Molecular spectra in the vacuum ultraviolet

JOURNAL

OF MOLECULAR

Molecular

SPECTROSCOPY

Spectra

6.

1-57 (1961)

in the Vacuum

Ultraviolet*

P. G. WILKINsoNt Laborator!/ of Molecular

Structure

and Spectra,

(‘hicago,

C’hicago,

Depnhtt~et,t oj

I’hysics,

l’h,e

f:niver.sit!g

of

Z/linois

Vacuum ultraviolet spectra of diatomic molecules since 1950, and polyatomic molecules since 1940 are reviewed in detail. In addition to a theoretical discussion, t,he data obtainable are presented in the form of tables which list the spectral region, molecular constants, characteristics of the transitions, intensity dat,a, ionization potentials, and references. A brief discussion of the application of the data to astrophysical problems is presented. I. ITTRODL~CTION n-e\\- methods and a renewd interest in vacuum ultraviolet spectroscopy have contjributed much new dat’a on molecular spectra especially since 1950. It has been shown possible to resolve rotational fine structure of diatomic and a few polyatomic molecules by employing higher orders of a concave diffraction grating. A rather large number of monochromators employing photoelectric detect#ion techniques have been built; these have eont’rihut,ed great,ly to intensity studies of molecular spectra. Finally, a series of new light, sources producing emission continua of t’he rare gases have made possihlc much improved absorption spectra. In this paper I plan to review the information obtained from the spectra of diatomic and small polyatomic molecules in the vapor stake. Some of these are: N, , 02 , NO, IT2, CO, CS, , CO2 , COS, CgHs , tizH4 , HCX, CH, , CH, , CH, , CsH6 , ?;&, (CH,),O and EOa . In N, electric quadrupole lines have been observed for the first time; in N2 and CO a number of forbidden band systems, some involving intersystem transitions, have been observed. This has contributed to a further understanding of transition probabilities of forbidden systems. In 01, the high resolution study has resulted in a much more precise dissociat(ion energy and in observation of predissociation processes at wavelengths longer than 1750 A. In NO numerous electronic t8ranxitions have been fully resolved and a study of the complex interactions of electronic states made. In I?? , CSz , COS, SzO, and C&He, to name a few, Rydberg series have heen observed leading to an

* This work was Cambridge Research AF 19(604)-3478 with 1 Present address:

assisted by the Geophysics Research Directorate of the Air Force Center, Air Research and Development Command under Contract the University of Chicago. U. S. Naval Research Laboratory, Washington 25, I).
2

WILKINSON

I400

1300

WAVELENGTH

IS00

1600

( A )

FIG. 1. Microphotometer

tracing of the krypton continuum excited by microwave power. Certain emission and absorption lines occurring in the light source are marked and serve as wavelength standards.

ionization potential. Of special importance are photoionization experiments which have established the ionization potentials of over 300 molecules. Absorption coefficients have been measured for a considerable number of molecules in the region 1000 A to 2000 A and for a few below 1000 A. Tables which give transitions, vibrational and rotational data, dissociation energies, ionization potentials, and intensity data are presented as a means of summarizing the data. The importance of the data to astrophysical problems is discussed. II. EXPERIMENTAL

TECHNIQUES

The recent discovery and development of rare gas continua as light sources have greatly contributed to absorption spectroscopy in the vacuum ultraviolet. The only other sources available producing continua below 2ooO A are hydrogen and the Lyman source.’ The rare gas sources (I, 2) have the following advantages: (1) simplicity of construction; (2) steadiness (suitable for photoelectric detection) ; (3) pure molecular continua; (4) short wavelength range (-600 A) ; therefore, they may be used in higher orders without separation of orders. However, the helium continuum is excited only by means of a condensed discharge; this makes it unsuitable for use with photoelectric detection. The neon 1For a survey of light sources of this type, see, e.g., Wilkinson

and Tanaka

(1).

MOLECULAR

SPECTRA

IN VACIJUM

ULTRAVIOLET

3

continuum is too short in wavelength coverage (300 A) and too weak t,o he useful except for special purposes. Figure 1 is a microphot,omet,er tracing of the krypt.on contBinuum from 1250 to 1600 A. Emission impurity lines of CI, 01, and the Xc1 (1470 A) resonance lint in absorption serve conveniently as wavelength st.andard$. The first caommercial vacuum ultSrzlvioletS monoahromat)or was built’ by the Baird Opt,ical Co. and its design and performance has been described (5). Wavelength s&&on at the exit. slit, is st,t.ained by moving t,he l-meter concave grating around the Rowland circle. A much simpler design of monochromator (4) allo!vs wnvclcngt h selection merely by rotat’ing the grating about its vertical axis. Thi:: inst.riunrnt, is also available commercially (darrell-Ash Co. and McPherson IrlstSrumt~llt Corp.). Light’ d&e&on in the \‘a(‘uurn ultraviolet is obtained b> mounting a photomult’iplicr t,uhe, with t’he envelope coat,rd wit’h a fluorescent m:Ltcri:rl (usually sodium salirglat’c), at the exit slit. An absorption crll may IX> moruitr4 dirc&ly in front of the photomultiplirr; sllc*h au arrnngcmcnt, rctducrs t hr tlcgrc~ of photolysis of the sample t#o a small fraction of a percent’.’ &Znimport,ant, advance in t,he technique for measuring ionization potentials of molecules was made by Wat’anabc et al. (6) in which the ahsorpt’ion cell at, the csit slit, is replaced by a cell containing two parallel charged plat,es. Wit,h a vapor in th(> ~~41,and as the spectrum is scanned, an incidcnc*cx of ion current is produced at :I wuvtlength corresponding to the ionizat,ion potcntinl. 0vcr SO0 molcc!l&~s ha\-c> thlls betIn stsldied (7 1. h further improvement has been effected by at.1:tchiiig thrb exit slit. of a Seya-Namioka sprctromcbtcr to a mass spcc+romctcr (8). The mass numbers of the photoionizLLt,ioll prodllcta (XII thus be determined. In the preceding discussion, only low resolution (-4.3 A) has bc>en emphasizc>tl. Howrvcr, in order to resolve rotational structjurc of molecules and complex atomic spc~ctnt, quite diffcrcnt8 ttlchniqucs arcs nccessnry. I-Terc>,t,hc trend has bcnl (~wxrd kwgc instruments wit,h long radius (S&l 0 tnct.nrs) concave gratings used in higher ord(>rs (9-11 ). It thus hwomw possible t’o resolve the rot,ational strutturcl of many diat,omic and some of the simpler polyat~omic moleculrs. ,I stir\-cy of vacuum ult.raviolct. spcrtroscopy has brcn given by Price (11~ I. II I. 1 ,IAT( )MIC ,\I( )Ll~X:UL1~8 z1 complct,cb survey of diatomit* spectra up to 1950 has been given by Herzberg (12) and, therefore, the present, paper will be devoted primarily to the work done sincsc that time. The data from tjhew rcseawhes are summarized in Tahlc .I which lists the molecule (column 1)) the transit,ion (column 3 j, the band system origin or spectral region in which observed (c*olumn 3), we (column 4 1, R, (column 5), i,he ground statr dissociation rnrqy if known (column 6) and the iolliz:ttion pot,ent,ial if known (column 7). In a few cases oscillator strengths are known and, in t,he case of nitrogen, r&tivc transit8ion probabilities may be given;

BF

or

66398 65862 63739

GlC+ -x1x+

x ?z+

F%-

- x lx-+

A 'II-X 'C+

x ?x+

A 'E-X %+ 0

50529.20

0

43948.5

(=I

I.P.

1401.14 1.5107

1264.96 1.4206

(4.3)

(2.5) 801.95 .5523 ~---____~~~_________~-----~~~~~~~~~~--

803.95

866.60 .5783

54282.45

B ‘C+

?x+

938.38

800

955

948

958

57755.2

..x

(3.1)

D(ev)

___~_____~_~_____~_----~~-~~~~~~~~~~~

482

we" =

537

377-

-1 (cm >

B e

c ?x+ -x ?x+

E

67390

6 systems

2075 A

me’

1820-

=

-1 (cm )

me

-1 (cm 1

region

spectral

V o.

H - X ?Z+

?

Transition

x ?x+ _---_________________

AlF

AlCl

PblC2CUle

Diatomic Molecules

Table A

Intensity

(86)

(85) (117)

(84)

Ref.

z tL

8

x

P, -

c12

6

‘c+

lx

H - X 'C+ g

x

x

-

0

7

X

M -

‘Z+ g lz+ g lx+

__

D(ev)

(ev)

I.P.

208 460

63975

323.2 0.08091 1.971 10.55 ____________________-_-__-------

480

293

426

281

330

120

-1 (cm )

Be

A-Cont,‘d.

150.5

-1 (cm )

cue

58454

0 _-_--_--

66500

62266

61444

60879

59855

x

L - x 1z+ g

-

.J -

I

56303

52090

lx+ g

x

H -

51802

47000

55534

x

-

II

?X+ g lx+ g 5;

-1 (cm )

region

spectral

" or 00

x %+ g

X

Transition

F -

___________

Br2

Fblecule

Table Intensity

(9Oa)

(9W

Ref.

E

00

or

67700 75000

0 - x 1x+ g

(cm-1)

region

spectral

*

I - x TX+ g

Transition

(cm-1)

cw

(cm-1)

B e

A-Cont’d.

‘Z; -

b 'II g

co

1515.61 1230.651 1093.993 [2082.07] [2133] [2134] 1739.25 1137.79

55353.9 64802.91 86917.8 91920.5 92923 48473.97 61784.7

a' 3x+ _ x.1x+

e 3z- - x lx+

B 'Z+ - x lx+

c ?Y+ - x Q+

E 'L"+- x 1x+

a +I= - x lx+

d 3a, - x lx+

1641.35

0

64746.5

1608.31

1671.50

?

?

46668.3

A fr _ x IX+

x 3rI u _______-________-___---_----

b 1x g

e

2.475

D(w)

11,48

(ev)

I.P.

1.2615

1.6810

1.9422

1.961

1.2663

1.3453

1.6116

1.6326 (3.6) -___~_____-_________~~~~~~~~~~~

0 564.9 0.2438 x lx+ g ~___~~____________________c____________~~~~___~_~__~________

%

MolC3lle

Table Illtensity

(29)

(12)

Ref.

cn

MOLECULAR

SPECTRA

IS VACUUM ULTRAVIOLET

WILKINSON

MOLECULAR

SPECTRA

IN VACUI:M ULTR.4VIOLET

-1 1

1.330

2373.6 1216.2

60021

B12A - x 211

many partially analyzed systems up to the I.P.

2.0026 1.9863

2323.90

53292.6 6oa62,al

2.002

2395

1.1265

1.9972

(a

D 2Cs - x 2n

52373.4

1038.41

2374.8

-1 (@J )

toe

E *Z+ - x 211

*c+- x *n

45486.1

B *n - x *II

c

44199.2

-1 (cm )

region

A *C+ - x *II

Transition

A %

- xlc+

73469.9

1608.8

1.587

2377.1 2.002 0 x lx+ _.____---________________________-I-___-__-______________--__.

NO+

1904.03 1.7046 0 x *II ~~~_~__~~~_~~~~__~~_~~~~~~~~~~~~~~~~_~~~~~~~~~~~~~--"~~~~~~-~

NO

MDlecule

Table A-Cont’d.

10.6

6.50

DC=)

9.24

(ev)

I.P.

f = .00240

f = .00151

Illtensity

Ref.

(95)

(94)

(14) (91) (92) (93) (94)

(12) (33a) (13)

XIOLECULAR

SPECTRA

IN VACUUM ULTRAVIOLET

11

WILKINSON

MOLECULAR

SPECTRA

IN VACUUM

ULTRAVIOLET

,

.

.I

13

WILKINSON

’ 2 I =1 I I

I * .

I

k-l I I

-

MOLECI’LAR

Frc;.

3. Al)sorption bands nitrogen temperat,ure (1)ottom). The WIT and

at liquid ture

SPECTRA

IN

of the P(f?II-S”II) (top) (courtesy f"Tl upper states

VACUUM

ULTRAVIOLET

system itnd the F(CSI-X’Q) system in SC) of Professor Miescher) , nnd nt. room trmper:b interact strongly.

these arc listed in column 8. Column 0 lists the reference from which the data were taken. Some of the dat#a, where contiguous, are taken from Herzberg ( 12 I. In all cases:, ground state dat,a are included. Where a considerable uncertainty exists, as in much of t,he dissociation energy data, parentheses are used. A brnc~ket is clmploycd when, e.g., AG1,? is list)ed instead of w, , or R,I instead of R, . Because of t,he increased interest, in upper atmosphere phenomena, much of the data is concerned with atmospheric molecules, i.e., molecules which occur in the atmosphere, and t,hose which could be formed photochemically in thr upper atmosphere (e.g., NO). ,4. SITRI~

OXIDE:

The ionization limit of NO is at 1342 A (9.24 ev ) : t,he band syst’ems observed in absorption in the vacuum ukrarioletI are extremely complicated even in the 1800-A region as may be seen in Fig. 2, in which a portion of the @(B’n--X?Ij and the 6(C2n-X211) systems are shown at’ liquid nitrogen temperature (upper photograph) and at room temperat#ure (bottom). Since the gromid st)ate of NO is 211with the difference *I13,z - ‘III,? being 121.1 cm-‘, absorption spectra

16

WILKINSON

I6891 I

FIG. 3. Microphotometer tracing ture (top) and at room temperature nitrogen should be noticed.

of the 6(3-O) band of IGO at liquid nitrogen tempera(bottom). The simplification produced by the liquid

taken at liquid nitrogen temperature are extremely simplified as compared to the room temperature photograph. A microphotometer tracing of t.he 6(3-O) band is shown in Fig. 3 at liquid nitrogen temperature (top) and at room t#emperature (bottom). Lagerqvist and Miescher (IS) have studied the /I and 6 systems and t’heir interactions in considerable detail. Rotational analyses were made for 5 6 and 15 @ bands. The &I and C*II excited states show a strong mutual perturbation (homogeneous) as a consequence of the crossing of their potential curves. This is an unusually fine example of a perturbation in band spectra. Ueda (14) has made a vibrational analysis of the absorption systems from 1300 A to 1800 A. Recently, at the University of Chicago, the region 1300 A to 1520 A has been photographed in the fourth order of the grating (21-ft radius) and an analysis is being carried out by T. Namioka. As might be expected as the ionization limit is approached, the spectra become even more complicated due to excessive overlapping of band systems.

MOLECULAR

SPECTRA

IN VACUUM

17

ULTRAVIOLET

Expos tip

8171

,7

I

3

i

,:2 __

CO

FIG. 4. Absorption photograph of the wl~~ e SIZE*, cc%,- + x’s,+, H’Q,‘- + S’Z,,‘, and n%, + S’Z,+ forbidden systems in nitrogen ftt low resolution. Court,esy of I)r. Y. Tanaka. B. NITROGEX

The following absorpt’ion syst,ems of nitrogen are shown at low resolut,ion in Fig. 4: w’A, +- Xl&+, a”S,,- c Xl&+, Br38,+- X'Z,,+, and a ‘I&, t S’B,+. The photograph was taken by filling the vacuum tank of a 2-meter vacuum spectrograph with purified nit’rogen. As may be seen, the Lyman-Birge-Hopfield system (LBH) (ccl& +- X’Z,+’ ) is by far the st,rongest’ and tends to obscure the other weaker forbidden systems. Herzberg (15) came to the conclusion that the LBH bands must be of type ‘rI-‘&,+ rather than ‘IIU-‘Zp+ on the basis of indirect arguments. This has been verified by a high resolution experiment (16). In Fig. 5, the l-0, 2-0, and 3-9 bands of the LBH system is shown under much higher resolution (fourt#h order of a 214% roncave grating) than in Fig. 4. In addition to the usual P, Q,and R branches, two additional branches are observed. From t,he precise rotat,ional

18

WILKINSON 7

FIG. 5. High resolution absorption photograph of the 1-0,2-O, and 3-O band of the LymanBirge-Hopfield (alII, +- Xl&+) system of Xx using the krypton continuum. The weak lines to shorter wavelengths of the main branches are electric yuadrupole lines (S branch, AJ = 2). See test for discussion.

constants obtained in t’he analysis of the P, Q, and R branches, it is possible to predict t’he positions of X(AJ = +2) lines and O(AJ = -2) lines. The predicted wave numbers for these lines agree closely with the experimental values as shown in Table I. There is, therefore, no question that these are quadrupole lines; it is further shown that the quadrupole contribution to the intensit’y of the LBH system is about 13% (wit,h the rest being magnetic dipole), and that the transition is indeed 1lI,-12 + in type. Since, in general, one would have expected that the quadrupole contribution would be about 1OF that of the magnetic dipole, it is of interest to examine t,he theory. Condon (17) has computed the relative transition probabilities (Q/M) of quadrupole (Q) to magnetic dipole (M) transitions as follows for an atomic case : Q/M = 3XY”/40R2,

(1)

MOLECULAR

SPECTRA

IX VACIJUM ULTRAVIOLET

1 !)

where X is t,he ratio (usually less than unity) between certain electric quadrupole and magnetic dipole matrix elements. v is the frequency of the line, and R is the Itydberg constant (109737.1 cm-‘). Q/M is so unusually large partly because v/R is near unity and also because X = 4.5, an exceptionally large value as compared with atomic cases. For many years the Vegard-Kaplan bands (A3Z,+-X1&+) were known only in emission (18). Recent’ly they were obtained in absorption by filling the tank of a 31-ft spectrograph with purified nitrogen (19). The positions of the absorpCon lines agreed closely with those values predicted from the emission data. Table I Wavenumbers of the S(J) and O(J) Quadrupole lines in the LBH Bands of Nitrogen

J

S(J) Obs.

S(J) Calc.(l)

O(J)

O(J)

ms.

Calc.

1-o 70627.11

0 1 2

32.66 70637.30

3 4

37.42 41.36

44.42s

5

44.52 46.86

6

48.45s

7

49.11*

49.15

a

49.11*

49.10

46.69*

11 12

70541.07

70541.09

13.09

13.18

482.09

482.08

65.62

65.33

72236.16

72235.42

48.23

9 10

48.40

46.57 44.10

40.90

40.85

13 2-o 0

72265.57*

1

71.06*

71.04

2

75.67X

75.65

72265.59

3

79.45*

79.42

4

a2.34*

82.35

26.06**

25.79

5

a4.41*

a4.45

17.78(T)

15.32

6

285.75s

285.70

7

203.98

204.02

86.13

a

85.75s

85.71

9

84.41+

84.46

10

82.34+

82.36

78.79x

78.91

50.34

50.46

20

WILKINSON Table

J

S(J) as.

11

S(J)

72279.45*

72279.44

75.67*

75.60

13

71.06*

71.08

14

65.57*

65.64

15

59.27*

59;34

16

52.34

52.26

17

44.61s

44.34

18

36.16(?)

35.57

*Blended with another

(1)

with a dipole

Calculated obtained

O(J)

Cslc. (1)

12

-Blended

I (Cont’d)

using

line

of

O(J)

Calc. (1)

72135.01

72134.99

18.66

18.70

the same branch.

line.

rotational

constants

and band origin

from the P, Q, and R branches.

Lofthus and Mulliken (20) investigated Kaplan’s first and second systems at high resolution and found them to be y?I,-u”Z,and y’II,w’A,, , By observation of a predissociation in v = 0 of state y, and assuming dissociation to ‘0 + ‘0 atoms, it seemed possible to locate on the energy scale (ev) the following “floating” states : 8.74 + C;

z = 14.38 + C;

w = 9.23 + c;

y = 14.50 + c;

a’ =

where C is a small positive constant associated with a potential hill in the predissociation process. More recently, however, the a”~,- c X’Z,+ system has been observed in absorption (21) by filling the tank of a 21-ft vacuum spectrograph with pure nitrogen. One band (5-O) of this system which consists of Q branches only is shown in Fig. 6 (compare also Fig. 4). Simultaneously a band system later identified as B’3zv- + X1&+ was found (Fig. 7). From the a’-X absorption systems the precise energy location of the a”SU-, w’AU , x12,-, and y’II, states becomes possible and these are given in Table II along with those of B’3T?J2Land a ‘II, . The new energy for &I, is 0.339 ev lower than the minimum value deemed possible by Lofthus and Mulliken (20) on the basis of the observed predissociation in v = 0 of state y. That predissociation is now reinterpreted as an accidental one resulting in the formation of 2P + 4S atoms instead of ‘0 + 2D atoms as formerly assumed (22). Potential curves are shown for a number of states, both known and predicted, near state y in Fig. 8.

MOLECULAR

SPECTRA

IN VACUUM

ULTRAVIOLET

21

From the path length required to produce various absorption systems in nitrogen, it is possible to list transition probabilities and, in the case of metastable states, to give mean lifetimes. These are given in Table III. All predicted states of electron configuration . . . ~~~~~~~~~ except 3Ah, have been found. These are b”r + A% + a”~ - Bf3Xu-, and w’A, (2s). Just why the 3A, state is as yet 11, dU , au3 unobserved is not clear. The spect,rum of N2+, especially the C’*Z,+-X*x,+ system has undergone a rather thorough investigat.ion. A low resolution photograph of these emission bands excited in mixtures of He + N2 and Ke + Nz by a condensed discharge is shown in Fig. 9. A high resolution photograph of the 4-10 and 3-9 bands is shown in Fig. 10. The most complete invest.igation of the C-X syst.em is by (‘arroll (24); others are by Tanaka (25) and Wilkinson (26). Carroll analyzed

V’

I~v’r~vv 6 8

IO

12

14

16 ,

1334

18

20

,22

1336

,24

J 26

1338

1340

FIG. 6. Microphotometer tracing of the 5-O band of the a’%,- + X1&+ system of N? in &sorption using the krypton continuum. Only the Q branch is observed in accordance with the selection rule AJ = 0.

'P2345

‘R 6

89

IO II

6

7

8

9

IO

I3

14

12

lITl-Krm

Q

2 3 4

5

6

7

8

9

IO

II

I2

I

I

I

1495

1486

1487

I6

FIG. 7. Microphotometer tracing of the 1-O band of the B’B,+ Xl&+ system of K\‘:! in absorption using the krypton continuum. The SR, OP, and the unresolved Q branches (QQ, QP, and 'JR)are identified corresponding to AJ = 2, -2, and 0.

22

WILKINSON Table Energies

Electronic

some electronic

State

states

8.164

lcu

8.398

” 1, x ?f Y ‘n a

a. a89

u

14.036

&

14.154

g

1

n z

1 ev = 8066.03 cm-1

of nitrogen.

Too (ev)*

B’ 3.X” a’

*

of

II

8.548

(Cohen,

Rev. Mod. Phys. &

DuMond, Layton,

and Rollet,

363 (1955).

the rot’ational structure of 15 bands of this system in 1650-2000 A region. Douglas (27) has discussed the dissociation energy of Ns+ in relation to that of Nz and favors the value, 8.723 ev. It is of interest also to note that Meinel (28) has found the A211,-X2&,+ bands in the aurora1 spectrum. These bands have also been observed and indeed analyzed in detail by Douglas (28~). C.

CARBON

MONOXIDE

The fourth positive bands of carbon monoxide have long been known and are indeed the most prominent feature of the spectrum. They are very easily produced and often occur, unwantedly, as impurity bands and frequently are confused with the LBH bands of nitrogen, with which they are analogous. They may be seen as impurity bands in Fig. 4. Of greater interest are the uf321+ +X’Zf, e32- +- X’B+ and d3A(?) +- X’Z’ (29, SO) forbidden systems. In order to obtain these bands in absorption, Herzberg and Hugo employed paths up to 400 cm-atmos in the 1230- to 1750-A region. A reproduction of one band of t#he d-X system is shown in Fig. 11. The nature of the upper electronic state is as yet unknown, but is believed to be 3A. in analogy with the as yet undiscovered ‘A,, state of nitrogen. The e3Z state is analogous with the nitrogen B’3~21- state and the appearance of the e-X bands is very similar to that of the nitrogen B’-X system. The e-X system (as well as the B’-X system) consists of 2 branches of the 0 and S form and 3 branches of Q form; it receives intensity by interaction with IZ and ‘n states nearby. Predissociations in CO have been discussed by Douglas and Moller (81) .

MOLECULAR SPECTRA IN VACUUM ULTRAVIOLET

FIG. 8. Potential to 15.5 ev. Courtesy

curves of Nz , both observed of Professor R. 8. Mulliken.

and predicted

23

in the energy range 12.5

24

WILKINSON Table III "Forbidden" transitions in nitrogen.

Transition

Path required (m-atm) P

FranckCondo; Factor F

Relative Transition Prob bility 10'4/F.P.

tiean Lifetime (set)

A 3i+ " --X 'Z-+(b)

4

0.09

2.8.10-5

2.6.10-2

a' lx- --X lE:(a) u

3.4

0.1

1.0.10-5(g)

4.0.10-2

B’

3.4

0.1

1.5.10-5

a 'I? --X 'Z+(e) (dgnetic dipole)

0.025

0.2

-3 2.10

a 'II --X 'Z;(d) e (electric quadrupole)

0.16

0.2

3.1.10-4

c 311 --x l?(f) "

0.058

0.47

3.6.10-4

Typical allowed transition

1o-5

3X

--x

Y(c)

u

1

t-4 Wilkinson and Mulliken, 3. Chem. Phys. 3l, 674 (1959). (b) P. G. Wilkinson, J. Chem. Phys. a

773 (1959).

Cc) P. G. Wilkinson, J. Chem. Phys. 32, 1061 (1960). Cd) Wilkinson and Mulliken, Astrophys. J. 126, 10 (1957). (e) P. G. Wilkinson, Astrophys. J. 126, 1, (1957). (f) Y. Tanaka, J. Opt. Sot. Am. 45, 663 (1955). (9) Since the a ' - X system consists of only one branch (Q), and, therefore, few lines to contribute to the overall intensity, the calculated transition probability based on appearance pressure has been multiplied by a factor of l/3.

D. CYANOGEN Carroll (32) has studied the emission of CN in the 1650-2100 region, especially the E2Z-X22, E22-A211, and J’A-A’II transitions. The constants of these stat,es are given in Table A.

MOLECULAR

SPECTRA

IN VACUUM

ULTRAVIOLET

25

FIG. 9. Emission spectra of the N2+C%<,+ -+ X22,+ system at low resolution in mixtures in a condensed discharge. Courtesy of Dr. Y. of He + N? , and Ne + Nz by excitation Tanaka.

*

I FIG. bands‘i.

E.

High resolution photograph of the C%,,- + X25,+ system of N?+ (J-10 and 3-9 Photograph t,aken in the first order of a 21.ft vacuum spectrograph.

10.

I”LUORISE

Iczkowski and Margrave (33) have observed a Rydberg series in fluorine (807-1035 A) with a limit at 15.7 ev. The dissociation energy was also obtained as 1.63 ev; the dissociation energy of F2+ as obtained is 3.3 ev.

26

WILKINSON

FIG. 11. High resolution absorption photograph of one d-X% band of CO taken in the fourth order of a 21.ft vacuum spectrograph. Absorption background is the krypton continuum. The upper state is believed to be d3A.

F. HYDROGEN

The Lyman bands of hydrogen have recently been investigated under high resolution in order to improve the ground state rotational and vibrational constants (%a). Over one hundred bands have been studied; both B,” and AG” curves have a point of inflection at 21”= 3, making the representation of AG” by the standard formula rather cumbersome. However B,” and ~y,~ may be given as 60.864 cm-’ and 3.0764 cm-‘, respectively. A rather large change in voofor the B’&+-X1x,+ system is found; the new value is 90203.35 cm-‘. G. OXYGEN The Schumann-Runge absorption bands (B3&- +- X32,-) of oxygen extend from 1750 A (dissociation limit) to 2008 A (O-O) ; below 1750 A a strong continuum arises with a maximum near 1420 A; at shorter wavelengths a number of sharp bands appear. Absorption coefficients in the Schumann-Runge system have been measured by a number of investigators (34-36). The oscillator strengths obtained in these experiments are in rather poor agreement’, covering the range 0.161 to 0.215. However, only the work by Watanabe et al. (N), was by photoelectric methods; their value is 0.161. The oscillator strength between 300 A and 1300 A was found (37) to be 6, in agreement with optical dispersion dat,a (5.93). The fine structure in the Schumann-Runge bands near t,he dissociation limit has been photographed at room and liquid nitrogen temperatures by Brix and Herzberg (3). Their analysis yielded valuable vibrational and rotational data as well as information on the triplet splitting. The value of the dissociation energy, which is the most precise ever recorded, is given as 5.1153 ev. The B”z,- state is predissociated at two positions (1792 A and 1920 A) above the 1750-A dissociation limit. These were found by noting abnormally large line widths in some of the bands (58, 59). A photograph of the 12-O (predissociated) and the 13-O band is shown in Fig. 12. While part of the broadening in the 12-O band is only apparent and is due to unresolved triplet components, nevertheless,

MOLECULAR

SPECTRA

IN VACUUM

ULTRAVIOLET

27

1782.982

I i-0

ii- 0

I&O

FIG. 12. Absorption photograph of the 12-O and 13-O bands of the Schumann-Runge system (BQ=- + X9-) of 02 taken in the first order of a 21.ft concave grating spectrograph (bottomi and in the third order (top 2). The increased resolution in the third order and the inrrenrc~d line width in the 12-O (predissociated) band should be noticed. it 1~1s possihle

to shokv that

t,he resolved

Pl

and

RI components are slightly

broadened in the 12-O band. Furt,hermore, microphotomet’er tracings between the discrete lines showed a background of continuous absorption; t’his is probably due to a direct, transit.ion ( %I,, + X3~,- ) into the repulsive stat.e which pauses at, least one of these observed predissociations. IV. POLE’ATOMIC

MOLECULES

In a classic paper, Sponer and Teller (/to) reviewed the literature up t.o that, t’ime and disrussed the theory of polyat’omic spectra. Since that work is adequate t’hcoretically, and since ot’her treat’ments (41, 42) are available, no at’tempt will be made here to repeat the t’heoretical considerations. The tables of Sponer and Teller are certainly comp1et.e through 1940; therefore our Table B att,empts to cover only the period since 1940 for polyatomic molecular dat’a in the region below 2000 A4. Certain except,ions will be found where ot#her dat,a are also list’ed

series. tinuum w. max. at 1800

valence vibr. 61OUOO

2100

CD 2

CHD

CH2

__-____________________c____________

CH3C=CH

B"=3.950

GO0

r" = l.O71,
If bent,

K; (CHD) = 1.034;

r; (CD2) = 1.029,

If linear,

CHD and CD2.

well resolved in

P and R branches

B'=3.595

1415.8

B"=5.333

Band at

B'4.808

- _

_ _ _________________---

Remarks

1415-5

10.36

10.19

I.P.

Band at

1414.5

Band at

series

2 Rydberg

_______________---_____c_____________-------

Con-

5 Rydberg

!J (a,)

1200-

Characteristics of Transitions

CH2=C=CH2

observed Vibrations

Spectral Region

Molecule

Polyatcmic Molecules

Table B

(124)

Ref.

MOLECULAR

SPECTRA

IN VACUUM

TJLTRAVIOLET

29

c2H2

C2H2

NoleCUle

vo0=42197

Y’U =1389 2g v”=608 26. 4 -’

1280-1520

1000-2000

1070-1540

per

.‘a =1047.70 3g

1970-2470

118498

B:

=l SQOH

=2303D

Y2ag=1342D

Vlag

=2808H

V4fig=302D

=393H

V2dg=1559D

=1781H

3R”v1ug,2053~

=2748M

3R:“2Bg=1720D

74747;

_

in

in

._- .-.

probably

(V*).

B

B

lA,.

lB,,

C2D2.

C-74735:

3R-65858;

probably C is

is

-74783

74653;

C2H2.

twisted-74622

and

C-trans-

3R’-

3R-

each.

B-trsnsbent

bent,

74498;

65814;

4 states:

7 bands

2 Ry series

Absorption

rCC=l. 388

1.0297

co=

Coefficients

BO=

g”44=o* 17

3R’

&-12.94,

11.41 1.1247,

Remarks

I. P.

rCH=1.07-1.09

.69

lA,.

up-

B--Cont’d.

,~“~~=3.29

state,

transbent

Observed Vibrations

Spectral Region

Characteristics of Transitions

Table

(48)

(52)

(51)

Ref.

$

MOLECULAR

SPECTRA4 IN VACUUM ULTRAVIOLET

R

(furan)

c4H40

-----

C6D6

'fjH6

MOl%XUlfZ

1050-2150

430-133s _~~~~~~~_-------

=334-3728

3rd series converging to 9.95~ -__------

v4=465 v5=2965

9.95 ___________--_______------

(133)

up to 10;

me&ers

v3=1068

(731 (132)

Absorption Coefficients

starting at 1980,

8.89

(63)

1320, 1050

(90) (64)

Abs.

(61) (131)

Coefficients, max.

Teller effect.

symmetry and Jahn-

v2=840

Over 100

(65)

Ref.

2 Ry series

bands

each.

' 18 indicates Dph

strong intensity of

Remarks

v1=1395

"20e2u =260-2891)

=302-343H

Y18e2g =30g-345D

9.247

up to 10 members, 9.251

4 Ry series,

=961-975H Y2alg =910-93813

1300-1850

I.P. (ev)

Characteristics of Transitions

Observed Vibrations

Spectral Region

Table B-Cont’d.

fi

8 p

N

w

*o

1050-1850

Spectral Region

CS2.

1553-1612

1650-2200

"=650

~~'-1670

y1

870, 830

f - .12

Max. 1121, 1332

1050-1200

1577, 1595, 1612

Bands at 1553,

Ry series --_------_-~--~~--_-

f = .0053

18.23

18.08(2oJ

diffuse bands

f = .004

13.78(lng) Absorption Coefficients

Absorption Coefficients

Remarks

Continuum and

1680. ____________________-------

Cont. max at

1383 and 1435.

10.565

I.P. (ev)

Max. 1475

Av=656,1547,

and 1713; 2 Ry

~~9720 series starting

starting at 1572

v3=1140

transitions

2

N-V

Characteristics of Transitions

~2-1460

Observed Vibraticm

1200-1400

1400-1750


co2

_____--_____________----

W2)

tilecule

Table H--Cont'tl.

(134)

(76) m-w

(134)

Ref.

.. z

P 2

P ;4

E;

34

WILKINSON

x

MOLECULAR

SPECTRA

IX VACUUM I_~LTRAVIOLET

<1800

D2Te

Observed Vibrations

resolved rotational

discrete bands;

1700-1840

Absorption

1150-1430

2 Ry series <1150 ~~_~_~__~___________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Continuous

1430-1860

H2°

strong bands

absorption

weak continuous

Ry series

1850-1950

2000-3000


(CH312S

3 Ry series

shows predissn.

rotational str.

Continuous abs.

1800-2000

Cl700 ~__~~_~_~~~~_~_~~~_~~~~~~~~~~~~~~~~~~~~~~

CH3HS

12.59

8.7 (?)

9.437

9.14

near 2000 Ry series with

9.138

I. P. (ev)

diffuse absorption

Characteristics of Transitions

B-Cont’d.

structure ~~____~______~__~___~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1880-2000

Spectral Region

H2Te

Wolecule

Table

f = .05

f = .041

Remarks

(140)

(139)

(138)

(138)

Ref.

No2

___~~_"~___~__

N2°

Molecule

2000-2700

687-1400


1380-1605

1600-2100

1080-2100

Spectral Region

B-Cont’d.

bands

Ry series

sharp

<'ONO=154°

<"oNo=154°

rA0=1.41

One resolved system

ri0=1.28

Diffused bands

Absorption Coefficients .__________--____---------

20.10

16.55

16.39

f = .l

1080 diffuse bands

f = .367

(80)

(144)

(76)

(143)

(142)

f = -0015

1285

Ref.

Remarks

f = .0211

12.94

I.P. (ev)

1450

at 1830

Continua with max.

Characteristics of Transitions

____________--_---.

wo=1005

wo=621.2

Observed Vibrations

Table

WILKINSON

38

8

” 8 2

I

8

0

m

iW~LECLJLAK SPECTRA

IN VACUUM IJLTRAVIOLET

N

8

co

8 2 ’

,o d

8

zv

I

0”

m

d ::

’ ’ ’ I

40

WILKINSON

for the sake of completeness. Platt. and Klevens (es) have reviewed the absorption spectra of organic molecules. Since emission spectra (except fluorescence) from polyatomic molecules are rare because of molecular decomposition, we must confine our interest principally to absorption spectra. In the region below 2000 A, the spectra of most polyatomic molecules consists of one or two 8-N type transitions and one or more Rydberg series. A V( a)-iV or V( 1)-N transit,ion is one of t,he strongest, if not the strongest in the spectrum. A “V” state is one which dissociates or tends to dissociate into ions. A series of Rydberg states extending to the ionization potential of the molecule involves excitation of a ?r electron to higher and higher orbitals. In general, the geometry of the Rydberg state will be similar to that of the molecular ion. However, perturbing effects of other nearby states can cause the geometry to differ slightly or greatly from that of t,he ion and that of the neutral ground state. In general, a Rydberg series may be represented: v = 1 -

R/(n

+ &

(2)

where v is the frequency of the transition, I is the ionization potential, R is the Rydberg constant for infinite mass (109737.3 cm-‘), n is the principal quantum number and a is the quantum defect which is itself a function of n. If 8-10 members are identified, it is generally possible to fit them to an equation of the form of Eq. (2)” and obtain a precise ionization potential. The molecular spectra of polyatomic molecules can become so complicated that a standardization of notation is necessary. Mulliken (45) has prepared such a report with recommendations on the use of certain conventions and symbols. The report was adopted by the Joint Commission for Spectroscopy (Lund, 1954). In only a few cases has the rot’ational structure of a polyatomic molecule been resolved in the vacuum ultraviolet. These are: HCN, CH2 , CH3 , H,S (only partially), H2Te, NH? , and C&H* (only partially). Some of these will be discussed below. It is considered that for most polyatomic molecules, the rotational structure is wiped out by predissociation; for some t,he rotational structure may be just too complicated to be resolved by present day equipment. A. THE RADICAL CH3 After considerable effort covering many years, Herzberg and Shoosmith (46) were able to observe in the flash photolysis of dimethyl mercury a Rydberg series in CHB starting at 1500 A and extending through 5 members. Later the same series was observed in the photolysis of other parent compounds. Corresponding deutero-compounds also showed a correspondingly slightly shifted series. Finally, an additional diffuse band was observed at 2160 A for CH, and a sharper band at 2144 for CD, (Fig. 13). From the Rydberg series, ionization potentials of 9.840 ev and 9.832 ev were obtained for CHS and CD, , respectively. These 3 Convenient

tables for fitting Rydberg

terms are given by Paschen

(44).

MOLECULAR

SPECTRA IN VACUUM l_JLTRAVlOLET

FIG. 13. Absorption bands of the CHB radical observed mercury. Courtesy of Dr. G. Herzberg.

by flash photolysis

il

of dimeth)

agree well with the electron impact value of 9.90 f 0.1 ev. The rotational analysis of the 2144 A band gave an angle, p = 75-90” (angle of the C-D bond with tbe symmetry axis), and TO(CD) = 1.066 f 0.006,4; i.e., the molecule is planar or very nearly so. B. THE RADICAL

CH,

Recently, Herzberg and Shoosmith (47) were able to find the following bands in the flash photolysis of diazomethane and the deuterated diazomethanes: 1411.5 A (CH?), 1415.5 A (CHD), and 1415.8 A (CD2). The CD:! band showed well developed and well resolved rotational structure and the analysis produced rotational constants for the upper and lower states (Table B). If linear, the ground statr is 3&- (1’mes of odd J strong, as observed) and r. (CH or CO) = 1.029 (CD,), 1.034 A (CHD). If nonlinear, TO= 1.071 A and
c2h

FIG. 14. Correlation of vibrations in the linear ground state and the tram-bent upper state (C&.

Dooh

(X2,+)

of CzHz (or CZDZ)

rotational or vibrational fine structure has ever been observed. This is in accordance with the theoretical conclusion that the excited states should be unstable (41) . D. CzHz The absorption spectra of CzHz have been carefully studied by Innes (51) and both &Hz and CzDz by Ingold and King (52) in the spectral region 19702470 A. The upper electronic state is bent with symmetry C2hand is designated probably as ‘A, . If the C-H distance is assumed to be between 1.07 and 1.09 A, then rcc = 1.388 A and
RIOLIXULAR

SPECTRA

IK VACUUM

ULTRAVIOLET

Xi

Thr correlation of vibrations in the linear ground state and the transbent upper statBe are shown in Fig. 14. Long progressions of the thermally excited v4" ( T,,) vibration (correlat,es \vith vaa,, in CSh’) are observed which is consistent with the symmetry assignment in the upper state. Vibrational const)ant,s in the upper state were obtained (Table B), and for the lower state, CQ” = 608.26, .rJ1 = 3.20, y,, = 0.17 cm-‘. Thercl is no reason to suppose that cis bent ‘A2 molecules cannot, exist ; however, the transition ‘A2 + ‘2, + is forbidden (53j. It may be predicted that ‘11, I,, + + ‘Zgt should be present, and be much Wongel (cis) t-- -II and IB, (tram) t,hat# these transitions can tx> than the ‘A-I,,(tram) + ‘2,+ system. It is probable identified with the complicated systems in the 1600P1900 X region. Two Rydberg series have been identified in acetylene (5,$, 5,5) starting at 1519 _I and l&l2 A. These, series have, been designated nkCil and nR’--4, rrspec*-

FIG. 15. Absorption hands of CJIn in the 1290-1370 A region t,aken with the krypton continuum and in the first order of a 21.ft vacuum spectrograph. Three systems observed (two of them bent) are the R’-,4-Z,+, R-X%,+, and C-X%,+. The appearance of “hot” bands of the Y*” (?T~)vibration should be noticed. The strong appearance of these bands provide :t clue to the (‘?h symmetry of the excited state.

WILKINSON

44

tively, where n is the whole number in the denominat)or of the Rydberg series term expression (Eq. (2))) and R and R’ designate Rydberg states. A is, of course, the linear ground state of symmetry ‘Z,+. Both these series lead to an ionization potential of 11.41 ev (54). In the same general region of the spectrum a third transition (C-A) involving a truns-bent ( C2h) upper st,ate at 1338 A (origin) and a fourth transition (B-A) at 1340 A (origin) involving a trans-bent and twisted upper state have been found. A photograph of the GDz spectrum in this region is shown in Fig. 15. The structure of the B-A and C-A bands is very similar to that of the ultraviolet system discussed above. A trans-bent state (B, C, ‘A,) of acetylene approximates a symmetric top with energy levels which may be expressed: F(J,

K)

= BJ(

J + 1) + (A - B)K2,

(3)

where B = +( B + 6) and A > B - 6. The rotational constants are related to the moments of inertia, I,, I,, and I, about the 2, y, and z axes (Fig. 14) as follows:

A = h/87&% ;

B = h/8?r2cIv ;

C = h/8&,

,

where the constants h and c are Planck’s constant and the velocity of light, respectively. Since A-B is rather large, a series of sub-bands representing transitions to different K are observed. In the case of the vacuum ultraviolet bands, B-A and C-A, (though not for the near ultraviolet bands, ‘A,-‘&+) the fine rotational structure is washed out by predissociation and only K subheads are obtained. The vibrational structure is shown schematically in Fig. 16 for the v~”O-l, O-2,0-3,0-4, and O-5 bands of CzDz . In the figure, the vibrational angular momentum quantum number L = (14 + Z5) is significant for the ground state, and the angular momentum quantum number, K, is significant for the upper state. The selection rule K - L = &l (perpendicular) bands applies to the strong transitions. Transitions corresponding to K - L = 0, f2 are also found occasionally, but these are weak and permitted probably because of mixing with the neighboring 3R’ state or because of a departure from a true symmetric top. From the substructure shown in Fig. 16, B - C - 1 cm-‘, A - 7 cm-’ (CZDZ) and -8 cm-’ (C&HZ). The ground state intervals in ~4” obtained from the above analysis agree well with those obtained from infrared data (56) and the data of Ingold and King (52). The upper state vibrational frequencies are given in Table B. It is of interest to compare the molecular orbitals of nitrogen and acetylene which are isoelectronic. The ground state of these two molecules may be expressed : ~,~,(~,1~,)2(~,1~,)*(2~~)2(2~~)2(3~~)~(1~~)~ HC,CH HC,CH

CC

%+.

MOLECULAR SPECTRA IN VACUUM ULTRAVIOLET n

A

45

6K

5

4

3 2 I 0

L 5,3#1 412,o 3,l 290 I

z

I

I

I

I

0

FIG. 16. Schematic energy level diagram for the ~4”O-1,O-2, O-3, O-4, and

C&D*. Mostly perpendicularbands are observed (K - L = fl).

O-5

bands in

In acet,ylene the bonding characteristics of the orbitals are indicated by the linkages HC and CC. The major difference ascribed to acetylene as compared t,o nitrogen is that the energies of the 3a, and lr, orbitals are reversed corresponding to the viewpoint (5s) that the ionization potential of a 3a, electron is 20 ev while that for a l*, electron is 11.41 ev (ionization potential of acetylene). Excitation of a I*, electron in acetylene to Rydberg orbitals (which are in general nearly nonbonding) corresponds t’o Rydberg transitions and subsequently

46

WILKINSON

leads to ionization. It is certain that the 3R st,ate of acetylene is of t,his t,ype and this indicates t’hat the ground state of the molecular ion is %I,, and linear. The other Rydberg state discussed here, 3R’, may be the first, member of a series leading to a low-lying excited state of the molecular ion which, like this st’ate, is no longer linear but very slightly bent. A more likely explanation is that since 3R’ lies so close to the bent B state there is a certain amount of interaction resulting in a bent configuration for 3R’. This necessarily implies that either the 3R’ and B st’ates have the same electronic symmetry or that excitation of vibrations in these states makes mixing possible. Mulliken (53) and Walsh (5s) have point,ed out that certain molecular orbitals have properties which favor bending and this has been demonstrated in AB, type molecules. In the V( 1~~~1~~) state of acetylene this orbital is rTgwhich becomes b, on bending (t’rans) and the bent st,at#eis therefore ‘B, (u,~,~?I~). Purt,hermore, it is not unreasonable t’hat t,he V stat,e may also be slight,ly twisted. E. CzH4 Price and Tutte (57) identified three Rydberg series leading to an ionization potential of 10.516 ev. Zelikoff and Watanabe (58) measured the absorption coefficients over the region 1060-2000 A; Wilkinson and Johnston made similar measurements from 1400 to 2000 A. The long wavelength absorption system of ethylene consists of a broad continuum with a maximum at about 1620 A and a long series of broad weak absorption bands extending to at least 2069 A. In C&D4some resolved structure is noted. This transition is the V(B,,,)-N(‘A,,) and involves an upper state with the CH2 groups at right angles; the long series of bands are an upper stat’e progression of v2 (C-C) ; the fine structure in t’he C2D4 spectrum is probably due to the v4 twisting vibration. Superposed on t’he V-N continuum are strong Rydberg bands (Fig. 17). Two vibrations are identified in the upper st,ate (3R) of this band and in bands to short,er wavelengths (59, 60). These are the v2 C-C stretching vibration and the v4 twisting vibration. If the upper state remains planar, then the v4 twisting vibration is the only vibration of species a, . Consequently (~4)~ 1 ( V~)LI, the ratio of the twisting frequency in CzH4 to that in C&D4can be accurately predicted as 1.414. Experimentally, this ratio is always greater than this; e.g., in the 3R state it is 1.67. Assuming again a planar upper state (&), a nontotally symmetric vibration should be weak and be observed only in jumps of 2 quanta (Au = 0,2, 4, etc.) ; if harmonic torsional oscillations are assumed, then the relative intensities corresponding to Au4 = 0, 2, and 4 should be 1:0.19: .06 for C2H4 and 1:0.23:0.08 for CzD4 (59). In actual fact the ratios are 1:0.6:0.2 for C&H4 and 1:2:0.1 for C&D4 (Fig. 17) f or state 3R. Corresponding barriers hindering free rotation vary from 500 to 1200 cm-’ depending on whether the molecule considered is C2H4 or C2D4 and on which Rydberg state is considered. It can

MOLECULAR

I

I

1560

* 1560

SPECTRA

IN

I

I

I

1600

1620

1640

VACUUM

I

I

1660 Wavelmgth

47

ULTRAVIOLET

I

I 1700

1660

1740

,720

(A)

1

:‘. d

I 1560

I

IS60

I

1600

I

1620

I 1640

I

I

,660 Wavrlmngth

1660 (A)

,700

,720

I

,740

FIN:. li. The 3R-A Rydberg absorption bands of CzHa (top) and C&d (bottom) using the Se continuum as the absorption background. The abnormal intensity distribution should I)e not,iced. The ratio (v,)H/iua)D is also abnormal. See text, for discussion.

only b(> concluded that the stable equilibrium configuration is not’ of symmetry Ij2h but is bent) or staggered in some fashion. 1’. CeH6 The absorption spectra of benzene and benzene-de were first photographed by Price rf al. (61) and two Rydberg series ident’ified, leading to an ionization po-

J

WILKINSON

1400

1450 WAVELENGTH

1500 (A,

FIG. 18. Rydberg

absorption bands of CeH6 in the 1340-1500 A region. The abnormal intensity distribution of the w(e211) vibration as compared to the ~~(al,) vibration is evidence of the Jahn-Teller effect. See text for discussion.

tential of 9.24 ev(CsHs). Absorption coefficients of benzene vapor were determined in the region 1700-2100 A by Morton and Stubbs (6%‘), Pickett et al. (66) and Romand and Vodar (64). The vibrational structure in the region 1300 to 1850 A has recently been examined in greater detail (65). A microphotometer tracing for the region 1300 to 1520 A is shown in Fig. 18. The absorption spectrum of benzene vapor in the vacuum ultraviolet may be divided into three regions: (1) a diffuse region from 1850 to 2000 A, which has been classified as due to the forbidden transition, ‘Blu-‘Alg , made allowed by vibrations of the type e2gand bZg(66) ; (2) a strong continuum with maxima at 1781 A and 1804 A, which is interpreted as the allowed lE1,-‘A1, transition (67) ; (3) an extensive series of Rydberg transitions, the first member of which lies at 1788 A, extending to the ionization limit at 1340 A. According to the Jahn and Teller theorem (65,68, 40) the stable equilibrium configuration of a degenerate state in a nonlinear molecule is not the symmetrical one. That the benzene Rydberg series exhibit the Jahn-Teller effect will be de-

MOLECULAR

SPECTRA

IK VACUUM ULTRAVIOLET

49

veloped in what follows. The ground state nuclear configuration of benzene belongs to point group D6h (69) and one might expect the Rydberg nuclear configuration to belong to Don also since only one weakly bonding elect’ron of a total of 18 bonding electrons has been excited. The symmetry of the vibrationless ground state is ‘A,, and the only allowed transitions are to states of symmetry ‘Elu and lAzzL; since the Rydherg transitions are quite strong, it is reasonable to at,tribute these to either lEIU-lA1g or ‘A2,-‘S1, . If the symmet,ry of the equilibrium nuclear positions is the same in both elect’ronic states (i.e., Doh), only totally symmetric vibrations will be excited strongly with the selection rule Au = 0, 1, 2, 3, etc. while the nont’ot,ally symmetric vibrations may occur with weakened intensity according to the select,ion rule Au = 0, 2, 4, etc. In the benzene Rydberg series the v20e2~vibration appears as l-l, 2-2, 3-3 bands; the y18eZg vibration appears strongly in a long progression as 2-0, 4-0, etc. bands and the totally symmetric “breathing” vibration v2algappears in accordance with the selection rule Au = 0, 1, 2, 3 etc. The ve bands should be far stronger than the others; in actual fact the v18eag vibration is by far the strongest. If the Rydberg st,ates are I&,, , the degeneracy may be removed by a nuclear displacement of oZysymmetry (68), resulting in a stable equilibrium configuration which is not, quite a symmetrical hexagon (D2h) ; such a nuclear displacement might’ be expected to enhance vibrations of symmetry e2, . The observation of the strong v18e2pprogressions is evidence of the Jahn-Teller effect. The Jahn-Teller effect. has been discussed theoretically by Liehr (70) and Longuet-Higgins et nl. (71 j . Four Rydberg series, ,nR-A, nR’-A, ,nR”-A, and nR”‘-A have been ident#ified in benzene; they lead to precise values of the ionization potent,ial; 9.2-17 ev (benzene) and 9.251 ev (benzene-do) which agree accurately with results obtained from photoionization experiment#s (9.245 ev) (72). (;.

FUnaN

(GH40)

The atmospheric absorption and photoionization coefficients of furan vapor have been det’ermined in the 1050- to 2150-A region (73). Two Rydberg series were found converging to 8.89 ev and one converging to 9.95 ev. H. ETHYLENE OXIDE (CH&O The absorpt’ion spectrum of ethylene oxide was investigated by Liu and Duncan (74) who found two Rydberg series leading to an ionization potential of 10.81 ev, which is 0.25 higher than that obtained by photoionization (75). Guided by the photoionization measurements and by the absorption intensities of the bands, Lowrey and Watanabe found a different arrangement of the bands which led to an ionization pot(entia1 of 10.565 ev. Two V-N type transitions are observed at 1572 A and 1713 A. I. Cop, CS2, COS AND T\;zo Tanaka et al. (76) have investigated Rydberg series in COZ , CS2 , COS, and X20 using the neon continuum as the absorpt,ion background. Photographs of

50

WILKINSON

I-

II

I

New

FIG. 19. Absorption is the neon continuum.

I

I

Series

spectra of CO2 showing two new Rydberg Courtesy of Dr. Y. Tanaka.

series. The background

four such series are shown in Fig. 19 for COz and two in Fig. 20 for CS, . The various ionization pot,entials obtained are given in Table B. By comparison of the results with theoretical predict’ions for COz (77)) it is possible to state that the first ionization potential at 13.78 ev corresponds to excitation and loss of the In, electron; that at’ 18.08 ev corresponds to ionization of the 2a, electron. Such similar identifications are possible for COS and CS, only if the Rydberg series in these molecules are analogous to CO2 . Absorption coefficients for COz are given by Wilkinson and Johnston (49) and Inn, Watanabe and Zelikoff (78). Higher ionization pot’ent’ials of NzO were also given by Tanaka et al. (76) (see Table B). J. HCN Four band systems in HCN and DCN have been found below 2000 A (79). Although the ground state is linear, at least three of the excited states are nonlinear. The rotational fine structure has been analyzed for the a-X and t’he P-X systems in the 1600-2000 A region; a photograph of the (030)‘-(00’0) band of the cu-X system in HCN is shown in Fig. 21. At shorter wavelengths predissocia-

-

xl

lQ FIN:. 20. Absorption is the neon continuum.

spectra of CS? showing two Courtesy of I)r. Y. Ttin:ks.

new

Rydherg

series.

The

txtckgro\ultl

P

i’

5

WOW.65 cm-1 FIG. 21. Rotation in HC?;. Background

IQ

20

I

a

546Ul.06 cm-” fine strllcture in the (030)‘-(OODO) I,and of the a-S atxwrption is the I,.~~~itn continuwn. Courksy of Dr. (;. Herzherg.

eastern

sets in gradually in t,he CL-X system, hut, at somewhat different5 energies in HCS and DCN. The geometrical parameters obtained are rO(CH ) = I. 110 :I, r,,( CK) = 1.297 A and
tion

52

WILKINSOX

the K-type doubling, it follows that both the a and the ,6 states belong to species A”. The electron configuration has been discussed by Mulliken (53). Ii. H&3, H2Se, H2Te, CH:HS,

AND

(CH3)2S

Well-developed Rydberg series leading to ionization potentials (Table B) are observed for all the above molecules. No excited state vibrations are observed; the spectra of the deuterides are almost identical with those of the hydrides, showing that virtually every band in the spectra is due to a separate electronic transition. This and the general nat’ure of the partially resolved rotational fine structure show that the transitions concerned are those of an electron being excited from a nonbonding ground state orbital, i.e., from the p-lone pair ground state orbital. The rotational fine structure in H&3, H2Te, and DITe would be well worth investigating under high resolution. Predissociation is observed in CHBHS and (CHs)*S. L. NO, In the 2000-2700 A region many diffuse bands and one well resolved system occur. The rotat.ional structure of the resolved bands has been analyzed by Harris et al. (80). The upper and lower state Y(N-O) distances are 1.41 A and 1.28 A, respectively; similarly, the O-N-O bond angles are found to be 154” in both states. At shorter wavelengths, certain regularities in the vibrational structure are observed (Table B). Absorption and ionizat’ion coefficients are given by Nakayama et al. (81) . Table IV Xnown Constituents of the Atmospheres of the Planets.

(lbject

VS?llUS

Mars

Molecule

co2 co2

Jupiter

CH4 and NH3

Saturn

cH4

UranUs

CH4 and H2

Neptune

CH4 and Ii2

Pluto Mercury

1 None

MOLECULAR

SPECTRA

IN VACUUM Table

ULTRAVIOLET

53

V

somepossible constituents of planetary atmospheres.

Kr

N2

m3

A

H2

cH4

Ne

O2

co2

HCN

He

xe

c2H4

V. ASTROPHYSICAL

APPLICATIONS

The possibility of spectroscopic studies of the sun, stars, plane@ aurorae, and the sky in the vacuum ultraviolet has been receiving increasing attent’ion. At the present t,he only vacuum ultraviolet experiments have been done on the sun and the aurorae by means of rockets (82). In the sun spectra many observed features are as yet unidentified. As rocket spectroscopy, and, later on, satellite spectroscopy progresses, additional features will undoubtedly be observed from many astrophysical objects which will require identification. Our knowledge of the atmospheres of the planets is extremely meager (Table IV). It has been possible to identify only CO2 , CH, , NH, , and Hz. The possibilities for other molecules is much greater as indicated in Table V. For some of these, the only known absorption spectrum lies in the vacuum ultraviolet. These are N2 , CO, Xe, Kr, Ne, and He. Ot’hers, e.g., H20, NH, , CH4 , CO2 , HCN, GHz , and C2H4 , while possessing infrared spectra, would be much more easily detectable in the vacuum ultraviolet. All molecules listed in Table V would be readily detectable in the vacuum ultraviolet. Spectra of many other molecules are to be expected from stellar atmospheres. Some of t(hese are Sic2 , CN, Cz , CH2 , CH3, C&H, and NH2 (83). Note

nitrogen

added

in proof:

Recent work has produced evidence for the existence of oxides of

in the atmosphere

of Mars

(83~). ACKNOWLEDGMENT

I wish to thank the many scientists lustrated in this paper. I am indebted the manuscript. RECEIVED:

September 9, 1960

who have supplied me with the spectrograms to Professor Mulliken for a thorough criticism

ilof

54

WILKINSON REFERENCES

1. P. G. WILKINSON AND Y. TANAKA, J. Opt. Sot. Am. 46, 344 (1955). 2. Y. TANAKA AND ZELIKOFF, J. Opt. Sot. Am. 44, 254 (1954); Y. TANAKA, J. Opt. Sot. Am. 46, 710 (1955); P. G. WILKINSON, J. Opt. Sot. Am. 46, 1044 (1955); TANAKA, JURSA AND LEBLANC, J. Opt. Sot. Am. 48, 304 (1958). 3. TOUSEY, JOHNSON,RICHARDSON,AND TORAN, J. Opt. Sot. Am. 41, 696 (1951). 4. M. SEYA, Science of Light 2, 8 (1952); T. NAMIOKA, Science of Light 3, 15 (1954); T. NAMIOKA, J. Opt. Sot. Am. 49, 951 (1959). 6. JOHNSON,K. WATANABE, AND TOUSEY, J. Opt. Sot. Am. 41, 702 (1951); K. WATANABE AND INN, J. Opt. Sot. Am. 43,32 (1953); CHUBB AND FRIEDMAN, Rev. Sci. In&r. 26, 493 (1955). 6. K. WATANABE, MARMO, AND INN, Phys. Rev. 91, 1155 (1953). K. WATANABE, J. Chem. Phys. 22, 1564 (1954). K. WATANABE AND MARMO, J. Chem. Phys. 26, 965 (1956). 7. K. WATANABE, J. Chem. Phys. 26, 542 (1957). 8. HURZELER, INGHRAM,AND MORRISON, J. Chem. Phys. 23, 76 (1958). 9. BRIX AND HERZBERG, Can. J. Phys. 32, 110 (1954). 10. P. G. WILKINSON, J. Mol. Spectroscopy 1, 288 (1957). 11. A. E. DOUGLAS (to be published). lla. W. C. PRICE, in “Advances in Spectroscopy,” Vol. I, p. 56. Interscience, New York, 1959. 12. G. HERZBERG, “Spectra of Diatomic Molecules,” 2nd ed. Van Nostrand, New York, 1950. 13. LAGERQVISTAND MIESCHER, Helv. Phys. Acta 31, 221 (1958). 14. M. UEDA, Science of Light 3, 143 (1955). 15. G. HERZBERG,Phys. Rev. 69, 362 (1946). 16. P. G. WILKINSON AND R. S. MULLIKEN, Astrophys. J. 126, 10 (1957). i7. E. U. CONDON, Astrophys. J. 79, 217 (1934). 18. G. HERZBERG, Trans. Roy. Sot. Canada 46, Section 3, 1 (1952). 19. P. G. WILICINSON,J. Chem. Phys. 30, 773 (1959). 20. A. LOFTHUSAND R. S. MULLIKEN, J. Chem. Phys. 26, 1010 (1957). 21. P. G. WILKINSON AND R. 8. MULLIKEN, J. Chem. Phys. 31, 674 (1959). 22. R. S. MULLIKEN, J. Chem. Phys. to be published. 23. R. S. MULLIKEN, “The Threshold of Space,” p. 169. Pergamon Press, New York, 1956. 24. P. Ii. CARROLL, Can. J. Phys. 37, 880 (1959). 25. Y. TANAKA, J. Chem. Phys. 21, 1402 (1953). 26. P. G. WILKINSON, Can. J. Phys. 34, 250 (1956). 27. A. E. DOUGLAS, Can. J. Phys. 30, 302 (1952). 28. A. B. MEINEL, Astrophys. J. 114, 431 (1951). 28~. A. E. DOOGLAS, Astrophys. J. 117, 380 (1953). 29. G. HERZBERGAND HUGO, Can. J. Phys. 33, 757 (1955). SO. Y. TANAKA, J~RSA, AND LEBLANC, J. Chem. Phys. 26, 862 (1957). 31. D~UCLAS AND M@LLER, Can. J. Phys. 33, 125 (1955). 32. P. Ii. CARROLL, Can. J. Phys. 34, 85 (1956). 33. ICZKOWSKIAND MARGRAVE, J. Chem. Phys. 30, 403 (1959). 33~. 6. HERZBERGAND HOWE, Can. J. Phys. 37, 636 (1959). 34. LADENBURGAND VAN VOORHIS, Phys. Rev. 43, 315 (1933). 35. DITCHBURNAND HEDDLE, Proc. Roy. Sot. A226, 509 (1954). 36. K. WATANABE, INN, AND ZELIKOFF, J. Chem. Phys. 21, 1026 (1953). $7. WEISSLER AND LEE, J. Opt. Sot. Am. 42, 200 (1952). 38. P. G. WILKIXSON AND R. S. MULLIKEN, Astrophys. J. 126, 597 (1957). 39. P. K. CARROLL,Astrophys. J. 129, 794 (1959).

MOLECULAR

SPECTRA

IN VACUUM

IJLTRAVIOLET

55

40. SPONERAND TELLER, Revs. Modern Phys. 13, 75 (1941). 41. R. S. MULLIKEN, Phys. Rev. 40, 55 (1932) ; Phys. Rev. 41, 49, 751 (1932); Phys. Rev. 43. 279 (1933) ; J. Chem. Phys. 1,492 (1933); J. Chem. Phys. 3,375,506,514,517, 564,573, 586, 635, 720 (1935); J. Chem. Phys. 7, 14, 20, 121, 339, 353, 356, 364, 570 (1939); J. Chem. Phys. 8, 234, 382 (1940). 42. A. D. WALSH, in “Annual Review of Physical Chemistry,” Vol. 5, p. 163, Annual Reviews, Stanford, 1954. 43. PLATT AND KLEVENS, Revs. Modern Phys. 16, 182 (1944). 44. F. PASCHEN,J. Opt. Sot. Am. and Rev. Sci. Instr. 16, 231 (1928). 45. R. S. MULLIKEN, J. Chem. Phys. 23. 1997 (1955). 46. G. HERZBERGAND ~HOOSMITH,Can. J. Phys. 34, 523 (1956). 47. (+. HERZBERGAND SHOOSMITH,Nature 183, 1801 (1959). 48. MOE AND I~UNCAN,J. Am. Chem. Sot. 74, 3136 (1952). 49. I’. G. WILKIXSON AND JOHXSTOX, J. Chem. Phys. 16, 190 (1950). 50. Sux AXD WEISSLER, J. Chem. Phys. 23, 1160 (1955). 51. Ii. K. INNES, J. Chem. Phys. 22, 863 (1954). 52. INGOLDAND KING, J. Chem. Sot. p. 2725 (1953). 53. R. S. MCLLIKEN, Can. J. C’hem. 36, 10 (1958); A. I>. WALSH, J. Chem. Sot. p. 2260 (1953). 64. W. C. PRICE, Phys. Rev. 47, 444 (1935). 55. I’. G. WILKINSON, J. ilfol. Spectroscopy 2, 387 (1958). 56. ALLEN, BLAINE AND PLYLER, J. Research Natl. Bar. Stundards 66, 279 (1956). 57. PRICE AND TUTTE, Proc. Roy. Sot. A174, 207 (1940). 58. ZPIJKOFF AND K. WATAXABE, J. Opt. Sot. dm. 43, 756 (1953). 59. I’. G. WILKINSON AND R. S. MULLIKEN, J. Chem. Phys. 23, 1895 (1955). 60. I’. G. WILIUNSON, Can. J. Phys. 34, 843 (1956). 61. W. C. PRIMEAND R. W. WOOD, J. Chem. Phys. 3, 439 (1935); W. C. PRICE AKD WALSH, Proc. Roy. Sot. A191, 22 (1947). 68. MORTON AND STUBBS, J. Chem. Sot. p. 1347 (1940). 65. PICKETT, MUNTZ, AND MCPHERSON,J. .4m. Chem. Sot. 73, 4862 (1951). 5’4. ROMANI)AND VODAR, Compt. rend. 233, 930 (1951). 65. P. G. WILKINSON, Can. J. Phys. 34, 596 (1956). 66. NORDHEI~X,SPONER,AND TELLER, J. Chem. Phys. 8, 455 (1940). 67. R. S. MULLIKEN, J. Chem. Phys. 7, 20 (1939); ROOTHAANAND R. s. MULLIKEN, J. (‘hem. Phys. 16, 118 (1948). 68. JAHN AND TELLER, Proc. Roy. Sot. A161, 220 (1937). 69. (;. HERZBERG, “Infrared and Raman Spectra.” Van Nostrand, New York, 1945. 70. A. I). LIEHR (to he published). 71. LOXGUET-HIGGINS, &II~, PRYCE, AND SACK, Proc. Roy. Sot. A244, 1 (1956). 72. K. WAT.GABE, J. rhem. Phys. 26, 542 (1957). 73. K. WAT~XABE AND NAKAYAMA, J. Chem. Phys. 29, 48 (1958). 74. LIU ANIUDTNCAN, J. Chem. Phys. 17, 241 (1949). 75. LOWREY AXVDK. WATANABE, J. Chem. Phys. 28, 208 (1958). 76. TANAKA, J~RSA, AND LEBLANC, J. Chem. Phys. 28, 350 (1958). 77. It. S. M~LLIKEN, J. Chem. Phys. 3, 720 (1935). J. F. MULLIGAN, J. Chem. Phys. 19, 347 (1951): A. D. MCLEAN, J. Chem. Phys. 32, 1595 (1960). 78. INN, K. WATANABE, AND ZELICOFF,J. Chem. Phys. 21, 1648 (1953). 79. Ci. HERZBERGAXD INNES, Can. J. Phys. 36, 842 (1957). 80. HARRIS, KING, BENEDICT,AND PEARSE, J. Chem. Phys. 8, 765 (1940). 81. NAKAYAMA, KITAMURA, AND K. WATANABE, J. Chem. Phys. 30, 1180 (1959). 82. H. FRIEI~YAN, Sci. American 200, No. 6, 52 (1959). 83. (:. HERZBERG,Mkm. sot. ray. xi. Lilge 141,18, 397. 8Sa. I?. C. KIESS, C. H. CORLISS, ANI) H. K. KIESS, Science 131.1319 (1960).

56

WILKINSON

84. S. P. REDDY AND P. T. RAO, Can. .I. Phys. 36, 912 (1957). 86. S. M. NAUDE AND T. J. HUGO, Can. J. Phys. 36, 64 (1957). 86. R. ONAKA, J. Chem. Phys. 27, 374 (1957). 87. H. SUN AND G. L. WEISSLER, J. Chem. Phys. 23, 1625 (1955). 88. A. L. G. REES, J. Chem. Phys. 26, 1567 (1957). 88. K. L. WRAY AND D. F. HORNIG, J. Chem. Phys. 24, 1271 (1956). 90. S. M. BUNCH, G. R. COOK, M. OGAWA, AND A. W. EHLER, J. Chem. Phys. 28,740 (1953). 90a.P. B. V. HARANATHAND P. T. RAO, J. Mol. Spectroscopy 2, 428 (1958). 90b.R. D. VERMA, J. Chem. Phys. 32, 738 (1966). 91. G. HERZBERG,A. LAGERQVIST,AND E. MIESCHER, Can. J. Phys. 34, 622 (1956). 92. L. H. SUITCLIFFAND A. D. WALSH, Proc. Phys. Sot. A66, 209 (1953). 95. D. MIGEOTTE AND B. ROSEN, Bull. Sot. Roy. Sci. LiBge 19, 343 (1959). 94. H. SUN AND G. L. WEISSLER, J. Chem. Phys. 23,1372 (1955); F. F. MARMO, J. Opt. Sot. Am. 43, 1186 (1953); G. BETHKE, J. Chem Phys. 31, 662 (1959). 96. E. MIESCHER, Can. J. Phys. 33, 355 (1958); Helv. Phys Acta 29, 135 (1956). 96. L. VEGARD, 2. Physik 76,30 (1932); J. KAPLAN, Phys. Rev. 44,947 (1933); 46,675 (1934); 0. R. WULF AND E. H. MELVIN, Phys. Rev. 66,687 (1939); J. JANIN, Ann. Physik (12) 1, 538 (1946). 97. P. G. WILKINSON, J. Chem. Phys. 32, 1061 (1960). 98. M. OGAWA AND Y. TANAKA, J. Chem. Phys. 32, 754 (1960). 99. P. G. WILKINSON, J. Chem. Phys. 31, 674 (1959). 100. M. OGAWA AND Y. TANAKA, J. Chem. Phys. 30, 1354 (1959). 101. P. G. WILKINSON, Astrophys. J. 126, 1, 10, (1957); A. LOFTHUS, Can. J. Phys. 34,736 (1956). 102. M. OGAWA AND Y. TANAKA, to be published. 108. Y. TANAKA, J. Opt. Sot. Am. 46, 663 (1955). 104. P. G. WILKINSON, J. Chem. Phys. 24, 528 (1956). 105. P. BRIX AND G. HERZBERG,Can. J. Phys. 32,110 (1956) ; W. A. RENSE AND R. MECURE, Bull. Am. Phys. Sot. [2], 30, 56 (1955); P. G. WILKINSON AND R. S. MULLIKEN, Astrophys. J. 126, 597 (1957); P. K. CARROLL,Astrophys. J. 129, 794 (1959); R. W. DITCHBURNAND D. W. D. HEDDLE, Proc. Roy. Sot. A226, 509 (1954). 106. Y. TANAKA, J. Chem. Phys. 20, 1728 (1952). 107. K. DRESSLER, Helv. Phys. Acta 28, 563 (1955). 108. K. DRESSLER (see Ref. 107). 109-111. K. DRESSLER (see Ref. 107). 112. J. W. C. JOHNS AND R. F. BARROW, Proc. Phys. Sot. 71, 475 (1958). IIS. Y. TANAKA, J. Opt. Sot. Am. 46, 710 (1955). 114. P. G. WILKINSON, J. Opt. Sot. Am. 46, 1044 (1955). 116. Y. TANAKA, J. Opt. Sot. Am. 46, 710 (1955). 116. Y. TANAKA, A. S. JURSA, AND F. J. LEBLANC, J. Opt. Sot. Am. 48, 304 (1958). 117. R. F. BARROWAND H. C. ROWLINSON,Proc. Roy. Sot. A224, 134 (1954). 118. A. E. DOUGLASAND P. M. ROUTLY, Astrophys. J. (Supp. Series) 1, 295 (1955). 119. R. F. BARROWAND H. C. ROWLINSOX, Proc. Roy. Sot. A224, 374 (1954). 120. J. W. C. JOHNS AND R. F. BARROW, Nature 179, 374 (1957). 121. R. F. BARROW AND E. MIESCHER, Proc. Phys. Sot. A70, 219 (1957). 122. G. W. BETHKE, J. Chem. Phys. 31, 662 (1959). 128. H. FREYMARK, Ann. Physik 8, 221 (1951). i2.4. L. H. SUITCLIFFE AND A. D. WALSH, J. Chem. Phys. 19, 1210 (1951). 126. T. NAMIOKA AND K. WATANABE, J. Chem. Phys. 24,915 (1956). 126. R. S. MULLIKEN AND E. TELLER, Phys. Rev. 61, 283 (1942). 127. G. MOE AND A. B. F. DUNCAN, J. Am. Chem. Sot. 74, 3149 (1952).

MOLECULAR

SPECTRA IN VACUUM

ULTRAVIOLET

57

128. W. MOFFETT AND J. SCANLAN, PTOC. Roy. Sot. A216,464(1953). 129.P. G. WILKINSON, J. Chem. Phys. 24, 917(1956). 130. W. C. PRICEAND A. D. WALSH, hoc. Roy. Sot. AlQl, 22 (1947). 131.H. C. LONOUET-HIGGINS,U., OPIK, M. H. L. PRYCE, AND R. A. SACK, PTOC. Roy. Sot. A244.1 (1958); A. LIEHR AND W. MOFFETT, J. Chem. Phys. 36, 1074 (1957). 132. L. W. PICKETT, J. Chem. Phys. 9, 293 (1940); L. W. PICKETT, N. J. HOEFLICH, ASD T. C. LIU, J. Am. Chem. Sot. 73, 4865 (1951). 133. W. C. PRICE AND A. D. WALSH, Proc. Roy Sot. A179,201(1941). 134.W. C. PRICE AND D. M. SIMPSON,PTOC. Roy. Sot. A169,501(1938). 135.G. S. FORBESAND J. E. CLINE, J. Am. Chem. Sot. 61, 151 (1939). 136. H. J. HILGENDORFF,2. Physik 96, 781 (1935). 137. W. C. PRICE, Phys. Rev. 46, 529 (1934). 138. W. L. PRICE, J. P. TEEGAN, AND A. D. WALSH, Proc. Roy. Sot. A201,600(1950). 139.Ii. WATANABE AND ZELIKOFF,J. Opt. Sot. Am. 43, 753 i1953). 140. W. C. PRICE, J. Chem. Phys. 4, 147 (1936). 141, J. J. HOPFIELD,Phys. Rev. 63, 931 (1938). 142. Nl. ZELIKOFF,K. WATANABE, AND E. C. W. INN, J. Chem. Phys. 21,1643 (1953). 143. -1. B. F. DUNCAN, J. Chem. Phys. 4, 638 (1936). 144. W. C. WALKER AND 6. L. WEISSLER, J. Chem. Phys. 23, 19G2 (1955). Z@. K. 310~1. Science of Light 4, 130 (1955). 146. W. C. PRICE AND D. M. SIMPSON,Trans. Faraday Sot. 37, 106 (1941). 147. F. TAXAKA, E. C. P. INN, AND K. WATANABE, J. Chem. Phys. 21, 1651 (1953). 1.48. M. ~G_~XVAAND G. R. COOK, J. Chem. Phys. 28, 173 (1958). 149. T.-Ii. LIL-, G. MOE, AND A. B. F. DUNCAN, J. Chem. Phys. 19, 71 (1951). 150. W. C. PRICE, J. P. TEEGAY, AND A. I). WALSH, J. Chem. Sot. p. 920 (1951).