PHOTOFRAGMENT SPECTROSCOPY OF MOLECULAR IONS USING FAST ION BEAMS

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PHOTOFRAGMENT SPECTROSCOPY OF

MOLECULAR IONS USING FAST ION BEAMS

J. Moseley, P. Cosby, J. Durup, J.-B. Ozenne

To cite this version:

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JOURNAL DE PHYSIQUE Colloque

C l ,

suppldment au n o 2, Tome 40, fdvrier

1979,

page

C1-46

PHOTOFRAGMENT SPECTROSCOPY OF MOLECUM IONS USING FAST I O N BEAMS J. T. Moseley and P. C. Cosby

Molecular Physics Laboratory, SRI Internationa1,Menlo Park, C a l i f o r n i a 94025 and

J. Durup and J.-B. Ozenne

Laborato.;Te des C o l l i s i o n s Atomiques e t Moleculaires, U n i v e r s i t e de Paris-Sud, 91405 Orsay, France

~'esum$

-

La spectroscopie des photofragments d ' i o n s a Ct6 un s u j e t d f i n t 6 r @ t c r o i s s a n t au cours des t r o i s d e r n i k r e s annges. Le dgveloppement de nouvelles f a c i l i t k s e t techniques expgrimentales a Qtendu l a l i s t e des i o n s gtudids, des H2+ originaux

5

~ r ~ + , Kr2+, 02+, 03+, C H ~ I + , e t peut-&re d ' a u t r e s . C e t t e revue s e concentrera s u r l e s ions dimkres de gaz r a r e s , dont l a spectroscopie des photofragments normale a s e m i

a

dkterminer l e s courbes de p o t e n t i e l , e t s u r 02+, pour l e q u e l des techniques des t r e s h a u t e r L s o l u t i o n o n t permis une gtude d k t a i l l g e des niveaux r o t a t i o n n e l s e t de s t r u c t u r e f i n e des e t a t s b'%

-

(v=4,5) e t de l a p r 6 d i s s o c i a t i o n de c e s niveaux.

-

g

Abstract

-

Ion photofragment spectroscopy has been a s u b j e c t of i n c r e a s i n g i n t e r e s t during t h e p a s t t h r e e y e a r s . Development of new experimental f a c i l i t i e s and techniques h a s extended t h e l i s t of ions s t u d i e d from t h e o r i g i n a l H2+ t o Ar2+, Kr2+, 02+, 03+, CH ₃I+, and perhaps o t h e r s . This review w i l l concentrate on t h e r a r e gas dimer i o n s , where normal photofragment spectroscopy has been used t o determine t h e p o t e n t i a l curves, and o n . 0 2 , where very h i g h r e s o l u t i o n techniques have allowed a d e t a i l e d study of the r o t a t i o n a l and f i n e s t r u c t u r e l e v e l s of t h e k4%'(v=4,5) s t a t e s , and of t h e p r e d i s s o c i a t i o n of these l e v e l s .

INTRODUCTION _{d i f f e r e n t l a b o r a t o r i e s , a l l of t h e experimental} The technique of ion photofragment spectroscopy techniques can be discussed with r e f e r e n c e t o Fig. 1. c o n s i s t s b a s i c a l l y of i n t e r a c t i n g a f a s t beam of This a p p a r a t u s , l1 r e c e n t l y constructed a t SRI, has molecular ions with photons, u s u a l l y from a l a s e r , t h e c a p a b i l i t y of performing a l l of t h e types of and d e t e c t i n g t h e ionized photofragments produced. photofragment spectroscopy experiments t h a t have The i n t e n s i t y and t h e energy and angular d i s t r i b u t i o n s been made t o date. I n t h i s apparatus, ions a r e of t h e photofragments can be measured a s functions of

ION SOURCE FOCUSING ANGULAR LASER ANGULAR PHOTOFRAGMENT ION wavelength. From t h e s e b a s i c measurements i t h a s MASS SELECTION E X T R A C ~ ~ ~ ~ , C~~~~~~~~~~ INTERACTION RESOLUTION ENERGY RESOLUTION AND DETECTION been p o s s i b l e t o measure v i b r a t i o n a l l e v e l spac-

1-3 3-5

ings, determine t r a n s i t i o n symmetries, con- s t r u c t diatomic i o n p o t e n t i a l

curve^,^'^

observe and explain t h e e f f e c t s of s p i n - o r b i t i n t e r a ~ t i o n , ~ ' ~ r e s o l v e r o t a t i o n a l s t r u c t u r e .of p r e d i s s o c i a t e d

s t a t e s ,6 determine bond d i s s o c i a t i o n energies, 4-7 measure t r a n s i t i o n e n e r g i e s with sub-Doppler reso- l u t i o n , 6 y 8 s 9 and study the l i f e t i m e s of predissoci'a- ted l e v e l s . 6,9,10 Examples of each of these types of measurements w i l l be discussed. EWERIMENTAU TECHNIQUES ~ l t h o u g h f a s t i o n photofragment spectroscopy TO MCA VELOCITY-TUNED SPECTRUM ENERGY SPECTRUM F i g . 1

-

Schematic of t h e photofragment spectrometer a t SRI, which can be used with both crossed experiments a r e now being ~ e r f o r m e d i n a t l e a s t f i v e and c o a x i a l beams.

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e x t r a c t e d from an ion source, a c c e l e r a t e d t o a few thousand v o l t s , and the species t o be s t u d i e d s e l e c - ted by a mass spectrometer. These parent ions a r e then collimated t o 2 mrad and bent through 90 degrees i n t o the l a s e r i n t e r a c t i o n region. Here t h e i n t r a c a v i t y photons from a fixed frequency o r tun- a b l e cw l a s e r can be e i t h e r c o a x i a l t o the i o n beam o r crossed with it. For photofragment spectroscopy, any photofragment i o n s produced i n the i n t e r a c t i o n region a r e bent through 90 degrees, and again c o l l i - mated t o 2 m a d so t h a t only photofragments d i r e c t e d forward o r backward along t h e beam e n t e r the energy analyzer. The energy analyzer i s capable of r e s o l - ving photofragment energies which correspond t o about 10 meV i n the c e n t e r of mass. The crossed beams arrangement can be used t o measure the angular d i s t r i b u t i o n of the photofragments by r o t a t i n g the l a s e r p o l a r i z a t i o n , and t o study p a r a l l e l t r a n s i t i m s . The coaxial beams arrangement is used t o maximize

the overlap between t h e photons and ions, and i s b e s t s u i t e d t o the study of perpendicular t r a n s i t i o n s , s i n c e t h e l a s e r p o l a r i z a t i o n i s n e c e s s a r i l y perpen- d i c u l a r t o t h e beam d i r e c t i o n . I n t h i s arrangement t h e e l e c t r o s t a t i c cage can be used t o vary t h e v e l o c i t y of t h e ion beam, thus varying the absorption wavelength.

It i s important t o n o t e t h a t , a s has been d i s - cussed elsewhere 1'2*L1 i n d e t a i l , the r e l a t i v e l y small c e n t e r of mass energies W a r e g r e a t l y ampli- f i e d i n t h e l a b o r a t o r y system. For a homonuclear diatomic, a photofragment r e s u l t i n g from a d i s s o c i a - t i v e t r a n s i t i o n such a s thbse i n d i c a t e d i n Fig. 1 w i l l appear a t 8 =

o0

i n t h e lab with an energy T

given by

1

.f.

T =

-

T

+

(WTO)

+

W/2

,

2 0 - (1)

where T i s the parent ion energy and ₀ W i s the t o t a l k i n e t i c energy of s e p a r a t i o n i n ihe c e n t e r of mass frame. Thus f o r T = 3000 eV, a photofragment cor-

Q

responding t o W = 0.1 eV appears i n the laboratory frame with an energy of about 1517 eV, compared w i t h 1500 eV f o r W = 0. It is a l s o important to note the high angular c o l l i m a t i o n of the p a r e n t ion beam and angular r e s o l u t i o n of the photofragments required s o t h a t one does n o t observe photofragments a t 0' i n

t h e l a b which a c t u a l l y r e s u l t from a t r a n s i t i o n of l a r g e r

and

8

#

0'. This imposes angular d e f i n i - t i o n requirements i n the range of m i l l i r a d i a n s i n the laboratory system.

The a b i l i t y t o o b t a i n sub-Doppler r e s o l u t i o n a r i s e s from t h e narrowing of the v e l o c i t y d i s t r i b u - t i o n i n an ion beam when i t i s accelerated. This a r i s e s very simply from the f a c t t h a t a population

of ions with an energy spread AE which i s e x t r a c t e d from an ion source and a c c e l e r a t e d t o an energy E maintains t h i s energy spread

AE;

however, s i n c e E = *vL, the v e l o c i t y spread a t an energy E

>>

AE must be s u b s t a n t i a l l y reduced from i t s value before a c c e l e r a t i o n . I n f a c t , an ion beam with a 1 eVenergy spread has a t 3 keV laboratory energy a v e l o c i t y spread corresponding t o l e s s than 1K.

NORMAL PHOTOFRAGMENT SPECTROSCOPY

The technique of photofragment spectroscopy a s o r i g i n a l l y conceived and used a t Orsayl and FOM 2 c o n s i s t s of c r o s s i n g a l a s e r beam with a n ion beam and measuring t h e energy d i s t r i b u t i o n of t h e photofragment ions. Figure 2 shows a photofragment k i n e t i c energy spectrum f o r the t r a n s i t i o n

4

s t u d i e d extensively a t ~ r s a y , ~ where the f Il s t a t e g was observed f o r t h e f i r s t time, and i t s general shape and l o c a t i o n determined. This work a l s o r e - s u l t e d i n t h e discovery12 of a very i n t e r e s t i n g pre- d i s s o c i a t i o n which w i l l be discussed l a t e r .

This spectrum can be understood by r e f e r e n c e t o

+

t h e O2 p o t e n t i a l curves shown i n Fig. 3, where the -

SEPARATION ENERGY W IrneV)

1000 500 250 100 25 5 0 5 25 100 250 500 10M)

I 1

I I 1 l l l l l I I I 1 1

10 1 I I I I I

1

1440 1460 1480 15W 1520 ₁₅₄₀ ₁₅₅₀

LABORATORY PHOTOFRAGMENT ENERGY lev)

Fig. 2

-

Photofragment k i n e t i c energy spectrum of

+

(4)

c1-48 JOURNAL DE PHYSIQUE

08 1.2 16 20 2 4 2 8 3 2

INTERNUCLEAR SEPARATION R

(A)

+

Fig. 3

-

Quartet potential curves of 0 relevant 2

to this discussion.

transition under consideration here is labeled I. The peaks on the right and left Side of separation energy

W

= 0 correspond to the forward and backward ejected ion photofragments originating from various vibrational levels of the ground state. The spacing of these levels, determined from spectra obtained at a number of wavelengths, identifies the ground state

4

of this photodissociation as the metastable

5 nu.

The fact that the angular distribution peaks for the laser polarization along the ion beam direction identifies the transition as parallel, l3 and thus the dissociative state as the previously unobserved

4ng.

The fact that the dissociation occurs for

W

= 0

4

shows that the ll state is bound. Thus this state g

has the general shape and location shown in Fig. 3, in agreement with recent calculations. 14

The high sensitivity of the apparatus shown in Fig. 1, due to the coaxial beams configurations and use of intracavity laser techniques, has permitted a

+

detailed study4 to be made of Ar2

.

This ion is of high fundamental interest, and has been the subject of many scattering experiments and calculations.

2

+

Thi6, study resulted in a determination of the _+.

_".

=u

,

2ng,

and 2~

'

potential curves to an accuracy of

g

about 20 meV, as well as explanation of the effects of spin-orbit interaction on the potential curves, the magnitude and wavelength dependence of the cross section; and the angular distributions of the photofragments.

An example of the photofragment energy spectra obtained in this work is shown in Fig,

4.

The smooth peaks represent the experimental data; the narrower, more structured peaks represent a calculation4 using

- a

_-

CROSSED BEAMS

.

-

- b COAXiAL BEAMS

-

- - I 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 PHOTODISSOCIATION ENERGY W (eV)

Fig.

4 -

Photofragment kinetic energy spectrum of

-I- -t

Ar2

+

hv - + A r

+

Ar.

the potentials determined in this work. Individual vibrational levels are not resolved for Ar + as they

2

are for 0 + since the rotational energy distribution 2

of the ions is as large as the vibrational spacing. Comparison of the crossed and coaxial beams data at 6471 shows that the highest energy peak is favored in crossed beams (parallel laser polarization, 8=0°), while the other peak is relatively favored in coaxial beams (perpendicular laser polarization, 8 = 90').

From this, the first peak can be identified with the

5

+ * 211 transition; the second with the 2~ + -.

g u

%u+. From data such as these at 14 different wave-

g

lengths using both crossed and coaxial beams, the potential curves shown in Fig. 5 were determined.

2 0 2 5 3 0 3 5 4 0

INTERNUCLEAR SEPARATION (dl

+

(5)

Of particular interest in this work was the observation of important effects of spin-orbit coupling on both the angular distribution and the magnitude of the cross section for the 2~ + -+ 2~ tran-

u g

sition.

Photofragment spectroscopy is thus now an established technique for the determination of potential

curves for diatomic ions, with an accuracy of about 20 meV. The only requirements are the ability to form a reasonably intense beam (- amp) of the ion, and the availability of lasers in the appropriate wavelength range.

PREDISSOCIATION PHOTOFRAGMENT SPECTROSCOPY

+

In addition to the transition in the 0

2 quartet

system described by Eq. (2) and by Transition I of Fig. 3, a second transition, 11,

can be observed for vibrational levels of the

b

state of

v'

= 4 or greater, since these levels are predissociated. These transitions were first observed by the Orsay with the correct identifi- cation made by the Southampton group. 8

A

detailed study of transitions to the v' = 4

and 5 levels of the

1!

state has been made at 0.2

2

resolution by Tadjeddine, Abouaf, Cosby, Huber and ~osele~.' The work was performed using the appara-

tus shown in Fig. 1. TWO CW jet-stream tunable dye lasers were used, one crossed and one coaxial with the ion beam, pumped by argon and krypton ionlasers. The polarization of the laser crossed with the ion beam was always parallel to the beam direction. This laser was arranged so that the ion beam passed through the laser cavity, in order to enhance the photofragment current. The polarization of the laser coaxial with the ion beam was necessatily always perpendicular to the beam direction. Proper accoant was taken of the approximately 2.6

11

Doppler shift with coaxial beams, and in order to have only one Doppler component, the extracavity laser beam was used. Each laser had a line width of about 0.2

2.

The wavelength was measured in air using a mono- chromator with a resolution of 30,000 (approximately 0.2

2

at 5800

2).

The primary experimental data were scans of the laser wavelength from 5750 to 5870 2, covering all of the expected

(4,4)

and (5,5) transitions while observing the photofragment current for fragment ions corresponding to total separation energies W centered at 0, 12, 25, 50 and 100 meV. Spectra at W = 0 were also obtained for the

(4,5)

transition (6090 to 6210

g)

and for the (4,3) transitions (5430 to 5550

g).

Figure

6

shows a predissociation spectrum obtained for W = 0 with crossed beams. The

W

= 0 spectrum obtained for coaxial beams is essentially identical, since the angular acceptance of the apparatus is such that there is no angular discrim- ination for photofragment ions whose energy is less than 12 meV. The resolution of the peaks in this spectrum is seen to be about 0.2

2,

and the uncer- tainty in the absolute wavelength scale is also about 0.2

2.

From the width and spacing of the peaks it is clear that rotational structure is resolved.

The first task is to identify the optical transition resulting in the predissociated levels.

8

Previous work suggests that the (4,4) band of the transition (3) is responsible, but rotationally- resolved spectra of this band have not been previously obtained, and there exist at least two other

12

types of transitions which could result in a

highly structured cross section near W = 0.

The wavelengths for the expected'transitions can be calculated using the work of Albritton, Schmeltekopf, Harrop, Zare and ~zarny.'~ Because of its high multiplicity, the structure of this band is quite complex. Figure 7 shows schematically the expected transitions, along with the relevant quantum numbers and their designations.

A

detailed discussion is given in Refs. 6 and 15. Transitions between the

b

and

a

states can be uniquely designated by P,

Q,

or

R,

depending on whether AJ is -1, 0

or +1 subscripted by

F'

FU, For example, the transition to the far left of Fig. 7, P24, corresponds to &J = -1,

F'

= 2, and FN =

4.

It can be seen from Fig. 7 that in principle

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Fig. 6

-

Predissociation photofragment spectrum of Photofragments corresponding to separation

02+ for the transition given by E l . (3). energies W between 0 and 20 meV are observed. tion rule N = 0,

+

1, and terminate in a given

d

level of the state. However, with the present resolution of 0.2

2,

only 24 branches can be resolved, 16 of these consisting of blends of two or three of the transitions of Fig.

7.

Even in the highest- resolution optical spectroscopy only 40 branches can be resolved.

In order to calculate the expected transition wavelengths for comparison with the data of Fig. 6,

the G(v,J) energies of the 2 and

b

states must be determined. The molecular constants of the v" =

4

and 5 levels of the 2 state are well-known;15 the constants for the v' =

4

and 5 levels of the

2

state

FU4

F",

FU2

-

FW1

6.5

4n-1/2

4n1/2

4n3/2 4n5/2

02+ (a4n,, v" = 4)

Fig.

7 -

Expected transitions for a band in the

+

First Negative System of 0

2 '

were determined using the Dunham coefficients from Ref. 15. The G(v,J) energies were then computed by diagonalization of the Hamiltonian. l5 All molecular constants used for the interpretation of the (4,4) and (5,5) bands are given in Ref.

6.

The results of these calculations for the

(4,4)

band, rotational levels

d

= 9, 11, 13 and 15, are shown in the upper part of Fig.

6.

All of the observed structure can be explained by the indicated transitions.

Characteristically, the peak corresponding to

d

= 9 is the most intense for each branch. This is not due primarily to population differences or vibrational line strengths but to the energy resolution function of the apparatus. With the energy resolution set nominally at W=O, the analyzer will dis- criminate more and more strongly against the higher rotational levels.

Before attempting to understand the dependence of the predissociation spectra on photofragment kinetic energy, it is important to establish the dissociation energies

w~(N'

, 3 ~ ) of the various

N'

32

+ 4 0

levels with respect to the O( P )

+

0 ( S ) dissocia-

2

tion limit. These dissociation energies can be calculated from the cycle

3

w~(N'

,3~2)

+

P1(O Pt)

+

Do(o2X)

+

(7)

+

where E(0 b,vl

=4,~')

is the energy of the designated 2

level above the v' = 0,

d

= 0 level of the

b

state. 3

Alternatively, we can determine W (9, P ) directly by

d 2

measurement of the photofragment kinetic energy spectrum with the laser tuned to one of the

N

'

= 9 peaks of Fig. 6. Such a measurement yielded7 1.1

2

3

1.1 meV for Wd(9, P2). This allows an accurate calculation of the other Wd values, using the molecular constants of Ref. 6.

3

This direct measurement of W (9, P ) allows a

d 2

determination of the dissociation energies for _-

+

4

02(x3r

-)

,'

O2

&

and 02+(b41: -) .6 Figure 8

g g

shows the cycle used to determine D (0 X). It can

0 2

be seen that the value determined here, 41262

2

10

-

1

cm

,

is in excellent agreement with the value of 41260

+

15 cm-I determined by Brix and Herzberg 16 using an entirely different cycle. In a similar manner, the bond energy of the

a

state was determined

-

1

to be 21276

+

10 cm

,

and of the

b

state t'o be 20411

+

10 cm-I.

The dependence of the spectra on photofragment kineaic energy has been discussed in detail in Ref. 6. Figure 9 shows this dependence for W values near 12, 25, 50 and 100 meV. Recall that in Fig. 6, for W = 0, rotational levels

N'

= 9, 11, 13 and 15 were observed. As the energy analyzer is centered to ob-

W ( N = ~ , ~ ? ) - 9 f 9 Present Study 1 --v.4, N.9-

--

-

t

o+I~s:,~) + o(~P,)

-f-v=O,N=O

₊

";,?I

=

0' E (0; b) = 146556 -L 2

Yosh~no 8 Tonoko E(O+ 4~:,,) _._.= 10983702 + 0 0 6

t

I

Moore

,

6

3 ~ s

I

-

15867862 Moore

)

n3r: D(OZ X) = 41262

*

I 0 Present Study D(0, X) = 61260 f 15 B r ~ x 8 Herzberq

Fig. 8

-

Cyclic determinations of the dissociation energy of ground state diatomic oxygen.

-

1 The energies are expressed in (cm ) and are not all to scale.

ABSORPTION WAVELENGTH (A1

Fig. 9

-

Predissociation photofragment spectra for

+

O2 obtained for W values near 12, 25, 50 and 100 meV.

serve higher energy photofragments, high rotational levels, and finally higher vibrational levels should be observed. Thus in the 12 meV spectrum

d

= 13, 15 and 17 are observed, in the 25 meV spectrum,

N

'

=

15, 17, 19 and 21 and in the 50 meV spectrum, N' =

(8)

nitude between the W = 0 and 50 meV spectra. This is due to the decrease in population of the higher rotational levels, and to the angular and energy resolution of the apparatus. Except for this filter of energy analysis, the forest of peaks would be much denser, complicating analysis. Further, a kinetic spectrum taken at the wavelength correspond-

ing to a particular peak gives an unequivocal identi- fication of the

N

'

level corresponding to that peak.

At W values near 100 meV, the v' = 5 level can be observed. The peaks in this spectrum correspond to

N'

levels 7, 9, 11, 13 and 15 of v' = 5.

PHOTOFRAGMENT SPECTROSCOPY WITH A COAXIAL SINGLE-MODE LASER

The use of a single mode laser coaxial with the ion beam provides both the ability to obtain sub- Doppler resolution and a convenient way of tuning the absorption wavelength, namely variation of the ion beam- velocity

The experimental data to be discussed here were obtained in the following way.

A

beam of 0 + ions

2

was established at an energy of 3600 eV. The single mode laser was tuned by tilting its etalons until its Doppler-shifted frequency was in the vicinity of one

r

of the absorptions already studied in detaild at lower resolution (0.2

2)

:), The velocity of the beam

was then scanned by varying the potential on the electrostatic cage. As the absorption wavelength was Doppler-tuned into resonance with a transition, pnotofragment ions appeared at the energy analyzer. If the energy analyzer was set to pass ions of the energy of these photofragments, counts could be observed at the detector.

A typical scan is shown in Fig. 10. Such a

-

1

scan, which,covers about 0.5 cm

,

required a variation in the ion beam energy of approximately 500 eV. The kinetic energy (and hence absorption wavelength) scan is controlled by a multichannel scaler, and thus each channel of the scaler corresponds to a specific absorption wavelength. The wavenumber listed at the center of Fig. 10 is that of the single mode laser beam. The abcissa of the figure gives the Doppler shift from that value due to the

I

I I I I I

I

8 7 8 6 8 5 8 4 8 3 8 2 8 1

DOPPLER SHIFT (cm-1)

Fig. 10

-

Typical wavelength (velocity) scan for the

+

4 + 4 -

band O2 @

II

,v1'=4) - 0 @

xg

,v'

=4)

-

u

+

2

0

+

0.

ion velocity. The absolute value of the wavelength 9

was determined using a digital wavemeter inteferom- eter, relative to the iodine absorptions near

-1

5145

2,

to an accuracy of 0.003 cm

.

The designa- tion of the peaks is as previously described, with the value of

N'

included here in brackets.

Data such as those shown in Fig. 10 have been obtained for over 1000 transitions in the (3,3),

(4,4),

(4,5), and (5,5) bands of normal9 and isotopic

02+.

A

statistical analysis of the data shows that the precision of the measurements for transitions

-1

terminating in v' =

4

is 0.003 cm

.

This is an order of magnitude better than the best previous optical spectroscopy on these transitions, which was limited by the normal Doppler width. The peaks labeled [9]Q21 and [9]P31 in Fig. 10 would blend in normal optical spectroscopy; here they are easily resolved.

The present data, which include higher vibrational levels than previously existing data on the b t- 3 transitions,15 and are of higher precision,

-

will allow an improvement in the Hamiltonian description of these states. In fact, Carrington, et al. report8 refined molecular constants for these states based on only 22 lines of the (4,l) band. Work is nearly complete9 on the analysis of the

+

bands listed above for normal O2

.

Substantial improvements are made in the molecular constants. 15

(9)

dominated by t h e apparatus-induced Doppler width, b u t c o n t a i n i n f o r m a t i o n about t h e n a t u r a l l i f e t i m e o f t h e l e v e l s . The Doppler c o n t r i b u t i o n t o t h e l i n e - w i d t h s can be accounted f o r i n a r e l a t i v e l y s t r a i g h t -

forward manner,'' and t h e l i f e t i m e of each p r e - d i s s o c i a t e d l e v e l obtained. The e x t e n s i v e n a t u r e o f

t h e p r e s e n t d a t a allowed a s t u d y l o of t h e v a r i a t i o n of l i f e t i m e w i t h quantum numbers v'

,

N'

,

and F

'

i s and w i t h i s o t o p i c composition. The r e s u l t s o f t h i s s t u d y w i l l be s u m a r i z e d h e r e .

The l i f e t i m e s d i d n o t vary s i g n i f i c a n t l y w i t h N. Thus r o t a t i o n a l c o u p l i n g can c o n t r i b u t e a t most a few p e r c e n t t o t h e observed p r e d i s s o c i a t i o n , and

t h e primary mechanism must be s p i n - o r b i t coupling. The r a t i o of t h e mean l i f e t i m e s T ( F F _{1' 4}) / ?(F F ) i s 1.8

_+

0.2, e s s e n t i a l l y independent of 2' 3 N'

,

v' o r i s o t o p i c c o n s t i t u t i o n . A r a t i o of a b o u t 0.67 would be expected8 f o r a d i s s o c i a t i o n by t h e 4 f

n

s t a t e ( s e e F i g . 3 ) , and of about 2.5' f o r d i s s o -

-

g c i a t i o n by t h e 4~ + s t a t e . Thus i t i s concluded 10 g t h a t t h e d i s s o c i a t i o n occurs by b o t h of t h e s e s t a t e s , 4

+

4 wit$ a branching r a t i o of 78% by C 22% by f

n

g g - The r a t i o s of t h e l i f e t i m e s %(F4)/C(F1) and Z ( F ~ ) / ~ ( F ~ ) a r e b o t h g r e a t e r than 1, and, p a r t i c u l a r - l y r ( F ) X ( F ), d e c r e a s e w i t h i n c r e a s i n g

d

.

The 4 1 d e t a i l e d r e s u l t s provide independent s u p p o r t f o r t h e branching r a t i o determined above.

The v a r i a t i o n o f l i f e t i m e s w i t h v i b r a t i o n a l l e v e l and i s o t o p i c composition i s c o n s i s t e n t w i t h a c l a s s i c a l c u r v e c r o s s i n g d e s c r i p t i o n , w i t h t h e c r o s s i n g n e a r t h e v = 5 l e v e l . ACKNOWLEDGEMENTS The a u t h o r s a r e p l e a s e d t o acknowledge t h e p a r - t i c i p a t i o n of o t h e r s a t SRI and Orsay i n p a r t s of t h e work p r e s e n t e d h e r e ; i n p a r t i c u l a r , M i r e i l l e Tadjeddine, Bernd Huber, Roberta Saxon, Abdallah Tab c h G - ~ o u h a i l l ' e , Robert Abouaf, C h r i s t k n e P e r n o t , Tom M i l l e r and James P e t e r s o n . The a s s i s t a t s e o f and h e l p f u l d i s c u s s i o n s w i t h D. L. A l b r i t t o n and R. N.

Zare a r e a p p r e c i a t e d . The work a t SRI was supported by t h e U.S. A i r Force O f f i c e of S c i e n t i f i c Research, t h e U.S. Army Research O f f i c e , and t h e N a t i o n a l Science Foundation under Grant No. CHE77-00428.

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-