Volume 167, number 6 CHEMICAL PHYSICS LETTERS 13 April 1990 FLUORESCENCE DECAY AND NON-RADIATIVE RELAXATION DYNAMICS OF THE A *Z+ STATES OF OH-Ar AND OD-Ar Sudbir K. KULKARNI, Yaomin LIN and Michael C. HEAVEN Department of Chemistry, Emory University, Atlanta, GA 30322, USA Received 16 January 1990 Fluorescence decay lifetimes of electronically excited OH-Ar and OD-Ar van der Waals complexes have been measured under collision-free conditions. The lifetimes for two vibronic levels of OD(A, a’ =O)-Ar were identical to that of OD(A, v’=I)), indicating that the A-X transition moment was unaffected by interactions between OD(A) and Ar. Lifetimes for two vibronic levels of OH( A, u’= O)-Ar were slightly shorter than that of OH( A, D’ = 0). This has heen interpreted in terms of a weak predis- sociation (OH (A, v’ =O)-Ar+OH(X, v” % 13) + Ar ). Implications ofthese lifetime measurements with regard to the assignment of recently reported LIF and emission spectra for OH/D-Ar are discussed. 1. Introduction Spectra of the A ‘Z’-X 21T transitions of OH-Ar and OD-Ar have been observed in the gas phase [ l- 6 ] and in cryogenic rare-gas matrices [ 731. Laser- induced fluorescence spectra of gas-phase OH/D-Ar showed two distinct vibrational progressions asso- ciated with motion of the Ar atom [ 1,2] (labeled A and U in ref. [ 2 ] 1. Rotationally resolved spectra for bands of the lower energy progression (A system) revealed contours that were consistent with linear- molecule 2E( b)-21T (a) linestrengths [ l-41. This progression, which was also observed for matrix-iso- lated OH/D-Ar [ 71, has been unambiguously as- signed to the OH (D )-Ar stretch mode of the A state. Rotational contours for members of the higher en- ergy progression (U system) were indicative of a ‘Il (b)-*II(a) transition, although the relative in- tersities of the Q, and QP,, transitions were signif- icantly smaller than the values obtained from simple linear molecule linestrength formulae [ 2,9]. The U system was not observed in the matrix studies, or given vibronic assignments in the previous gas-phase work. The lifetime measurements reported here were made, in part, to permit comparison of the fluores- cence decay characteristics of the A and U bands. The idea being that such comparisons could provide sup- porting evidence for or against assignment of the Zl bands to the A 2E+-X ‘Il transition. Further motivation for lifetime measurements originated from the recent work of Lemaire et al. [ 61, who reported an emission spectrum that was attrib- uted to OD-Ar (A-X). In the analysis of this spec- trum they [ 61 suggested that short-range interac- tions between Ar and OD (A) may considerably enhance the OD A-X transition moment. This could occur as a consequence of mixing between the neu- tral OD(A)-Ar and ionic OD--Ar+ states. It has been proposed that the unusual stability of electron- ically excited OH/D-Ar is a result of such mixing [ 71. The transition moment between the ionic and ground states is expected to be large (comparable to that of the analogous ArF B-X transition). Hence, a small admixture of the ionic configuration could influence the OH/D-Ar A-X transition moment. This would be observed as a modification of the OH/ D-Ar A state lifetime, relative to that of free OH/ D. 2. Experithental F’luorescence decay lifetime measurements were made for various vibronic levels of the A 2E+ states of OH, OD, OH-Ar and OD-Ar. A detailed descrip 0009-2614/90/S 03.50 0 Elsevier Science Publishers B.V. (North-Holland ) 597