Theoretical study of the photodetachment spectroscopy of ClH  2 Subhas Ghosal, Susanta Mahapatra * School of Chemistry, University of Hyderabad, P.O. Central University, Hyderabad 500 046, Andhra Pradesh, India Received 2 June 2004; in final form 24 June 2004 Available online 22 July 2004 Abstract The photodetachment spectrum of ClH  2 , probing the van der Waals well region of the coupled multi-sheeted reactive Cl + H 2 potential energy surfaces, is theoretically calculated with the aid of a time-dependent wave packet approach. The theoretical findings are compared with the available experimental results [J. Chem. Phys. 117 (2002) 8181]. The calculated photodetachment spectrum reveals resolved structures that can be attributed to the low-frequency van der Waals progression. The spectral envelopes are 0.1 eV apart in energy (nearly equal to the spin–orbit splitting of 0.109 eV of atomic Cl) in accord with the experiment. The congestion of the spectral lines in the coupled states results mainly arises from the non-adiabatic effects due to spin–orbit coupling. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction The Ôsimplest Cl reactionÕ, Cl+H 2 !HCl+H has been a benchmark for the basic understanding of chem- ical reaction dynamics [1,2]. Recent experimental and theoretical studies on this reaction indicate that the iso- topic variant Cl+HD supports a shallow van der Waals well (0.51 kcal/mol) in the reagent asymptote that has an important role in its reaction dynamics [3]. The greater reactivity of the spin–orbit (SO) excited Cl * ( 2 P 1/2 ) compared to SO ground Cl ( 2 P 3/2 ) as indicat- ed by LiuÕs experiment remains to be a debatable issue until today [3–5]. The approach of an H 2 molecule to the Cl atom splits the threefold degeneracy of the 2 P state. Two electronic states, 1 2 A 0 and 1 2 A 00 (correlating to the 2 R and 2 P species, respectively, in the linear geome- try), correlate adiabatically to the ground state Cl * ( 2 P 3/2 ) atom, while the third state 2 2 A 0 ( 2 P species in the linear geometry) correlates adiabatically to the excited state Cl * ( 2 P 1/2 ) atom. Of these, only the 1 2 A 0 electronic state correlates with the electronic ground state of the products ½HClð ~ X 1 R þ Þþ Hð 2 SÞ. The re- maining two electronic states correlate with the elec- tronically excited state of the product [HCl( 3 P) + H( 2 S)] [3,6–8]. The two 2 A 0 states correlating with the Cl ( 2 P 3/2 ) asymptote form a conical intersection at the linear geometry [8]. The SO coupling of the Cl atom further splits these states; hereafter, we refer to them as 2 R 1/2 , 2 P 3/2 (correlating with the Cl( 2 P 3/2 ) asymptote) and 2 P 1/2 (correlating with the Cl * ( 2 P 1/2 ) asymptote). Remarkable progress has been made over the past decades to resolve the complex electronic structure of the ClH 2 system and to develop global three-dimen- sional potential energy surfaces of its low-lying elec- tronic states [8–10]. The recently developed diabatic surfaces and their non-adiabatic and SO coupling sur- faces by Cappechi and Werner are the most accurate and complete so far [6,7]. The availability of this bench- mark electronic structure result enables us and others to undertake the challenging task of investigating its complex reaction dynamics on coupled multi-sheeted PESs. The reaction Cl + H 2 is slightly endothermic (en- dothermicity 0.045 eV) and proceeds on a late-barrier type of surface (barrier height 0.366 eV) [7]. Unlike the exothermic F + H 2 reaction, which proceeds on an 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.135 * Corresponding author. Fax: +91-40-23012460. E-mail address: smsc@uohyd.ernet.in (S. Mahapatra). www.elsevier.com/locate/cplett Chemical Physics Letters 394 (2004) 207–214