The mechanisms of collision-induced dissociation

Volume 48A, number 6

PHYSICS LETTERS

29 July 1974

THE MECHANISMS OF COLLISION-INDUCED DISSOCIATION S.J. ANDERSON* and LB. SWAN Department of Physics, University of Western Australia, Nedlands, 6009, Australia Received 30 December 1972 The energy distributions of Ht and Dt fragments from collision-induced dissociation of 0.2—2.6 keV Ht and D~ ions reveal that dissociation occurs via polaiisation-induced vibrational-rotational excitation and rotational predissociation, in addition to the well-known electronic excitation.

The most rewarding investigations of collision-induced dissociation are those in which the momentum distribution of the fragment ions is measured, and such experiments have been performed in this laboratory to establish the dominant dissociation mechanism for 0.2—2.6 keY H~and D~ions incident upon inert gas targets. Although one-step processes such as stripping reactions have been postulated [2], moderate and high energy results support the two-step dissociation model. Schopman and Los [3] have observed the dissociation of the (HeH’)~ion via vibrationalrotational excitation produced by polarisation-induced forces [1]; in this case theofhornonuclear polarisation force varies 5, whereas in the case diatomic as R the absence of a permanent dipole moment dicions tates that the relevant polarisation force vary as R6. If the two-step model is adopted, one may transform the observed laboratory-frame (LF) energy spectrum of the fragment ions into the centre-of-momenturn frame (CMF) of the mcident ions. This has been done in the present work, taking into account also the finite bandwidth and finite acceptance angle of the electrostatic energy spectrometer and the inelasticity of the collision [4]. In general, two separate components of the CMF spectra thus obtained are distinguishable, one at W 0.1 eV and another at W 2 eV, where W is the energy liberated in the dissociation of the excited ion. The dependence of the spectra upon the incident ion energy, V 0, is consistently characterised by a reduction in the low-W component and an enhancement of the high-W component as V0 increas-

es (fig. 1). For a particular V0, the relative intensities of the two components vary dramatically with the atomic number (and hence the polarisabiity) of the target atom; thus with helium the low energy feature has all but vanished for V0 > 800 eV, while with xenon it stifi predominates at much higher incident energies (fig. 2). Also shown in fig. 2 is an example of the fine structure observed for both H~and D~with the heavier target atoms. The energies of the fine-structure COUNTS H2

boo iv

1800

W

~

Ec •

10

5

2

2 05 01

01 05

2

5

10

W (iv) *

Now at W.apons Research Establishment, Salisbury, South

Fig. 1. Incident-energy dependence of the CMF distributions

Australia.

of protons from H~collisions with argon.

435

Volume 48A, number 6

PHYSICS LETTERS

29 July 1974

Table 1 Comparison of experimental and Russek-theoretical values of the ratio avi1~rot.:celec at 1600 eV He Experiment

0.03

±100%

Russek theory

0.014

Ne

Ar

Kr

Xe

0.18 ±30%

0.33 ±20%

0.62 ±20%

1.10 ±20%

0.030

0.33

0.62

1.20

COUNTS H~He 400 eV

panied by simultaneous excitation of the target atom [6], yet in the present results the LF energy of the central peak corresponds to 0.5 ±0.5 eV which implies that the target atom remains in itsshould groundcontnstate. Furthermore, impulsive excitation

(Arb. Units) D~-XC 2000 .v

INN

H~.X. I4()O eV

bute symmetrically about W = 0 since, in the Russek theory, it is accounted for by the Born approximation but one observed that the low-W component is enhanced forE < V0 (where Eis the LF energy of the fragment ion). 1~heratios of the cross-sections of the two components are compared with theoretical values in table 1. The reasonable agreement for the heavier target atoms together with the consistent trends observed in the CMF spectra provide compelling evidence in support of the Russek polarisation mechanism of dissociation. This work was supported by the Australian Research Committee. The assistance of Dr. A. Otto in

E>~~ 0

5

2

05 01 0 OP 05

2

5

10

~ (IV)

Fig. 2. Fine structure and the dependence on target gas of the CMF spectra.

the construction of the ion source and the planning of the experiments is gratefully acknowledged. References

peaks appear to increase parabolically, consistent with rotational pre-dissociation from a projectile-target cornplex formed in close encounters, analogous to the resonant state of(HeH)~[3]. The identification of the 2pa~state high-W component with [5] is readily electronic to the method [4], and it can establishedexcitation by a comparative be demonstrated by a similar argument and symmetry considerations that this mechanism is not responsible for the low-W component. Impulsive excitation to the vibrational continuum [5] is expected to be accom-

436

Lii A. Russek, Physica 48 (1970) 165. [2] M.H. Cheng, M. Chiang, E.A. Gislason, B.H. Mahan, C.W. Tsao and A.S. Werner, J. Chem. Phys. 52(1970) 5518. [3] S.J. 1. Schopman J. Los, Pllysica 48 (1970) 190. [4] Anderson,and Ph.D. Thesis (University of Western Australia, 1972). [5] E.E. Salpeter, Proc. Phys. Soc. A63 (1950) 1295. [6] P. Fournier, de doctorat de sp~cia1itI (Orsay, (1969). J. Durup, P. These Fournier and P. D&ng, mt. J. Mass. Spect. ~ Ph~.2 (1969) 311.