Volume 132, number 2 CHEMICAL PHYSICS LETTERS 5 December I986 PHOTOFRAGMENTATION DYNAMICS OF H,O, AT 193 nm Axe1 Ulrich GRUNEWALD, Karl-Heinz GERICKE and Franz Josef COMES Instltut ftir Physikalische und Theoretische Chemie an der Universitlit Frankfurt am Main, N~ederurseler Hang D-6000 Frankfurt am Main SO, Federal Republic of Germany Received 16 September 1986 The photofragmentation dynamics of Hz02 at 193 nm have been analyzed by probing the OH products by Doppler spectroscopy Using laser-induced fluorescence. The excess energy (Eav = 4 17 kJ/mol) appears mostly as translational mo- tion of the fragments with ft = 0.84. No vibrational excitation was observed cfv < 0.003). The rotational state distribution is strongly inverted, peaking for N” = 12 which results in fr = 0.16. Our results indicate that previously published rotation- al state distributions are not nascent in character, but are perturbed by rotational relaxation. Individual fragment recoil ve- locities obtained from Doppler lineshapes are u = 4950 m s-l [OH(N” = 4)] and v = 4550 m s-l [OH(N” = lo)]. The ef- fective anisotropy parameter Peff is slightly negative at high N” (Jeff = 0.35 at N” = 10). 1. Introduction The tetraatomic H20z is a useful molecule for the study of photofragmentation processes and their dynamics. Photofragmentation of H202 leads to two molecular fragments which are chemically equivalent and which can both be monitored by the same spec- troscopic techniques. Moreover, the spectroscopy of OH radicals is well known so that the internal state distribution of the fragments can be completely es- tablished. The absorption spectrum of H202 in the W is structureless and entirely continuous [l] which in- dicates a fast dissociation process. This makes the study of its photodissociation particularly suitable for extracting fragment vector properties using po- larization and Doppler spectroscopy [2-4]. Photo- fragmentation at wavelengths longer than 172 mn leads to (electronic) ground state OH radicals [ 11. At shorter wavelengths one of the OH fragments is produced in its fust excited electronic state, 2 Z+ , which demonstrates the existence of a new dissocia- tion channel at higher energies. Studies of the photo- dissociation at 157 nm and shorter wavelengths showed that the electronically excited fragments are formed with an inverted rotational state distribution [5]. Moreover, a strong rotational alignment is also observed [6]. At the longer wavelengths used to study H202 photodissociation, the OH fragments were observed using the sensitive LIF technique. Due to the high resolution of the applied dye lasers fine details such as spin state and A state populations could be studied as well as the partitioning of the excess energy into the rotational and vibrational degrees of freedom of the fragments. The application of polarization and Doppler spectroscopy clearly demonstrated that on absorption of radiation at 266 and 248 nm the only excited electronic state is the A lA state of H202 K-1. Using the excitation wavelength of the ArF laser at 193 nm this picture of only one excited electronic state changes. The rotational state distribution was no longer representable by a Boltzmann-like distribu- tion. Instead a bimodal rotational distribution was found indicating the excitation of more than one elec- tronic state [7]. As in the case for the longer excita- tion wavelengths at 248 and 266 run, no vibrational excitation of the fragments was observed and the bulk of the excess energy is transferred into fragment recoil translation. In the present study, the measurement of the pho- tofragmentation of H202 at 193 nm has been re- peated. From the experimental conditions used in the earlier study [7] and from information obtained 121 0 009-26 141861%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)