Emission and excitation spectra of CCl2 in solid argon

JOURNAL

OF MOLECULAR

SPECTROSCOPY

64, 180-183

Emission and Excitation

(1977)

Spectra of CCI, in Solid Argon

V. E. BONDYBEY Bell Laboratories,

Murray

Hill, New Jersey 07974

Studies of the CClz excitation spectra in solid argon give values of 624 and 304 cm-1 for the upper-state vibrational modes ~1 and ~2, respectively. The OOWlOO band of the electronic transition occurs at 17 092 cm-l. Vibrational relaxation in the upper state is fast compared with the 3.6 Nsec emission lifetime, and only vibrationally relaxed emission is observed.

The CC12 radical has been prepared by Milligan and Jacox by matrix reactions of Cl2 with carbon atoms (I, 2). They have studied extensively its infrared spectrum and assigned both of the ground-state stretching vibrational fundamentals. They also observed a progression of broad absorption bands in the visible with an average spacing of 306 cm+ and assigned them to a progression in the upper-state bending frequency. Shirk has succeeded in exciting the CC& emission spectrum in solid argon by the 5145 A argon ion laser line (3) and observed a‘long progression in the groundstate bending fundamental. More recently, Andrews et al. have prepared CC12 by the reaction of carbon tetrachloride with Li atoms (4), and demonstrated that progressions in both ~1 and v2 appear in the emission spectrum. The initial intent of the present study was to study vibrational relaxation following selective excitation of the upper-state vibrational levels of CC&. However, our first experiments have already demonstrated that the emission in CC12 occurs from the totally vibrationally relaxed level and that vibrational relaxation is fast compared with the radiative lifetime. Study of matrix excitation spectra can provide detailed information about the upper-state vibrational potential function, The role of Freons as potential pollutants for the upper atmosphere (5) enhances the interest in the spectral properties of their radical fragments, and we therefore report the results of our study in the present paper. EXPERIMENTAL

DETAILS

Our experimental techniques were described in detail previously (6). Briefly, the spectra were excited using a tunable dye laser (Chromatix CMX4), which was slightly modified to permit wavelength scanning. The sample reemission was resolved by SPEX 14018 double monochromator, and detected by an RCA C31034 photomultiplier. CClz was prepared by in situ photolysis of CH2C12 in the matrix with the Lyman Q! line (1216 A). Typically, 2 mmole of the sample (1:2000-l : 10 000 CH,C% in argon) was deposited at 4.2 K over a period of 1 hr on a Pt mirror. 180 Copyright

Q 1977 by Academic

AU rights of reproduction

Press. Inc.

in any form reserved.

CClz SPECTR.4

: bb

IN

SOLID

181

ARGON

C~~CL"CP

I

Y

FIG. 1. CC12 emission

spectra

RESULTS

[cd] argon: (a) CY12;

in solid AND

(b) CXY’CI.

DISCUSSION

Figure 1 shows typical Ccl? emission spectra. Panel (a) shows a part of the C3T12 emission ; the same section of the spectrum of the C3T137C1 species is shov,n in panel (b). Figure 2 shows the excitation spectrum for the major C3W2 isotope. It was previously demonstrated (#) that the emission spectrum consists of progressions in both groundstate symmetric vibrations, ~1 and ~2. One can therefore expect both the stretching and bending vibrations to occur also in the excitation spectrum. ‘Ihis is confirmed

v

FIG. 2. CC12 excitation

spectrum;

[cm-l]

spectrum

is not corrected

for laser power.

182

V. E. BONDYBEY

_Band

r

in the !:-' 'I?$ Excitation ?pectrum [cm-l airj

“;/vi

0

2

17':192

177x

18333

1

173%

1;,>2:<

18637

2

1';7on

18324

lR941

3

1811,'<

15121-

1924i-

4

18309

1893,~

5

18613

6

18917

0

by our results

3

1

and the observed

19251

bands and their assignments

are listed in Table

1.

The w+‘, wz”, xrl”, and ~12~constants can be derived from the spectrum and are listed in Table 2. Similar to the ground state, the upper-state bending fundamental v2 also shows a negligible anharmonicity. The sharpness of the zero phonon lines in the excitation permits

selective

by the partial

excitation

of individual

isotopic

spectrum

(FWHM

emission spectra in Figs. la and lb. The observed

isotope and their assignment

are shown in Table

-3

cm+)

species of CC12, as is demonstrated bands for the major

3. The molecular

constants

for the

ground state are also listed in Table 2. The ground-state stretching fundamental implied by our data, y1 = 722 cm-l, is in excellent agreement with the infrared value of 721 cm-’ Tevault

(1, 2). Our value of ~2, 333 cm-‘,

is -7

cm-l

higher than reported

by

et al. (4).

It was previously

suggested

in Ref.

(4) that the CC& emission occurs from vibra-

tionally quenched states. In that work CCI, was prepared by the reaction of CC14 with alkali metals. The alkali halide molecules which are thus formed in the vicinity of the CCIZ radicals would, with their low frequency of the vibrational

energy.

The present

modes, act as efficient acceptors

work, where the CC12 is formed

by H&J&

photolysis, might offer a better chance for observing vibrationally unrelaxed emission. We find, however, that identical spectra are observed regardless of which of the bands and which of the upper-state confirm the fully vibrationally

vibrational

modes was initially

relaxed nature of the emission. TABLE 2

Summary of the Molecular Constants of Ccl2 in Argon -1 [cm 1 4

4

G

lower state

726

333

745

upper state

626

309.5

v

00

17093 cm1

?

0 Y1

x22

-3.7

-2.2

-0.2

-2.0

-0.4

0.0

0

32

excited,

and we

183

Ccl, SPECTRA IN SOI,ID ARCO?i

i5 Fmission band:: ot' C- ':12 in Argon rem-' air1 ”

“/V

2

0

”

1

1.

2

3

4

5

12908

0

17'i92

1r: 3h 9

1

16759

1COII;)

15321;

1’!619

2

16427

157”)?

11’997

14293

13595

3

1csgs

15377

lkhg

13967

13272

4

15763

15040

14341

13641

12951

5

15429

lh719

1401.5

13315

13688

13931

6

In a recent work (6) we have shown that in CF2 the vibrational relaxation is competitive with the radiative lifetime. In the present case we find for CC12 emission in argon a lifetime of 3.6 bsec, considerably longer than the 27 nsec value found for CF2. Considering this long lifetime and lower vibrational spacing in Ccl?, the absence of appreciable unrelaxed emission is understandable. The lowest energy band in the excitation spectrum coincides with the first band in emission and must therefore be the Ooo-OOO band. This band therefore lies at 17 092 cm-r, and the vibrational assignment in Ref. (3) is in error by one quantum. Comparison of the molecular constants in Table 2 shows that both the symmetric vibrational frequencies in the upper state are slightly reduced from their ground-state values and indicate weaker bonding in the excited electronic state. Finally, one may infer from the appearance of long progressions in both the bending and stretching fundamentals in our spectra that not only the Cl-C-Cl valence angle but also the interatomic distances change in the transition.

In the present work we study the excitation spectra of CC12 in solid argon and find values of 624 and 304 cm-‘, respectively, for the upper-state symmetric vibrations, ~1 and ~2. The 000-000 transition is found to occur at 17 092 cm-‘. The study also reveals that vibrational relaxation in the upper state is fast compared with the observed 3.6 psec lifetime, and all emission occurs from the lowest vibrational level. RECEIVED:

July 12, 1976 REFERENCES

1. 2. 3. -I. 5.

D. E. MILLICAN AND M. E. JACOX, J. Chem. Phys. 47, 703 (1967). M. E. JACOX AND D. E. MILLIGAN, J. Chm. Phys. 53, 2688 (1970). J. S. SHIRK, J. Chem. Phys. 55, 3608 (1971). D. E. TEVAULT AND L. ANDREWS, J. Mol. Spectrosc. 54, 110 (1975). D. D. DAVIS, J. F. SCHMIDT,C. M. NEELEY, AND R. J. HANRAHAN, J. Phys.

Chem. 79, 11 (1975).

6. V. E. BONDYBEY, Vibrationally unrelaxed fluorescence in matrix isolated CF2,” to appear.