Field predissociation

Volume 5, number 4

CHEMICAL PHYSICS LETTERS

FIELD A NEW

L April 1970

PREDISSOCIATION.

PHENOMENON

IN MOLECULAR

SPECTROSCOPY

F. J. COMES and U. WENNING Institut fiiv Physikaliscke

Ckenzie der Urziuersitiit

Bonn, Germany

Received 19 November 1969

In the presence oi a weak electric

field molecules in special excited states are forced to dissociate. This

effect is due to a new couplin,m mechanism induced by the field.

Recent experiments on the absorption and fluorescence of hydrogen yielded an absolute determination of the dissociation cross section [ 1,2]. From these measurements conclusions could be drawn concerning the collisional behavior of excited hydrogen atoms [3,4]. This is of importance in the discussion of a new molecular phenomenon. In the following we will show in the case of H2 that excited molecules can be forced to dissociate in weak electric fields. This phenomenon which has not yet been reported in the literature we will call Field-Predissociation. As we cannot prove this phenomenon from line broadening of isolated molecular states due to lack of experimental resolving power, the dissociation due to the field will be shown by the occurrence of the atomic fluorescence, the Ly@-radiation. As the dissociation of the molecular states discussed in this paper will always lead to an excited atom in its first state of excitation each Lycr-quantum is a representative of one dissociation process. Excited hydrogen atoms can also be formed by normal predissociation. We are therefore going to show that the field dependence of the measured Ly@-radiation is not due to the behavior of excited atoms in the field but r&her to that of molecules. We therefore have to prove that the field effect on atoms is quite different from that on molecules, namely much smaller. On the other hand collision-induced effects have to be discussed. This will further prove the effect to be present at isolated molecules (independent of pressure). There are other experimental results which show that in the absence of a field the excited states exhibit reactive (H+formation)and absorption properties (satura 3-ion effects) characteristic of their molecular nz&ure.

discussed the photo-dissociation of the hydrogen molecule. The excited particle formed by this decay is mostly the H(2S) metastabIe atom. Collisional induced fluorescence of Lycr as well as reactive processes occur if the photodissociation is carried out at hydrogen pressures large enough to allow, at least to some extent, collisions between these metastables and molecules. Thus a partial deactivation of the excited atoms takes place. By the application of an eIectric field the lifetime of the metastables is shortened so that the competition between collisional deactivation and spontzineous emission is now in favour of the latter process. For kinetic energies of the metastables if formed by dissociation near the ionization limit only 3 OF i of the excited atoms is deactivated in reactive collisions. This gives rise to an increase of the Ly,-radiation of 20-308 if measured in the field [3]. Investigating the dissociation behavior of H2molecules we found excited states which exhibit a much larger effect in the electric field (table 1). These states are mostly near the ionization limit and even above it. In fig. la two different curves are given. They Table 1 Ly+ntensity normalized to the zero field case at five different positions in the spectrum. 3 represents the minimum next to position 2, 5 a position in the ioniza-

tion region

-

State

A&)

‘ZOO/IO

D-X(5,0) : Rl lopqo,o) : Rl minimum D-X(6,0) :Ql ion. region

814.6 811.3 8Ll.8 804.75

L.31 2.67 L .I5 L.21

79X-92

L52

In several preceding papers [l-6] we have 195

Volume 5, number 4

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CHEMICAL PHYSICS LETTERS

$4

,a

npq;Rl

,I2

I12

JO

,n

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Ill

1 April 1970

s-

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Tar

b-

id

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2-

I-

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Fig. 2. LyU-intensity as a iunction of H2-pressure at 0 volt and 100 voli/cm: xvavelen h‘ of primary radiation 811.3 f a) 5'

it to be independent

of the H2-pressure.

This

is a clear indication of the fact that the dissociation occurs at isolated particles only. On account of the low concentration of excited species in the system (105 - 106 quanta absorbed per second)

1x10-Ycrr

only collisions

with unexcited

molecules

have to

be considered. This is therefore experimental proof of a dissociation of the molecule taking place in an electric field. Other experimental results confirm that the excited atoms are produced due to the field and that the effect

Fig. 1. (a) Lye-intensity as a function of primary e.xcitation, taken with a bandwidth of 0.25A. Curve 1: without any field applied: Curve 2: with 200 volt/cm applied. (b) Measured H-i- current as a function of primary excitation (PHI : 0.~ tori-), taken with a bandwidth of o. ZOA.

represent the Lye-fluorescence with (2) and without (1) a field. Positions in the spectrum where a large increase in radiation is observed are clearly visible. As the field effect due to excited atoms is at most 20-300/o at these wave1engths'Lhe much larger increase must be due to excited molecules dissociating under the influ-

ence of the field. In the system

a dissociation

may be caused

by

collisions if the lifetime of the excited molecules is sufficiently long. The lifetimes are not known. But a collision-induced effect has to be pressure dependent. Fig. 2 shows the pressure dependence of the measured radiation for one representative position in the spectrum. The field effect is pronounced only in the low-pressure region showing 196

is caused

by a new coupling

mech-

anism. As is shown in fig. lb there is a formation of Hi-ions in that region of the spectrum -where the field effect occurs. Not taking into account the intensity of the ion peaks which is a function of several parameters, the positions of the peaks are coincident with those where a large field effect upon the Ly,-radiation is measured. Although the coincidence is not complete, at least at all positions with the field effect Hs-ions are formed. This is only true for wavelengths longer than 808A. For shorter wavelengths Hi-formation starts, thus forming Hz-ions by an ion molecule reaction

which has a large

cross

section.

As the

Ri-iormation above that WaVelength iS due to excited molecular states - the cross section for atomic formation is much too low [5] - further evidence is given that the particles causing the field effect are molecules. The pressure dependence of the atomic fluorescence (fig. 2) is a further proof that the absorbing molecular states showing the field effect are rather stable.without a field applied. It can easily be seen from this figure that the region of high fluorescence increase is only represented

Volume 5, number 4

CHEMICAL PHYSICSLETTERS

by the first part of the curve. The second part shows a smaller effect. It should be mentioned that the second part of the curve is a linear function of pressure over a much larger region than shown in the figure. It is obvious that the curve consists of two different parts. One part shows a saturation due to total absorption starting at about 3 X 10m2 torr, whereas the other part is caused by a more continuous absorption having its saturation at much higher pressure. This saturation effect at 3 X 10m2 is an indication for the fact that the absorbing states are represented by narrow absorption lines which probably are not broadened by any configuration interaction. The increase in radiation due to the field at higher pressures as shown in fig. 2 is larger than that to be expected from the action of purely atomic states. This is understandable in view cf the strong overlapping of states in this region of the spectrum [7]. At those positions in the spectralrn where preferentially absorption due to a IHi-state (Q-term) is to be expected, only an increase of about 20% is measured (table 1). These Q-terms

were

always

found

not to be dissociative.

The increase of 20% is the amount which is to be expected for metastable atoms only if formed with this primary energy [3]. In an attempt to make a decision regarding which peaks are predissociating under the influence of the field we find that there is strong evi$en+ce for the fact that the higher members of the C,-Rydberg series in their vibrationless state are mainly responsible for the effect as indicated by fig. 1. There are still other lines but at the position of the C-terms it is most pronounced. It may be concluded from our own measurements [6] as well as from those of other authors [7,9] that in the H2-spectrum an uncoupling mechanism occurs leading from Hunds case (b) to case (d). On the other hand if an electric field is applied the symmetry is destroyed. As an example the lZ&states may couple with the lC;- or the lllg-continuum. As the altered selection rules allow transitions in which the total angular momentum changes by 0 or +l new decay channels are opened. Due to the perturbation by the electric field a mixing of states occurs. The matrix elements are only non-vanishing if the field-free states have opposite parity. As a degeneracy always exists for the mixing states first-order perturbation theory gives an interaction which is linear in field strength as is the case for the hydrogen atom. The dependence on field strength is shown in fig. 3 for different positions where the field effect occurs as compared with the behavior of the

1 April L970

O-X(5.0)

l.0

100

50

150

Rl. ~~l.6r10-zTorr

200

E &‘/cm

1

Fig. 3. Lya-intensity as a function of field strength (volt/cm) for three different positions in the spectrum, normalized to the measured intensity at zero

field.

radiation from the D-X(5.0)-band. The latter state is well known for predissociating without any field thus showing the quenchi= for metastables onI7

[3,61.

We are, dictions:

therefore,

making the folloting

pre-

(1) The molecular fluorescence of the excited HZ-states showing field predissociation will be missing in an electric field because for these states the measured cross sections for both absorption and dissociation are identical if a field is applied. (2) In an electric field the Hs-peaks due tc molecular formation will be quenchable. The extent to which this occurs will certainly be different for the different lines measured because field predissociation is not pronounced at all positions in the spectrum where Hi-formation is found to occur. (3) We believe tbat the extent of ionization by autoionization will be influenced by the appIication of an electric field favouring predissociation. The effect may be small so that the experimental detection may be quite difficult. The authors are grateful to Dr. E. Frenkel for several discussions about the Syminetq CO&~Merations.

REFERENCES t11 I?. J. Comes and H.O.Wellern. (1968) 881. 121F. J. Comes,

2. Naturforsch.

23a

B. Schmitz. H.O. Wellem and U. WenPhysik. Chem. 72 (1968) 986. and LJ.Wenning. Z _ Naturforsch. 24a

ning. Ber. Bunsenges.

[31 F. J. Comes

(1969) 587.

197

Volume 5. number 4 [4] F.J.Comes Chem.,

to

and U.Wenning, be

Ber, Bunsenges. Physik.

published.

[5] F. J. Comes and U.Wenning. 2. Naturforsch. 24a (i969) 1227. [6] F. J-Comes and U.Wenning, to be published.

198

1 April 1970

CHEMICAL PHYSICS LETTERS (71 Y.Tanaka and S.Takezawa,

private communication.

[S]

to

F.J.Comes

and

B.Schmitz,

be

published.

[9] W. A.Chupka and LBerkowitz. J. Chem. Phys. 48 (1968) 5726 and private communication.