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The near ultraviolet spectra of benzene in inert solids
JOURNAL
The
OF MOLTSCULAR SPFCTROSCOPY
Near
Ultraviolet GARY
Department
SMITH,
of Physics,
36,61-65
(1970)
Spectra SUSAN
of Benzene
HENRY,
Texas Christian
AND
University,
in Inert
Solids’
C. E. BLOUNT Port
Worth, Texas
Y612.9
The near ultraviolet absorption spect,ra of benzene in argon, krypton, and nitrogen have been recorded for deposit temperatures from 4.2 to 30°K. The spectra exhibit multiple structure that are dependent on the deposit t,emperature. These multiples can be explained by assuming one component is caused by isolat.ed benzene molecules in the host solid and the remaining components are due to int,eracting pairs of benzene molecules trapped at nonnearest-neighbor sites in the host. INTRODUCTION
Several authors (1-5) have reported the existence of multiple structure in the near ultraviolet absorption spectrum of benzene trapped in inert solids at IOK t)emperatures. Diamant, et al. (2) concluded that the multiple components found for benzene in argon at 20.4”1< resulted from the trapped benzene molecules having sharply defined but different environments in the host solid. The possibility of the coexistence of two crystalline phases, cubic and hexagonal, of the host lattice in the presence of the benzene impurity was discussed. The multiple structure observed in the spectrum of benzene in argon at 20.4% ~vas not observed in the spectrum of benzene trapped in nitrogen at 20.4”K. Marusak and Rlount (,/c) reported the existence of multiple structure in the spectrum of benzene trapped in argon and in nitrogen at 4.-‘>“I<. From the temperature dependence of the spectrum of benzene in nitrogen for temperatures between 4.2 :md 30°K (5) multiple structure was observed in the spectrum of trapped benzene :tt deposit temperatures as high as 17.4%. The object of this paper is to show that the observed multiple structure can be explained by assuming the existence of both isolated benzene molecules in tllrb solid and interacting benzene molecules trapped at nonneighboring sitc,s in ttle llost lattice. EXPERIMENTAL
METHODS
The spectrum of trapped benzene was obtained on a 2.2-m ~\IcPhersor~ spec:trograph using Ilford Q-2 plates. A hydrogen source and mercury reference lines 1Work supported
by the Office of Naval
Research 61
and The Robert
A. Welch Folmdxtiorr.
SMITH,
62
HENRY,
AND
BLOUNT
were used. An Air Products AC3L Cryo-tip was used to cool a sapphire window. The temperature of the window was measured with a gold-iron versus normal silver thermocouple. Spectra grade benzene (vacuum distilled) was premixed with Air Reduction research grade argon and krypton in molar ratios of 250: 1 to 2000: 1. The gas mixture was deposited at a controlled rate using a Granville-Phillips variable leak. The thickness of the deposit was determined using an interference method. Deposit thickness of from 7 to 26 p were measured depending on the molar ratio of the gas mixture.
L 390
395
20-K
390 ( X lO*CM-‘)
FIG. 1. Microdensitometer in argon at 20, 25, and 30°K.
tracings of an absorption
band (A00 band in vapor) of benzene
NEAR UV SPECTRA OF BENZENE IN INERT 80LIl)s
63
DATA ANL) RESULTS The absorption spectrum of benzene in argon was obtained for deposit temperatures of 4.2,17,SO,25,and 30°K.The absorption spectrum at 30°K consists of a broad band and a sharp band to higher energy (the 0 + 516 transition along with the 924 vibration and its overtones), Fig. 1.At 25°K the width of the broad band is reduced. Three bands are observed at 20°K; a doublet having a separation of 7 and 86 cm-l lower in frequency than the sharp band observed at 30 and 25°K. The sharp band observed in the spectrum of benzene for deposit temperatures of 20, 25, and 30°K results from transition of isolated benzene molecules in the argon host. The broad band at 25 and 3O”K, and the doublet at 20°K result from transitions of interacting benzene molecules. At deposit temperatures of 25 and 30°K the argon host is not sufficiently rigid to give the well defined separation between interacting benzene molecules found at 20°K. The results are tabulated in Table I. The dependence of the multiple structure of benzene in argon on the deposit temperature is similar to results obtained for mercury in argon (6) where the high energy component was found to be due to transitions of isolated mercury and the low energy members were due to transition of interacting pairs of mcrcury atoms trapped at nonneighboring sites in the argon host. Further evidence that the multiple structure in the spectrum of trapped benzene is due to both isolated benzene molecules and benzene molecules trapped at TABLE: I _____ Vapor
___
Assignment -.. ___ .-
‘40°
AI”
A20
A30 -. _______a Freqtlency
SPECTRUM
Frequency (cm-‘)
OF
BENZENE
-.-I _.-Argon; frequency (cm-l) ~_ __ ._~__~~ _-.
-
(OK ): 20 _-.
2.5 (38 495)”
(38 495P
38 612
38 498 38 505 38 584
38 581
38 582
(38 420)
(38 420)
39 534
39 425 39 432 39 511
39 506
39 5oti
(40 350)
(40 350)
40 456
40 353 10 360 40 439
40 432
40 331
41 377.9 of broad band in parentheses.
-
41 280 (41 280) 41 287 41 366 41 357 ---__--_~..-_-~__-.
.~_.
30 ._.~ ____-
(41 280, ‘41 358 ~~~_~__~~._
64
SMITH,
1
38
.
i
I
I
39
HENRY,
AND
._
40
BLOUNT
1
1
38
FIG. 2. Photoelectric traces of benzene in argon at 20.4%: source; (B) after 20 min of exposure to HP source.
tracings of an absorption
.
I
(XIOG’,
(Xl&M-‘)
FIG. 3. Microdensitometer in krypton at 20 and 25°K.
I
39
(A) just, after exposure to HZ
band (Am0band in vapor)
of benzene
XEAR UV SPECTRA OF BENZENE IN INERT SOLIDS
(i.j
nonneighboring sites in the argon host was obtained from the bleaching (decrease in intensity of an absorption band from prolonged exposure to light at its nbsorption frequency) of the low energy members, Fig. 2. The bleaching of a band is interpreted to be the results of the migration of a pair of nonnearest-neighbor benzene molecules to form a dimer, a pair of benzene molecules at neighboring sites. This will result if the intermolecular potential between a pair of molecules is more attractive with one molecule of the pair in an excited state rather tjhan both of the molecules in their ground state. The spectrum of benzene in krypton was obtained for deposit temperatures of 4.2, 17, 20, and 25°K. The multiple structure observed in argon at 20°K is present in krypton for deposit temperatures of 20 and 25X, Fig. 3. The doublets have a separation of S cm-’ and are 90 cm-’ lower in frequency than the isolakd band. The presence of the doublets in krypton at 25% is due to the fact that at 25°K the krypton lattice is still rigid while the argon lattice is not. The upper limit on the deposit temperature for well defined separation between interacting benzene molecules of 17°K for nitrogen (6), 20% for argon, and 25% for krypton are about 12°K below the upper limit for the temperature of these materials for a matrix [35”K for argon, 30% for nitrogen, and 40°K for krypton (Y)]. RECEIVED:
December
S, 1969 REFERENCES
1. M. MCC.ARTY AND G. W. I~OIXNSON, Mol. Phy. 2,415 (1959). 2. Y. I)IAM:ANT, R. M. HEXTER, AND 0. SCHNEPP, J. Mol. Spedrose. 3. 4. 5. 6. 7.
E. G. R. R. E.
18, 157 (1965). HOLLIER AND C. BLOUNT, J. Mol. Spectrosc. 19, 456 (1966). MARUSAK AND C. BLOUNT, Bull. Amer. Phys. Sot-. 12, 200 (1967). B. MERRITHEW, G. V. MARUSBK, AND C. BLOUNT, J. Mol. Spectrosc. 26, 269 (1968). B. MERRITHEW, G. V. MARUSAP, AND C. BLOUNT, J. IWOZ.Spectrosc. 29, 54 (1969). II. BECKER.AND G. C. PIMENTAL, J. Chenr. Phys. 26, 224 (1956).
The
OF MOLTSCULAR SPFCTROSCOPY
Near
Ultraviolet GARY
Department
SMITH,
of Physics,
36,61-65
(1970)
Spectra SUSAN
of Benzene
HENRY,
Texas Christian
AND
University,
in Inert
Solids’
C. E. BLOUNT Port
Worth, Texas
Y612.9
The near ultraviolet absorption spect,ra of benzene in argon, krypton, and nitrogen have been recorded for deposit temperatures from 4.2 to 30°K. The spectra exhibit multiple structure that are dependent on the deposit t,emperature. These multiples can be explained by assuming one component is caused by isolat.ed benzene molecules in the host solid and the remaining components are due to int,eracting pairs of benzene molecules trapped at nonnearest-neighbor sites in the host. INTRODUCTION
Several authors (1-5) have reported the existence of multiple structure in the near ultraviolet absorption spectrum of benzene trapped in inert solids at IOK t)emperatures. Diamant, et al. (2) concluded that the multiple components found for benzene in argon at 20.4”1< resulted from the trapped benzene molecules having sharply defined but different environments in the host solid. The possibility of the coexistence of two crystalline phases, cubic and hexagonal, of the host lattice in the presence of the benzene impurity was discussed. The multiple structure observed in the spectrum of benzene in argon at 20.4% ~vas not observed in the spectrum of benzene trapped in nitrogen at 20.4”K. Marusak and Rlount (,/c) reported the existence of multiple structure in the spectrum of benzene trapped in argon and in nitrogen at 4.-‘>“I<. From the temperature dependence of the spectrum of benzene in nitrogen for temperatures between 4.2 :md 30°K (5) multiple structure was observed in the spectrum of trapped benzene :tt deposit temperatures as high as 17.4%. The object of this paper is to show that the observed multiple structure can be explained by assuming the existence of both isolated benzene molecules in tllrb solid and interacting benzene molecules trapped at nonneighboring sitc,s in ttle llost lattice. EXPERIMENTAL
METHODS
The spectrum of trapped benzene was obtained on a 2.2-m ~\IcPhersor~ spec:trograph using Ilford Q-2 plates. A hydrogen source and mercury reference lines 1Work supported
by the Office of Naval
Research 61
and The Robert
A. Welch Folmdxtiorr.
SMITH,
62
HENRY,
AND
BLOUNT
were used. An Air Products AC3L Cryo-tip was used to cool a sapphire window. The temperature of the window was measured with a gold-iron versus normal silver thermocouple. Spectra grade benzene (vacuum distilled) was premixed with Air Reduction research grade argon and krypton in molar ratios of 250: 1 to 2000: 1. The gas mixture was deposited at a controlled rate using a Granville-Phillips variable leak. The thickness of the deposit was determined using an interference method. Deposit thickness of from 7 to 26 p were measured depending on the molar ratio of the gas mixture.
L 390
395
20-K
390 ( X lO*CM-‘)
FIG. 1. Microdensitometer in argon at 20, 25, and 30°K.
tracings of an absorption
band (A00 band in vapor) of benzene
NEAR UV SPECTRA OF BENZENE IN INERT 80LIl)s
63
DATA ANL) RESULTS The absorption spectrum of benzene in argon was obtained for deposit temperatures of 4.2,17,SO,25,and 30°K.The absorption spectrum at 30°K consists of a broad band and a sharp band to higher energy (the 0 + 516 transition along with the 924 vibration and its overtones), Fig. 1.At 25°K the width of the broad band is reduced. Three bands are observed at 20°K; a doublet having a separation of 7 and 86 cm-l lower in frequency than the sharp band observed at 30 and 25°K. The sharp band observed in the spectrum of benzene for deposit temperatures of 20, 25, and 30°K results from transition of isolated benzene molecules in the argon host. The broad band at 25 and 3O”K, and the doublet at 20°K result from transitions of interacting benzene molecules. At deposit temperatures of 25 and 30°K the argon host is not sufficiently rigid to give the well defined separation between interacting benzene molecules found at 20°K. The results are tabulated in Table I. The dependence of the multiple structure of benzene in argon on the deposit temperature is similar to results obtained for mercury in argon (6) where the high energy component was found to be due to transitions of isolated mercury and the low energy members were due to transition of interacting pairs of mcrcury atoms trapped at nonneighboring sites in the argon host. Further evidence that the multiple structure in the spectrum of trapped benzene is due to both isolated benzene molecules and benzene molecules trapped at TABLE: I _____ Vapor
___
Assignment -.. ___ .-
‘40°
AI”
A20
A30 -. _______a Freqtlency
SPECTRUM
Frequency (cm-‘)
OF
BENZENE
-.-I _.-Argon; frequency (cm-l) ~_ __ ._~__~~ _-.
-
(OK ): 20 _-.
2.5 (38 495)”
(38 495P
38 612
38 498 38 505 38 584
38 581
38 582
(38 420)
(38 420)
39 534
39 425 39 432 39 511
39 506
39 5oti
(40 350)
(40 350)
40 456
40 353 10 360 40 439
40 432
40 331
41 377.9 of broad band in parentheses.
-
41 280 (41 280) 41 287 41 366 41 357 ---__--_~..-_-~__-.
.~_.
30 ._.~ ____-
(41 280, ‘41 358 ~~~_~__~~._
64
SMITH,
1
38
.
i
I
I
39
HENRY,
AND
._
40
BLOUNT
1
1
38
FIG. 2. Photoelectric traces of benzene in argon at 20.4%: source; (B) after 20 min of exposure to HP source.
tracings of an absorption
.
I
(XIOG’,
(Xl&M-‘)
FIG. 3. Microdensitometer in krypton at 20 and 25°K.
I
39
(A) just, after exposure to HZ
band (Am0band in vapor)
of benzene
XEAR UV SPECTRA OF BENZENE IN INERT SOLIDS
(i.j
nonneighboring sites in the argon host was obtained from the bleaching (decrease in intensity of an absorption band from prolonged exposure to light at its nbsorption frequency) of the low energy members, Fig. 2. The bleaching of a band is interpreted to be the results of the migration of a pair of nonnearest-neighbor benzene molecules to form a dimer, a pair of benzene molecules at neighboring sites. This will result if the intermolecular potential between a pair of molecules is more attractive with one molecule of the pair in an excited state rather tjhan both of the molecules in their ground state. The spectrum of benzene in krypton was obtained for deposit temperatures of 4.2, 17, 20, and 25°K. The multiple structure observed in argon at 20°K is present in krypton for deposit temperatures of 20 and 25X, Fig. 3. The doublets have a separation of S cm-’ and are 90 cm-’ lower in frequency than the isolakd band. The presence of the doublets in krypton at 25% is due to the fact that at 25°K the krypton lattice is still rigid while the argon lattice is not. The upper limit on the deposit temperature for well defined separation between interacting benzene molecules of 17°K for nitrogen (6), 20% for argon, and 25% for krypton are about 12°K below the upper limit for the temperature of these materials for a matrix [35”K for argon, 30% for nitrogen, and 40°K for krypton (Y)]. RECEIVED:
December
S, 1969 REFERENCES
1. M. MCC.ARTY AND G. W. I~OIXNSON, Mol. Phy. 2,415 (1959). 2. Y. I)IAM:ANT, R. M. HEXTER, AND 0. SCHNEPP, J. Mol. Spedrose. 3. 4. 5. 6. 7.
E. G. R. R. E.
18, 157 (1965). HOLLIER AND C. BLOUNT, J. Mol. Spectrosc. 19, 456 (1966). MARUSAK AND C. BLOUNT, Bull. Amer. Phys. Sot-. 12, 200 (1967). B. MERRITHEW, G. V. MARUSBK, AND C. BLOUNT, J. Mol. Spectrosc. 26, 269 (1968). B. MERRITHEW, G. V. MARUSAP, AND C. BLOUNT, J. IWOZ.Spectrosc. 29, 54 (1969). II. BECKER.AND G. C. PIMENTAL, J. Chenr. Phys. 26, 224 (1956).