Volume 146, number 3,4 CHEMICAL PHYSICS LETTERS 6 May 1988 DISSOCIATION OF DOUBLY EXCITED STATES OF H2 INTO RYDBERG ATOMS J.W.J. VERSCHUUR, L.D. NOORDAM, J.H.M. BONNIE and H.B. VAN LINDEN VAN DEN HEUVELL FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJAmsterdam. The Netherlands Received 23 December 1987; in final form 9 February 1988 We report the dissociation of doubly excited states of molecular hydrogen into an excited and a ground-state atom. After three- photon excitations to various vibrational levels in the B ‘Z: state, one- and two-photon excitations result in doubly excited states with a (2~0,) core of both gerade and ungerade symmetry. For the first time, excitation of Q, states with all principal quantum numbers is observed. Dissociation of these states results in a complete Rydberg series of atomic fragments, which are photoion- ized. The energy of the photoelectrons is measured. The series is characterized by the lower states, which are energetically resolved up to n = 5, and the limit n-co of the series. 1. Introduction The study of doubly excited states is a major topic in the molecular dynamics of diatomic molecules. Electronic autoionization and neutral dissociation can have their origins in doubly excited states. Due to this common origin the two processes will com- pete, i.e. the ionization continuum and the neutral dissociation continuum will couple [ 1,2 1. It is clear that the exact position and symmetry of the doubly excited state is of importance for the decay pro- cesses. Glass-Maujean et al. [ 3 ] and Arai et al. [ 41 measured dissociation of H2 by means of fluores- cence of the excited fragments. Electronic autoion- ization results in a non-Franck-Condon contribution to the direct ionization process, as measured by Pratt et al. [ $61 in H2. Recently, Xu et al. [ 71 and O’Halloran et al. [ 81 performed experiments with H2 using the C ‘l-l, state as an intermediate state, in which the dissociation channel was explicitly observed. The combination of a low dissociation limit for the ionic ground state, and low ionization energies of the excited neutral fragments makes molecular hy- drogen a favoured candidate to study the decay mechanisms of doubly excited states. Ionization and dissociation take place in the same energy region. As mentioned, doubly excited states play a major role in the coupling of these two continua. This coupling can for instance be observed in dissociative recom- bination of Hz with slow electrons [ 9, lo]. Another consequence of the fact that both ionization and dis- sociation continua are in the same energy region is that the ionization signal can be used as a reference for the dissociation signal. An additional advantage of the study of dissociating states of Hz by means of multiphoton excitation is that the excited fragment can be ionized by absorbing a single additional pho- ton. Of course molecular hydrogen is also interesting from the theoretical point of view, since it is a sys- tem suitable for detailed calculations. Potential cal- culations of the lowest doubly excited states of Hz have been done by Bottcher [ 111, Hazi [ 121, and more recently by Guberman [ 13 ] and Sato et al, [ 141. The former separated the states into two groups, characterized by the core state; states with a (2~0,) core were labeled Q, states, and states with a (2~7~“) core were labeled QZ states. In this Letter we present a resonantly enhanced multiphoton ionization photoelectron spectroscopy (REMPI PES) study of some of the doubly excited Q1 states in Hz. The excitation is resonantly en- hanced by the vibrational levels of the B 5: state, after absorption of three photons. The emphasis will be on the dissociation channels, which are detected 0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division ) 283