ELI, H’“A, and G*Z:- + .Y%. In the gas there are irregular vibrational spacGngs in the BzIl and B’?A st,atcs corresponding to the crossing of energy
162
RONCIN,
FIG. 9. Absorption
spectra
DAMANY,
AND
of solid NO-Ar
ROMAND
(4.2%)
5000 to 15 000 d thick
.__.. _
FIG. 10. Absorption
spectra
of solid NO-Ne
(1.5%)
potential curves between these valence states and Rydberg states of the same symmetry. On the contrary, the series become perfectly regular in the solid as there seems to be no Rydberg st,ates to cause perturbations. In Table III we summarize the displacement of each series in the solid with respect to the deperturbed ones in the gas. On changing the matrix from Ar to Ne the series B12Aand G% sharpen while the inverse is true for the series B2K This might involve a correlation between the different lattice structure of the two matrices and the different orbital extension of the two configurations . . . (~2p)(&p)~ (ii2p)’ for B’ and G and . . . (~&$)~(7r2p)~(ii2p)~for B. The disappearance of the Rydberg transitions can be qualitatively explained by the large size of the
ITV ABSORPTION
OF TRAPPP:I> TABLE
SIIIFT Transition “K 1.5 -4.2
SOLII)
SPECIES
163
III
AND HALF-WIDTH OF THE VIBRATIONAL TERMS OF TTALENCE TRANSITIONS OF SOLID NO BT-SW
Cl?-'
Yx-Yg
PiO-Ne NO-Kr NObAr Nt )bAr
-60 -355 -205 - 175
B’V~.Y*II. Au1 i? 100
60 (30 10
Y*-Vg
-5x - 700 -305 -4Y5
THE
G2Y- -5*11
A Y, ,'?
30 150 120
vr-rrl
&Ii4
- 70 -Y70 -100 - GO5
30 150 120
Rydherg orbit:& as compared to those of the valence orbitals (nearly 3 t,imes larger). This will considerably increase the probnbilit’y of transitjion of the clcc*t,ron to the valence orbit& which are less affect.ed by the environment. E’urt#her the configuration . . . (?&I)” of the valence stat,es has a much more compac*t charge distribution wit,h its two elec.trons t,han the (*onfiguration . . . Xnl(n 2 3) of the Rydberg states. I\.. CoNCLuGIos Our work was not’ exhaustive and, in a general way, it, would be useful to rcexnmine the spectra of t,he molecules in the solid state under higher resolution with t#hc view of observing the isotopic2 shifts, and also in the exkeme ultraviolet in order to look for Rydberg states in solid CO and iY?. Further the experiments should be undertaken with temperature, concaentration and evaporat’ion &es varying c~ont~inuously over large ranges. It would be also ext’remely worthwhile to st,udy t)he crys:talline structure of the deposits and to prepare if possible monocsrystalline layers for polarization studies t,o make easier the int’erpretation 01 thr specka in t,he light of solid state theory.
The authors are grateful to Prof. E. Miescher (University of Basel) for nlunicating his latest results of the analysis of 30 gas hefore publication and express their sincere thanks to him as well as to Prof. K. Dressier (Princetjon 1)r. H. Lefebvre-Brion, and l)r. C. Moser (Centre de MCcanique Ondulatoire Paris) all:1 Dr. V. Chandrasekharan (Lahoratoire des Hantes Pressions) for discussions we had with them. REC’EIVEI)
: August
kindly comthey wish to IJniversity), Appliqnbr. the helpfttl
15, 1966 REFERENCES
1. J. (;KANIEIG%AYENCE AND J. ROMAND, Compl. Bend. 268, 1148 (lY53). d. K. I)KEsSLER, J. Quant. Spectry. & l?adiative Transfer 2, 683 (19G2). 3. B. \.oDAIL, Proc. k!ovl. ,%c. A266, 44 (19cio). 4. H. P. BILOIDA, dnn. I\‘~w l*ork ~lcad. Sci. 67, 530 (1957). ii. H. I)AMASY, J.-Y. HoN(-IN, AND N. I)AMANY-AYTOIN, _!ppl. Opt. 6, 237 (1Yfiti) fi. .J. I:~MANI> AXI) (+. BALLOFFET, Spectrochim. .Aclu 18, 791 (1962). 7. J.-Y. I
164
RONCIN,
DAMANY,
AND
ROMAND
J. ROBIN, J. Rech. Centre Natl. Rech. Sci., Lab. Bellevue (Paris) 47, 89 (1959). M. MCCARTY AND G. W. ROBINSON, J. Chem. Phys. 66, 723 (1959). L. BREW-ER,B. MEYER, AND G. D. BRABSON,J. Chem. Phys. 43, 3973 (1965). 8. ROBIN AND S. ROBIN, Rev. Opt. 37(4), 161 (1958). J.-Y. RONCIN, V. CHANDRASEKHARAN, AND N. DAMANY, Compt. Rend. 268, 2513 (1964) S. ROBIN AND J. ROMAND, Compt. Rend. 231, 145Fj (1950). 14. G. BALDINI, Phys. Rev. 137, A 508 (1965). 16. J. C. PHILLIPS, Phys. Rev. 136, 1714 (A) (1964). 16. J.-Y. HONCIN, N. DAMANY, AND B. VODAR, Cow~pt. Rend. 260, 96 (1965). f7. G. BALDINI, Phys. Rev. 136, 248 (A) (1964). 18. T. H. KEIL AND A. GOLD, Phys. Rev. 136, 252 (A) (1964). 19. M. BRITH AND 0. SCHNEPP,Mol. Phys. 9, 473 (1965). 20. J.-Y. RONCIN AND N. DAMANY, Compt. Rend. 260, 6069 (1965). 21. J.-Y. RONCINAND N. DAMANY, Compt. Rend. 261, 4697 (1965). 22. J.-Y. RONCINAND N. DAMANY, Compt. Rend. B262, 1436 (1966). 23. Y. TANAKA, M. OGAQ-A, AND A. S. JURSA, J. Chem. Phys. 40, 3600 (1964). 24. K. DRESSLERAND E. MIESCHER, Astrophys. J. 142, 1660 (1965). 25. A. LAGERQVISTAND E. MIESCHER, Can. J. Phys. (to be published). 8. 9. 10. 11. 12. IS.
Far Ultraviolet Trapped
(1967)
Absorption Spectra in Rare Gas Matrices
of Atoms and Molecules at Low Temperature
JEAN-YVEH RONCIN, NICOLE DAMANY, I,abomtoire
des Hastes
Pressions,
Centre
National
AND JACQUES
de la Recherche
ROMAND
Scienti$qiqPLe, Rellevue,
France Several absorption spectra are present,ed which were taken in the spectral range 2600-1050 8 with atoms (Hg, Xe), radical (H) and molecules (CO, Nz, NO) in pure solid form or trapped in solid rare gas matrices at temperatures between 20 and 1.5”K. Shifts of resonance lines of Hg and Xe are compared with those obtained under high pressure in gaseous state. The fourth positive group of CO and the Lyman-Birge-Hopfield bands of NP are perturbed in the pure solid but not in matrices. The intensit.y of the zolA,+ X1 Z,+ transition of N? is enhanced in the pnre solid but not in matrix and the progression is observed up t,o v’ = 17. Rydberg states disappear in solid dilute NO and vibrational spacings of the valence states become regular as the perturbating interactions of the two types of states no longer exist. I. INTRODUCTION
The earliest studies of the optical absorption of pure solid films obtained by condensation of a gaseous beam were first made in our laboratory by Romand and Granier-Mayence down to 1600 i and at a temperature of 20°K (1). Later, as part, of t’he program of research undertaken by the National Bureau of Standards on trapped radicals, Schnepp and Dressler extended the investigations to 1200 8 and 4.2% (2). Since then, our apparatus permitted us to work at 1.5% and to go to 1050 d (the limit’ of transparency of LiF) using a triggered vacuum spark, developed at Bellevue, as the source of continuum. The presentation of the results obtained by us will show that the study of the absorption of condensed gases has a t.hreefold interest: (i) The study may be regarded as an extension of t,he observations of pressure effects on spect,ral lines under high pressure of foreign gas, as outlined by Vodar (3). (ii) It allows one to stabilize free radicals [in the sense of unstable chemical species as defined by Broida (d)] in the solid state during the time necessary for their observation. (iii) Finally the work in t,he Schumann region permits a more complete characterization of the electronic tran&ions of molecules. 154
UV ABSORPTION
OF TRAPPED
SOLID
SPECIES
1.G
II. ESPRRIMENTAL
The great,cr part of the spectra presented here were taken with the experiment,al set,up represented in IPig. 1. The cont’inuous spectrum emit’ted by the source (S) is focussed by means of a toroidal mirror (n/r) on t’he entrance slit (E) of a vacuum normal incidence one-meter spectrograph (R) and t,hc spec+rn recorded photographicaally (1’). In the path of the incident light, bct,ween Al and E, is interposed t.he absorbing deposit formed on a knsparrnt mindon- (1:) caooled by a cryostat (C). The source described elsewhere (5) is a modified t.hree-elec*trode va~um spark whosc continuum extends from the visible t.o t.he soft, x-rays (6). The spcc*trograph has a dispersion of X.5 A/mm and a slit width of 70 p. About 50 sparks, at a rate of 5 per second, are necOessary to give a good blackening of the Kodak I’ath6 SC 5 film down t,o 1050 A, because of the presence of three cleaved I,ilf windows in the light path. We successively used two cryostats: t,he first was limited to work at. 4.2’~ while the second permitted us to work successfully at l.:i”Ii by decrckng the pressure above liquid helium down to 3 t.orr. l’or producing H at,oms an Ar-H?O mixture was ndmit’ted in a Wood’s tubtl. The discharge products were condut4ed to the cryos;tat through a t&on-c*oaterl I’yrex tube cooled with liquid S? or possibly with liyuid Hz. III. A.
RESULTS
AXI)
~lSClX3ION
COMPAHISON WITH HIGH I’RESSUHE EFFECTS IS GASES
(1) .l/ewwy
resonawe
lines
At first we st)udied the two resonanc~c lines “P, t lSO (2537 A) and 11’~+ IS,, (IS.50 8) of Hg trapped in an Ar matrix at 20°K (7). The two lines are shifted and broadened as shown in Table I. An extra line a,ppears whkh might be due to the forbidden t,ransition “ZJ2+ ‘So (2270 8). In Fig. 2 we represent the extrap olnt.ion of Robin’s curve for t’hc displacement, of the line 3.537 8 and its s:itcllitc>s versus the densit,y of Ar (8). The point. for Hg-Ar at 20’3 (*orresponding to :I
FIO. 1. I)rawing of the assembly. (S) Three electrode vacuum spark source. (M) toroidal mirror. (E) F,ntranre slit of the spectrograph (Ii) Conc:tve grating. (I’) Photographic film. CC) Liyuid helium cryostat. (I,) LiF window.
156
RONCIN,
DAMANY,
AND
ROMAND
TABLE I POSITION AND SHIFT OF THE 3 BANDS IN SOLID Hg-Ar
Hg
gas
Hg-Ar
20°K
hA
Y cm-
Y’
/-Y
2537 2270 1850
39 410 44 055 54 055
40 320 48 500 55 700
910 4450 1650
Transition
aP1-‘so 3Py’s” ‘PI-‘So
x4, 2540 2530 2520 2510
2490 2460 2470 2460 2450 --.--~~
/
2440
/ 200
400
600
1 ‘~ 1 \ / ‘a.
1 p(amagatsl
000 989 993 1200
FIG. 2. Shift of the satellite of the 3P1 +- ‘SO line of Hg in compressed Ar (Robin). Extrapolation; 0, McCarty’s result (Hg-Ar: 4.2”K) ; +, our result (Hg-Ar: 20°K).
--
density of 989 amagats obtained by us is in good agreement wit,h the extrapolated part of the S1 curve deduced from the high pressure experiment. The discrepancy between our results and those of McCarty and Robinson (9) could be ascribed t,o differences in temperature and concentration in the light of a recent work (10) undertaken to study the systematic influencezf various factors on the format,ion of the layer. Although the shift of the 1850-A line perturbed by compressed Ar is only known up to a density of 400 amagats (ll), the shift in the solid appears to fit’ the extrapolated curve. (2) Xenon
resonance
lines
We observed the resonance doublet of xenon [9$Jlo+- ‘So (1470 A) and [$$Jl” + ‘X0 (1296 A) in the pure solid and in matrices of Kr, Ar, and Ne at 20°K or at,
UV ABSORPTION
OF TRAPPED
SOLID
SPECIES
FIG. 3. Absorption spect.ra of solid Kr (20°K). For all the figures the thickness layer illcreases from the top to the bottom.
1.57
of the
(12). We obtained the displacement of the t’wo lines and verified the xppearunce of extra bands in the solid. ,4s in the c*ase of Hg, the shift could be wnlpared with the curve giving the displacement of the line 1470 d of Xe perturbed by Ar up to a density of 500 amagats (13). Our point is then located on the curve extrapolated t’o 989 amagat’s according to a p3 law. These results, as well as those concerning solid Kr in the pure state and in matrices of Ar and Ne (Fig. 3), are very close to those obtained simultaneously by Rnldini using a line source and a monochromat’or and direct’ly condensing on the fluorescent support which serves as the receptor (14). Baldini att,ributes the new bands of the solid t’o hydrogen-type series due to exciton levels in the crystal. Phillips gave a det’ailed interpretation of the pure solid Xe spect,rum in terms of band struct’ure (16).
I.YI<
K.
ST.IBILIZATION OF H RADICAL IN AN AR MATRIX AT 4.2”Ii
We suwessfully obtained t’he absorption spectrum of H in solid Ar at, a cow wntration of about l:%O (16). The Lyman (Y resonance line (1216 A) of atomit hydrogen is shifted towards short wavelengths by 45 A (Av - 2340 cm-l)
158
HONCIN,
DAMANY,
AND ROMAND
(Fig. 4). Our results are very close to those obtained by Baldini using a similar technique (17). The large half-widt’h (Avl,z - 1300 cm-‘) might be due to the great mobility of H atoms at 4.2%. Keil and Gold have calculated theoretically t.he energy of the Is-2p transit’ion of H atom in a substitutional site of solid Ar using the tight binding approximation (18). It might perhaps be possible to get a sharper band by cooling H in Ne down to 1.5% for which the isotopic shift would not be masked by the bandwidth, as judging from our results with CO-Ne and NO-Ne (see sect’ion B(3)). B. ELECTRONICSTATES OF DIATOWC MOLECULES Dressler gave preliminary results about pure solid CO and Nz at 4.2% down to 1200 B (2) and recently Brith and Schnepp reexamined the problem (19). Extending the spectral region and the range of temperature we studied solid CO, Kg, and NO both in the pure state and in various matrices (%‘O-.Z%). (1) Fourth Positive Group of CO Figures 5-7 exhibit the absorption spectra of solid CO pure and in matrices of Ar and Ne. Table II summarizes the results. The splitting clearly observed for the most’ intense terms of the pure solid is ascribed to be of the Davydov type; it arises from the coupling between molecules occupying translationally nonequivalent sit’es. The splitting is approximately proportional to the Franck--
FIG. 4. Absorption
spectra
of solid H-Ar
(4.2”K)
IJY ABSORPTION
OF TRAPPED
SOLIJ> SPE( XES
159
FIG. 5. Absorption spectra of pure solid CO c4.a°Kj 70 to 4000 A thick. The weak bands in the long wavelength side of the four first bands may be ascribed eit,her tu the PO’” isotope of CO or IO a secondary trapping site in solid Ne. Condon fxtors. In various mat.rices the interaction between CO molecules vanishes so t,hat the bands become sharper and the shift is constant for each vibrational term, which indicates that. there is no deformation of t,he pot,ential energy curve hut only a vert)ical Cranslation. The films absorb in t,he short wavelength region but not compleOely as can be seen on the first spectrum, reprotlucaed in lcig. 7, obtained with longer exposure time. Although the longer wavclengt,h bands clearly prove the presence of CO no trace is found of t,he Rydberg transitions like RIZ+ +- >Y IS+ But, it,s absence is not conclusive since it is quite possible that the system would be either shiftSed by about 100 w towards shorter wavelengths as in t.he case of Rydberg series of atomic: lines, so that t,his system would be situated below the experimental limit, of observation (1050 8), or broadened so much that. it gives rise to a weak cont,inuous absorption diffic*ult 10 cleteclt .
(2) Forbidden Transitions
of i’-C?
The transit#ions of NZ being forbidden in this spectral region their int,ensities are very low so that’ a relatively thick film (of the order of 0.8 cc) is necessary for getting the bands. The results obtained with pure solid iY2 at 4.2% are shown iI1 I’ig. S. Although the bands are sharper, the split,ting is smaller than in the case of CO so that it is not clear. The series w-S is more int,ense; this means that thr w -37 transition is comparatively enhanced by a facator of lo3 as it becomes
160
RONCIN,
FIG. 6. Absorption
DAMANY,
AND
spectra of solid CO-Ar
FIG. 7. Absorption
spectra
ROMAND
(4.2”K) 5000 to 12 000 .& thick
of solid CO-Ne
(15°K)
dipole-allowed in the solid (19). The observation of this transition in the solid brings an additional support to the identification proposed by Tanaka (W). In spite of the great thickness of t’he layers required for observing absorption bands of N, in a matrix (from 10 to 50 p) we recently succeeded in this attempt, by condensing N, in X’e at 1.5% with the time of evaporation ranging from 20 min to 1 hr. The bands are very much sharper than in the pure solid, the two series are shifted very little as in the case of CO in Ke and the intensities are similar to that in the gas since w-X remains dipole forbidden in the matrix. The remark made with regard to the Rydberg transition of CO still remains valid for the C3n, t X’Z,+ transition of N2.
UV ABSOR.PTION
OF TRAPPED TABLIX
,SHJF.TAND
HALF-WIDTH
OF THE
SOLII)
II
\:IBKA.I'IONAL TERMS
POSI'rIVE t:ROUP
OF SOLID
4.2’K
“,--yfl Av1/2
-350 330
FIG. 8. Absorption
(13) Zhited
States
-590 160
spect,ra
OF THE
of pl~re solid
CO-Ne - 65 110
N?(4.2”K)
~TH
co
l.S”K C0m.k
pure co
161
SPECIES
-___
CO-Kr
CO-Se
- 1000 130
- 1twJ 400
3500 to 50 000 i
thick
oj” NO
While no definite conclusion could be drawn regarding the Rydberg transitions of CO and X2 in the solid due to t,heir t,oo high energy levels, the X0 molecule affords, in contrast, a particularly favorable case, sincae its Rydberg states and also it,s valence states lie het’ween 2300 and 1300 ,k. Partly because of it.s odd number of electrons, t-he behavior of NO is cornplet,ely different from that. of CO and X 2; as a matter of fact in the pure solid stat,e only a large absorption band ascribed to X\;?O?has been observed at, 2100 K (1). We did not observe any mw st,ructure at short wavelengths. In contrast, on isolating SO in a rare gas matrix, we find some 40 bands as shown in Figs. 9 and 10. The remarkable feature of t’hese spectra is the sharpness of the NO-Nc hands which is only limited by the resoluGon of the speckograph (0.6 A). Thanks to the recent analysis of the spectrum of X0 gas by Miescher (k$, 25) we have been able to ascribe prakcally all the bands of the solid t,o the three valence st.at.es ELI, H’“A, and G*Z:- + .Y%. In the gas there are irregular vibrational spacGngs in the BzIl and B’?A st,atcs corresponding to the crossing of energy
162
RONCIN,
FIG. 9. Absorption
spectra
DAMANY,
AND
of solid NO-Ar
ROMAND
(4.2%)
5000 to 15 000 d thick
.__.. _
FIG. 10. Absorption
spectra
of solid NO-Ne
(1.5%)
potential curves between these valence states and Rydberg states of the same symmetry. On the contrary, the series become perfectly regular in the solid as there seems to be no Rydberg st,ates to cause perturbations. In Table III we summarize the displacement of each series in the solid with respect to the deperturbed ones in the gas. On changing the matrix from Ar to Ne the series B12Aand G% sharpen while the inverse is true for the series B2K This might involve a correlation between the different lattice structure of the two matrices and the different orbital extension of the two configurations . . . (~2p)(&p)~ (ii2p)’ for B’ and G and . . . (~&$)~(7r2p)~(ii2p)~for B. The disappearance of the Rydberg transitions can be qualitatively explained by the large size of the
ITV ABSORPTION
OF TRAPPP:I> TABLE
SIIIFT Transition “K 1.5 -4.2
SOLII)
SPECIES
163
III
AND HALF-WIDTH OF THE VIBRATIONAL TERMS OF TTALENCE TRANSITIONS OF SOLID NO BT-SW
Cl?-'
Yx-Yg
PiO-Ne NO-Kr NObAr Nt )bAr
-60 -355 -205 - 175
B’V~.Y*II. Au1 i? 100
60 (30 10
Y*-Vg
-5x - 700 -305 -4Y5
THE
G2Y- -5*11
A Y, ,'?
30 150 120
vr-rrl
&Ii4
- 70 -Y70 -100 - GO5
30 150 120
Rydherg orbit:& as compared to those of the valence orbitals (nearly 3 t,imes larger). This will considerably increase the probnbilit’y of transitjion of the clcc*t,ron to the valence orbit& which are less affect.ed by the environment. E’urt#her the configuration . . . (?&I)” of the valence stat,es has a much more compac*t charge distribution wit,h its two elec.trons t,han the (*onfiguration . . . Xnl(n 2 3) of the Rydberg states. I\.. CoNCLuGIos Our work was not’ exhaustive and, in a general way, it, would be useful to rcexnmine the spectra of t,he molecules in the solid state under higher resolution with t#hc view of observing the isotopic2 shifts, and also in the exkeme ultraviolet in order to look for Rydberg states in solid CO and iY?. Further the experiments should be undertaken with temperature, concaentration and evaporat’ion &es varying c~ont~inuously over large ranges. It would be also ext’remely worthwhile to st,udy t)he crys:talline structure of the deposits and to prepare if possible monocsrystalline layers for polarization studies t,o make easier the int’erpretation 01 thr specka in t,he light of solid state theory.
The authors are grateful to Prof. E. Miescher (University of Basel) for nlunicating his latest results of the analysis of 30 gas hefore publication and express their sincere thanks to him as well as to Prof. K. Dressier (Princetjon 1)r. H. Lefebvre-Brion, and l)r. C. Moser (Centre de MCcanique Ondulatoire Paris) all:1 Dr. V. Chandrasekharan (Lahoratoire des Hantes Pressions) for discussions we had with them. REC’EIVEI)
: August
kindly comthey wish to IJniversity), Appliqnbr. the helpfttl
15, 1966 REFERENCES
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164
RONCIN,
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AND
ROMAND
J. ROBIN, J. Rech. Centre Natl. Rech. Sci., Lab. Bellevue (Paris) 47, 89 (1959). M. MCCARTY AND G. W. ROBINSON, J. Chem. Phys. 66, 723 (1959). L. BREW-ER,B. MEYER, AND G. D. BRABSON,J. Chem. Phys. 43, 3973 (1965). 8. ROBIN AND S. ROBIN, Rev. Opt. 37(4), 161 (1958). J.-Y. RONCIN, V. CHANDRASEKHARAN, AND N. DAMANY, Compt. Rend. 268, 2513 (1964) S. ROBIN AND J. ROMAND, Compt. Rend. 231, 145Fj (1950). 14. G. BALDINI, Phys. Rev. 137, A 508 (1965). 16. J. C. PHILLIPS, Phys. Rev. 136, 1714 (A) (1964). 16. J.-Y. HONCIN, N. DAMANY, AND B. VODAR, Cow~pt. Rend. 260, 96 (1965). f7. G. BALDINI, Phys. Rev. 136, 248 (A) (1964). 18. T. H. KEIL AND A. GOLD, Phys. Rev. 136, 252 (A) (1964). 19. M. BRITH AND 0. SCHNEPP,Mol. Phys. 9, 473 (1965). 20. J.-Y. RONCIN AND N. DAMANY, Compt. Rend. 260, 6069 (1965). 21. J.-Y. RONCINAND N. DAMANY, Compt. Rend. 261, 4697 (1965). 22. J.-Y. RONCINAND N. DAMANY, Compt. Rend. B262, 1436 (1966). 23. Y. TANAKA, M. OGAQ-A, AND A. S. JURSA, J. Chem. Phys. 40, 3600 (1964). 24. K. DRESSLERAND E. MIESCHER, Astrophys. J. 142, 1660 (1965). 25. A. LAGERQVISTAND E. MIESCHER, Can. J. Phys. (to be published). 8. 9. 10. 11. 12. IS.