International Journal of Mass Spectrometry and Ion Processes, 65 (1985) 231-233 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

231

Short Communication

ENERGY DEPENDENCE OF THE FRAGMENTATION OF THE Z-PHENYLETHANOL MOLECULAR ION

ELVIRA WEGER, KARSTEN LEVSEN

Institut ftir Physikalische Chemie, D 5300 Bonn I (F.R.G.)

JAN ZWINSELMAN and ALEX G. HARRISON

Department of Chemistry University of Toronto, Toronto M5S IA1 (Canada)

(Received 18 December 1984)

The energy dependence of the fragmentation of the n-butylbenzene molec- ular ion by the competing reaction channels

C,H,C,H;’ + C,Hf + C,H; (1)

+ C,H,+’ + C,H, (2)

as elucidated by charge exchange mass spectrometry [l], is in disagreement with the energy dependence as elucidated by photodissociation experiments on a high-energy beam of molecular ions in a sector mass spectrometer [2]. Although this discrepancy has been rationalized in terms of incomplete energy interconversion following photoexcitation [3], the recent detailed study of the photodissociation of n-butylbenzene molecular ions by FTICR techniques [4] provides results in excellent agreement with the charge ex- change data.

Although this recent photodissociation study suggests that the major reason for the discrepancy of the earlier photodissociation data lies in the range of internal energies possessed by the ions subjected to photodissocia- tion, and not to incomplete energy interconversion, it is important to ascertain whether photodissociation studies on high-energy beams in sector instruments can give reliable energy-resolved data. To provide further infor- mation on this matter, we have studied a closely related system, ionized 2-phenylethanol, which also dissociates exclusively to give C,Hq and C,Hl’ fragment ions at low excitation energies. Photodissociation data were ob- tained with a krypton ion laser operated, for intensity reasons, in the broad band mode [5]. The charge exchange mass spectra were determined as described previously [6] using (recombination energy in brackets) COS+’ (11.2 ev), Xe+’ (12.5 ev), N20+’ (12.9 ev), CO+’ (14.0 ev), NT’ (15.3 eV)

0168-1176/85/$03.30 0 1985 Elsevier Science Publishers B.V.

232

LO-

0.9 -

0.8-

:

G 0.7-

5 2 0.6-

s ; 0.5-

?

z O.L-

.g 0.3- z e lL 0.2-

0.1 -

I I I I I I I 1 I 2 3 4 5 6 7

EIeV)

Fig. 1. Breakdown diagram of 2-phenylethanol. 0, A, Charge exchange; 0, A, photodissocia- tion.

and Ar+’ (15.8 eV). The breakdown graphs were constructed using an ionization energy of 8.8 eV for 2-phenylethanol (i.e. the same as ethylben- zene 171).

The results of both the photodissociation and charge exchange experi- ments are shown in Fig. 1. Very similar curve shapes are observed from both experiments, however, the ,photodissociation data are shifted to lower en- ergies by 0.6-0.8 eV. This shift is not unexpected since, in the photodissocia- tion experiments, the molecular ions of 2-phenylethanol are formed by 70 eV electron excitation and thus are likely to possess internal excitation prior to photon impact. The present results show that photodissociation experiments on high-energy beams of ions in sector mass spectrometers can provide reliable energy-resolved data although the results will be shifted on the energy axis by the average internal excitation imparted to the ions in the formation process. In particular, there is no evidence in this system for incomplete energy interconversion prior to photodissociation.

ACKNOWLEDGEMENT

The work at the University of Toronto was supported by the Natural Science and Engineering Research Council of Canada.

233

REFERENCES

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Phys., 37 (1981) 159. (b) I.W. Griffiths, E.-S. Mukhtar, R.E. March, F.M. Harris and J.H. Beynon, Int. J. Mass Spectrom. Ion Phys., 39 (1981) 125.

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