9 July 1999 Ž . Chemical Physics Letters 307 1999 385–390 www.elsevier.nlrlocatercplett On the H-atom formation after Lyman-a excitation of CHF Cl 2 Lih-Huey Lai, Yen-Tsung Hsu, Kopin Liu ) ( ) Institute of Atomic and Molecular Sciences IAMS , Academia Sinica, P.O. Box 23-166, Taipei 10764, Taiwan, ROC Received 25 January 1999; in final form 8 March 1999 Abstract Ž . Contrary to what has previously been suggested from an often observed bimodal Doppler profile, three distinct features are actually revealed in the H-fragment speed distribution for the title process via the Doppler-selected time-of-flight measurement. In combination with the results from an additional VUV photoionization measurement, the underlying dissociation pathways are elucidated. An alternative reaction mechanism is proposed, which is subject to future experimental verifications. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction There has been considerable interest in photo- Ž . chemistry of hydrochlorofluorocarbon HCFC and Ž . hydrofluorocarbon HFC compounds in recent years w x 1–7 . This interest arises from the imminent phase Ž . out of ozone-destroying chorofluorocarbons CFCs and their replacement by HCFCs and HFCs, which have been suggested as ‘environmentally friendly’ w x substitutes 8,9 . The absorption spectra of the HCFCs exhibit a broad and featureless band in the 180–240 w x U nm range 10 which is assigned as a n ™ s Cl transition, a nonbonding to an antibonding excitation w x in the C–Cl bond 11 . The main photolytic process is then expected to be the C–Cl bond rupture. At 193 nm the HrCl branching ratio of 0.19 " 0.06 was wx measured 2, which further established a mixed C–H and C–Cl character for the s U antibonding orbital. At shorter wavelengths the Rydberg-type bands start to come into play and the spectra become ) Corresponding author. E-mail: kpliu@gate.sinica.edu.tw w x richer 12,13 . All absorption bands up to ; 120 nm have been assigned as the transition from the chlo- rine lone pair orbital. The presence of the fluorine atom causes a shift to either higher or lower energies of the Rydberg orbitals. One of the often-used technique to elucidate the underlying photodissociation mechanism is the Doppler profile measurement of photofragments, in w x particular the H-atom. In several recent studies 1–6 of VUV photodissociations of methyl halides and HCFCs, etc., it was shown that the observed H-frag- ment Doppler profile could be well represented by either a Gaussian profile or a mixture of a Gaussian and a non-Gaussian ones. In the latter case, two different fragmentation pathways were then invoked in the interpretation of dissociation mechanism, fol- wx lowing the initial suggestion by Tonokura et al. 6. As exemplified in Fig. 1 for a typical H-fragment Doppler profile from the process of CHF Cl q L 2 a Ž . 121.6 nm , it indeed can be represented by a bi- modal profile. A similar Doppler profile has recently been reported for that process by Brownsword et al. wx 1 , and their partition of the observed bimodal pro- 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00548-5